Peter Deák

The mechanism of defect creation and passivation at the SiC/SiO2 interface

From the viewpoint of application in power electronics, SiC possesses the greatest advantage of having SiO2 as its native oxide. Unfortunately, the usual thermal oxidation produces an unacceptably high density of interface states, with a complex energy distribution. Deep states are assumed to be caused by carbon excess at the interface, while the slow electron traps, called NIT, with especially high density near the conduction band of 4H-SiC (which would be the best polytype for power devices), are expected to originate from oxide defects near the interface. Unlike the case of the Si/SiO2 interface, simple hydrogen passivation does not help to reduce the high trap density. A possible passivation method for both deep states and NIT is post-oxidation annealing or oxidation in the presence of NO or N2O molecules. Here we present systematic and sophisticated theoretical calculations on a model of the 4H-SiC/SiO2 interface, in order to establish the main reaction routes and the most important defects that are created during dry oxidation, and may give rise to the observed interface traps. We also investigate the effect of nitrogen in suppressing them.

Proper Surface Termination for Luminescent Near-Surface NV Centers in Diamond

By accurate quantum mechanical simulations, we show that typical diamond surfaces possess image states with sub-bandgap energies, and compromise the photostability of NV centers placed within a few nm of the surface. This occurs due to the mixture of the NV-related gap states and the surface image states, which is a novel and distinct process from the well-established band bending effect. We also find that certain types of coverages on the diamond surface may lead to blinking or bleaching due to the presence of acceptor surface states. We identify a combination of surface terminators that is perfect for NV-center based nanoscale sensing.

First principles theoretical study of the hole-assisted conversion of CO to CO2 on the anatase TiO2(101) surface.

First principles density functional theory calculations were carried out to investigate the adsorption and oxidation of CO on the positively charged (101) surface of anatase, as well as the desorption of CO(2) from it. We find that the energy gain on adsorption covers the activation energy required for the oxidation, while the energy gain on the latter is sufficient for the desorption of CO(2), leaving an oxygen vacancy behind. Molecular dynamics simulations indicate that the process can be spontaneous at room temperature. The oxidation process described here happens only in the presence of the hole. The possibility of a photocatalytic cycle is discussed assuming electron scavenging by oxygen.

Theoretical study of the luminescent substoichiometric silicon oxides (SiOx)

Density-functional based tight-binding molecular-dynamics and semiempirical molecular orbital calculations are applied to identify the 1.9 eV luminescence center in SiOx. The identification is made possible by an 890 cm−1 vibrational line that is correlated to the luminescence peak. Two possible models are examined: (i) Si6 rings in substituted siloxenes are known to show 1.8 eV emission. A model SiO crystal is built based upon Si rings interconnected by bridging O atoms. The structure is found to be stable up to 1000 K, but the correlated vibration is not observed. (ii) a-SiO is exposed to simulated annealing and the quenched amorphous structure is analysed for occurance of Si rings and the previously proposed non-bridging oxygen hole centers (NBOHCs). Localized stretching vibrations of the NBOHCs are found to be in the experimentally detected frequency range, thus explaining the correlation with the observed and calculated photoluminescence peak.

Defect states of substitutional oxygen in diamond

Different charged forms of substitutional oxygen in diamond are examined using ab initio plane-wave pseudopotential calculations. The results show that two defect levels are associated with substitutional oxygen and that the charged form of the defect depends sensitively on the Fermi level. One of the defect states lies well in the energy gap and the other is located near the conduction band edge. The positively charged form of the oxygen defect is suggested to be responsible for an optical absorption band occurring at 2.6 eV. It is shown that oxygen in the lattice vacancy exhibits an amphoteric behaviour in diamond.

Modelling of stress-induced diamond nucleation

Abstract PM3 semi-empirical calculations have been used to model the primary diamond nucleation in the graphite-like initial carbon layer usually produced on substrates other than diamond. The effect of quenched-in density increase was simulated by compressing the graphitic environment (represented by a small graphite cluster with cyclic boundary conditions) of a carbon atom with an additional methyl group in its vicinity. The critical (linear) compression for the “sp2 → sp3 phase transformation” of the fourfold coordinated carbon was estimated to be 3%.

Influence of oxygen on the absorption of silicon carbide nanoparticles

We have investigated the absorption of 0.9, 1.4 nm silicon carbide nanoparticles (SiC NPs) by time-dependent density functional calculations, focusing on the effect of different oxygen adsorbates of the surface. We have found that negatively charged Si-O−, Si-COO− defects dramatically lower the optical gap of SiC NPs. Our findings can help interpret recent controversary experiments on colloidal SiC NPs.

Calculation of migration barriers on hydrogenated diamond surfaces

Abstract A very important question for modeling diamond growth is whether long-range surface migration should be taken into account as a limiting step in the growth on differently oriented surfaces. The migration of active surface sites as well as that of layerbuilding adsorbants (CH3, bridging CH2, etc.) is of interest. Calculation of migration barriers for polyatomic units is an extremely demanding computational task, especially since the surface has to be simulated appropriately. The PM3 semi-empirical method has already proven its usefulness in growth modeling studies. This method was used to calculate migration barriers in a static approximation (T = 0 K) on thin hydrogenated diamond slabs. Results are presented for C(111):H as well as for 2 × 1 reconstructed C(001):H surfaces.

Time-Dependent Density Functional Calculations on Hydrogenated Silicon Carbide Nanocrystals

The electronic structure and absorption spectrum of hydrogenated silicon carbide nanocrystals (SiC NC) have been determined by first principles calculations. We show that the reconstructed surface can significantly change not just the onset of absorption but the shape of the spectrum at higher energies. We compare our results with two recent experiments on ultrasmall SiC NCs.

Theoretical Investigations of Complexes of p-Type Dopants and Carbon Interstitial in SiC: Bistable, Negative-U Defects

Interaction of boron and aluminum with interstitial carbon is studied using first principles calculations. It is shown that carbon can form very stable complexes with Al and B, forming a family of negative-U bistable defects with deep levels. The influence of this effect on the activation rate of p-type implants is discussed.