Nitride crystal substrate and method for manufacturing the same

A nitride crystal substrate constituted by group-III nitride 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=NeKλa equation (1),
• where 1.5×10−19≤K≤6.0×10−19, a=3,
• a wavelength being λ (μm), an absorption coefficient of the nitride crystal substrate at 27° C. being α (cm−1), a carrier concentration in the nitride crystal substrate being Ne (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 ±0.1 α, in a reflection spectrum measured by irradiating the nitride crystal substrate with infrared light, there is no peak within a wavenumber range of 1,200 cm−1 or more and 1,500 cm or less, and
• a maximum concentration of oxygen in the nitride crystal substrate is less than 1×1017 at ·cm3.