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.     

Effect of thermal treatment on structure and properties of a-Si:H films obtained by cyclic deposition

The effect of thermal treatment in a vacuum on the structure and properties of amorphous hydrogenated silicon (a-Si:H) films obtained by cyclic deposition with intermediate annealing in hydrogen plasma was studied. a-Si:H films deposited under optimal conditions are characterized by the nonuniform distribution of the nanocrystalline phase (<1 vol %) across the film thickness and have the optical gap E _g=1.85 eV, the activation energy of conductivity E _a =0.91 eV, and a high photosensitivity (σ_ph/σ_d≈10⁷ under illumination of 100 mW/cm² in the visible spectral range). Transmission electron microscopy studies demonstrated that thermal treatment in a vacuum leads to blurring of the initial layered structure of a-Si:H films and to a somewhat higher amount of nanocrystalline inclusions in the amorphous phase matrix. Thermal treatment above 350°C causes a dramatic increase in the dark conductivity, as well as the resulting decrease in the photosensitivity, of a-Si:H films.     

Resonant inelastic light scattering and photoluminescence in isolated nc-Si/SiO2 quantum dots

Observation at the room temperature the spectra of the resonant inelastic light scattering by the spatially confined optical phonons as well as the excitonic luminescence caused by confinement effects in the ensemble of isolated quantum dots (QDs) nc-Si/SiO₂ is reported. It is shown that the samples investigated are high purity and high crystalline perfection quality nc-Si/SiO₂ QDs without amorphous phase α-Si and contaminants. Comparison between the experimental data obtained and phenomenological model of the strong space confinement of optical phonons revealed the need of the more accurate form of the weighted function for the confinement of optical phonons. It is shown that simultaneous detection of the inelastic light scattering by the confinement of phonons and the excitonic luminescence spectra by the confined electron-hole pairs in the nc-Si/SiO₂ QDs allows selfconsistently to determine more accurate values of the diameter of the nc-Si/SiO₂ QDs.     

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.     

Properties of periodic α-Si:H/a-SiNx:H structures obtained by nitridization of amorphoussilicon layers

The kinetics of nitridization of a-Si:H layers, the properties of the structures that are formed and a-Si:H in them have been investigated. The changes occurring in the resistance of the a-Si:H layers in the course of nitridization are described in terms of the competition between doping, transport, and change in the thickness of the remaining a-Si:H layer. The experimental data on the band spectrum of superlattices with a-Si:H and a-SiN_x:H layer thicknesses ～35 Å and ～5 Å, respectively, are in agreement with calculations in a model of interacting quantum wells with m*=(0.36±0.1)m ₀. Comparison of the properties of superlattices obtained by deposition of successive layers and nitridization of the a-Si:H layers showed that the latter can have a higher “structural perfection.”     

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.     

A quantum-well model and the optical absorption edge in structurally nonuniform a-Si:H-based alloys

This paper describes studies of the microstructure and optical absorption edge of films of a-Si: H and a-SiN_x: H alloys (x=0.0–0.72), obtained by decomposing gas mixtures in an rf glow discharge. For the a-Si: H films, the gas mixture was monosilane diluted by hydrogen, for the a-SiN_x: H alloys it was SiH₄+NH₃. Structurally nonuniform films with “device-quality” optoelectronic characteristics were obtained when the rates of growth were increased (to 8 Å/s). Atomic force microscopy and infrared absorption spectroscopy are used to identify a characteristic feature of the microstructures of these films: the presence of islets ～500 Å in diameter, whose boundaries are formed by clusters of hydrogen atoms (in the case of a-Si: H) or of hydrogen and nitrogen (in the case of a-SiN_x: H). In this case the optical band gap of a-Si: H (a-SiN_x: H) is determined by the concentration of SiH (SiN) bonds in the interior of the islets and is not sensitive to changes in the content of hydrogen (nitrogen) at their boundaries. This result is explained by a quantum-well model which takes into account the characteristic sizes of the microstructures formed by hydrogen or nitrogen atoms.     

Luminescence spectra and efficiency of GaN-based quantum-well heterostructure light emitting diodes: Current and voltage dependence

Luminescence spectra and quantum yield in light emitting diodes (LEDs) based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells (MQWs) were studied in the range of currents J=10⁻⁶–10⁻¹ A. Minor spread in the quantum yield at operating currents (±15% at J≈10 mA) was observed in these LEDs, which were fabricated by Hewlett-Packard. The spread is due to differences in the current and voltage dependences of the diode emission intensity, caused by differences in the charged center distribution across the space-charge region (SCR) of the structures and in the role of the tunnel current component at low voltages. In the diodes with a thin (≲120 nm) SCR, a tunnel emission band was observed for J≲100 μA; the peak energy of this band ℏω_max=1.92–2.05 eV corresponds to the voltage applied. At low currents (J=0.05–0.5 mA), the spectral position of the main peak ℏω_max=2.35–2.36 eV is independent of the voltage and is determined by the radiative transitions between the localized states. At J>1 mA, this band shifts with the current (ℏω_max=2.36–2.52 eV). Its shape corresponds to the model for the occupation of states in the two-dimensional energy band tails, which are caused by the microscopic potential fluctuations. The four parameters in this model are related to the calculated energy band diagram of the MQW structure. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 35, No. 7, 2001, pp. 861–868. Original Russian Text Copyright ? 2001 by Kudryashov, Mamakin, Turkin, Yunovich, Kovalev, Manyakhin. Part of this study was reported at the 3rd All-Russia Workshop on Structures and Devices Based on Gallium, Indium, and Aluminum Nitrides (Moscow State University, 1999); the 3rd International Conference on Nitride Semiconductors (Montpellier, 1999); and the 4th European Gallium Nitride Workshop (Nottingham, 2000).     

Absorptivity of semiconductors used in the production of solar cell panels

The dependence of the absorptivity of semiconductors on the thickness of the absorbing layer is studied for crystalline silicon (c-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), copper indium diselenide (CuInSe₂, CIS), and copper gallium diselenide (CuGaSe₂, CGS). The calculations are performed with consideration for the spectral distribution of AM1.5 standard solar radiation and the absorption coefficients of the materials. It is shown that, in the region of wavelengths λ = λ _g = hc/E _g , almost total absorption of the photons in AM1.5 solar radiation is attained in c-Si at the thickness d = 7−8 mm, in a-Si at d = 30–60 μm, in CdTe at d = 20−30 μm, and in CIS and CGS at d = 3−4 μm. The results differ from previously reported data for these materials (especially for c-Si). In previous publications, the thickness needed for the semiconductor to absorb solar radiation completely was identified with the effective light penetration depth at a certain wavelength in the region of fundamental absorption for the semiconductor.     

A Comparison of ZnO Nanowires and Nanorods Grown Using MOCVD and Hydrothermal Processes

A comparison of ZnO nanowires (NWs) and nanorods (NRs) grown using metalorganic chemical vapor deposition (MOCVD) and hydrothermal synthesis, respectively, on p-Si (100), GaN/sapphire, and SiO₂ substrates is reported. Scanning electron microscopy (SEM) images reveal that ZnO NWs grown using MOCVD had diameters varying from 20 nm to 150 nm and approximate lengths ranging from 0.7 μm to 2 μm. The NWs exhibited clean termination/tips in the absence of any secondary nucleation. The NRs grown using the hydrothermal method had diameters varying between 200 nm and 350 nm with approximate lengths between 0.7 μm and 1 μm. However, the NRs grown on p-Si overlapped with each other and showed secondary nucleation. x-Ray diffraction (XRD) of (0002)-oriented ZnO NWs grown on GaN using MOCVD demonstrated a full-width at half-maximum (FWHM) of 0.0498 (θ) compared with 0.052 (θ) for ZnO NRs grown on similar substrates using hydrothermal synthesis, showing better crystal quality. Similar crystal quality was observed for NWs grown on p-Si and SiO₂ substrates. Photoluminescence (PL) of the NWs grown on p-Si and SiO₂ showed a single absorption peak attributed to exciton–exciton recombination. ZnO NWs grown on GaN/sapphire had defects associated with oxygen interstitials and oxygen vacancies.     

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.     

The influence of erbium on electrical and photoelectric properties of amorphous silicon produced by radio-frequency silane decomposition

Thermal evaporation of tris(2,2,6,6-tetramethyl-2,5-heptadionato) Er(III) inside the plasma gap was used to introduce erbium into amorphous hydrogenated silicon (a-Si:H) obtained by radio-frequency silane decomposition. The samples obtained had a pronounced layered structure due to exhaustion of the erbium source. The layer nearest to the substrate was enriched with erbium, oxygen, and carbon; gave rise to luminescence with a wavelength of 1.535 µm characteristic of ⁴ I _13/2 → ⁴ I _15/2 intra-atomic transitions of erbium; and contained a large number of defects. The top layer contained much fewer defects, was close to undoped a-Si:H in the photoelectric characteristics, and was responsible for photoconductivity in the samples obtained. The experimental data are analyzed in the context of the models for doping of a-Si:H with Er with the resulting emergence of n-type conduction and formation of heterojunction as the film grows.     

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.     

The influence of local surroundings of Er atoms on the kinetics of decay of Er photoluminescence in amorphous hydrogenated silicon

Kinetics of the decay of photoluminescence of Er impurity in the films of amorphous hydrogenated Si a-Si:H〈Er〉 was studied for the first time. The films were obtained either by cosputtering of Si and Er targets with the use of the technology of dc silane decomposition in a magnetic field (MASD) or by radio-frequency decomposition of silane. In the second case, an Er(TMHD)₃ polymer powder was used as the source of Er. It is shown that, at room temperature, the a-Si:H〈Er〉 films obtained by the MASD method feature the characteristic times of Er photoluminescence decay equal to 10–15 μs, which is 20 times smaller than in the case of Er-doped crystalline Si (c-Si〈Er, O〉) as measured at liquid-nitrogen temperature. For the a-Si:H〈Er〉 films obtained by radio-frequency decomposition of silane, the decay times of Er photoluminescence amount to 2 μs. The difference in the photoluminescence decay times is related to dissimilarities in the local surroundings of Er atoms in the a-Si:H〈Er〉 films obtained by different methods. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 34, No. 1, 2000, pp. 90–92. Original Russian Text Copyright ? 2000 by Terukov, Kudoyarova, Kon’kov, Konstantinova, Kamenev, Timoshenko.     

The effect of oxygen on segregation-induced redistribution of rare-earth elements in silicon layers amorphized by ion implantation

A model of segregation-induced redistribution of impurities of rare-earth elements during solid-phase epitaxial crystallization of silicon layers amorphized by ion implantation is developed. This model is based on the assumption that a transition layer with a high mobility of atoms is formed at the interphase boundary on the side of a-Si; the thickness of this layer is governed by the diffusion length of vacancies in a-Si. The Er concentration profiles in Si implanted with both erbium and oxygen ions are analyzed in the context of the model. It shown that, in the case of high doses of implantation of rare-earth ions, it is necessary to take into account the formation of R_m clusters (m = 4), where R denotes the atom of a rare-earth element, whereas, if oxygen ions are also implanted, formation of the complexes RO_n (n = 3–6) should be taken into account; these complexes affect the transition-layer thickness and segregation coefficient.     

Effect of end-substitutions of distyryl-oligothiophenes by hexyl chains on environmental stability in organic thin film transistors

In this study, solution and solid-state properties as well as the organic thin film transistor (OTFT) behaviour of two α,ω-hexyl-distyryl-oligothiophenes (DH-DSnT, n = 2, 4) are presented. The optical and electrochemical properties of the oligomers in solution were investigated by UV–vis absorption and photoluminescence spectroscopies, and cyclic voltammetry. Liquid crystal properties were studied by differential scanning calorimetry (DSC) and optical polarising microscopy (OPM). High-vacuum evaporated thin films were studied by optical absorption, X-ray diffraction and atomic force microscopy (AFM), and implemented as p-type semiconducting layers into organic thin film transistors (OTFTs). The results are investigated to study the influence of alkyl chains on the α,ω-end positions of the distyryl-oligothiophene skeleton (DSnT). Furthermore, detailed shelf-live tests of hole mobility (μ), threshold voltage (V_t), on current (I_ON) and off current (I_OFF) under atmospheric conditions (air, temperature) of OTFTs based on DS4T and DH-DS4T show that the presence of alkyl chains in α,ω-end positions of distyryl-oligothiophenes is detrimental to the environmental stability of OTFTs over time.     

Mixing of nickel and silicon by incoherent-light-pulse annealing

Mixed layers of Ni and Si were formed by pulsed incoherent-light annealing of Ni thin films evaporated on Si substrates. Incoherent light pulse of 36 μs produced in an arc-discharge plasma system was used as the energy source for annealings. A thin layer (300 Å) of amorphous silicon (a-Si) deposited on 800–1000 Å thick Ni films was found to increase light absorption significantly. The light energy needed to form uniform mixing was measured to be about 23 J/cm². Rutherford backscattering and Auger electron spectroscopy techniques were used for qualitative analysis of a-Si/Ni/Si samples. Optimum operating conditions of the light source was determined.     

Influence of the channel layer thickness on electrical properties of indium zinc oxide thin-film transistor

Thin-film transistors (TFTs) were fabricated on SiO₂/n⁺-Si substrates using amorphous binary In₂O₃-ZnO (a-IZO) films with different thickness for active channel layers deposited by the rf magnetron sputtering at room temperature. The performance of devices was found to be thickness dependent. With the active layer thickness from 33 to 114 nm, the field-effect mobility μ_FE increased from 1.60 to 4.59 cm²/V s, the threshold voltage V_TH decreased from 62.26 to 20.82 V, and the subthreshold voltage swing S decreased from 4.06 V/decade to 1.30 V/decade. Further, the dependence of TFTs’ electrical properties on active layer thickness was investigated in detail on the basis of free carrier density and interface scattering.     

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.