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Oriented thin films of β-(Ga1−x Fe x )2O3 were deposited by radio frequency magnetron sputtering on c-Al2O3 and GaN substrates. The itinerant character of the Fe 3d states forming the top of the valence band (VB) of the Fe-substituted β-Ga2O3 thin films has been determined from resonant photoelectron spectroscopy. Further, the admixture of the itinerant and localized characters of these Fe 3d states has been obtained for larger binding energies; i.e. deeper in the VB. The bottom of the conduction band (CB) for β-(Ga1−x Fe x )2O3 has been also found to have strongly hybridized states involving Fe 3d and O 2p states compared to that of Ga 4s in pristine β-Ga2O3. This suggests that β-Ga2O3 transforms from a band-like system to a charge-transfer system with Fe substitution. Furthermore, the bandgap red shifts with Fe composition, which has been found to be primarily related to the shift of the CB edge.

Lattice distortion and epitaxial nature of β-Ga2O3 layers deposited on GaN templates using radio-frequency magnetron sputtering have been evaluated. Determined values of out-of-plane (tensile type) and in-plane (compressive type) strain are found to decrease with increase in deposition temperature indicating better relaxation of unit cell lattice at higher temperatures. The obtained value of angle β is invariably higher than its bulk value suggesting a distortion in the unit cell of β-Ga2O3 layer. The columnar type of growth for β-Ga2O3 layer on GaN template has been observed. Analysis of epitaxial nature reveals that six domains of grown layer are in-plane rotated by 60o ± δo (δ=2-3o) with each other, which is explained due to the presence of three non-equivalent oxygen atoms on (-201) plane of β-Ga2O3. The calculated values of δ nearly match with experimentally observed values; however, smaller difference between the two is related to residual strain in the layer. The out-of-plane and in-plane epitaxial relationship for β-Ga2O3 layer with respect to GaN are (-201)Ga2O3 || (0001)GaN, (010)Ga2O3 || (11-20)GaN respectively.

Some of the published experimental results on the thermoluminescence and isothermal decay of LiF TLD-100 have been explained on the basis of a continuous uniform trap distribution within a finite range ( Delta E) assuming first-order kinetics. This provides credibility to the results of two rival groups of workers who, in the recent part have severely critized the conclusions of each other in a number of publications.

The accuracy of the initial rise (IR) method and the variable heating rate (VHR) method when the frequency factor depends on temperature as Ta (with -2

Temperature dependence of the photoluminescence (PL) peak energy of bulk and quantum well (QW) structures is studied by using a new phenomenological model for including the effect of localized states. In general an anomalous S-shaped temperature dependence of the PL peak energy is observed for many materials which is usually associated with the localization of excitons in band-tail states that are formed due to potential fluctuations. Under such conditions, the conventional models of Varshni, Vi?a and Passler fail to replicate the S-shaped temperature dependence of the PL peak energy and provide inconsistent and unrealistic values of the fitting parameters. The proposed formalism persuasively reproduces the S-shaped temperature dependence of the PL peak energy and provides an accurate determination of the exciton localization energy in bulk and QW structures along with the appropriate values of material parameters. An example of a strained InAs0.38P0.62/InP QW is presented by performing detailed temperature and excitation intensity dependent PL measurements and subsequent in-depth analysis using the proposed model. Versatility of the new formalism is tested on a few other semiconductor materials, e.g. GaN, nanotextured GaN, AlGaN and InGaN, which are known to have a significant contribution from the localized states. A quantitative evaluation of the fractional contribution of the localized states is essential for understanding the temperature dependence of the PL peak energy of bulk and QW well structures having a large contribution of the band-tail states.

Light-hole tunnelling to the surface states is studied using photoluminescence (PL) spectroscopy and transient reflectivity measurements in the tensile-strained GaAsP/AlGaAs near-surface quantum well (NSQW) samples by reducing the top barrier layer thickness from 275 to 5 nm. The ground state transition (e1–lh1) remains excitonic even at room temperature (RT) for a buried quantum well sample with 275 nm thick top barrier. When the top barrier thickness is reduced to 50 nm the same transition is found to be excitonic only at low temperatures but changes to free-carrier recombination at higher temperatures. When the top barrier layer thickness is further reduced to 5 nm, the ground state transition is no longer excitonic in nature, where it shows free-carrier behaviour even at 10 K. We therefore find a clear relationship between the character of the ground state transition and the top barrier layer thickness. Light-hole excitons cannot be formed in NSQW samples when the top barrier layer thickness is kept reasonably low. This is attributed to the quantum mechanical tunnelling of free light holes to the surface states, which is found to be faster than the exciton formation process. A tunnelling time of ~500 fs for light holes is measured by the transient reflectivity measurements for the NSQW sample with a 5 nm top barrier. On the other hand, heavy-hole-related transitions in NSQW samples are found to be of excitonic nature even at RT because of the relatively large tunnelling time. It supports the dominance of excited state feature over the ground state transition in PL measurements at temperatures higher than 150 K.

Micro-Raman imaging along with other techniques are applied to study the morphology, structure and crystalline quality of various types of InAs nanowires (NWs). The NWs of low and high densities are formed using metal organic vapor phase epitaxy. Raman mapping is effectively used as a local probe to gain information about the structure and crystalline quality of low-density NWs where the conventional characterization techniques are not very useful. However, for high-density NWs, the image and crystalline quality obtained from the LO phonon strongly corroborate with scanning electron microscopy and x-ray diffraction (XRD) results, respectively. These low-density (104 cm−2) and high-density (108 cm−2) NWs are grown on Si(0 0 1) under various growth conditions such as catalyst-assisted and catalyst-free growth, growth on native oxide-covered and oxide-cleaned Si, grooved Si surfaces and also varying the V/III ratio and growth temperature. NWs (1 µm long and 50–100 nm wide) with high density and tapered NWs (50–80 µm long and 200–500 nm wide at the tip) with low density are formed under different growth conditions. The growth of hillock- and wire-like structures is observed under the same growth condition. Raman, XRD, scanning electron microscopy and atomic force microscopy analyses confirm that the hillocks are grown along the 〈0 0 1〉 direction, whereas the wires are grown along [1 1 0] directions in the plane of Si(0 0 1). Furthermore, the Raman analysis of these NWs confirms that the smaller NWs have much better crystalline quality (half-width of LO phonon frequency ∼6 cm−1) compared to the larger NWs (half-width of LO phonon frequency ∼15 cm−1) although both NWs are oriented with the Si(0 0 1) surface.

Elastic-relaxation-induced barrier layer thickness fluctuations are observed for InP/GaAs type-II quantum well (QW) superlattice structures. Crystalline and interfacial quality of superlattice structures is reasonable for thick barrier layers of GaAs, and strain is easily accommodated in the GaAs barrier layers, thus confirming the pseudomorphic growth. However, for thin GaAs barrier layers, interfacial and crystalline quality of the superlattice samples is poor, because the strain between adjacent InP QW is effectively coupled. It leads to the partial relaxation of the superlattice structures without introducing either appreciable defects/dislocations or formation of three-dimensional quantum dots. On the other hand, relaxation introduces clear undulations in GaAs barrier layer thickness for the superlattice structures with relatively thicker InP QWs and thinner GaAs barrier layers. Such undulations seem to propagate up to the surface of the superlattice structures and degrade the surface morphology of the partially relaxed short-period superlattice samples.

The charge carrier distribution profile, carrier dynamics, barrier height and conduction band offset for asymmetric InAs/InxGa1−x As/GaAs dot-in-well (DWELL) structures are evaluated from temperature-dependent electronic transport and spectroscopic measurements. The charge carrier leakages via defects lead to less charge carrier accumulation in the DWELL architecture in comparison with the associated QWs. Furthermore, the differential capacitance–voltage measurements show the resonance of charge carrier escape rate with the ground and excited states of the DWELL structure. The peak of the charge carrier escape rate related to the excited state is almost independent of the temperature while that of the ground state changes significantly with heating. It is explained on the basis of changes in the accumulated carrier density due to carrier localization and nonradiative recombination processes, via defects/dislocations/Auger recombination, which are dominant at moderate and high temperatures, respectively. Furthermore, the obtained values of barrier height and conduction band offsets for asymmetric InAs/InxGa1−x As/GaAs (x = 0.18) DWELL structures are 194 ± 10 meV and 287 ± 10 meV, respectively. Downward transitions from both ground and excited states of quantum dots (QDs) are observed in photoluminescence while upward transitions only to the excited state energy levels of QDs are observed in photoreflectance spectroscopy. This is due to the filling of the ground state energy levels of QDs, which is confirmed by the electron transport measurements. Hence, it is concluded that interband absorption occurs to the excited states, whereas intersubband absorption predominantly takes place from the ground states to the continuum of the QD ensemble.

Temperature dependence of the photoluminescence (PL) spectra has been investigated for InP/GaAs type-II ultrathin quantum wells (QWs). Room temperature PL has been observed for 1.43 monolayer thick ultrathin QW. Fitting parameters of the Bose–Einstein empirical relation for ultrathin QWs show that the temperature dependence of PL peak energy is similar to the temperature dependence of the band gap for InP and GaAs materials. In addition, we have also determined the PL quenching mechanism from the Arrhenius-like plot of integrated PL intensity. Thermal escape of carriers from these ultrathin QWs into the GaAs barrier is mainly responsible for the PL quenching with temperature. This is also supported by the observation that the PL intensity related to the GaAs barrier increases with increasing temperature.

Gallium phosphide (GaP) homoepitaxial layers are optimized by varying the growth temperature, growth rate and V/III ratio in a low-pressure metal organic vapour phase epitaxy (MOVPE) process. It is observed that we need high growth temperature, low growth rate and an optimum value of the V/III ratio in order to obtain a mirror finish surface morphology. The surface morphology is mirror finish for an epitaxial layer grown ≥820 °C when viewed under a stereo-zoom microscope for a V/III ratio of about 100 or more. A sharp and intense excitonic photoluminescence (PL) feature and a fine structure associated with donor–acceptor pair recombination confirm a high optical quality of the grown layer, which is supported by high-resolution x-ray diffraction measurements. For silicon doping of GaP using silane in low-pressure MOVPE, our results confirm theoretical predictions available in the literature and the values of a distribution coefficient lie in a similar range known for other conventional semiconductors. This optimization provides high-quality GaP (1 1 1)B epilayers needed for the development of either nanostructure-based optoelectronic devices on transparent substrates or polarization-sensitive infrared photodetectors based on two-photon absorption.