Doppler Broadening Spectroscopy of Positron Annihilation near Ge and Si (001) Single Crystal Surfaces

Abstract

carrier mobility compared with Si. 1 Although closely related to Si, in some aspects Ge shows a quite different behavior. This should be taken into account when processing steps are developed for the production of electronic devices with active Ge layers, in which the Ge layer surfaces and interfaces will play an important role. 2 In this letter, we investigated the effect of the free (001) surface on positron annihilation spectroscopy (PAS) results, comparing Si and Ge substrates. PAS is a powerful tool for the study of vacancy-type defects in semiconductors, 3,4 e.g. in irradiated Si and Ge. Positron trapping at vacancies can be observed by the narrowing of the Doppler-broadened 511 keV annihilation γ-ray peak. By varying the incident positron energy, one can probe different sample depths in the submicron range. This method is called Doppler Broadening Spectroscopy (DBS). The so called S-parameter, extracted from DBS, corresponds to positron annihilation by the valence electrons and is sensitive to open volume defects. An increase of the S-parameter can be simply interpreted as an increase of the number of vacancy-type defects. The S-parameter obtained for an incident positron energy lower than 2 keV, however, in general differs from the one obtained for positron energies above 15 keV. Incident positrons with lower energy will indeed mainly interact with the near surface area of the sample, and will be sensitive for thin native oxide layers, surface contamination, surface roughness, surface defects, etc. The S-parameter decreases in general toward the surface for most materials due to positron diffusion toward and annihilation at the surface. 5 The present letter reports on the different behavior of Si and Ge with this respect. While in case of Si, the S-parameter decreases considerably near the Si (001) surface as expected and as is the case for other semiconductor materials, in the case of a Ge (001) surface, the S-parameter shows no remarkable dependence on depth although both are group IV elements and have the same diamond structure. For the PAS measurements, moderately doped p-type Si and n-type Ge (001) wafers were used with a resistivity at room temperature in the range of 10–100 ohm corresponding with a carrier concentration at room temperature in the range of 10 13 ∼10 15 cm −3 . As a reference for the Si bulk and at the same time the near surface of Ge, a Geon-Insulator (GOI) wafer with a un-doped Ge top-layer produced by the Ge condensation technique, was also included in the study. The carrier concentration in the n-type Ge film of the GOI wafer that