CN114262937B - Preparation method of gallium nitride epitaxial structure - Google Patents
Preparation method of gallium nitride epitaxial structure Download PDFInfo
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Abstract
The invention provides a preparation method of a gallium nitride epitaxial structure, which comprises the steps of providing a silicon carbide substrate and pretreating the silicon carbide substrate; and sequentially growing a graphene film, a diamond film, a three-dimensional aluminum nitride buffer layer, a gallium nitride buffer layer, an AlGaN barrier layer with gradient components and a gallium nitride cap layer on the pretreated silicon carbide substrate, wherein the three-dimensional aluminum nitride buffer layer is formed by firstly growing an aluminum nitride polycrystalline film, then annealing the aluminum nitride polycrystalline film to grow an aluminum nitride single crystal nucleus, and then continuously growing the three-dimensional aluminum nitride buffer layer on the basis of the aluminum nitride single crystal nucleus. According to the method, the graphene and the diamond are used as the rapid heat conducting layer, the thin gallium nitride buffer layer greatly shortens the heat transmission distance of the radio frequency device, the working thermal resistance of the device can be effectively reduced, and the working efficiency of the HEMT device is improved.
Description
Technical Field
The invention relates to the technical field of semiconductor epitaxial growth, in particular to a preparation method of a gallium nitride epitaxial structure.
Background
Gallium nitride (GaN) materials are widely used in the fields of high frequency, high temperature, high voltage, high power radio frequency devices and the like due to their good thermal and electrical properties and chemical stability, such as wide forbidden band width (3.4 ev), high thermal conductivity, high electron saturation drift velocity, high breakdown electric field corrosion resistance, radiation resistance and the like.
The AlGaN/GaN heterojunction has extremely strong piezoelectric polarization and spontaneous polarization effects, two-dimensional Electron gas (2 DEG) with High Mobility and High concentration can be formed at the heterojunction interface, and an A1GaN/GaN High Electron Mobility Transistor (HEMT) can be prepared by utilizing the characteristics. A1GaN/GaN HEMTs are widely used in radar, 5G base stations, and the like.
At present, substrates suitable for epitaxial growth of GaN materials mainly comprise a monocrystalline silicon substrate, a sapphire substrate and a silicon carbide (SiC) substrate, and because the SiC substrate has good thermal conductivity and small lattice mismatch with GaN, siC is an ideal substrate for preparing GaN-based HEMT. In recent years, with the increase of the requirement on the working power of GaN-based HEMT power radio frequency devices, the high-power tube of 500 w or even 1000 w is required, and the devices are required to have better heat dissipation capability. Under high power operation conditions, a large amount of heat is generated in a channel layer of the device in a short time, and if the heat cannot be effectively conducted out, the efficiency of the device is rapidly reduced, and even the device is possibly burnt. 3.5% of lattice mismatch and 24% of thermal mismatch exist between SiC and GaN, a thicker aluminum nitride (AlN) nucleation layer and a thicker GaN buffer layer are often grown on a SiC substrate in the traditional process, so that the thickness of an epitaxial structure is thicker, and in the working process of an HEMT device, the transmission path of heat generated by 2DEG is longer, so that the heat transmission capability of the device cannot meet the requirement of a high-power device, the electrical performance of the device is seriously improved, and the reliability of the device is reduced. Therefore, the reduction of the thermal resistance of the HEMT epitaxial structure has great significance for improving the reliability performance of the HEMT device under the high-power condition.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a preparation method of a gallium nitride epitaxial structure, which introduces diamond with high thermal conductivity into an epitaxial layer and is used for solving the problem of poor heat transfer capability of a gallium nitride-based HEMT device.
In order to achieve the above objects and other related objects, the present invention provides a method for preparing a gallium nitride epitaxial structure, comprising at least the following steps:
providing a silicon carbide (SiC) substrate and pretreating the SiC substrate;
growing a graphene film on the SiC substrate;
growing a diamond film on the graphene film;
growing an AlN polycrystalline film on the diamond film;
annealing the AlN polycrystalline film to grow an AlN single crystal nucleus;
continuously growing a three-dimensional AlN buffer layer on the basis of the AlN single crystal nucleus;
growing a gallium nitride (GaN) buffer layer on the three-dimensional AlN buffer layer;
growing an AlGaN barrier layer with gradient composition on the GaN buffer layer;
and growing a GaN cap layer on the AlGaN barrier layer with the gradient composition.
In an embodiment of the present invention, the pre-treatment includes performing surface cleaning treatment on the silicon carbide substrate to remove an oxide layer on a surface of the substrate and performing surface reconstruction of an auxiliary silicon beam on the surface of the cleaned silicon carbide substrate to obtain a flat and uniform SiC layered structure; wherein the surface cleaning treatment is to place the silicon carbide substrate in a hydrogen atmosphere and keep the silicon carbide substrate at 1100-1200 ℃ for 5-20 min; the surface reconstruction is to keep the temperature of the silicon carbide substrate at 1000-1100 ℃, so that the evaporation rate of the silicon beam on the surface is 0.3-0.5 nm/min, and the surface reconstruction time is 3-10 min.
In an embodiment of the present invention, growing the graphene film on the silicon carbide substrate includes performing a high temperature annealing process on the silicon carbide substrate in a hydrogen atmosphere, wherein an annealing temperature is 1250 to 1400 ℃, a flow rate of hydrogen is 50 to 80L/min, an annealing time is 1 to 3min, and a thickness of the obtained surface graphene is 1 to 3 layers.
In an embodiment of the present invention, the growing the diamond film on the graphene film includes introducing methane (CH) into the reaction chamber at a temperature of 1100-1200 ℃ 4 ) And hydrogen (H) 2 ) Wherein the ratio of methane/hydrogen is 2-4%, the growth time is 8-12 hours, and the thickness of the grown diamond is 100-200 nm.
In one embodiment of the present invention, growing an aluminum nitride polycrystalline film on the diamond film comprises: regulating the temperature of the reaction chamber to 800-900 ℃, and introducing trimethyl aluminum (TMAl) and ammonia gas (NH) into the reaction chamber 3 ) And growing an AlN polycrystalline film with the thickness of 3-5 nm.
In one implementation of the present inventionIn one example, annealing the AlN polycrystalline thin film includes introducing NH into the reaction chamber 3 And the temperature of the reaction chamber is adjusted to 1300-1350 ℃ under NH 3 And annealing the AlN polycrystalline film for 5-10 min under protection to grow a monocrystal AlN crystal nucleus.
In an embodiment of the present invention, growing a three-dimensional AlN buffer layer on the basis of the AlN single crystal nucleus includes: adjusting the temperature of the reaction chamber to 1000-1100 ℃ and the pressure to 50-100 mbar; in the presence of hydrogen (H) 2 ) Introducing TMAl with the flow rate of 200-300 sccm into the reaction chamber under the atmosphere, and pausing after introducing for 10-20 seconds; then NH with the flow rate of 15-25 slm is introduced into the reaction chamber 3 The time is stopped after the time is 10 to 20 seconds; with TMAl and NH 3 And alternately introducing the AlN buffer layer into the reaction chamber for one growth period, wherein the growth period is 15-20 periods, and the thickness of the grown three-dimensional AlN buffer layer is 30-50 nm.
In an embodiment of the present invention, growing a GaN buffer layer on the three-dimensional AlN buffer layer includes:
growing a GaN transition layer of 200-300 nm on the three-dimensional AlN buffer layer; and
and growing a 100-150 nm unintentionally doped GaN channel layer on the GaN transition layer.
In an embodiment of the present invention, the growing a 200-300 nm thick GaN buffer layer on the three-dimensional AlN buffer layer includes: the temperature of the reaction chamber is adjusted to 1020-1050 ℃, the pressure is adjusted to 250-400 mbar, H 2 Introducing trimethyl gallium (TMGa) with the flow rate of 200-300 sccm and NH with the flow rate of 15-20 slm into the reaction chamber under the atmosphere 3 The ratio of V/III (the molar ratio of the V group element to the III group element) is controlled between 500 and 800 during the growth process.
In an embodiment of the present invention, the growing of the 100-150 nm unintentionally doped GaN channel layer on the GaN transition layer includes: the temperature of the reaction chamber is adjusted to 1020-1050 ℃, the pressure is adjusted to 100-150 mbar in H 2 Introducing TMGa with the flow rate of 150-200 sccm and NH with the flow rate of 30-40 slm into the reaction chamber under the atmosphere 3 The V/III ratio is controlled between 1500 and 2000 in the growth process.
In one embodiment of the present invention, growing the gradient composition AlGaN barrier layer on the GaN buffer layer comprises growing 5nm Al on the GaN buffer layer x Ga 1-x N layers; and in Al x Ga 1-x Al with the thickness of 10-20 nm grows on the N layer y Ga 1-y N layers, wherein the aluminum component x is 5-10%, and the y is 20-30%; the growth temperature is 1000-1050 ℃, the growth pressure is 50-100mbar 3 The flow rate of the AlGaN layer is 3000-6000 sccm, the V/III ratio in the growth process is controlled to be 1000-1500, and the components and the growth rate of the AlGaN layer are controlled by adjusting the flow rate ratio of TMGa and TMAl.
In an embodiment of the invention, the step of growing the gallium nitride cap layer on the gradient-composition AlGaN barrier layer is to stop introducing the aluminum source after the growth of the AlGaN barrier layer is completed, and continue to grow the gallium nitride cap layer with the thickness of 2 to 3nm under the unchanged other conditions.
The epitaxial structure prepared by the method comprises the following steps:
a SiC substrate;
the graphene film is formed on the SiC substrate;
a diamond film formed on the graphene film;
a three-dimensional AlN buffer layer formed on the diamond film;
a GaN buffer layer formed on the three-dimensional AlN buffer layer;
an AlGaN barrier layer formed on the GaN buffer layer; and
and the GaN cap layer is formed on the AlGaN barrier layer.
The thickness of the graphene is 1-3 layers, and the graphene is formed by annealing the SiC substrate at a high temperature; the thickness of the diamond is 100-200 nm; the thickness of the three-dimensional AlN buffer layer is 30-50 nm; the GaN buffer layer comprises a GaN transition layer with the thickness of 200-300 nm and a GaN channel layer with the thickness of 100-150 nm; the AlGaN barrier layer comprises 5nm of Al x Ga 1-x N layer and 10-20 nm Al y Ga 1-y N layers, wherein the aluminum component x is 5-10%, and the y is 20-30%; the thickness of the GaN cap layer is 2-3 nm.
In addition, a graphene film and a diamond film are grown between the silicon carbide substrate and the aluminum nitride buffer layer, the thermal resistance of the interface between the substrate and the buffer layer is obviously reduced by utilizing the high thermal conductivity of the graphene film and the diamond film, the thermal conductivity of the interface between the substrate and the buffer layer is improved, and the performance and the long-term reliability of the gallium nitride-based device are improved; aluminum source TMAl and nitrogen source NH in preparation process of three-dimensional AlN buffer layer 3 The mode of alternately introducing the AlN single crystal into the reaction chamber is adopted, so that the migration time of Al atoms can be prolonged, the migration distance of the Al atoms is increased, the transverse growth rate of AlN is increased, and the island-shaped AlN buffer layer is promoted to be continuously grown on an AlN single crystal nucleus in a three-dimensional form. In the preparation process of the GaN buffer layer, the GaN transition layer grows in a three-dimensional island mode in a high-pressure low V/III ratio mode, gaps between islands in the three-dimensional AlN buffer layer can be filled and leveled, and then a high-quality GaN channel layer grows in a two-dimensional mode in a low-pressure high V/III ratio mode.
Drawings
The characteristics and advantages of the invention will be better understood by reference to the attached drawings, which are schematic and should not be understood as imposing any limitation on the invention, and from which other drawings may be derived by those skilled in the art without inventive step.
FIG. 1 is a flow chart illustrating the steps of a method for fabricating an epitaxial structure according to the present invention;
FIG. 2 is a flowchart of step S1 in FIG. 1;
FIG. 3 is a schematic view of an epitaxial structure corresponding to step S3 in FIG. 1;
FIG. 4 is a schematic view of an epitaxial structure corresponding to step S4 in FIG. 1;
FIG. 5 is a schematic view of an epitaxial structure corresponding to step S5 in FIG. 1;
FIG. 6 is a schematic view of an epitaxial structure corresponding to step S6 in FIG. 1;
FIG. 7 shows TMAl and NH as growth sources in step S6 of FIG. 1 3 A flow-time diagram of (a);
FIG. 8 is a schematic view of an epitaxial structure corresponding to step S7 in FIG. 1;
FIG. 9 is a schematic view of an epitaxial structure corresponding to step S8 in FIG. 1;
FIG. 10 is a schematic view of an epitaxial structure corresponding to step S9 in FIG. 1 according to the present invention;
fig. 11 shows a hall test position mark diagram for the epitaxial structure of the present invention.
Reference numerals
1. A SiC substrate; 2. a graphene film; 3. a diamond film; 4. an AlN polycrystalline thin film; 5. AlN single crystal nuclei; 6. a three-dimensional AlN buffer layer; 7. a GaN buffer layer; 8. an AlGaN barrier layer; 9. a GaN cap layer.
Detailed Description
The embodiments of the present invention are described below with reference to specific embodiments, and other advantages and effects of the present invention will be easily understood by those skilled in the art from the disclosure of the present specification. The invention is capable of other and different embodiments and of being practiced or of being carried out in various ways, and its several details are capable of modification in various respects, all without departing from the spirit and scope of the present invention.
It is to be noted that the features in the following embodiments and examples may be combined with each other without conflict. It is also to be understood that the terminology used in the examples is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Test methods in which specific conditions are not specified in the following examples are generally carried out under conventional conditions or under conditions recommended by the respective manufacturers.
When numerical ranges are given in the examples, it is understood that both endpoints of each of the numerical ranges and any number between the two endpoints are optional unless otherwise specified in the invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs and are intended to be open ended, i.e., to include any methods, devices, and materials similar or equivalent to those described in the examples.
The invention provides a preparation method of a gallium nitride epitaxial structure, which adopts TMAl, TMGa and NH 3 As aluminum (Al), gallium (Ga) and nitrogen (N) sources, H 2 Or N 2 As a carrier gas.
Referring to fig. 1 to 10, the method for fabricating a gan epitaxial structure of the present invention at least includes the following steps:
s1, providing a SiC substrate 1, and preprocessing the SiC substrate 1;
s2, growing a graphene film 2 on the pretreated SiC substrate 1;
s3, growing a diamond film 3 on the graphene film 2;
s4, growing an AlN polycrystalline film 4 on the diamond film 3;
s5, annealing the AlN polycrystalline film 4 to grow an AlN single crystal nucleus 5;
s6, growing a three-dimensional AlN buffer layer 6 by taking the AlN single crystal nucleus 5 as a nucleation point;
s7, growing a GaN buffer layer 7 on the three-dimensional AlN buffer layer 6;
s8, growing an AlGaN barrier layer 8 with gradient components on the GaN buffer layer 7;
and S9, growing a GaN cap layer 9 on the AlGaN barrier layer 8 with the gradient composition.
Referring to fig. 1 to 2, the step S1 of pretreating the SiC substrate 1 includes:
s11, performing surface cleaning treatment on the SiC substrate 1; and
and S12, performing surface reconstruction of the auxiliary silicon beam on the surface of the cleaned SiC substrate 1.
Wherein step S11 is specifically included in H 2 The SiC substrate 1 is subjected to annealing treatment in an atmosphere to remove an oxide layer and oil stains on the surface of the substrate. In one embodiment, the selected SiC substrate 1 is first placed on a susceptor in a reaction chamber of an MOCVD apparatus, and the temperature of the reaction chamber is adjusted to be loweredThe fire temperature is for example 1100-1200 ℃, and H is introduced into the reaction chamber 2 Introduction of H 2 The flow rate is 130-160L/min, and the annealing time is controlled between 5-20 min. When the SiC substrate 1 is annealed, attention should be paid to an annealing process, and excessive treatment adversely affects the growth quality of the subsequent epitaxial layer. The step S12 specifically comprises the steps of adjusting the temperature of the SiC substrate 1 to 1000-1100 ℃, and introducing a silicon source, so that the evaporation rate of a silicon beam is 0.3-0.5 nm/min, and the surface reconstruction time is 3-10 min, thereby obtaining a flat and uniform SiC laminated structure and providing a flat plane for the growth of the graphene film.
Referring to fig. 1 and 3, in step S2, the graphene film 2 is grown on the SiC substrate 1 in H 2 Carrying out high-temperature annealing treatment on the SiC substrate 1 in the atmosphere to obtain 1-3 layers of surface graphene 2, specifically, adjusting the temperature of a reaction chamber to 1250-1400 ℃, and introducing H 2 The flow rate of the annealing furnace is 50-80L/min, and the annealing is carried out for 1-3 min under the environment. Wherein the annealing temperature can be 1250 deg.C, 1350 deg.C, 1400 deg.C, H 2 The flow rate of the annealing furnace can be selected to be 50-80L/min, 60L/min or 80L/min, and the annealing time can be selected to be 1min, 2min or 3min. Both endpoints of the above range of values and any value therebetween can be selected.
Referring to fig. 1 and 3, the step S3 of growing the diamond film 3 on the graphene film 2 specifically includes adjusting the temperature of the reaction chamber to 1100-1200 ℃, and introducing CH 4 And H 2 Maintaining CH 4 /H 2 The ratio of the diamond film to the substrate is 2-4%, the diamond film grows for 8-12 hours under the condition, and the thickness of the obtained diamond film 3 is 100-200 nm. Both endpoints of the above range of values and any value therebetween can be selected.
Referring to fig. 1 and 4, the step S4 of growing the AlN polycrystalline thin film 4 on the diamond thin film 3 specifically includes: firstly, the temperature of a reaction chamber is adjusted to 800-900 ℃, and then Al source TMAl and N source NH are introduced into the reaction chamber 3 An AlN polycrystalline thin film 4 of 3 to 5nm is grown on the SiC substrate 1.
Referring to fig. 1 and 5, step S5 is to perform an annealing process on the AlN polycrystalline thin film 4, which specifically includes: NH is introduced into the reaction chamber 3 And the reaction chamberThe temperature is controlled between 1300 and 1350 ℃ under NH 3 The AlN polycrystalline film 4 is annealed under the protection of the protective layer, and the annealing time is controlled to be 5 to 10min. For example, the annealing temperature may be 1300 ℃, 1325 ℃ or 1350 ℃, and the annealing time may be 5min, 8min, 10min, etc. At the temperature of 1300-1350 ℃, the migration of Al atoms can be increased, the AlN polycrystalline film is promoted to be recrystallized to form a plurality of AlN single crystal nuclei 5, and the size and the density of the AlN single crystal nuclei 5 can be influenced by the annealing temperature and the annealing time, so that the quality of a subsequent epitaxial layer is influenced. Any of the values given above for the annealing temperature and annealing time can be selected at either end of the ranges and between the two ends.
Referring to FIGS. 1, 5 and 7, step S6 is to grow a three-dimensional AlN buffer layer 6 on the basis of the AlN single crystal nuclei 5 by alternately introducing Al source TMAl and N source NH into the reaction chamber 3 Mode growth, TMAl and NH because Al atoms are more difficult to migrate 3 The alternate introduction is equivalent to increase the migration time of Al atoms, improve the migration distance of the Al atoms and promote the AlN to continue to grow in a three-dimensional island form on the AlN single crystal nucleus. The method specifically comprises the following steps: at H 2 Under the atmosphere, the temperature of the reaction chamber is adjusted to 1000-1100 ℃, and the pressure of the reaction chamber is adjusted to 50-100 mbar; introducing TMAl into the reaction chamber, wherein the flow rate of TMAl is 200-300 sccm, the gas introduction time is 10-20 seconds, then suspending the introduction of TMAl, and introducing NH into the reaction chamber 3 ，NH 3 The flow rate of (2) is 15-25 slm, and the ventilation time is 10-20 seconds; so far, the growth period is one; repeating the steps until the growth period is 15-20. Under the growth conditions described above, the thickness of the three-dimensional AlN buffer layer 4 is 30 to 50nm. In some embodiments, the flow rate of TMAl in one growth cycle can be selected to be 200sccm, 230sccm, 260sccm, 300sccm, etc., the ventilation time can be selected to be 10 seconds, 15 seconds, 20 seconds, etc., NH 3 The flow rate of (a) can be selected to be 15slm, 20slm, 25slm, etc., the aeration time can be selected to be 10 seconds, 15 seconds, 20 seconds, etc., and the growth cycle can be selected to be 15, 17, 20, etc. Both endpoints of the ranges given by the above parameters and any value between the endpoints are optional, and AlN buffer layers prepared according to the ranges given by the parameters of the present inventionThe thickness of 4 is kept between 30 and 50nm.
Referring to fig. 1 and 8, the step S7 of growing the GaN buffer layer 7 on the three-dimensional AlN buffer layer 6 includes a first growth stage and a second growth stage, wherein the first growth stage adopts a high-pressure and low V/III ratio mode to grow a GaN transition layer in a three-dimensional island-shaped mode to fill and level up the three-dimensional AlN buffer layer 6, so as to provide a flat growth platform for the subsequent growth of the GaN channel layer; in the second growth stage, the GaN channel layer is grown in a two-dimensional layered mode in a low-pressure and high-V/III-ratio mode, the high-quality GaN channel layer can be obtained, a GaN/AlGaN heterojunction is formed with the AlGaN barrier layer, and a large amount of 2DEG is generated.
Referring to FIGS. 1 and 8, the first growth stage of the GaN buffer layer 7 specifically includes growth at H 2 The temperature of the reaction chamber is adjusted to 1020-1050 ℃ under the atmosphere, the pressure of the reaction chamber is adjusted to 250-400 mbar, and then TMGa with the flow rate of 200-300 sccm and NH with the flow rate of 15-20 slm are introduced into the reaction chamber 3 The V/III ratio is controlled between 500 and 800 in the growth process; under the growth condition, the growth rate of the GaN is 25-35 nm/min, the grown GaN is a three-dimensional GaN layer without intentional doping, and the growth thickness is 200-300 nm, such as 200nm, 250nm, 300nm and the like. In some embodiments, the growth pressure in the first growth stage may be selected from 250mbar, 300mbar, 350mbar, 400mbar, etc., and the flow rate of TMGa may be selected from 200sccm, 250sccm, 300sccm, etc.; NH (NH) 3 The flow rate of (c) may be selected to be 15slm, 18slm, 20slm, etc., and any value between the two endpoints of the ranges given by the above parameters may be selected.
Referring to FIGS. 1 and 8, the second growth stage of the GaN buffer layer 7 specifically includes maintaining the temperature of the reaction chamber constant, adjusting the pressure of the reaction chamber to 100-150 mbar, and then introducing TMGa with a flow rate of 150-200 sccm and NH with a flow rate of 30-40 slm into the reaction chamber 3 The V/III ratio is controlled between 1500 and 2000 in the growth process; the growth rate of GaN under the growth conditions is 15-20 nm/min, and the thickness of the grown GaN channel layer is 100-150 nm, such as 100nm, 120nm, 150nm, etc. In some embodiments, the growth pressure of the second growth stage can be selected to be 100mbar, 125mbar, 150mbar, etc., and the flow rate of TMGa can be selected to be 150sccm, 175sccm, 200sccm, etc.; NH (NH) 3 The flow rate of (a) may be selected to be 30slm, 35slm, 40slm, etc., and any value between the two endpoints of the ranges given by the above parameters may be selected.
Referring to fig. 1 and 9, step S8 is to grow an AlGaN barrier layer 8 with a gradient composition on the GaN buffer layer 7, which specifically includes: first, 5nm of Al is grown on the GaN buffer layer 7 x Ga 1-x N layer on Al x Ga 1-x Al with the thickness of 10-20 nm is grown on the N layer y Ga 1-y And N layers, wherein the aluminum component x is 5-10%, and the y is 20-30%. I.e. at H 2 Regulating the temperature of the reaction chamber to 1000-1050 ℃ and the pressure of the reaction chamber to 50-100 mbar under the atmosphere, and introducing TMGa, TMAl and NH into the reaction chamber 3 Wherein NH 3 The flow rate of the AlGaN layer is 3000-6000 sccm, the V/III ratio is controlled to be 1000-1500 in the growth process, and the components and the growth rate of the AlGaN layer are controlled by adjusting the flow rate ratio of TMGa and TMAl. In some embodiments, NH 3 The flow rate of the (C) can be selected to be 3000sccm, 4000sccm, 5000sccm, 6000sccm and the like, the V/III ratio can be selected to be 1000, 1200 or 1500 and the like, and any value between two endpoints of the range given by the parameters can be selected. The composition and growth rate of the AlGaN layer are controlled by adjusting the flow ratio of TMGa to TMAl.
Referring to fig. 1 and 10, in step S9, after the growth of the AlGaN barrier layer 8 is finished, the introduction of the Al source is stopped, other growth conditions are kept unchanged, and the GaN cap layer 9 of 2 to 3nm is continuously grown.
Referring to fig. 10, the epitaxial structure prepared by the preparation method of the present invention includes:
a SiC substrate 1 having a silicon carbide substrate,
a graphene film 2 formed on the SiC substrate 1;
a diamond film 3 formed on the graphene film 2;
a three-dimensional AlN buffer layer 6 formed on the diamond film 3;
a GaN buffer layer 7 formed on the three-dimensional AlN buffer layer 6;
an AlGaN barrier layer 8 formed on the GaN buffer layer 7; and
and a GaN cap layer 9 formed on the AlGaN barrier layer 8.
The graphene film 2 is 1-3 layers thick and is formed by annealing the SiC substrate 1 at a high temperature. The thickness of the diamond film 3 is 100 to 200nm, and may be, for example, 100nm, 150nm, or 200nm. The three-dimensional AlN buffer layer 6 is formed by continuously growing on the basis of the AlN single crystal nucleus 5, and the AlN single crystal nucleus 5 is formed by annealing the AlN polycrystalline film 4 at high temperature; the thickness of the three-dimensional AlN buffer layer 4 is 30 to 50nm, and may be, for example, 30nm, 40nm, or 50nm. The GaN buffer layer 7 includes a three-dimensional GaN transition layer and a two-dimensional GaN channel layer formed on the three-dimensional GaN transition layer without intentional doping, wherein the thickness of the three-dimensional GaN transition layer is 200 to 300nm, for example, 200nm, 250nm, 300nm, etc.; the thickness of the two-dimensional GaN channel layer without intentional doping is 100-150 nm, for example, 150nm, 175nm, 200nm, etc. The AlGaN barrier layer 8 includes 5nm of Al x Ga 1-x N layer and 10-20 nm Al y Ga 1-y And N layers, wherein the aluminum component x is 5-10%, and the y is 20-30%. The thickness of the GaN cap layer 9 is 2 to 3nm.
The invention utilizes the three-dimensional island-shaped AlN as the buffer layer to prepare the gallium nitride epitaxial structure, so that the thickness of an epitaxial wafer is obviously reduced, the transmission distance of heat generated in the 2DEG working process is greatly shortened, and the thermal resistance caused by the thickness of the epitaxial layer is reduced. In addition, a graphene film and a diamond film are grown between the silicon carbide substrate and the aluminum nitride buffer layer, the interface thermal resistance between the substrate and the buffer layer is obviously reduced by utilizing the high thermal conductivity of the graphene film and the diamond film, the thermal conduction between interfaces is improved, and the performance and the long-term reliability of the gallium nitride-based device are improved.
In an embodiment, an epitaxial structure is prepared on a SiC substrate by using the preparation method of the present invention, and the hall effect tester is used to test the electron mobility and the electron concentration of different positions of the epitaxial wafer, and the test positions and the test results are shown in fig. 11 and table 1.
FIG. 11 is a schematic diagram showing the use of Hall effectThe position marking chart of the epitaxial structure of the invention when the Hall test is carried out by a tester, four test points are respectively taken along the X-axis direction and the Y-axis direction of the wafer and respectively marked as X 1 、X 2 、X 3 、X 4 、Y 1 、Y 2 、Y 3 、Y 4 The intersection of the X axis and the Y axis is marked as O, the test data of the electron mobility and the carrier density of each test point are shown in table 1, and the minimum value of the electron mobility is 1738cm from table 1 2 The maximum value of the concentration of the sodium hydroxide can reach 1948.8cm 2 A v.s value of 1877.47cm on average 2 V.s; maximum carrier density (8.21E + 12) cm -2 Minimum is (6.7E + 12) cm -2 The average value is (7.569E +12) cm -2 . Hall tests show that the HEMT epitaxial structure prepared by the invention can generate higher 2DEG, and the generation of the 2DEG is not influenced by a thin epitaxial layer. Therefore, the epitaxial structure provided by the invention is applied to the HEMT device, so that the self-heating phenomenon of the device can be obviously reduced, and the working efficiency of the device is improved.
Table 1: hall test results of HEMT epitaxial structure
In summary, the invention provides a method for preparing a gallium nitride epitaxial structure, which uses three-dimensional island AlN as a buffer layer to provide a high-quality growth platform for the growth of a GaN/AlGaN heterojunction, and the total thickness of the epitaxial structure prepared by the method is greatly reduced compared with the thickness of a conventional HEMT epitaxial structure, thereby shortening the distance of heat transmission from the top of the epitaxial layer to a lower substrate when a device works, reducing the thermal resistance caused by the thickness of the epitaxial layer, and improving the working efficiency of the device. The graphene film and the diamond film grow between the silicon carbide substrate and the aluminum nitride buffer layer, the thermal resistance of the interface between the substrate and the buffer layer is obviously reduced by utilizing the high thermal conductivity of the graphene film and the diamond film, the thermal conduction between the interfaces is improved, and the performance and the long-term reliability of the gallium nitride-based device are improved. Therefore, the invention effectively overcomes some practical problems in the prior art, thereby having high utilization value and use significance.
The foregoing embodiments are merely illustrative of the principles of this invention and its efficacy, rather than limiting it, and various modifications and variations can be made by those skilled in the art without departing from the spirit and scope of the invention, which is defined in the appended claims.
Claims (9)
1. A preparation method of a gallium nitride epitaxial structure is characterized by at least comprising the following steps:
providing a silicon carbide substrate and pretreating the silicon carbide substrate;
growing a graphene film on the silicon carbide substrate;
growing a diamond film on the graphene film;
growing an aluminum nitride polycrystalline film on the diamond film;
annealing the aluminum nitride polycrystalline film to grow the aluminum nitride polycrystalline film into an aluminum nitride single crystal nucleus;
growing a three-dimensional aluminum nitride buffer layer on the basis of the aluminum nitride single crystal nucleus;
growing a gallium nitride buffer layer on the three-dimensional aluminum nitride buffer layer;
growing an AlGaN barrier layer with gradient composition on the gallium nitride buffer layer;
growing a gallium nitride cap layer on the AlGaN barrier layer with the gradient composition;
wherein growing a three-dimensional aluminum nitride buffer layer on the basis of the aluminum nitride single crystal nuclei comprises: regulating the temperature of the reaction chamber to 1000-1100 ℃ and the pressure to 50-100 mbar; introducing trimethylaluminum with the flow of 200-300 sccm into the reaction chamber in a hydrogen atmosphere, and suspending after 10-20 seconds;
then ammonia gas with the flow of 15-25 slm is introduced into the reaction chamber, and the reaction chamber is suspended after 10-20 seconds;
introducing trimethylaluminum and ammonia gas into the reaction chamber alternately once to form a growth period, wherein the growth period is 15-20 periods;
the thickness of the three-dimensional aluminum nitride buffer layer is 30-50 nm.
2. The production method according to claim 1, wherein the pretreatment comprises subjecting the silicon carbide substrate to a surface cleaning treatment by placing the silicon carbide substrate in a hydrogen atmosphere at a temperature of 1100 to 1200 ℃ for 5 to 20min and subjecting the cleaned silicon carbide substrate to a surface reconstruction of an auxiliary silicon beam flow; the surface reconstruction is to keep the temperature of the silicon carbide substrate at 1000-1100 ℃, the evaporation rate of the silicon beam is 0.3-0.5 nm/min, and the surface reconstruction time is 3-10 min.
3. The preparation method according to claim 1, wherein the growing of the graphene film on the silicon carbide substrate comprises annealing the silicon carbide substrate at a high temperature in a hydrogen atmosphere, wherein the annealing temperature is 1250-1400 ℃, the flow rate of hydrogen is 50-80L/min, and the annealing time is 1-3 min, so that 1-3 layers of surface graphene is obtained.
4. The preparation method according to claim 1, wherein the growing of the diamond film on the graphene film comprises introducing methane and hydrogen into a reaction chamber at a temperature of 1100-1200 ℃, wherein the methane/hydrogen is 2-4%, the growth time is 8-12 hours, and the growth thickness is 100-200 nm.
5. The method of claim 1, wherein growing an aluminum nitride polycrystalline thin film on the diamond thin film comprises: regulating the temperature of the reaction chamber to 800-900 ℃, and introducing trimethyl aluminum and ammonia gas into the reaction chamber to grow the aluminum nitride polycrystalline film with the thickness of 3-5 nm.
6. The preparation method according to claim 1, wherein the annealing treatment of the aluminum nitride polycrystalline thin film comprises introducing ammonia gas into the reaction chamber, controlling the temperature of the reaction chamber to be 1300-1350 ℃, and carrying out annealing treatment under the protection of the ammonia gas for 5-10 min to convert the aluminum nitride polycrystalline thin film into aluminum nitride single crystal nuclei.
7. The method of claim 1, wherein growing a gallium nitride buffer layer on the three-dimensional aluminum nitride buffer layer comprises: growing a 200-300 nm gallium nitride transition layer on the three-dimensional aluminum nitride buffer layer; and
growing a 100-150 nm thick unintentionally doped gallium nitride channel layer on the gallium nitride transition layer;
wherein, when growing the gallium nitride transition layer, the V/III ratio is controlled to be 500-800, and when growing the gallium nitride channel layer, the V/III ratio is controlled to be 1500-2000.
8. The method of claim 1, wherein growing a graded composition AlGaN barrier layer on the gallium nitride buffer layer comprises: growing 5nm of Al on the GaN buffer layer x Ga 1-x N layers; and in Al x Ga 1-x Al with the thickness of 10-20 nm grows on the N layer y Ga 1-y N layer, wherein, the aluminum component x is 5-10%, and y is 20-30%.
9. The preparation method of claim 1, wherein the step of growing the gallium nitride cap layer on the gradient-composition AlGaN barrier layer comprises stopping introducing an aluminum source after the growth of the AlGaN barrier layer is completed, and continuously growing the gallium nitride cap layer with the thickness of 2-3 nm under the unchanged other conditions.
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