D. F. Williams

Properties of zinc oxide films prepared by the oxidation of diethyl zinc

Polycrystalline transparent semiconducting zinc oxide films have been deposited by the oxidation of diethyl zinc. The film growth rate is controlled by a complex multistep oxidation process which is dominated by radical reactions. The effect of substrate temperature and gas pressures have been studied. Samples deposited between 280 and 350 °C have a conductivity varying from 10−2 to 50 Ω−1 cm−1. The electrical properties of the films which are typical of polycrystalline material with small crystallites are shown to depend very closely on the film growth conditions. A study of oxygen chemisorption at grain boundaries confirms the importance of grain boundary effects in ZnO polycrystalline films.

Radiationless Transitions in Polyatomic Molecules. III. Anharmonicity, Isotope Effects, and Singlet‐to‐Ground‐State Transitions in Aromatic Hydrocarbons

The work on Franck–Condon factors for radiationless transitions in polyatomic molecules, as reported in Parts I and II of this series, is extended and generalized, starting from the theoretical expression for Franck–Condon factors in aromatic hydrocarbons, which was derived and applied to triplet‐to‐ground‐state transitions in Part II. The role played in this expression by the anharmonicity and symmetry of CH‐stretching modes is clarified. As a preliminary step towards application of this expression to singlet‐to‐ground‐state transitions in aromatic hydrocarbons, the appropriate Franck–Condon factors are studied spectroscopically by measuring the long‐wavelength part of the fluorescence spectra of anthracene‐h10 and ‐d10. These experimental Franck–Condon factors are used to determine the anharmonicity parameter in the theoretical expression for the singlet‐to‐ground‐state radiationless‐transitions rate constant. Theoretical rate constants for the transition calculated on this basis compare favorably with ...

Transparent and highly conductive films of ZnO prepared by rf reactive magnetron sputtering

Highly conductive films of zinc oxide have been prepared by reactive rf magnetron sputtering from an oxide target. Film conductivities ranging from ∼10−8 Ω−1 cm−1 to 5×102 Ω−1 cm−1 can be obtained depending on the sputter conditions. Films with sheet resistivities of 85 Ω/⧠ showed little absorption and ∼90% transmission between λ = 4000→8000 A. A second low power discharge at the substrate is used to initiate growth of the highly conducting material on room‐temperature substrates. Thus, during the deposition of insultating ZnO, turning on this second discharge causes the deposition to ’’switch’’ from low conductivity to high conductivity material. This is of particular interest in the fabrication of semiconductor‐insulator‐semiconductor solar cells where precise control over the thickness of the insulating layer is necessary and where a highly transparent and conductive window‐junction layer is required.

Preparation of conducting and transparent thin films of tin‐doped indium oxide by magnetron sputtering

High‐quality 800‐A‐thick films of tin‐doped indium oxide have been prepared by magnetron sputtering. It is shown that films with low resistivity (∼4×10−4 Ω cm) and high optical transmission (≳85% between 4000 and 8000 A) can be prepared on low‐temperature (40–180 °C) substrates with O2 partial pressures of (2–7)×10−5 Torr.

Absorption edge shift in ZnO thin films at high carrier densities

The optical absorption edge has been measured as a function of carrier concentration for thin films of zinc oxide prepared by organometallic chemical vapour deposition and reactive R.F. magnetron sputtering. Large shifts of the absorption edge have been observed which are only a function of the carrier concentration. Below n = 3 × 1019 cm-3 the shifts are well described by the Burstein-Moss model. For carrier concentrations between 3–5 ×1019cm-3, the shift decreases very rapidly, finally increasing again with further increases in the carrier density. These effects are consistent with a merging of the donor band with the conduction band following a semiconductor-metal transition.

The influence of target oxidation and growth-related effects on the electrical properties of reactively sputtered films of tin-doped indium oxide☆

Abstract The electrical properties of tin-doped indium oxide films prepared by reactive r.f. magnetron sputtering were studied. The properties vary markedly with the added oxygen pressure and also with the film thickness owing to growth effects and target oxidation. The growth effects were analysed in terms of the grain size effects and of the target oxidation by a simple kinetic model. We show that films exhibiting a wide range of electrical properties can be prepared if the deposition conditions are carefully controlled. Films with conductivities as high as 2.5 × 10 3 ohms −1 cm −1 and with optical transmission greater than 85% averaged over the visible spectrum were deposited.

Electrolytic Preparation and EPR Study of Crystalline Pyrene, Perylene, and Azulene Perchlorates

The preparation of crystalline pyrene, perylene, and azulene perchlorates is reported. Details of the novel method, growth on the anode in an electrolytic cell, are given. Crystalline samples of each compound give EPR resonances with very narrow linewidths, about 0.1 G. The resonances from the pyrene and perylene salts each consist of two components. One follows a Curie law and dominates at low temperature. It is attributed to “impurities,” probably lattice defects. The other component, dominating at high temperature, arises from thermally generated Wannier triplet excitons. The excitation energies are 0.18 and 0.27 eV for the pyrene and perylene salts, respectively. The azulene salt yielded only the “impurity” signal within the available temperature range.

dc and Pulsed Electroluminescence in Anthracene and Doped Anthracene Crystals

The current–voltage characteristics of anthracene crystals have been studied under dc and pulsed conditions in the temperature range 350–100°K. The crystals are provided with contact combinations consisting of an electron injecting contact together with either a hole injecting contact or an individually blocking contact. Intense electroluminescence is observed from all contact combinations, and at high crystal currents, ∼ 10−6 A cm−2, each combination shows similar steady‐state characteristics. Pulsed measurements illustrate that different mechanisms for hole injection are involved, at high and low current densities, and a possible mechanism for enhanced hole injection under high electron current densities is discussed. The spectral distribution of the electroluminescence is found to be independent of contact material, but does depend upon crystal preparation.

Epitaxial growth of (100)CdTe on (100)GaAs induced by pulsed laser evaporation

Epitaxial (100) CdTe films were grown on the (100) GaAs surface by pulsed laser evaporation. The growth was achieved on substrates held at 260 °C and under a pressure of about 8×10−8 Torr. High‐energy electron diffraction, x‐ray diffraction, and UV reflectivity studies have shown that epilayers of high crystalline quality were obtained. The surface morphology of films thicker than about 0.4 μm approached atomic smoothness. No incorporated impurities, including oxygen and carbon, were found by Auger electron spectroscopy in the films studied.

Site selection spectroscopy luminescence of solutions with laser excitation

Abstract The narrow line emission observed by Personov et al. at low temperature in a glassy matrix from perylene has been shown to be observable from many other molecules. Spectra of 1-chloronaphthalene and tetracene are presented. Narrow line phosphorescence was not observed. Narrow line fluorescence was also absent when the excitation energy was 1500 cm −1 greater than the 0–0 fluorescence level and in these two cases it is suggested that heating of the matrix by radiationless processes causes the spectral broadening.