OPTIMIZING GROWTH METHOD FOR IMPROVING QUALITY OF MOCVD EPITAXIAL THIN FILMS

Abstract

The present invention provides an optimizing growth method for improving quality of MOCVD epitaxial thin films, including the following method: step 1, putting a substrate and a thin film A to a reaction chamber of an MOCVD equipment; and feeding a compound containing an element X as an X source under the condition that the reaction chamber is filled with H2; configuring a temperature, reaction chamber pressure and deposition time within a parameter range where the gaseous compound can decompose X atoms; pre-depositing an X atomic layer on a surface of the substrate or the thin film A; the X atomic layer is adsorbed on the substrate or thin film A at this time; and the X atomic layer can be reacted with other compounds to generate a thin film B component in the follow-up process, or can directly form a thin film B component with the thin film A.

Claims

1. An optimizing growth method for improving quality of an MOCVD epitaxial thin film, comprising the following steps: step 1, putting a substrate and a thin film A to a reaction chamber of an MOCVD equipment; and feeding a compound containing an element X as an X source under the condition that the reaction chamber is filled with H.sub.2; configuring a temperature, reaction chamber pressure and deposition time within a parameter range where the gaseous compound is capable of decomposing X atoms; pre-depositing an X atomic layer on a surface of the substrate or the thin film A, wherein the X atomic layer is adsorbed on the substrate or thin film A at this time; and the X atomic layer is reacted with other compounds to generate a thin film B component in the follow-up process, or is directly form a thin film B component with the thin film A; and step 2, after completing the growth of the above pre-deposited X atomic layer, and subjecting the thin film B to growth; simultaneously feeding all gaseous compounds required by epitaxial growth of the thin film B under the condition that the reaction chamber is filled with H.sub.2; configuring a temperature, reaction chamber pressure and deposition time within a parameter range capable of achieving epitaxial growth of the thin film B; subjecting the thin film B to epitaxial growth on the X atomic layer, wherein the pre-deposited X atomic layer is firstly reacted with the gas during such process, thus providing nucleation sites for the thin film B, and then the thin film B grows up with these nucleation sites as starting points; or wherein the pre-deposited X atomic layer has generated the thin film B component with the thin film A as nucleation sites; and at this time, the thin film B grows up with these nucleation sites as starting points; during such growing process, the pre-deposited X atomic layer disappears and becomes a portion of the thin film B.

2. The optimizing growth method according to claim 1, wherein in the step 1, the temperature is controlled within a range from 800° C. to 1400° C.; the reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and the time is controlled within a range from 0 s to 300 s.

3. The optimizing growth method according to claim 1, wherein, subjecting an AlN buffer layer and a GaN thin film to epitaxial growth on a Si substrate, comprising the following preparation method: (1) pretreating the Si substrate, comprising a cleaning process and a desorption process; (2) pre-depositing an Al atomic layer, putting the Si substrate to a reaction chamber of a MOCVD equipment, feeding TMAl as an Al source under the condition that the reaction chamber is filled with H.sub.2; wherein a surface temperature of the Si substrate is controlled within a range from 800° C. to 1400° C., a reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and time is controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Al atomic layer, wherein the pre-deposited Al atomic layer is adsorbed on the Si substrate; (3) growing the AlN buffer layer, feeding TMAl as an Al source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; wherein during such process, the pre-deposited Al atomic layer is firstly reacted with NH.sub.3 to form AlN nucleation sites, then AlN nucleation sites grow up to thereby forming an AlN thin film, and during such growing process, the pre-deposited Al atomic layer disappears and becomes a portion of the AlN thin film; and (4) growing a GaN epitaxial layer, feeding TMGa as a Ga source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2.

4. The optimizing growth method according to claim 1, wherein, subjecting an AlGaN buffer layer and a GaN thin film to epitaxial growth on an AlN thin film, comprising the following preparation method: (1) growing an AlN epitaxial layer on a Si substrate, feeding TMAl as an Al source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; (2) pre-depositing a Ga atomic layer, putting an AlN thin film to a chamber, feeding TMGa as a Ga source under the condition that the reaction chamber is filled with H.sub.2; wherein a surface temperature of AlN is controlled within a range from 800° C. to 1400° C., a reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and time is controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Ga atomic layer, wherein the pre-deposited Ga atomic layer is adsorbed on the AlN thin film to form AlGaN nucleation sites; (3) growing the AlGaN buffer layer, feeding TMAl as an Al source, feeding TMGa as a Ga source, and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; wherein during such process, the pre-deposited AlGaN nucleation sites grow up to thereby forming an AlGaN thin film, and during such growing process, the pre-deposited Ga atomic layer disappears and becomes a portion of the AlGaN thin film; and (4) growing a GaN epitaxial layer, feeding TMGa as a Ga source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2.

5. The optimizing growth method according to claim 1, wherein, subjecting an Al.sub.yGa.sub.1-yN buffer layer and a GaN thin film to epitaxial growth on an Al.sub.xGa.sub.1-xN thin film, comprising the following preparation method, wherein 1>x>y>0: (1) growing an AlN and Al.sub.0.45Ga.sub.0.55N epitaxial layers on a Si substrate, feeding TMAl as an Al source, feeding TMGa as a Ga source, and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; (2) pre-depositing a Ga atomic layer, putting the Al.sub.0.45Ga.sub.0.55N thin film to a chamber, feeding TMGa as a Ga source under the condition that the reaction chamber is filled with H.sub.2; wherein a surface temperature of Al.sub.0.45Ga.sub.0.55N is controlled within a range from 800° C. to 1400° C., a reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and time is controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Ga atomic layer; wherein the pre-deposited Ga atomic layer can be adsorbed on the Al.sub.0.45Ga.sub.0.55N thin film, thus rendering the components thereof to be gradually close to an Al.sub.0.25Ga.sub.0.75N-grown thin film; (3) growing an Al.sub.0.25Ga.sub.0.75N buffer layer, feeding TMAl as an Al source, feeding TMGa as a Ga source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; during such process, a surface of the Al.sub.0.45Ga.sub.0.55N thin film contains more and more Ga component, such that the components thereof are closer and closer to the Al.sub.0.25Ga.sub.0.75N-grown thin film, thereby finally forming a stable Al.sub.0.25Ga.sub.0.75N-grown thin film; wherein during such growing process, the pre-deposited Ga atomic layer disappears and becomes a transition portion grown with two thin films of Al.sub.0.45Ga.sub.0.55N and Al.sub.0.25Ga.sub.0.75N; and (4) growing a GaN epitaxial layer, feeding TMGa as a Ga source and feeding NH3 as a N source under the condition that the reaction chamber is filled with H.sub.2.

6. The optimizing growth method according to claim 1, wherein, subjecting a GaN thin film to epitaxial growth on an AlGaN thin film, comprising the following preparation method: (1) growing AlN and AlGaN epitaxial layers on a Si substrate, feeding TMAl as an Al source, feeding TMGa as a Ga source, and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; (2) pre-depositing a Ga atomic layer, putting an AlGaN thin film to a chamber, feeding TMGa as a Ga source under the condition that the reaction chamber is filled with H.sub.2; wherein a surface temperature of AlGaN is controlled within a range from 800° C. to 1400° C., a reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and time is controlled within a range from 0 s to 300 s; wherein the pre-deposited Ga atomic layer can be adsorbed on the AlGaN thin film to form an AlGaN atomic layer with a higher component and reach a saturation point rapidly, thereby abstracting N atoms and forming GaN nucleation sites; and (3) growing a GaN buffer layer, feeding TMGa as a Ga source, and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; wherein during such process, the pre-deposited GaN nucleation sites grow up, thereby forming a GaN thin film, and during such growing process, the pre-deposited Ga atomic layer disappears and becomes a portion of the GaN thin film.

7. The optimizing growth method according to claim 2, wherein, subjecting an AlN buffer layer and a GaN thin film to epitaxial growth on a Si substrate, comprising the following preparation method: (1) pretreating the Si substrate, comprising a cleaning process and a desorption process; (2) pre-depositing an Al atomic layer, putting the Si substrate to a reaction chamber of a MOCVD equipment, feeding TMAl as an Al source under the condition that the reaction chamber is filled with H.sub.2; wherein a surface temperature of the Si substrate is controlled within a range from 800° C. to 1400° C., a reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and time is controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Al atomic layer, wherein the pre-deposited Al atomic layer is adsorbed on the Si substrate; (3) growing the AlN buffer layer, feeding TMAl as an Al source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; wherein during such process, the pre-deposited Al atomic layer is firstly reacted with NH.sub.3 to form AlN nucleation sites, then AlN nucleation sites grow up to thereby forming an AlN thin film, and during such growing process, the pre-deposited Al atomic layer disappears and becomes a portion of the AlN thin film; and (4) growing a GaN epitaxial layer, feeding TMGa as a Ga source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2.

8. The optimizing growth method according to claim 2, wherein, subjecting an AlGaN buffer layer and a GaN thin film to epitaxial growth on an AlN thin film, comprising the following preparation method: (1) growing an AlN epitaxial layer on a Si substrate, feeding TMAl as an Al source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; (2) pre-depositing a Ga atomic layer, putting an AlN thin film to a chamber, feeding TMGa as a Ga source under the condition that the reaction chamber is filled with H.sub.2; wherein a surface temperature of AlN is controlled within a range from 800° C. to 1400° C., a reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and time is controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Ga atomic layer, wherein the pre-deposited Ga atomic layer is adsorbed on the AlN thin film to form AlGaN nucleation sites; (3) growing the AlGaN buffer layer, feeding TMAl as an Al source, feeding TMGa as a Ga source, and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; wherein during such process, the pre-deposited AlGaN nucleation sites grow up to thereby forming an AlGaN thin film, and during such growing process, the pre-deposited Ga atomic layer disappears and becomes a portion of the AlGaN thin film; and (4) growing a GaN epitaxial layer, feeding TMGa as a Ga source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2.

9. The optimizing growth method according to claim 2, wherein, subjecting an Al.sub.yGa.sub.1-yN buffer layer and a GaN thin film to epitaxial growth on an Al.sub.xGa.sub.1-xN thin film, comprising the following preparation method, wherein 1>x>y>0: (1) growing an AlN and Al.sub.0.45Ga.sub.0.55N epitaxial layers on a Si substrate, feeding TMAl as an Al source, feeding TMGa as a Ga source, and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; (2) pre-depositing a Ga atomic layer, putting the Al.sub.0.45Ga.sub.0.55N thin film to a chamber, feeding TMGa as a Ga source under the condition that the reaction chamber is filled with H.sub.2; wherein a surface temperature of Al.sub.0.45Ga.sub.0.55N is controlled within a range from 800° C. to 1400° C., a reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and time is controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Ga atomic layer; wherein the pre-deposited Ga atomic layer can be adsorbed on the Al.sub.0.45Ga.sub.0.55N thin film, thus rendering the components thereof to be gradually close to an Al.sub.0.25Ga.sub.0.75N-grown thin film; (3) growing an Al.sub.0.25Ga.sub.0.75N buffer layer, feeding TMAl as an Al source, feeding TMGa as a Ga source and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; during such process, a surface of the Al.sub.0.45Ga.sub.0.55N thin film contains more and more Ga component, such that the components thereof are closer and closer to the Al.sub.0.25Ga.sub.0.75N-grown thin film, thereby finally forming a stable Al.sub.0.25Ga.sub.0.75N-grown thin film; wherein during such growing process, the pre-deposited Ga atomic layer disappears and becomes a transition portion grown with two thin films of Al.sub.0.45Ga.sub.0.55N and Al.sub.0.25Ga.sub.0.75N; and (4) growing a GaN epitaxial layer, feeding TMGa as a Ga source and feeding NH3 as a N source under the condition that the reaction chamber is filled with H.sub.2.

10. The optimizing growth method according to claim 2, wherein, subjecting a GaN thin film to epitaxial growth on an AlGaN thin film, comprising the following preparation method: (1) growing AlN and AlGaN epitaxial layers on a Si substrate, feeding TMAl as an Al source, feeding TMGa as a Ga source, and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; (2) pre-depositing a Ga atomic layer, putting an AlGaN thin film to a chamber, feeding TMGa as a Ga source under the condition that the reaction chamber is filled with H.sub.2; wherein a surface temperature of AlGaN is controlled within a range from 800° C. to 1400° C., a reaction chamber pressure is controlled within a range from 20 mbar to 200 mbar; and time is controlled within a range from 0 s to 300 s; wherein the pre-deposited Ga atomic layer can be adsorbed on the AlGaN thin film to form an AlGaN atomic layer with a higher component and reach a saturation point rapidly, thereby abstracting N atoms and forming GaN nucleation sites; and (3) growing a GaN buffer layer, feeding TMGa as a Ga source, and feeding NH.sub.3 as a N source under the condition that the reaction chamber is filled with H.sub.2; wherein during such process, the pre-deposited GaN nucleation sites grow up, thereby forming a GaN thin film, and during such growing process, the pre-deposited Ga atomic layer disappears and becomes a portion of the GaN thin film.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] FIG. 1 is a structure diagram of a method for MOCVD epitaxial growth of a thin film, where (a) denotes a conventional method; and (b) denotes an optimization method.

[0036] FIG. 2 is a diagram showing a growth process of MOCVD epitaxial growth of a thin film in the conventional method.

[0037] FIG. 3 is a diagram showing a growth process of MOCVD epitaxial growth of a thin film in the optimization method.

[0038] FIG. 4 shows OM pictures of a GaN thin film grown on an AlN buffer layer prepared by different methods, where (a) denotes that there is no pre-deposited Al atomic layer; and (b) denotes that there is a pre-deposited Al atomic layer.

[0039] FIG. 5 shows AFM pictures of a GaN thin film grown on an AlN buffer layer prepared by different methods, where (a) denotes that there is no pre-deposited Al atomic layer; and (b) denotes that there is a pre-deposited Al atomic layer.

[0040] FIG. 6 shows intensity of an XRD swing curve of a GaN (0002) surface grown on an AlN buffer layer prepared by different methods.

[0041] FIG. 7 shows OM pictures of a GaN thin film grown on an AlGaN buffer layer prepared by different methods, where (a) denotes that there is no pre-deposited Ga atomic layer; and (b) denotes that there is a pre-deposited Ga atomic layer.

[0042] FIG. 8 shows AFM pictures of a GaN thin film grown on an AlGaN buffer layer prepared by different methods, where (a) denotes that there is no pre-deposited Ga atomic layer; and (b) denotes that there is a pre-deposited Ga atomic layer.

[0043] FIG. 9 shows intensity of an XRD swing curve of a GaN (0002) surface grown on an AlGaN buffer layer prepared by different methods.

[0044] FIG. 10 shows OM pictures of a GaN thin film grown on an Al.sub.0.25Ga.sub.0.75N buffer layer prepared on Al.sub.0.45Ga.sub.0.55N by different methods, where (a) denotes that there is no pre-deposited Ga atomic layer; and (b) denotes that there is a pre-deposited Ga atomic layer.

[0045] FIG. 11 shows AFM pictures of a GaN thin film grown on an Al.sub.0.25Ga.sub.0.75N buffer layer prepared on Al.sub.0.45Ga.sub.0.55N by different methods, where (a) denotes that there is no pre-deposited Ga atomic layer; and (b) denotes that there is a pre-deposited Ga atomic layer.

[0046] FIG. 12 shows intensity of an XRD swing curve of a GaN (0002) surface grown on an Al.sub.0.25Ga.sub.0.75N buffer layer prepared on Al.sub.0.45Ga.sub.0.55N by different methods.

[0047] FIG. 13 shows OM pictures of a GaN thin film when GaN is prepared on AlGaN by different methods, where (a) denotes that there is no pre-deposited Ga atomic layer; and (b) denotes that there is a pre-deposited Ga atomic layer.

[0048] FIG. 14 shows AFM pictures of a GaN thin film when GaN is prepared on AlGaN by different methods, where (a) denotes that there is no pre-deposited Ga atomic layer; and (b) denotes that there is a pre-deposited Ga atomic layer.

[0049] FIG. 15 shows intensity of an XRD swing curve of a corresponding GaN (0002) surface when GaN is prepared on AlGaN by different methods.

DESCRIPTION OF THE EMBODIMENTS

Embodiment 1

[0050] By referring to FIGS. 1-3, the optimizing growth method for improving quality of an MOCVD epitaxial thin film by a pre-deposited nucleation layer has an optimized structure as shown in FIG. 1(b). A preparation method of growing a thin film B on a substrate or a thin film A (as shown in FIG. 3) has the following steps:

[0051] step 1, pre-depositing an X atomic layer: a substrate or a thin film A was put to a reaction chamber of an MOCVD equipment; and a compound containing an element X was fed as an X source under the condition that the reaction chamber was filled with H.sub.2; a temperature, reaction chamber pressure and deposition time were configured within a parameter range where the gaseous compound could decompose X atoms; an X atomic layer was pre-deposited on a surface of the substrate or the thin film A, where the X atomic layer was adsorbed on the substrate or thin film A; and the X atomic layer can be reacted with other compounds to generate a thin film B component in the follow-up process, or directly formed a thin film B component with the thin film A;

[0052] step 2, growing a thin film B: after completing the growth of the above pre-deposited X atomic layer, the thin film B was grown; all gaseous compounds required by epitaxial growth of the thin film B were fed simultaneously under the condition that the reaction chamber was filled with H.sub.2 (for example, NH.sub.3 was fed as a N source; TMX was fed as an X source; TMY was fed as a Y source; and TMZ was fed as a Z source); a temperature, reaction chamber pressure and deposition time were configured within a parameter range capable of achieving epitaxial growth of the thin film B; the thin film B was subjected to epitaxial growth on the X atomic layer, where the pre-deposited X atomic layer was firstly reacted with the gas during the process, thus providing nucleation sites for the thin film B, and then the thin film B grew up with these nucleation sites as starting points; or where the pre-deposited X atomic layer had generated a thin film B component with the thin film A as nucleation sites; and at this time, the thin film B grew up with these nucleation sites as starting points; during such growing process, the pre-deposited X atomic layer disappeared and became a portion of the thin film B. The obtained structure was shown in FIG. 3(b).

[0053] Preferably, in the step 1, the temperature was controlled within a range from 800° C. to 1400° C.; the reaction chamber pressure was controlled within a range from 20 mbar to 200 mbar; and the time was controlled within a range from 0 s to 300 s.

EXAMPLE 2

[0054] By referring to FIGS. 4-6, an AlN buffer layer and a GaN thin film were subjected to epitaxial growth on a Si substrate, and the GaN thin film was represented and analyzed by an optical microscope (OM), an atomic force microscope (AFM), and an x-ray diffraction (XRD), thus judging the effect of the optimization method.

[0055] A preparation method for growing AlN and GaN thin films on a Si substrate has the following steps.

[0056] Step 1, pretreating the Si substrate: including a cleaning process and a desorption process (growth parameters are common knowledge in the art, and thus are not specified any more).

[0057] Step 2 pre-depositing an Al atomic layer: the Si substrate was put to a chamber, TMAl was fed as an Al source under the condition that the reaction chamber was filled with H.sub.2; where a surface temperature of the Si substrate was controlled within a range from 800° C. to 1400° C., a reaction chamber pressure was controlled within a range from 20 mbar to 200 mbar; and time was controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Al atomic layer. The pre-deposited Al atomic layer might be adsorbed on the Si substrate.

[0058] Step 3, growing an AlN buffer layer (growth parameters are common knowledge in the art, and thus are not specified any more): TMAl was fed as an Al source and NH.sub.3 was fed as a N source under the condition that the reaction chamber was filled with H.sub.2. During such process, the pre-deposited Al layer was firstly reacted with NH.sub.3 to form AlN nucleation sites, and the AlN nucleation sites grew up, thus forming an AlN thin film. During such growing process, the pre-deposited Al layer disappeared and became a portion of the AlN thin film.

[0059] Step 4, growing a GaN epitaxial layer (growth parameters are common knowledge in the art, and thus are not specified any more): TMGa was fed as a Ga source and NH.sub.3 was fed as a N source under the condition that the reaction chamber was filled with H.sub.2.

[0060] The present invention was further proved by contrastive analysis on the conventional method and optimization method to have the following beneficial effects below.

[0061] After an AlN buffer layer was grown by two methods of a conventional method (a pre-deposited Al atomic layer was not taken) and an optimization method (a pre-deposited Al atomic layer was taken), by making a comparison to the GaN thin film grown on the AlN buffer layer, it was found that the AlN buffer layer prepared by the optimization method greatly improved the homogeneity and crystal quality of the GaN thin film thereon.

[0062] It was found (FIG. 4) through OM observation that the GaN thin film grown by the conventional method showed a large number of holes and flaws; while the GaN thin film grown by the optimization method was smooth, and cracks could be found.

[0063] It was found (FIG. 5) through AFM observation that the GaN thin film grown by the conventional method had poor quality and could not obtain useful signals; while the GaN thin film grown by the optimization method was rough and uneven microscopically, and holes could be found.

[0064] It was found (FIG. 6) through XRD detection results that the GaN thin film grown by the conventional method could not obtain effective XRD data, which meant that the crystal quality was far below the GaN thin film grown by the optimization method.

[0065] To sum up, the new optimization method for epitaxial growth of an AlN thin film on a Si substrate could improve the homogeneity and surface quality of the GaN thin film grown thereon.

EXAMPLE 3

[0066] By referring to FIGS. 7-9, an AlGaN buffer layer and a GaN thin film were subjected to epitaxial growth on an AlN thin film, and the GaN thin film was represented and analyzed by OM, AFM and XRD, thus judging the effect of the optimization method.

[0067] A preparation method for growing AlGaN and GaN thin films on an AlN thin film has the following steps.

[0068] Step 1, growing an AlN epitaxial layer (growth parameters are common knowledge in the art, and thus are not specified any more) on a Si substrate: TMAl was fed as an Al source and NH.sub.3 was fed as a N source under the condition that the reaction chamber was filled with H.sub.2.

[0069] Step 2, pre-depositing a Ga atomic layer: the AlN thin film was put to a chamber, TMGa was fed as a Ga source under the condition that the reaction chamber was filled with H.sub.2; where a surface temperature of AlN was controlled within a range from 800° C. to 1400° C., a reaction chamber pressure was controlled within a range from 20 mbar to 200 mbar; and time was controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Ga atomic layer. The pre-deposited Ga atomic layer might be adsorbed on the AlN thin film to form AlGaN nucleation sites.

[0070] Step 3, growing an AlGaN buffer layer (growth parameters are common knowledge in the art, and thus are not specified any more): TMAl was fed as an Al source, TMGa was fed as a Ga source, and NH.sub.3 was fed as a N source under the condition that the reaction chamber was filled with H.sub.2. During such process, the pre-deposited AlGaN nucleation sites grew up, thus forming an AlGaN thin film. During such growing process, the pre-deposited Ga layer disappeared and became a portion of the AlGaN thin film.

[0071] Step 4, growing a GaN epitaxial layer (growth parameters are common knowledge in the art, and thus are not specified any more): TMGa was fed as a Ga source and NH.sub.3 was fed as a N source under the condition that the reaction chamber was filled with H.sub.2.

[0072] The present invention was further proved by contrastive analysis on the conventional method and optimization method to have the following beneficial effects:

[0073] After an AlGaN buffer layer was grown by two methods of a conventional method (a pre-deposited Ga atomic layer was not taken) and an optimization method (a pre-deposited Ga atomic layer was taken), by making a comparison to the GaN thin films grown thereon, it was found that the AlGaN buffer layer prepared by the optimization method greatly improved the homogeneity and crystallization quality of the GaN thin film thereon.

[0074] It was found (FIG. 7) through OM observation that the GaN thin film grown by the conventional method had relatively dense cracks; while the GaN thin film grown by the optimization method had far fewer cracks.

[0075] It was found (FIG. 8) through AFM observation that the GaN thin film grown by the conventional method was rough and uneven, and had obvious cracks and holes; while the GaN thin film grown by the optimization method was rough and uneven microscopically, and precious little holes could be found.

[0076] It was found (FIG. 9) through XRD detection results that peak intensity of the two methods was very close; that is, the crystal quality of the GaN thin film grown by the conventional method was slightly lower than that of the GaN thin film grown by the optimization method, but both had been very close.

[0077] To sum up, the new optimization method for epitaxial growth of an AlGaN thin film on an AlN thin film could improve the homogeneity and surface quality of the GaN thin film grown thereon.

EXAMPLE 4

[0078] By referring to FIGS. 10-11, an Al.sub.yGa.sub.1-yN buffer layer and a GaN thin film were subjected to epitaxial growth of on an Al.sub.xGa.sub.1-xN thin film, where 1>x>y>0, such that the thin film gradually contained more Ga from containing less Ga. In this case, x=0.45 and y=0.25. Effect analysis was performed by using influences of a GaN thin film on the optimization method.

[0079] A preparation method for growing an Al.sub.0.25Ga.sub.0.75N and GaN thin films on an Al.sub.0.45Ga.sub.0.55N thin film has the following steps.

[0080] Step 1, growing AlN and Al.sub.0.45Ga.sub.0.55N epitaxial layers on a Si substrate (growth parameters are common knowledge in the art, and thus are not specified any more): TMAl was fed as an Al source, TMGa as a Ga source, and NH.sub.3 was fed as a N source under the condition that the reaction chamber is filled with H.sub.2.

[0081] Step 2, pre-depositing a Ga atomic layer: the Al.sub.0.45Ga.sub.0.55N thin film was put to a chamber, TMGa was fed as a Ga source under the condition that the reaction chamber was filled with H.sub.2; where a surface temperature of Al.sub.0.45Ga.sub.0.55N was controlled within a range from 800° C. to 1400° C., a reaction chamber pressure was controlled within a range from 20 mbar to 200 mbar; and time was controlled within a range from 0 s to 300 s, thus obtaining a pre-deposited Ga atomic layer. The pre-deposited Ga atomic layer might be adsorbed on the Al.sub.0.45Ga.sub.0.55N thin film, rendering the component thereof to be gradually close to Al.sub.0.25Ga.sub.0.75N.

[0082] Step 3, growing an Al.sub.0.25Ga.sub.0.75N buffer layer (growth parameters are common knowledge in the art, and thus are not specified any more): TMAl was fed as an Al source, TMGa was fed as a Ga source and NH.sub.3 was fed as a N source under the condition that the reaction chamber was filled with H.sub.2. During such process, the surface of the Al.sub.0.45Ga.sub.0.55N thin film contained more and more Ga; therefore, the component thereof is closer and closer to Al.sub.0.25Ga.sub.0.75N, thus finally forming a stable Al.sub.0.25Ga.sub.0.75N thin film. During such growing process, the pre-deposited Ga layer disappeared and became a transition portion of the two thin films of Al.sub.0.45Ga.sub.0.55N and Al.sub.0.25Ga.sub.0.75N.

[0083] Step 4, growing a GaN epitaxial layer (growth parameters are common knowledge in the art, and thus are not specified any more): TMGa was fed as a Ga source and NH.sub.3 was fed as a N source under the condition that the reaction chamber was filled with H.sub.2.

[0084] The present invention was further proved by contrastive analysis on the conventional method and optimization method to have the following beneficial effects below.

[0085] After a high-component Al.sub.0.25Ga.sub.0.75N buffer layer was grown on a low-component Al.sub.0.45Ga.sub.0.55N by two methods of a conventional method (a pre-deposited Ga atomic layer was not taken) and an optimization method (a pre-deposited Ga atomic layer was taken), by making a comparison to the GaN thin films grown thereon, it was found that the Al.sub.0.25Ga.sub.0.75N buffer layer prepared by the optimization method greatly improved the homogeneity and crystallization quality of the GaN thin film thereon.

[0086] It was found (FIG. 9) through XRD detection results that the GaN thin film grown by the conventional method had crystal quality inferior to the GaN thin film grown by the optimization method.

[0087] It was found (FIG. 10) through OM observation that the GaN thin film grown by the conventional method had a little cracks; while no crack was found on the GaN thin film grown by the optimization method.

[0088] It was found (FIG. 11) through AFM observation that the GaN thin film grown by the conventional method was rough and uneven, a little holes could be found; while the GaN thin film grown by the optimization method was rough and uneven microscopically, and precious little holes could be found.

[0089] It was found (FIG. 12) through XRD detection results that the GaN thin film grown by the optimization method has a slightly higher peak intensity, that is, the crystal quality was higher than that of the GaN thin film grown by the conventional method.

[0090] To sum up, the new optimization method for epitaxial growth of a high-component AlGaN thin film on a low-component AlGaN thin film could improve the homogeneity and surface quality of the GaN thin film grown thereon.

EXAMPLE 5

[0091] By referring to FIG. 12-15, a preparation method for epitaxial growth of a GaN thin film on an AlGaN thin film has the following steps.

[0092] Step 1, growing AlN and AlGaN epitaxial layers on a Si substrate (growth parameters are common knowledge in the art, and thus are not specified any more): TMAl was fed as an Al source, TMGa as a Ga source, and NH.sub.3 was fed as a N source under the condition that the reaction chamber is filled with H.sub.2.

[0093] Step 2, pre-depositing a Ga atomic layer: the AlGaN thin film was put to a chamber, TMGa was fed as a Ga source under the condition that the reaction chamber was filled with H.sub.2; where a surface temperature of AlGaN was controlled within a range from 800° C. to 1400° C., a reaction chamber pressure was controlled within a range from 20 mbar to 200 mbar; and time was controlled within a range from 0 s to 300 s, thus obtaining a pre-paved Ga atomic layer. The pre-paved Ga atomic layer might be adsorbed on the AlGaN thin film to form an AlGaN atomic layer with a higher component and reach a saturation point rapidly, thus abstracting N atoms and forming GaN nucleation sites.

[0094] Step 3, growing a GaN buffer layer (growth parameters are common knowledge in the art, and thus are not specified any more): TMGa was fed as a Ga source and NH.sub.3 was fed as a N source under the condition that the reaction chamber was filled with H.sub.2. During such process, the pre-paved GaN nucleation sites grew up, thus forming a GaN thin film. During such growing process, the pre-paved Ga layer disappeared and became a portion of the GaN thin film.

[0095] The present invention was further proved by contrastive analysis on the conventional method and optimization method to have the following beneficial effects below.

[0096] After a GaN buffer layer was grown by two methods of a conventional method (a pre-deposited Ga atomic layer was not taken) and an optimization method (a pre-deposited Ga atomic layer was taken), by making a comparison to the GaN thin film, it was found that the GaN thin film prepared by the optimization method had improved homogeneity and crystal quality.

[0097] It was found (FIG. 12) through XRD detection results that the GaN thin film grown by the conventional method had crystal quality inferior to the GaN thin film grown by the optimization method.

[0098] It was found (FIG. 13) through OM observation that no crack was found on the GaN thin film grown both by the conventional method and the optimization method.

[0099] It was found (FIG. 14) through AFM observation that no holes were found on GaN thin film both grown by the conventional method and the optimization method; but the GaN thin film grown by the optimization method had neater and longer grains.

[0100] XRD detection results (FIG. 15) showed that the GaN thin film grown by the optimization method had a slightly higher peak intensity, that is, the crystal quality was higher than that of the GaN thin film grown by the conventional method.

[0101] To sum up, the new optimization method for epitaxial growth of a GaN thin film on an AlGaN thin film could improve the homogeneity and surface quality.

[0102] What is mentioned above is construed as limiting the prevent invention in any form; the prevent invention has been disclosed above by the preferred embodiments, but is not used to limit the present invention. A person skilled in the art can make some alterations or embellishments as equivalent embodiments by means of the structures and technical contents disclosed above within the scope of the technical solution of the present invention. Moreover, any simple modification or equivalent variation and embellishment made to the above examples based on the technical spirit of the present invention within the technical solution of the present invention shall fall within the scope of the technical solution of the present invention.