Thermally stimulated current spectroscopy and photoluminescence of carbon‐doped semi‐insulating GaN grown by ammonia‐based molecular beam epitaxy

Abstract

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.

Deep traps in AlGaN/GaN heterostructures studied by deep level transient spectroscopy: Effect of carbon concentration in GaN buffer layers

“…In a subsequent TSC study of carbondoped semi-insulating GaN, grown by ammonia-based molecular beam epitaxy ͑MBE͒, trap B x ͑0.50 eV͒ was found to dominate in the high-carbon sample ͓͑C͔ =8ϫ 10 18 cm −3 ͒ and was tentatively assigned to C Ga . 11 The impact of carbon on trap states in n-type GaN grown by MOCVD was also studied by both DLTS and DLOS. 12 From a comparison of the DLOS measurements on low-pressure grown codoped GaN:C:Si and atmospheric-pressure grown UID GaN, two deep levels, at 1.35 and 3.28 eV below the conduction band, were observed to have a direct relation with excess carbon incorporation.…”

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

“…It is important to note that the photo response ͑or photocurrent per 1 V͒ in both types of samples is many orders of magnitude higher than that in carbon-doped semi-insulating GaN materials. In a previous study, 9 the carbon-doped GaN layers showed saturated photocurrent of less than 0.2 pA under 100 V bias and 360 nm light excitation. This indicates that the UID semi-insulating/ resistive GaN materials have significantly longer effective carrier lifetime than the carbon-doped GaN materials.…”

“…The luminescence properties of the carbon-doped GaN have been reported and discussed in a previous paper. 9 Because of the high carbon ͑in the order of 10 19 cm −3 ͒ and associated oxygen ͑in the order of 10 18 cm −3 ͒ concentrations, radiative recombination of excitons is suppressed by prevailing nonradiative recombination centers that were introduced by the carbon-doping process. The nonradiative centers may involve carbon and oxygen impurities and their complexes formed with native defects.…”

“…The deep acceptors are formed by Ga vacancies ͑V Ga ͒, or more likely V Ga -O N complexes. 9,[31][32][33] The second model ascribes the YL to a transition from the split deep levels ͑1.6 and 2.3 eV above valence band͒ formed by carbon interstitials ͑C I ͒ in carbon-doped GaN materials to shallow acceptors or the valence band. [34][35][36] Substitutional C N atoms form shallow acceptor states at about 0.2 eV above the valence band.…”

“…The heavily doped semi-insulating GaN materials are mainly used as thick buffer layers in GaN HEMT devices. However, concern still exists as to possible deterioration of electrical or optical performance associated with carrier trapping, short effective carrier lifetime, 9 growth quality degradation, or memory effect of the compensation impurities. In that regard, unintentionally doped ͑UID͒ semi-insulating or high-resistivity GaN have advantages especially when used for the active regions of devices such as HEMT, PIN diodes, and high voltage Schottky diodes, and when efficiency associated with long carrier lifetime is required.…”