阅读说明：本技术 一种硅基无磷激光器及其制备方法 (Silicon-based non-phosphorus laser and preparation method thereof ) 是由 欧欣 梁好 林家杰 游天桂 王庶民 于 2020-04-14 设计创作，主要内容包括：本申请提供一种硅基无磷激光器及其制备方法,该方法包括：在InP衬底上生长InGaAs外延层；利用离子注入的方式在所述InGaAs外延层中注入离子形成损伤层；将所述InGaAs外延层与硅衬底键合；通过加热的方式将所述InGaAs外延层于所述损伤层处剥离,得到硅基InGaAs衬底；将所述硅基InGaAs衬底的顶层抛光后外延InGaAs缓冲层、超晶格位错阻挡层、腐蚀阻挡层以及激光器结构。本申请的硅基无磷激光器及其制备方法通过键合技术直接获得高质量的硅基InGaAs衬底,可以避免在硅衬底上直接外延生长III-V族化合物所产生的的失配位错、缺陷、反相畴等影响后续材料外延生长质量的问题。(The application provides a silicon-based non-phosphorus laser and a preparation method thereof, wherein the method comprises the following steps: growing an InGaAs epitaxial layer on the InP substrate; implanting ions into the InGaAs epitaxial layer by using an ion implantation mode to form a damaged layer; bonding the InGaAs epitaxial layer with a silicon substrate; stripping the InGaAs epitaxial layer at the damage layer in a heating mode to obtain a silicon-based InGaAs substrate; and polishing the top layer of the silicon-based InGaAs substrate, and then extending an InGaAs buffer layer, a superlattice dislocation barrier layer, an etching barrier layer and a laser structure. According to the silicon-based non-phosphorus laser and the preparation method thereof, the high-quality silicon-based InGaAs substrate is directly obtained through the bonding technology, and the problem that the epitaxial growth quality of subsequent materials is affected by misfit dislocation, defects, anti-phase domains and the like generated by directly epitaxially growing III-V group compounds on the silicon substrate can be solved.)

1. A preparation method of a silicon-based non-phosphorus laser is characterized by comprising the following steps:

growing an InGaAs epitaxial layer on the InP substrate;

implanting ions into the InGaAs epitaxial layer by using an ion implantation mode to form a damaged layer;

bonding the InGaAs epitaxial layer with a silicon substrate;

stripping the InGaAs epitaxial layer at the damage layer in a heating mode to obtain a silicon-based InGaAs substrate;

and polishing the top layer of the silicon-based InGaAs substrate, and then extending an InGaAs buffer layer, a superlattice dislocation barrier layer, an etching barrier layer and a laser structure.

2. The method for preparing a silicon-based non-phosphorus laser as claimed in claim 1, wherein the InP substrate is a (100) type InP single crystal substrate, and the growth thickness of the InGaAs epitaxial layer is 50-2000 nm.

3. The method of claim 1, wherein the implanted ions comprise H⁺And He⁺At least one of them, the implantation depth is 50 nm-1000 nm.

4. The method for preparing a Si-based non-phosphorus laser according to claim 1, wherein the steps of polishing the top layer of the Si-based InGaAs substrate, epitaxially growing an InGaAs buffer layer, a superlattice dislocation barrier layer and an etching barrier layer comprise:

removing the surface damage of the silicon-based InGaAs substrate, polishing the surface of the silicon-based InGaAs substrate until the roughness is less than 1nm, and then removing surface residues through chemical cleaning;

epitaxially growing an InGaAs buffer layer on the silicon-based InGaAs substrate;

epitaxially growing a superlattice dislocation blocking layer with a thickness of 5-10 nm, wherein the superlattice dislocation blocking layer has at least ten periods and is made of InGaAs and InAlAs matched with InP lattices;

and epitaxially growing an etching barrier layer on the superlattice dislocation barrier layer, wherein the etching barrier layer adopts high-doped InGaAs and has a thickness of not less than 500 nm.

5. The method for preparing a Si-based non-phosphorus laser according to claim 4, wherein the step of polishing the top layer of the Si-based InGaAs substrate and then epitaxially growing the laser structure comprises:

and epitaxially growing an InAlAs lower waveguide layer, an InAlGaAs lower limiting layer, an InGaAs quantum well, an InAlGaAs upper limiting layer, an InAlAs upper waveguide layer and an InGaAs contact layer on the corrosion barrier layer in sequence.

6. The method of claim 5, wherein the InAlAs lower waveguide layer is doped with Si at a concentration of 2x10¹⁸cm^-3Linearly down to 1x10¹⁸cm^-3；

The InAlAs upper waveguide layer adopts Be as a doping source with the doping concentration from 1x10¹⁸cm^-3Increase linearly to 2x10¹⁸cm^-3。

7. The method of claim 1, further comprising:

etching the laser structure to form a ridge waveguide;

growing silicon dioxide insulating layers on two sides and the upper surface of the ridge waveguide and the surface of the laser structure;

forming an electrode window on the ridge waveguide and the laser structure;

and preparing an electrode at the electrode window to obtain the silicon-based non-phosphorus laser.

8. A silicon-based non-phosphorus laser is characterized by comprising a silicon-based InGaAs substrate, an InGaAs buffer layer, a superlattice dislocation barrier layer, an etching barrier layer and a laser structure;

the silicon-based InGaAs substrate comprises a substrate silicon layer and an InGaAs layer;

the InGaAs buffer layer, the superlattice dislocation barrier layer, the corrosion barrier layer and the laser structure are sequentially formed on the silicon-based InGaAs substrate from bottom to top.

9. The silicon-based phosphorus-free laser of claim 8, wherein the laser structure comprises an InAlAs lower waveguide layer, an inalgas lower confinement layer, an InGaAs quantum well, an inalgas upper confinement layer, an inalgas upper waveguide layer, and an InGaAs contact layer from bottom to top.

10. The silicon-based phosphorus-free laser of claim 9, wherein the active region of the laser comprises the inalgas lower confinement layer, three periods of the InGaAs quantum well, and the inalgas upper confinement layer.

Technical Field

The invention relates to the field of semiconductors and photoelectric integration, in particular to a silicon-based phosphorus-free laser and a preparation method thereof.

Background

Silicon is the semiconductor material with the largest market share, the most mature process and the lowest cost at present, and the realization of silicon-based photoelectron integration is an important development direction for breaking through Moore's law and promoting the semiconductor industry to a further extent in the future. Limited by the physical properties of silicon (indirect bandgap), the fabrication of silicon-based light sources is a significant impediment to the realization of silicon-based optical interconnects. III-V group compounds have been widely researched in the aspect of preparing semiconductor lasers, silicon-based III-V group compound semiconductor lasers are one of solutions for preparing silicon-based light sources, and the development of silicon-based 1550nm lasers with optical communication bands has great significance in preparing silicon-based light sources and realizing silicon-based optical interconnection.

To realize the silicon-based optical communication waveband laser, a high-quality III-V/Si substrate is obtained firstly, and due to the fact that lattice mismatch of Si and III-V group materials is large, heteroepitaxy can generate a large number of dislocation and anti-phase domains, so that the quality of an epitaxial layer is influenced, and the performance of a device is limited.

Currently, the main methods for the epitaxy of phosphorus-containing materials include Metal-organic Chemical Vapor Deposition (MOCVD) and Molecular Beam Epitaxy (MBE). However, the development of the phosphorus source is hindered by safety problems caused by the extremely toxic and flammable characteristics of the phosphorus alkane serving as the source material and environmental protection and cost problems of tail gas treatment, and the solid phosphorus source is complex in structure and high in cost and is difficult to popularize.

The above problems all increase the difficulty in implementing the silicon-based communication band laser, and limit future mass production and cost reduction of the silicon-based communication band laser, so that the silicon-based communication band laser must solve the problems.

Disclosure of Invention

In view of the above-mentioned shortcomings of the prior art, the present application aims to provide a silicon-based non-phosphorus laser and a method for preparing the same, which are used to solve the problem of lattice mismatch of heteroepitaxy in the prior art.

In order to solve the technical problem, the embodiment of the application discloses a preparation method of a silicon-based non-phosphorus laser, which comprises the following steps:

growing an InGaAs epitaxial layer on the InP substrate;

implanting ions into the InGaAs epitaxial layer by using an ion implantation mode to form a damaged layer;

bonding the InGaAs epitaxial layer with a silicon substrate;

stripping the InGaAs epitaxial layer at the damage layer in a heating mode to obtain a silicon-based InGaAs substrate;

and polishing the top layer of the silicon-based InGaAs substrate, and then extending an InGaAs buffer layer, a superlattice dislocation barrier layer, an etching barrier layer and a laser structure.

Optionally, the InP substrate is a (100) type InP single crystal substrate, and the growth thickness of the InGaAs epitaxial layer is 50 to 2000 nm.

Optionally, the implanted ions comprise H⁺And He⁺At least one of them, the implantation depth is 50 nm-1000 nm.

Optionally, the step of polishing the top layer of the silicon-based InGaAs substrate and then epitaxially growing the InGaAs buffer layer, the superlattice dislocation barrier layer, and the etching barrier layer includes:

removing the surface damage of the silicon-based InGaAs substrate, polishing the surface of the silicon-based InGaAs substrate until the roughness is less than 1nm, and then removing surface residues through chemical cleaning;

epitaxially growing an InGaAs buffer layer on the silicon-based InGaAs substrate;

epitaxially growing a superlattice dislocation blocking layer with a thickness of 5-10 nm, wherein the superlattice dislocation blocking layer has at least ten periods and is made of InGaAs and InAlAs matched with InP lattices;

and epitaxially growing an etching barrier layer on the superlattice dislocation barrier layer, wherein the etching barrier layer adopts high-doped InGaAs and has a thickness of not less than 500 nm.

Optionally, the polishing the top layer of the silicon-based InGaAs substrate and then epitaxially growing the laser structure includes:

and epitaxially growing an InAlAs lower waveguide layer, an InAlGaAs lower limiting layer, an InGaAs quantum well, an InAlGaAs upper limiting layer, an InAlAs upper waveguide layer and an InGaAs contact layer on the corrosion barrier layer in sequence.

Optionally, the InAlAs lower waveguide layer adopts Si as a doping source, and the doping concentration is from 2 × 10¹⁸cm^-3Linearly down to 1x10¹⁸cm^-3；

The InAlAs upper waveguide layer adopts Be as a doping source with the doping concentration from 1x10¹⁸cm^-3Increase linearly to 2x10¹⁸cm^-3。

Optionally, the preparation method of the silicon-based phosphorus-free laser further includes:

etching the laser structure to form a ridge waveguide;

growing silicon dioxide insulating layers on two sides and the upper surface of the ridge waveguide and the surface of the laser structure;

forming an electrode window on the ridge waveguide and the laser structure;

and preparing an electrode at the electrode window to obtain the silicon-based non-phosphorus laser.

On the other hand, the embodiment of the application provides a silicon-based phosphorus-free laser, which comprises a silicon-based InGaAs substrate, an InGaAs buffer layer, a superlattice dislocation barrier layer, an etching barrier layer and a laser structure;

the silicon-based InGaAs substrate comprises a substrate silicon layer and an InGaAs layer;

the InGaAs buffer layer, the superlattice dislocation barrier layer, the corrosion barrier layer and the laser structure are sequentially formed on the silicon-based InGaAs substrate from bottom to top.

Optionally, the laser structure sequentially comprises an InAlAs lower waveguide layer, an inalgas lower limiting layer, an InGaAs quantum well, an inalgas upper limiting layer, an inalgas upper waveguide layer and an InGaAs contact layer from bottom to top.

Optionally, the active region of the laser includes the inalgas lower confinement layer, the InGaAs quantum well of three periods, and the inalgas upper confinement layer.

By adopting the technical scheme, the application has the following beneficial effects:

1) the high-quality silicon-based InGaAs substrate is directly obtained by the bonding technology, so that the problem that the epitaxial growth quality of subsequent materials is influenced by misfit dislocation, defects, anti-phase domains and the like generated by directly epitaxially growing III-V compounds on the silicon substrate can be avoided;

2) through optimizing the growth temperature, the thickness, the source furnace beam current and other parameters of the InGaAs buffer layer, the superlattice dislocation barrier layer and other structures, the threading dislocation can be reduced, the interface quality can be improved, and the device performance can be improved;

3) the laser structure design adopting InAlAs as the waveguide layer and InAlGaAs as the active region avoids the use of a phosphorus source in the growth process, can reduce the growth cost and improve the experimental safety.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

Fig. 1 is a flow chart of an alternative method for fabricating a silicon-based non-phosphorus laser according to an embodiment of the present disclosure;

FIG. 2 is a schematic diagram of an alternative InGaAs epitaxial layer grown on an InP substrate according to an embodiment of the present invention;

FIG. 3 is a schematic view of the InGaAs epitaxial layer of FIG. 2 after ion implantation to form a damage layer;

FIG. 4 is a schematic view of the InGaAs epitaxial layer of FIG. 3 bonded to a silicon substrate;

FIG. 5 is a schematic view of an alternative silicon-based InGaAs substrate in accordance with an embodiment of the present application;

FIG. 6 is a schematic view of an InGaAs buffer layer, a superlattice dislocation barrier layer, an etch barrier layer, and a laser structure after epitaxy on a silicon-based InGaAs substrate;

fig. 7 is a schematic structural diagram of an alternative laser according to an embodiment of the present application.

The following is a supplementary description of the drawings:

1-InP substrate; 2-InGaAs epitaxial layer; 201-damage layer; 301-a silicon substrate; 3-silicon-based InGaAs substrate; 4-InGaAs buffer layer; 5-superlattice dislocation barrier layers; 6-corrosion barrier layer; 7-a laser structure; 701-ridge waveguide; 702-a silicon dioxide insulating layer; 703-electrode window.

Detailed Description

The technical solution in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

Reference herein to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic may be included in at least one implementation of the invention. In the description of the present invention, it is to be understood that the terms "upper", "lower", "top", "bottom", and the like, indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, are only for convenience in describing the present invention and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed in a specific orientation, and be operated, and thus, should not be construed as limiting the present invention. Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Moreover, the terms "first," "second," and the like are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used is interchangeable under appropriate circumstances such that the embodiments of the invention described herein are capable of operation in sequences other than those illustrated or described herein.

Fig. 1 is a flowchart of a method for manufacturing an alternative silicon-based phosphorus-free laser according to an embodiment of the present application, including the following steps:

s1 growing InGaAs epitaxial layer 2 on InP substrate 1 (as shown in FIG. 2);

s2, implanting ions into the InGaAs epitaxial layer 2 by ion implantation to form a damaged layer 201 (as shown in FIG. 3);

s3, bonding the InGaAs epitaxial layer 2 with a silicon substrate 301 (shown in figure 4);

s4, stripping the InGaAs epitaxial layer 2 from the damage layer 201 by heating to obtain a silicon-based InGaAs substrate 3 (as shown in FIG. 5);

and S5, polishing the top layer of the silicon-based InGaAs substrate 3, and then extending the InGaAs buffer layer 4, the superlattice dislocation barrier layer 5, the corrosion barrier layer 6 and the laser structure 7 (as shown in figure 6).

In an alternative embodiment, the InP substrate 1 in step S1 is a (100) type InP single crystal substrate, and the InGaAs epitaxial layer 2 is grown to a thickness of 50 to 2000 nm.

As an alternative embodiment, the step S2 of injecting ions includes H⁺And He⁺At least one of them, the implantation depth is 50 nm-1000 nm.

As an alternative embodiment, the bonding manner in step S3 includes direct bonding and dielectric layer bonding;

as an alternative embodiment, the step S5 of polishing the top layer of the silicon-based InGaAs substrate 3 and then epitaxially growing the InGaAs buffer layer 4, the superlattice dislocation barrier layer 5 and the corrosion barrier layer 6 includes:

s501, removing surface damage of the silicon-based InGaAs substrate 3, polishing the surface of the silicon-based InGaAs substrate 3 until the roughness is less than 1nm, and removing surface residues through chemical cleaning;

s502, epitaxially growing an InGaAs buffer layer 4 on the silicon-based InGaAs substrate 3;

s503, epitaxially growing a superlattice dislocation barrier layer 5 with not less than ten periods on the InGaAs buffer layer 4, wherein the superlattice dislocation barrier layer 5 adopts InGaAs and InAlAs matched with InP lattices and has the thickness of 5 nm-10 nm;

and S504, epitaxially growing an etching barrier layer 6 on the superlattice dislocation barrier layer 5, wherein the etching barrier layer 6 adopts the highly doped InGaAs and has the thickness of not less than 500 nm.

The InGaAs buffer layer 4 and the superlattice dislocation barrier layer 5 can improve the wafer quality of the silicon-based InGaAs substrate 3, and the corrosion barrier layer 6 can avoid an electric contact layer under the influence of corrosion in the subsequent process.

It should be noted that the polishing of the top layer of the silicon-based InGaAs substrate includes mechanical polishing and chemical etching.

As an alternative embodiment, the step S5 of polishing the top layer of the silicon-based InGaAs substrate 3 to epitaxially grow the laser structure 7 includes:

and S505, epitaxially growing an InAlAs lower waveguide layer, an InAlGaAs lower limiting layer, an InGaAs quantum well, an InAlGaAs upper limiting layer, an InAlAs upper waveguide layer and an InGaAs contact layer on the corrosion barrier layer 6 in sequence.

In the specific implementation, the growth of the laser structure mainly adopts molecular beam epitaxy growth, an InAlAs lower waveguide layer with the thickness of 1um is epitaxially grown on the corrosion barrier layer 6, Si is adopted as a doping source, and the doping concentration is 2x10¹⁸cm^-3Linearly down to 1x10¹⁸cm^-3(ii) a Then growing an InAlGaAs lower limiting layer with the thickness of 250nm, wherein the material components are matched with the crystal lattices of the waveguide layer; then growing three lnGaAs quantum wells with the period of 8 nm; then growing an InAlGaAs upper limit layer with the thickness of 250 nm; regrowing an InAlAs upper waveguide layer with the thickness of 1um, adopting Be as a doping source and having the doping concentration of 1x10¹⁸cm^-3Increase linearly to 2x10¹⁸cm^-3(ii) a Finally growing an InGaAs contact layer with the thickness of 100nm and the doping concentration of 2x10¹⁹cm^-3。

As an optional implementation manner, the method for preparing a silicon-based phosphorus-free laser according to the embodiment of the present application further includes:

s6, etching the laser structure to form a ridge waveguide 701;

s7, growing silicon dioxide insulating layers 702 on the two sides and the upper surface of the ridge waveguide 701 and the surface of the laser structure 7;

s8, forming an electrode window 703 on the ridge waveguide 701 and the laser structure 7;

s9 electrodes are prepared at the electrode window 703 to obtain a silicon-based non-phosphorus laser (as shown in FIG. 7).

The embodiment of the application also provides a silicon-based non-phosphorus laser, which comprises a silicon-based InGaAs substrate, an InGaAs buffer layer, a superlattice dislocation barrier layer, an etching barrier layer and a laser structure;

the silicon-based InGaAs substrate comprises a substrate silicon layer and an InGaAs layer;

the InGaAs buffer layer, the superlattice dislocation barrier layer, the corrosion barrier layer and the laser structure are sequentially formed on the silicon-based InGaAs substrate from bottom to top.

As an optional implementation manner, the laser structure sequentially comprises an inalgas lower waveguide layer, an inalgas lower limiting layer, an InGaAs quantum well, an inalgas upper limiting layer, an inalgas upper waveguide layer and an InGaAs contact layer from bottom to top.

As an alternative embodiment, the active region of the laser comprises an InAlGaAs lower limiting layer, three periods of InGaAs quantum wells and an InAlGaAs upper limiting layer.

Table 1 is a structural parameter table of a silicon-based phosphorus-free laser according to an embodiment of the present disclosure, in which a second column represents components of each part of the silicon-based phosphorus-free laser, a third column represents deposition thickness of each part of the silicon-based phosphorus-free laser, and a fourth column represents doping concentration of each part of the silicon-based phosphorus-free laser.

Specifically, the first column in the table lists a specific structure of a silicon-based non-phosphorus laser in an embodiment of the present application, including: a silicon-based InGaAs substrate; an InGaAs buffer layer formed on the substrate, the InGaAs buffer layer having a thickness of 1 μm, and using Si as a doping source with a doping concentration of 5 × 10¹⁸cm^-3(ii) a A superlattice dislocation blocking layer formed on the InGaAs buffer layer, wherein the superlattice dislocation blocking layer adopts InGaAs and InAlAs matched with InP lattices, the thicknesses of the superlattice dislocation blocking layer and the InAs are both 10nm, and the superlattice dislocation blocking layer grows for 10 periods; an InAlAs lower waveguide layer formed on the superlattice dislocation barrier layer, the InAlAs lower waveguide layer has a thickness of 1 μm, adopts Si as a doping source, and has a doping concentration of 1x10¹⁸cm^-3(ii) a The InAlGaAs lower limiting layer is formed on the InAlGaAs lower waveguide layer and has the thickness of 250 nm; an InGaAs quantum well formed on the InAlGaAs lower limiting layer, the thickness of the InGaAs quantum well is 8nm, and the InGaAs quantum well grows for 3 weeksThe semiconductor device further comprises an InAlGaAs isolation layer with 2 periods and used for isolating the 3 InGaAs quantum wells, wherein the thickness of the InAlGaAs isolation layer is 8 nm; the InAlGaAs upper limit layer is formed on the InGaAs quantum well, and the thickness of the InAlGaAs upper limit layer is 250 nm; an InAlAs upper waveguide layer formed on the InAlGaAs upper limiting layer, the InAlAs lower waveguide layer has a thickness of 1.4 μm, and adopts Be as doping source with a doping concentration of 2 × 10¹⁸cm^-3(ii) a An InGaAs contact layer formed on the InAlAs lower waveguide layer, the thickness of the InGaAs contact layer is 100nm, Be is used as a doping source, and the doping concentration is 2x10¹⁹cm^-3。

Table 1:

	composition (I)	Thickness (nm)	Doping concentration
				InGaAs contact layer	P+InGaAs：Be	100	2x1019cm-3
InAlAs upper waveguide layer	PInAlAs：Be	1400	2x1018cm-3
				InAlGaAs upper limiting layer	InAlGaAs	250
InGaAs quantum well	InGaAs	8
				InAlGaAs isolation layer	InAlGaAs	8
InGaAs quantum well	InGaAs	8
				InAlGaAs isolation layer	InAlGaAs	8
InGaAs quantum well	InGaAs	8
				InAlGaAs lower limiting layer	InAlGaAs	250
InAlAs lower waveguide layer	N InAlAs：Si	1000	1x1018cm-3
				Superlattice dislocation barrier layer	N+InAlAs/InGaAs：Si	10/10*10
InGaAs buffer layer	N+InGaAs：Si	1000	5x1018cm-3
				InGaAs substrate	InGaAs/Si

By using the technical scheme of the embodiment of the application, the method has the following beneficial effects:

1) the high-quality silicon-based InGaAs substrate is directly obtained by the bonding technology, so that the problem that the epitaxial growth quality of subsequent materials is influenced by misfit dislocation, defects, anti-phase domains and the like generated by directly epitaxially growing III-V compounds on the silicon substrate can be avoided;

2) through optimizing the growth temperature, the thickness, the source furnace beam current and other parameters of the InGaAs buffer layer, the superlattice dislocation barrier layer and other structures, the threading dislocation can be reduced, the interface quality can be improved, and the device performance can be improved;

3) the laser structure design adopting InAlAs as the waveguide layer and InAlGaAs as the active region avoids the use of a phosphorus source in the growth process, can reduce the growth cost and improve the experimental safety.

The above description is only for the purpose of illustrating the preferred embodiments of the present invention and is not to be construed as limiting the invention, and any modifications, equivalents, improvements and the like that fall within the spirit and principle of the present invention are intended to be included therein.