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Site preference next to germanium atom of gold and platinum impurities in SiGe alloy
Abstract
The technique of Laplace transform deep level transient spectroscopy has been used to study the acceptor levels of platinum and gold diffused into dilute (0–5% Ge) SiGe alloys. The high-resolution spectra obtained display a fine structure that we interpret as the effect of alloy splitting in terms of the relative number of silicon and germanium atoms in the immediate proximity of the transition metal. We show that Ge atoms in the first and in the second shell of atoms surrounding the impurity perturb the electronic properties of the well-known Au and Pt acceptor defects. For both defects the spectral distributions indicate an overpopulation of Ge-perturbed sites as compared to randomly occupied sites. This can be quantitatively interpreted in terms of an enthalpy difference of ∼60meV between configurations with zero or one Ge in the first or second shell surrounding the impurity.
The high resolution Laplace deep level transient spectroscopy (LDLTS) was used to study the defects of silicon materials and devices. This technique was used to detect the defects in hydrogenated transition metals, implant damage, effects of the local environment on impurities in SiGe and the use of uniaxial stress with LDLTS to determine defect symmetry. With this technology, by carefully designed experimental system and the transient averaging, transients can be separated with more than an order of magnitude better resolution than DLTS.
Deep-level transient spectroscopy (DLTS) studies of 2-MeV proton-induced defect electronic levels in strained epitaxial Si1-xGex, 0≤x≤0.13, layers have been performed. It is found that the irradiation results in the formation of a dominant peak in the DLTS spectra for all compositions. Isochronal 20-min annealing studies of the observed deep level have revealed that the peak anneals out at 100-200 °C. This peak is identified as the vacancy-phosphorous (VP) pair. The compositional dependence of the activation enthalpy of the VP pair is nonlinear, with a sharp increase for x = 0.04 and little variation for higher Ge concentrations. After annealing of the VP pair, the dominating defect level is assigned to the single acceptor state of the divacancy [V2(-/0)]. The double-acceptor state [V2(=/-)] is observed to be strongly suppressed in the strained Si1-xGex layers. The compositional dependence of the activation enthalpy of V2(-/0) is relatively weak, with a small decrease in the activation enthalpy. It is found that formation of V2 in the strained Si1-xGex layers is enhanced with respect to that in Si. This is associated with an increased concentration of vacancies in the strained layer, acting as a sink for vacancies migrating in the substrate.
High resolution Laplace minority carrier transient spectroscopy (LMCTS)
has been used to study defects in gas source molecular beam epitaxy
grown Si/Si0.855Ge0.145 strained quantum wells. In
LMCTS, the minority carrier emission transient is analysed and detailed
emission properties of minority carrier traps which have similar energy
can be characterised. The technique was evaluated by comparing LMCTS of
a Au:H hole trap, G3, in n-type silicon with Laplace deep level
transient spectroscopy of the same trap in p-type silicon. Both
techniques confirm that this level consists of two states, as previously
suggested in the literature. MCTS was then applied to the n-type Si/SiGe
quantum well sample, and five-hole traps were observed. A level at 95K
in the MCTS spectrum was identified as a possible candidate for hole
emission from the quantum wells. LMCTS showed that the emission rate of
this hole trap exhibited only a slight temperature dependence compared
to that exhibited by hole states associated with isolated point defects.
This is attributed to holes tunnelling out of the quantum well assisted
by the electric field present in the experiment.
This paper is to commemorate the work of Leszek Dobaczewski 1 who devoted much of his life to the development and application of high resolution DLTS. Under good experimental conditions Laplace DLTS provides an order of magnitude higher energy resolution than conventional DLTS techniques. This has had a profound effect on electrical defect spectroscopy enabling the effect of external probes, such as uniaxial stress, and internal perturbations, such as the proximity of atoms isovalent with the host, to be quantified in terms of electronic behaviour. Laplace DLTS provides a synergy with other techniques that was difficult or impossible to achieve previously. In this paper we present an overview of the development of LDLTS and illustrate some of its uses by describing its application in a number of key areas of defect research. Leszek Dobaczewski was born on 25th December 1954. He received his education in Warsaw taking his PhD in 1986 with Jerzy Langer at the Institute of Physics on ''Recombination Processes at defects with the large lattice relaxation''. He held a research position at the institute in Warsaw until he came to Manchester in 1990 and thereafter alternated between Manchester and Warsaw. He worked primarily on the develop-ment and application of high resolution DLTS. He was awarded the degree of DSc in 1994 for his work on DX centres and held an appointment as full professor in Warsaw with Visiting Professor posts at Manchester and Aarhus. & 2011 Elsevier B.V. All rights reserved. Leszek Dobaczewski (1954 to 2010).
Deep-level transient spectroscopy (DLTS) studies of 2-MeV proton-induced defect electronic levels in strained epitaxial Si1-xGex, 0<~x<~0.13, layers have been performed. It is found that the irradiation results in the formation of a dominant peak in the DLTS spectra for all compositions. Isochronal 20-min annealing studies of the observed deep level have revealed that the peak anneals out at 100–200 °C. This peak is identified as the vacancy-phosphorous (VP) pair. The compositional dependence of the activation enthalpy of the VP pair is nonlinear, with a sharp increase for x=0.04 and little variation for higher Ge concentrations. After annealing of the VP pair, the dominating defect level is assigned to the single acceptor state of the divacancy [V2(-/0)]. The double-acceptor state [V2(=/-)] is observed to be strongly suppressed in the strained Si1-xGex layers. The compositional dependence of the activation enthalpy of V2(-/0) is relatively weak, with a small decrease in the activation enthalpy. It is found that formation of V2 in the strained Si1-xGex layers is enhanced with respect to that in Si. This is associated with an increased concentration of vacancies in the strained layer, acting as a sink for vacancies migrating in the substrate.
We have developed a high resolution technique for examining minority carrier emission from defect states in semiconductors called Laplace minority carrier transient spectroscopy (LMCTS). The experiment uses the same analytical approach to the capacitance transient as Laplace deep level transient spectroscopy (LDLTS), but minority carriers are injected into the depletion region by application of a suitable light pulse. The combination of LDLTS and LMCTS means that detailed emission properties of closely spaced majority and minority carrier traps across the whole band gap can be now characterized. The technique has been used to study minority carrier traps in gas source molecular beam epitaxy-grown Si/Si0.86Ge0.14 strained quantum wells. Initially the technique was evaluated by comparing LMCTS of a hole trap associated with the gold–hydrogen complex in n-type silicon with LDLTS of the same trap in p-type silicon. Both techniques confirm that this level consists of two states, as previously suggested in the literature. LMCTS was then applied to an n-type multiquantum well Si/SiGe layer. We have been able to measure directly the emission rate of holes out of SiGe quantum wells using LMCTS. The emission rate exhibited only slight temperature dependence, in strong contrast to that of holes which are thermally emitted from isolated point defects. We show that in the particular case of LMCTS, a temperature invariant emission rate out of quantum wells is to be expected, and this is consistent with theoretical predictions. © 2002 American Institute of Physics.
The quality of molecular-beam epitaxy (MBE) grown SiGe alloys layers is discussed from a defect-study point of view. It is shown that strain-free Si1-xGex alloy layers of x≤0.50 can be grown of a quality sufficient for many types of defect studies. Two examples of the use of MBE grown SiGe layers for defect studies are discussed, namely the diffusion of Sb and the pinning behaviour of the Sb-vacancy pair.
Many physical phenomena are described by first-order differential equations whose solution is an exponential decay. Determining the time constants and amplitudes of exponential decays from the experimental data is a common task in semiconductor physics (deep level transient spectroscopy), biophysics (fluorescence decay analysis), nuclear physics and chemistry (radioactive decays, nuclear magnetic resonance), chemistry and electrochemistry (reaction kinetics) and medical imaging. This review article discusses the fundamental mathematical limitations of exponential analysis, outlines the critical aspects of acquisition of exponential transients for subsequent analysis, and gives a comprehensive overview of numerical algorithms used in exponential analysis. In the first part of the article the resolution of exponential analysis as a function of noise in input decays is discussed. It is shown that two exponential decays can be resolved in a transient only if the ratio of their time constants is greater than the resolution limit, which can be explicitly calculated from the signal-to-noise ratio in the transient. Although the signal-to-noise ratio is generally limited by the sensitivity of the equipment, it is shown that digitalization of the decays may be a major source of noise. The requirements for type of analog-to-digital converter, number of digitized data points and duration of digitized transients, which must be met to obtain the theoretical resolution limit and to improve stability of the exponential analysis, are formulated. The second part of the review article gives an overview and comparison of major numerical techniques of exponential analysis, such as the nonlinear least squares fit, the Prony method, the method of modulating functions, the method of moments, the Laplace–Padé approximation, the Tikhonov regularization method, the Gardner transformation, the method of maximum entropy and others. © 1999 American Institute of Physics.
A quantitative improvement in deep‐level transient spectroscopy (DLTS) resolution has been demonstrated by using Laplace transform method for the emission rate analysis. Numerous tests performed on the software used for the calculations as well as on the experimental setup clearly demonstrated that in this way the resolution of the method can be increased by more than an order of magnitude. Considerable confidence in this approach was gained through measurements of a selection of well‐characterized point defects in various semiconductors. The results for platinum in silicon and EL2 in GaAs are presented. For each of these cases conventional DLTS give broad featureless lines, while Laplace DLTS reveals a fine structure in the emission process producing the spectra.
High‐resolution Laplace‐transform deep level transient spectroscopy has been used to study the influence of the defect local environment on electron emission from the DX centers related to group‐IV (silicon) donors in Al x Ga 1-x As (0.20≪x≪0.76) and δ‐doped GaAs and group‐VI (tellurium) donor elements in Al x Ga 1-x As (0.25≪x≪0.73) and GaAs 0.35 P 0.65 . The experimental evidence that substitutional–interstitial atom motion is responsible for DX behavior and for the associated metastability effects is presented. The atom which is subjected to this transition is for DX(Si) silicon itself, as in the spectra only one group of peaks in Al x Ga 1-x As is observed, while for DX(Te) it can be either gallium or aluminum, producing two groups of peaks in Al x Ga 1-x As and three or four broad emission bands in GaAs 0.35 P 0.65 . The present results rule out a possibility that the DX‐type defect states are formed by a donor atom in a stable substitutional position with a small lattice relaxation or with a fully symmetric large lattice relaxation effect. © 1995 American Institute of Physics.
A high-resolution Laplace-transform deep-level-transient spectroscopy has been used to study electron emission from the DX centers related to group-IV (silicon) and group-VI (tellurium) donor elements in AlxGa1-xAs (0.25 < x < 0.76). This provides experimental evidence that substitutional-interstitial atom motion is responsible for DX behavior and for the associated metastability effects. The atom which is subjected to this transition is, for DX(Si), silicon itself, and so only one group of peaks is observed in the spectra; while for DX(Te), it can be either gallium or aluminum, producing two groups of peaks.
It is demonstrated that relaxed Si1−xGex of 0≤x≤0.25 grown by molecular beam epitaxy, using the compositional grading technique, can be doped with gold by a high‐temperature indiffusion process from a deposited gold layer. Substitutional gold in both p‐ and n‐type alloy layers has been identified and characterized by deep level transient spectroscopy, and the donor and acceptor levels of neutral, substitutional gold in the SiGe alloys were studied as a function of the Ge content. The data suggest that both the donor and acceptor levels are pinned to the conduction band. © 1996 American Institute of Physics.
The use of compositionally graded buffer layers in the growth of fully relaxed epitaxial Si1−xGex alloy layers has led to a major improvement in crystalline quality. A considerable reduction in the density of the threading dislocations has become possible, facilitating point defect studies in these materials. The issues addressed in this review are inherent to the coupling between band gap engineering and defect-related levels. Among them, the pinning behaviour, charge state effects and their consequence upon the thermal stability of point defects are discussed together with the impact of the fluctuation in Ge distribution
Evaluation of data obtained from deep level transient spectroscopy (DLTS) is often based on the assumption that the transients are exponential. The applicability of DLTS to the study of deep energy levels in semiconductor alloys has therefore been questioned since thermal transients are often nonexponential in these materials. In this paper we present calculated DLTS spectra in a simple model for broadened defect levels. The calculated spectra are compared with experimental data for a deep electron trap in GaAs 1-x P x . The main result is that, within the model, DLTS‐deduced activation energies and thermal emission rates are, indeed, relevant even when the transients are strongly nonexponential as a result of alloy broadening. A method of estimating the corrected concentration of deep levels and the distribution in binding energies is also presented.
Si‐doped GaAs and dilute Al x Ga 1-x As alloys under hydrostatic pressure have been studied using deep level transient spectroscopy (DLTS). In GaAs the DLTS spectrum of the DX center is a single peak. In AlGaAs however, multiple peaks, resulting from different thermal emission rates from donors having different numbers of Al atoms as near neighbors, are observed. The pressure dependence of the electron occupation of individual DX levels shows that the larger the number of Al atoms near the Si donor, the lower the energy position of the DX level.
Using EPR we have resolved the question of whether the dominant Pt- defect in silicon consists of an isolated platinum ion or a platinum-platinum pair. We have measured the uniaxial-stress-induced shifts in the g values and find that the stress-coupling tensor shows the defect symmetry to be C2v. By selective doping with 195Pt and 198Pt, we demonstrate that there is no hyperfine evidence of a second platinum. We therefore conclude that the original model of Woodbury and Ludwig of an isolated substitutional platinum impurity that spontaneously distorts off center in a 〈100〉 direction is correct. We have also measured the defect realignment under stress, which occurs even at pumped-liquid-helium temperatures. We find that the sense of the alignment for this platinum defect is opposite to that for the negative vacancy. In addition, we have fully resolved the superhyperfine interaction involving the two nearest-neighbor silicon atoms. Lastly, we observe a shift in the g values depending upon the nuclear isotopes of these two nearest-neighbor silicon atoms.
Detailed information on the electronic structure of the neutral substitutional gold center in silicon (Au0) has been revealed from Zeeman studies of the donor and acceptor excitation spectra at 793 and 611 meV, respectively. The center is paramagnetic, S=1/2, with g||~=2.8 and g⊥~=0, and has a static tetragonal distortion. Reorientation between different equivalent distortions is observed even at 1.9 K which establishes the single substitutional nature for this gold center. The fact that g⊥~=0 explains the failure to detect Au0 by EPR.