LATTICE-CONSTANT FORMATTED EPITAXIAL TEMPLATE FOR LIGHT EMITTING DEVICES AND A METHOD FOR MAKING THE SAME
A lattice constant formatted epitaxial template for light emitting devices includes a starting epitaxial template having a base, a plurality of alternately arranged protrusions and depressions on the base; first material portions epitaxially formed on top of the protrusions and second material portions epitaxially formed in the depressions, wherein lattice constants of the first material portions on the protrusions are different from those of the second material portions in the depressions. A method for making a lattice constant formatted epitaxial template is provided. Also provided is a light emitting device containing a lattice constant formatted epitaxial template.
The present invention relates in general to a lattice-constant formatted epitaxial template for semiconductor light emitters, more particularly for group III nitride compound semiconductor ultraviolet light or visible light emitters, a method of forming the same, and a light emitting device containing a lattice constant formatted epitaxial template.
1. DESCRIPTION OF THE RELATED ARTNitride compound semiconductors such as InN, GaN, AlN, and their ternary and quaternary alloys are viewed as very important optoelectronic materials. Depending on alloy composition, nitride compounds can enable ultraviolet (UV) emissions ranging from 410 nm down to approximately 200 nm. This includes UVA (400-315 nm), UVB (315-280 nm), and part of UVC (280-200 nm) regimes. UVA emissions are leading to revolutions in curing industry, and UVB and UVC emissions owing to their germicidal effect are looking forward to general adoption in food, water, and surface disinfection businesses. Compared to the traditional UV light sources, such as mercury lamps, UV light emitters made of nitride compounds offer intrinsic merits. In general, nitride UV emitters are robust, compact, spectrum adjustable, and environmentally friendly. They offer high UV light intensity & dosage, which is ideal treatment for fresh food, water and surface storage, disinfection, and sterilization. Further, the light output can be modulated at frequencies up to a few hundreds of mega-hertz, promising them innovative light sources for covert communication and bio-chemical detection.
At the present, commercially available UVB and UVC light-emitting diodes (LEDs) commonly adopt the laminate structure containing a c-plane sapphire as UV transparent substrate, an AlN layer coated over the substrate serving as epitaxy template, and a set of AlN/AlGaN superlattice for dislocation and strain management. The utilization of AlN template and AlN/AlGaN superlattice enables the growth of high-quality high-conductivity n-type AlGaN electron supplier layer, which injects electrons into the following AlGaN-based multiple quantum well (MQW) active-region. On the other side of the MQW active-region are an AlGaN electron-blocking layer, an AlGaN hole injection layer, a hole supplier layer and a p-type GaN or InGaN layer for ohmic contact formation. The prior art AlGaN UV LED structures can be found in the reference. (e.g., “Milliwatt power deep ultraviolet light-emitting diodes over sapphire with emission at 278 nm”, J. P. Zhang, et al, APPLIED PHYSICS LETTERS 81, 4910 (2002), the content of which is incorporated herein by reference in its entirety.).
Prior art UV LEDs suffer from low light output efficiency. Firstly, there is no lattice matched substrate for AlGaN based devices, which means strain management is essential for AlGaN device performance. When using UV transparent sapphire as substrate, an AlN template is inevitable. The AlN template is preferably to be thick, so as to maintain high structural quality. Thick AlN template or bulk AlN substrate exerts large biaxial compression on the overlying AlGaN based device structure, resulting in additional dislocation generation and surface roughness leading to inferior device performance. When forming AlGaN based UV emitters on GaN substrate or template, the UV emitters experience strong biaxial tension from the larger GaN lattice constants. Biaxial tensile strain not only generates dislocations to deteriorate device internal quantum efficiency but also lead to surface cracks which are deadly device defects. In the past, AlN/AlGaN superlattice had been applied to manage strain for AlGaN heteroepitaxy over foreign substrates such as sapphire and GaN (e.g. “Crack-free thick AlGaN grown on sapphire using high temperature AlN/AlGaN superlattice for strain management”, J. P. Zhang, et al, Appl. Phys. Lett. 80, 3542 (2002); “AlGaN layers grown on GaN using strain-relief interlayers”, C. Q. Chen, et al, Appl. Phys. Lett. 81, 4961 (2002), and U.S. Pat. No. 7,547,925.). And pulsed source supply method was also investigated for high quality AlN and AlGaN epitaxial growth (e.g., U.S. Pat. No. 7,811,847, and “Pulsed Atomic Layer Epitaxy of Ultrahigh-Quality AlxGa1-xN structures for Deep Ultraviolet Emissions below 230 nm”, J. P. Zhang et al, Appl. Phys. Lett. 81, 4392 (2002)). The other drawback of the prior art UV LEDs comes from light extraction, since the employment of UV absorbing p-GaN or p-InGaN contact layer and the large differences in the refractive indexes among air, sapphire, AlN, and AlGaN make light extraction out of the solid state device marginal, usually limiting light extraction efficiency to as small as 6%-10%.
The present invention discloses UV LED structures with greatly improved strain status and improved internal quantum and light extraction efficiencies and the method to form these UV LEDs.
2. SUMMARY OF THE INVENTIONOne aspect of the present invention is directed to a light emitting device, which includes:
an n-type layer;
a p-type layer;
an active region sandwiched between the n-type layer and the p-type layer; and
a lattice constant formatted epitaxial template;
wherein the lattice constant formatted epitaxial template has a growing surface on which the n-type layer grows, and the growing surface has a plurality of first material portions and a plurality of second material portions with each of the first material portions being alternately arranged with each of the second material portions, and wherein lattice constants of the first material portions are different from those of the second material portions.
Another aspect of the present invention is directed to a lattice constant formatted epitaxial template, which includes:
a starting epitaxial template comprising a base, a plurality of alternately arranged protrusions and depressions on the base;
first material portions epitaxially formed on top of the protrusions and second material portions epitaxially formed in the depressions, wherein lattice constants of the first material portions on the protrusions are different from those of the second material portions in the depressions.
Another aspect of the present invention is directed to a light emitting device, which includes:
an n-type layer;
a p-type layer;
an active region sandwiched between the n-type layer and the p-type layer;
wherein the n-type layer contains alternatively arranged first portions and second portions, and lattice constants of the first portions are smaller than lattice constants of the second portions.
Another aspect of the present invention is directed to a light emitting device, which includes:
a p-type layer;
a light emitting active region; and
the above described lattice constant formatted epitaxial template;
wherein the active region is directly formed on the first material portions and the second material portions of the lattice constant formatted epitaxial template.
Another aspect of the present invention is directed to a method of making a lattice constant formatted epitaxial template, which includes:
providing a starting epitaxial template having a plurality of alternately arranged protrusions and depressions;
simultaneously and epitaxially growing first material portions on the protrusions and second material portions in the depressions from the same feed material containing at least two elements with different surface diffusion lengths, wherein composition segregation occurs between the first material portions and the second material portions due to the difference in the surface diffusion lengths of the at least two elements and the dimensions of the protrusions and the depressions.
The method may further include:
providing an epitaxial template substrate; and
forming, via a nanoimprint or photolithography and etch process, the protrusions and the depressions on the epitaxial template substrate to obtain the starting epitaxial template, wherein the protrusions have a lateral dimension of 100-3000 nm, a height of 200-5000 nm, and a pitch of 200-3500 nm.
In the above method, the first material portions and the second material portions are made of AlGaN via the feed material including trimethylaluminium (TMA), trimethylgallium (TMG), and ammonia.
The method may further include:
forming an n-type layer on the first and second material portions;
forming an active region over the n-type layer; and
forming a p-type layer over the active region.
The method may further include removing the starting epitaxial template, the first material portions and the second material portions so as to expose the n-type layer.
The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this application, illustrate embodiments of the invention and together with the description serve to explain the principle of the invention. Like reference numbers in the figures refer to like elements throughout, and a layer can refer to a group of layers associated with the same function.
The present invention discloses a lattice constant formatted epitaxial template for a light emitting device with improved internal quantum and light extraction efficiencies. Throughout the specification, the term III-nitride or nitride in general refers to metal nitride with cations selecting from group MA of the periodic table of the elements. That is to say, III-nitride includes AlN, GaN, InN and their ternary (AlGaN, InGaN, InAlN) and quaternary (AlInGaN) alloys. In this specification, a quaternary can be reduced to a ternary for simplicity if one of the group III elements is significantly small. For example, if the In-composition in a quaternary AlInGaN is significantly small, smaller than 1%, then this AlInGaN quaternary can be shown as ternary AlGaN for simplicity. Using the same logic, a ternary can be reduced to a binary for simplicity if one of the group III elements is significantly small. For example, if the In-composition in a ternary InGaN is significantly small, smaller than 1%, then this InGaN ternary can be shown as binary GaN for simplicity. III-nitride or nitride can also include small compositions of transition metal nitride such as TiN, ZrN, HfN with molar fraction not larger than 10%. For example, III-nitride or nitride may include AlxInyGazTi(1-x-y-z)N, AlxInyGazZr(1-x-y-z)N, AlxInyGazHf(1-x-y-z)N, with (1−x−y−z)≦10%. A III-nitride layer or active-region means that the layer or active-region is made of III-nitride semiconductors.
In the following contents, wurtzite c-plane nitride light-emitting devices or structures are used as embodiments to elucidate the principle and spirit of the present invention. Those of ordinary skills in the field can apply the teachings in this specification and given by the following embodiments to non-c-plane nitride semiconductors, II-VI semiconductors and other lattice-mismatched light-emitting devices or semiconductor devices without creative work.
Illustrated in
According to an embodiment of the present invention, starting epitaxial template 20′ is intended for high-quality AlGaN layer epitaxy (ternary AlGaN or with a trace amount of Indium as previously explained). Hence starting epitaxial template 20′ is preferably made of materials suitable for AlGaN epitaxy, such as crystal sapphire, AlN, SiC, GaN, AlGaN, Si et al. Starting epitaxial template 20′ can be a self supporting substrate made of the abovementioned materials, or can be a thin film made of the abovementioned materials formed over a supporting substrate (though for simplicity the substrate is not explicitly shown in
It is well known to the field of interest that there is no lattice matched substrate for AlGaN based UV light emitters. So, AlGaN based UV devices have to be heteroepitaxially formed over foreign substrate or template. Shown in
where sij are the elastic compliance coefficients of epilayer E1, and the strain ε is,
So, the total strain energy within epilayer E1 is,
Estr=U×A×t
where A, t are the area and thickness of epilayer E1, respectively. As seen, the total strain energy is proportional to epilayer's area and thickness. The strain energy gets larger as the epilayer gets thicker. If it is larger than the dislocation formation energy, dislocations will be generated within epilayer E1; if it is tensile strain and larger than the crack formation energy, cracks will be generated within epilayer E1. Generally speaking, crack formation energy is much higher than dislocation formation energy. The total strain energy equation tells that to achieve high-quality epilayer with thick thickness without dislocation or crack generation, a promising approach is to limit the epilayer area thus limiting the strain energy.
The lattices of portions 22 and 23 are preferably to be relaxed or partly relaxed, via misfit dislocation generation at the interfaces between portions 22 and protrusions 22′, and between portions 23 and base 21′, respectively. The threading dislocations associated with the above mentioned misfit dislocations are provided with termination sites along the sidewalls of protrusions 22′ and portions 22 and 23. To further facilitate threading dislocation termination to the sidewalls, dislocation bending structure such as an AlN inserting layer, or a set of AlN/AlGaN superlattice or multiple layers is formed on starting epitaxial template 20′, as shown in
Lattice constant formatted templates 20A and 20B as shown in
Since templates 20A and 20B shown in
According to some embodiments of the present invention, starting epitaxial template 20′ is made of AlN. It can be a bulk AlN substrate, or can be an AlN thin film with thickness preferably greater than 2 μm, formed over another substrate such as sapphire, SiC, or Si. The protrusions 22′ are made of AlN, arranged over the base 21′ also being AlN, according to the patterns shown in
The laminate structure of a UV light emitter according to one embodiment of the present invention is illustrated in
According to some other embodiments of the present invention, starting epitaxial template 20′ is made of GaN. It can be a bulk GaN substrate, or can be a GaN thin film with thickness preferably greater than 4 μm, formed over another substrate such as sapphire, SiC, or Si. The protrusions 22′ are made of GaN, arranged over the base 21′ also being GaN, according to the patterns shown in
The laminate structure of a UV light emitter according to an embodiment of the present invention is illustrated in
Further, for the UV emitter structure shown in
According to still other embodiments of the present invention, starting epitaxial template 20′ is made of sapphire, Si, or SiC substrate. In an exemplary embodiment according to this aspect of the present invention, starting epitaxial template 20′ is made of sapphire substrate. The protrusions 22′ are made of sapphire, arranged over the base 21′ also being sapphire, according to the patterns shown in
The embodiments disclosed above take AlGaN lattice constant formatted epitaxial templates and UV light emitters as example. However, the present invention is not limited to AlGaN lattice constant formatted epitaxial templates and UV light emitters. Similarly, InGaN materials (or AlInGaN materials) are suitable for making lattice constant formatted epitaxial template for InGaN thin film epitaxy, in view of the large difference in surface diffusion lengths of In and Ga adatoms under optimal InGaN growth conditions. The optimal InGaN epitaxial growth conditions include high growth pressure of 200-760 torr, lower growth temperatures of 600-850° C., and high V/III ratio of 2000-30000. Under the optimal InGaN growth conditions, Al and Ga adatoms possess small even vanishing surface diffusion lengths, as compared to that of In adatoms. When performing InGaN or AlInGaN epitaxial growth on a starting epitaxial template 20′ under the optimal InGaN growth conditions, material portions 22 formed on protrusion 22′ will be In-deficient and have smaller lattice constants as compared to material portions 23 formed in depressions 23′. InGaN or AlInGaN lattice constant formatted epitaxial templates are ideal for high-In-content InGaN epitaxy for green, amber and red InGaN light emitters.
According to this aspect of the present invention, starting epitaxial template 20′ is made of GaN. It can be a bulk GaN substrate, or can be a GaN thin film with thickness preferably greater than 4 μm, formed over another substrate such as sapphire, SiC, or Si. The protrusions 22′ are made of GaN, arranged over the base 21′ also being GaN, according to the patterns shown in
The laminate structure of a visible light emitter according to an embodiment of the present invention is illustrated in
When the active region is directly formed on an InGaN or AlGaN lattice constant formatted epitaxial template, material portions 22 and 23 of lattice constant formatted epitaxial template 20A or 20B may function as n-type layer and, for that purpose, material portions 22 and 23 can be properly doped.
The principal of the present invention can be readily extended to other semiconductor devices as long as there is strain to mediate. According to the teachings taught above, using a lattice constant formatted epitaxial template can split the otherwise large-area, substantially uniform, biaxially compressive or tensile strain into numerous miniature areas of strains with reduced and opposite-signed strains. The general steps to form a lattice constant formatted epitaxial template consist:
Identifying a starting epitaxial template material and an epitaxial material that can be epitaxially formed over the template material, whereas the epitaxial material shall contain at least two different elements which possess substantially different diffusion lengths when performing epitaxial growth of the epitaxial material on the epitaxial template;
Forming protrusions and depressions on the epitaxial template material to make a starting epitaxial template. The dimensions of the protrusions and depressions shall take the advantage of the different diffusion lengths of the at least two different elements of the epitaxial material, to make the epitaxially formed materials on top of the protrusions and depressions have different compositions thus different lattice constants.
Performing epitaxial growth on the starting epitaxial template. The growth conditions shall facilitate the composition segregation due to the difference of the at least two different elements' surface diffusion lengths. The lattice constant formatted template will be accomplished as the starting epitaxial template approaches surface planarization.
The present invention has been described using exemplary embodiments. However, it is to be understood that the scope of the present invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangement or equivalents which can be obtained by a person skilled in the art without creative work or undue experimentation. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and equivalents.
Claims
1. A light emitting device comprising:
- an n-type layer;
- a p-type layer;
- an active region sandwiched between the n-type layer and the p-type layer; and
- a lattice constant formatted epitaxial template;
- wherein the lattice constant formatted epitaxial template has a growing surface on which the n-type layer grows, and the growing surface comprises a plurality of first material portions and a plurality of second material portions with each of the first material portions being alternately arranged with each of the second material portions, and wherein lattice constants of the first material portions are different from those of the second material portions.
2. The light emitting device according to claim 1, wherein each of the first material portions and each of the second material portions are of lateral dimension from 100 nm to 3000 nm in at least one lateral direction, respectively.
3. The light emitting device according to claim 1, wherein the first and second material portions are made of AlGaN, the Al-composition of the first material portions is higher than the Al-composition of the second material portions, thus the lattice constants of the first material portions are smaller than those of the second material portions.
4. The light emitting device according to claim 3, wherein the n-type layer is made of AlGaN, the Al-composition of the n-type layer is lower than the Al-composition of the first material portions and higher than the Al-composition of the second material portions, thus the lattice constants of the n-type layer are larger than the lattice constants of the first material portions and smaller than the lattice constants of the second material portions.
5. The lattice constant formatted epitaxial template according to claim 1, wherein there is a height difference between the first material portions and the second material portions in a layer growing direction, and the height difference is larger than 100 nanometers.
6. Light emitting device according to claim 1, wherein the n-type layer is an n-type AlGaN layer and the active region emits ultraviolet emissions in the wavelengths of 230-365 nm.
7. A lattice constant formatted epitaxial template comprising:
- a starting epitaxial template comprising a base, a plurality of alternately arranged protrusions and depressions on the base;
- first material portions epitaxially formed on top of the protrusions and second material portions epitaxially formed in the depressions, wherein lattice constants of the first material portions on the protrusions are different from those of the second material portions in the depressions.
8. The lattice constant formatted epitaxial template according to claim 7, wherein the protrusions are arranged on the base in a one-dimensional or two-dimensional periodic pattern, and the protrusions are of lateral dimension of 100-3000 nm, height of 200-1500 nm and pitch of 200-3500 nm.
9. The lattice constant formatted epitaxial template according to claim 7, wherein the lattice constants of the first material portions are smaller than those of the second material portions.
10. The lattice constant formatted epitaxial template according to claim 7, wherein the starting epitaxial template is made of AlN, and the first and second material portions are made of AlGaN, wherein the Al-composition of the first material portions is higher than the Al-composition of the second material portions, thus the lattice constants of the first material portions are smaller than those of the second material portions.
11. The lattice constant formatted epitaxial template according to claim 7, wherein the starting epitaxial template is made of GaN, and the first and second material portions are made of AlGaN, wherein the Al-composition of the first material portions is higher than the Al-composition of the second material portions, thus the lattice constants of the first material portions are smaller than those of the second material portions.
12. The lattice constant formatted epitaxial template according to claim 7, wherein the starting epitaxial template is made of AlGaN, sapphire, SiC, or Si, and the first and second material portions are made of AlGaN, wherein the Al-composition of the first material portions is higher than the Al-composition of the second material portions, thus the lattice constants of the first material portions are smaller than those of the second material portions.
13. The lattice constant formatted epitaxial template according to claim 7, wherein the first and second material portions are simultaneously formed on the protrusions and in the depressions, respectively, from the same feed material via composition segregation.
14. The lattice constant formatted epitaxial template according to claim 7, wherein there is a height difference between the first material portions and the second material portions in a layer growing direction, and the height difference is larger than 100 nanometers.
15. A light emitting device comprising:
- an n-type layer;
- a p-type layer;
- an active region sandwiched between the n-type layer and the p-type layer;
- wherein the n-type layer contains first portions and second portions alternately arranged along a lateral direction of the n-type layer, and lattice constants of the first portions are smaller than lattice constants of the second portions.
16. The light emitting device according to claim 15, wherein each of the first portions and each of the second portions has a lateral dimension from 100 nm to 3000 nm in at least one lateral direction, respectively.
17. The light emitting device according to claim 15, wherein the n-type layer is an unrelaxed layer.
18. The light emitting device according to claim 15, wherein the n-type layer is a fully relaxed layer, and the first and second portions are formed within a sublayer of the fully relaxed n-type layer.
19. The light emitting device according to claim 15, wherein the n-type layer is a AlGaN layer or an InGaN layer.
20. A light emitting device comprising:
- a p-type layer;
- a light emitting active region; and
- the lattice constant formatted epitaxial template according to claim 7;
- wherein the active region is directly formed on the first material portions and the second material portions of the lattice constant formatted epitaxial template.
21. The light emitting device according to claim 20, wherein the first and second material portions are made of InGaN and the In-composition of the second material portions is higher than that of the first material portions.
22. The light emitting device according to claim 21, wherein the active region is made of InGaN light emitting quantum wells for emitting visible light.
Type: Application
Filed: Mar 27, 2015
Publication Date: Sep 29, 2016
Inventors: JIANPING ZHANG (SAN JOSE, CA), HONGMEI WANG (SAN JOSE, CA)
Application Number: 14/671,925