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ABSTRACT: Structural variations of SiOx matrix have been studied with Fourier Transform Infrared Spectroscopy (FTIR) during the formation of Si and Ge nanocrystal. Two frequently used methods, magnetron sputtering and ion implantation have been employed to form SiOx matrix containing excess Si and Ge. The Si-O-Si stretching mode has been deconvoluted to monitor the evolution of SiOx films during the annealing process. The integrated area and the shift in the SiOx peak positions are found to be well correlated with the change of the film stoichiometry and nanocrystal formation. It is shown that the nonstoichiometric SiOx matrix turns into stoichiometric SiO2 as the excess Si and Ge atoms precipitate to form nanocrystals. This process takes place at much lower temperatures for Ge than Si for both ion implantation and magnetron sputtering. FTIR technique is shown to be useful to study the matrix hosting nanocrystals to monitor nanocrystal formation.
Journal of Nanoscience and Nanotechnology 01/2010; 10(1):525-31. · 1.56 Impact Factor
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D. I. Tetelbaum,
A. N. Mikhaylov,
A. I. Belov,
A. V. Ershov,
E. A. Pitirimova,
S. M. Plankina,
V. N. Smirnov,
A. I. Kovalev, R. Turan,
S. Yerci,
T. G. Finstad,
S. Foss
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ABSTRACT: Photoluminescence, infrared Fourier spectroscopy, Raman scattering, transmission electron microscopy, and electron diffraction
were used to study the luminescent, optical, and structural properties of aluminum oxide layers (sapphire and films of Al2O3 deposited on silicon) implanted with Si+ to produce silicon nanocrystals. It is established that, in both cases, a high-temperature annealing of heavily implanted
samples brings about the formation of silicon nanocrystals. However, the luminescent properties of the nanocrystals are strongly
dependent on the type of pristine matrix; namely, nanocrystals in Al2O3 films emit light in the spectral range typical of Si quantum dots (700–850 nm), whereas in sapphire this photoluminescence
is not observed. This difference is interpreted as being due to the fact that local stresses arise in the nanocrystal/sapphire
system and break chemical bonds at the interface between the phases, whereas in Al2O3 films stresses are relaxed.
Physics of the Solid State 01/2009; 51(2):409-416. · 0.71 Impact Factor
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ABSTRACT: SiGe nanocrystals have been formed in SiO <sub>2</sub> matrix by cosputtering Si, Ge, and SiO <sub>2</sub> independently on Si substrate. Effects of the annealing time and temperature on structural and compositional properties are studied by transmission electron microscopy, x-ray diffraction (XRD), and Raman spectroscopy measurements. It is observed that Ge-rich Si <sub>(1-x)</sub> Ge <sub>x</sub> nanocrystals do not hold their compositional uniformity when annealed at high temperatures for enough long time. A segregation process leading to separation of Ge and Si atoms from each other takes place. This process has been evidenced by a double peak formation in the XRD and Raman spectra. We attributed this phase separation to the differences in atomic size, surface energy, and surface diffusion disparity between Si and Ge atoms leading to the formation of nonhomogenous structure consist of a Si-rich SiGe core covered by a Ge-rich SiGe shell. This experimental observation is consistent with the result of reported theoretical and simulation methods.
Journal of Applied Physics 01/2009; · 2.17 Impact Factor
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ABSTRACT: Nanocrystalline Ge films were prepared by isotropic chemical etching on single-crystalline Ge substrates with 100 and 111 orientations. The structural and optical properties have been investigated by transmission electron microscopy (TEM), electron diffraction (ED), Raman photoluminescence (PL), and infrared spectroscopy. The average size of nanocrystals (NCs) was estimated by fitting of the Raman spectra using a phonon-confinement model developed for spherical semiconductor NCs. Considered collectively TEM, ED, and Raman results indicate that all films contain high density of 3–4 nm diameter, diamond-structured Ge NCs with disordered surfaces. There are indications that surface of nanoparticles is mainly hydrogen terminated even for air-stabilized samples. Red PL is observed at room temperature upon excitation by 1.96 eV with peak energy of ∼1.55 eV and correlates well with recent theoretical calculations of the enlarged optical gap in Ge NCs of similar size.
Journal of Applied Physics 07/2008; · 2.17 Impact Factor
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ABSTRACT: In this study, e-beam evaporated Ag–In–Se (AIS) thin films were doped by the implantation of boron (B) ions at 75 keV with a dose of 1 × 1015 ions cm−2 and a subsequent annealing process was applied to the doped AIS films at different temperatures under nitrogen atmosphere. The effects of implantation and annealing on the electrical and photoelectrical properties of AIS thin films were investigated through temperature dependent conductivity, spectral photoresponse and photoconductivity measurements under different illumination intensities. The electrical conductivity measurements showed that the room temperature conductivity values were determined as 2.4 × 10−7 (Ω cm)−1, 1.7 × 10−6 (Ω cm)−1 and 8.9 × 10−5 (Ω cm)−1 for B-doped films (B0), B-doped and annealed films at 200 °C (B2) and at 300 °C (B3), respectively. It was observed that the electrical conductivity improved as the annealing temperature increased up to 400 °C at which the AIS thin films showed degenerate semiconductor behaviour. The spectral distribution of the photoresponse curves indicated three local maxima located at 1.63, 1.79 and 2.01 eV for B0 type films, 1.65, 1.87 and 2.07 eV for B2 type films and 1.73, 2.02 and 2.32 eV for B3 type films at room temperature. These three different energy values were ascribed to the splitting of the valence band due to spin–orbit interaction and crystalline lattice field effects. The first energy values of each set were determined to be energy band gaps of the AIS thin films. The photoconductivity measurements as a function of temperature and illumination intensity were performed on the B-doped AIS thin films in order to determine the nature of recombination processes in the films. The photoconductivity values were found to be thermally quenched for all types of thin films and the variation of photocurrent as a function of illumination intensity showed that the dependence of photocurrent on the intensity was supralinear. The two-centre recombination model was applied successfully in order to explain the photoconductivity behaviours of the films.
Journal of Physics D Applied Physics 05/2008; 41(11):115308. · 2.54 Impact Factor
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ABSTRACT: Ge nanocrystals were formed in Al2O3 matrix by implantation of Ge ions into sapphire (alpha-Al2O3) substrates and subsequent annealing. Diagnostic techniques, Raman spectroscopy, XRD, TEM, EDS, and SAED were employed to monitor and study formation of Ge nanocrystals and their evolution during heat treatments. TEM and EDS analysis revealed the diffusion of Ge ions into the substrate during annealing process. While Ge nanocrystals with mean sizes of 15 nm were observed in the heavily implanted region small nanocrystals with mean sizes of 4 nm were identified underneath this region. Some grains of transition aluminas were formed in the implanted region which was amorphized during the implantation. Extensive stress between the transition aluminas and sapphire matrices and its effects on the matrix were detected. The effect of stress on the Raman and XRD spectra of Ge nanocrystals was discussed.
Journal of Nanoscience and Nanotechnology 03/2008; 8(2):759-63. · 1.56 Impact Factor
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ABSTRACT: Depth profiles of Si nanocrystals formed in sapphire by ion implantation and the effect of charging during X-ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) measurements have been studied. Atomic concentration and the chemical environment of Si, Al, and O have been measured as a function of depth from the sample surface by SIMS and XPS. Both as-implanted and annealed samples have been analyzed to understand the effect of nanocrystal formation on the depth distribution, chemical structure, and the charging effect before and after the formation process. SIMS measurements have revealed that the peak position of the Si concentration shifts to deeper values with implantation dose. This is explained by the fact that the structure of the matrix undergoes a phase transformation from pure sapphire to a Si rich amorphous Al2O3 with heavy dose implantation. Formation of Si nanocrystals has been observed by XPS by an increase in the Si-Si signal and a decrease in Si-O bond concentrations after the annealing. Variation in binding energies of Si and O with Si concentration (i.e., with depth) has been studied in terms of chemical environments and charging effects. It is found that binding energy of these elements shifts to lower values with increasing Si content. This is a result of less charging due to the presence of easy discharge paths in the Si rich regions of the matrix. Nanocrystal formation leads to even less charging which is probably due to the further increase in conductivity with the formation.
Journal of Applied Physics 07/2007; 102(2):024309-024309-5. · 2.17 Impact Factor
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ABSTRACT: Electroluminescence (EL) and photoluminescence (PL) measurements were conducted on Si-implanted SiO2 layers as a function of process and measurement parameters. Measurable light emission was observed from the metal oxide semiconductor light emitting diode (MOS-LED) when holes are injected from the substrate. It was shown that major PL and EL emissions have the same origin. However, two important differences were observed between EL and PL spectra. The first one is the light emission from the Si substrate due to the recombination of electrons supplied by the front contact and holes that were accumulated in the inversion region at the substrate/SiO2 interface. This might be a factor reducing the contribution of Si nanocrystals to the EL emission of the MOS-LED structure as a result of decrease in the number of holes in the inversion layer. The second difference is that EL emission peaks stay at a slightly higher energy than PL peaks. It was observed that the EL peak shifts towards the PL peak with increasing bias voltage. This behaviour is explained by considering the size distribution of nanocrystals formed by ion implantation.
Semiconductor Science and Technology 09/2006; 21(12):1527. · 1.72 Impact Factor
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ABSTRACT: Si nanocrystals in thermal oxide films (~250 nm) were fabricated by 100 keV Si ion implantation at various doses followed by high temperature annealing. After annealing a sample implanted with a dose of 1×1017 cm−2 at 1050°C for 2 h, a broad photoluminescence peak centred around 880 nm was observed. A dose of 5×1016 cm−2 yields a considerable blue shift of about 100 nm relative to the higher dose. Transmission electron microscopy and atomic force microscopy (AFM) are used to characterize the microstructures in the SiO2 film. The limitations of these techniques for the study of the nanostructures are addressed in this paper and it is suggested that AFM combined with etching can yield a structural spectroscopy with very good sensitivity.
Physica Scripta 08/2006; 2006(T126):77. · 1.20 Impact Factor
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ABSTRACT: Photoluminescence (PL) properties of various Ge-based nanostructured materials, prepared using methods such as chemical etching, ion implantation and spark processing, have been studied. The PL was much stronger from the samples containing Ge nanocrystals, in comparison with their counterparts with no or less Ge nanoparticles, hinting at the likely role of the quantum confinement in Ge nanocrystals (NCs) in the light emission process. On the other hand, defects in the oxides or Ge–O bonded material alone are also shown to emit light in the same energy zone as with the Ge NCs. Discussion on these results is presented. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 05/2005; 202(8):1472 - 1476. · 1.46 Impact Factor
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ABSTRACT: Nanocrystals embedded in SiO2 films are the subject of a number of recent works, mainly because of their potential usefulness in the fabrication of optoelectronic devices and nanocrystal memory structures. One interesting method for the fabrication of such nanocrystals is the ion implantation of segregating species into SiO2 films followed by heat treatment in order to induce nanocrystal formation. This method is both relatively simple and also compatible with the current MOS (metal-oxide-semiconductor) device technology. An unintentional effect can occur during the fabrication of nanocrystals using this method, namely a significant diffusion of the implanted species during annealing, away from the regions with the highest concentration. The Si∕SiO2 interface can be exposed to this diffusion flux. This can result in an altered interface and have a significant influence on electronic devices. Here, we report on ion implantation of Ge into SiO2 on Si followed by annealing under conditions, resulting in Ge accumulation at the Si∕SiO2 interface as determined by secondary-ion mass spectroscopy analysis, transmission electron microscopy with energy dispersive analysis of x-rays, and Rutherford backscattering spectrometry. The accumulation of Ge at the Si∕SiO2 interface has also been reported before. The resulting effect on the electronic structure of the interface is a priori unknown. We have fabricated MOS capacitors on the sample structures and their capacitance-voltage characteristics were measured and analyzed. We measure an interface state density around 1×1012 cm−2, which is high compared to standard Si MOS devices. We discuss the results in terms of the previous electrical measurements on Ge-oxide interfaces and SiGe interfaces, which also can yield a high interface state density. The specific conditions we report result in a sufficiently low Ge concentration that nanocrystals are not segregated in the SiO2 film, while Ge still accumulates at the Si∕SiO2 interface after annealing.
Journal of Applied Physics 10/2004; 96(8):4308-4312. · 2.17 Impact Factor
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ABSTRACT: Ge nanocrystals formed in a SiO2 matrix by ion implantation were studied by Raman spectroscopy. It is shown that Raman analysis based on the phonon confinement model yields a successful explanation of the peculiar characteristics resulting from the nanocrystals. A broadening and a shift in the Raman peak are expected to result from the reduced size of the crystals. Asymmetry in the peak is attributed to the variations in the size of the nanocrystals. These effects were observed experimentally for the Ge nanocrystals prepared by ion implantation and explained theoretically by incorporating the effect of size and size distribution into the theoretical description of the Raman shift. A comparison with the transmission electron microscopy images indicated that this analysis could be used to estimate the structural properties of nanocrystals embedded in a host matrix. The evolution of nanocrystal formation with annealing temperature, i.e. the size growth, was monitored by Raman spectrometry for several samples and the corresponding nanocrystal sizes were estimated using the phonon confinement model.
Semiconductor Science and Technology 11/2003; 19(2):247. · 1.72 Impact Factor
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ABSTRACT: Metal-oxide semiconductor (MOS) structures fabricated on high dose Ge-implanted Si by thermal oxidation have been studied. From the analysis of capacitance-voltage (C-V) characteristics, the electrical properties of the interface between oxide and Si substrate were found to be greatly modified by a SiGe layer which forms by segregation of Ge during oxidation. The C-V curves are consistent with a model incorporating a narrower band gap than that of Si close to the interface. An electronic band model which includes a Si/SiGe heterojunction under the grown oxide is therefore used to interpret the experimental results. For n-type samples the C-V curves are analysed in terms of interface trap states. Variations in the density of interface states with the Ge implantation dose are examined. The effects of different oxidation conditions and N2 annealing on the C-V characteristics and trap density of the n-type samples are studied.
Semiconductor Science and Technology 12/1998; 7(1):75. · 1.72 Impact Factor
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ABSTRACT: Ge diffusion after solid‐phase epitaxy of Si amorphized by Ge<sup>+</sup><sup>+</sup> implantation has been measured by backscattering spectrometry. Asymmetrically enhanced diffusion is observed for 〈111〉‐oriented samples annealed at 1050 °C. A high concentration of twins observed by cross‐section transmission electron microscopy seems to be connected to the enhancement. There is a large difference in Ge diffusion as well as defect structure between 〈100〉‐ and 〈111〉‐oriented samples. Hole mobilities and sheet resistivities have been measured and compared for B‐ and B+Ge‐implanted samples. The effect of Ge on the electrical properties is small.
Journal of Applied Physics 09/1989; · 2.17 Impact Factor
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ABSTRACT: Embedded Si nanocrystals in SiO2 have a large current interest due to the prospects for practical applications. For most of these it is essential to characterize and ultimately control the nanocrystal size, size distribution and spatial distribution. Here we present a study of Si nanocrystals and clusters in SiO2 studied by atomic force microscopy (AFM). Since it is an indirect method, it requires several other methods to establish a reliable description of the structure of the samples. We here compare the AFM results with photoluminescence (PL) and transmission electron microscopy (TEM). Si nanocrystals in thermal oxide films (∼ 250 nm) were fabricated by 100 keV Si ion implantation at a dose of 1 × 1017 cm− 2 followed by high temperature annealing. AFM micrographs were taken after different etching times of the oxide and compared to TEM measuerements of the nanocrystal size and distribution. The correlations observed strongly indicate AFM signatures connected to the nanocrystals. We have analyzed and modeled the etch sectioning technique. Comparisons with the experiments let us conclude that the sectioning technique has some memory effect, but yields a distribution of nanocrystals with depth. A dose of 5 × 1016 cm− 2 yields a PL blue shift of about 100 nm relative to the higher dose. No nanocrystals are observed with TEM in this latter case. However distinct signatures can be observed with AFM and is tentatively attributed to the presence of non-crystalline Si-rich nanoclusters.
Thin Solid Films.
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ABSTRACT: Si nanocrystals in thermal oxide films (∼ 250 nm) were fabricated by 100 keV Si ion implantation followed by high temperature annealing. Two different doses were compared after annealing at 1050 °C for 2 h. A sample implanted with a dose of 1 × 1017 cm− 2 shows a broad photo luminescence peak centered around 880 nm after annealing. A dose of 5 × 1016 cm− 2 yields a considerable blue shift of about 100 nm relative to the higher dose as well as a reduction in intensity. Transmission electron microscopy study reveals a difference in the microstructure of the SiO2 films. Nanocrystals are clearly identified in the middle of the film for the highest dose, but not for the lower dose. The difference is discussed in terms of concentration dependent nucleation rate and differences in defect concentration. It is argued that the latter effect has a strong effect on the depth distribution of nanocrystals.
Surface and Coatings Technology 201:8482-8485. · 1.87 Impact Factor
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ABSTRACT: Si nanoclusters were formed by 28Si ion implantation into SiO2 matrix and subsequently annealed at 1050 °C for 2 h under N2 ambient. The photoluminescence (PL) characteristics depend on the 28Si fluence. The PL signals seen at around 775 nm and 825 nm are assigned to light emitting centers connected to formation of Si clusters, for 5 × 1016 cm−2 and 1 × 1017 cm−228Si fluences, respectively. Defect-related light emission observed at around 625 nm emerges only in the case of high implantation fluence. These sample sets were post implanted with 28Si ions at fluences between 1 × 1012 and 1 × 1014 cm−2 to monitor the variations in PL emission as a function of post implantation fluence. The PL emission decreases and exhibits a blue-shift with increasing post implantation fluence and quenches totally when the fluence exceeds a certain value, while the peak seen at 625 nm which is related to matrix defects is enhanced after the post implantation. Upon thermal annealing, the PL peaks measured at 775 nm and 825 nm recovered and reached the intensity of the peak before the post implantation. The variations in the PL peak with the implantation fluence and annealing conditions are ascribed to the different structure of Si nanoinclusions in the SiO2 matrix.
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 254(1):87-92. · 1.21 Impact Factor
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ABSTRACT: Si nanocrystals embedded in a wide bandgap material have been of interest for various electronic devices, including third-generation solar cells with efficiency values exceeding theoretical limits. In this work, Si-rich amorphous SiC layers with different Si contents were fabricated by the RF magnetron sputtering deposition technique. Si nanocrystal formation was induced by a high-temperature annealing process in a series of samples with different Si contents controlled by the DC power applied to the Si target during the sputtering. Nanocrystal formation was monitored to understand the basic kinetics as a function of process parameters, such as DC power and annealing temperature. In the SiC films containing excess Si, nanocrystal formation was clearly identified by Raman spectroscopy and high-resolution Transmission Electron Microscopy (TEM). Si nanocrystals with a mean size of 2 nm were imaged by TEM in the samples annealed at 1100 °C. The presence of Si–Si bonds was also detected by XPS through a series of experiments, including depth profiling of the chemical bonds of O, Si and C as a function of depth from the surface. Infrared spectroscopy was employed to study the stoichiometry of the SiC matrix with or without nanocrystals.
Physica E: Low-dimensional Systems and Nanostructures.
