R.F. reactive sputter deposition of hydrogenated amorphous silicon carbide films

The preparation of hydrogenated amorphous silicon carbide films by r.f. reactive sputtering of a silicon target in Ar-CH₄ gas mixtures with and without an r.f. bias on the substrates was studied. Starting with a pure silicon target and increasing monotonically the CH₄ percentage from 0% to about 10%, films with 1 ⩾ x ⩾ 0 were obtained at decreasing deposition rates. After sputtering for some hours in methane-rich gas mixtures, carbon atoms were incorporated into the silicon target surface, probably as a result of atomic peening, and nearly stoichiometric SiC films were prepared by sputtering of such a target in pure argon. The different mechanisms of film formation, deposition rate, composition, hardness, friction coefficient and stresses in the films as functions of the partial pressure of methane and the value of the r.f. bias were investigated. The IR spectra offilms with different carbon contents were analysed. The greatest hardness was found for nearly stoichiometric SiC films deposited with a bias.     

Room temperature photoluminescence spectrum modeling of hydrogenated amorphous silicon carbide thin films by a joint density of tail states approach and its application to plasma deposited hydrogenated amorphous silicon carbide thin films

Room temperature photoluminescence (PL) spectrum of hydrogenated amorphous silicon carbide (a-SiC_x:H) thin films was modeled by a joint density of tail states approach. In the frame of these analyses, the density of tail states was defined in terms of empirical Gaussian functions for conduction and valance bands. The PL spectrum was represented in terms of an integral of joint density of states functions and Fermi distribution function. The analyses were performed for various values of energy band gap, Fermi energy and disorder parameter, which is a parameter that represents the width of the energy band tails. Finally, the model was applied to the measured room temperature PL spectra of a-SiC_x:H thin films deposited by plasma enhanced chemical vapor deposition system, with various carbon contents, which were determined by X-ray photoelectron spectroscopy measurements. The energy band gap and disorder parameters of the conduction and valance band tails were determined and compared with the optical energies and Urbach energies, obtained by UV-Visible transmittance measurements. As a result of the analyses, it was observed that the proposed model sufficiently represents the room temperature PL spectra of a-SiC_x:H thin films.     

Analysis on the interfacial properties of transparent conducting oxide and hydrogenated p-type amorphous silicon carbide layers in p-i-n amorphous silicon thin film solar cell structure

Quantitative estimation of the specific contact resistivity and energy barrier at the interface between transparent conducting oxide (TCO) and hydrogenated p-type amorphous silicon carbide (a-Si_1 − xC_x:H(p)) was carried out by inserting an interfacial buffer layer of hydrogenated p-type microcrystalline silicon (μc-Si:H(p)) or hydrogenated p-type amorphous silicon (a-Si:H(p)). In addition, superstrate configuration p-i-n hydrogenated amorphous silicon (a-Si:H) solar cells were fabricated by plasma enhanced chemical vapor deposition to investigate the effect of the inserted buffer layer on the solar cell device. Ultraviolet photoelectron spectroscopy was employed to measure the work functions of the TCO and a-Si_1 − xC_x:H(p) layers and to allow direct calculations of the energy barriers at the interfaces. Especially interface structures were compared with/without a buffer which is either highly doped μc-Si:H(p) layer or low doped a-Si:H(p) layer, to improve the contact properties of aluminum-doped zinc oxide and a-Si_1 − xC_x:H(p). Out of the two buffers, the superior contact properties of μc-Si:H(p) buffer could be expected due to its higher conductivity and slightly lower specific contact resistivity. However, the overall solar cell conversion efficiencies were almost the same for both of the buffered structures and the resultant similar efficiencies were attributed to the difference between the fill factors of the solar cells. The effects of the energy barrier heights of the two buffered structures and their influence on solar cell device performances were intensively investigated and discussed with comparisons.     

Optical properties of doped hydrogenated amorphous silicon films

The effect of doping on the reflectance and on the imaginary part of the dielectric function in the UV-visible range was investigated for a series of hydrogenated amorphous silicon films. The samples, doped with phosphorus or boron, were prepared by a reactive evaporation technique. Doping, with either p-or n-type dopants, strongly affects the spectra causing both a red shift and a decrease in peak height. Analysis of the results in terms of the oscillator strength and the excitation energy indicates that there is a decrease in the short-range order which, according to the literature, is attributed to a degree of structural disorder induced by the dopants.     

Photovoltaic characterization of amorphous hydrogenated and chlorinated silicon films

Hydrogenated and chlorinated silicon films were used to deposit Schottky barrier solar cells. Photovoltaic characterization, together with the results of electronic transport measurements, led to the conclusion that the presence of chlorine is detrimental to the properties of this kind of device.     

Microhardness and other properties of hydrogenated amorphous silicon carbide thin films formed by plasma-enhanced chemical vapor deposition

The objective of this work was to correlate thin film microhardness with composition and other properties of the Si-C system. Compositions varying from 80% Si to 100% C were formed using plasma-enhanced chemical vapor deposition on Si〈100〉 substrates. As the fraction of carbon increases from 23 to 100 at.%, the hardness increases from about 2300 to more than 7000 kgf mm^-2 (Knoop hardness) at 10% penetration of the indenter. The hardness for stoichiometric SiC is also about 7000 kgf mm^-2. At a particular composition the hardness can be maximized by adjusting the deposition parameters to decrease the hydrogen content and to increase the silicon-to-carbon bonding.     

Thermal annealing and hydrogenation of chlorinated hydrogenated amorphous silicon films

The effects of thermal annealing and hydrogenation on the electrical and optical properties of chlorinated hydrogenated amorphous silicon (a-Si:H:Cl) films were studied to understand the role of hydrogen in this material. It was shown that hydrogen determines the gap state density and the structural disorder in a-Si:H:Cl to an extent comparable with that in a-Si:H.     

Nanostructure, electrical and optical properties of p-type hydrogenated nanocrystalline silicon films

In this paper, p-type hydrogenated nanocrystalline (nc-Si:H) films were prepared on corning 7059 glass by plasma-enhanced chemical vapor deposition (PECVD) system. The films were deposited with radio frequency (RF) (13.56 MHz) power and direct current (DC) biases stimulation conditions. Borane (B₂H₆) was a doping agent, and the flow ratio η of B₂H₆ component to silane (SiH₄) was varied in the experimental. Films’ surface morphology was investigated with atomic force microscopy (AFM); Raman spectroscopy, X-ray diffraction (XRD) was performed to study the crystalline volume fraction X_c and crystalline size d in films. The electrical and optical properties were gained by Keithly 617 programmable electrometer and ultraviolet-visible (UV-vis) transmission spectra, respectively. It was found that: there are on the film surface many faulty grains, which formed spike-like clusters; increasing the flow ratio η, crystalline volume fraction X_c decreased from 40.4% to 32.0% and crystalline size d decreased from 4.7 to 2.7 nm; the optical band gap E_g^opt increased from 2.16 to 2.4 eV. The electrical properties of p-type nc-Si:H films are affected by annealing treatment and the reaction pressure.     

Properties of rapidly deposited amorphous hydrogenated carbon films

The properties of hydrogenated amorphous carbon films prepared by the rapid and low-temperature decomposition of toluene in an r.f. glow discharge have been investigated. The films are composed of a mixture of sp² and sp³ bonded carbons. Measurements of the optical gap show that the optical gap decreases with an increase of the r.f. power and increases with increasing pressure. The room-temperature dielectric constant decreases with increasing pressure with a dielectric loss tangent of 1%.     

Applications of microcrystalline hydrogenated cubic silicon carbide for amorphous silicon thin film solar cells

We demonstrated the fabrication of n-i-p type amorphous silicon (a-Si:H) thin film solar cells using phosphorus doped microcrystalline cubic silicon carbide (μc-3C-SiC:H) films as a window layer. The Hot-wire CVD method and a covering technique of titanium dioxide TiO₂ on TCO was utilized for the cell fabrication. The cell configuration is TCO/TiO₂/n-type μc-3C-SiC:H/intrinsic a-Si:H/p-type μc- SiC_x (a-SiC_x:H including μc-Si:H phase)/Al. Approximately 4.5% efficiency with a V_oc of 0.953 V was obtained for AM-1.5 light irradiation. We also prepared a cell with the undoped a-Si_1−xC_x:H film as a buffer layer to improve the n/i interface. A maximum V_oc of 0.966 V was obtained.     

Optical and structural features of silicon-rich hydrogenated amorphous silicon nitride thin films

A systematic study of fabrication and effect of post-deposition processing on the optical and structural features of silicon-rich hydrogenated amorphous silicon nitride thin films deposited by Hg-sensitized Photo-Chemical Vapour Deposition technique is presented. Both deposition parameters and post-deposition thermal treatment resulted into substantial change in the refractive index associated with the densification of the film. Our studies reveal that the presence of hydrogen and its out-diffusion upon thermal treatment play a crucial role in the overall structural evolution, specially the stabilization of individual phases such as Si and Si3N4. We further report the room-temperature photoluminescence from as-deposited films, which is due to formation of silicon nanostructures in crystalline and amorphous forms. These studies are of great interest from the prospective of commercially viable Si-based technology.     

Large-grain polycrystalline silicon thin films obtained by low-temperature stepwise annealing of hydrogenated amorphous silicon

Large grained polycrystalline silicon thin films have been prepared by low-temperature solid phase crystallisation of sputter-deposited hydrogenated amorphous silicon (a-Si:H), with relatively short processing times, and a considerably low thermal budget. Various a-Si:H samples, deposited under different conditions and with varying hydrogen concentrations and hydrogen bonding configurations, were simultaneously annealed. Only a particular set of deposition conditions led to crystallisation. The a-Si:H thin film which was successfully crystallised was prepared in an argon-hydrogen mixture, in which the last few minutes of film deposition occurred in a hydrogen-rich atmosphere. For that film, the hydrogen concentration profile resulted in a much higher hydrogen content on the sample surface than in the bulk, and H-Si bonds were predominantly of the weak type. Crystallisation was accomplished by low-temperature stepwise annealing from 200°C to 600°C at 100°C steps, with samples being cooled down to room-temperature between each annealing step. This resulted in large grained (> 10 μm range) polycrystalline silicon after the 600°C annealing step for a 1.1 μm thick sample. Fourier transform infrared (FTIR) spectroscopy, elastic recoil detection analysis (ERDA) and scanning electron microscopy (SEM) techniques were used to analyse samples before and after crystallisation.     

Characterization of pulsed laser deposited hydrogenated amorphous silicon films by spectroscopic ellipsometry

The wide absorption band of hydrogenated amorphous silicon (a-Si:H) is being realized as a key component of solar cells on glass. In this study, a-Si:H films were prepared by reactive pulsed laser deposition onto silicon and glass substrates. Ellipsometry showed that the optical properties of the films are effectively independent on the choice of substrate. According to the optical properties, the character of the films changes from amorphous silicon to dielectric as the hydrogen background pressure increases from 0 to 25 Pa. This observation was attributed to oxygen incorporation indicated by Rutherford Backscattering Spectrometry. Furthermore, a refractive index gradient in depth was revealed, which was attributed to the oxygen concentration gradient.     

Improvement on the conventional MWECR-CVD system and preparation of hydrogenated amorphous silicon films

In order to obtain a compact-structure, saving-energy and good-property system based on the conventional microwave electron-cyclotron resonance chemical vapor deposition (MWECR-CVD) system, we proposed a new technique using a magnetic field combination of an electromagnetic coil and a permanent magnet unit, as well as adopting a dedicated microwave coupling structure. The experiment results showed that by using this new optimized system the deposition rate can reach more than 2.5 nm/s, and the photosensitivity of a-Si:H films can be up to 1.1×10⁵.     

Anodic etching of p-type cubic silicon carbide

p-Type cubic silicon carbide was anodically etched using an electrolyte of HF:HCl:H₂O. The etching depth was determined versus time with a fixed current density of 96.4 mA cm^–2. It was found that the etching was very smooth and very uniform. An etch rate of 22.7 nm s^–1 was obtained in a 1:1:50 HF:HCl:H₂O electrolyte.     

Structural relaxation of amorphous silicon carbide thin films in thermal annealing

Amorphous Si_0.4C_0.6 thin films were deposited by radio frequency magnetron sputtering onto non-heated single crystal Si substrates, followed by annealing at 800 °C or 1100 °C in the vacuum chamber. The chemical bond properties and atomic local ordering as a function of the annealing temperature were characterized by Auger electron spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, Infrared absorption spectroscopy, X-ray diffraction, and Raman spectroscopy measurements. We have examined the evolution of microstructure in annealing-induced relaxation process, and investigated the initial stages of thermal crystallization of amorphous Si_0.4C_0.6. Meanwhile, the structure of excess C in the films also has been studied.     

Properties of nitrogen diluted hydrogenated amorphous carbon (n-type a-C:H) films and their realization in n-type a-C:H/p-type crystalline silicon heterojunction diodes

Nitrogen incorporated hydrogenated amorphous carbon (a-C:N:H) films were grown in an asymmetric rf PECVD system using C₂H₂ and N₂ gaseous mixture. Deposition rate, stress, hardness, optical bandgap, refractive index, and electrical characteristics have been studied as a function of self bias. Microstructures of these films were also studied using LASER Raman technique. Finally nitrogen diluted a-C:H films were realized as n-type semiconductor in n-type a-C:H/p-type crystalline silicon hetrojunction diodes. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics have also been studied as a function of self bias on these heterojunction diodes.