Abstract:
An epitaxial structure includes a substrate, a first epitaxial layer and a second epitaxial layer. The substrate has a surface, and the first epitaxial layer is disposed over the substrate and defines a plurality of slanting air voids tapering away from the substrate and an opening over each of the slanting air voids. The second epitaxial layer is disposed on the first epitaxial layer and collectively defines the slanting air voids in a shape of trapezoid with the surface and the first epitaxial layer.

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 13/932,665, filed on Jul. 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. 
    
    
     BACKGROUND 
     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. 
     SUMMARY 
     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 III nitride semiconductor. 
     According to one embodiment of the present disclosure, the second epitaxial layer comprises a group III 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 sidewall, 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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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: 
         FIG. 1  is a flow chart showing an epitaxial growth method according to one embodiment of the present disclosure; 
         FIG. 2A  to  FIG. 2F  are cross-sectional views illustrating the process steps of the epitaxial growth method  100 ; 
         FIG. 2G  is a top view schematically illustrating the cavities formed in step  150  according to one embodiment of the present disclosure; 
         FIG. 3A  to  FIG. 3D  are cross-sectional views illustrating the process steps of an epitaxial growth method according to another embodiment of the present disclosure; 
         FIG. 3E  to  FIG. 3H  are cross-sectional views illustrating the process steps of an epitaxial growth method according to still another embodiment of the present disclosure; 
         FIG. 4A to 4F  are cross-sectional views illustrating the process steps of an epitaxial growth method according to still another embodiment of the present disclosure; and 
         FIGS. 5A and 5B  are cross-sectional views illustrating the process steps of an epitaxial growth method according to still another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  is a flow chart showing an epitaxial growth method  100  according to one embodiment of the present disclosure. The method  100  at least includes step  110  to step  160 .  FIG. 2A  to  FIG. 2F  are cross-sectional views illustrating the process steps of the epitaxial growth method  100 . The epitaxial growth method  100  disclosed herein may be applied in manufacturing LEDs or flat display panels. 
     In step  110 , a substrate  210  is provided, as depicted in  FIG. 2A . The substrate  210  may be a sapphire substrate, a silicon substrate or other substrates suitable for epitaxial growth. 
     In step  120 , a sacrifice layer  220  is formed on the substrate  210 , as depicted in  FIG. 2B . In one embodiment, the sacrifice layer  220  is a layer of inorganic material. The sacrifice layer  220  may comprise inorganic material such as silicon oxide or silicon nitride. 
     In step  130 , the sacrifice layer  220  is patterned to form a plurality of bumps  222 , as depicted in  FIG. 2C . The bumps  222  are spaced apart from each other on the substrate  210 . The spacing interval between two bumps  222  allows a surface of the substrate  210  to be exposed. 
     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 α 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  FIG. 2D , a first epitaxial layer  231  is epitaxially formed on the exposed portion of the substrate  210  such that the first epitaxial layer  231  covers a portion of each of the bumps  222 , but a top surface  222   t  of each of the bumps  222  is exposed out of the first epitaxial layer  231 . In particular, when the first epitaxial layer  231  is epitaxially grown, the first epitaxial layer  231  crawls along the surface of the bump  222 . The first epitaxial layer  231  would completely covers the bumps  222  if the process of the epitaxial growth is not stopped. Accordingly, one feature of the present disclosure relays on that the epitaxial growth of the first epitaxial layer  231  is sopped before the first epitaxial layer  231  completely covers the bumps  222  so that the top surface  222   t  of each of the bumps  222  is exposed out of the first epitaxial layer  231 . 
     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  FIG. 2E . Since the top surface  222   t  of each of the bumps  222  is exposed out of the first epitaxial layer  231 , the bumps  222  may be removed by wet etching processes. For instance, the etchant may be a mixed solution containing ammonium fluoride (NH4F) and hydrogen fluoride (HF). Significantly, the position, volume and shape of the cavities  224  substantially depend upon that of the bumps  222  as well as the coverage level that the first epitaxial layer  231  covers the bumps  222 . Accordingly, the morphologies of the cavities  224  are controlled in advance of step  140  because the shape and the volume of each of the bumps  222  as well as the arrangement of these bumps  222  are precisely controlled in step  130  by the process of patterning the sacrifice layer  220 , according to the embodiments of the present disclosure. 
       FIG. 2G  is a top view schematically illustrating the cavities  224  formed in step  150  according to one embodiment of the present disclosure. In this embodiment, when the bumps  222  are removed, each of the cavities  224  has an opening  224   a  and a bottom portion  224   b , in which the area of each of the openings  224   a  is about 5% to about 50% of the area of the corresponding bottom portion  224   b , specifically about 15% to about 40%. When the area of the opening  224   a  is less than about 5% of the area of the bottom portion  224   b , it is difficult to rapidly remove the bumps  222  in step  150 . On the other hands, when the area of the opening  224   a  is greater than about 50% of the area of the bottom portion  224   b , the coverage of the first epitaxial layer  231  over the bumps  222  is insufficient and is unfavorable to the subsequent step  160 . Accordingly, the area of each of the opening  224   a  is preferably about 5% to about 50% of the area of the corresponding bottom portion  224   b , more preferably about 15% to about 40%, according to the embodiments of the present disclosure. 
     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  FIG. 2F . In other words, by epitaxially forming the second epitaxial layer  232 , the cavities  224  become closed air voids embedded in the first epitaxial layer  231  and the second epitaxial layer  232 . The method of forming the second epitaxial layer  232  may be the same as these described hereinbefore in connection with the first epitaxial layer  231 . In one embodiment, the second epitaxial layer  232  includes a group III nitride semiconductor. In one example, the material of the second epitaxial layer  232  may be the same as that of the first epitaxial layer  231 . For example, both the first and the second epitaxial layer  231 ,  232  may be made of gallium nitride. In other embodiments, the refractive index of the second epitaxial layer  232  may be different from that of the first epitaxial layer  231  when taking the entire optical path into consideration. 
     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 .  FIG. 3A  to  FIG. 3D  are cross-sectional views illustrating the process steps of an epitaxial growth method according to another embodiment of the present disclosure. In the embodiment shown in  FIG. 3A  to  FIG. 3D , a number of bumps  222 , each having a trapezoidal cross-section, are formed, as shown in  FIG. 3A . Thereafter, a first epitaxial layer  231  is epitaxially formed to cover a portion of each of the bumps  222 , as shown in  FIG. 3B . Subsequently, the bumps  222  are removed to form a plurality of cavities  224 , as shown in  FIG. 3C . And then, a second epitaxial layer  232  is epitaxially formed on the first epitaxial layer  231  to enclose the cavities  224  with trapezoidal cross-sections, as shown in  FIG. 3D . Similarly, the embodiment shown in  FIG. 3E  to  FIG. 3H , the cross-sections of the cavities  224  may be formed like stair. 
       FIG. 4A to 4F  are cross-sectional views illustrating the process steps of an epitaxial growth method according to still another embodiment of the present disclosure. In  FIG. 4A , a substrate  210  is provided. In  FIG. 4B , a buffer layer  212  is formed on a surface of the substrate  210 . In  FIG. 4C , a plurality of bumps  222  are formed on the buffer layer  212 . Thereafter, as shown in  FIG. 4D , a first epitaxial layer  231  is epitaxially formed on the buffer layer  212 . Subsequently, as shown in  FIG. 4E , the bumps  222  are removed to form a plurality of the cavities  224 . And then, as shown in  FIG. 3D , a second epitaxial layer  232  is epitaxially formed on the first epitaxial layer  231  to enclose the cavities  224 . 
       FIGS. 5A and 5B  are cross-sectional views illustrating the process steps of an epitaxial growth method according to still another embodiment of the present disclosure. In this embodiment, step  110  to step  140  may be the same as these described hereinbefore in connection with  FIG. 2A  to  FIG. 2D . Thereafter, as shown in  FIG. 5A , only a portion of each of the bumps  222  is removed in step  150 , and another portion of each of the bumps  222  remains. In particular, the process of etching the bumps  222  is stopped before the bumps  222  are completely removed so that a portion  222   a  of each of the bumps  222  is remained on the substrate. Subsequently, as shown in  FIG. 5B , a second epitaxial layer  232  is epitaxially formed on the first epitaxial layer  231  in step  160  such that the cavities  224  are enclosed by the first and the second epitaxial layers  231 ,  232 . In other words, the remained portions  222   a  of the bumps may be formed in the closed air voids that are embedded in the first and the second epitaxial layers  231 ,  232  according to this embodiment. In one example, a refractive index of the remained portions  222   a  of the bumps is different from that of the first epitaxial layer  231  and/or the second epitaxial layer  232 , and therefore the remained portions  222   a  serves as an optical medium to change the optical path and the optical characteristics of the structure shown in  FIG. 2F . 
     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.