a photoluminescence study of cadmium and aluminium-related defects in silicon

Photoluminescence measurements of cadmium-implanted and aluminium-doped oxygen-rich silicon are presented in this thesis. Temperature dependence, uniaxial stress and Zeeman measurements were carried out on all the defects reported. In the case of cadmium-implanted silicon, three new cadmium related defects are reported with zero phonon lines at ~ 1026meV, 983 meV and 935 meV. Temperature dependence studies indicate that the thermal binding energies of all the defects are in the range 13-17meV and no excited thermalising states are seen for the 983meV and 935meV lines. The 1026meV line shows clear evidence of an excited state at higher energy (-1.1 meV). Uniaxial stress measurements reveal rhombic I (C2v) symmetry for the 1026 meV and 983 meV lines. The 935 meV line is tentatively ascribed to monoclinic I symmetry. None of the cadmium-related lines show either a shift or splitting in magnetic fields up to 5 Tesla. Cadmium isotope substitution studies reveal the involvement of a single cadmium atom in each defect. Circumstantial evidence indicates the involvement of oxygen in the defects, as none of the defects are observed in oxygen lean silicon. The relationship between these cadmium-related defects and previously reported zinc-related defects is discussed. In the case of aluminium-doped silicon, a previously reported defect with a zero phonon line at 922meV was investigated further. Measurements as a function of temperature reveal that the thermal binding energy is ~15meV and no evidence of thermalising states is seen. Uniaxial stress measurements reveal a low symmetry configuration for the 922meV line (monoclinic I, Clh). This line neither shifts nor splits under magnetic fields of up to 5 Tesla. Similiarities in the behaviour under uniaxial stress of this defect and a previously reported aluminium-related defect are presented

ZnO wide bandgap semiconductor nanostructures: growth, characterisation and applications

The compound ZnO, or zincite, has a long, fascinating and diverse history displaying a number of peaks and troughs in terms of the degree of research interest, numbers of publications per annum and so forth. These peaks have in some cases been associated with the discovery of a new aspect of its behaviour which is relevant to some scientific or technological focus (such as UV light emission) and the ambitions associated with these technologies. Some of these ambitions have been realized and some, to date, remain ambitions. The general public will most probably know of this material from some of its earlier applications. Zinc oxide was initially known by a variety of names, some of rather unclear origin, including “nihil album”, “flowers of zinc”, “chinese white” and “philosopher’s wool”. The usage of these terms has obviously declined since the standardization of chemical nomenclature since the early 1800’s as discussed by Kent (1958). ZnO is, or has been, used as a pigment in paints and enamel coatings (hence the name “chinese white”) and also as an ingredient in cements, glass, tires, glue, matches, white ink, reagents, photocopy paper, flame retardant, fungicides, cosmetics and dental cements and ~ 100,000 tonnes of ZnO is produced per annum as reported by Klingshirn (2007). These diverse applications rely on various properties of ZnO such as the white colour of the material, its chemical activity, UV blocking capability, heat conductivity and bioactivity. ZnO is used extensively in various pharmaceutical and cosmetic products including ointments and sunscreen preparations (including an appearance in the Hollywood movie “Jaws”, where Brody’s wife enquires if he has remembered to bring the zinc oxide sunscreen before he boards the Orca). ZnO is a material which is used in a very wide variety of applications in a diverse range of technological spaces. In addition to this already impressive technological resume, ZnO is used widely in the semiconductor industry, primarily in varistor manufacture, but also as a transparent conducting oxide, a photoconductor and a phosphor, see e.g. Klingshirn (2007), Minami (2005), Monroy et al. (2003) and Heiland et al. (1959)

Photoluminescence due to Group IV impurities in ZnO

We report the results of photoluminescence measurements on ZnO bulk crystals implanted with both stable and radioactive species involving the group IV impurities Ge, Si and Sn. We previously confirmed the identity of a line emerging at 3.3225 eV as being related to Ge and present here uniaxial stress data which show that the defect responsible has trigonal symmetry. Experiments with Si provide circumstantial evidence of a connection with the well-known line at 3.333 eV. Our measurements indicate that for the case of Sn on the Zn site luminescence is not observed. We also confirm that the I9 and I2 lines are due to substitutional In impurities

Dellafossite CuAlO2 film growth and conversion to Cu–Al2O3 metal ceramic composite via control of annealing atmospheres

In this work we demonstrate simple techniques to form well crystallised CuAlO2 powders and thick films from CuO and boehmite or alumina, using a novel molten salt painting process. We examine the formation mechanism using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray 15 spectroscopy and in situ high temperature X-ray diffraction and find that the annealing atmosphere plays a critical role. From this we develop a method to create Cu-Al2O3 conductive metal-ceramic composite materials with novel morphologies via the thermal decomposition of CuAlO2 precursor films

Influence of ZnO nanowire array morphology on field emission characteristics

In this work the growth and field emission properties of vertically aligned and spatially ordered and unordered ZnO nanowires are studied. Spatially ordered nanowire arrays of controlled array density are synthesised by both chemical bath deposition and vapour phase transport using an inverse nanosphere lithography technique, while spatially unordered arrays are synthesised by vapour phase transport without lithography. The field emission characteristics of arrays with 0.5 µm, 1.0 µm, and 1.5 µm inter-wire distances, as well as unordered arrays, are examined, revealing that with the range of values examined field emission properties are mainly determined by variations in nanowire height, and show no correlation with nanowire array density. Related to this, we find that a significant variation in nanowire height in an array also leads to a reduction in catastrophic damage observed on samples during field emission because arrays with highly uniform heights are found to suffer significant arcing damage. We discuss these results in light of recent computational studies of comparable nanostructure arrays and find strong qualitative agreement between our results and the computational predictions. Hence the results presented in this work should be useful in informing the design of ZnO nanowire arrays in order to optimise their field emission characteristics generally

Effects of excitonic diffusion on stimulated emission in nanocrystalline ZnO

We present optically-pumped emission data for ZnO, showing that high excitation effects and stimulated emission / lasing are observed in nanocrystalline ZnO thin films at room temperature, although such effects are not seen in bulk material of better optical quality. A simple model of exciton density profiles is developed which explains our results and those of other authors. Inhibition of exciton diffusion in nanocrystalline samples compared to bulk significantly increases exciton densities in the former, leading, via the nonlinear dependence of emission in the exciton bands on the pump intensity, to large increases in emission and to stimulated emission

Structural, optical and magnetic properties of Zn1−xMnxO micro-rod arrays synthesized by spray pyrolysis method

Undoped and Mn-doped ZnO micro-rod arrays were fabricated by the spray pyrolysis method on glass substrates. X-ray diffraction and scanning electron microscopy showed that these micro-rod arrays had a polycrystalline wurtzite structure and high c-axis preferred orientation. Photoluminescence studies at 10 K show that the increase of manganese content leads to a relative decrease in deep level band intensity with respect to undoped ZnO. Magnetic measurements indicated that undoped ZnO was diamagnetic in nature whereas Mn-doped ZnO samples exhibited ferromagnetic behavior at room temperature, which is possibly related to the substitution of Mn ions (Mn2+) for Zn ions in the ZnO lattice

Highly transparent and reproducible nanocrystalline ZnO and AZO thin films grown by room temperature pulsed-laser deposition on flexible zeonor plastic substrates

Zeonor plastics are highly versatile due to exceptional optical and mechanical properties which make them the choice material in many novel applications. For potential use in flexible transparent optoelectronic applications, we have investigated Zeonor plastics as flexible substrates for the deposition of highly transparent ZnO and AZO thin films. Films were prepared by pulsed laser deposition at room temperature in oxygen ambient pressures of 75, 150 and 300 mTorr. The growth rate, surface morphology, hydrophobicity and the structural, optical and electrical properties of as grown films with thicknesses∼65–420 nm were recorded for the three oxygen pressures. The growth rates were found to be highly linear both as a function of film thickness and oxygen pressure, indicating high reproducibility. All the films were optically smooth, hydrophobic and nanostructured with lateral grain shapes of∼150 nm wide. This was found compatible with the deposition of condensed nanoclusters, formed in the ablation plume, on a cold and amorphous substrate. Films were nanocrystalline (wurtzite structure), c-axis oriented, with average crystallite size∼22 nm for ZnO and∼16 nm for AZO. In-plane compressive stress values of 2–3 GPa for ZnO films and 0.5 GPa forAZO films were found. Films also displayed high transmission greater than 95% in some cases, in the 400–800 nmwavelength range. The low temperature photoluminescence spectra of all the ZnO and AZO films showed intense near band edge emission. A considerable spread from semi-insulating to n-type conductive was observed for the films, with resistivity∼103 Ω cm and Hall mobility in 4–14 cm2 V−1 s−1 range, showing marked dependences on film thickness and oxygen pressure. Applications in the fields of microfluidic devices and flexible electronics for these ZnO and AZO films are suggested

The identification and nature of bound exciton I-line PL systems in ZnO

The chemical identification of donor bound excitons in ZnO has been studied using radioactive ions. Implantations of 117Ag – which decays to radioactive Cd and In – have enabled the identification of the I2 optical feature as being the ionized donor counterpart of I9, one of the most prominent optical features in the photoluminescence spectrum of ZnO. Both of these lines are consistent with In occupying a Zn site

Chemical identification of luminescence due to Sn and Sb in ZnO

We show that the I10 line in ZnO photoluminescence is associated with Sn impurities. The evidence comes from material implanted with radioactive 117Ag, which decays through 117Cd and 117In to stable 117Sn, and 121Ag which decays through the same chain to stable 121Sb. Supporting evidence is provided by ZnO:Sn prepared by the in-diffusion of stable Sn. The I2 and I9 lines are also shown conclusively to be due to In, confirming earlier identifications in the literature. We also observe shallow bound exciton emission at 3.3643(3) eV due to Sb impurities produced at the end of the decay chain of 121Ag