A method for manufacturing a group III nitride semiconductor substrate, that includes: growing a first AlN buffer layer on an Si substrate at a first growth temperature; growing a second AlN buffer layer on the first AlN buffer layer at a second growth temperature higher than the first growth temperature; and growing a group III nitride semiconductor layer on the second AlN buffer layer, wherein an Al raw material and an N raw material are alternately repeatedly fed in the growing the first AlN buffer layer.

A method for producing a power semiconductor device with heat dissipating capability includes epitaxially growing a GaN-based buffer layer on a first surface of a sapphire substrate, epitaxially growing a Ga.sub.2O.sub.3 semiconductor layer on the GaN-based buffer layer, forming a source and a drain, a gate dielectric layer, a first gate, an insulator layer, and a metal adhesive layer in sequence, removing part of the metal adhesive layer, the insulator layer, and the gate dielectric layer to expose one of the source and the drain, forming a heat sink which covers the metal adhesive layer, the insulator layer, the gate dielectric layer, and the one of the source and the drain, and conducting a laser lift-off process through a second surface of the sapphire substrate to remove the sapphire substrate and the GaN-based buffer layer.

The present invention is a nitride semiconductor wafer, including: a silicon single-crystal substrate; and a device layer composed of a nitride semiconductor above the silicon single-crystal substrate, wherein the silicon single-crystal substrate is a CZ silicon single-crystal substrate, and has a resistivity of 1000 .Math.cm or more, an oxygen concentration of 5.010.sup.16 atoms/cm.sup.3 (JEIDA) or more and 2.01.0.sup.17 atoms/cm.sup.3 (JEIDA) or less, and a nitrogen concentration of 5.010.sup.14 atoms/cm.sup.3 or more. This provides a nitride semiconductor wafer that hardly causes plastic deformation even using a high-resistant low-oxygen silicon single-crystal substrate produced by the CZ method, which is suitably used for a high-frequency device, and that can reduce warpage of the substrate.