Method of processing a surface of group iii nitride crystal and group iii nitride crystal substrate

A method of processing a surface of a group III nitride crystal, comprising the steps of:

• polishing a surface of a group III nitride crystal with a polishing slurry containing abrasive grains;
• and
• thereafter polishing said surface of said group III nitride crystal with a polishing liquid at least once, wherein:
• each step of polishing with said polishing liquid employs one of a basic polishing liquid and an acidic polishing liquid as said polishing liquid;
• and
• said basic polishing liquid contains at least one type selected from the group consisting of a base having at least any of C, O, H and N as a constituent element, a salt thereof and an oxidizer having at least any of C, O, H and N as a constituent element and has a pH of at least 8.5 and at most 14, and if said pH is represented by a value x and said basic polishing liquid has an oxidation reduction potential represented by a value y represented in mV, said values x and y satisfy the following expression:
• y>−50x+800 (i);
• said acidic polishing liquid contains at least one type selected from the group consisting of an acid having at least any of C, O, H and N as a constituent element, a salt thereof and an oxidizer having at least any of C, O, H and N as a constituent element and has a pH of at least 1.5 and at most 6, and if said pH is represented by a value x and said acidic polishing liquid has an oxidation reduction potential represented by a value y represented in mV, said values x and y satisfy the following expression:
• y>−50x+800 (i).