Rasit Turan

Effect of electroless etching parameters on the growth and reflection properties of silicon nanowires

Vertically aligned silicon nanowire (Si NW) arrays have been fabricated over large areas using an electroless etching (EE) method, which involves etching of silicon wafers in a silver nitrate and hydrofluoric acid based solution. A detailed parametric study determining the relationship between nanowire morphology and time, temperature, solution concentration and starting wafer characteristics (doping type, resistivity, crystallographic orientation) is presented. The as-fabricated Si NW arrays were analyzed by field emission scanning electron microscope (FE-SEM) and a linear dependency of nanowire length to both temperature and time was obtained and the change in the growth rate of Si NWs at increased etching durations was shown. Furthermore, the effects of EE parameters on the optical reflectivity of the Si NWs were investigated in this study. Reflectivity measurements show that the 42.8% reflectivity of the starting silicon wafer drops to 1.3%, recorded for 10 µm long Si NW arrays. The remarkable decrease in optical reflectivity indicates that Si NWs have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.

Effect of particle properties and light polarization on the plasmonic resonances in metallic nanoparticles

The resonance behavior of localized surface plasmons in silver and gold nanoparticles was studied in the visible and near-infrared regions of the electromagnetic spectrum. Arrays of nano-sized gold (Au) and silver (Ag) particles with different properties were produced with electron-beam lithography technique over glass substrates. The effect of the particle size, shape variations, period, thickness, metal type, substrate type and sulfidation were studied via transmission and reflectance measurements. The results are compared with the theoretical calculations based on the DDA simulations performed by software developed in this study. We propose a new intensity modulation technique based on localized surface plasmons in nanoparticles with asymmetric shapes.

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.

Silicon nanowire - poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) heterojunction solar cells

Radial heterojunctions are known to exhibit magnificent anti-reflectivity and enhanced carrier collectivity due to short carrier diffusion distances. In this work, silicon nanowire-poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) radial heterojunction solar cells are presented. Both layers of the heterojunction are fabricated using simple and cost-effective methods. Radial heterojunctions showed remarkable improvements in solar cell characteristics compared to planar heterojunctions, fabricated under the same conditions. The highest solar cell efficiency of 5.30% is obtained. The cells exhibit external quantum efficiency of 77% at 500 nm wavelength and harvest light over the entire 300-1200 nm spectral bandwidth. The effect of nanowire length on device performance is also determined.

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.

Silicon nanowire network metal-semiconductor-metal photodetectors

We report on the fabrication and characterization of solution-processed, highly flexible, silicon nanowire network based metal-semiconductor-metal photodetectors. Both the active part of the device and the electrodes are made of nanowire networks that provide both flexibility and transparency. Fabricated photodetectors showed a fast dynamic response, 0.43 ms for the rise and 0.58 ms for the fall-time, with a decent on/off ratio of 20. The effect of nanowire-density on transmittance and light on/off behavior were both investigated. Flexible photodetectors, on the other hand, were fabricated on polyethyleneterephthalate substrates and showed similar photodetector characteristics upon bending down to a radius of 1 cm.

“N” structure for type-II superlattice photodetectors

In the quest to raise the operating temperature and improve the detectivity of type II superlattice (T2SL) photodetectors, we introduce a design approach that we call the “N structure.” N structure aims to improve absorption by manipulating electron and hole wavefunctions that are spatially separated in T2SLs, increasing the absorption while decreasing the dark current. In order to engineer the wavefunctions, we introduce a thin AlSb layer between InAs and GaSb layers in the growth direction which also acts as a unipolar electron barrier. Unlike the symmetrical insertion of AlSb into GaSb layers, N design aims to exploit the shifting of the electron and hole wavefunctions under reverse bias. With cutoff wavelength of 4.3 μm at 77 K, temperature dependent dark current and detectivity measurements show that the dark current density is 3.6 × 10−9 A/cm2, under zero bias. Photodetector reaches background limited infrared photodetection (BLIP) condition at 125 K with the BLIP detectivity (D*BLIP) of 2.6 × 1010 ...

Electrical transport at a non-ideal CrSi2-Si junction

Electrical transport through a CrSi2/n-Si Schottky junction was investigated by internal photo emission spectroscopy and electrical current–voltage (I–V) techniques in a wide temperature range. When the thermionic emission theory is used in its common form the apparent barrier height is found to be strongly temperature dependent. Internal photoemission measurements yielded a barrier height weakly temperature dependent for these samples. This difference between optical and electrical results shows that the optical transport is dominated by a single photoemission over the expected barrier at the junction while the electrical transport is determined by more than one current mechanism. Two different models based on the presence of more than one current channel and mechanism are developed in order to describe measured I–V curves. In both approaches, it was assumed that the junctions interface contains small local regions through which charge carriers can flow. In the first model, the current flow through these small regions is assumed to be of the tunneling type. Experimental results agree reasonably well by using this approach. In the second model, the barrier height in these small regions is assumed to vary statistically according to a distribution function. The result of this latter model is not satisfactory in the whole temperature range.

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.

On the origin of the 2.2-2.3 eV photoluminescence from chemically etched germanium

The photoluminescence (PL) at B2.2–2.3 eV from Ge-based nanocrystalline materials is described in the literature as nanocrystal size-independent. We have observed visible luminescence from two different types of stain-etched Ge samples, one prepared after Sendova-Vassileva et al. (Thin Solid Films 255 (1995) 282) in a solution of H2O2:HF at 50:1 volume ratio, and the other in a solution of HF:H3PO4:H2O2 at 34:17:1 volume ratio. Energydispersive X-rayanalysis (EDX), Raman and FTIR spectroscopy, and the near edge X-ray absorption structure (XANES), indicate that the chemically etched Ge layers of the former type of samples are composed of non-stoichometric Ge oxides, i.e. GeOx (0oxo2), and free from anyGe nanoconstructions. It is also suggested from XANES that the latter type of chemically etched Ge samples comprise 8–9 nm nanocrystals of Ge, surface-covered with mainly oxygen. Photoluminescence occurred at B2.3 eV for all samples. The PL behavior of the latter type of chemically etched Ge on annealing in different chemical environments (air or H) allowed us to conclude that the PL from these materials, as well as that from those Ge-based nanocrystalline materials reported in the literature, is from GeOxs. r 2002 Elsevier Science B.V. All rights reserved.