Semiconductors

A study of the temperature dependence of the photoluminescence spectra from Zn1-xMnxS (x = 0.01, 0.03, 0.05, 0.10, 0.20, 0.25, 0.30, 0.35 and 0.50) with different excitation energies (514.5, 488.0, 457.9, 405.5 and 365.9 nm) is given. A new band is reported in addition to the four usual emission bands. The unusual shift of the 2.1 eV (YEL) band energy is first observed in our experiments. Models for YEL, ORG (2.0 eV) and RED (1.7 eV) emission bands have been proposed. The quenching with concentration and temperature and the influence of excitation energy on the photoluminescence are discussed. The spin-spin exchange interaction between the nearest neighbor Mn^+2 ions (d-d electrons), and the exchange interaction between 3d electrons of Mn^+2 ions and the conduction or valence band electrons (d-s or d-p electrons) are introduced to interpret the shift of the bands. The strong electron-phonon interaction is also analyzed, and an effective phonon energy and a Huang-Rhys factor are obtained.

Measurements of the Hall Effect and conductivity at between 20 and 500K
and optical absorption measurements at 77 and 20K have completed the
characterization of manganese as a deep acceptor in gallium arsenide.
The photo-ionization cross-section maxium (about 8.5x10^-17 cm^2) agreed
well with the Lucovsky delta function potential model, while the
spectral dependence agreed best with the quantum defect model with a
coulomb continuum final state. Impurity concentrations and activation
energies were determined from Hall Effect data and the correlation of
activation energy with Na and Nd determined. Impurity conduction
appearing below 100K was characterized in heavily doped (Na = 2x10^19 cm^-3 )
a moderately compensated (K=0.1) samples. Both metallic type impurity
conduction and the thermally activated type were seen. AMott
transition was found that occurs within a very narrow range of impurity
concentrations.

Classical trajectory molecular dynamics methods are used to investigate the critical strain of single-walled carbon nanotubes ("SWT") and the strength and extent of the interactions between 3D Ge structures on the surface of Si(001). The discrete model is capable of giving some insight into the critical strain of the SWT's beyond the limits of the continuous model and allow us to investigate the effects of lattice distortion due to the placement of Ge structures on the surface of a Si substrate. Total energy calculations performed using classical three-body interatomic potentials with appropriate boundary conditions for each case are used to investigate the two systems. We discuss the development of a parallel code to simulate short-ranged empirical potentials such as those of Stillinger and Weber, Tersoff, and Tersoff-Brenner. We then use the Tersoff potential to model C and Si/Ge system. Data collected are used to examine the behavior of the two systems.

High density storage mechanisms are generally created using either magnetic or optical implementation techniques. Both of these techniques require mechanical transport of the medium and, therefore, have low reliability factors. These devices also generate unwanted low level ambient noise, which is of particular concern when considering modern quiet office standards. Additionally, optical techniques tend to be read-only in nature. Both mechanisms exhibit random access times that are measured in milli-seconds, rather than in micro-seconds. Therefore, the creation of a non-volatile random access memory as a replacement for the above mentioned storage techniques would be of great advantage in terms of access time, reliability, and ambient noise level. Described within are the device and circuit modeling and fabrication techniques used to develop a non-volatile random access memory cell from an amorphous silicon thin-film transistor based technology. Amorphous silicon thin-film transistors are fabricated by depositing the metal, the insulator and the semiconductor materials with a sputtering mechanism in a vacuum at 220 degrees centigrade, rather than by diffusion at 2000 degrees centigrade, as is done with crystalline silicon. By depositing a metal in the insulator, which is located between the gate and the channel, and by using an insulator material with extremely high resistivity, one can store charge in the gate region for a long period of time without external power. For example, this period of time can be as little as one week or as long as over one year. With a periodic refresh, one can extend the memory time of this storage mechanism indefinitely. Thin-film transistors can be deposited on a variety of materials such as glass, quartz or plastic by means of a stationary or continuous motion fabrication system. This material can be either rigid or flexible, and can be comparatively large in size. This allows for much greater circuit density than a standard crystalline silicon chip that contains devices of a comparable channel length. Ten-thousand mega bytes, or more, of virtual storage could become common place. In summary, this approach represents a large scale, high density, high speed "non-volatile" storage device, with read-write random access capability, without moving parts.