Glow Discharge Decomposition Of Silane Research Articles

Compositional dependence of the optical gap in amorphous silicon–chalcogen alloys

In this work we present a model to explain the compositional dependence of the optical (Tauc) gap for a-Si, S:H and a-Si, Se:H alloys. Thin films of these materials were grown by a capacitively coupled r.f. glow-discharge decomposition of silane and either hydrogen sulfide or hydrogen selenide gases. The compositional dependence was obtained by varying the gas volume ratio R v = H 2 S or H 2 S e / S i H 4 . Optical transmission measurements were employed to measure the optical (Tauc) gap of both alloys as a function of the gas volume ratio. The compositional dependence exhibits an inverted S-shaped variation. A model was developed that fits accurately the experimental data. It was shown that the parameters of the fitting curve are directly and accurately related to some fundamental atomic constant.

Thin film silicon solar cells grown near the edge of amorphous to microcrystalline transition

Thin films of silicon can be grown by glow-discharge decomposition of silane and hydrogen. As the hydrogen dilution increases, a transition from amorphous to microcrystalline phase takes place. The regime below and above this edge has been found to be most suitable for making high efficiency amorphous and microcrystalline solar cells. In this paper I outline the status of material research, cell performance and manufacturing of thin film silicon grown near the edge of amorphous to microcrystalline transition.

Correlation between the opto-electronic and structural parameters of amorphous semiconductors

The variation of opto electronic parameters like the optical band-gap (Tauc gap E opt and the isoabsorption gap E 04), and structural parameters like the Tauc slope parameter B, of the hydrogenated amorphous silicon carbon alloy films prepared by r.f. glow discharge decomposition of silane and acetylene as well as silane and ethylene, are discussed for various compositions and also for different annealing temperatures. We observe, for the first time, an interesting new correlation between ( E 04− E opt) and the Tauc slope parameter B of amorphous semiconductors.

Plasma enhanced growth, composition and refractive index of silicon oxynitride films

Silicon oxynitride films of various compositions have been prepared by the glow discharge decomposition of silane (SiH 4) and nitrous oxide (N 2O). The refractive index of the films has been measured using ellipsometry, while the composition of the films has been determined using Rutherford backscattering (RBS) and elastic backscattering (EBS) techniques. This study has demonstrated that the measurement of composition using EBS technique is distinctly superior to RBS technique, particularly for the films like silicon oxynitride containing multicomponent light elements in low concentration.

Effect of ammonia plasma pretreatment on the plasma enhanced chemical vapor deposited silicon nitride films

Silicon-rich, nearly stoichiometric and nitrogen-rich silicon nitride films have been prepared by the glow-discharge decomposition of silane and ammonia with nitrogen dilution, with and without ammonia plasma pre-treatment of the silicon substrate. A considerable improvement in the silicon nitride/silicon interface, as evident from the large reduction in the minimum interface state density ( D it) min, due to ammonia plasma pretreatment is observed.

Hydrogenated amorphous silicon deposited by glow discharge of SiH4 diluted with He: photoluminescence and electroluminescence in the visible region

Samples of a-Si:H having a wide gap (≥2.1 eV) were prepared using routine rf glow discharge decomposition of silane (SiH4) strongly diluted with He. Also, microwave electron–cyclotron–resonance plasma-enhanced chemical-vapour-deposition was used to prepare wide-gap a-Si:H via decomposition of He diluted SiH4. The wide-gap a-Si:H samples, containing above 30 at.% of hydrogen, exhibit visible photoluminescence at room temperature. From the temperature dependence of photoluminescence intensity and from the observed shift of the maximum with temperature we deduce that the customary model of radiative recombination of carriers localised in tail states is not directly applicable to the case of wide-gap a-Si:H. Instead, we propose that the photoluminescence arises in parallel from tail states and from isolated (extrinsic) centres. Electroluminescence results obtained on a p–i–n structure with wide-gap a-Si:H are reported.

Annealing study of the optical properties of hydrogenated amorphous silicon–carbon alloy film

Hydrogenated amorphous silicon–carbon alloy films were prepared by the glow discharge decomposition of silane and ethylene. The real and imaginary parts of the dielectric constants ϵ 1 and ϵ 2 of the film were determined at room temperature for the photon energies between 1.38 and 4.86 eV for the as-deposited ( T s=250 °C) as well as for subsequent anneals at different temperatures up to 1200 °C. The optical bandgap ( E opt) was determined from the Tauc's relation while the characteristic optical parameters such as the dispersion energy ( E d), average excitation energy ( E o), plasmon energy ( ℏ w p) and the valence electron density ( n v) were obtained from the Wemple–DiDomenico model. We observe an interesting correlation between E opt and n v which seems to indicate the coupling of the valence and conduction band through single-particle excitation. © 1997 Elsevier Science S.A. All rights reserved.

Effect of argon dilution on the structure of microcrystalline silicon deposited from silane

The structure of the hydrogenated amorphous and microcrystalline silicon films deposited by radio-frequency (rf) glow discharge decomposition of silane diluted in argon has been studied by transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy techniques. It has been observed that argon acts not only as a passive diluent gas but also plays an important role in the growth of the amorphous or microcrystalline network. Calculation of the variation of equilibrium concentration of Ar*(3P0,2) and Ar+ with argon dilution shows that at high argon dilution (≳90%) equilibrium concentration of Ar* rises sharply with increase in argon dilution. Variation of structural properties with argon dilution suggests involvement of these excited states of argon in the growth process. A model has been proposed based on the energy exchange between the Ar* states and the growth zone of the materials to explain the structural changes observed in the presence of argon in the plasma.

In situ ellipsometric study of the influence of powder formation on the initial growth of glow discharge a− Si: H

Even under standard preparation conditions for amorphous hydrogenated silicon ( a− Si: H) by r.f. glow discharge decomposition of silane, powder formation can occur leading to an initial transient behaviour of silane discharges. Phase-modulated real-time in situ ellipsometry is used to study the influence of the discharge behaviour on the initial stage of growth of a− Si: H after ignition and subsequent to intentional discharge interruptions. A clear correlation between the powder-determined time dependences of the discharge parameters pressure and self-bias voltage and of the ellipsometric angles is confirmed. In the discharge regime where this initial transient behaviour due to powder formation is observed, the corresponding Δ - ψ trajectories can be attributed to the initial growth of less dense material. The time-dependent Δ signal of the growing film is found to be very sensitive even to minor discharge perturbations. The influence of controlled temporal variations of preparation parameters on the growing a− Si: H film can be simultaneously monitored by real-time in situ ellipsometry. This offers an opportunity for interface optimization in a− Si: H structures.

Structural investigations on low-frequency, low-temperature, plasma-deposited a-SiN x: H films

Hydrogenated amorphous a-SiN x : H films were prepared using a 35 kHz low-frequency glow discharge decomposition of silane and ammonia gas mixture without thermal activation. Samples with different silicon contents were studied by AES and FTIR. Silicon LVV and KLL peak profiles were analyzed as functions of the silicon content of the layer. The structure of the silicon-rich films was found to be highly disordered with several kinds of tetrahedron around the silicon atoms. The structure of the nitrogen-rich films appears more ordered and consists mainly of SiN bonds and can be described as a modification of the c-Si 3N 4 structure in which SiN bonds are replaced by SiH bonds.

Systematic study of light-induced effects in hydrogenated amorphous silicon.

A systematic investigation of the origin of the photoinduced changes in the defect density of states in the pseudogap of undoped hydrogenated amorphous silicon (a-Si:H) has been carried out on films deposited by glow-discharge decomposition of silane under different conditions. We used several complementary techniques, such as junction capacitance versus temperature, electrical conductivity, electron-spin resonance, and photothermal deflection spectroscopy, to determine the shifts in the Fermi-level (${\mathit{E}}_{\mathit{F}}$) position as well as the variation of the defect density of states, within the forbidden gap, after annealing (state A) and light-soaking (state B) treatments. The total hydrogen concentration and the hydrogen bonding configurations of each sample were analyzed by elastic-recoil-detection analysis (ERDA) and infrared (ir) transmission measurements, respectively, both for state A and state B. The experimental results show in all cases an increase in the bulk density of states accompanied by a shift of ${\mathit{E}}_{\mathit{F}}$ towards the valence band in the light-soaked state B with respect to the annealed state A, but these changes depend considerably on the deposition conditions of the films. On the other hand, no change is detected either in the ERDA or in the ir spectra. Moreover, it is found that when the saturated light-soaked samples are partially annealed at temperatures ranging from 110 \ifmmode^\circ\else\textdegree\fi{}C to 130 \ifmmode^\circ\else\textdegree\fi{}C and for times less than 15 min, and then left in the dark at room temperature for 24 h and more, a long-time process, characterized by a variation of the density of states as well as by a motion of ${\mathit{E}}_{\mathit{F}}$ within the gap, occurs in these films. The observed behavior is completely different from one sample to another. From the analysis of the results, it is suggested that in some samples the photodegradation is mainly due to a trapping of the excess free carriers at charged spinless defects, which must be present in the sample at equilibrium in addition to the neutral dangling bonds.

PREPARATION AND CHARACTERIZATION OF GRADED BANDGAP AMORPHOUS HYDROGENATED SILICON-SULFUR THIN FILMS

Compositionally graded hydrogenated amorphous silicon-sulfur alloys ( a-Si 1−x S x: H ) were grown by RF glow discharge decomposition of silane and hydrogen sulfide gases. Infrared spectra show clear evidence for the incorporation of sulfur in the form of Si-S bonds in the material. The graded bandgap films possess optical bandgaps ranging from 1.91 to 2.05 eV depending on the RF power. The compositionally graded layers display high photosensitivities indicating that they may be ideal candidates for use in amorphous silicon based tandem cells.

Fabrication of sub-100nm linewidth x-ray masks and replication using synchrotron radiation at super-ACO

A high resolution x-ray mask technology has been developed based on amorphous silicon carbide covered with stress minimized W and Au patterns. SiC blanks are fabricated from silicon wafers coated with low stress stoechiometric SiC deposited by rf low frequency (110 kHz) glow discharge decomposition of silane and methane (PECVD). Mask absorber pattern fabrication is performed by electron- beam lithography with a single resist layer process. These masks have been tested in the lithography station implemented by our laboratory at Super-ACO using synchrotron radiation in the spectral range 0.5–1.5 nm. Sub-100 nm replication patterns obtained by contact printing in PMMA resist are presented. Results of the evaluation of the x-ray resists SAL 601 and SAL 603 used with the Karl Suss stepper XRS 200 are also described.

Thin film hydrogenated silicon-sulphur alloys as promising materials for solar cell applications

Compositionally graded hydrogenated amorphous silicon-sulphur alloys were grown by r.f. glow discharge decomposition of silane and hydrogen sulphide at a substrate temperature of 250°C. The composition of the alloys was controlled by changing the gas volume ratio R v = [ H 2 S] [ SiH 4] . IR spectra reveal clear evidence for the incorporation of sulphur in the form of Si—S bonds in the material. Although there is a decrease in the photoconductivity σ ph with increasing E opt, the photosensitivity σ ph/σ d remains high, exceeding 10 3 throughout the whole alloy range. The compositionally graded layers display high photosensitivities, indicating that they may be of use in amorphous-silicon-based solar cells.

Influence of pressure and radio frequency power on deposition rate and structural properties of hydrogenated amorphous silicon thin films prepared by plasma deposition

The influence of radio frequency (rf) power and pressure on deposition rate and structural properties of hydrogenated amorphous silicon (a-Si:H) thin films, prepared by rf glow discharge decomposition of silane, have been studied by phase modulated ellipsometry and Fourier transform infrared spectroscopy. It has been found two pressure regions separated by a threshold value around 20 Pa where the deposition rate increases suddenly. This behavior is more marked as rf power rises and reflects the transition between two rf discharges regimes (α and γ). The best quality films have been obtained at low pressure and at low rf power but with deposition rates below 0.2 nm/s. In the high pressure region, the enhancement of deposition rate as rf power increases first gives rise to a reduction of film density and an increase of content of hydrogen bonded in polyhydride form because of plasma polymerization reactions. Further rise of rf power leads to a decrease of polyhydride bonding and the material density remains unchanged, thus allowing the growth of a-Si:H films at deposition rates above 1 nm/s without any important detriment of material quality. This overcoming of deposition rate limitation has been ascribed to the beneficial effects of ion bombardment on the a-Si:H growing surface by enhancing the surface mobility of adsorbed reactive species and by eliminating hydrogen bonded in polyhydride configurations.

Deposition of silicon nitride by glow-discharge decomposition of silane–nitrogen mixtures in a saddle-field plasma

High-quality silicon nitride films have been deposited onto a variety of substrates including c-Si, GaAs, and InP at temperatures in the range 50–350 °C by the dc glow-discharge decomposition of silane–nitrogen mixtures in a saddle-field cavity. The effect of the N2–SiH4 flow ratio [Formula: see text] and the effect of a dc bias Vb applied to the substrate holder on the properties of the resulting films were systematically investigated. The N/Si ratio in the resulting films, as determined by Auger analysis, could be varied from 0 to 1.8 by adjusting [Formula: see text], with [Formula: see text] yielding stoichiometric silicon nitride. The total hydrogen content, as determined by 15N profiling, decreased linearly with [Formula: see text]. Far infrared absorption measurements indicated that hydrogen was incorporated mainly into Si–H sites at lower N2–SiH4 flow ratios in the range 1–2; higher N2–SiH4 flow ratios of 5–15 resulted in approximately equal concentrations of Si–H and N–H sites with a total bonded hydrogen content of about 12 at.% for films deposited at 250 °C. Ellipsometry measurements indicated that a refractive index as low as 1.82 at 632.8 nm was achieved. Refractive index and thickness variations of less than 5% were observed in films deposited on 4 in (1 in = 2.54 cm) diameter c-Si wafers. Scanning electron microscopy and photoluminescence studies of the thermal stability of SiNH films deposited onto semiinsulating GaAs indicated improved annealing behaviour with positive substrate bias.

Wide Bandgap Fluorinated Silicon-Carbide-Nitride Films Using NF3

ABSTRACTAmorphous hydrogenated silicon carbide has been studied extensively because of its properties as a wide bandgap material. However, a large amount of methane is needed to deposit the material. Also, the high carbon content of these films poses some problems. The addition of NF3 to the gas stream results in wide bandgap films with a substantial reduction in the required CH4 flow for deposition. Amorphous SiCx Ny :H:F films were prepared using rf glow discharge decomposition of silane, methane, and nitrogen trifluoride in a parallel-plate stainless steel reactor. Gas flow rate and power density were varied. For a gas mixture containing 6% NF3 and 78% CH4, FTIR measurements reveal a reduction in C-H peak heights at 2960 cm-1 and 2880 cm-1 with respect to the Si-H peak at 2080 cm-1 indicating a smaller carbon content in the film. The C-H peaks shift to higher wavenumbers with increasing NF3. The use of NF3 increases the bandgap from 2.6 to 3.14 eV while reducing the refractive index from 2.12. to 1.87. A maximum deposition rate of 625 A/min was achieved. This should be compared to the very low deposition rate of 18 A/min for comparable bandgap Si-C films deposited using 97% methane in silane. Increasing the deposition power density resulted in a larger bandgap and a smaller refractive index.

Annealing of “intrinsic” and photo-induced defects in hydrogenated amorphous silicon

Electronic defects in hydrogenated amorphous silicon thin films, deposited from the glow discharge decomposition of silane in the temperature range between about 225 °C and 325 °C and having approximately 10–15 at.% hydrogen, undergo a thermally activated relaxation during film deposition. We determine the kinetics of this relaxation process in films with similar hydrogen concentrations deposited by reactive magnetron sputtering at a substrate temperature of about 40 °C, and annealed at temperatures greater than 150 °C. We compare the kinetics of this process with the annealing of photo-induced defects in low defect density (less than 10 16 cm −3) films. We also present a quantitative relationship between the relaxation time and the deposition and/or annealing conditions required to produce low defect density material.

Properties of plasma-deposited hydrogenated amorphous silicon prepared under visible light illumination

Properties of hydrogenated amorphous silicon (a-Si:H) prepared by glow discharge decomposition of silane with visible light illumination on a growing surface have been described. The visible light illumination during growth has resulted in smaller light-induced changes, slightly lower photoconductivity before the prolonged illumination, lower content of bonded hydrogen, and lower ratio of the number of silicon dihydrides to that of total silicon-hydrogen bonds. Formation of Si network and consequent structural disorder of SiSi bonds, on the other hand, has not been affected by the illumination during the growth. The improved stability against the long exposure to light can be ascribed to the lower bonded hydrogen content and the reduced density of SiH2 bonds, while the structural disorder of the Si network does not have a direct relationship with the light-induced changes.

Present and future applications of amorphous silicon and its alloys

Thin films of amorphous silicon (a-Si:H) and its alloys prepared by the glow discharge decomposition of silane and appropriate gases, are presently incorporated into six commercial products and approximately twenty other applications have been proposed for these materials. This presentation will review the present applications and a number of those proposed for the future. Finally, some of the material and technological issues of current importance will be discussed.