The fabrication of low-resistance and thermal stable ohmic contacts is important for realization of reliable SiC devices. For then-type SiC, Ni-based metallization is most commonly used for Schottky and ohmic contacts. Many experimental studies have been performed in order to understand the mechanism of ohmic contact formation and different models were proposed to explain the Schottky to ohmic transition for Ni/SiC contacts. In the present review, we summarize the last key results on the matter and post open questions concerning the unclear issues of ohmic contacts ton-type SiC. Analysis of the literature data and our own experimental observations have led to the conclusion that the annealing at high temperature leads to the preferential orientation of silicide at the heterointerface (0001)SiC//(013)δ-Ni₂Si. Moreover, we may conclude that onlyδ-Ni₂Si grains play a key role in determining electrical transport properties at the contact/SiC interface. Finally, we show that the diffusion barriers with free diffusion path microstructure can improve thermal stability of metal-SiC ohmic contacts for high-temperature electronics.

We present a comparative study of the strain relaxation of GaN/AlN short-period superlattices (SLs) grown on two different III-nitride substrates introducing different amounts of compensating strain into the films. We grow by plasma-assisted molecular beam epitaxy (0001)-oriented SLs on a GaN buffer deposited on GaN(thick)-on-sapphire template and on AlN(thin)-on-sapphire template. The ex-situ analysis of strain, crack formation, dislocation density, and microstructure of the SL layers has established that the mechanism of strain relaxation in these structures depends on the residual strain in substrate and is determined mainly by the lattice mismatch between layers. For growth on the AlN film, the compensating strain introduced by this film on the layer prevented cracking; however, the densities of surface pits and dislocations were increased as compared with growth on the GaN template. Three-dimensional growth of the GaN cap layer in samples with pseudomorphly grown SLs on the AlN template is observed. At the same time, two-dimensional step-flow growth of the cap layer was observed for structures with non-pseudomorphly grown SLs on the GaN template with a significant density of large cracks appearing on the surface. The growth mode of the GaN cap layer is predefined by relaxation degree of top SL layers.

The effects of lattice misfit strain in epitaxial GeSn/Ge/Si(001) heterostructures on Sn incorporation, misfit dislocations (MDs), and the critical thickness were investigated using high-resolution x-ray diffraction. By performing a simulation of the x-ray reciprocal space maps measured in the vicinity of an asymmetrical reflection, we determined the strong correlation between the strain relaxation, density of MDs, and Sn content in the GeSn alloy for a compositional range of 4 to 15 at. %. Herein, we quantitatively describe the phenomenon of strain-suppressed Sn incorporation in GeSn, leading to the formation of top Sn-rich, middle compositionally graded, and bottom Sn-poor regions at fixed growth conditions. It is shown that the thicknesses of the bottom Sn-poor region at which the composition spontaneously changes are correlated with the theoretically predicted critical thickness for the nucleation of MDs. Depending on the degree of strain relaxation in the bottom GeSn layer, the density of MDs at the GeSn/Ge interface varies in the range of $\sim$ 2–5 × 10⁵ cm⁻¹ as the Sn content increases from 4 to 12 at. %. The thickness of the compositionally graded region, 160 ± 25 nm, corresponds to the thickness at which the GeSn layer fully relaxes through the interaction/multiplication of MDs. These results contribute to the fundamental understanding that strain engineering is not only crucial to tune the bandgap of Sn-containing group-IV semiconductors but also a key factor for the growth of Sn-rich metastable GeSn layers for mid-infrared photonic devices.

A mechanism of charge transport in Au-TiB _x -n-GaN Schottky diodes with a space charge region considerably exceeding the de Broglie wavelength in GaN is studied. Analysis of temperature dependences of current-voltage (I–V) characteristics of forward-biased Schottky barriers showed that, in the temperature range 80–380 K, the charge transport is performed by tunneling along dislocations intersecting the space charge region. Estimation of dislocation density ρ by the I–V characteristics, in accordance with a model of tunneling along the dislocation line, gives the value ρ ≈ 1.7 × 10⁷ cm⁻², which is close in magnitude to the dislocation density measured by X-ray diffractometry.