Neutral Impurity Research Articles

Magnetic circular dichroism and site-selective optically detected magnetic resonance of the deep amphoteric vanadium impurity in 6H-SiC

Magnetic-circular-dichroism (MCD) absorption and optically detected electron-spin-resonance (ESR) data for the neutral vanadium impurity, ${\mathrm{V}}^{4+}$(3${\mathit{d}}^{1}$) on the \ensuremath{\alpha}, \ensuremath{\beta}, and \ensuremath{\gamma} sites in 6H-SiC are presented and supplemented by linearly polarized absorption and luminescence spectra. An analysis of the linearly polarized spectra yields crystal-field-level schemes in zero magnetic field and an intensity parameter u for ${\mathrm{V}}^{4+}$ on each of the three Si sites. This information is used to account for the ${\mathrm{V}}^{4+}$ MCD zero-phonon-line positions, signs, and relative intensities without an adjustable parameter. Evidence of Jahn-Teller effects is found in both the ${\mathrm{V}}^{4+}$ $^{2}$E ground and the $^{2}$${\mathit{T}}_{2}$ excited states. The MCD-ESR technique, in the present case, is site selective. The ${\mathrm{V}}^{4+}$ resonances associated with the \ensuremath{\beta} and the \ensuremath{\gamma} site are virtually identical and cannot be distinguished by conventional ESR. The g factors for all three sites are satisfactorily explained within the framework of crystal-field theory.

Capture of hot holes by shallow acceptors in p-type GaAs studied by picosecond infrared spectroscopy.

Picosecond recombination of free holes with shallow acceptors in p-doped GaAs is directly monitored via spectrally and temporally resolved infrared studies. Neutral Zn impurities are photoionized by picosecond excitation in the wavelength range around 5 \ensuremath{\mu}m. The recombination dynamics of the free carriers with ionized acceptors is measured via transient changes of the acceptor deionization band at photon energies close to the band gap. Hole capture is observed on a time scale of up to 100 ps following nonexponential kinetics. The data are analyzed with the help of model calculations considering single-step as well as multiple-step trapping mechanisms. Emission of single longitudinal-optical phonons is found to be the main mechanism of picosecond recombination.

Study of the effect of ion implantation on the electrical and microstructural properties of tin-doped indium oxide thin films

Ion implantation of H2+ or O+ ions in the range 0–1.7×1015 and 0–1.3×1015/cm2, respectively, was used to investigate the effect of implant-induced damage on the electrical properties of Sn-doped In2O3 (ITO) films deposited by electron-beam evaporation on SiO2-coated soda-lime glass substrates. The films were characterized as a function of implant dose using low-temperature Hall effect, resistivity, optical transmissivity, x-ray diffraction, and transmission electron microscopy (TEM). A systematic decrease in both carrier density (n) and Hall mobility (μ) was observed with increasing dose of either implant species. The electronic results were analyzed using charged and neutral impurity scattering models which suggest that the observed changes are due to the degradation of electrically active donor centers and the generation of the neutral scattering centers. The microstructure of the implanted films, as revealed by TEM and x-ray diffraction, is consistent with the presence of significant dynamic recovery during implantation.

Energy dependence of the carrier capture coefficient for neutral shallow impurities in semiconductors

The authors' consider the effect of carrier resonance scattering in slightly doped compensated semiconductors on the carrier capture by neutral shallow impurities, resulting in the formation of H--like centres. The energy dependence of the capture coefficient alpha ( epsilon ) exhibits a weak maximum at Ei, the carrier affinity to the H--like centre. The photoconductivity and photo Hall effect are studied for Si:B samples with a boron concentration NA<1016 cm-3 and a degree of compensation K<10-3 at liquid helium temperature in 'heating' electric fields. The obtained energy dependence of the hole capture coefficient for neutral boron atoms is in agreement with the authors' calculations.

Raman Light Scattering Due to Collisions of Wannier‐Mott Excitons with Impurities and between Them

AbstractThe theory of Raman light scattering due to Coulomb (exchange) interaction of an exciton in an intermediate state with neutral impurities in semiconductors is developed. The consideration is based on the third‐order perturbation theory. The possibility of such an approach is proved even for conditions of resonant Raman light scattering in direct‐gap semiconductors. The theoretical estimations for scattering cross‐section and scattering efficiency are well fitted with experimental data obtained by Ulbrich et al. on CdTe crystals. However, full explanation of these data must involve exciton scattering on bound excitons or biexcitons.

Study of the effect of Sn doping on the electronic transport properties of thin film indium oxide

High quality, low resistivity (2.6×10−4 Ω cm), 0.32-μm-thick amorphous and polycrystalline, pure and Sn-doped, In2O3 films prepared by high density plasma-assisted electron beam evaporation were used to investigate the effect of Sn doping on the electronic transport properties of this material. Amorphous films with high carrier density in the as-deposited state showed no effect of Sn doping on resistivity (ρ), Hall mobility (μ), or carrier density (n) over the range 0 to 5.3 wt % Sn. After recrystallization by annealing in air at 180 or 250 °C for 20 min, n, μ, and ρ were seen to be strongly dependent on Sn concentration in the range 0 to 1.5 wt % with a decreasing effect of Sn doping in the range 1.5 to 5.3 wt %. The data presented in this study were analyzed based on charged and neutral impurity scattering models and suggest that increasing Sn concentration leads to the formation of defect complexes which act as scattering centers but which do not contribute carriers to the material.

Crystallization kinetics of boron- and germanium-implanted 〈100〉 Si: a balance between doping and strain effects

Electrical and strain arguments were used to explain the differences in growth rate observed for silicon solid phase epitaxy in the presence of electrically active dopants and neutral impurities. In the case of dopants, strain effects seem to be absent or at least to play a negligible role, while for neutral impurities strain seems to be very important. The purpose of this work was to study the role of dopants in the presence of intentionally strained silicon films. The studies were performed by comparing the growth rates measured in samples implanted with boron and germanium, and the growth rates obtained in samples implanted with only germanium or only boron. With respect to intrinsic silicon, boron produces a rate enhancement, germanium a decrease and boron-germanium a slight increase. The results are interpreted by taking into account the strain and by assuming that strain and electrical effects are independent.

Spin-spin scattering in a silicon two-dimensional electron gas.

The scattering of conduction electrons by neutral impurities in Si has been demonstrated through measurements of spin-dependent transport (SDT) in a two-dimensional electron gas (2DEG). SDT was observed by monitoring the conductivity of a specially fabricated Si accumulation layer transistor operating at 4 K while modulating the electron-spin populations. By utilizing the differences in the singlet and triplet state scattering cross sections, the technique provides the first direct measure of neutral impurity scattering. The SDT spectrum consists of a pair of hyperfine peaks separated by 42 G which are assigned to the impurity electron system and a broad central feature which is attributed to both the conduction spins and donor spins with strong exchange interactions. A phenomenological theory for the resonant change in device current is developed and comparison of theory with experiment indicates that the interaction of a 2DEG with neutral impurities must be treated two dimensionally; scaling the known 3D scattering cross sections to their 2D counterparts is inappropriate. Numerical simulations of the SDT line shape are obtained by modeling the spin dynamics. The possibility of alternative explanations for the observed signal, such as bolometric detection of the electron-spin resonance, are explored and eliminated. The SDT spectrum is characterized as a function of temperature, microwave power, Fermi energy of the 2DEG, and magnetic-field orientation. A lower bound for 〈${\mathrm{\ensuremath{\Sigma}}}_{\mathit{s}}$-${\mathrm{\ensuremath{\Sigma}}}_{\mathit{t}}$〉, the difference in singlet and triplet scattering cross sections integrated over angle and position of the donor impurity, is obtained as a function of the Fermi energy. A spin-dependent signal from \ensuremath{\sim}${10}^{8}$ spins was observed, which demonstrates the enhanced sensitivity of SDT over conventional electron-spin-resonance methods.

Measurements and modeling of helium and carbon ion plumes (abstract)

Following charge exchange between fast neutral atoms and impurities, emission from the charge exchange product can take two forms. The first is prompt radiation due to rapid cascading of the excited electron following the charge exchange event. In addition, as a result of electron impact excitation from the ground state before ionization, the impurity can subsequently radiate at the chosen wavelength. Depending on the beam-sightline geometry, plasma density, and species observed, the emission of this ‘‘ion plume’’ can rival the prompt emission, making it a potentially important consideration for ion temperature and density measurements. Direct measurements of this ion plume radiation after the charge exchange event have been made for He+ ions (n=4–3, 4686 Å) and C5+ ions (n=8–7, 5292 Å). The ratios of plume radiation to prompt charge exchange radiation for these transitions is compared to predictions from modeling, and good agreement is found. This work was supported by U.S. Department of Energy Contract No. DE-AC02-76-CHO-3073.

Effects of divertor plate biasing on radial and poloidal edge fluxes in the TdeV

The divertor plates of TdeV, a tokamak with a double-null divertor and closed divertor chambers, have been electrically biased with respect to the walls. The authors discuss the resulting effects on the edge electron density profile, on the neutral pressures and impurity fluxes in the main vacuum chamber and the divertor chambers, and on the plasma flow to the divertors. As a function of the bias voltage, which was varied between-180 V and +160 V, the electron density scrape-off width and the wall impurity influxes increase monotonically; the flows to the top and bottom divertors vary strongly, in qualitative agreement with an E*B/B2 rotation, but not symmetrically. With negative biasing, the electrostatic barrier and the rotation combine to give a strong improvement of the divertor efficiency

Picosecond capture of photoexcited holes by shallow acceptors in p-type GaAs.

Ultrafast recombination of holes with shallow acceptors in a III-V semiconductor is directly observed for the first time, by means of picosecond infrared spectroscopy. Neutral impurities in p-doped GaAs at low temperature are photoionized by picosecond infrared excitation. The recombination of free holes with negatively charged acceptors---monitored via absorption changes below the band edge---occurs on a time scale of several tens of picoseconds, following a nonexponential kinetics. Emission of longitudinal-optical phonons by the free holes is found to be the dominant mechanism of recombination.

Thin pyrite (FeS 2) films prepared by magnetron sputtering

Thin pyrite films have been prepared by sulphur-assisted magnetron sputtering at deposition temperatures between 88 and 390 °C. The films were microcrystalline, with an average grain size of about 400 Å and a preferential orientation changing from (200) to (111) on going to higher deposition temperatures. The electrical conductivity was degenerate, with σ RT = 4 Ω −1 cm −1 , p = 5 × 10 18 cm −3 and μ HRT = 5 cm 2 V −1 s −1. A positive sign was observed in both Hall and Seebeck measurements, indicating holes to be the predominant current carrier. μ H varied approximately linearly with temperature, consistent with contributions from both ionized and neutral impurity scattering.

Acceptor-bound excitons in GaAs/AlxGa1−xAs symmetric coupled-double-quantum-well structures

The effect of an electric-field perturbation on excitons bound to Be acceptors in symmetric coupled-double-quantum-well (CDWQ) structures has been studied with photoluminescence (PL) and photoluminescence excitation spectroscopy. The Be acceptors were positioned in the central region of each well. With an electric field perpendicular to the well layers, interwell and intrawell free excitons are observed in the PL spectra of CDQW's. In weakly coupled DQW's, interwell and intrawell excitons bound to neutral acceptors can be observed simultaneously. Only one (interwell) bound exciton is observed in a strongly coupled system. Interwell and intrawell bound excitons are found to have the same bound-exciton (BE) binding energies. This indicates that it is mainly the hole of the exciton which is attracted by the neutral acceptor impurity. No variations of BE binding energies with the strength of the electric field were observed for the weakly coupled DQW's, while the binding energy increased with the electric field in a strongly coupled DQW. It was found that the BE binding energies at zero electric field in CDQW's are equal to those obtained for center-doped single-quantum-well structures with the same well widths.

Infrared absorption spectrum of neutral magnesium donors in silicon

The infrared absorption spectrum of neutral magnesium donor impurities in silicon has been investigated under the high resolution of a Fourier-transform infrared spectrometer. The absorption spectrum measured at liquid helium temperature are observed to be clearly better than those reported earlier in the literature. Several new lines corresponding to excitation from ground state to higher excited states have been observed and identified.

Quantum-well laser with integrated passive waveguide fabricated by neutral impurity disordering

Double quantum-well lasers have been fabricated with an integrated low-loss waveguide in which the quantum wells have been intermixed by neutral impurity disordering. Threshold currents of devices with integrated 600- mu m passive guides show an increase of less than 10% over normal Fabry-Perot lasers of the same active length, confirming that the propagation loss in the passive wavelength is significantly reduced by disordering to 4.5+or-2 cm/sup -1/. Comparison with unimplanted devices suggests that the implanted dopant also passivates the waveguide electrically.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Temperature dependence of the linewidth of shallow impurity spectral lines in lightly doped weakly compensated semiconductors

The broadening of spectral lines of intradonor absorption in n-GaAs is determined by random electric fields of charged donors and acceptors. The temperature dependence of the spectral line width reflects a redistribution of electrons over donors. At low temperatures the relative positions of charged and neutral impurities are strongly correlated by the Coulomb interactions in the system, whereas at high enough temperatures the distribution of electrons over impurities is random. This effect was recently suggested for quantitative characterization of semiconductors and a few theoretical attempts were made to describe the temperature dependence of the width of the spectral lines due to the transition from a correlated distribution of electrons to a random distribution in the low compensated material. We report here on the results of a Monte Carlo computer simulation of the transition. It is shown that the available analytical theories are not accurate enough and the reasons for such an inaccuracy are discussed.

Chaotic carrier transport induced by impact ionization in semiconductors

Chaotic carrier transport induced by impact ionization of neutral impurities (i.e. impurity avalanche breakdown at low temperature) in semiconductors (e.g. GaAs Ge, InSb) has been summarized. Viewed over the wide spectrum of the observations, a universality class of low-dimensional dissipative chaos is recognized in the related nonlinear carrier transport. Among various types of the current instabilities reviewed, driven chaos and crossover instability in n-GaAs have been used as examples to show typical low-dimensional chaos. High-dimensional complex behaviour of the current instability has also been discussed.

Electrical properties of indium selenide single crystals doped with tin

Resistivity, Hall effect and deep level transient spectroscopy measurements have been performed on Sn-doped InSe single crystals grown by the Bridgman-Stockbarger method. The electrical properties of the investigated samples are dominated by a donor level attributed to the dopant at about 50 meV from the conduction band. The temperature dependence of electron Hall mobility is explained by combining the optical phonon, the ionized and the neutral impurity scatterings. According to Schimd's model an electron-phonon coupling constant g2 = 0.065 is determined. Finally, an electron trap has been detected at about 0.62 eV below the conduction band with a capture cross section of about 5 × 10−16cm2.

Photoassisted MBE growth of doped CdTe and CdMnTe thin films and their characterization

We report experimental results obtained on CdTe and CdMnTe thin films grown by photoassisted molecular beam epitaxy (PAMBE) which give insights in the mechanisms underlying the PAMBE growth process. The magnitude of compensation in doped CdTe thin films is derived by transport and photoluminescence measurements. As a result the In-doped CdTe films grown by conventional MBE are highly compensated, whereas comparable PAMBE-grown In-doped CdTe films are less than 20% compensated. A theoretical analysis of the temperature dependance of free carrier mobilities shows that the scattering from neutral impurities gets less important for samples grown with laser illumination. It appears that laser illumination improves the quality of CdMnTe films and increases the Mn content. This shows that illumination increases the Cd/Te ratio at the growth surface. For CdMnTe films grown with laser illumination we find a growth rate which is up to 20% higher compared to conventionally grown films. This indicates that laser illumination increases the sticking coefficient and/or the surface diffusion on the growth surface.

Spin dependent transport in a two-dimensional electron gas

Scattering by neutral impurities in Si has been demonstrated through measurements of spin dependent transport (SDT) in a two-dimensional electron gas (2DEG). SDT was observed by monitoring the conductivity of a specially fabricated Si accumulation layer transistor operating at 4K while modulating the electron spin populations. By utilizing the differences in the singlet and triplet scattering cross sections, the technique provides the first direct measure of neutral impurity scattering. A signal from ∼10 8 spins was observed, which demonstrates the enhanced sensitivity of SDT over conventional electron spin resonance methods.