Charles W. Myles

Framework Contraction in Na-Stuffed Si(cF136)

Systematic crystal structure refinements from powder X-ray diffraction data as well as density functional theory calculations demonstrate that the silicon clathrate II Si(cF136) exhibits a lattice contraction as Na is introduced solely into the Si(28) cages. When the Si(20) cages, in addition, begin to be filled with Na, a contrasting lattice expansion results. The nonmonotonic structural response to filling is an indication of markedly dissimilar guest-framework interactions for Na@Si(20) and Na@Si(28).

Identification of defect centers in Hg1−xCdxTe using their energy level composition dependence

We have extended the Kobayashi, Sankey, and Dow [Phys. Rev. B 25, 6367 (1982)] theory of deep levels in Hg1−xCdxTe to include (vacancy, impurity) nearest‐neighbor pairs. In qualitative agreement with the results obtained by these workers for isolated point defects, we find that the composition dependences (dE/dx) of the defect energy levels associated with such complexes depend on the site occupied by the impurity atom. Furthermore, we find that the composition dependences of some of the defect levels produced by such a complex are very different than the dE/dx of levels associated with the corresponding isolated point defects. We thus suggest that this theory can often be used as an aid in the identification of the defect center producing an observed energy level. In particular, it can be used to obtain site information about an observed level and, in favorable cases, to distinguish between levels produced by isolated point defects and those produced by complexes. As an example, we compare our theoretica...

Steady-state properties of lock-on current filaments in GaAs

Collective impact ionization has been used to explain lock-on in semi-insulating GaAs under high-voltage bias. We have used this theory to study some of the steady-state properties of lock-on current filaments. In steady state, the heat gained from the field is exactly compensated by the cooling due to phonon scattering. In the simplest approximation, the carrier distribution approaches a quasi equilibrium Maxwell-Boltzmann distribution. In this report, we examine the validity of this approximation. We find that this approximation leads to a filament carrier density that is much lower than the high density needed to achieve a quasi-equilibrium distribution. Further work on this subject is in progress.

Lock‐on effect in pulsed‐power semiconductor switches

Certain high‐voltage pulsed‐power switches based on semi‐insulating GaAs or InP exhibit a ‘‘lock‐on’’ effect. In this paper, this effect is argued to be fundamentally a transferred‐electron effect, and its experimentally observed characteristics are explained. The lock‐on effect causes high forward drop and high power dissipation for certain pulsed‐power switches based on GaAs and various other direct‐gap materials.

Prediction of Giant Thermoelectric Power Factor in Type-VIII Clathrate Si 46

Clathrate materials have been the subject of intense interest and research for thermoelectric application. Nevertheless, from the very large number of conceivable clathrate structures, only a small fraction of them have been examined. Since the thermal conductivity of clathrates is inherently small due to their large unit cell size and open-framework structure, the current research on clathrates is focused on finding the ones with large thermoelectric power factor. Here we predict an extraordinarily large power factor for type-VIII clathrate Si46. We show the existence of a large density of closely packed elongated ellipsoidal carrier pockets near the band edges of this so far hypothetical material structure, which is higher than that of the best thermoelectric materials known today. The high crystallographic symmetry near the energy band edges for Si46-VIII clathrates is responsible for the formation of such a large number of carrier pockets.

Avalanche breakdown in p‐n AlGaAs/GaAs heterojunctions

Avalanche breakdown in abrupt p‐n AlGaAs/GaAs heterojunctions is invesigated, and the breakdown voltage, the maximum electric field, and the depletion layer width are calculated as functions of the doping densities, the temperature, and the AlAs mole fraction in AlGaAs. The model employed is an extension of Hauser’s model of homojunction breakdown [Appl. Phys. Lett. 33, 351 (1978)], and it includes the effects of the band offsets at the interface.

Vibrational spectra of one-dimensional mass-disordered quaternary alloys

Abstract The spectral density of vibrational states for one-dimensional mass-disordered quaternary alloys A y B 1− y C 1− x D x is evaluated using the negative eigenvalue theorem. Spectra for masses corresponding to Al y Ga 1−y As 1−x P x are presented.

A collective impact ionization theory of lock-on [in pulsed power switches]

Photoconductive semiconductor switches (PCSSs), such as optically-triggered GaAs switches, have been developed for a variety of pulsed power applications. Such switches exhibit unique properties associated with lock-on, a phenomenon associated with bistable switching. In this paper, lock-on is explained in terms of collective impact ionization.

Thermal properties of guest-free Si136 and Ge136 clathrates: A first-principles study

We have used the generalized gradient approximation (GGA) to density functional theory to study the vibrational and thermal properties of guest-free Si136 and Ge136 clathrates. In order to study the effects of supercell size on our results, we have performed both 34 and 136 atom supercell calculations for each material. We find that the 34 atom supercell calculations predict a small frequency downshift (in comparison with the 136 atom supercell calculations) in the vibrational density of states of both materials. The GGA-predicted Γ phonon frequency of Si136 (480 cm−1 at T=0 K) obtained from the 136 atom calculations is in very good agreement with the experimental value for Na1Si136 (484 cm−1 at T=300 K). Using the results from our 136 atom calculations, we have also calculated the temperature dependence of the vibrational contributions to the Helmholtz free energy, the entropy, and the specific heat (CV) of the guest-free Si136 and Ge136 clathrates. The predicted and experimental heat capacities of Si136...

Comparison of Si, GaAs, SiC AND GaN FET-type switches for pulsed power applications

Among the present limitations on the peak voltage of traditional Si-MOSFET switches are fundamental materials properties that are related both to intrinsic properties (such as bandgap), and to defects. Switches fabricated from semiconductors such as GaAs, SiC and GaN hold promise if hold-off voltages of several kilovolts and fast rise rates are needed. High power and short pulse (< 1/spl mu/s) applications require both fast switching speed and great current handling capability. The question arises whether any single material has all of these desired properties or whether there are intrinsic limitations. In order to investigate this, we have performed simulations of the electrical properties of FET-type switches fabricated from each of these materials. Both perfect material properties and the effects of defects have been included. The simulation results show that deep level defects degrade the device performance. Based on our simulations and on the available data, in the near term, 4H-SiC is the most attractive of the four materials for pulsed power applications.