Ab initio density-functional calculations using Gaussian orbitals are carried out on large Si and Ge supercells containing oxygen defects. The formation energies, local vibrational modes, and diffusion or reorientation energies of ${\mathrm{O}}_i,$ ${\mathrm{O}}_{2i},$ $V\mathrm{O},$ $V\mathrm{OH},$ and $V{\mathrm{O}}_2$ are investigated. The piezospectroscopic tensors for ${\mathrm{O}}_i,$ $V\mathrm{O},$ and $V{\mathrm{O}}_2$ are also evaluated. The vibrational modes of ${\mathrm{O}}_i$ in Si are consistent with the view that the defect has effective $D_{3d}$ symmetry at low hydrostatic pressures but adopts a buckled structure for large pressures. The anomalous temperature dependence of the modes of ${\mathrm{O}}_{2i}$ is attributed to an increased buckling of Si-O-Si when the lattice contracts. The diffusion energy of the dimer is around 0.8 eV lower than that of ${\mathrm{O}}_i$ in Si and 0.6 eV in Ge. The dimer is stable against $V\mathrm{O}$ or $V{\mathrm{O}}_2$ formation and the latter defect has modes close to the reported $894-{\mathrm{cm}}^{-1}$ band. The reorientation energies for O and H in $V\mathrm{O}$ and $V\mathrm{OH}$ defects are found to be a few tenths of an eV and are greater when the defect has trapped an electron.

The term junction spectroscopy embraces a wide range of techniques used to explore the properties of semiconductor materials and semiconductor devices. In this tutorial review, we describe the most widely used junction spectroscopy approaches for characterizing deep-level defects in semiconductors and present some of the early work on which the principles of today's methodology are based. We outline ab-initio calculations of defect properties and give examples of how density functional theory in conjunction with formation energy and marker methods can be used to guide the interpretation of experimental results. We review recombination, generation, and trapping of charge carriers associated with defects. We consider thermally driven emission and capture and describe the techniques of Deep Level Transient Spectroscopy (DLTS), high resolution Laplace DLTS, admittance spectroscopy, and scanning DLTS. For the study of minority carrier related processes and wide gap materials, we consider Minority Carrier Transient Spectroscopy (MCTS), Optical DLTS, and deep level optical transient spectroscopy together with some of their many variants. Capacitance, current, and conductance measurements enable carrier exchange processes associated with the defects to be detected. We explain how these methods are used in order to understand the behaviour of point defects and the determination of charge states and negative-U (Hubbard correlation energy) behaviour. We provide, or reference, examples from a wide range of materials including Si, SiGe, GaAs, GaP, GaN, InGaN, InAlN, and ZnO.

In this paper we study the formation of energy gaps in the spectrum of graphene and its bilayer when both these materials are covered with water and ammonia molecules. The energy gaps obtained are within the range 20–30 meV, values compatible to those found in experimental studies of graphene bilayer. We further show that the binding energies are large enough for the adsorption of the molecules to be maintained even at room temperature.

Reliable single-photon emission is crucial for realizing efficient spin-photon entanglement and scalable quantum information systems. The silicon vacancy ( $${V}_{{\rm{Si}}}$$VSi ) in 4H-SiC is a promising single-photon emitter exhibiting millisecond spin coherence times, but suffers from low photon counts, and only one charge state retains the desired spin and optical properties. Here, we demonstrate that emission from $${V}_{{\rm{Si}}}$$VSi defect ensembles can be enhanced by an order of magnitude via fabrication of Schottky barrier diodes, and sequentially modulated by almost $$50 \%$$50% via application of external bias. Furthermore, we identify charge state transitions of $${V}_{{\rm{Si}}}$$VSi by correlating optical and electrical measurements, and realize selective population of the bright state. Finally, we reveal a pronounced Stark shift of 55 GHz for the V1′ emission line state of $${V}_{{\rm{Si}}}$$VSi at larger electric fields, providing a means to modify the single-photon emission. The approach presented herein paves the way towards obtaining complete control of, and drastically enhanced emission from, $${V}_{{\rm{Si}}}$$VSi defect ensembles in 4H-SiC highly suitable for quantum applications.