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Because of its high diffusivity in silicon, aluminum is best suited for deep diffusions often required in high-voltage-power semiconductor devices. The ion implantation technique allows the reproducible low dosage doping necessary, e.g., for the new concepts of junction termination systems. The most important drawback of using aluminum as a p-type dopant in silicon is its low electrical activity after the anneal. In order to obtain a deeper insight into the mechanisms responsible for the loss of the electrical activity, we have studied the states of aluminum implanted into silicon before and after annealing by means of spreading resistance, secondary-ion mass spectroscopy, transmission electron microscopy, and energy-dispersive x-ray techniques. The case study presented here [Czochralski grown (100) silicon, implanted dose 3×1015 cm−2, junction depth 6 μm] reveals that the major source for the loss of the electrical activity is out-diffusion, i.e., segregation into the native silicon oxide layer and/or evaporation into the vacuum. In addition, the activity is reduced by the formation of aluminum oxide precipitates. The results are discussed in the light of optical studies on the same materials performed previously as well as on the basis of a diffusion model which allows for out-diffusion. The large rate constant for out-diffusion indicates that the native oxide layer represents a highly reactive surface for aluminum. From the diffusion model it is possible to calculate an approximate electrical activity Ã(xj) as a function of junction depth xj, which qualitatively reproduces well the observed activity A(xj). This demonstrates that our case study is representative for a large number of samples which were implanted and annealed under widely different conditions. Some technical processes which could possibly enhance the electrical activity are discussed.

The local magnetic field at a stopped positive muon has been measured in single crystals of Co as a function of temperature between 4 and 1100 K and of external magnetic field between 0 and 2 kG. The measurements demonstrate the importance of a dipolar contribution to the local field and give a strong indication that the muon occupies the octahedral interstitial site.In this Letter, the results of a muon spin rotation (MSR) experiment performed at the Swiss Institute for Nuclear Research (SIN) on single crystals of ferromagnetic cobalt are reported. The MSR technique enables one to measure the magnitude and direction of the local magnetic field at the site of the stopped muon. Two possible sites in the Co lattice were considered: the octahedral and tetrahedral interstitial sites. The hyperfine field arising from the contact interaction of the conduction electrons with the muon was computed from the measured local field by taking account of the field contribution from the dipolar interaction of the muon with the neighboring Co ions. The temperature dependence of the local magnetic field B^ is most consistently understood by assuming that the muon occupies the octahedral interstitial site. A similar analysis allows one to predict correctly the observed external-field dependence of the local field.The MSR technique has been described in detail elsewhere. 1 Briefly, it consists of stopping a polarized beam of positive muons in the sample to be investigated and observing the timedependent angular distribution of decay positrons. Because the positron emission is correlated with the muon spin direction, one may observe the precession of the muon in its local field. A typical precession pattern is shown in Fig. 1. From the precession frequency and the known gyromagnetic ratio, the magnitude of the local field is determined. The sign of the local field is determined from the initial phase of the precession observed by a counter perpendicular to the initial muon polarization. A damping of the precession signal describable by a dephasing time T 2 is generally observed.The temperature-dependent measurements of the local field were made at zero external field on an approximately ellipsoidal single crystal. The sample was cooled in a He flow cryostat and heated in an evacuated oven by noninductive electrical heating elements. Measurements were also made at room temperature in various external fields on a crystal which had been machined TIME (;JS FIG. 1. A MSR time histogram taken with a cobalt single crystal at 4 K in zero external field. 1644

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