Search Results (11 total)

A study has been made of the effects of uniaxial stress on the photocapacitance quenching phenomenon associated with the EL2 center in GaAs. The results show directly that the defect is a complex with C_3v (trigonal) site symmetry. The internal optical excitation assigned to the EL2 center is shown not to be directly related to the photoquenching transition. The implications of these results with respect to models of EL2 and its photoquenching mechanism are discussed.

We report a study of the effects of electron-hole recombination on the stability of the EL2 center in GaAs. In addition to possible recombination-enhanced diffusion, we have examined the effects of recombination on the low-temperature metastable optical properties of the EL2 center. No defect migration or annealing was observed, and no changes in metastable properties were evident for diode injection currents up to 60 Acm^-2 at temperatures up to 400°C. The implications of these results with respect to the identity of the EL2 defect are discussed.

We explore the possibility that surface charge induced by the scanning tunneling microscope will influence the structure of the surface under investigation. In general, we find that the emission currents limit the induced charge densities and preclude major structural modifications on the more stable surfaces. However, the possibility of modifying less stable structures or of reducing the transition temperatures for transformation between different surface phases does exist and is discussed in detail.

We report the observation of a defect in Fe-doped InP which can exist in either of two configurations, for the same charge state. Each configuration exhibits distinct electronic and optical properties. Thermally stimulated capacitance, capacitance transient spectroscopy, and photocapacitance were used to study the properties of the defect in each configuration, and the kinetics of the reversible transformations between configurations. The unique properties of the defect are discussed.

A fascinating electron-irradiation-induced defect (the M center) in n-type InP has been reported recently. Two entirely different capacitance transient spectra can be obtained for this center in the same sample (a p-n junction diode) depending upon its bias condition during the cool down to the initial measurement temperature (~ 30 K). We present a conceptually simple configuration-coordinate model that explains the unusual properties of this center. In our model the metastable center can exist in either of two configurations, one which displays a very large lattice relaxation, and another ordinary (no anomalously large lattice relaxation) configuration. Photoionization rates have been measured for the defect levels of the two different defect configurations as a function of photon energy. These data are used to confirm the qualitative properties of our model and to determine its parameters. Classification of the M-center behavior within the general context of selftrapped configurations is proposed.

Angle-resolved high-resolution electron-energy-loss spectroscopy has been used to study electronic excitations on both the clean and contaminated Si(100)(2×1) surfaces. Absorption edges on the clean surface were observed near 0.4 and 1.1 eV corresponding to transitions from the bulk valence band to the surface conduction band and to the bulk conduction band, respectively. The bulk-to-surface absorption edge was found to move both as a function of primary energy and angle of incidence. The resulting dispersion relationship can be explained without invoking phonon-mediated transitions. This implies that the minimum in the unoccupied surface state is at the Γ point, that the true unit cell is larger than the (2×1), and that the reconstruction of the surface is more extensive than a simple, asymmetric dimerization of the surface atoms. One of the more interesting results of this study is the absence of observable direct transitions from the occupied to the unoccupied surface state. Transitions from the occupied surface state to the bulk conduction band are, however, observed.

The M center is an unusual metastable defect which can exist in either of two configurations, each with distinct electronic properties. These properties, together with the thermally stimulated configurational transformation kinetics, lead to a model of charge-state—controlled structural relaxation involving a shallow-donor—intrinsic-defect complex. We present studies of the electronically and optically stimulated configurational transformations. These include unique pulsed optical experiments, which are made possible by the particular properties of the defect, and which provide information on the rate of lattice relaxation associated with the transformation. The results lead to a more complete understanding of the nature of the configurational instability for the M center than for any other covalent semiconductor defect exhibiting large lattice relaxation.

Existing approaches are unsuccessful in explaining the anomalous persistent photocapacitance quenching effect associated with the EL 2 center in GaAs. Here a model is presented which invokes a simple physical mechanism to account in a consistent way for all aspects of this behavior. The model consists of a charge-state-controlled, electrostatic and lattice-strain driven, structural rearrangement of a defect complex. This rearrangement results in two defect configurations, each with distinct electronic and optical properties. It is proposed that this type of configurational instability may be common, but rarely detected, in covalent semiconductors.

We report the observation of a reversible charge-state—dependent defect reaction in 1- MeV-electron—irradiated n-type InP. The reaction, observed by capacitance transient spectroscopy and thermally stimulated capacitance, is characterized by metastable defect configurations at T≲160 K. The electronic properties of the two configurations, together with the reaction kinetics, provide evidence regarding the nature of the defects involved. The phenomenon can be described by a model involving the interaction of a shallow donor with an intrinsic defect or defect complex. The defect charge state controls an electrostatically driven pairing-dissociation reaction which results in changes in the observed defect electronic properties.

Topological particle theory is applied to the computation of elementary-hadron coupling constants. All are determined through topological supersymmetry by a single dimensionless "zero-entropy" constnat g₀. The predicted coupling-constant ratios encompass and justify those of Mandelstam. The universality conjecture, g₀=e, is supported (to 6%) by the accurately measured value of the pion-nucleon coupling constant. An explanation emerges for the experimental failure to find baryonium.

Pure, perfect, single-crystal ZrV₂ does not undergo a structural phase transformation but all other sample modifications, including twinned single crystal, do transform (∼100 K) from the cubic to a rhombohedral phase. All samples exhibit, in addition, an electronic instability (also ∼100 K). An anomaly in the ultrasonic velocity occurs at higher temperatures (∼150-180 K) for pure polycrystalline ZrV₂ samples.