JP4904726B2 - Semiconductor epitaxial wafer and method for manufacturing semiconductor epitaxial wafer for HEMT - Google Patents

Description

  The present invention relates to a compound semiconductor epitaxial wafer, and more particularly to a semiconductor epitaxial wafer suitable for realizing a high-frequency electronic device having GaN as a channel layer.

  Group III-V nitride semiconductor epitaxial wafers in which GaN, AlN, InN, and mixed crystals thereof are laminated and grown in an optimum structure have already been put on the market as blue LED crystals. An ultraviolet LED epitaxial wafer and the like are also being developed. However, with the demand for high output transistors, not only optical devices but also electronic devices are expected. Therefore, in recent years, GaN-HEMT (High Electron Mobility Transistor: high electron mobility transistor) has been developed at various research institutions.

  Usually, the growth of gallium nitride (GaN) on these semiconductor epitaxial wafers for optical / electronic devices is performed on a sapphire or SiC substrate by metal organic vapor phase epitaxy (MOVPE method). It is relatively difficult to grow a thin film. As the cause,

  (i) Deterioration of the surface state of the thin film crystal due to the difference in lattice constant from the substrate, and increase in transition

(ii) Wafer warpage and cracking due to difference in thermal expansion coefficient,
There are two.

  The problem (i) can be solved by selecting a growth condition and growing a thickness of about several μm. However, the above problem (ii) becomes noticeable as the film thickness increases. The problem (ii) becomes more serious as the wafer diameter increases.

  In order to solve this problem, various strain relaxation buffer layer structures have been proposed. Usually, AlN (aluminum nitride) is used because of the advantage that the buffer layer formation → nitride thin film growth can be performed together in one apparatus. Is grown as a first buffer layer on a substrate, and then a required GaN (gallium nitride) buffer layer structure is grown.

  FIG. 8 shows the structure of a conventional semiconductor epitaxial wafer for HEMT.

  On the sapphire substrate 10, an AlN buffer layer (low temperature deposition layer) 12 and a thick GaN buffer layer 13 of about several tens of nm are grown as a buffer layer 20, and a nitride mixed crystal is formed thereon as a channel layer. A gallium nitride based electronic device structure 30 is formed. Here, a high electron mobility transistor (HEMT) structure in which GaN is the channel layer 4 and AlGaN is the carrier supply layer 5 is formed.

  FIG. 9 shows the warpage of a HEMT semiconductor epitaxial wafer having a diameter of about 10.16 cm (4 inches) grown by the MOVPE method using the above-described conventional growth conditions and epitaxial wafer structure. As an index indicating warpage, BOW (a numerical value for evaluating warpage at a relative position of the wafer center with respect to the wafer periphery, the smaller the warpage, the smaller the warpage) is used. The warpage of a conventional semiconductor epitaxial wafer for HEMT is as follows: BOW shows a very large value of 47 μm.

  In this way, when warping exceeding 40 μm is generated in a 4-inch semiconductor epitaxial wafer, lithography for realizing a short gate length necessary for a high-frequency electronic device as it is and for obtaining a good ohmic electrode It is difficult to perform heat treatment.

Conventionally, the following is known as a technique for constructing the buffer layer of the high-frequency electronic device described above.
(A) JP 2004-048076 A (Patent Document 1)

Japanese Patent Application Laid-Open No. 2004-048076 of Patent Document 1 discloses a first buffer layer formed on a substrate at a low temperature lower than the single crystal growth temperature, and a Ga formed at the single crystal growth temperature and in contact with the first buffer layer. And a second buffer layer having a layer made of nitride not containing In and in this order are disclosed. Specifically, the first buffer layer is formed of a laminated film in which an AlN film having a thickness of about 2.5 nm and a GaN film having a thickness of about 2.5 nm are alternately laminated for four periods. The second buffer layer is composed of an AlN film having a thickness of about 0.1 μm.
(B) JP 2003-218127 A (Patent Document 2)

In Japanese Patent Application Laid-Open No. 2003-218127 of Patent Document 2, an AlN layer on which a two-dimensional nucleus is grown is provided on a SiC substrate. An epitaxial wafer for a field effect transistor provided with a gallium nitride based field effect transistor structure having a mixed crystal as a channel layer is disclosed. This is a technology that solves the problem of substrate warpage by reducing the thickness of the epitaxial layer based on the premise that a large warpage of the wafer occurs at room temperature due to the difference in linear expansion coefficient between the substrate and the GaN-based compound semiconductor. Is also disclosed.
(C) JP 2003-218128 A (Patent Document 3)

  Japanese Patent Application Laid-Open No. 2003-218128 also discloses that the thickness of the epitaxial layer is reduced based on the premise that a large warp occurs in the wafer at room temperature due to the difference in linear expansion coefficient between the substrate and the GaN-based compound semiconductor. Thus, a technique for solving the problem of substrate warpage is disclosed. However, in this Patent Document 3, by using an InGaN low temperature deposition layer instead of an AlN low temperature deposition layer under the GaN buffer layer, the total film thickness of the epitaxial layer of the GaN field effect transistor structure is reduced to 1 μm or less, and the amount of warpage To 20 μm or less. It is said that the warpage amount that does not affect the device process is 20 μm or less, and the thickness of the epitaxial layer for that purpose must be 1 μm or less.

Japanese Patent Laying-Open No. 2004-048076 (paragraph numbers 0013 and 0038, FIG. 1) Japanese Patent Laying-Open No. 2003-218127 (paragraph numbers 0003, 0010, 00003, FIG. 2) Japanese Patent Laying-Open No. 2003-218128 (paragraph numbers 0003, 0006, 0031, FIG. 2)

  However, Patent Document 1 has no description regarding warpage. That is, there is no disclosure about the mixed crystal ratio of the layer inserted below the AlN layer (AlGaN layer), the relationship between the film thickness and the warp, and the case where the first buffer layer mainly has a periodic structure of an AlN film and a GaN film. It is mainly explained.

  Further, both Patent Documents 2 and 3 are based on the premise that a large warp occurs in the wafer at room temperature due to the difference in linear expansion coefficient between the substrate and the GaN-based compound semiconductor. This technology solves the problem of warping. It does not disclose a technique for reducing the warpage by specifying the growth conditions of a layer (AlGaN layer) or a GaN buffer layer inserted below the AlN layer.

  In a field effect transistor, it is known that shortening the gate length and reducing the source / drain resistance are effective in improving high frequency characteristics. For this purpose, fine lithography for realizing a shorter gate and a process for lowering the resistance by subjecting the source / drain electrodes to a thermal alloy are indispensable.

  However, as described with reference to FIGS. 8 and 9, it has been difficult to apply fine lithography with a shallow depth of focus and heat treatment that requires uniform back contact in the conventional GaN-based semiconductor epitaxial wafer that has greatly warped. .

  Accordingly, an object of the present invention is to solve the above problems and reduce the warpage of the semiconductor epitaxial wafer by the presence of the AlGaN buffer layer inserted below the AlN buffer layer or by the growth conditions of the GaN buffer layer added thereto. Accordingly, an object of the present invention is to provide a semiconductor epitaxial wafer capable of easily producing a device having a short gate and a low ohmic resistance.

  In order to achieve the above object, the present invention is configured as follows.

The semiconductor epitaxial wafer according to the invention of claim 1 is made of Al _x Ga _1-x N (provided that 0.3 ≦ x ≦ 0.7) on the substrate and has a thickness of 0 . A first buffer layer of 07 to 0.15 μm, a second buffer layer made of AlN formed on the first buffer layer, and any one of GaN, AlGaN or InGaN grown two-dimensionally on the second buffer layer And a gallium nitride electronic device structure having a nitride mixed crystal as a channel layer is provided on the third buffer layer.

According to a second aspect of the present invention , in the semiconductor epitaxial wafer according to the first aspect, the substrate is a sapphire substrate.

According to a third aspect of the present invention, in the semiconductor epitaxial wafer according to the second aspect, the mixed crystal ratio x of the first buffer layer made of Al _x Ga _1-x N is 0.5 and the film thickness is 0.1 μm. It is characterized by being.

According to a fourth aspect of the present invention, there is provided a method of manufacturing a semiconductor epitaxial wafer for HEMT in which a semiconductor layer is formed on a substrate by vapor phase epitaxy. On the substrate, Al _x Ga ₁₋ a step _{of x} N (provided that 0.3 ≦ x ≦ 0.7) Tona RimakuAtsu form a first buffer layer of 0.07～0.15Myuemu, to the first buffer layer, a single crystal growth temperature Forming a second buffer layer made of AlN on the second buffer layer, and forming a third buffer layer made of GaN on the second buffer layer at a single crystal growth temperature lower than that of the second buffer layer by two-dimensional growth . And a step of forming an electronic device structure including a channel layer made of GaN and a carrier supply layer made of AlGaN on the third buffer layer.

According to a fifth aspect of the present invention, in the HEMT semiconductor epitaxial wafer according to the fourth aspect, the third buffer layer is formed at a growth pressure of 13332 Pa (100 Torr) or less and a growth temperature of 1060 ° C. or more.

<Key points of the invention>
The main point of the present invention is to reduce the warpage of the semiconductor epitaxial wafer by adjusting the structure of the buffer layer and the growth conditions.

As shown in FIG. 1, a typical configuration of a semiconductor epitaxial wafer according to the present invention is formed on an sapphire substrate 10 with Al _x Ga _1-x N (where 0.3 ≦ x ≦ 0.7, preferably x = A first buffer layer 1 having a thickness of 0.07 to 0.15 μm, preferably about 0.1 μm, and a second buffer layer 2 made of AlN formed thereon, A buffer layer 20 having a three-layer structure composed of a third buffer layer 3 made of GaN grown in a two-dimensional direction is provided. The device structure 30 includes a channel layer 4 made of GaN and a carrier supply layer 5 made of AlGaN.

  The basis for the above numerical limitation is as follows.

  FIG. 3 shows an AlGaN layer (first layer) in a semiconductor epitaxial wafer in which a gallium nitride-based electronic device structure 30 is provided on a sapphire substrate 10 via a buffer layer 20 having a three-layer structure of GaN layer / AlN layer / AlGaN layer. The relationship between the layer thickness of the buffer layer) and the warpage of the epitaxial wafer is shown.

  In this warpage evaluation sample, an AlN layer having a thickness of 0.4 μm as a second buffer layer and an un-GaN layer having a thickness of 2 μm as a third buffer layer are provided on the AlGaN layer (first buffer layer). Further, a sample of a semiconductor epitaxial wafer having a HEMT structure composed of a GaN channel layer and an AlGaN carrier supply layer was used as the electronic device structure 30 thereon.

Here, when the mixed crystal ratio x of Al _x Ga _1-x N in the AlGaN layer (first buffer layer) is fixed to x = 0.5 and the thickness is changed in the range of 0 to 0.25 μm FIG. 3 shows how the warpage changes.

  In FIG. 3, when the film thickness of the AlGaN layer (first buffer layer) is 0 μm, that is, the bow value of warpage is very large at 47 μm under the conventional conditions, and the BOW value decreases as the film thickness increases. When the film thickness is 0.10 μm, the BOW value is 35 μm, which is the minimum. When the film thickness is in the range of 0.07 to 0.15 [mu] m, the BOW value is less than 40.0 [mu] m, and the warpage is smaller than in the conventional case. Therefore, the appropriate value of the film thickness of the AlGaN layer (first buffer layer) is in the range of 0.07 to 0.15 μm, and the optimum value is about 0.1 μm.

FIG. 4 shows the dependence of BOW on the mixed crystal ratio x of Al _x Ga ₁ -xN. That is, FIG. 4 shows how the wafer warps when the Al _x Ga _1-x N mixed crystal ratio (Al composition ratio) x of the first buffer layer is changed in the range of x = 0 to 0.9. It shows how it changes. As can be seen from FIG. 4, when the mixed crystal ratio x is in the range of x = 0.3 to 0.7, the value of BOW is less than 40 μm, and the warpage is reduced. In particular, when x = 0.5, the value of BOW is a minimum of 35 μm, and the warp is minimum. Therefore, there is an appropriate value for the mixed crystal ratio x, which is in the range of x = 0.3 to 0.7, and the optimum value is x = 0.5. FIG. 3 is a graph obtained by fixing the mixed crystal ratio x of Al _x Ga _1-x N to the optimum value of 0.5.

  Next, in the present invention, the third buffer layer is made of GaN that promotes two-dimensional growth under growth conditions of a growth pressure of about 13332 Pa (100 Torr) or less and a growth temperature of 1060 ° C. or more. Reduce warpage. The basis for this is as follows.

  FIG. 5 shows the dependence of the bow (BOW) on the growth pressure of the GaN layer (third buffer layer). From the figure, it can be seen that the bow value of the warp is further reduced by setting the growth pressure to about 13332 Pa (100 Torr) or less. This is because the growth at a low growth pressure promotes two-dimensional growth of the GaN layer (third buffer layer), and dislocations are introduced into the GaN / AlN interface, thereby reducing the strain that causes warping. It is believed that there is.

  In order to further promote this, the dependence of bow (BOW) on the growth temperature of the GaN layer (third buffer layer) (it is generally well known that the higher the growth temperature, the more the two-dimensional growth is promoted). FIG. 6 shows the result of the investigation. When the growth temperature of the GaN layer (third buffer layer) is 1060 ° C. or higher, the bow value of warping tends to decrease. However, when the growth temperature of the GaN layer (third buffer layer) exceeded 1090 ° C., the surface of the semiconductor epitaxial wafer could not be mirrored. Therefore, it can be seen that the growth temperature of the third buffer layer is 1060 ° C. or higher, preferably 1060 ° C. or higher and 1090 ° C. or lower.

According to the present invention, the AlGaN layer of the first buffer layer is inserted below the AlN layer of the second buffer layer, and the mixed crystal ratio x of Al _x Ga _1-x N is 0.3 to 0.7, Since the film thickness is specified in the range of about 0.07 to 0.15 μm, it is possible to provide a semiconductor epitaxial wafer capable of reducing the warpage of the semiconductor epitaxial wafer and easily manufacturing a device having a short gate and a low ohmic resistance. Can do.

  Further, according to the present invention, in addition to the above, the third buffer layer is made of GaN that promotes two-dimensional growth under growth conditions of a growth pressure of about 13332 Pa (100 Torr) or less and a growth temperature of 1060 ° C. or more. Accordingly, the warpage can be further reduced.

  Therefore, according to the present invention, it is possible to obtain a semiconductor epitaxial wafer having a small warp, and thus a device having excellent high frequency characteristics.

  Hereinafter, the present invention will be described based on illustrated embodiments.

As shown in FIG. 1, the HEMT semiconductor epitaxial wafer according to this example is formed on a sapphire substrate 10 having a diameter of about 10.16 cm (4 inches) with Al _x Ga _1-x N (x = about 0.5). A first buffer layer 1 having a thickness of about 0.1 μm, a second buffer layer 2 made of AlN formed thereon, and a third buffer layer 3 made of GaN grown two-dimensionally thereon. A buffer layer 20 having a three-layer structure is provided, and a channel layer 4 made of GaN and a carrier supply layer 5 made of AlGaN are provided as the HEMT electronic device structure 30 on the buffer layer 20 having the three-layer structure.

  Each epitaxial layer of this HEMT semiconductor epitaxial wafer was grown as follows using the MOVPE method. That is, as the raw material used for the growth, TMG was used as the gallium raw material, TMA was used as the aluminum raw material, and ammonia gas was used as the nitrogen raw material. Further, hydrogen was used as the carrier gas, and monosilane was used as the n-type dopant.

  Epitaxial growth was performed using a face-up heater-heated decompression furnace with the growth surface of the wafer facing upward, and in the growth other than the third buffer layer 3 of GaN, the pressure in the furnace was set to about 13332 Pa (100 Torr). .

  Regarding the buffer layer 20, the growth temperature of the first buffer layer 1 of AlGaN is 520 ° C., the second buffer layer 2 of AlN is 1100 ° C., and the third buffer layer 3 of GaN is 1080 ° C. (after optimization). grown. For the electronic device structure 30 thereon, the GaN channel layer 4 and the AlGaN carrier supply layer 5 were grown at 1040 ° C., respectively.

  Regarding the buffer layer 20, the AlGaN first buffer layer 1 was 0.1 μm, the AlN second buffer layer 2 was 0.4 μm, and the GaN third buffer layer 3 was 0.5 μm. Further, regarding the electronic device structure 30 thereon, the GaN channel layer 4 was 1.5 μm, and the AlGaN carrier supply layer 5 was 30 nm (0.03 μm).

  When the above structure is viewed by paying attention to temperature, the first buffer layer 1 made of AlGaN formed on the sapphire substrate 10 at a low temperature lower than the single crystal growth temperature (about 1000 to 1200 ° C.), and the single crystal growth A second buffer layer 2 made of AlN formed at a temperature and a third buffer layer 3 made of GaN formed at a single crystal growth temperature lower than that of the second buffer layer, on the third buffer layer As a gallium nitride electronic device structure 30 having a nitride mixed crystal as a channel layer, a HEMT electronic device structure having a channel layer 4 made of GaN and a carrier supply layer 5 made of AlGaN is provided.

  FIG. 7A shows the warpage of the HEMT semiconductor epitaxial wafer configured as described above. In this example, a HEMT semiconductor epitaxial wafer having a bow of 26 μm and a small warp could be obtained.

  For comparison, FIG. 7B shows the warpage of a HEMT semiconductor epitaxial wafer having a diameter of about 10.16 cm (4 inches) grown by the MOVPE method using the conventional growth conditions and the epitaxial wafer structure (FIG. 8). Indicates. The warpage of the conventional HEMT semiconductor epitaxial wafer is 47 μm in BOW, which is very large, whereas the warpage of the HEMT semiconductor epitaxial wafer in this embodiment is 26 μm in BOW, which can be made very small. done.

  FIG. 2 shows in-plane distribution measurement values of the sheet resistance of the semiconductor epitaxial wafer for GaN-HEMT according to this example. In this measured value, the average value is 487.0 Ω / sq. However, the minimum value is 466.5 Ω / sq. The maximum value is 508.3Ω / sq. It can be said that the in-plane sheet resistance variation is very small, which is industrially advantageous. This good result is considered to be because the amount of warpage of the wafer is reduced.

  In the above embodiment, GaN is used as the material of the third buffer layer, but AlGaN or InGaN can also be used in addition to GaN, and this can also provide the same warp reduction effect as in the above embodiment.

  In the above embodiment, the HEMT structure is formed as the gallium nitride based electronic device structure 30, but the channel structure may be an FET (field effect transistor) and is not limited to the HEMT structure. In addition to GaN, InGaN or AlGaN can be used as the material of the channel layer.

It is a schematic diagram which shows the structure of the semiconductor epitaxial wafer for HEMT which concerns on one Example of this invention. It is a sheet resistance map of the semiconductor epitaxial wafer for HEMT which concerns on one Example of this invention. It is a graph which shows the relationship between the layer thickness of the 1st buffer layer which consists of AlGaN, and curvature. Is a graph showing a mixed crystal ratio x and the warping of the relationship between Al _x Ga _1-x N of the first buffer layer made of AlGaN. It is a graph which shows the relationship between the growth pressure and curvature of the 3rd buffer layer which consists of GaN. It is a graph which shows the relationship between the growth temperature and curvature of the 3rd buffer layer which consists of GaN. It is the figure which showed the curvature (a) of the semiconductor epitaxial wafer for HEMT of this invention compared with the curvature (b) of the conventional semiconductor epitaxial wafer for HEMT. It is a schematic diagram which shows the structure of the conventional semiconductor epitaxial wafer for HEMT. It is a figure which shows the curvature of the conventional semiconductor epitaxial wafer for HEMT.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st buffer layer 2 2nd buffer layer 3 3rd buffer layer 4 Channel layer 5 Carrier supply layer 10 Sapphire substrate 20 Buffer layer 30 Gallium nitride system electronic device structure

Claims (5)

A first buffer layer made of Al _x Ga _1-x N (where 0.3 ≦ x ≦ 0.7) and having a thickness of 0.07 to 0.15 μm is formed on the first buffer layer. A second buffer layer made of AlN and a third buffer layer made of GaN, AlGaN, or InGaN grown two-dimensionally on the second buffer layer,
A semiconductor epitaxial wafer characterized in that a gallium nitride electronic device structure having a nitride mixed crystal as a channel layer is provided on the third buffer layer. The semiconductor epitaxial wafer according to claim 1,
A semiconductor epitaxial wafer, wherein the substrate is a sapphire substrate. The semiconductor epitaxial wafer according to claim 2,
A semiconductor epitaxial wafer characterized in that the mixed crystal ratio x of the first buffer layer made of Al _x Ga _1-x N is 0.5 and the film thickness is 0.1 μm. In a method for manufacturing a semiconductor epitaxial wafer for HEMT, in which a semiconductor layer is formed on a substrate by vapor deposition,
On the _{substrate, Al x Ga 1-x N} ( provided that 0.3 ≦ x ≦ 0.7) at a temperature lower than the single crystal growth temperature Tona RimakuAtsu first buffer is 0.07~0.15μm Forming a layer;
Forming a second buffer layer made of AlN at a single crystal growth temperature on the first buffer layer;
Forming a third buffer layer made of GaN at a single crystal growth temperature lower than that of the second buffer layer by two-dimensional growth on the second buffer layer;
Forming an electronic device structure including a channel layer made of GaN and a carrier supply layer made of AlGaN on the third buffer layer;
A method of manufacturing a semiconductor epitaxial wafer for HEMT, comprising: In the manufacturing method of the semiconductor epitaxial wafer for HEMT of Claim 4,
A method of manufacturing a semiconductor epitaxial wafer for HEMT, wherein the third buffer layer is formed at a growth pressure of 13332 Pa (100 Torr) or less and a growth temperature of 1060 ° C. or more.