Electrical Properties of <Emphasis Type="Italic">p</Emphasis>-NiO/<Emphasis Type="Italic">n</Emphasis>-Si Heterostructures Based on Nanostructured Silicon

Silicon nanowires are formed on n-Si substrates by chemical etching. p-NiO/n-Si heterostructures are fabricated by reactive magnetron sputtering. The energy diagram of anisotype p-NiO/n-Si heterostructures is constructed according to the Anderson model. The current–voltage and capacitance–voltage characteristics are measured and analyzed. The main current-transport mechanisms through the p-NiO/n-Si heterojunction under forward and reverse biases are established.     

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.     

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.     

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.     

Mechanism of formation of the response of a hydrogen gas sensor based on a silicon MOS diode

Experimental data on the dependence of the flat-band voltage and relaxation time for the capacitance of the space-charge region in an MOS diode (Pd-SiO₂-n-Si) on the hydrogen concentration in a hydrogen/air gaseous mixture are discussed. It is assumed that variation in the flat-band voltage U _fb in an MOS structure with the thickness d = 369 nm subjected to a hydrogen/air gaseous mixture can be accounted for by the formation of dipoles in the Pd-SiO₂ gap due to polarization of hydrogen atoms (H _a ). An analytical expression describing the dependence of variation in the flat-band voltage ΔU _fb on the hydrogen concentration \(n_{H_2 } \) was derived. In MOS structures with d ≤ 4 nm (or MOS diodes), the value of ΔU _fb is mainly controlled by passivation of the centers responsible for the presence of the surface acceptor-type centers at the SiO₂-n-Si interface by hydrogen atoms. Analytical expressions describing the dependences of ΔU _fb and the capacitance relaxation time in the space-charge region on \(n_{H_2 } \) are derived. The values of the density of adsorption centers and the adsorption heat for hydrogen atoms at the Pd-SiO₂ and SiO₂-n-Si interfaces are found.     

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%.     

Structural and Electrical Properties of Ag/<Emphasis Type="Italic">n</Emphasis>-TiO<Subscript>2</Subscript>/<Emphasis Type="Italic">p</Emphasis>-Si/Al Heterostructure Fabricated by Pulsed Laser Deposition Technique

We have investigated the structural and electrical characteristics of the Ag/n-TiO₂/p-Si/Al heterostructure. Thin films of pure TiO₂ were deposited on p-type silicon (100) by optimized pulsed laser ablation with a KrF-excimer laser in an oxygen-controlled environment. X-ray diffraction analysis showed the formation of crystalline TiO₂ film having a tetragonal texture with a strong (210) plane as the preferred direction. High purity aluminium and silver metals were deposited to obtain ohmic contacts on p-Si and n-TiO₂, respectively. The current–voltage (I–V) characteristics of the fabricated heterostructure were studied by using thermionic emission diffusion mechanism over the temperature range of 80–300 K. Parameters such as barrier height and ideality factor were derived from the measured I–V data of the heterostructure. The detailed analysis of I–V measurements revealed good rectifying behavior in the inhomogeneous Ag/n-TiO₂/p-Si(100)/Al heterostructure. The variations of barrier height and ideality factor with temperature and the non-linearity of the activation energy plot confirmed that barrier heights at the interface follow Gaussian distributions. The value of Richardson’s constant was found to be 6.73 × 10⁵ Am^?2 K^?2, which is of the order of the theoretical value 3.2 × 10⁵ Am^?2 K^?2. The capacitance–voltage (C–V) measurements of the heterostructure were investigated as a function of temperature. The frequency dependence (Mott–Schottky plot) of the C–V characteristics was also studied. These measurements indicate the occurrence of a built-in barrier and impurity concentration in TiO₂ film. The optical studies were also performed using a UV–Vis spectrophotometer. The optical band gap energy of TiO₂ films was found to be 3.60 eV.     

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.     

Photoinduced conductivity change in erbium-doped amorphous hydrogenated silicon films

Changes in the dark conductivity of erbium-doped amorphous hydrogenated silicon (a-Si:H(Er)) films after their preliminary illumination at room temperature have been studied. The effect of a compensating boron impurity on the photoinduced change in the conductivity of a-Si:H(Er) films is analyzed. It is established that the magnitude and the sign of the change in conductivity depend on the duration of illumination and position of the Fermi level in the mobility gap. Possible mechanisms leading to a photoinduced change in the conductivity of a-Si:H(Er) films are discussed.     

A study of the effect of oxygen on the intensity of erbium photoluminescence in amorphous SiO<Subscript>x</Subscript>:(H,Er) films formed by DC magnetron sputtering

The effect of oxygen on the intensity of erbium photoluminescence at λ=1.54 μm in amorphous a-SiO_x:H(Er) films formed by dc magnetron sputtering was studied. The oxygen content in the gaseous phase ranged from 0.1 to 12 mol %, with other parameters of deposition remaining constant. Analysis of an a-Si:(H, Er, O) system showed that the range of homogeneity of amorphous a-SiO_x:H(Er) is retrograde (T=const). The range of homogeneity can be conventionally divided into two portions, each of which should contain either of two differently charged [Er-O] ⁿ⁻ and [Er-O-Si-O] ^m− clusters (m>n). This inference is confirmed experimentally: in the range of oxygen concentrations amounting to 5.5–8 mol % in the plasma, unusual associative processes take place probably directly above the growing film surface; these processes are caused by the appearance of [Er-O-Si-O] ^m− clusters in the plasma and at the surface. It is these processes that account for the intensification of erbium photoluminescence as the oxygen content increases above 5.5 mol %. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 7, 2003, pp. 853–859. Original Russian Text Copyright ? 2003 by Undalov, Terukov, Gusev, Kudoyarova.     

The effect of low-field injection of charge carriers on the electrical properties of the metal-oxide-semiconductor structures

The method of high-frequency capacitance-voltage characteristics was used to study the effect of low-field injection of charge carriers on the electrical properties of metal-SiO₂-Si structures with n- and p-type substrates. It is shown that in all cases of injection (irrespective of the polarity of the voltage at the gate), an effective positive charge is generated in the oxide; after completion of the injection, this charge relaxes with characteristic times that depend on the bias voltage applied to the gate and the type of the metal-oxide-semiconductor structure. In the structures with p-Si substrates, in the case of a positive voltage applied to the gate, a capacitance minimum appeared in the inversion portion of the capacitance-voltage characteristics in the course of injection; this effect became more pronounced as the gate voltage was increased. After the injection, the capacitance gradually approached the initial value (before injection).     

The density of states in the mobility gap of amorphous hydrogenated silicon doped with erbium

The effect of doping films of amorphous hydrogenated silicon (a-Si:H) with erbium on the density of the states in the mobility gap is studied. The data obtained are compared with those for a-Si:H films doped with arsenic. The data on the density of the states in the lower and upper halves of the mobility gap are determined from measurements of the spectral dependences of the absorption coefficient and the temperature dependences of the constant and modulated components of the photoconductivity in films exposed to modulated light, respectively. It is shown that doping the a-Si:H films with erbium leads to an increase in the density of states both in the lower and upper halves of the mobility gap.     

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.     

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.     

Specific features of light current-voltage characteristics of <Emphasis Type="Italic">p-i-n</Emphasis> structures based on amorphous silicon in the case of the tunnel-drift mechanism of dark current transport

Current-voltage (I–V) characteristics of p-i-n structures based on amorphous silicon (α-Si:H) with small hole diffusion lengths (shorter than the thickness of the i-layer of a p-i-n structure) have been experimentally studied with and without illumination. It is shown that forward I–V characteristics of structures of this kind can be described by a dependence inherent in diodes, with a diode ideality factor two-three times the maximum value of 2, theoretically predicted for generation-recombination currents in p-n junctions. The dark current is always substantially lower than the photocurrent in a cell biased with a voltage approximately equal to the opencircuit voltage of the photocell. Dark currents cannot contribute to the I–V characteristic under illumination. The photocurrent decreases with increasing photovoltage at a bias lower than the open-circuit voltage because of a decrease in the collection coefficient and the increasingly important role of back diffusion of electrons into the p-contact, rather than as a result of the dark injection. In the case of biases exceeding the open-circuit voltage, back diffusion becomes the predominant component of the current.     

GaAs/InGaAsN heterostructures for multi-junction solar cells

Solar-cell heterostructures based on GaAs/InGaAsN materials with an InAs/GaAsN superlattice, grown by molecular beam epitaxy, are studied. A p-GaAs/i-(InAs/GaAsN)/n-GaAs p–i–n test solar cell with a 0.9-μm-thick InGaAsN layer has an open-circuit voltage of 0.4 V (1 sun, AM1.5G) and a quantum efficiency of >0.75 at a wavelength of 940 nm (at zero reflection loss), which corresponds to a short-circuit current of 26.58 mA/cm² (AM1.5G, 100 mW/cm²). The high open-circuit voltage demonstrates that InGaAsN can be used as a material with a band gap of 1 eV in four-cascade solar cells.     

Photoconductivity of thin <Emphasis Type="Italic">a</Emphasis>-Si:H films

Photoelectric and optical properties of a-Si:H films with a thickness of 60–100 nm were studied. Temperature dependences of photoconductivity in the temperature range 130–440 K and spectral dependences of the absorption coefficient near the absorption edge were measured. The results of comparative measurements of the room-temperature conductivity and the absorption coefficient in the defect-related subgap spectral range (photon energy hν = 1.2 eV) indicate that the recombination of nonequilibrium carriers and, accordingly, the photoconductivity of a-Si:H films with a thickness of ～100 nm are determined by the defect concentration 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.     

Charge state distribution in the <Emphasis Type="Italic">a</Emphasis>-Si:H mobility gap

A photoconductivity simulation technique taking into account the hopping mechanism of carrier transport is presented. The technique allows for the estimation of the charge state distribution in the a-Si:H mobility gap. Simulation of the photoconductivity of a-Si:H thin films grown at increased rates made it possible to determine the charge state distribution and analyze the processes of charge redistribution in the mobility gap of the amorphous semiconductor.     

Computationally Efficient Motion Estimation Algorithm for HEVC

In this paper, we present a novel computationally efficient motion estimation (ME) algorithm for high-efficiency video coding (HEVC). The proposed algorithm searches in the hexagonal pattern with a fixed number of search points at each grid. It utilizes the correlation between contiguous pixels within the frame. In order to reduce the computational complexity, the proposed algorithm utilizes pixel truncation, adaptive search range, sub-sampling and avoids some of the asymmetrical prediction unit techniques. Simulation results are obtained by using the reference software HM (e n c o d e r_l o w d e l a y_P_m a i n and e n c o d e r_r a n d o m a c c e s s_m a i n profile) and shows 55.49% improvement on search points with approximately the same PSNR and around 1% increment in bit rate as compared to the Test Zonal Search (TZS) ME algorithm. By utilizing the proposed algorithm, the BD-PSNR loss for the video sequences like B a s k e t b a l l P a s s_416 × 240@50 and J o h n n y_1280 × 720@60 is 0.0804 dB and 0.0392 dB respectively as compared to the HM reference software with the e n c o d e r_l o w d e l a y_P_m a i n profile.