57 research outputs found
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
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
Ab-initio study of germanium di-interstitial using a hybrid functional (HSE)
In this work, we present ab-initio calculation results of Ge di-interstitials (I2(Ge)) in the framework of the density functional theory (DFT) using the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional. The formation energy, transition levels and minimum energy con gurations were obtained for I2(Ge) -2, -1, 0, +1 and +2 charge states. The calculated formation energies shows that for all charge states of I2(Ge), the double tetrahedral (T) con guration formed the most stable defect with a binding energy of 1.24 eV in the neutral state. We found the (+2/+1) charge state transition level for the T lying below the conduction band minimum and (+2/+1) for the split[110]-tetrahedral con guration lying deep at 0.41 eV above the valence band maximum. The di -interstitials in Ge exhibited the properties of both shallow and deep donor levels at (+2/+1) within the band gap and depending on the con gurations. I2(Ge) gave rise to negative-U, with e ective-U values of -0.61 and -1.6 eV in di erent con gurations. We have compared our results with calculations of di -interstitials in silicon and available experimental data.National Research foundation (NRF) of South Africa (Grant specific unique reference number (UID) 78838).http://www.elsevier.com/locate/physb2017-01-31hb201
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