Patent Application
20140225123
This invention relates in general to the formation of a template for the growth of GaN on a silicon substrate and more specifically to the formation of a DBR as the REO in a REO/aluminum oxide/aluminum nitride template.
In the semiconductor industry, it is known that growing a III-N material, such as GaN, on a silicon substrate is difficult due in large part to the large crystal lattice mismatch (−16.9%) and the thermal mismatch (53%) between silicon and GaN. It is also known that LED devices built on silicon substrates suffer from absorption of emitted light by the silicon substrate. Thus, some type of buffer layer or layers is generally formed on the silicon substrate and the III-N material is grown on the buffer layer. It is also known that during much of the growth process there must ideally be no exposed silicon surface due to detrimental reaction between silicon and the various MBE process gasses, i.e. N2 plasma, NH3 and metallic Ga. Also in the case where other growth processes are used, such as MOCVD process gasses (NH3, H2, TMGa, etc.). Reaction of silicon with process gasses usually results in etching of silicon (H2), formation of nitrides (NH3), or severe reaction and blistering (Ga precursors).
In the prior art, one method of solving the light absorption problem is to fabricate the LED on a silicon substrate and then bond the finished LED on a reflective coating and remove the silicon substrate. Generally, the top layer of the resulting structure is roughened to improve light extraction efficiency. However, this is a long and work intensive process.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide new and improved methods for the formation of a REO/aluminum oxide/aluminum nitride template on a silicon substrate.
It is another object of the present invention to provide new and improved methods for the formation of a template that includes eliminating or greatly reducing the problem of possible damage to the silicon substrate with process gasses.
It is another object of the present invention to provide a new and improved REO/aluminum oxide/aluminum nitride template on a silicon substrate.
It is another object of the present invention to provide new and improved LED structures on a template on a silicon substrate.
It is another object of the present invention to provide a new and improved DBR/aluminum oxide/aluminum nitride template on a silicon substrate.
Briefly, the desired objects and aspects of the instant invention are also realized in accordance with a specific crystal lattice matched template on a single crystal silicon substrate. The template includes a Distributed Bragg Reflector positioned on the silicon substrate. The Distributed Bragg Reflector is substantially crystal lattice matched to the surface of the silicon substrate. An aluminum oxide layer is positioned on the surface of the Distributed Bragg Reflector and substantially crystal lattice matched to the surface of the Distributed Bragg Reflector. A layer of aluminum nitride (AlN) is positioned on the surface of the aluminum oxide layer and substantially crystal lattice matched to the surface of the aluminum oxide layer. A III-N LED structure including at least one III-N layer can then be grown on the aluminum nitride layer and substantially crystal lattice matched to the surface of the aluminum nitride layer. The DBR redirects all downwardly directed light from the LED upwardly to substantially improve the efficiency of the LED.
The desired objects and aspects of the instant invention are further achieved in accordance with a preferred method of fabricating a template on a silicon substrate including the steps of providing a single crystal silicon substrate and epitaxially growing a Distributed Bragg Reflector on the silicon substrate. The Distributed Bragg Reflector is substantially crystal lattice matched to the surface of the silicon substrate. The method further includes the steps of epitaxially growing an aluminum oxide layer on the surface of the Distributed Bragg Reflector substantially crystal lattice matched to the surface of the Distributed Bragg Reflector and epitaxially growing a layer of aluminum nitride (AlN) on the surface of the aluminum oxide layer substantially crystal lattice matched to the surface of the aluminum oxide layer. A III-N LED structure including at least one III-N layer can then be epitaxially grown on the aluminum nitride layer and substantially crystal lattice matched to the surface of the aluminum nitride layer. The DBR redirects all downwardly directed light from the LED upwardly to substantially improve the efficiency of the LED.
The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the drawings, in which:
Turning to
A layer 11 of rare earth oxide (REO) is epitaxially grown on silicon substrate 10. Various rare earth oxides have a crystal lattice spacing that can be matched to silicon with very little strain. For example, Gd2O3 has a crystal lattice spacing (a) of 10.81 Å, Er2O3 has a crystal lattice spacing (a) of 10.55 Å, Nd2O3 has a crystal lattice spacing (a) of 11.08 Å, and silicon has a double spacing (2a) of 10.86 Å. Further, the crystal lattice spacing of REO layer 11 can be varied by varying the composition of the constituents, which allows for strain engineering of the silicon wafers. Generally, the REO material closest to or adjacent silicon substrate 10 will have a crystal spacing closest to the crystal spacing of silicon while REO materials adjacent the opposite side of layer 11 will have a crystal spacing closer to the crystal spacing of materials grown on the surface. Strain engineering mitigates the stresses formed during growth of III-N materials on these substrates.
In a typical example, layer 11 includes Gd2O3 epitaxially grown on silicon substrate 10 with Er2O3 epitaxially grown adjacent the opposite (upper) side. The REO materials can be grown in a graded fashion bridging the two compositions or split to have an abrupt change in the composition and/or constituents of layer 11. Also, while two constituents are used in this example other and/or additional rare earth oxides can be included in layer 11.
A layer 14 of aluminum oxide is grown on the surface of REO layer 11. Aluminum oxide layer 14 is grown epitaxially and is mostly single crystal material substantially crystal lattice matched to silicon substrate 10. It will be understood that Al2O3 is the normal proportion required (stoichiometric) but non-stoichiometric compounds (e.g. Al2-xO3-y) may be used in specific applications. Also, aluminum oxide layer 14 may include aluminum oxynitride (AlxOyN), which is intended to come within the definition of “aluminum oxide” for purposes of this invention.
It should be noted that REO materials and aluminum oxide are impervious to MBE process gasses, i.e. N2 plasma, NH3 and metallic Ga, which is the preferred growth process in this invention. Also, in the event that other growth processes are used, such as the MOCVD process, the aluminum oxide is also impervious to MOCVD process gasses (NH3, H2, TMGa, etc.). Reaction of silicon with process gasses usually results in etching of silicon (H2), formation of nitrides (NH3), or severe reaction and blistering (Ga precursors). Thus silicon substrate 10 is protected from damage caused by generally all growth process gasses by both REO layer 11 and aluminum oxide layer 14. Preferably, aluminum oxide layer 14 is a few nanometers (nm) thick but for certain applications thicker or thinner films can be grown. Also, aluminum oxide layer 14 can be formed with a single continuous composition or it can be graded, in linear, stepwise or any similar schemes.
An aluminum nitride (AlN) layer 16 is epitaxially grown on aluminum oxide layer 14 preferably by an MBE process. The combination of aluminum oxide layer 14 and aluminum nitride layer 16 results in a base for the further growth of III-N materials. REO layer 11, aluminum oxide layer 14, and aluminum nitride layer 16 form template 12 which substantially crystal lattice matches the III-N materials to the silicon substrate and greatly reduces any thermal mismatch. Also, template 12 imparts chemical stability to the process due to the nature of the materials.
Turning to
Turning to
One important embodiment for the REO/aluminum oxide/aluminum nitride template described above is the formation of a DBR as the REO in the template. This is especially true when forming a LED or other photonic device in or on the final III-N layer. It is known in the semiconductor industry that the fabrication of LEDs on silicon substrates is the most efficient because of the expense and wide use and established technology in the use of silicon. However, as stated above, it is also known that LED devices built on silicon substrates suffer from absorption of emitted light by the silicon substrate. LEDs emit light in all directions and any light directed at the silicon substrate is substantially lost since it is absorbed by the silicon substrate. Prior art has placed reflective surfaces on one side of the LED and removed the substrate so that substantially all light is emitted in one direction. This however is a very tedious and work intensive process.
Turning to
As is known in the art, DBRs consist of a plurality of pairs of layers of material, with each pair forming a partial mirror that reflects some of the light incident upon it. In
Referring additionally to the chart of
Generally layers 115 and 116 are grown epitaxially on silicon substrate 110 and on each other as layers of single crystal material. Various rare earth oxides have a crystal lattice spacing that can be substantially matched to silicon with very little strain. For example, Gd2O3 has a crystal lattice spacing (a) of 10.81 Å, Er2O3 has a crystal lattice spacing (a) of 10.55 Å, Nd2O3 has a crystal lattice spacing (a) of 11.08 Å, and silicon has a double spacing (2a) of 10.86 Å. Thus, REOa˜Si2a herein defined as a “substantial crystallographic match”. Further, the crystal lattice spacing of the REO layers can be varied by varying the composition of the constituents.
Because the REO layers and the Si layers are substantially lattice matched, the first and last layers of DBR 112 can be either a REO layer or a Si layer. Also, it should be noted that because the Si layers in DBR 112 are very thin very little impinging light will be absorbed. In the example illustrated, pairs 114 of layers 115 and 116 are repeated three times, which forms a DBR mirror that is highly effective (90% of incident light is reflected) due to the larger refractive index difference between REO and silicon. It will be understood that more or fewer pairs 114 can be incorporated if a greater or lesser effective reflection is desired.
Turning to
While aluminum oxide can be grown on DBR 112, in some specific applications it may be desirable to include a graded or stepped layer of REO with an upper material having a lattice spacing more closely matching the lattice spacing of aluminum oxide, as explained in more detail above.
It should be noted that REO materials and aluminum oxide are impervious to MBE process gasses, i.e. N2 plasma, NH3 and metallic Ga, which is the preferred growth process in this invention. Also, in the event that other growth processes are used, such as the MOCVD process, the aluminum oxide is also impervious to MOCVD process gasses (NH3, H2, TMGa, etc.). Reaction of silicon with process gasses usually results in etching of silicon (H2), formation of nitrides (NH3), or severe reaction and blistering (Ga precursors). Thus silicon substrate 110 is protected from damage caused by generally all growth process gasses by both the REO layers and aluminum oxide layer 120. Preferably, aluminum oxide layer 120 is a few nanometers (nm) thick but for certain applications thicker or thinner films can be grown. Also, aluminum oxide layer 20 can be formed with a single continuous composition or it can be graded, in linear, stepwise or any similar schemes to aid in relief of stress.
An aluminum nitride (AlN) layer 122 is epitaxially grown on aluminum oxide layer 120 preferably by an MBE process. The combination of aluminum oxide layer 120 and aluminum nitride layer 122 results in a base for the further growth of III-N materials. DBR 112, aluminum oxide layer 120, and aluminum nitride layer 122 form template 130 which substantially crystal lattice matches the III-N materials to the silicon substrate and greatly reduces any thermal mismatch. Also, template 130 imparts chemical stability to the process due to the nature of the materials.
Template 130 is illustrated with a III-N LED structure 135 formed thereon. Structure 135 is illustrated as a single layer for convenience but it should be understood that III-N LED structure 135 is any convenient LED and may include for example the epitaxial growth of one or more typical layers i-GaN, n-GaN, active layers such as InGaN/GaN, electron blocking layers, p-GaN, and other inter-layers used in the formation and performance of LED devices. As explained above, III-N LED structure 135 will emit light downwardly as well as upwardly in
Because pairs 114 of layers 115 and 116 of DBR 112 are substantially crystal lattice matched to substrate 110 and because aluminum oxide layer 120, aluminum nitride layer 122 and LED 135 are all formed of single crystal material substantially crystal lattice matched to reduce strain, the entire structure can be easily and conveniently grown epitaxially and in many instances in a continuous growth process. Therefore, incorporating DBR 112 into the structure is relatively inexpensive and simple, compared to prior art methods and apparatus.
Thus, new and improved methods for the formation of a DBR/aluminum oxide/aluminum nitride template on a silicon substrate are disclosed. The main purpose of the DBR is to reflect light from an LED grown on the template upwardly so the light is not absorbed in silicon substrate 10. The new and improved methods for the formation of the template include incorporating the growth of the DBR into the normal fabrication of the LED and eliminating or greatly reducing the problem of possibly damaging the silicon substrate with subsequent process gasses. The invention also includes a new and improved DBR/aluminum oxide/aluminum nitride template on a silicon substrate with strain engineering to mitigate stresses formed during growth of III-N materials. Because of the strain engineering, new and improved LED structures can be substantially lattice matched and thermally matched by the new template on a silicon substrate.
Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. To the extent that such modifications and variations do not depart from the spirit of the invention, they are intended to be included within the scope thereof which is assessed only by a fair interpretation of the following claims.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13717211 | Dec 2012 | US |
| Child | 14180079 | US |
