阅读说明：本技术 X射线和伽玛射线光电二极管 (X-ray and gamma-ray photodiodes ) 是由 安娜·梅根·巴内特 西尔维亚·布泰拉 于 2018-02-28 设计创作，主要内容包括：公开一种在检测X射线和/或伽玛射线中使用的光电二极管。所述光电二极管包括InGaP,InGaP被布置和配置为吸收入射到所述光电二极管上的X射线和/或伽玛射线,并且响应于此产生电荷-载流子。所述检测器可以被提供于X射线或伽玛射线光子计数谱仪中。(A photodiode for use in detecting X-rays and/or gamma rays is disclosed. The photodiode includes InGaP arranged and configured to absorb X-rays and/or gamma rays incident on the photodiode and generate charge-carriers in response thereto. The detector may be provided in an X-ray or gamma-ray photon counting spectrometer.)

1. A photodiode for use in detecting X-rays and/or gamma rays, the photodiode comprising InGaP arranged and configured to: absorb X-rays and/or gamma rays incident on the photodiode and generate charge-carriers in response thereto.

2. The photodiode of claim 1 comprising an envelope or shield arranged and configured to block photons other than X-rays and/or gamma rays from reaching the InGaP and/or arranged and configured to block radioactive β particles and/or α particles from reaching the InGaP.

3. The photodiode of claim 2 wherein the housing or shield is a metal, such as a metal foil.

4. The photodiode of any of claims 1, 2, or 3 comprising a PIN junction or a p-n junction formed from the InGaP.

5. The photodiode of claim 4 wherein the i layer of the PIN junction has a thickness of: not less than 5 μm, not less than 10 μm, not less than 15 μm, not less than 20 μm, not less than 25 μm, not less than 30 μm, not less than 35 μm, not less than 40 μm, not less than 45 μm, or not less than 50 μm.

6. The photodiode of claim 4 or 5 wherein the p-layer and/or n-layer of the PIN junction has the following thicknesses: less than or equal to 0.5 μm, less than or equal to 0.4 μm, less than or equal to 0.3 μm, less than or equal to 0.2 μm or less than or equal to 0.1 μm.

7. The photodiode of any one of claims 4, 5 or 6 comprising electrodes on either side of the PIN junction or p-n junction for applying a voltage across the junction and/or for measuring photo-generated charge-carriers generated in the junction, wherein at least one of the electrodes does not cover a portion of the one side of the junction where it is located, such that X-rays and/or gamma rays can enter the junction through the one side without passing through the at least one electrode.

8. The photodiode of claim 7 wherein the at least one electrode is annular, apertured, recessed or staggered so as not to cover the entire one side and to allow the X-rays and/or gamma rays to enter the junction without passing through the material forming the electrode.

9. The photodiode of any one of claims 4-8 comprising a voltage source configured and arranged to: and applying a reverse bias voltage on the PIN or the p-n junction, wherein the voltage is more than or equal to 2V, more than or equal to 3V, more than or equal to 4V, more than or equal to 4.5V or more than or equal to 5V.

10. The photodiode of any preceding claim, wherein the InGaP is a crystal structure having a lattice composition in0.5ga0.5p.

11. A photodiode according to any preceding claim, wherein the photodiode has a mesa diode structure.

12. The photodiode of any of claims 1-10 wherein the InGaP material is provided within a substantially planar structure, and wherein the planar structure comprises Schottky contacts, and/or wherein ions have been implanted within the device to form resistive regions.

13. An X-ray and/or gamma-ray detector comprising a photodiode according to any preceding claim and comprising electronics for processing electrical signals generated by the photodiode to determine whether the electrical signals are at least partially due to the generation of photo-generated charge-carriers.

14. An X-ray and/or gamma-ray photon counting spectrometer comprising a detector according to claim 13 and comprising a processor arranged and configured to: determining from the electrical signal the energy of individual X-ray and/or gamma ray photons detected by the detector and/or determining from the signal the number of X-ray and/or gamma ray photons detected by the detector.

15. A system, comprising: a source of X-rays and/or gamma rays; and a photodiode, detector or spectrometer according to any preceding claim for detecting the X-rays and/or gamma-rays from the source. A nuclear battery or radioisotope battery, comprising: the source of X-rays and/or gamma rays; and the photodiode, is used for changing the said X-ray and/or gamma ray into the electric current.

16. The system of claim 15, wherein the system is a nuclear battery or a radioisotope battery comprising: the source of X-rays and/or gamma rays; and the photodiode, is used for changing the said X-ray and/or gamma ray into the electric current.

17. The system of claim 16, wherein the source is a radioactive material.

18. The system of claim 16 or 17, wherein the source and photodiode are housed within a housing, the housing optionally being arranged and configured to: the X-rays and/or gamma rays from the source are substantially prevented from exiting the housing.

19. A method of detecting X-rays and/or gamma rays, comprising:

exposing the X-ray and/or gamma-ray detector of claim 13 to a source of X-rays and/or gamma-rays; and

determining from the signal generated by the photodiode that X-rays and/or gamma rays have been received at the detector.

20. The method of claim 19 preceded by the steps of: the source of X-rays and/or gamma rays is selected and the detector is moved towards the source.

21. The method of claim 19, further comprising: detecting the presence and/or location of the source of X-rays and/or gamma-rays using the detector.

22. A method of counting X-ray and/or gamma ray photons, comprising:

exposing an X-ray and/or gamma ray spectrometer according to claim 14 to a source of X-rays and/or gamma rays; and

based on the electrical signals generated by the photodiodes, the energy of individual X-ray and/or gamma-ray photons detected by the detector is determined from the electrical signals, and/or the number of X-ray and/or gamma-ray photons that have been detected at the detector is determined.

23. A photon or particle detection apparatus comprising:

a diode comprising InGaP arranged and configured to: absorbing and/or interacting with the photons or particles and generating charge-carriers in response thereto; and

electronics to process an electrical signal generated by the diode to determine whether the electrical signal is due, at least in part, to the generation of the charge-carriers.

24. The instrument of claim 23, further comprising: a user interface, such as a display or speaker; and a controller for controlling the interface to notify a user that the photon or particle has been detected.

25. A photon or particle counting spectrometer comprising an instrument according to claim 23 or 24 and comprising a processor arranged and configured to: determining from the electrical signal the energy of individual photons or particles detected by the instrument and/or determining from the signal the number of photons or particles detected by the instrument.

26. A system, comprising: a source of photons or particles; and an instrument or spectrometer according to claim 23, 24 or 25 for detecting photons or particles from said source.

27. A method of detecting photons or particles, comprising:

exposing the instrument of claim 23 to a source of the photons or particles; and

determining from the signal generated by the diode that a photon or particle has been received at the instrument.

28. A method of counting photons or particles, comprising:

exposing the spectrometer of claim 25 to a source of photons or particles; and

determining from the electrical signal, based on the electrical signal generated by the diode, the energy of individual photons or particles detected by the instrument, and/or the number of photons or particles that have been detected at the instrument.

29. A nuclear or radioisotope battery, comprising:

a converter for absorbing or interacting with X-rays and/or gamma rays and generating other types of photons or particles in response thereto; and

a diode comprising InGaP arranged to receive the photons or particles and convert them to an electrical current.

Technical Field

The present invention relates generally to photodiodes for detecting photons and, in particular, to photodiodes having improved performance.

Background

Photodiodes are commonly used as detectors of X-rays, gamma rays (gamma rays) and other radiation. Most photodiodes for X-ray and gamma-ray applications are made of silicon. In recent years, much effort has been expended in attempting to develop alternative materials, other than silicon, that can operate at high temperatures in strongly radiant environments and are more efficient (i.e., detect a greater proportion of incident radiation). Many materials have been studied, including GaAs, AlGaAs, AlInP, SiC, CdTe, and CdZnTe, among some of the most promising materials. However, it has been found that these materials all suffer from significant problems, limitations or compromises. For example, while SiC is the most mature "modern" material for these photodiodes, and it can operate in relatively high temperature and strong radiation environments, it is not as efficient as silicon.

Photodiodes are expected to be used, for example, in X-ray and gamma-ray photon counting spectrometers. These spectrometers are required for a number of applications in science and industry. By way of example, there is currently significant commercial pressure to develop detectors for inhibiting contraband materials (e.g., nuclear weapons, "dirty bombs," etc.). However, the requirements of these photon counting spectrometers place extreme demands on the materials used in photodiodes.

It is desirable to provide an improved photodiode.

Disclosure of Invention

The present invention provides a photodiode for use in detecting X-rays and/or gamma rays, the photodiode comprising InGaP arranged and configured to: absorb X-rays and/or gamma rays incident on the photodiode and generate charge-carriers in response thereto.

The inventors have realized that the use of InGaP (also known as GaInP) as the active material in a photodiode enables the photodiode to be used for the detection of X-rays and gamma-rays. This is particularly surprising because InGaP is a ternary compound of InP and GaP that is known to perform relatively poorly as an active material in both X-ray and gamma-ray photodiodes. For example, GaP has previously been found to be not spectral at X-ray energy, and InP has previously been found to be spectral only at low temperatures (≦ -60℃) at X-ray energy. Since InGaP is a ternary compound of InP and GaP, InGaP would traditionally be expected to have even worse performance than either of its binary equivalents.

The use of InGaP photodiodes is particularly advantageous in X-ray spectroscopy and photon counting X-ray spectroscopy. InGaP detectors have been found to perform significantly better than the corresponding binary compounds GaP and InP, and have been found to have sufficiently high energy resolution to allow photon counting X-ray spectroscopy at room temperature (which is not true for GaP and InP based devices).

In addition, InGaP has low leakage current, and thus, photodiodes according to embodiments herein may operate at room temperature and above (i.e., > 20℃.), without utilizing a cooling system. This results in X-ray and gamma-ray detection systems having relatively low mass, volume and power requirements. The system may thus be relatively inexpensive, compact and temperature resistant, and is therefore particularly useful in applications such as, for example, space missions outside a laboratory environment or land applications (e.g. underwater exploration). Instruments that include a cooling system to allow the photodiode to operate in very high temperature environments are also contemplated. Due to the use of InGaP, for example, the amount of cooling required will be zero or relatively low compared to silicon-based detectors.

In contrast to other conventional aluminum-containing semiconductor materials (e.g., AlInP), the use of InGaP in photodetectors is advantageous because it enables the elimination of aluminum from the detector. This is advantageous because aluminum, like silicon, is a material of frequent interest in planetary and geological X-ray fluorescence spectroscopy (XRF), and therefore, it is desirable to eliminate it from the detector to reduce the complexity of the spectral analysis by removing these lines from the autofluorescence of the detector.

Due to InGaP (e.g. In) compared to some other wide band gap materials (e.g. GaAs, AlGaAs, and AlInP)_0.5Ga_0.5P) can be produced relatively high due to the reduced volume of the semiconductor material as well as the wide band gapThin InGaP detectors and improved high temperature performance can be achieved.

InGaP photodiodes also provide an alternative to GaAs, CdTe, and CdZnTe for hard X-ray and gamma detection.

In addition, InGaP may be lattice matched to commercial substrates (e.g., GaAs substrates) and may be processed using common commercial techniques.

Absorption of X-rays and/or gamma rays by InGaP materials photo-generated electron-hole pairs.

The photodiode may include a housing or shield arranged and configured to block photons other than X-rays and/or gamma rays from reaching InGaP and/or arranged and configured to block radioactive β particles and/or α particles from reaching InGaP.

The housing or shield may be configured to: substantially blocking the passage of photons having a lower frequency than X-rays or gamma rays. For example, the housing or shield may be arranged and configured to: blocking blue light from reaching InGaP.

The housing or shield may be a metal (e.g., a metal foil). For example, the foil may be an aluminum foil or a beryllium foil. However, it is contemplated that other metals or materials may be used.

The photodiode may include a PIN junction or a p-n junction formed from the InGaP.

PIN (e.g. p)⁺-i-n⁺) The junction may be formed from an undoped layer of InGaP disposed directly between a p-doped layer of InGaP and an n-doped layer of InGaP.

The inclusion of an intrinsic i layer in a PIN junction may serve to provide a relatively large depletion region, and hence a relatively large volume, to absorb X-rays and/or gamma rays and generate charge-carriers.

However, it is also contemplated that InGaP may be provided in the form of a p-n junction. The p-n junction may be formed by contacting a p-doped layer of InGaP with an n-doped layer of InGaP.

The i-layer of the PIN junction may have the following thickness: not less than 5 μm, not less than 10 μm, not less than 15 μm, not less than 20 μm, not less than 25 μm, not less than 30 μm, not less than 35 μm, not less than 40 μm, not less than 45 μm, or not less than 50 μm. Providing this relatively thick i-layer enables a relatively high proportion of incident X-rays and/or gamma rays to be absorbed, thereby generating a relatively high number of charge-carriers, and the photodiode is relatively efficient in generating electrical current from the X-rays and/or gamma rays.

The p-layer and/or n-layer of the PIN junction may have the following thicknesses: less than or equal to 0.5 μm, less than or equal to 0.4 μm, less than or equal to 0.3 μm, less than or equal to 0.2 μm or less than or equal to 0.1 μm. Providing these relatively thin layers limits the absorption of X-rays and/or gamma rays by these layers. For example, if a p-layer (or n-layer) is disposed on the side of the junction facing the source of X-rays and/or gamma rays, then using a relatively thin p-layer (or n-layer) will absorb a relatively small proportion of the photons, especially for lower energy photons, so that a greater proportion of incident X-rays and/or gamma rays can enter the i-layer of the PIN junction to be absorbed therein, and generate charge-carriers (although it is contemplated herein that portions of the p-layer may also contribute effectively to generating useful charge-carriers). If an n-layer (or p-layer) is arranged on the side of the junction remote from the source of the X-rays and/or gamma rays, then using a relatively thin n-layer (or p-layer) will reduce photon absorption and hence charge collection from that layer. This may for example improve the spectral performance of the instrument.

The photodiode may comprise electrodes on either side of the PIN junction or the p-n junction for applying a voltage across the junction and/or for measuring photo-generated charge-carriers generated in the junction, wherein at least one of the electrodes does not cover a portion of one side of the junction where it is located, such that X-rays and/or gamma rays may enter the junction through said one side without passing through said at least one electrode.

The at least one electrode may be annular, apertured, recessed or staggered so as not to cover the entire one side and to allow the X-rays and/or gamma rays to enter the junction without passing through the material forming the electrode.

The photodiode may include a voltage source configured and arranged to: and applying a reverse bias voltage on the PIN or the p-n junction, wherein the voltage is more than or equal to 2V, more than or equal to 3V, more than or equal to 4V, more than or equal to 4.5V or more than or equal to 5V. Providing this reverse bias voltage can provide a relatively large depletion depth because it sweeps free charge-carriers out of the i-layer. This may also provide relatively low noise for certain noise sources and enable relatively high resolution to be achieved, for example, when the photodiode is used in an X-ray and/or gamma-ray spectrometer. However, it is also contemplated that no reverse bias may be applied.

InGaP may be In having a lattice composition_0.5Ga_0.5The crystal structure of P. These compositions of InGaP enable the material to be lattice matched to and grown on a conventional substrate (e.g., GaAs). These ratios also enable InGaP to be grown with high crystalline quality and/or to a relatively large thickness. This is particularly useful because, as noted above, it may be desirable to provide a relatively thick layer of InGaP to increase the probability that any given X-ray or gamma-ray photon will be absorbed in the material.

However, it is contemplated that the InGaP material may be In with a lattice composition_xGa_1-xP, wherein x is a value other than 0.5.

InGaP may have been formed on a substrate, desirably a semiconductor substrate (e.g., GaAs).

The photodiode may have a mesa diode structure.

The photodiode may include one or more layers for forming electrical contacts on each side of the InGaP material. For example, the one or more layers may include at least one metal layer and/or at least one semiconductor layer. Examples of metal layers include gold and titanium. Examples of the semiconductor layer include inp.

A bonding layer may be provided between the one or more layers used to form the electrical contact and the InGaP to enable good bonding of the electrical contact to the InGaP layer. The bonding layer may be a semiconductor (e.g., GaAs). One of the bonding layers may be a substrate forming InGaP.

The InGaP material may be provided in a substantially planar structure and the planar structure may include Schottky contacts and/or ions may have been implanted into the device to form resistive regions.

Ions may be implanted to form resistive regions that electrically isolate portions of the semiconductor wafer. For example, if multiple diode structures are provided, ions may be implanted to form a region that prevents the voltage on one diode structure from spreading to an adjacent diode.

The photodiode may be an avalanche diode or a non-avalanche diode.

The photodiode is ideally a single crystal InGaP detector, i.e. InGaP is ideally in the form of a single lattice of InGaP. However, it is also contemplated that the InGaP material may be polycrystalline, i.e., include many crystals in the material that may have random orientations.

A photodiode may include a heterostructure formed of layers or regions of different semiconductors, and thus include a heterojunction. As described above, InGaP materials absorb X-rays and/or photons to generate charge carriers, but may also include different semiconductor layers for other functions. For example, a layer of another semiconductor (e.g., AlInP) may be included to receive charge-carriers from the InGaP material and generate secondary charge carriers, e.g., to expand the number of charge-carriers and form an avalanche photodiode. However, it is also contemplated that the InGaP photodiode may include one or more homojunctions, thereby providing an avalanche effect.

The present invention also provides an X-ray and/or gamma ray detector comprising a photodiode as described herein; and electronic means for processing the electrical signal generated by the photodiode to determine whether the electrical signal is at least partially attributable to the generation of the photo-generated charge-carriers.

The photodiode forms part of the circuitry in the detector and the electronics can be arranged and calibrated to detect the signal resulting from the generation of said charge-carriers. For example, in a current measurement mode, the portion of the signal above the dark current may be determined to be due to X-rays and/or gamma rays being received at the detector.

The present invention also provides an X-ray and/or gamma ray photon counting spectrometer comprising a detector as described herein; and comprising a processor arranged and configured to: determining from the electrical signal the energy of individual X-ray and/or gamma ray photons detected by the detector and/or determining from the signal the number of X-ray and/or gamma ray photons detected by the detector.

For photon counting spectroscopy, the detector may be connected to a chain of electronics, which typically includes a charge sensitive preamplifier, a shaped amplifier, a multichannel analyzer, and a computer. When an X-ray or gamma ray photon is absorbed by an InGaP material, some charge (multiple electrons and holes) is generated in the material. The amount of charge generated is proportional to the energy of the photons. The charge is swept to the contacts of the detector and during their travel their motion induces a charge on the contacts of the detector. One of the contacts is connected to a charge sensitive preamplifier so that the charge sensitive preamplifier sees charge at its input, which is converted to a voltage tail pulse proportional to the charge it receives. The shaping amplifier takes the trailing pulse (which has a fast rise and a slow fall) and changes the shape of the pulse so that it is more easily measured by the multi-channel analyzer. A multi-channel analyzer takes the output of the shaping amplifier and measures the height of the pulse (which is proportional to the energy of the photon) and constructs a histogram that includes the pulse and other pulses it receives, thus producing a spectrum that can be viewed on a computer.

The present invention also provides a system comprising: a source of X-rays and/or gamma rays; and a photodiode, detector or spectrometer as described herein for detecting X-rays and/or gamma rays from the source.

The source of X-rays and/or gamma rays referred to herein may be the primary source (e.g., radioactive source) of X-rays and/or gamma rays. Alternatively, the source of X-rays and/or gamma rays referred to herein may be a source of fluorescent X-rays and/or gamma rays. Embodiments of the present invention thus extend to X-ray and/or gamma-ray spectroscopy and X-ray and/or gamma-ray fluorescence spectroscopy of photons directly from a primary source.

The system may be a nuclear battery or a radioisotope battery, comprising: the source of X-rays and/or gamma rays; and the photodiode, is used for changing the said X-ray and/or gamma ray into the electric current.

The photodiode is configured to: operation is performed in photovoltaic mode.

The nuclear battery or radioisotope battery may be a nuclear microbattery or radioisotope microbattery.

The cell may be configured such that most or substantially all of the generated current (i.e., not the dark current) comes from the photodiodes that convert X-rays and/or gamma rays from the source into current, e.g., rather than from α particles and β particles.

The source may be a radioactive material.

The source and photodiode may be housed within a housing, the housing optionally being arranged and configured to: the X-rays and/or gamma rays from the source are substantially prevented from exiting the housing.

While the source of X-rays and/or gamma rays has been described as being within the same housing as the photodiodes, it is alternatively contemplated that one or more of the sources may not be located in a housing with photodiodes. For example, the photodiode may clear X-rays and/or gamma rays from the environment in which the photodiode is located. The following examples are contemplated: where the source is nuclear waste that emits X-rays and/or gamma rays, and the photodiodes in the cells convert these rays to electricity.

The photodiode may have any of the features described elsewhere herein. However, instead of reverse biasing, a zero or forward bias may be applied to the PIN or p-n junction.

The present invention also provides a method of detecting X-rays or gamma rays, comprising:

exposing an X-ray and/or gamma-ray detector described herein to a source of X-rays and/or gamma-rays; and

determining from the signal generated by the photodiode that X-rays and/or gamma rays have been received at the detector.

These method steps may be preceded by the steps of: the source of X-rays and/or gamma rays is selected and the detector is moved towards the source. For example, the method may be used to specifically analyze a particular source that has been selected.

The method may further comprise: the presence and/or location of the source of X-rays and/or gamma rays is detected using a detector. For example, the method may be used to detect nuclear or radioactive materials.

The present invention also provides a method of counting X-ray and/or gamma ray photons, comprising:

exposing an X-ray and/or gamma ray spectrometer described herein to a source of X-rays and/or gamma rays; and

based on the electrical signal generated by the photodiode, determining from the electrical signal the energy of individual X-ray and/or gamma-ray photons detected by the detector and/or determining the number of X-ray and/or gamma-ray photons that have been detected at the detector.

The various devices and methods described herein may be used in a wide variety of fields, including scientific research, medicine, defense, safety, food processing, aerospace, and the like.

The photodiodes described herein may be used in a detector in X-ray spectrometry/spectrometry (e.g., X-ray fluorescence spectrometry/spectrometry).

Although photodiodes, detectors, spectrometers, systems and methods have been described for detecting X-rays and/or gamma rays and/or for use with sources of such X-rays and/or gamma rays, it is contemplated that the instrument or method may detect other types of photons or detect particles (e.g., electrons, ions, α particles or β particles).

Accordingly, the present invention also provides a photon or particle detection apparatus comprising:

a diode comprising InGaP arranged and configured to: absorbing and/or interacting with the photons or particles and generating charge-carriers in response thereto; and

electronics to process an electrical signal generated by the diode to determine whether the electrical signal is due, at least in part, to the generation of the charge-carriers.

The diode may have the features of the photodiodes described herein, except that the diode may interact with particles or photons to generate charge carriers.

The apparatus may comprise: a user interface, such as a display or speaker; and a controller for controlling the interface to notify a user that the photon or particle has been detected.

The invention also provides a photon or particle counting spectrometer comprising an instrument as described herein; and comprising a processor arranged and configured to: determining from the electrical signal the energy of individual photons or particles detected by the instrument and/or determining from the signal the number of photons or particles detected by the instrument.

The spectrometer may output the energy or amount to the user interface.

The present invention also provides a system comprising: a photon or particle source; and an instrument or spectrometer as described herein for detecting photons or particles from the source.

The present invention also provides a method of detecting photons or particles, comprising:

exposing an instrument described herein to a source of said photons or particles; and

determining photons or particles that have been received at the instrument from the signal generated by the diode.

The method may include: the output is sent to an electronic user interface (e.g., a display or speaker) to inform the user that the photon or particle has been detected.

The invention also provides a method for counting photons or particles, comprising the following steps:

exposing the spectrometer described herein to a source of photons or particles; and

based on the electrical signal generated by the diode, determining from the electrical signal the energy of individual photons or particles detected by the instrument, and/or determining the number of photons or particles that have been detected at the instrument.

Although nuclear cells have been described that use photodiodes to convert X-rays and/or gamma rays directly to electrical power (via InGaP), it is contemplated that the cells may first convert X-rays and/or gamma rays to other types of photons or particles, and those other photons or particles may impinge on the InGaP material to generate charge carriers.

Accordingly, the present invention also provides a nuclear battery or radioisotope battery comprising:

a converter for absorbing or interacting with X-rays and/or gamma rays and generating other types of photons or particles in response thereto; and

a diode comprising InGaP arranged to receive the photons or particles and convert them to an electrical current.

The battery may include a source of X-rays and/or gamma rays, for example, in the same housing as the diodes, or alternatively receive X-rays and/or gamma rays from the local environment.

Drawings

Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings, in which:

1A-1B illustrate how the current generated by two differently sized photodiodes varies as a function of applied reverse bias;

2A-2B illustrate how the capacitance of two differently sized photodiodes varies as a function of applied reverse bias;

3A-3B illustrate how the depletion depths of two differently sized photodiodes vary as a function of applied reverse bias;

FIG. 4 shows how the doping concentration in a photodiode varies as a function of depletion depth;

5A-5B show X-ray spectra obtained from two differently sized photodiodes;

6A-6B illustrate parallel white noise, serial white noise, and 1/f noise as a function of applied reverse bias for two differently sized photodiodes;

FIG. 7 shows the equivalent noise charge as a function of applied reverse bias for two differently sized photodiodes; and

figures 8A-8B illustrate how the detection efficiency of a PIN structure with layers of different thicknesses varies as a function of the energy of photons incident on the structure.

Detailed Description

Exemplary embodiments of InGaP photodiodes will now be described, by way of example only, to assist in understanding the present invention. It is to be understood that the invention is not limited to the specific construction of all or various layers described including in this example.

Referring to table 1 below, an exemplary photodiode includes nine layers as shown. At n⁺In growth by Metal Organic Vapor Phase Epitaxy (MOVPE) on doped GaAs substrate (layer 7 In table 1)_0.5Ga_0.5P wafer (layers 4-6 in table 1), thereby forming P⁺-i-n⁺And (5) structure. In is grown on GaAs substrate In sequence_0.5Ga_0.5P layers of wafer, thereby forming a wafer having a 2 x 10 thickness¹⁸cm^-3And In of a thickness of 0.1 μm_0.5Ga_0.5Silicon doped n of P⁺Layer (layer 6 In Table 1), followed by In with no doping and a thickness of 5 μm_0.5Ga_0.5Intrinsic layer of P (layer 5 in Table 1), followed by a layer with 2X 10¹⁸cm^-3And In of a thickness of 0.2 μm_0.5Ga_0.5Zinc P of P⁺Doped layer (layer 4 in table 1). In_0.5Ga_0.5P of P⁺On top of the doped layer (layer 4 in table 1) grown with 1 × 10¹⁹cm^-3And a high doping concentration of 0.01 μm of GaAs with zinc p⁺Layer (layer 3 in table 1) was doped to promote ohmic contact.

Chemical wet etching techniques were then used to etch the layers to produce circular mesa photodiodes with diameters of 200 μm and 400 μm. K₂Cr₂O₇:HBr:CH₃The 1:1:1 solution of COOH was followed by H₂SO₄:H₂O₂:H₂O in a 1:8:80 solution for 10 seconds to etch a mesa photodiode. In_0.5Ga_0.5The P device is unpassivated.

An ohmic top contact was formed on the highly doped layer of GaAs (layer 3 in table 1) by depositing a layer of gold (layer 2 in table 1) having a thickness of 0.2 μm and then depositing a layer of titanium (layer 1 in table 1) having a thickness of 0.02 μm. The top ohmic contact has an annular shape allowing photons to pass through the aperture in the ohmic contact. The annular ohmic contacts cover 33% and 45% of the top surface in 400 μm and 200 μm diameter photodiodes, respectively. An ohmic back contact was formed on the back of the GaAs substrate (layer 7 in table 1) by depositing a layer of ingre (layer 8 in table 1) having a thickness of 0.02 μm and then depositing a layer of gold (layer 9 in table 1) having a thickness of 0.2 μm.

TABLE 1

For the area of the photodiode not covered by the top annular ohmic contact, the X-ray Quantum Efficiency (QE) was determined to be 53% and 44% at energies of 5.9keV and 6.49keV, respectively (calculated using Beer-Lambert's law and assuming complete charge collection in the p-layer and i-layer). This is reduced to 44% and 38% for the area covered by the annular top contact, respectively. The linear attenuation coefficients used in the QE calculation were 0.132 μm at 5.9keV and 6.49keV, respectively^-1And 0.102 μm^-1. These values are higher than other semiconductors (e.g. Si, GaAs and Al)_0.52In_0.48P) is higher.

The electrical characteristics of 200 μm and 400 μm diameter InGaP photodiodes were investigated.

Under dark conditions (i.e., without an X-ray or gamma-ray source) and under⁵⁵The current generated by InGaP photodiodes as a function of the reverse bias applied across the photodiodes was studied under irradiation with Fe radioisotope X-rays (Mn K α ═ 5.9keV, Mn K β ═ 6.49keV)<5%) of the photodiodes. A Keithley6487 picometer/voltage source was used during the experiment. The uncertainty associated with the current reading is 0.3% of its value plus 400fA, while the uncertainty associated with the applied bias voltage is 0.1% of its value plus 1 mV.

Fig. 1A shows the current curve as a function of reverse bias for a 200 μm diameter photodiode. The lower graph formed by the blank squares shows the current as a function of reverse bias when the photodiode is in a dark condition, while the upper graph formed by the filled squares shows the current as a function of reverse bias when the photodiode is illuminated by an X-ray source. Fig. 1B shows the current curve as a function of reverse bias for a 400 μm diameter photodiode. The lower graph formed by the blank squares shows the current as a function of reverse bias when the photodiode is in a dark condition, while the upper graph formed by the filled squares shows the current as a function of reverse bias when the photodiode is illuminated by an X-ray source. As can be seen from FIGS. 1A-1B, the dark current values for the photodiodes of both diameters are less than 0.22pA (corresponding to 6.7X 10 for 200 μm and 400 μm diameter devices, respectively^-10A/cm²And 1.7X 10^-10A/cm²Current density of). For 200 μm and 400 μm diameter photodiodes, illumination currents of 3.5pA and 7pA, respectively, were observed at a reverse bias of 30V. By subtracting the dark current from the illumination current, it was determined that photocurrents of 3.3pA and 6.5pA were generated at 30V reverse bias for 200 μm and 400 μm diameter devices, respectively.

The capacitance of the photodiode as a function of reverse bias was also studied from 0V to 30V using an HP 4275A multi-frequency LCR meter. The test signal is a sinusoid with a magnitude of 50mV rms and a frequency of 1 MHz. The uncertainty associated with each capacitance reading was-0.12% plus experimental repeatability error (+ -0.07 pF). The uncertainty associated with the applied bias voltage is 0.1% plus 1mV of its value. The capacitance of the same blank package is also measured and subtracted from the measured capacitance of each packaged photodiode to determine the capacitance of the device itself.

Fig. 2A shows the capacitance as a function of applied reverse bias for a 200 μm diameter photodiode, while fig. 2B shows the capacitance as a function of applied reverse bias for a 400 μm diameter photodiode.

Then according to W ═ epsilon₀ε_rA) Calculating the depletion depth (W) of each photodiode, wherein ∈₀Is the permittivity of a vacuum,. epsilon_rIs In_0.5Ga_0.5P dielectric constant, a is the device area, and C is the capacitance.

Fig. 3A shows the depletion depth as a function of applied reverse bias for a 200 μm diameter photodiode, while fig. 2B shows the depletion depth as a function of applied reverse bias for a 400 μm diameter photodiode. From

3A-3B, at low reverse bias, the depletion depth increases as the reverse bias increases. Above a reverse bias voltage of about 5V, the depletion depth remains almost constant as the reverse bias increases (due to the i-layer in the PIN structure completely sweeping away charge carriers at these biases). Depletion depths of 4.0 μm 0.5 μm and 4.6 μm 0.2 μm were measured for 200 μm and 400 μm diameter devices, respectively, at a reverse bias voltage of 30V.

P as a function of depletion depth (W) is calculated according to^+-Doping concentration under the i-junction (i.e., the junction between layers 4 and 5 in table 1 above):

wherein q is the charge,. epsilon₀Is the permittivity of a vacuum,. epsilon_rIs In_0.5Ga_0.5P relative permittivity, a is the device area, and C is the capacitance.

FIG. 4 shows In for a diameter of 400 μm_0.5Ga_0.5P device obtained doping concentration n (w).

A (non-avalanche) mesa photodiode with a diameter of 200 μm and 400 μm was connected to low noise, charge sensitive preamplifier electronics to implement an X-ray spectrometer. By way of example, for a 200 μm photodiode operating at reverse bias above 5V, the instrument has the capability of system energy resolution of 900eV at 5.9 keV.

Collection of the effluent from the reactor was performed using 200 μm and 400 μm diameter devices⁵⁵Fe radioisotopeAn X-ray spectrum of the X-ray source. The distance between the top surface of the InGaP photodiode and the X-ray source was 6 mm. A low noise, charge sensitive preamplifier with a feedback resistorless design (similar to that in Bertuccio, p.rehak, and d.xi, nuclear. instrum. meth. phys. res.a326,71 (1993)) is connected to each InGaP diode. The signal from the preamplifier was amplified and shaped using an Ortec 572a shaping amplifier whose output was connected to an Ortec Easy-MCA-8K multichannel analyzer. The spectra were accumulated by reverse-biased diodes at 0V, 5V, 10V and 15V. A 10 mus shaping time and 100 second lifetime limit for each spectrum was used.

Fig. 5A and 5B show the X-ray spectra obtained by reverse bias of 5V for 200 μm and 400 μm diameter devices, respectively. In each spectrum, observed⁵⁵The Fe photopeaks are combinations of Mn K α and Mn K β lines at 5.9keV and 6.49keV, respectively, consider appropriate ratios at 5.9keV and 6.49keV⁵⁵Relative X-ray emission rates of Fe radioisotope X-ray sources [ U.S. Shotzig, Applied Radiation and Isotips 53, 469(2000)]And the relative difference in efficiency of the detector at these X-ray energies, a gaussian fit is the combined peak.

The InGaP spectrometer energy resolution was studied as a function of detector reverse bias, as quantified by FWHM at 5.9 keV. At a reverse bias of 0V, the FWHM at 5.9keV is the worst obtained (FWHM at 5.9keV of 1keV and 1.4keV were observed for 200 μm and 400 μm diameter devices, respectively). This is due to the increased contribution of reduced incomplete charge collection noise at higher reverse bias. With further increasing reverse bias in the range studied, at reverse bias of 5V and above, the charge collection efficiency increased (incomplete charge collection noise decreased) and the FWHM at 5.9keV improved and remained constant. At reverse bias of 5V, FWHM at 5.9keV of 0.9keV and 1.2keV were observed for 200 μm and 400 μm diameter devices, respectively.

Noise analysis is performed to identify the contributions of different noises to FWHM broadening. The spectral resolution of a non-avalanche photodiode X-ray spectrometer is given by:

where Δ E is FWHM, ω is electron-hole pair generation energy, F is Fano factor, E is the energy of the absorbed X-ray photon, R and A are electronic noise and incomplete charge collection noise, respectively [ G.Lioliou and A.M.Barnett, Nucl.Instrum.meth.Phys.Res.A 801, 63(2015)]. Assume In of 4.8eV_0.5Ga_0.5P electron-hole pairs generate energy (2.5 times the band gap) and Fano factor of 0.12, In_0.5Ga_0.5The basic "Fano limited" energy resolution of P (i.e., R0 and a 0) is estimated to be 137 eV. This noise contribution takes into account the statistical properties of the ionization process in the semiconductor X-ray detector. Since the measured FWHM is greater than 137eV, contributions from other noise sources need to be considered. The electronic noise of the system includes parallel white noise, serial white noise, induced gate current noise, 1/f noise, and dielectric noise. The leakage current of the input JFET of the detector and the preamplifier is a driving factor of the parallel white noise, and the capacitance of the input JFET of the detector and the preamplifier determines the serial white noise and the 1/f noise. The serial white noise is adjusted for induced gate current noise.

FIGS. 6A and 6B show parallel white noise, series white noise, and 1/f noise calculated as a function of detector reverse bias for a 200 μm diameter device and a 400 μm diameter device, respectively. The parallel white noise contribution is shown as a circle, the series white noise contribution is shown as a triangle, and the 1/f noise contribution is shown as a square. It can be seen that at each reverse bias, the parallel white noise contribution is similar for the 200 μm and 400 μm diameter devices. This is due to similar dark current in the two sized devices, as shown in fig. 1A-1B. In contrast, series white noise and 1/f noise are larger for 400 μm diameter devices compared to 200 μm diameter devices. This is due to the larger capacitance measured with larger diameter devices, as shown in fig. 2A-2B. The increased FWHM observed for 400 μm diameter devices can be explained in part by considering the increased series white noise and 1/f noise contribution.

Fano noise, parallel white noise, series white noise and 1/f noise contribution at 5.9keV were orthogonally subtracted from the measured FWHM at 5.9keV to calculate the combined contribution of dielectric noise and incomplete charge collection noise at 5.9 keV.

Fig. 7 shows the equivalent noise charge of dielectric noise and incomplete charge collection noise as a function of reverse bias for a spectrometer with 200 μm devices (plotted as a cross) and 400 μm devices (plotted as a diamond). The combined contribution of dielectric noise and incomplete charge collection noise is greater for 400 μm devices than for 200 μm devices at all reverse biases. At 0V reverse bias, the combined contributions have equivalent noise charges of 123e-rms and 87e-rms for 400 μm and 200 μm devices, respectively. At reverse bias above 5V, equivalent noise charges of 105e-rms and 78e-rms were calculated for 400 μm and 200 μm devices, respectively. Since dielectric noise is independent of detector bias, the difference in combined equivalent noise charge from dielectric electroacoustic and incomplete charge collection noise observed at 0V compared to ≧ 5V can be attributed to the incomplete charge collection noise at 0V. Thus, it can be said that at 0V, there are 18e-rms and 9e-rms of incomplete charge collection noise using 400 μm and 200 μm devices, respectively, and that incomplete charge collection noise is insignificant at reverse bias ≧ 5V.

In FIG. 7, the equivalent noise charge at reverse bias ≧ 5V is attributed to the dielectric contribution. Dielectric Equivalent Noise Charge (ENC)_D) Given by:

wherein q is a charge, A₂Is a constant (1.18) [ e.gatti, p.f. manfredi, m.sampietro and v.speziali, nuclear.instrum.meth.phys.res., a 297, 467(1990) depending on the type of signal shaping]K is the boltzmann constant, T is temperature, D is dissipation factor, C is capacitance [ g.liolou and a.m.barnett, nuclear.instrum.meth.phys.res.a 801, 63(2015)]. Using the ENC given above_DThe equation of (2) and the experimental data of FIG. 7, the effective dielectric dissipation factor is as high as (4.2. + -. 0.4). times.10^-3. It should be noted that this does not directly correspond to In_0.5Ga_0.5The dissipation factor of P, but it indicates the effective combined dissipation factor of all dielectric materials contributing to the noise, as it is analyzed here.

Although an example of energy resolution (FWHM) has been given herein, higher resolution is achievable, for example, by using a preamplifier with lower electronic noise.

Also study on In_0.5Ga_0.5P p⁺-i-n⁺The thickness of the intrinsic layer in the structure results in an effect on the detection efficiency. Suppose GaAs p⁺The side cap is arranged at p⁺-i-n⁺P of structure⁺And photons are incident on that side of the device. Also assume that p⁺-i-n⁺Only the intrinsic layer in the structure is effective.

FIG. 8A shows four p for intrinsic layers having thicknesses of 5 μm, 10 μm, 30 μm, and 50 μm⁺-i-n⁺Structure, p as a function of the energy of photons incident on the structure⁺-i-n⁺Detection efficiency of the structure. Each p⁺-i-n⁺In the structure of p⁺The thickness of the layer was 0.2 μm. It can be seen that at relatively low photon energies, the detection efficiency is substantially the same for all structures even though they have intrinsic layers of different thicknesses. However, structures with thinner intrinsic layers have lower detection efficiency as photon energy increases.

FIG. 8B shows p corresponding to FIG. 8A⁺-i-n⁺The detection efficiency of the structure is different in that p is⁺-i-n⁺P in the structure⁺The thickness of the layer is 0.1 μm (instead of 0.2 μm). The plot in fig. 8B follows the same pattern as the plot in fig. 8A, except that in fig. 8B, the detection efficiency is higher at lower photon energies.

From FIGS. 8A-8B, it can be seen that at low photon energies, the detection efficiency is affected by p⁺-i-n⁺P in the structure⁺The thickness of the layer dictates. In other words, at low photon energies, p⁺-i-n⁺P in the structure⁺The thicker the layer, the lower the detection efficiency. However, at higher photon energies, detectionEfficiency of measurement is measured by⁺-i-n⁺The thickness of the intrinsic layer in the structure dictates. In other words, the thicker the intrinsic layer, the higher the detection efficiency at higher photon energies.

Although not shown in fig. 8A-8B, detection efficiency at photon energies of 59.5keV is of particular interest because this is from²⁴¹Am, energy of the gamma ray. At these high energies, p in GaAs side caps⁺Layer and p⁺-i-n⁺P in the structure⁺Both layers are more or less transparent to photons, and the detection efficiency is therefore limited by p⁺-i-n⁺The thickness of the intrinsic layer in the structure. For example, for photons having an energy of 59.5keV, the detection efficiency of a 5 μm thick intrinsic layer would be 0.005, while the detection efficiency of a 50 μm thick intrinsic layer would be 0.05.

High resolution X-ray astronomy and X-ray fluorescence spectroscopy have been made possible because of the use of photon counting X-ray spectrometers. For example, in an aerospace mission, the ability to determine the energy of individual X-ray photons at a particular energy and the number of X-ray photons detected may be necessary. These attributes are particularly useful for studying planetary surfaces, magnetoballoons and sun physics and for terrestrial applications such as industrial monitoring and non-destructive testing. As mentioned above, the use of wide bandgap materials in these spectrometers is attractive because these materials can have low thermally generated leakage currents; as such, they can operate at high temperatures without the need for cooling systems, thus resulting in more compact, lower mass, and lower power instrumentation.

High energy resolution and temperature tolerant photon counting X-ray spectrometers have been reported that use various wide bandgap semiconductor detectors coupled with low noise preamplifier electronics. However, embodiments of the present invention provide an improved such apparatus.

Referring to table 2 below, heavily doped n is epitaxially grown by low pressure (150 torr) metalorganic vapor phase epitaxy using trimethylgallium, trimethylindium, arsine and phosphine as precursors and hydrogen as a carrier gas⁺Growing In on GaAs substrate_0.5Ga_0.5P p⁺-i-n⁺An epitaxial layer. Second stepSilane and dimethyl zinc: triethylamine was used for n-doping and p-doping, respectively. The epitaxial surface of the substrate has an orientation<111>Orientation of (100) at a miscut angle of 10 ° of a. An unintentionally doped i-layer (thickness of 5 μm) on top p⁺Layer (thickness of 0.2 μm; 2X 10)¹⁸cm^-3Doping concentration) and bottom n⁺Layer (thickness of 0.1 μm; 2X 10)¹⁸cm^-3Doping concentration of). It must be noted that p⁺Layer and n⁺The thickness of the layers is as thin as possible to reduce absorption in these layers. Based on high quality In_0.5Ga_0.5Empirical selection of growth of P for P⁺Layer (0.2 μm) and n⁺Thickness of the layer (0.1 μm). The thickness of the i-layer is instead thick to increase absorption in this layer and thus increase quantum efficiency. It must be emphasized that In_0.5Ga_0.5The P device is the thickest i-layer In reported so far_0.5Ga_0.5P photodiode, but an i layer thicker than 5 μm may be provided. In_0.5Ga_0.5P p⁺-i-n⁺On top of the epitaxial layer, a thin p is grown⁺GaAs layer (thickness of 0.01 μm; 1X 10)¹⁹cm^-3Doping concentration) to help achieve good top ohmic contact. n-type GaAs, n-type In_0.5Ga_0.5P and unintentionally doped In_0.5Ga_0.5P is grown at a temperature of 700 c and the subsequent P-doped layer is grown at 660 c. In grown at room temperature_0.5Ga_0.5P has a photoluminescence peak energy of 1.89 eV. This energy is in good agreement with the band gap of materials with suppressed spontaneous long range order in the group III sublattice. p is a radical of⁺The ohmic contact on top of the GaAs layer is formed of Ti (thickness of 20 nm) and Au (thickness of 200 nm). Is deposited to n⁺The ohmic back contact on the back of the GaAs substrate is formed of both ingre (thickness of 20 nm) and Au (thickness of 200 nm). In_0.5Ga_0.5The P photodiode is unpassivated. Using chemical wet etching technique (1:1: 1K)₂Cr₂O₇:HBr:CH₃COOH solution, followed by 1:8:80H₂SO₄:H₂O₂:H₂10s final etch In O solution) to prepare 200 μm diameter In for use In the study_0.5Ga_0.5A P mesa device. The device layers, their relative thicknesses and materials are summarized in table 2 below:

TABLE 2

192MBq⁵⁵An Fe radioisotope X-ray source (Mn K α ═ 5.9keV, Mn K β ═ 6.49keV) was positioned 200 μm diameter In_0.5Ga_0.5The top surface of the P mesa photodiode was 5mm away to study the detector performance under illumination.

Using the beer-lambert law and assuming complete charge collection In the p-and i-layers, the In through the optical window (the area not covered by the contact) of the device was calculated_0.5Ga_0.5P X Quantum Efficiency (QE) of the radiation.

FIG. 10 shows In as a function of photon energy up to 10keV_0.5Ga_0.5P X quantum efficiency of the radiation. The X-ray Quantum Efficiency (QE) was calculated for the structure at 53% at 5.9keV and 44% at 6.49 keV. Table 3 below shows the results for In_0.5Ga_0.5Attenuation coefficients at 5.9keV and 6.49keV for P and other different materials. The attenuation coefficients for binary and ternary compounds are estimated from their single element attenuation coefficients, appropriately weighted.

TABLE 3

In_0.5Ga_0.5The P device was installed inside a TAS Micro MT climate box for temperature control. The temperature was initially set at 100 ℃ and was gradually reduced to 20 ℃ at 20 ℃. The device was left for 30 minutes to ensure stability before any measurements were taken at each temperature. Measuring In as a function of reverse bias using a Keithley6487 Peak Meter/Voltage Source_0.5Ga_0.5P leakage current. The uncertainty associated with the individual current readings is 0.3% of their value plus 400fA, while the uncertainty associated with the applied bias voltage is 0.1% of their value plus 1 mV.Measurement of In as a function of reverse bias Using an HP 4275A Multi-frequency LCR Meter_0.5Ga_0.5And P capacitance. The uncertainty associated with each capacitance reading is 0.12%, while the uncertainty associated with the applied bias voltage is 0.1% plus 1mV of its value. The test signal is a sinusoid with a magnitude of 50mV rms and a frequency of 1 MHz. In both leakage current and capacitance measurements, the reverse bias voltage (in 1V increments) increases from 0V to 15V.

Use of⁵⁵The Fe radioisotope X-ray source obtains X-ray spectrum to irradiate 200 μm diameter In at a temperature of from 100 deg.C to 20 deg.C_0.5Ga_0.5And P devices. The experimental setup utilized a custom charge sensitive preamplifier with a feedback resistor-less design. The preamplifier operates at the same temperature as the photodiode. The signal from the preamplifier was shaped by an Ortec 572a shaping amplifier and digitized by a multichannel analyzer (Ortec Easy-MCA-8K). The spectra were accumulated at shaping times of 0.5. mu.s, 1. mu.s, 2. mu.s, 3. mu.s, 6. mu.s and 10. mu.s and analyzed. In each case, In_0.5Ga_0.5The P devices are reverse biased at 0V, 5V, 10V and 15V. The lifetime of each spectrum is 200 seconds. In a dry nitrogen atmosphere (relative humidity)<5%) was performed.

The leakage currents of the packaged devices measured at 100 ℃ and 80 ℃ are shown in fig. 11; leakage currents at temperatures below 80 ℃ are not reported because they are below the background noise of the picoampere meter. Measurement of leakage current as a function of reverse bias of the blank package shows that the package of the diode is contributing significantly to the measured leakage current. The packaged device (defined as the combined semiconductor and package) had leakage currents of 1.5pA and 0.5pA, respectively, at reverse bias of 10V at 100 ℃ and 80 ℃. The blank package has leakage currents of 1.1pA and 0.2pA, respectively, under the same temperature and reverse bias conditions. When the reverse bias was increased to 15V in each case, the leakage currents measured with the packaged device and the blank package became indistinguishable at these two temperatures. In view of the uncertainty associated with the leakage current measurement, the leakage current from the diode itself may be considered negligible compared to the leakage current from the package.

In of the package was measured as a function of reverse bias voltage at different temperatures_0.5Ga_0.5The capacitance of the P detector. The capacitance of a blank package of the same type was also measured at different temperatures and was measured from the In of the package_0.5Ga_0.5The measured capacitance of the P photodiode is subtracted. Measuring the capacitance a plurality of times at each temperature; the mean and its relative standard deviation are considered. In the temperature range studied, In was found_0.5Ga_0.5The capacitance (C) of the P-detector itself is temperature invariant. In FIG. 12, the 1/C as a function of reverse bias at 100 deg.C and 80 deg.C is shown²Similar results were found at temperatures ≦ 60 ℃. 1/C was found at reverse bias below 3V²Dependence on reverse bias; 1/C²Is constant at reverse bias above 3V.

Use of⁵⁵The Fe radioisotope X-ray source obtains an X-ray spectrum. Although temperatures above 100 ℃ may be used, these results are not described herein. At 100 ℃, the diode is stable throughout the spectrum acquisition time. After these temperatures were used, the diodes did not degrade.

An improvement in energy resolution (quantified by FWHM at 5.9keV) was observed when increasing the applied reverse bias from 0V to 5V. The result may be interpreted in view of the reduction of the capacitance of the detector and possibly improved charge collection. When biased in the reverse direction>When the detector was operated at 5V, no further change in FWHM was observed. In can be considered_0.5Ga_0.5The P photodiode is fully depleted above 5V to explain the latter behavior.

The optimum forming time (i.e., the forming time that produces the minimum FWHM) varies with temperature, as shown in fig. 13. Because In is at these temperatures_0.5Ga_0.5Lower leakage current of P-photodiode and Si JFET, so FWHM decreases at lower temperature. Fig. 13 shows the minimum observed FWHM of the 5.9keV peak as a function of temperature at the optimum forming time when the detector is reverse biased at 5V.

FIGS. 14A and 14B present 100 deg.C and 20 deg.C, respectively, with the photodiode reverse biased at 5VThe spectrum with the best energy resolution (minimum FWHM) is also shown in each spectrum, as are the deconvoluted Mn K α (dashed line) and Mn K β (dotted line) peaks⁵⁵Fe photopeak is⁵⁵A combination of characteristic Mn K α (5.9keV) and Mn K β (6.49keV) lines of a Fe radioisotope X-ray source to determine the FWHM of the 5.9keV peak in FIGS. 13 and 14, a Gaussian fit is performed to the peaks, the Mn K α and Mn K β peaks are deconvoluted from the detected combined photopeak.

The energy resolution (FWHM) of non-avalanche X-ray photodiode spectrometers is degraded by Fano noise, charge trapping noise, and electronic noise. Fano noise is due to the statistical nature of the ionization process. At each temperature studied, the observed FWHM was greater than the desired Fano limited energy resolution, indicating that noise sources other than the statistical charge generation process were significant. In a photodiode X-ray spectrometer, electronic noise is generated by five different components: parallel white noise, serial white noise, induced gate current noise, 1/f noise, and dielectric noise. The leakage current of the Si input JFET of the detector and preamplifier (operating uncooled at each temperature) affects the parallel white noise. The capacitance of the input JFET of the detector and preamplifier affects the series white noise and 1/f noise. The parallel white noise and the serial white noise are respectively in direct proportion and inverse proportion to the forming time; while 1/f noise and dielectric noise are independent of the shaping time.

The calculated parallel white noise, serial white noise (adjusted with respect to induced gate current noise and 1/f noise at shaping times of 0.5 μ s, 1 μ s and 10 μ s in the case of a photodiode reverse-biased at 5V) are shown in fig. 15A, 15B and 15C, respectively. In the graph, parallel white noise (white circles), serial white noise (white squares) and 1/f noise (white triangles) contributions are shown. The high parallel white noise observed at increased temperature and increased shaping time is not due to the high leakage current of the detector, but rather to the higher current of the uncooled Si input JFET of the preamplifier.

As described above, (1) the FWHM of the photopeak does not decrease at reverse bias >5V, (2) the centroid channel number of the photopeak does not increase as the reverse bias increases beyond 5V, and (3) the spectrum shown in fig. 14 fits well by gaussian without shoulder (shoulder) or other distortion exceeding the expected low energy tail. Thus, it can be assumed that the quadrature difference between the FWHM at 5.9keV and the calculated noise contributions (Fano noise, parallel white noise, serial white noise, and 1/f noise) can be attributed to dielectric noise (i.e., under this condition, the charge trapping noise is negligible).

The temperature dependence of the dielectric noise is shown in fig. 16. Fig. 16 shows the equivalent noise charge of dielectric noise at 5.9keV as a function of temperature when the electro-optic diode is reverse biased at 5V. In the temperature range 100 ℃ to 20 ℃, the dielectric noise contribution at 5.9keV decreases linearly with decreasing temperature: calculating to obtain a value of 94e-rms +/-15 e-rms at 100 ℃; and at 20 ℃ was determined as a value of 68 e-rms. + -. 7 e-rms. A linear least squares fit is performed on the squared relationship between dielectric equivalent noise charge and temperature. Accordingly, the combined dissipation factor associated with all dielectricity in the spectrometer was estimated to be (8.5 ± 0.8) × 10^-3. Comparison of the standard deviation of the fit with the experimental uncertainty shows that a linear fit is appropriate, validating the value of the calculated combined dissipation factor.

FIG. 17 shows the squared equivalent noise charge (ENCD2) of dielectric noise at 5.9keV as a function of temperature. Also shown is the line of best fit calculated by a linear least squares fit.

In_0.5Ga_0.5The P spectrometer allows high temperature operation (up to the maximum of 100 ℃ under investigation). And use of Al_0.52In_0.48P and Al_0.8Ga_0.2It exhibits a better FWHM than that achieved by As spectrometers. The ability to operate at such high (100 ℃) temperatures, together with their large X-ray attenuation coefficients, allows In_0.5Ga_0.5The P spectrometer is superior to the more recently reported GaAs spectrometer with a maximum operating temperature of 60 ℃. Compared with the Al reported previously_0.52In_0.48P X radiation spectrometer, In_0.5Ga_0.5P X radiation spectrometers perform better at 100 c. For In_0.5Ga_0.5FWHM at 5.9keV for P device is 1.27keV at 100 deg.C, vs. Al for electronic device read out using similar device_0.52In_0.48P1.57 keV for the device. In_0.5Ga_0.5P also has a specific Al content_0.52In_0.48P greater linear attenuation coefficient.

Since the read-out electronics used to characterize these materials are already broadly comparable, the difference in the resulting FWHM for these materials (GaAs, AlInP, InGaP) can be accounted for by the different electron-hole pairs of these materials (GaAs, AlInP, InGaP) accounting for the differences in the energy generation and noise contribution of the read-out electronics at high temperatures. The total noise at the input of the preamplifier of 86e-rms corresponds, for example, to 840eV, In GaAs_0.5Ga_0.51.00keV and Al in P_0.52In_0.481.08keV in P. For In_0.5Ga_0.5The FWHM of 1.02keV at 5.9keV at 60 ℃ observed by the P-spectrometer is very close to the expected value. Therefore, the total noise In e-rms is In GaAs spectrometers and In_0.5Ga_0.5The P-spectrometer is similar because the preamplifier is limited to noise other than detector leakage current at these temperatures. It should also be noted that In_0.5Ga_0.5The X-ray attenuation coefficient of P is much larger than with other materials (e.g., SiC). Therefore, even In_0.5Ga_0.5The ultimate achievable energy resolution In the case of P is lower than for other materials, In_0.5Ga_0.5P may still be preferred for low-throughput, high-energy applications.

The Fano limited energy resolution is related to the charge generation process upon absorption of X-ray photons and is not the statistically limited energy resolution of avalanche X-ray photodiode spectrometers. The Fano limited energy resolution (FWHM in eV) can be calculated using equation 1 below:

where ε is the semiconductor electron-hole pair generation energy, F is the Fano factor, and E is the energy of the X-ray photon. Different semiconductors have different Fano-limited energy resolutions at the same energy of the X-ray photon. This is because the Fano-limited energy resolution at each energy depends on the physical material properties (average electron-hole pair generation energy and Fano factor).

For room temperature (20 ℃) electron-hole pair generation energy measurements, the measurement is from the charge generated In the reference 200 μm GaAs mesa photodiode_0.5Ga_0.5In P photodiode⁵⁵The charge generated by the absorption of X-rays by the Fe radioisotope X-ray source. The structure of the GaAs device is summarized in table 4 below. In_0.5Ga_0.5The P and GaAs detectors are connected in parallel to a custom low noise charge sensitive preamplifier.

TABLE 4

For In_0.5Ga_0.5Both the P photodetector and the GaAs photodetector are independently reverse biased at 10V. In is irradiated individually In turn by a radioisotope X-ray source_0.5Ga_0.5The spectra were accumulated by P device and GaAs device Gaussian fitting of the detected Mn K α (5.9keV) and Mn K β (6.49keV) peaks of the accumulated spectra, shown In FIG. 18 for In_0.5Ga_0.5Accumulated of P-detector and GaAs reference photodetector⁵⁵Fe X-ray spectrum and fitted 5.9keV peak.

FIG. 18 shows⁵⁵In is used under irradiation of an X-ray source of Fe radioisotope_0.5Ga_0.5P-device and GaAs reference photodetector at 10V reverse bias integrated X-ray spectra. Also shown is for In_0.5Ga_0.5The fitted 5.9keV line for the P device (dotted line) and GaAs reference photodetector (dashed line.) for clarity, the fitted 6.49keV Mn K β peak is not shown, but is suitably included in the fit.

Zero noise energy peak using preamplifier and position of 5.9keV peak detected by GaAs reference photodiodeThe amount of charge corresponding to each MCA channel was calculated. In this calculation, the GaAs electron-hole pair generation energy (4.184 eV. + -. 0.025eV) was also used. In is then determined using equation 2 below_0.5Ga_0.5P electron-hole pair generation energy (epsilon InGaP):

wherein ε GaAs is the electron-hole pair generation energy In GaAs, and NGaas and NInGaP are the GaAs reference detector and In, respectively_0.5Ga_0.5The amount of charge generated in the P detector. Experimental values of 4.94 eV. + -. 0.06eV were measured for ε InGaP at room temperature (20 ℃). To check operation of In at higher reverse bias_0.5Ga_0.5The effect of the P detector, increasing the reverse bias to 15V, and repeating the experiment. In this example, an electron-hole pair generation energy of 4.90 eV. + -. 0.04eV is measured. The similarity of the values further confirms that charge trapping is negligible. If charge trapping is significant, a substantial reduction in the energy generated by the electron-hole pairs is observed at higher reverse bias due to improved charge transport at higher electric fields.

Investigation of In across the temperature range 100 ℃ to 20 ℃_0.5Ga_0.5The P electron-hole pair generates an energy dependence on temperature. For this set of measurements, the detectors were individually connected to a custom low noise charge sensitive preamplifier (i.e., without a GaAs reference detector) and passed through⁵⁵Fe radioisotope X-ray source. The change in conversion factor of the preamplifier itself with temperature is measured across a temperature range by connecting a stabilized pulse generator (BerkeleyNucleonics Corporation model BH-1) to the test signal input of the preamplifier. The change in the centroid position of the pulse generator peak allows the preamplifier performance to be decoupled from the change in the photodiode electron-hole pair generation energy with changes in temperature. The change in the centroid position of the pulser peak is appropriately corrected for the change in test capacitance with temperature. Spectra were collected and the light peaks and peaks from the pulse generator were Gaussian fitted to determine their centroids relative to zero noiseThe position of the peak. X-ray photon In_0.5Ga_0.5The charge generated in the P photodiode is related to the relative change in the position of the photopeak on the scale of the charge of the MCA. The latter is corrected for the change in conversion factor of the preamplifier with temperature (determined from the pulser peak). In_0.5Ga_0.5The change in the P electron-hole pair generation energy (epsilon InGaP) causes different amounts of charge to be generated at different temperatures. At each temperature, the absolute value of ε InGaP is then calculated using the previously determined room temperature ε InGaP.

In as a function of temperature is reported In FIG. 19_0.5Ga_0.5The P electron-hole pairs generate energy. The finding indicates that In_0.5Ga_0.5The P electron-hole pairs give rise to a clear slight tendency to increase in energy with increasing temperature: at 100 deg.C,. epsilon.InGaP ═ 5.02 eV. + -. 0.07eV, and at 20 deg.C,. epsilon.InGaP ═ 4.94 eV. + -. 0.06 eV. This trend is surprising. It is conventionally considered that the average electron-hole pair generation energy linearly decreases with increasing temperature. This reduction can be understood by considering the dependence of the electron-hole pair generation energy on the material bandgap energy. According to previous reports, the empirical relationship between electron-hole pair generation energy and bandgap energy in semiconductors is linear. Similar behavior is expected for electron-hole pair generation energy due, at least in part, to changes in the band gap as the band gap increases as temperature decreases. Theoretical monte carlo calculations previously performed on silicon predict a reduction in the Si electron-hole pair generation energy as a function of temperature. In this model, physical mechanisms are considered as X-ray absorption, atomic relaxation, and electron energy loss.

From In_0.5Ga_0.5The expected Fano limited energy resolution (FWHM) at 5.9keV for a P-made X-ray detector can be calculated to be 139eV at 20 ℃. This is estimated using equation 1 for a determined value of electron-hole pair generation energy and assumes a Fano factor of 0.12. Negligible changes over the temperature range of 20 ℃ to 100 ℃ were observed.

Electron-hole pair generation energies obtained at 27 deg.C (300K) (4.95 eV. + -. 0.03eV) vs. In by empirical Bertuccio-Maiiocchi-Barnett (BMB) relationship_0.5Ga_0.5The values for P prediction (4.83 eV. + -. 0.21eV) are well in agreement.

FIG. 20 shows temperature at 300K for Ge, Si, GaAs, Al_0.2Ga_0.8As、Al_0.8Ga_0.2As (filled circle) and In_0.5Ga_0.5The average electron-hole pairs of P (filled squares) as a function of their respective bandgap energies generate energy.

Linear least squares fit of the data shows that it is possible for In_0.5Ga_0.5P refines the previously reported BMB dependence between electron-hole pair generation energy and bandgap energy using new data. The new relationship is epsilon-AEg + B, where a-8962 + -0.08 and B-eV (1.79 + -0.13).

For the first time, the inventors have shown that X-ray spectrometers with InGaP detectors are used across a temperature range of 100 ℃ to 20 ℃. The spectrometer is characterized by different shaping times and detector reverse bias. The best energy resolution (minimum FWHM) at 5.9keV at 100 deg.C using a shaping time of 0.5 μ s is 1.27keV, and when the InGaP detector is reverse biased at 5V, it improves to 840eV at 20 deg.C (using a shaping time of 10 μ s). An improvement in energy resolution (quantified by FWHM at 5.9keV) was observed when increasing the applied reverse bias from 0V to 5V. The better results obtained at 5V can be explained by considering improved charge collection in larger electric field strengths. Similar FWHM as measured at 5V was observed at 10V and 15V, indicating that the charge trapping noise above 5V was negligible. System noise analysis shows that the observed FWHM is higher than the likelihood statistics limited energy resolution (i.e., Fano limited energy resolution). Parallel white noise, serial white noise, 1/f noise, and dielectric noise are calculated. The higher parallel white noise observed at the increased temperature is generated by the Si input JFET of the preamplifier rather than the photodetector.

Although circular photodiodes have been discussed herein (from top to bottom), other geometries of photodiodes are contemplated. Circular devices may be preferred for single pixel detectors because their ratio of top surface area to side wall area is maximized. For a pixel array detector comprising a plurality of these photodiodes arranged in a 1D or 2D array, a circular or other shaped device may be desired. For example, it may be desirable to use devices having a checkerboard shape (e.g., square or polygonal devices).

Various embodiments have been described in which photodiodes are used in spectroscopy or to determine the source and/or position of an X-ray and/or gamma-ray source. However, it is contemplated that the photodiode may be used in other instruments. For example, photodiodes may be used in radioisotope microbatteries (also known as nuclear microbatteries).

Nuclear microbatteries include a radioactive source for emitting radioactive particles (i.e., α particles or β particles) or photons, and a detector that receives those particles or photons and converts them into electrical current these devices are desirable because they have a relatively long life span (e.g., >10 years), high energy density, and small size.

The effect of operating temperature on the performance of microbattery photovoltaic cells is important for many target applications because temperature can significantly affect the performance of voltaic cells.

The inventors have recognized that photodiodes comprising InGaP are particularly useful in core microbatteries because, as noted above, InGaP is surprisingly effective at converting X-rays and gamma-rays to charge carriers. Furthermore, the use of InGaP enables the microbattery to operate in a relatively wide temperature range and at high doses of radiation with relatively low performance losses. These characteristics, combined with relatively high conversion efficiency and relatively low production cost (e.g., it can be grown on commercial GaAs substrates using common growth methods) make InGaP particularly suitable for use in X-ray and/or gamma-ray nuclear microbatteries.

It has also been recognized that the use of nuclear microbatteries including radioisotope X-ray sources reduces the risks associated with device damage compared to radioisotope β particle sources, for example, because soft X-ray sources (e.g., photon energies <10keV) can be shielded relatively easily to provide a safe condition.

Exemplary embodiments of a nuclear microbattery will now be described, by way of example only, to assist in understanding the present invention. It is to be understood that the invention is not limited to the specific compositions comprising all of the layers described or the various layers in this example.

A microbattery is formed that includes a 400 μm diameter mesa photodiode described with respect to Table 1 above and 206MBq for illuminating the photodiode⁵⁵An Fe radioisotope X-ray source (Mn K α ═ 5.9keV, Mn K β ═ 6.49 keV.) the X-ray source was positioned 5mm away from the top surface of the detector (i.e. with the PIN structure between the X-ray source and the substrate (layer 7 in table 1)).

The microbattery was placed inside a TAS MicroMT climate box with a dry nitrogen atmosphere (relative humidity < 5%). The current characteristics as a function of applied forward bias (from 0V to 1V in 0.01V increments) were measured using a Keithley6487 picometer/voltage source over a temperature range of-20 ℃ to 100 ℃.

Fig. 9 shows the current generated by the microbattery as a function of the forward bias across the photodiode for various different temperature conditions.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the following claims.