Group III nitride substrate with oxygen gradient, method of making, and method of use

A free-standing crystal, comprising a group III metal and nitrogen, wherein the free-standing crystal comprises:

• a wurtzite crystal structure;
• a first surface having a maximum dimension greater than 40 millimeters in a first direction and a crystallographic orientation within 10 degrees of (0 0 0 1);
• and
• a second surface on the opposite side of the crystal from the first surface, wherein a separation between the first surface and the second surface is between about 200 micrometers and about 2000 micrometers,
• wherein an average oxygen concentration within a depth of 2 to 10 micrometers from the first surface, measured at at least four regions, is between 1×1016 cm−3 and 5×1019 cm−3 and is greater, by a factor between about 1.1 and about 10, than the average oxygen concentration within a depth of 2 to 10 micrometers from the second surface, measured at at least four regions, as quantified by calibrated secondary ion mass spectrometry.