Papers by Author: Valentin V. Emtsev

Abstract: The free volume of the thermally stable vacancy center in n-FZ-Si:P has been probed by positrons. The defects were produced with 15 MeV protons, and then the irradiated material was subjected to the isochronal annealing. The positron lifetime has been determined over the temperature range ~ 30 K – 300 K; the samples-satellites have been characterized by Hall effect measurements. The microstructure of the center involves, at least, one atom of phosphorus and it manifests itself as a deep donor. The center is singly negatively charged and the cascade phonon-assisted trapping of positron proceeds over the length characteristic of the point defect, l₀ ~3.62 a. u. Obeying ~ T ^–3 law, the positron trapping cross section ranges 3∙10^–12 cm² (66K) to 2.5∙10^–14 cm² (266 K). The positron lifetimes ranging from ~240 ps to ~280 ps suggest that the atomic relaxation is directed inward towards the free volume of the deep donor involving, at least, two vacancies.

Abstract: The recovery of shallow donor states of the atoms of phosphorus in n-FZ-Si:P material irradiated at the room temperature with 15 MeV protons was studied in the course of isochronal annealing. This process was investigated by the positron annihilation lifetime (PAL) spectroscopy and by low-temperature electrical measurements. The positron traps of a vacancy type manifesting themselves as deep donors have been revealed. These defects begin to anneal at ~ 593 K– 613 K; roughly estimated activation energy of the process is Ea ≈ 0.59 eV under the first order of reaction. The results suggest the involvement, at least, of one vacancy and the impurity atom of phosphorus in the microstructure of the deep donor.

Abstract: The changes of the positron lifetime and loss/recovery of shallow donor states in n-FZ-Si:P material irradiated at the room temperature with 15 MeV protons have been investigated in the course of isochronal annealing. Thermally stable point radiation defects which begin to anneal at ~ 300 C° – 340 C° have been revealed; they manifest themselves as deep donors. It is argued that these defects involve, at least, more than one vacancy and the impurity atom (s) of phosphorus.

Abstract: This paper reports experimental results on the production and annealing of oxygen-vacancy related (VO_n, 1<n<5) and carbon-related (C_iO_i, C_iO_iI, and C_iC_s) defects in Ge-doped Czochralski-grown silicon (Cz-Si) materials containing carbon. The samples were irradiated by 2 MeV fast electrons and the behavior of radiation-produced defects is studied by means of infrared (IR) spectroscopy, monitoring the relevant bands in spectra. Regarding the VO_n family, it was found that the presence of Ge affects the annealing temperature of VO defects as well as their fraction that is converted to VO₂ defects. Both effects are discussed in relation with an impact of Ge on the concentration of self-interstitials that take part in the annealing of VO defects via two reaction paths VO + I → O_i and VO + O_i → VO₂. Furthermore, two bands at 1037 and 1051 cm^-1 are attributed to the VO₅ defect, although three other bands at 762, 967 and 1005 cm^-1 are believed to be associated with V_nO_m clusters containing carbon, most likely having a VO_nC_s structure. Regarding carbon-related complexes, it has been established that the annealing of the 862 cm^-1 band belonging to the C_iO_i defect is accompanied by the emergence of the 1048 cm^-1 band previously assigned to the C_sO_2i center. The evolution of the C_iC_s and the C_iO_iI bands is monitored and the identification of bands at 947, 967 and 1020 cm^-1 making their appearance in IR spectra over the temperature range where C_iC_s and C_iO_iI defects are annealed out is discussed.

Abstract: Si_1-xGe_x alloys with small atomic fraction of Ge, x≤ 0.05 are investigated. The Hall mobility of electrons in n-type materials was measured at cryogenic temperatures, T≤ 100 K. Taking into account the partial mobility due to charge carrier scattering by ionized centers and phonons it is possible to estimate the partial mobility associated with alloy scattering. It appears that this contribution to the electron mobility in n-Si_1-xGe_x at low temperatures is important even at x≈0.01. The obtained results can be useful for understanding the nature of SiGe alloys and their transport properties.

Abstract: The electron momentum distribution and microstructure of centers incorporating a vacancy (vacancies) and a group-V-impurity atom (P, As, Sb, or Bi) in oxygen-lean n-Ge crystals have been investigated by means of the angular correlation of the annihilation radiation (ACAR). The vacancy-group-V-impurity atom complexes have been induced by irradiation with 60Co γ – rays at Tirr. ≈ 280K. A split between the intensities of the high-momentum emission of the annihilation radiation measured before and after n-p-conversion has been revealed for the complexes containing smaller ion cores (P, As) and the larger ones (Sb, Bi), respectively. After n-p-conversion the electron density decreases slightly (but markedly) around the positron localized at the vacancy complexes incorporating P, Sb, and Bi impurity atom. This decrease is accompanied by a lessening of intensity of the high-momentum emission of the annihilation radiation thus bringing in a direct evidence of a multi-vacancy structure of the vacancy-group-V-impurity atom complexes after n-p-conversion; the electron density was found to be affected by the localized deep acceptor states related to these centers. The relaxation inward open volume is a common feature which is pronounced for As-containing complexes. Subvalent band states are suggested to contribute the high-momentum annihilation most markedly. The electron momentum density around the positron is due to rather by the elemental specificity of the surrounding atoms than by changes of the electron-positron many-body interaction in the vacancy-group-V-impurity atom complexes.

Abstract: We present infrared (IR) spectroscopy measurements on carbon-rich, germanium-doped Czochralski-grown (Cz-Si) subjected to irradiation with 2 MeV electrons. The study is focused on the effect of germanium doping on the production of carbon-related defects CiCs, CiOi and CiOi(SiI). For carbon concentrations [Cs] up to 11017 cm-3 the production of the defects increases with the increase of Ge content, for [Ge] up to 11020 cm-3. However, for carbon concentrations around 21017 cm-3 the production of these defects shows a decrease for samples with [Ge]=21020 cm-3 in comparison with those of [Ge]=21019 cm-3. The results are discussed taking into account the effect of germanium on the annihilation of vacancies and self-interstitials in the course of irradiations. In the first case, due to the temporary trapping of vacancies by Ge atoms in the course of irradiation, more self-interstitials are available for the production of carbon interstitials (Cs+ SiI Ci), leading finally to an increase of the carbon-related defects. In the second case, and for [Ge] of the order of ~1020 cm-3 or higher, Ge atoms tend to form large clusters. These clusters attract primary defects facilitating their annihilation on them. As a result, the availability of self-interstitials decreases, which finally leads to a decrease of the carbon-related defects.

Abstract: We report studies of defects in electron-irradiated Czochralski-grown silicon (Cz-Si) subjected to thermal treatments at 1000oC and 1130oC with or without the application of high hydrostatic pressure of ~ 11 Kbars, prior to irradiation. The work is primarily focused on the impact of the pre-treatments on the production rate of the VO defect and its conversion to the VO2 defect. To this end, IR spectroscopy measurements were carried out and the amplitudes of the VO band (830 cm-1) and the VO2 band (888 cm-1) were monitored in the course of an isochronal anneal sequence up to ~ 550oC. Thermal treatments at 1000oC result in a reduction of the production rate of the VO defect. This rate however increases when pressure is applied during the treatment. The opposite behavior is observed for thermal treatments at 1130oC. The production rate of the VO increases slightly in heat treated samples but decreases substantially when high pressure is applied. Similar trends show the conversion of the VO to the VO2 defect for the corresponding treatments. The results are discussed taking into account the oxygen precipitates formed at the various treatments and their impact on the amount of primary defects available during irradiation which affects the production of the vacancy-oxygen defects.

Abstract: Angular correlation of annihilation radiation technique (ACAR) has been used for studying a microstructure of the vacancy-group-V-impurity complexes (DV) formed by irradiation with 60Co γ – rays at Tirr. ≈ 280K in oxygen-lean n-Ge doped with group-V-impurity atoms As, Sb, and Bi. The probability of annihilation of positrons with the core electrons of DV complexes to be reconstructed from ACAR spectra has been analyzed on the basis of Chapman-Kolmogorov formalism; the Coulomb repulsion is proved to regulate the penetration of a positron into Ge4+ and D5+ ion cores. In passing from AsV to SbV and BiV complexes the ion cores D5+ are found to contribute more effectively to the probability of the positron annihilation in the core region. These data correlate well with the augmentation of the entropy of ionization (4S ~ 2,9 ÷ 4,2K) observed by means of capacitance transient techniques with the use of Au-Ge Schottky barriers in the same row of a similar vacancy-impurity complexes. The results obtained by ACAR spectroscopy suggest the full-vacancy configuration of DV pair with relaxation of atoms inward towards the vacancy.

Abstract: A probing of the atomic environment of positron in Cz-Si single crystal heat-treated at T=600C and T=450C has been performed by one-dimensional angular correlation of annihilation radiation (ACAR). It has been established that positrons get trapped by the oxygen-related complexes. The penetration of positrons into the core region of surrounding atoms results in emission of the elementally specific high-momentum annihilation radiation. The processes of expelling of positron from ion cores and its penetration into the core region are regulated by the potential barrier (to be considered as the Coulomb’s one as a first approximation). The characteristic electron-positron ion radius and the probabilities of correlated events of the highmomentum annihilation are due to the chemical nature of the ion cores of atoms involved in the composition of the oxygen-related complexes. The interpretation of the results is based on the notion of the positron localization in the field of negative effective charge resulted from comparatively high electron affinity of the oxygen impurity atom. The presence of a free volume (perhaps, a vacancy) as well as the carbon atom in the microstructure of the oxygen-related positron-sensitive thermal defects is briefly discussed.