H. Wolf

Doping of the nanocrystalline semiconductor zinc oxide with the donor indium

Doping of the nanocrystalline semiconductor ZnO with the donor 111In was achieved by the incorporation of 111In atoms during the growth process followed by a hydrothermal treatment at 473 K. The incorporation of 111In on substitutional Zn sites was shown by the perturbed γγ angular correlation technique. The structural quality of nanocrystalline ZnO with a mean grain size of 11 nm is significantly improved by annealing at 473 K, as revealed by x-ray diffraction, transmission electron microscopy, optical absorption measurements, and photoluminescence spectroscopy. It is shown that the incorporation of 111In on undisturbed Zn sites in nanocrystalline ZnO seems to be supported by the onset of crystal growth and by the removal of intrinsic defects.

Characterisation of ZnSe and other II–VI semiconductors by radioactive dopants

Abstract In the II–VI compounds ZnS, CdS, ZnSe, CdSe, ZnTe and CdTe doped with In, the formation of In M -V M pairs in shown to occur using the radioactive dopant 111 In along with the perturbed γγ angular correlation technique. For CdS and ZnSe, the migration energy of the metallic vacancy defect V M and its binding energy to the donor In is determined. The creation of V M defects under different experimental conditions is investigated, such as stoichiometry, temperature, electron irradiation and doping with Li atoms.

Investigation of cadmium‐donor pairs in silicon

The formation of indium‐donor pairs (donors: P, As, Sb) in silicon is identified by perturbed angular correlation spectroscopy. After the electron capture decay of the 111 In probe atoms to 111 Cd, the electric field gradient (EFG) is measured at the corresponding cadmium‐donor pairs. For all three complexes a similar temperature dependence of the EFG is observed which can be explained quantitatively by a model based on the charge state of the cadmium‐donor acceptors. The corresponding energy levels are given.

ATOMIC CONFIGURATIONS OF GROUP V ACCEPTORS IN ZNSE, ZNTE, AND CDTE

The formation of donor–acceptor pairs was detected by perturbed γγ angular correlation spectroscopy. In ZnSe, ZnTe, and CdTe crystals doped with the donor 111In and one of the acceptors N, P, As, or Sb the strength of the electric field gradient shows a systematic correlation of the bond length between the respective acceptor and the neighboring cation. For N acceptors, an inward relaxation of the neighboring cations is concluded to occur.

Vacancies in CdTe: Experiment and theory

Ab initio calculations in the framework of density functional theory are employed to complement experimental PAC and EPR data on vacancies in CdTe. The Te vacancy is found to be a negative-U centre with a large lattice relaxation in the neutral and the doubly charged state. The electronic state introduced by VTe lies below the valence band maximum for VTe 0 and shifts to above the conduction band minimum for VTe ++ . An experimentally observed electric field gradient is interpreted in terms of acceptor compensation by VTe ++ . Experimental data available for the Cd vacancy is discussed in the context of the calculated electric field gradients. A Jahn-Teller effect for V Cd is not confirmed. PACS: 61.72.Vv, 71.55.Gs, 76.80.+y

Defect complexes formed with Ag atoms in CdTe, ZnTe, and ZnSe

Using the radioactive acceptor 111 Ag for perturbed γγ-angular correlation (PAC) spectroscopy for the first time, defect complexes formed with Ag are investigated in the II-VI semiconductors CdTe, ZnTe, and ZnSe. The donors In, Br and the Te-vacancy were found to passivate Ag acceptors in CdTe via pair formation, which was also observed in In-doped ZnTe. In undoped or Sb-doped CdTe and in undoped ZnSe, the PAC experiments indicate the compensation of Ag acceptors by the formation of double broken bond centres, which are characterised by an electric field gradient with an asymmetry parameter close to η = 1. Additionally, a very large electric field gradient was observed in CdTe, which is possibly connected with residual impurities.

Identification of Ag-acceptor related photoluminescence in 111Ag doped CdTe

Bridgman-grown, nominally undoped CdTe crystals were doped with Ag by implanting radioactive 111Ag. Photoluminescence spectra of the crystals show a donor-acceptor pair (DAP) line at 1.491 eV. The decrease of the intensity of this line with a half life of T1/2=(7.2±0.4) d is in good agreement with the half life of the β− decay of 111Ag to 111Cd of 7.45 d. This decrease is not caused by the aging behavior of Ag which was reported in the literature. The data show that the involved acceptor defect contains exactly one Ag atom and confirm the earlier assignment of the acceptor to the AgCd defect. Based on the DAP line at 1.491 eV, the spectra did not reveal a contamination of the CdTe crystals by stable Ag.

Annealing of Cd-implanted GaAs: Defect removal, lattice site occupation and electrical activation

A systematic investigation of the behavior of Cd‐implanted GaAs after rapid thermal annealing is presented. The use of various experimental techniques gives a detailed picture regarding the annealing process in the low‐dose regime (1012 and 1013 cm−2) on a microscopic as well as on a macroscopic scale. Perturbed angular correlation experiments, using the radioactive probe 111mCd, yield information on the immediate environment of the Cd implant on an atomic scale. Rutherford backscattering channeling and photoluminescence spectroscopy give complementary information concerning the overall damage level in the implanted layer, Hall measurements are used to determine the degree of electrical activation of the implanted Cd acceptors. The outdiffusion of the implanted radioactive Cd atoms is also investigated. The removal of defects in the next‐nearest neighborhood of the Cd atoms takes place after annealing at 700 K and is accompanied by a general recovering of the crystal lattice. Between 600 and 900 K more di...

Cd‐H pairs in GaAs: Identification and stability

The Cd‐H complex in 111mCd‐doped GaAs implanted with low‐energy (150–400 eV) hydrogen atoms is identified and studied by perturbed angular correlation spectroscopy using radioactive 111mCd as a probe. By measuring the fraction of Cd‐H pairs in an isochronal annealing experiment, the stability of the pairs is deduced yielding a dissociation energy of ED=1.35(10) eV. After 111mCd implantation but preceding the H loading, the GaAs samples have to be annealed at temperatures exceeding 900 K in order to form Cd‐H pairs. These temperatures are in agreement with the temperature range required for electrical activation of Cd implants, suggesting that a Coulombic interaction is responsible for the formation of Cd‐H pairs in GaAs.

Acceptor-donor pairs in germanium

The formation of acceptor-donor pairs in germanium was studied by perturbed angular correlation spectroscopy. At the single acceptor In the donors P and As were trapped, resulting in In-P and In-As pairs as well as in In-P2 complexes. At the double acceptor Cd three P and two As correlated configurations were formed, i.e. Cd-P and Cd-As pairs, Cd-P2, Cd-As2 and Cd-P3 complexes. In Ge:Sb and Ge:Se the pairing of Cd acceptors with Sb or Se donors was detected. As predicted by calculations of the Coulomb binding energy the stability of the complexes decreases with increasing number of involved donor atoms, and a configuration formed at the double acceptor Cd reveals a higher stability than the analogous one at the single acceptor In. The weak temperature dependence of the corresponding field gradients indicates either electrical inactivity of Cd-donor complexes or the generation of shallow levels.