Correlation between plasma chemistry, microstructure and electronic properties of Si:H thin films prepared with hydrogen dilution (original) (raw)
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Silicon, 2016
Investigation of carrier transport in hydrogenated amorphous silicon (a-Si:H) thin films deposited at various pressures (0.03-0.53 Torr) using 27.12 MHz assisted high frequency Plasma Enhanced Chemical vapor Deposition (PECVD) process is presented. From results of Steady State Photocarrier Grating (SSPG) the carrier diffusion length was found to vary from 0.098-0.189 μm. Moreover a direct influence of ambipolar diffusion length was observed with the transport mechanism for deposition pressure in the range (0.13-0.53 Torr). There was a correlation observed for photosensitivity and microstructure parameter with mobility lifetime (μτ) product and diffusion length of carriers. Diffusion length and μτ product were observed to be maximum (0.189 μm and 0.471 x 10 −8 cm 2 V −1) for the film having high photosensitivity (7.2x10 3) deposited at a rate ∼1.39Å/s at 0.53 Torr deposition pressure. In addition to electrical transport properties, the effect of deposition pressure on structural and optical properties was also studied using various characterization tools such as Raman, UV-Vis and infrared spectroscopy.
Solar Energy Materials and Solar Cells, 2008
Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited from pure silane (SiH 4) and hydrogen (H 2) gas mixture by conventional plasma enhanced chemical vapour deposition (PE-CVD) method at low temperature (200 1C) using high rf power. The structural, optical and electrical properties of these films are carefully and systematically investigated as a function of hydrogen dilution of silane (R). Characterization of these films with low angle X-ray diffraction and Raman spectroscopy revealed that the crystallite size in the films tends to decrease and at same time the volume fraction of crystallites increases with increase in R. The Fourier transform infrared (FTIR) spectroscopic analysis showed at low values of R, the hydrogen is predominantly incorporated in the nc-Si:H films in the mono-hydrogen (Si-H) bonding configuration. However, with increasing R the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (Si-H) to di-hydrogen (Si-H 2) and (Si-H 2) n complexes. The hydrogen content in the nc-Si:H films decreases with increase in R and was found less than 10 at% over the entire studied range of R. On the other hand, the Tauc's optical band gap remains as high as 2 eV or much higher. The quantum size effect may responsible for higher band gap in nc-Si:H films. A correlation between electrical and structural properties has been found. For optimized deposition conditions, nc-Si:H films with crystallite size 7.67nmhavinggooddegreeofcrystallinity(7.67 nm having good degree of crystallinity (7.67nmhavinggooddegreeofcrystallinity(84%) and high band gap (2.25 eV) were obtained with a low hydrogen content (6.5 at%). However, for these optimized conditions, the deposition rate was quite small (1.6 Å /s).
Effect of substrate on hydrogen in and out diffusion from a-Si:H thin films
Journal of Materials Science: Materials in Electronics, 2007
We present a detailed study on the effect of the substrate on the structure and hydrogen evolution from p-type hydrogenated amorphous silicon thin films co-deposited on the grounded and RF electrodes of an asymmetric radio frequency glow discharge reactor, as well as the similar films exposed to an hydrogen plasma. We used spectroscopic ellipsometry and hydrogen evolution measurements to analyze the effects of the substrate, ion energy and hydrogen plasma on the films microstructure, thickness, hydrogen content, hydrogen binding and hydrogen evolution. The hydrogen evolution spectra show a strong substrate dependence. In particular on crystalline silicon substrate, we observe the formation of bubbles. For the various substrates, ion energy and hydrogen plasma treatment do not affect the hydrogen evolution spectra. These results indicate that the action of hydrogen in a-Si:H is modified by the nature of the substrate.
Vacuum, 2009
Thin film silicon Medium-range order Grain size Hydrogen dilution Microstructure X-ray diffraction Raman analysis FTIR spectrometry a b s t r a c t Plasma enhanced chemical vapour deposition (PECVD) has been used to prepare hydrogenated amorphous silicon (a-Si:H) thin films at different hydrogen dilution of silane source gas. The films were deposited on Corning glass 1737 substrate and on (100) oriented c-Si wafers and characterized by XRD diffraction, micro-Raman and FTIR spectrometry. Experimental data show evolution from amorphous to nanocrystalline silicon and contain the medium-range order (MRO) with varying hydrogen dilution during deposition. From X-ray diffraction and Raman analysis, it is found that the presence of crystalline phase depends on the kind of substrate and on the dilution scale.
Solar Energy Materials and Solar Cells, 2008
The characteristics of 13.56-MHz discharged SiH 4 +Ar+H 2 plasma at high pressure (2-8 Torr), used for the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films in a capacitively coupled symmetric PECVD system, has been investigated. Plasma parameters such as average electron density, sheath field and bulk field are extracted from equivalent circuit model of the plasma using outputs (current, voltage and phase) of RF V-I probe under different pressure conditions. The conditions of growth in terms of plasma parameters are correlated with properties of the hydrogenated nanocrystalline silicon films characterized by Raman, AFM and dc conductivity. The film deposited at 4 Torr of pressure, where relatively low sheath/bulk field ratio is observed, exhibits high crystallinity and conductivity. The crystalline volume fraction of the films estimated from the Raman spectra is found to vary from 23% to 79%, and the trend of variation is similar to the RF real plasma impedance data.
Hydrogen evolution during deposition of microcrystalline silicon by chemical transport
Philosophical Magazine, 2008
We have exposed a freshly deposited boron-doped hydrogenated amorphous silicon (a-Si:H) layer to a hydrogen plasma under conditions of chemical transport. In situ spectroscopic ellipsometry measurements revealed that atomic hydrogen impinging on the film surface behaves differently before and after crystallization. First, the plasma exposure increases the hydrogen solubility in the a-Si:H network leading to the formation of a hydrogen-rich subsurface layer. Then, once the crystallization process engages, the exceeding hydrogen starts to leave the sample. We have attributed this unusual evolution of the exceeding hydrogen to the role of the grown hydrogenated microcrystalline (µc-Si:H) layer that gradually prevents the atomic hydrogen coming from the plasma to reach the µc-Si:H/a-Si:H interface. Consequently, the hydrogen solubility, initially increased by the hydrogen plasma, recovers its initial value of an untreated a-Si:H material. To support the idea that the out-diffusion is a consequence and not the cause of the growth of the µc-Si:H layer, we have solved the combined diffusion and trapping equations that govern the hydrogen diffused into the sample,
physica status solidi (c), 2012
The exact role of hydrogen in the crystallization process is still a subject of broad controversies due to the complexity of the overall plasma enhanced chemical vapor deposition (PECVD) process. We have investigated by ellipsometry the amorphous-to-microcrystalline the phase transition in intrinsic and doped hydrogenated amorphous silicon (a-Si:H) thin films during their exposure to a hydrogen plasma in conditions of chemical transport. The whole ellipsometry diagnostics reveal that, while intrinsic and phosphorus-doped a-Si:H present a similar trend during the plasma treatment, boron-doped a-Si:H differs by special features such as a rapid formation of the hydrogen-rich subsurface layer and an early amorphous-tomicrocrystalline phase transition. The particular behavior of boron-doped material is also pointed out through the time-evolution of the self-bias voltage on the radiofrequency electrode during the hydrogen plasma treatment.
Journal of Physics D: Applied Physics, 2009
The effect of incorporation of oxygen into the initially crystalline-like Si : H network and its gradual increase up to 20 at% on the optical, electrical and structural properties of the material has been studied systematically. The prime objective of the experiment was to investigate the contribution of He as the diluent to the SiH4 plasma in modifying the SiO : H network when CO2 was used as the source of oxygen, and to extract some ideas that might be extended in the future development of nc-SiO : H network from a similar plasma with suitably modified parameters in RF-PECVD. Incorporation of oxygen invariably widened the optical gap; however, it reduced the electrical conductivity by several orders of magnitude with a significant increase in its activation energy. A sharp increase in the bonded H content (C-H) of the film was identified at the initial stage of oxygen incorporation and that occurred mostly due to the gross change in the network structure from the crystalline-like to an amorphous-like phase. However, on further addition of oxygen C-H gradually decreased, bonded mostly in the polyhydride configuration and the overall film surface roughness diminished. Systematic reduction in C-H identified a dehydrogenation process occurring in the Si network during the gradual inclusion of oxygen, induced by the presence of He as the diluent to the plasma. The result seems opposite to the conventional H-2-diluted plasma and appears significantly favourable for the future development of nc-SiO : H materials under optimized plasma parameters.