Rf Glow Discharge Decomposition Research Articles

Characterization of Germanium Nanocrystallites Grown on SiO2 by a Conductive AFM Probe Technique

Hydrogenated germanium films were fabricated in the thickness range of 7-98 nm on SiO 2 at 150°C by an rf glow discharge decomposition of 0.25% GeH 4 diluted with H 2 , and the nucleation and growth of Ge nanocrystallites were measured from topographic and current images simultaneously taken by a conductive AFM probe after Cr contact formation on films so prepared. We have demonstrated that current images show fine grains in comparison with topographic images and the lateral evolution of the Ge grains with progressive film growth. The contrast in current images can be interpreted in terms of the difference in electron concentration between nanocrystalline grains and their boundaries.

04/02885 Competitive solar heat engines: Valdès, L.-C. Renewable Energy, 2004, 29, (11), 1825–1842

Hydrogenated amorphous silicon-germanium alloy films have been prepared by rf glow discharge decomposition of a mixture of silane, germane and hydrogen varying the germane concentration in the gas mixture and the substrate temperature. The opto-electronic properties of a-SiGe: H films deteriorate sharply as the germane concentration is increased in the source gas mixture. This deterioration is attributed to an increase in defect density with Ge incorporation. The substrate temperature, Ts, was varied in the range 100–350°C. Due to the healing effect on the amorphous network at higher temperature, the opto-electronic properties of a-SiGe: H films improve up to Ts ∼ 250°C. At higher Ts, the properties deteriorate due to the breaking of weak GeH bonds. Infrared vibrational spectra show formation of polyhydrides and higher Ge concentration at lower Ts. The increase in density of states with higher Ge content is shown by ESR data.

Electrical characterization of Ge microcrystallites by atomic force microscopy using a conducting probe

We have studied the nucleation and growth of Ge microcrystallities on Si(100) or evaporated Cr substrates from an rf glow discharge decomposition of GeH 4 highly-diluted with H 2, where the crystallinity, the surface microroughness and the local electric transport of the films have been measured as a function of the film thickness. For the film growth thicker than ∼65 nm, Raman scattering spectra show that the evolution of the microcrystalline phase tends to be saturated. In the thickness range of 7–65 nm, the nucleation and/or microcrystalline grain formation with progressive film growth and corresponding significant difference in the electrical conductivity in the direction of the film thickness between the grains and their boundaries have been demonstrated from topographic and current images taken simultaneously by an atomic force microscope with a conducting probe.

Steady-state and transient photoconductivity in hydrogenated amorphous silicon nitride films

Abstract Amorphous SiNx:H films were prepared by the rf glow-discharge decomposition of ammonia/silane gas mixture with varying nitrogen content. The steady-state photoconductivity and its dependence on light intensity have been investigated in a-SiNx:H as a function of temperature between 100 and 420 K. The electron drift mobility of a set of SiNx:H samples has been determined from their steady-state photoconductivity and response time measurements. The results suggest that electron drift mobility of the samples was nearly unchanged for a low nitrogen content. Two samples containing lowest nitrogen showed higher photoconductivity than that of unalloyed sample within a temperature range including the room temperature.

Optical bandgap and quantum well model in hydrogenated amorphous silicon carbon alloy films

Hydrogenated amorphous silicon carbon alloy (a-SiC : H) films of various composition were prepared by the rf glow discharge decomposition of acetylene and silane as well as ethylene and silane and were also annealed for various annealing temperatures. Careful and detailed IR absorption and the optical bandgap (Eopt) measurements were undertaken for a-SiC : H films of different composition (x) and annealing temperatures (Ta). Interesting variation of Eopt with x and Ta have been observed. Our study demonstrates, for the first time, that the models involving alloying or/and ordering break down and the proposed quantum well model based on heterogeneity is found to be the only model capable of explaining the observed variation of Eopt. This, consequently, may have considerable implications.

Optical, electrical and structural properties of amorphous SiCN:H films prepared by rf glow-discharge decomposition

Amorphous quaternary alloy (a-SiCN:H) films composed of silicon, carbon, nitrogen and hydrogen have been prepared by the rf glow-discharge decomposition in a gas mixture of methane, silane, nitrogen and helium. Effects of nitrogen addition on the optical, electrical, structural and optoelectronic properties of the films have been investigated. Optical bandgap remains almost constant for a wide range of nitrogen addition. Dark and photoconductivity becomes greater by the nitrogen addition than those of undoped a-SiC:H films. Incorporation of a small amount of nitrogen in the film is likely to reduce the structural disorder and/or the density of defects, and some nitrogen atoms seem to work as a dopant. On the other hand, large incorporation deteriorates these properties. The nitrogen addition effect appears more remarkably in the films prepared under the low flow rate ratio of methane to silane.

Electrical conduction studies of plasma enhanced chemical vapor deposited silicon nitride films

Current conduction mechanisms have been studied for three representative films, namely, silicon-rich, nearly stoichiometric and nitrogen-rich silicon nitride films, prepared by rf glow-discharge decomposition of silane and ammonia with nitrogen dilution. Ohmic conduction has been observed for all the films at low electric fields. The dominance of Poole–Frenkel conduction at intermediate fields and Fowler–Nordheim conduction at high fields has been observed both for the nitrogen-rich and the nearly stoichiometric films. However, for the silicon-rich films, the Poole–Frenkel conduction mechanism dominates both for the intermediate as well as the higher fields. This study indicates that the silicon-rich films have the highest density of traps and the nitrogen-rich films have the lowest, which may be ascribed to the effect of nitrogen dilution.

Infrared attenuated-total-reflection spectroscopy of microcrystalline silicon growth

In situ Fourier transform infrared attenuated-total-reflection (FT-IR-ATR) spectroscopy has been applied to study the growth of μc-Si:H from an rf glow discharge decomposition of H 2-diluted SiH 4 and from a layer-by-layer (LbL) process in which 3 nm-thick Si:H deposition and H 2-plasma annealing are alternately repeated. The results indicate that the FT-IR-ATR technique enables us to measure the nucleation and the grain growth with a submonolayer sensitivity. During the μc-Si:H growth, the time evolution of the absorption intensity of the stretching modes due to interior silicon hydrides is suppressed and distinct absorption peaks due to surface hydrides emerge, which are not observable in the case for a-Si:H film growth. Temporal changes in the ATR spectrum during the LbL process at 250°C have shown that the hydrogen desorption from a near surface region, especially the grain boundary and amorphous phase, caused by H 2-plasma exposure affects the crystalline nucleation and the grain growth.

A study of plasma-deposited amorphous SiO x:H [formula omitted] films using infrared spectroscopy

The Si–O stretching mode in amorphous SiO x :H (a-SiO x :H) films, prepared by rf glow discharge decomposition of a SiH 4–O 2 mixture at 300°C, has been investigated by infrared absorption measurements as a function of the O content x. It was found that the ratio, I AS 1 /N Si , of the absorption intensity of the Si–O stretching mode centered around 1000 cm −1 to the density of Si atoms increased linearly with increasing x, up to x=0.6. Above x=0.6, the rate of increase of the I AS 1 /N Si values became slower. However, the ratio, I sum /N Si , of sum of the absorption intensities for both 1000 and 1150 cm −1 bands to the density of Si atoms increased linearly with increasing x for x>0.6. We obtained the proportionality coefficient, A SiO, of the Si–O stretching mode to be 1.48×10 19 cm −2. The characteristic of the Si–O stretching mode and the A SiO are discussed.

Synthesis of nanosize Si–C–N powder in low pressure plasmas

Multicomponent powders based on the B–C–N–Si system are of great interest as starting materials for sintering advanced ceramics and composites with improved properties. The square-wave modulation of the electrical power supplied to low pressure (<100 Pa) radio frequency (rf) plasmas has been shown as a suitable method to produce high purity silicon and binary powders, such as SiC, SiN and BN. The present study investigates the synthesis of silicon carbonitride (SiCN) nanometric powder in a plasma-enhanced chemical vapour deposition reactor, by rf glow discharge decomposition of CH 4, SiH 4 and NH 3 gases at room temperature. The output of the rf source (13.56 MHz) was square-wave modulated at a period of 20 s with plasma-on times between 0.05 and 5 s. Transmission electron microscopy showed that the SiCN nanopowder was amorphous and that the average particle size increased from 9 to 100 nm as the plasma-on period increased. The chemical composition of the powder was analyzed by X-ray photoelectron spectroscopy and elemental analysis. Infrared spectroscopy revealed the presence of Si–C, Si–N, C–N and hydrogenated bonds. The infrared absorptions of hydrogenated bonds decreased as the plasma-on time increased, volume ratio with the increase in particle size.

Relaxation processes and metastability in amorphous hydrogenated silicon investigated with differential scanning calorimetry

In this work we investigated the thermodynamics and kinetics of structural relaxation in a-Si:H films using differential scanning calorimetry (DSC). The relation between structural relaxation and light-induced metastability (Staebler–Wronski effect, SWE) has also been investigated. The a-Si:H films were prepared by RF (13.56 MHz) glow discharge decomposition of the mixtures (10% SiH 4+90% H 2) and (5% SiH 4+95% He), and by 55 kHz plasma-enhanced chemical-vapor deposition (PECVD) of pure silane for high deposition rates. Detailed analysis of the peaks on DSC curves shows that the low-temperature effect (LTEP) is caused by the relaxation of weak Si bonds and that the high-temperature effect is caused by hydrogen effusion with subsequent relaxation of the microstructure. A method for the calculation of kinetic parameters of structural relaxation using DSC data is proposed. We found from the joint investigation of DSC and SWE that both the light-induced defect generation and the LTEP on DSC curves have a common nature and are described by the same reaction between weak Si–Si bonds and dangling bonds. We show that the rates of this reaction in both the forward and the reverse directions are controlled by hydrogen microstructure.

Deposition and properties of germanium/carbon films deposited from tetramethylgermanium in a parallel plate RF discharge

Thin germanium/carbon films were prepared by an RF glow discharge decomposition of tetramethylgermanium in a stream of argon using typical parallel plate reactor. Both the deposition yield and the physical properties of the films strongly depended on the power input used. Soft and semitransparent polymer-like materials, characterized by a strong dependence of their physical properties on the energetic conditions of deposition, were obtained at low power magnitudes. High power loads yielded hard, glassy, dark brown to black films of relatively high electrical conductivity (up to 4×10 −3 S/m) and refractive index (approximately 3.0), low value of optical gap (1.3 eV) and density amounting to about 2600 kg/m 3 with the above values for a relatively large range remaining independent of deposition conditions. Hard films exhibited extremely homogeneous surface (down to several nm) while that of soft films was characterized by circular inhomogeneities at the level of several dozen nanometres. FTIR spectroscopy revealed an incorporation of certain amount of oxygen, particularly in the case of the latter materials. Although there are differences in physical properties such as density or optical gap, as well as several structural differences between the resulting materials, the process to a large extent reminisces that of the deposition of Ge/C films from tetraethylgermanium in the same equipment.

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.

Optical and transport properties of amorphous and microcrystalline silicon films prepared by excimer laser assisted rf glow-discharge deposition

Hydrogenated amorphous and microcrystalline silicon films were deposited on glass substrates at 250 °C by combining rf glow discharge decomposition of silane–helium gas mixtures and ultraviolet laser irradiation on the surface of the growing film. The effect of the laser fluence on the optical and electrical properties of the films was investigated with combined in situ ellipsometry and ex situ techniques. Particular attention was paid to the properties of the films deposited at low and high laser fluences. At a low laser fluence, the resulting hydrogenated amorphous silicon films display a reduced defect density as compared to unirradiated ones. At laser fluences above the melting threshold, we obtain microcrystalline silicon films with a high surface roughness, as observed by scanning electron microscopy, and high electron mobility, as deduced from time resolved microwave conductivity measurements. The enhanced optical absorption produced by the roughness along with the high electron mobility make these films excellent candidates for photodetection and photovoltaic devices.

Initial Growth of Microcrystalline Silicon on Atomically Flat Hetero-Substrate

ABSTRACTInitial growth of microcrystalline silicon μ-Si:H) deposited on an atomically flat GaAs (001) wafer using a RF glow-discharge decomposition of hydrogen diluted monosilane gas mixture has been studied by means of atomic force microscope (AFM), Auger electron spectroscopy (AES), and cross-sectional transmitssion electron microscopy (XTEM).It is shown that the initial growth of μc-Si:H deposited at a substrate temperature of 50∼250°C consists of four successive stages, i.e., (1) a layer-by-layer growth of a-Si:H up to d ∼5 Å, (2) island formation of a-Si:H, (3) the coalescence of the islands and the nucleation of microcrystallite at d∼ 10–40 Å depending on the growth temperature, and (4) a rapid roughening with microcrystalline growth.

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.

Nitrogen Incorporation in a-Ge:H Produced in High-Hydrogen-Dilution Plasma

The network of hydrogenated amorphous germanium (a-Ge:H) has been modified with nitrogen by employing the rf glow discharge decomposition of a hydrogen-diluted gas mixture of NH3 and GeH4. It is found that the hydrogen dilution of the material gas hinders the incorporation of three-fold coordinated nitrogen atoms in the network and causes optical bandgap shrinkage. Dramatic enhancement of the conductivity and decrease of the activation energy are observed in the high-hydrogen-dilution regime, and are explained by the incorporation of four-fold coordinated nitrogen in the relaxed network.

A structural interpretation of SiOSi vibrational absorption of high-photoconductive amorphous a-SiOx:H films

The SiOSi stretching vibrational absorption in high-photoconductive amorphous silicon-oxygen alloy films prepared by rf glow discharge decomposition of SiH4CO2 gas mixtures is investigated as a function of oxygen concentration. With increasing oxygen content, the peak position of the SiOSi bond stretching mode increases from 980 cm−1 to 1080 cm−1. The change of the absorption spectra with oxygen concentration is well analyzed on the basis of a two-phase structure model, in which each Si-rich phase (mostly a-Si:H) is surrounded by transition oxide SiO to SiO2 phase.

Small-angle X-ray scattering studies of a-Si:C:H

Small-angle X-ray scattering studies have been performed on a series of four a-Si:C:H alloys, prepared by rf glow discharge decomposition of varying proportions of propane and silane, in an attempt to elucidate their mesoscopic structure. The observed broad scattering peak has been interpreted as originating from irregular, elongated voids with a repeat distance of about 20 Å and correlation length of about 25 Å. The implications of this result in explaining the photo-oxidation properties of the material are also discussed.

Substrate temperature effect on the stability of hydrogenated amorphous silicon films deposited at high rates

The metastability of the electronic properties of hydrogenated amorphous silicon (a-Si:H) films deposited at high rates by the rf glow discharge decomposition of mixtures containing 40% silane in helium is compared with that of device quality a-Si:H material deposited at 250 °C under standard low deposition rate conditions. The density of states above the Fermi level of the films obtained under helium dilution decreases for deposition temperatures increasing from 250 to 350 °C, both after annealing and after light soaking. At 350 °C, the density of states becomes comparable in both states to that of device quality low deposition rate a-Si:H. We observe a correlation between these results and the degradation of the photoconductivity and the below-Fermi-level defect density measured in situ during light soaking.