Surface passivation of semiconducting oxides by self-assembled nanoparticles

Physiochemical interactions which occur at the surfaces of oxide materials can significantly impair their performance in many device applications. As a result, surface passivation of oxide materials has been attempted via several deposition methods and with a number of different inert materials. Here, we demonstrate a novel approach to passivate the surface of a versatile semiconducting oxide, zinc oxide (ZnO), evoking a self-assembly methodology. This is achieved via thermodynamic phase transformation, to passivate the surface of ZnO thin films with BeO nanoparticles. Our unique approach involves the use of BexZn1-xO (BZO) alloy as a starting material that ultimately yields the required coverage of secondary phase BeO nanoparticles, and prevents thermally-induced lattice dissociation and defect-mediated chemisorption, which are undesirable features observed at the surface of undoped ZnO. This approach to surface passivation will allow the use of semiconducting oxides in a variety of different electronic applications, while maintaining the inherent properties of the materials

Optical and electronic properties of defects and dopants in oxide semiconductors

Interest in semiconductor materials has continually grown over the past 60 years due to their potential use in electronic and optoelectronic device structures. Oxide semiconductors are a particular class of materials that also combine conductivity with optical transparency, properties not usually found in the same material. These transparent conducting oxides (TCOs) have been among the first oxide materials to benefit from the availability of improved epitaxial growth techniques, although perovskite oxides and heterostructures have also proved to be opening a new era of high mobility structures based on oxide materials. Optical and electronic properties of binary oxides, specifically, high quality CdO and SnO2 epi- taxial films have been investigated in this thesis. The main band structure quantities, the band gap and band edge effective mass of CdO has long been a subject of controversy due to the degeneracy of this material. The lowest carrier concentration for an as-grown CdO film is 1-2×1019 cm−3. This brings about further difficulties in determining the optoelectronic properties due to conduction band filling and many body effects. The effective mass value is of particular importance in carrier mobility studies. Simulation and analysis of data collected from Hall effect, mid- and near-infrared reflectance measurements together with optical absorption spectroscopy enabled the band gap and band edge effective mass values to be determined at room temperature. Variations of the band gap, band edge effective mass, high frequency dielectric constant and the Fermi level with temperature and carrier concentration, and taking into ac- count the non-parabolicity of the conduction band, the Burstein-Moss shift and band gap renormalization, revealed the 0 K band gap and band edge effective mass values of 2.31 eV and 0.266m0 at the limit of zero carrier concentration in CdO. With the emergence of sophisticated growth techniques (MBE), high quality growth has become a key property in semiconductor research as it enables further investigation into the intrinsic characteristics of these materials. Carrier mobilities in high quality SnO2(101) films grown on r-plane sapphire by molecular beam epitaxy were studied. Transmission electron microscopy revealed a high density of dislocations at the interface due to the large lattice mismatch of -11.3%, along the direction, between the films and the substrate, with an exponential decrease towards the surface of the films. Carrier mobility modelling proved to be impossible if a constant density of threading dislocations was assumed, however, by introducing a layer-by-layer model for the simulation of the mobility as a function of carrier concentration, the donor nature of dislocations in epitaxial SnO2 films was revealed. The deformation potential produced by the presence of these defects has been shown to be the dominant scattering mechanism for carrier concentrations above the Mott transition level of SnO2. Finally, the surface electronic structure of antimony-doped SnO2 films has been studied by the Hall effect, infrared reflectance, X-ray photoemission spectroscopy and electrochemical capacitance-voltage measurements. The bulk Fermi level was determined by carrier statistics calculations and used to obtain the degree of surface band bending. Modelling the surface energy bands through the capacitance-voltage spectra, revealed that SnO2 has downward band bending and surface electron accumulation. The respective variations were attained as a function of depth and composition of the samples

Temperature dependence of the direct bandgap and transport properties of CdO

Temperature-dependent optical absorption, Hall effect, and infrared reflectance measurements have been performed on as-grown and post-growth annealed CdO films grown by metal organic vapor phase epitaxy on sapphire substrates. The evolution of the absorption edge and conduction electron plasmon energy with temperature has been modeled, including the effects arising from the Burstein-Moss shift and bandgap renormalization. The zero-temperature fundamental direct bandgap and band edge effective mass have been determined to be 2.31+/-0.02 eV and 0.27+/-0.01m(0), respectively. The associated Varshni parameters for the temperature dependence of the bandgap are found to be a alpha = 8 x 10(-4) eV/K and beta = 260 K

Valence-band density of states and surface electron accumulation in epitaxial SnO2 films

The surface band bending and electronic properties of SnO2(101) films grown on r-sapphire by plasma-assisted molecular beam epitaxy have been studied by Fourier-transform infrared spectroscopy (FTIR), x-ray photoemission spectroscopy (XPS), Hall effect, and electrochemical capacitance-voltage measurements. The XPS results were correlated with density functional theory calculation of the partial density of states in the valence-band and semicore levels. Good agreement was found between theory and experiment with a small offset of the Sn 4d levels. Homogeneous Sb-doped SnO2 films allowed for the calculation of the bulk Fermi level with respect to the conduction-band minimum within the k⋅p carrier statistics model. The band bending and carrier concentration as a function of depth were obtained from the capacitance-voltage characteristics and model space charge calculations of the Mott-Schottky plots at the surface of Sb-doped SnO2 films. It was quantitatively demonstrated that SnO2 films have downward band bending and surface electron accumulation. The surface band bending, unoccupied donor surface-state density, and width of the accumulation region all decrease with increasing Sb concentration

Impact of degenerate n-doping on the optical absorption edge in transparent conducting cadmium oxide

In order to facilitate the development of next-generation display devices or modern solar cells, material performance is critically important. A combination of high transparency in the optical spectral range and high electrical conductivity under ambient conditions is attractive, if not crucial, for many applications. While the doping-induced presence of free electrons in the conduction bands of CdO can increase the conductivity up to values desired for technological applications, it is, however, expected to impact the optical properties at the same time. More specifically, variations of the band gap, effective electron mass, and optical-absorption onset have been reported. In this work recent results from modern theoretical spectroscopy techniques are compared to experimental values for the optical band gap in order to discuss the different effects that are relevant for an accurate understanding of the absorption edge in the presence of free electrons with different concentrations. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only

Electron mobility in CdO films

Electron mobility in degenerate CdO thin films has been studied as a function of carrier concentration. The "optical" mobility has been determined from infrared reflectance measurements of the conduction band plasmon lifetime. The acquired values vary from similar to 209 to similar to 1116 cm(2) V(-1) s(-1) for carrier concentrations between 2.5 x 10(20) and 2.6 x 1019 cm(-3). Ionized impurity scattering is shown to be the dominant effect reducing the intra-grain mobility of the electrons at room temperature. The transport mobilities from Hall effect measurements range between similar to 20 and similar to 124 cm(2) V(-1) s(-1) which are much lower than the optical mobilities. Simulation of grain boundary scattering-limited mobility is commonly based on models that assume a depletion layer at the boundaries which causes an inter-grain potential barrier. These models are found not to be applicable to CdO as it has been previously shown to have surface electron accumulation. Therefore, simulation of the transport mobility has been performed using the Fuchs-Sondheimer and Mayadas-Shatzkes models to take into account the grain boundary and surface scattering mechanisms, in addition to intra-grain scattering. The results indicate that electron scattering at grain boundaries with similar to 95 % reflection is the dominant mechanism in reducing the mobility across the layer. The effect of surface scattering plays only a minor role in electron transport. (C) 2011 American Institute of Physics. [doi:10.1063/1.3562141

Recrystallization of highly-mismatched BexZn1–xO alloys : formation of a degenerate interface

We investigate the effect of thermally induced phase transformations on a metastable oxide alloy film, a multiphase BexZn1–xO (BZO), grown on Al2O3(0001) substrate for annealing temperatures in the range of 600–950 °C. A pronounced structural transition is shown together with strain relaxation and atomic redistribution in the annealed films. Increasing annealing temperature initiates out-diffusion and segregation of Be and subsequent nucleation of nanoparticles at the surface, corresponding to a monotonic decrease in the lattice phonon energies and band gap energy of the films. Infrared reflectance simulations identify a highly conductive ZnO interface layer (thicknesses in the range of ≈10–29 nm for annealing temperatures ≥800 °C). The highly degenerate interface layers with temperature-independent carrier concentration and mobility significantly influence the electronic and optical properties of the BZO films. A parallel conduction model is employed to determine the carrier concentration and conductivity of the bulk and interface regions. The density-of-states-averaged effective mass of the conduction electrons for the interfaces is calculated to be in the range of 0.31m0 and 0.67m0. A conductivity as high as 1.4 × 103 S·cm–1 is attained, corresponding to the carrier concentration nInt = 2.16 × 1020 cm–3 at the interface layers, and comparable to the highest conductivities achieved in highly doped ZnO. The origin of such a nanoscale degenerate interface layer is attributed to the counter-diffusion of Be and Zn, rendering a high accumulation of Zn interstitials and a giant reduction of charge-compensating defects. These observations provide a broad understanding of the thermodynamics and phase transformations in BexZn1–xO alloys for the application of highly conductive and transparent oxide-based devices and fabrication of their alloy nanostructures

Recrystallization of Highly-Mismatched Be_<i>x</i>Zn_1–<i>x</i>O Alloys: Formation of a Degenerate Interface

We investigate the effect of thermally induced phase transformations on a metastable oxide alloy film, a multiphase Be_<i>x</i>Zn_1–<i>x</i>O (BZO), grown on Al₂O₃(0001) substrate for annealing temperatures in the range of 600–950 °C. A pronounced structural transition is shown together with strain relaxation and atomic redistribution in the annealed films. Increasing annealing temperature initiates out-diffusion and segregation of Be and subsequent nucleation of nanoparticles at the surface, corresponding to a monotonic decrease in the lattice phonon energies and band gap energy of the films. Infrared reflectance simulations identify a highly conductive ZnO interface layer (thicknesses in the range of ≈10–29 nm for annealing temperatures ≥800 °C). The highly degenerate interface layers with temperature-independent carrier concentration and mobility significantly influence the electronic and optical properties of the BZO films. A parallel conduction model is employed to determine the carrier concentration and conductivity of the bulk and interface regions. The density-of-states-averaged effective mass of the conduction electrons for the interfaces is calculated to be in the range of 0.31<i>m</i>₀ and 0.67<i>m</i>₀. A conductivity as high as 1.4 × 10³ S·cm^–1 is attained, corresponding to the carrier concentration <i>n</i>_Int = 2.16 × 10²⁰ cm^–3 at the interface layers, and comparable to the highest conductivities achieved in highly doped ZnO. The origin of such a nanoscale degenerate interface layer is attributed to the counter-diffusion of Be and Zn, rendering a high accumulation of Zn interstitials and a giant reduction of charge-compensating defects. These observations provide a broad understanding of the thermodynamics and phase transformations in Be_<i>x</i>Zn_1–<i>x</i>O alloys for the application of highly conductive and transparent oxide-based devices and fabrication of their alloy nanostructures