In-situ UV-visible ellipsometry and Kelvin probe measurements were performed to study the growth of boron-doped microcrystalline silicon (μc-Si:H) thin films and the band profiling at the p- (μc-Si:H)/i-(a-Si:H) interface. The in-situ UV-visible spectroscopic ellipsometry measurements, combined with dark conductivity measurements, performed at different stages of the growth show that p-type μc-Si:H formation can be achieved for a film thickness below 10 nm. These analyses also reveal that both the optical absorption, and the dark conductivity do not change significantly for a crystalline volume fraction above 50%. Moreover, the contact potential as measured by in- situ Kelvin probe shows a saturation just after the percolation threshold. These results indicate that highly crystallized doped layers are not necessary in device applications. From the Kelvin probe measurements, the potential profile through the p-(μc-Si:H)/ i-(a-Si:H) interface was measured. The microcrystalline silicon p-layers were successfully incorporated in single junction solar cells.

We investigated p-i-n solar cells with microcrystalline absorber but amorphous contact layers. Fill factor and open circuit voltage depend sensitively on the p/i interface. Using an optimized design of the p/i interface, cells with fill factors up to 65% and open circuit voltages of 0.45 V were deposited on amorphous p-layers. They are comparable to cells on micro- crystalline p-layers. A further increase of the open circuit voltage was achieved by variation of the p/i interface treatment but up to now it was accompanied by a decrease of the fill factor. We attribute this effect to a thin undoped amorphous layer at the p/i interface. Under non-optimized deposition conditions an amorphous instead of a microcrystalline silicon layer is grown at the p/i interface which can be detected by Raman measurements on cell structures. While the proper design of the p/i interface is crucial for the cell performance we did not observe significant differences between cells with amorphous and microcrystalline n-layers. The results reveal that the open circuit voltage is limited by the bulk properties of the undoped microcrystalline silicon.

A-Si:H films were deposited in a PECVD system using 5k to 5 Hz and 25 to 75% duty cycle square-wave pulse modulation RF power and with 80% hydrogen or 80% helium dilution. The transmission and the microstructure of the hydrogen bonding configuration were measured using NIR-VIS-UV and FTIR spectrometers. The powder formation in the reactor chamber was low for a deposition rate less than 5.87 nm/min and was completely suppressed by the 500 Hz pulse RF power with a duty cycle less than 33.3%. The low microstructure R ratio is the result of the reduction of the polymization reaction in the plasma for a low deposition process. A R ratio less than 14% could be obtained using 500 Hz with a duty cycle less than 33.3% for helium dilution or using 5k to 50 Hz with a duty cycle less than 50% for hydrogen dilution. The refractive index of the films is high for the films with low R ratio. The more compact and higher quality films exhibit greater SiH bonding configurations in the a-Si:H films.

The hydrogen and silicon density of a-Si:H-films deposited by an expanding thermal plasma have been investigated for a wide range of substrate temperatures and growth rates by infrared absorption spectroscopy in combination with elastic recoil detection and Rutherford backscattering. The study reveals that, despite the increasing atomic hydrogen interaction and high substrate temperatures, the a-Si:H remains purely amorphous at low growth rates as concluded from Raman spectroscopy. Additionally, the infrared spectroscopy proportionality constants of the silicon-hydrogen and silicon-deuterium bondings have been recalibrated.

We have identified longitudinal and transverse optical modes of Si-N bond in vibrational absorption spectrum of hydrogenated amorphous silicon nitride thin films. The locations of longitudinal optical resonances coincide with transverse mode of Si-O and closely neighbor bending modes of N-H, Si-H2 bond. Furthermore, the conventionally assigned asymmetric and symmetric stretching modes of Si-N bond are merely transverse modes of Si-N bond. Features of pure longitudinal vibrational modes can be revealed by subtracting appropriately scaled transverse mode components from the vibrational absorption spectrum. Analysis of the longitudinal spectrum indicates that, in addition to their association with Si-N bond, the density of the peak located at about 1215 cm−1 is also well correlated to hydrogen and nitride content in the film, while the density of the peak located at about 880 cm−1 is closed related to silicon density in the film. Peak located at about 1040 cm−1 is the longitudinal mode associated with Si-N bond.

Two aspects of VHF plasma deposition in a large area reactor are investigated: 1) Experiments and model show that voltage inhomogeneities become serious at high frequency but can be reduced by suitable RF connection configuration. These voltage inhomogeneities are the main factor limiting the excitation frequency in order to satisfy film thickness uniformity requirements; 2) the effect of the frequency on the plasma properties has been studied between 13.56 MiHz and 70 MHz. The results show that an increase of the electron density with the frequency leads to a more efficient dissociation of silane. This increase of the gas phase reactivity of the plasma is largely responsible for the higher deposition rates observed athigh frequency.

The choice of the excitation frequency for a given application is a compromise between the gain of higher deposition rate and the frequency limit imposed by homogeneity requirements.

Particle contamination formed in reactive plasmas imposes an upper limit on the rate for particle-free deposition. Conversely, these plasmas could be exploited to produce nanometric clusters and particles for various applications. Infrared absorption spectroscopy has been applied to analyse the chemical composition of suspended particles. Mass spectrometry was also used to investigate cluster formation in these deposition plasmas. In pure silane plasmas, a random model reproduces the measured mass spectra, whereas the rich plasma chemistry in silane/oxygen mixtures shows a remarkable tendency to produce silasesquioxane anions.

The pulsed plasma technique has been shown to increase the deposition rate without an increase in the particulate count in the plasma which is an important factor determining the yield of commercial products such as active matrix displays. In this paper, we report the deposition of amorphous silicon at deposition rates of up to 15 Å/sec, using a modulation frequency in the range of 1-100kHz. These materials have been incorporated into a simple p/i/n solar cell and thin film transistor (TFT) configurations. We report on the effect of the conversion efficiency as a function of the modulation frequency, which in turn is related to the deposition rate. We also report on the TFT performance with modulation frequency and compare the results with devices made under the conventional continuous wave PECVD plasma at 13.56MHz.

A comparative study of DC, RF and VHF excitation for the plasma enhanced chemical vapor deposition (PECVD) of intrinsic layers of a-Si:H is presented, with special emphasis on the effects of hydrogen dilution. Growth rates at comparable plasma power, for substrate temperatures between 100°C and 300°C and for various H2 dilution ratios are presented, along with optical bandgap, H content, and electronic transport properties in the light-soaked state.

H2 strongly reduces the growth rate for all techniques. The growth rate for the highest H2 dilution ratio is approximately four times higher for VHF than for DC or RF excitation. In all three cases increasing the substrate temperature reduces the optical gap and the H content CH. Raising the substrate temperature slightly enhances the stability of undiluted films. H2 dilution increases the optical gap for all three techniques. The H content of RF- and VHF-deposited samples increases with increasing H2 dilution ratio, while in DC deposition it produces an initial drop of the H content, followed by an increase.

In all three cases, H2 dilution improves the electronic transport properties of the material by roughly a factor of two. The gain in stability is most pronounced for relatively small dilution; in the case of DC deposition, too strong a dilution even has an adverse effect on stability.

Thin films of hydrogenated amorphous silicon (a-Si:H) have been deposited by plasma-enhanced chemical vapour deposition (PECVD), and the resulting topography measured in-situ on a nanometre scale using a scanning tunnelling microscope (STM). An island structure is observed on the surface of device quality a-Si:H, which can be quantitatively analysed using a one dimensional Fourier transform of the topography. Results suggest that deposition is limited by the creation of dangling bonds on the a-Si:H surface and not by the surface transport of SiH 3 radicals at the deposition temperature (598 K). Island nucleation takes place through the abstraction of hydrogen atoms from the a-Si:H surface by plasma etching and the subsequent attachment of an SiH 3 radicals to the available sites. A thermally activated hydrogen effusion process around the edge of each island, where the step edge causes a high local hydrogen concentration, then creates further dangling bonds which allow the islands to grow. A simulation has been constructed, which confirms this two stage mechanism.

The crystallization of amorphous silicon (a-Si:H) deposited by plasma enhanced chemical vapor deposition (PECVD) by thermal annealing is of great interest for display and silicon-on-insulator (SOI) technologies, though long anneal times (about 20 hrs) at 600 °C are typically required. We report that a room temperature hydrogen plasma exposure in a parallel plate diode type Reactive Ion Etcher (RIE) can reduce this crystallization time by a factor of five. This plasma enhanced crystallization can be spatially controlled by masking with patterned oxide, so that both amorphous and polycrystalline areas can be realized simultaneously at desired locations. This effect is due to the creation of seed nuclei at the surface, which enhance crystallization rates.

We present internal friction and shear modulus measurements of amorphous silicon (a-Si) and germanium (a-Ge) films. The temperature independent plateau in internal friction below 10 K, common to all amorphous solids, also exists in these films. However, its magnitude which depends critically on the deposition method is smaller than found for all other amorphous solids. In particular, hydrogenated a-Si with about 1 at. % H prepared by hot-wire chemical-vapor-deposition leads to an internal friction nearly three orders of magnitude smaller than observed for all other amorphous solids. The internal friction increases after the hydrogen is removed by effusion.

We have studied the elastic properties of hydrogenated amorphous silicon (a-Si:H) prepared by hot wire chemical-vapor deposition (HWCVD). With 1 at.% H, this material has been found to be the only amorphous solid which has a low-temperature internal friction more than two orders of magnitude smaller than all other amorphous solids studied to date, as reported recently. As the hydrogen concentration increases above 1 at.%, a broad relaxation peak in internal friction around 5 K is observed. Even more striking is an extremely narrow peak in internal friction accompanied by a discontinuous change in the sound velocity at 13.8 K, which coincides with the triple point temperature of molecular hydrogen. Evidences are provided to show that this anomaly is caused by bulk molecular hydrogen which undergoes a liquid-solid phase transition. This is the first observation for the existence of bulk H2 in HWCVD a-Si:H.

Infrared absorption data of various Si:H samples involving several series of a-Si:H films as well as µc-Si:H and c-Si:H samples were analyzed to obtain ratios of the absorption strengths A of the Si-H stretching modes at 2000 and 2100 cm−1 and of the Si-H wagging mode at 640 cm−1. Hydrogen effusion measurements were used to obtain the absolute values of A. The results suggest essentially equal absorption strengths for the two Si-H stretching modes and a ratio of wagging and stretching absorption strengths independent of hydrogen density NH. Films with predominant 2100 cm−1stretching absorption show a stronger sample dependence of A than observed for samples with predominant 2000 cm−1absorption. A slight increase of the absorption strength of the Si-H wagging and stretching modes with rising hydrogen concentration is observed for a-Si:H films and is attributed to the decrease of the refractive index. Enhanced A values of µc-Si:H samples compared to a-Si:H samples point to the presence of molecular hydrogen.

This work reports on the performances of undoped and n doped amorphous/nano-crystalline silicon films grown by hot wire plasma assisted technique. The film's structure (including the presence of several nanoparticles with sizes ranging from 5 nm to 50 nm), the composition (oxygen and hydrogen content) and the transport properties are highly dependent on the filament temperature and on the hydrogen dilution. The undoped films grown under low r.f. power (≍ 4 mWcm−2) and with filament temperatures around 1850 °K have dark conductivities below 10−1Scm−1, optical gaps of about 1.5 eV and photo-sensitivities above 105, (under AM3.5), with almost no traces of oxygen content. N- doped silicon films were also fabricated under the same conditions which attained conductivities of about 10−2Scm−1.

Deuteron magnetic resonance (DMR) has been used to further examine hydrogen (deuteron) populations in amorphous-silicon (a-Si) and in n-type crystalline silicon (x-Si). In both a-Si and x-Si DMR shows central components arising at least in part from isolated molecular deuterium and sharp doublet features from Si-bonded hydrogen (D). Our new results include the observation in x-Si of molecule-specific DMR multiple echoes from trapped ortho-D2. A second new result is the observation in x-Si of a substantialn Si-bonded D population with splittings between 46 and 80 kHz and perhaps arising from (deuterated) hydrogen bond-centered and antibonding configurations.

A study of a-Si:H structure on the scales of short- and medium- range order has been carried out by the Raman spectroscopy method. Undoped a-Si:H films were deposited by the conventional rf- PECVD with some special variations in the process conditions ( pseudodoping technique); doping with boron was performed from a gas phase as well as by ion implantation. Changes in short- and medium- range order induced by light-soaking, pseudodoping and doping with boron were determined and compared.

Light-soaking kinetics of photoconductivity at room temperature for undoped a-Si:H films deposited by rf-plasma enhanced chemical vapor deposition and by dc-magnetron assisted decomposition at T8 = 300 and 350°C have been investigated. The rate of degradation has been expressed by the value of exponent of the power law: σph∼t−y. The hydrogen content in the films, the type of Si-H bonds and their contribution as well as the Fermi level position were controlled by changes in the deposition conditions. Hydrogen's effect on the y value was shown for the different cases. The films, having γ=0, were identified.

Grazing-incidence measurements of polarized electroabsorption (EA) in p-i-n solar cells based on hydrogenated amorphous silicon (a-Si:H) are presented. We confirm polarized electroabsorption effect of a-Si:H with the present “sandwich” electrodes, in fact, we find a significantly stronger polarization dependence compared with earlier work based on electroabsorption detected using coplanar electrodes on a-Si:H thin films. We did not reproduce the significant dependence of the polarized electroabsorption upon light soaking, which was found in previous work with coplanar electrodes. We speculate the difference between two electrode geometries is due to the space charge and two dimensional fields.

We show how positron annihilation can distinguish vacancies in undoped hydrogenated amorphous silicon by performing Variable Energy Positron Annihilation Spectroscopy experiments before and after light soaking. We find that vacancy clusters, di-vacancies and a new type of single vacancies are created in undoped as-grown a-Si:H thin film by light illumination. The fact that the vacancy clusters are eliminated by the thermal annealing suggests that the Staebler-Wronski effect is closely related to vacancy clusters in a-Si:H material. The creation of vacancy clusters and redistribution of di-vacancies and even single vacancies probably result in photo-induced structural changes in this material.