JP2006128536A - Semiconductor epitaxial wafer and semiconductor device cut therefrom - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor epitaxial wafer in which substrate surface is thermally kept in extremely homogeneous state, and provide a semiconductor device processed therefrom. <P>SOLUTION: The semiconductor epitaxial wafer consists of a solid crystal substrate and a gallium nitride system compound semiconductor film grown up to the solid crystal substrate by MOVPE method. The gallium nitride system compound semiconductor film is grown up in an atmosphere containing hydrogen by the MOVPE method. When the above-mentioned solid crystal substrate is profiled in circle, and has a diameter of ≥70.0 mm; the above-mentioned solid crystal substrate 301 has a thickness of ≥380 μm. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a structure of a semiconductor epitaxial wafer, and more particularly to a semiconductor epitaxial wafer having a gallium nitride compound semiconductor film formed by a MOVPE (Metal Organic Vapor Phase Epitaxy) method and a semiconductor element cut out from the semiconductor epitaxial wafer.

  Gallium nitride-based compound semiconductors are used as light-emitting element materials that cover most regions from ultraviolet to visible light by controlling the composition ratio of group III elements. In addition, gallium nitride compound semiconductors have high saturation electron velocity and high breakdown voltage, so they are also used as high frequency and high output electronic device materials.

  The gallium nitride compound semiconductor is formed of a group III element such as gallium, aluminum, or indium and nitrogen belonging to the group V. Since a gallium nitride compound semiconductor has a high vapor pressure of nitrogen at normal pressure, it is very difficult to grow a single crystal from a solution. Therefore, a gallium nitride compound semiconductor is usually grown as a single crystal by vapor phase growth methods such as MOVPE, MBE (Molecular Beam Epitaxy), and HVPE (Hydride Vapor Phase Epitaxy).

When a gallium nitride compound semiconductor is formed on a solid crystal substrate by the MOVPE method, the solid crystal substrate is set on a substrate holder (susceptor) heated to about 1,000 ° C. with a heater or the like. Here, if there is a foreign substance between the substrate and the substrate holder, there is a problem that the substrate is not heated uniformly and the temperature of the substrate becomes non-uniform. Therefore, while adopting a method of heating the substrate by radiant heat, at least one of the group consisting of Ga, Al, In, and B is added to the substrate body to obtain a substrate having a high radiation absorption rate. Has been proposed (see, for example, Patent Document 1).
JP 2001-237192 A

  However, it is desired to provide not only radiant heat but also other means for heating the substrate using a heating mechanism based on heat conduction and radiant heat such as an ordinary heater to make the temperature of the substrate more uniform.

Therefore, in the case of producing a gallium nitride compound semiconductor by the MOVPE method, ammonia is used as a group V material, and trimethyl gallium (TMG), trimethyl indium (TMI), and trimethyl aluminum (TMA) are used as group III materials. It is common to use. Further, in order to control the conductivity type of the gallium nitride compound semiconductor, monosilane (SiH ₄ ) is used when an n-type semiconductor is manufactured, and bis (cyclopentadienyl) magnesium (when a p-type semiconductor is manufactured. Cp ₂ Mg) is often used. At the time of manufacture, a desired thin film of a gallium nitride compound semiconductor is formed on the surface of the substrate by spraying the above raw material on the heated substrate in a reaction vessel.

Among the above raw materials, particularly Group III raw materials and Cp ₂ Mg are liquid or solid at room temperature, so it is difficult to supply them to the reaction vessel as they are. For this reason, normally, nitrogen gas or hydrogen gas is first blown into the raw material container, saturated in the gas into which molecules such as Group III raw material and Cp ₂ Mg are blown, and then this gas is taken out and supplied into the reaction container. The method is used. In semiconductor manufacturing, materials are required to be highly purified, and hydrogen gas that can be easily purified by using a palladium film is generally used as the carrier gas.

By the way, this hydrogen gas is a gas having an extremely large thermal conductivity and heat capacity. For example, the thermal conductivity near room temperature is 2.40 × 10 ⁻² Wm ⁻¹ K ^{−1 for} nitrogen and 2.18 × 10 ⁻² Wm ⁻¹ K ^{−1 for} ammonia, whereas hydrogen is 16.82 × 10 ⁻² Wm ⁻¹ K ⁻¹ and very large. The specific heat near room temperature is 1.04 × 10 ³ Jkg ⁻¹ K ^{−1 for} nitrogen and 1.97 × 10 ³ Jkg ⁻¹ K ⁻¹ for ammonia, whereas 14.32 × 10 ^{3 for} hydrogen. Jkg ^-1 K ^-1, which is also very large. Thus, hydrogen is a special gas with extremely large thermal conductivity and heat capacity, so when hydrogen gas is blown onto the surface of a substrate or the like, the amount of heat is orders of magnitude greater than that of nitrogen or ammonia. It will be taken away from the surface of the substrate or the like.

  As described above, when a gallium nitride compound semiconductor is formed on a solid crystal substrate by the MOVPE method, the solid crystal substrate is set on a substrate holding table heated to about 1,000 ° C. by a heater or the like. Yes. At this time, the lower side of the solid crystal substrate in contact with the substrate holding table is maintained at the same high temperature as the substrate holding table. On the other hand, the carrier gas is sprayed on the upper side of the solid crystal substrate, and particularly when hydrogen is mixed in the carrier gas, heat is violently taken away from the upper surface of the substrate by the gas. Tends to be lower than the lower side (vertical temperature difference).

  When a temperature difference occurs between the upper and lower sides of the solid crystal substrate because hydrogen is mixed in the carrier gas, a phenomenon that the substrate is warped starts to occur. Specifically, since the lower side of the substrate is thermally expanded and the upper side of the substrate is contracted, the substrate is protruded downward and warped in a bowl shape. When the substrate starts to warp, it is not completely in close contact with the substrate holder except in the vicinity of the center of the substrate, and a gap is formed between the back surface of the substrate and the substrate holder depending on the distance from the center of the substrate. Then, the amount of heat transport from the substrate holder to the substrate is large at the center of the substrate and is small at the outer periphery. For this reason, the central part of the substrate is strongly heated to a high temperature, while the outer peripheral part of the substrate tends to be a low temperature (lateral temperature difference).

  When a temperature difference occurs in the plane of the substrate in this way, there is a negative effect that the components and characteristics of the gallium nitride compound semiconductor formed on the substrate have a distribution according to the temperature difference. For example, InGaN used as an active layer of a blue light-emitting LED is composed of a GaN component and an InN component that is relatively easy to evaporate, while the In concentration in InGaN increases at a low temperature portion on the same substrate, Since the evaporation of InN is promoted at a high portion, the In concentration in InGaN becomes low. That is, a large distribution of In concentration occurs on the same substrate. As a result, different wavelengths of light emitted from the InGaN layer can be obtained at different positions on the substrate, and the characteristics of optical devices such as LEDs themselves have a large non-uniformity in the substrate plane.

  As a similar example, AlGaN used as a barrier layer in a GaN field effect transistor is composed of a GaN component and an AlN component that is relatively difficult to evaporate. On the other hand, since the evaporation of GaN is promoted at a high temperature portion, the Al concentration in AlGaN becomes high. That is, a large distribution of Al concentration occurs on the same substrate. As a result, the two-dimensional electron gas induced by spontaneous polarization and piezo polarization of the AlGaN layer has different concentrations at different positions on the substrate, and the characteristics of electronic devices such as field-effect transistors are large in the substrate plane. Will have non-uniformity.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above problems and provide a semiconductor epitaxial wafer in which the substrate surface is kept in a thermally extremely homogeneous state and a semiconductor element cut out therefrom.

  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 a semiconductor epitaxial wafer having a diameter of 70.0 mm or more, comprising a solid crystal substrate and a gallium nitride compound semiconductor film grown on the surface of the solid crystal substrate by MOVPE. The growth of the gallium nitride compound semiconductor film by an MOVPE method is performed in an atmosphere containing hydrogen, and the solid crystal substrate having a thickness of 380 microns or more is used.

  According to a second aspect of the present invention, in the semiconductor epitaxial wafer according to the first aspect of the present invention, the growth of hydrogen by the MOVPE method in an atmosphere containing hydrogen causes the hydrogen to occupy the total flow rate of the inflowing gas that is the sum of the source gas and the carrier gas. The ratio is 8% or more.

  According to a third aspect of the present invention, in the semiconductor epitaxial wafer according to the first or second aspect, the solid crystal substrate is sapphire or silicon carbide (SiC).

  A semiconductor device according to a fourth aspect of the present invention is cut out from the semiconductor epitaxial wafer according to any one of the first to third aspects.

  The semiconductor epitaxial wafer of the present invention is a semiconductor epitaxial wafer having a diameter of 70.0 mm or more, comprising a solid crystal substrate and a gallium nitride compound semiconductor film grown on the surface of the solid crystal substrate by MOVPE. The gallium compound semiconductor film is grown by the MOVPE method in an atmosphere containing hydrogen, and the solid crystal substrate having a thickness of 380 microns or more is used. Therefore, according to the present invention, it is possible to improve the uniformity of the thickness, doping concentration, and group III atomic composition ratio of the gallium nitride compound semiconductor film in the wafer surface of the semiconductor epitaxial wafer.

  The present invention will be described based on the following specific prototype examples.

[Comparative example]
A layer structure (comparative example) of a typical epitaxial wafer for GaN-HEMT is shown in FIG.

  In FIG. 3, 101 is a sapphire substrate (3 inch (76.2 mm) diameter, 330 microns thick), 102 is an InGaN buffer layer, 103 is an un-GaN layer (undoped GaN layer), and 104 is an un-AlGaN layer (undoped). AlGaN layer). We grow such a layer structure in a MOVPE apparatus. The MOVPE apparatus used for growth has the ability to charge six 3 inch substrates, and the stage on which the epitaxial wafer is set is revolved by a driving mechanism. Inside the reactor of the MOVPE apparatus, a large number of shower-like fine holes are provided on the upper facing surface of the epitaxial wafer, and the carrier gas containing the raw material is blown out from the shower-like holes. The stage on which the epitaxial wafer inside the reactor is set is heated by a resistance heating method, and this heat is applied to the sapphire substrate through the stage.

  In the growth process, a sapphire substrate is carried into the reactor internal stage of the MOVPE apparatus, and first heated to 1200 ° C. in a hydrogen atmosphere, and surface treatment called so-called thermal cleaning is performed for 10 minutes. Thereafter, the substrate temperature is lowered to 500 ° C. by controlling the heater power. When the substrate temperature is stabilized at 500 ° C., trimethylgallium, trimethylindium and ammonia gas are supplied to the reactor using hydrogen as a carrier gas. By such a method, the InGaN buffer layer (nucleation layer) 102 can be formed on the sapphire substrate 101. Specifically, the thickness of the InGaN buffer layer 102 is desirably about 20 nm, but actually, even if the thickness is about 1 nm to 50 nm, it functions as a buffer layer (nucleation layer).

When the growth of the InGaN buffer layer 102 is finished, the substrate temperature is raised to 1090 ° C. When the substrate temperature is stabilized at 1090 ° C., hydrogen is used as a carrier gas, and trimethyl gallium (TMG) and ammonia gas (NH ₃ ) are in a molar ratio V / III = V / III = about 5,000. More specifically, it is supplied to the reactor at a rate of NH ₃ : 10 SLM, TMG: 8.9 × 10 ⁻⁵ mol / sec. By such a method, an un-GaN layer 103 of about 2 mm is formed on the sapphire substrate 101 via the buffer layer (nucleation layer) 102.

  After the growth of the un-GaN layer 103 is completed, trimethylaluminum, trimethylgallium, and ammonia are supplied to the reactor using hydrogen as a carrier gas. The un-AlGaN layer 104 is formed by such a method. The thickness of the un-AlGaN layer 104 needs to be changed depending on the intended device characteristics, but in practice, it is generally 25 nm to 45 nm.

  When AlGaN is coherently formed on GaN, a two-dimensional electron gas is induced on the GaN side of the AlGaN / GaN heterointerface due to spontaneous polarization and piezoelectric polarization of AlGaN. Since this two-dimensional electron gas travels in the undoped, ie, GaN layer into which no impurities are introduced, it has the advantage that it has high electron mobility without being affected by ionized impurity scattering. A field effect transistor structure like this structure is called a HEMT (High Electron Mobility Transistor), and in this case, GaN is used as a material, so that it becomes a GaN-HEMT.

The concentration of the two-dimensional electron gas is mainly determined by the thickness of the un-AlGaN layer 104 and the aluminum composition ratio in the AlGaN. In a typical structure, the sheet carrier is about 1.0 × 10 ¹³ cm ⁻² . Concentration. The electron mobility in the GaN-HEMT structure largely depends on the density of dislocations contained in the crystal, but a typical value is about 1,200 cm ² V ⁻¹ s ⁻¹ . Therefore, the sheet resistance value of a normal GaN-HEMT epitaxial wafer is about 500 Ω / sq. Before and after.

  The in-plane distribution of the sheet resistance of the epitaxial wafer grown by the above method can be measured in a non-destructive and non-contact manner by a method using eddy current. FIG. 4 shows LEHIGHTON ELECTRONICS INC. The sheet resistance in-plane distribution of the 3-inch (76.2 mm) diameter epitaxial wafer obtained by a non-contact sheet resistance measuring apparatus manufactured by the company is shown.

  In this measured value, the average value is 534.5Ω / sq. However, the minimum value is 451.4Ω / sq. The maximum value is 680.8 Ω / sq. And the standard deviation is 78.11 Ω / sq. In addition, in-plane sheet resistance variation is very large, which is not industrially preferable.

  As described above in the background of the sheet resistance variation in the surface of the sheet resistance, as described in the above-mentioned section “Problems to be solved by the invention”, the warpage of the epitaxial wafer and the epitaxial wafer surface resulting therefrom The authors thought that there was an internal substrate temperature distribution. That is, a thin sapphire substrate having a thickness of 330 microns in a hydrogen carrier and at a high temperature exceeding 1,000 ° C. is extremely warped. When the epitaxial wafer is warped, it is not completely in contact with the substrate holder except in the vicinity of the center of the substrate, and a gap is formed between the back surface of the substrate and the substrate holder depending on the distance from the center of the substrate. Then, the amount of heat transport from the substrate holder to the substrate is large at the center of the substrate and is small at the outer periphery. For this reason, the central portion of the substrate is strongly heated to a high temperature, while the outer peripheral portion of the substrate is a low temperature. Then, the Al concentration in the AlGaN is low because the GaN component is difficult to evaporate at the low temperature part on the same substrate, while the Al concentration in the AlGaN is high because the evaporation of GaN is promoted at the high temperature part. turn into. As a result, it is considered that the sheet resistance value of the epitaxial wafer for GaN-HEMT affected by the film thickness of AlGaN and the aluminum composition is low at the central part and high at the outer peripheral part.

  Such a phenomenon becomes more prominent as the diameter of the epitaxial wafer is larger.

[Example]
The method invented by the present inventors in order to avoid the inconvenience as described above uses a thicker sapphire substrate than before. More specifically, when a gallium nitride compound semiconductor film is grown by an MOVPE method in an atmosphere containing hydrogen using a sapphire or SiC substrate having a diameter of 70.0 mm or more, it has a thickness of 380 microns or more. Choose what you have.

  Specific examples (examples) will be described below.

  The layer structure (Example) of the epitaxial wafer for GaN-HEMT improved by us is shown in FIG.

  In FIG. 1, 301 is a sapphire substrate (3 inch (76.2 mm) diameter, 430 microns thick), 302 is an InGaN buffer layer, 303 is an un-GaN layer, and 304 is an un-AlGaN layer. The thickness of the InGaN buffer layer 302, the un-GaN layer 303, and the un-AlGaN layer 304 is about the same as that in FIG. 3, but is the only sapphire substrate (3 inch (76.2 mm) diameter, thickness 430 microns). Only the thickness of 301 is greatly increased. Such an epitaxial wafer is formed by the MOVPE method in the same manner as described above, like the epitaxial wafer shown in FIG.

  FIG. 2 shows in-plane distribution measurement values of the sheet resistance of the GaN-HEMT epitaxial wafer according to this example. In this measured value, the average value is 527.8 Ω / sq. However, the minimum value is 521.1 Ω / sq. The maximum value is 538.4 Ω / sq. And the standard deviation is 4.79 Ω / sq. Therefore, it can be said that the in-plane sheet resistance variation is very small, which is industrially advantageous.

  The above-mentioned good result is that since a sufficiently thick substrate of 430 microns is used, the amount of elastic warpage of the epitaxial wafer is reduced even with respect to the heat distribution in the vertical direction. This is probably because no gap was generated between the holding tables, and the epitaxial wafer surface was kept in a thermally extremely homogeneous state. In the present invention, utilizing this natural phenomenon, the thickness of the gallium nitride compound semiconductor film, the doping concentration, and the uniformity of the group III atomic composition ratio within the epitaxial wafer surface have been successfully improved.

It is the figure which showed the cross-section of the epitaxial wafer for GaN-HEMTs using the sapphire substrate of 430 micron thickness obtained by this invention. It is the figure which showed the result of the sheet resistance distribution measurement of the epitaxial wafer for GaN-HEMT by this invention. It is the figure which showed the cross-section of the epitaxial wafer for GaN-HEMT using the 330-micron-thick sapphire substrate by a comparative example. It is the figure which showed the result of the sheet resistance distribution measurement of the epitaxial wafer for GaN-HEMT of FIG. 3 which concerns on a comparative example.

Explanation of symbols

301 Sapphire substrate (3 inch (76.2 mm) diameter, thickness 430 microns)
302 InGaN buffer layer 303 un-GaN layer 304 un-AlGaN layer

Claims (4)

In a semiconductor epitaxial wafer having a diameter of 70.0 mm or more, comprising a solid crystal substrate and a gallium nitride compound semiconductor film grown on the surface of the solid crystal substrate by MOVPE,
Growth of the gallium nitride compound semiconductor film by MOVPE is performed in an atmosphere containing hydrogen, and the solid crystal substrate having a thickness of 380 microns or more is used. Epitaxial wafer. The semiconductor epitaxial wafer according to claim 1,
A semiconductor epitaxial wafer characterized in that a proportion of hydrogen in a total flow rate of an inflowing gas, which is a sum of a source gas and a carrier gas, is 8% or more by growth by the MOVPE method in an atmosphere containing hydrogen. In the semiconductor epitaxial wafer according to claim 1 or 2,
A semiconductor epitaxial wafer, wherein the solid crystal substrate is sapphire or silicon carbide.   A semiconductor element cut out from the semiconductor epitaxial wafer according to claim 1.

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