M.M. Kamble

Hydrogenated Nanocrystalline Silicon Thin Films Prepared by Hot-Wire Method with Varied Process Pressure

Hydrogenated nanocrystalline silicon films were prepared by hot-wire method at low substrate temperature (200∘C) without hydrogen dilution of silane (SiH4). A variety of techniques, including Raman spectroscopy, low angle X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, atomic force microscopy (AFM), and UV-visible (UV-Vis) spectroscopy, were used to characterize these films for structural and optical properties. Films are grown at reasonably high deposition rates (>15 Å/s), which are very much appreciated for the fabrication of cost effective devices. Different crystalline fractions (from 2.5% to 63%) and crystallite size (3.6–6.0 nm) can be achieved by controlling the process pressure. It is observed that with increase in process pressure, the hydrogen bonding in the films shifts from Si–H to Si–H2 and complexes. The band gaps of the films are found in the range 1.83–2.11 eV, whereas the hydrogen content remains <9 at.% over the entire range of process pressure studied. The ease of depositing films with tunable band gap is useful for fabrication of tandem solar cells. A correlation between structural and optical properties has been found and discussed in detail.

Hydrogenated Silicon Carbide Thin Films Prepared with High Deposition Rate by Hot Wire Chemical Vapor Deposition Method

Structural, optical, and electrical properties of hydrogenated silicon carbide (SiC:H) films, deposited from silane (SiH4) and methane (CH4) gas mixture by HW-CVD method, were investigated. Film properties are carefully and systematically studied as function of deposition pressure which is varied between 200 mTorr and 500 mTorr. The deposition rate is found to be reasonably high (9.4 nm/s 15.54 nm/s). Formation of SiC:H films is confirmed by FTIR, Raman, and XPS analysis. XRD and Raman analysis revealed that with increasing deposition pressure amorphization occurs in SiC:H films. FTIR spectroscopy analysis shows that bond density of C–H decreases while Si–C and Si–H bond densities increase with increasing deposition pressure. Total hydrogen content increases with increasing deposition pressure and was found to be <20 at.%. The absence of band ~1300–1600 cm−1 in the Raman spectra implies negligible C–C bond concentration and formation of nearly stoichiometric SiC:H films. The band gap shows increasing trend with increasing deposition pressure. The high value of Urbach energy suggests increased structural disorder in SiC:H films. Finally, it has been concluded that CH4 can be used as effective carbon source in HW-CVD method to prepare stoichiometric SiC:H films.

Highly conducting phosphorous doped n-type nc-Si:H films by HW-CVD for c-Si heterojunction solar cells

Phosphorous doped hydrogenated nanocrystalline silicon (nc-Si:H) films were prepared using the hot wire chemical vapor deposition (HW-CVD) method at a low substrate temperature of 200 °C. The microstructure and opto-electrical properties of these films were systematically studied using Raman spectroscopy, low angle XRD, high resolution transmission electron microscopy (HR-TEM), UV-Visible spectroscopy, Fourier transform infrared (FTIR) spectroscopy, dark conductivity and its activation energy measurements and Hall measurement as a function of PH3 gas-phase ratio. It has been found that with an increase in the PH3 gas-phase ratio, both the volume fraction of the crystallites and its size decrease, signifying that the phosphorous atom favors the growth of amorphization in the nanocrystalline Si network. At the optimized PH3 gas-phase ratio we have obtained n-type nc-Si:H films with a band gap of ∼1.84 eV, high dark conductivity (∼6.78 S cm−1) with low hydrogen content (∼1.72 at. %), at a reasonably high deposition rate (∼10 A s−1). Finally, Al/ZnO:Al/n-nc-Si:H/buffer a-Si:H/p-c-Si/Al heterojunction solar cells were fabricated using the optimized n-layer, showing excellent photovoltaic performance with Voc = 719 mV, Jsc = 9.94 mA cm−2, FF = 53.8%, and an energy conversion efficiency of 5.2%. These are very encouraging results for the future fabrication of high efficiency silicon heterojunction solar cells and thin film tandem solar cells.

Synthesis of Cubic Nanocrystalline Silicon Carbide (3C-SiC) Films by HW-CVD Method

Cubic nanocrystalline silicon carbide (3C-SiC) films have been deposited by using the hot wire chemical vapor deposition (HW-CVD) method at a low substrate temperature and at high deposition rate. Structural, optical and electrical properties of these films have been investigated as a function of H2 dilution ratio. The formation of 3C-SiC films has been confirmed from low angle XRD analysis, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, x-ray photoelectron spectroscopy (XPS) and dark and photoconductivity measurements. The FTIR spectroscopy analysis revealed that the bond densities of Si-H and C-H decrease while that of Si-C increases with increase in the H2 dilution ratio. The total hydrogen content decreases with increase in H2 dilution ratio and was found < 15 at. % over the entire range of H2 dilution ratio studied whereas the band gap show an increasing trend with increase in the H2 dilution ratio.

Structural and optical investigations of nc‐Si:H thin films prepared by hot‐wire method

Structural and optical properties of hydrogenated nanocrystalline silicon (nc‐Si:H) films have been carefully studied as a function of hydrogen dilution of silane (RH.) Raman spectroscopic analysis showed that with increase in RH, the crystalline fraction in the films increases whereas crystallite size remains almost constant (∼8.7 nm). Also, the Raman spectra shows a blue shift of transverse optic (TO) phonon mode indicating that the films are stressed and the induced stress is compressive. The FTIR spectroscopic analysis revealed that the hydrogen predominantly incorporated in Si‐H2 and (Si‐H2)n bonding configuration. We have obtained high band gap (1.88‐2.07 eV) at low hydrogen content (< 2.5 at. %) over the entire range of RH studied at reasonably high deposition rate (7.4‐9.5 A/s).

Boron Doped p‐type Hydrogenated Nanocrystalline Silicon Films Grown by Hot Wire Chemical Vapor Deposition

In this paper we report on the effect of diborane (B2H6) flow rate on the microstructural and opto‐electrical properties of p‐type nc‐Si:H films grown by HW‐CVD method. An attempt has been made to elucidate the boron doping mechanism of the p‐type nc‐Si:H films. The correlation between B2H6 gas flow rate (FB2H6) and material properties including crystalline volume fraction (XRaman), crystallite size (dRaman), band gap (Eg) and hydrogen content (CH) has been established. We obtained p‐type nc‐Si:H films with high dark conductivity (≤0.2 S/cm), high Eg (>2 eV) at low CH (<3.6 at. %). The employment of these films in nc‐Si:H based p‐i‐n solar cell as a p‐type window layer could have better collection of charge carriers when illuminated from p‐side.

High growth rate of a-SiC:H films using ethane carbon source by HW-CVD method

Hydrogenated amorphous silicon carbide (a-SiC:H) thin films were prepared using pure silane (SiH4) and ethane (C2H6), a novel carbon source, without hydrogen dilution using hot wire chemical vapour deposition (HW-CVD) method at low substrate temperature (200 °C) and at reasonably higher deposition rate (19·5 Å/s < rd < 3·2 Å/s). Formation of a-SiC:H films has been confirmed from FTIR, Raman and XPS analysis. Influence of deposition pressure on compositional, structural, optical and electrical properties has been investigated. FTIR spectroscopy analysis revealed that there is decrease in C–H and Si–H bond densities while, Si–C bond density increases with increase in deposition pressure. Total hydrogen content drops from 22·6 to 14·4 at.% when deposition pressure is increased. Raman spectra show increase in structural disorder with increase in deposition pressure. It also confirms the formation of nearly stoichiometric a-SiC:H films. Bandgap calculated using both Tauc’s formulation and absorption at 104 cm−1 shows decreasing trend with increase in deposition pressure. Decrease in refractive index and increase in Urbach energy suggests increase in structural disorder and microvoid density in the films. Finally, it has been concluded that C2H6 can be used as an effective carbon source in HW-CVD method to prepare stoichiometric a-SiC:H films.

BORON DOPED nc-Si:H WINDOW LAYER PREPARED BY HW-CVD FOR SOLAR CELL APPLICATIONS

In this work, we report on synthesis of boron doped hydrogenated nanocrystalline silicon (p-nc-Si:H) films by HW-CVD method. Films were prepared at low substrate temperature (165 °C) and low process pressure (20 mTorr) by varying diborane gas phase ratio [defined as RB2H6 = (FB2H6/FSiH4)×100%]. The material properties of these films are studied using micro-Raman spectroscopy, low angle X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV-visible spectroscopy, dark conductivity measurements etc. The correlation between RB2H6 and resulting material properties such as crystalline volume fraction, crystallite size, band gap and hydrogen content has been established. We have obtained high band gap (~ 2.48 eV) p-nc-Si:H films having dark conductivity (~ 0.6 S/cm) with low hydrogen content (~ 1.8 at. %) at high deposition rate (~19.2 A/s). The employment of these films in a-Si:H based p-i-n solar cell as a p-type window layer could have low absorption losses thereby enhancing the current density which i...

Synthesis of Hydrogenated Nanocrystalline Silicon Films By HW‐CVD Without Hydrogen Dilution of Silane

In this work we report synthesis and characterization of nc‐Si:H films by HW‐CVD method. The role of filament temperature (Tfil) in controlling the material properties has been carefully and systematically investigated. Characterization of these films with Raman spectroscopy and XRD revealed that increase in Tfil endorses the growth of crystallinity in the films. Furthermore, crystallites in the films have preferential orientation in (111) direction. The hydrogen content in the films shows decreasing trend with increase in Tfil but it remains <6 at. % over the entire range of Tfil studied. However, band gap remains as high as 2.1 eV. We attribute high band gap at low hydrogen content to increase in crystalline fraction with Tfil. From the present study we conclude that Tfil is a key process parameter in HW‐CVD to induce crystallinity in Si:H films without hydrogen dilution of Silane.