JP5711001B2 - Manufacturing method of nitride semiconductor device - Google Patents

Description

The present invention relates to a method for manufacturing a nitride semiconductor equipment, in particular to a method for manufacturing a nitride semiconductor equipment provided with a high-resistance buffer layer of gallium nitride.

  FIG. 8 is a cross-sectional view of a high electron mobility transistor (HEMT) which is an example of a nitride semiconductor device. As shown in FIG. 8, a low-temperature deposition layer 12 made of gallium nitride (GaN) and a buffer layer 13 made of GaN are sequentially stacked on a sapphire substrate 11, and an electron transit layer 14 made of GaN, aluminum gallium nitride ( The heterostructure has an electron supply layer 15 made of AlGaN. A two-dimensional electron gas is formed immediately below the heterointerface. A source electrode 16, a drain electrode 17, and a gate electrode 18 are formed on the electron supply layer 15.

  In this type of nitride semiconductor device, it is necessary to increase the resistance of the buffer layer 13 in order to suppress the leakage current of the buffer layer 13. In general, since GaN has n-type conductivity without doping, a p-type impurity doping method is employed for high resistance. For example, Patent Document 1 discloses a technique for increasing the resistance by doping carbon in a buffer layer made of GaN. In Patent Document 1, the growth rate is adjusted so that the carbon concentration falls within a predetermined range, and p-type impurities are doped to increase the resistance of the GaN layer.

JP 2007-251144 A

As disclosed in Patent Document 1, a high-resistance buffer layer can be formed by doping carbon and compensating carriers. However, since group IV carbon is an amphoteric impurity in GaN, it can be either a donor impurity or an acceptor impurity. Therefore, there is a problem that a high-resistance buffer layer cannot be formed even if GaN is doped only with carbon. The present invention aims to provide a nitride semiconductor equipment manufacturing method which includes a high-resistance buffer layer which can be formed by a simple method.

The invention according to claim 1 of the present invention provides a method for manufacturing a nitride semiconductor device including a step of stacking a nitride semiconductor layer on a substrate via a buffer layer made of gallium nitride, and supplying organometallic gallium as a source gas. However, the epitaxial growth temperature is set to 850 ° C. to 1000 ° C., and the supply amount of the doping gas of the n-type impurity is controlled, so that the n-type impurity is doped and the Fermi level moves to the conduction band side. The method includes a step of epitaxially growing a gallium nitride layer having a desired resistivity by being doped with carbon caused by being doped at a nitrogen position or an interstitial position as a buffer layer .

Since the nitride semiconductor device according to the method for manufacturing a nitride semiconductor device of the present invention is doped with carbon during the epitaxial growth of the buffer layer made of n-type GaN, the doped carbon is not only a nitrogen position but also a lattice. The probability of occupying the interposition is high. As a result, carriers are effectively compensated and a high-resistance buffer layer can be formed.

  The method for manufacturing a nitride semiconductor device of the present invention can be easily achieved by setting the growth temperature in the range of 850 ° C. to 1000 ° C. and adjusting the supply amount of the doping gas to a desired range by a normal MOVPE method. A resistive buffer layer can be formed, which is very convenient.

It is a figure explaining the principle of this invention. It is a figure explaining the characteristic of the buffer layer of this invention. It is a graph illustrating the relationship between SiH ₄ flow rate and resistivity. SiH ₄ is a graph illustrating the relationship between flow rate and Si concentration and the carrier concentration. It is a figure explaining that the buffer layer of this invention is thermally stable. It is a figure explaining that the buffer layer of this invention is thermally stable. It is a figure explaining the Id-Vg characteristic of HEMT provided with the buffer layer of this invention. It is sectional drawing of HEMT which is an example of the nitride semiconductor device.

  First, the principle of the present invention will be described. When carbon is doped in GaN, carbon that is a group IV element becomes an amphoteric impurity and can be either a donor type or an acceptor type. When doped carbon occupies a gallium position, a nitrogen position, or an interstitial position, the formation energy and the charge state depend on the Fermi level, so the ease of formation differs depending on the position of the Fermi level. Fig. 1 shows the results of Seager et al.'S first-principles calculations to determine the Fermi level dependence of the formation energy and charge state of carbon occupying each position (CH Seager et al, J. Appl. Phys., 92 , 2002, p6553).

In FIG. 1, the position of the Fermi level 0.0 eV at the left end represents the valence band (VB) upper end, and the position of the right end Fermi level 3.5 eV represents the lower end of the conduction band (CB). For example, carbon occupying a nitrogen position (shown as C _N ) decreases in formation energy as it approaches the conduction band side, and the charged state becomes 1−. Similarly, carbon occupying the interstitial position (shown as C _i ) decreases in formation energy as it approaches the valence band or conduction band, and the charged state changes to 4+ near the valence band and 2− near the conduction band. To do. Further, carbon occupying the gallium position (denoted as C _Ga ) indicates that the formation energy decreases as the valence band approaches, and the charged state becomes 1+.

  In the present invention, the position of the Fermi level is moved to the conduction band side so that carbon occupies the nitrogen position or the interstitial position. That is, by doping the n-type GaN with carbon, the resistance of the epitaxial layer is effectively increased by the carbon having a charged state of 1- or 2-.

  Such control of the state can be realized by adjusting the growth conditions of epitaxial growth. Hereinafter, the manufacturing process will be described in detail.

In order to describe in detail the characteristics of the buffer layer, which is the main configuration of the present invention, a case where a semiconductor substrate having the structure shown in FIG. 2 is formed will be described as an example. As shown in FIG. 2, a Si-doped GaN layer 23 (corresponding to the buffer layer of the present invention) is formed on a sapphire substrate 21 via a low-temperature deposition film 22 made of GaN. Specifically, a low-temperature deposited film 22 having a thickness of 25 nm is grown on the sapphire substrate 21 by MOVPE at a growth temperature of 500 ° C. using trimethyl gallium (TMGa) and ammonia (NH ₃ ) as source gases. Subsequently, a Si-doped GaN layer 23 having a thickness of 700 nm is grown on the low-temperature deposited film 22 by adjusting the growth temperature and the supply amount of the doping gas.

First, it is shown that the resistivity changes depending on the epitaxial growth temperature and the supply amount of the doping gas. In the following description, the supply amount of the doping gas is “SiH ₄ ”, which is a gas obtained by diluting SiH ₄ gas to 10 ppm with nitrogen gas as a dilution gas. FIG. 3 shows SiH used as a doping gas when the growth temperature of the Si-doped GaN layer 23 is 950 ° C. (indicated by a circle) and when the normal epitaxial growth temperature is 1125 ° C. (indicated by a triangle). _{4 The} result of changing the flow rate is shown. As shown in FIG. 3, when the epitaxial growth temperature was set to 950 ° C., it was confirmed that the Si-doped GaN layer 23 having a high resistivity could be formed as compared with the case of growing at 1125 ° C. In particular, when the epitaxial growth temperature is 950 ° C. and the SiH ₄ flow rate is 1 sccm or more and 3 sccm or less, the resistivity is in the order of 10 ⁴ Ω · cm to 10 ⁵ Ω · cm, and it is used as a buffer layer when forming a nitride semiconductor device. It was confirmed that this was suitable. The epitaxial growth temperature is not limited to 950 ° C., and similar results can be obtained in the range of 850 ° C. to 1000 ° C. Here, if the epitaxial growth temperature is less than 850 ° C., good epitaxial growth cannot be performed, and if it exceeds 1000 ° C., it has been confirmed that it becomes difficult to capture carbon and the effects of the present invention cannot be obtained. It is necessary to set the temperature range.

Next, it is shown that the carrier concentration varies depending on the supply amount of the doping gas. FIG. 4 shows the measurement results of the carrier concentration when the epitaxial growth temperature is 950 ° C. and the SiH ₄ flow rate is changed. For comparison, the Si concentration by SIMS analysis is indicated by ◯. As shown in FIG. 4, when the flow rate of SiH ₄ used as a doping gas is 10 sccm, the carrier concentration decreases rapidly, and when it is 3 sccm or less, it is below the measurement limit. Further, the inert Si concentration calculated from the Si concentration and the carrier concentration by SIMS analysis was higher than the carbon concentration. Therefore, by doping Si, the carrier concentration was compensated more than the carbon concentration. This shows that carbon not only occupied the nitrogen position but also occupied the interstitial position, compensated for carriers, and formed a high-resistance GaN layer. This result was not limited to the epitaxial growth temperature of 950 ° C., and similar results could be obtained in the range of 850 ° C. to 1000 ° C.

  The carbon introduced into the epitaxial growth layer is likely to be taken in from the methyl group contained in the organic metal gallium as a raw material by lowering the growth temperature, and thus is caused by the epitaxial growth raw material gas. In this way, the method of introducing from the source gas makes it possible to dope n-type impurities and take in carbon.

Next, although the buffer layer of the present invention is grown at a temperature lower than the normal epitaxial growth temperature, it will be described that it is thermally stable in the temperature range used as a buffer layer of a normal nitride semiconductor device. FIG. 5 shows the change in carrier concentration after the buffer layer of the present invention epitaxially grown at 950 ° C. (SiH ₄ flow rate = 10 sccm: indicated by the mark ●) is heat-treated at a temperature of 1125 ° C. for a predetermined time. FIG. 6 shows the change in carrier concentration after heat treatment for 300 seconds with a heat treatment time of 300 seconds. For comparison, the case where the SiH ₄ flow rate is 20 sccm and 100 sccm is also shown. 5 and 6, it was confirmed that even when the heat treatment time and the heat treatment temperature were changed, the carrier concentration did not change and was thermally stable. The buffer layer of the present invention includes a case where the buffer layer is grown at a flow rate lower than the SiH ₄ flow rate shown in FIGS. 5 and 6, but it has been confirmed that the buffer layer is thermally stable even in a change in resistivity. .

  As described above, in the present invention, it was confirmed that a thermally stable high-resistance buffer layer can be formed by adjusting the epitaxial growth temperature and the flow rate of the doping gas. Note that the optimum values of the epitaxial growth temperature and the flow rate of the doping gas vary depending on other epitaxial growth conditions, and are thus set as appropriate. Hereinafter, examples of the present invention will be described according to the manufacturing process of the nitride semiconductor device.

Examples of the present invention will be described by taking an AlGaN / GaN HEMT as an example. As in the conventional example shown in FIG. 8, a low-temperature deposition layer 12 made of GaN having a thickness of 25 nm is formed on a sapphire substrate 11 at an epitaxial growth temperature of 500 ° C., and then a buffer layer 13 made of GaN having a thickness of 700 nm is sequentially stacked. . Here, the buffer layer 13 is epitaxially grown to a predetermined thickness at 950 ° C. while supplying a gas obtained by mixing SiH ₄ with nitrogen as a diluent gas as a doping gas at a flow rate of 1 sccm. Thereafter, the substrate temperature is raised to 1125 ° C., the electron transit layer 14 made of non-doped GaN is grown to a thickness of 300 nm, the substrate temperature is lowered to 1050 ° C., and the electron supply layer 15 made of AlGaN is grown to a thickness of 25 nm. After forming an ohmic electrode (titanium / aluminum / titanium / gold laminated film) on the electron supply layer 15 by vapor deposition, heat treatment is performed at 850 ° C. to form the source electrode 16 and the drain electrode 17 on the electron supply layer 15. To do. Furthermore, a gate electrode 18 is formed by vapor-depositing a Schottky electrode (nickel / gold laminated film) between the source electrode 16 and the drain electrode 17.

The measurement result of the Id-Vg characteristic of the HEMT formed in this way is shown in FIG. For comparison, the case where the SiH ₄ flow rate is changed is also shown. As shown in FIG. 7, when the SiH ₄ flow rate is 1 sccm, the drain current (leakage current) is 10 ⁻⁴ mA / mm, and the pinch-off characteristics are improved by two orders of magnitude compared to the SiH ₄ flow rate of 0 sccm. On the other hand, when the SiH ₄ flow rate is 10 sccm and 100 sccm, as described with reference to FIG. 3, the resistivity of the buffer layer 13 decreases, so that the leakage current flowing through the buffer layer 13 increases and the pinch-off operation is not performed.

  As described above, the HEMT has been described as an example. However, depending on the semiconductor device formed on the nitride semiconductor layer formed on the buffer layer 13, the allowable value of the leakage current varies, so that a condition for achieving a desired resistivity is set. The present invention can be applied.

  Moreover, although the case where silicon is introduced as an n-type impurity has been described, not only silicon but also other n-type impurities such as tin, sulfur, selenium can be used as a dopant.

11, 21: Sapphire substrate, 12, 22: Low temperature deposited film, 13: Buffer layer, 14: Electron travel layer, 15: Electron supply layer, 16: Source electrode, 17: Drain electrode, 18: Gate electrode, 23: Si Doped GaN layer

Claims (1)

In a method for manufacturing a nitride semiconductor device including a step of stacking a nitride semiconductor layer on a substrate via a buffer layer made of gallium nitride,
While supplying the organometallic gallium as the source gas, the epitaxial growth temperature is set to 850 ° C. to 1000 ° C., and the supply amount of the doping gas of the n-type impurity is controlled, so that the n-type impurity is doped and the Fermi level position is on the conduction band side. And a step of epitaxially growing, as a buffer layer, a gallium nitride layer having a desired resistivity by being moved and doped with carbon caused by the organometallic gallium and disposed at a nitrogen position or an interstitial position. A method for manufacturing a nitride semiconductor device.

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