ANISOTROPIC MOMENTUM DISTRIBUTION OF POSITRON-ANNIHILATION RADIATION IN SEMICONDUCTORS

Momentum distributions of positron-annihilation radiation with electrons in semiconductors are studied by use of first-principles calculations of the annihilation rates. For perfect crystals of Si and GaAs, the present calculation successfully reproduces the observed momentum distribution, in which a valley along the [100] line and a dip around the GAMMA-point exist. An analysis based on group theory shows that both the valley and dip are consequences of the fact that the upper two valence bands make no contribution to the annihilation rates on the [100] line and at the GAMMA-point. Furthermore, the case that a positron is captured by a neutral monovacancy in Si is studied by employing a supercell model in which the cell contains 53 Si atoms and one vacancy site. It is found that the valley and the dip in the momentum distribution observed in the perfect crystal disappear. It is proposed, from the drastic change in the momentum distribution upon vacancy introduction into the perfect crystal, that the angular correlation of annihilation radiation in the positron-annihilation technique is an effective tool for the detection of vacancies and for obtaining microscopic information about their electronic states. Finally, the lifetime of the captured positron is found to be sensitive to the small relaxation of the host atoms around the vacancy.