Nitride crystal substrate, semiconductor laminate, method of manufacturing semiconductor laminate and method of manufacturing semiconductor device

1. A nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities,

wherein an absorption coefficient α in a wavelength range of at least 1 μm or more and 3.3 μm or less

satisfies equation (1)′,

1.5×10 −19 nλ 3 ≤α≤6.0×10 −19 nλ 3   (1)′,

wherein, λ is a wavelength expressed in μm,

α is absorption coefficient of the nitride crystal substrate at 27° C. expressed in cm −1 , and

n is a free electron concentration in the nitride crystal substrate expressed in cm −3 .

2. A nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities, wherein an absorption coefficient α is approximately expressed by equation (1)″ in a wavelength range of at least 1 μm or more and 3.3 μm or less according to the least-squares method, and

an error of the measured absorption coefficient relative to the absorption coefficient α obtained from equation (1)″ is within ±0.1α at the wavelength of 2 μm:

α=2.2×10 −19 nλ 3   (1)″,

wherein, λ is a wavelength expressed in μm,

α is absorption coefficient of the nitride crystal substrate at 27° C. expressed in cm −1 , and

n is a free electron concentration in the nitride crystal substrate expressed in cm −3 .

3. The nitride crystal substrate according to claim 1 , wherein the absorption coefficient α and Δα satisfy equations (2) and (3) in a wavelength range of at least 1 μm or more and 3.3 μm or less:

α≥0.15λ 3   (2)

Δα≤1.0  (3)

wherein, Δα is a difference between a maximum value and a minimum value of the absorption coefficient α in a main surface of the nitride crystal substrate.

4. A nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities,

wherein a concentration of boron in the nitride crystal substrate is less than 1×10 15 at·cm −3 ,

wherein an absorption coefficient of the nitride crystal substrate at a wavelength of 2 μm is 1.2 cm −1 or more, and

a difference between a maximum value and a minimum value of the absorption coefficient at a wavelength of 2 μm in a main surface of the nitride crystal substrate is within 1.0 cm −1 .

5. The nitride crystal substrate according to claim 1 , wherein a concentration of intrinsic carriers thermally excited between bands of the nitride crystal substrate under a temperature condition of 27° C. or more and 1250° C. or less, is lower than a concentration of free electrons generated in the nitride crystal substrate by doping with the n-type impurities under a temperature condition of 27° C.

6. The nitride crystal substrate according to claim 1 ,

wherein a concentration of free electrons generated in the nitride crystal substrate by doping with the n-type impurities, is 1×10 18 cm −3 or more under a temperature condition of 27° C., and

a difference between a maximum value and a minimum value of the free electron concentration in a main surface of the nitride crystal substrate is within 8.3×10 17 cm −3 .

7. The nitride crystal substrate according to claim 1 ,

wherein a concentration of the n-type impurities in the nitride crystal substrate is 1×10 18 at·cm −3 or more, and

a difference between a maximum value and a minimum value of the concentration of the n-type impurities in a main surface of the nitride crystal substrate is within 8.3×10 17 at·cm −3 .

8. The nitride crystal substrate according to claim 1 , wherein a concentration of oxygen in the nitride crystal substrate is 1/10 or less relative to a total concentration of silicon and germanium in the nitride crystal substrate.

9. A semiconductor laminate, comprising:

a nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities; and

a semiconductor layer comprising group-III nitride semiconductor and provided on the nitride crystal substrate,

wherein an absorption coefficient α

in a wavelength range of at least 1 μm or more and 3.3 μm or less satisfies equation (1)′,

1.5×10 −19 nλ 3 ≤α≤6.0×10 −19 nλ 3   (1)′,

wherein, λ is a wavelength expressed in μm,

α is absorption coefficient of the nitride crystal substrate at 27° C. expressed in cm −1 , and

n is a free electron concentration in the nitride crystal substrate expressed in cm −3 .

10. A semiconductor laminate, comprising:

a nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities; and

a semiconductor layer comprising group-III nitride semiconductor and provided on the nitride crystal substrate,

wherein a concentration of boron in the nitride crystal substrate is less than 1×10 15 at·cm −3 ,

wherein an absorption coefficient of the nitride crystal substrate at a wavelength of 2 μm is 1.2 cm −1 or more, and

a difference between a maximum value and a minimum value of the absorption coefficient at a wavelength of 2 μm in a main surface of the nitride crystal substrate is within 1.0 cm −1 .

11. The semiconductor laminate according to claim 9 , wherein a reflectance of a surface of the semiconductor layer is 5% or more and 30% or less in a wavelength range of at least 1 μm or more and 3.3 μm or less.

12. A method for manufacturing a semiconductor laminate, comprising:

preparing a nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities; and

irradiating the nitride crystal substrate with at least infrared rays, to heat the nitride crystal substrate,

wherein, in the preparation of the nitride crystal substrate,

a substrate with an absorption coefficient α in a wavelength range of at least 1 μm or more and 3.3 μm or less satisfying equation (1)′, is prepared as the nitride crystal substrate:

1.5×10 −19 nλ 3 ≤α≤6.0×10 −19 nλ 3   (1)′,

wherein, λ is a wavelength expressed in μm,

α is absorption coefficient of the nitride crystal substrate at 27° C. expressed in cm −1 , and

n is a free electron concentration in the nitride crystal substrate expressed in cm −3 .

13. A method for manufacturing a semiconductor laminate, comprising:

preparing a nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities; and

irradiating the nitride crystal substrate with at least infrared rays, to heat the nitride crystal substrate,

wherein, in the preparation of the nitride crystal substrate,

a substrate in which an absorption coefficient at a wavelength of 2 μm is 1.2 cm −1 or more and a difference between a maximum value and a minimum value of the absorption coefficient at a wavelength of 2 μm in a main surface is within 1.0 cm −1 , is prepared as the nitride crystal substrate.

14. The method for manufacturing the semiconductor laminate according to claim 12 ,

wherein the heating of the nitride crystal substrate comprises: epitaxially growing a semiconductor layer including group-III nitride semiconductor on the nitride crystal substrate.

15. The method for manufacturing the semiconductor laminate according to claim 14 , comprising:

epitaxially growing a p-type semiconductor layer containing p-type impurities as a layer constituting the semiconductor layer, on the nitride crystal substrate,

wherein the heating of the nitride crystal substrate comprises: heating the nitride crystal substrate, to activate the p-type impurities in the p-type semiconductor layer.

16. The method for manufacturing the semiconductor laminate according to claim 14 , comprising:

ion-implanting predetermined impurities into the semiconductor layer,

wherein the heating of the nitride crystal substrate comprises: heating the nitride crystal substrate, to activate the impurities in the semiconductor layer.

17. A method for manufacturing a semiconductor laminate, comprising:

preparing a nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities; and

irradiating the nitride crystal substrate with at least infrared rays, to heat the nitride crystal substrate,

wherein the preparation of the nitride crystal substrate comprises:

measuring an absorption coefficient of the nitride crystal substrate in an infrared region at 27° C.; and

determining whether the absorption coefficient α in a wavelength range of at least 1 μm or more and 3.3 μm or less satisfies equation (1)′ based on the measured absorption coefficient of the nitride crystal substrate:

1.5×10 −19 nλ 3 ≤α≤6.0×10 −19 nλ 3   (1)′,

wherein, λ is a wavelength expressed in μm,

α is absorption coefficient of the nitride crystal substrate at 27° C. expressed in cm −1 , and

n is a free electron concentration in the nitride crystal substrate expressed in cm −3 .

18. A method for manufacturing a semiconductor laminate, comprising:

preparing a nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities; and

irradiating the nitride crystal substrate with at least infrared rays, to heat the nitride crystal substrate,

wherein the preparation of the nitride crystal substrate comprises:

measuring absorption coefficients in an infrared region at least at two or more points in a main surface of the nitride crystal substrate; and

determining whether the absorption coefficient of the nitride crystal substrate at a wavelength of 2 μm is 1.2 cm −1 or more and whether a difference between a maximum value and a minimum value of the absorption coefficient at a wavelength of 2 μm in the main surface of the nitride crystal substrate is within 1.0 cm −1 , based on the measured absorption coefficient of the nitride crystal substrate.

19. A method for manufacturing a semiconductor device, comprising:

preparing a nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities; and

irradiating the nitride crystal substrate with at least infrared rays, to heat the nitride crystal substrate,

wherein, in the preparation of the nitride crystal substrate,

a substrate with an absorption coefficient α in a wavelength range of at least 1 μm or more and 3.3 μm or less satisfying equation (1)′, is prepared as the nitride crystal substrate:

1.5×10 −19 nλ 3 ≤α≤6.0×10 −19 nλ 3   (1)′,

wherein, λ is a wavelength expressed in μm,

α is absorption coefficient of the nitride crystal substrate at 27° C. expressed in cm −1 , and

n is a free electron concentration in the nitride crystal substrate expressed in cm −3 .

20. A method for manufacturing a semiconductor device, comprising:

preparing a nitride crystal substrate comprising group-III nitride crystal and containing n-type impurities; and

irradiating the nitride crystal substrate with at least infrared rays, to heat the nitride crystal substrate,

wherein, in the preparation of the nitride crystal substrate,

a substrate in which an absorption coefficient at a wavelength of 2 μm is 1.2 cm −1 or more and a difference between a maximum value and a minimum value of the absorption coefficient at a wavelength of 2 μm in a main surface is within 1.0 cm −1 , is prepared as the nitride crystal substrate.

21. The nitride crystal substrate according to claim 1 , wherein a concentration of boron in the nitride crystal substrate is less than 1×10 15 at·cm −3 .