V. Ostheimer

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.

MOCVD grown CdTe investigated by photoluminescence and PAC

Abstract Comparative investigations, using perturbed γγ-angular correlation and photoluminescence spectroscopy, were performed on CdTe films grown on GaAs substrates by MOCVD. The incorporation of the radioactive probe atoms 111 In into the CdTe films was done via diffusion, implantation, and, for the first time, by in situ doping during MOCVD growth. The latter process was realised by using a modified MOCVD reactor with the precursor trimethylindium (TMIn) labelled with the radioactive isotope 111 In. The influence of the doping procedure and the thermal treatment, including annealing under Cd or Te overpressure, was investigated. For two donor-acceptor pairs, involving the donor In and the acceptors As or the Cd vacancy, the results of both techniques were correlated.

Photoluminescence study of II-VI semiconductors by using radioactive 71As dopants

By means of radioactive dopants, photoluminescence signals related to the respective chemical element are identified. In the present work, the isotope 71 As (t 1/2 = 65.3 h) which decays to 71 Ge (t 1/2 = 11.2 d) and subsequently to 71 Ga was implanted into CdTe and ZnSe. In CdTe, the (D, A) transition related to the As Te acceptor is identified in agreement with the literature. In addition, a new (D, A) transition caused by a shallow Ga acceptor is identified. The corresponding acceptor level having an ionisation energy of E i = 42 meV is still shallower than the effective-mass like state (E i = 57 meV). It is interpreted in terms of a 71 Ga Te antisite defect in a Jahn-Teller distorted configuration. Ab initio calculations within the framework of density functional theory confirm the existence of a Jahn-Teller relaxed stable configuration for Ga Te . In ZnSe, no clear evidence of a (D, A As ) transition is found, but like in CdTe, a new shallow acceptor is observed which is supposed to originate from a 71 Ga Se defect.

Identification of Ag-acceptors in 111Ag/111Cd doped ZnTe and CdTe

Nominally undoped ZnTe and CdTe crystals were implanted with radioactive 111 Ag, which decays to 111 Cd, and investigated by photoluminescence spectroscopy (PL). In ZnTe, the PL lines caused by an acceptor level at 121 meV are observed: the principal bound exciton (PBE) line, the donor-acceptor pair (DAP) band, and the two-hole transition lines. In CdTe, the PBE line and the DAP band that correspond to an acceptor level at 108 meV appear. Since the intensities of all these PL lines decrease in good agreement with the half-life of 111 Ag of 178.8 h, both acceptor levels are concluded to be associated with defects containing a single Ag atom. Therefore, the earlier assignments to substitutional Ag on Zn- and Cd-lattice sites in the respective II-VI semiconductors are confirmed. The assignments in the literature of the S 1 , S 2 , and S 3 lines in ZnTe and the X Ag 1 , X Ag 2 /C Ag 1 , and C Ag 2 lines in CdTe to Ag-related defect complexes are not confirmed.

Luminescence and influence of defect concentration on excitons in 197Hg/197Au-doped CdTe

Abstract CdTe, implanted with 197Hg ions, which decay to 197Au with a half-life of 64.1 h, was investigated by photoluminescence (PL) spectroscopy. The results unambiguously verify the assignments of both, the donor–acceptor pair transition at 1.335 eV, which corresponds to an acceptor level with EA=263 meV, and the recombination of excitons bound to neutral acceptors at 1.57606 eV to single Au atoms on Cd sites. In addition, the dependence of the intensities of excitonic lines on the defect concentration was investigated quantitatively. The observed intensities are well explained, assuming that a defect can only bind an exciton if there is no additional defect within the volume of the bound exciton. The ratio between the exciton radii of the Cu and Au-bound excitons RexcCu/RexcAu=1.2±0.2 obtained from this model is in good agreement with the ratio derived from the diamagnetic shift parameters of the two corresponding PL lines.

Hydrogen-related photoluminescence in CdTe

CdTe, nominally undoped, was exposed to a hydrogen plasma and to low energy H+ implantation. Under both conditions, seven typical photoluminescence lines are observed in the excitonic region. They are assigned to the presence of hydrogen in CdTe.

Defect identification by means of electric field gradient calculation

Abstract The electric field gradients (EFG) caused by group V acceptors, group Ib acceptors and the cadmium vacancy in CdTe are calculated using the linearised augmented plane wave method and compared to the corresponding experimental values. Experimentally, the EFG are determined by the perturbed γγ-angular correlation spectroscopy with the radioactive probe isotopes 111 In and 77 Br. Besides the chemical nature, information about the charge state and the lattice relaxation associated with each defect is obtained.