DX Centers In GaAs Research Articles

X-ray excited spectroscopy of defects and impurities in compound semiconductors

Analysis of local structures around impurities and defects in compound semiconductors and insulator by X-ray excited spectroscopy, namely, XAFS with fluorescence X-ray detection technique (fluorescence-XAFS) and with photoluminescence detection technique (PL-XAFS), is presented. Those two techniques that use the surface incidence and surface detection are essentially important for samples with a thin epitaxial layer, where the impurities and defects exist, grown on a thick substrate. Er in InP, Er and O in GaAs, Tb in SiO 2 and DX-center in GaAs are taken as examples and local structures around these impurities are elucidated.

Metastable population of self-organized InAs/GaAs quantum dots

In the present work, we report on the investigation of a p-n heterostructure with InAs/GaAs quantum dots (QD) by capacitance-voltage and deep level transient spectroscopy. We have observed controllable and reversible metastable population of the energy states of quantum dots and interface in the structure containing one plane of InAs QDs as a function of temperature of isochronous annealing as well as under bias-on-bias-off cooling conditions and white light illumination. This effect was attributed to the change in the Fermi level position due to the hole capture on self-trapped defects similar to the DX center in GaAs after isochronous annealing and white light illumination.

Persistent photoconductivity in Si delta-doped GaAs at low doping concentration

In addition to the persistent photoconductivity (PPC) attributed to DX centers in GaAs delta-doped with Si, a weak PPC (WPPC) with a PPC carrier density independent of Si-doping concentration has been generally reported at ambient atmosphere, but the nature of the deep states responsible has not been elucidated. Here, we present the results of a detailed study of the WPPC in δ-GaAs:Si at low-doping densities, NSi≈1–3×1012 cm−2, and ambient pressure. It is concluded that the WPPC does not arise from DX centers but from another deep defect, which is DX-like in the sense that it can be metastably excited. The presence of two distinct DX-like states is apparent from two separate annealing temperatures of the PPC, Ta≈50 K and Tb≈230 K; to the best of our knowledge, the latter is the highest annealing temperature observed in the AlGaAs:Si system.

Far Infrared Spectroscopy of Semiconductors at Large Hydrostatic Pressures

AbstractThe combination of high resolution far infrared spectroscopy and large hydrostatic pressures produced by diamond anvil cells (DAC) offers a unique approach to the study of dopants, defects, and bandstructure related properties of semiconductors. The problem of small optical throughputs typical of DACs has been solved. Using this unique tool, we have obtained several new results, including the discovery of a local vibrational mode of the DX center in GaAs:Si and the demonstration that the DX center is a negative‐U defect. We have also discovered the first evidence of DX centers in InP and studied the ground to bound excited state transitions of shallow impurities in GaAs at pressures at which GaAs is an indirect bandgap semiconductor with its conduction band minimum near the X symmetry point.

Localized vibrational modes of DX centres in Si-doped GaAs under hydrostatic pressure-the Green function approach

Localized vibrational modes (LVMs) of Si DX centres in GaAs are worked out for different pressures using the Green function technique, introducing lattice relaxation. This yields a very good agreement with experimental results for LVMs of DX centres.

Enhancement of persistent photoconductivity in insulating high-Tc thin films.

Persistent photoconductivity, characterized by a long-term increase in the conductivity due to illumination of oxygen-deficient high-temperature superconductors, is greatly enhanced in R${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$ (R=Y or Gd) films as x decreases to 6.1. These results are in qualitative agreement with a model similar to that used to explain the existence of DX centers in GaAs. However, they cast doubts over several models commonly used to explain the mechanism behind this phenomenon and that of photoinduced superconductivity.

The peculiarities of the tunneling conductance and phonon induced features in tetrahedrically bonded semiconductors under pressure

Abstract Summary The 30 kbar high pressure unit for low temperatures and the digital processing have been applied to the tunneling investigations of Ge and GaAs. The obtained results include the values of the mode Gruneisen constants for zone boundary phonons in direction in Ge, the irregular optical phonons behavior in GaAs, the bistability of tunneling voltage-current curve for GaAs at elevated pressure and the observation of optically induced oscillations on the tunneling conductance of GaAs Schottky barrier. The semi-quantitative model involving the conductance band structure change under pressure and DX-centers in GaAs(Te) is used for the tentative explanation.

Interpretation of x-ray rocking-curve broadening caused by lattice relaxation around metastable point defects.

The x-ray rocking-curve broadening accompanying the transfer to and from the metastable state of EL2 and DX centers in GaAs and ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As has recently been observed experimentally [Leszczynski et al., Semicond. Sci. Technol. 6, B66 (1991)]. This paper gives a more quantitative analysis of the experimental results. Computer simulations of rocking curves based on the dynamical theory of x-ray diffraction for various models of the real crystal structure made it possible to evaluate the conditions in which the lattice relaxation could be observed in experiment. The general conclusion is that in all the materials examined, the inhomogeneities played a decisive role. The possible range of the inhomogeneities and the strains around EL2 and DX centers is discussed in relation to their microscopic models.

Chemical trends and s-p hybridization in the DX center in GaAs.

The Sn- and S-related DX centers in GaAs are studied from first-principles calculations based on supercell models within the local-density approximation. The calculated emission barrier for the Sn-related DX center with the broken-bond configuration is lower by 0.18 eV than that in Si, in good agreement with the observed value (0.16 eV) from the deep-level transient spectroscopy. This considerably low emission barrier in the Sn-related DX center is attributed to weak s-p hybridization of the Sn dangling bond in the broken-bond configuration. In the case of S, the first-neighbor Ga distorted broken-bond configuration (${\mathrm{Ga}}_{\mathrm{I}}$${\mathrm{V}}_{\mathrm{Ga}}$${\mathrm{S}}_{\mathrm{As}}$${)}^{\mathrm{\ensuremath{-}}1}$ has lower energy than the S-distorted broken-bond configuration (${\mathrm{S}}_{\mathrm{I}}$${\mathrm{V}}_{\mathrm{As}}$${)}^{\mathrm{\ensuremath{-}}1}$, and is confirmed to have the lowest energy among the negatively charged geometries. The broken-bond geometry (${\mathrm{Ga}}_{\mathrm{I}}$${\mathrm{V}}_{\mathrm{Ga}}$${\mathrm{S}}_{\mathrm{As}}$${)}^{\mathrm{\ensuremath{-}}1}$ is found to have a surprisingly (0.11 \AA{}) shorter Ga-S bond length than the shallow center. This prominent feature of GaAs:S provides important insight for the explanation of results of an extended x-ray-absorption fine-structure measurement. Finally, we reveal that the chemical trend in the stability of the broken-bond geometries of the DX center is closely related to the degree of the s-p hybridization inherent to the distorted atom.

Observation of the DX center in Pb-doped GaAs

Pb in GaAs introduces a DX center-like (metastable) defect level that is (229±16) meV above the Γ-conduction-band edge, in a similar energetic position to the Si and Sn related DX centers in GaAs. The persistent photoconductivity effect quenches at approximately 50 K, indicating that the barrier to capture for the Pb and Sn dopants are similar. Despite the quite different atomic parameters of the Pb atom compared with the Sn atom, no significant chemical shifts have been found.

Energetics and local vibrations of the DX center in GaAs.

The negatively charged broken-bond geometry is a strong candidate for the microscopic origin of the DX center in GaAs but the validity of the broken-bond model is still controversial. We thus examine the model for the Si-related DX center, performing large-scale supercell calculations within the local-density approximation. The highly efficient conjugate-gradient minimization technique is combined with the norm-conserving-pseudopotential method in order to include both short-range and medium-range lattice relaxations. First, we compare the total energy of the broken-bond geometry with that of the negatively charged on-site geometry, which is also a candidate for the DX center. It is found that the broken-bond geometry has a lower total energy, supporting the broken-bond model for the DX center. Second, the barrier height for the broken-bond geometry is found to be comparable to observed emission barriers for the DX center. Third, the broken-bond model is shown to be consistent with results of a Fourier-transform infrared-absorption measurement for the local vibrational frequency of Si in pressurized GaAs, although the neutral on-site model also reproduces the experimental results within the calculational accuracy. Most of the results in this work indicate that the broken-bond model is consistent with experiments. Finally, we find that the broken-bond model with the neutral charge state is unstable and thus expect that the Si atom moves to the substitutional site after the DX center is photoexcited.

Probing the DX Center in GaAs and Related Alloys by Capacitance Transient Measurements under Stress

This paper will review some recent results in the study of the DX center in GaAs and AlGaAs using capacitance transient techniques under hydrostatic and uniaxial stress. These measurements have established the DX center as the deep state of substitutional donors in GaAs or AlGaAs which is lowered into the band gap by alloying or pressure.When compared with the predictions of various models of the DX center, the stress results are shown to be consistent with the DX centers having a two electrons negatively charged ground state (negative U).Models of the DX center as effective mass state of donor associated with the L conduction band valleys are found to be incon-sistent with stress experiments.

Spatial Correlation of Occupied DX Centres in GaAs and its Destruction by Thermal and Optical Excitation

We have studied the population and depopulation processes of Si-DX levels and the mobility enhancement due to the spatial correlation of charged centres in heavily doped GaAs. The destruction of the correlated occupation of DX centres has been investigated using two methods; (i) optical destruction by sub-bandgap photons, (ii) thermal destruction using time dependent relaxation processes. Depending on the degree of correlation the mobility at a particular temperature and at atmospheric pressure can vary over a range of values for a fixed value of carrier concentration.

M�ssbauer spectroscopy investigation of the persistent photoionization of the DX-center in129mTe-implanted GaAs

Persistent photoionization of the DX-center in GaAs implanted with129mTe isotopes is observed with Mossbauer spectroscopy. The time constant of the relaxation from the donor site to the DX-center defect site, after the illumination is stopped, is measured as a function of temperature. A small energy barrier for capture (1 meV) is derived from these measurements.

Atomic model for hydrogen passivation of DX centers in GaAs and Al xGa 1−xAs alloys

We present the atomic model for the hydrogen passivation of DX centers in GaAs and Al xGa 1−xAs alloys doped with Si and S donors using an ab initio pseudopotential method. In GaAs, the DX centers with the negative- U property are found to be stabilized at pressures above 22 kbar. Both the shallow donors and deep DX centers are neutralized by hydrogen via the formation of donor-hydrogen complexes which are neutral in a charge state. In Al xGa 1−xAs alloys, we study the effects of the Al atoms on the interactions between H and its neighboring atoms. The calculated dissociation energies for the donor-hydrogen complexes agree with measured values.

Monte Carlo simulations of spatial correlation effects of charged centres in delta -doping layers

This paper presents results of Monte Carlo simulations of a system of charged donors in delta -doping layers, aiming at elucidating the effects related to a spatial correlation of donor charges in a 2D configuration. The simulations yielded several quantities describing the system, among them the pair correlation functions, gain in the energy of the system due to correlation of charge locations and density of impurity states, all as functions of temperature. The calculations have direct application in analysis of the behaviour of the DX centres in GaAs planarly doped with Si.

Vibrational Mode Fourier Transform Spectroscopy with a Diamond Anvil Cell: Modes of the Si DX Center and S Related Centers in GaAs

The authors report the first use of the diamond anvil cell to observe the local modes of impurities in GaAs under large hydrostatic pressure by Fourier Transform Spectroscopy. These observations were accomplished by constructing a monolithic assembly which has a light concentrating cone in front of the diamond anvil cell and a Ge:Be photoconductor detector designed to operate at low photon fluxes mounted directly behind the cell. This technique has been used to discover a local vibrational mode of the DX center in GaAs:Si and, in combination with Hall effect and resistivity analysis, to infer that the charge state of this defect is negative. This new spectroscopic tool has also been used to identify new LVMs in heavily doped GaAs:S which are due to sulfur related centers.

A Direct Observation of the DX Center in GaAs Using Optoelectronic Modulation Spectroscopy Above the Bandedge

Optoelectronic modulation spectroscopy has been used to examine sulfur‐doped VPE and tin‐doped MBE using photon energies in excess of the fundamental energy gap. A peak in the modulated electrical admittance at 1.56 eV is attributed to the DX level and locates the level 1.56 eV above the valence band maximum, resonant with the conduction band. Transitions to it from the valence band seem to be indirect, and the level appears to have an optical cross section which increases with photon energy. This is the first direct measurement of its energy. The method avoids the necessity for high doping and the use of hydrostatic pressure to move the level into the energy gap.

High Pressure Studies of Electronic States with Small Lattice Relaxation of DX-Centres in GaAs

High Pressure Studies of Electronic States with Small Lattice Relaxation of DX-Centres in GaAs

The effect of high hydrostatic pressure on DX centres in GaAs and GaAsP

The authors report results concerning the identification of DX centres obtained from high-pressure experiments. The sulphur-related DX centre in GaAsP alloys is characterized by two levels 40 meV apart which follow the X conduction band minimum. Properties of the DX centres in GaAs and GaAsP are compared.