Method for making gallium nitride epitaxial layer by silicon substrate

The disclosure relates to a method for making gallium nitride (GaN) epitaxial layer by silicon substrate is related. The method includes: providing a silicon substrate; providing a carbon nanotube structure comprising a plurality of carbon nanotubes and defining a plurality of holes; forming the carbon nanotube structure on a surface of the silicon substrate so that portions of the silicon substrate are exposed; dry etching the silicon substrate using the carbon nanotube structure as mask to obtain a patterned silicon substrate having a pattern surface comprising a plurality of bulges; and growing the GaN epitaxial layer using the patterned silicon substrate as an epitaxial substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims all benefits accruing under 35 U.S.C. § 119 from China Patent Application No. 201710017094.7, filed on Jan. 10, 2017, in the China Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND

1. Technical Field

The present disclosure relates to a method for making gallium nitride (GaN) epitaxial layer by silicon substrate.

2. Description of Related Art

Light emitting devices such as light emitting diodes (LEDs) based on group III-V nitride semiconductors (e.g. GaN) have been put into practice.

Since wide GaN substrate cannot be produced, the LEDs have been produced on a heteroepitaxial substrate such as sapphire. The use of sapphire substrate is problematic due to lattice mismatch and thermal expansion mismatch between GaN and the sapphire substrate. One consequence of thermal expansion mismatch is bowing of the GaN/sapphire substrate structure, which leads to cracking and difficulty in fabricating devices with small feature sizes. A solution for this is to form a plurality of grooves on the surface of the sapphire substrate by lithography before growing the GaN layer. However, the process of lithography is complex, high in cost, and will pollute the sapphire substrate, specifically, the GaN epitaxial layer grown on the sapphire substrate is more preferred over the GaN epitaxial layer grown on the silicon substrate.

What is needed, therefore, is a method for making GaN epitaxial layer by silicon substrate that overcomes the problems as discussed above.

DETAILED DESCRIPTION

Several definitions that apply throughout this disclosure will now be presented. The term “coupled” is defined as connected, whether directly or indirectly through intervening components, and is not necessarily limited to physical connections. The connection can be such that the objects are permanently connected or releasably connected. The term “outside” refers to a region that is beyond the outermost confines of a physical object. The term “inside” indicates that at least a portion of a region is partially contained within a boundary formed by the object. The term “substantially” is defined to essentially conforming to the particular dimension, shape or other word that substantially modifies, such that the component need not be exact. For example, substantially cylindrical means that the object resembles a cylinder, but can have one or more deviations from a true cylinder. The term “comprising” means “including, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in a so-described combination, group, series and the like. It should be noted that references to “an” or “one” exemplary embodiment in this disclosure are not necessarily to the same exemplary embodiment, and such references mean at least one.

References will now be made to the drawings to describe, in detail, various exemplary embodiments of the present methods for making GaN epitaxial layers on a silicon substrate.

Referring toFIG. 1, a method for making a GaN epitaxial layer14on a silicon substrate12of one exemplary embodiment includes the following steps:

step (S10), providing the silicon substrate12;

step (S11), providing a carbon nanotube composite structure110, wherein the carbon nanotube composite structure110includes a carbon nanotube structure112and a protective layer114coated on the carbon nanotube structure112; the carbon nanotube structure112includes a plurality of intersected carbon nanotubes and defines a plurality of openings116;

step (S12), forming the carbon nanotube composite structure110on a surface121of the silicon substrate12, wherein portions of the surface121are exposed from the plurality of openings116;

step (S13), forming a patterned silicon substrate12ahaving a bulged pattern122by dry etching the surface121using the carbon nanotube composite structure110as a mask, wherein the bulged pattern122includes a plurality of strip-shaped bulges intersected with each other as shown inFIG. 13; and

step (S14), epitaxially growing a GaN epitaxial layer14on the patterned silicon substrate12a.

In step (S10), the silicon substrate12is a single crystal silicon substrate. The silicon substrate12can be an intrinsic silicon substrate, a P-type doped silicon substrate, or a N-type doped silicon substrate. In one exemplary embodiment, the silicon substrate12is an intrinsic silicon wafer with a thickness of 300 micrometers.

In step (S11), the carbon nanotube structure112is a free-standing structure. The term “free-standing structure” includes that the carbon nanotube structure112can sustain the weight of itself when it is hoisted by a portion thereof without any significant damage to its structural integrity. Thus, the carbon nanotube structure112can be suspended by two spaced supports.

The plurality of carbon nanotubes can be single-walled carbon nanotubes, double-walled carbon nanotubes, or multi-walled carbon nanotubes. The length and diameter of the plurality of carbon nanotubes can be selected according to need. The diameter of the single-walled carbon nanotubes can be in a range from about 0.5 nanometers to about 10 nanometers. The diameter of the double-walled carbon nanotubes can be in a range from about 1.0 nanometer to about 15 nanometers. The diameter of the multi-walled carbon nanotubes can be in a range from about 1.5 nanometers to about 50 nanometers. In one exemplary embodiment, the length of the carbon nanotubes can be in a range from about 200 micrometers to about 900 micrometers.

The plurality of carbon nanotubes are orderly arranged to form an ordered carbon nanotube structure. The plurality of carbon nanotubes extend along a direction substantially parallel to the surface of the carbon nanotube structure112. The term ‘ordered carbon nanotube structure’ includes, but is not limited to, a structure wherein the plurality of carbon nanotubes are arranged in a consistently systematic manner, e.g., the plurality of carbon nanotubes are arranged approximately along the same direction.

The carbon nanotube structure112defines a plurality of apertures. The aperture extends throughout the carbon nanotube structure112along the thickness direction thereof. The aperture can be a hole defined by several adjacent carbon nanotubes, or a gap defined by two substantially parallel carbon nanotubes and extending along axial direction of the carbon nanotubes. The hole shaped aperture and the gap shaped aperture can exist in the carbon nanotube structure112at the same time. Hereafter, the size of the aperture is the diameter of the hole or width of the gap. The sizes of the apertures can be different. The average size of the apertures can be in a range from about 10 nanometers to about 500 nanometers. For example, the sizes of the apertures can be about 50 nanometers, or 100 nanometers.

In one exemplary embodiment, the carbon nanotube structure112includes a single drawn carbon nanotube film. The drawn carbon nanotube film can be drawn from a carbon nanotube array that is able to have a film drawn therefrom. The drawn carbon nanotube film includes a plurality of successive and oriented carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. The drawn carbon nanotube film is a free-standing film. Referring toFIG. 2, each drawn carbon nanotube film includes a plurality of successively oriented carbon nanotube segments joined end-to-end by van der Waals attractive force therebetween. Each carbon nanotube segment includes a plurality of carbon nanotubes parallel to each other, and combined by van der Waals attractive force therebetween. As can be seen inFIG. 2, some variations can occur in the drawn carbon nanotube film. The carbon nanotubes in the drawn carbon nanotube film are oriented along a preferred orientation. The drawn carbon nanotube film can be treated with an organic solvent to increase the mechanical strength and toughness and reduce the coefficient of friction of the drawn carbon nanotube film. A thickness of the drawn carbon nanotube film can range from about 0.5 nanometers to about 100 micrometers. The drawn carbon nanotube film defines a plurality of apertures between adjacent carbon nanotubes.

The carbon nanotube structure112can include at least two stacked drawn carbon nanotube films. In other exemplary embodiments, the carbon nanotube structure112can include two or more coplanar carbon nanotube films, and can include layers of coplanar carbon nanotube films. Additionally, when the carbon nanotubes in the carbon nanotube film are aligned along one preferred orientation (e.g., the drawn carbon nanotube film), an angle can exist between the orientation of carbon nanotubes in adjacent films, whether stacked or adjacent. Adjacent carbon nanotube films can be combined by only the van der Waals attractive force therebetween. An angle between the aligned directions of the carbon nanotubes in every two adjacent carbon nanotube films can range from about 0 degrees to about 90 degrees. When the angle between the aligned directions of the carbon nanotubes in adjacent stacked drawn carbon nanotube films is larger than 0 degrees, a plurality of micropores is defined by the carbon nanotube structure112. In one exemplary embodiment, the carbon nanotube structure112has the aligned directions of the carbon nanotubes between adjacent stacked drawn carbon nanotube films at 90 degrees. Stacking the carbon nanotube films will also add to the structural integrity of the carbon nanotube structure112.

The more the carbon nanotube films are stacked, the less apertures the carbon nanotube structure112has, and the less openings116the carbon nanotube composite structure110has. In one exemplary embodiment, the number of the stacked carbon nanotube films can be in a range from about 2 to about 4. When two carbon nanotube films are stacked, the angle between the aligned directions of the carbon nanotubes in the two drawn carbon nanotube films is about 90 degrees. When three carbon nanotube films are stacked, the angle between the aligned directions of the carbon nanotubes in every two adjacent drawn carbon nanotube films is about 60 degrees. When four carbon nanotube films are stacked, the angle between the aligned directions of the carbon nanotubes in every two adjacent drawn carbon nanotube films is about 45 degrees.

As shown inFIGS. 3-4, in one exemplary embodiment, only two carbon nanotube films are stacked with each other, and aligned directions of the carbon nanotubes in the two drawn carbon nanotube films are substantially perpendicular with each other.

The carbon nanotube composite structure110can be made by applying a protective layer114on a surface of the carbon nanotube structure112. The carbon nanotube structure112can be suspended in a depositing chamber during depositing the protective layer114so that two opposite surfaces of the carbon nanotube structure112are coated with the protective layer114. In some exemplary embodiments, each of the plurality of carbon nanotubes is fully enclosed by the protective layer114. In one exemplary embodiment, the carbon nanotube structure112is located on a frame so that the middle portion of the carbon nanotube structure112is suspended through the through hole of the frame. The frame can be any shape, such as a quadrilateral. The carbon nanotube structure112can also be suspended by a metal mesh or metal ring.

The method of depositing the protective layer114can be physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer deposition (ALD), magnetron sputtering, or spraying.

The plurality of openings116are formed because of the plurality of apertures of the carbon nanotube structure112. The plurality of openings116and the plurality of apertures have the same shape and different size. The size of the plurality of openings116is smaller than that of the plurality of apertures because the protective layer114is deposited in the plurality of apertures.

The thickness of the protective layer114is in a range from about 3 nanometers to about 50 nanometers. In one exemplary embodiment, the thickness of the protective layer114is in a range from about 3 nanometers to about 20 nanometers. If the thickness of the protective layer114is less than 3 nanometers, the protective layer114cannot prevent the carbon nanotubes from being destroyed in following etching process. If the thickness of the protective layer114is greater than 50 nanometers, the plurality of apertures may be fully filled by the protective layer114and the plurality of openings116cannot be obtained.

The material of the protective layer114can be metal, metal oxide, metal nitride, metal carbide, metal sulfide, silicon oxide, silicon nitride, or silicon carbide. The metal can be gold, nickel, titanium, iron, aluminum, titanium, chromium, or alloy thereof. The metal oxide can be alumina, magnesium oxide, zinc oxide, or hafnium oxide. The material of the protective layer114is not limited to the materials above and can be any material as long as the material can be deposited on the carbon nanotube structure112, would not react with the carbon nanotubes and would not be etched easily in following drying etching process. The protective layer114is combined with the carbon nanotube structure112by van der Waals attractive force therebetween only.

As shown inFIGS. 5-6, in one exemplary embodiment, an alumina layer of 5 nanometers thickness is deposited on two stacked drawn carbon nanotube films by electron beam evaporation. The angle between the aligned directions of the carbon nanotubes between the two stacked drawn carbon nanotube films is 90 degrees. As shown inFIGS. 7-8, in one exemplary embodiment, an alumina layer of 10 nanometers thickness is deposited on three stacked drawn carbon nanotube films by electron beam evaporation. As shown inFIG. 9, each of the plurality of carbon nanotubes is entirely enclosed by the alumina layer.

In step (S12), the carbon nanotube composite structure110can be in direct contact with the surface121of the silicon substrate12or suspended above the surface121of the silicon substrate12by a support. In one exemplary embodiment, the carbon nanotube composite structure110is transferred on the surface121of the silicon substrate12through the frame.

In one exemplary embodiment, the formation of the carbon nanotube composite structure110on the surface121further comprises solvent treating the silicon substrate12with the carbon nanotube composite structure110thereon. Because there is air gap between the carbon nanotube composite structure110and the surface121of the silicon substrate12, the solvent treatment can exhaust the air and allow the carbon nanotube composite structure110to be closely and firmly adhered on the surface121of the silicon substrate12. The solvent treating can be applying a solvent to entire surface of the carbon nanotube composite structure110or immersing the entire silicon substrate12with the carbon nanotube composite structure110in a solvent. The solvent can be water or volatile organic solvent such as ethanol, methanol, acetone, dichloroethane, chloroform, or mixtures thereof. In one exemplary embodiment, the organic solvent is ethanol.

In the step (S13), the dry etching can be plasma etching or reactive ion etching (ME). In one exemplary embodiment, the dry etching is performed by applying plasma energy on the entire or partial surface of the surface121via a plasma device. The plasma gas can be an inert gas and/or etching gases, such as argon (Ar), helium (He), chlorine (Cl2), hydrogen (H2), oxygen (O2), fluorocarbon (CF4), ammonia (NH3), or air.

In one exemplary embodiment, the plasma gas is a mixture of chlorine and argon. The power of the plasma device can be in a range from about 20 watts to about 70 watts. The plasma flow of chlorine can be in a range from about 5 sccm to about 20 sccm, such as 10 sccm. The plasma flow of argon can be in a range from about 15 sccm to about 40 sccm, such as 25 sccm. When the plasma is produced in vacuum, the work pressure of the plasma can be in a range from about 3 Pa to 10 Pa, such as 6 Pa. The time for plasma etching can be in a range from about 10 seconds to about 60 seconds, such as 45 seconds.

In the etching process, the etching gas reacts with the silicon substrate12, but does not react with the protective layer114or react with the protective layer114at a speed much less than that of the reaction between the etching gas and the silicon substrate12. Thus, the exposed portion of the silicon substrate12would be etched gradually and the portion of the silicon substrate12that are shielded by the carbon nanotube composite structure110would not be etched.

The bulged pattern122and the carbon nanotube composite structure110substantially have the same pattern. When the carbon nanotube structure112includes a plurality of intersected drawn carbon nanotube films, the bulged pattern122includes a plurality of strip-shaped bulges intersected with each other to form a net structure as shown inFIGS. 10-13.FIGS. 10-11show a SEM image of one exemplary embodiment of a patterned silicon substrate viewed from above.FIG. 12is a SEM image of one exemplary embodiment of a cross-section of the patterned silicon substrate.

Referring toFIG. 13, the patterned silicon substrate12acomprises a base123and a bulged pattern122located on a surface of the base123. The bulged pattern122comprises a plurality of strip-shaped bulges intersected with each other to form a net structure and defines a plurality of holes124. The bottom surfaces of the plurality of holes124are defined as an epitaxial growth surface126.

Each of the plurality of strip-shaped bulges has a length less than or equal to the width of length of the base123. The plurality of strip-shaped bulges comprises a plurality of first strip-shaped bulges and a plurality of second strip-shaped bulges. The plurality of first strip-shaped bulges are substantially parallel with each other and extends along the first direction, and the plurality of second strip-shaped bulges are substantially parallel with each other and extends along the second direction that is different from the first direction. The angle between the first direction and the second direction is greater than 0 degrees and less than or equal to 90 degrees. In one exemplary embodiment, the angle between the first direction and the second direction is greater than 30 degrees.

The width of the plurality of strip-shaped bulges can be in a range from about 20 nanometers to about 150 nanometers. In one exemplary embodiment, the width of the plurality of strip-shaped bulges can be in a range from about 20 nanometers to about 100 nanometers. In one exemplary embodiment, the width of the plurality of strip-shaped bulges can be in a range from about 20 nanometers to about 50 nanometers. The distance between every two adjacent of the plurality of strip-shaped bulges can be in a range from about 10 nanometers to about 300 nanometers. In one exemplary embodiment, the distance between every two adjacent of the plurality of strip-shaped bulges can be in a range from about 10 nanometers to about 100 nanometers. In one exemplary embodiment, the distance between every two adjacent of the plurality of strip-shaped bulges can be in a range from about 10 nanometers to about 50 nanometers. The height of the plurality of strip-shaped bulges can be in a range from about 50 nanometers to about 1000 nanometers. In one exemplary embodiment, the height of the plurality of strip-shaped bulges can be in a range from about 500 nanometers to about 1000 nanometers. The average diameter of the plurality of holes124can be in a range from about 10 nanometers to about 300 nanometers, and the depth of the plurality of holes124can be in a range from about 50 nanometers to about 1000 nanometers. In one exemplary embodiment, the ratio between the depth and the average diameter is greater than 5. In one exemplary embodiment, the ratio between the depth and the average diameter is greater than 10.

After coating with the protective layer114, the diameter of the carbon nanotubes are about tens of nanometers, and distance between every two adjacent carbon nanotubes are about tens of nanometers. Thus, the width and distance of the plurality of strip-shaped bulges are also tens of nanometers, and the average diameter of the plurality of holes124are also tens of nanometers. The density of the strip-shaped bulges and the holes124would be increased. For example, when both the width and distance of the plurality of strip-shaped bulges are 20 nanometers, the number of the strip-shaped bulges and the holes124would be 50 within 1 micrometer. The conventional photolithography method cannot make all the strip-shaped bulges in nanoscale and obtain this density due to the resolution limitation.

In step (S14), the GaN epitaxial layer14can be grown by a method such as molecular beam epitaxy, chemical beam epitaxy, reduced pressure epitaxy, low temperature epitaxy, select epitaxy, liquid phase deposition epitaxy, metal organic vapor phase epitaxy, ultra-high vacuum chemical vapor deposition, hydride vapor phase epitaxy, or metal organic chemical vapor deposition (MOCVD).

The GaN epitaxial layer14is a single crystal layer. The thickness of the GaN epitaxial layer14can be prepared according to need. The thickness of the GaN epitaxial layer14can be in a range from about 100 nanometers to about 500 micrometers. For example, the thickness of the GaN epitaxial layer14can be about 200 nanometers, 500 nanometers, 1 micrometer, 2 micrometers, 5 micrometers, 10 micrometers, or 50 micrometers.

Referring toFIG. 14, step (S14) includes the following substeps:

step (141), nucleating on the epitaxial growth surface126and growing a plurality of epitaxial crystal grains142along the direction substantially perpendicular to the epitaxial growth surface126;

step (142), forming a continuous epitaxial film144by making the epitaxial crystal grains142grow along the direction substantially parallel to the epitaxial growth surface126; and

step (143), forming the GaN epitaxial layer14by making the epitaxial film144grow along the direction substantially perpendicular to the epitaxial growth surface126.

In step (S141), the epitaxial crystal grains142cannot grow from the top surface of the bulged pattern122because of the baffle of the carbon nanotube composite structure110. The epitaxial crystal grains142may grow also from the side surface of the plurality of holes124.

In one exemplary embodiment, the GaN epitaxial layer14is grown on the patterned silicon substrate12aby MOCVD method. The nitrogen source gas is high-purity ammonia (NH3), the Ga source gas is trimethyl gallium (TMGa) or triethyl gallium (TEGa), and the carrier gas is hydrogen (H2). The patterned silicon substrate12ais placed in a vacuum reaction chamber, and the vacuum reaction chamber is heated to a temperature of about 1100° C. to about 1200° C. The hydrogen gas is introduced in the vacuum reaction chamber, and the reaction chamber is kept at the temperature of about 1100° C. to about 1200° C. for about 200 seconds to about 1000 seconds. The reaction chamber is cooled down to a temperature of about 500° C. to about 650° C., and the nitrogen source gas and the Ga source gas are introduced in the vacuum reaction chamber to grow a buffer layer with a thickness of 10 nanometers to about 50 nanometers. Ga source gas is no longer introduced, however, the nitrogen source gas continues to input, and the reaction chamber is heated to the temperature of about 1100° C. to about 1200° C. again and kept at the temperature of about 1100° C. to about 1200° C. for about 30 seconds to about 300 seconds. Keep introducing the nitrogen source gas, and the nitrogen source gas is introduced in the vacuum reaction chamber again to grow the GaN epitaxial layer14.

The GaN epitaxial layer124grown on the patterned silicon substrate12ais as good as the GaN epitaxial layer grown on the sapphire substrate.

Referring toFIG. 15, a method for making a GaN epitaxial layer14on a silicon substrate12of one exemplary embodiment includes the following steps:

step (S20), providing the silicon substrate12;

step (S21), providing a carbon nanotube composite structure110, wherein the carbon nanotube composite structure110includes a carbon nanotube structure112and a protective layer114coated on the carbon nanotube structure112, and the carbon nanotube structure112includes a plurality of intersected carbon nanotubes and defines a plurality of openings116;

step (S22), forming the carbon nanotube composite structure110on a surface121of the silicon substrate12, wherein portions of the surface121are exposed from the plurality of openings116;

step (S23), forming a patterned silicon substrate12ahaving a bulged pattern122by dry etching the surface121using the carbon nanotube composite structure110as a mask, wherein the bulged pattern122includes a plurality of strip-shaped bulges intersected with each other;

step (S24), removing the carbon nanotube composite structure110from the patterned silicon substrate12a; and

step (S25), epitaxially growing a GaN epitaxial layer14on the patterned silicon substrate12a.

The method ofFIG. 15is similar to the method ofFIG. 1except that the carbon nanotube composite structure110is removed from the patterned silicon substrate12abefore epitaxially growing the GaN epitaxial layer14.

In step (S23), the patterned silicon substrate12ais formed and observed by SEM. The SEM images are shown inFIGS. 16-18.

In step (S24), the method of removing the carbon nanotube composite structure110can be ultrasonic method, or adhesive tape peeling, oxidation. In one exemplary embodiment, the patterned silicon substrate12awith the carbon nanotube composite structure110thereon is placed in an N-methyl pyrrolidone solution and ultrasonic treating for several minutes. In another one exemplary embodiment, the carbon nanotube composite structure is entirely removed from the patterned silicon substrate by blowing as shown inFIG. 19because the carbon nanotube composite structure is still a free standing structure after dry etching.

In step (S25), the epitaxial crystal grains142are simultaneously grown from both the top surface of the bulged pattern122and the bottom surface of the plurality of holes124along the direction substantially perpendicular to the epitaxial growth surface126. The epitaxial crystal grains142grows along the direction substantially parallel to the epitaxial growth surface126to form the epitaxial film144. The epitaxial film144grows along the direction substantially perpendicular to the epitaxial growth surface126to form the GaN epitaxial layer14.

Referring toFIG. 20, a method for making a GaN epitaxial layer14on a silicon substrate12of one exemplary embodiment includes the following steps:

step (S30), providing the silicon substrate12;

step (S31), providing a carbon nanotube composite structure110, wherein the carbon nanotube composite structure110includes a carbon nanotube structure112and a protective layer114coated on the carbon nanotube structure112, and the carbon nanotube structure112includes a plurality of intersected carbon nanotubes and defines a plurality of openings116;

step (S32), forming the carbon nanotube composite structure110on a surface121of the silicon substrate12, wherein portions of the surface121are exposed from the plurality of openings116;

step (S33), forming a patterned silicon substrate12ahaving a bulged pattern122by dry etching the surface121using the carbon nanotube composite structure110as a mask, wherein the bulged pattern122includes a plurality of strip-shaped bulges intersected with each other;

step (S34), depositing a baffle layer16to cover both the carbon nanotube composite structure110and the bulged pattern122;

step (S35), obtaining a patterned baffle layer17by removing the carbon nanotube composite structure110from the patterned silicon substrate12a; and

step (S36), epitaxially growing a GaN epitaxial layer14on the patterned silicon substrate12a.

The method ofFIG. 20is similar to the method ofFIG. 15except that the baffle layer16is deposited on the patterned silicon substrate12ato cover both the carbon nanotube composite structure110and the bulged pattern122.

In step (S33), the patterned silicon substrate12ais formed and observed by SEM. The SEM images are shown inFIGS. 21-24.

In step (S34), the baffle layer16can be deposited by electron beam evaporation, ion beam sputtering, atomic layer deposition, magnetron sputtering, thermal vapor deposition, or chemical vapor deposition. A first portion of the baffle layer16is deposited on the surface of the carbon nanotube composite structure110, and a second portion of the baffle layer16is deposited on the bottom surface of the plurality of holes124. The thickness of the baffle layer16is less than the depth of the holes124so the first portion of the baffle layer16and the second portion of the baffle layer16are spaced from each other to form a discontinuous structure. Thus, the first portion of the baffle layer16would be removed from the patterned silicon substrate12atogether with the carbon nanotube composite structure110. The second portion of the baffle layer16would be remained on the bottom surface of the plurality of holes124to form the patterned baffle layer17. The material of the baffle layer16can be any material not able to be used to grow the GaN epitaxial layer14. The material of the baffle layer16can be silicon dioxide (SiO2) or silicon nitride (Si3N4).

In step (S36), the epitaxial crystal grains142are vertically grown from the top surface of the bulged pattern122, and cannot grow from the bottom surface of the plurality of holes124because the baffle of the patterned baffle layer17. The epitaxial crystal grains142then laterally grows along the direction substantially parallel to the epitaxial growth surface126to form the epitaxial film144. The epitaxial film144grows along the direction substantially perpendicular to the epitaxial growth surface126to form the GaN epitaxial layer14. The plurality of holes124are not filled by the GaN epitaxial layer14, and a hollow space is formed between the patterned baffle layer17and the GaN epitaxial layer14. Thus, the mismatch between the GaN epitaxial layer14and the patterned silicon substrate12awould be further reduced.

Referring toFIG. 25, a method for making a GaN epitaxial layer14on a silicon substrate12of one exemplary embodiment includes the following steps:

step (S40), providing the silicon substrate12;

step (S41), providing a carbon nanotube structure112, wherein the carbon nanotube structure112includes a plurality of intersected carbon nanotubes and defines a plurality of openings116;

step (S42), forming the carbon nanotube structure112on a surface121of the silicon substrate12, wherein portions of the surface121are exposed from the plurality of openings116;

step (S43), forming a patterned silicon substrate12ahaving a bulged pattern122by dry etching the surface121using the carbon nanotube structure112as a mask, wherein the bulged pattern122includes a plurality of strip-shaped bulges intersected with each other;

step (S44), epitaxially growing a GaN epitaxial layer14on the patterned silicon substrate12a.

The method ofFIG. 25is similar to the method ofFIG. 1except that the carbon nanotube structure112only including the plurality of intersected carbon nanotubes is used as the mask for dry etching.

Referring toFIG. 26, a method for making a GaN epitaxial layer14on a silicon substrate12of one exemplary embodiment includes the following steps:

step (S50), providing the silicon substrate12;

step (S51), providing a carbon nanotube composite structure110, wherein the carbon nanotube composite structure110includes a carbon nanotube structure112and a protective layer114coated on the carbon nanotube structure112, and the carbon nanotube structure112includes a plurality of intersected carbon nanotubes and defines a plurality of openings116;

step (S52), forming the carbon nanotube composite structure110on a surface121of the silicon substrate12, wherein portions of the surface121are exposed from the plurality of openings116;

step (S53), forming a patterned silicon substrate12ahaving a bulged pattern122by dry etching the surface121using the carbon nanotube composite structure110as a mask, wherein the bulged pattern122includes a plurality of strip-shaped bulges intersected with each other;

step (S54), depositing a baffle layer16to cover both the carbon nanotube composite structure110and the bulged pattern122, wherein the baffle layer16is deposited on the surface of the carbon nanotube composite structure110and the bottom surface of the plurality of holes124so that portions of the side surface of the bulged pattern122is exposed; and

step (S55), epitaxially growing a GaN epitaxial layer14on the patterned silicon substrate12a.

The method ofFIG. 26is similar to the method ofFIG. 20except that the carbon nanotube composite structure110is kept on the bulged pattern122during epitaxially growing the GaN epitaxial layer14so that the GaN epitaxial layer14the carbon nanotube composite structure110is enclosed by the GaN epitaxial layer14.

Referring toFIG. 27, step (S55) includes the following substeps:

step (551), nucleating and laterally growing a plurality of epitaxial crystal grains142on the side surface of the bulged pattern122along the direction substantially parallel to the bottom surface of the plurality of holes124;

step (552), forming a continuous epitaxial film144by vertically growing the epitaxial crystal grains142along the direction substantially perpendicular to the bottom surface of the plurality of holes124and then joining the epitaxial crystal grains142; and

step (553), forming the GaN epitaxial layer14by making the epitaxial film144vertically grow along the direction substantially perpendicular to the bottom surface of the plurality of holes124.

It is to be understood that the above-described exemplary embodiments are intended to illustrate rather than limit the disclosure. Any elements described in accordance with any exemplary embodiments is understood that they can be used in addition or substituted in other exemplary embodiments. Exemplary embodiments can also be used together. Variations may be made to the exemplary embodiments without departing from the spirit of the disclosure. The above-described exemplary embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure.