Ferd Williams

Liquid water as a lone‐pair amorphous semiconductor

We approximate liquid water as a lone‐pair amorphous semiconductor. Since electronic orbital times are short compared to the periods of atomic and molecular motion, properties dependent on electronic states can be interpreted to determine short‐time intermediate range order. An anomalous temperature dependence of the Urbach exponential absorption edge for intramolecular electronic excitation is noted and interpreted in terms of the perturbation of the lone‐pair valence band by hydrogen bonding. The extrinsic optical absorptions with I− and Br− doped water are observed to have Urbach tails. An approximate electronic band structure for pure and doped liquid water is presented.

Optical Absorption and Energy Levels of Manganese in ZnS : Mn Crystals

The energy of the 6S state of the 3d5 configuration of substitutional manganese is located with respect to the band structure of zinc sulfide by approximate theoretical methods and found to be consistent with a tight‐binding, crystal‐field analysis for the states of this impurity. In addition, the perturbation by the manganese of the band structure is estimated and found to be negligible. Optical absorption measurements are reported on a series of manganese‐activated crystals. The five absorption bands are ascribed to intercombination transitions in the 3d5 configuration of manganese. The intensities of these bands are linear in manganese concentration. The peak absorption energies are analyzed using the Racah coefficients and the crystal field as parameters, and the most probable identification of the transitions determined. The energy of the 6S state of substitutional chromium is also estimated and found to agree with experiment, thus supporting the approximate theoretical methods used for locating thes...

High-field electroluminescence☆

Abstract The theoretical efficiency for the direct conversion of electrical energy into luminescent emission, that is, electroluminescence (EL), is first evaluated from thermodynamics. High-field, collisional excitation EL is then introduced and compared with low-field, minority carrier injection EL and with cathodoluminescence. Some general characteristics of thin-film high-field EL are discussed. The basic mechanism of high-field EL is reviewed. It consists of three steps occuring in sequence: generation of charge carriers, acceleration of these carriers to optical energies, and their inelastic collisions to excite luminescent centers. Ballistic, streaming and Maxwellian hot carriers are distinguished, and their distribution functions discussed. The incompatibility of large cross sections for collisional excitation and stability of the excited luminescent centers in high fields is considered, taking account of the energies of the electronic states of dopants with respect to band structure. The present understanding of the structure and electronic states of transition metal and rare earth (RE) luminescent centers in II–VI and II–VII 2 semiconductors is evaluated. Particular attention is given to probable structures of molecular dopants such as REF 3 in ZnS. Recent advances in thin-film, high-field EL are summarized. The emphasis has been on ZnS:Mn and ZnSe:Mn films sandwiched between insulating films such as Y 2 O 3 and operated with a.c. voltages. All three steps of the high-field EL mechanism occur in the II–VI layer, with some carrier generation at the II–VI/insulator junction. The spatial separation of the sequential steps in contiguous layers in composite EL cells is proposed, and evidence for separation of the acceleration and collision excitation in SiO/ZnF 2 :Mn/SiO cells is presented. Some quantum optical effects in thin-film EL cells are discussed. Finally, some basic problems of high-field EL are reviewed. These include: the origin of charge carriers, the distribution function of the energetic carriers, structure and energy levels of complex dopants, and sources of inefficiencies and of saturation. Possible methods for achieving higher efficiencies than the current (1%) are proposed.

Mechanism of thin-film electroluminescence

Experimental and theoretical evaluations of cross sections for hot carrier collisional excitation of manganese and rare-earth dopants in thin-film electroluminescent devices are reported. The hot carriers are identified experimentally as electrons. Energies of the hot electrons are estimated, and possibilities for improvements in efficiency are discussed.

A new thin‐film electroluminescent material—ZnF2 : Mn

Both ac and dc orange (580 nm) electroluminescence (EL) are reported for thin films of ZnF2 : Mn sandwiched between SiO semi‐insulating films. All layers are formed by vacuum evaporation and no postdeposition annealing is required. Unique power input and efficiency‐vs‐frequency characteristics are observed, in part due to the 0.1‐s lifetime of excited Mn in ZnF2; hysteresis in the brightness‐voltage characteristic occurs. The brightness and power efficiency are found to be, respectively, 10 f L and 0.5% under suitable operating conditions.

Analysis of lead azide thin films by Rutherford backscattering

The Rutherford backscattering technique using 1‐MeV protons has been employed to analyze pure and doped lead azide thin films of various thicknessess which had been prepared by vacuum evaporation followed by vapor conversion. The films were found to be approximately homogeneous in composition with depth within the limits of the depth resolution of the experiment. The energy rate loss dE/dx of the films was found to be about 60 keV/μm. The Tl‐doped films showed a somewhat different backscattering spectrum from the undoped films.

Growth and Electrical Properties of Zinc‐Cadmium Sulfide Graded‐Band‐Gap Crystals

Large quasi‐single crystals of ZnxCd1−xS with increasing x along the growth direction, have been successfully grown by vapor‐growth techniques. Typical dimensions are 15×3×4 mm. The growth mechanism is studied and, as a result, it is believed that the thermal conductivity plays a decisive role. Position‐dependent optical and electrical studies on these crystals have revealed that the crystals with a slowly varying composition can be described by a graded band gap while the properties of crystals having a more abrupt change in composition are determined by lattice defects. Most of the crystals exhibit photoluminescence and high‐field electroluminescence at low temperatures. From the optical and electrical data an energy‐band diagram for graded ZnxCd1−xS is deduced with the position‐dependent band gap accounted for by the position‐dependent electron affinity.

Photodecomposition and electronic structure of lead azide

Photodecomposition, optical absorption, and conductivity measurements were performed on thin films of lead azide in order to determine interrelation between electronic structure and chemical instability. Both pure lead azide and thallium and bismuth doped films were investigated. The azide films are prepared by chemical conversion of evaporated metallic films by the vapor of hydrazoic acid and are found to consist of 1 μm2 platelets which lie parallel to the substrate and are optically active. No effects of doping on the Fermi level are observed and we conclude that the dopants are self‐compensated by native defects. The observed changes in ultraviolet and infrared absorption during thermal or photodecomposition can be attributed to the formation of azide vacancies. Thallium doping results in changes in optical properties similar to those obtained on decomposition. The initial step in photodecomposition appears to be the creation of charge‐transfer excitons.

Infrared absorption spectra of doped and undoped lead azide

The far and near infrared absorption of thin films of pure and doped lead azide are reported. The effects of thallium and bismuth doping were investigated. The far ir lattice modes are essentially unaffected by doping; the near ir intra‐azide ion modes are markedly affected by Tl doping. These effects are related to changes in the electronic properties with doping and are attributed to azide vacancies which compensate for, and pair with, substitutional Tl acceptors.

Collision excitation cross-sections and energy levels of deep and very deep centers in electroluminescence

Abstract We have investigated the collision excitation mechanism of EL and report preliminary results of calculations of cross sections for excitation within the 3d5 configuration of Mn2+ in sulfides and fluorides. The energies E of the hot carrier investigated are: Ee