Theory of shallow donor impurity near-surface states in Si and Ge

Previously developed methods for determining the wave functions and energy levels of shallow donor impurities on the surface or in the bulk of a semiconductor have been extended to the case of impurities at a finite depth from the surface. Calculations were performed for impurities near either a (100) or (111) surface in Sci and for a (111) surface in Ge. The ground state and the excited state having the largest electric dipole coupling to the ground state wete determined. In addition, the optical absorption cross section for transitions from the ground state to this excited state were calculated for surface and near surface states. These results were used to calculate a theoretical absorption spectrum for Si doped uniformly to a depth of 10 atomic layers.     

Final states in Si and GaAs via RF μSR spectroscopy

The ionization of muonium centers in Si and GaAs have been studied using radio frequency (RF) resonant techniques. In Si all three muonic centers are detectable by RF. No evidence was found for delayed Mu and Mu^* states at any temperature. However, our results on the diamagnetic final state (μ _f ⁺ ) show that it is composed of prompt fractions (as seen by conventional μSR) and delayed fractions arising from the ionization of Mu^* and Mu. We observe a full μ _f ⁺ fraction at 317 K when the Mu relaxation rate is above 10 μs⁻¹. GaAs differs from the situation in Si in that we observed only a partial conversion of Mu^* and Mu to a μ⁺ final state up to 310 K in spite of the fact that the transverse field relaxation rates become very high at 150 and 250 K respectively.     

Photothermal ionization fourier transform spectroscopy of shallow donor states in III–V semiconductors

The technique of photothermal ionization Fourir transform spectroscopy of shallow donors is very useful for the detection and identification of residual impurity species in very high purity compound semiconductors, especially GaAs. However, the use of this technique in less pure or intentionally doped samples has resulted many incorrect impurity identifications. In this paper we show how the photothermal ionization spectra change with impurity concentration, thickness, and magnetic field, and present a model of the dielectric response of shallow impurity states which explains many of the previously anomalous results.     

Confined electron and shallow donor states in graded GaAs/Al Ga As spherical quantum dots

A theoretical study is performed on the confined electron and shallow donor states properties in graded GaAs/Al_xGa_1-xAs spherical quantum dots. The two lowest energy levels of a confined electron are obtained taking into account the dependence of the electron effective mass on the spatial profile of the Al molar fraction. The ground state of a single Si shallow donor, which may be located at an arbitrary position in the structure, is calculated through a variational approach. Depending on the dot interface width and localization, we find that the energy levels of the electron and donor states for the system under study can be blue or red shifted appreciably in comparison to those calculated within the sharp interface picture. We show that it is necessary to have accurate information concerning the interface of semiconductor dots whose samples are used in the experiments, in order to achieve a better understanding of their optical properties. Received 31 May 1999     

Quantum diffusion of muonium in GaAs with shallow donor impurities

Muonium diffusion was studied in silicon doped GaAs by means of muon spin relaxation in a longitudinal field. The muonium hopping frequencies in two samples with n-type carrier concentration of 10¹²¹⁴ cm^–3 and 8×10¹⁶ cm^–3 were deduced by using the model of fluctuating effective local fields. We found that muonium diffusion is strongly influenced by the dilute Si impurity in both samples in the temperature range belowT 30K. The absence of such a behavior in compensated high-resistivity samples indicates that the presence of shallow donor levels plays a decisive role for the tunneling diffusion of muonium in semiconductors.We would like to thank Drs. N.V. Prokof'ev and R.F. Kiefl for helpful discussion.After submission of this paper it was revealed by the field dependence of the LF relaxation rate that the observed relaxation rate in GaAs:Si-B is due to spin-exchange interaction of Mu^*.     

Experimental evidence of dislocation related shallow states in p-type Si

Theory, models, and experimental phenomena provide evidence of the existence of shallow bands in silicon induced by the dislocation strain field. Nevertheless, only deep bands, likely associated with contamination at dislocations, have been detected up to now by junction spectroscopy. Here we present the first experimental result by junction spectroscopy that assesses the existence of the dislocation related shallow states. These are found to be located at 70 and 60 meV from the valence and conduction band edge, respectively.     

Direct evidence for shallow acceptor states with nonspherical symmetry in GaAs

We investigate the energy and symmetry of Zn and Be dopant-induced acceptor states in GaAs using cross-sectional scanning tunnelling microscopy (STM) and spectroscopy at low temperatures. The ground and first excited states are found to have a nonspherical symmetry. In particular, the first excited acceptor state has a T(d) symmetry. Its major contribution to the STM empty-state images allows us to explain the puzzling triangular shaped contrast observed in the empty-state STM images of acceptor impurities in III-V semiconductors.     

First-principles supercell calculations for simulating a shallow donor state in Si

We present first-principles simulations of As-doped Si carried out using several cubic supercells of up to 10 648 atoms. The 1s As donor level in each supercell splits into three states, which have A₁, T₂, and E symmetries, respectively. The 1s(A₁) wavefunction is well converged in the largest cell, and its spread is close to those of the effective-mass theories. However, the calculated binding energies are smaller than experimental values. This discrepancy would be due to the self-interaction error within the approximated exchange-correlation density functional used in this calculation. Therefore, we also show perturbative calculations based on an impurity potential without the self-interaction error to estimate the binding energies of the 1s(A₁) donor state. The estimated binding energy in the largest supercell agrees well with the experimental value.     

The screening influence of the conduction electrons on the shallow donor levels in n-type GaAs

Using the approximative formula found by the author for the eigenvalues of Schrödinger equation for an electron in the screened coulombic field and the condition of the charge neutrality in partially compensated semiconductor, the ground-state and the first excited-state energy levels of shallow donors in n-type GaAs are computed for various donor concentrations and compensation ratios. The energy levels are found to be temperature dependent, which enables to explain the discrepancy between the ionization energies experimentally determined by optical methods at 4·2 K and those found by fitting the experimental temperature variation of the carrier concentration. The different values of the energies for the transition from the ground-state to the first excited-state at 4·2 K and at 15 K experimentally determined by other authors also confirm the temperature dependence of shallow donor levels which can be explained by the screening effect of conduction electrons.     

Impurity states in GaAs:O

The ground state energy E, wavefunction, and wavevector dependence of the probability density of GaAs:O are presented. E is found to be 0.785 eV from the bottom of the conduction band.     

Structure of28Si from the spectroscopy of high-spin states

A search for high-spin states in²⁸Si has been performed byn?y coincidence measurements in the²⁵Mg(α,nγy) reaction atE _α=14 and 15.5 MeV. Spin-parity assignments of the observed levels were obtained fromn?γ angular correlation and lifetime measurements atE _α=14.5 MeV. Theγ-decay of the 9,164 keV level was investigated separately with the²⁷Al(p, γ) reaction at theE _p=2,160 and 2,312 keV resonances. Rotational bands withK ^π=3^? (comprising levels atE _x=6,879, 8,413, 10,188 and 12,204 keV),K ^π =5^? (comprising levels atE _x=9,702, 11,577 and 13,741 keV) andK ^π=0⁺ (comprising levels atE _x=6,691, 7,381, 9,164 and 11,509 keV) were observed. The finding of the latter band supports the idea of coexisting oblate and prolate shapes in²⁸Si. A level at 14,643 keV excitation energy has the properties of theI ^π=8⁺ member of the ground state band. There are additional positive-parity high-spin states which do not fit into rotational bands. All types of positive-parity states are well accounted for by shell model calculations.     

Magnetic field and finite barrier-height effects on the polarizability of a shallow donor in a GaAs quantum wire

The binding energy and the polarizability are estimated for a shallow donor confined to move in a GaAs quantum well wire (QWW) with a rectangular and square cross-section under the action of an axial magnetic field. In this work, the Hass variational method within the effective mass approximation is used in the case of infinite and finite barrier QWWs. We present our results as a function of the size of the wire and for several values of the magnetic field strength. It is found that the magnetic field strongly reduces the polarizability. The finite barrier-height effect is important for smaller well widths. For higher fields and large wire, the effects of the magnetic field are predominant and the barrier potential is a small perturbation.     

Study of shallow impurity states in compound semiconductors using high resolution photoluminescence spectroscopy

In this paper we review briefly the use of high resolution photoluminescence to study the behavior of shallow impurity states in compound semiconductors. As an illustration we focus our review on GaAs. The binding energies of the ground state and of several low-lying excited states of the impurity centers are determined by studying the radiative transitions associated with excitons bound to neutral donors or acceptors. The difference between the binding energies of different donors in GaAs is rather small. Thus to resolve transitions associated with different chemical donors a magnetic field is used. This has the effect of sharpening the transitions as well as increasing the separation between them. One can identify donors in samples with total impurity concentrations as high as 5X10¹⁵/cm³. The binding energies of different chemical acceptors in GaAs are much higher. Thus the radiative transitions associated with excitons bound to neutral acceptors can be resolved in zero magnetic field. Energy levels of shallow donors and acceptors in GaAs are reviewed.     

Behaviour of surface states on a GaAs/AlGaAs heterostructure investigated by capacitance spectroscopy

Low temperature depletion capacitance-voltage data, taken from a novel open design of surface superlattice fabricated on a GaAs/AlGaAs heterostructure, are compared with two theoretical models. These assume either that the Fermi level on the free semiconductor surface is pinned, or that the surface charge state is unaltered (frozen) when the superlattices are biased. The data are in much better agreement with the frozen surface model.