Table of contents

It is now widely accepted that DX levels in n-type Al_xGa_1-xAs are ground states of isolated substitutional donors in distorted configurations which are stabilized by trapping two electrons. The leading model is that the distortion occurs when the group IV donor substituted on a group III lattice site, moves along a <111> axis towards an interstitial site. In the case of a group VI donor which substitutes on a group V lattice site, one of the group III neighbours moves toward the interstitial site. Recent work suggests that localized excited states of both donor configurations play an important role in electron transitions between DX levels and the conduction band. The research leading to this view of substitutional donors is reviewed. Some implications of this model for technological applications of Al_xGa_1-xAs and related alloys are discussed.

Three structural calculations of the DX centre presented at Thessaloniki are of especial interest. Although they were carried out using nominally the same calculational apparatus, namely first-principles pseudopotentials, local density approximation, large unit cells etc, the three reached vastly different conclusions about the nature of the DX centre. In this paper the author studies these calculations in terms of their internal consistency and find that only one is essentially error-free in the picture it presents. The author's analysis suggests that the size and possibly the sign of the U of the DX centre may well depend on alloy concentration or pressure. The negative-U property may therefore not be as universal as heretofore assumed.

The available theoretical calculations concerning deep donors in GaAlAs compounds are discussed. The duality of the deep and shallow behaviours is analysed and shown to result in an inverted A₁-T₂ splitting for the L-derived states. This is applied to the interpretation of the stress splitting of the optical absorption of EL2. The electron-lattice interaction is then considered, with a comparison of the different possible ground state configurations for DX. Finally the information provided by recent magnetic resonance data is taken into consideration and the conclusions concerning the possible level scheme are drawn.

A new model for DX centres is proposed and shown to be the only model which agrees with DLTS, metastability and ballistic phonon scattering experiments. In this model an As antisite complex, formed from a substitutional donor by the exchange of two adjacent atoms, binds two electrons. Furthermore, if these electrons are assumed to occupy a triplet spin state, the model can explain the paramagnetic susceptibility reported for filled DX centres. The stability of such a configuration could be due to the unusual properties of the antisite complex. Extension of this analysis to the related EL2 defect in GaAs suggests that the normal EL2 state may be ³T₂(A₂) and the metastability of the excited state due, in part, to spin conservation.

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.

Due to the vanishing of both electron emission and capture rates, the DX centre shows a non-equilibrium occupancy at low temperature. It is shown that this effect results in a non-uniform free-electron density profile that can be controlled by biasing the junction during cooling. Preliminary results are reported showing that the I-V characteristics of Schottky barriers on AlGaAs (x=0.25) are affected by the DX centre occupation.

Measurements of the Hall coefficient and the conductivity in n-GaAs heavily doped with silicon were performed at 4.2 K under hydrostatic pressure applied at an elevated temperature. The procedure ensures that a metastable occupation of the DX centres by electrons is induced. As shown earlier, electric charges on DX centres form, under these conditions, a spatially correlated system of charges. The correlation marks its presence as an enhancement of the electron mobility. Here the authors study the destruction of the spatial correlation of donor charges by illumination with an LED. A calculation within the short-range correlation model accounts well for the decrease of the mobility seen after illumination.

DX centres, as well as other charged impurities with random location in the host crystal, give rise to fluctuations in the local potential. These fluctuations are comparable in amplitude to the level splittings of the DX centres due to different numbers of Al neighbours in Al_xGa_1-xAs:Si and also to the variation in the quasi-Fermi level in experiments involving the kinetics of electron capture and emission processes. Thus, to describe the situation in Al_xGa_1-xAs:Si in a quantitative way the authors develop a self-consistent model of the fluctuations. The resulting broadening of the DX level is taken into account in evaluating transport experiments. The proposed model yields the ground state energies and the barrier height for the four types of Si DX centre as functions of alloy composition and hydrostatic pressure. They explain also the non-exponential behaviour of the capture kinetics as well as the mobility in terms of the impurity level broadening and self-screening which results from a minimization of the Coulomb energy of interacting charges localized on donors.

For heavily doped samples of Al_xGa_1-xAs:Si (x equivalent to 0.3) the persistent photoconductivity is limited by the finite height of the capture barrier. This effect allows the determination of the capture barrier. The result is compatible with literature data for the emission barrier only for a negative intra-atomic Hubbard correlation energy. Modelling shows that the thermal emission from the Si DX centre is a two-electron process.

In this paper the consequences of an idea in which DX centres in semiconductors form a negative-U system for the approach to commonly used DLTS and photoionization measurements are discussed. For the negative-U model of DX centres to be valid the carrier exchange between the DX states and the conduction band must occur via an intermediate one-electron D⁰ state. In such a system the neutral D⁰ state must be thermodynamically unstable, but obviously should play a role in all carrier capture and emission processes (D^- from or to D⁰+e^- from or to D⁺+2e^-). The experimental evidence for the existence of such an intermediate state is presented. It is based upon the detailed observations of the temperature evolution of the photoionization transients of the DX centres in Al_xGa_1-xAS:Te as well as the capacitance transients of the thermal emission process from DX centres in GaSb:S. Rate equations for photoionization and isothermal DLTS experiments on a defect forming a negative-U system are presented.

The authors present new experimental data on the capture of electrons onto Si-induced impurity states, which give evidence of the existence of several configurations for the DX centre with a large lattice relaxation. The investigated samples were Si-doped GaAlAs alloys and GaAs/AlAs short-period superlattices, grown by molecular beam epitaxy, with different aluminium contents. Persistent photoconductivity in the samples has been investigated by means of Hall measurements under pressures up to 15 kbar, between 4.2 and 400 K. In GaAlAs alloys several steps have been observed on the thermostimulated capture curves between 90 and 140 K. The mechanism responsible for the presence of these steps is not only related to capture processes, but also results from the equilibrium between capture and emission processes, involving the different states of the DX centre. Their data confirms that the DX centre presents several configurations, each of them with discrete emission and capture barriers, related to the local environment of the Si atom. For the short-period superlattices selectively doped in GaAs layers or in AlAs layers, the thermal annealing curves decrease monotonically, as expected for a single barrier between 90 and 140 K. In uniformly doped GaAs/AlAs superlattices the n(T) curve shows a two-step behaviour, indicating that two barriers are involved in the capture process. These results confirm the existence of several configurations for the DX centres in short-period superlattices. The selective doping of short-period superlattices in AlAs layers made it possible to study the all-Al environment of the Si atom, unobserved until now.

Direct investigations of the relaxation of the persistent photoconductivity (PPC) in Al_0.29Ga_0.71As:Te have been performed with the use of hydrostatic pressure P. The rate of free-electron relaxation, dn_e/dt, exhibits a dramatic decrease while the Fermi energy of the electrons, E_F, lowers at constant temperature T. In a wide range of E_F the dependence In(dn_e/dt) appears to be linear in E_F with the slope about 3 kT^-1. The height of the capture barrier E_B has been found to increase linearly with reduction of E_F, the - Delta E_B/ Delta E_F ratio being 3.2 at P=1 bar and 2.5 at P=4 kbar, i.e. larger than expected for both the neutral and negatively charged DX centre models.

The use of an X-ray diffractometer is proposed for examining the lattice relaxation effects accompanying the transfer of DX centres and EL2 defects between their stable and metastable states. The comparison of experimental results for GaAlAs:Te (with DX centres), semi-insulating GaAs and low-temperature GaAs (with EL2 defects) is given.

The bistable character of Sn donors Al_xGa_1-xAs for x>0.2 or in GaAs under pressure >2.4 GPa has been studied by ¹¹⁹Sn Mossbauer spectroscopy (MS). The shallow Sn donor state and the deep Sn DX state are observed to exist simultaneously and are readily distinguishable due to significantly different electronic configurations as manifested in their isomer shifts. An upper limit on the amount of non-cubic local lattice distortion at the Sn DX centre has been obtained through establishment of an upper limit on the local electric field gradient as determined from the quadrupole interaction. The MS data from GaAs under pressure, coupled with magnetotransport data, provide strong evidence that the Sn DX centre localizes at least two electrons in its ground state.

The authors report the observation of a new local vibrational mode (LVM) in hydrostatically stressed Si-doped GaAs. The corresponding infrared absorption peak is distinct from the Si_Ga shallow-donor LVM peak, which is the only other LVM peak observed in their samples, and is assigned to the Si DX centre. Analysis of the relative intensities of the Si DX LVM and the Si shallow-donor LVM peaks has been combined with Hall effect and resistivity analysis to infer that the Si DX centre is negatively charged.

A new, light-induced electron paramagnetic resonance (EPR) signal with apparent tetragonal symmetry has been observed in n-type Al_0.68Ga_0.32As: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.

In a thick (100 mu m) Sn-doped Al_0.35Ga_0.65As layer and an 11 mu m Si-doped Al_0.41Ga_0.59As layer the magnetic circular dichroism (MCDA) of the optical absorption, the optically detected electron spin resonance (ODEPR) and the photoconductivity (only in the Sn layer) have been measured simultaneously. All signals appear together after the photoionization of the DX centre and are directly correlated to each other. The ODEPR spectra measured in the MCDA bands show the hyperfine doublets due to the magnetic ¹¹⁷Sn and ¹¹⁹Sn isotopes (I=1/2) in Sn-doped samples and an ODEPR line (I=0) in Si-doped layers. The 4.7% abundant ²⁹Si (I=1/2) isotopes could not be observed. The authors show that the MCDA is due to a neutral (Si⁰, Sn⁰) donor, a deep level which is stable at 1.6 K in the dark. This DX⁰ state needs thermal energy or light to capture an electron to form the negative-U DX^- ground state which contains one donor dopant in a large lattice configuration.

Magneto-optical absorption spectra of a 0.4 mm thick, single-crystal Al_0.35Ga_0.65As:Te sample give evidence for two bleachable absorbers, one of which is identified as the DX centre. The bleached-state absorption coefficient and magnetic circular dichroism (MCD), measured from 0.66 to 2.2 mu m at 1.7 K, are adequately described by the Drude free-electron model, in terms of which a value for the electron effective mass is obtained. Cooling the sample in darkness leads to transmission transients, from which the absorption coefficient and optical-conversion cross section for the bleachable deep DX ground state are derived. The MCD at the beginning of each transient is identified also with the DX ground state, and its temperature dependence reveals that the bulk of it has a non-paramagnetic origin. The authors conclude that the paramagnetic contribution to the MCD from the DX ground state is very small, being less than 0.004% of its peak absorption coefficient. This provides strong support to the diamagnetic ground-state, negative-U model of Chadi and Chang (1988, 1989). The origin of the second bleachable absorber (threshold approximately 1.6 eV) has not been established.

Optically detected magnetic resonance experiments with uniaxial stress along the (110) and (100) directions have been performed on Si-doped epitaxial layers of AlAs and Al_0.4Ga_0.6As grown on (001) GaAs substrates. These studies enable the symmetry of the donor wavefunction to be probed in detail. The results obtained with T mod mod (100) confirm the independent valley description for Si donor states in AlAs. The lack of a response to uniaxial stress along (110) provides evidence against the contribution of donor states derived from the L-point conduction band minima and against a purely spin-valley coupled state at x=0.4.

Photoluminescence and optically detected magnetic resonance (ODMR) have been studied for three Sn-doped Al_xGa_1-xAs samples with mole fractions from 0.45 to 1.0. The near-band-edge photoluminescence is weak and broad but strong deep bands occur for each sample. The ODMR is dominated by broad, luminescence-enhancing signals with g=2.00. The results are quite different from those for Si or other dopants. These experiments reveal that Sn is producing an optically active deep level in these samples.

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.

A review is given of defects that display metastability in silicon, with emphasis on those that have been identified and the various mechanisms that they reveal for the phenomenon. Pair defects described include interstitial-iron-substitutional-group-III-acceptors and ones formed by interstitial carbon with substitutional group V donors or substitutional carbon. Interstitial hydrogen, boron and silicon and substitutional nitrogen and oxygen are taken as examples of isolated single-atom defects that display on-centre to off-centre instabilities. It is argued that this single-atom instability can be understood in terms of a predictable Jahn-Teller effect and that this concept may provide useful insight into the DX and EL2 phenomena in the III-V materials and their alloys.

Recent infrared absorption studies of semi-insulating GaAs have revealed an electrically active, oxygen-related defect with remarkable spectroscopic and microscopic properties. This defect, the structural analogue of the oxygen vacancy centre in Si, occurs in three charge states, the zero-, one- and two-electron states. The experimental fingerprint for each charge state is the local mode frequency which shows a characteristic charge-state-induced shift. Dependent on the Fermi potential, at thermal equilibrium only the local modes corresponding to the zero- or the two-electron state are experimentally observable. The metastable one-electron state disproportionates spontaneously into the zero- and the two-electron states. The energy positions of the associated gap levels are at E_c-0.14 eV and at E_c-0.58 eV for the first and the second electron, respectively. These assignments are derived from the thermally activated decay of the local mode lines and, independently, from the threshold energies of the optical transitions sigma _p⁰(1) and sigma _n⁰(2). Through a comparative deep-level transient spectroscopy and infrared absorption study on neutron-transmutation-doped, n-type samples the second electron level is identified as the well known EL3 level.

A metastable defect in silicon is discussed. The defect appears after irradiation with electrons at room temperature with 2 MeV electrons, and is studied by means of infrared absorption spectroscopy. An absorption spectrum with a lowest no-phonon line at 615.0 meV is associated with a metastable configuration of the defect. This spectrum is initially not observable when the sample is cooled down to temperatures below approximately 65 K. It is created, however, after the sample is irradiated with light below this temperature. The spectrum disappears when the sample is heated in darkness at temperatures exceeding 70 K. It is concluded that the transformation to the infrared-active metastable configuration is induced by a capture or recombination process involving charge carriers. The observation of an isotope shift of the no-phonon line at 615.0 meV confirms that carbon is one of the constituents of the defect.

The effect of metastability of the 'red' photoluminescence band (hv_max=1.68 eV) is studied in detail in electron-irradiated CdS. Spectral, temperature and polarization measurements are performed. A new mechanism of optical pumping and stimulation of luminescence intensity is proposed. This mechanism is attributed to the complex triple-luminescence centre and the recharging of its components under light illumination.

High-pressure experiments with thin slabs of silicon donors in GaAs are employed to search for the correlation effects expected from the formation of D⁺D^- pairs expected from the negative-U model of DX centres. The mobilities in the individual subbands are very dependent on subband energy but a preliminary analysis does not require the existence of such pairs.

The authors present measurements under hydrostatic pressure on Si delta-doped GaAs. From the measurements they conclude that the energy position of the DX level is shifted away from the Gamma conduction band minimum at high doping concentrations. This shift is consistent with measurements carried out on bulk GaAs heavily doped with silicon.

The pressure dependences of the electrical conductivity and Hall coefficient have been studied for GaAs and Al_xGa_1-xAs (x=0.20 and 0.25) heavily doped with Te. The results obtained show that for a GaAs crystal the resonant donor level related to Te is located approximately 0.45 eV above the conduction band minimum. The effect of persistent photoconductivity observed at high pressures proves the DX-like character of this donor state. Comparison of the data on the energetic position of the DX level illustrates its strong chemical energetic dependence (for Si, Sn and S dopants an E_DX value of around 0.3 eV has been reported previously). It appears that Te represents an impurity which is very suitable for testing the microscopic models of the DX centre. Studies of processes of the thermal recovery after high pressure freeze-out of electrons on the metastable states of the DX centres clearly demonstrate the multicomponent structure of the DX state in AlGaAs samples.

The authors report results concerning the identification of DX centres obtained from high-pressure experiments. The sulphur-related DX centre in GaAsP alloys is characterized by two levels 40 meV apart which follow the X conduction band minimum. Properties of the DX centres in GaAs and GaAsP are compared.