A. Burchard

Experimental evidence of the self-compensation mechanism in CdS

Abstract Microscopic origin of full electrical compensation of donor doped CdS was analyzed with perturbed angular correlation and Hall-effect measurements. Single crystals were implanted with radioactive 111 In and stable 115 In ions. Total In concentration ranged from 1016 to 1020/cm3. A strong correlation was observed between electrical self-compensation and the formation of (InCd–VCd) pairs (A centers) as a result of thermal annealings. It is shown that the presence of In donors during thermal treatment under the S pressure provokes spontaneous formation of (doubly) ionized cation vacancies [VCd]. During cooling, these vacancies form pairs with In donors (A center), which compensate the rest of the donors, leading to highly resistive material. The experiments presented provide direct evidence for self-compensation: doped crystals spontaneously create just a matching concentration of native point defects needed to compensate foreign doping atoms electrically. This holds for over four orders of magnitude of In concentrations.

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.

First Microscopic Observation of Cadmium-Hydrogen Pairs in GaN

The formation and properties of acceptor-hydrogen pairs in GaN have been studied using radioactive {sup 111m}Cd acceptors and the perturbed {gamma}{gamma} angular correlation spectroscopy (PAC). After H-loading by low energy implantation (100 eV) at temperatures between 295K and 473 K, the formation of two Cd-H complexes involving about 30% of the Cd-acceptors is observed. The complexes have been identified as single hydrogen atoms bound to the Cd acceptor in two different configurations. The dissociation enthalpies of these configurations have been determined as 1.1(1) eV and 1.8(1) eV, respectively.

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.

Microscopic Study Of The Hydrogen Diffusion In III-V Semiconductors

The authors report on experiments which observe on a microscopic scale the migration of isolated hydrogen in InP, GaAs, and InAs. Using the radioactive acceptor {sup 117}Cd, Cd-H pairs have been formed in these III-V semiconductors. After the decay of {sup 117}Cd to {sup 117}In, H is no longer bound to an acceptor and can diffuse freely. This diffusion has been observed by perturbed {gamma}{gamma} angular correlation (PAC) spectroscopy. At 10 K, the occupation of two different lattice sites by hydrogen has been observed. First results on the diffusion of hydrogen will be discussed.

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 recovery of ion-implanted CdTe

The defect recovery of Bridgman and MBE grown CdTe implanted with In and Cd ions (E = 60-350 keV) has been studied using the perturbed γγ angular correlation technique (PAC) sensitive to defects present in the immediate neighborhood of the implanted ions and photoluminescence spectroscopy (PL). Both the implanted species and the material influence the annealing behavior. Emission channeling experiments with implanted 111m Cd ions show that already after implantation at 295 K the major fraction of implanted ions occupies substitutional lattice sites. The observed differences between Bridgman and MBE grown material indicate that the annealing behavior depends on the purity and crystal quality of the material. PL measurements on In-implanted CdTe show that for the overall recovery of the implanted layer annealing temperatures up to 800 K are necessary.

The Incorporation and Complex Formation of Ag Acceptors in CdTe

Using the radioactive isotope 111 Ag, the incorporation of Ag into CdTe is investigated by photoluminescence spectroscopy (PL) and the perturbed yy-angular correlation technique (PAC). PL is used to demonstrate the incorporation as a substitutional acceptor (Ag Cd ) by monitoring the intensity of the donor-acceptor transition, corresponding to an acceptor level of E V + 108 meV, which decreases just with the half life of the radioactive decay of the 111 Ag isotope. Since the isotope 111 Ag also serves as a probe atom for PAC experiments, it is possible to confirm the incorporation of Ag atoms on lattice sites with cubic symmetry. Additionally, in CdTe a defect complex is detected, which is assigned to a Ag Cd -V Te pair. A second defect complex is observed in CdTe doped with In, which is assigned to an In Cd -Ag Cd pair. By PAC experiments with the probe 111 In that are performed in Ag doped CdTe, the formation of the In Cd -Ag Cd pair is confirmed besides the well known formation of the In Cd -V Cd pair. The migration energy of the cation vacancy is estimated on the basis of an isothermal annealing sequence.

Implantation and diffusion of 73As in GaAs and GaP

Self-diffusion on the As sublattice in intrinsic GaAs and foreign-atom diffusion on the P sublattice in intrinsic GaP were investigated in a direct way by As tracer diffusion measurements using the radioisotope 73As. For this purpose 73As was implanted in both materials at the ISOLDE facility of CERN. Then diffusion annealings were performed followed by serial sectioning and counting of the radioactivity in each section. The resulting profiles were simulated within a computer model which accounts for the observed loss of tracer to the diffusion ambient. The so-obtained diffusion coefficients for As in GaAs and GaP are compared with existing diffusivities in these compounds.