阅读说明：本技术 使用相变材料的光电器件 (Photovoltaic device using phase change material ) 是由 赛义德·加齐·萨尔瓦特 热拉尔多·罗德里格斯·赫南德斯 哈里什·巴斯卡兰 于 2018-05-03 设计创作，主要内容包括：一种用于检测光的装置(200),包括检测器(100)和读出电路(150)。检测器(100)包括：基板(10)、由基板(10)支撑的相变材料层(16)、与相变材料层(16)电接触的第一电极(14)、以及与相变材料层(16)电接触的第二电极(18)。第一电极和第二电极(14、18)可操作以通过由于第一电极与第二电极(14、18)之间的偏置电压使电流穿过相变材料而偏置相变材料(16)。读出电路(15)被配置为：通过在第一电极与第二电极(14、18)之间施加偏置电压而偏置相变材料层(16)；通过检测由于相变材料层(16)中的相变而导致的相变材料层(16)的电阻改变来检测入射到相变材料(16)上的光；在相变材料层(16)中的相变之后,通过经由第一电极和第二电极(14、18)施加重置脉冲来重置相变材料层(16)。(An apparatus (200) for detecting light includes a detector (100) and a readout circuit (150). The detector (100) comprises: the phase change memory device includes a substrate (10), a phase change material layer (16) supported by the substrate (10), a first electrode (14) in electrical contact with the phase change material layer (16), and a second electrode (18) in electrical contact with the phase change material layer (16). The first and second electrodes (14, 18) are operable to bias the phase change material (16) by passing a current through the phase change material as a result of a bias voltage between the first and second electrodes (14, 18). The readout circuit (15) is configured to: biasing the phase change material layer (16) by applying a bias voltage between the first and second electrodes (14, 18); detecting light incident on the phase change material (16) by detecting a change in resistance of the phase change material layer (16) due to a phase change in the phase change material layer (16); after a phase change in the phase change material layer (16), the phase change material layer (16) is reset by applying a reset pulse via the first and second electrodes (14, 18).)

1. An apparatus for detecting light comprising a detector and a readout circuit, wherein the detector comprises:

a substrate, a phase change material layer supported by the substrate, a first electrode in electrical contact with the phase change material layer, and a second electrode in electrical contact with the phase change material layer, wherein the first and second electrodes are operable to bias the phase change material by passing a current through the phase change material due to a bias voltage between the first and second electrodes;

and the readout circuitry is configured to:

biasing the phase change material layer by applying a bias voltage between the first electrode and the second electrode;

detecting light incident on the phase change material by detecting a change in resistance of the phase change material layer due to a phase change in the phase change material layer;

resetting the phase change material layer by applying a reset pulse through the first electrode and the second electrode after the phase change occurs in the phase change material layer.

2. The device of claim 1, wherein the first and second electrodes are disposed below and above the phase change material layer, respectively, the first electrode being in electrical contact with a lower surface of the phase change material layer and the second electrode being in electrical contact with an upper surface of the phase change material layer.

3. The apparatus of claim 2, wherein the detector comprises a mirror layer arranged to reflect light through the phase change material layer so as to increase the absorbance of incident light by the detector.

4. The apparatus of any preceding claim, wherein the detector is configured to act as a resonant optical cavity so as to maximise the absorbance of light of a selected wavelength by the detector.

5. The apparatus of claim 4, wherein the selected wavelength is in a range of 400nm to 700 nm.

6. The device of any one of the preceding claims, wherein the first and second electrodes comprise an at least partially transparent conductive material.

7. The apparatus of claim 6, wherein the first and second electrodes comprise indium tin oxide, graphene, multi-layer graphene, graphite, gold, or PEDOT.

8. The device according to any of the preceding claims, wherein the phase change material comprises a compound or alloy of a combination of elements selected from the list of combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AgInSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb.

9. The apparatus of any preceding claim, wherein the readout circuit is operable in a count rate mode in which the rate at which the detector is reset by the readout circuit infers the light flux on the detector.

10. The apparatus of any preceding claim, wherein the readout circuitry is operable in a sub-threshold mode in which the light flux on the detector is inferred from the resistance of the phase change material in the amorphous state.

11. The apparatus of any preceding claim, wherein the readout circuitry is configured to adjust the bias voltage to change a sensitivity and/or a dynamic range of the detector.

12. The apparatus of claim 9, wherein the readout circuit is configured to adjust the bias voltage to change a sensitivity of a device in response to a reset count rate of the detector.

13. The apparatus of claim 10, wherein the readout circuitry is configured to adjust the bias voltage to change a dynamic range of the detector in response to a reset event to change an amount of light incident on the detector required to cause a phase change in the phase change material layer.

14. The apparatus of any preceding claim, wherein the detector comprises a plurality of pixels, each pixel comprising:

a phase change material layer, a first electrode in electrical contact with the phase change material layer, and a second electrode in electrical contact with the phase change material layer, wherein, for each pixel, the first and second electrodes are operable to bias the phase change material by passing a current through the phase change material due to a bias voltage between the first and second electrodes such that each pixel individually responds to light; and is

Wherein the readout circuitry is configured to, for each pixel:

biasing the phase change material layer by applying a bias voltage between the first electrode and the second electrode;

detecting light incident on the phase change material by detecting a change in resistance of the phase change material layer due to a phase change in the phase change material layer;

resetting the phase change material layer by applying a reset pulse via the first electrode bias and the second electrode bias after the phase change occurs in the phase change material layer.

15. A method of detecting radiation incident on a detector, the detector comprising: a phase change material layer, a first electrode in electrical contact with the phase change material layer, and a second electrode in electrical contact with the phase change material layer, the method comprising:

biasing the phase change material by applying a bias voltage between the first electrode and the second electrode;

detecting light incident on the phase change material by detecting a change in resistance of the phase change material layer due to a phase change in the phase change material layer;

resetting the phase change material layer by applying a reset pulse through the first electrode and the second electrode after the phase change occurs in the phase change material layer.

16. The method of claim 15, comprising using the apparatus of any one of claims 1 to 14.

Technical Field

The present invention generally relates to an optoelectronic device and a method of operating the same. More particularly, but not exclusively, the present invention relates to a phase change material based optoelectronic device and its use in optical detection applications.

Background

The development of inexpensive optical sensors with high light sensitivity and reduced size is motivated by their positive impact on the wide range of applications from medical devices to consumer electronics. Commercially available optical sensors that operate at room temperature in the visible near Infrared (IR) spectral range are typically based on semiconductor materials such as silicon (Si) and indium gallium arsenide (InGaAs). Image sensors commonly used today in cell phone cameras and web cameras are based on Si active pixel sensor arrays fabricated using CMOS technology. Such sensors provide high optical sensitivity but are expensive to manufacture, have limited dynamic range and are not easily scaled down in size. In most conventional semiconductor-based optical sensors that respond to visible near-IR light, the optical response results from the separation and drift of photoexcited carriers (photo-carriers) in the electric field present between the terminals of the device. The electric field is any one of: internal/built-in electric fields such as those present at p-n junctions; an external electric field, such as generated by an applied bias voltage; or a combination of the two. Thus, for a given incident optical power, the optical response is largely determined by the material specific properties (such as optical absorption and carrier mobility) that govern the generation and drift of photo-carriers. These properties are fixed. The dynamic range (i.e., the effective range of optical power over which the device can operate) is also limited by material specific properties, meaning that typical optical devices saturate at a fixed and relatively low light level. In contrast, the human eye has a wide dynamic range, since the human eye can adapt to varying levels of light intensity via the constriction surrounding the pupil.

Alternative optoelectronic devices and methods of photodetection are desired, preferably with improved adaptability to light levels.

Disclosure of Invention

According to a first aspect, there is provided an apparatus for detecting light, comprising a detector and a readout circuit. The detector includes: the substrate, a phase change material layer supported by the substrate, a first electrode in electrical contact with the phase change material layer, and a second electrode in electrical contact with the phase change material layer the first and second electrodes are operable to bias the phase change material by passing a current through the phase change material due to a bias voltage between the first and second electrodes. The readout circuitry is configured to:

biasing the phase change material by applying a bias voltage between the first electrode and the second electrode;

detecting light incident on the phase change material by detecting a change in resistance of the phase change material layer due to a phase change in the phase change material layer; and is

After the phase change in the phase change material layer, the phase change material layer is reset by applying a reset pulse through the first electrode and the second electrode.

The term "light" is used in this specification in a non-limiting sense and generally refers to any form of electromagnetic radiation. Some embodiments may be adapted to detect light at wavelengths in the range of 10nm to 1mm or 400nm to 700 nm.

Detecting the change in resistance may include detecting a change in current flowing through the phase change material due to the bias voltage.

The first and second electrodes may be disposed below and above the phase change material layer, respectively. The first electrode may be in electrical contact with a lower surface of the phase change material layer, and the second electrode may be in electrical contact with an upper surface of the phase change material layer.

The detector may comprise a mirror layer arranged to reflect light through the layer of phase change material so as to increase the absorbance of the incident light by the detector.

The detector may be configured to act as a resonant optical cavity so as to maximize the absorbance of light of a selected wavelength by the detector.

The selected wavelength may be in the range of 400nm to 700 nm.

The first and second electrodes may comprise an at least partially transparent conductive material.

The first and second electrodes may comprise indium tin oxide, graphene, multilayer graphene, graphite, gold, or PEDOT.

The phase change material may comprise a compound or alloy of a combination of elements selected from the list of combinations: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AgInSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb.

The readout circuit may operate in a count rate mode in which the rate at which the readout circuit resets the detector infers the light flux on the detector.

The readout circuit may operate in a sub-threshold mode in which the light flux on the detector is inferred from the resistance of the phase change material in the amorphous state.

The readout circuit may be configured to adjust the bias voltage to change the sensitivity and/or dynamic range of the detector.

The readout circuit may be configured to adjust the bias voltage to change the sensitivity of the device in response to a reset count rate of the detector.

The readout circuit may be configured to adjust the bias voltage in response to a reset event to change the dynamic range of the detector, thereby changing the amount of light incident on the detector that is required to cause a phase change in the phase change material layer.

The detector may comprise a plurality of pixels, each pixel comprising:

a phase change material layer, a first electrode in electrical contact with the phase change material layer, and a second electrode in electrical contact with the phase change material layer, wherein, for each pixel, the first and second electrodes are operable to bias the phase change material by passing a current through the phase change material due to a bias voltage between the first and second electrodes such that each pixel individually responds to light; and is

Wherein the readout circuitry is configured to, for each pixel:

biasing the phase change material layer by applying a bias voltage between the first electrode and the second electrode;

detecting light incident on the phase change material by detecting a change in resistance of the phase change material layer due to a phase change in the phase change material layer;

after the phase change in the phase change material layer, the phase change material layer is reset by applying a reset pulse via the first electrode bias and the second electrode bias.

According to a second aspect, there is provided a method of detecting radiation incident on a detector, the detector comprising: a phase change material layer, a first electrode in electrical contact with the phase change material layer, and a second electrode in electrical contact with the phase change material layer, the method comprising:

biasing the phase change material by applying a bias voltage between the first electrode and the second electrode;

detecting light incident on the phase change material by detecting a change in resistance of the phase change material layer due to a phase change in the phase change material layer;

after the phase change in the phase change material layer, the phase change material layer is reset by applying a reset pulse through the first electrode and the second electrode.

The method may be performed using an apparatus according to the first aspect, including any of its optional features in any combination.

In general, features described in the context of different aspects and embodiments of the invention may be used together and/or interchangeably. Similarly, where features are, for brevity, described in the context of a single embodiment, these features may also be provided separately or in any suitable subcombination. Features described in connection with the device may have corresponding features which may be defined with respect to the method and embodiments are specifically contemplated.

Drawings

In order that the invention may be well understood, embodiments will now be discussed, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic top view of a thin-film device and a cross-section of a layer structure;

fig. 2 is a schematic view of an apparatus according to an embodiment;

fig. 3 is a graph showing current-voltage characteristics of a detector in an amorphous state used in an embodiment measured in the dark;

FIG. 4 is a graph showing current as a function of incident light power (at 632 nm) when the PCM is in the amorphous state;

FIG. 5 is a graph showing current as a function of incident light power (at 632 nm) when the PCM is in the crystalline state;

FIG. 6 is a graph showing the excitation at 1592mW/cm under constant light excitation at 532nm²A graph of the current-voltage sweep of the detector of power density measurements showing switching to the crystalline state.

FIG. 7 is a flow chart illustrating a method according to an embodiment;

FIG. 8 is a graph illustrating a repeating cycle of phase changes performed on a phase change material; and

fig. 9 is a schematic diagram of an apparatus for performing a test using a laser light source according to an embodiment.

Detailed Description

Phase Change Materials (PCMs) have been the subject of active research and development in the past decade, for example, in the context of electronic memory.

The electrical and optical characteristics of the PCM may exhibit a high contrast between the crystalline and amorphous phases. In particular, a PCM (e.g., a chalcogenide-based PCM, such as GST) may have the ability to switch between these two states in response to an appropriate thermal stimulus (resulting in crystallization) or melt quenching process (resulting in amorphization). At the crystallization temperature T_CThe phase transition takes place. Initially, below T_CThe PCM may be in an amorphous state at the temperature of (a). When heating to above T_CAt a temperature of (2), the PCM switches to a crystalline state and when it cools back below T_CIt is kept in a crystalline state. By heating the PCM to melting point T_MAbove, then rapidly cooled back to T_CIn the following, the PCM may be "reset" back to the amorphous state.

These PCMs (including tellurides andantimonide) switchable on a sub-nanosecond time scale with high reproducibility, which can be up to 10 deg.f using contemporary materials¹²Achieve ultra-fast operation within the switching period of (c). New and improved PCM materials, such as the so-called phase change superlattice materials, are expected to provide better performance in the future.

In addition to the change in conductivity, many PCMs also exhibit a significant change in refractive index (optical reflection/index) in the visible, while varying more in the near infrared wavelength range. In particular, the amorphous state may have low conductivity and low reflectivity, and the crystalline state may have relatively high conductivity and high reflectivity.

FIG. 1 illustrates a detector 100 suitable for use in embodiments of the present invention. The detector 100 comprises a layer stack 20 deposited onto a substrate 10. The layer stack 20 comprises a plurality of layers stacked on top of each other. Each layer may comprise a different material, or two or more layers may comprise substantially the same material. The layer stack 20 comprises a layer of PCM 16 sandwiched between a first electrode 14 and a second electrode 18. The layer stack 20 may also include a mirror layer 12. The mirror layer 12 may be disposed between the first electrode 12 and the substrate 10.

Although the device employs a vertical structure in which the first electrode 14 is a lower electrode and the second electrode 18 is an upper electrode, and the PCM layer is sandwiched between the lower electrode 14 and the upper electrode 18, other embodiments may employ a lateral structure that includes electrodes configured to pass current laterally through the PCM layer. Such lateral electrodes may be patterned starting from a single layer and/or may be in electrical contact with only one side of the PCM layer. In other embodiments, the lateral electrodes may be in contact with both sides of the PCM layer.

The detector may include another encapsulation layer (not shown) which may include an oxide or polymer, for example, to protect the layer stack 20 from degradation.

It will be understood that the use of terms such as "upper" and "lower" do not limit the orientation of the detector 100 in use, and are used in a relative sense.

The substrate 10 may be substantially opaque in the visible spectrum and may be capable of absorbing light. Alternatively, the substrate 10 may be substantially transparent in the visible spectrum. In case the substrate 10 is substantially transparent in the visible spectrum, the mirror layer 12 may be positioned to allow illumination of the PCM layer 16 from the backside (through the substrate 10). For example, in the case of a vertical device, the mirror layer 12 may be located above the upper electrode 18.

In the exemplary embodiments described below, PCM 16 is a well-studied germanium antimony tellurium (GST) compound or alloy Ge₂Sb₂Te₅Because it has proven to be chemically and solid-state stable down to nanoscale dimensions and has the potential for device miniaturization. GST has T_C150 ℃ and T_M～600℃。

In other embodiments, the PCM 16 may be or include a material of a compound or alloy comprising a combination of elements selected from the list of combinations of: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AgInSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb. The PCM may be doped with any element (e.g., C, Ni, Ce, Si, etc.).

The PCM layer 16 may have a thickness in the range of 10nm to 50 nm. In other embodiments, the PCM layer 16 may have a thickness in a range of 10nm to 20nm, 20nm to 30nm, 30nm to 40nm, or 40nm to 50 nm.

The PCM layer 16 may have a switching time of less than 1 microsecond or preferably less than 1 nanosecond. The PCM layer 16 may have a T of less than 200 ℃ and preferably less than 150 ℃_C. The PCM layer 16 may have a thickness T_MAnd T_CThe large difference between them is characterized by high phase stability. The PCM layer 16 may have a T in the range of 100 to 200 ℃, 200 to 300 ℃, 300 to 400 ℃, or 400 to 500 ℃_M-T_CValue or T of more than 500 ℃_M-T_CThe value is obtained.

The upper electrode 18 and the lower electrode 14 comprise a conductive material. In an embodiment, the electrodes 14, 18 may have a size greater than 1x10³Conductivity of S/cm. In other embodiments, the electrodes 14, 18 may have a size greater than 1x10²Conductivity of S/cm or 10S/cm.

The upper electrode 18 and/or the lower electrode 14 are at least partially transparent in the visible spectrum. In an embodiment, the upper electrode 18 and the lower electrode 14 have a transmittance of at least 50% in the visible spectrum (e.g., 400nm to 700nm), or an average transmittance of at least 50%. In other embodiments, the electrodes 14, 18 have a minimum transmission of at least 60%, 70%, 80%, or 90%.

The upper electrode 18 and/or the lower electrode 14 may be or may include a metal, metal alloy, semi-metal alloy, semiconductor compound, oxide, or polymer. Suitable materials for the upper electrode 18 and the lower electrode 14 may include, but are not limited to: indium Tin Oxide (ITO), graphene, multilayer graphene, graphite, gold, PEDOT (poly (3, 4-ethylenedioxythiophene) polystyrene sulfonate)

The upper electrode 18 and the lower electrode 14 may not include the same material or the same electrical/optical characteristics.

In the exemplary embodiments described below, ITO is used because of its easy manufacturing and well-controlled optical and electrical properties.

The upper electrode 18 and/or the lower electrode 14 may have a thickness in a range of 10nm to 50 nm. The thicknesses of the upper electrode 18 and the lower electrode 14 may not be the same. In other embodiments, the upper electrode 18 and/or the lower electrode 14 may have a thickness in a range of 10nm to 20nm, 20nm to 30nm, 30nm to 40nm, or 40nm to 50 nm.

The mirror layer 12 may have a reflectivity of greater than 90% in the visible spectrum. In another embodiment, the mirror layer can have a reflectivity of greater than 85%, 80%, or 75% across the visible spectrum. The mirror layer may be or may include a material that does not absorb light in the wavelength range of interest. In an embodiment, the mirror layer may be or may include a metal layer. For example, the mirror layer 12 may be or may include aluminum.

The layer stack 20 of the detector 100 comprises a plurality of partially transparent thin films (14, 16, 18) and a substrate 10 stacked on top of each other. The layer stack may or may not include the mirror layer 12. The layer stack 20 may be designed to provide a maximum (enhanced) optical absorption in the PCM layer 16 for certain wavelengths in the visible spectrum, while providing a minimum optical absorption in the PCM layer 16 for other visible spectra. This can be achieved by using for example thin film interference effects,the thin film interference effects can be simulated using well known techniques, for example, using commercially available software such as COMSOL Multiphysics. Thus, the detector 100 may be configured to be wavelength selectable such that it operates only at certain wavelengths. In the detector 100 showing the results herein, for a 15nm thick GST layer 16, an absorption of 40-55% is obtained at a wavelength of 632nm, with the layer stack 20 at 300nm/10 μm thick SiO₂The upper part of the/Si substrate 10 has a 20nm thick ITO layer 18, a 15nm thick GST layer 16 and a 20nm thick ITO layer 14. Since the optical response is proportional to the optical absorption, the layer stack 20 may provide both wavelength selectivity and enhanced optical response at that wavelength.

The detector 100 may be fabricated using a number of processing steps known in the art. The PCM layer 16 and/or the electrodes 14, 18 may be deposited using any suitable technique, for example using physical or chemical methods such as thermal evaporation, e-beam evaporation, sputtering, chemical vapor deposition, atomic layer deposition, etc., depending on the desired material. In the following exemplary embodiment, RF sputtering is used to deposit the PCM layer 16 and the electrodes 14, 18.

The detector may be fabricated using a "bottom-up" approach, in which the PCM layer 16 and/or electrodes 14, 18 are defined by a deposition and lift-off process. Alternatively, the detector 100 may be fabricated using a "top-down" approach, wherein the PCM layer 16 and/or electrodes 14, 18 are defined by a deposition and reduction etch process. Detector fabrication may involve a combination of lift-off and etch processes. Detector fabrication may involve lithographic and/or e-beam lithographic steps. The features and steps of the manufacturing process are merely exemplary, and embodiments may be manufactured in other ways.

The upper electrode 18 and the lower electrode 14 provide electrical contact to the upper side and the lower side of the PCM layer 16, respectively. In an embodiment, the upper and lower electrodes may provide ohmic contact with the PCM layer 16 with low contact resistance. In an embodiment, the contact resistance may be less than 1x10^-3Ωcm². In other embodiments, the contact resistance may be less than 1x10^-4Ωcm²、1x 10^-5Ωcm²、1x 10^-6Ωcm²、1x 10^-7Ωcm²Or 1x10^-8Ωcm²。

The upper electrode 18 and the lower electrode 14 are further configured to be connected to a readout circuit 150, as explained more fully with reference to fig. 2.

Fig. 2 shows an apparatus 200 for detecting light according to an embodiment, comprising a detector 100 and a readout circuit 150.

The detector 100 is described with reference to fig. 1. The readout circuit 150 includes a voltage source, shown schematically as a battery. The voltage source may be a programmable voltage source or an additional control element may be provided which is operable to vary the bias voltage V applied to the PCM layer 16 via the first and second electrodes 14, 18_B. The voltage source is electrically connected to the first and second electrodes 14, 18. The readout circuit 150 also includes a current detector a for monitoring the amount of current flowing through the PCM layer 16 due to the applied bias voltage. The voltage detector V may also be arranged to monitor the bias voltage.

The voltage source may comprise a low noise DC power source (such as a battery). Alternatively, the voltage may be obtained from an AC power source. The current may be monitored by any suitable means, such as using a current preamplifier or measuring the voltage across a series resistor of known resistance. In the case of an AC power supply, the AC current or voltage may be measured using known phase sensitive detection techniques, such as a lock-in amplifier.

The detector 100 may be exposed to the light flux 32 of the light source 30. In the example shown in fig. 2, the light source 30 is a laser. However, source 30 may be any light source.

When the detector 100 is exposed to the light flux 32, the incident light is absorbed in the PCM layer 16. The generation of photo-carriers may increase the electrical conductivity of the PCM layer 16 and/or may only heat the PCM layer 16. When a constant bias voltage is applied to the PCM layer 16, a change in conductivity due to the incident light flux will result in an increase in the current through the PCM layer 16.

In a first readout mode (count rate mode), the readout circuitry 150 is configured to apply a relatively high bias voltage to the PCM layer 16, which is initially in the amorphous state, such that a relatively small amount of incident light on the PCM layer 16 will cause the temperature to rise above Tc, resulting in a phase change of the PCM layer 16 to the crystalline state. With a sufficiently high bias voltage, the detector 100 can be made very sensitive to incident light. A lower bias voltage will mean that each reset event corresponds to a higher luminous flux. The phase change will cause the resistivity of the PCM layer 16 to decrease, resulting in an increase in current that the readout circuitry is configured to detect. Readout circuitry 150 then resets detector 100 through amorphized PCM layer 16, which may occur very rapidly (e.g., less than 1ms, 100 μ s, 10 μ s, or 1 μ s). The detector 100 is then ready to be crystallized by another incident light pulse.

The rate of reset events is a measure of the intensity of light 32 incident on the detector 100. Since the sensitivity of the detector 100 depends on the applied bias voltage V_BThe sensitivity of the detector 100 may thus be dynamically adjusted, for example, in response to the rate of reset events. The sensing circuit 150 may be configured to vary the bias voltage according to a predetermined relationship to the count rate (e.g., as defined in a mathematical expression or look-up table). A higher rate of reset events may result in a reduced bias voltage, while a lower rate of reset events may result in an increased bias voltage.

In the second readout mode (sub-threshold mode), the readout circuitry 150 may be configured to detect light incident on the detector based on a change in resistance of the PCM layer prior to the phase change. In this sub-threshold readout mode, detector reset may occur when an abnormally high rate of light flux is incident on the device.

The PCM layer 16 may have different optical properties in the crystalline state (compared to the amorphous state), which may result in a change in the wavelength absorbed by the detector 100 after a phase change.

Due to the threshold switching mechanism (the pull-frack effect), the current-voltage (or IV) characteristics of PCMs (amorphous and crystalline) can be generally split into two parts: ohmic current at low bias voltage and nonlinear current at high bias voltage. Fig. 3 shows the DC IV characteristics of detector 100 in the amorphous state measured in the dark. In the amorphous state, detector 100 is typically characterized by a high resistance (R) on the order of tens of megaohms. In contrast, in the crystalline state, detector 100 is characterized as having a relatively low resistance on the order of hundreds of kiloohms (not shown).

Fig. 4 and 5 show the measured current in the detector 100 as a function of incident optical power at 632nm at a bias voltage of 1.4V when in the amorphous and crystalline states, respectively. The layer stack 20 is designed to provide 40% absorption in the PCM layer 16 at 632 nm. The data show the general trend of increasing current with increasing luminous flux. The change in resistance with increasing light flux may be due to a negative temperature coefficient of resistivity of the PCM layer and/or the photoconductive response of the PCM layer. Due to the increased conductivity, the current is greater at zero incident power in the crystalline state.

In addition to changing the temperature of the surrounding environment, the temperature of the PCM layer 16 may be changed by electrical and optical mechanisms.

Power consumption in the detector (and I) when the detector 100 is electrically biased²R proportional) increases the temperature of the layers of the detector 100 by joule heating. Since the electrical resistance of the PCM layer 16 is relatively high compared to the electrodes 14, 18, most of the power is dissipated in the PCM layer 16. Thus, the PCM layer 16 is heated higher than the electrode layers 14, 18.

Furthermore, photo-excitation of the detector 100 with light in the visible spectrum generates photo-carriers in the PCM layer 16 that initially have high energy. The photo-carriers undergo rapid energy relaxation via interaction with phonons, which transfer energy to the crystal lattice and raise the temperature of the PCM layer 16.

Any of these mechanisms may be sufficient to switch the detector 100 from the amorphous state to the crystalline state. The detector 100 according to the present invention may operate using both mechanisms.

FIG. 6 shows the optical density at 1592mW/cm under constant light excitation at 532nm²Current-voltage sweep of the detector 100 for power density measurement. Before measurement, the PCM is in the amorphous state. The voltage is swept from 0V to 4V and then back to 0V. Since the compatibility limit of the current measuring device is reached (the current is limited to a maximum of 5 μ a), there is a significant cut-off at 5 μ a. Detector 100 exhibits a clear-to-switch event from the amorphous state to the crystalline state, which is achieved by hysteresis behaviorTo verify. As shown in fig. 3 and 4, no matter at 1592mW/cm²Nor is the photo-excitation of up to 4V biased sufficient to cause phase change. This demonstrates the combined effect of light and current when switching the PCM 16. Providing a bias voltage close to 4V would mean that a relatively small incident light flux would result in a phase change of the PCM.

As shown in fig. 6, when illuminated in the amorphous state, the detector 100 is beyond the switching (threshold) voltage V at which the detector 100 switches from the amorphous state to the crystalline state_TBoth front and rear exhibit strong non-linearity. At a value exceeding V_TThereafter, when the voltage is reduced, the detector 100 remains in a crystalline state.

In use, the detector 100 may be biased to be exactly at V_TAt or below the voltage of the nonlinear switching region. Optical excitation at or near the wavelength at which the layer stack 20 has been optimized increases the current flowing through the detector 100 (by the negative TCR in combination with the effect of temperature rise due to light absorption, and/or by the photoconductive effect). This causes a phase switching event in the PCM 16, which is represented by a jump in the measured current or conductance peak. Optical excitation increases the current, which in turn causes a phase change in the PCM 16, which further increases the current. The change in current caused by the phase change may be only greater than the TCR/photoconductive response in the amorphous state.

Can be adjusted by adjusting the applied bias voltage V_BTo control the amount of light or luminous flux required to cause the switching event. Bias voltage step (lower than) V_TThe further away, the more light flux is required to switch the detector 100. Conversely, for higher sensitivity, near V may be applied_TIs electrically biased. In this way, the sensitivity of the detector 100 or the range of optical power that the detector 100 can sense may be adjusted/controlled.

The conductance peak or rapid increase in current that occurs when PCM 16 switches from the amorphous state to the crystalline state may be used as a trigger signal in a feedback circuit. The feedback circuit may be used as an automatic brightness adjuster.

A method of operating the detector 100 is shown in fig. 7. In step S1, a bias is applied to the detector. In step S2, the current and/or voltage is measured. In step S3, the measured current value is compared with a predetermined threshold value. When the measured current value is smaller than the threshold value, in step S4, the process returns to step S2. When the measured current exceeds the threshold, in step S4, a "reset" process may be initiated (step S5) to transition PCM 16 back to the amorphous state. After reset, in step S6, the bias voltage may be changed, e.g., based on the detected reset count rate, to adjust the optical power required to cause switching or reach the threshold current (as already discussed above). The current is then measured again in step S2 and the process continues. In this manner, detector 100 may continue to function to capture information about the intensity of incident light when in the amorphous state.

The reset procedure may be electrically driven. For example, a sufficiently large current may be applied to detector 100 to heat PCM 16 to T for a short period of time_MAnd allow the material to cool back to the amorphous state.

Fig. 8 shows a plurality of "reset" cycles that are performed throughout the time that the detector 100 has been electrically driven between the amorphous (high resistance) and crystalline (low resistance) states. A reset period may be advantageous/necessary to adjust and/or stabilize the detector before the optical measurement starts.

The vertical stack arrangement of the detector 100 may reduce the transport length of photo-carriers through the PCM 16 and may also reduce the transit time of the photo-carriers, i.e., the time it takes for the photo-carriers (traveling at drift velocity) to pass through the PCM 16. This may enhance the optical response by providing optical gain, whereby the photo-carriers may pass the detector 100 multiple times before recombining.

It will be appreciated that since the layer stack 20 is designed to absorb a particular wavelength or range of wavelengths of interest in the visible spectrum, the detector will not operate as expected for wavelengths outside the particular range. In other words, the layer stack 20 functions as a color filter. For example, if the detector 100 is designed to detect red light, it will not detect green or blue light. The wavelength selectivity and scalability of the detectors 100 means that they are well suited for use in image sensor arrays to capture 2D images with color information. The controllable dynamic gain may be particularly suitable for applications where the optical level varies. Applications may include artificial retinas.

Image sensors typically use a color filter array on a pixel sensor array to filter the wavelengths of light detected by the pixel sensors. A common example is a bayer filter that gives information about the intensity of red, green and blue (RGB) wavelengths. The RGB data can be converted to a full color image using a suitable demosaicing algorithm. Since the detector 100 may include a color filter, an array of detectors 100 designed for each particular color may be used without a color filter array.

An apparatus 200 operating in a mixed mode opto-electronic configuration has been described. The apparatus 200 operates in an intrinsic negative feedback loop in which the conductivity of the detector is modulated by the optical input flux and the output current signal acts as a self-modulating trigger. This essentially replicates the function of the human eye in which the amount of light striking the retina is controlled by the collapsible structure surrounding the pupil. The detector 100 is very fast, robust, sensitive and wavelength selective. In addition, detector 100 can be easily scaled down in size, enabling very high detector densities to be achieved, thereby achieving high spatial image resolution. Furthermore, unlike commonly used Charge Coupled Device (CCD) and Complementary Metal Oxide Semiconductor (CMOS) CCD image sensors, the detector 100 is low in energy, relatively simple in their construction and operation, and low in manufacturing cost.

Other variations and modifications will be apparent to the skilled person upon reading the present disclosure. Such variations and modifications may involve equivalent and other features which are already known in the art, and which may be used instead of or in addition to features already described herein.

Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature, any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention.

Features which are described in the context of a single embodiment can also be provided in combination in a single embodiment. Conversely, various features that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable subcombination.

For the sake of completeness it is also noted that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, and any reference signs in the claims shall not be construed as limiting the scope of the claims.