Polycrystalline silicon films (5 to 30 μm thick) have been deposited on glass substrates at low temperatures (400–550 °C) with a rate of 15 Å/s by hot-wire chemical vapour deposition (HWCVD). The homogeneity of the deposited layer is ±5% on a 8 cm diameter substrate. The films have columnar microstructure and a textured surface. The undoped films (carrier concentration, 1011 cm−3) have a resistivity of 105-106 Ω-cm, activation energy of 0.50 ± 0.05 eV and Hall mobility of 14 ± 4 cm2 /V.s. By in situ gas phase doping, resistivity can be varied by six to seven orders of magnitude. Incorporation of dopant atoms such as boron into the film, strongly influences its morphological and crystallographic structure. The mobility lifetime product of undoped films is low (10−8 cm2/V), possibly due to the presence of a high density of dangling bonds defects and broad band-tails. This product can be improved by a factor of 5 to 10 by using in-situ hydrogen passivation in the same reactor at lower temperature (350–400 °C) within one hour. The results of many complementary experiments suggest that hydrogen treatment mainly improves carrier mobility by a factor of 3 to 4 by passivating extended defects. Preliminary results on application of these types of materials in unoptimized P-I-N solar cells on c-Si and glass substrates are presented.

The electronic and optical properties of microcrys tall ine silicon films prepared by plasma enhanced chemical vapour deposition are investigated with Hall-effect, electrical conductivity, photothermal deflection spectroscopy and photoluminescence measurements. In particular, the influence of the grain size and the crystalline volume fraction on the conductivity, the carrier density and the Hall mobility is investigated in highly doped films. A percolation model is proposed to describe the observed transport data. Photoluminescence properties were studied in un-doped films. It is proposed that the photoluminescence is due to recombination at structural defects similar to those observed in crystalline silicon.

We report on electroabsorption spectra for plasma deposited thin films of hydrogenated silicon ranging from amorphous (a-Si:H) to microcrystalline (μc-Si-H) structures. The EA spectrum of a-Si:H deposited from silane with low hydrogen dilution were consistent with previous works; material prepared with high hydrogen dilution showed a 0.07 eV blue shift of the spectrum and somewhat stronger electroabsorption. μc-Si-H specimens have a sharp peak at 1.19eV; the spectrum is blue shifted by 0.03 eV and is significantly stronger than electroabsorption reported in single crystal silicon. Spectral features which have no correspondence to single crystal silicon were also observed in μc-Si-H. Specimens deposited using “cyclic” deposition and chemical annealing had electroabsorption spectra with both the 1.19 eV, crystalline feature and a band peaking at 2.02 eV which we attribute to strongly hydrogenated a-Si:H. We discuss applications of electroabsorption to determining the crystal fraction of microcrystalline material and to determining grain size distributions.

Electronic transport parallel and perpendicular to growth direction has been studied in a series of microcrystalline silicon samples obtained by various dilutions of silane in hydrogen. It is clearly shown that the transport properties (dark conductivity, drift mobility, ambipolar diffusion length and photoconductivity) under dark and under illumination conditions are enhanced as the dilution is increased. Furthermore, these films exhibit no degradation upon light-soaking. X-Ray diffraction patterns of the samples confirm that there is a correlation between the amount of crystalline fraction in the samples and the transport properties, as well as a preferential orientation along the growth direction. A similar correlation is found with the shift of the Si-H stretching mode peak of the infrared spectra (IR). Because transport properties have been measured by different techniques (dark conductivity, ambipolar length and photoconductivity in the direction perpendicular to growth direction, drift mobility in the direction parallel to growth direction), no statement can be made about a possible anisotropy in transport, as it would be expected from the columnar shape of the crystallites.

Recently, we have proposed a cyclic method, referred to as Closed Chamber CVD (CC-CVD), for the preparation of μc-Si films of high crystalline fraction at increased deposition rates. In this work we first report new process conditions of CC-CVD, which result in growth of highly crystalline films with a sharp interface on a foreign substrate. Then these conditions are further used together with a pulsed injection of B2H6 in an appropriate moment of each cycle, so that the disturbance of the crystallization process is prevented. A series of ultrathin μc-Si films, doped by this technique, is characterized by conductivity measurements, SEM, Raman Scattering, optical transmission and UV reflection. A strong reduction of the transient interface layer is achieved and conductivity as high as 2 S/cm <p-type> with an activation energy of 27 meV is reached.

Nanostructured silicon thin films have been deposited by plasma enhanced chemical vapor deposition at low substrate temperature (100 °C) in the presence of silicon nanoparticles. The nanostructure of the films was revealed by transmission electron microscopy, Raman spectroscopy and X-ray diffraction, which showed ordered silicon domains (1–2 nm) embedded in an amorphous silicon matrix. These ordered domains are due to the particles created in the discharge that contribute to the film growth. One consequence of the incorporation of nanoparticles is the accelerated crystallization of the nanostructured silicon thin films when compared to standard a-Si:H, as shown by the electrical characterization during the annealing.

The influence of grain size on the enhanced optical absorption of μc-Si has been investigated using films of various grain sizes prepared by solid phase crystallization. We show that we can control this grain size and therefore the degree of absorption changes. For grain sizes below a threshold range significant absorption enhancement can be seen in the photon energy range of 1 to ∼3 eV and the absorption characteristics of these films show that the dominant mode of optical transitions is indirect. A correlation between first order Raman peak broadening and enhanced absorption was found suggesting both effects are related to confinement. A simple model was developed to see how confinement in the crystallites could influence indirect optical transitions.

We deposit phosphorus-doped, amorphous Si by low pressure chemical vapor deposition and subsequently crystallize the films by furnace annealing at a temperature of 600°C. Optical in-situ monitoring allows one to control the crystallization process. Phosphorus doping leads to faster crystallization and a grain size enhancement with a maximum grain size of 15 μm. Using transmission electron microscopy we find a log-normal grain size distribution in our films. We demonstrate that this distribution not only arises from solid phase crystallization of amorphous Si but also from other crystallization processes based on random nucleation and growth. The log-normal grain size distribution seems to be a general feature of polycrystalline semiconductors.

The preparation of seed lattices, using three interfering beams (TIB) from a pulsed Nd:YAG laser in a-Si layers of 100 to 400 nm thickness is introduced and applied for seeded laser or thermally induced crystallization of a-Si on Corning 7059 glass. The structural and electronic properties of the μc-Si layers are investigated by X-ray, electron- and atomic force microscopy, Hall and conductivity measurements. In highly boron-doped μc-Si, grains up to 1.3 μm in diameter are detected, giving rise ti conductivities of ≃ 2000 S/cm and hole mobilities of ≃ 10cm2/Vs.

Uniform gratings of sharply defined polycrystalline silicon lines with micrometer-sized periods were created by laser interference crystallization of amorphous silicon. Atomic force microscopy (AFM) reveals that lines fabricated with high pulse energies (380 mJ/cm2) contain large grains (dimensions up to 1.5 μm), growing in a direction perpendicular to the lines. We assign this strong lateral growth to the melting of the material in the center of the lines combined with the presence of small grains, which act as nuclei, at the interfaces with the amorphous regions. Spatially resolved Raman spectroscopy shows that size effects dominate the Raman line shape at the edge of the line, confirming the AFM results, while stress increases towards the center of the line. The spectra measured in the middle of lines created with high energies show doping effects caused by the diffusion of boron atoms from the substrate upon exposure.

The diffusion and effusion of hydrogen in hydrogenated microcrystalline silicon films deposited in an electron cyclotron resonance reactor were studied for various deposition temperatures Ts. For deposition temperatures below 250°C, hydrogen effusion is found to be dominated by desorption of hydrogen from internal surfaces followed by rapid out-diffusion of H2. Higher substrate temperatures result in an increased hydrogen stability suggesting the growth of a more compact material. For this latter type of samples, a hydrogen diffusion coefficient similar as in compact plasma-grown a-Si:H films is found despite a different predominant bonding of hydrogen according to infrared absorption.

Polycrystalline SiGe (poly-SiGe) film growth by reactive thermal CVD with a gaseous mixture of Si2H6 and GeF4 was investigated on various substrates such as Al,Cr, Pt, Si, ITO, ZnO and thermally grown SiO2.

In Ge-rich film growth, SEM observation in the early stage of the film growth revealed that direct nucleation of crystallites took place on the substrates. The nucleation was governed by two different mechanisms: one was a heterogeneous nucleation on the surface and the other was a homogeneous nucleation in the gas phase. In the former case, the selective nucleation was observed at temperatures lower than 400°C on metal substrates and Si, where the activation of adsorbed GeF4 on the surface played a major role for the nuclei formation, leading to the selective film growth.

On the other hand, the direct nucleation did not always take place in Si-rich film growth irrespective of the substrates and depended on the growth rate. In a growth rate of 3.6nm/min, the high crystallinity of poly-Si0.95Ge0.05in a 220nm-thick film was achieved at 450°C due to the no initial deposition of amorphous tissue on SiO2 substrates.

We have studied the effects of intense x-ray irradiation on the stucture of amorphous Si films. The films were obtained by either physical vapor deposition or by implantation of high energy ions into crystalline Si. They were exposed to different total doses of synchrotron x-rays. From the EXAFS and EXELFS measurements we find that an exposure to x-rays increases the Si coordination number. Also in the PVD films a prolonged x-ray exposure enlarges, by about 2 %, the Si-Si bond length. Raman spectroscopy shows that Si amorphized with high energy ions contains small residual amounts of crystalline material. Irradiation of such films with x-rays annihilates those crystallites resulting in homogenously amorphous layer with a close to four-fold coordination of Si atoms. This rearangement of the local structure has a pronounced effect on the crystallization process of the amorphous films. Thermal annealing of x-ray irradiated ion amorphized films leads to nearly defect free solid phase epitaxy at 500°C. Also we observe a delay in the onset of the crystallization process in x-ray irradiated PVD films. We associate this with a reduced concentration of nucleation centers in the x-ray treated materials.

We present a detailed study of ESR and spin-dependent transport (EDMR) on μc-Si. We identify to different types of defects at g=2.0055(±3) and g=2.0044(±5) and study their influence on transport and recombination by stepwise annealing the samples. We find that transport is not controlled by defects if ND<1018 cm−3. For ND>1018cm−3 a dramatic decrease of the conductivity is found and we identify a hopping contribution in transport. To explain our ESR and EDMR results we propose a simple model where most defects are distributed at the surface of the columns and transport is along percolation paths. We also observe minor metastable changes of the defect density which are assigned to adsorption of atmospheric oxygen.

We report the fabrication and electrical characteristics of an ultrathin nanocrystalline-silicon/amorphous silicon oxide double barrier resonant tunneling diode. The cross-section high resolution TEM photographs indicate the layered structure and the crystallinity of nanocrystalline Si with an average grain size of 10 nm. In this prototype device, a series of reproducible conductance peaks have been observed as a function of the applied gate bias at room temperature. The real voltage spacing of 0.38V between the peaks is determined from an equivalent circuit. The results are in agreement with the theory of Coulomb blockade effect using 1.1 aF of capacitance in single Si nanocrystallite. We conclude that the observed behavior results from resonant tunneling, which is strongly influenced by Coulomb blockade effect, through the zero-dimensional states in the well isolated silicon quantum dots.

Nano-crystalline GaN (nc-GaN) and hydrogenated nc-GaN (nc-GaN:H) thin films and thin film transistors (TFT) prepared by a reactive sputtering method have been studied. Hydrogen incorporation in nc-GaN film induces localized states at mid-gap energy. Thermal annealing at 400 °C and 600 °C creates mid-gap states which is detectable by electron spin resonance. Further thermal annealing treatment at 800 °C reduces the deep states in nc-GaN and nc-GaN:H. Photoluminescence spectra of the nc-GaN film have two broad peaks at 2.4 eV and 3.2 eV. The source-drain current voltage characteristics of the nc-GaN TFT is demonstrated for the first time. The obtained field effect mobility is 10−4 cm2/V-s. Thermal annealing at 800 °C improves the field effect mobility to 10−2 cm2/V-s.

Amorphous SiOx alloys containing Ge or GeO2 nanocrystals are produced by dc-magnetron sputtering and controlled crystallization. The samples are investigated by Raman scattering, transmission electron microscopy, photo luminescence and excitation spectroscopy. Under UV excitation, both types of films luminesce around 3.1 eV, with identical PL line shapes and subnanosecond PL dynamics. The strongest PL intensity is found for the films containing GeO2 crystals and for the largest nanocrystals. These results are a clear indication that although the blue luminescence is without a doubt correlated with the formation of Ge (or GeO2) nanocrystals, it is not produced by the radiative recombination of excitons confined in the nanocrystals. Possible mechanisms for the luminescence are discussed, including defects at the nanocrystal/matrix interface or in the matrix itself.

Initial 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.

The surface morphology and preferential orientation of μc-Si:H has been studied on c-Si substrate in relation to the Si-H bonding mode. Epitaxy-like growth is observed on Si (001) substrate at a moderate temperature and under low ion bombardment, while the surface is significantly roughened. With increasing or decreasing the temperature or increasing the ion bombardment, the lattice orientation is randomized and an amorphous component is increased, resulting in the reduction of the surface roughness. The growth mode is intimately related to the Si-H bonding configuration. The epitaxy-like growth is collapsed accompanying the disappearance of the surface Si-H 2,3 mode. The resemblance of these phenomena to the low temperature MBE is discussed.

A novel plasma enhanced vapor deposition (PECVD) technique employing biased wall (BW) method has been developed for the enhanced growth rate of the hydrogenated microcrystalline silicon (μc-Si:H) films. Using this method, we have achieved a growth rate of more than 6Å/sec for the formation of μc-Si-H having an average grain size of 200Å; at 350°C.