ALBERT

Notes: An analysis of silicon on insulator structures obtained by single and multiple implants by means of Raman scattering and photoluminescence spectroscopy is reported. The Raman spectra obtained with different excitation powers and wavelengths indicate the presence of a tensile strain in the top silicon layer of the structures. The comparison between the spectra measured in both kinds of samples points out the existence in the multiple implant material of a lower strain for a penetration depth about 300 nm and a higher strain for higher penetration depths. These results have been correlated with transmission electron microscopy observations, which have allowed to associate the higher strain to the presence of SiO2 precipitates in the top silicon layer, close to the buried oxide. The found lower strain is in agreement with the better quality expected for this material, which is corroborated by the photoluminescence data.

Notes: In the present work, an analysis of the dark and optical capacitance transients obtained from Schottky Au:GaAs barriers implanted with boron has been carried out by means of the isothermal transient spectroscopy (ITS) and differential and optical ITS techniques. Unlike deep level transient spectroscopy, the use of these techniques allows one to easily distinguish contributions to the transients different from those of the usual deep trap emission kinetics. The results obtained show the artificial creation of the EL2, EL6, and EL5 defects by the boron implantation process. Moreover, the interaction mechanism between the EL2 and other defects, which gives rise to the U band, has been analyzed. The existence of a reorganization process of the defects involved has been observed, which prevents the interaction as the temperature increases. The activation energy of this process has been found to be dependent on the temperature of the annealing treatment after implantation, with values of 0.51 and 0.26 eV for the as-implanted and 400 °C annealed samples, respectively. The analysis of the optical data has corroborated the existence of such interactions involving all the observed defects that affect their optical parameters.

Notes: The optical and electrical recovery processes of the metastable state of the EL2 defect artificially created in n-type GaAs by boron or oxygen implantation are analyzed at 80 K using optical isothermal transient spectroscopy. In both cases, we have found an inhibition of the electrical recovery and the existence of an optical recovery in the range 1.1–1.4 eV, competing with the photoquenching effect. The similar results obtained with both elements and the different behavior observed in comparison with the native EL2 defect has been related to the network damage produced by the implantation process. From the different behavior with the technological process, it can be deduced that the electrical and optical anomalies have a different origin. The electrical inhibition is due to the existence of an interaction between the EL2 defect and other implantation-created defects. However, the optical recovery seems to be related to a change in the microscopic metastable state configuration involving the presence of vacancies.

Notes: The correlation between the structural (average size and density) and optoelectronic properties [band gap and photoluminescence (PL)] of Si nanocrystals embedded in SiO2 is among the essential factors in understanding their emission mechanism. This correlation has been difficult to establish in the past due to the lack of reliable methods for measuring the size distribution of nanocrystals from electron microscopy, mainly because of the insufficient contrast between Si and SiO2. With this aim, we have recently developed a successful method for imaging Si nanocrystals in SiO2 matrices. This is done by using high-resolution electron microscopy in conjunction with conventional electron microscopy in dark field conditions. Then, by varying the time of annealing in a large time scale we have been able to track the nucleation, pure growth, and ripening stages of the nanocrystal population. The nucleation and pure growth stages are almost completed after a few minutes of annealing time at 1100 °C in N2 and afterward the ensemble undergoes an asymptotic ripening process. In contrast, the PL intensity steadily increases and reaches saturation after 3–4 h of annealing at 1100 °C. Forming gas postannealing considerably enhances the PL intensity but only for samples annealed previously in less time than that needed for PL saturation. The effects of forming gas are reversible and do not modify the spectral shape of the PL emission. The PL intensity shows at all times an inverse correlation with the amount of Pb paramagnetic centers at the Si–SiO2 nanocrystal–matrix interfaces, which have been measured by electron spin resonance. Consequently, the Pb centers or other centers associated with them are interfacial nonradiative channels for recombination and the emission yield largely depends on the interface passivation. We have correlated as well the average size of the nanocrystals with their optical band gap and PL emission energy. The band gap and emission energy shift to the blue as the nanocrystal size shrinks, in agreement with models based on quantum confinement. As a main result, we have found that the Stokes shift is independent of the average size of nanocrystals and has a constant value of 0.26±0.03 eV, which is almost twice the energy of the Si–O vibration. This finding suggests that among the possible channels for radiative recombination, the dominant one for Si nanocrystals embedded in SiO2 is a fundamental transition spatially located at the Si–SiO2 interface with the assistance of a local Si–O vibration. © 2002 American Institute of Physics.

Notes: Si1−(x+y)GexBy strained layers on Si (x≤3.4%, y≤0.4%) have been analyzed by Raman spectroscopy. Stress in the layers has not been observed to affect the Fano interaction parameters of the first-order Si–Si Raman line. These parameters have been determined in the range of B concentrations from 5×1019 to 2×1020 cm−3 and Ge fractions from 1% to 3.4%. The observed shift in the spectra has been found to depend linearly on both the germanium and boron contents. These data have been correlated with the stress measured in the layers by mechanical wafer bow measurements. The dependence of the Raman shift on the germanium content and strain agrees with that previously reported for strained SiGe layers. According to these data, Raman spectroscopy appears as an interesting tool for the nondestructive assessment of stress and composition of these layers. © 1996 American Institute of Physics.

Notes: The validity of optical absorption (OA) as a technique for the measurement of strain e11, alloy composition x, and relaxation in InxGa1−xAs epilayers on InP has been examined by comparison with similar measurements by double-crystal x-ray diffraction (DCXD). Provided that the strain arising from differences in the thermal contraction of the substrate and epilayer are taken into account, measurements of strain by OA show good agreement with DCXD results, with a dispersion of Δe11=±0.27×10−3. Comparison of alloy compositions given by the two techniques shows similarly good agreement, with a dispersion in the values of x of less than Δx=±0.7%. OA may also be used to determine lattice relaxation. The degree of uncertainty in the measurement of this parameter increases as lattice match is approached and decreases as the lattice relaxes. Our studies indicate that OA may be used as an independent technique to evaluate strain, alloy composition, and the degree of lattice relaxation in InxGa1−xAs epilayers. © 1995 American Institute of Physics.

Notes: High-dose carbon-ion-implanted Si samples have been analyzed by infrared spectroscopy, Raman scattering, and x-ray photoelectron spectroscopy (XPS) correlated with transmission electron microscopy. Samples were implanted at room temperature and 500 °C with doses between 1017 and 1018 C+/cm2. Some of the samples were implanted at room temperature with the surface covered by a capping oxide layer. Implanting at room temperature leads to the formation of a surface carbon-rich amorphous layer, in addition to the buried implanted layer. The dependence of this layer on the capping oxide suggests this layer to be determined by carbon migration toward the surface, rather than surface contamination. Implanting at 500 °C, no carbon-rich surface layer is observed and the SiC buried layer is formed by crystalline β-SiC precipitates aligned with the Si matrix. The concentration of SiC in this region as measured by XPS is higher than for the room-temperature implantation. © 1995 American Institute of Physics.

Notes: The analysis of SiC films obtained by carbon ion implantation into amorphous Si (preamorphized by Ge ion implantation) has been performed by infrared and Raman scattering spectroscopies, transmission electron microscopy, Rutherford backscattering, and x-ray photoelectron spectroscopy (XPS). The data obtained show the formation of an amorphous Si1−xCx layer on top of the amorphous Si one by successive Ge and C implantations. The fitting of the XPS spectra indicates the presence of about 70% of Si–C bonds in addition to the Si–Si and C–C ones in the implanted region, with a composition in the range 0.35〈x〈0.6. This points out the existence of a partial chemical order in the layer, in between the cases of perfect mixing and complete chemical order. Recrystallization of the layers has been achieved by ion-beam induced epitaxial crystallization (IBIEC), which gives rise to a nanocrystalline SiC layer. However, recrystallization is not complete, observing still the presence of Si–Si and C–C bonds in an amorphous phase. Moreover, the distribution of the different bonds in the IBIEC processed samples is similar to that from the as-implanted ones. This suggests that during IBIEC homopolar bonds are not broken, and only regions with dominant Si–C heteropolar bonds recrystallize. © 1996 American Institute of Physics.

Notes: In this letter, we report the ion-beam synthesis and the structural characterization of ZnS nanocrystals in SiO2. Both electron diffraction and x-ray diffraction measurements show the precipitation of ZnS nanocrystals having a wurtzite–2H structure and infrared spectroscopy confirms the presence of Zn–S bonding. Upon annealing, transmission electron microscopy observations show the Ostwald ripening of the precipitates coupled with a self-organization in two layers parallel to the free surface. This self-organization has been also detected by secondary ion mass spectroscopy, and its origin is discussed in terms of a pure Ostwald ripening process and/or a consequence of the implantation damage. © 1998 American Institute of Physics.

Notes: The size distribution, band gap energy, and photoluminescence of silicon nanocrystals embedded in SiO2 have been measured by direct and independent methods. The size distribution is measured by coupling high-resolution and conventional electron microscopy in special imaging conditions. The band gap is calculated from photoluminescence excitation measurements and agrees with theoretical predictions. Their correlation allows us to report the experimental Stokes shift between absorption and emission, which is 0.26±0.03 eV, independent of average size. This is almost exactly twice the energy of the Si–O vibration (0.134 eV). These results suggest that the dominant emission is a fundamental transition spatially located at the Si–SiO2 interface with the assistance of a local Si–O vibration. © 2000 American Institute of Physics.