GaN HEMT (High Electron Mobility Transistor) epitaxial structure with stress release layer and preparation method

Abstract

The invention discloses a GaN HEMT epitaxial structure with a stress release layer and a preparation method, the epitaxial structure comprises an AlGaN buffer transition layer located on a substrate, the AlGaN buffer transition layer comprises n AlGaN layers, n is an integer greater than or equal to 2, and the carbon atom content of the (n-1) th AlGaN layer located below the nth layer is greater than the carbon atom content of the nth AlGaN layer by more than one order of magnitude. According to the invention, by changing growth conditions, a gradient in which the carbon concentration is reduced from bottom to top is formed in the AlGaN buffer transition layer, so that stress can be well released.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor technology, and in particular relates to a GaN HEMT epitaxial structure with a stress release layer and a preparation method thereof. Background Art

[0002] The description in this section merely provides background information related to the disclosure of the present application and does not constitute prior art.

[0003] Figure 1 It is a typical GaN HEMT device (gallium nitride high electron mobility transistor) epitaxial structure, which includes, from bottom to top: a substrate 1, an AlGaN buffer transition layer 2, a high-resistance buffer layer 3, a channel layer 4, a barrier layer 5 and a cap layer 6.

[0004] The substrate 1 is generally made of silicon, SiC, sapphire, etc., which is used to provide physical support and electrical insulation. The high-resistance buffer layer 3 is generally made of carbon-doped GaN, etc., which is used to block the two-dimensional electron gas (2-DEG) from diffusing toward the substrate. The channel layer 4 and the barrier layer 5 are used to form a polarization effect to generate a two-dimensional electron gas. The cap layer 6 is used to cover the barrier layer 5 to prevent the barrier layer 5 from oxidizing. However, except for the use of GaN substrates, there are problems such as lattice mismatch and thermal expansion coefficient mismatch between other substrates and the high-resistance buffer layer. The introduction of the AlGaN buffer transition layer can reduce the above problems. The lattice coefficient and thermal expansion coefficient of AlGaN are between the two, so that a good transition buffer effect can be obtained.

[0005] The AlGaN buffer transition layer 2 generally includes multiple layers, and the Al content gradually decreases from bottom to top. By slowly changing the Al content, the above problems can be solved more effectively. Figure 1 As shown, the AlGaN buffer transition layer consists of three layers, namely a first AlGaN layer 21 , a second AlGaN layer 22 , and a third AlGaN layer 23 . Figure 2 The AlGaN buffer transition layer shown is also three layers.

[0006] However, this structure still has the problem of large stress caused by lattice mismatch and different thermal expansion coefficients, which leads to quality problems such as edge cracks and warping of the epitaxial wafer, and further causes problems such as decreased electron mobility and leakage in GaN HEMT devices.

[0007] On the other hand, Figure 2As shown, in order to further enhance the electrical performance of GaN HEMT, a superlattice layer 7 is introduced into the epitaxial structure. The introduction of the superlattice layer increases the epitaxial thickness. Although it can increase the insulation of the bottom layer, reduce the probability of vertical leakage, and increase the BV (breakdown voltage) strength, the thickening of the epitaxial structure will increase the stress accumulation and make it more likely to cause quality problems such as edge cracks and warping. Summary of the invention

[0008] The purpose of the present invention is to provide a GaN HEMT epitaxial structure with a stress release layer and a preparation method thereof in order to solve the deficiencies of the prior art.

[0009] On the one hand, the present application provides a GaN HEMT epitaxial structure with a stress release layer, including an AlGaN buffer transition layer located on a substrate, wherein the AlGaN buffer transition layer includes n AlGaN layers, wherein n is an integer ≥ 2, wherein the carbon atom content of the n-1th AlGaN layer located below the nth layer is greater than the carbon atom content of the nth AlGaN layer by more than one order of magnitude.

[0010] Preferably, the carbon atom content of the n-1th AlGaN layer is greater than the carbon atom content of the nth AlGaN layer by 1 to 3 orders of magnitude;

[0011] Preferably, the carbon atom content of the n-th AlGaN layer is 1018-1019 / cm3, and the carbon atom content of the n-1-th AlGaN layer is 1020-1022 / cm3.

[0012] Preferably, the thickness of the n-th AlGaN layer is 300-1000 nm.

[0013] Preferably, the nth AlGaN layer comprises a plurality of layers;

[0014] Preferably, the thickness of each layered AlGaN layer is ≥100 nm.

[0015] Preferably, n is an integer ≥ 3; preferably, n is 4;

[0016] From bottom to top, the Al content of the first layer to the n-1th AlGaN layer gradually decreases;

[0017] The Al content of the nth AlGaN layer is ≠ the Al content of the n-1th AlGaN layer;

[0018] Preferably, the Al content of the n-th AlGaN layer is less than the Al content of the n-2-th AlGaN layer.

[0019] Preferably, the Al content of the n-th AlGaN layer is less than the Al content of the n-1-th AlGaN layer.

[0020] Preferably, the epitaxial structure provided in the present application further includes a superlattice layer located on the AlGaN buffer transition layer.

[0021] Preferably, the nth AlGaN layer is doped with iron.

[0022] Preferably, the epitaxial structure provided in the present application further comprises a high-resistance buffer layer, a channel layer, a barrier layer, and a cap layer sequentially located on the AlGaN buffer transition layer;

[0023] When the superlattice layer exists, the superlattice layer is located between the AlGaN buffer transition layer and the high-resistance buffer layer;

[0024] Preferably, the superlattice layer is any one selected from AlN / AlGaN, AlN / GaN, AlGaN / GaN, AlN / AlGaN / GaN;

[0025] The high-resistance buffer layer is a C-GaN layer;

[0026] The barrier layer is an AlGaN layer;

[0027] The cap layer is a GaN layer.

[0028] On the other hand, the present application also provides a method for preparing the GaN HEMT epitaxial structure with a stress release layer as described above, wherein the AlGaN buffer transition layer is deposited on the substrate by a MOCVD method;

[0029] The deposition parameters of the nth AlGaN layer are: growth temperature ≥1000° C. and <1100° C., pressure >80 mbar and ≤200 mbar, V / III ratio >300 and ≤1500, and growth rate ≥0.5 um / h and <1 um / h;

[0030] The deposition parameters of the n-1th AlGaN layer are: growth temperature ≥1100° C. and ≤1200° C., pressure ≥30 mbar and ≤80 mbar, V / III ratio ≥10 and ≤300, and growth rate ≥1 um / h and ≤5 um / h.

[0031] The present application forms a gradient of decreasing carbon concentration from bottom to top in the AlGaN buffer transition layer by changing the growth conditions, thereby effectively releasing stress. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1This is a typical schematic diagram of the epitaxial structure of a GaN HEMT device;

[0033] Figure 2 This is a typical schematic diagram of the epitaxial structure of a GaN HEMT device with a superlattice layer;

[0034] Figure 3 This application is in Figure 1 Schematic diagram of the epitaxial structure of a GaN HEMT device with a fourth AlGaN layer inserted into the base;

[0035] Figure 4 This application is in Figure 2 Schematic diagram of the epitaxial structure of a GaN HEMT device with a fourth AlGaN layer inserted into the base;

[0036] Figure 5 is a curve diagram of curvature (warpage) and reflectivity change during the growth process of Example Sample 1;

[0037] Figure 6 is a curve diagram of curvature (warpage) and reflectivity change during the growth process of Example Sample 2;

[0038] Figure 7 is a graph showing the change in curvature (warpage) and reflectivity during the growth of comparative sample 1;

[0039] Figure 8 is the X-ray diffraction pattern of Example samples 1-2 and Comparative sample 1;

[0040] Fig. 9 is a curve diagram of curvature (warpage) and reflectivity change during the growth process of Example Sample 3;

[0041] Fig.10 is a graph showing the change in curvature (warpage) and reflectivity during the growth of comparative sample 2;

[0042] Fig.11 is a SIMS distribution curve of C concentration versus depth of Example Sample 3;

[0043] Fig.12 is the X-ray diffraction pattern of Example Sample 3 and Comparative Sample 2;

[0044] Fig.13 is a curve diagram of curvature (warpage) and reflectivity change during the growth process of Example Sample 4;

[0045] Fig.14 is a curve diagram of the curvature (warpage) and reflectivity change during the growth of comparative sample 3;

[0046] Fig.151 is the X-ray diffraction pattern of Example Sample 4 and Comparative Sample 3;

[0047] Description of reference numerals:

[0048] 1-substrate; 2-AlGaN buffer transition layer; 21-first AlGaN layer; 22-second AlGaN layer; 23-third AlGaN layer; 24-fourth AlGaN layer; 3-high resistance buffer layer; 4-channel layer; 5-barrier layer; 6-cap layer; 7-superlattice layer. DETAILED DESCRIPTION

[0049] The present application is further described below in conjunction with the accompanying drawings.

[0050] like Figures 1-2 As shown in FIG. 1 , a typical GaN HEMT epitaxial structure uses multiple AlGaN buffer transition layers with gradually decreasing Al content from bottom to top to solve the technical problems of lattice mismatch between silicon substrate and group III nitride high-resistance buffer layer and large difference in thermal expansion coefficient. However, this structure still has large stress accumulation, especially for Figure 2 The GaN HEMT epitaxial structure with a thickened superlattice layer structure is shown in Fig. Figure 1 Has greater stress accumulation.

[0051] In response to the above problems, the applicant has found through research that under conventional growth conditions, the carbon content in each AlGaN layer increases slowly from bottom to top. By changing the growth conditions, a gradient with a decreasing carbon concentration from bottom to top is formed in the AlGaN buffer transition layer, which can effectively release stress. Specifically:

[0052] The GaN HEMT epitaxial structure with a stress release layer provided in the present application includes an AlGaN buffer transition layer located on a substrate, wherein the AlGaN buffer transition layer includes n AlGaN layers, n is an integer ≥ 2, wherein the carbon content of the n-1 AlGaN layer located below the n layer is greater than the carbon content of the n AlGaN layer, and specifically, the carbon content of the n-1 AlGaN layer located below the n layer is greater than the carbon atom content of the n AlGaN layer by more than one order of magnitude. Under such conditions, the high carbon concentration of the n-1 AlGaN layer will provide a very large compressive stress, and then there is a concentration gradient with the low carbon concentration of the n AlGaN layer, and this gradient will be converted into stress regulation when the epitaxial growth is completed and cooled to room temperature, and the key lies in the role of the high compressive stress reflected between the high carbon concentration gradient difference.

[0053] The nth AlGaN layer can be obtained by adjusting the carbon concentration of the original AlGaN layer located at the top, such as Figure 3As shown, by changing the growth conditions of the upper half of the original third AlGaN layer, two AlGaN layers with different carbon concentrations can be deposited, namely, the fourth AlGaN layer 24 located at the top and the third AlGaN layer 23 located at the bottom. Alternatively, based on the original AlGaN layers, new growth conditions can be used to deposit a new AlGaN layer with a reduced carbon concentration, such as Figure 4 As shown, in the original Figure 2 On the basis of the three AlGaN layers shown, a fourth AlGaN layer 24 with a reduced carbon concentration is deposited using new growth conditions.

[0054] Preferably, the carbon atom content of the n-1th AlGaN layer located below the nth layer is greater than the carbon atom content of the nth AlGaN layer by 1 to 3 orders of magnitude. Furthermore, the carbon atom content of the nth AlGaN layer is 10 18 ~10 19 / cm 3 The carbon content of the n-1th AlGaN layer is 10 20 ~10 22 / cm 3 .

[0055] Preferably, n is an integer ≥ 2, that is, the total AlGaN layer including the nth AlGaN layer is at least 2 layers, more preferably 4 to 5 layers, and further preferably 4 layers. If the number of layers is too small, the original buffer transition effect cannot be achieved. If the number of layers is too large, the growth cycle will be too long and the device thickness will also be thick.

[0056] Except for the nth AlGaN layer, the Al contents of other AlGaN layers gradually decrease from bottom to top.

[0057] The Al content of the nth AlGaN layer has been found to be greater than that of the n-1th AlGaN layer, or less than that of the n-1th AlGaN layer, and both can achieve a certain stress release effect. When the Al content of the two AlGaN layers is different, combined with the carbon concentration gradient, a certain compressive stress is generated, resulting in a change in the curvature during the growth process compared to the prior art. When the curvature recorded by the in-situ monitoring is negative at the end of the epitaxial growth, it is relatively in a compressive stress state. Compared with the tensile stress state when the curvature is positive, the compressive stress state makes the epitaxial wafer relatively less likely to crack, thereby effectively improving product quality.

[0058] Preferably, the Al content of the n-th AlGaN layer is less than the Al content of the n-2-th AlGaN layer.

[0059] Preferably, the Al content of the nth AlGaN layer is less than the Al content of the n-1th AlGaN layer, which has a better stress release effect than the Al content of the nth AlGaN layer is greater than the Al content of the n-1th AlGaN layer. Figure 2 and Figure 4 As shown, when the superlattice layer is introduced, the epitaxial structure is thickened, and the total thickness of the epitaxial structure including the nth AlGaN layer is ≥5um. It is preferred to adopt the technical solution that the Al content of the nth AlGaN layer is less than the Al content of the n-1th AlGaN layer. However, the combination of high or low Al content still varies depending on the overall epitaxial structure design, and the key is still the existence of the carbon concentration gradient of the present invention.

[0060] Preferably, the thickness of the nth AlGaN layer is 300-1000 nm. When the thickness is too low, the stress release effect is not achieved, and when the thickness is too high, the overall epitaxial structure is also thicker, which leads to an accumulated increase in epitaxial stress and affects the stress release effect.

[0061] Preferably, the nth AlGaN layer may include a plurality of layers, preferably 2 to 3 layers, and further preferably, the thickness of each AlGaN layer is ≥100 nm.

[0062] Preferably, the nth AlGaN layer is doped with iron to increase insulation and reduce vertical leakage.

[0063] The epitaxial overall structure can be formed using conventional techniques. This application provides a preferred GaN HEMT epitaxial structure, such as Figure 1 As shown, from bottom to top, it includes a substrate, an AlGaN buffer transition layer, a high-resistance buffer layer, a channel layer, a barrier layer, and a cap layer, wherein the AlGaN buffer transition layer adopts the structure as described above.

[0064] The present application provides another preferred GaN HEMT epitaxial structure, such as Figure 3 As shown, a superlattice layer is introduced between the AlGaN buffer transition layer and the high-resistance buffer layer.

[0065] Further preferably, the substrate of the present application may be made of silicon, silicon carbide, sapphire, etc., the superlattice layer is any one selected from AlN / AlGaN, AlN / GaN, AlGaN / GaN, AlN / AlGaN / GaN; the high-resistance buffer layer is a C-GaN layer; the barrier layer is an AlGaN layer; and the cap layer is a GaN layer.

[0066] As those skilled in the art can understand, in order to obtain other technical effects, other corresponding structures can be introduced based on the epitaxial structure of the present application.

[0067] As described above, the carbon concentration gradient can be formed directly by adjusting the growth conditions of the AlGaN buffer transition layer without introducing a new carbon source to adjust the carbon doping concentration, which can reduce costs and simplify the process. Specifically:

[0068] An AlGaN buffer transition layer is deposited on a substrate by using a MOCVD (metal organic chemical vapor deposition) method;

[0069] The deposition parameters of the nth AlGaN layer are: growth temperature ≥1000°C and <1100°C, pressure >80mbar and ≤200mbar, V / III ratio (5:3 ratio, molar ratio of nitrogen element and aluminum gallium element) >300 and ≤1500, and growth rate ≥0.5um / h and <1um / h;

[0070] The deposition parameters of the n-1th AlGaN layer are: growth temperature ≥1100° C. and ≤1200° C., pressure ≥30 mbar and ≤80 mbar, V / III ratio ≥10 and ≤300, and growth rate ≥1 um / h and ≤5 um / h.

[0071] Research has found that within a suitable temperature and pressure range, on the one hand, the C content can be suppressed by increasing the V / III ratio and increasing the amount of N source introduced. On the other hand, the C content can also be suppressed by reducing the growth rate. Therefore, by adjusting the growth conditions, the purpose of forming a carbon concentration gradient can be achieved without introducing a new carbon source to adjust the carbon doping concentration.

[0072] Example 1

[0073] A silicon substrate is used, and the first AlGaN layer (AlGaN1 for short), the second AlGaN layer (AlGaN2 for short), the third AlGaN layer (AlGaN3 for short), the fourth AlGaN layer (AlGaN4 for short), a C-GaN high-resistance buffer layer (C-GaN buffer), a GaN channel layer (GaN channel), an AlGaN barrier layer (AlGaN barrier) and a GaN cap layer (GaN cap) are sequentially deposited on the substrate by a conventional MOCVD method.

[0074] Among them, the growth conditions of the third AlGaN layer are: growth temperature of 1150°C, pressure of 55mbar, V / III ratio of 40.5:1, and growth rate of 2.5um / h; by adjusting the amount of Al source when growing the first to third AlGaN layers, the Al content of the first AlGaN layer to the third AlGaN layer is gradually reduced.

[0075] The growth conditions of the 4th AlGaN layer are: growth temperature of 1055°C, pressure of 140mbar, V / III ratio of 1000:1, growth rate of 0.75um / h; deposition thickness of 470nm; and Fe doping; the amount of Al source used when growing the 4th AlGaN layer is adjusted to obtain different GaN HEMT epitaxial structure samples, referred to as Example Sample 1 (the Al content of AlGaN4 layer is higher than that of AlGaN3 layer) and Example Sample 2 (the Al content of AlGaN4 layer is lower than that of AlGaN3 layer).

[0076] The third and fourth AlGaN layers are deposited under conventional growth conditions (the deposition conditions of the third and fourth AlGaN layers are the same as the growth conditions of the third AlGaN layer of Example Samples 1 and 2), and the rest are the same as Example Sample 2, to obtain Comparative Sample 1.

[0077] In-situ monitoring of the curvature and reflectivity changes of Example samples 1-2 and Comparative sample 1 during the growth process, the results are as follows Figures 5 to 7 shown.

[0078] After the samples were cooled to room temperature, the performance of Example Samples 1 to 2 and Comparative Sample 1 were tested using conventional methods. The results are shown in Tables 1 and Figure 8 shown.

[0079] As shown in Table 1, all samples were compared under the same epitaxial thickness. Compared with Sample 1, which did not use AlGaN3 and AlGaN4 with different growth conditions, and did not have a high carbon concentration gradient between different AlGaNs, we found that Example Sample 1 and Example Sample 2 can improve the edge cracks, curvature and warpage by using the AlGaN4 insertion layer. Among them, Example Sample 1, we grew high-aluminum AlGaN4 such as Figure 8 As shown (the diffraction peak of AlGaN4 is on the right side of AlGaN3, that is, the diffraction angle is greater than that of AlGaN3, which means that the aluminum content of AlGaN4 is higher than that of AlGaN3), we can see from Figures 5 to 7 The reflectivity map shows the presence of the AlGaN4 insertion layer (the cyan rectangular background represents the AlGaN4 layer growth process, Figure 5 and Figure 6 The number of oscillation peaks relative to Figure 7 Less, indicating Figure 5 and Figure 6The AlGaN4 layer of Example Samples 1 and 2 grows at a slower rate). It can be observed from the curvature diagram that the insertion of AlGaN4 causes the curvature to go up because its aluminum content is higher than that of AlGaN3, which represents the accumulation of tensile stress. However, the in-situ monitored curvature value at the end of growth is lower than that of Comparative Sample 1, which represents the compressive stress generated by the high carbon concentration gradient, which can produce a stress adjustment effect, thereby improving edge cracks. This carbon concentration gradient is attributed to the different growth conditions of AlGaN3 and AlGaN4. Similarly, in Example Sample 2, we grow low-aluminum AlGaN4 (aluminum is lower than AlGaN3), and the insertion of AlGaN4 causes the curvature to go down because its aluminum content is lower than that of AlGaN3, which represents the accumulation of compressive stress. It is easier to obtain an in-situ monitored curvature value at the end of growth that is lower than that of Comparative Sample 1, which is in line with general expectations. Therefore, regardless of the combination of high aluminum or low aluminum, we can prove the fact that stress is adjusted by the characteristics of having a high carbon concentration gradient.

[0080] Table 1

[0081] project Example Sample 1 Example Sample 2 Comparative sample 1 Total thickness / um 4.48 4.44 4.48 Edge crack(mm) 1.5 4～5 3.5～4 Curvature(um) -51.75 -38.71 -55.07 Warpage(um) 60.51 48.15 66.95

[0082] In the table, edge cracks were detected by OM (metallographic microscope), and bow and warp were detected by PL detector.

[0083] It can be seen from Table 1 that by adopting the method of the embodiment of the present application, a gradient of decreasing carbon concentration is formed in the 3rd AlGaN layer and the 4th AlGaN layer. When the Al content of the 4th AlGaN layer is higher than that of the 3rd AlGaN layer (Example Sample 1), the edge crack can be effectively improved, and the bow and warp of the product can also be improved to a certain extent. When the Al content of the 4th AlGaN layer is lower than that of the 3rd AlGaN layer (Example Sample 2), the bow and warp of the product can be greatly improved, thus proving that the present application can achieve the effect of stress release.

[0084] Example 2

[0085] A silicon substrate is used, and the first AlGaN layer (AlGaN1), the second AlGaN layer (AlGaN2), the third AlGaN layer (AlGaN3), the fourth AlGaN layer (AlGaN4), an AlN / AlGaN / GaN superlattice layer, a C-GaN high-resistance buffer layer (C-GaN buffer), a GaN channel layer (GaN channel), an AlGaN barrier layer (AlGaNbarrier) and a GaN cap layer (GaN cap) are sequentially deposited on the substrate by a conventional MOCVD method.

[0086] The growth conditions of the third AlGaN layer are: growth temperature 1120°C, pressure 40mbar, V / III ratio 100:1, and growth rate 3.5um / h.

[0087] The growth conditions of the fourth AlGaN layer are: growth temperature 1025°C, pressure 100mbar, V / III ratio 1496:1, growth rate 0.6um / h; deposition thickness 470nm;

[0088] By adjusting the amount of Al source used when growing the first to fourth AlGaN layers, the Al content of the first to fourth AlGaN layers is gradually reduced, thereby obtaining Example Sample 3.

[0089] The fourth AlGaN layer was not deposited as a control, and the other conditions were the same as Example Sample 3 to obtain Comparative Sample 2.

[0090] The curvature and reflectivity changes of Example Sample 3 and Comparative Sample 2 during the growth process were monitored. The results are as follows: Figures 9-10 shown.

[0091] from Figures 9-10 It can be seen from the curvature curve that the curvature value of the in-situ monitoring at the end of the growth process of Example Sample 3 is lower than that of Comparative Sample 2, and the curvature is negative, which is not easy to produce cracks. Figures 9-10 From the reflectivity curve, it can be seen that the embodiment sample 3 has an AlGaN4 layer with a slower growth rate inserted than the comparison sample 2.

[0092] The performance of Example Sample 3 and Comparative Sample 2 was tested using conventional methods. The results are as follows: Figures 11-12 And as shown in Table 2.

[0093] from Fig.11 It can be seen that the C content of the fourth AlGaN layer of Example 3 is about 2 orders of magnitude lower than that of the third AlGaN layer. Indeed, by using different growth conditions of AlGaN3 and AlGaN4, a highly differentiated carbon concentration gradient can be designed and grown, and the existence of this gradient achieves the effect of the embodiment of the present invention. Fig.12 It can be seen that the Al content of the fourth AlGaN layer of sample 3 in this embodiment is lower than that of the third AlGaN layer in the experimental design.

[0094] Table 2

[0095] project Example Sample 3 Comparative sample 2 Total thickness / um 5.38 4.91 Edge crack(mm) 0.5～1 1～2 Curvature(um) 1.33 -20.45 Warpage(um) 13.18 27.68

[0096] Due to the addition of the superlattice layer, the sample structure of Example 2 is thicker than that of Example 1, especially after the introduction of the fourth AlGaN layer that forms a carbon concentration gradient, the total thickness of the epitaxial structure exceeds 5um. This increase in thickness theoretically leads to an increase in stress accumulation, which makes it easy to produce edge cracks and greater bending and warping when the temperature is lowered to room temperature after the epitaxial growth is completed.

[0097] pass Figures 9-11 As can be seen from Table 2, by adopting the method of the present application, a fourth AlGaN layer (Example Sample 3) is inserted to form a carbon concentration gradient. Even if the epitaxial structure is thickened, the curvature and warping of the product are greatly improved, and the edge cracks are not worsened compared to when the fourth AlGaN layer is not introduced (Comparative Sample 2). Therefore, through the present application, a product with a thicker epitaxial structure, good bottom layer insulation, and good stress release effect can be obtained, which has important application value.

[0098] Example 3

[0099] A silicon substrate is used, and the first AlGaN layer (AlGaN1 for short), the second AlGaN layer (AlGaN2 for short), a C-GaN high-resistance buffer layer (C-GaN buffer), a GaN channel layer (GaNchannel), an AlGaN barrier layer (AlGaN barrier) and a GaN cap layer (GaN cap) are deposited on the substrate in sequence by a conventional MOCVD method.

[0100] The growth conditions of the first AlGaN layer, i.e., AlGaN1 layer, are as follows: growth temperature of 1180°C, pressure of 75 mbar, V / III ratio of 200:1, and growth rate of 2.2 um / h;

[0101] The growth conditions of the second AlGaN layer, i.e., AlGaN2 layer, are as follows: growth temperature of 1075°C, pressure of 180 mbar, V / III ratio of 800:1, growth rate of 0.9 um / h, and deposition thickness of 470 nm.

[0102] Due to different growth conditions and by adjusting the amount of Al source used when growing the first and second AlGaN layers so that the Al content of the first AlGaN layer is greater than that of the second AlGaN layer, Example Sample 4 was obtained.

[0103] The first AlGaN layer and the second AlGaN layer were deposited according to the growth conditions of the second AlGaN layer, except for the difference in the amount of Al source used, to obtain comparative sample 3.

[0104] The curvature and reflectivity changes of Example Sample 4 and Comparative Sample 3 during the growth process were monitored. The results are as follows: Figures 13-14 shown.

[0105] The performance of Example Sample 4 and Comparative Sample 3 were tested using conventional methods. The results are as follows: Fig.15 And as shown in Table 3.

[0106] Table 3

[0107] project Example Sample 4 Comparative sample 3 Total thickness / um 2.23 2.24 Edge crack(mm) <1 4 Curvature(um) -10.89 -10.62 Warpage(um) 16.9 16.3

[0108] As described above, we also conducted experiments with a thin epitaxial structure with a total thickness of about 2.3 um, in which Example Sample 4 and Comparative Sample 3 only had two AlGaN buffer transition layers.

[0109] As shown in Table 3, when Example Sample 4 has the same thickness as Comparative Sample 3, by forming a carbon concentration gradient, the edge cracks of Example Sample 4 are reduced to <1, which is significantly improved compared to the original result of Comparative Sample 3 of 4 mm, and the curvature and warpage shown are not much different between the two.

[0110] like Figures 13-14 , Comparative sample 3 uses the same growth conditions as AlGaN2 to grow two AlGaN buffer transition layers, such as Fig.15 Among them, the comparison sample 3 only has a high Al content at the beginning and a low Al content at the end. Since AlGaN uses the same growth conditions, there is no aforementioned carbon concentration gradient. Its epitaxial result is not as good as that of the embodiment sample 4, and the edge crack can be reduced to less than 1mm. This reflects the effect of the carbon concentration gradient, which allows the stress to be adjusted through such a design and shortens the edge crack.

[0111] Therefore, in the embodiments that echo each other, such as the comparison sample 1 of embodiment 1, in which AlGaN 3 and AlGaN 4 are both grown under low-pressure conditions (i.e., comparison sample 1 adopts growth conditions that form a high carbon content to grow AlGaN 3 and AlGaN 4), and the comparison sample 3 of embodiment 3, in which AlGaN 1 and AlGaN 2 are both grown under high-pressure conditions (i.e., comparison sample 3 adopts growth conditions that form a low carbon content to grow AlGaN 1 and AlGaN2), the improvement effect can be obtained by the existence of a high carbon concentration gradient in the embodiment samples (regardless of whether the carbon content of the two AlGaN buffer transition layers is higher or lower, the stress release effect cannot be achieved, and the stress release effect can only be achieved by forming a carbon concentration gradient between the two AlGaN buffer transition layers).

[0112] Although preferred embodiments of the present invention have been described, additional changes and modifications may be made to these embodiments by those skilled in the art once the basic inventive concepts are known. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention. Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A GaN HEMT epitaxial structure with a stress release layer, comprising an AlGaN buffer transition layer located on a substrate, characterized in that: The AlGaN buffer transition layer includes n AlGaN layers, where n is an integer ≥ 2, and the carbon atom content of the n-1th AlGaN layer located below the nth layer is greater than the carbon atom content of the nth AlGaN layer by more than one order of magnitude.

2. The GaN HEMT epitaxial structure with a stress release layer according to claim 1, characterized in that: The carbon atom content of the n-1th AlGaN layer is greater than the carbon atom content of the nth AlGaN layer by 1 to 3 orders of magnitude; Preferably, the carbon atom content of the nth AlGaN layer is 10 18 ~10 19 / cm 3 The carbon atom content of the n-1th AlGaN layer is 10 20 ~10 22 / cm 3 .

3. The GaN HEMT epitaxial structure with a stress release layer according to claim 1, characterized in that: The thickness of the nth AlGaN layer is 300-1000 nm.

4. The GaN HEMT epitaxial structure with a stress release layer according to claim 1, characterized in that: The nth AlGaN layer includes a plurality of layers; Preferably, the thickness of each layered AlGaN layer is ≥100 nm.

5. The GaN HEMT epitaxial structure with a stress release layer according to claim 1, characterized in that: Said n is an integer ≥ 3; preferably, said n is 4; From bottom to top, the Al content of the first layer to the n-1th AlGaN layer gradually decreases; The Al content of the nth AlGaN layer is ≠ the Al content of the n-1th AlGaN layer; Preferably, the Al content of the n-th AlGaN layer is less than the Al content of the n-2-th AlGaN layer.

6. The GaN HEMT epitaxial structure with a stress release layer according to claim 5, characterized in that: The Al content of the n-th AlGaN layer is less than the Al content of the n-1-th AlGaN layer.

7. The GaN HEMT epitaxial structure with a stress release layer according to claim 1, characterized in that: The invention also includes a super lattice layer located on the AlGaN buffer transition layer.

8. The GaN HEMT epitaxial structure with a stress release layer according to claim 1, characterized in that: The nth AlGaN layer is doped with iron.

9. The GaN HEMT epitaxial structure with a stress release layer according to any one of claims 1 to 8, characterized in that: It also includes a high-resistance buffer layer, a channel layer, a barrier layer, and a cap layer which are sequentially located on the AlGaN buffer transition layer; When the superlattice layer exists, the superlattice layer is located between the AlGaN buffer transition layer and the high-resistance buffer layer; Preferably, the superlattice layer is any one selected from AlN / AlGaN, AlN / GaN, AlGaN / GaN, AlN / AlGaN / GaN; The high-resistance buffer layer is a C-GaN layer; The barrier layer is an AlGaN layer; The cap layer is a GaN layer.

10. The method for preparing a GaN HEMT epitaxial structure with a stress release layer according to any one of claims 1 to 9, characterized in that: Depositing the AlGaN buffer transition layer on the substrate using a MOCVD method; The deposition parameters of the nth AlGaN layer are: growth temperature ≥1000° C. and <1100° C., pressure >80 mbar and ≤200 mbar, V / III ratio >300 and ≤1500, and growth rate ≥0.5 um / h and <1 um / h; The deposition parameters of the n-1th AlGaN layer are: growth temperature ≥1100° C. and ≤1200° C., pressure ≥30 mbar and ≤80 mbar, V / III ratio ≥10 and ≤300, and growth rate ≥1 um / h and ≤5 um / h.

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