CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation of U.S. application Ser. No. 15/061,156, filed Mar. 4, 2016, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 62/128,112, filed Mar. 4, 2015, the entire content of which is hereby incorporated herein by reference.
STATEMENT REGARDING GOVERNMENT SUPPORT This invention was made with government support under U.S. National Science Foundation Award Nos. ECCS-1408051 and DMR-1505122. The U.S. government has certain rights in the invention.
FIELD OF INVENTION In general, the invention relates generally to III-Nitride materials and fabrication methods. In more detail, the invention relates to a material structure and method for generating a III-Nitride digital alloy.
BACKGROUND III-Nitride materials have been extensively studied and implemented in advanced solid state lighting technologies in recent decades. The III-Nitride platform has also attracted tremendous efforts in developing high performance active region for optoelectronic devices including detectors and solar energy convertors. Specifically, the demand for integrating devices covering a broad spectral regime in a single nitride-based material platform drives the further pursuit of III-Nitride materials with a tunable band gap property.
The identification of the narrow bandgap in InN binary alloys (˜0.64 eV) and large bandgap in AlN binary alloys (˜6 eV) has enabled access to broad energy gap coverage by utilizing corresponding ternary and quaternary alloys with different Indium (In)/Gallium (Ga)/Aluminum (Al) composition. For example, varying the Indium (In) composition in InGaN ternary alloy from very low to high In-content provides the ability to cover a broad optical regime from ˜3.4 eV (GaN) to ˜0.64 eV (InN). Similarly, tuning the Aluminum (Al) composition in the AlGaN ternary alloy allows the transition energy to change from ˜3.4 eV (GaN) to ˜6 eV (AlN).
The InGaN ternary alloy with high In content has been recognized for its importance in achieving optical emission and absorption devices covering the visible spectral regime from blue to red emission, while the AlGaN ternary alloy is critical for application in deep-UV regime. However, the experimental realization of such material systems has been limited by the challenges in growing conventional ternary and quaternary alloys with high indium and aluminum composition.
In particular, the conventional epitaxy of InGaN alloy with high In composition results in a phase separated material system, which leads to detrimental issues in the electronics and optoelectronic properties of this alloy. The limitation of growing high quality InGaN alloy with high In content has been one of the major barriers in the realization of high performance optoelectronic devices employing indium rich InGaN alloys for longer wavelength applications. Therefore, new strategies are necessary to access the epitaxy of high crystalline quality III-Nitride quaternary and ternary material systems and eventually achieve the broad tunability of optoelectronic properties in the III-Nitride platform.
SUMMARY OF INVENTION The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is not intended to identify key/critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
According to one embodiment, a method of forming a III-Nitride quaternary digital alloy (“DA”) of AlGaInN comprises generating a periodic structure of closely separated binary alloy layers, each of said binary alloy layers comprising one of AlN, GaN and InN, wherein each of said binary alloy layers has a respective thickness of 1-2 monolayers (“ML”s) and said periodic structure of binary alloy layers has a total thickness of between 10-50 periods.
A method of forming a III-Nitride ternary DA comprises generating a periodic structure of closely separated binary alloy layers of a first type and a second type, each of said first type of binary alloy layer comprising one of AlN, GaN and InN and each of said second type of binary alloy layer comprising one of AlN, GaN and InN, wherein the first type of binary alloy layer is different from the second type of binary alloy layer and each of said binary alloy layers has a respective thickness of 1-4 MLs and said periodic structure of binary alloy layers has a total thickness of between 10-50 periods.
A III-Nitride quaternary DA of AlGaInN comprises a periodic structure of closely separated binary alloy layers, each of said binary alloy layers comprising one of AlN, GaN and InN, wherein each of said binary alloy layers has a respective thickness of 1-2 monolayers (“ML”s) and said periodic structure of binary alloy layers has a total thickness of between 10-50 periods.
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic illustration showing how various ternary and quaternary DAs can be achieved according to one embodiment.
FIG. 2A is a schematic illustration of a quaternary III-Nitride digital alloy achieved by generating a periodic structure of closely separated binary alloy layers according to one embodiment.
FIG. 2B is a schematic illustration of a ternary III-Nitride digital alloy achieved by generating a periodic structure of two different closely separated binary alloys according to one embodiment.
FIG. 3 illustrates one particular embodiment of a quaternary AlGaInN DA according to one embodiment.
FIGS. 4A illustrates a particular embodiments of a ternary DA of InGaN according to one embodiment.
FIGS. 4B illustrates a particular embodiments of a ternary DA of AlGaN according to one embodiment.
FIGS. 4C illustrates a particular embodiments of a ternary DA of AlInN according to one embodiment.
FIG. 5A is a plot illustrating Indium (In)-content in InGaN DA as a function of thickness of each binary alloy layer respectively according to one embodiment.
FIG. 5B is a plot illustrating Aluminum (Al)-content in AlGaN DA as a function of thickness of each binary alloy layer respectively according to one embodiment.
FIG. 6A is a plot illustrating calculated miniband structures of InGaN DA utilizing 2 ML GaN with 2 ML InN as one single period element according to one embodiment.
FIG. 6B is a plot illustrating calculated miniband structures of InGaN DA utilizing 2 ML GaN with 4 ML InN as one single period element according to one embodiment.
FIG. 6C is a plot illustrating calculated miniband structures of InGaN DA utilizing 4 ML GaN with 2 ML InN as one single period element according to one embodiment.
FIG. 6D is a plot illustrating calculated miniband structures of InGaN DA utilizing 4 ML GaN with 4 ML InN as one single period element according to one embodiment.
FIG. 7A is a plot illustrating calculated miniband structures of AlGaN DA utilizing 2 ML GaN with 2 ML AlN as one single period element according to one embodiment.
FIG. 7B is a plot illustrating calculated miniband structures of AlGaN DA utilizing 4 ML GaN with 2 ML AlN as one single period element according to one embodiment.
FIG. 7C is a plot illustrating calculated miniband structures of AlGaN DA utilizing 2 ML GaN with 4 ML AlN as one single period element according to one embodiment.
FIG. 7D is a plot illustrating calculated miniband structures of AlGaN DA utilizing 4 ML GaN with 4 ML AlN as one single period element according to one embodiment.
FIG. 8A shows tunable energy gaps of InGaN DAs formed by M ML GaN and N ML InN ultra-thin binary-alloy layers according to one embodiment.
FIG. 8B shows tunable energy gaps of AlGaN DAs formed by M ML AlN and N ML GaN ultra-thin binary-alloy layers according to one embodiment.
FIG. 9A show calculated wave functions overlap of ground-state electron and heavy hole as a function of the thickness of each binary-alloy layer for InGaN DA.
FIGS. 9B show calculated wave functions overlap of ground-state electron and heavy hole as a function of the thickness of each binary-alloy layer for AlGaN DA.
FIG. 10 illustrates the calculated band edge energy position of the valence sub-bands (HH and CH bands) in AlGaN DA as a function of the thickness of each ultra-thin binary alloy layer.
DETAILED DESCRIPTION Applicants have devised a method and system for accessing all possible ternary and quaternary III-Nitride alloys without the need for employing high In content and/or high Al content in III-Nitride structures. According to one embodiment, a set of artificially engineered nano-structures based on finite short-period superlattice structures in which different III-Nitride ultra-thin binary-alloys are utilized to overcome conventional limitations in growing high quality III-Nitride alloys. To this end, Applicants have devised a structure, herein referred to as a III-Nitride Digital Alloy (“DA”) comprising a set of artificially engineered nano-structures, which are based on finite short-period superlattices formed by closely-separated binary alloy layers.
According to embodiments described herein, DAs provide an artificial engineered material structure exhibiting a large tunability in their respective optoelectronic properties. Based on the concept of DA, the phase separation issue of conventional ternary alloys is avoided naturally in this nano-structure through the alternate epitaxy of high quality binary alloys. Moreover, employing very thin GaN and InN binary layers introduces strong inter-well resonant coupling effect within the superlattice structure and therefore forms miniband structures. Taking advantage of such resonant coupling effect, miniband engineering can be performed by carefully designing the DA nano-structure and controlling the thickness of those binary thin layers during epitaxy. In this fashion, an effective “digital alloy” can be achieved with tunable optoelectronic properties comparable to that of bulk alloy. The thickness of each binary layer in the DA, represented by a monolayer (ML), determines the tunable optoelectronic properties of the resultant material. In particular, according to one embodiment, employing ultra-thin binary layers with thickness ranging from 1 to 4 MLs introduces a strong inter-well resonant coupling effect within the superlattice structure resulting in the formation of miniband structures.
DAs can be deposited by an epitaxial method employing alternate growth of ultra thin layers of high crystalline quality AlN, GaN, and InN binary alloys. By designing the combination of these binary alloys, a quaternary DA of AlGaInN and a ternary DA of AlGaN, InGaN, and AlInN can be obtained.
FIG. 1 is a schematic illustration showing how various ternary and quaternary DAs can be achieved according to one embodiment. In particular, the center of the triangle shown in FIG. 1 illustrates generation of a quaternary DA alloy. The sides of the triangle shown in FIG. 1 illustrate the generation of a ternary DA. Thus, as shown in FIG. 1 a quaternary alloy of AlGaInN may be obtained by a periodic structure of closely separated, thin binary layers of InN, GaN and AlN. A ternary DA of AlInN to be generated via a periodic structure of closely separated thin binary layers of AlN and InN. A ternary DA of InGaN may be generated via a periodic structure of close separated thin binary layers of InN and GaN. A ternary DA of AlGaN may be generated via a periodic structure of closely separated thin binary layers of AlN and GaN.
By performing an alternate epitaxy of high quality ultra-thin III-Nitride binary alloys, the growth issues of conventional III-Nitride alloys are naturally avoided using the DA method described herein. In particular, FIG. 2A is a schematic illustration of a quaternary III-Nitride digital alloy achieved by generating a periodic structure of closely separated binary alloy layers according to one embodiment. As shown in FIG. 2A, a quaternary DA 110 is achieved by generating a periodic structure utilizing a periodic element 102, wherein periodic element 102 comprises a group of three different ultra-thin binary-alloy layers (104, 106 and 108), which are closely-separated from one another. According to one embodiment, the binary alloy layers comprising periodic element 102 may be AlN, GaN and InN. By combining all three ultra-thin binary-alloy layers (e.g. AlN, GaN, and InN) to form a short-period superlattice, an AlGaInN quaternary DA can be achieved. Thus, referring to FIG. 2A, a quaternary DA may be achieved via a periodic structure of P periods of periodic elements 102(1)-102(P), wherein each periodic element comprises three digital alloys (e.g., 104(1), 106(1) and 108(1)-104(P), 106(P) and 108(P).
Further, each of the binary alloys (e.g., AlN, GaN and InN) within a periodic element 102 is associated with a respective thickness represented in monolayer (“ML”) units. Thus, referring again to FIG. 2A, the AlN binary layers may exhibit a thickness of L ML, the GaN digital alloy may exhibit a thickness of M ML and the AlN digital alloy may exhibit a thickness of N ML. By varying the respective thicknesses of the digital alloys (i.e., M, L and N) within a periodic element, the optoelectronic properties of the DA may be tuned.
Quaternary DA 110 is also associated with a total thickness T. According to one embodiment, the total thickness T must be finite and is determined in order to preserve a coherency of a wave function in quaternary DA 110.
FIG. 2B is a schematic illustration of a ternary III-Nitride digital alloy achieved by generating a periodic structure of two different closely separated binary alloys according to one embodiment. According to one embodiment, the binary alloy layers may be two different alloys of AlN, GaN, and InN. In particular, FIG. 2B is a schematic illustration of a ternary III-Nitride digital alloy achieved by generating a periodic structure of closely separated binary alloy layers according to one embodiment. As shown in FIG. 2B, ternary DA 112 is achieved by generating a periodic structure utilizing a periodic element 102, wherein periodic element 102 comprises a group of two different ultra-thin binary-alloy layers (104, 106), which are closely-separated from one another. According to one embodiment, the binary alloy layers comprising periodic element 102 may be any two different ones from the group of AlN, GaN and InN. Thus, according to one embodiment, by combining two ultra-thin binary-alloy layers (e.g. two from the group of AlN, GaN, and InN) to form a short-period superlattice, either an AlGaN, AlInN or InGaN ternary DA 112 may be achieved. Thus, referring to FIG. 2B, a ternary DA may be achieved via a periodic structure of P periods of periodic elements 102(1)-102(P), wherein each periodic element comprises two digital alloys (e.g., 104(1), 106(1)-104(P), 106(P)).
Further, each of the binary alloys (e.g., AlN, GaN and InN) within a periodic element 102 is associated with a respective thickness represented in monolayer units. Thus, referring again to FIG. 2B, the AlN binary layers may exhibit a thickness of L monolayers, the GaN digital alloy may exhibit a thickness of M ML and the AlN digital alloy may exhibit a thickness of N ML. By varying the respective thicknesses of the digital alloys within a periodic element (i.e., M and L), the optoelectronic properties of the DA may be tuned.
Ternary DA 112 is also associated with a total thickness T. The total thickness T must be finite and is determined according to one embodiment in order to preserve a coherency of the wave function in the structure.
FIG. 3 illustrates one particular embodiment of a quaternary AlGaInN DA according to one embodiment. Referring to FIG. 3 AlGaInN DA 302 comprises a periodic structure of periodic elements, wherein each periodic element comprises three binary digital alloys of AlN, GaN and InN. As shown in FIG. 3, AlGaInN DA 302 is achieved by generating a periodic structure utilizing a plurality of periodic elements 102 (only one specific periodic element 102 is called out in FIG. 3), wherein periodic element 102 comprises a group of three different ultra-thin binary-alloy layers of AlN, GaN, and InN, which are closely-separated from one another.
As shown in FIG. 3, each periodic element, 102, comprises L ML layers of InN, M ML of GaN and N ML of AlN. According to one embodiment, the thicknesses of each layer of InN, GaN and AlN in ML may be varied to tune the optoelectric properties of quaternary DA 302. That is, the variables L, M, N representing the number of ML of InN, GaN and AlN may be varied to tune the optoelectrical properties of AlGaInN DA 302.
FIGS. 4A-4C illustrate particular embodiments of ternary DAs of AlGaN, InGaN and AlInN respectively according to one embodiment. Referring to FIG. 4A InGaN DA 402 comprises a periodic structure of periodic elements, wherein each periodic element comprises two binary alloys of InN and GaN. InGaN DA 402 is achieved by generating a periodic structure utilizing a plurality of periodic elements 102 (only one specific periodic element 102 is called out in FIG. 4A), wherein periodic element 102 comprises a group of two ultra-thin binary-alloy layers of GaN, and InN, which are closely-separated from one another.
As shown in FIG. 4A, each periodic element, 102, comprises M ML layers of GaN and N ML of InN. According to one embodiment, the thicknesses of each layer of GaN and InN in ML may be varied to tune the optoelectric properties of ternary DA 402. That is, the variables M and N representing the number of ML of GaN and InN may be varied to tune the optoelectrical properties of DA 402.
According to one embodiment, M and N are varied between 1-4 ML. Further, a total thickness T of 10-50 periods is used.
Referring to FIG. 4B AlGaN DA 404 comprises a periodic structure of periodic elements, wherein each periodic element comprises two binary alloys of AlN and GaN. AlGaN DA 404 is achieved by generating a periodic structure utilizing a plurality of periodic elements 102 (only one specific periodic element 102 is called out in FIG. 4B), wherein periodic element 102 comprises a group of two ultra-thin binary-alloy layers of AlN, and GaN, which are closely-separated from one another.
As shown in FIG. 4B, each periodic element, 102, comprises M ML layers of GaN and N ML of AlN. According to one embodiment, the thicknesses of each layer of GaN and AlN in ML may be varied to tune the optoelectric properties of ternary DA 404. That is, the variables M and N representing the number of ML of GaN and AlN may be varied to tune the optoelectrical properties of AlGaN DA 404.
According to one embodiment, M and N are varied between 1-4 ML. Further, a total thickness T of 10-50 periods is used.
Referring to FIG. 4C AlInN DA 406 comprises a periodic structure of periodic elements, wherein each periodic element comprises two binary alloys of AlN and InN. AlInN DA 406 is achieved by generating a periodic structure utilizing a plurality of periodic elements 102 (only one specific periodic element 102 is called out in FIG. 4C), wherein periodic element 102 comprises a group of two ultra-thin binary-alloy layers of AlN, and InN, which are closely-separated from one another.
As shown in FIG. 4C, each periodic element, 102, comprises M ML layers of InN and N ML of AlN. According to one embodiment, the thicknesses of each layer of InN and AlN in ML may be varied to tune the optoelectric properties of ternary DA 406. That is, the variables M and N representing the number of ML of InN and AlN may be varied to tune the optoelectrical properties of DA 406.
According to one embodiment, M and N are varied between 1-4 ML. Further, a total thickness T of 10-50 periods is used.
FIGS. 5A-5B illustrate Indium (In)-content in InGaN DA 402 and Aluminum (Al)-content in AlGaN DA 404 as a function of the thickness of each binary alloy layer respectively. According to one embodiment, the In-content x in the InxGa1-xN DA (and the Al-content x in the AlxGa1-xN) is determined by the duty cycle x=n/(m+n). Referring to FIGS. 5A-5B, the In-content and Al-content can both be tuned from 20% to 80% by varying the thickness of each layer from 1 to 4 MLs in InGaN DA 402 and AlGaN DA 404.
FIGS. 6A-6D illustrate calculated miniband structures of four exemplary InGaN DAs according to one embodiment. In particular, FIG. 6A shows an InGaN DA 402 utilizing 2 ML GaN with 2 ML InN as one single period element. FIG. 6B shows an InGaN DA 402 utilizing 2 ML GaN with 4 ML InN as one single period element. FIG. 6C shows an InGaN DA 402 utilizing 4 ML GaN with 2 ML InN as one single period element. FIG. 6D shows an InGaN DA 402 utilizing 4 ML GaN with 4 ML InN as one single period element.
FIGS. 6A-6D illustrate that the effective energy gap between the ground-state miniband in conduction band (C-1) and the ground-state miniband in valence band (HH-1) is reduced as the thickness of those InN binary layers increased. Further, as the thickness of the GaN and InN layers is reduced, the bandwidth of each miniband increases. These trends suggest that the energy gap as well as the optoelectronic properties of the DA can be engineered by simply tuning the thickness of each ultra-thin binary-alloy layer in the DAs.
Similar phenomenon can be observed in the AlGaN DA as shown in FIGS. 7A-7D. In particular, FIGS. 7A-7D illustrate calculated miniband structures of four exemplary AlGaN DAs according to one embodiment. In particular, FIG. 7A shows an AlGaN DA 404 utilizing 2 ML GaN with 2 ML AN as one single period element. FIG. 7B shows an AlGaN DA 404 utilizing 2 ML AN with 4 ML GaN as one single period element. FIG. 7C shows an AlGaN DA 404 utilizing 4 ML AN with 2 ML GaN as one single period element. FIG. 7D shows an AlGaN DA 404 utilizing 4 ML GaN with 4 ML AN as one single period element.
FIG. 8A shows tunable energy gaps of InGaN DAs formed by M ML GaN and N ML InN ultra-thin binary-alloy layers according to one embodiment. In particular, according to one embodiment, the effective energy gap of InGaN DAs 402 can be engineered from 0.63 eV (with 1 ML GaN and 4 MLs InN) to 2.4 eV (with 4 MLs GaN and 1 ML InN). Correspondingly, according to one embodiment, a transition wavelength of InGaN DA 402 varied between ˜510 nm to ˜1900 nm covering the green up to infrared regime.
FIG. 8B shows tunable energy gaps of AlGaN DAs formed by M ML AlN and N ML GaN ultra-thin binary-alloy layers according to one embodiment. As shown in FIG. 8B, the tunable energy gap be engineered from 3.96 eV (with 1 ML AlN and 4 MLs GaN) to 5.20 eV (with 4 MLs AlN and 1 ML GaN). The corresponding transition wavelength of AlGaN DA 404 according to this embodiment ranged from 18 240 nm to ˜310 nm covering the deep-UV regime. The broad tunability of energy gaps covered by InGaN DAs and AlGaN DAs implies great potential of such III-Nitride DAs as nano-engineered active regions for optoelectronics applications.
FIG. 9A show calculated wave function overlap of ground-state electron and heavy hole as a function of the thickness of each binary-alloy layer for InGaN DA. As shown in FIG. 9A, the ground state carrier wavefunction overlap in InGaN DA is varied from 86% to 99% while the thickness of the GaN and InN binary-alloy layer changes from 1 to 4 MLs.
FIG. 9B show calculated wave function overlap of ground-state electron and heavy hole as a function of the thickness of each binary-alloy layer for AlGaN DA. As shown in FIG. 9B, the ground-state carrier wavefunction overlap in the AlGaN DA ranges from 75% to 97% while the thickness of the AlN and GaN binary-alloy layer changes from 1 to 4 MLs.
As shown in FIGS. 9A-9B, for InGaN and AlGaN, the polarization induced charge separation issue is effectively suppressed within the III-Nitride DA structures by employing ultra-thin binary-alloy layers. Eventually, the entire III-Nitride DA performs as a complete “active alloy” that exhibits comparable characteristics of a conventional alloy. The large overlaps observed in these DAs provide a strong suggestion that these nano-structures behave as an effective “alloy”.
By employing a AlGaN DA structure, the valence band cross over issue in the conventional AlGaN ternary alloy with high Al-content can be solved. The valence band cross over issue is attributed to relocation of the crystal-field spilt-off hole (CH) band sufficiently higher than the heavy hole (HH) band. Thus the dominant transition in the conventional AlGaN active region will switch from C—HH to C—CH leading to a dominant TM-polarized emission. Such dominant TM-polarized emission is not preferable in the top emitter application due to its low extraction efficiency.
FIG. 10 illustrates the calculated band edge energy position of the valence sub-bands (HH and CH bands) in AlGaN DA as a function of the thickness of each ultra-thin binary alloy layer. Referring to FIG. 10, it is clear to see that the HH band is always located sufficiently high above the CH band due to the valence band rearrangement. Such phenomenon indicates that the dominant transition in the AlGaN DA can be always the C—HH transition leading to the dominant TE-polarized emission with high efficiency for the top emitter application.
These and other advantages maybe realized in accordance with the specific embodiments described as well as other variations. It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments and modifications within the spirit and scope of the claims will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.
