Solid phase crystallization (SPC) of a-Si: H at 600°C was investigated by transmission electron microscopy (TEM) and Raman spectroscopy in a cantilever structure, where the underlying SiO2 was removed prior to the crystallization. The absence of the underlying oxide leads to both a higher grain size and a lower intragranular defect density. The grain size increases from 0.6 μm in regions with the underlying oxide to 3.0 μm without the underlying oxide, and the intragranular defect density decreases one order of magnitude from ∼ 1011 cm-2 to ∼ 1010 cm-2. The improvements in material quality without the lower a-Si/SiO2 interface are thought to be due to a lower nucleation rate and a lower tensile stress with an easier silicon atomic rearrangement at the lower silicon interface.

A new band gap profile (exponential profile) for the active layer of the a-SiGe:H single junction solar cell deposited by plasma enhanced CVD (PECVD) has been designed and experimentally demonstrated. In this work we study the difference between the experimental results and the computer simulation. Using Rutherford Backscattering Spectrometry (RBS) significant differences were found between the composition profile as expected from variations in the gas phase ratio GeH4/SiH4 during the deposition and the actual Ge/Si depth profile as determined by RBS. This has important implications for the design of bandgap-graded a-SiGe:H cells. Among others, a delayed response of the plasma conditions due to the changes in the GeH4 flow is studied.

The descriptive scattering parameters, haze and angular distribution functions of textured ZnO:Al transparent conductive oxides with different surface roughness are measured. An approach to determine the scattering parameters of all internal interfaces in p-i-n a-Si:H solar cells deposited on the glass/ZnO:Al substrates is presented. Using the determined scattering parameters as the input parameters of the optical model, a good agreement between the measured and simulated quantum efficiencies of the p-i-n a-Si:H solar cells with different interface roughness is achieved.

Amorphous intrinsic silicon carbide (a-SiCx:H(i)) films and amorphous phophorous doped silicon carbide (a-SiCx:H(n)) films deposited by plasma enhanced chemical vapor deposition (PECVD) from silane/methane mixtures provide excellent electronic passivation of p-type c-Si. Effective surface recombination velocities (Seff) lower than 23 cm s-1 have been reported for a- SiCx:H(i) films and Seff < 11 cm s-1 for a-SiCx:H(n) films. The analysis of the dependence of Seff on the injection level indicates that the good electronic passivation is due to field-effect passivation resulting from a high fixed charge (Qf) created in the a-SiCx:H film. In this work the absorption of SiH bonds in infrared transmission spectra of a-SiCx:H films is quantitatively analysed resulting in about 30% smaller amount of SiH bonds in phosphorous doped films compared to intrinsic films. Furthermore, a strong reciprocal correlation of the hydrogen content in the films and the Qf created at the a-SiCx:H/c-Si interface is observed.

We present detailed studies of the initial relaxation processes of photoexcited carriers in hydrogenated amorphous silicon. We have carried out time-resolved measurements of the photoexcited carrier response in HWCVD a-Si:H thin films using a wavelength-resolved femtosecond pump-probe technique, in which an intense 35-fs pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in optical properties as a function of time delay following the pump pulse. Measurements of the transient optical absorbance were carried out as a function of the density of excited carriers, sample temperature, and probe wavelength. These studies indicate fast carrier thermalization via phonon emission on a ∼ 150 fs time scale and rapid phonon equilibration on a ∼ 230 fs time scale.

In order to determine the mechanism or mechanisms of hydrogen transport in a-Si:H one must identify the reactions that bring deeply bonded H (on dangling bonds) to the transport band which is some form of interstitial hydrogen. Further, one must have reliable estimates of the energy difference between these deep states and transport states as well as the barrier energies among transport sites. In this paper we consider all of these ingredients in order to partially explain how hydrogen migrates in a-Si:H.

Silicon carbide (SiC) thin films are attractive for a wide range of applications ranging from microelectronic and opto-electronic devices to protective and tribological coatings. In this paper, we will demonstrate that silicon carbide films can be successfully deposited by pulsed laser deposition (PLD) technique over large areas, with good uniformity in thickness, composition, and film-specific properties.

Amorphous SiC films were grown on silicon wafers of 75-mm diameter over a temperature range of 25 – 650°C using a KrF excimer laser at a wavelength 248 nm and a repetition rate of 100 Hz. The large-area uniform coverage was obtained by rastering the laser beam over the radius of a rotating SiC target of 90-mm diameter, while the substrate was rotated simultaneously. The uniformity of film composition over the 75-mm wafers was characterized by Auger electron spectroscopy (AES), while the crystallinity of films was investigated by X-ray diffraction (XRD). The morphology of the films was evaluated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The thickness of the coatings, and the index of refraction, n, along the wafer radii were measured optically using a spectrophotometer.

Hydrogenated amorphous silicon samples have been deposited by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD), using a square-wave amplitudemodulated radio-frequency excitation. It is observed that the gas-utilization efficiency improves by a considerable amount when amplitude modulation is combined with a reduction in the gas flows. Using a conventional continuous wave (cw) 50 MHz plasma with SiH4 and H2 gas flows of 30 sccm each at a pressure of 0.2 mbar, the gas-utilization efficiency is about 8%. It increases up to 50%, by modulating the amplitude of the radio-frequency excitation signal and reducing both gas flows to 10 sccm, keeping the pressure constant. In this case, the deposition rate amounted to 0.55 nm/s; which is twice as large as compared to the deposition rate of a cw deposition. Device-quality opto-electronic properties are obtained under these conditions. The refractive index at 2 eV is about 4.25 and the microstructure parameter has a value around 0.02. The materials exhibited a low defect density (CPM) which is in the order of 3-8x1015 per cubic centimeter and photo-to-dark-conductivity ratio of 4-6x106. N-i-p solar cells of size 0.16 cm2 deposited on 10cmx10cm stainless steel (SS) substrate in the configuration SS/n-a-Si:H/i-a- Si:H/buffer/p-μc-Si/ITO/Ag grid (without back reflector) using amorphous silicon i-layer made by amplitude-modulated VHF plasma CVD showed an efficiency of 6.5%. This is a similar efficiency to the cell with standard device-quality cw a-Si:H in the same n-i-p structure, but at a high growth of 0.55 nm/s and gas utilization of ∼50%.

The minority-carrier diffusion length in thin silicon films can be extracted from the electrically-detected transient grating method, EDTG, by a simple ambipolar analysis only in the case of lifetime dominated carrier transport. If the dielectric relaxation time, τdiel, is larger than the photocarrier response time, τR, then unexpected negative transient signals can appear in the EDTG result. Thin silicon films deposited by hot-wire chemical vapor deposition (HWCVD) near the amorphous-to-microcrystalline transition, where τR varies over a large range, appeared to be ideal candidates to study the interplay between carrier recombination and dielectric response. By modifying the ambipolar description to allow for a time-dependent carrier grating build-up and decay we can obtain a good agreement between analytical calculation and experimental results.

In order to obtain a large Si grain and to control the location of grain boundary in a Si film thermally melting-crystallized by pulse laser, we have proposed to use a periodic thermal distribution spontaneously induced by a linearly polarized laser beam. The lateral grain growth of polycrystalline Si thin films, which is controlled by the laser-induced periodic thermal distribution, was analyzed numerically by two-dimensional finite element computer simulations. From this analysis, it can be conclude that the laser irradiation should be performed to melt not all but most of Si film or melt it partially, making the large difference between the maximal and minimal temperature in the thermal distribution. It was also found that the temperature difference was increased with the optical absorption in the Si film and the fluence.

The technology of large area electronics has made significant progress in recent years because of the fast maturing excimer laser annealing process. The new thin film transistors based on laser processed poly silicon provide unprecedented performance over the traditional thin film transistors using amorphous silicon. They open up the possibility of building flat panel displays and imagers with higher integration and performance. In this paper, we will review the progress of poly-Si thin film transistor technology with emphasis on imager applications. We also discuss the challenges of future improvement of flat panel imagers based on this technology.

We have made a series of a-SiSx:H based solar cells, with a pin structure, in a multichamber plasma enhanced chemical vapor deposition (PECVD) system. The sulfur concentration ranges from zero to 5 x 1018 cm-3 as measured by secondary ion mass spectroscopy. The initial conversion efficiencies of cells in this series with sulfur concentrations ≤ 1018 cm-3 are approximately 7%. The time constants for degradation increase with increasing sulfur concentration, but not fast enough to be of practical importance in photovoltaic devices.

We have developed a new method of eliminating defect states in Si. This method called cyanide treatment simply includes immersion of Si in KCN solutions followed by the rinse. The contamination by potassium ions can be completely prevented by the inclusion of 18-crown-6 in the KCN solutions (crown-ether cyanide treatment). When the crown-ether cyanide treatment was performed on intrinsic amorphous Si (a-Si) films, decreases in the photo-and dark current densities with the irradiation time were completely prevented. When cyanide treatment using aqueous KCN solutions was applied to pin-junction a-Si solar cells, the conversion efficiencies measured before and after light-induced degradation became higher than those with no treatment. These improvements are attributed to the elimination of defect and defect precursor states by the reaction with cyanide ions, resulting in the formation of Si-CN bonds. From density functional calculations, Si-CN bonds are found to possess a high bond energy of 4.5 eV. Due to the high bond energy, the bonds are not ruptured by heat treatment at 800°C and upon irradiation, resulting in the thermal and irradiation stability of the cyanide treatment.

We recently developed an atomistic model of metastability of a-Si:H, where defect creation is driven by the breaking of weak silicon bonds. The kinetics of degradation in this model is simulated with coupled rate equations that show t1/3 kinetics of defect creation and saturation behavior similar to experiment. Saturated defect densities of neutral dangling bonds are accompanied by a much smaller density of negatively charged floating bonds and positively charged dangling bonds (D+). We propose a two-step annealing mechanism where the positively charged D+ dangling bonds are annealed at low temperature and the D0 at higher temperature -which accounts for hysteresis in mobility lifetime products.

We have been developing a model to study the nucleation and growth of particles in silane plasmas. Presently we are experimenting with parameters that could potentially be important. It has been found that an increase in the heavy species gas temperature leads to a delay in the nucleation of particles in low pressure silane plasmas. This effect could be useful in tailoring processes for the manufacture of new materials such as polymorphous silicon. We have made an effort to study this effect using a plasma chemistry model. The model includes gas temperature dependent electron attachment and vibrational relaxation of excited species with a view to study their importance in relation to the gas temperature. Preliminary simulation results predict that these two processes do not contribute substantially to slowing the nucleation process when the gas temperature is increased. Extra efforts must be made to study this potentially important effect.

Silicon nitride films have been grown by hot-wire chemical vapor deposition and film properties have been characterized as a function of SiH4/NH3 flow ratio. Quadrupole mass spectrometry measurements revealed that NH3 should be present in large excess relative to SiH4 (<10), due to its lower decomposition probability on the wire. Silicon nitride films were produced with refractive indices ranging from 1.8-2.5 and H-content from 9-18 atomic % as the flow ratio increased from 1% to 8%. Fourier Transform Infrared Spectroscopy revealed a change from predominantly N-H to Si-H bonding as the flow ratio increases beyond 6%. Subsequent annealing studies showed different kinetics for H release from Si versus N. Films grown with a low SiH4/NH3 ratio were found to oxidize readily (23 atomic %), while larger ratios yielded no oxygen incorporation.

During metal induced lateral crystallization (MILC) of amorphous silicon (a-Si) the size and quality of obtained film depend on nickel penetration and it is very important to know about nickel diffusion at recrystallization process. The nickel has penetrated during annealing on surface of a-Si inducing the recrystallization process, which has changed the mechanism of diffusion on surface of a-Si to the mechanism of diffusion on surface of single crystal silicon and in single crystal silicon. Also the effect of thickness of nickel and a-Si film are discussed.

We report on the growth and properties of amorphous Silicon-Germanium [a–(Si,Ge):H] films and devices fabricated at growth rates of ∼ 5 Å/sec using a remote ECR plasma growth process. The films and devices were made using mixtures of germane and silane along with dilution with hydrogen and helium. The addition of He to the gas mixture significantly increased the growth rates. It was found that hydrogen was always necessary in order to achieve the best film and device properties. Films and devices were made across the entire bandgap range, from a-Si to a-Ge. High ratios of photo/dark conductivity and low values of Urbach energy ( > 50 meV) indicate good film properties. The defect densities were measured using space charge limited current techniques. The defect densities were in the range of 1-2 x 10 16/cm 3 –eV, about 5 times higher than for a-Si:H. Electron mobility-lifetime products were measured and found to be in the range of 2-4 x 10-7 cm2/V, even for low gap materials (1.35 eV). Single and graded gap devices were fabricated in these materials. Device fill factors of ∼ 70% were obtained in graded gap devices.

Electron-cyclotron-resonance (ECR) plasma are used extensively to deposit amorphous and microcrystalline Si and (Si,Ge) films, and for depositing oxides and nitrides of Si. In this paper, we discuss the results of optical emission spectroscopy, Langmuir probe measurements and mass-spectrometery measurements of the ECR plasma when used to deposit a-Si films. We study He and H diluted plasmas. We find that the addition of hydrogen to the gas mixture immediately reduces the flux of the active (e.g. SiH, SiH2,SiH3) radicals and ions that are responsible for the growth of films. Thus, introducing hydrogen in the mixture reduces the growth rate significantly, not only because it etches the film during growth, but primarily because it reduces the production of the needed growth radicals. We find an excellent correlation between growth rates and SiH intensity signal. A surprising result is that the densities of all three ions, SiH, SiH2 and SiH3, arriving at the substrate are comparable in magnitude in this low pressure reactor, with the SiH ion density becoming larger than the SiH3 density at higher powers. This observation raises some doubt about the standard model for growth that states that SiH3 is the dominant radical responsible for growth during plasma CVD processes. We also find that decreasing the pressure increases the ratio of H/H2 arriving at the substrate, which in turn means that more H ions and atoms are available to do etching of the growing film and change its properties at lower pressures.

Growth of intrinsic micro-crystalline silicon layers by means of VHF-PECVD, assisted by remote microwave (MW) plasma has been investigated. The aim of the MW plasma is to enhance the deposition rate by introducing excited hydrogen and Ar atoms from the MW plasma in the VHF deposition zone. For this purpose a remote microwave plasma source was constructed in which a H2/Ar plasma is generated in a 20 mm diameter quartz tube. A gasshower has been constructed for homogeneous distribution of the flow of excited gas species from the microwave source into the deposition zone of the VHF-PECVD reactor where the dissociation of silane takes place. In a first series of experiments we applied high microwave power (< 500 W) and pure hydrogen in the MW source. This resulted in a larger deposition rate, but all layers – even grown at low silane concentrations – were amorphous and had a high oxygen content. The oxygen contamination was partly due to reduction of the quartz tube by the hydrogen plasma. In a second series of experiments Ar dilution and reduced MW power were used to eliminate the effect of etching of the tube by the microwave hydrogen plasma. In this series of experiments an increase of the growth rate of micro-crystalline silicon by about 15 % due to assistance of the microwave plasma was found. Optical emission spectroscopy indicates that – in these experiments – the main mechanism for the increased dissociation of silane is through molecular quenching reaction of Ar* metastables.