Acceptors In Semiconductors Research Articles

Topological states in honeycomb arrays of implanted acceptors in semiconductors

We study a neutral honeycomb array of acceptors in a Group IV semiconductor. Tight-binding exhibits a band gap from different hopping of angular momentum ±32 and ±12 , forming a topological insulator (TI). The hopping is even under separate reversal of spatial and spin motions, unlike spin–orbit coupling. The TI is robust to Coulomb interactions and realistic electronic structure calculations show it survives for a range of spacings and distortions commensurate with the silicon growth surface, but has an instability towards spin density wave formation at large separations.

Screened Potential by Electron Gas

Donors and acceptors in semiconductors are charged impurities with a Coulomb potential and, therefore, are screened by free charge carriers – electrons. An analytical description of the oscillations of the screened external potential by the electron gas is considered in this paper. Using the inverse transformation of the Fourier component, the screened Coulomb Potential U(r) and the value of the screening radius rs for the degenerate and nondegenerate electron gas are found. The dependence of rs on potentials is also given. It is shown that the screening radius decreases with increasing the electron density.

Direct measurement of the density and energy level of compensating acceptors and their impact on the conductivity of n-type Ga2O3 films

Intrinsic and extrinsic point defects often act as electron traps in oxide-based semiconductors and significantly impact their electrical and optical properties. Here, we show how to measure the density, energy level, and trapping cross section of the compensating acceptors that act as electron traps in Ga2O3 films, and we introduce the sheet trap number or the sheet compensating acceptor number as an essential parameter to fully describe the electrical transport properties of semiconductors. Si-doped β-Ga2O3 thin films were fabricated homoepitaxially by metalorganic chemical vapor deposition and studied by thermally stimulated luminescence spectroscopy, temperature dependent Hall-effect measurements, and secondary ion mass spectroscopy to investigate the compensating acceptor defects responsible for suppressing conductivity in the films. A deep level defect of energy in the range of 0.50–0.65 eV was identified as a compensating acceptor. The correlation between the electrical properties and its concentration and characteristics was established. This work shows how to quantify the density of compensating acceptors in semiconductors and directly relate it to the electrical transport properties, which should significantly advance the development of semiconductors and devices.

Heitler-London model for acceptor-acceptor interactions in doped semiconductors

The interactions between acceptors in semiconductors are often treated in qualitatively the same manner as those between donors. Acceptor wave functions are taken to be approximately hydrogenic and the standard hydrogen molecule Heitler-London model is used to describe acceptor-acceptor interactions. But due to valence band degeneracy and spin-orbit coupling, acceptor states can be far more complex than those of hydrogen atoms, which brings into question the validity of this approximation. To address this issue, we develop an acceptor-acceptor Heitler-London model using single-acceptor wave functions of the form proposed by Baldereschi and Lipari, which more accurately capture the physics of the acceptor states. We calculate the resulting acceptor-pair energy levels and find, in contrast to the two-level singlet-triplet splitting of the hydrogen molecule, a rich ten-level energy spectrum. Our results, computed as a function of inter-acceptor distance and spin-orbit coupling strength, suggest that acceptor-acceptor interactions can be qualitatively different from donor-donor interactions, and should therefore be relevant to the control of two-qubit interactions in acceptor-based qubit implementations, as well as the magnetic properties of a variety of p-doped semiconductor systems. Further insight is drawn by fitting numerical results to closed-form energy-level expressions obtained via an acceptor-acceptor Hubbard model.

Relationship between persistent phosphorescence and electric conductivity in transparent conductive oxide β- Ga_2O_3

The luminescence spectra in β-Ga2O3 (4N) and β-Ga2O3:Si single crystals excited with the band-edge energy consist of UV/blue and blue/green broad bands. The luminescence with decay times (>1 s) is called persistent phosphorescence (PP). The PP spectra observed in both β-Ga2O3 single crystals at 15 K are in agreement with the blue/green band. The PP intensities decrease gradually by increasing the temperature from 15 K and disappear above ~150 K. The decay curves of the luminescence in wide time range between 4 and 100 μs and between 1 and 103 s at 15 K fit a power function of t-n with n~1.1 and 0.9, respectively. The decay occurs through the same mechanism as the recombination of donors and acceptors in semiconductors. Thermal excitation of shallowly trapped electrons in β-Ga2O3 into the conduction band leads to the decrease of the PP intensity and the increase of the electrical conductivity and the photocurrent.

Reccurent sequenses in solving the Schrödinger equation

An explicit numerical-analytical method is demonstrated for accurate solving the Schrödinger equation in those cases when this equation reducible to a system of n coupled ordinary differential equations with singular points. Fundamental system of solutions is constructed as algebraic combinations of power series, power functions and logarithmic function in the neighbourhood of the regular singular point and as asymptotic expansions of solutions in the neighbourhood of the irregular singular point. The method is based on the calculation of recurrent sequences of constant matrices of coefficients in power series and in inverse power series in asymptotic expansions using derived recurrent relations, that makes possible to calculate solutions at any given point using only algebraic computations and elementary functions. In turn it makes possible to solve accurately the eigenvalue problem and scattering problem and to derive analytical expressions for the wavefunctions. The method is applied to calculations of energies and wavefunctions of the discrete spectrum and wave functions of the continuous spectrum of the hydrogen-like atoms and of acceptors in semiconductors.

First‐principles theory of acceptors in nitride semiconductors

AbstractAcceptor defects and impurities play a critical role in the performance of GaN‐based devices. Mg is the only acceptor impurity that gives rise to p‐type conductivity, while other acceptors (such as C impurities and defects) act as sources of compensation and trapping. From the point of view of theory, understanding the physics of acceptor species in GaN has long been a challenge. In the past, limitations of computational techniques made it difficult to quantitatively predict crucial quantities such as thermodynamic and optical transition levels. However, advances in first‐principles calculations, including the use of hybrid functionals in density functional theory, have led to a resurgence in efforts to understand properties of acceptors in nitrides. After briefly discussing advances in theoretical techniques, we review recent computational work on acceptor impurities in GaN and compare theoretical results with the available experimental data. We also present new hybrid density functional calculations on the transition levels of and its complexes with O and H impurities. The results indicate that donor impurities significantly lower transition levels, and that and complexes give rise to yellow luminescence. We also discuss the properties of acceptor impurities in AlN and InN.

Determination of concentrations of donors and acceptors in many-valley semiconductors

A possibility of separate determination of concentrations of donors and acceptors in manyvalley semiconductors on the basis of temperature dependence of total concentration of charge carriers is shown. As distinct from a single-valley semiconductor, it is necessary to take into account that effective mass of density of states and effective ionization energy are not constant, but depend on temperature.

Effects of hole localization on limiting p-type conductivity in oxide and nitride semiconductors

We examine how hole localization limits the effectiveness of substitutional acceptors in oxide and nitride semiconductors and explain why p-type doping of these materials has proven so difficult. Using hybrid density functional calculations, we find that anion-site substitutional impurities in AlN, GaN, InN, and ZnO lead to atomic-like states that localize on the impurity atom itself. Substitution with cation-site impurities, on the other hand, triggers the formation of polarons that become trapped on nearest-neighbor anions, generally leading to large ionization energies for these acceptors. Unlike shallow effective-mass acceptors, these two types of deep acceptors couple strongly with the lattice, significantly affecting the optical properties and severely limiting prospects for achieving p-type conductivity in these wide-band-gap materials.

Tight-binding description of impurity states in semiconductors

Introductory textbooks in solid state physics usually present the hydrogenic impurity model to calculate the energy of carriers bound to donors or acceptors in semiconductors. This model treats the pure semiconductor as a homogeneous medium and the impurity is represented as a fixed point charge. This approach is only valid for shallow impurities and it also departs from the conventional view in solid state physics, where carriers move in a crystal lattice. As an alternative description of impurities in semiconductors, we present a minimal one-dimensional lattice model within the tight-binding approximation. The lattice model is valid for deep and shallow impurities. In the latter case, the results are in agreement with the predictions of the hydrogenic impurity model. The underlying ideas are simple and knowledge of advanced quantum mechanics is not required. Thus, this alternative model could be suitable for introductory courses in solid state physics and materials science.

Shallow versus Deep Nature of Mg Acceptors in Nitride Semiconductors

We investigate the properties of Mg acceptors in nitride semiconductors with hybrid functional calculations. We find that although the thermodynamic transition level is relatively close to the valence band in GaN (260 meV), Mg(Ga) exhibits key features of a deep acceptor: the hole is localized on a N atom neighboring the Mg impurity, inducing a large local lattice distortion and giving rise to broad blue luminescence. We show that the ultraviolet photoluminescence peak attributed to Mg acceptors in GaN is likely related to Mg-H complexes, explaining the results of photoluminescence and electron paramagnetic resonance experiments. Predictions for Mg acceptors in AlN and InN are also presented.

Shallow acceptor impurities in diamond-like semiconductors studied by polarized negative muons

Abstract The results of μ SR study of the behavior of shallow acceptor centers (AC) in diamond, silicon, and germanium are presented. It was found that the muonic atom, which imitates acceptor in semiconductors, is formed in the paramagnetic state in silicon and germanium at low temperatures whereas in diamond it is formed in the diamagnetic state. The hyperfine interaction constant for the aluminium AC in silicon was estimated for the first time. It was shown that the main contribution to the relaxation of the magnetic moment of the AC is due to interaction of the AC with the phonon via the Roman process in non-degenerated silicon at temperatures ⩽ 50 K . In the case of germanium, there is an experimental evidence for a change in the contribution of different phonon processes to the relaxation of the AC at temperature ∼ 10 K . The hole capture rate by the ionized boron acceptor in diamond was determined.

Acceptorlike behavior of nitrogen deep traps inGaAs:N

We report the observation of excited states of holes for acceptorlike excitons bound to isolated nitrogen impurities in $\mathrm{GaAs}:\mathrm{N}$ under high hydrostatic pressures. Appearance of a large absorption resonance $(5\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ in optical transmission and photoluminescence excitation spectroscopies leads to the identification of the $2S$ excited hole state associated with the ground-state nitrogen isoelectronic bound exciton---commonly known as the ${\mathrm{N}}_{X}$ state from earlier $\mathrm{GaAs}$-based alloy studies. Comparison with the well-established effective-mass theory of Baldereschi and Lipari for excited-state $S$-type spectra of acceptors in semiconductors provides good qualitative agreement with our data. We thus deduce at $26.9\phantom{\rule{0.3em}{0ex}}\mathrm{kbar}$ the ground-state hole ionization energy as being $\ensuremath{\sim}19.2\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$, which in turn leads to a unique electron binding energy of $\ensuremath{\sim}10.3\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ for isolated nitrogen traps in $\mathrm{GaAs}:\mathrm{N}$ at this pressure. These data and accompanying interpretations may provide for further understanding of the deep-level behavior of isoelectronic binding mechanisms in semiconductors.

Polarons in crystalline and non-crystalline materials

The current state of polaron theory as applicable to transition metal oxides is reviewed, including problems such as impurity conduction where disorder plays a role. An estimate is given of the conditions under which polaron formation leads to an enhancement of the mass but no hopping energy. The binding energy of a polaron to a donor or acceptor in narrow-band semiconductors is discussed. The experimental evidence about the conductivity of TiO 2 and NiO is reviewed. Impurity conduction in NiO and conduction in glasses containing transition metal ions is discussed and it is emphasized that the activation energy for hopping nearly all vanishes at low temperatures. Pollak's theory of a.c. impurity conductivity is reviewed and applied to the problem of dielectric loss in these materials.

Zeeman effect of electronic Raman lines of acceptors in elemental semiconductors: Boron in blue diamond

The Zeeman effect of the electronic Raman transition from ${1s(p}_{3/2}):{\ensuremath{\Gamma}}_{8}$ to the ${1s(p}_{1/2}):{\ensuremath{\Gamma}}_{7}$ spin-orbit partner $({\ensuremath{\Delta}}^{\ensuremath{'}})$ of boron acceptors in diamond is studied with magnetic field $\mathit{B}$ along [001], [111], or [110]. As many as eight Zeeman components of ${\ensuremath{\Delta}}^{\ensuremath{'}}$ and, in addition, four Raman lines ascribed to transitions between the Zeeman sublevels of ${\ensuremath{\Gamma}}_{8}$ [Raman--electron-paramagnetic-resonance (Raman-EPR) transitions] are observed with the polarizations expected from the polarizability tensors that characterize them. These tensors are formulated in terms of ${\ensuremath{\gamma}}_{1},{\ensuremath{\gamma}}_{2},$ and ${\ensuremath{\gamma}}_{3},$ the Luttinger parameters characterizing the ${p}_{3/2}$ and ${p}_{1/2}$ valence band maxima. The selection rules and relative intensities of the Zeeman components and of the Raman-EPR lines, observed in diverse polarization configurations and scattering geometries, have led to determination of (1) the assignments of magnetic quantum numbers; (2) the level ordering of the Zeeman sublevels, or, equivalently, the magnitudes and signs of ${g}_{1}$ and ${g}_{2},$ the orbital and spin g factors of the acceptor-bound hole; (3) the extreme mass anisotropy as reflected in the ratio $({\ensuremath{\gamma}}_{2}/{\ensuremath{\gamma}}_{3})=0.08\ifmmode\pm\else\textpm\fi{}0.01.$ Magnetic-field-induced mixing of zero field states, time reversal symmetry, and the diamagnetic contributions that characterize the different sublevels are fully taken into account in the interpretation of the experimental results. These include the striking mutual exclusion of the Stokes spectrum from its anti-Stokes counterpart in specific polarization configurations.

Defects, quasibound states, and quantum conductance in metallic carbon nanotubes

The effects of impurities and local structural defects on the conductance of metallic carbon nanotubes are calculated using an ab initio pseudopotential method within the Landauer formalism. Substitutionally doped boron or nitrogen produces quasibound impurity states of a definite parity and reduces the conductance by a quantum unit (2e(2)/h) via resonant backscattering. These resonant states show strong similarity to acceptor or donor states in semiconductors. The Stone-Wales defect also produces quasibound states and exhibits quantized conductance reduction. In the case of a vacancy, the conductance shows a much more complex behavior than the prediction from the widely used pi-electron tight-binding model.

Enhancement of deep acceptor activation in semiconductors by superlattice doping

The thermal activation of acceptors in wide-gap semiconductors can be very low due to large acceptor activation energies. It is shown that superlattice doping, i.e., the composition modulation of a uniformly doped ternary semiconductor, can enhance the acceptor activation by more than one order of magnitude.

Dynamic Jahn-Teller effect for a double acceptor or acceptor-bound exciton in semiconductors: Mechanism for an inverted level ordering.

The dynamic Jahn-Teller effect provides a simple mechanism for inverting the ordering of the J=0,2 levels of the two equivalent holes in a neutral double acceptor or an exciton bound at a neutral acceptor. Whereas Hund's rule places the J=2 level below J=0, Jahn-Teller coupling to E and/or ${\mathit{T}}_{2}$ vibrational modes tends to shift J=0 below J=2, while splitting J=2 into its ${\mathrm{\ensuremath{\Gamma}}}_{3}$ and ${\mathrm{\ensuremath{\Gamma}}}_{5}$ components. Alternative proposed explanations involving central-cell, Stark, and strain effects fail to introduce the attractive interaction needed to offset the holes's Coulomb repulsion.

Theory of the piezo-Zeeman effect for shallow acceptors in group-IV semiconductors.

Theory is presented for the energy splittings and transition intensities for a single hole system initially of ${\mathit{T}}_{\mathit{d}}$ symmetry which is then subject to the simultaneous application of a uniaxial stress and uniform magnetic field. Four combinations of stress and field directions are considered.

A +— center and exciton bound to neutral acceptor in diamond-like semiconductors

The shallow positive charged acceptor center as well as the exciton bound to a neutral acceptor have been considered in the semiconductors with a degenerate valence band. The multiplet structure of the charged acceptor center has been investigated and the binding energies have been calculated as functions of the light hole to heavy hole effective mass ratio. The dissociation energies of bound exciton — neutral acceptor complex have been calculated in donor — like model.