having the etch temperature be 1400 u00b0 c and the etch time be 60 minutes etching was done in the same manner as in embodiment 1 to remove the 400 u03bcm thick aln substrate and a portion of the al 0 25 ga 0 75 n crystal a section thereof from its interface with the substrate to a thickness of 250 u03bcm yielding an al 0 25 ga 0 75 n crystal of 1850 u03bcm thickness in respect of 100 al 0 25 ga 0 75 n crystal samples obtained by the same process steps as just detailed the process damaged layer thickness was 0 u03bcm warpage in the crystal was 6 5 u03bcm and the cracking incidence was 15 the results are tabulated in table v reference is made to fig1 onto one principal face 10 m of an gan substrate group iii nitride substrate 10 of 5 08 cm 2 inch diameter and 400 u03bcm thickness al 0 25 ga 0 75 n crystal iii nitride crystal 20 of 2100 u03bcm thickness doped with 7 u00d7 10 18 cm u2212 3 o atoms was grown by hvpe in this process the al 0 25 ga 0 75 n crystal growth temperature was let be 1150 u00b0 c the partial pressure of al chloride gas and the partial pressure of ga chloride gas which were the group iii precursor gases were let be 250 pa and 750 pa.