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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. In compositions with nitrogen of the same content, bandgap reduction was higher in the rutile phase in comparison to the anatase phase. In general, overall results revealed that with a higher loading fraction of ammonia, the N content increases, while Cr addition prevents nitrogen loss even up to high post annealing temperatures, i.e., 850 °C. [ABSTRACT FROM AUTHOR]

A detailed photoluminescence study of the effects of Al doping on GaAs grown by molecular-beam epitaxy was conducted. Photoluminescence spectra indicated three major peaks. A near-band-edge exciton peak, a carbon acceptor-related free-to-bound transition and a defect complex-related luminescence peak were identified as the primary features of the luminescence spectra. The integrated luminescence intensity indicated a strong quenching with the increase in Al doping.

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 semiconductor is very important for room temperature spintronics applications and optical devices. [ABSTRACT FROM AUTHOR]

This paper reports a detailed study of the effects of irradiation and thermal annealing on deep levels in Rh-doped p-type GaAs grown by low-pressure metal-organic chemical-vapor deposition, using deep level transient spectroscopy (DLTS) technique. It is found upon irradiation with alpha particles that, in addition to the radiation-induced defect peaks, all the Rh-related peaks observed in majority, as well as minority-carrier emission DLTS scans show an increase in their respective concentrations. The usually observed α-induced defects Hα1, Hα2, and Hα3 are found to have lower introduction rates in Rh-doped samples, as compared to reference samples (not doped with Rh). Alpha-irradiation has been found to decompose the two minority carrier emitting bands (one at low temperature ∼150 K and the other at ∼380 K) observed prior to irradiation into distinct peaks corresponding to deep levels Rh1 and Rh2 and EL2 and Rh3, respectively. A similar effect is also observed for the majority-carrier emitting band composed of hole emission from deep levels RhA and RhB, which separate out well upon irradiation. Further, from the double-correlation DLTS measurements, the emission rates of carriers from the radiation-enhanced peaks corresponding to deep levels Rh1, Rh2, Rh3, and RhC were found to be dependent on junction electric field. For RhC, the field dependence data have been analyzed in terms of the Poole-Frenkel model employing a 3-dimensional Coulomb potential with q = 2e (electronic charge). Temperature dependence of the hole capture cross-sections of the levels RhA and RhC was also studied quantitatively. The observed dependence of the hole capture cross-section of RhC on temperature can be interpreted in terms of multiphonon capture model, yielding a capture barrier of 0.2 eV and σ(∞) = 2.3 × 10-14 cm2. The results of irradiation and isochronal thermal annealing study, in combination with the theoretical analysis of the field dependence of hole emission data, lead us to interpret the levels RhA and RhB as charged centers and probably complexes of arsenic anti-site defect (AsGa) with Rh-impurity, (AsGa-Rh). The levels Rh1 and Rh2 are also proposed to be complexes, albeit of a different type, of arsenic anti-site defects (AsGa) and Rh-impurity (AsGa-Rh), while the level RhC is likely to be a doubly-charged complex center composed of arsenic interstitial (IAs) and Rh-impurity (IAs-Rh). [ABSTRACT FROM AUTHOR]

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 origin of these inadvertent (intrinsic) defects, thermal annealing behavior of these levels has also been studied. Analyses of field dependence and hole capture data, in combination with the annealing study, suggest that the dominant level is associated with an arsenic-antisite (AsGa) defect. Probable association of this dominant level with the doubly charged state of the well-known EL2 defect has been discussed in detail. [ABSTRACT FROM AUTHOR]

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 levels, labeled RhA and RhB, in the majority-carrier emission spectrum of p-type GaAs. In addition, a small hole emission peak, RhC, at the high-temperature end of the spectrum of p-type material corresponds to a deep level at Ev+0.87 eV. Data on directly measured carrier capture cross sections of deep levels are reported, wherever experimentally possible. Detailed double-correlation DLTS measurements are performed to investigate the characteristics of the observed Rh-related deep levels. The broad deep-level band in the upper-half bandgap, as well as the lower-half bandgap levels, RhA and RhB, are found to exhibit pronounced dependence of the thermal emission rates on electric field. While the former band cannot be resolved into component peaks, detailed data on the electric field dependence of hole emission rates are reported for the levels RhA and RhB over a range of measurement temperatures and analyzed in the light of the available theories. This analysis suggests the deep level RhB to be associated with a charged center while the deep level RhA is probably associated with an autolocalized self-trapping center. [ABSTRACT FROM AUTHOR]

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 all three levels have been obtained. The hole capture cross section for the levels Hα1 and Hα5 are found to be temperature independent, while for HSA, the hole capture data show a dependence on temperature. The dependence of hole capture cross section of HSA on temperature has been explained in terms of multiphonon capture mechanism, yielding a capture barrier of 0.13 eV and σ(∞)=1.5×10-14 cm2. These analyses lead us to conclude that the levels Hα1 and HSA are associated with a charged center, while the level Hα5 is most likely a substitutional defect in GaAs. [ABSTRACT FROM AUTHOR]

Presents information on a study which investigated the effects of aluminum as an isoelectronic dopant on the deep level defects in gallium arsenide grown by molecular beam epitaxy using deep level transient spectroscopy. Methods; Results; Discussion.

Abstract: A preliminary study of growth interruption effects on the GaAs/AlGaAs layers is reported. Deep level transient spectroscopy (DLTS) is performed on Al Schottky barrier devices fabricated on Si-doped, isotype GaAs/Al0.78Ga0.22As heterostructures grown by molecular beam epitaxy (MBE), both with (200s) and without, growth interruption. Our spectra extending from ∼15 to 300K show four peaks corresponding to deep level defects with thermal activation energies (E A): 0.09, 0.32, 0.42 and 0.52eV, which are detected in both types of samples (close to and) above liquid nitrogen temperature, although with different relative concentrations. Comparison with reported work on GaAs/Al x Ga1− x As samples with different compositions (x) shows that latter two of these deep levels are counterparts of the levels, P3–P4, earlier observed in MBE grown Al x Ga1− x As materials with Al content different than that used here. However, the deep level spectra of our samples show dramatic differences for the growth interrupted and uninterrupted samples in the temperature range below 77K. Whereas the samples prepared without growth interruption show a prominent, hitherto unobserved, positive (majority carrier emission) peak, P0 (E A=0.029eV), with shoulders on both higher and lower temperature flanks, the samples with growth interruption show a pronounced negative peak, apparently corresponding to a deep level with E A=0.031eV, in the same temperature region, under similar measurement conditions. The deep-level characteristics, including emission rate signatures and apparent capture cross-sections, are determined for the observed features using the thermal emission rate data. Some interesting additional features related to depth profiles of deep levels, bias effects and charge-induced metastability effects are also reported. [Copyright &y& Elsevier]