The effects of deposition parameters on a-Si:H films fabricated by microwave glow discharge techniques

Hydrogenated amorphous silicon (a-Si:H) films have been fabricated by a novel method of microwave glow-discharge deposition from SiH₄ and H₂, operating at 2.45 GHz. The properties of the deposited films are dependent upon the confinement of the microwave plasma by a magnetic field, and upon the orientation of the substrates with respect to the electric field. The quality of these materials is comparable to that of films deposited in conventional radio-frequency glow-discharge systems.     

The interpretation of the electric and optical properties of a-Si:H films produced by rf glow discharge through dark conductivity,photoconductivity and pulse controlled capacitance-voltage measurements

This paper deals with the interpretation of transport properties of amorphous silicon hydrogenated films (a-Si:H) through dark conductivity, photoconductivity and pulse controlled capacitance-voltage measurements. a-Si:H films were produced by rf glow discharge coupled either inductively or capacitively to a 3% SiH₄/Ar mixture at different crossed electromagnetic static fields. The data concerned with the dark activation energy, photoactivation energy, variation of the density of localized states and photosensitivity, (σ_ph/σ_d)_25°C, of a-Si:H films can account for their optoelectronic properties which are strongly dependent on the deposition parameters. We also observed that crossed electromagnetic static fields applied during film formation influences hydrogen incorporation in a different manner than previously proposed.     

The location,evolution, and role of fluorine in glow discharge a-Si:F

Amorphous Si:F films werer prepared by d.c. glow-discharge decomposition of a mixture of SiF₂+SiF₄ gas. Fluorine content, infra-red absorptance, electron spin density and electronic transport properties were determined as a function of both deposition and annealing temperatures. The role of fluorine in a-Si:F is discussed on the basis of the results.     

Mass-transfer analysis of a:Si:H deposition by the glow discharge process

The deposition rate and the gas efficiencies in the glow discharge plasma process are calculated by solving the convective diffusion equations for the precursor gas and the condensable species which are created by the discharge. The theoretical predictions are compared with experimental results obtained with a cross-flow reactor.     

Transport properties of amorphous germanium prepared by the glow discharge technique

The paper deals with conductivity, thermoelectric power and field effect measurements on amorphous Ge specimens prepared by the decomposition of germane gas in a rf glow discharge. Substrate temperatures T_d of 300, 400 and 500 K were used during deposition. The sign of the thermoelectric power S is negative throughout the temperature range investigated (200–500 K). Above 300 K, the conductivity activation energy in specimens prepared at T_d = 500 K lies between 0.40 and 0.43 eV; it is equal to the gradient of the S versus 1/T curves, suggesting transport in the extended electron states. Below room temperature there is an increasing contribution in all specimens from electron hopping transport in localized states lying about 0.25 eV below ?_C. Both conductivity and thermoelectric power results can be interpreted satisfactorily in terms of these two current paths. Hopping at the Fermi level has not been observed. The preliminary field effect measurements indicate that, as in amorphous Si, ?_f lies near a density of state minimum. The density of states at ?_f is appreciably higher than that in similarly prepared Si specimens.     

Influence of hydrogen on the thermomechanical properties of a-CNx:H and a-CNx films deposited by glow discharge and ion beam assisted deposition

The coefficient of thermal expansion (CTE), Young’s modulus, Poisson’s ratio, stress and hardness of a-CN_x and a-CN_x:H were investigated as a function of nitrogen concentration. Hydrogenated films were prepared by glow discharge, GD, and unhydrogenated films were prepared by ion beam assisted deposition, IBAD. Using nanohardness measurements and the thermally induced bending technique, it was possible to extract separately, Young’s modulus and Poisson’s ratio. A strong influence of hydrogen, in a-CN_x:H films, was observed on the CTE, which reaches about ∼9 × 10⁻⁶ C⁻¹, close to that of graphite (∼8 × 10⁻⁶ C⁻¹) for nitrogen concentration as low as 5 at.%. On the other hand, the CTE of unhydrogenated films increases with nitrogen concentration at a much lower rate, reaching 5.5 × 10⁻⁶ C⁻¹ for 33 at.% nitrogen.     

Transport properties of hydrogenated amorphous silicon deposited by dc glow discharge decomposition

Conductivity and thermoelectric power measurements have been made as a function of temperature on a series of hydrogenated amorphous silicon samples. The samples were prepared by the dc glow discharge decomposition of silane and silane phosphine mixtures. The activation energy for conduction varied with the substrate temperature and discharge condition for undoped specimens. The difference in the activation energy for conduction as well as the dependence of photoconductivity and optical gap on the activation energy for conduction among undoped specimens can be explained by introducing centers acting as donors or by change transfer between the island and hydrogen rich interfacial region. The kinks in the log σ versus inverse temperature curves always appear at about 430 K for the undoped specimens prepared at 300°C, while they are absent for low substrate temperature specimens. The downward kinks with increasing temperature can be explained by a two-phase material model. A revised two-channel conduction path model including material heterogeneity is applied to interpret the conductivity and thermopower versus inverse temperature curves of doped a-Si:H films, and to determine the position of phosphorus donor levels. The levels are found to lie at about 0.47 eV below E_c, the mobility edge at the conduction band.     

Optical and electrical properties of chlorinated and hydrogenated amorphous silicon prepared by glow discharge

Chlorinated and hydrogenated amorphous silicon films were prepared by glow discharge of a SiCl₄/H₂ mixture. Infrared spectra of these films show that, in addition to the hydrogen induced bands, two new modes appear at 545 cm^?1 (SiCl stretching) and 500 cm^?1 (Si TO modes induced by chlorine). Observation of the 545 cm^?1 band proves that chlorine is able to act as a dangling bond terminator in an amorphous silicon matrix. A good agreement is found between the total amount of chlorine determined by electron microprobe analysis and the value estimated from the integrated strength of the SiCl stretching mode. The relatively high value of the optical band gap (1.80 eV) of our material containing only 5 at.% bonded hydrogen shows that chlorine plays a major role in the optical gap value. Electrical conductivity, photoconductivity and luminescence properties are qualitatively similar to that of a: SiH films.     

Defects and microvoids in a-Si and a-Si:H

We report the first measurements of positron-annihilation spectra of samples of both pure and hydrogenated amorphous silicon. Comparison of these spectra with that of crystalline silicon indicates that the lowest-lifetime component can be identified as the contribution mainly from valence-band electrons. Both the pure (a-Si) and the hydrogenated (a-Si:H) samples exhibit a component with intermediate lifetime, which we attribute to small vacancies consisting of about 4 missing atoms. Finally, only a-Si:H shows a significant long-lived line (τ > 5 ns), which arises from large microvoids, with ～ 100 missing atoms. The existence of these microvoids in a-Si:H is consistent with recent reports of the presence of occluded H₂ gas under high pressure in such films.     

Microscopic properties of silicon dihydride bonding in a-Si:H

We use proton nuclear magnetic resonance to probe stable silicon dihydride existing in high defect density hydrogenated amorphous silicon. The silicon dihydride line shape exhibits the well-known ‘Pake’ doublet form resulting from a dipole interaction between two protons. We simulate the line shape using the relevant Hamiltonian and broadening effects known to occur in this amorphous system and discuss the sets of simulation parameters that reproduce the experimental data. The sets share a common feature – they require the proton–proton separation to be approximately 1.8 Å, not the 2.4 Å determined by recent theoretical calculations.     

Complex study of mechanical properties of a-Si:H and a-SiC:H boron doped films

The aim of the present work is to provide the complex study of the mechanical properties of p-doped a-Si:H and a-SiC:H thin films prepared under different plasma conditions. For the investigation of the samples we used mainly the continuous depth sensing indentation technique (DSI), pin-on-disc test and internal stress measurement. The morphology of the thin film surface and the indentation prints are studied using optical microscopy, scanning electron microscopy (SEM) and topography mode of atomic force microscopy (AFM). The dependence of the mechanical parameters upon the deposition conditions were compared with the optoelectronic properties of studied films.     

Formation kinetics and control of microcrystallite in μc-Si:H from glow discharge plasma

The formation kinetics of μc-Si:H has been investigated through the film depositions and plasma diagnoses in widely-scanned glow discharge plasma conditions; RF power density, SiH₄/H₂ ratio and substrate temperature. The roles of H and SiH_x adsorbed on the surface as well as impinging ions have been discussed in relation to volume fraction and crystallite size of μc films, and continuous control of crystallite size has been demonstrated using a triode system. Hall mobility of the deposited μc-Si:H films has also been presented as a function of the volume fraction of μc.     

Local bonding of hydrogen in a-Si:H,a-Ge:H and a-Si,Ge:H alloy films

This paper reviews the local bonding of hydrogen (and deuterium) in a-Si:H(D), a-Ge:H(D) and a-Si, Ge:H(D) alloy films. We specify the types of atomic environments that have been identified through vibrational spectroscopy, primarily infrared (IR) absorption. We emphasize local modes and discuss the atomic motions that are responsible for the various spectral features. We discuss correlations between the occurrence of specific local bonding groups, e.g., polysilane and polygermane configurations, and the deposition techniques and parameters, including the substrate temperature (T_s), the gas mixtures and the RF power into a glow discharge, etc. We include a discussion of the theoretical approaches that have been used to treat vibrational modes in these materials. We emphasize the approximations that are valid because of the relatively light mass of the hydrogen and deuterium atoms compared with those of the silicon and germanium atoms. Finally we highlight the effects of neighboring alloy or impurity atoms on the frequencies of hydrogen vibrations in a-Si:H, and point out the differences between oxygen atom incorporation in a-Si and a-Ge alloys.     

Gap states in a-Si:H by photoconductivity and absorption

The spectral dependence of photoconductivity in a series of a-Si:H samples has been analyzed in order to derive the absorption coefficient α. It has been found that the exponent β which relates photoconductivity and generation rate varies with photon energy and chopping frequency. Its critical influence upon the value of α is established. The Urbach tail in the region 1.4–1.8 eV has been studied and found to be independent of extrinsic gap states and hydrogen content. Finally an inverse correlation has been determined between the photoconductivity lifetime and the extrinsic gap-state density, as measured from the corresponding integrated area of the absorption coefficient.     

Doping of microcrystalline Si:H,Cl films in R.F. glow discharge

The deposition rate of n-doped μc-Si:H,Cl GD films has been found to increase with the gas phase phosphine content, while the opposite behaviour has been observed for p-doped specimens. The optical gap ranges between 1.8 ÷ 2.6 eV and its variation seems related to hydrogen and chlorine contents. The effectiveness of the doping has been tested by electrical conductivity measurements.     

Characterization and luminescence of a-Si:H:Cl films

a-Si:H:Cl films have been deposited by glow-discharge and characterized by infrared transmission, optical absorption and photoluminescence. The influence of growth parameters on the H and Cl content has been investigated. The luminescence spectra show that three different radiative transitions can occur, at 0.75, 0.95 and ～1.3 eV. These bands have been interpreted respectively in terms of the following recombinations: defect to defect, defect to band tail, band tail to band tail.     

Preparation and analysis of reactively evaporated a-Si:H

a-Si:H formed by the method of reactive evaporation of silicon with supplying hydrogen ion has much the same properties as films prepared by silane grow discharge (GD) decomposition. Growth kinetics in this method is described. Hydrogen ion suppresses the oxidation of silicon and it is incorporated as mono-hydride, whose content is proportional to hydrogen ion current. Adsorption energy of hydrogen ion is some 130 meV. Dark conductivity of a-Si:H depends mainly on energy gap associated with hydrogen content. Spin density is determined by the collision phenomenon between Si-H and spin in depositing.