Impurities In GaAs Research Articles

Comparison of the dilute bismide and nitride alloys GaAsBi and GaAsN

AbstractDilute III–V alloys containing N or Bi share many features that are common, but some that are distinct. In GaP and GaAs, both the substituent species N and Bi behave as isoelectronic impurity traps and both lead to a giant bandgap bowing phenomenon. The isolated N and Bi impurities generate bound states in GaP but resonant states in GaAs. N impurity pairs have been observed as bound states in GaP and in GaAs whereas Bi impurity pairs have not been observed as bound states in GaP nor in GaAs. Low temperature photoluminescence studies on GaAs1–x Bix show undulations in the spectra but these are not associated with Bi–Bi pairs. Theoretical arguments for the differing behaviour of the N and Bi isolated impurities in GaAs as a function of pressure are provided. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Role of impurities in GaAs affecting the quantisation of electrons passage through island in single electron transistor

We show that the presence of inevitable impurities in the semi-insulating GaAs domains when manufacturing a single electron transistor (SET) alters the quantisation mechanism of the single electron tunnelling through its course inside the island. Moreover, these impurities affect the amount of energy needed to change the number of electrons on the island. This decreases drastically the quality of the SET. The published experimental data (I-V characteristics) of GaAs nano-crystals has been well fitted to our model.

Inter-impurity and impurity–host magnetic correlations in semiconductors with low-density transition–metal impurities

Experiments on (Ga,Mn)As in the low-doping insulating phase have shown evidence for the presence of an impurity band at 110 meV above the valence band. The motivation of this paper is to investigate the role of the impurity band in determining the magnetic correlations in the low-doping regime of the dilute magnetic semiconductors. For this purpose, we present results on the Haldane–Anderson model of transition–metal impurities in a semiconductor host, which were obtained by using the Hirsch–Fye quantum Monte Carlo (QMC) algorithm. In particular, we present results on the impurity–impurity and impurity–host magnetic correlations in two- and three-dimensional semiconductors with quadratic band dispersions. In addition, we use the tight-binding approximation with experimentally determined parameters to obtain the host band structure and the impurity–host hybridization for Mn impurities in GaAs. When the chemical potential is located between the top of the valence band and the impurity bound state (IBS), the impurities exhibit ferromagnetic (FM) correlations with the longest range. We show that these FM correlations are generated by the antiferromagnetic coupling of the host electronic spins to the impurity magnetic moment. Finally, we obtain an IBS energy of 100 meV, which is consistent with the experimental value of 110 meV, by combining the QMC technique with the tight-binding approach for an Mn impurity in GaAs.

Modeling the compositional dependence of electron diffraction in dilute GaAs- and GaP-based compound semiconductors

The change in the intensity of the (200) electron-beam reflection induced by the incorporation of isovalent impurities in GaAs and GaP is studied theoretically. Calculations are performed in the framework of the kinematical scattering theory. The structure factor of random alloys was obtained using atomic form factors calculated with the density-functional theory and an empirical-potential valence force-field model for the structure relaxation. The calculations include the effect of redistribution of the electron density on the electron-scattering amplitudes as well as the effect of the local lattice distortions associated with the impurity sites. We propose a way to calculate these distortions analytically and to introduce them in a simple form to the expression for the structure factor. This method is an alternative to the simulations, which invoke demanding computations for atomic relaxation, and enables quantitative prediction of the compositional variation of the structure factor for dilute alloys taking into account static atomic displacements. The implications of the results for quantification of composition fluctuations in heterostructures using dark-field transmission electron microscopy are discussed. The effect of nitrogen and boron incorporation on the intensity of the (200) reflection is found to be partly compensated by the static atomic displacements they cause. Neglecting this effect would lead to an underestimation of the impurity content by approximately a factor of two. The redistribution of the electron density is found to be less crucial for the evaluation of the chemical composition leading to a relative error in the (200) scattering amplitude of about 16%.

MAGNETIC STUDIES OF METAL–INSULATOR TRANSITION IN A QUANTUM WELL SYSTEM

Metal–insulator transition in doped semiconductors is investigated in the presence of intense magnetic fields. A variational procedure within the effective mass approximation is employed using the Thomas–Fermi screening function and the exact quasi-Q2D Lindhard dielectric function. The Hubbard model results are justified using an effective mass that depends on interimpurity separation. The nonparabolicity of the subband is included using an energy-dependent effective mass. Though an increase of ionization energy with a magnetic field is observed for isolated donor models, the metallization occurs with an intense magnetic field at a higher concentration for a particular well width. The diamagnetic susceptibility of a hydrogenic donor impurity in GaAs / Ga 1 - x Al x As quantum well systems is discovered in the observation of metal–insulator transition. It is shown that the diamagnetic susceptibility diverges for all critical concentrations for a given well width. The large diamagnetic susceptibility (> 6) is observed at the transition. All the calculations are carried out for infinite and finite barriers, and the results are compared with the existing literature.

Effect of the state of vacancy equilibrium on diffusion of chromium impurity in gallium arsenide

The results of studying the diffusion of Cr impurity in GaAs according to electrical measurements are reported. Dependences of the diffusion coefficient and limiting solubility of electrically active Cr atoms in GaAs on temperature (at fixed pressures of As vapors) and on the pressure of As vapors (at fixed temperatures) are determined. The dependence of the Cr diffusion coefficient in GaAs on the ratio between the volume of the sample under study to the volume of the cell in the case of pronounced deviation from the crystal’s stoichiometry towards Ga excess is established. The obtained experimental data are analyzed on the basis of concepts concerning the dissociative mechanism of migration of Cr atoms in the GaAs crystal lattice; according to this mechanism, the diffusion coefficient depends heavily on the concentration of Ga vacancies.

Bi isoelectronic impurities in GaAs

$\mathrm{Ga}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{Bi}}_{x}$ is a mixed-anion semiconductor alloy. In the As-rich regime $(0.4%lxl4%)$, isovalent Bi creates a series of bound states but this alloy nonetheless exhibits properties characteristic of regular semiconductors. The dual impurity-alloy character can be tuned by varying the temperature. Below $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, multiple Bi bound states appear at low energy in the luminescence spectrum. These states are associated with the pseudodonor potential created by isovalent Bi impurities. Taking into consideration the concentration regime at which these bound states are observed, they likely involve excitons bound to clusters composed of a few substitutional Bi atoms, indirectly implying that the isolated Bi state is resonant with the valence band. At ambient temperature, these localized states are strongly suppressed and luminescence from the band edges is measured. The important Bi-induced atomic disorder creates a strong intraband coupling resulting in an important reduction of the band gap for a relatively small alloy concentration. These results on a pseudodonor isoelectronic alloy are reminiscent of the better known pseudoacceptor GaAsN, but offer a complementary view of this intriguing and yet little understood class of alloys.

Study of photoexcited plasma in p-doped GaAs beveled structures by micro-Raman spectroscopy

The generation and properties of photoexcited steady-state plasma of electrons and holes in bevel-shaped p-type GaAs structures were studied by micro-Raman spectroscopy. The best correspondence of theoretical calculations with experimental spectra was obtained by using of photoexcited carrier concentration of 1.1 × 10 17 cm −3 and mobility 600 and 40 cm 2/V s for the photoexcited steady-state electrons and holes, respectively. The analysis of the plasma behavior and its coupling with longitudinal optical phonons at different positions along the bevel shows that the mode resulting from this coupling causes the changes of Raman intensities recorded in frequency positions of transversal (TO) and longitudinal (LO) optical phonon peaks. These changes were further studied and physical interpretation was provided. The dependence of their ratio in the region affected by surface depletion layer can be fitted by linear function very well. The linearity was observed at all studied structures. This behavior on beveled structures prepared by special treatment with very low bevel angle can be used for analysis of the p-type GaAs nanostructures, particularly for measurement and extraction of a doping profile of p-type impurities in GaAs with very high resolution in nm scale.

Spin-orbit splitting of a hydrogenic donor impurity in GaAs∕GaAlAs quantum wells

The electronic states of a hydrogenic donor impurity in GaAs∕GaAlAs quantum wells are investigated theoretically in the framework of effective-mass envelope function theory, including the effect of Rashba spin-orbit coupling. The splits of electron energy levels are calculated. The results show that (1) the split energy of the excited state is larger than that of the ground state; (2) the split energy peak appears as the GaAs well width increases from zero; and (3) the maximum split energy reaches about 1.6meV. Our results are useful for the application of Rashba spin-orbit coupling to photoelectric devices.

Polaron effects on the energy of a hydrogenic donor impurity in GaAs–(Ga,Al)As quantum-well wires

In this work using the effective-mass approximation within a variational approach we study the effect of the 3D longitudinal optical phonons on the energy of a shallow donor impurity in a GaAs–(Ga,Al)As quantum-well wire with a magnetic field applied in the wire-axis direction. We obtain the energy of the lower impurity states as a function of the wire radius, the donor position inside the wire, and under the effect of the applied magnetic field. The polaron effects on the impurity energy are studied using second order perturbation theory. We include non-parabolicity effects to explain the optical experiments under applied high magnetic fields.

Hydrostatic pressure effects on the Γ–X conduction band mixing and the binding energy of a donor impurity in GaAs–Ga1–xAlxAs quantum wells

Mixing between Γ and X valleys of the conduction band in GaAs–Ga1–x Alx As quantum wells is investigated taken into account the effect of applied hydrostatic pressure. This effect is introduced via the pressure-dependent values of the corresponding energy gaps and the main band parameters. The mixing is considered along the lines of a phenomenological model. Variation of the confined ground state in the well as a function of the pressure is reported. The dependencies of the variationally calculated binding energy of a donor impurity with the hydrostatic pressure and well width are also presented. It is shown that the inclusion of the Γ–X mixing explains the non-linear behavior in the photoluminescence peak of confined exciton states that has been observed for pressures above 20 kbar. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Characterizing In and N impurities in GaAs fromab initiocomputer simulation of (110) cross-sectional STM images

Computer simulations of cross-sectional scanning-tunneling microscopy images of GaAs(110) are performed by means of density-functional theory, with the aim of characterizing ${\mathrm{In}}_{\mathrm{Ga}}$ and ${\mathrm{N}}_{\mathrm{As}}$ substitutional impurities. In some cases the comparison between an experimental image taken at a single negative bias with the results of our simulations is sufficient to discriminate among different types and locations of impurities. When ambiguities arise, a combined positive/negative scan of the same region will help resolve them. We find that ${\mathrm{In}}_{\mathrm{Ga}}\text{\ensuremath{-}}{\mathrm{N}}_{\mathrm{As}}$ neighboring pairs are energetically favored relative to the isolated impurities. The main features of the images of these pairs are determined by the location of the nitrogen atoms, but details due to neighboring indium atoms are also meaningful and recognizable in the experimental images.

Electromagnetically induced transparency and slow light with n-doped GaAs

The suitability of bound exciton system in semiconductors is studied for use in nonlinear optical schemes based on EIT, such as “slow” or “stored” photons. We match the desired properties of such a system exhibiting EIT with the known physical realities of a semiconductor system, and suggest, in particular, two suitable schemes using donor impurities in GaAs. In addition to generic properties, we also focus on the influence of many neighboring levels and continuum levels, and on the effect of strong hole-mixing.

Enhancement of interactions between magnetic ions in semiconductors due to declustering

It is often assumed that the exchange interaction between two magnetic ions in a semiconductor host depends only on the distance and orientation of the magnetic ions. Using first-principles electronic structure calculations of Mn impurities in GaAs, we show that the exchange interaction between two magnetic ions depends also on the concentration and spatial arrangement of the other, ``spectator'' magnetic ions. Thus, such systems cannot be described by a Heisenberg Hamiltonian with fixed exchange interactions. Specifically, we find that at fixed Mn concentration, association (``clustering'') of Mn impurities leads to a decrease of the Curie temperature, while dissociation (``declustering'') leads to higher Curie temperatures. We conclude that clustering is the major impediment to achieve high Curie temperatures in Mn-doped GaAs.

Magnetic properties of 3d impurities in GaAs

Electronic structure, thermodynamic, and magnetic properties of 3d-transition metal (TM) impurities in GaAs have been studied from first principles using Green's function approach. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The possibility of raising the Curie temperature T C in (GaMn)As by co-doping it with Cr impurities was examined on the basis of total energy difference between the disordered local moment (DLM) and the ferromagnetically ordered (FM) spin configurations. The calculated Curie temperature and magnetic moment have maxima for GaAs doped with Cr and Mn. The magnetic properties of Mn-doped GaAs are shown to be more sensitive to antisite As defects than those of Cr-doped GaAs. However, the Cr impurities are sensitive to the presence of acceptor defects, such as vacancies on the Ga sublattice. The investigation of the electronic structure of pseudo-ternary alloys (Ga (1− x − y )Mn x Cr y )As has shown a mutual compensation of Mn and Cr impurities. Therefore, in order to reach the highest critical temperature, GaAs has to be separately doped with Cr or Mn impurities. The GaAs doped with Fe is found to be non-ferromagnetic.

Electronic structure of rare-earth impurities in GaAs and GaN

The electronic structures of substitutional rare-earth (RE) impurities in GaAs and cubic GaN are calculated. The total energy is evaluated with the self-interaction corrected local spin density approximation, by which several configurations of the open 4f shell of the rare-earth ion may be investigated. The defects are modelled by supercells of type REGa$_{n-1}$As$_n$, for n=4, 8 and 16. The preferred defect is the rare-earth substituting Ga, for which case the rare-earth valency in intrinsic material is found to be trivalent in all cases except Ce and Pr in GaN. The 3+ --> 2+ f-level is found above the theoretical conduction band edge in all cases and within the experimental gap only for Eu, Tm and Yb in GaAs and for Eu in GaN. The exchange interaction of the rare-earth impurity with the states at both the valence band maximum and the conduction band minimum is weak, one to two orders of magnitude smaller than that of Mn impurities. Hence the coupling strength is insufficient to allow for ferromagnetic ordering of dilute impurities, except at very low temperatures.

Influence of the electron-hole equilibrium shift on transition-metal diffusion in GaAs

Diffusion of impurities of transition metals Fe, Cu, and Cr in heavily doped p +-, n +-, and intrinsic (at diffusion temperature) GaAs is studied. A technique in which impurity diffuses into GaAs-based structures with heavily doped layers (p +-n or n +-n) was used. It is shown that the impurity diffusivity values in p +-GaAs and n +-GaAs are significantly higher and lower, respectively, than for i-GaAs. The results obtained are discussed taking into account the effect of the electron-hole equilibrium shift in semiconductors on the diffusion of impurities migrating according to the dissociative mechanism. The interstitial-component concentration for Fe, Cu, and Cr impurities in GaAs was determined at the diffusion temperature.

Energetics, electronic structure and local magnetism of single 3 d impurity in GaAs

A real-space first-principles method based on density functional theory has been used to study the energetics, the electronic structure and the magnetic property of single 3 d impurity in a GaAs host (with Ga substituted by Cr, Mn, Fe, Co, and Ni, respectively). It is found that Mn exhibits the largest exchange splitting in 3 d orbitals, while Fe, Co, and Ni are almost nonmagnetic. The calculated substitutional formation energies are 3.87, 3.21, 1.36, 1.41, 0.37 eV, and the calculated local magnetic moments are 3.25, 4.26, −0.01, −0.03 and −0.01 μ B , for Cr, Mn, Fe, Co and Ni, respectively. For a good understanding on the local magnetism of the 3 d impurity, we analyzed the bonding characteristics and the charge transfer property in detail, and found that the hybridizations between 3 d impurity and its neighboring As atoms play an important role.

Zeeman spectroscopy of Be impurity in GaAs to [formula omitted

Absorption measurements have been made in fields to 30 T of the far-infrared optical transitions associated with the Be impurity in GaAs. The order and magnitude of the splitting of the ground state has been clarified by low-field (to 6 T ) photo-thermal ionisation spectroscopy measurements of the C line. In light of the new high-field data the G line is now believed to comprise two unresolved components. At high magnetic field (above 25 T ) a new feature appears which increases in energy with field at a much greater rate than the other transitions; this is thought to originate in valence-band Landau levels.

Intrinsic defects and transition metal impurities in GaAs

Thermodynamics of intrinsic point defects, 3d-transition metal (TM) impurities, and various defect pairs in GaAs have been studied by means of the locally self-consistent Green's function method. Antisite defects, As Ga, are found to be the most energetically favorable defects in the As-rich GaAs. The studied TM impurities (V, Cr, Mn, and Fe) are found to form substitutional alloys on the Ga sublattice. The magnetic moments of TM impurities in the GaAs host are calculated under the assumption that orbital moments of TM are quenched and the total magnetization is due to spin ordering of electrons in the crystal. Thermodynamic possibility of formation of complexes between TM-atoms is investigated.