Technical Field
The present disclosure relates to an epitaxial structures and an epitaxial growth method for forming an epitaxial layer with cavities.
Description of Related Art
There has been rapid progress in technologies of light emitting diodes (LEDs) in recent years. For the purpose of increasing the light-extraction ratio of LEDs, techniques such as patterned sapphire substrates have been widely applied in LEDs, which include gallium nitride. For further increasing the light-extraction ratio, it have been purposed to form cavities or pores in epitaxial gallium nitride layers of LEDs. In this regard, it is difficult to well control the shape and the volume of the cavities in LEDs, and therefore the quality of LEDs is unstable in prior art. In view of the above, there exists a need of a new method that would improve the drawbacks in prior art.
An epitaxial growth method for forming an epitaxial layer with cavities is provided. The method includes the steps of: providing a substrate; forming a sacrifice layer on the substrate; patterning the sacrifice layer to form a plurality of bumps spaced apart from each other on the substrate, wherein a portion of the substrate between the bumps is exposed; epitaxially forming a first epitaxial layer on the exposed portion of the substrate, wherein the first epitaxial layer covers a portion of each of the bumps, and a top surface of each of the bump is exposed; removing the bumps to form a plurality of cavities; and epitaxially forming a second epitaxial layer on the first epitaxial layer such that the cavities are enclosed by the first epitaxial layer and the second epitaxial layer.
According to one embodiment of the present disclosure, the substrate is a sapphire substrate or a silicon substrate.
According to one embodiment of the present disclosure, the first epitaxial layer comprises a group Ill nitride semiconductor.
According to one embodiment of the present disclosure, the second epitaxial layer comprises a group Ill nitride semiconductor.
According to one embodiment of the present disclosure, each of the first and the second epitaxial layers comprises gallium nitride.
According to one embodiment of the present disclosure, each of the first and the second epitaxial layers is formed by a hydride vapor phase epitaxy process, a metal organic chemical vapor deposition process or a molecular beam epitaxy process.
According to one embodiment of the present disclosure, a horizontal growth rate of the second epitaxial layer is greater than a horizontal growth rate of the first epitaxial layer.
According to one embodiment of the present disclosure, a temperature of the second epitaxial layer in the step of epitaxially forming the second epitaxial layer is greater than a temperature of the first epitaxial layer in the step of epitaxially forming the first epitaxial layer.
According to one embodiment of the present disclosure, a pressure in the step of epitaxially forming the second epitaxial layer is less than a pressure in the step of epitaxially forming the first epitaxial layer.
According to one embodiment of the present disclosure, the sacrifice layer is an inorganic material layer.
According to one embodiment of the present disclosure, the sacrifice layer comprises silicon oxide or silicon nitride.
According to one embodiment of the present disclosure, each of the bumps has a maximum height of about 0.5 μm to about 3 μm, and each of the bumps has a bottom width of about 0.5 μm to about 5 μm.
According to one embodiment of the present disclosure, each of the bumps has a taper angle of less than or equal to about 90 degrees in the step of forming the bumps.
According to one embodiment of the present disclosure, the step of patterning the sacrifice layer comprises etching the sacrifice layer by a process of inductively coupled plasma reactive ion etching (ICP-RIE).
According to one embodiment of the present disclosure, each of the bumps is removed by a wet etching process.
According to one embodiment of the present disclosure, an etchant of the wet etching process comprises ammonium fluoride (NH4F) and hydrogen fluoride (HF).
According to one embodiment of the present disclosure, each of the cavities has an opening, and an area of the opening is about 5% to about 50% of a bottom area of each of the cavities in the step of removing the bumps to form the cavities.
According to one embodiment of the present disclosure, the substrate comprises a buffer layer formed thereon.
According to another aspect of the present disclosure, an epitaxial structure is provided. The epitaxial structure includes a substrate, a first epitaxial layer, a second epitaxial layer and a closed air void. The first epitaxial layer is disposed over the substrate. The second epitaxial layer is disposed on the first epitaxial layer. The closed air void is embedded in the first epitaxial layer and the second epitaxial layer. The closed air void has a bottom portion and a top portion respectively formed in the first epitaxial layer and the second epitaxial layer, and a width of the top portion is less than a width of the bottom portion.
In one embodiment of the present disclosure, the top portion of the closed air void includes an arc surface, and the bottom portion of the closed air void is a substantially flat surface.
In one embodiment of the present disclosure, the closed air void has a cross-section in a shape of trapezium.
In one embodiment of the present disclosure, the closed air void includes a stair-like sideman, and the top portion and the bottom portion are two opposite flat surfaces.
In one embodiment of the present disclosure, the epitaxial structure further includes a buffer layer disposed between the substrate and the first epitaxial layer, and the bottom portion of the closed air void is formed on the buffer layer.
In one embodiment of the present disclosure, the epitaxial structure further includes a patterned sacrifice layer disposed on the substrate, and a top surface of the patterned sacrifice layer forms the bottom portion of the closed air void.
In one embodiment of the present disclosure, the closed air void has a height of about 0.5 μm to about 3 μm, the bottom portion of the closed air void has a width of about 0.5 μm to about 5 μm.
It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed.
The invention can be more fully understood by reading the following detailed description of the embodiments, with reference made to the accompanying drawings as follows:
Reference will now be made in detail to the present embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. However, it will be apparent, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawings.
In step 110, a substrate 210 is provided, as depicted in
In step 120, a sacrifice layer 220 is formed on the substrate 210, as depicted in
In step 130, the sacrifice layer 220 is patterned to form a plurality of bumps 222, as depicted in
In one embodiment, a patterned photoresist layer is formed on the sacrifice layer 220, in which the photoresist layer covers the regions that are desired to form bumps 222, and the other portions of the sacrifice layer 220 is exposed. Thereafter, etching processes may be employed to remove the exposed portion of the sacrifice layer 220, and thereby forming the bumps 222, Specifically, the pattern of the photoresist layer dominates the contour of each of the bumps 222 in top view. Regarding the cross-sectional shape of each bump 222, it may be well controlled by suitable etching techniques. In one example, the exposed portion of the sacrifice layer 220 is etched by the technique of inductively coupled plasma reactive ion etching (ICP-RIE). The ICP-RIE technique may simultaneous provide anisotropic etching and isotropic etching, respectively contributed by ion bump and reactive ions, and therefore each bump 222 may be formed in a shape of hemisphere. The etching rates associated with the ion bump and the reactive ion may be respectively controlled by modulating the process parameters, and thus a variety of cross-sectional shapes of the bumps 222 may be formed. For instance, each bump 222 may have a cross-section in a shape of trapezium or rectangle, which are described in detail hereinafter.
In a preferred embodiment, a taper angle a of each of the bumps 222 is less than or equal to 90 degrees. Each of the bumps 222 has a maximum height H of about 0.5 μm to about 3 μm, and each of the bumps 222 has bottom width W of about 0.5 μm to about 5 μm. Significantly, the contours of bumps 222 dominant the shapes of the cavities formed in the following steps, which are described in detail hereinafter.
In step 140, as shown in
In one embodiment, the first epitaxial layer 231 includes a group III-nitride semiconductor, such as gallium nitride. The first epitaxial layer 231 may be formed by techniques such as hydride vapor phase epitaxy processes, metal organic chemical vapor deposition processes or molecular beam epitaxy processes.
In step 150, the bumps 222 are removed by etching approaches to form a plurality of cavities 224 exposing a surface of the substrate 210. as depicted in
In step 160, a second epitaxial layer 232 is epitaxially formed on the first epitaxial layer 231 such that the cavities 224 are enclosed by the first epitaxial layer 231 and the second epitaxial layer 232, as shown in
In another embodiment, the horizontal growth rate in the growth of the second epitaxial layer 232 is greater than the horizontal growth rate in the growth of the first epitaxial layer 231. The horizontal growth rate may be controlled by the temperature and the pressure of the epitaxial growth process. In one example, a temperature of the second epitaxial layer 232 in the growth of the second epitaxial layer 232 is greater than a temperature of the first epitaxial layer 231 in the growth of the first epitaxial layer 231, and a pressure of forming the second epitaxial layer 232 is less than a pressure of forming the first epitaxial layer 231. In other words, the first epitaxial layer 231 is epitaxially grown in an environment at a low temperature and under a high pressure such that the first epitaxial layer 231 has a better three-dimensional structure. As compared to the first epitaxial layer 231, the second epitaxial layer 232 is epitaxially grown in an environment at a higher temperature and under a lower pressure, such that the formation of the second epitaxial layer 232 exhibits an excellent planar characteristic and a rapid growth rate in a horizontal direction.
As described hereinbefore in connection with step 130 and step 140, the cavities 224 may be formed in a variety of shapes by controlling the contours and the cross-sectional shapes of the bumps 222.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims.
Number | Date | Country | Kind |
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101124450 | Jul 2012 | TW | national |
The present application is a divisional application of U.S. application Ser. No. 14/694,617, filed on Apr. 23, 2015, which is a continuation of U.S. application Ser. No. 13/932,665, filed on July 1, 2013, which was based on, and claims priority from, Taiwan Patent Application Serial Number 101124450, filed Jul. 6, 2012, the disclosure of which is hereby incorporated by reference herein in its entirely.
Number | Date | Country | |
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Parent | 14694617 | Apr 2015 | US |
Child | 15428128 | US |
Number | Date | Country | |
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Parent | 13932665 | Jul 2013 | US |
Child | 14694617 | US |