U. Serincan

Characterization of Ge nanocrystals embedded in SiO2 by Raman spectroscopy

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.

Formation of silicon nanocrystals in sapphire by ion implantation and the origin of visible photoluminescence

Silicon nanocrystals, average sizes ranging between 3 and 7 nm, were formed in sapphire matrix by ion implantation and subsequent annealing. Evolution of the nanocrystals was detected by Raman spectroscopy and x-ray diffraction XRD. Raman spectra display that clusters in the matrix start to form nanocrystalline structures at annealing temperatures as low as 800 ° C in samples with high dose Si implantation. The onset temperature of crystallization increases with decreasing dose. Raman spectroscopy and XRD reveal gradual transformation of Si clusters into crystalline form. Visible photoluminescence band appears following implantation and its intensity increases with subsequent annealing process. While the center of the peak does not shift, the intensity of the peak decreases with increasing dose. The origin of the observed photoluminescence is discussed in terms of radiation induced defects in the sapphire matrix.

Structural and optical properties of porous nanocrystalline Ge

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.

Effect of the Passivation Layer on the Noise Characteristics of Mid-Wave-Infrared InAs/GaSb Superlattice Photodiodes

The authors describe the noise characterization of a mid-wavelength-infrared (MWIR) photodiode based on indium arsenide and gallium antimonide (InAs/GaSb) superlattice (SL), addressing the influence of different passivation layers applied to the surface of the device. The MWIR InAs/GaSb SL design structure is based on p-i-n configuration grown by the molecular beam epitaxy on a (001) n-GaSb substrate. The SiO2-passivated SL photodiodes demonstrated a Schottky-limited noise up to a bias voltage of -0.1 V where the measured peak responsivity is 1.37 AAV with a cut-off wavelength of 4.9 μm and the specific detectivity as high as 1.23 × 1012 cm. Hz1/2 /W, demonstrating the high quality of the fabricated MWIR SL photodiodes. The noise measurements exhibited a frequency-dependent plateau (i.e., 1/f noise) for unpassivated and Si3N4-passivated samples, whereas 1/f-type noise suppression (i.e., frequency-independent plateau) with a noise current reduction at about 30 Hz of more than one order of magnitude was observed for the SiO2-passivated ones.

Introduction of Si∕SiO2 interface states by annealing Ge-implanted films

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 a...

Electroluminescence generated by a metal oxide semiconductor light emitting diode (MOS-LED) with Si nanocrystals embedded in SiO2 layers by ion implantation

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.

Nanocrystal and nanocluster formation and oxidation in annealed Ge-implanted SiO2 films

Abstract There has been much interest in semiconductor nanocrystals embedded in oxides and their interesting optical and electrical properties, which can potentially be utilised in future devices. We have studied the effects of different processing parameters on the formation of Ge nanocrystals in SiO2 prepared by ion implantation followed by heat treatment. We implanted Ge doses of between 2×1015 and 1×1017 cm−2 at implantation energy of 30 or 100 keV and used a range of annealing temperatures and times. The samples were investigated using transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDS), secondary ion mass spectroscopy (SIMS) and Raman spectroscopy. Ge nanocrystals were observed for doses of 3×1016 and 1×1017 cm−2 at 100 keV after annealing at 800 °C. Annealing of similar samples at 1000 °C yielded no nanocrystals. We believe that diffusion of oxidising species from the atmosphere is important and that this process is faster at 1000 than at 800 °C, creating Ge-rich amorphous oxides rather than Ge nanocrystals at the higher temperature. This oxidation process also explains the absence of Ge nanocrystals in SiO2 films implanted with Ge at 30 keV after annealing. Electron beam-induced precipitation was observed in samples with amorphous Ge-rich layers under intense electron irradiation in the TEM. Accumulation of Ge at the Si/SiO2 interface was observed in samples implanted at 100 keV and annealed at 1000 °C or higher temperatures.

The quantum confined Stark effect in silicon nanocrystals

The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures. It is shown that the PL peak position shifts to higher wavelengths with increasing applied electric field, which is expected from carrier polarization within the quantum dots. It is observed that the effect is more pronounced at lower temperatures due to the improved carrier localization at the lowest energy states of the quantum dots. Experimental results are shown to be in good agreement with phenomenological model developed for the QCSE model.

Depth profile investigations of silicon nanocrystals formed in sapphire by ion implantation

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 concentratio...

Luminescence from silicon nanoparticles in SiO2: atomic force microscopy and transmission electron microscopy studies

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.