D. Forkel

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.

PAC STUDIES OF ION IMPLANTED SILICON

The annealing behaviour of radiation induced defects in ion implanted silicon is studied by the perturbed angular correlation method (PAC). Between 700 K and 1000 K the trapping and detrapping of vacancy-oxygen complexes is observed. In annealed p-Si a well defined, axially symmetric electric field gradient (EFG) appears at low temperatures. This EFG is oriented to the surface and not to any crystallographic direction. The size of the EFG depends strongly on the surface charge.

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.

Perturbed angular correlation spectroscopy of hydrogen-passivated indium acceptors in silicon

Abstract Hydrogen-passivated 111 In atoms in boron-doped silicon samples are investigated with respect to their thermal properties by the perturbed angular correlation method. The temperature dependence of one of the electric field gradients induced by the formation of close 111 InH pairs reflects the electron capture decay of 111 In into 111 Cd. A model based on the thermally activated population of a 111 CdH acceptor fits quantitatively to the temperature data, yielding an acceptor level close to 0.1 eV above the valence band.

Measurements of the electric field gradient at cadmium in the high Tc superconductor YBa2Cu3O6+x

The electric field gradient (EFG) at111Cd probe atoms in YBa2Cu3O6+x has been studied by the perturbed angular correlation (PAC) spectroscopy. After annealing at temperatures below 1000 K the probe atoms are located at the Cu(1) site; above 1100 K they preferentially occupy the Y site, whereas in the intermediate temperature range they also reside on the Ba site. A reduction of the oxygen content from x=1.0 to 0.2 resulted in a decrease of the asymmetry parameter measured at the Ba site, but in an increase of it at the Y site, indicating there a local distortion of the YBa2Cu3O6+x lattice by the Cd impurity. Two different surroundings for the Cd atoms at the Cu(1) site were observed.

Investigation of cadmium-hydrogen complexes in silicon

Formation, stability and electric properties of (In-H) and (Cd-H) complexes in Si are studied by the perturbed angular correlation spectroscopy (PAC). The trapping of hydrogen at the acceptors results in three different defect specific electric field gradients (EFGs), which are studied as function of temperature, doping concentration and annealing time. Two of the three observed hydrogen-related EFGs are identified as different charge states of one (Cd-H) acceptor. Within the framework of a model, which takes into account the dynamics of charge fluctuations and Shockley, Read and Hall statistics, the energy level of this (Cd-H) acceptor at E = EV + 60 meV is deduced. Furthermore, the dynamical behavior of the hydrogen atom within the (Cd-H) complexes is studied. From the relaxation of the PAC signal the hopping rate of the hydrogen atom is extracted. This rate is thermally activated with an activation energy of E = 0.20 eV.

Influence of grain boundaries on the recombination rate in germanium

The electronic properties of the In (→ Cd) acceptor are studied in polycrystalline germanium by the perturbed angular correlation method (PAC). The time independent anisotropy reveals a strong temperature dependence. This behaviour can be explained quantitatively by a model based on recombination processes at the Cd acceptor. The recombination rates depend on the annealing procedure of the samples, thus indicating a strong influence of the grain boundaries.

PAC measurements in titanium and titanium-hydrogen alloys

Measurements have been performed with the PAC method in pure hcp titanium and highly concentrated titanium hydrogen alloys to investigate the temperature dependence of the electric field gradient (EFG) in both systems. In pure titanium it can be explained by a recent theoretical model; in the titanium hydride the EFG reveals a strong decrease with temperature, which indicates enhanced local vibration amplitudes. Furthermore the interaction of hydrogen and oxygen impurities with the111In probe atoms have been studied.

In-beam studies of defect cascades in metals

Abstract The PAD method allows the investigation of recoil-induced defect cascades during the first 10 −9−10 −5 s after their production by nuclear reactions. A short review of experimental results and their interpretation is given. Recent very accurate measurements of 69Ge in a 66Zn single crystal performed in the temperature range between 25 K and 580 K allow one to draw conclusions concerning the dynamical rearrangement of the radiation damage in the vicinity of the primary recoil atom. The measured spectra for temperatures below 500 K deviate from the modulation pattern expected for static defect distributions around the probe atoms. This proves that the temperature dependence of the fraction of probe atoms located at unperturbed substitutional lattice sites has to be explained in terms of trapping and detrapping of defects. No distinct defect induced quadrupole interaction frequency could be resolved. The increase in the observed anisotropy between 400 K and 500 K is analyzed in the framework of a new extended model for detrapping processes in noncubic metals. This model reproduces the observed temperature dependence of the different harmonics of the perturbation function. It is shown that the center of the efg distribution caused by the trapped defects is shifted to a value close to zero. It thus deviates significantly from the value of the substitutional lattice site. The dissolution energy of the probe-defect complex was deduced to be 0.54(5) eV.

DIRECT OBSERVATION OF ACCEPTOR-DONOR PAIRING IN SEMICONDUCTORS BY THE PAC-SPECTROSCOPY

The formation of close acceptor-donor complexes was studied in Si, Ge and III-V compounds like GaAs, InP and GaP by means of the Perturbed Angular Correlation method. In Ge the trapping of P and As atoms at the single acceptor In was observed, resulting in two In-P complexes and one In-As pair, respectively. The double acceptor Cd, however, forms up to three configurations with P and two with As donors. All acceptordonor pairs in Ge are characterized by a weak temperature dependence of the corresponding electric field gradients, in contrast to analogous pairs in Si. Complexing of Cd acceptors with S and Se donors could be detected in GaAs. In plasma exposed GaAs, InP and GaP distinct Cd-defect complexes are formed, whose similar PAC data indicate the trapping of the same defect.