Donor impurity-related linear and nonlinear intraband optical absorption coefficients in quantum ring : effects of applied electric field and hydrostatic pressure

ABSTRACT: The linear and nonlinear intraband optical absorption coefficients in GaAs three-dimensional single quantum rings are investigated. Taking into account the combined effects of hydrostatic pressure and electric field, applied along the growth direction of the heterostructure, the energies of the ground and first excited states of a donor impurity have been found using the effective mass approximation and a variational method. The energies of these states are examined as functions of the dimensions of the structure, electric field, and hydrostatic pressure. We have also investigated the dependencies of the linear, nonlinear, and total optical absorption coefficients as a function of incident photon energy for several configurations of the system. It is found that the variation of distinct sizes of the structure leads to either a redshift and/or a blueshift of the resonant peaks of the intraband optical spectrum. In Maddition, we have found that the application of an electric field leads to a redshift, whereas the influence of hydrostatic pressure leads to a blueshift (in the case of on-ring-center donor impurity position) of the resonant peaks of the intraband optical spectrum

Persistent photoconductivity and the metal-insulator transition in Cd(_1-x)Mn(_x)Te:In

The persistent photoconductivity (PPC) effect in the diluted magnetic semiconductor Cd(_1-x)Mn(_x)Te:In has been studied in detail. Electrical transport measurements have been made on a large number of samples to build up an understanding of the phototransport properties of this material. In particular, the compositional dependence of the phototransport parameters has been measured up to x ≈ 0.2. Several samples exhibit an elevated temperature PPC effect which has been interpreted in terms of the formation of multiple DX centres. These samples can have a quenching temperature of up to 190 K, suggestmg that Cd(_1-x)Mn(_x)Te:In could be an interesting material in terms of applications of room temperature persistent photoconductors. The low field magnetoresistance has been measured and analysed quantitatively in order to attempt to identify the origin of the magnetoresistive effects in insulating and metallic samples. The positive magnetoresistance has been found to be linked with the magnetization of the sample. An anomalous negative magnetoresistance has been observed tinder certain experimental conditions. This negative magnetoresistance has been interpreted in terms of the formation of bound magnetic polarons and their contribution to spin-disorder scattering. The main body of this thesis is concerned with the study of the Metal-Insulator Transition (MIT). The PPC effect allows us to study the MIT in a continuous fashion by fine timing the carrier density by illumuiation. In this way we have made the first zero magnetic field study of the MIT in a magnetic semiconductor. The critical behaviour has been found to be consistent with the scaling theory of electron localization, which predicts a critical form σ = σ(_0)(n/n(_c) – 1)(^v). The critical conductivity exponent, v was determined to be close to one, while the critical carrier density, n(_e), was found to be ~ 2 x 10(^17) cm(^-3), for x = 0.08. The temperature dependence of the conductivity has been quantitatively analysed m both the metallic and insulating phases. On the insulating side of the transition, variable range hopping (VRH) conduction has been observed at low temperatures (down to 300 mK). The temperature dependence is consistent with VRH conduction with electron-electron interaction effects taken into account. In the metallic phase the temperature dependence of the conductivity (up to ~ 1 K) is consistent with a model where the zero temperature value of the conductivity is corrected by electron-electroninteraction effects, and the effects of weak localization. The magnitudes of these corrections are found to be in reasonable agreement with theoretical predictions. The electrical transport has also been studied in the weakly localized regime in Cd(_1-x)Mn(_x)Te:In and Cd(_1-x)Mn(_x)Te:In, Al. A rapid decrease in the conductivity occurs at low temperatures ( < 1.5 K). This is interpreted in terms of the effect of the s-d exchange interaction, which leads to the formation of bound magnetic polarons. It is suggested that this drop in conductivity can only be observed in the paramagnetic phase, and that spinglassordering has a significant effect on the temperature dependence of the conductivity at low temperatures

Optical spectroscopy of thin film semiconductor structures

This thesis consists of a study of several thin film semiconductor structures of practical technological use either presently or in the near future. The first system studied is an ultra thin film single crystal gallium arsenide layer. The absorption spectra of these layers are measured and transitions at both the F- point and L-point of the Brillouin Zone are observed, the latter are not normally measurable in thicker layers. The observed shift in the F-point absorption edge is attributed to contributions from the Franz-Keldysh Effect and the Moss-Burstein Effect. The temperature dependence of the L-point energy gap is measured and compared with previous data. The next system investigated is an n-type porous silicon layer coated with p-type polyaniline. Both photoluminescence and electroluminescence spectra and the electrical characteristics have been measured for this system. The interface between the two layers is found to be a rectifying junction consistent with a potential barrier formed at the interface. In forward bias, it is possible to generate electroluminescence in the visible and near infra red regions. The final structure studied is a thin film cadmium sulphide-cadmium telluride solar cell structure. The cells are found to have a low efficiency of around 1% as grown, but a process of treatment with cadmium chloride and annealing in air improves this by a factor of approximately ten. Photoluminescence measurements on the back surface of the cadmium telluride revealed three major emission bands at 1.59 eV, 1.55 eV and 1.45 eV. By varying temperature and incident laser power, attempts at assigning the bands to specific impurity centres in the cadmium telluride is made Using a novel bevelling etch technique to prepare samples, depth dependent measurement of the photoluminescence is possible. This reveals that the major changes associated with the improvement in efficiencies occurs at the interface between the CdS and the CdTe

Far-infrared laser spectroscopy of neutral and negatively charged shallow donors in GaAs and InP

An optically pumped far-infrared laser and superconducting magnet have been used to perform high resolution studies of the energy levels of neutral and negatively charged shallow donors in high purity n-GaAs and n-InP in magnetic fields where the dimensionless magnetic field gamma is approximately one (where gamma=hoc/(2R*), hoc is the cyclotron energy and R* is the Coulomb binding energy). The central cell structure caused by the presence of different shallow donor species has been studied on the 1s-2p+1,0 transitions of neutral shallow donors in undoped GaAs samples grown by molecular beam epitaxy, liquid phase epitaxy and vapour phase epitaxy (VPE). VPE material showed two new shallow donor species with negative central cell shifts. The ls-2p-1 transition at magnetic fields where gamma>1 shows exceptionally well resolved central cell structure. Detailed structure at magnetic fields below the 1s-2p+2 transition is due to transitions from the is to higher excited states. Samples of undoped high purity InP grown by the VPE, metal organic chemical vapour deposition and bulk growth techniques have been studied. VPE samples always show a strong component related to sulphur though some also show a strong silicon related component, and some show up to 7 components. A bulk, sample showed two strong components shallower than silicon which may have negative central cell shifts. Transitions between the excited states of neutral shallow donors in GaAs have been studied. Recent theoretical work by Makado (1982) describes the transition energies very well. Clearly resolved central cell structure is observed on inter-excited state transitions involving the 2s state. The first unambiguous observation of negatively charged shallow donors (D-states) in GaAs is reported. Simultaneous observations of transitions involving D-states, the cyclotron resonance and inter-excited state transitions of neutral donors over a wide magnetic field range, 0.03<gamma<3.5, highlight the differences between the transitions and the relative effects of optical excitation, temperature, magnetic field and electric field bias

Theoretical investigation of the quantum-confined Stark effect

The two main objectives of this dissertation are the systematic development of explicitly correlated electron-hole wave function based methods and the application of these methods to chemical systems with an emphasis on nanoparticles. The understanding of the basic physics of excited electronic states is an important consideration when developing new methods and applications. In this dissertation, excited electronic states were studied using the electron-hole quasiparticle representation. Theoretical treatment of electronic excitation in large quantum dots and nanoparticles is challenging because of the large number of electrons in the system. The quasiparticle representation provides an alternative representation that can partially alleviate the computational bottleneck associated with investigating these systems. However, in this representation, the effects of electron-hole correlation must be understood in order to accurately describe the system\u27s optical and electronic properties. The electron-hole wave function consists of two separate mathematical components which are the explicitly correlated part of the wave function and the reference wave function which is operated on by the explicitly correlated operator. This dissertation presents theoretical development of both of these components. In the first part, a systematic formulation for deriving the explicitly correlated form of the electron-hole wave function was performed. Towards that goal, the electron-hole correlation length was defined using the electron-hole cumulant. The construction of explicitly correlated wave function was improved by the introduction of the electron-hole correlation length which was determined using the electron-hole cumulant. The electron-hole correlation length allowed the determination of parameters in the explicitly correlated operator without the performance of energy minimizations. In the second part, the electron-hole reference wave function was improved by combining full configuration electron-hole wave function with the explicitly correlated operator. The developed methods were used to investigate the quantum-confined Stark effect (QCSE) and the effect of pH on the optical properties of quantum dots. The effect of applied electric fields on nanoparticles is known as the quantum-confined Stark effect. In this dissertation, the effect of both homogeneous and inhomogeneous electric fields on the optical and electronic properties of quantum dots was investigated. The effect of electric fields on the optical and electronic properties of a GaAs quantum dot was determined by combining the variational polaron transformation with the explicitly correlated electron-hole wave function. The presence of charged ligands also influenced the optical properties of quantum dot and this effect is known as the ligand-induced quantum-confined Stark effect. In this dissertation, the effect of pH on the optical properties of functionalized quantum dots were investigated by first calculating the charged states of the surface ligands at a given pH and then performing electron-hole explicitly correlated wave function based calculations in the electrostatic field generated by the charged ligands. Theoretical methods developed in this dissertation have impacted the field of computational nanoscience by reducing the computational bottleneck to investigate nanoparticles and by providing novel avenues for improving accuracy of existing methods

Electrical and optical properties of zinc oxide for scintillator applications

Zinc oxide (ZnO) is a wide-band-gap semiconductor suitable for many optical and optoelectronic applications. Among these is to use single crystal, powder, or ceramic forms of ZnO as a fast UV scintillator. In this work, the electrical and optical properties of ZnO were studied using photoluminescence, X-ray-induced luminescence, optical absorption, and Hall Effect techniques. This study included single crystal ZnO and ZnO:Ga samples grown from high-pressure-melt (HPM), seeded chemical-vapor-transport (SCVT), and hydrothermal (HYD) techniques; powder samples synthesized using both solution and solid-state processes, and purchased from different commercial sources; and ceramic samples prepared by hot-uni-axial-pressing and spark-plasma-sintering methods. Temperature-dependent PL and Hall measurements were combined to establish the luminescence origins in the n-type ZnO and ZnO:Ga single crystals. Based on a PL line-shape analysis, including band-gap renormalization, the direct (e,h) transition is the main luminescent channel in highly n-type ZnO:Ga, while FX and FX-LO recombinations are responsible for the UV PL from as-grown ZnO. An intrinsic mobility limit for n-type ZnO was established by including three major phonon-scattering mechanisms. Analysis of Hall data from single-crystal samples including both neutral- and ionized-impurity scatterings provided donor and acceptor concentrations and energy levels. High n-type single-crystal ZnO samples prepared either by Ga doping and co-doping, or by after-growth treatments, were also studied. Absorption and reflectance data were used to obtain free carrier concentrations from the Ga-doped and co-doped crystals, and it was found that several samples with n ∼ high-1018 to low-1019 cm -3 had optimum UV luminescence. Anneal treatments in reducing atmospheres increased free carrier concentrations in HPM and HYD samples, but an induced absorption band due to oxygen vacancies limited the UV emission from these samples. PL and X-ray-induced luminescence studies on powder ZnO:Ga samples demonstrated that high Ga-doping levels and H-anneal treatments can improve UV emission, while impurities such as Cu and Li enhance the lower energy visible emissions and affect the UV output. For ceramic forms of ZnO, reduction of scattering losses remains as the main challenge for improved scintillation

The characterization of bulk as-grown and annealed ZnO by the Hall effect

A fully automated Temperature Dependent Hall (TDH) measurement setup has been assembled for the purposes of this study. This TDH setup is capable of measuring samples in the 20 K to 370 K temperature range. Sample sizes of up to 20 mm × 20 mm can be accommodated by the custom designed and manufactured sample holder. Samples with a resistance in the 1Ω to 250 MΩ range can be measured with this setup provided that the mobility of the sample is greater than 1 cm²/Vs. The computer program controlling the automated measurement processwas written in LabView™ version 6.1. Single crystal Zinc Oxide (ZnO) was the material under investigation in this study. Bulk ZnO samples grown by three different methods, namely pressurized melt growth, seeded chemical vapor transport (SCVT) growth and hydrothermal growth, were measured in the 20 K to 370 K range. The effect of annealing in argon atmosphere in the 550 ºC to 930 ºC range was investigated on all three ZnO types. In addition, hydrogen-implanted layers on semi-insulating hydrothermally grown ZnO were studied. These samples were annealed in the 200 ºC to 400 ºC range and Hall measurements in the 20 K to 330 K range were performed. Programs were written to fit, wherever possible, the obtained temperature dependent carrier concentration and mobility profiles to suitable theoretical models. The carrier concentration data was fitted to a multi-donor single acceptor charge balance equation for the purpose of extracting donor concentrations and activation energies. Before fitting, the data was corrected for the Hall scattering factor and, where necessary, for two-layer effects particularly a degenerate surface conduction channel that developed through annealing on the SCVT-grown and hydrothermally grown samples. The acceptor concentrations of the samples were obtained by fitting the mobility data to a model based on D.L. Rode’s method of solving the Boltzmann transport equation. Scattering mechanisms included in the model were piezoelectric and deformation potential acoustic modes, polar optic modes and ionized impurity scattering. It was found that the mobility data did not fit the model very well without assigning questionable values to other parameters, in this case the deformation potential. Plausible values for the acceptor concentration were however obtained. The carrier concentration data fitted the model well, but due to the large number of parameters to be extracted (up to six parameters in the case of three donors) there was often not much certainty in the extracted values This study shows that TDH analysis is a valuable tool to assess the quality of semiconductors. Bulk and degenerate surface (or interfacial) conduction are separated with relative ease, and shallow defect concentrations as well as compensation level concentrations could be extracted. The generally observed uncertainty in values obtained in the multi-parameter regression of carrier concentration data indicates that supplementary techniques such as photoluminescence are needed to support results obtained by the TDH technique.Dissertation (MSc (Physics))--University of Pretoria, 2007.PhysicsMScunrestricte

High Pressure Optical Studies of Semiconductors

Until this work was completed no detailed studies of the low-temperature emission of A10.48gIn0.52As under high pressures were available to the best of our knowledge. We investigated the low-temperature emission of Al0.48gIn0.52AS under high pressures from 1 bar up to 92 kbar, especially with respect to the changes in luminescence mechanisms that occur concurrently with the crossover between the direct- and indirect-related bands. By investigating the temperature and excitation power dependence of the photoluminescence (PL) together with the photoluminescence excitation (PLE), we demonstrated that the low-temperature emission of Al0.48In0.52As is not excitonic but due to (