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.

The properties of carbon-doped GaN epilayers grown by molecular-beam epitaxy have been studied by temperature-dependent resistivity, Hall-effect measurements, x-ray diffraction, and by photoluminescence spectroscopy. Carbon doping was found to render the GaN layers highly resistive (>108 Ω cm) and quench the band edge excitonic emissions. Yellow luminescence is still present in carbon-doped GaN layers. The highly resistive state is interpreted as being caused by direct compensation by the carbon acceptors and by the consequently enhanced potential barrier at the subgrain boundaries. Evidence of dislocations joining to form potential barriers along the subgrain boundaries was observed in photoassisted wet etching experiments on electrically conducting GaN layers. GaN films grown on insulating carbon-doped base layers are of excellent transport and optical properties.

Semi-insulating GaN samples, grown by ammonia-based molecular beam epitaxy and doped with carbon, were investigated by thermally stimulated current spectroscopy and photoluminescence at 4.2 K. In addition to a dominant trap at 0.90 eV, thought to be related to the N interstitial, a trap at 0.50 eV, presumably related to C Ga , was observed in the samples with high carbon concentrations. For all of the carbon-doped samples, strong photoluminescence (PL) bands were observed in the yellow (YL), blue (BL), and nearband-edge regions, with the YL dominating, and the BL decreasing as the carbon concentration increased. Besides the PL and trap properties, the carbon doping also influenced the resistivity and effective carrier lifetime. 1 Introduction Carbon is often a major background impurity in GaN grown by metal-organic vapor phase epitaxy (MOCVD), and can be controlled to produce semi-insulating (SI) films, useful as buffer layers in the fabrication of AlGaN/GaN high electron mobility transistors [1]. However, for SI film growth by molecular beam epitaxy (MBE), the C must be specifically added. In spite of a large body of experimental and theoretical research on C in GaN (for example, see Ref.[2] for a theoretical study), several properties, including the formation of deep traps, remain poorly understood. In a recent work, SI GaN samples grown by MOCVD and by plasma-assisted (PA) MBE under Ga-rich conditions were characterized by thermally stimulated current (TSC) spectroscopy and a new carbon-related trap B x (0.50 eV) was clearly observed [3]. In this study, C-doped GaN grown by ammonia-based (AB) MBE under N-rich conditions was characterized by TSC and 4.2-K photoluminescence (PL). It was found that the C incorporation had a strong influence on resistivity, trap species, effective carrier lifetime, and blue luminescence band.