Research Articles (Danie Auret Collection)

Permanent URI for this collectionhttp://hdl.handle.net/2263/1190

Collection of articles published by Prof F.D. Auret.

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Kalibrering van 'n opto-elektroniese stelsel vir die karakterisering van ultraviolet-sensitiewe fotodiodes
(Suid Afrikaanse Akademie vir Wetenskap & Kuns, 2011) Van Schalkwyk, Louwrens; Meyer, Walter Ernst; Nel, Jacqueline Margot; Auret, Francois Danie; louwrens.vanschalkwyk@up.ac.za
Spectral and electrical characteristion of ultraviolet (UV) sensitive photodiodes requires a calibrated optoelectronic system. For spectral characteristion, the irradiance of the UV light source, after the light passed through a monochromator and optical fiber, was calibrated for wavelengths ranging from 200 nm to 400 nm. Commercially available AlGaN-based photodiodes was characterised and the obtained parameters were compared to that specified.
A compact streak camera for 150 fs time resolved measurement of bright pulses in ultrafast electron diffraction
(American Institute of Physics (AIP), 2010-10) Kassier, Gunther Horst; Haupt, K.; Erasmus, N.; Rohwer, E.G.; Von Bergmann, H.M.; Schwoerer, H.; Coelho, Sergio M.M.; Auret, Francois Danie
We have developed a compact streak camera suitable for measuring the duration of highly charged subrelativistic femtosecond electron bunches with an energy bandwidth in the order of 0.1%, as frequently used in ultrafast electron diffraction UED experiments for the investigation of ultrafast structural dynamics. The device operates in accumulation mode with 50 fs shot-to-shot timing jitter, and at a 30 keV electron energy, the full width at half maximum temporal resolution is 150 fs. Measured durations of pulses from our UED gun agree well with the predictions from the detailed charged particle trajectory simulations.
Correlation between barrier heights and ideality factors of Ni/n-Ge (100) schottky barrier diodes
(Korean Physical Society, 2010-12) Chawanda, Albert; Nel, Jacqueline Margot; Auret, Francois Danie; Mtangi, Wilbert; Nyamhere, Cloud; Diale, M. (Mmantsae Moche); Leach, Lindsay Josephine; mmantsae.diale@up.ac.za
We computed the homogeneous Schottky barrier height (SBH) at ideality factor (n) = 1.0 of Ni/n- Ge (100) Schottky diodes (SDs). The SDs were identically prepared by using resistive evaporation of Ni on n-Ge (100). The SBHs and n of these diodes (24 dots) were calculated from their experimental forward bias current-voltage (I-V ) and reverse bias capacitance-voltage (C-V ) measurements at room temperature. Even though the Schottky diodes were identically prepared, the values of the SBH from the I-V characteristics varied from 0.487 to 0.508 eV, the ideality factor varied from 1.34 to 1.53, and the SBH from the C−2-V characteristics varied from 0.358 to 0.418 eV. The Gaussian fits of the experimental SBH distributions obtained from the C−2-V and the I-V characteristics yielded mean SBH values of 0.401 ± 0.015 and 0.503 ± 0.006 eV, respectively. Furthermore, a homogeneous SBH value of approximately 0.535 eV was also computed from an extrapolation of a linear plot of the experimental SBHs versus the ideality factors. The homogeneous SBHs, rather than the effective SBHs, of individual contacts or mean values should be used to discuss the theories and the physical mechanisms that determine the SBHs of SDs.
Determination of the laterally homogeneous barrier height of palladium Schottky barrier diodes on n-Ge (1 1 1)
(Elsevier, 2011) Chawanda, Albert; Roro, K.T.; Auret, Francois Danie; Mtangi, Wilbert; Nyamhere, Cloud; Nel, Jacqueline Margot; Leach, Lindsay Josephine; albert.chawanda@up.ac.za
We have studied the experimental linear relationship between barrier heights and ideality factors for palladium (Pd) on bulk-grown (1 1 1) Sb-doped n-type germanium (Ge) metal-semiconductor structures with a doping density of about 2.5×1015 cm−3. The Pd Schottky contacts were fabricated by vacuum resistive evaporation. The electrical analysis of the contacts was investigated by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements at a temperature of 296 K. The effective barrier heights from I–V characteristics varied from 0.492 to 0.550 eV, the ideality factor n varied from 1.140 to 1.950, and from reverse bias capacitance–voltage (C−2–V) characteristics the barrier height varied from 0.427 to 0.509 eV. The lateral homogenous barrier height value of 0.558 eV for the contacts was obtained from the linear relationship between experimental barrier heights and ideality factors. Furthermore the experimental barrier height distribution obtained from I–V and (C−2−V) characteristics were fitted by Gaussian distribution function, and their mean values were found to be 0.529 and 0.463 eV, respectively.
Analysis of current voltage measurements on Au/Ni/n-GaN Schottky contacts in a wide temperature range
(Elsevier, 2010-07) Mtangi, Wilbert; Janse van Rensburg, Pieter Johan; Diale, M. (Mmantsae Moche); Auret, Francois Danie; Nyamhere, Cloud; Nel, Jacqueline Margot; Chawanda, Albert
Current-voltage characteristics of Au/Ni/n-GaN Schottky contacts have been measured in the 60-320 K temperature range. The zero bias barrier height, bo  and ideality factor, n have been studied as a function of temperature. The sharp increase in ideality factor at low temperatures has been explained as an effect of thermionic field emission. The deviation of the characteristics from the ideal thermionic behaviour are more pronounced with a decrease in temperature, in which the results obtained indicate the presence of other current transport mechanisms in the 60-280 K temperature range and the dominance of pure thermionic emission current at 300 K. The increase in barrier height with increasing temperature has been explained as an effect of barrier inhomogeneities.
Defect production in strained p-type Si1-xGex by Er implantation
(American Institute of Physics, 2011-01) Mamor, M.; Pipeleers, B.; Auret, Francois Danie; Vantomme, M.
Strained p-Si1−xGex (x = 5.3%, 10.2%, and 15.4%) was irradiated at room temperature with 160 keV 166Er2+ ions to a fluence of 1×1010 or 3×1013 Er/cm2. The defects induced by ion implantation were investigated experimentally using high-resolution x-ray diffraction, Rutherford backscattering and channeling spectroscopy, and deep level transient spectroscopy. X-ray diffraction indicates that the damage induced by Er implantation produces a slight perpendicular expansion of the SiGe lattice. For all compositions, channeling measurements reveal that Er implantation in p-Si1−xGex to a fluence of 3×1013 Er/cm2 induces an amorphous region below the Si1−xGex surface. Annealing at 850 °C for 30 s, results in a reduction in damage density, a relaxation of the implantation-induced perpendicular expansion of the SiGe lattice in the implanted region, while a more pronounced relaxation of the compressive strain SiGe is observed for higher Ge content (x = 0.10 and 0.15). On the other hand, for the annealed SiGe samples that were implanted with Er at the fluence of 1010 Er/cm2, the compressive strain in the SiGe layer is nearly completely retained. Deep level transient spectroscopy studies indicate that two prominent defects with discrete energy levels above the valence band are introduced during Er implantation. Their activation energy was found to decrease with increasing Ge content. However, the relatively large local strain induced by high fluence Er implantation reduces the activation energy by 40 meV with respect to the low fluence Er implanted p-Si1−xGex. This shift (40 meV) in the activation energy remains constant regardless of the Ge content, suggesting that the Si1−xGex layers remained fully strained after Er implantation. The observed defects are further compared to those introduced by alpha particle irradiation and electron beam metal deposition. The results indicate that defects introduced by Er implantation have similar electronic properties as those of defects detected after electron beam deposition and alpha particle irradiation. Therefore, it is concluded that these defects are due to the Er implantation-induced damage and not to the Er species specifically.
Lithium and electrical properties of ZnO
(American Institute of Physics, 2010-03) Vines, L.; Monakhov, E.V.; Schifano, R.; Mtangi, Wilbert; Auret, Francois Danie; Svensson, B.G.
Hydrothermally grown n-type ZnO samples have been investigated by deep level transient spectroscopy (DLTS), thermal admittance spectroscopy (TAS), temperature dependent Hall effect (TDH) temperature, and secondary ion mass spectrometry (SIMS) after thermal treatments up to 1500◦C, in order to study the electrical properties of samples with different lithium content. The SIMS results showed that the most pronounced impurities were Li, Al, Si, Mg, Ni and Fe with concentrations up to ∼ 5 × 1017cm−3. The Li concentration was reduced from ∼ 1017 cm−3 in as-grown samples to ∼ 1015 cm−3 for samples treated at 1500◦C, while the concentration of all the other major impurities appeared stable. The results from DLTS and TAS displayed at least five different levels having energy positions of Ec − 20 meV , Ec − 55 meV , Ec − 0.22 eV , Ec − 0.30 eV , and Ec − 0.57 eV (Ec denotes the conduction band edge), where the Ec − 55meV level is the dominant freeze out level for conduction electrons in samples treated at temperatures < 1300◦C, while higher annealing temperatures revealed the shallower (Ec − 20 meV ) level. The TDH measurements showed a pronounced increase in the electron mobility for the heat treated samples, where a peak mobility of 1180 cm2/V s was reached for a sample treated at 1300◦C. The results provide strong evidence that Li in hydrothermal ZnO is almost exclusively in the substitutional configuration (LiZn), supporting theoretical predictions that the formation of LiZn prevails over Li on the interstitial site for Fermi level positions at and above the middle of the band gap.
Effect of alpha-particle irradiation on the electrical properties of n-type Ge
(Elsevier, 2009) Roro, K.T.; Janse van Rensburg, Pieter Johan; Auret, Francois Danie; Coelho, Sergio M.M.; kittessa.roro@up.ac.za
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Effects of chemical treatment on barrier height and ideality factors of Au/GaN Schottky diodes
(Elsevier, 2009) Diale, M. (Mmantsae Moche); Auret, Francois Danie; mmantsae.diale@up.ac.za
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Microstructural and surface characterization of thin gold films on n-Ge (111)
(Elsevier, 2009) Nel, Jacqueline Margot; Chawanda, Albert; Auret, Francois Danie; Jordaan, W.; Odendaal, R.Q. (Quintin); Hayes, M.; Coelho, Sergio M.M.; Jackie.Nel@up.ac.za
Thin gold films were fabricated by vacuum resistive deposition on the n-Ge (111) wafers. The films were annealed between 300 and 600°C. These resulting thin films were then characterised using scanning electron microscopy (field emission and back-scattering modes), Rutherford back scattering spectroscopy and time of flight secondary ion mass spectroscopy (TOF-SIMS). For temperatures below the eutectic temperature the distribution of both the gold and the germanium on the surface are uniform. Above the eutectic temperature, the formation of gold rich islands on the surface of the Germanium were observed. These changes in the microstructure were found to correspond to changes in the electrical characteristics of the diodes.
Damage formation in Ge during Ar+ and He+ implantation at 15 K
(Elsevier, 2009) Hayes, M.; Schroeter, A.; Wendler, E.; Wesch, W.; Auret, Francois Danie; Nel, Jacqueline Margot
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The dependence of barrier height on temperature for Pd Schottky contacts on ZnO
(Elsevier, 2009) Mtangi, Wilbert; Auret, Francois Danie; Nyamhere, Cloud; Janse van Rensburg, Pieter Johan; Chawanda, Albert; Diale, M. (Mmantsae Moche); Nel, Jacqueline Margot; Meyer, Walter Ernst; wilbert.mtangi@up.ac.za
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Current–temperature measurements of a SBD evaporated onto inductively coupled plasma cleaned germanium
(Elsevier, 2009) Coelho, Sergio M.M.; Auret, Francois Danie; Myburg, G.; Janse van Rensburg, Pieter Johan; Meyer, Walter Ernst; sergio@up.ac.za
Inductively coupled plasma (ICP) etching has been used primarily on compound semiconductors. There are however compelling reasons to study the effects of ICP etching on Ge. Pd Schottky barrier diodes (SBDs) were resistively evaporated onto Ge (111) that was ICP etched at a rate of 60 Å per minute for three or ten minute intervals. Although plasma cleaning is known to introduce defects that were observed with DLTS, the diodes exhibited excellent current–voltage characteristics when cooled down to 80K. Current–temperature(IT) scans that were recorded from 20K up to 300K after cooling under reverse bias showed no effect of recombination/generation(RG). On the other hand, IT scans that were recorded after cooling under zero or forward bias clearly exhibited RG effects in the 100–240K temperature range. This effect was found to be completely reversible. In addition, ICP etching leads to superior devices when compared to devices manufactured by RF sputter deposition.
Electronic and annealing properties of the E0.31 defect introduced during Ar plasma etching of germanium
(Elsevier, 2009) Auret, Francois Danie; Coelho, Sergio M.M.; Myburg, G.; Janse van Rensburg, Pieter Johan; Meyer, Walter Ernst; danie.auret@up.ac.za
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Characterization of defects introduced in Sb doped Ge by 3keV Ar sputtering using deep level transient spectroscopy (DLTS) and Laplace-DLTS (LDLTS)
(Elsevier, 2009) Nyamhere, Cloud; Das, A.G.M.; Auret, Francois Danie; Chawanda, Albert; Mtangi, Wilbert; Odendaal, R.Q. (Quintin); Carr, Alan; cloud.nyamhere@up.ac.za
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Electrical characterization of rare-earth implanted GaN
(Elsevier, 2009) Janse van Rensburg, Pieter Johan; Auret, Francois Danie; Matias, V.S.; Vantomme, A.; jvr@up.ac.za
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Radiation damage in ZnO ion implanted at 15 K
(Elsevier, 2009) Wendler, E.; Bilani, O.; Gartner, K.; Wesch, W.; Hayes, M.; Auret, Francois Danie; Lorenz, K.; Alves, E.; elke.wendler@uni-jena.de
Commercial O-face (0 0 0 1) ZnO single crystals were implanted with 200 keV Ar ions. The ion fluences applied cover a wide range from 5 x 1011 to 7 x 1016 cm-2. The implantation and the subsequent damage analysis by Rutherford backscattering spectrometry (RBS) in channelling geometry were performed in a special target chamber at 15 K without changing the target temperature of the sample. To analyse the measured channelling spectra the computer code DICADA was used to calculate the relative concentration of displaced lattice atoms. Four stages of the damage evolution can be identified. At low ion fluences up to about 2 x 1013 cm-2 the defect concentration increases nearly linearly with rising fluence (stage I). There are strong indications that only point defects are produced, the absolute concentration of which is reasonably given by SRIM calculations using displacement energies of Ed(Zn) = 65 eV and Ed(O) = 50 eV. In a second stage the defect concentration remains almost constant at a value of about 0.02, which can be interpreted by a balance between production and recombination of point defects. For ion fluences around 5 x 1015 cm-2 a second significant increase of the defect concentration is observed (stage III). Within stage IV at fluences above 1016 cm-2 the defect concentration tends again to saturate at a level of about 0.5 which is well below amorphisation. Within stages III and IV the damage formation is strongly governed by the implanted ions and it is appropriate to conclude that the damage consists of a mixture of point defects and dislocation loops.
Electrical characterization of defects introduced during metallization processes in n-type germanium
(Elsevier, 2009) Auret, Francois Danie; Coelho, Sergio M.M.; Janse van Rensburg, Pieter Johan; Nyamhere, Cloud; Meyer, Walter Ernst
We have studied the defects introduced in n-type Ge during electron beam deposition (EBD) and sputter deposition (SD) by deep-level transient spectroscopy (DLTS) and evaluated their infleunce on the rectification quality of Schottky contracts by current-voltage (I=V) measurements. I=V measurements demonstrated that the quality of sputter-deposited diodes are poorer than those of diodes formed by EBD. The highest quality Schottky diodes were formed by resistive evaporation that introduced no defects in Ge. In the case of EBD of metals the main defect introduced during metallization was the V=Sb complex, also introduced during by electron irradiation. The concentrations of the EBD-induced defects depend on the metal used: metals that required a higher electron beam intensity to evaporate, e.g. Ru, resulted in larger defect concentrations than metals requiring lower electron beam intensity, e.g. Au. All the EBD-induced defects can be removed by annealing at temperatures above 325º C. Sputter deposition introduces several electronically active defects near the surface of Ge. All these defects have also been observed after high-energy electron irradiation. However, the V=Sb centre introduced by EBD was not observed after sputter deposition. Annealing at 250º C in Ar removed all the defects introduced during sputter deposition.
Thermal stability of Co, Ni, Pt or Ru Schottky contacts on n-Si and defects introduced thereon during contacts fabrication using electron beam deposition
(Elsevier, 2007-12-15) Nyamhere, Cloud; Chawanda, Albert; Das, A.G.M.; Auret, Francois Danie; Hayes, M.
When using Schottky barrier diodes (SBDs) on silicon (Si) to study the thermal stability of radiation-induced defects, point defects injection into the silicon substrate can occur at temperatures where silicidation occurs. These injected point defects can react with the radiation-induced defects and may lead to an incorrect picture of annealing studies of these defects. In order to overcome this problem, we have annealed (1) ruthenium (Ru), cobalt (Co), nickel (Ni) and platinum (Pt) SBDs to form stable silicides on phosphorus (P) doped Si and (2) have measured the electrical characteristics of defects introduced during diodes fabrication by electron beam deposition (EBD), using conventional and (high resolution) Laplace (L-) deep level transient spectroscopy (DLTS). A primary electron trap at 0.48 eV below the conduction band was observed after EBD processing of the contacts. Isochronal annealing of the SBDs at 350 °C, annealed-in defects 0.05, 0.09, 0.18 and 0.28 eV below the conduction band. All the EBD-induced defects were removed after annealing at 600 °C. Primary defect depth profile versus annealing temperature results are also presented in this study.
Electronic properties of defects in pulsed-laser deposition grown ZnO with levels at 300 and 370 meV below the conduction band
(Elsevier, 2007-12) Auret, Francois Danie; Meyer, Walter Ernst; Janse van Rensburg, Pieter Johan; Hayes, M.; Nel, Jacqueline Margot; Von Wenckstern, Holger; Schmidt, H.; Biehne, G.; Hochmuth, H.; Lorenz, M.; Grundmann, Marius
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