Electrically active defects in the band gap of semiconductors can have a profound effect on the performance of devices. In the case of silicon, SiGe and the GaAs & InAs families of semiconductors the use of deep level transient spectroscopy (DLTS) to study defect levels is well established. The situation is nowhere near as satisfactory in the wider band gap materials. In this paper we describe work at Manchester on developing electrical techniques to study defects such materials with an emphasis on SiC and GaN. We discuss the general problem of defect characterisation in wider gap materials and the the use of high resolution (Laplace DLTS) to study radiation damage in SiC. We then discuss measurements of defect states using DLTS and LDLTS measurements on electron irradiated GaN. Finally, we describe a novel technique we are developing under a UK EPSRC contract to better study wide gap materials.

We report about cathodoluminescence characterization directly performed in scanning transmission electron microscopy (STEM-CL) of AlGaN layers grown by sputtering deposition on AlN/Si(111)-templates.
AlGaN bilayer structure was grown on top of the template. STEM images exhibit dislocation density comparable to that of the template for the first AlGaN layer. A columnar growth is observed for the second AlGaN layer. Integral spectra reveal a recombination at 331 nm and defect related emission at ~500 nm. In highly spatially resolved spectral linescans locally strong wavelengths variations of the 331 nm band can be found.
STEM-CL investigations on sputtered AlGaN/AlN multiple quantum wells were carried out. The panchromatic CL intensity images exhibit a spotty intensity distribution. Broad luminescence emitting at 290 nm is observed in overview spectra. The spectral evolution in growth direction shows a strong variation of the peak wavelength mainly caused by the strong quantum well thickness fluctuations.

We demonstrate that 3.5% in-plane lattice mismatch between GaN (0001) epitaxial layers and SiC (0001) substrates can be accommodated without triggering extended defects over large areas using a grain-boundary-free AlN nucleation layer (NL). Defect formation in the initial epitaxial growth phase is thus significantly alleviated, confirmed by various characterization techniques. As a result, a high-quality 0.2-μm thin GaN layer can be grown on the AlN NL and directly serve as a channel layer in power devices, like high electron mobility transistors (HEMTs). The channel electrons exhibit a state-of-the-art mobility of >2000 cm2/V-s, in the AlGaN/GaN heterostructures without a conventional thick C- or Fe-doped buffer layer. The highly scaled transistor processed on the heterostructure with a nearly perfect GaN–SiC interface shows excellent DC and microwave performances. A peak RF power density of 5.8 W/mm was obtained at VDSQ = 40 V and a fundamental frequency of 30 GHz. Moreover, an unpassivated 0.2-μm GaN/AlN/SiC stack shows lateral and vertical breakdowns at 1.5 kV. Perfecting the GaN–SiC interface enables a GaN–SiC hybrid material that combines the high-electron-velocity thin GaN with the high-breakdown bulk SiC, which promises further advances in a wide spectrum of high-frequency and power electronics.

To meet the tight requirements for GaN RF devices in space and defence applications, a repeatable GaN epitaxial process with sufficient throughput is needed. Recent developments using our existing capability have addressed the material requirements, including surface defect density and cross-wafer uniformity while the addition of a new multi-wafer reactor addresses volume.

In this paper we will review progress, which has, for example, simultaneously reduced the surface defect density and wafer uniformity on the existing capability. Device uniformity for Vth and Ron of wafers processed at BeMiTeC have been below 2% , consistent with non-destructive measurements at IQE. In addition very respectable maximum output powers of ≈5 W/mm at 10GHz for a 1mm device at Vds of 28V have been demonstrated. In the non-optimised transfer of the epitaxial process to the multi-wafer reactor, identical structures have delivered sheet resistivity uniformities below 2% and surface defect densities of <10 cm-2.

While GaN is gradually introduced in RF power amplifiers, advanced GaN material solutions are strongly desired to address higher frequency bands up to mm-wave, such as desired for 5G wireless communication, satellite communication, or autonomous driving. Increasing the transconductance, while simultaneously broadening useful range of device operation shall also be addressed at the material level, with the development of ultra-thin FET barriers (<4nm): either InAlN/GaN or SiN/AlN/GaN active layers. While these can be deposited on SiC or Si substrates, we will show GaN-on-Si RF buffers with leakage current below 1uA/mm up to 200V, low dispersion (<10%) and low RF losses (below 0.4dB/mm at 40GHz). Remarkably, these RF losses remain low up to 150°C (base plate temperature) increasing up to 0.6dB/mm only at 200°C.
Finally, we will show these epiwafers can be grown on large wafer diameters either up to 150mm on SiC substrates or on 200mm High Resistive Si substrates.

We report on the exploration of the use of Atomic Layer Deposition for the synthesis of ZnO / GaN heterojunctions. Such II-VI / III-V heterojunctions are intended for use in future new vertical devices for microwave amplifiers and / or power electronics.
On the ( 0001-Ga ) surface of MOCVD grown GaN, smooth ZnO surface morphology is obtained, associated with ( 0001 ) orientation. X-ray diffraction and TEM images show clear evidences of epitaxial ordering of the ZnO onto the GaN.
The ZnO layer can be made highly n-type conductive. The lowest measured resistivity is close to 4-5 milliohm.cm for undoped ZnO and to 1-1.5 milliohm.cm for Al doped ZnO.
The maximum conductivity of the ALD deposited ZnO layers has been obtained so far for layers grown at low growth rate, typically 80-90 pm / cycle, instead of the more typical 110-120 pm/cycle rate in the literature.