Impurities In GaAs Research Articles

Multiband tight-binding model of local magnetism in Ga1-xMnxAs.

We present the spin and orbitally resolved local density of states (LDOS) for a single Mn impurity and for two nearby Mn impurities in GaAs. The GaAs host is described by a sp(3) tight-binding Hamiltonian, and the Mn impurity is described by a local p-d hybridization and on-site potential. Local spin-polarized resonances within the valence bands significantly enhance the LDOS near the band edge. For two nearby parallel Mn moments the acceptor states hybridize and split in energy. Thus scanning tunneling spectroscopy can directly measure the Mn-Mn interaction as a function of distance.

A universal model for trends in A1-type defect states in zincblende and diamond semiconductor structures

We present a simple and accurate 4-level model derived from sp3s∗ tight-binding calculations on a 64-atom unit cell that quantitatively describes A1-type defect states and their coupling to the lowest Γ1c conduction band state in zincblende and diamond semiconductors. We use a realistic Hamiltonian that quantifies the chemical differences between the impurity and the host atom which it replaces, and that also incorporates local strain effects due to any size discrepancy between the host and impurity atom. The defect state is constructed from a basis that involves only three states: namely an A1-symmetric combination of the host L, X and Σ states, with the matrix elements between the basis states dependent only on the free-atom orbital self-energies and on the material lattice constants. We apply the model to analyse general group III and group V impurities in GaAs, and show that N is the only impurity that generates a strong perturbation. We identify the important role played by each of the three main contributions to the defect Hamiltonian in determining the N resonant state energy and the magnitude of its interaction with the conduction band Γ minimum as well as identifying why the interplay of these different effects leads to boron only weakly influencing the energy gap of GaAs. Finally, we derive simple expressions for the resonant state energy EN and its coupling βN to the Γ minimum, obtaining results in very good agreement with full sp3s∗ tight-binding calculations.

Diffusion of nitrogen in gallium arsenide

Diffusion of N in GaAs was studied in GaAs films grown by molecular beam epitaxy containing a buried N doping layer with a maximum concentration of about 1019cm−3. The as-grown films were annealed at temperatures between 724°C and 922°C in an As-rich ambient and subsequently analysed by secondary ion mass spectrometry. We observed a remarkably fast, non-Gaussian broadening of the as-grown N distribution. Evaluation of the profile shape provides strong evidence for a kick-out diffusion mechanism on the As sublattice involving not only interstitial and substitutional N but also interstitial As as native point defect. Fitting within this framework yields the diffusion coefficient of N in GaAs as a function of temperature. It obeys an Arrhenius equation with an activation energy of 2.27eV and a pre-exponential factor of 6.5×10−3cm2s−1. The present N data are compared with reported diffusivities of the host atom As and some selected impurities in GaAs.

Origin of the nitrogen-induced optical transitions inGaAs1−xNx

The temperature and concentration dependence of optical transitions in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ $(<3.8\mathrm{eV})$ are studied by electromodulated reflectance. These studies suggest that the ${E}_{+}$ transition involves the valence-band maximum at $\ensuremath{\Gamma}$ and a singlet state originating from the splitting of the quadruply degenerate conduction band at L. Such a transition, forbidden in pure GaAs, becomes allowed in ${\mathrm{GaAs}}_{1\ensuremath{-}x}{\mathrm{N}}_{x}$ due to the strong perturbation of nitrogen doping to the band structure. A similar analysis applied to the transition ${E}_{*}$ suggests that it is related either to resonant states evolving from the level created by a single isolated nitrogen impurity in GaAs, or to the splitting of the triplet originating from the L conduction band, induced by a further reduction in symmetry associated with nitrogen pairs.

Structure and electrical activity of rare-earth dopants in selected III-Vs

ABSTRACTDensity functional theory is used to investigate Eu, Er and Tm rare earth (RE) impurities in GaAs, GaN and AlN. The most stable site is when the RE is located at a group III substitutional site but in GaN and GaAs these defects do not then possess any gap levels, unlike AlN. RE-VN defects in GaN are shown to possess levels which could act as traps for excitons. The interaction of oxygen with substitutional REs is also considered.

Enhanced impurity incorporation by alternate Te and S doping in GaAs prepared by intermittent injection of triethylgallium and arsine in ultra high vacuum

Enhanced incorporation of doped impurity in GaAs by alternate doping of Te and S is reported. When diethyltelluride (DETe) was injected on GaAs surface covered with S, the incorporation of Te was enhanced and it is shown that the Te concentration was increased according to the S coverage. Mechanism of enhanced impurity incorporation is discussed in view of the charge distribution in the molecules and the electronegativity difference.

BINDING ENERGY FOR A SHALLOW DONOR IMPURITY IN GaAs–(Ga, Al)As QUANTUM WELLS UNDER HYDROSTATIC PRESSURE AND APPLIED ELECTRIC FIELD

The present work investigates the effects of the hydrostatic pressure and the external applied electric field on the binding energy for shallow donor impurities in GaAs–Ga 1 - x Al x As quantum wells. The effective mass approximation is used and a trial envelope wave function is adopted for the impurity carrier. For fixed well width and applied electric field, the binding energy of the shallow donor impurity is enhanced by increasing the external hydrostatic pressure, and for fixed well width and hydrostatic pressure, the binding energy decreases by increasing the external electric field.

Spin Polarization Dependent Far Infrared Absorption in Ga1-xMnxAs

Infrared and far infrared transmission spectra have been measured for the ferromagnetic Ga1-xMnxAs (x=0.034 and x=0.050) between 10 and 12000 cm-1. A broad peak is observed around 1600 cm-1, and this energy is close to the deep acceptor level of Mn impurity in GaAs. A finite featureless absorption, spreading between this peak and the band gap, due to inter valence band transitions is also observed. At the same time, a non-zero absorption is observed in the lowest frequency studied, which points to the absorption by mobile holes related to the metallic DC conductivity. The frequency dependence in this range, which differs markedly from the Drude model, demonstrates a strong influence of electrostatic and magnetic disorder on the optical conductivity. In particular, the existence of a strong coupling between the Mn spins and the holes explains the considerable increase of the far infrared absorption below the ferromagnetic ordering temperature.

Distribution of defects and impurities in gallium arsenide wafers after surface gettering

Gettering of defects and impurities in GaAs using a heat treatment (HT) of the yttrium-coated wafers has been investigated. The gettering has been established to be of a volume character. That has allows high-resistivity material to be obtained with the near-uniform distribution both of electron concentration and of minority carrier lifetime in the GaAs wafers with thickness up to 1.6 mm. These in-depth distribution profiles have been first presented. Moreover, low-temperature photoluminescence (PL) of the GaAs wafers has been studied.

Uniaxial stress dependence of the binding energy of shallow donor impurities in GaAs–(Ga,Al)As quantum dots

We have studied the effects of an uniaxial stress on the binding energy of a shallow donor impurity in a parallelepiped-shaped GaAs–(Ga,Al)As quantum dot. In the calculations we have used a variational technique within the effective-mass approximation. The stress was applied in the z direction and the donor impurity was located at various positions along the z axis. Our results show that the donor binding energy increases with increasing stress and for decreasing sizes of the quantum dot. Also, we have found that the binding energy for various values of the donor position along the z axis for constant quantum well box size increases with the proximity of the impurity to the center of the structure. Moreover, we obtain the shallow-donor binding energies as functions of uniaxial stress in the limit in which the quantum dot turns into either a quantum well or a quantum-well wire.

Characteristics of Si and Be δ-codoped GaAs grown by MBE

Simultaneous doping (codoping) of n- and p-type impurities in GaAs is investigated in both uniformly and δ-doped structures using Si and Be as n- and p-type dopants. In highly Si-doped GaAs layers, the electrical activation ratio of Si-donors is much less than unity because of the formation of compensating centers such as antisite Si (Si AS), Si-clusters and Ga vacancies (V Ga). Although no enhancement is observed in the electrical activation, it is found that most of these compensating centers disappear as a result of codoping regardless of the doping structures. The integrated photoluminescence intensity in the codoped GaAs layers is much higher than that in the solely Si-doped samples, indicating the reduction of nonradiative recombination centers. The uniformly doped samples show very large spectrum widths, while the δ-doped samples keep narrow line with even for increased Be concentration.

Coherent manipulation of semiconductor quantum bits with terahertz radiation.

Quantum bits (qubits) are the fundamental building blocks of quantum information processors, such as quantum computers. A qubit comprises a pair of well characterized quantum states that can in principle be manipulated quickly compared to the time it takes them to decohere by coupling to their environment. Much remains to be understood about the manipulation and decoherence of semiconductor qubits. Here we show that hydrogen-atom-like motional states of electrons bound to donor impurities in currently available semiconductors can serve as model qubits. We use intense pulses of terahertz radiation to induce coherent, damped Rabi oscillations in the population of two low-lying states of donor impurities in GaAs. Our observations demonstrate that a quantum-confined extrinsic electron in a semiconductor can be coherently manipulated like an atomic electron, even while sharing space with approximately 10(5) atoms in its semiconductor host. We anticipate that this model system will be useful for measuring intrinsic decoherence processes, and for testing both simple and complex manipulations of semiconductor qubits.

The Binding Energies of Shallow Donor Impurities in GaAs-(Ga,Al)As Coaxial Quantum-Well Wires

The Binding Energies of Shallow Donor Impurities in GaAs-(Ga,Al)As Coaxial Quantum-Well Wires

Isoelectronic impurity states in GaAs:N

Using the one-band one-site Koster-Slater model, we explain the different behavior of isoelectronic impurities in GaAs:N and GaP:N in terms of their band-structure difference. We show that the two lowest nitrogen bound states, ${\mathrm{NN}}_{1}$ and ${\mathrm{NN}}_{2},$ are associated with the [220] and [110] nitrogen pairs, respectively, that the optical transition of the former is dipole allowed whilst the latter is forbidden in both systems, and that the order of the [220] and [110] pair levels are reversed in the two systems.

Structural properties and energetics of oxygen impurities in GaAs

We investigate the structural properties, formation energies, and electronic structure of oxygen impurities in GaAs using first-principles total-energy calculations. Five charge states of oxygen occupying an arsenic site $({\mathrm{O}}_{\mathrm{As}})$ and various interstitial sites $({\mathrm{O}}_{i})$ were considered. For the ${\mathrm{O}}_{\mathrm{As}}$ defect in negative charge states we find off-center configurations with ${C}_{2v}$ symmetry as reported experimentally. Our results for the formation energies reveal a negative-$U$ behavior for the ${\mathrm{O}}_{\mathrm{As}}$ defect, in which the paramagnetic $2\ensuremath{-}$ charge states is never stable. For the ${\mathrm{O}}_{i}$ defect, we find three equilibrium configurations for the O atom, which are present in all the charge states investigated. The stable configuration for the neutral defect shows the O atom between an As-Ga pair forming the As-O-Ga structure. However, for the negative charge states, the stable configuration shows the O atom exactly at the tetrahedral interstitial site, bonding with four gallium first neighbors. Further, we find that the $1\ensuremath{-}$ charge state is never stable, suggesting that the interstitial defect also exhibits a negative-$U$ behavior. Based on our results, we suggest that the $A, {B}^{\ensuremath{'}},$ and B bands of the local-vibrational-mode absorption spectrum of oxygen in GaAs are due to the off-center ${\mathrm{O}}_{\mathrm{As}}$ defect. Also we show that this spectrum cannot be associated with an interstitial-oxygen configuration as previously proposed.

First-principles dynamics of defect reactions triggered by electronic excitation

We investigated the influence of electronic excitation on reactivation of H-passivated impurities in GaAs. By applying the recently developed scheme for the first-principles molecular dynamics coupled with the real-time electron dynamics, the electronic excitation that significantly reduces the dissociation barrier heights of a SiGa–H complex in GaAs was found. This fact indicates strong enhancement of H-dissociation upon the excitation and subsequent reactivation of the Si donors. Moreover, the reactivation mechanisms of the C acceptor are partially reviewed.

Characteristics of Molecular Beam Epitaxy Grown GaAs Simultaneously Doped with Si and Be

Simultaneous doping (codoping) of n- and p-type impurities in GaAs is investigated using Si and Be as n- and p-type dopants. In highly Si-doped GaAs layers, the electrical activation ratio of Si-donors is much less than unity because of the formation of compensating centers such as antisite Si (SiAs), Si-clusters and Ga vacancies (VGa). Although no enhancement is observed in the electrical activation, it is found that most of these compensating centers disappear as a result of codoping. The integrated photoluminescence intensity in the codoped GaAs layers is much higher than that in the solely Si-doped samples, indicating that simultaneous Si and Be doping decreases the concentration of nonradiative recombination centers created by high Si doping, without lowering the electron concentration.

Zeeman spectroscopy of the Be acceptor in GaAs to intermediate fields

Magneto-spectroscopy to 17.5 T of the Be acceptor impurity in GaAs in the Faraday configuration using unpolarised radiation is reported. One strong component of the G-line is observed. This moves slowly with field at ∼0.24 cm −1/T . All eight transitions expected for the D-line are observed, allowing some of the splittings of the field-induced components of the ground and the second excited states to be determined.

Effect of low-energy nitrogen molecular-ion impingement during the epitaxial growth of GaAs on the photoluminescence spectra

Low-energy (∼100 eV) nitrogen molecular ions (N2+) were impinged during molecular beam epitaxial growth of GaAs at the substrate temperature of 550 °C. In the low-temperature (2 K) photoluminescence (PL) spectra, extremely sharp N-related emissions (Xi, i=1, 2, and 5) were observed in as-grown condition. These emissions were roughly two orders of magnitude stronger than those formed by the impingement of nitrogen atomic ions (N+). The results indicate that nitrogen (N) atoms are in situ substituted at As sites without inducing large structural damages and become quite efficient radiative recombination centers as isoelectronic impurities in GaAs. Further, to study the substitutional condition of N isoelectronic impurity, N isotope (15N) doped GaAs was grown by N152+ ion impingement. When N15 is doped, PL peak energy of X5 shifted towards higher energy side by 1.8 meV. The value is fairly close to the expected one of 1.9 meV when N15 replaces N14N. Together from energy separation between X2 emission (∼60 meV), origin of X5 was ascribed to the local vibrational mode of X2 emission.

Getting to grips with light-atom impurities

The use of localized vibrational mode (LVM) absorption, a type of IR absorption, allows quantitative measurements of the concentration of light-atom impurities. Such impurities, e.g. H, N and C, are extremely difficult to measure using mass spectrometric techniques, particularly where concentrations are low. Here, Mike Brozel explains the mechanisms of LVM absorption and, using Si impurities in GaAs as an Example, shows the type of information that can be obtained.