Dietrich Meyerhofer

Current Flow in Very Thin Films of Al2O3 and BeO

This paper describes a study of current flow through very thin insulating films. Several possible conduction mechanisms are presented and their properties are discussed. The dominant conduction process in the layers studied is quantum‐mechanical tunneling. Previous work on this mechanism is extended by an exact calculation which shows the current‐voltage characteristic to be divided into three regions of different logarithmic curvature. The dielectric constant of the insulator is found to have a large effect, via the image force, on the tunneling current. The theoretical results are fitted to measurements made on insulating layers less than 100 A thick, sandwiched between metal electrodes. For Al2O3 layers the fit is excellent, resulting in barrier heights of 2.0–2.5 eV and thicknesses approximately 25 A less than the values determined from capacitance, as expected. In contrast the conduction characteristics of BeO layers can be explained by tunneling only in the low‐voltage region; barrier height and thi...

Properties of p‐Type GaAs Prepared by Copper Diffusion

Purification of arsenic by hydrogen sublimation and gallium by vacuum annealing resulted in the preparation of high resistivity (∼106 ohm‐cm), n‐type GaAs crystals by the horizontal Bridgman technique. Conversion of this material to p‐type was accomplished by the inward diffusion of copper from the crystal surface. Measurements of Hall mobility as a function of hole concentration in the range, 2×1016 cm−3 to 3×1017 cm−3, at room temperature showed a dependence which is consistent with theory. The analysis suggests a lattice mobility for holes of 450 cm2 V−1 sec−1 at 300°K. The mobility varied approximately as T−2.3 in the range 76°–300°K. At the low temperatures the number of ionized impurities ranged from 2×1015 cm−3. The energy levels associated with copper in GaAs are 0.023 ev and 0.15 ev above the valence band.

Elastic and dielectric constants in mixtures of nematic liquid crystals

The dielectric constants and two elastic constants were measured for a series of mixtures of two nematic liquid crystals, MBBA and PEBAB. The materials covered the range from 0 to 10% PEBAB. The parameters were calculated from the measured voltage dependence of capacitance. The dielectric constants vary linearly with PEBAB concentration and the anisotropy changes from negative to positive at 1.7%. The elastic constants remain fixed over most of the range, but are reduced in mixtures of small anisotropy. The results differ significantly from recent measurements of Michel and Smith, the probable reason being that we take any nonideal alignment at the surface explicitly into account in the calculations.

New technique of aligning liquid crystals on surfaces

A new evaporation procedure for preparing the surfaces of liquid crystal cells is described. It causes the liquid crystal to align uniformly, but with a small tilt bias. This is the optimum configuration for the important field effect display devices. The exact dependence of the orientation on processing parameters and the stability of the surfaces to high temperatures are described.

Electro‐optic and hydrodynamic properties of nematic liquid films with free surfaces

The electro‐optic and hydrodynamic properties of unconstrained films of MBBA have been studied using both freely suspended films with two free surfaces and suspended layers having a single free surface. Orientation in the bulk can be different from that at the surface and is influenced by magnetic fields and by flow. Changes in orientation near the surface can account for the observed optical rotation, domain boundaries, and electro‐optic effects. At frequencies below the dielectric relaxation frequency, electric fields also give rise to hydrodynamic flow and flow alignment which is demonstrated visually.

The Effect of Host on Pleochroic Dye Order Parameter

Abstract The order parameters of three dyes in seven nematic liquid crystalline hosts have been measured. A correlation between a dyes length and its order parameter could be seen. Variations of as much as 50% in a dyes order parameter occurred on changing host.

Spatial resolution of relief holograms in dichromated gelatin.

The properties of relief phase holograms in unhardened dichromated gelatin have been investigated. Relief holograms are particularly attractive when it is desired to duplicate the stored information. Unhardened layers of gelatin were exposed with two collimated laser beams and developed by washing away the unexposed material. This resulted in high quality gratings with very low scattering. By varying the spatial frequency, it was found that the depth of modulation starts to decrease approximately linearly at a few microns grating spacing. This limited spatial resolution was shown not to be a property of the basic photochemical process of polymerization but due to a development step. Larger modulation and diffraction efficiencies can be produced at the higher frequencies by other development procedures, but they also result in increased scattering which is incompatible with high quality holograms.

Optical transmission of liquid‐crystal field‐effect cells

The relationships between the optical transfer characteristics of twisted‐nematic liquid‐crystal devices and the material and cell parameters were investigated. The dependence of transmission on wavelength, birefringence, cell thickness, and surface alignment was measured as functions of light direction and operating conditions. The experimental results were confirmed by calculations of the transmission using Berreman’s numerical procedure for anisotropic media. The angle over which good contrast is observed increases with increasing wavelength and decreasing cell thickness and birefringence. This is accompanied by a decreasing value of optical threshold. The most uniform response and widest viewing angle is obtained with materials of small birefringence in cells with near‐planar alignment.

Holographic and Interferometric Viewing Screens

Novel kinds of rear-projection viewing screens are described. The light incident on each part of the screen is deflected and scattered by a prescribed amount, so that the picture presented to the viewer is optimized. In addition, the amount of light needed can be minimized. The screens are. produced in dichromated gelatin by photographic techniques with coherent light. In one case, a hologram is formed that allows any kind of characteristic to be produced but suffers from less than 100% efficiency; in the other case, an interferometric exposure with appropriate development results in a screen with almost ideal characteristics for a short-focal-length viewer, such as a microfilm reader. The processing techniques and results are discussed, and second kind of screen.

A Light‐Activated Semiconductor Switch

A GaAs p‐n‐i‐n diode which operates as a light‐activated light‐coupled switch is described. The i‐region is compensated semi‐insulating GaAs and is obtained by diffusing Cu, Fe, or Cr into low‐doped n‐GaAs. The electrical and optical properties of this diode have been studied as well as its switching behavior. The diode exhibits a current‐controlled negative resistance region with ratio of off‐ to on‐resistance as high as 106. Turn‐on voltages range from 10 to 200 V, and the turn‐on time can be made as short as 10 nsec with sufficient overvoltage. The current is limited in the high‐resistance mode by intrinsic or one‐carrier space‐charge‐limited conduction in the i region. At higher currents the light emission at the p‐n junction causes photoconduction in the i region which is enhanced by the trapping of holes at acceptor levels. In the low‐resistance mode all the traps (approximately 1016 cm−3) are filled and remain that way because of the high efficiency of light emission from the junction at the high c...