Novel MBE III-Nitride Heterostructure Devices Enabled by Bulk Substrates

III-Nitride semiconductors have been of great interest and have been studied extensively for the applications in high frequency and high power devices. So far, most III-Nitride devices are grown and fabricated on sapphire, SiC, Si and other substrates. The epitaxially grown materials on these substrates suffer from high defect densities due to the lattice mismatch. Dislocations and other defects degrade performance, especially for devices utilizing a vertical current path. Recently single-crystal bulk gallium nitride (GaN) and aluminum nitride (AlN) substrates with very low defect densities have become available, opening the door for fundamental advances in epitaxy, and physical re-evaluation of the III-Nitrides, and also potentially new applications in optical and electronic devices.

In this work, we have explored various III-Nitride heterostructures epitaxially grown on single-crystal bulk AlN and GaN substrates. MBE grown vertical p-n junctions enabled by n⁺ GaN bulk substrates are studied for high power applications. GaN p-n junctions with off-state leakage current as low as 3 nA/cm², an on-resistance of 0.23 mΩ·cm² and a breakdown field of ~3.1 MV/cm are achieved. These leakage and breakdown characteristics represent the highest performance metrics in GaN p-n junction diodes grown by MBE. To further improve the high field properties of p-n junctions, polarization-induced AlGaN (Pi-AlGaN) p-n junctions are grown by MBE. Compared to GaN p-n homojunction diodes, the Pi-AlGaN p-n diodes take advantages of both polarization-induced doping and the larger band gap of AlGaN, to achieve improved breakdown properties but still with a low resistance. The fabricated polarization-induced Al_0.23GaN p-n diodes show a highest breakdown field of ~3.8 MV/cm. The breakdown field is among the highest reported values for III-Nitrides. Based on the observed electroluminescence from the GaN p-n diodes, we propose optical cooling for power electronic devices. The excellent high field properties together with unique optical cooling of III-Nitrides makes the Pi-AlGaN heterostructures candidates for a new generation of power electronic devices.

MBE growths of AlN/GaN/AlN quantum well (QW) heterostructures on single-crystal bulk AlN were studied. The QW structure has shown attractive properties such as the presence of a 2DEG whose charge can be varied by changing the QW thickness as well as the barrier thickness. The AlN barrier of the QW induces the maximal carrier densities while providing the best confinement for GaN channels at the same time. Bulk single crystal AlN substrates offer the highest thermal conductivity possible in the III-Nitride materials and a low dislocation density platform. By the growth optimization of GaN QWs on single crystal AlN substrates, an n-channel 2DEG with an electron mobilities of 601/1380 cm²/V·s with a 2DEG densities of 3.2/2.6×10¹³ cm^-2 and sheet resistance of 327/171 Ω/□ were measured at 300/77 K. The mobilities at 300 K and 77 K are both the highest among GaN QW heterostructures on the AlN platform. The resulting overall electrical properties make the QW heterostructures well suited for high-frequency field effect transistors. Ultra-thin body (UTB) GaN on AlN platform also enables high density of two-dimensional hole gases (DHG), by polarization-induced p-type doping. The growth of the UTB GaN/AlN heterosturcture is studied on single-crystal bulk AlN as well, and 2DHG is measured experimentally.

The strained GaN layer QW layer in the AlN/GaN/AlN QW structures discussed above were demonstrated as optical markers for Raman characterization. The GaN QW structure is ideally suited as an optical marker because all other regions – the barrier and the underlying buffer have much larger bandgaps. Together with isotope ¹⁵N as a second optical marker, a new dual marker method for Raman spectroscopy is enabled and demonstrated. We demonstrate the effectiveness of dual optial marker Raman spectroscopy in studying strain in both vertical and horizontal directions.