In this study the parameters characterizing the electronic transport in glow-discharge deposited amorphous (a) Si:H, a-Si:C:H and a-Si:Sn:H films are investigated For the investigation five experimental techniques are employed. These are the photocurrent reversal, time-of-flight, charge collection, junction recovery and surface photovoltage experiments. In the photocurrent reversal experiment the on-going dispute concerning the magnitude of the drift mobilities in a-Si:H is addressed. It is established that the current transients observed in this type of experiment are due to the drift of photogenerated excess carriers. The details of the results are in excellent agreement with the low mobility picture which has been gained by means of transient photoconductivity experiments. In the time-of-flight and charge collection measurements, the drift mobilities, carrier ranges, capture cross-sections for shallow and deep trapping, and the distribution of bandtail states are determined. The origin of the deep trapping states and their role in the Staebler-Wronski effect are discussed. For the first time a detailed description of the junction recovery experiment applied to a-Si:H p-i-n type diodes is given. The recombination process relevant to this experiment is discussed and hole recombination times of the order of 10 ns are measured for double injection conditions. These values are consistent with typical hole diffusion lengths measured in the surface photovoltage method. On the basis of the transport parameters derived a possible distribution of the energy levels for the dangling bond states in a-Si:H is discussed The a-Si:Sn:H and a-Si:C:H films are investigated by means of the charge collection technique. A comparison to the results obtained with the a-Si:H films reveals a strong degradation of the electronic properties in the alloy films and indicates the emergence of a new defect center in the atomic network of these alloys / acase@tulane.edu

Explicit functions are widely used to interpolate, extrapolate, and differentiate theoretical or experimental data on the equation of state (EOS) of a solid. Two new theoretically motivated EOS functions are presented here. The simplest realistic model for a simple metal, the stabilized jellium (SJ) or structureless pseudopotential model, is the paradigm for the Stabilized Jellium Equation Of State (SJEOS). A simple metal with exponentially-overlapped ion cores is the paradigm for an augmented version (ASJEOS) of SJEOS. For the three solids tested (Al, Li, Mo), ASJEOS matches all-electron calculations better than prior equations of state. Like most of the prior EOS's, ASJEOS predicts pressure P as a function of compressed volume v from only a few equilibrium inputs: the volume v 0, the bulk modulus B0, and its pressure derivative B1. Under expansion, the cohesive energy serves as another input. An advantage of the new equation of state is that these equilibrium properties other than v0 may be found by linear fitting methods. The SJEOS can be used to correct B0 and the EOS found from an approximate density functional, if the corresponding error in v0 is known. The typically-small contribution of phonon zero-point vibration to the EOS is also estimated, as well as it is shown that the physical hardness Bv does not maximize at equilibrium, and that the 'ideal metal' of Shore and Rose is the zero-valence limit of stabilized jellium / acase@tulane.edu

The jellium surface energy has long been of interest to the Density Functional Theory (DFT) community, since the jellium surface is the simplest example of a strongly inhomogeneous density. Over three decades, various methods have been employed to investigate this problem, and the answers differ significantly from one method to another. In recent years, the appearance of the Quantum Monte Carlo surface energies, which are significantly higher than those of DFT, has raised a serious concern for density functional theorists, since the results from Quantum Monte Carlo are usually considered 'exact'. In this work, we investigate the problem with: (1) a long-range correction to the generalized gradient approximation (GGA); and (2) a GGA short-range correction to the random phase approximation (RPA). The results from these two approaches and those from another advanced density functional, the PKZB meta-GGA, agree among themselves within one percent. Our consistent results strongly support the older and less-sophisticated DFT estimates of jellium surface energies, and not the Quantum Monte Carlo values Our GGA's for the exchange-correlation energy within and beyond RPA are constructed by a real-space cutoff of the spurious long-range contribution to the second-order gradient expansion for the exchange-correlation hole around an electron. In a loosely related study, we show that the sixth-order gradient expansion for the kinetic energy, which diverges for surfaces and finite systems, provides useful accuracy for bulk solid / acase@tulane.edu

The geometrical and electronic surface properties of In2O 3 and Sn-doped In2O3 (ITO) have been investigated. Sn-doped In2O3 is widely used as a transparent conducting oxide in flat panel displays, organic-light-emitting-diodes, solar cells, and electrochromic windows. Despite the fact that surface and interfaces are important in all these applications, a fundamental understanding of the surface properties of this material is lacking. Meaningful surface investigations are best conducted on single-crystalline samples, thus epitaxial thin films of In2O3 and ITO were grown and used as samples for the surface investigations This work focuses on two low-index surfaces of ITO, the non-polar (111) orientation and the (100) orientation, which, in its bulk-terminated form, is polar. The epitaxial films were grown with oxygen-plasma assisted molecular beam epitaxy (MBE) on yttria-stabilized zirconia, which exhibits a cube-on-cube epitaxy as well as a small lattice mismatch with respect to In2O 3 The YSZ(111) substrate was characterized with Re ection-high-electron-energy-diffraction (RHEED) and Low-energy-electron-diffraction (LEED) and its surface was found to be (1x1) terminated. RHEED and LEED measurements on the substrate were possible if the substrate was kept at 300°C in order to avoid charging effects of this insulating material. RHEED exhibited 2-dimensional growth mode for the Sn-doped In2O3 thin films. Using LEED it was found that the surface of In2O3 and Sn-doped In 2O3 poses a (1x1) terminated surface. A de-convolution of X-ray core level photoemission (XPS) of In 3d peaks; into one component that is due to regular photoemission and one that is due to interaction of core holes with electronic plasmons, provided the plasmon energy, E p; From Ep the electron density n of the doped films was obtained. For an ITO film with 6.2 at% of Sn, it was found that 1/3 of the Sn atoms contribute one electron to the conduction band. Scanning-tunneling-microscopy (STM) was measured and atomic resolution was achieved. Bright and dark atomic features were assigned to a surface with bulk (1x1) termination. Density-functional-theory (DFT) calculations performed by collaborators confirmed that indium atoms are being imaged bright and dark in empty-states STM, depending on the configuration of their oxygen neighbors; bright when neighboring three-fold and dark when neighboring four-fold oxygen atoms Epitaxial (100)-oriented thin films of In2O3 and Sn-doped In2O3 were also grown. The YSZ substrate was characterized using RHEED and LEED and was found to have a (2x2) reconstructed surface. RHEED showed that the Sn-doped In2O3 grows in a 3-dimensional growth mode. Using LEED it was found that the surface of Sn-doped In2O3 exposes a surface with a c(1x1) termination. Considering symmetry arguments and the systematic absence of LEED spots, it was concluded that the surface can have only an oxygen termination. Due to the polarity of this orientation, it was found that unusually high concentrations of tin, ca. 15.6 at%, are needed to render the surface flat. Comparing STM measurements with DFT calculations published in the literature, it was concluded that surface oxygen atoms are dimerized, with all O dimers present for a high Sn concentration in ITO. At low Sn-concentrations the observed STM images are consistent with a model where only 2/3 of the oxygen atoms are present, again consistent with published DFT predictions. It is thus concluded that both, the Sn concentration and oxygen dimerization, are instrumental in stabilizing the polar ITO (001) surface / acase@tulane.edu

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