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

Abstract

Electrical properties, including leakage currents, threshold voltages, and deep traps, of AlGaN/GaN heterostructure wafers with different concentrations of carbon in the GaN buffer layer, have been investigated by temperature dependent current-voltage and capacitance-voltage measurements and deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs). It is found that (i) SBDs fabricated on the wafers with GaN buffer layers containing a low concentration of carbon (low-[C] SBD) or a high concentration of carbon (high-[C] SBD) have similar low leakage currents even at 500 K; and (ii) the low-[C] SBD exhibits a larger (negative) threshold voltage than the high-[C] SBD. Detailed DLTS measurements on the two SBDs show that (i) different trap species are seen in the two SBDs: electron traps Ax (0.9 eV), A1 (0.99 eV), and A2 (1.2 eV), and a holelike trap H1 (1.24 eV) in the low-[C] SBD; and electron traps A1, A2, and A3 (∼1.3 eV), and a holelike trap H2 (>1.3 eV) in the high-[C] SBD; (ii) for both SDBs, in the region close to GaN buffer layer, only electron traps can be detected, while in the AlGaN/GaN interface region, significant holelike traps appear; and iii) all of the deep traps show a strong dependence of the DLTS signal on filling pulse width, which indicates they are associated with extended defects, such as threading dislocations. However, the overall density of electron traps is lower in the low-[C] SBD than in the high-[C] SBD. The different traps observed in the two SBDs are thought to be mainly related to differences in microstructure (grain size and threading dislocation density) of GaN buffer layers grown at different pressures.