Single crystalline aluminum nitride substrate and optoelectronic devices made therefrom

A method for forming an aluminum nitride single crystalline substrate,

• comprising:
• placing a single crystal AlN seed and an AlN source material in spaced relationship within a crucible;
• heating the crucible in a furnace such that a temperature gradient is formed between the single crystal AlN seed and an AlN source, resulting in a portion of the AlN source material being sublimed and deposited on the single crystal AlN seed to form an AlN single crystalline boule, wherein the crystal growth temperature within the crucible proximal to the single crystal AlN seed during said heating is maintained at about 2200° C. or less;
• cooling the AlN single crystalline boule in three phases comprising a first cooling phase from the crystal growth temperature to a first intermediate temperature of about 1900° C. to about 1800° C., a second cooling phase from the first intermediate temperature to a second intermediate temperature of about 1500° C. to about 1400° C., wherein the second cooling phase is characterized by a cooling rate of 5.0° C. per minute or less, and a third cooling phase from the second intermediate temperature to room temperature, wherein the cooling rate of one or both of the first and third cooling phases is greater than 5.0° C. per minute.