Study of the correlation properties of the surface structure of <Emphasis Type="Italic">nc</Emphasis>-Si/<Emphasis Type="Italic">a</Emphasis>-Si:H films with different fractions of the crystalline phase

The correlation properties of the structure of nc-Si/a-Si:H films with different volume fractions of the crystalline phase are studied using 2D detrended fluctuation analysis. Study of the surface relief of experimental samples showed that with increasing in volume fraction of the crystalline phase in the nc-Si/a-Si:H films, the size and number of nanoclusters on their surface grow. The size of Si nanocrystals in the a-Si:H matrix (6–8 nm) indicates the formation of coarse nanoclusters due to the self-organization of Si nanocrystals in groups under laser radiation. According to 2D detrended fluctuation analysis data, the number of correlation vectors (harmonic components) in the nc-Si/a-Si:H film structure increased with an increase in the nanocrystal fraction in the films.     

Optical properties of <Emphasis Type="Italic">p–i–n</Emphasis> structures based on amorphous hydrogenated silicon with silicon nanocrystals formed via nanosecond laser annealing

Silicon nanocrystals are formed in the i layers of p–i–n structures based on a-Si:H using pulsed laser annealing. An excimer XeCl laser with a wavelength of 308 nm and a pulse duration of 15 ns is used. The laser fluence is varied from 100 (below the melting threshold) to 250 mJ/cm² (above the threshold). The nanocrystal sizes are estimated by analyzing Raman spectra using the phonon confinement model. The average is from 2.5 to 3.5 nm, depending on the laser-annealing parameters. Current–voltage measurements show that the fabricated p–i–n structures possess diode characteristics. An electroluminescence signal in the infrared (IR) range is detected for the p–i–n structures with Si nanocrystals; the peak position (0.9–1 eV) varies with the laser-annealing parameters. Radiative transitions are presumably related to the nanocrystal–amorphous-matrix interface states. The proposed approach can be used to produce light-emitting diodes on non-refractory substrates.     

ESR studies of nanocrystalline silicon films obtained by pulsed laser ablation of silicon targets

Nanocrystalline silicon films formed using laser ablation of silicon targets were studied using electron spin resonance. The measurements were performed in the X band with modulation of the magnetic field at a frequency of ～100 kHz at temperatures of 300 and 77 K. Two types of spectra were observed. The first type of spectra is related to the high concentration of dangling silicon bonds in Si nanocrystals and SiO_x sheaths of nanocrystals and are inherent in nanocrystalline silicon (nc-Si) films that do not exhibit photoluminescence in the visible region of the spectrum. The second type of spectra is related to the presence of E′ centers, nonbridging oxygen hole centers (NBOHC), and peroxide radicals and is characteristic of films with photoluminescence in the visible region of the spectrum, which indicates that high-barrier SiO₂ layers exist in these films. An increase in the photoluminescence intensity and a decrease in the signal of electron spin resonance were observed in porous nc-Si films exposed to atmospheric air for a long time.     

Study of Deep Levels in a HIT Solar Cell

The results of studying a HIT (heterojunction with an intrinsic thin layer) Ag/ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n⁺)/ITO/Ag solar cell by the capacitance–voltage characteristic and current deep-level relaxation transient spectroscopy methods are presented. The temperature dependence of the capacitance–voltage characteristics of the HIT structure and deep-energy-level parameters are studied. The results of comprehensive studies by the above methods are used to determine the features of the energy-band diagram of actual HIT structures.     

Formation and study of <Emphasis Type="Italic">p–i–n</Emphasis> structures based on two-phase hydrogenated silicon with a germanium layer in the <Emphasis Type="Italic">i</Emphasis>-type region

Four pairs of p–i–n structures based on polymorphous Si:H (pm-Si:H) are fabricated by the method of plasma-enhanced chemical vapor deposition. The structures in each pair are grown on the same substrate so that one of them does not contain Ge in the i-type layer while the other structure contains Ge deposited by molecular-beam epitaxy as a layer with a thickness of 10 nm. The pair differ from one another in terms of the substrate temperature during Ge deposition; these temperatures are 300, 350, 400, and 450°C. The data of electron microscopy show that the structures formed at 300°C contain Ge nanocrystals (nc-Ge) nucleated at nanocrystalline inclusions at the pm-Si:H surface. The nc-Ge concentration increases as the temperature is raised. The study of the current–voltage characteristics show that the presence of Ge in the i-type layer decreases the density of the short-circuit current in p–i–n structures when they are used as solar cells, whereas these layers give rise to an increase in current at a reverse bias under illumination. The obtained results are consistent with known data for structures with Ge clusters in Si; according to these data, Ge clusters increase the coefficient of light absorption but they also increase the rate of charge-carrier recombination.     

Characterization of an <Emphasis Type="Italic">a</Emphasis>-Si:H/<Emphasis Type="Italic">c</Emphasis>-Si interface by admittance spectroscopy

The capabilities of admittance spectroscopy for the investigation of a-Si:H/c-Si heterojunctions are presented. The simulation and experimental results, which compare very well, show that the admittance technique is sensitive to the parameters of both the a-Si:H layer and the a-Si:H/c-Si interface quality. In particular, the curves showing capacitance versus temperature have two steps, accompanied by two bumps in the temperature dependence of the conductance. The first step, occurring in the low temperature range (100–200 K), is related to the transport and response of gap states in the a-Si:H layer. The second step, occurring at higher temperatures (>200 K), is caused by a carrier exchange with interface states and appears when the interface defect density exceeds 5 × 10¹² cm^?2. Then, the interface defects affect band bending, and, thus, the activation energy of de-trapping, which favors exchange with electrons from a-Si:H and holes from c-Si, respectively, for an increasing defect density.     

The electronic and emissive properties of Au-doped porous silicon

The temperature dependences of photovoltage induced by intense pulses of red and white light, along with the time-resolved spectral dependences of photoluminescence, are studied for porous silicon structures por-Si/p-Si). These structures have been obtained by anode etching of p-Si with subsequent Au doping from an aqueous solution with Au ion concentrations of 10^?4 and 10^?3 M. The current-voltage characteristics and electroluminescence of the resulting por-Si/p-Si and por-Si:Au/p-Si structures are also studied after a deposition of semitransparent Au electrodes on por-Si. It is shown that the Au doping changes the sign of the boundary potential of p-Si from positive to negative, alters the magnitude and sign of the photovoltage in the por-Si films, and eliminates photomemory phenomena, which are associated with the capture of nonequilibrium electrons at grain-boundary traps and por-Si traps. The formation of Au nanocrystals in por-Si substantially affects the current-voltage and photoluminescence characteristics. Electroluminescence is observed for the Au/por-Si:(Au, 10^?3 M)/p-Si/Al structures and is attributed to emission from the nanocrystals.     

Polarized photoluminescence of nc-Si–SiO x nanostructures

The effect of photoluminescence polarization memory in nc-Si–SiO _x light-emitting structures containing Si nanoparticles (nc-Si) in an oxide matrix is for the first time studied. The polarization properties of continuous and porous nanostructures passivated in HF vapors (or solutions) are studied. It is established that the polarization memory effect is manifested only after treatment of the structures in HF. The effect is also accompanied by a shift of the photoluminescence peak to shorter wavelengths and by a substantial increase in the photoluminescence intensity. It is found that, in anisotropic nc-Si–SiO _x samples produced by oblique deposition in vacuum, the degree of linear photoluminescence polarization in the sample plane exhibits a noticeable orientation dependence and correlates with the orientation of SiO _x nanocolumns forming the structure of the porous layer. These effects are attributed to the transformation of symmetrically shaped Si nanoparticles into asymmetric elongated nc-Si particles upon etching in HF. In continuous layers, nc-Si particles are oriented randomly, whereas in porous structures, their preferential orientation coincides with the orientation of oxide nanocolumns.     

Laser ultrasonic study of porous silicon layers

Propagation of ultrasonic signals induced by nanosecond laser pulses in porous silicon (por-Si) is considered from both the theoretical and the experimental viewpoints. The experimental samples are por-Si layers with 5-to 40-µm thickness and porosity of 50–75%; these layers were formed on a single-crystal silicon substrate by electrochemical etching. It is shown that the suggested ultrasonic laser method allows both the thickness and the porosity of a layer to be determined with the respective accuracies of no worse than 1 µm and 5%.     

Spectral photosensitivity of <Emphasis Type="Italic">a</Emphasis>-SiGe:H/<Emphasis Type="Italic">c</Emphasis>-Si heterostructures

Characteristics of a-SiGe:H/c-Si heterostructures produced by rapid plasma-chemical low-frequency (55 kHz) deposition are studied. High photosensitivity of a-SiGe:H films is established. The spectral position of the maximum of photosensitivity of a a-SiGe:H/c-Si heterostructure can be varied from 830 to 920 nm by increasing the content of germanium in an a-SiGe:H alloy and decreasing its band gap.     

Influence of the doping type and level on the morphology of porous Si formed by galvanic etching

The formation of porous silicon (por-Si) layers by the galvanic etching of single-crystal Si samples (doped with boron or phosphorus) in an HF/C₂H₅OH/H₂O₂ solution is investigated. The por-Si layers are analyzed by the capillary condensation of nitrogen and scanning electron microscopy (SEM). The dependences of the morphological characteristics of por-Si (pore diameter, specific surface area, pore volume, and thickness of the pore walls), which determine the por-Si combustion kinetics, on the dopant type and initial wafer resistivity are established.     

SiO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> layer formation during plasma sputtering of Si and SiO<Subscript>2</Subscript> targets

Deposition of SiO _x layers of variable composition onto silicon wafers was performed by co-sputtering of spaced Si and SiO₂ targets in argon plasma. Coordinate dependences of the thickness and refractive index of separately deposited Si and SiO₂ layers and the SiO _x layer grown during co-sputtering of targets were determined using optical techniques. It was shown that the SiO _x layer composition is not equal to a simple sum of thicknesses of separately deposited Si and SiO₂ layers. The coordinate dependences of the Si and SiO₂ layer thicknesses were calculated. To fit the calculated and experimental data, it is necessary to assume that no less than 10% of silicon is converted to dioxide during co-sputtering. A comparison of the coordinate dependences of the IR absorbance in SiO₂ and SiO _x layers with experimental ellipsometric data confirmed the presence of excess oxygen in the SiO _x layer. Taking into account such partial oxidation of sputtered silicon, composition isolines in the substrate plane were calculated. After annealing of the SiO _x layer at 1200°C, photoluminescence was observed in a wafer area predicted by calculations, which was caused by the formation of quantum-size Si nanocrystallites. The photoluminescence intensity was maximum at x = 1.78 ± 0.3, which is close to the composition optimum for ion-beam synthesis of nanocrystals.     

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.     

Enhancement of spontaneous erbium emission near the photonic band edge of distributed Bragg reflectors based on <Emphasis Type="Italic">a</Emphasis>-Si:H/<Emphasis Type="Italic">a</Emphasis>-SiO<Subscript>x</Subscript>:H

Results obtained in an experimental study of spontaneous emission from erbium ions in a spectral range corresponding to the lower photonic band edge of distributed Bragg reflectors (1D photonic crystals) are presented. The photonic crystals were constituted of alternating quarter-wave a-Si:H and a-SiO_x:H layers grown by PECVD. Erbium was introduced into the a-Si:H layers by magnetron sputtering of an erbium target in the course of structure growth. The change observed in the intensity of spontaneous emission is due to the nonmonotonic behavior of the density of optical modes near the photonic band edge.     

A new type of high-efficiency bifacial silicon solar cell with external busbars and a current-collecting wire grid

Results regarding bifacial silicon solar cells with external busbars are presented. The cells consist of [n⁺p(n)p⁺] Cz-Si structures with a current-collecting system of new design: a laminated grid of wire external busbars (LGWEB). A LGWEB consists of a transparent conducting oxide film deposited onto a Si structure, busbars adjacent to the Si structure, and a contact wire grid attached simultaneously to the oxide and busbars using the low-temperature lamination method. Bifacial LGWEB solar cells demonstrate record high efficiency for similar devices: 17.7%(n-Si)/17.3%(p-Si) with 74–82% bifaciality for the smooth back surface and 16.3%(n-Si)/16.4%(p-Si) with 89% bifaciality for the textured back surface. It is shown that the LGWEB technology can provide an efficiency exceeding 21%.     

Fast exothermic processes in porous silicon

It is found that rapid oxidation of porous Si (por-Si) layers in air may occur in the form of combustion or explosion. Combustion occurs in por-Si layers thinner than 60 μm and impregnated with potassium nitrate, while explosion is observed in thicker porous layers. It is suggested that explosion develops by a thermal mechanism resulting from an exponential increase in the reaction rate with temperature.     

Direct exchange between silicon nanocrystals and tunnel oxide traps under illumination on single electron photodetector

In this paper we present the trapping of photogenerated charge carriers for 300 s resulted by their direct exchange under illumination between a few silicon nanocrystals (ncs-Si) embedded in an oxide tunnel layer (SiO_x = 1.5) and the tunnel oxide traps levels for a single electron photodetector (photo-SET or nanopixel). At first place, the presence of a photocurrent limited in the inversion zone under illumination in the I–V curves confirms the creation of a pair electron/hole (e–h) at high energy. This photogenerated charge carriers can be trapped in the oxide. Using the capacitance-voltage under illumination (the photo-CV measurements) we show a hysteresis chargement limited in the inversion area, indicating that the photo-generated charge carriers are stored at traps levels at the interface and within ncs-Si. The direct exchange of the photogenerated charge carriers between the interface traps levels and the ncs-Si contributed on the photomemory effect for 300 s for our nanopixel at room temperature.     

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.     

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.     

Study of photocapacitance in diodes fabricated from silicon doped with vanadium

The levels of vanadium in the band gap of n-and p-Si were determined using photocapacitance measurements. It is shown that vanadium introduces levels only in the upper half of the band gap of n-Si; these levels have ionization energies of about E _c ?0.21 eV, E _c ?0.32 eV, and E _c ?0.52 eV. By contrast, V levels are located both in the upper and lower halves of the p-Si band gap: E _c ?0.26 eV, E _v +0.52 eV, E _v +0.42 eV, and E _v +0.31 eV. It is ascertained that the photoionization cross sections of all vanadium levels are larger for electrons than for holes. It is shown that the concentration of electrically active vanadium centers in n-and p-Si depends on both the concentration of shallow-level impurities and the time of vanadium diffusion into Si.