Neutral Impurity Research Articles

Dissipation due to a "valley wave" channel in the quantum Hall effect of a multivalley semiconductor.

When the quantized Hall effect occurs at a semiconductor surface such as Si(110), where the carriers have a time-reversal valley degeneracy, there should be a spontaneous valley polarization at appropriate values of the filling factor \ensuremath{\nu}. There can then be dissipation at T=0 due to radiation of Goldstone bosons (``valley waves'') at impurity sites, provided that the current density exceeds a critical value ${\mathrm{j}}_{\mathrm{c}}$ determined by the intervalley electron-electron scattering or other terms which modify the valley-wave dispersion at long wavelengths. The dissipation above ${\mathrm{j}}_{\mathrm{c}}$ is described by a constant resistivity ${\mathrm{\ensuremath{\rho}}}_{\mathrm{xx}}$, which should be small but measureable, and sensitive to the density of neutral impurities.

Central-cell and screening effects on the binding energies of neutral chalcogen impurities in silicon.

Central-cell and screening effects on the binding energies of neutral chalcogen impurities in silicon.

Green's-function-quantum-defect treatment of impurity photoionization in semiconductors.

An analytic expression is derived for the photoionization cross section of impurity atoms in semiconductors. The use of a quantum-defect wave function for the impurity ground state permits impurity atoms to be conveniently characterized in terms of ground-state energies. The Coulomb Green's-function and time-dependent perturbation theory are used to calculate the wave function of photoexcited electrons or holes from which the photoionization cross section is obtained. The results apply to shallow or deep levels associated with carriers bound to charged impurity centers. Results for photodetachment of carriers bound to neutral impurities are obtained in the limit where the impurity charge goes to zero. Theoretical cross-section maxima are found to be in remarkably good agreement with experimental results, considering that no free parameters or empirical effective field ratios are employed.

Determination of neutral manufacturing impurities in heroin by capillary gas chromatography with electron capture detection after reduction with lithium aluminum hydride and derivatization with heptafluorobutyric anhydride.

Determination of neutral manufacturing impurities in heroin by capillary gas chromatography with electron capture detection after reduction with lithium aluminum hydride and derivatization with heptafluorobutyric anhydride.

Neutral impurity scattering in AlGaAs

In the alloy system AlGaAs, a crossover from a direct (Γ) to an indirect (X) conduction-band minimum occurs when the AlAs mole fraction approaches 0.45. For AlAs mole fractions greater than about 0.2, the ground-state energy of substitutional donors becomes deep relative to the direct minimum. The donor level achieves a maximum depth of 0.16 eV near the crossover and it is not uncommon to find the donor impurity concentration exceeding the free-electron concentration by a factor of 10 at room temperature and a factor of 20 at lower temperatures. For large impurity to electron ratios and doping levels on the order of 1018/cm3, neutral impurity scattering proves to be the dominant scattering mechanism at all temperatures.

Transport coefficients for neutral impurity scattering

Transport coefficients for neutral impurity scattering

Interstitial hydrogen and neutralization of shallow-donor impurities in single-crystal silicon.

It is demonstrated for the first time that hydrogen can passivate shallow-donor impurities in n-type single-crystal silicon, and a novel chemical-bonding model is proposed to explain the phenomenon. Phosphorus greatly slows the bulk diffusion of hydrogen at \ensuremath{\sim}150 \ifmmode^\circ\else\textdegree\fi{}C. Conductivity and Hall measurements show that the room-temperature electron density is decreased by hydrogenation, but the mobility is increased, so that compensation is ruled out. Total-energy calculations predict an energy minimum for H at the antibonding site of a silicon nearest neighbor of substitutional P. Known solubilities imply \ensuremath{\sim}1-eV binding for atomic H in pure silicon.

A tunneling theory of exciton photoluminescence for neutral acceptors in silicon: A study of the systems Si: (B, In), Si: (Al, In), Si: (Ga, In), Si: (B, Al), Si: (B, Ga), and Si: (Al, Ga)

Rate equations for the densities of free excitons and excitons bound to two different neutral acceptors in silicon are solved for steady state in the absence of saturation. These rate equations explicitly include terms for forward and reverse tunneling of bound excitons from one type of neutral impurity to another. Both tunneling rates are calculated using a simple model of an exciton in a one-dimensional semi-infinite double potential well. The steady-state solutions of the rate equations yield an expression for the ratio of the bound exciton luminescence intensity as a function of the impurity concentrations. The relative photoluminescence intensities for the systems Si: (B, In), Si: (Al, In), Si: (Ga, In), Si: (B, Al), Si: (B, Ga), and Si: (Al, Ga) are calculated for the relevant free-exciton capture cross-section ratios. This model predicts no exciton tunneling for any of the above systems for the low-impurity concentration range of 1012–1013 cm−3. For those systems with large differences in the bound exciton energy levels such as Si: (B, In), Si: (Al, In), and Si: (Ga, In), and having indium concentrations exceeding 1015 cm−3, it predicts quenching of shallow impurity bound exciton luminescence because the forward exciton tunneling rate from the shallow level to the deep level of indium dominates and the reverse exciton tunneling rate from indium to the shallow impurities is negligible. For the systems with small differences in the bound exciton energy levels such as Si: (B, Al) and Si: (B, Ga), the theory predicts enhancement of shallow impurity bound exciton luminescence beyond certain concentrations depending upon the free-exciton capture cross-section ratios because in these cases the reverse exciton tunneling rate dominates. For the system Si: (Al, Ga) in which the difference in the bound exciton energy levels is very small, gallium bound exciton luminescence dominates when the gallium concentration exceeds 1016 cm−3.

Dielectric properties of a disordered Bose condensate.

The frequency-dependent and wave-number-dependent dielectric properties of a condensate of a boson gas with static disorder are evaluated. For weak disorder the condensate is superfluid and the disorder induces a mass enchancement. For strong disorder the condensate is insulating and the static dielectric function is finite. Numerical solutions for the frequency dependence of the density relaxation function and the dynamical conductivity are given. The dielectric properties of a charged Bose gas with charged impurities and a repulsive Bose gas with neutral impurities are evaluated explicitly.

The equilibrium and time evolution of the parameters of the Tokamak de Varennes

The equilibrium and time evolution of the density, current, and temperature profiles for a tokamak are numerically analyzed using the one-dimensional transport code VARTOK1. The numerical model contains the full neoclassical transport, with coefficients covering transitions between regimes, and includes the Ware pinch and bootstrap currents, empirical anomalous particle and electron heat transports based on Alcator scaling, neutral particle transport, oxygen impurity, and diffusion of the poloidal magnetic field. The solution uses a time- and space-centered Crank–Nicolson scheme. The equilibrium profiles are first determined for a constant current of 200 kA by running the code at constant current. Then the current is ramped down to zero in 20 ms, and the time evolution of the different parameters are followed.

Electron mobility in nInSb under pressure

The low electric field Hall electron mobility has been measured in undoped n-type InSb as a function of temperature and pressure. As long as the semiconductor-insulator transition region is avoided ( P > 7.5 kbar and T < 100 K) the mobility below 50 K appears to be dominated by neutral impurity scattering instead of the expected ionized impurity scattering. The mobility and its pressure derivative agree with the predictions of Ehrenreich for temperatures above 77 K.

Electric field dependence of hydrogen neutralization of shallow-acceptor impurities in single-crystal silicon

An electric field applied to p-type single-crystal silicon during hydrogenation dramatically affects the neutralization of shallow-acceptor impurities (e.g., boron). This is demonstrated with capacitance-voltage and secondary-ion mass spectrometry measurements on n+-p junction diodes that were reverse biased during exposure to monatomic deuterium at elevated temperatures. The results reveal that there is no neutralization within the field-induced space-charge layer, which extends from the metallurgical p-n junction into the p-type material. Rather, deuterium-boron pairing occurs in the electrically neutral material beginning at the edge of the depletion layer. Consequently, the distribution of deuterium (hydrogen) is also altered by the applied field. The effect of an electric field on hydrogen-boron pairing in p-type silicon is discussed in terms of the role of free holes in the chemical reaction and possible charge states of the diffusing hydrogen atoms.

Optical detection of cyclotron resonance in GaP and ZnTe

Optical detection of magnetic resonance has been used to observe cyclotron resonance of electrons and holes in GaP and ZnTe. The effective mass values determined from these measurements agree with those already found by more conventional techniques. The effects of cyclotron carrier heating on exciton formation and recombination are analyzed and it is seen that trapping and recombination at neutral impurity atoms is most strongly affected by heating of the carrier type that is trapped by the neutral impurity. In particular the energy dependence of the resonant trapping cross section of electrons at isoelectronic N atoms in GaP is found to be in good agreement with theoretical predictions.

Elastic scattering of exciton polaritons by neutral impurities.

A model based on the elastic scattering of exciton polaritons from residual neutral impurities is proposed to account for the line shape of the exciton-polariton photoluminescence emission in GaAs at low temperatures. The good agreement obtained with experimental spectra emphasizes the importance of impurity scattering on exciton-polariton transport for modes in the vicinity of the exciton energy. This model is applicable to any solid possessing dipole-active excitons.

Influence of elastic scattering from neutral impurities on the exciton-polariton photoluminescence lineshape in GaAs

The low temperature photoluminescence spectral lineshape of the free exciton in undoped bulk GaAs, a broad weak doublet whose shape is commonly assumed a consequence of intrinsic processes, is experimentally demonstrated to be of extrinsic origin. In particular, the doublet shape is associated with the presence of residual donor impurities. A model, based on elastic scattering from neutral impurities and incorporating the polariton nature of the exciton, was developed. The excellent agreement between the model calculations and the experimental spectra demonstrates the importance of scattering processes on exciton-polariton transport. This model is applicable to any material with a dipole active exciton and may account for the structure observed in the lineshapes of free excitons in other compound semiconductors which have previously been attributed to other mechanisms.

Generation of new bound exciton lines by lithium implantation of CdS

Sharp new bound exciton optical absorption lines are observed following very low fluence (φ&amp;lt;5×1012 cm−2 room-temperature 7Li implantation of high quality cadmium sulfide platelets. A substantial broadening of the free exciton resonance by the lithium neutral donor impurities is observed. A model of the line shape is proposed.

Cadmium Concentration Dependence of Mobility in Cadmium‐Doped Lead Telluride

AbstractLead telluride doped with cadmium is grown in a two temperature furnace. The cadmium content is controlled through cadmium vapor pressure. Electronic concentration and Hall mobility are measured for cadmium concentrations up to 1.65 × 1021 cm−3 at 77 and 300 K. Mobility variations are fairly explained by a model taking into account dispersion on acoustic and optical phonons, and neutral impurities.

Neutral beam injector oxygen impurity measurements and concentration reduction via gettering processes

We have measured the reduction of oxygen impurity levels by means of gettering within the arc chambers of the TMX-U neutral-beam injectors using the TMX-U neutral-beam test stand. Our analysis incorporated silicon surface-probe measurements and optical Doppler-shift measurements of the hydrogen alpha spectra of deuterium atoms with energies appropriate for D2O parentage. Without gettering, the Auger electron spectroscopy analysis of an exposed silicon sample showed a large oxygen peak below the surface peak with a concentration equivalence of approximately 2% for an accelerated beam. After gettering, with either titanium or chromium getters, optical monochromator data indicated a reduction in the oxygen concentration of at least a factor of 10, whereas Auger spectroscopy data showed at least a factor of 8 reduction. Other metallic impurities remained below the level of detection even after gettering. Additional effects observed during this study include a change in the accelerated deuterium species concentrations, loss of gettering activity, loss of arc operation, and a change in arc performance due to arc chamber gas absorption during operation.

Edge diagnostics for tandem mirror machines (invited)

The edge plasma in a tandem mirror machine shields the plasma core from cold neutral gas and impurities. A variety of diagnostics are used to measure the fueling, shielding, and confinement of the edge plasma in both the end plug and central-cell regions. Fast ion gauges and residual gas analyzers measure the gas pressure and composition outside of the plasma. An array of Langmuir probes is used to measure the electron density and temperature. Extreme ultraviolet (EUV) and visible spectroscopy are used to measure both the impurity and deuterium densities and to estimate the shielding factor for the core plasma. The linear geometry of a tandem mirror also allows direct measurements of the edge plasma by sampling the ions and electrons lost out of the ends of the machine. Representative data obtained by these diagnostics during operation of the Tandem Mirror Experiment (TMX) and Tandem Mirror Experiment-Upgrade (TMX-U) experiments are presented. Diagnostics that are currently being developed to diagnose the edge plasma are also discussed.

Theory of charge-state splittings of deep levels.

The Green's-function method, with an empirical tight-binding basis, is used to determine the deep levels of the singly ionized and neutral impurities S, Se, and Te in Si. The impurity potentials are determined self-consistently. The resulting theory accounts for the observed charge-state splittings of neutral and singly ionized ${A}_{1}$ deep levels, obtaining, for S, Se, and Te, 0.23, 0.22, and 0.19 eV (to be compared with experimental values of 0.30, 0.29, and 0.21 eV, and with a self-consistent local-density-theory value for S of 0.20).