M. Deicher

Indium-Defect Complexes in Silicon Studied by Perturbed Angular Correlation Spectroscopy

The formation of molecule-like complexes, consisting of a defect and a radioactive111In atom, is studied using the perturbedγγ angular correlation technique (PAC). The complexes are characterized by their defect specific electric field gradients which also contain information on the geometry of the formed complexes. Whereas the complex is formed with the111In atom, its electric field gradient is measured after the decay of the radioactive111In atom to111Cd. Formation and dissolution of the molecule-like complexes is pursued for a variety of different conditions, such as sample temperature, dopant concentration and position of the Fermi level. In particular, the interaction of In atoms with the following defects in Si was investigated: Intrinsic defects, created by particle irradiation; substitutional donor atoms (P, As, Sb, Bi); and interstitial impurity atoms (Li, H, and an unidentified X defect); especially, the latter ones are known to passivate acceptor atoms in Si. Methodology and specific properties of the PAC technique will be illustrated with the help of these examples.

Contributions of different defects to the recovery stage at 250 K in gold

Abstract Gold doped with radioactive 111In was irradiated with electrons, protons and Au ions. The trapping of defects at the In atoms during isochronal annealing in stage III was investigated using the perturbed γγ-angular correlation technique (PAC). Via their characteristic electric field gradients four defect configurations were observed. Together with the results of quenched gold specimens we show that the recovery around 250 K is effected by at least two defect types, a small defect cluster being mobile at 195 K and the monovacancy migrating at about 245 K. Additionally, the formation of a large vacancy cluster at the probe impurity is observed at the end of stage III. The occurrence of the different defects is strongly influenced by the mass of the irradiating particles. The results are compared with the information supplied by other experimental methods, especially by resistivity measurements.

Intrinsic limitations of doping diamonds by heavy‐ion implantation

The local environment of 111In implanted into diamonds has been studied by means of the perturbed angular correlation technique as a function of annealing temperature and of implantation dose. It is found that even when the diamond is heated under vacuum to the highest possible temperature before graphitization occurs (2100 K), the implants are not driven to spherically symmetric sites in the lattice. This lack of annealing, if common to all heavy implants, may have serious consequences on the usefulness of doping of diamond by heavy‐ion implantation.

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.

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...

Detection of electronic perturbations in silicon after EC decay of 111In observed by PAC

Abstract Using the perturbed γ-γ angular correlation technique (PAC), the structural and electronic surroundings of a probe atom can be monitored on a microscopic scale. Si crystals have been doped by implantation with radioactive 111 In atoms. Electronic perturbations produced by the EC decay of 111 In to 111 Cd (“decay after-effect”) have been studied as a function of temperature and carrier concentration in intrinsic, p-type and n-type Si. This effect may be a tool to study the carrier concentration and mobility in semiconductors in an atomistic, time differential way.

Nucleation of He clusters at111In in metals

Helium has been implanted in copper. Its interaction with111In atoms during isochronal annealing is observed by the perturbed γγ angular correlation (PAC) technique. The results reveal that the nucleation of He-clusters at the111In site is accompanied by reactions of intrinsic defects. Therefore, in the case of Cu, where the knowledge of intrinsic defects is nearly complete, the successive building-up of He-clusters can be pursued.

Structural lattice defects in silicon observed at111in by perturbed angular correlation

Probing of structural defects in silicon by the perturbed γγ angular correlation (PAC) technique is demonstrated between 77 K and 1300 K. The behaviour of radioactive111 In probe atoms implanted at 295 K, is monitored during isochronal annealing in n-type, p-type and intrinsic Si. Trapping of defects, produced by the111In implantation itself or by postirradiation is studied in P-doped crystals (1016/cm3-1017/cm3).

Solid state physics at ISOLDE

Radioactive atoms have been used in solid state physics and in materials science for decades. Besides their classical applications as tracers for diffusion studies, nuclear techniques such as Mössbauer spectroscopy, perturbedγγ angular correlation,β-NMR, and emission channeling make use of nuclear properties (via hyperfine interactions or emittedα orβ particles) to gain microscopic information on structural and dynamical properties of solids. During the last decade, the availability of many different radioactive isotopes as clean ion beams at ISOL facilities like ISOLDE/CERN has triggered a new era involving methods sensitive to the optical and electronic properties of solids, especially in the field of semiconductor physics. This overview will browse through ongoing solid state physics experiments with radioactive ion beams at ISOLDE. A wide variety of problems is under study, involving bulk properties, surfaces and interfaces in many different systems like semiconductors, superconductors, magnetic systems, metals and ceramics.