Nitride crystal substrate and method for manufacturing the same
There is provided a nitride crystal substrate constituted by group-III nitride crystal, containing n-type impurities, with an absorption coefficient α being approximately expressed by equation (1) by a least squares method in a wavelength range of at least 1 μm or more and 3.3 μm or less. α = N e K λ a ( where 1.5 × 10 - 19 ≤ K ≤ 6. × 10 - 19 , a = 3 ) , ( 1 ) here, a wavelength is λ (μm), an absorption coefficient of the nitride crystal substrate at 27° C. is α (cm −1 ), a carrier concentration in the nitride crystal substrate is N e (cm −3 ), and K and a are constants, wherein an error of an actually measured absorption coefficient with respect to the absorption coefficient α obtained from equation (1) at a wavelength of 2 μm is within +0.1α, and in a reflection spectrum measured by irradiating the nitride crystal substrate with infrared light, there is no peak with a peak top within a wavenumber range of 1,200 cm −1 or more and 1,500 cm −1 or less.
1. A GaN free-standing substrate comprising GaN single crystal, containing n-type impurities, with an absorption coefficient α being approximated by equation (1) by a method of least squares in a wavelength range of at least 1 μm or more and 3.3 μm or less:
α
=
N
e
K
λ
a
,
equation
(
1
)
where 1.5×10 −19 ≤K≤6.0×10 −19 , a=3,
a wavelength being λ(μm), an absorption coefficient of the GaN free-standing substrate at 27° C. being α(cm −1 ), a carrier concentration in the GaN free-standing substrate being N e (cm −3 ), and K and a being constants,
wherein an error of an actually measured absorption coefficient with respect to the absorption coefficient α obtained from equation (1) at a wavelength of 2 μm is within ±0.1α, and
in a reflection spectrum measured by irradiating the GaN free-standing substrate with infrared light, there is a peak within a wavenumber range of 1,200 cm −1 or more and 1,500 cm −1 or less, and a peak height which is a difference between an intensity reflectance of the peak and an intensity reflectance of a baseline of the reflection spectrum is 35% or less, the baseline of the reflection spectrum is a reflection spectrum in a case where there is no point defect in the GaN free-standing substrate.
2. The GaN free-standing substrate according to claim 1 , where Δα represents a difference between a maximum value and a minimum value of the absorption coefficient α, when analyzed in a main surface of the GaN free-standing substrate by irradiation, the value of Δα≤1.0 and the wavelength range is 1 μm or more and 3.3 μm or less.
3. A method for manufacturing a GaN free-standing substrate, comprising:
preparing a GaN free-standing substrate constituted by GaN single crystal, containing n-type impurities, with an absorption coefficient α being approximated by equation (1) by a method of least squares in a wavelength range of at least 1 μm or more and 3.3 μm or less:
α
=
N
e
K
λ
a
,
equation
(
1
)
where 1.5×10 −19 ≤K≤6.0×10 −19 , a=3,
a wavelength being λ(μm), an absorption coefficient of the GaN free-standing substrate at 27° C. being α(cm −1 ), a carrier concentration in the GaN free-standing substrate being N e (cm −3 ), and K and a being constants,
wherein an error of an actually measured absorption coefficient is within ±0.1α with respect to the absorption coefficient α obtained from equation (1) at a wavelength of 2 μm; and
measuring a reflection spectrum by irradiating the GaN free-standing substrate with infrared light, and inspecting whether there is a peak within a wavenumber range of 1,200 cm −1 and 1,500 cm −1 or less and a peak height which is a difference between an intensity reflectance of the peak and an intensity reflectance of a baseline of the reflection spectrum is 35% or less, the baseline of the reflection spectrum is a reflection spectrum in a case where there is no point defect in the GaN free-standing substrate.
4. The method for manufacturing a GaN free-standing substrate according to claim 3 , comprising: measuring the reflection spectrum for each of a plurality of positions in a main surface of the GaN free-standing substrate; obtaining a peak height, which is a difference between an intensity reflectance of the detected peak and an intensity reflectance of a baseline of the reflection spectrum; and comparing the peak heights obtained for each of the plurality of positions.
5. The method for manufacturing a GaN free-standing substrate according to claim 3 , wherein a point defect density of the GaN free-standing substrate is estimated based on a peak height, which is a difference between an intensity reflectance of the detected peak and an intensity reflectance of a baseline of the reflection spectrum.