No description available.

No description available.

No description available.

Metal-semiconductor contacts are of fundamental importance to the operation of all semiconductor devices. There are many competing theories of Schottky barrier formation but as yet no quantitative predictive model exists to adequately explain metal-semiconductor interfaces. The II-VI compound semiconductors CdTe, CdS and ZnSe have recently come to the fore with the advent of high efficiency photovoltaic cells and short wavelength light emitters. Major problems still exist however in forming metal contacts to these materials with the desired properties. This work presents results which make a significant contribution to the theory of metal/II-VI interface behaviour in terms of Schottky barriers to n-type CdTe, CdS and ZnSe. Predominantly aqueous based wet chemical etchants were applied to the surfaces of CdTe, CdS and ZnSe which were subsequently characterised by X-ray photoelectron spectroscopy. The ionic nature of these II-VI compounds meant that they behaved as insoluble salts of strong bases and weak acids. Acid etchants induced a stoichiometric excess of semiconductor anion at the surface which appeared to be predominantly in the elemental or hydrogenated state. Alkaline etchants conversely induced a stoichiometric excess of semiconductor cation at the surface which appeared to be in an oxidised state. Metal contacts were vacuum-evaporated onto these etched surfaces and characterised by current-voltage and capacitance-voltage techniques. The surface preparation was found to have a clear influence upon the electrical properties of Schottky barriers formed to etched surfaces. Reducing the native surface oxide produced near ideal Schottky diodes. An extended study of Au, Ag and Sb contacts to [mathematical formula] substrates again revealed the formation of several discrete Schottky barriers largely independent of the metal used; for [mathematical formula]. Deep levels measured within this study and those reported in the literature led to the conclusion that Fermi level pinning by native defects is a dominant mechanism in Schottky barrier formation in these systems.

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

No description available.

Thesis (Ph. D.)--University of Hong Kong, 1994. / Includes bibliographical references.

Electrical, forming and switching characteristics of metal-semiconductor-metal (MSM) memory switches of ion bombarded hydrogenated amorphous silicon (a-Si:H) and its alloys are presented. MSM devices for memory switching applications are known to be characterised by instabilities as well as non-uniformity and irreproducibility of the forming and switching characteristics. It is believed that the presence of defect states in the semiconductor layer plays a significant role in the observation of memory switching in these MSM devices. Gas-phase doping and current stressing of the semiconductor are some of the techniques that have been used in the past to introduce mid-gap defect states. In this work, we use for the first time, ion bombardment as a novel tool for defect introduction into the semiconductor material of the MSM devices and we compare the electrical, forming and switching characteristics of these devices to those fabricated using the previous techniques mentioned above. A significant observation is that as the density of defects is increased in the semiconductor film with increasing implantation dose, conduction in the devices changes from barrier-controlled thermionic emission to bulk controlled where carriers hop through the defect states in a Poole-Frenkel manner. This transformation eliminates problems associated with Schottky barriers such as quality of contacts, oxidation, etc. In the forming characteristics, not only do we report enhanced uniformity of the forming voltages (VF) but also the magnitude of VF is observed to vary systematically with the implantation dose used. The ON states and switching characteristics in the bombarded devices are also observed to be much more stable presumably as a result of the uniformity of the defects introduced by ion bombardment. We report also an enhanced switching ratio in the ion bombarded devices, especially after partial annealing of the bombardment induced Si dangling bond defects.

The extraordinary magnetoresistance (EMR) in metal-semiconductor hybrid structures was first demonstrated using a van der Pauw configuration for a circular semiconductor wafer with a concentric metallic inclusion in it. This effect depends on the orbital motion of carriers in an external magnetic field, and the remarkably high magnetoresistance response observed suggests that the geometry of the metallic inclusion can be optimized to significantly enhance the EMR. Here we consider the theory and simulations to achieve this goal by comparing both two-dimensional as well as three-dimensional structures in an external magnetic field to evaluate the EMR in them. Examples of structures that are compatible with present day technological capabilities are given together with their expected responses in terms of EMR. For a 10 micron 2D square structure with a square metallic inclusion, we see a MR up to 10^7 percent for an applied magnetic field of 1 Tesla.

The long-term goal of this research project is the development of solution-based inorganic dielectric and semiconductor materials for inkjet printed electronics. The main focus of this thesis involves testing of the materials and devices under development. A new solution-based inorganic dielectric material (HfOSO₄), given the name hafsox, is developed and shows excellent dielectric properties. Hafsox with the addition of lanthanum, to improve film dehydration, has successfully been demonstrated as a gate dielectric. Metal-insulator-metal (MIM) capacitance testing of hafsox with lanthanum, has resulted in a low loss tangent of 0.30% at 1 kHz, a relative permittivity of 11.47 at 1 kHz, a breakdown voltage of 6.30 MV cm⁻¹, and a leakage current density of 4.38 nA cm⁻² at 1 MV cm⁻¹. Progress has also been achieved in the development of solution-based semiconductor materials. To date the most successful of these materials is zinc indium oxide (ZIO), which has been demonstrated as a thin-film-transistor (TFT) channel material. This ZIO TFT is a depletion-mode device with a turn-on-voltage of V[subscript on]~ -19 V, a threshold voltage of V[subscript T] ~-16 V, and a drain current on-to-off ratio of ~10³. Mobilities extracted from this ZIO TFT include an incremental mobility of μ[subscript inc] ~0.05 cm² V⁻' sec⁻', an effective mobility of μ[subscript eff] ~0.02 cm² V⁻' sec⁻', and an average mobility of μ[subscript avg] ~0.02 cm² V⁻' sec⁻' at V[subscript GS]=20 V. The development of metal-semiconductor field-effect transistors (MESFET) TFTs is also investigated as a means of eliminating the need for a dielectric material in order to reduce the complexity of fabricating circuits. MESFETs are attempted with semiconductor materials such as CdS that is deposited by chemical bath deposition (CBD) and SnO₂ that is deposited by RF magnetron sputtering, but with little success. The most successful MESFET-like device fabricated, employing SnO₂ as the channel material, is a strong depletion-mode device with a small amount of gate voltage modulation. / Graduation date: 2006