Nanoscale investigation of AlGaN/GaN-on-Si high electron mobility transistors

The n-Si/p-CVD Diamond Heterojunction

Due to the possible applications, materials with a wide energy gap are becoming objects of interest for researchers and engineers. In this context, the polycrystalline diamond layers grown by CVD methods on silicon substrates seem to be a promising material for engineering sensing devices. The proper tuning of the deposition parameters allows us to develop the diamond layers with varying crystallinity and defect structure, as was shown by SEM and Raman spectroscopy investigations. The cathodoluminescence (CL) spectroscopy revealed defects located just in the middle of the energy gap of diamonds. The current–voltage–temperature, I−V−T characteristics performed in a broad temperature range of 77–500 K yielded useful information about the electrical conduction in this interesting material. The recorded I−V−T in the forward configuration of the n–Si/p–CVD diamond heterojunction indicated hopping trough defects as the primary mechanism limiting conduction properties. The Ohmic character of the carriers flux permitting throughout heterojunction is intensified by charges released from the depletion layer. The magnification amplitude depends on both the defect density and the probability that biasing voltage is higher than the potential barrier binding the charge. In the present work, a simple model is proposed that describes I−V−T characteristics in a wide range of voltage, even where the current saturation effect occurs.

In situ transmission electron microscopy of transistor operation and failure

Microscopy is typically used as a post-mortem analytical tool in performance and reliability studies on nanoscale materials and devices. In this study, we demonstrate real time microscopy of the operation and failure of AlGaN/GaN high electron mobility transistors inside the transmission electron microscope. Loading until failure was performed on the electron transparent transistors to visualize the failure mechanisms caused by self-heating. At lower drain voltages, thermo-mechanical stresses induce irreversible microstructural deformation, mostly along the AlGaN/GaN interface, to initiate the damage process. At higher biasing, the self-heating deteriorates the gate and catastrophic failure takes place through metal/semiconductor inter-diffusion and/or buffer layer breakdown. This study indicates that the current trend of recreating the events, from damage nucleation to catastrophic failure, can be replaced by in situ microscopy for a quick and accurate account of the failure mechanisms.

Nanoscale electrical and structural modification induced by rapid thermal oxidation of AlGaN/GaN heterostructures

In this paper, the structural and electrical modifications induced, in the nanoscale, by a rapid thermal oxidation process on AlGaN/GaN heterostructures, are investigated. A local rapid oxidation (900 ° C in O2, 10 min) localized under the anode region of an AlGaN/GaN diode enabled a reduction of the leakage current with respect to a standard Schottky contact. The insulating properties of the near-surface oxidized layer were probed by a nanoscale electrical characterization using scanning probe microscopy techniques. The structural characterization indicated the formation of a thin uniform oxide layer on the surface, with preferential oxidation paths along V-shaped defects penetrating through the AlGaN/GaN interface. The oxidation process resulted in an expansion of the lattice parameters due to the incorporation of oxygen atoms, accompanied by an increase of the crystal mosaicity. As a consequence, a decrease of the sheet carrier density of the two-dimensional electron gas and a positive shift of the threshold voltage are observed. The results provide useful insights for a possible future integration of rapid oxidation processes during GaN device fabrication.