Small Lattice Relaxation Research Articles

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.

Charge redistribution in the multiple levels of the DX center

The multiple DX levels are studied by using the rate-window scan method of deep level transient spectroscopy. It is found that the electron concentrations in the individual DX levels do not always increase monotonically with filling time, showing a charge redistribution effect. This charge redistribution in the DX levels indicates that the DX center is negatively charged and that each defect site is able to generate multiple states in different levels. The charge redistribution effect unambiguously rules out all existing small lattice relaxation models, while it is understood within the broken-bond negative U model.

Influence of electric field on electron emission from DX(Te) centres in Al0.55Ga0.45As

The authors show that the electric field strongly influences the electron emission from the effective-mass, metastable, excited states of DX centres in Al0.55Ga0.45As, while up to the field of 4.5*105 V cm-1 no influence on the emission from the group, localized state of DX centres has been detected. Such behaviour is inconsistent with the small lattice relaxation models of the DX centre and can be understood if the DX centres are bistable donors and/or of the negative-U type.

Li impurity in ZnSe: Electronic structure and the stability of the acceptor.

We have performed density-functional calculations of a Li impurity in ZnSe using the dynamical simulated-annealing scheme with the supercell method. We find that Li at a substitutional Zn site has a higher free energy than the sum of those of a Zn vacancy and interstitial Li when the chemical potential is located below 0.8 eV measured from the valence-band maximum. The calculated result shows that, in the case of the substitutional Li impurity, there occurs an inward breathing relaxation and a Jahn-Teller distortion around the impurity site. It is also found that the interstitial Li acts as a shallow donor with small lattice relaxation (0.1 \AA{}).

Confirmation of large lattice relaxation of theDX center by extended photo-ionization cross-section measurements

Although the large lattice relaxation model (LLR) for electron capture at the donor relatedDX center in AlxGa1-xAs has seen wide acceptance over the last 12 years, there have been some recent proposals which have attempted to explain the experimental data with models that only require small lattice relaxation (SLR). One key piece of evidence supporting LLR is the large observed difference (in the case of Si-doped AlxGa1-xAs) between the optical (∼1.4 eV) and thermal (∼0.2 eV) ionization energies. The SLR model proposed that the lowest energy optical ionization was a very weak process, and that the optical transition which had been observed previously is a transition to a higher band. These arguments were supported by photoconductivity data showing a finite photo-ionization rate at energies as low as 200 meV. To resolve this question we have measured the photo-ionization cross section over 7 to 8 orders of magnitude using a tunable infrared laser as a source. A consistent optical ionization energy of about 1.4 eV was observed for 4 samples of differing alloy compositions and doping levels.In no case was there any detectable photo- ionization below 0.8 eV. A detailed discussion of these experiments examines the difficulty in obtaining such a large dynamic range optical spectrum. Of particular relevance are the issues of ionization detection, and the brightness of purity of the optical source. A thorough review of these issues and their impact on previous studies of theDX center is presented.

Magneto-optical spectrum of donors in AlxGa1-xAs and its implications on the DX center.

The 1s-2p intracenter optical transitions of shallow donors in n-type ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As (x=0.18 and 0.24) were investigated by far-infrared photoconductivity in magnetic fields of up to 10 T. The spectra could be well fitted by the hydrogenic effective-mass model, and were analyzed in terms of the effective mass ${\mathit{m}}^{\mathrm{*}}$ and the linewidth. The result indicates (1) the inadequacy of the usual linear interpolation scheme for ${\mathit{m}}^{\mathrm{*}}$(x) and (2) the importance of alloy disorder in line broadening. No evidence was seen for the interaction of the shallow-donor ground state with the DX center, which is inconsistent with the model based on the DX center being an ${\mathit{A}}_{1}$ state with small lattice relaxation.

Mid-gap level in SiGe alloys

Junction space charge measurements were used to study a mid-gap level in chemical vapour deposition (CVDS) grown Si1-xGex alloys of three different compositions. Evidence is given for the thermal and optical capacitance transients being fairly exponential. Absolute values of emission rates and capture cross sections for electrons, as well as enthalpies and entropies, are presented. The results suggest that this defect exhibits only a small lattice relaxation effect.

A deep level in Zn-doped InGaAlP

A deep level in Zn-doped In0.5(Ga1−xAlx)0.5P (x=0–1.0) has been studied by using pulsed-bias transient capacitance measurements and photocapacitance measurements. One major deep level was found for the entire alloy composition range (x=0–1.0). The deep-level concentration was proportional to Zn concentration for x=0.4. The ratio of the deep-level concentration to Zn concentration increased slightly with x. The energy depth from the valence-band maximum increased with x, from 0.04 eV at x=0 to 0.42 eV at x=1.0. The energy position, referred to as the vacuum state, is essentially invariable for 0<x<1. This level was considered to have a small lattice relaxation. It seemed that this level originates from Zn-related defects.

Luminescence of the D X center in AlGaAs

Low-temperature photoluminescence of the DX center in the near band edge and in the near-infrared region is interpreted within the small lattice relaxation model. The 1.5 μm luminescence band is attributed to an internal transition between the excited DX state and its ground-state level.

Electric Field Enhancement of Electron Emission from DX Centers and Consequences

AbstractWe present data which show that electron emission from the DX center is sensitive to the Poole-Frenkel effect. We demonstrate that this result implies that the DX center is an L effective-mass state of the donor impurity accompanied by a small lattice relaxation. We show that all the observations so far obtained on this center are in agreement with this model.

The Metastability of the EL2 and DX Defects in GaAs and 3-5 Alloys

AbstractThe metastability of the EL2 and DX defects in GaAs and Ga1-x AlxAs alloys is discussed in the context of recent hydrostatic pressure and photo-EPR results. A unified model is presented based on the metastable trapping of carriers in excited effective-mass (EM) states derived from secondary conduction band (CB) minima. The metastability is attributed to small lattice relaxation effects.

Photoionization threshold of the deep donor in Si-doped AlxGa1-xAs.

In order to clear up the controversy about the magnitude of the lattice relaxation of the deep donor in $n$-type ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$, samples were exchanged between Philips and IBM. The present photocapacity study on an IBM sample definitely shows the existence of two photoionization processes: one with a threshold between 200 and 300 meV, representing an effective-mass-type deep donor with small lattice relaxation, and another one with a threshold near 700 meV, representing the so-called $\mathrm{DX}$ center with large lattice relaxation. These results suggest either two distinct donor configurations or a bistable character of the deep donor.

Comment on ‘‘New evidence of small lattice relaxation for the D X center in AlxGa1−xAs’’ [Appl. Phys. Lett. 5 1, 1358 (1987)

Deep level transient spectroscopy has been performed on the DX center in the Ga0.65Al0.35As:Te as a function of pressure. The lattice relaxation we observed showed that the results of Talwar et al. (small lattice relaxation) are inconsistent and inconclusive. Our results significantly weaken their evidence in favor of small lattice relaxation. (AIP)

Small lattice relaxation at the D X center as studied by extended x-ray absorption fine structure on Se-doped AlGaAs

Lattice relaxation at the Se DX center in Al0.38Ga0.62As was determined by the extended x-ray absorption fine structure (EXAFS). During the EXAFS measurements the deep and shallow (metastable) states of the DX center were prepared and the resultant difference of the nearest-neighbor distance around Se between the two electronic states was found to be quite small (less than 0.04 Å). The result is indicative of small lattice relaxation at the DX center.

New evidence of small lattice relaxation for the D X center in AlxGa1−xAs

Local structure of isolated Si impurity in GaAs and AlAs is studied using a parameter-free semi-empirical tight binding method. It is predicted that nearest neighbor As atom around the impurity moves toward SiGa(Al) causing a 6.54% (5.73%) change in GaAs(AlAs) bond length. An estimation of lattice distortion energy 0.02±0.003 eV (0.025±0.003 eV) for GaAs:Si (AlAs:Si) is found in good qualitative agreement with the value obtained by J. C. M. Henning and J. P. M. Ansems [Semicond. Sci. Technol. 2, 1 (1987)] from the photoionization of the DX center in lightly doped Si impurities in Al0.33Ga0.67As. The fits for the observed maxima in the optical cross section and the calculation of the pressure-dependent thermal barrier energy lend support for the small lattice relaxation models and cast doubt on the validity of those with the large lattice relaxation usually recommended for this class of centers.

A new proposed method for determining inner or outer crossing lattice relaxation of DX centers in Al xGa l−xAs based on pressure effects

It is proposed that hydrostatic pressure measurements can be used to settle the recent controversies surrounding the model of the DX center in Al xGa l−xAs alloy. The method can in general distinguish the two possible cases of lattice relaxations in defect configuration coordinate diagrams: one with large lattice relaxation which we labelled as “outer crossing,” and the other with small lattice relaxation labelled as “inner crossing”.

Hot-carrier effects in non-radiative multiphonon capture by deep traps in semiconductors

For any lattice temperature T<or approximately=103K and different charge states Z of the centre the authors describe the dependences of non-radiative multiphonon carrier capture coefficients CZ(T; E) on the carrier energy E by products of the familiar Sommerfeld factors sZ(E) with corresponding energy-loss factors C0(T; E). As the carrier energy E is increased these 'neutral parts' C0(T; E) of capture coefficients decrease or increase (exponentially) in the alternative regimes of small and large lattice relaxation. Their maximum enhancements Cmax0/C0(0;0), which can be produced in the latter regime, at low lattice temperatures, are shown to be approximately exp(S(1+ gamma ln gamma - gamma )) (approaching eS in the gamma to 0 limit) where S represents the Huang-Rhys factor and gamma the ratio between thermal depth and lattice relaxation energy. For values of S of the order of 10 this 'enhanced hot-carrier capture due to large lattice relaxation' effect can thus amount to several orders of magnitude and should hence be of considerable importance for interpretations of corresponding hot-carrier capture data.

Nonradiative multiphonon capture of thermal and hot carriers by deep traps in semiconductors for the alternative regimes of small and large lattice relaxation

Nonradiative multiphonon capture of thermal and hot carriers by deep traps in semiconductors for the alternative regimes of small and large lattice relaxation

Electron temperature dependences of nonradiative multiphonon hot-electron capture coefficients of deep traps in semiconductors—I: Small lattice relaxation

Generalizing the theory of nonradiative multiphonon capture of thermal electrons for cases where the effective temperature T e of conduction electrons differs from lattice temperature T L , we develop a corresponding theory of hot electron capture. Its principal novelty is due to an exponential decrease ∝ exp(− E/ k B τ) of the efficiency of this energy loss mechanism, at increasing electron energy E, for the usual regime of small lattice relaxation characterized by a corresponding “capture extinction temperature” τ. The resulting T e -dependences of hot electron capture coefficients at sufficiently low T e ( ▪ τ, i.e. in the “Sommerfeld factor regime”) are controlled by the net charge of the centre which means C( T e ) ∝ C(T e) ∝ T e − 1 2 , ∝ T e 0, or ∝ T e 2 3 exp [−3·(θ/T e) 1 3 ] in cases of attraction, neutrality, and repulsion, respectively. At high T e ▪ τ, i.e. in the “energy-loss factor regime”) these relations reduce to the form C( T e ) ∝ T e − 3 2 which is simply due to an increasing depopulation of the relevant region of low-lying band states. This theory is applied to electron capture by a singly charged repulsive gold centre and a doubly charged repulsive copper centre in germanium. The corresponding theoretical maximum values C max. (−1) = 5.2 · 10 −11 cm 3 s −1 and ifC ( rnmax.) (−2) = 2.4 · 10 −12 cm 3 s −1 of hot electron capture coefficients are in good agreement with experimen observations.

Deep level transient spectroscopy of electron traps and sensitizing centers in undoped CdS single crystals

Data are presented on the characterization of deep electron traps in six different undoped single crystals of CdS and in one alloy of CdS96Se4. The obtained spectra by the capacitance deep level transient spectroscopy (DLTS) of the seven samples are very similar with only two or three distinct maxima observed between liquid nitrogen and room temperature. Detailed analysis of the DLTS and admittance spectroscopy reveals that three deep levels:EL1 (200±10 meV, Se ≃10−14 cm−2), EL2 (430±30 meV, Se≃10−12 cm2), and EL4 (630±30 meV, Se ≃10−14 cm2), are present in almost all samples. Direct measurements of the capture cross sections indicate that the capture of an electron occurs with a small lattice relaxation in the case of EL1 and a large lattice relaxation in the case of EL2. Two other trap levels, EL3 (520±30 meV, Se ≃10−11 cm2) and EL5 (750±40 meV, Se ≃10−11 cm2), are also detected in two respective samples. Comparisons are made with previous data obtained in semi-insulating and semiconducting CdS crystals. Illumination of the sample at low temperatures induces a permanent capacitance change due to electron excitation from a very deep level which cannot be electrically refilled at liquid-nitrogen temperature. The energy threshold E≃1.4 eV of the spectral response of this emptying process is an indication that this level is probably the same as the sensitizing center. Direct measurements of the temperature dependence of its capture cross section, Sc =4×10−23 exp (−0.042/kT) cm2 is direct of the doubly ionized acceptor -like imperfection, probably associated to a cadmium vacancy VCd.