Effective-mass States Research Articles

Electron Paramagnetic Resonance Study of Se-Doped AlSb: Evidence for Negative-U of the DX Center

A metastable electron-paramagnetic-resonance (EPR) signal is observed in Se-doped AlSb. When cooled in the dark, the sample is nonparamagnetic. After illumination with white light from a tungsten source a strong, persistent EPR signal with an isotropic {ital g} factor of 1.949 is observed. The metastability of the signal is consistent with earlier work done on {ital DX}-like centers. The absence of any EPR when cooled in the dark is direct evidence for the negative-{ital U} model. The EPR arises from the effective-mass state of the defect, which is not filled at thermal equilibrium. A detailed analysis of the line shape reveals that the linewidth is determined by hyperfine interactions. The extent of the wave function is found to be comparable to the predicted value of the effective-mass state.

Electron-paramagnetic-resonance study of Se-doped AlSb: Evidence for negative U of the DX center.

A metastable electron-paramagnetic-resonance (EPR) signal is observed in Se-doped AlSb. When cooled in the dark, the sample is nonparamagnetic. After illumination with white light from a tungsten source a strong, persistent EPR signal with an isotropic {ital g} factor of 1.949 is observed. The metastability of the signal is consistent with earlier work done on {ital DX}-like centers. The absence of any EPR when cooled in the dark is direct evidence for the negative-{ital U} model. The EPR arises from the effective-mass state of the defect, which is not filled at thermal equilibrium. A detailed analysis of the line shape reveals that the linewidth is determined by hyperfine interactions. The extent of the wave function is found to be comparable to the predicted value of the effective-mass state.

Optically detected magnetic resonance of GaN films grown by organometallic chemical-vapor deposition.

Photoluminescence (PL) and optically detected magnetic resonance (ODMR) experiments have been performed on a set of GaN epitaxial layers grown by organometallic chemical-vapor deposition. Both undoped and Mg-doped GaN films were investigated. Samples were studied from three laboratories to obtain general trends and behavior. The ODMR experiments on the undoped films reveal resonances from both effective-mass and deep-donor states. These two features appear in all of the undoped GaN films and are most likely associated with an intrinsic point defect or defects. The same two donor resonances and a Mg-related quasideep acceptor resonance are found from the ODMR experiments on the Mg-doped samples. The energy level of the deep-donor state is found from ODMR spectral studies to be \ensuremath{\sim}1 eV below the conduction-band minimum. The PL experiments provide evidence for shallow acceptors in the undoped films and in a GaN sample lightly doped by residual Mg in the growth reactor. Models are proposed that describe the capture and recombination among these shallow- and deep-donor and acceptor states. The assignments of the magnetic resonance features are compared with the latest theoretical calculations of defect states in GaN.

Native defects and dopants in gan studied through photoluminescence and optically detected magnetic resonance

Native defects and dopants in GaN grown by organometallic chemical vapor deposition have been studied with photoluminescence and optically detected magnetic resonance. For undoped samples, the combined results indicate the presence of residual shallow donors and acceptors and deep donors. A model for the capture and recombination among these defects is developed. For Mg-doped samples, the experiments reveal shallow and perturbed acceptors and shallow and deep donors. Hence, shallow and deep states for the native donor or donors appear in all samples. The Mg-acceptor is perturbed from its effective-mass state by nearby point defects.

Pressure dependence of deep centers in II–VI semiconductors: Theory

In contrast to shallow acceptors or donors levels, the binding energies of deep impurity levels are sensitive to pressure. Transitions between shallow and deep levels can often be induced by pressure and useful information on energy levels and structural metastabilities can be derived. It is not generally well known that pressure has an opposite effect on deep donor states than on deep acceptor states. In both III–VI and II–VI semiconductors, pressure increases the binding energy of deep donors with respect to the Γ lc conduction band and, at sufficiently high pressures, can make the effective-mass state unstable relative to a deep state. In contrast, the results of recent experiments by Weinstein and co-authors [ High-Pressure Science and Technology, Vol. 1, p. 141, Plenum, NY (1979); J. Crystal Growth 138, 318 (1994); Phys. Rev. B (submitted)] show that the binding energies of deep acceptor states in ZnSe decrease with pressure. In this paper we present the results of ab initio pseudopotential calculations on the pressure dependence of the shallow and deep levels of Al, Ga and In donor impurities in CdTe, and of As and P acceptor impurities in ZnSe. The experimental results are proposed to arise from an upward shift in the energies of both the valence and conduction band edges with pressure. Deep states have a large degree of atomic-like character and tend to have a smaller pressure dependence than the band edges. The upward shift of the band edges, therefore, increases the binding energies of deep donors and decreases those of deep acceptors.

Detection of Magnetic Resonance on Shallow Donor - Shallow Acceptor and Deep (2.2 eV) Recombination from GaN Films Grown on 6H-SiC

ABSTRACTPhotoluminescence (PL) and optically-detected magnetic resonance (ODMR) experiments have been performed on undoped GaN epitaxial layers grown on 6H-SiC substrates. The defects observed in these films are compared with those found from previous ODMR studies of undoped GaN layers grown on sapphire substrates. Strong, sharp donor-bound exciton bands at 3.46 -3.47 eV and weak, broad emission bands at 2.2 eV were observed from several 0.7 and 2.6 μm-thick films. In addition, fairly strong shallow donor - shallow acceptor (SD-SA) recombination with a zero-phonon-line at 3.27 eV was found for GaN layers less than 1 μm-thick. The first observation of magnetic resonance on this SD-SA recombination from undoped GaN is reported in this work. Two magnetic resonance features attributed to effective-mass (EM) and deep-donor (DD) states were detected on the 2.2 eV emission bands from all the GaN/6H-SiC films. These resonances were observed previously on similar emission from undoped GaN layers grown on sapphire substrates. The same EM donor resonance, though much weaker, was also found on the SD-SA recombination. However, a resonance associated with shallow acceptor states was not observed on this emission. The weakness of the donor resonance arises from the weak spin-dependence of the recombination mechanism involving spin-thermalized shallow acceptors. The absence of an EM acceptor is due to the broadening of the resonance through the spreading of the acceptor g-values by random strains in these films.

Magnetic freezeout effect in Al0.32Ga0.68As

The behaviour of the thermal ionization energy of the Gamma effective-mass state under a magnetic field has been studied near the Gamma -L-X crossover of AlGaAs. The results deviate significantly from the typical behaviour and it is shown that a better agreement can be obtained if screening effects and an anticrossing with a resonant state of a1 symmetry are considered.

Self-compensation in nitrogen-doped ZnSe

The structural and electronic properties of column V acceptor impurities in ZnSe are reviewed with special emphasis on N. The results of our calculations indicate that As and P acceptors possess two atomic configurations: a metastable effective-mass state with a small lattice relaxation and a stable deep state with a large lattice relaxation. Substitutional N gives rise to shallow acceptor states in either configuration. The low 20–30% doping efficiency is proposed to be caused by N bonding at interstitial sites. Various interstitial bonding configurations are found to give both shallow donor and shallow acceptor states leading to self-compensation. Post growth in diffusion of N is proposed to be useful in increasing the concentration of shallow acceptor states.

DX Centers in AlGaAs and Pressurised GaAs

ABSTRACTIn this paper, we present results of our investigations on some aspects of the DX centers. It is shown from the low temperature Hall mobility measurements that the charge state of the DX center is neutral supporting the positive U model for the DX center. Hall and DLTS measurements on Al-rich Al0.8Ga0.2As:Te sample show a reversal in the ordering of the energies of the hydrogenic and deep states of the Te donor, with the DX state lying higher than the X valley related effective mass state. Thermal emission properties of the pressure induced DX states of Ge and Se donors in neutron transmutation doped (NTD) GaAs are discussed.

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.

Investigation of Shallow States Related to Si-DX Centers in AlGaAs near the \\varGamma-X-L Crossover

Using pressure for tuning the band structure we have demonstrated that Si-DX centers in AlGaAs create shallow effective mass states linked to each of the minima (Γ ,L and X) of the conduction band. The presence of the state which we attribute to the L minimum is also confirmed by magnetic freezeout measurements, revealing an interaction (anticrossing effect) between the Γ and L-Like states.

Uniaxial stress dependence of the properties of the DX center in n-Al0.32Ga0.68As

We have measured the emission energy, Ee, and the capture energy, Ec, and derived the thermal activation energy of the DX center, EDX, in n-Al0.32Ga0.68As as a function of uniaxial stresses along the 〈100〉 and the 〈111〉 directions. We found that the stress coefficients of EDX change sign when the band gap of Al0.32Ga0.68As changes from direct to indirect for both stress directions, and the stress coefficients of EDX in the indirect region are about same for both stress directions. These results agree with the model proposed by Chadi and Chang which assumes that the DX center is a highly localized center and disagree with the model which takes the DX center to be the effective-mass state associated with the L minima.

Column V acceptors in ZnSe: Theory and experiment

First-principles pseudopotential calculations are used in conjunction with extensive experimental data on P and As-derived acceptor states in ZnSe to develop a microscopic theory of their atomic and electronic properties. A structural model that explains the presence of both shallow and deep acceptor states, the thermal and optical quenching of photoluminescence lines, and the strong C3v symmetry of the electron-spin-resonance (ESR) active state is derived. The primary result of the calculations is that a neutral acceptor possesses two atomic configurations: a metastable effective-mass state with a small lattice relaxation labeled a0, and a deep A0 state with a large lattice distortion which is responsible for most of the observed properties of acceptors in ZnSe. Nitrogen impurities are proposed to give rise to a shallow acceptor state in either the small or large-lattice-relaxed limits. Extension of the results to ZnTe is discussed.

Excited states of DX in Ga1-xAlxAs.

The photoexcited neutral states of the Sn donors in ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As alloys are studied both experimentally and theoretically. Two types of donor states are evidenced: a deep strongly localized one, ${\mathit{D}}^{0}$, as well as a delocalized effective-mass state ${\mathit{d}}^{0}$. Both belong to the principal donor ${\mathrm{Sn}}_{\mathrm{Ga}}$, as confirmed by EPR measurements on $^{119}\mathrm{enriched}$ samples. The ${\mathit{D}}^{0}$ state is consistently modeled as the ${\mathit{A}}_{1}$(ab) antibonding donor state. Theory predicts the coexistence of both states ${\mathit{D}}^{0}$,${\mathit{d}}^{0}$ in an intermediate alloy-composition range. This level scheme also explains the results of the Si and Te donor reported previously.

Magnetic resonance studies of group IV and VI donors in Ga1-xAlxAs

The magnetic resonance results for the group IV (Si, Sn) and group VI (S, Se, Te) donors in GaAlAs alloys are reviewed. For the donors Si, S, Se, Te the DX deep donor state has apparently not been observed; the paramagnetic state generated by photoexcitation has been attributed to the X conduction band derived effective-mass (EM) state. On the contrary recent results for Sn, where the central hyperfine interaction could be resolved, demonstrate clearly that for Sn the paramagnetic donor state is not EM-like; it is attributed to the neutral charge state of the DX centre. A comparison of the photoexcitation process and the associated persistent photoconductivity gives a strong indication for a negative-U DX ground state. It is shown that recent deep-level transient spectroscopy results further support this assignment. However, it is unclear why in some cases (Si, S, Se, Te) apparently only the EM states are observed and not the deep donor state, whereas in the case of Sn the situation is reversed.

Hall measurements under weak persistent photoexcitation in Si-doped AlxGa1-xAs

The low-temperature Hall mobility of photoexcited electrons has been measured in Si-doped MBE AlGaAs samples. Different fractions of occupied DX centres were obtained by selecting different free-electron densities: possible systematic errors in Hall measurements due to the method of photoexcitation are demonstrated and critically analysed. Using suitable values for the acceptor density and the alloy scattering potential a fair fitting of the experimental data was achieved within both negative-U and positive-U models for the DX centre. Discrepancies between calculated and experimental mobility versus temperature curves are observed, which are more evident the lower the free-electron densities. They are tentatively explained as being due to electrons in an impurity band originated by the shallow effective-mass state related to the Gamma minimum of the conduction band.

Electron paramagnetic resonance of the shallow Sn donor in GaAs/Al0.68Ga0.32As:Sn heterostructures

A new, light-induced electron paramagnetic resonance (EPR) signal with apparent tetragonal symmetry has been observed in n-type Al0.68Ga0.32As:Sn layers grown on Sl-GaAs. A comparison with previous magnetic resonance data for the shallow Si donor indicates that the signal corresponds to the shallow, X-valley-associated effective-mass state of the Sn donor. The optical behaviour of the new signal shows that the shallow state is a metastable state of the Sn DX centre. The nearly vanishing EPR intensity for magnetic fields in the layer is attributed to the large spin-valley interaction expected for the shallow Sn donor in indirect AlGaAs.

Occupancy of the DX center in n-Al0.32Ga0.68As under uniaxial stress

We have used the deep level transient spectroscopy signal height as a function of applied stress data and the statistics of the occupancy of the DX center to obtain the stress dependence of the thermal binding energy of the neutral DX center, EDX. We find that EDX decreases with about the same rate for uniaxial stresses along 〈100〉 and 〈111〉 directions. Our results confirm that the DX center is a highly localized center as proposed by Chadi and Chang and disagree with the model assuming the DX center being an effective mass state of the doping impurity associated with the L band.

Donor states in GaAs under hydrostatic pressure.

Spectroscopic studies have been carried out for GaAs crystals under hydrostatic pressure, intended for the investigation of effective-mass donor levels associated with different conduction-band minima and the DX center. Our results reveal the existence of three donor states appearing in the band gap. These are labeled ${\mathit{D}}_{\mathrm{\ensuremath{\Gamma}}}$, ${\mathit{D}}^{\mathrm{*}}$, and ${\mathit{D}}_{\mathit{X}}$. The donor state ${\mathit{D}}_{\mathrm{\ensuremath{\Gamma}}}$, normally observed at atmospheric pressure, successively crosses two other donor states (${\mathit{D}}^{\mathrm{*}}$ and ${\mathit{D}}_{\mathit{X}}$) as the hydrostatic pressure exceeds 28 kbar. ${\mathit{D}}^{\mathrm{*}}$, in turn, crosses ${\mathit{D}}_{\mathit{X}}$ at pressures of about 44 kbar. From their corresponding donor-acceptor pair luminescence, we know that ${\mathit{D}}_{\mathrm{\ensuremath{\Gamma}}}$ and ${\mathit{D}}_{\mathit{X}}$ have pressure dependences that track the \ensuremath{\Gamma}- and X-band minimum, and are believed to be effective-mass donor states associated with the \ensuremath{\Gamma}- and X-band edges, respectively. The third donor state ${\mathit{D}}^{\mathrm{*}}$ has a pressure dependence that does not seem to agree with any known conduction-band minimum. However, it is in many respects similar to the DX center, although it has not, in studies of radiative emission, revealed evidence of a large lattice relaxation and photoquenching effects.

Electron-paramagnetic-resonance study of the Te donor inGa0.70Al0.30As

We report an electron-paramagnetic-resonance (EPR) study of a group-VI donor in ${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Al}}_{\mathit{x}}$As (x=0.30). No EPR spectrum associated with the DX ground state could be detected. After photoexcitation with E\ensuremath{\ge}0.6 eV an EPR spectrum is observed, which is attributed to the ${\mathit{A}}_{1}$(1s) effective-mass state derived from the secondary ${\mathit{X}}_{1}$ conduction-band minimum. The occupation of this state results from hot-electron capture and is metastable below 40 K. The photoexcitation spectrum of this excited donor state is identical to the photoionization spectrum of the DX ground state previously determined by photocapacitance measurements. The asymmetrical line shape and the absence of the Te hyperfine doublets demonstrate modification of the isolated donor properties by donor-donor interaction.