Growth kinetics and electronic properties of unintentionally doped semi-insulating GaN on SiC and high-resistivity GaN on sapphire grown by ammonia molecular-beam epitaxy

Growth of unintentionally doped (UID) semi-insulating GaN on SiC and highly resistive GaN on sapphire using the ammonia molecular-beam epitaxy technique is reported. The semi-insulating UID GaN on SiC shows room temperature (RT) resistivity of 1011 Ω cm and well defined activation energy of 1.0 eV. The balance of compensation of unintentional donors and acceptors is such that the Fermi level is lowered to midgap, and controlled by a 1.0 eV deep level defect, which is thought to be related to the nitrogen antisite NGa, similar to the “EL2” center (arsenic antisite) in unintentionally doped semi-insulating GaAs. The highly resistive GaN on sapphire shows RT resistivity in range of 106–109 Ω cm and activation energy varying from 0.25 to 0.9 eV. In this case, the compensation of shallow donors is incomplete, and the Fermi level is controlled by levels shallower than the 1.0 eV deep centers. The growth mechanisms for the resistive UID GaN materials were investigated by experimental studies of the surface kinetics during growth. The required growth regime involves a moderate growth temperature range of 740–780 °C, and a high ammonia flux (beam equivalent pressure of 1×10−4 Torr), which ensures supersaturated coverage of surface adsorption sites with NHx radicals. Such highly nitrogen rich growth conditions lead to two-dimensional layer by layer growth and reduced oxygen incorporation.