Edge photoluminescence of single-crystal silicon at room temperature

The edge photoluminescence of single-crystal silicon (c-Si) with a peak at ～1.09 eV at room temperature is observed for structures that consist of nanocrystalline silicon (nc-Si) and c-Si. The structures are obtained by pulsed-laser deposition of an nc-Si film onto a c-Si substrate. The photoluminescence signal increases as both the density of surface states at the nc-Si/c-Si boundary and the scattering of the edge emission from c-Si in the nc-Si film decreases.     

Photoluminescent and electronic properties of nanocrystalline silicon doped with gold

Doping nanocrystalline silicon (nc-Si) films grown by laser ablation with gold leads to a considerable suppression of the nonradiative recombination of the charge carriers and excitons, an increase in the intensity and stability of the visible photoluminescence, and enhancement of the low-energy (1.5–1.6 eV) photoluminescence band. In Au-doped samples, the magnitude of the photovoltage and the rate of electron capture by traps in the film are reduced, and the density of boundary electron states and the concentration of deep electron traps at the single-crystal silicon (c-Si) substrate are decreased as well. The observed effect of doping on the photoluminescent and electronic properties of nc-Si films and nc-Si/c-Si structures is caused by the passivation of dangling Si bonds with Au and by the further oxidation of silicon at the surface of nanocrystals, which results in the formation of high-barrier SiO₂ layers.     

Enhancement of the Raman scattering in grooved silicon structures

The Raman effect in grooved silicon structures consisting of an array of cavities (grooves) and silicon layers is studied. It is found that the intensity of the Stokes component increases severalfold when the thickness of the silicon layers (1–2 μm) is close to the excitation wavelength. The results obtained, interpreted as a manifestation of the effects of localization of light, suggest that grooved structures of this kind offer promise as matrices providing for enhanced efficiency of the Raman scattering.     

Changes in properties of a 〈porous silicon〉/silicon system during gradual etching off of the porous silicon layer

Scanning electron microscopy has shown etching of a porous silicon layer in an HF solution to be irregular. The intensity of porous silicon photoluminescence significantly decreases during gradual etching off, and its peak initially shifts to shorter and then to longer wavelengths. Under red-light pulse excitation, photo-voltage measurements have shown that the boundary potential ?_s of the p-Si substrate is positive, and ?_s grows with the etching time and as the temperature decreases from 300 to 200 K. At T<230 K, the photomemory of ?_s caused by nonequilibrium electron capture by p-Si boundary traps is observed. The concentration of shallow traps and boundary electron states in p-Si increases as porous silicon is etched. At T<180 K, the system of boundary electron states is rearranged. Photovoltage measurements with white-light pulses have revealed electron capture at oxide traps of aged porous silicon.     

Enhancement of photoluminescence and raman scattering in one-dimensional photonic crystals based on porous silicon

In porous-silicon-based multilayered structures that exhibit the properties of one-dimensional photonic crystals, an increase in the photoluminescence and Raman scattering intensities is observed upon optical excitation at the wavelength 1.064 μm. When the excitation wavelength falls within the edge of the photonic band gap of the structures, a multiple increase (by a factor larger than 400) in the efficiency of Raman scattering is detected. The effect is attributed to partial localization of excitation light and, correspondingly, to the much longer time of interaction of light with the material in the structures.     

Charge carrier transport in a structure with silicon nanocrystals embedded into oxide matrix

Current-voltage characteristics of Al/SiO₂/c-Si structures with silicon nanocrystals (nc-Si) in the oxide layer are studied in a wide temperature range. The analysis based on the experimental data has shown that thermostimulated tunneling via electron states in nc-Si is a most probable mechanism of charge carrier transport in these structures.     

Photosensitivity of a-C:H/c-Si heterojunctions

Layers of a-C:H were grown on c-Si wafers by the glow discharge method in a CH₄ + Ar gaseous mixture. The electrical and photoelectric properties of a-C:H/c-Si heterojunctions were studied. It was found that the heterojunctions display rectification and broad-band photovoltaic effects. It is shown that the polarization sensitivity in these structures occurs at an oblique incidence of linearly polarized light under the illumination of the surface coated with a-C:H layers. The observed oscillations in the spectrum of the coefficient of induced photopleochroism are attributed to the interference of light in these layers.     

Transport properties and photosensitivity of metal/porous-silicon/c-Si structures

The current-voltage characteristics, photosensitivity, and impedance of p-type Al/porous-silicon/c-Si structures with 0.2 to 6-μm-thick porous layers of 80% porosity are studied. It is shown that at reverse and small forward bias voltages the current is determined by the potential barrier of the c-Si substrate at the isotypic porous-silicon/c-Si heterojunction. The photosensitivity is determined by the absorption of light in the c-Si substrate. The potential barrier of the metal/porous-silicon contact does not influence the photosensitivity or the currentvoltage characteristics of the structures. The experimental plots of the dependence of the impedance on applied forward bias, thickness of porous silicon layer, and frequency agree well with the theoretical dependences, if an equivalent circuit including two RC circuits connected in series and comprised of the resistance and geometric capacitance of the porous silicon layer and the resistance and capacitance of the potential barrier of the c-Si substrate is used. Fiz. Tekh. Poluprovodn. 33, 211–214 (February 1999)     

a-SiOx:H passivation layers for Cz-Si wafer deposited by hot wire chemical vapor deposition

In order to get the high photoelectric conversion efficiency a-Si:H/c-Si solar cells, high quality intrinsic hydrogenated passivation layer between the a-Si:H emitter layer and the c-Si wafer is necessary. In this work, hot wire chemical vapor deposition (HWCVD) is used to deposite intrinsic oxygen-doped hydrogenated amorphous silicon (a-SiO_x:H) and hydrogenated amorphous silicon (a-Si:H) films as the intrinsic passivation layer for a-Si:H/c-Si solar cells. The passivation effect of the films on the c-Si surface is shown by the effective lifetime of the samples that bifacial covered by the films with same deposition parameters, tested by QSSPC method. The imaginary part of dielectric constant (ε₂) and bonds structure of the layers are analyzed by Spectroscopic Ellipsometry(SE) and Fourier Transfom Infrared Spectroscopy(FTIR). It is concluded that: (1) HWCVD method can be used to make a-SiO_x:H films as the passivation layer for a-Si:H/c-Si cells and the oxidation of the filament can be overcome by optimizing the deposition parameters. In our experiments, the lowest surface recombination velocity of the c-Si wafer is 3.0 cm/s after a-SiO_x:H films passivation. (2) Oxygen-doping in the amorphous silicon layers can increase H content and the band-gap of films, similar as the phenomenon of the films deposited by PECVD.     

Impact ionization of excitons in single-crystal silicon and its effect on the exciton concentration and luminescence near the fundamental absorption edge

The effect of impact ionization of excitons by free charge carriers on the exciton concentration in single-crystal silicon (c-Si) at room temperature and at a high injection level is investigated. At sufficiently high concentrations of free electrons (n), the impact ionization dominates over thermal ionization. At such n, the effect results in much lower exciton concentrations (n _ex) compared to those with disregard of it and linear or almost linear portions in the dependences n _ex(n) and the dependences of the near-band-edge luminescence intensity in c-Si on the intensity of its excitation. The proposed technique for calculating n _ex can be developed for other semiconductors at other temperatures.     

Mechanism of the subband excitation of photoluminescence from erbium ions in silicon under high-intensity optical pumping

The photoluminescence (PL) excitation spectra of erbium and band-to-band silicon in Si:Er/Si epitaxial structures under high-intensity pulsed optical excitation are studied. It is shown that the nonmonotonic dependence of the PL intensity on the excitation wavelength ??_ex near the absorption edge of silicon is due to inhomogeneity in the optical excitation of the Si:Er active layer. The sharp rise in the erbium PL intensity in the spectral range ??_ex = 980?1030 nm is due to an increase in the excited part of the Si:Er emitting layer on passing to subband light pumping (??_ex > 980 nm) with a low absorption coefficient in silicon because of the effective propagation of the excitation light in the bulk of the structures under study. It is shown that, under the subband optical pumping of Si:Er/Si structures, as also in the case of interband pumping, the exciton mechanism of erbium ion excitation is operative. Excitons are generated under the specified conditions as a result of a two-stage absorption process involving impurity states in the band gap of silicon.     

Photoelectric properties of ZnO/CuPc/Si structures

n-ZnO:Al/CuPc/n(p)-Si structures were formed using vacuum-evaporation deposition of copper phthalocyanine (CuPc) layers on the surface of n-and p-Si wafers with subsequent magnetron-sputtering deposition of ZnO:Al layers on the CuPc surface. It is shown that the structures obtained exhibit a high photosensitivity (～80 V/W at T=300 K) in the spectral range 1.65–3.3 eV. The rectification factor and photovoltaic effect in these structures are discussed in relation to the properties of silicon substrates. It is concluded that the contact of phthalocyanine with diamond-like semiconductors (e.g., silicon) are promising for application in wide-band high-efficiency photovoltaic converters.     

Photoelectric phenomena in (μc xa1−x )-Si:H/c-Si heterostructures

Heterostructures (μc _xa_1−x )-Si:H/c-Si with a various volume fraction x of microcrystalline and amorphous Si phases were obtained by plasmochemical deposition. The fraction variation was achieved by changing the silane content in H₂. The steady-state current-voltage characteristics and spectral dependences of photosensitivity of the structures obtained were investigated. The latter dependences were recorded by exposing the samples to the natural and linearly polarized light in relation to the phase composition of thin (d ₁ ≅ 0.6–0.8 μm) films of microcrystalline and amorphous Si. The photovoltaic effect and induced photopleochroism of the structures obtained were detected. The prospects of using a new type of heterojunction in photoconverters of natural and linearly polarized light were assessed. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 11, 2001, pp. 1316–1319. Original Russian Text Copyright ? 2001 by Mell, Nikolaev, V. Rud’, Yu. Rud’, Terukov.     

Erbium ion electroluminescence in p ++/n +/n-Si:Er/n ++ silicon diode structures

Results of experimental studies of erbium ion electroluminescence in p ⁺⁺/n ⁺/n-Si:Er/n ⁺⁺ silicon diode structures grown by sublimation molecular-beam epitaxy are discussed. The distinctive feature of these structures is that the regions of electron flux formation of (n ⁺-Si) and impact excitation of erbium ions (n-Si:Er) are spaced. The influence of the n ⁺-Si layer thickness on electrical and electroluminescent properties of diodes was studied. It was shown that n ⁺-Si layer thinning causes the transformation of the structure breakdown mechanism from tunneling to avalanche. The dependence of the Er³⁺ ion electroluminescence on the thickness of the heavily doped n ⁺-Si region is bell-shaped. At the n ⁺-Si-layer doping level n ≈ 2 × 10¹⁸ cm^?3, the maximum electroluminescence intensity is attained at an n ⁺-Si layer thickness of ～23 nm.     

Optical and electrical properties of thin wafers fabricated from nanocrystalline silicon powder

The infrared transmittance spectra and dark conductivity of wafers fabricated from nanocrystalline silicon powder (nc-Si) have been studied. The initial nc-Si powder is first synthesized by laser-induced silane dissociation, with the temperature of the surrounding buffer gas varied from 20 to 250°C, and then compacted at pressures from 10⁸ to 10⁹ Pa. It is found that compaction of the nc-Si powder results in formation of Si-H, Si-CH_x, and O_y-Si-H_x structures (x, y=1–3). The formed structures break down under annealing, with the Si-H and Si-CH_x complexes disintegrating at the lowest annealing temperature (t = 160°C). The dark conductivityof the nc-Si wafers is shown to increase along with the buffer gas temperature at which the starting powder was prepared. Two temperature regions are found in which the dark conductivity behaves in radically different ways. At wafer temperatures T ≥ 270 K, conductivity is mediated by free carriers, whereas, at lower temperatures, electron transport is governed by hopping conduction over localized states in the band gap.     

基于不同单光子能量拉曼谱的氢化硅薄膜微观特性研究

用等离子体增强化学汽相沉积法（PECVD）在玻璃和单晶硅（c-Si）衬底上分别制备了氢化纳米硅（nc-Si：H）和非晶硅（a-Si：H）薄膜,用紫外、可见和近红外3种不同波长的激光线对不同形态的Si薄膜进行拉曼散射实验研究。研究发现,这些Si薄膜在不同的单光子能量的激光线激发下的拉曼谱线形也不同。进而通过对Si薄膜材料...     

Effect of the Distribution of the Radiation Defect on the Field-Emission Properties of Silicon Crystals

The correlated regularities of the variations in the structural and phase composition and morphological and field-emission characteristics of the surface-structured р- and n-Si crystals upon stepwise highdose ion-beam carbon processing are studied. It is shown that the stepwise high-dose ion implantation of carbon atoms into the surface of silicon wafers structured using nonlithographic carbon mask coatings makes it possible to reduce field-emission thresholds and increase the densities of the maximum field-emission currents by more than two orders of magnitude relative to the values for emitter arrays fabricated using traditional microelectronic technologies. The physicochemical mechanisms responsible for modifying the surface properties of silicon structures upon carbon ion implantation are discussed.     

Distribution of the density of electronic states in the energy gap of microcrystalline hydrogenated silicon

Photomodulation spectroscopy was applied to a study of the distribution of the density of electronic states in the energy gap of microcrystalline hydrogenated silicon (μc-Si:H) with a varied level of boron doping. The information on the density-of-states distribution was extracted by analyzing temperature dependences of the constant and modulated components of photoconductivity in a sample exposed to modulated light. The distributions of the density of electronic states in the upper and lower halves of the energy gap of μc-Si:H were determined. The study demonstrated that the tail of the density-of-states distribution near the valence band is less steep than that near the conduction band.     

Electrical properties of Si:H/<Emphasis Type="Italic">p</Emphasis>-Si structures fabricated by hydrogen implantation

Hydrogenated silicon (Si:H) layers and Si:H/p-Si heterostructures were produced by multiple-energy (3–24 keV) high-dose (5×10¹⁶–3×10¹⁷ cm^?2) hydrogen implantation into p-Si wafers. After implantation, current transport across the structures is controlled by the Poole-Frenkel mechanism, with the energy of the dominating emission center equal to E _c ?0.89 eV. The maximum photosensitivity is observed for structures implanted with 3.2×10¹⁷ cm^?2 of hydrogen and annealed in the temperature range of 250–300°C. The band gap of the Si:H layer E _g ≈2.4 eV, and the dielectric constant ?≈3.2. The density of states near the Fermi level is (1–2)×10¹⁷ cm^?3 eV^?1.     

Optical and structural properties of thin films precipitated from the sol of silicon nanoparticles

A new technique of growing nanocrytalline silicon (nc-Si) thin films is suggested. The technique involves the centrifuge-assisted size-selective deposition of nanoparticles from a colloidal solution (sol) containing nc-Si powders. The structural and optical parameters of the initial nc-Si powders and films deposited by the newly suggested procedure are studied by transmission electron microscopy and analysis of absorption spectra and Raman spectra. The absorption coefficient of the nc-Si films increases with decreasing dimensions of the constituent nanoparticles. The experimentally measured band gap of the films, E _g, is widened from 1.8 to 2.2. eV on etching the nc-Si powders used for deposition of the corresponding films. On the basis of the analysis of the Raman spectra, it is suggested that the amorphous component is involved in the nc-Si powders and films due to oxygen atoms arranged at the nanoparticle surface.