Umar S. Qurashi

Defects induced luminescence and tuning of bandgap energy narrowing in ZnO nanoparticles doped with Li ions

Microstructural and optical properties of Zn1−yLiyO (0.00 ≤ y ≤ 0.10) nanoparticles are investigated. Li incorporation leads to substantial changes in the structural characterization. From micro-structural analysis, no secondary phases or clustering of Li was detected. Elemental maps confirmed homogeneous distribution of Li in ZnO. Sharp UV peak due to the recombination of free exciton and defects based luminescence broad visible band was observed. The transition from the conduction band to Zinc vacancy defect level in photoluminescence spectra is found at 518 ± 2.5 nm. The yellow luminescence was observed and attributed to Li related defects in doped samples. With increasing Li doping, a decrease in energy bandgap was observed in the range 3.26 ± 0.014 to 3.17 ± 0.018 eV. The bandgap narrowing behavior is explained in terms of the band tailing effect due to structural disorder, carrier-impurities, carrier-carrier, and carrier-phonon interactions. Tuning of the bandgap energy in this class of wide bandgap...

Electrical characterization of alpha radiation-induced defects in p-GaAs grown by metal-organic chemical-vapor deposition

Investigations of the alpha particle irradiation-induced defects in low-pressure metal-organic chemical-vapor deposition grown p-GaAs have been carried out. By employing deep-level transient spectroscopy, at least seven radiation-induced deep-level defects have been observed in the lower half of the band gap in the temperature range of 12−475 K. Double-correlation deep-level transient spectroscopy measurements show three prominent levels: two known radiation-induced levels namely, Hα1 and Hα5, and one inadvertent center HSA, present before irradiation, to exhibit a significant dependence of thermal emission rate on the junction electric field. For Hα1 and HSA the field-enhanced emission data are well fitted with a Poole-Frenkel model, using a three-dimensional square-well potential with radius r=3.2 and 1.43 nm, respectively. The field effect for Hα5 has been explained by a square-well potential in combination with a phonon-assisted tunneling process. Detailed data on the carrier capture cross section for...

Characteristics of Alpha-Radiation-Induced Deep Level Defects in p-Type InP Grown by Metal-Organic Chemical Vapor Deposition

Room temperature storage and/or minority carrier injection behaviors of three prominent majority carrier levels H3 (Ev+0.34 eV), H4 (Ev+0.39 eV) and H5 (Ev+0.58 eV) and three minority carrier levels, 0.22, 0.29 and 0.35 eV below the conduction band edge in metal-organic chemical vapor deposition (MOCVD) grown p-type InP crystals produced by alpha radiation have been studied using deep levels transient spectroscopy. In particular, H5, absent immediately after irradiation, is found to grow with storage at room temperature after irradiation with no change in the other two majority carrier levels. Minority carrier injection saturates H5 while H3 and H4 continue to decay after injection. These observations are interpreted to mean that H4 and H5 are unrelated levels with no or very little mutual conversion proposed in some previous irradiation studies. Detailed production rate data for H3, H4 and H5 (post-injection) over a range of alpha-particle doses have also been provided for the first time.

Arsenic antisite defects in p-GaAs grown by metal-organic chemical-vapor deposition and the EL2 defect

Epitaxial layers of p-GaAs grown on p+-GaAs substrates by low-pressure metal organic chemical vapor deposition have been investigated using deep level transient spectroscopy (DLTS). One dominant peak and other relatively small peak, corresponding to deep levels at Ev+0.55 eV and Ev+0.96 (low field energies), respectively, have been observed in the lower half of the band gap. Investigation with double-correlation DLTS reveals that the measured thermal emission rate of holes from the dominant level is strongly dependent on the junction electric field. Detailed data on this field enhancement have been analyzed in terms of different available theoretical models. The hole capture cross section for the dominant deep level has been found to be temperature dependent. Detailed data on the temperature dependence of the hole capture cross section have been interpreted in terms of the multiphonon carrier capture mechanism, yielding a capture barrier of 0.11 eV. In order to get deeper insight into the nature and origi...

4d transition-metal impurity rhodium in GaAs grown by metal-organic chemical vapor deposition

Deep levels associated with 4d transition-metal impurity rhodium have been investigated in GaAs epitaxial layers grown by metal-organic chemical vapor deposition, using deep level transient spectroscopy (DLTS) technique. A comprehensive study of deep levels in the entire bandgap of GaAs has been carried out using both n- and p-type GaAs crystals over a wide temperature scan range, 12–470 K. In the n-type, Rh-doped material, two prominent broadbands of deep levels are found to be associated with Rh impurity; one in the upper-half and the other in the lower-half bandgap. In addition, one relatively small peak at the high temperature end of the majority-carrier emission spectra, corresponds to a deep level at Ec−0.92 eV, is also detected in the n-type Rh-doped samples. The minority carrier (hole) emission band in n-type material is found to consist of a doublet of hole emitting deep levels, labeled RhA and RhB, in the lower-half bandgap. This doublet is clearly observed to correspond to two Rh-related deep l...

Interaction of iron with transition metals and alpha radiation in thermally quenched p-silicon

Iron-induced deep levels have been studied in p-type silicon quenched from high temperatures, using deep-level transient spectroscopy. The interstitial iron donor and the iron - boron pair defect are observed and identified by a number of their characteristics including their mutual transformation. Our recent work revealed an interesting new property of the interstitial iron donor defect, namely its complete bleaching by the transition metals Ag, Au, Pt and Pd in samples which have received thermal treatment identical to the undoped samples. The study reported here was intended to investigate the relationship of this effect to the well known iron - boron complex defect known to act as a source of the interstitial iron defect by injection-induced dissociation in boron doped p-silicon. Our results show that both the iron - boron complex and the isolated interstitial iron defects disappear concurrently in our transition-metal-doped samples. Thus the bleaching of the iron interstitial in the presence of the transition metals used is found to be associated with the bleaching of this source rather than due to some unknown complex mechanism inhibiting the dissociation of these pairs in the presence of the transition metals. In addition, the response of the iron interstitial defect to irradiation with alpha-particles and its thermal annealing behaviour have been studied. Alpha-irradiation is seen to lead to a significant suppression of the iron interstitial defect, in agreement with the hitherto reported results of electron irradiation, without causing any significant change in its thermal annealing characteristics.

Photoluminescence study of Al doping in GaAs grown by molecular‐beam epitaxy

A detailed photoluminescence investigation of the effects of Al doping on GaAs grown by molecular‐beam epitaxy is reported. Materials with 0.1%, 1%, and 3% Al doping have been studied. Viewed as an AlxGa1−xAs alloy semiconductor, our study on these materials presents results on the optical characteristics of the lowest Al composition material reported to date. Photoluminescence spectra show three major peaks. Detailed measurements on the dependence of these spectra on temperature and excitation power have been carried out to characterize the various transitions responsible for the luminescence peaks. A near‐band‐edge exciton peak, a carbon acceptor‐related free‐to‐bound transition, and a defect complex‐related luminescence peak, probably involving silicon, are identified as the main features of our luminescence spectra. The integrated luminescence intensity shows a strong quenching with the increase in Al doping, thus supporting the increase of nonradiative deep‐level centers with Al doping, concluded fro...

Effect of Cr-N codoping on structural phase transition, Raman modes, and optical properties of TiO2 nanoparticles

Noncompensated cation-anion codoping in TiO2 nanoparticles has been achieved by a chemical synthesis route. Significant reduction in the optical bandgap and enhancement in the absorption of visible light have been observed. Structural phase transformation has been tracked in detail as a function of doping and heat treatment temperature. Anatase to rutile phase transition temperature for doped samples was higher in comparison to the pure TiO2 nanoparticles. Nitrogen and chromium addition increases the phase transformation barrier, where the effect of the former dopant is of more significance. The Raman results showed an increase in the oxygen content with higher post annealing temperatures. With Cr incorporation, the peak associated with the Eg mode has been found to shift towards a higher wave number, while with nitrogen incorporation, the shift was towards a lower wave number. A decrease in reflectance with N co-doping for all samples, irrespective of phase and annealing temperatures, has been observed. ...

Osmium Related Deep Levels in Indium Phosphide

Deep energy levels arising from doping of n- and p-type InP with the 5d transition metal Os are studied, using electrical and optical deep level transient spectroscopy. Electrically active Os-related deep level concentrations up to 8.2 x 10 15 cm -3 are observed representing up to 20% of the chemical concentration. Three prominent levels with low-electric-field activation energies of E V + 0.29, E C - 0.32 and E V + 0.53 eV, labelled OsA, Os1 and OsB, respectively, are identified. Double-correlation deep level transient spectroscopy measurements show strong dependence of carrier emission rates on the electric field for all the three levels. All three levels exhibit a stronger field effect than that reported for any other deep level in InP. The field effect can be interpreted in terms of a phonon-assisted tunneling mechanism, merging in a direct tunneling process at high field strengths for both Os1 and OsA. Detailed data are obtained on the carrier capture cross-sections and their temperature dependence for all three levels. In the case of OsA the data can be interpreted in terms of a multiphonon capture model.

Deep levels in nitrogen-doped MBE-grown p-ZnSe

Deep-level transient spectroscopy (DLTS) has been used to investigate and characterize deep-level defects in nitrogen-doped p-type ZnSe layers grown by molecular beam epitaxy on GaAs substrates. At least three prominent levels, H1, H3 and H4, are found in the lower-half bandgap with thermal activation energies 0.31 eV, 0.63 eV and 0.94 - 0.99 eV respectively. Comparison of their characteristics with the two previous reports shows that, with the exception of H3, these are new levels. In addition, some evidence is presented for three minority carrier (electron) peaks for the first time.