Films were prepared under argon and xenon dilution with silane concentrations ranging from 3 to 100% using the very high frequency glow discharge (VHF-GD) technique. The H-content and the microstructure were determined by IR-spectroscopy. A surface profiler was used to measure the stress and thickness of the films. The surface roughness of the films was evaluated by the UV-light reflectance loss. The samples were further characterized by dark- and photoconductivity, by CPM and by PDS, both in annealed and light-soaked state.

Down to silane concentrations of about 10–20% film properties change only little; however, for both dilution series the microstructure parameter shows a minimum and the internal stress a maximum near 20% silane concentration. At still higher rare gas dilution the film properties change drastically. Surprisingly the photoconductivity remains almost constant for all gas dilutions. It is shown how these changes in the film properties are linked with the light induced degradation.

The use of Very High Frequency (VHF) glow discharge for the deposition of amorphous silicon films is one approach to influencing the deposition conditions.

For the experiments a capacitatively-coupled parallel plate reactor with two aluminium electrodes of 125 mm in diameter is used. The VHF is generated by means of an all-solid state amplifier which has a frequency response ranging from 250 kHz to 150 MHz. Experiments with an nitrogen discharge within the range from 5 to 150 MHz are an principle in agreement with the results in the literature. The results show, that the frequency dependence of the discharge strongly depends on the design and the external electrical connection of the VHF-reactor. In the examined range of frequency resonances appear, which are probably responsible for the maximum of the deposition rate. These resonances are not essentially resultant from changes in the physics of the discharge but a result of the reactor construction and the external electrical connection. Possibilities which influence this frequencies of resonance will be discussed.

Results for an electrical network model will be discussed in the comparison with different reactors and the optimization of the behaviour of the deposition process.

a-Si: H was deposited in a VHF-plasma in a wide frequency range up to 250MHz. The deposition rate was found to increase with excitation frequency up to 180MHz, where values of 35Å are achieved maintaining good film properties

The onset of this increase occurs near 60MHz, when the excitation frequency exceeds the inelastic electron-neutral collision rate. From optical emission of hydrogen and a He/Ar line pair the high energy tail of EEDF was probed. It is found, that the rise in deposition rate is solely due to the increased plasma density and not to an enhancement of the high energy tail of the EEDF, as postulated previously.

The materials quality (uniformity and homogeneity) produced by PECVD technique depends on the plasma behaviour. Problems related to the gas dissociation ratio and powder formation are responsible for materials costs and the appearance of voids. In this paper, we present data concerning the role of the plasma and reactor design, on the deposition conditions, suggesting ways to improve them for large area device (macroelectronics) applications.

The effects of annealing on undoped a-Si:H films were studied. Films of a-Si:H were prepared by mercury-sensitized photochemical vapor deposition, and were annealed immediately. The hydrogen configuration was altered in the annealing process below 200°C, although films were prepared at a substrate temperature of 230°C. At the same time, electron drift mobility greatly increased. These observations indicate a reduction in the number of tail states in the a-Si:H band-gap. Thus the annealing below the substrate temperature clearly influences not only the bonded-hydrogen atoms but also the Si-Si network in the films to some extent.

A new PHOTO-CVD apparatus has been built in order to deposit a – Si : H films and other kinds of amorphous thin films by a technique which is both simple and versatile. This apparatus is composed of three chambers connected together: a load-lock chamber, a process chamber and a third chamber for in-situ analysis of deposited films. A peculiarity of the lamp, a dielectric discharge lamp which can work with noble gases like Xe or Kr, is that it can be completely dismounted without breaking the vacuum in order to clean the optical MgF2 window. By this method, the deposition chamber can be kept in very clean conditions. In this apparatus, we started to deposit a – SixC1−x: H of very good quality, taking their thickness into account. These films have been completely characterized by chemical (RBS, ERDA) and optical (PDS) methods. Their quality can be compared with quality of a – Si : H samples of the same thickness obtained by PECVD.

The magnetic field profile of an electron cyclotron resonance (ECR) microwave plasma was systematically altered to determine subsequent effects on a-Si:H film quality. Films of a-Si:H were deposited at pressures of 0.7 mTorr and 5 mTorr with a H2/SiH4 ratio of approximately three. The mobility gap density of states ND, deposition rate and light to dark conductivity were determined for the a-Si:H films. This data was correlated to the magnetic field profile of the plasma, which was characterized by Langmuir probe measurements of the ion current density. By variation of the magnetic field profile ND could be altered by more than an order of magnitude, from 1×1016 to 1×1017 at 0.7 mTorr and 1×1016 to 5×1017 at 5 mTorr. Two deposition regimes were found to occur for the conditions of this study. Highly divergent magnetic fields resulted in poor quality a-Si:H, while for magnetic field profiles defining a more highly confined plasma, the a-Si:H was of device quality and relatively independent of the magnetic field configuration.

We have deposited microcrystalline silicon (μc-Si) at low temperature by reactive dc magnetron sputtering. The structure and crystallinity of the films are analyzed by in situ spectroscopie ellipsometry. μc-Si growth occurs at deposition conditions with substrate temperature between 150 – 300 °C and hydrogen partial pressure above 4 mtorr. We have also observed that the deposition rate strongly affects the interface structure. At a rate of 21 À/min, a 230 Å thick amorphous layer appears at the interface of film and glass substrate. The interface layer thickness decreases with deposition rate, and becomes undiscernible for growth rates ∼ 2 Å/min. The thickness of the interface layer is also found to depend on substrates.

We have studied the effects of hydrogen plasma treatments of a-Si:H at elevated temperatures using conductivity, photoconductivity, and spin density measurements. Effects were measured as a function of temperature and of treatment time; control specimens were subjected to comparable treatments without the plasma. In no case did plasma treatment improve the specimen photoconductivity over the as-deposited state. At treatment temperatures (230 °C < T < 460 °C) plasma treatment reduced the photoconductivity below that of the control specimen. This effect may be evidence for plasma-assisted dehydro-genation of the specimen. The defect structural equilibration determined by hydrogen chemical potential depending on temperature and hydrogen plasma is also addressed.

The microstructure of high-density amorphous hydrogenated silicon (a-S.i:H) films deposited at 50°C substrate temperature was revealed by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies to be similar to that of “device-quality” a-Si:H films deposited at standard “optimum” conditions. However, optical absorption measurements of these low microstructure 50°C films with photothermal deflection spectroscopy indicate that they have higher densities of gap state defects and localized band tail states than “device-quality” films deposited at standard substrate temperatures. The correlation between the amount of microstructure and electronic properties is not unique. A low amount of microstructure is a necessary, but not sufficient, requirement for high electronic quality a-Si:H films.

A model for correlating the observed properties of hydrogenated amorphous silicon (a-Si:H) with the underlying hydrogen microstructure is reviewed. The model provides a unified description of defect equilibration, hydrogen evolution, rehydrogenation and hydrogen diffusion measurements.

By using light soaking and partial dark annealing to vary the Fermi level in n-type a-Si:H, we have examined the thermal emission of electrons from the dangling bond (D) defect. We find optical evidence for a change in the configuration of the D defect when EF = Ec-0.55±0.08eV. We find that the relaxation rate increases with temperature and increases as EF is brought closer to Ec. Voltage-pulse photocapacitance and depletion-width-modulated ESR show emission is predominantly from D° defects for short emission times and short filling pulse widths. With longer emission times and longer filling pulse widths, emission from D-dominates. We also find that the charge emission transient fits a universal scaling law under a variety of pulsing conditions, temperatures, and anneal states.

The use of X-ray absorption spectroscopy and related techniques in the investigation of the structure of amorphous group IV based semiconductor alloys is briefly reviewed, with emphasis on recent results.

In these materials the single scattering contribution to the x-ray absorption signal contains information mainly on first shell coordination; this is illustrated by showing trends in the local structure of amorphous semiconductor alloys. This type of analysis can be complemented by using other methods. For example, X-ray scattering yields information on second shell structure which reflects the influence of bond angle disorder.

Recent results on a-Si-1-xCx:H are then reported. Presence of both carbidic and graphitic carbon, interatomic distances which are constant with composition and a clear tendency towards chemical order are found. These results are compared to some recent structural models.

Electron density fluctuations associated with microstructural features on a scale from about 1 to 25 nm in glow-discharge-deposited a-Si1-xGex:H films were studied by the technique of small-angle x-ray scattering (SAXS). Films prepared in four different deposition systems (in different laboratories) have been characterized and a general increase in the SAXS signal with increasing x is observed. Density deficiencies determined from film flotation measurements lead to the correlation of the increased scattering intensities with increases in the volume fractions of micro voids. Modeling of the data yields void size distributions that demonstrate significantly more of the larger voids (2 to 6 nm) than found at x=0 (around 1 nm). For the alloys with x>0.4, the scattering at the smallest angles was observed to decrease substantially upon tilting of the sample relative to the x-ray beam. This result contrasts with the small or no changes in SAXS upon tilting device-quality x=0 films. This anisotropie scattering associated with the tilting experiments has been modeled with distributions of ellipsoidal microvoids that are preferentially oriented with their major axes normal to the film plane. This latter result is consistent with a columnar-like microstructure. However, one film with x=0.37 shows no evidence for such microstructure.

The optical properties of thin film microcrystalline silicon (μc-Si:H) prepared by plasma-enhanced chemical vapor deposition (PECVD) have been studied by real time spectroscopie ellipsometry in the nucleation regime as isolated crystalline particles increase in size. A simple geometric model of nucleation allows us to remove the dominant effect of voids and extract the dielectric functions of the crystallites themselves. We find that the results can be understood in terms of a classical size effect whereby limitations on the electron mean free path by scattering at crystallite surfaces control the absorption onset from 2.0 to 3.0 eV. Finally, we describe how well-ordered, continuous 15 Å c-Si films can be prepared on metal substrates.

The density of occupied states was investigated in a number of intrinsic a-Si:H samples by means of photoelectron yield spectroscopy operated in the constant final state mode. Valence band edge, exponential tail and deep state distribution were analyzed and compared with the bulk density of states as determined by spectral dependence of photoconductivity. By evaporating Cs atoms on the clean surface the Fermi level position was progressively shifted toward the conduction band and the evolution of localized states was investigated. On the assumption that these near-surface states are paramagnetic centers with positive correlation energy, the results agree with the prediction of the “defect pool” model.

Surface oxidation and surface defect creation processes in a-Si:H films have been studied in detail by means of electron spin resonance(ESR) and X-ray photoelectron spectroscopy(XPS). It is found that Si dangling bonds created by the surface oxidation distribute far wider than the thickness of the SiO2 layer. These defects are also found to be removed out by annealing at around 100°C. These defects are proposed to be created by a stress in a-Si:H induced by the surface oxidation. Moreover, the presence of the surface defects unrelated to oxidation is shown for the first time by the present experiment. The origin of these defects, however, are not clear at present.

We have recently produced experimentally plausible structural models of 12a-Si:H. We discuss the structural, vibrational and electronic properties of these models, and indicate how such structures can be obtained.

The cross-correlation of intensity fields of different regions of an HREM micrograph can identify regions of structural order in an apparendy random matrix. This paper presents results of cross-correlations on computer-simulated HREM images of a diin Si crystallite located on a relatively thick slab of amorphous Si. These simulations confirm the efficacy of cross-correlations in retrieving atomic order from die image of an apparently amorphous material and thus confirm the interpretation of experimental results obtained earlier.

We have examined the distribution of the neutral dangling bond defect states, T30, as a function of the local disorder. T30 defects in a-Si play an important role in many of the current models of the metastable photoconductivity. To understand the relationship between the T30 defect and its bonding environment, the Local Density of States (LDOS) for under-coordinated Si atoms in disordered environments are calculated using the cluster-Bethe lattice method, CBLM. Our Hamiltonian employs the tight-binding parameters of an sp3 orbital basis containing both 1st and 2nd nearest-neighbor interaction terms fit to c-Si band structure. Averaged LDOS of atoms with various bond angle distortions are calculated in order to demonstrate the relationship between the standard deviation in bond angle and the width of the defect states. The CBLM is also used to determine the extent of the valence band tails as a function of the standard deviation in bond angle. In addition, the LDOS of clusters with 2 dangling bonds are examined to determine the degree that their energy levels split due to their interaction with each other.