WO2017169176A1 - 窒化物半導体基板、半導体装置、および窒化物半導体基板の製造方法 - Google Patents

窒化物半導体基板、半導体装置、および窒化物半導体基板の製造方法 Download PDF

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WO2017169176A1
WO2017169176A1 PCT/JP2017/004983 JP2017004983W WO2017169176A1 WO 2017169176 A1 WO2017169176 A1 WO 2017169176A1 JP 2017004983 W JP2017004983 W JP 2017004983W WO 2017169176 A1 WO2017169176 A1 WO 2017169176A1
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drift layer
concentration
substrate
carbon
donor
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French (fr)
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好伸 成田
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Sumitomo Chemical Co Ltd
Sciocs Co Ltd
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Sumitomo Chemical Co Ltd
Sciocs Co Ltd
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Priority to US16/088,221 priority Critical patent/US10818757B2/en
Priority to CN201780020285.7A priority patent/CN109075212B/zh
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Definitions

  • the present invention relates to a nitride semiconductor substrate, a semiconductor device, and a method for manufacturing a nitride semiconductor substrate.
  • Group III nitride semiconductors such as gallium nitride have a higher saturation free electron velocity and higher breakdown voltage than silicon. For this reason, nitride semiconductors are expected to be applied to power devices that control power and the like, and high-frequency devices such as mobile phone base stations.
  • Specific examples of the device include semiconductor devices such as a Schottky barrier diode (SBD) and a pn junction diode. In these semiconductor devices, a thick drift layer with a low donor concentration is provided in order to improve the breakdown voltage when reverse bias is applied (see, for example, Patent Document 1).
  • carbon can be incorporated due to the Group III organometallic raw material during crystal growth. At least a part of the carbon taken into the nitride semiconductor functions as an acceptor. For this reason, in an n-type nitride semiconductor to which a donor is added, at least a part of carbon captures electrons from the donor and compensates the donor.
  • the donor concentration in the drift layer is set low in order to improve the breakdown voltage. For this reason, for example, in a low concentration region such as 5 ⁇ 10 16 pieces / cm 3 or less, even if the donor concentration in the drift layer is set to a predetermined value, the influence of a part of carbon to compensate a small amount of donor is large. In some cases, a desired free electron concentration cannot be obtained in the drift layer. As a result, the performance of the semiconductor device may be degraded.
  • An object of the present invention is to provide a technique capable of improving the performance of a semiconductor device.
  • a substrate made of an n-type semiconductor A drift layer provided on the substrate and made of gallium nitride containing donor and carbon; Have The concentration of the donor in the drift layer is 5.0 ⁇ 10 16 atoms / cm 3 or less, and the concentration of the carbon that functions as an acceptor in the drift layer is greater than or equal to the entire area of the drift layer. Yes, The difference obtained by subtracting the concentration of the carbon functioning as the acceptor in the drift layer from the concentration of the donor in the drift layer is gradually decreased from the substrate side toward the surface side of the drift layer.
  • a physical semiconductor substrate is provided.
  • a substrate made of an n-type semiconductor A drift layer provided on the substrate and made of gallium nitride containing donor and carbon; Have The concentration of the donor in the drift layer is 5.0 ⁇ 10 16 atoms / cm 3 or less, and the concentration of the carbon that functions as an acceptor in the drift layer is greater than or equal to the entire area of the drift layer. Yes, The difference obtained by subtracting the concentration of the carbon functioning as the acceptor in the drift layer from the concentration of the donor in the drift layer is gradually decreased from the substrate side toward the surface side of the drift layer.
  • An apparatus is provided.
  • a drift layer made of gallium nitride containing a donor and carbon on a substrate made of an n-type semiconductor In the step of forming the drift layer, The concentration of the donor in the drift layer is 5.0 ⁇ 10 16 pieces / cm 3 or less, and the concentration of the carbon that functions as an acceptor in the drift layer over the entire drift layer
  • a nitride semiconductor that gradually reduces a difference obtained by subtracting the concentration of the carbon functioning as the acceptor in the drift layer from the concentration of the donor in the drift layer from the substrate side toward the surface side of the drift layer.
  • the performance of the semiconductor device can be improved.
  • FIG. 1 It is a schematic band diagram near the drift layer. It is sectional drawing which shows the semiconductor device which concerns on one Embodiment of this invention.
  • (A) is a diagram showing a difference obtained by subtracting the concentration N A of the carbon that acts as an acceptor in the drift layer from the donor concentration N D in the drift layer of the modified example 1
  • (b) the drift layer of the second modification it is a diagram illustrating a difference obtained by subtracting the concentration N a of the carbon that acts as an acceptor from the donor concentration N D in the drift layer in the.
  • 11 is a cross-sectional view showing a nitride semiconductor substrate according to Modification 3.
  • FIG. 10 is a cross-sectional view showing a semiconductor device according to Modification 3.
  • FIG. 1 is a cross-sectional view showing a nitride semiconductor substrate according to this embodiment.
  • a nitride semiconductor substrate (nitride semiconductor laminate, nitride semiconductor epitaxial substrate) 10 of this embodiment is a nitride semiconductor layer for manufacturing a semiconductor device 20 as a pn junction diode described later.
  • Is formed as an epitaxially grown wafer and includes, for example, a substrate 100, a base n-type semiconductor layer 120, a drift layer 140, a first p-type semiconductor layer 220, and a second p-type semiconductor layer 240.
  • the “stacking direction” refers to stacking a nitride semiconductor layer such as the base n-type semiconductor layer 120 from the substrate 100 side upward in the figure (a direction away from the main surface of the substrate 100). It refers to the direction.
  • the “stacking direction” can be rephrased as “the direction from the substrate 100 side toward the surface side of the drift layer 140”.
  • the surface (second main surface) of drift layer 140 is the surface of drift layer 140 opposite to the surface (first main surface) on the substrate 100 side.
  • the substrate 100 is configured as, for example, an n-type single crystal gallium nitride (GaN) substrate (a self-standing GaN substrate) including a predetermined donor.
  • the donor in the substrate 100 include silicon (Si) or germanium (Ge).
  • the donor concentration in the substrate 100 is, for example, 5.0 ⁇ 10 17 pieces / cm 3 or more and 5.0 ⁇ 10 18 pieces / cm 3 or less.
  • a donor concentration, the carbon concentration mentioned later, etc. can be measured by secondary ion mass spectrometry (SIMS: Secondary Ion Mass Spectrometry), for example.
  • SIMS Secondary Ion Mass Spectrometry
  • the plane orientation of the main surface of the substrate 100 is, for example, the c plane ((0001) plane).
  • the GaN crystal constituting the substrate 100 may have a predetermined off angle with respect to the main surface of the substrate 100.
  • the off-angle is an angle formed between the normal direction of the main surface of the substrate 100 and the c-axis of the GaN crystal constituting the substrate 100.
  • the off angle of the substrate 100 is, for example, not less than 0.15 ° and not more than 0.8 °. If the off angle of the substrate 100 is less than 0.15 °, the concentration of carbon (C) added when a nitride semiconductor layer such as the drift layer 140 is grown on the substrate 100 may increase.
  • the concentration of carbon added when growing a nitride semiconductor layer such as the drift layer 140 on the substrate 100 is set to a predetermined amount or less. can do.
  • the off angle of the substrate 100 is more than 0.8 °, the morphology of the main surface of the substrate 100 may be deteriorated.
  • the morphology of the main surface of the substrate 100 can be flattened by setting the off angle of the substrate 100 to 0.8 ° or less.
  • the dislocation density in the main surface of the substrate 100 is set to 1 ⁇ 10 7 pieces / cm 2 or less, for example.
  • the dislocation density in the main surface of the substrate 100 is more than 1 ⁇ 10 7 / cm 2 , dislocations that reduce local breakdown voltage increase in a nitride semiconductor layer such as the drift layer 140 formed on the substrate 100.
  • the concentration of impurities added unintentionally when growing a nitride semiconductor layer on the substrate 100 for example, carbon The concentration tends to be high.
  • the dislocation density on the main surface of the substrate 100 is set to 1 ⁇ 10 7 pieces / cm 2 or less, so that the nitride semiconductor such as the drift layer 140 formed on the substrate 100 is formed. An increase in dislocation that lowers the local breakdown voltage in the layer can be suppressed. Further, by setting the threading dislocation density on the main surface of the substrate 100 to 1 ⁇ 10 7 pieces / cm 2 or less, the concentration of impurities added unintentionally during the growth of the nitride semiconductor layer can be reduced. it can.
  • the base n-type semiconductor layer 120 is provided between the substrate 100 and the drift layer 140 as a buffer layer that inherits the crystallinity of the substrate 100 and stably epitaxially grows the drift layer 140.
  • the base n-type semiconductor layer 120 is configured as an n + -type GaN layer containing a donor having a concentration similar to that of the substrate 100.
  • the donor in the base n-type semiconductor layer 120 for example, Si or Ge can be used as in the donor in the substrate 100.
  • the donor concentration in the base n-type semiconductor layer 120 is, for example, 5.0 ⁇ 10 17 pieces / cm 3 or more and 5.0 ⁇ 10 18 pieces / cm 3 or less, similarly to the donor concentration in the substrate 100. .
  • the underlying n-type semiconductor layer 120 contains carbon added (auto-doped) due to the group III organometallic raw material used during the crystal growth.
  • the total concentration of carbon in the base n-type semiconductor layer 120 is, for example, 1.0 ⁇ 10 15 pieces / cm 3 or more and 5.0 ⁇ 10 16 pieces / cm 3 or less.
  • the “total concentration of carbon” means the concentration of all carbons including carbon that does not function as an acceptor as well as carbon that functions as an acceptor, as will be described later.
  • the n-type nitride semiconductor layer such as the base n-type semiconductor layer 120
  • at least a part of carbon functions as an acceptor (compensation dopant) to compensate the donor.
  • the effective free electron concentration in the base n-type semiconductor layer 120 is obtained as a difference obtained by subtracting the carbon concentration that functions as an acceptor from the donor concentration.
  • the donor concentration is high and the carbon concentration functioning as an acceptor is relatively low so as to be negligible.
  • the free electron concentration in the base n-type semiconductor layer 120 can be regarded as substantially equal to the donor concentration. For example, 5.0 ⁇ 10 17 atoms / cm 3 or more and 5.0 ⁇ 10 18 atoms / cm 3. It is as follows.
  • the donor concentration and the total carbon concentration in the base n-type semiconductor layer 120 are substantially constant in the stacking direction.
  • the base n-type semiconductor layer 120 only needs to include a region in which each additive concentration is constant in the stacking direction, and the base n-type semiconductor layer 120 is near each of the substrate 100 side and the drift layer 140 side. A region where the additive concentration is inclined may be included.
  • the thickness of the base n-type semiconductor layer 120 is thinner than the thickness of the drift layer 140 described later, for example, 0.1 ⁇ m or more and 3 ⁇ m or less.
  • the drift layer 140 is provided on the base n-type semiconductor layer 120 and is configured as an n-type GaN layer containing a low concentration donor.
  • Examples of the donor in the drift layer 140 include Si or Ge, similarly to the donor in the base n-type semiconductor layer 120.
  • the donor concentration in the drift layer 140 is lower than the donor concentration of the substrate 100 and the donor concentration of the underlying n-type semiconductor layer 120, for example, 1.0 ⁇ 10 15 pieces / cm 3 or more and 5.0 ⁇ 10 16 pieces / cm. 3 or less. If the donor concentration is less than 1.0 ⁇ 10 15 / cm 3 , the drift layer 140 may have a high resistance. On the other hand, when the donor concentration is 1.0 ⁇ 10 15 atoms / cm 3 or more, it is possible to suppress an excessive increase in the resistance of the drift layer 140. On the other hand, if the donor concentration is more than 5.0 ⁇ 10 16 / cm 3 , the withstand voltage when a reverse bias is applied may be reduced. On the other hand, when the donor concentration is 5.0 ⁇ 10 16 pieces / cm 3 or less, a predetermined breakdown voltage can be secured.
  • the drift layer 140 also contains carbon added due to the group III organometallic raw material used at the time of crystal growth, and at least a part of the carbon in the drift layer 140 functions as an acceptor to compensate the donor. Yes.
  • the donor concentration is high on the order of 10 18 .
  • the carbon concentration relative to the donor concentration is so low that it can be ignored.
  • the donor concentration is as low as 5.0 ⁇ 10 16 ions / cm 3 or less.
  • the carbon concentration with respect to the donor concentration cannot be ignored, and the free electron concentration in the drift layer 140 is easily affected by compensation of a small amount of donor by using a part of carbon as an acceptor. Therefore, in the drift layer 140, a desired free electron concentration distribution cannot be obtained unless the relative relationship between the donor concentration and the concentration of carbon functioning as an acceptor is controlled.
  • the donor concentration in the drift layer 140 is adjusted to be equal to or higher than the concentration of carbon that functions as an acceptor in the drift layer 140 over the entire drift layer 140.
  • the difference obtained by subtracting the concentration of carbon that functions as an acceptor in the drift layer 140 from the concentration of the donor in the drift layer 140 gradually decreases from the substrate 100 side toward the surface side of the drift layer 140 (that is, toward the stacking direction). It has been adjusted. Thereby, a desired free electron concentration distribution can be obtained in the drift layer 140. Details of the relative relationship between the donor concentration and the carbon concentration in the drift layer 140 will be described later.
  • the drift layer 140 is provided thicker than the base n-type semiconductor layer 120 in order to improve the breakdown voltage when a reverse bias is applied.
  • the thickness of the drift layer 140 is, for example, 3 ⁇ m or more and 40 ⁇ m or less. If the thickness of the drift layer 140 is less than 3 ⁇ m, the breakdown voltage when a reverse bias is applied may be reduced. In contrast, by setting the thickness of the drift layer 140 to 3 ⁇ m or more, a predetermined breakdown voltage can be ensured. On the other hand, if the thickness of the drift layer 140 is more than 40 ⁇ m, the on-resistance when a forward bias is applied may increase. On the other hand, by setting the thickness of the drift layer 140 to 40 ⁇ m or less, it is possible to suppress an excessive increase in on-resistance when a forward bias is applied.
  • the first p-type semiconductor layer 220 is provided on the drift layer 140 and configured as a p-type GaN layer including an acceptor.
  • An example of the acceptor in the first p-type semiconductor layer 220 is magnesium (Mg).
  • the acceptor concentration in the first p-type semiconductor layer 220 is, for example, 1.0 ⁇ 10 17 pieces / cm 3 or more and 2.0 ⁇ 10 19 pieces / cm 3 or less.
  • the second p-type semiconductor layer 240 is provided on the first p-type semiconductor layer 220 and is configured as a p + -type GaN layer containing a high concentration of acceptor.
  • the acceptor in the second p-type semiconductor layer 240 for example, Mg may be used as in the first p-type semiconductor layer 220.
  • the acceptor concentration in the second p-type semiconductor layer 240 is higher than the acceptor concentration in the first p-type semiconductor layer 220, for example, 5.0 ⁇ 10 19 pieces / cm 3 or more and 2.0 ⁇ 10 20 pieces / cm. 3 or less.
  • FIG. 2 (a) is a diagram showing a difference obtained by subtracting the concentration N A of the carbon that acts as an acceptor in the drift layer from the donor concentration N D of the drift layer.
  • the horizontal axis indicates the position (depth) from the surface side of the drift layer 140.
  • the donor concentration in the drift layer 140 and N D the total concentration of carbon in the drift layer 140 (the concentration of all carbon in the drift layer 140) and N C, in the drift layer 140 carbon the concentration of carbon which serves as an internal acceptor to N a.
  • the vertical axis the donor concentration in the drift layer 140 N difference by subtracting the concentration N A of the carbon that acts as an acceptor in the drift layer 140 from the D N D -N A (hereinafter, in the drift layer 140 Density difference N D -N A ).
  • the concentration difference N D -N A in the drift layer 140 can be considered as a difference obtained by subtracting the amount of free electrons captured from the donor by carbon as an acceptor from the total amount of free electrons obtained from the donor. Accordingly, the concentration difference N D -N A of the drift layer 140 corresponds to the effective free electron concentration in the drift layer 140.
  • the donor concentration N D in the drift layer 140, over the entire region of the drift layer 140, and the carbon concentration N A or functioning as an acceptor in the drift layer 140 (N D ⁇ N A ). If the donor concentration N D in the drift layer 140 at least a portion of the drift layer 140 is less than the concentration N A of the carbon that acts as an acceptor in the drift layer 140, a region where free electrons is not generated in a portion of the drift layer 140 May occur. In contrast, the donor concentration N D in the drift layer 140, over the entire region of the drift layer 140, by a higher concentration N A of the carbon that acts as an acceptor in the drift layer 140, a donor concentration of 5.
  • the drift layer 140 can function as an n-type layer.
  • density difference N D -N A of the drift layer 140 toward the substrate 100 side to the surface side of the drift layer 140 (i.e. toward the stacking direction ) It is gradually decreasing.
  • the concentration difference N D -N A of the drift layer 140 has decreased monotonically toward the stacking direction.
  • the concentration N A of the carbon that acts as an acceptor is made at least 1/3 or more of the total concentration N C carbon (N C / 3 ⁇ N A ⁇ N C ).
  • the donor concentration N D in the drift layer 140 in consideration of the proportion of carbon which acts as an acceptor as described above, the donor concentration N D in the drift layer 140, over the entire region of the drift layer 140, in at least the drift layer 140 carbon
  • the total concentration N C is 1/3 times or more (N D ⁇ N C / 3). Since the donor concentration N D in the drift layer 140 is less than 1/3 of the total concentration N C carbon, many donors in the drift layer 140 is compensated by the carbon that acts as an acceptor, the drift layer 140 There is a possibility that a predetermined amount of free electrons will not be generated. For this reason, the drift layer 140 is not n-type, and the drift layer 140 may have a high resistance.
  • the donor concentration N D in the drift layer 140 by the above 1/3 of the total concentration N C carbon, the donor amount in the drift layer 140, the amount to be compensated by the carbon as acceptor And a predetermined amount of free electrons can be generated in the drift layer 140.
  • the drift layer 140 can function as an n-type layer, and the resistance of the drift layer 140 can be suppressed from becoming excessively high.
  • the donor concentration N D in the drift layer 140 if a 1/3 or more of the total concentration N C of carbon in the drift layer 140, a donor concentration N D in the drift layer 140 in the drift layer 140 It may be lower than the total carbon concentration N C.
  • the donor concentration N D in the drift layer 140 over the entire region of the drift layer 140, and more preferably in a total concentration N C or more carbons in the drift layer 140.
  • the donor amount in the drift layer 140 can be surely made larger than the amount compensated by carbon as an acceptor.
  • the drift layer 140 can be made to function stably as an n-type.
  • FIG. 3 is a schematic band diagram in the vicinity of the drift layer.
  • the conduction band of the drift layer 140 A predetermined amount of free electrons is generated. Also, by gradually decreasing the density difference N D -N A of the drift layer 140 toward the substrate 100 side to the surface side of the drift layer 140, the drift layer 140 is free electron concentration in the drift layer 140 from the substrate 100 side It gradually increases toward the surface side of. For this reason, the substrate 100 side of the drift layer 140 is a high free electron concentration region in the drift layer 140 having a low free electron concentration, while the surface side of the drift layer 140 is the drift layer 140 having a low free electron concentration. Among them, it is a low free electron concentration region.
  • the conduction band of the drift layer 140 is inclined so as to gradually rise in the stacking direction.
  • the free electron concentration of the drift layer 140 gradually increases as it approaches the base n-type semiconductor layer 120, and the free electron concentration of the base n-type semiconductor layer 120. It is close to. As a result, the conduction band of drift layer 140 and the conduction band of underlying n-type semiconductor layer 120 are gently joined, and the energy barrier between the conduction band of drift layer 140 and underlying n-type semiconductor layer 120 is reduced. . As a result, when a forward bias is applied, electrons can be smoothly moved from the base n-type semiconductor layer 120 toward the drift layer 140, and the on-resistance can be reduced.
  • the free electron concentration of the drift layer 140 gradually decreases as it approaches the first p-type semiconductor layer 220. It is lower than the pore concentration.
  • the depletion layer in the vicinity of the junction interface does not spread so much from the junction interface toward the first p-type semiconductor layer 220 side, but extends from the junction interface toward the drift layer 140 side. Thereby, the inclination (electric field strength) of the conduction band in the vicinity of the junction interface is gentle.
  • the depletion layer expands further from the state of the depletion layer before the reverse bias is applied toward the base n-type semiconductor layer 120 side.
  • the gradient of the conduction band in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 is the largest.
  • the free electron concentration on the surface side of the drift layer 140 becomes low, and the depletion layer spreads in the drift layer 140. Therefore, even when a reverse bias is applied, conduction near the junction interface is performed.
  • the band inclination is suppressed from becoming excessively steep. As a result, the occurrence of avalanche breakdown in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 can be suppressed, and the breakdown voltage can be improved.
  • density difference N D -N A of the drift layer 140, (linearly) in a straight line toward the stacking direction has decreased.
  • the inclination of the conduction band can be made smooth and gentle over the entire drift layer 140.
  • the on-resistance when a forward bias is applied can be stably reduced, and the breakdown voltage when a reverse bias is applied can be stably improved.
  • the absolute value of the slope of the D -N a is, for example, 5.0 ⁇ 10 14 atoms / cm 3 ⁇ ⁇ m -1 or more 3.0 ⁇ 10 16 atoms / cm 3 ⁇ ⁇ m -1 or less.
  • the absolute value of the slope of the N D -N A is less than 5.0 ⁇ 10 14 atoms / cm 3 ⁇ ⁇ m -1, when a low N D -N A includes a drift layer 140 and the underlying n-type semiconductor layer 120 There is a possibility that the energy barrier of the conduction band becomes large at the junction interface, and the on-resistance becomes higher when a forward bias is applied.
  • N D -N A is high, the slope of the conduction band in the vicinity of the junction interface between the drift layer 140 first 1p-type semiconductor layer 220 is increased, the possibility that the withstand voltage at the time of applying a reverse bias decreases is there.
  • the absolute value of the slope of N D -N A is less than 5.0 ⁇ 10 14 pieces / cm 3 ⁇ ⁇ m ⁇ 1 , the on-resistance is reduced when the forward bias is applied, and the reverse bias is applied. It is difficult to achieve both improvement in breakdown voltage.
  • the absolute value of the slope of the N D -N A 5.0 ⁇ 10 14 atoms / cm 3 ⁇ ⁇ m -1 or more, the bonding interface between the drift layer 140 and the underlying n-type semiconductor layer 120
  • the inclination of the conduction band in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 can be reduced while reducing the energy barrier of the conduction band.
  • FIG. 2B is a diagram showing donor and carbon concentrations in the drift layer.
  • the horizontal axis indicates the position (depth) from the surface side of the drift layer 140, as in FIG. 2A.
  • the vertical axis represents the donor and carbon concentrations in the drift layer 140. Note that, as described above, such as the donor concentration N D and total concentration N C of carbon in the drift layer 140, for example, it can be measured by SIMS.
  • the donor concentration N D in the drift layer 140 As shown in FIG. 2 (b), the donor concentration N D in the drift layer 140, for example, have decreased linearly toward the stacking direction. As described above, the maximum and minimum values of the donor concentration N D in the drift layer 140, the 1.0 ⁇ 10 15 atoms / cm 3 or more 5.0 ⁇ 10 16 atoms / cm 3 within the following ranges has entered, donor concentration N D in the drift layer 140, over the entire region of the drift layer 140, has a 1/3 or more concentration N C of carbon in the least drift layer 140.
  • donor concentration N D in the drift layer 140 is at 5.0 ⁇ 10 16 atoms / cm 3 or less, the acceptor in the drift layer 140 donor concentration N D in the drift layer 140 over the entire area of the drift layer 140 If the three conditions that the concentration difference N D -N A in the drift layer 140 is not less than the concentration N A and the concentration difference N D -N A gradually decreases in the stacking direction are satisfied, The total concentration N C of carbon can be arbitrarily distributed with respect to the stacking direction.
  • total concentration N C of carbon in the drift layer 140 may be gradually increased toward the stacking direction. That is, the total concentration N C of carbon in the drift layer 140 may be changed in the opposite direction with respect to the donor concentration N D in the drift layer 140.
  • the number of free electrons in the vicinity of the junction interface increases, and the energy barrier of the conduction band can be reduced at the junction interface between the drift layer 140 and the base n-type semiconductor layer 120.
  • the on-resistance when a forward bias is applied can be reduced.
  • the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 there are few donors and carbon as an acceptor increases. Thereby, free electrons in the vicinity of the junction interface (on the surface side of the drift layer 140) are reduced, and the inclination of the conduction band in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 can be reduced. As a result, the breakdown voltage when a reverse bias is applied can be improved.
  • the total concentration N C of carbon in the drift layer 140 may be constant toward the stacking direction.
  • the total concentration N C of carbon in the drift layer 140 may be constant toward the stacking direction.
  • by way of growth condition only by changing the flow rate of the donor material can be a density difference N D -N A of the drift layer 140 and a predetermined distribution. That is, it is not necessary to adjust growth conditions other than the flow rate of the donor raw material (for example, a growth rate described later) in order to control the carbon concentration, and control during growth can be simplified.
  • the carbon concentration N C in the drift layer 140 may be gradually decreased in the stacking direction.
  • the amount of donors compensated by carbon as an acceptor decreases.
  • a predetermined amount of free electrons can be ensured on the surface side of the drift layer 140 although it is small.
  • the crystallinity on the surface side of the drift layer 140 is improved by reducing the carbon concentration N C on the junction interface side (the surface side of the drift layer 140) between the drift layer 140 and the first p-type semiconductor layer 220. The loss on the surface side of the drift layer 140 in the semiconductor device 20 to be described later can be reduced.
  • the total concentration N C of carbon in the drift layer 140 is, for example, 5.0 ⁇ 10 16 atoms / cm 3 or less. If the carbon concentration N C in the drift layer 140 is more than 5.0 ⁇ 10 16 atoms / cm 3 , the crystallinity of the drift layer 140 may decrease, and the loss of the semiconductor device 20 described later may increase. On the other hand, when the carbon concentration N C in the drift layer 140 is 5.0 ⁇ 10 16 pieces / cm 3 or less, the crystallinity of the drift layer 140 is improved and the semiconductor device 20 is reduced in loss. Can do. Note that the lower the carbon concentration N C in the drift layer 140, the better. Therefore, the lower limit value of the carbon concentration N C is not particularly limited.
  • the drift layer 140 contains hydrogen (H) in addition to donor and carbon. Hydrogen is taken into the drift layer 140 due to, for example, a group III organometallic material or a donor material used during crystal growth of the drift layer 140.
  • the hydrogen concentration in the drift layer 140 is, for example, 5.0 ⁇ 10 16 atoms / cm 3 or less, preferably 1.0 ⁇ 10 16 atoms / cm 3 or less. If the hydrogen concentration in the drift layer 140 is more than 5.0 ⁇ 10 16 atoms / cm 3 , the crystallinity of the drift layer 140 may decrease, and the loss of the semiconductor device 20 described later may increase.
  • the lower limit value of the hydrogen concentration is not particularly limited.
  • FIG. 4 is a cross-sectional view showing the semiconductor device according to the present embodiment.
  • the semiconductor device 20 is configured as a vertical pn junction diode manufactured using the above-described nitride semiconductor substrate 10, for example, a substrate 100 and a base n-type semiconductor layer. 120, a drift layer 140, a first p-type semiconductor layer 220, a second p-type semiconductor layer 240, an anode 310, an insulating film 400, and a cathode 360.
  • the drift layer 140, the first p-type semiconductor layer 220, and the second p-type semiconductor layer 240 form a mesa structure 180.
  • the mesa structure 180 has, for example, a quadrangular frustum shape or a truncated cone shape, and the cross-sectional area of the mesa structure 180 in plan view gradually decreases in the stacking direction. Accordingly, the mesa structure 180 has a forward tapered side surface.
  • a portion of a region in which the concentration difference N D -N A of the drift layer 140 is gradually decreased toward the stacking direction a portion of the mesa structure 180 Is configured.
  • electric field concentration is likely to occur in the vicinity of the pn junction interface in the vicinity of the side surface of the mesa structure 180.
  • density difference N D -N A of the drift layer 140 is gradually decreased toward the stacking direction.
  • the depletion layer extends from the pn junction interface toward the drift layer 140, and the electric field in the region is relaxed.
  • a reverse bias when a reverse bias is applied, it is possible to suppress the occurrence of avalanche breakdown in a region near the pn junction interface near the side surface of the mesa structure 180 and improve the breakdown voltage of the semiconductor device 20.
  • the first anode (p-type contact electrode) 320 of the anode (p-side electrode) 310 is provided on the upper surface of the mesa structure 180, that is, on the second p-type semiconductor layer 240.
  • the first anode 320 is made of a material that is in ohmic contact with the second p-type semiconductor layer 240.
  • the insulating film 400 is provided so as to cover the surface of the drift layer 140 outside the mesa structure 180, the side surface of the mesa structure 180, and part of the surface of the second p-type semiconductor layer 240 (around the top surface of the mesa structure 180). ing.
  • the insulating film 400 functions to insulate the drift layer 140 and the like from a second anode 340 described later and protect the drift layer 140 and the like.
  • the insulating film 400 has an opening for contacting the first anode 320 and a second anode 340 described later.
  • the insulating film 400 of this embodiment has, for example, a two-layer structure, and includes a first insulating film 420 and a second insulating film 440.
  • the first insulating film 420 is made of, for example, an SOG (Spin On Glass) film formed by a coating method such as a spin coating method.
  • the second insulating film 440 is made of, for example, a silicon oxide (SiO 2 ) film formed by sputtering or the like.
  • a second anode (p-side electrode pad) 340 of the anode 310 is in contact with the first anode 320 in the opening of the insulating film 400 and extends outward from the first anode 320 on the insulating film 400 to have a mesa structure.
  • 180 is provided so as to cover 180.
  • the second anode 340 includes a part of the surface of the drift layer 140 outside the mesa structure 180, a side surface of the mesa structure 180, and an upper surface of the mesa structure 180 when the semiconductor device 20 is viewed from above. It is provided so that it may overlap.
  • the second anode 340 is made of, for example, an alloy (Ti / Al) of titanium (Ti) and aluminum (Al).
  • the cathode 360 is provided on the back side of the substrate 100.
  • the cathode 360 is made of a material that is in ohmic contact with the substrate 100 made of n-type GaN, and is made of, for example, Ti / Al.
  • Nitride semiconductor substrate manufacturing method semiconductor device manufacturing method
  • Step 1 Preparation of substrate
  • a substrate 100 as an n-type single crystal GaN substrate is prepared.
  • Step 2 Formation of underlying n-type semiconductor layer
  • MOVPE metal organic vapor phase epitaxy
  • the substrate 100 is carried into the processing chamber of the MOVPE apparatus.
  • hydrogen gas or a mixed gas of hydrogen gas and nitrogen gas
  • the substrate 100 is heated to a predetermined growth temperature (for example, 1000 ° C. or higher and 1100 ° C. or lower).
  • a predetermined growth temperature for example, 1000 ° C. or higher and 1100 ° C. or lower.
  • TMG trimethylgallium
  • NH 3 ammonia
  • SiH 4 monosilane
  • the base n-type semiconductor layer 120 as an n + -type GaN layer is epitaxially grown on the substrate 100 made of n-type single crystal GaN. Since the crystal growth at this time is homoepitaxial growth in which the same GaN crystal is grown in the stacking direction, the base n-type semiconductor layer 120 with good crystallinity can be formed on the substrate 100.
  • a drift layer 140 as an n ⁇ type GaN layer is epitaxially grown on the underlying n type semiconductor layer 120.
  • the donor concentration N D in the drift layer 140 becomes 5.0 ⁇ 10 16 atoms / cm 3 or less
  • the acceptor in the drift layer 140 donor concentration N D in the drift layer 140 over the entire area of the drift layer 140 functions become more concentration N a of the carbon as a further density difference N D -N a of the drift layer 140, so as to decrease gradually in the stacking direction to adjust the various growth conditions.
  • the donor concentration N D in the drift layer 140 is gradually reduced toward the stacking direction in the range of 5.0 ⁇ 10 16 atoms / cm 3 or less
  • the flow rate of the donor material is gradually decreased as the drift layer 140 is grown.
  • the carbon donor concentration N D in the drift layer 140 over the entire area of the drift layer 140 of total concentration N C 1/3 The flow rate of the donor material and other growth conditions are relatively adjusted so as to be twice or more. Specifically, by adjusting the flow rate (growth rate) of TMG during the growth of the drift layer 140, the V / III ratio (ratio of the flow rate of the group V raw material to the flow rate of the group III organometallic raw material), the growth temperature, etc.
  • the total carbon concentration N C can be adjusted.
  • the donor concentration N D in the drift layer 140 is at 5.0 ⁇ 10 16 atoms / cm 3 or less, the donor concentration N D in the drift layer 140 over the entire area of the drift layer 140 drift and a layer 140 of carbon concentration N a or functioning as an acceptor in further, if three conditions are met that the density difference N D -N a of the drift layer 140 is gradually decreased toward the stacking direction
  • the total concentration N C of carbon in the drift layer 140 can be arbitrarily distributed with respect to the stacking direction.
  • the growth conditions may be adjusted so that the total concentration N C of carbon in the drift layer 140 gradually increases in the stacking direction.
  • the flow rate of TMG (the growth rate of the drift layer 140) is gradually increased.
  • the V / III ratio is gradually reduced as the drift layer 140 is grown.
  • the growth temperature is gradually lowered as the drift layer 140 is grown.
  • Such growth conditions can be gradually increased toward the total concentration N C of carbon in the drift layer 140 in the stacking direction. Incidentally, it is necessary to vary the total concentration N C carbon in a trace amount, it is preferable not to adjust the growth pressure.
  • the growth conditions other than the donor flow rate are maintained so that the total concentration N C of carbon in the drift layer 140 becomes constant in the stacking direction. May be.
  • the growth rate, V / III ratio, growth temperature, and growth pressure of the drift layer 140 are kept constant.
  • Such growth conditions may be a constant total concentration N C of carbon in the drift layer 140 toward the stacking direction.
  • the growth conditions may be adjusted so that the total concentration N C of carbon in the drift layer 140 gradually decreases in the stacking direction.
  • the flow rate of TMG (the growth rate of the drift layer 140) is gradually reduced.
  • the V / III ratio is gradually increased as the drift layer 140 is grown.
  • the growth temperature is gradually increased as the drift layer 140 is grown.
  • Such growth conditions can be gradually reduced toward the total concentration N C of carbon in the drift layer 140 in the stacking direction. Also in this case, since it is necessary to vary the total concentration N C carbon in a trace amount, it is preferable not to adjust the growth pressure.
  • Step 4 Formation of the first p-type semiconductor layer
  • the first p-type semiconductor layer 220 as a p-type GaN layer is epitaxially grown on the drift layer 140.
  • Cp 2 Mg biscyclopentadienyl magnesium
  • Step 5 Formation of second p-type semiconductor layer
  • the second p-type semiconductor layer 240 as the p + -type GaN layer is epitaxially grown on the first p-type semiconductor layer 220 by the same processing procedure as in Step 4.
  • Step 6 Unloading
  • the atmosphere in the processing chamber of the MOVPE apparatus is replaced with N 2 gas to return to atmospheric pressure, and the temperature in the processing chamber is lowered to a temperature at which the substrate can be unloaded.
  • the nitride semiconductor substrate 10 of the present embodiment is manufactured. Thereafter, the nitride semiconductor substrate 10 is supplied as an epi wafer for manufacturing the semiconductor device 20 to a manufacturer of the semiconductor device 20.
  • Step 7 Manufacture of semiconductor device
  • the second p-type semiconductor layer 240, the first p-type semiconductor layer 220, and a part of the drift layer 140 are etched by, for example, RIE (Reactive Ion Etching).
  • RIE Reactive Ion Etching
  • the mesa structure 180 is formed in the second p-type semiconductor layer 240, the first p-type semiconductor layer 220, and the drift layer 140.
  • a Pd / Ni film is formed by sputtering, for example, so as to cover the surfaces of the mesa structure 180 and the drift layer 140, and is patterned into a predetermined shape.
  • the first anode 320 is formed on the upper surface of the mesa structure 180, that is, on the second p-type semiconductor layer 240.
  • an SOG film is formed by, for example, spin coating so as to cover the surfaces of the mesa structure 180 and the drift layer 140, and an SiO 2 film is formed thereon by, for example, sputtering, and patterned into a predetermined shape.
  • the first insulation is provided so as to cover the surface of the drift layer 140 outside the mesa structure 180, the side surface of the mesa structure 180, and a part of the surface of the second p-type semiconductor layer 240 (around the top surface of the mesa structure 180).
  • An insulating film 400 including the film 420 and the second insulating film 440 is formed.
  • a Ti / Al film is formed by sputtering, for example, so as to cover the first anode 320 and the insulating film 400 in the opening of the insulating film 400 and patterned into a predetermined shape.
  • the second anode 340 is formed so as to be in contact with the first anode 320 within the opening of the insulating film 400 and to extend outside the first anode 320 on the insulating film 400 and cover the mesa structure 180.
  • a cathode 360 is formed by forming a Ti / Al film on the back side of the substrate 100 by, for example, sputtering.
  • the semiconductor device 20 of this embodiment is manufactured.
  • N D ⁇ N A concentration of the carbon that acts as an acceptor in the drift layer 140
  • the drift layer A predetermined amount of free electrons can be generated over the entire area 140.
  • the drift layer 140 can function as an n-type layer.
  • the density difference N D -N A of the drift layer 140 such a predetermined distribution, as at least part of the carbon in the drift layer 140 had to compensate the donor, the desired free electron concentration distribution Obtainable.
  • the free electron concentration can be gradually decreased from the substrate 100 side toward the surface side of the drift layer 140.
  • the drift layer 140 gradually decreases toward the surface side of the drift layer 140 from the substrate 100 side, the junction between the drift layer 140 and the 1p-type semiconductor layer 220 Near the interface, the free electron concentration of the drift layer 140 gradually decreases as it approaches the first p-type semiconductor layer 220.
  • the depletion layer in the vicinity of the junction interface between the drift layer 140 and the first p-type semiconductor layer 220 spreads from the junction interface toward the drift layer 140 side, and the inclination of the conduction band (electric field strength) in the vicinity of the junction interface is It has become moderate.
  • the concentration N A of the carbon that serves as an acceptor since it has at least 1/3 or more of the total concentration N C carbon, donor concentration in the drift layer 140 of this embodiment N D is a 1/3 or more of the total concentration N C of carbon in the drift layer 140 over the entire region of the drift layer 140.
  • the donor amount in the drift layer 140 can be made larger than the amount compensated by carbon as an acceptor, and a predetermined amount of free electrons can be generated in the drift layer 140.
  • the drift layer 140 can function as an n-type layer, and the resistance of the drift layer 140 can be suppressed from becoming excessively high.
  • the hydrogen concentration in the drift layer 140 is 5.0 ⁇ 10 16 atoms / cm 3 or less. Thereby, the crystallinity of the drift layer 140 can be improved, and the loss of the semiconductor device 20 can be reduced.
  • FIG. 5 (a) is a diagram showing a difference obtained by subtracting the concentration N A of the carbon that acts as an acceptor in the drift layer from the donor concentration N D in the drift layer of the first modification.
  • Figure 5 As shown in the first modification of (a), density difference N D -N A of the drift layer 140 may be decreased stepwise toward the surface side of the drift layer 140 from the substrate 100 side.
  • the drift layer 140 is composed of a plurality layers, be considered to N D -N A of the upper layer of the adjacent two layers is lower than N D -N A of the lower layer Good. According to the first modification, the same effect as that of the above-described embodiment can be obtained.
  • the growth conditions can be switched in stages as the drift layer 140 is grown, and the growth conditions can be easily controlled.
  • the energy barrier of the conduction band occurs at the step portion of the N D -N A, increases the on-resistance when applying a forward bias fear There is.
  • the inclination of the conduction band at the step portion of the N D -N A becomes steep, the breakdown voltage may be reduced at the time of applying a reverse bias. Therefore, better density difference N D -N A of the drift layer 140 is reduced in a linear shape toward the surface side of the drift layer 140 from the substrate 100 side as in the embodiment described above (FIG.
  • the inclination of the conduction band of the drift layer 140 can be made smooth and gentle, and it is possible to achieve both a reduction in on-resistance when a forward bias is applied and an increase in breakdown voltage when a reverse bias is applied. ,preferable.
  • 5 (b) is a diagram showing a difference obtained by subtracting the concentration N A of the carbon that acts as an acceptor in the drift layer from the donor concentration N D in the drift layer of the second modification.
  • the density difference N D -N A of the drift layer 140 may be decreased nonlinearly toward the surface side of the drift layer 140 from the substrate 100 side.
  • N D slope of -N A gradually increases toward the middle position of the drift layer 140 from the substrate 100 side, the drift layer from the intermediate position side slope of the drift layer 140 of N D -N A It gradually decreases toward the surface side of 140.
  • N D -N A may be a multi-order function, a logarithmic function, an exponential function, or a combination thereof.
  • the same effect as that of the above-described embodiment can be obtained.
  • N D -N A had varied linearly growth conditions according gradually grown drift layer 140, N D -N A so as to correspond to the growth conditions do not change linearly Sometimes. In such a case, it becomes difficult to change N D -N A linearly.
  • the change in N D -N A as in the modified example 2 is nonlinear Even so, the free electron concentration of the drift layer 140 is gradually decreased as it approaches the first p-type semiconductor layer 220, and the free electron concentration of the drift layer 140 is gradually increased as it approaches the base n-type semiconductor layer 120. Can do. Thereby, also in the modification 2, it becomes possible to make compatible the reduction
  • the nitride semiconductor substrate 10 is configured as a wafer for manufacturing a pn junction diode and the semiconductor device 20 is configured as a pn junction diode in the above-described embodiment, the nitride semiconductor substrate is described. Further, the following modification 3 may be applied to the semiconductor device.
  • FIG. 6 is a cross-sectional view showing a nitride semiconductor substrate according to Modification 3.
  • the nitride semiconductor substrate 12 of Modification 3 is configured as a wafer for manufacturing a Schottky barrier diode (SBD).
  • SBD Schottky barrier diode
  • the concentration difference N D ⁇ N A in the drift layer 142 gradually decreases from the substrate 102 side toward the surface side of the drift layer 142.
  • FIG. 7 is a cross-sectional view showing a semiconductor device according to Modification 3. As shown in FIG.
  • the semiconductor device 22 of Modification 3 is configured as an SBD manufactured using the nitride semiconductor substrate 12 described above, and includes, for example, a substrate 102, a base n-type semiconductor layer 122, and a drift layer. 142, an insulating film 402, an anode 312, and a cathode 362.
  • the mesa structure as in the above-described embodiment is not formed.
  • the insulating film 402 is provided on the flat surface of the drift layer 142.
  • the insulating film 402 has an opening for bringing the drift layer 142 and the anode 312 into contact with each other.
  • the anode 312 is configured as a so-called field plate electrode.
  • the anode 312 is in contact with the drift layer 142 in the opening of the insulating film 402 and extends on the insulating film 402 to the outside of the opening of the insulating film 402. Thereby, it is possible to suppress the concentration of the electric field at the end portion of the region where the anode 312 and the drift layer 142 are in contact with each other.
  • the anode 312 is configured to form a Schottky barrier with the drift layer 140, and is made of, for example, Pd, Pd / Ni, or Ni / Au.
  • the cathode 362 is provided on the back side of the substrate 102. According to the modification 3, even if the semiconductor device 22 is SBD, the same effect as the above-described embodiment can be obtained.
  • the SBD such as the semiconductor device 22 of this modification has a lower breakdown voltage than the pn junction diode, but according to this modification, the concentration difference N D ⁇ N A in the drift layer 142 is reduced.
  • the breakdown voltage of the semiconductor device 22 as the SBD can be improved.
  • the substrate 100 is an n-type GaN substrate.
  • the substrate 100 may be configured as a semiconductor substrate other than GaN.
  • the substrate may be configured as an n-type SiC substrate, for example.
  • the substrate is preferably an n-type GaN substrate.
  • the base n-type semiconductor layer 120 is interposed between the substrate 100 and the drift layer 140 has been described, but the base n-type semiconductor layer may not be provided. That is, the drift layer may be provided directly on the substrate.
  • the nitride semiconductor layer such as the drift layer 140 is formed using the MOVPE apparatus.
  • the hydride vapor phase epitaxy (HVPE) apparatus is used to form the drift layer.
  • a nitride semiconductor layer such as 140 may be formed.
  • a hydrocarbon gas is supplied to the substrate 100 as a carbon source, and the total carbon concentration N C in the drift layer 140 is adjusted by adjusting the flow rate of the carbon source.
  • (Appendix 1) a substrate made of an n-type semiconductor; A drift layer provided on the substrate and made of gallium nitride containing donor and carbon; Have The concentration of the donor in the drift layer is 5.0 ⁇ 10 16 atoms / cm 3 or less, and the concentration of the carbon that functions as an acceptor in the drift layer is greater than or equal to the entire area of the drift layer. Yes, The difference obtained by subtracting the concentration of the carbon functioning as the acceptor in the drift layer from the concentration of the donor in the drift layer is gradually decreased from the substrate side toward the surface side of the drift layer. Semiconductor substrate.
  • Appendix 2 The nitride semiconductor substrate according to appendix 1, wherein the concentration of the donor in the drift layer is not less than 1/3 times the total concentration of the carbon in the drift layer throughout the drift layer.
  • the drift layer includes hydrogen;
  • drift layer made of gallium nitride containing a donor and carbon on a substrate made of an n-type semiconductor;
  • concentration of the donor in the drift layer is 5.0 ⁇ 10 16 pieces / cm 3 or less, and the concentration of the carbon that functions as an acceptor in the drift layer over the entire drift layer
  • a nitride semiconductor that gradually reduces a difference obtained by subtracting the concentration of the carbon functioning as the acceptor in the drift layer from the concentration of the donor in the drift layer from the substrate side toward the surface side of the drift layer.
  • the concentration of the donor in the drift layer is 5.0 ⁇ 10 16 pieces / cm 3 or less, and the concentration of the carbon that functions as an acceptor in the drift layer over the entire drift layer
  • a semiconductor device that gradually decreases a difference obtained by subtracting the concentration of the carbon functioning as the acceptor in the drift layer from the concentration of the donor in the drift layer from the substrate side toward the surface side of the drift layer. Production method.

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