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The microscopic structure of the paramagnetic anion antisite defect in semi-insulating GaAs was determined by optically detected electron-nuclear double resonance (ODENDOR). It is an arsenic-antisite-arsenic-interstitial (Aso, -As; ) pair. It is shown, by optically detected ESR and ODENDOR experiments, that its energy levels and optical properties in the diamagnetic state are those of the EL2 defect.The dominant midgap level EL2 in undoped GaAs is responsible for its semi-insulating properties. It is due to a defect of yet unknown microscopic structure, which has unique and fascinating properties. At low temperature it can be photoexcited into a metastable state which is responsible for the observed persistent quenching of the photocapacitance. It returns to the ground state only after thermal activation at 140 K. ESR investigations of crystals grown under different melt stoichiometry conditions show that EL 2 is related to an anion antisite (Aso, ) ESR signal.Therefore, many defect models exist involving an anion antisite defect, isolated or combined with vacancies or an interstitial.The identification of EL 2 with an AsG, -As; complex in Ref. 7 was based on a combination of ESR and deep-level transient spectroscopy measurements (DLTS) of electron-irradiated n-type GaAs. In the ESR experiments, however, only the hyperfine interaction with the central nucleus is resolved, whereas the hyperfine interactions and symmetry of the ligands cannot be deduced from the ESR spectrum. Therefore, the identification of EL2 in Ref. 7 was a very indirect one: the role of the As; defect and the assignment of its charge state and position were based on theoretical predictions, a comparison of diffusion data, and speculations.In this Rapid Communication we report on a direct experimental structure determination of the EL2 defect by optically detected electron-nuclear double-resonance (ODENDOR) experiments on the arsenic antisite defect in semi-insulating GaAs. A correlation between the energy levels and optical properties and the ESR and ENDOR spectra (i.e., its microscopic structure) is made by photoexcitation experiments in p-type GaAs. The EL2 defect is indeed an arsenic-antisitearsenic-interstitial pair.From the ENDOR data the location and charge state of As; can be inferred. The OD ENDOR signals are measured as radiofrequency-(rf) induced changes of the magnetic circular dichroism (MCD) under microwave resonance conditions.Compared to our earlier ODENDOR investigations the rf strength and the frequency range have been extended. The positions of the ENDOR lines were determined using digital filtering algorithms and an automatic peak-search program. ' Undoped semi-insulating GaAs with an arsenic fraction in the melt of 0.5 ([As]i' 120 110 100 N 90 80 C 70 CD 60 50 80 0 I&1O[I 10 ZO 30 40 angle (degree) FIG. 1. Angular dependence of the ODENDOR lines of Asg, in semi-insulating GaAs:V for rotation of the crystal in a (110) plane from [110] (0') towards [100]. Solid lines and dashed drawn lines correspond to the four nearest As l...
ENDOR experiments on paramagnetic thermal donors are presented for the first time. They show that the ESR spectra of all thermal donors identified by ir bands are superimposed in one narrow line ("#£8"). Only 29 Si superhyperfine interactions were found and determined for up to seven neighbor shells of four thermal donors. From the analysis of the data with effective-mass theory it is concluded that the thermal-donor defect most probably does not contain a central Si atom assumed in most current models. It is tentatively suggested that it is an O2 molecule.PACS numbers: 71.55.Fr, 61.70.At, 76.30.Da, 76.70.Dx Oxygen is one of the major impurities incorporated into silicon during Czochralski (Cz) crystal growth in quartz crucibles. Cz-Si is used for fabrication of many devices. Oxygen easily forms aggregates at elevated temperatures, which is of great importance for the silicon device technology. Heat treatment of oxygen-rich Si at about 450 °C leads to the formation of oxygenrelated, electrically active shallow donors ["thermal donors" (TD)]. 1 Although this has already been known for over thirty years, so far no details of their microscopic structure are known. They are assumed to consist of small oxygen complexes. Apart from the technological aspect it is of principal interest to unravel the geometric structure of such a cluster in a solid, which has not yet been reported. There is also particular interest in the electronic structure, especially to understand the origin of their electrical activity, which must be a cluster property. Existing theories for shallow donors so far have been applied only to describe point defects such as single impurity atoms. It is of general interest to study how a cluster defect becomes a shallow donor and how it can be understood with present theories. For this the details of the clusterdefect structure must be investigated. Electron-nuclear double resonance (ENDOR) has proved to be a useful tool for determining structures of point defects by resolving ligand hyperfine (hf) structures. This paper presents the first attempt to apply ENDOR to determine the complicated microscopic structure of a cluster in a solid.Infrared absorption showed that upon increase of annealing time several TD's are generated. 2 A first ESR investigation of paramagnetic TD + 's was performed by Muller et al? on one Cz-grown and several floating-zone samples, into which oxygen was diffused. Nine different ESR spectra (labeled JVL8-10, NL 13-18) were observed. No resolved hf lines could be observed. The g tensor always had C 2v symmetry.In order to resolve 29 Si hf interactions to obtain more information about the TD structure, ENDOR measurements were performed on Cz-grown B-doped Si.Before annealing of the Cz-grown Si samples, containing -10 18 /cm 3 O and 4.5 x 10 15 /cm 3 B, up to 8 h at 460 °C, they were preannealed at 770 °C for 10 min as was done for the ir measurements. 2 Only the NL 8 ESR spectrum was observed. A search for resolved 29 Si hf lines ( 29 Si abundance 4.7%) with a detection limit of ...
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