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.