Papers by Author: Charalamos A. Londos

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: It has been shown by means of EPR and NMR technique that at the Si-SiO2 interface at appropriate oxidation temperature (time) local dynamical equilibrium may be achieved. At oxidation temperature 1130oC the dencity of point defects is less than at lower and higher temperature (1100oC and 1200°C) and the content of absorbed impurities (hydrogen, oxygen) diminishes.

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: Fourier transform infrared absorption spectroscopy was used to study the evolution of multivacancy-oxygen-related defects in the temperature range 200-300 °C in Czochralski-grown Si samples irradiated with MeV electrons or neutrons. A clear correlation between disappearance of the divacancy (V2) related absorption band at 2767 cm-1 and appearance of two absorption bands positioned at 833.4 and 842.4 cm-1 at 20 K (at 825.7 and 839.1 cm-1 at room temperature) has been found. Both these two emerging bands have previously been assigned to a divacancy-oxygen defect formed via interaction of mobile V2 with interstitial oxygen (Oi) atoms. The present study shows, however, that the two bands arise from different defects since the ratio of their intensities depends on the type of irradiation. The 842.4 cm-1 band is much more pronounced in neutron irradiated samples and we argue that it is related to a trivacancy-oxygen defect (V3O) formed via interaction of mobile V3 with Oi atoms or/and interaction of mobile V2 with VO 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: Fast neutron irradiations on pre-treated Cz-grown silicon were carried out. The pretreatments involved thermal anneals at 450 oC and 650 oC under high hydrostatic pressure. We mainly examined, by means of IR spectroscopy, the effect of pre-treatments on the production of the oxygen-vacancy (VO) pair. The amplitude of the VO band was found independent on the 450 oC treatment although the amplitudes of the TDs bands were reduced. On the other hand, the amplitude of the VO band was found lower in the samples treated at 650 oC, indicating an influence on the production of the oxygen-vacancy defects. The results are discussed and explanations are suggested concerning possible interactions between thermal and radiation defects.

Abstract: Local vibrational mode (LVM) spectroscopy has been used to study the evolution of vacancy-oxygen-related defects (VOn) in the temperature range 300-700°C in carbon-lean Cz-Si samples irradiated with MeV electrons or neutrons. New experimental data confirming an attribution of the absorption bands at 910, 976 and 1105 cm-1 to the VO3 complex are obtained. In particular, a correlated generation of VO3 and the oxygen trimer is observed upon irradiation of Cz- Si crystals in the temperature range 300-400°C. Strong evidence for the assignment of the bands at 991 and 1014 cm-1 to a VO4 defect is presented. The lines are found to develop very efficiently in the VO2 containing materials enriched with the oxygen dimer. In such materials the formation of VO4 is enhanced due to occurrence of the reaction O2i+VO2 ⇒ VO4. Annealing of the VO3 and VO4 defects at T ≥ 550C °C is found to result in the appearance of new defects giving rise to a number of O-related LVM bands in the range 990-1110 cm-1. These bands are suggested to arise from VO5 and/or VO6 defects. Similar bands also appear upon the annihilation of oxygen-related thermal double donors at 650°C in Cz-Si crystals pre-annealed at 450°C.

Abstract: Cz-grown, carbon-doped silicon samples were irradiated by fast neutrons. We investigated the annealing behaviour of oxygen-related defects, by infrared spectroscopy. We studied the reaction channels leading to the formation of various VmOn defects and in particular the VOn defects formed by the accumulation of oxygen atoms and vacancies in the initially produced by the irradiation VO defects, as the annealing temperature ramps upwards. We mainly focused on bands appearing in the spectra above 450 oC. A band at 1005 cm-1 is found to be the convolution of two bands at 1004 and 1009 cm-1. The latter band has the same thermal stability with the 983 cm-1 of the VO4 defect and therefore is also attributed to this defect. The former band has the same thermal stability with three other bands at 965, 1034 and 1048 cm-1. These four bands may be attributed to VOn (n=5,6) defects, although other VmOn complexes are also potential candidates. Furthermore, we found that pre-treatments of the samples at 1000 oC, with or without the application of high hydrostatic pressure lead to an increase in the concentration of the VO2, VO3 and generally VOn defects in comparison with that of the untreated samples.

Abstract: Effects of compressive stress on oxygen agglomeration processes in Czochralski grown silicon heat treated at T= 450OC, used as a reference temperature, and T= 600OC to 800OC are investigated in some detail. Compressive stresses of about P= 1 GPa lead to enhanced formation of Thermal Double Donors in materials annealed over a temperature range of T= 450OC – 600OC. It has been shown that the formation of thermal donors at T= 450OC under normal conditions and compressive stress is accompanied with loss of substitutional boron. In contrast, the concentration of the shallow acceptor states of substitutional boron in silicon annealed under stress at T≥ 600OC remains constant. An enhancement effect of thermal donor formation is gradually weakened at T≥ 700OC. The oxygen diffusivity sensitive to mechanical stress is believed to be responsible for the observed effects in heat-treated silicon.

Abstract: Oxygen precipitation and creation of defects in Czochralski grown silicon with interstitial oxygen concentration 9.4·1017 cm-3, subjected to irradiation with neutrons (5 MeV, dose 1x1017 cm-2) and subsequently treated for 5 h under atmospheric and high hydrostatic pressures (HP, up to 1.1 GPa) at 1270 / 1400 K, were investigated by spectroscopic and X - Ray methods. Point defects created by neutron irradiation stimulate oxygen precipitation and creation of dislocations under HP, especially at 1270 K. The effect of pressure treatment is related to changed concentration and mobility of silicon interstitials and vacancies as well as of the VnOm – type defects.