Digital DLTS studies on radiation induced defects in Si, GaAs and GaN

Since the development of deep level transient spectroscopy (DLTS) in the 1970’s by Lang and others, the technique has become a powerful analytical tool to characterise the electrical properties of defects in semiconductors. With the development of more powerful computers and improved data acquisition systems, it has become possible to replace the original analogue boxcar analysers and lock-in amplifiers that were commonly used in early DLTS systems with digitisers and digital signal processing equipment. The use of a computer for signal processing allows for much more flexibility in the DLTS system. For instance, a digital DLTS system is capable of measuring a much wider range of emission rates than an analogue system. Furthermore, since the digital DLTS system does not rely on a repetitive signal, such a system can observe phenomena such as defect metastability that cannot be observed in an analogue system. In this thesis, the design and characterisation of a digital DLTS system is described. The results of a number of experiments that illustrated the capabilities of the system are reported. The extended range of emission rates that could be measured by the system were illustrated by the measuring of the EL2 defect in GaAs over the temperature range 270 – 380 K (corresponding to emission rates ranging from less than 10–3 s–1 to more than 103 s–1). The results compared well with previous results obtained by means of an analogue DLTS system. Further low temperature measurements on the E2 defect in GaAs showed that in the low temperature region, thermal radiation from the cryostat shroud influenced carrier emission. The field dependence of the emission rate of a number of defects, including defects in as-grown n-GaN, He-ion irradiated n-GaN and Si, was investigated as well. The ability of the digital DLTS system to measure single transients was used to investigate configurationally bistable defects in He-ion irradiated p-Si and a sputter-induced defect with negative-U properties in n-GaN. In both of these cases, the results proved far superior to those obtained by means of an analogue system.Thesis (PhD (Physics))--University of Pretoria, 2007.Physicsunrestricte

A hybrid functional calculation of Tm3+ defects in germanium (Ge)

Please read abstract in the article.National Research foundation (NRF) of South Africa (Grantspecific unique reference number (UID) 78838).http://www.elsevier.com/locate/mssp2017-03-31hb201

The carbon-substitutional-carbon-interstitial (CsCi) defect pair in silicon from hybrid functional calculations

Using both the Perdew–Burke–Ernzerhof (PBE) functional and the hybrid functional of Heyd–Scuseria– Ernzerhof (HSE06), the metastability of the carbon-substitutional–carbon-interstitial (CsCi) defect in silicon has been investigated within density functional theory using the two experimentally proposed configurations of the defect. While the PBE functional predicted the defect complex to have both donor and acceptor levels, it did not predict any form of charge-state controlled metastability as was observed experimentally. In the case of HSE06 functional, the defect was found to exhibit charge-state controlled metastability in the 0 and 1 charge states with no metastability predicted for +1 charge state. The calculated binding energies for the neutral charge state indicate that the defect is a stable bound defect complex.National Research Foundation of South Africa.http://www.elsevier.com/locate/com/matsci2017-06-30hb2016Physic

Hybrid functional study of hydrogen passivation in carbon-oxygen related defect complexes in silicon

Please read abstract in the article.The National Research foundation (NRF) of South Africa (Grant specific unique reference number (UID) 98961).http://www.elsevier.com/locate/physbhj2020Physic

Ab initio study of metastability of Eu3+ defect complexes in GaN

Density functional theory (DFT) within the generalized gradient approximation (GGA) has been used to study the structural and electronic properties of Eu3þ defect complexes in GaN under Ga-rich conditions. Two distinct configurations of the EuGaVN defect complex, the axial and basal configuration, have been investigated. We report two forms of metastable defects namely; the Negative U defect in the lower half of the GaN band-gap and a metastable defect with two distinct configurations each with levels at EC 0.46 eV and 0.56 eV in the upper half of the GaN band-gap.http://www.elsevier.com/locate/physbhb201

Surface Structure of Bi(111) from Helium Atom Scattering Measurements. Inelastic Close-Coupling Formalism

8 págs.; 4 figs.; 2 tabs.; Open Access funded by Creative Commons Atribution Licence 4.0© 2015 American Chemical Society. Elastic and inelastic close-coupling (CC) calculations have been used to extract information about the corrugation amplitude and the surface vibrational atomic displacement by fitting to several experimental diffraction patterns. To model the three-dimensional interaction between the He atom and the Bi(111) surface under investigation, a corrugated Morse potential has been assumed. Two different types of calculations are used to obtain theoretical diffraction intensities at three surface temperatures along the two symmetry directions. Type one consists of solving the elastic CC (eCC) and attenuating the corresponding diffraction intensities by a global Debye-Waller (DW) factor. The second one, within a unitary theory, is derived from merely solving the inelastic CC (iCC) equations, where no DW factor is necessary to include. While both methods arrive at similar predictions for the peak-to-peak corrugation value, the variance of the value obtained by the iCC method is much better. Furthermore, the more extensive calculation is better suited to model the temperature induced signal asymmetries and renders the inclusion for a second Debye temperature for the diffraction peaks futile.This research was supported by the European Commission and the Styrian Government within the ERDF program. S.M.A. acknowledges MICINN (Spain) through Grant No. FIS2011- 29596-C02-01. A.T. acknowledges financial support provided by the FWF (Austrian Science Fund) within the project J3479- N20.Peer Reviewe

Rare earth interstitials in Ge : a hybrid density functional theory study

In this work, the results of density functional theory calculations for rare earth (Ce, Pr, Eu, and Er) interstitials in Ge are presented. We employed the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06) for all the calculations. We calculated the formation energies and charge state transition levels for the tetrahedral (T) and hexagonal (H) configurations of the Ce, Pr, Eu, and Er interstitials in Ge. While for the T configuration, the charge states of the Ce and Pr did not induce any thermodynamic accessible transition state level within the band gap of Ge, for both the T and H configurations the Eu and Er interstitials in Ge induce deep levels in the band gap. The H configuration of the Ce interstitial in Ge induces a shallow donor level at 0.03 eV below the conduction band. The Eu interstitial exhibits negative-U properties for the (+2/2) transition level and the Er interstitial displays characteristics of charge state controlled metastability.Supported partly by National Research Foundation (NRF) of South Africa (Grant speci c unique reference number (UID) 98961).http://link.springer.com/journal/116642018-02-28hb2017Physic

Ab initio study of aluminium impurity and interstitial-substitutional complexes in Ge using a hybrid functional (HSE)

The results of an ab initio modelling of aluminium substitutional impurity (AlGe), aluminium interstitial in Ge [IAl for the tetrahedral (T) and hexagonal (H) configurations] and aluminium interstitial-substitutional pairs in Ge (IAlAlGe) are presented. For all calculations, the hybrid functional of Heyd, Scuseria, and Ernzerhof in the framework of density functional theory was used. Defects formation energies, charge state transition levels and minimum energy configurations of the AlGe, IAl and IAlAlGe were obtained for 2, 1, 0, þ1 and þ2 charge states. The calculated formation energy shows that for the neutral charge state, the IAl is energetically more favourable in the T than the H configuration. The IAlAlGe forms with formation energies of 2.37 eV and 2.32 eV, when the interstitial atom is at the T and H sites, respectively. The IAlAlGe is energetically more favourable when the interstitial atom is at the T site with a binding energy of 0.8 eV. The IAl in the T configuration, induced a deep donor (þ2/þ1) level at EV þ 0:23 eV and the AlGe induced a single acceptor level (0/1) at EV þ 0:14 eV in the band gap of Ge. The IAlAlGe induced double-donor levels are at EV þ 0:06 and EV þ 0:12 eV, when the interstitial atom is at the T and H sites, respectively. The IAl and IAlAlGe exhibit properties of charge state-controlled metastability.This work is based on the research supported partly by the National Research foundation (NRF) of South Africa [Grant-specific Unique Reference Number (UID) 98961].http://link.springer.com/journal/116642018-07-30hb2016Physic

Ab initio study of the effect of hydrogen passivation on boron-oxygen-carbon related defect complexes in silicon

Please read abstract in the article.The National Research Foundation (NRF) of South Africa (Grant specific unique reference number (UID) 98961).http://www.elsevier.com/locate/mssphj2020Physic

First-principles study of the impact of hydrogen passivation on the charge state transition levels of the CiOi(Sii)n defect complexes in silicon

Please read abstract in the article.The National Research foundation (NRF) of South Africa (Grant specific unique reference number (UID) 98961).https://www.springer.com/journal/126332021-07-16hj2020Physic