High Breakdown Voltage and Low Buffer Trapping in Superlattice GaN-on-Silicon Heterostructures for High Voltage Applications.

The aim of this work is to demonstrate high breakdown voltage and low buffer trapping in superlattice GaN-on-Silicon heterostructures for high voltage applications. To this aim, we compared two structures, one based on a step-graded (SG) buffer (reference structure), and another based on a superlattice (SL). In particular, we show that: (i) the use of an SL allows us to push the vertical breakdown voltage above 1500 V on a 5 µm stack, with a simultaneous decrease in vertical leakage current, as compared to the reference GaN-based epi-structure using a thicker buffer thickness. This is ascribed to the better strain relaxation, as confirmed by X-Ray Diffraction data, and to a lower clustering of dislocations, as confirmed by Defect Selective Etching and Cathodoluminescence mappings. (ii) SL-based samples have significantly lower buffer trapping, as confirmed by substrate ramp measurements. (iii) Backgating transient analysis indicated that traps are located below the two-dimensional electron gas, and are related to C defects. (iv) The signature of these traps is significantly reduced on devices with SL. This can be explained by the lower vertical leakage (filling of acceptors via electron injection) or by the slightly lower incorporation of C in the SL buffer, due to the slower growth process. SL-based buffers therefore represent a viable solution for the fabrication of high voltage GaN transistors on silicon substrate, and for the simultaneous reduction of trapping processes.