JP2021027296A - Nitride semiconductor substrate - Google Patents

Abstract

To provide a technique for increasing a breakdown voltage as to a nitride semiconductor substrate arranged by use of a silicon substrate.SOLUTION: A nitride semiconductor substrate comprises: a silicon substrate; and a nitride semiconductor layer laminated on the silicon substrate. The nitride semiconductor layer includes a first layer formed by AlxGa1-xN(0.9≤x≤1), an AlGaN-containing second layer having an Al composition smaller than an Al composition x of the first layer, and a third layer arranged by laminating a plurality of nitride semiconductor crystal layers different in composition in order from a silicon substrate side. An oxygen density in the second layer is lower than an oxygen density in the third layer.SELECTED DRAWING: Figure 5

Description

The present invention relates to a nitride semiconductor substrate.

As a semiconductor substrate, there is a nitride semiconductor substrate in which a nitride semiconductor is epitaxially grown on a base substrate. The nitride semiconductor is a wide bandgap semiconductor and has excellent dielectric breakdown strength. Therefore, application of nitride semiconductors to semiconductor devices that require high withstand voltage is being studied. As a base substrate for epitaxially growing a nitride semiconductor, it has been studied to use a silicon substrate from the viewpoint of increasing the diameter (see, for example, Patent Document 1).

JP-A-2018-88501

When the nitride semiconductor is epitaxially grown on a silicon substrate, a laminated structure such as a reaction suppressing layer, an intermediate layer, and a stress generating layer is formed as described later. According to the knowledge obtained by the inventor of the present application, the oxygen concentration in the laminated structure has a large influence on the withstand voltage of the nitride semiconductor substrate using the silicon substrate, and therefore it is preferable that the oxygen concentration is controlled so as to improve the withstand voltage. ..

An object of the present invention is to provide a technique for improving the withstand voltage of a nitride semiconductor substrate using a silicon substrate.

According to one aspect of the invention
A nitride semiconductor substrate in which a nitride semiconductor layer is laminated on a silicon substrate.
The nitride semiconductor layer is formed in order from the silicon substrate side.
The first layer composed of Al _x Ga _1-x N (0.9 ≦ x ≦ 1) and
The second layer containing AlGaN having an Al composition smaller than the Al composition x of the first layer, and
A third layer formed by laminating a plurality of nitride semiconductor crystal layers having different compositions is provided.
Provided is a nitride semiconductor substrate in which the oxygen concentration in the second layer is lower than the oxygen concentration in the third layer.

A technique for improving the withstand voltage of a nitride semiconductor substrate using a silicon substrate is provided.

FIG. 1 is a schematic cross-sectional view of a nitride semiconductor substrate according to an embodiment of the present invention. FIG. 2 is a schematic view of a vapor phase growth apparatus used for manufacturing a nitride semiconductor substrate according to an embodiment. 3A and 3B are timing charts conceptually showing how to control the growth conditions in the growth process of the reaction suppressing layer, the intermediate layer and the stress generating layer according to the embodiment. 4 (a) and 4 (b) are timing charts conceptually showing how to control the growth conditions in the growth process of the reaction suppressing layer, the intermediate layer and the stress generating layer according to the embodiment. FIG. 5 shows the profile of the nitride semiconductor substrate of the example in the thickness direction of the oxygen concentration measured by secondary ion mass spectrometry. FIG. 6 shows the current-voltage characteristics measured for the nitride semiconductor substrate of the example.

<One Embodiment of the present invention>
Hereinafter, the nitride semiconductor substrate 100 according to the embodiment of the present invention will be described. FIG. 1 is a schematic cross-sectional view of the nitride semiconductor substrate 100. The nitride semiconductor substrate 100 (hereinafter, also referred to as epi substrate 100) is configured by laminating a nitride semiconductor layer 120 (hereinafter, also referred to as epi layer 120) made of a group III nitride semiconductor on a silicon substrate 110. ..

(Silicon substrate)
The silicon (Si) substrate 110 is a base substrate for heteroepitaxially growing the epi layer 120. As the Si substrate 110, for example, one having a large diameter of 150 mm or more can be used.

(Epi layer)
The epi layer 120 includes a reaction suppression layer 130, an intermediate layer 140, a stress generation layer 150, and a device forming layer 160 in this order from the Si substrate 110 side. As will be described later, in the epi substrate 100 according to the present embodiment, the oxygen concentration in the intermediate layer 140 during the lamination of the reaction suppressing layer 130, the intermediate layer 140 and the stress generating layer 150 is suppressed to be low, so that the thickness direction ( The pressure resistance in the vertical direction) has been improved. Hereinafter, the withstand voltage in the thickness direction of the epi substrate 100 is simply referred to as a withstand voltage.

(Reaction suppression layer)
The reaction suppression layer 130 is a layer (preferably a layer composed of AlN) provided on the Si substrate 110 and containing AlN (aluminum nitride). The reaction suppressing layer 130 suppresses the reaction (alloying) between Si contained in the Si substrate 110 and gallium (Ga) contained in the intermediate layer 140 by interposing between the Si substrate 110 and the intermediate layer 140. .. As a material constituting the reaction suppression layer 130, Al _x Ga _1-x N (0.9 ≦ x ≦ 1) may be used. _{Further, a layer made of SiN y} (1 ≦ y ≦ 2) may be sandwiched between the reaction suppression layer 130 and the Si substrate 110. The reaction suppression layer 130 also has a function of forming an initial nucleus of a nitride semiconductor layer grown above the reaction suppression layer 130. The thickness of the reaction suppression layer 130 is, for example, 30 nm or more and 300 nm or less.

(Middle layer)
The intermediate layer 140 is provided on the reaction suppression layer 130 and is a layer containing AlGaN (aluminum gallium nitride) (a layer composed of AlGaN containing Al and Ga as group III elements). The intermediate layer 140 expands the initial nuclei formed in the reaction suppression layer 130 and has a flat upper surface than the upper surface of the reaction suppression layer 130, so that the intermediate layer 140 is under the stress generating layer 150 grown above the intermediate layer 140. Make up the ground. The Al composition of the intermediate layer 140 is selected to be smaller than the Al composition of the reaction suppression layer 130. The thickness of the intermediate layer 140 is, for example, 40 nm or more and 1000 nm or less. The intermediate layer 140 preferably grows coherently with respect to the reaction suppression layer 130, but it is not essential that the intermediate layer 140 grows completely coherently with respect to the reaction suppression layer 130, and lattice relaxation may occur.

(Stress generation layer)
The stress generating layer 150 is located between the silicon (Si) substrate 110 and the device forming layer 160, and is a layer that generates compressive stress for reducing the warping of the entire epi substrate 100. The stress generation layer 150 is formed by repeatedly stacking a plurality of nitride semiconductor crystal layers having different AlGaN compositions. By repeatedly laminating nitride semiconductor crystal layers having different AlGaN compositions, compressive stress can be generated in the layer formed above the stress generating layer 150, and the warpage of the entire epi substrate 100 can be reduced. it can.

As shown in FIG. 1, for example, the stress generation layer 150 is configured by alternately laminating first and second nitride semiconductor crystal layers 151 and 152 having different AlGaN compositions, and repeatedly laminating multiple crystal layers 153. To. The first nitride semiconductor crystal layer 151 is formed from, for example, a group III nitride crystal having a lattice constant of a1 in a bulk crystal. The second nitride semiconductor crystal layer 152 is formed from, for example, a group III nitride crystal whose lattice constant in the bulk crystal is a2 (a1 <a2) larger than that of the first nitride semiconductor crystal layer. The AlGaN compositions of the first and second nitride semiconductor crystal layers 151 and 152 are not particularly limited as long as the relationship of the lattice constants is a1 <a2. For example, the AlGaN composition of the first nitride semiconductor crystal layer 151 _{is preferably Al q} Ga _1-q N (0.9 ≦ q ≦ 1). Further, the second nitride semiconductor crystal layer 152 has a higher ga than the first nitride semiconductor crystal layer 151 from the viewpoint of making the lattice constant a2 larger than the lattice constant a1 of the first nitride semiconductor crystal layer 151. The ratio is preferably high, and for example, the AlGaN composition thereof is preferably Al _p Ga _1-p N (0 ≦ p ≦ 0.3).

In the stress generation layer 150, the thicknesses of the first and second nitride semiconductor crystal layers 151 and 152 and the number of repetitions of the multiple crystal layer 153 are not particularly limited, and the device forming layer formed on the stress generation layer 150. It may be appropriately changed according to the thickness and composition of 160. For example, the thickness of the first nitride semiconductor crystal layer 151 is preferably 1 nm or more and 20 nm or less, and more preferably 5.0 nm or more and 20 nm or less. The thickness of the second nitride semiconductor crystal layer 152 is preferably 5 nm or more and 300 nm or less, and more preferably 10 nm or more and 300 nm or less. The number of layers of the multilayer crystal layer 153 is, for example, 2 to 500.

The stress generating layer 150 does not have to be a structure in which a plurality of multiple crystal structures 153 are laminated, and may be a single layer. Further, for example, the stress generating layer 150 is a stack of three layers in addition to the first and second nitride semiconductor crystal layers and a third nitride semiconductor crystal layer having a lattice constant of a3 (a2 <a3) in the bulk crystal. It may be a structure. Further, for example, the stress generation layer 150 may be composed of a graded crystal layer in which the lattice constant in the bulk crystal increases continuously or stepwise as the distance from the Si substrate 110 increases (as it goes upward).

(Device cambium)
The device forming layer 160 is provided on the stress generating layer 150 and is a layer on which a device structure such as a transistor or an LED is formed. For example, the device forming layer 160 used for manufacturing a high electron mobility transistor (HEMT) includes an active layer 161 and a Schottky layer 162. The active layer 161 is, for example, an Al _a Ga _1-a N (0 ≦ a <1) layer, preferably a GaN layer. The thickness may be, for example, 100 nm or more and 2000 nm or less. The Schottky layer 162 is, for example, an Al _b Ga _1-b N (0 <b ≦ 1, a <b) layer, preferably an Al _b Ga _1-b N (0.1 <b ≦ 0.3) layer. is there. The thickness may be, for example, 10 nm or more and 100 nm or less.

(Oxygen concentration in the epi layer)
In the growth process, the epi layer 120 is mixed with oxygen or the like as impurities in the raw material or derived from the inner wall of the crystal growth apparatus. As described below, the inventor of the present application improves the withstand voltage of the epi-board 100 by suppressing the oxygen concentration in the intermediate layer 140 during the lamination of the reaction suppressing layer 130, the intermediate layer 140 and the stress generating layer 150 to a low level. I got the finding that it can be done.

FIG. 5 is an oxygen concentration profile (hereinafter, also referred to as an oxygen concentration profile of the example) in the Si substrate 110, the reaction suppression layer 130, the intermediate layer 140, and the stress generating layer 150 of the epi substrate 100 produced as an example. The oxygen concentration profile of the examples was measured by secondary ion mass spectrometry (SIMS). Note that FIG. 5 shows a profile in which fine irregularities such as noise are leveled. Hereinafter, the oxygen concentration conditions preferable for improving the pressure resistance of the epi substrate 100 according to the present embodiment will be described while exemplifying the oxygen concentration profile of the examples.

The oxygen concentration in each of the reaction suppressing layer 130, the intermediate layer 140, and the stress generating layer 150 is defined as follows. The oxygen concentration in each layer is located in the center of each layer in the thickness direction, and is the thickness in the portion having a thickness of 10% of the thickness of each layer (the portion having a thickness of 5% above and below the center). It is defined by the oxygen concentration averaged in the direction.

As can be seen from FIG. 5, in the oxygen concentration profile of the example, the dent in the intermediate layer 140 is characteristically observed. On the other hand, in the conventional oxygen concentration profile, the oxygen concentration in each layer decreases monotonically from the reaction suppressing layer 130 toward the stress generating layer 150, and the dent in the intermediate layer 140, that is, the stress generating layer 150 The feature that the oxygen concentration in the intermediate layer 140 was lower than the oxygen concentration in the intermediate layer 140 was not observed.

Therefore, a feature of the oxygen concentration of the epi layer 120 of the present embodiment is that the oxygen concentration in the intermediate layer 140 is lower than the oxygen concentration in the stress generating layer 150. Further, the oxygen concentration in the intermediate layer 140 is lower than the oxygen concentration in the reaction suppression layer 130. The oxygen concentration in the stress generating layer 150 is lower than the oxygen concentration in the reaction suppressing layer 130. That is, the oxygen concentration in the stress generating layer 150 is higher than the oxygen concentration in the intermediate layer 140, and the oxygen concentration in the reaction suppressing layer 130 is higher than the oxygen concentration in the stress generating layer 150.

The withstand voltage of the epi substrate 100 (hereinafter, also referred to as the conventional epi substrate 100) provided with the epi layer 120 having the conventional oxygen concentration profile was about 840 V at the maximum. On the other hand, the withstand voltage of the epi substrate 100 having the epi layer 120 having the oxygen concentration profile of the example (hereinafter, also referred to as the epi substrate 100 of the embodiment) was improved to about 880 V. Further, in the conventional epi-board 100, the withstand voltage varies widely among the epi-boards 100, and some show a low withstand voltage of 200 V or less, whereas in the epi-board 100 of the example (the epi-board 100 of the example). It was also found that, in the plurality of epi-boards 100 having the same oxygen concentration profile as above), the variation in withstand voltage for each epi-board 100 is suppressed, and it is difficult to generate such a low withstand voltage. The measurement was performed at room temperature.

FIG. 6 shows the current-voltage characteristics of the epi-board 100 produced as an example. The horizontal axis of FIG. 6 is a voltage expressed in V units. The vertical axis of FIG. 6 is the current density expressed in units of ^{A / mm 2.} The applied voltage when the device forming layer 160 side applies a positive DC voltage between the device forming layer 160 and the Si substrate 110 of the epi substrate 100 and the current density reaches 1 × 10 ^-6 A / mm ^2. , Withstand voltage. In the example shown in FIG. 6, a high withstand voltage of about 880 V is obtained.

Although the mechanism by which the effect of improving the withstand voltage was obtained in the epi substrate 100 of the embodiment as compared with the conventional epi substrate 100 is not clear, the concentration profile by SIMS of various impurities is improved with the conventional epi substrate 100. As a remarkable difference, the above-mentioned difference was found in relation to the oxygen concentration profile when compared with the epi-board 100 (of the example) in which the above was observed. That is, in the epi substrate 100 of the example, a difference was found in that the oxygen concentration in the intermediate layer 140 was lower than the oxygen concentration in the stress generating layer 150. Therefore, it is considered that making the oxygen concentration in the intermediate layer 140 lower than the oxygen concentration in the stress generating layer 150 is a preferable feature for improving the withstand voltage of the epi substrate 100.

Further, the conditions of the oxygen concentration preferable for improving the withstand voltage of the epi substrate 100 will be described. The guideline for the ratio of the oxygen concentration in the stress generating layer 150 to the oxygen concentration in the intermediate layer 140 is, for example, more than 1 time (more preferably more than 1.5 times) and 10 times or less. Further, the guideline of the ratio of the oxygen concentration in the reaction suppressing layer 130 to the oxygen concentration in the intermediate layer 140 is, for example, more than 5 times and 100 times or less.

The guideline for the upper limit of the oxygen concentration in the intermediate layer 140 is, for example, 1 × 10 ¹⁸ cm ^-3 or less. The guideline for the upper limit of the oxygen concentration in the stress generation layer 150 is, for example, 5 × 10 ¹⁸ cm ^-3 or less. The guideline for the upper limit of the oxygen concentration in the reaction suppression layer 130 is, for example, 5 × 10 ¹⁹ cm ^-3 or less. The lower limit of the oxygen concentration in the intermediate layer 140, the oxygen concentration in the stress generating layer 150, and the oxygen concentration in the reaction suppressing layer 130 is not particularly limited (as long as the above-mentioned relationship is satisfied).

The above-mentioned regulation of oxygen concentration in each layer can be regarded as the average oxygen concentration in the central portion in the thickness direction of each layer. The reaction suppression layer 130 has a tendency that the oxygen concentration (not averaged) decreases from the Si substrate 110 side toward the intermediate layer 140 side. This tendency is that the oxygen concentration in the reaction suppression layer 130 (the average oxygen concentration in the central portion in the thickness direction) is lower than the oxygen concentration at the interface position of the reaction suppression layer 130 with the Si substrate 110, and the reaction suppression layer 130. It can be defined as a relationship that the oxygen concentration at the interface position of the reaction suppression layer 130 with the intermediate layer 140 is lower than the oxygen concentration (average oxygen concentration in the central portion in the thickness direction).

The oxygen concentration (not averaged) at each depth of the stress generating layer 150 tends to be a substantially constant level, and the oxygen concentration at each depth of the stress generating layer 150 tends to be at any depth. Also, it tends to be higher than the oxygen concentration in the intermediate layer 140 (the average oxygen concentration in the central portion in the thickness direction). Further, the (non-averaged) oxygen concentration at each depth of the reaction suppression layer 130 is higher than the oxygen concentration at the intermediate layer 140 (average oxygen concentration at the central portion in the thickness direction) at any depth. Has a high tendency.

(Manufacturing method of epi substrate)
Next, a method of manufacturing the epi substrate 100 according to the present embodiment will be described. The Si substrate 110 is prepared. Above the Si substrate 110, the reaction suppression layer 130, the intermediate layer 140, the stress generation layer 150, and the device forming layer 160, which are the layers constituting the epi layer 120, are grown by, for example, organic metal vapor phase growth (MOVPE). Then, the epi substrate 100 is formed. As the Ga raw material gas among the group III raw material gases, for example, trimethylgallium (Ga (CH3) ₃ , TMG) gas is used. As the Al source gas among the group III source gases, for example, trimethylaluminum (Al (CH ₃ ) ₃ , TMA) gas is used. As the nitrogen (N) raw material gas which is a group V raw material gas, for example, ammonia (NH ₃ ) is used. As the carrier gas, for example, _{at least one of nitrogen gas (N 2} gas) and hydrogen gas (H ₂ gas) can be used. _{For NH 3} , N ₂ , and H ₂ supplied to the reactor, the concentration of background impurities taken into the epi layer 120 is reduced by sufficiently reducing the impurities (moisture) in the gas using a gas purification device. Can be done. The growth temperature can be selected in the range of 900 ° C. to 1260 ° C., and the V / III ratio, which is the flow rate ratio of the group V raw material gas to the group III raw material gas, can be selected in the range of 10 to 10,000, for example. is there. The ratio of the supply amount of each raw material gas is adjusted according to the composition of each layer to be formed. For the thickness of each layer to be formed, for example, the growth time corresponding to the design thickness can be calculated from the growth rate obtained in the preliminary experiment, and the thickness can be controlled by the growth time.

FIG. 2 is a schematic view of the MOVPE device 200, which is a crystal growth device used in the method for manufacturing the epi substrate 100. The MOVPE apparatus 200 includes a reactor 210, and a susceptor 220 for mounting a Si substrate 110 is arranged in the reactor 210. Above the susceptor 220, a top plate 242 made of quartz (silicon oxide) is supported and fixed so as to face the upper surface of the susceptor 220 at a distance from each other.

The space 250 partitioned between the susceptor 220 and the top plate 242 serves as a flow path for supplying the group III raw material gas Ga1, the group V raw material gas Ga2, and the carrier gas Gb1. The carrier gas supplied together with the group III raw material gas Ga1 and the carrier gas supplied together with the group V raw material gas Ga2 are collectively referred to as a carrier gas Gb1. The space 251 between the top plate 242 and the inner wall of the reactor 210, which is arranged on the opposite side of the top plate 242 to the susceptor 220, serves as a flow path to which the gas Gb2 for cooling the top plate 242 is supplied. As the gas Gb2, for example, _{at least one of N 2} gas and H ₂ gas can be used. The flow rate and composition of each of the gases Ga1, Ga2, Gb1 and Gb2 are adjusted by a flow rate adjusting mechanism controlled by the control device 270. In FIG. 2, gass Ga1, Ga2, Gb1 and Gb2 flow from left to right on the paper.

A susceptor cover 241 made of quartz is placed on the susceptor 220. The susceptor cover 241 has a function of covering and protecting the susceptor 220 so that the susceptor 220 is not contaminated by the raw material gas. The susceptor cover 241 is provided with an opening 231, and the Si substrate 110 is placed in the opening 231. FIG. 2 illustrates a mode in which the Si substrate 110 is placed via the rotation mechanism 230. The rotation mechanism 230 allows the layers 130 to 160 constituting the epi layer 120 to grow while rotating the Si substrate 110.

The susceptor 220 is provided with an outer peripheral side heater 261 and an inner peripheral side heater 262. The outer peripheral side heater 261 can heat the susceptor cover 241 mounted on the susceptor 220. The inner peripheral side heater 262 can heat the Si substrate 110 mounted on the susceptor 220 via the rotation mechanism 230. Each of the outer peripheral side heater 261 and the inner peripheral side heater 262 is controlled by the control device 270 so as to reach a predetermined temperature at a predetermined timing.

In the manufacturing process of the epi-board 100 of the present embodiment, the susceptor cover 241 is placed on the susceptor 220, the top plate 242 is arranged facing the susceptor, and the Si substrate 110 is placed in the opening 231 of the susceptor cover 241. By supplying (at least) the raw material gases Ga1 and Ga2 to the space 250 between the susceptor cover 241 and the top plate 242 while heating the Si substrate in the step of preparing the prepared state, the epi layer is formed on the Si substrate. It has a step of forming 120. In the step of forming the epi layer 120, the reaction suppressing layer 130, the intermediate layer 140, and the stress generating layer 150 are formed in this order from the Si substrate 110 side.

As described above, in the epi substrate 100 according to the present embodiment, the oxygen concentration in the intermediate layer 140 during the lamination of the reaction suppressing layer 130, the intermediate layer 140 and the stress generating layer 150 is suppressed to be low. Hereinafter, a method for suppressing the oxygen concentration of the intermediate layer 140 to be low in the step of forming the epi layer 120 will be described.

According to the study by the inventor of the present application, it has been found that the quartz constituting the susceptor cover 241 and the top plate 242 is one of the factors for generating oxygen mixed in the epi layer 120. Since quartz contains oxygen, when the susceptor cover 241 or top plate 242 made of quartz becomes hot, the quartz is reduced by the atmosphere to generate oxygen, and the amount of generation is temperature-dependent (the higher the temperature, the more oxygen is generated). (Large amount) is known. By reducing the amount of oxygen generated from the susceptor cover 241 or the top plate 242 during the growth of the intermediate layer 140, or by reducing the amount of oxygen generated from the susceptor cover 241 or the top plate 242 mixed into the intermediate layer 140. It was found that the oxygen concentration of the intermediate layer 140 could be reduced.

As another factor for generating oxygen mixed in the epi layer 120, oxygen-containing components such as oxygen and water remaining in the reaction furnace 210 can be mentioned. When impurities (moisture) in the gas supplied to the reactor are sufficiently reduced by using the gas purification apparatus, the oxygen taken into the epi layer 120 is mixed from the oxygen-containing components remaining in the reactor 210. Is dominant, so it is considered that the epilayer 120 gradually decreases from the start of growth. In the conventional oxygen concentration profile, the tendency of the oxygen concentration in each layer to decrease monotonically from the reaction suppressing layer 130 to the stress generating layer 150 is considered to correspond to such a decrease in the amount of oxygen mixed.

When the conventional epi substrate 100 was manufactured, the growth conditions in the growth process of the reaction suppressing layer 130, the intermediate layer 140, and the stress generating layer 150 were kept constant. That is, the amount of oxygen generated from the susceptor cover 241 or the top plate 242, or the amount of oxygen generated from the susceptor cover 241 or the top plate 242, is the amount of the reaction suppressing layer 130, the intermediate layer 140, and the stress generating layer 150. The epi layer 120 was grown under growth conditions that did not change in the growth process. Therefore, in the conventional oxygen concentration profile, the oxygen in each layer is reflected from the reaction suppression layer 130 to the stress generation layer 150, reflecting the change in the amount of oxygen mixed from the oxygen-containing components remaining in the reaction furnace 210. It is probable that the concentration tended to decrease monotonically.

On the other hand, when manufacturing the epi-board 100 according to the present embodiment, the amount of oxygen generated from the susceptor cover 241 or the top plate 242, or the amount of oxygen generated from the susceptor cover 241 or the top plate 242 is mixed. The growth conditions are controlled so as to be reduced in the growth process of the intermediate layer 140. As a result, the oxygen concentration in the intermediate layer 140 can be made lower than the oxygen concentration in the stress generating layer 150.

Hereinafter, with reference to the timing charts (FIGS. 3A to 4B) conceptually showing how to control the growth conditions in the growth process of the reaction suppressing layer 130, the intermediate layer 140 and the stress generating layer 150, more This will be described in detail.

First, a method of reducing the oxygen concentration of the intermediate layer 140 by reducing the amount of oxygen generated from the susceptor cover 241 (per unit time) will be described with reference to the timing chart of FIG. 3A. The first timing chart from the top of FIG. 3A shows the temperature of the susceptor cover 241. In the growth step of the intermediate layer 140, the temperature of the susceptor cover 241 is lowered as compared with the growth step of the reaction suppression layer 130 and the stress generating layer 150. As a result, the amount of oxygen generated from the susceptor cover 241 can be reduced in the growth step of the intermediate layer 140.

The second, third, and fourth timing charts from the top of FIG. 3A exemplify some specific embodiments for lowering the temperature of the susceptor cover 241 in the growth step of the intermediate layer 140. The second, third, and fourth timing charts from the top of FIG. 3A show the temperature of the outer peripheral heater 261 and the ratio of H _{2 gas to N 2 gas in the carrier gas Gb1 (H 2} / N ₂ ratio), respectively. ), And the flow rate of the carrier gas Gb1.

In order to lower the temperature of the susceptor cover 241, in the growth step of the intermediate layer 140, for example, the temperature of the outer peripheral side heater 261 is lowered as compared with the growth step of the reaction suppression layer 130 and the stress generating layer 150, and H in the carrier gas Gb1. _At least one of increasing the 2 / N ₂ ratio and increasing the flow rate of the carrier gas Gb1 is performed.

Since the outer peripheral side heater 261 is provided for heating the susceptor cover 241, the temperature of the susceptor cover 241 can be lowered by lowering the temperature of the outer peripheral side heater 261. Since the H ₂ gas has high heat transfer property _{, increasing the H 2} / N ₂ ratio in the carrier gas Gb1 increases the amount of heat that the carrier gas Gb1 takes from the susceptor cover 241. Therefore, the temperature of the susceptor cover 241 can be lowered. it can. Further, by increasing the flow rate of the carrier gas Gb1, the amount of heat that the carrier gas Gb1 takes from the susceptor cover 241 increases, so that the temperature of the susceptor cover 241 can be lowered.

As described with reference to FIG. 2, in the present embodiment, the susceptor cover 241 is directly mounted on the susceptor 220, and the Si substrate 110 is indirectly mounted on the susceptor 220 via the rotation mechanism 230. The mode in which it is placed is illustrated. Therefore, when the outer peripheral side heater 261 is heated under the same conditions as the inner peripheral side heater 262, the susceptor cover 241 is more likely to be heated than the Si substrate 110, so that the temperature tends to be higher.

Next, with reference to the timing chart of FIG. 3B, a method of reducing the oxygen concentration of the intermediate layer 140 by reducing the amount of oxygen generated from the top plate 242 (per unit time) will be described. .. The first timing chart from the top of FIG. 3B shows the temperature of the top plate 242. In the growth step of the intermediate layer 140, the temperature of the top plate 242 is lowered as compared with the growth step of the reaction suppressing layer 130 and the stress generating layer 150. As a result, the amount of oxygen generated from the top plate 242 can be reduced in the growth step of the intermediate layer 140.

The second and third timing charts from the top of FIG. 3B exemplify some specific embodiments for lowering the temperature of the top plate 242 in the growth step of the intermediate layer 140. Second and third timing chart from the top in FIG. 3 (b), respectively, _H 2 / _{N 2} ratio in the gas Gb2, and show the flow rate of the gas Gb2.

In order to lower the temperature of the top plate 242, in the growth step of the intermediate layer 140, for example, the H ₂ / N ₂ ratio in the gas Gb2 is increased as compared with the growth step of the reaction suppression layer 130 and the stress generating layer 150, and At least one of increasing the flow rate of the gas Gb2 is performed.

_{By increasing the H 2} / N ₂ ratio in the gas Gb2, the amount of heat that the gas Gb1 takes from the top plate 242 increases, so that the temperature of the top plate 242 can be lowered. Further, by increasing the flow rate of the gas Gb2, the amount of heat taken by the gas Gb2 from the top plate 242 increases, so that the temperature of the top plate 242 can be lowered.

Next, referring to the timing charts of FIGS. 4 (a) and 4 (b), the amount of oxygen generated from the susceptor cover 241 or the top plate 242 mixed (incorporated) into the intermediate layer 140 is reduced. A method of reducing the oxygen concentration of the intermediate layer 140 will be described. By this method, the amount of oxygen generated from factors other than the susceptor cover 241 or the top plate 242 can be reduced in the intermediate layer 140.

The first timing chart from the top of FIG. 4A shows the temperature of the Si substrate 110. In the growth step of the intermediate layer 140, the temperature of the Si substrate 110 is raised as compared with the growth step of the reaction suppression layer 130 and the stress generating layer 150. As a result, oxygen (O) can be made difficult to be taken into the intermediate layer 140, so that the amount of oxygen mixed in the intermediate layer 140 can be reduced.

The second timing chart from the top of FIG. 4A shows a mode in which the temperature of the inner peripheral side heater 262 is raised as a specific mode for raising the temperature of the Si substrate 110 in the growth step of the intermediate layer 140. Since the inner peripheral side heater 262 is provided for heating the Si substrate 110, the temperature of the Si substrate 110 can be raised by raising the temperature of the inner peripheral side heater 262.

In the method described with reference to FIG. 3A, the temperature of the susceptor cover 241 may be lowered in the growth step of the intermediate layer 140, and the temperature may be excessively lowered to the temperature of the Si substrate 110. Therefore, the temperature drop of the Si substrate 110 may be suppressed by raising the temperature of the inner peripheral side heater 262.

The timing chart of FIG. 4B shows the growth rate of the reaction suppression layer 130 and the like grown on the Si substrate 110. In the growth step of the intermediate layer 140, the growth rate is increased as compared with the growth step of the reaction suppression layer 130 and the stress generating layer 150. As a result, the proportion of oxygen in the intermediate layer 140 is relatively reduced, so that the amount of oxygen mixed in the intermediate layer 140 can be reduced. The growth rate can be adjusted by the V / III ratio, for example, when the supply amount of the nitrogen (N) raw material gas is constant.

The method of increasing the flow rate of the carrier gas Gb1 in the growth step of the intermediate layer 140 shown in the fourth timing chart from the top of FIG. 3A reduces the gas phase concentration of oxygen generated from the susceptor cover 241. It also has the effect of suppressing the uptake of oxygen at high flow rates. Therefore, this method can also reduce the amount of oxygen mixed in the intermediate layer 140. The same applies to the method of increasing the flow rate of the gas Gb2 in the growth step of the intermediate layer 140 shown in the third timing chart from the top of FIG. 3B.

As described above, any one of the methods illustrated in FIGS. 3A to 4B may be carried out alone, or a plurality of methods may be carried out in combination. Various growth conditions are based on, for example, preliminary experiments so that the oxygen concentration in the intermediate layer 140 during the lamination of the reaction suppressing layer 130, the intermediate layer 140, and the stress generating layer 150 becomes the predetermined conditions as described above. , May be adjusted as appropriate.

In order to obtain the feature that the oxygen concentration in the intermediate layer 140 is lower than the oxygen concentration in the stress generating layer 150, at least in the growing step of the intermediate layer 140, the susceptor cover is compared with the growing step of the stress generating layer 150. It is preferable to control the growth conditions so as to reduce the amount of oxygen generated from the 241 or the top plate 242, or to reduce the amount of oxygen mixed from the susceptor cover 241 or the top plate 242.

As described above, according to the present embodiment, the epi-board 100 with improved withstand voltage can be obtained. As a result, the withstand voltage of the semiconductor device manufactured by using the epi substrate 100 of the present embodiment can be improved, and the quality of the semiconductor device composed of the group III nitride semiconductor and excellent in energy saving effect and the like can be improved. Can be done.

<Preferable Aspect of the Present Invention>
Hereinafter, preferred embodiments of the present invention will be added.

(Appendix 1)
A nitride semiconductor substrate in which a nitride semiconductor layer is laminated on a silicon substrate.
The nitride semiconductor layer is formed in order from the silicon substrate side.
The first layer composed of Al _x Ga _1-x N (0.9 ≦ x ≦ 1) and
The second layer containing AlGaN having an Al composition smaller than the Al composition x of the first layer, and
A third layer formed by laminating a plurality of nitride semiconductor crystal layers having different compositions is provided.
A nitride semiconductor substrate in which the oxygen concentration in the second layer is lower than the oxygen concentration in the third layer.

(Appendix 2)
The nitride semiconductor substrate according to Appendix 1, wherein the oxygen concentration in the second layer is lower than the oxygen concentration in the first layer.

(Appendix 3)
The nitride semiconductor substrate according to Appendix 1 or 2, wherein the oxygen concentration in the third layer is higher than the oxygen concentration in the second layer, and the oxygen concentration in the first layer is higher than the oxygen concentration in the third layer.

(Appendix 4)
Any one of Appendix 1 to 3, wherein the oxygen concentration in the third layer is more than 1 time (more preferably more than 1.5 times) and 10 times or less with respect to the oxygen concentration in the second layer. The nitride semiconductor substrate according to.

(Appendix 5)
The nitride semiconductor substrate according to any one of Supplementary note 1 to 4, wherein the oxygen concentration in the first layer is more than 5 times and 100 times or less with respect to the oxygen concentration in the second layer.

(Appendix 6)
The nitride semiconductor substrate according to any one of Supplementary note 1 to 5, wherein the oxygen concentration in the second layer is 1 × 10 ¹⁸ cm ^{-3 or less.}

(Appendix 7)
The nitride semiconductor substrate according to any one of Supplementary note 1 to 6, wherein the oxygen concentration in the third layer is 5 × 10 ¹⁸ cm ^{-3 or less.}

(Appendix 8)
The nitride semiconductor substrate according to any one of Supplementary note 1 to 7, wherein the oxygen concentration in the first layer is 5 × 10 ¹⁹ cm ^{-3 or less.}

(Appendix 9)
The nitride semiconductor substrate according to any one of Supplementary note 1 to 8, wherein the first layer is configured such that the oxygen concentration decreases from the silicon substrate side toward the second layer side.

(Appendix 10)
The third layer is formed by alternately laminating first and second nitride semiconductor crystal layers having different compositions.
The first nitride semiconductor crystal layer is formed of Al _q Ga _1-q N (0.9 ≦ q ≦ 1).
The nitride semiconductor substrate according to any one of Supplementary note 1 to 9, wherein the second nitride semiconductor crystal layer is formed of Al _p Ga _{1-p N (0 ≦ p ≦ 0.3).}

(Appendix 11)
A step of preparing a state in which a quartz susceptor cover is placed on the susceptor, a top plate made of quartz is placed facing the susceptor, and a silicon substrate is placed in an opening provided in the susceptor cover. ,
It has a step of forming a nitride semiconductor layer on the silicon substrate by supplying a raw material gas to a space formed between the susceptor cover and the top plate while heating the silicon substrate. ,
In the step of forming the nitride semiconductor layer,
From the silicon substrate side, _{a first layer composed of Al x} Ga _1-x N (0.9 ≦ x ≦ 1), and a first layer containing AlGaN having an Al composition smaller than the Al composition x of the first layer. A third layer formed by laminating two layers and a plurality of nitride semiconductor crystal layers having different compositions is formed.
In the formation of the second layer, the amount of oxygen generated from the susceptor cover or the top plate is reduced as compared with the formation of the third layer, or the oxygen generated from the susceptor cover or the top plate is formed. A method for manufacturing a nitride semiconductor substrate, in which growth conditions are controlled so as to reduce the amount of oxygen mixed.

100 ... Nitride semiconductor substrate, 110 ... Silicon substrate, 120 ... Nitride semiconductor layer, 130 ... Reaction suppression layer, 140 ... Intermediate layer, 150 ... Stress generation layer, 151 ... First nitride semiconductor crystal layer, 152 ... 2 nitride semiconductor crystal layer, 160 ... device forming layer, 161 ... active layer, 162 ... Schottky layer

Claims (9)

A nitride semiconductor substrate in which a nitride semiconductor layer is laminated on a silicon substrate.
The nitride semiconductor layer is formed in order from the silicon substrate side.
The first layer composed of Al _x Ga _1-x N (0.9 ≦ x ≦ 1) and
The second layer containing AlGaN having an Al composition smaller than the Al composition x of the first layer, and
A third layer formed by laminating a plurality of nitride semiconductor crystal layers having different compositions is provided.
A nitride semiconductor substrate in which the oxygen concentration in the second layer is lower than the oxygen concentration in the third layer. The nitride semiconductor substrate according to claim 1, wherein the oxygen concentration in the second layer is lower than the oxygen concentration in the first layer. The nitride semiconductor substrate according to claim 1 or 2, wherein the oxygen concentration in the third layer is higher than the oxygen concentration in the second layer, and the oxygen concentration in the first layer is higher than the oxygen concentration in the third layer. .. The nitride semiconductor substrate according to any one of claims 1 to 3, wherein the oxygen concentration in the third layer is more than 1 times and 10 times or less with respect to the oxygen concentration in the second layer. The nitride semiconductor substrate according to any one of claims 1 to 4, wherein the oxygen concentration in the first layer is more than 5 times and 100 times or less with respect to the oxygen concentration in the second layer. The nitride semiconductor substrate according to any one of claims 1 to 5, wherein the oxygen concentration in the second layer is 1 × 10 ¹⁸ cm ^{-3 or less.} The nitride semiconductor substrate according to any one of claims 1 to 6, wherein the oxygen concentration in the third layer is 5 × 10 ¹⁸ cm ^{-3 or less.} The nitride semiconductor substrate according to any one of claims 1 to 7, wherein the oxygen concentration in the first layer is 5 × 10 ¹⁹ cm ^{-3 or less.} The nitride semiconductor substrate according to any one of claims 1 to 8, wherein the first layer is configured such that the oxygen concentration decreases from the silicon substrate side toward the second layer side.