Leakage currents and Fermi-level shifts in GaN layers upon iron and carbon-doping

Abstract

Semi-insulating GaN is a prerequisite for lateral high frequency and high power electronic devices to isolate the device region from parasitic conductive channels. The commonly used dopants for achieving semi-insulating GaN, Fe, and C cause distinct properties of GaN layers since the Fermi-level is located either above (Fe) or below (C) the midgap position. In this study, precursor-based doping of GaN in metalorganic vapor phase epitaxy is used at otherwise identical growth conditions to control the dopant concentrations in the layer. Using electric force microscopy, we have investigated the contact potentials of Fe- and C-doped samples with respect to a cobalt metal probe tip in dependence of on the dopant concentration. While in Fe-doped samples the sign of the contact potential is constant, a change from positive to negative contact potential values is observed at high carbon concentrations, indicating the shift of the Fermi-level below the midgap position. In vertical transport measurements, C-doped GaN layers with a dopant concentration of 4.6 × 1018 cm−3 exhibit up to 5 orders of magnitude lower dark current at room temperature and significantly lower temperature dependence than Fe-doped samples with a similar dopant concentration. Therefore, precursor-based carbon doping is the superior doping technique to achieve semi-insulating GaN.

“Kink” in AlGaN/GaN-HEMTs: Floating Buffer Model

“…In that case, the SIMS background carbon density was 10 17 cm −3 with an unknown donor density, which presumably must have been less than that value. Scanning probe measurements in carbon-doped GaN have shown that the material can change from p-type at high C density to n-type at low density [20]. Koller et al [21] measured the built-in voltage in a p-n diode and showed that high C density material is p-type.…”

Investigation of Defect Characteristics and Carrier Transport Mechanisms in GaN Layers With Different Carbon Doping Concentration

“…Above the TL, a GaN buffer layer is grown, which is significant for high-power and RF devices due to its impact on the vertical leakage, the breakdown behavior, and the current collapse (CC). Carbon doping is usually employed to achieve a high resistivity GaN buffer layer, which, reportedly, can compensate the initial n-type background doping of GaN, resulting in a semi-insulating layer [6], [7]. However, the carbon doping also induces defects to the GaN layer, which may capture electrons under OFF-state switching and degrade the ONstate resistance.…”

Reliability comparison of AlGaN/GaN HEMTs with different carbon doping concentration

“…Fe acceptors have been reported to be related to the presence of traps located at 0.72 eV or 0.63 eV below conductance band [11], [12], but it may lead to kink effects in the devices under certain conditions [13]. Carbon doping is used both for power switching and RF GaN HEMTs and it has been proved to be effective in reducing leakage current and increasing breakdown voltage [14]. Due to its self-compensation effect [15], [16], it can replace Ga or N, working as donor or acceptor trapping centres.…”