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.

μc-Si has traditionally been deposited by Hot Wire CVD at a low filament temperature. At these temperatures, silicides rapidly form on the filament surface, leading in the case of a tungsten filament to both film reproducibility and filament lifetime issues. By depositing films consecutively using identical deposition parameters, these issues are chronicled for a filament temperature of ∼ 1750°C. Upon increasing the filament temperature to ∼ 1825-1850°C, these reproducibility and lifetime issues disappear and, by lowering both the substrate temperature and chamber pressure, device quality μc-Si is deposited at high deposition rates in a filament regime where tungsten silicide formation is minimal. Both single junction and tandem solar cells are fabricated using this material, confirming the validity of this approach.

We present femtosecond time-resolved studies of the photoexcited carrier response in the far-infrared spectral range in PECVD a-Si:H and a-SiGe:H thin films. The experiments are carried out using an optical pump / terahertz (THz) probe technique, in which a femtosecond pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in the far-infrared optical properties as a function of time delay following the excitation. These measurements are sensitive to carrier processes at low energy, corresponding to a range of approximately 1 - 10 meV, a key energy scale in these materials. We find that the observed photoexcited carrier dynamics are consistent with trapping of carriers into band tail states on a picosecond time scale.

We measured the low temperature internal friction of a variety of hydrogenated amorphous, nanocrystalline, polycrystalline, and epitaxial silicon thin films. Most of the films studied are prepared either by hot-wire chemical-vapor deposition (HWCVD) or by plasma-enhanced chemical-vapor deposition (PECVD). We show that structural changes with varying modes of deposition can be monitored by the internal friction measurements, which are a sensitive probe to structural disorder resulted from atomic tunneling states below 10 K. For hydrogenated amorphous silicon films, the measurements are also sensitive to the content of atomic hydrogen and bulk molecular hydrogen trapped in the films. With H2 dilution of the silane during deposition, a transition from amorphous to nanocrystalline phase takes place as H2 dilution increases. We show that with increasing volume of nanocrystallinity, the internal friction increases in contrast to our common expectation. The results suggest a large structural disorder in the nanocrystalline phase that is at least comparable to that of amorphous phase exists. The observation of internal friction peaks in amorphous HWCVD films and in nanocrystalline PECVD films indicates that the microstructures of the HWCVD and PECVD films are different.

We studied the growth of silicon films deposited by hot-wire chemical vapor deposition under different values of filament current, substrate temperature, and hydrogen dilution ratio. The physical and electrical properties of the films were studied by Raman spectroscopy, x-ray diffraction, atomic force microscopy, conductive-atomic force microscopy, and transmission electron microscopy. There is an interdependence of the growth parameters, and films grown with different parameters can have similar structures. We discuss why this interdependence occurs and how it influences the properties of the deposited films, as well as the deposition rate. In general, the films have a complex structure, with a mixture of amorphous, (220)-oriented crystalline and nanocrystalline phases present in most cases. The amount of each phase can be controlled by the variation of one or more of the growth parameters at a time.

Real-time spectroscopic ellipsometry (RTSE) has been used to characterize the optical and structural properties of hot-wire CVD (HWCVD) deposited amorphous and nanocrystalline silicon as a function of hydrogen dilution, substrate temperature, and gas pressure. Throughout the range of parameters investigated in this three-dimensional parameter space we find good correlations between post-deposition conductivity measurements and the thickness of the film at the transition from amorphous to nanocrystalline growth, as indicated by a smoothering transition in the surface roughness measured by RTSE during film growth. These results validate the use of RTSE as an in-situ diagnostic to elucidate the nature of HWCVD film growth, particularly as it relates to maximization of photovoltaic device efficiency. (C) 2003 Elsevier B.V. All rights reserved.

In situ real-time spectroscopic ellipsometry (RTSE) provides detailed information on the evolution of the structural and optical properties of Si:H films during growth [Collins et al., Appl. Surf. Sci. 154-155, 217 (2000)]. We have used in situ RTSE to characterize the morphology and crystallinity of hot-wire chemical vapor deposition (HWCVD) Si:H films grown on native-oxide covered crystal silicon substrates as a function of substrate temperature Ts, hydrogen dilution R=[H2/SiH4], and film thickness db. Transitions from one mode of film growth to another are correlated with abrupt changes in the magnitude of the surface roughness during film growth. The degree of crystallinity of the film can be determined from the shape of the dielectric spectrum. We have studied the growth parameter space consisting of R from 0 to 14, Ts of 250 and 500 °C, and db from 0 to 1 μm. We have mapped out the crystallinity versus R, Ts, and db based on our analysis of the RTSE data. The transition between a-Si:H and μc-Si:H for growth on crystalline silicon substrates is near the R=3 to R=4 dividing line and is also a strong function of film thickness. Initial coalescence of purely μc-Si:H material does not occur until R⩾10. These results have been corroborated using Raman scattering and atomic force microscopy to characterize the crystallinity and surface morphology of the films.

We deposited microcrystalline silicon (μc-Si) by hot-wire chemical vapor deposition (HWCVD) at different thickness and dilution ratio, with and without seed layer. As the dilution ratio increased, we observed an increase in the amount of microcrystalline phase in the film, a change in the structure of the grains and a loss of the (220) preferential orientation. The films deposited over a seed layer had a larger fraction of crystalline phase than films deposited with the same parameters but without a seed layer. For high dilution ratios (R=100), most of the film grows epitaxially at the interface with the Si substrate, but a microcrystalline film slowly replaces the single-crystal phase. For low dilution ratios (R=14), the film starts growing mostly amorphously, but the amount of crystalline phase increases with thickness.

We have applied real time spectroscopic ellipsometry and secondary ion mass spectrometry to study the growth of amorphous silicon by hot-wire chemical vapor deposition. Differences in temperature and hydrogen content affect the optical properties of the film. These effects provide valuable insight into the growth process. We have compared a-Si:H films grown at two different temperatures to better understand these effects. Our studies reveal the presence of a distinct 100–200-thick layer at the top of the growing film. The properties of this layer are primarily determined by the ambient conditions in the growth chamber and appear relatively independent of substrate temperature. In contrast, the properties of the bulk of the film are strongly influenced by substrate temperature. These results imply that differences in film properties associated with substrate temperature are the result of subsurface reconstruction and diffusion processes.

We have improved the electronic properties of narrow-bandgap (Tauc gap below 1.5 eV) amorphous-silicon germanium alloys (a-SiGe:H) grown by hot-wire chemical vapor deposition (HWCVD) by lowering the substrate temperature and deposition rate. Prior to this work, we were unable to grow a-SiGe:H alloys with bandgaps below 1.5 eV that had photo-to-dark conductivity ratios comparable with our plasma-enhanced CVD (PECVD) grown materials [B.P. Nelson et al., Mater. Res. Soc. Symp. 507 (1998) 447]. Decreasing the filament diameter from our standard configuration of 0.5 mm to 0.38 or 0.25 mm provides first big improvements in the photoresponse of these alloys. Lowering the substrate temperature from our previous optimal temperatures (Tsub starting at 435 °C) to at 250 °C provides additional photo-to-dark conductivity ratio increasing by two orders of magnitude for growth conditions containing 20–30% GeH4 in the gas phase (relative to the total GeH4+SiH4 flow).

Several a-Si:H and a-Si:D films prepared by hot-wire chemical vapor deposition have been examined by small-angle neutron scattering (SANS) to search for H non-uniformity in this material. The SANS measurements were supplemented by small-angle X-ray scattering measurements. The differences in H/D detection sensitivity of these two techniques allow distinction of the scattering mechanisms. Two- or three-phase models are used to interpret the results quantitatively. Significant H non-uniformity, as well as a small fraction of microvoids, was found in the best-quality material. Samples grown with higher deposition rates or lower substrate temperatures have much larger void fractions. The size scale of the heterogeneity spans a range from 2 nm to more than 50 nm, with the largest features assigned to surface roughness.

We have used in-situ real-time spectroscopic ellipsometry (RTSE) to characterize the morphology and crystallinity of hot-wire CVD (HWCVD) Si:H films as a function of hydrogen dilution R=[H]/[H+SiH/sub 4/], substrate temperature T/sub s/, and film thickness d/sub b/. Transitions from one mode of film growth to another are correlated with changes in the magnitude of the surface roughness during growth. The degree of crystallinity of the film can be determined from the form of the dielectric function. We have studied the growth parameter space for R from 0 to 14, T/sub s/ of 250/spl deg/C and 500/spl deg/C, and db from 0 to 1 /spl mu/m. We have mapped out the crystallinity vs. R, T/sub s/, and d/sub b/ based on our analysis of the RTSE data. These results have been corroborated using Raman scattering and atomic force microscopy to characterize the crystallinity and surface morphology of the films.

We observe that the charge state of diffusing hydrogen depends upon the electronic Fermi level (Ef) in hydrogenated amorphous silicon (a-Si:H). We incorporate a thin layer of deuterium (2H) at various positions between the n- and p-layers of i–n–i–p–i structures on crystalline silicon substrates. The electric field (F) is above 6×104 V/cm at each 2H layer. After annealing, marked asymmetries in the secondary ion mass spectrometry profiles of diffused deuterium are observed. With the 2H layer placed near the p-layer (Ef near the valence band), diffusion is into the p-layer, indicating H+ moving with F. With the 2H layer near the n-layer (Ef near the conduction band), most diffusion is into the n-layer, indicating H− moving against F. Because the Si–H bond is neutral, the charged diffusing species must be emitted mobile H. We estimate an effective correlation energy of 0.4±0.1 eV for the mobile H.

The charge transport properties and microstructure of hydrogenated amorphous silicon–germanium alloys (a-SiGe:H) prepared by the hot-wire chemical vapor deposition (HWCVD) process as a function of alloy composition have been investigated in detail by employing the photoconductive frequency mixing and small angle X-ray scattering techniques. Evidence for the presence of long-range potential fluctuations in a-SiGe:H alloys is revealed from the measurements of electric field dependence of the drift mobility. The effect of the long-range potential fluctuations is enhanced by the addition of Ge to the alloy system that results in the deterioration of the opto-electronic properties of a-SiGe:H alloys. Through the drift mobility field dependence, the depth and range of the potential fluctuations as a function of alloy composition are determined, and subsequently the charged defect density. It was found that at a composition of ∼10% Ge in Si, the photoresponse begins to decrease monotonically with increasing Ge content due to the decreases in the drift mobility and lifetime as a result of an increase in the concentration of charged defects, which lead to the long-range potential fluctuations whose depth increases, while the range decreases. The sharp changes in these parameters are demonstrated to be attributed to a concurrent abruptly increased structural heterogeneity due to the introduction of microvoids.

Journal Article Tem Study of The Microstructure of Si Thin Films Deposited by Hot Wire CVD Get access KM Jones, KM Jones National Renewable Energy Laboratory, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar MM Al-Jassim, MM Al-Jassim National Renewable Energy Laboratory, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar DH Levi, DH Levi National Renewable Energy Laboratory, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar BP Nelson BP Nelson National Renewable Energy Laboratory, Golden, Colorado 80401 Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 8, Issue S02, 1 August 2002, Pages 1196–1197, https://doi.org/10.1017/S1431927602107859 Published: 01 August 2002

Microcrystalline silicon films were grown with different thicknesses and different hydrogen dilution ratios on glass and Si substrates. Some films were deposited with a seed layer, whereas others were deposited directly on the substrate. We used atomic force microscopy, scanning electron microscopy, and X-ray diffraction to study the morphology and crystalline structure of the samples. We did not find a significant influence of the different substrates on the morphology or crystalline structure. The presence of the seed layer enhanced the crystallization process, decreasing the amount of amorphous layer present in the films. The microstructure of most films was formed by grains, with a subgrain structure. Films grown with low values of dilution ratio had (220) texture and elongated grains, whereas films deposited with high values of dilution ratio were randomly oriented and had an irregular shape.

Hydrogenated amorphous-silicon (a-Si:H) is grown by hot-wire chemical vapor deposition (HWCVD) at deposition rates (Rd) exceeding 140 Å/s (∼0.8 μm/min). These high rates are achieved by using multiple filaments and deposition conditions different than those used to produce our standard 20 Å/s material. With proper deposition parameter optimization, an AM1.5 photo-to-dark-conductivity ratio of 105 is maintained at an Rd up to 130 Å/s, beyond which it decreases. In addition, the first saturated defect densities of high Rd a-Si:H films are presented. These saturated defected densities are similar to those of the best HWCVD films deposited at 5–8 Å/s, and are invariant with Rd up to 130 Å/s.

The structure of a-Si:H, deposited at rates in excess of 100 Å/s by the hot wire chemical vapor deposition technique, has been examined by x-ray diffraction (XRD), Raman spectroscopy, H evolution, and small-angle x-ray scattering (SAXS). The films examined in this study were chosen to have roughly the same bonded H content CH as probed by infrared spectroscopy. As the film deposition rate Rd is increased from 5 to >140 Å/s, we find that the short range order (from Raman), the medium range order (from XRD), and the peak position of the H evolution peak are invariant with respect to deposition rate, and exhibit structure consistent with a state-of-the-art, compact a-Si:H material deposited at low deposition rates. The only exception to this behavior is the SAXS signal, which increases by a factor of ∼100 over that for our best, low H content films deposited at ∼5 Å/s. We discuss the invariance of the short and medium range order in terms of growth models available in the literature, and relate changes in the film electronic structure (Urbach edge, as-grown defect density) to the increase in the SAXS signals. We also note the invariance of the saturated defect density versus Rd, measured after light soaking, and discuss possible reasons why the increase in the microvoid density apparently does not play a role in the Staebler–Wronski effect for this type of material.

We grow hydrogenated amorphous silicon (a-Si:H) solar cells in a device structure denoted as SS/n–i–p/ITO. We grow all the a-Si:H layers by hot-wire chemical vapor deposition (HWCVD) and the indium-tin-oxide (ITO) by reactive evaporation. We are able to grow HWCVD i-layer materials that maintain an AM1.5 photoconductivity-to-dark-conductivity ratio of 105 at deposition rates up to 130 Å/s. We have put these high-deposition rate i-layer materials into SS/n–i–p/ITO devices and light-soaked them for ≥1000 h under AM1.5 conditions. We obtain stabilized solar cell efficiencies of 5.5% at 18 Å/s, 4.8% at 35 Å/s, 4.1% at 83 Å/s and 3.8% at 127 Å/s.