Carbon‐doping in the concentration range from [C] = 5 × 1017 to 1.2 × 1019 cm−3 is employed to achieve semi‐insulating properties of GaN layers as required for electronic power devices. Using propane as a carbon precursor, an independent analysis of the carbon incorporation during growth and its impact on electrical properties of the layers was obtained as growth parameters for optimum GaN quality could be applied. We observe that C is within precision of measurements fully incorporated in GaN as compensating deep acceptor. In a series of Si + C co‐doped samples, semi‐insulating properties were obtained for [C] > [Si] and the compensation efficiency for electrons is around unity. Through the extrinsic C‐doping technique previous ambiguous results on electrical and optical properties of GaN:C layers are clarified.
Carbon-doping is proposed to reduce the dislocation-mediated leakage currents in the GaN buffer layers. GaN:C grown by metalorganic vapor phase epitaxy using propane shows excellent quality up to [C] = 6.7 × 1018 cm−3. Locally probing dislocations by surface scanning potential microscopy reveal a transition from mostly neutral or weakly charged regions to dominantly negatively charged regions relative to the surrounding area at high doping levels. A relation between leakage currents and the relative dislocation charge state exists. Minimum leakage current is achieved if the dominant charge state of dislocation regions becomes negative against the surrounding.
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.
In this work, we have designed excellent performance GaN-based Schottky barrier diode (SBD) with a sandwich structure by inserting a graphene (Gr) interlayer. The electrical properties of Pt/Gr/GaN and Pt/GaN SBDs have been systematically investigated by the temperature-dependent current—voltage (I–V) and capacitance–voltage measurements in order to explore the effects of Gr on main diode parameters. At room temperature, the Pt/Gr/GaN SBD exhibited lower turn-on voltage (Von), ideality factor (n), differential specific on-resistance (Ron), and higher Schottky barrier height (SBH), signifying enhanced device attributes. Furthermore, the ideality factor and SBH for the Pt/Gr/GaN SBD were found to be insensitive to temperature from the temperature-dependent I–V analysis. The results revealed a highly homogeneous Schottky barrier interface in the case of Pt/Gr/GaN SBD. This facile strategy opened a pathway to improve the performance of the nitride Schottky rectifiers.
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