A method of growing semi-insulating GaN epilayers by ammonia molecular beam epitaxy through intentional doping with carbon is reported. Thick GaN layers of high resistivity are an important element in GaN-based heterostructure field-effect transistors. A methane ion source was used as the carbon dopant source. The cracking of the methane gas by the ion source was found to be the key to the effective incorporation of carbon. High-quality C-doped GaN layers with resistivities greater than 106 Ω cm have been grown with high reproducibility and reliability. AlGaN/GaN heterostructures grown on the C-doped semi-insulating GaN-based layers exhibited a high-mobility two-dimensional electron gas at the heterointerface, with room-temperature mobilities typically between 1000 and 1200 cm2/V s, and liquid-nitrogen-temperature mobilities up to 5660 cm2/V s. The carrier density was almost constant, with less than 3% change over the measured temperature range.

A simple UV photo-enhanced wet etch has been developed for GaN. Unlike photoelectrochemical wet etching, this technique does not require an electrical contact to be made to the sample, and nitrides deposited on insulating substrates (such as sapphire) can be etched. The technique relies on adding an appropriate oxidizing agent, in this case, peroxydisulfate to KOH solutions. Maximum etch rates are observed at pH 12.4. The etch rate increases linearly with light intensity at 365 nm up to intensities of 25 mW/cm 2 , where etch rates of up to 50 nm/min are observed.

Growth kinetics and electronic properties of unintentionally doped semi-insulating GaN on SiC and high-resistivityGrowth of unintentionally doped ͑UID͒ 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 ͑RT͒ resistivity of 10 11 ⍀ 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 N Ga , similar to the "EL2" center ͑arsenic antisite͒ in unintentionally doped semi-insulating GaAs. The highly resistive GaN on sapphire shows RT resistivity in range of 10 6 -10 9 ⍀ 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 ϫ 10 −4 Torr͒, which ensures supersaturated coverage of surface adsorption sites with NH x radicals. Such highly nitrogen rich growth conditions lead to two-dimensional layer by layer growth and reduced oxygen incorporation.

No abstract