Tapati Jana

Changes in electric and optical properties of intrinsic microcrystalline silicon upon variation of the structural composition

Device-grade microcrystalline silicon prepared by plasma enhanced chemical vapor deposition is investigated with respect to its structural, electronic and optical properties in order to identify material parameters with relevance to the solar cell efficiency. All sample series show a similar increase of dark conductivity with increasing crystalline volume fraction, suggesting a close relationship between electrical transport and the structural details of the material. Conditions yielding the highest solar cell performance, i.e. close to the transition to amorphous growth, are characterized by the lowest dark conductivity values together with the maximum photosensitivity in the whole crystalline range.

Transition from amorphous to microcrystalline Si:H: effects of substrate temperature and hydrogen dilution

Abstract Structural changes in silicon films with variations of hydrogen dilution and substrate temperature have been studied. At a critical value of hydrogen dilution in silane (φ) a sharp transition from amorphous to microcrystalline phase has been observed. An increase of substrate temperature from 170 to 340 °C shifts the transition point from φ=95.5% to 93.5%. The change in dark conductivity and absorption corroborate with the change in crystalline volume fraction in the films. The grain size varies from 45 to 360 A depending on deposition conditions. Optical absorption and hydrogen content in the film decreases drastically with formation of microcrystalline structure. Silicon films developed at 340 °C show moderate photosensitivity together with low light induced degradation at low crystallinity, which might be suitable properties for solar cell application.

Transparent polymer and diamond-like hydrogenated amorphous carbon thin films by PECVD technique

Optical, electrical and structural properties of hydrogenated amorphous carbon films deposited by radio frequency plasma enhanced chemical vapour deposition technique (RF-PECVED) have been investigated thoroughly. The films were deposited using a mixture of hydrogen and acetylene gases at a low substrate temperature of 200 °C. The electrical resistivities of the films are ≥ 109 Ω cm. The ratio of hydrogenated sp3 to sp2 bonded carbon atoms in the films is obtained by deconvoluting the infrared spectra over the 2800 cm−1–3100 cm−1 range. Polymer-like hydrogenated amorphous carbon films are grown at a low power (50 mW cm−2) having a high hydrogen content (30.3–46.1 at%). Increase in hydrogen dilution increases the total hydrogen content as well as the ratio of hydrogenated sp3 to sp2 carbon bonding and it becomes a maximum at an optimum hydrogen dilution. Similarly the grain density also increases with the increase in hydrogen dilution. On the other hand, at a higher RF power of 150 mW cm−2 much stronger C–C sp3 and C–C sp2 bonds are formed and the total hydrogen content drastically reduces to a very low value of 1.1–1.8 at%. This indicates the growth of hydrogenated diamond-like carbon (DLC) structure. The stronger C–C sp3 bonds present in the DLC film make the films harder as well as Raman active.

Silicon oxide thin films prepared by a photo-chemical vapour deposition technique

Wide band gap a-SiOx:H films have been prepared by the photochemical decomposition of a SiH4, CO2 and H2 gas mixture. Deposition parameters namely the CO2 to SiH4 gas flow ratio, H2 dilution and chamber pressure were optimized in order to achieve highly photoconducting (1 × 10-6 S cm-1) films with an optical gap of 1.99 eV. The optical gap was found to increase with an increase in the CO2 to SiH4 flow ratio. A decrease in the photoconductivity, refractive index, spin g-value and a simultaneous increase in the spin density are attributed to an incorporation of oxygen into the films. Upon hydrogen dilution the photoconductivity of a-SiOx:H films was observed to improve along with an increase of the optical gap. The spin density of a-SiOx:H films was of the order of 1017 cm-9. The optoelectronic properties of the films have been correlated with the bonding configurations in the film, deposition parameters and the growth kinetics.

Microcrystalline silicon phase in silicon oxide thin films developed by photo-CVD technique

Abstract Intrinsic and p -type μc-Si/a-SiO x :H films have been developed by photochemical vapor deposition using SiH 4 , CO 2 , B 2 H 6 and H 2 gases. The level of boron doping [ f =B 2 H 6 /(SiH 4 +CO 2 )] and hydrogen dilution [ y =H 2 /(SiH 4 +CO 2 )] significantly affected the structural and optoelectronic properties of the material. The effect of chamber pressure was also studied. The crystalline planes of silicon were identified by X-ray diffraction and transmission electron microscopy. No evidence of crystalline silicon oxide was observed, i.e. oxygen incorporation occurred only in the amorphous phase. The fraction of crystallinity in the films was calculated from Raman spectra. Boron incorporation increased the conductivity, but beyond a certain level it decreased due to deterioration in the crystallinity. By the optimization of deposition parameters, high conducting (2.2×10 −1 Scm −1 ) p -type microcrystalline silicon–oxygen alloy films were developed for an optical gap ( E 04 ) of 1.95 eV. This would be useful for a window layer or the tunnel junction of solar cell.

Development of low temperature silicon oxide thin films by photo-CVD for surface passivation

Low temperature (250°C) silicon oxide (a-SiOx:H) films have been developed for surface passivation as well as antireflection coating in silicon solar cell. Films have been fabricated by ion damage free photochemical vapor decomposition technique using SiH4, N2O, and H2 gas mixture. In this paper we have reported the effect of N2O to SiH4 ratio (R) on optoelectronic and structural properties of the films. The bonding configurations of Si and H were investigated in detail by IR absorption measurement. The Si–H stretching mode supports the presence of H–(Si3-nOn)(n=0–3) structural unit, which is also present in the Si–O stretching mode. Developed silicon oxide films also have been studied on the c-Si solar cell. A substantial enhancement (11.2%) in efficiency has been achieved.Low temperature (250°C) silicon oxide (a-SiOx:H) films have been developed for surface passivation as well as antireflection coating in silicon solar cell. Films have been fabricated by ion damage free photochemical vapor decomposition technique using SiH4, N2O, and H2 gas mixture. In this paper we have reported the effect of N2O to SiH4 ratio (R) on optoelectronic and structural properties of the films. The bonding configurations of Si and H were investigated in detail by IR absorption measurement. The Si–H stretching mode supports the presence of H–(Si3-nOn)(n=0–3) structural unit, which is also present in the Si–O stretching mode. Developed silicon oxide films also have been studied on the c-Si solar cell. A substantial enhancement (11.2%) in efficiency has been achieved.

Optoelectronic and Structural Properties of Undoped Microcrystalline Silicon Thin Films: Dependence on Substrate Temperature in Very High Frequency Plasma Enhanced Chemical Vapor Deposition Technique

The effects of substrate temperature on optoelectronic and structural properties of undoped microcrystalline silicon thin films have been investigated. The undoped silicon films have been deposited by the very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) technique using a SiH4 and H2 gas mixture at 105 MHz plasma excitation frequency and moderately low power density of 70 mW/cm2. The effect of the systematic variation of substrate temperature (from 180°C to 370°C) on film properties has been studied, while keeping the other parameters constant. The deposition rate is considerably high (2.6 As-1) at 180°C and remains almost constant over the whole temperature range. Dark conductivity for all the films lies around ~10-6 Scm-1. Low subband gap absorption has been observed by photothermal deflection spectroscopy for these microcrystalline films. Increase of substrate temperature improves the microcrystallinity of the film, which is confirmed from structural studies. Crystalline grain size also increases with increase of substrate temperature and a maximum of 460 A has been achieved at 370°C. A satisfactory correlation is observed among the results of different structural studies: Raman spectroscopy, infrared spectroscopy, X-ray diffraction, transmission electron microscopy and atomic force microscopy.

Boron-doped a-SiOx:H films prepared by photo-CVD technique

Abstract The optoelectronic and structural properties of p-type a-SiOx:H films have been studied. The deposition parameters e.g. chamber pressure and diborane to silane ratio are optimized to get a film with dark conductivity (σd) 7.9×10−6 S cm−1 and photoconductivity 9.3×10−6 S cm−1 for an optical gap (E04) of 1.94 eV. The decrease of optical gap accompanied by the increase of conductivity is due to less oxygen incorporation in the film, which is substantiated by the decrease of the intensity of SiO absorption spectra. The properties are very much effected by the chamber pressure and diborane to silane ratio.

Development of p-type microcrystalline silicon carbon alloy films by the very high frequency plasma-enhanced chemical vapor deposition technique

We developed p -type μc-silicon carbon alloy thin films by the very high frequency plasma-enhanced chemical vapour deposition technique using a SiH 4 , H 2 , CH 4 , and B 2 H 6 gas mixture at low power (55 mW/cm 2 ) and low substrate temperatures (150–250 °C). Effects of substrate temperature and plasma excitation frequency on the optoelectronic and structural properties of the films were studied. A film with conductivity 5.75 Scm −1 and 1.93 eV optical gap ( E 04 ) was obtained at a low substrate temperature of 200 °C using 63.75 MHz plasma frequency. The crystalline volume fractions of the films were estimated from the Raman spectra. We observed that crystallinity in silicon carbon alloy films depends critically on plasma excitation frequency. When higher power (117 mW/cm 2 ) at 180 °C with 66 MHz frequency was applied, the deposition rate of the film increased to 50.7 A/min without any significant change in optoelectronic properties.