Surface or interface roughness can impact optical, electronic, and MEMS applications of thin a-Si:H films. Deposition at lower temperatures can be advantageous for some applications of a-Si:H, but lower temperature deposition generally leads to rougher films. We have found that the evolution of surface roughness growth can be modified substantially by ion bombardment due to the self-bias of the plasma during Plasma-Enhanced Chemical Vapor Deposition (PECVD). Notable differences in the surface roughness evolution and deposition rate are shown for films deposited in “cathodic” versus “anodic” mode – where the substrate is placed on the powered and grounded electrode respectively. Suppression of surface roughness growth of a-Si:H can be achieved under conditions of relatively high ion bombardment even at deposition temperatures as low as 75 C. Atomic force microscopy (AFM) was used to measure the relative surface roughness profile as a function of deposition time. Analysis of the power spectral density of the roughness yielded important statistical surface parameter information. Based on these observations, insight is given into growth mechanisms under the two deposition conditions.

Amorphous and microcrystalline silicon films were deposited by HWCVD under different deposition conditions and the gas phase chemistry was studied by in situ Quadrupole Mass Spectrometry. Attempt is made to correlate the properties of the films with the gas phase chemistry during deposition. Interestingly, unlike in PECVD, partial pressure of H2 is higher than any other species during deposition of a-Si:H as well as μc-Si:H. Effect of hydrogen dilution on film properties and on concentration of various chemical species in the gas phase is studied. For low hydrogen dilution [H2]/ [SiH4] from 0 to 1 (where [SiH4] is 10 sccm), all films deposited are amorphous with photoconductivity gain of ∼ 106. During deposition of these amorphous films SiH2 was dominant in gas phase next to [H2]. Interestingly [Si]/[SiH2] ratio increases from 0.4 to 0.5 as dilution increased from 0 to 1, and further to more than 1 for higher hydrogen dilution leading to [Si] dominance. At hydrogen dilution ratio 20, consequently films deposited were microcrystalline.

We employ first principles methods to explore the coupling between electrons and the lattice in amorphous silicon (a-Si). First we compute the adiabatic electronic response to phonon modes in a realistic model of a-Si. Then, we present a simulation of the electron dynamics of localized edge states in a-Si at room temperature by integrating the time dependent Schrödinger equation. We study the character of the spatial and spectral diffusion of the localized states and directly simulate and reveal the nature of thermally driven hopping. Phonon-induced resonant mixing leads to rapid electronic diffusion if states are available nearby in energy and real-space. We believe that many of the results we obtain are central to modeling transport involving localized states.

We observe improved photoconductivity stability against light-soaking in hydrogenated amorphous silicon thin films as a result of an ultraviolet (UV) illumination and etch treatment. UV-etch treated samples begins with red-light photoconductivities inferior to that of a control sample which is only etched. After less than an hour of 1 sun red light-soaking, the photoconductivity of the etched-only control falls below that of the UV-etch treated sample. After 2 to 3 days light soaking, the UV-etch films can have a photosensitivity 20 to 38% above their control. We observe no corresponding improvement of defect optical absorption by constant photocurrent method spectroscopy. The UV-etch treatment also produces small improvements in the stabilized open-circuit voltage of Schottky barier solar cells. We speculate that mobile hydrogen produced during UV illumination is penetrating the film and improving stability.

The choice of filament material has an effect on the decomposition of silane during the hot wire chemical vapor deposition (HW-CVD) of amorphous and microcrystalline silicon films. The Si radicals produced from W, Re, Mo and Ta filament materials have been probed by laserbased single photon ionization (SPI) as a function of hot wire temperature. The Si radical profiles are shown to demonstrate two distinct regimes: a regime below 1600°C-1800°C (depending on filament material) limited by surface reaction at the filament in which Si concentration increases monotonically; and a mass transfer limited regime above 1600°C-1800°C where Si intensity saturates. The apparent activation energy of Si radical production in the surface reaction regime from Ta (140-170 kcal/mol) is found to be close to the corresponding Si thermal desorption energy from a Ta surface, suggesting that the Si production is controlled by the desorption process from the bare metal. On the other hand, the Si activation energies from W and Re (30-60 kcal/mol) are lower than the related desorption energies, suggesting that other rate limiting reactions play a role for these materials. The apparent activation energy for the Mo surface (60-90 kcal/mol) is intermediate between the other metal values. In addition to the Si radical study, corresponding film deposition is detected in situ by multiple internal reflection infrared (MIR-IR) spectroscopy. The IR measurements have been used to estimate the growth rate of a-Si:H deposited on a Ge substrate. The results show similar activation energies for both the growth rate and the Si formation from a W filament, implying that Si radical production and subsequent film growth may be dominated by the same elementary reactions within the decomposition and film growth processes at low pressure.

The surface roughness evolution during a-Si:H film growth from a SiH3 beam under purely chemical deposition conditions is discussed. The data is explained in terms of the different universality classes proposed in literature. It is argued that roughness evolution during a-Si:H growth shows great similarity with MBE growth, which belongs to a universality class studied extensively by Das Sarma and coworkers. The activation energy for surface diffusion extracted from simulating the growth exponent β vs. temperature reveals an activation energy of about 1 eV. This result suggests that the surface roughness evolution is not controlled by the weakly adsorbed SiH3 radical.

We have studied hydrogenated microcrystalline silicon, μc-Si:H, using dark and lightinduced electron spin resonance (ESR). In dark ESR measurements only one center is observed. The g values obtained empirically, assuming axial symmetry, from powder pattern lineshape simulations are g⊥∥ = 2.0096 and gτ = 2.0031. We suggest that this center is related to defects in the crystalline phase. Another signal, which occurs only after illumination, at low temperatures, is best described by two powder patterns indicating the presence of two centers. One center is slightly asymmetric (g∥ = 1.999, g⊥ = 1.996) and the other has a large unresolved broadening such that unique g-values cannot be obtained. The average g value for this center is 1.998. We interpret the light-induced electron spin resonance (LESR) signal as coming from electrons and holes trapped in the band tails at the crystalline grain boundaries.

We have explored which deposition parameters in Hot Wire CVD have the largest impact on the quality of microcrystalline silicon (μc-Si) made at deposition rates (Rd) < 10 Å/s for use in thin film solar cells. Among all parameters, the filament temperature (Tfil) appears to be crucial for making device quality films. Using two filaments and a filament-substrate spacing of 3.2 cm, μc-Si films, using seed layers, can be deposited at high Tfil (∼2000°C) with a crystalline volume fraction < 70-80 % at Rd's < 30 Å/s. Although the photoresponse of these layers is high (< 100), they appear not to be suitable for incorporation into solar cells, due to their porous nature. n-i-p cells fabricated on stainless steel with these i-layers suffer from large resistive effects or barriers, most likely due to the oxidation of interconnected pores in the silicon layer. The porosity is evident from FTIR measurements showing a large oxygen concentration at ∼1050 cm-1, and is correlated with the 2100 cm-1 signature of most of the Si-H stretching bonds. Using a Tfil of 1750°C, however, the films are more compact, as seen from the absence of the 2100 cm-1 SiH mode and the disappearance of the FTIR Si-O signal, while the high crystalline volume fraction (< 70-80 %) is maintained. Using this Tfil and a substrate temperature of 400°C, we obtain an efficiency of 4.9 % for cells with a Ag/ZnO back reflector, with an i-layer thickness of only ∼0.7 μm. High values for the quantum efficiency extend to very long wavelengths, with values of 33 % at 800 nm and 15 % at 900 nm, which are unequalled by a-SiGe:H alloys. Further, by varying the substrate temperature to enable deposition near the microcrystalline to amorphous transition (‘edge’) and incorporating variations in H2 dilution during deposition of the bulk, efficiencies of 6.0 % have been obtained. The Rd's of these i-layers are 8-10 Å/s, and are the highest to date obtained with HWCVD for microcrystalline layers used in cells with efficiencies of ∼6 %.

Plasma and in situ film studies have been applied to the expanding thermal plasma to obtain basic insight into the deposition of a-Si:H and μc-Si:H at high rates (> 10 Å/s). A study of the density of plasma radicals (Si, SiH, SiH3) and of the radicals' surface reactivity has revealed that SiH3 is the most important radical for the growth of both materials. In situ attenuated total reflection infrared spectroscopy and spectroscopic ellipsometry have revealed a thick interface layer and consequently long incubation time for the materials deposited at a high deposition rate.

We report spatial arrangement of self-assembled Ge quantum dots on patterned structures on Si (001) without using complicated lithography. Starting from stripes and mesas fabricated by conventional lithography and plasma etching, we prepare sinusoidal Si stripes with narrow ridges and mesas with humped edges via high-temperature annealing. Deposited Ge self-assembles into coherent nanocrystals that align along the narrow ridges of the stripes and the sloped mesa edges. Enhanced strain relief at the ridges due to elastic relaxation and the high step densities on the shallow slopes at the edges are likely causes of nanocrystal alignment.

We have investigated epitaxial growth of dysprosium (Dy)-doped silicon (Si:Dy) thin films for fabrication of 1.3 μm light-emitting diodes (LEDs) using supersonic-free-jet CVD. The 700-nm-thick Si:Dy films are grown on p+-type, p-type, and n-type Si(100) at a substrate temperature of 800° for 90 min. The doping concentrations of Dy atoms range from 1018 to 1019 cm-3 at the surface region of the film measured by the secondary ion mass spectrometry (SIMS). The particles with average size ~200 nm form at the surface of the film and it suggests the possibility to form Si-nanoparticle (Si quantum dots) from atomic force microscopy (AFM) observation. From the electrical measurements at room temperature, the metal (Al)-Si:Dy junction is a Schottky contact and Dy introduces donor states in Si.

Low-temperature growth of nano-crystalline silicon film and silicon nanorods by hot-wire chemical vapor deposition (HWCVD) using SiCl4/H2 gases are reported here. Nano-crystalline silicon films were deposited at a filament temperature of 1800°C, SiCl4/H2 flow rate ratio of 8/37 sccm and a substrate temperature of 150°C. The volume fraction of the crystallinity of the film characterized by Raman spectroscopy is 82%. The TEM analysis reveals that the average size of nano-crystalline silicon film is 5nm. Silicon nanorods with an average diameter of 80 nm were obtained as the SiCl4/H2 flow rate ratio was reduced to 5/30 sccm at substrate temperatures lower than 200°C. TEM analyses reveal that Si nanocrystals are embedded in an amorphous nanorods with fish-born like structure.

Measurements of the second spectra and correlation coefficients of conductance fluctuations are reported for a series of n-type doped a-Si:H films for which the deposition conditions are systematically varied. The non-Gaussian character of the 1/f noise is found to decrease with decreasing deposition temperature, which is also correlated with an increase in the difference between the conductivity and thermopower activation energies (the Q-function). The decrease in non-Gaussian statistics is interpreted as a decrease in the interactions between inhomogeneous current filaments due to increasing long-range disorder in the material.

We produce a novel form of amorphous silicon through ultra-high-vacuum molecular beam epitaxy. By depositing silicon atoms onto a fused quartz substrate at temperatures between 98 and 335°C, we obtain a silicon-based material that lacks the characteristic periodicity of crystalline silicon but nevertheless has 98% of its density. The impurity content of this material is studied through infrared and secondary ion mass spectroscopies. The primary impurity found is oxygen, with hydrogen and carbon atoms also being found at trace levels. The Raman spectra of the amorphous silicon films are measured and the results, as they relate to the presence of disorder, are interpreted. We also use this molecular beam epitaxy method to fabricate a number of amorphous silicon superlattices, comprised of thin layers of amorphous silicon separated with even thinner layers of SiO2. The optical properties of the films and superlattices are contrasted.