Partha Chaudhuri

Effect of argon dilution on the structure of microcrystalline silicon deposited from silane

The structure of the hydrogenated amorphous and microcrystalline silicon films deposited by radio‐frequency (rf) glow discharge decomposition of silane diluted in argon has been studied by transmission electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy techniques. It has been observed that argon acts not only as a passive diluent gas but also plays an important role in the growth of the amorphous or microcrystalline network. Calculation of the variation of equilibrium concentration of Ar*(3P0,2) and Ar+ with argon dilution shows that at high argon dilution (≳90%) equilibrium concentration of Ar* rises sharply with increase in argon dilution. Variation of structural properties with argon dilution suggests involvement of these excited states of argon in the growth process. A model has been proposed based on the energy exchange between the Ar* states and the growth zone of the materials to explain the structural changes observed in the presence of argon in the plasma.

Nanostructures and defects in silicon–hydrogen alloys prepared by argon dilution

Abstract Nanostructural heterogeneity of silicon–hydrogen (Si:H) alloy films deposited in a conventional radio frequency plasma-enhanced chemical vapour deposition unit from silane argon mixture has been studied by small-angle X-ray scattering (SAXS). The densities of defect states of Si:H films have been estimated by dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS) and the modulated photocurrent (MPC) method. From the structural and defect studies we identify two regions of Ar dilution, where the structure of the films are distinctly different. Up to 90% Ar dilution, the nanostructural as well as the large-scale (>30 nm) structural heterogeneities in the amorphous Si:H (a-Si:H) network decrease. A lowering of the bulk defect density has also been observed in this Ar dilution region. Increasing Ar dilution to greater than 90% of the mixture, the a-Si:H films show some dense regions embedded in the amorphous matrix. The high- and low-density amorphous structures within the films can explain the experimental results obtained from SAXS, DBP and PDS. A negligible conduction band tail, as observed from MPC result, suggests the formation of high degree of crystallinity in the film deposited with 99% Ar dilution and higher rf power density (80 mW/cm 2 ).

Control of Microstructure and Optoelectronic Properties of Si:H Films by Argon Dilution in Plasma-Enhanced Chemical Vapor Deposition from Silane

Structural and optoelectronic properties of thin films of silicon-hydrogen binary alloy (Si:H) deposited from silane and argon mixture in a rf glow discharge plasma have been studied for different argon dilutions and rf powers. It has been observed that with low rf power density ( 30-70 mW/cm3) increase of argon dilution up to 95% reduces the microstructure in the films, as determined from IR absorption spectra. Simultaneously, increase in refractive index and decrease in ESR spin density have been observed. Above 95% argon dilution or with higher rf powers, transmission electron microscopy (TEM) studies reveal a dominance of the columnar growth mechanism, and the optoelectronic properties of the films deteriorate. At 99% argon dilution, microcrystallites appear to form within columnar regions. Addition of a small amount of hydrogen to the silane-argon plasma improves the network significantly, which is manifested by the changes in the dark conductivities in the different rf power regimes.

Properties of undoped and p-type hydrogenated amorphous silicon carbide films

The properties of undoped and p-type hydrogenated amorphous silicon carbide (a Si C:H) films were studied. Thea Si C:H films were prepared under different deposition conditions by the r.f. glow discharge decomposition of gas mixtures of silane and methane in an inductively coupled system. The p-typea Si C:H films were prepared from mixtures of silane, methane and diborane gases. The dark conductivity, photoconductivity, optical absorption, band gap and spectral response of these films were studied. By analysing the dark and photoconductivity data, information about the transport mechanism and the recombination processes in these films was obtained. It is suggested that there is possibly a significant change in the structure ofa Si C:H films with an increase in the carbon concentration that influences various properties of the material.

The influence of deposition parameters on the properties of amorphous silicon thin films produced by the magnetron sputtering method

The influence of various deposition parameters on the properties of amorphous silicon (a-Si) thin films produced by r.f. magnetron sputtering was studied. The following parameters were varied: the incident r.f. power, the argon pressure in the sputtering chamber and the substrate temperature. The films were characterized by studying the dark conductivity and optical absorption from which the values of the band gaps were obtained. The most interesting result was that a-Si films prepared at high argon pressures showed significant photoconductivity. The results show that the magnetron sputtering method has good potential for producing high quality a-Si films.

The effect of mixing hydrogen with silane on the electronic and optical properties of hydrogenated amorphous silicon thin films

The electronic and optical properties of a-Si:H films prepared by the r.f. glow discharge method from a mixture of SiH4 and H2 were studied. The effects of variation in the concentration of H2 in the mixture were also investigated. The dark conductivity of a-Si:H films reaches a minimum at a low concentration of H2 and then increases with increasing concentration of H2. In addition, the photoconductive gain shows a maximum at a low concentration of H2. The band gap, however, remains virtually constant up to 25% H2 and then increases significantly as the H2 concentration is increased further. The activation energy increases slowly with H2 concentration up to 25% and then decreases with a further increase in the H2 concentration. IR vibrational spectra show an increase in the amount of Si—H bonding as the H2 concentration increases to 25% and then a decrease with a further increase in the H2 concentration.

Hydrogenated amorphous silicon films with low defect density prepared by argon dilution: application to solar cells

Abstract Structural study of several amorphous silicon (a-Si:H) films deposited by rf-PECVD from a silane–argon mixture have revealed that the 90% argon diluted sample has the lowest microstructure and defect density. This sample has been further compared with a-Si:H layer deposited from undiluted silane by using each of them as the intrinsic (i) layer in p–i–n solar cells. The output characteristics of these devices have been studied experimentally and by theoretical modeling. These studies demonstrate that the improvement in the stabilized output characteristics of the cell with argon diluted i-layer results both from the structural improvement of the a-Si:H layer, as well as a reduction of the p/i interface defects.

Correlation of Nanostructural Heterogeneity and Light Induced Degradation in a-Si:H Solar Cells

The small angle X-ray scattering (SAXS) method was adopted to estimate the nanostructural heterogeneity in hydrogenated amorphous silicon (a-Si:H) materials deposited by rf plasma enhanced chemical vapor deposition (PECVD) process from silane-argon mixtures at different volume ratios. The performance of the solar cells fabricated by using the same materials as the intrinsic layer has been correlated with the integrated SAXS intensity of the intrinsic layer. The change in the density of states due to light soaking has been measured in solar cell structure by a dual beam photoconductivity method. We have observed a systematic increase in the photoinduced degradation of the photoconductivity, defect density and the solar cell parameters with the increase in the structural heterogeneities in the film. Modification of the growth kinetics due to bombardment of the metastable argon (Ar*) has been found to control the amount of nanostructural heterogeneity in the material.

Performance dependence of a-Si/a-Si double junction solar cell on the quality of the a-Si:C:H buffer layer at p+-Si:C:H/i-a-Si:H interface

Abstract The use of a highly photoconductive a-SiC:H buffer layer at the p+-SiC:H and i-Si:H interface of an a-Si/a-Si double junction tandem cell results in a high value for the fill factor of the cell. It has been observed that the collection efficiency of the bottom cell measured under a forward bias condition greatly depends upon the quality of the a-SiC:H buffer layer. With no a-SiC:H buffer layer at p+-SiC:H and i-Si:H junction or with an inferior quality a-SiC:H buffer the collection efficiency of the bottom cell has been observed to decrease at any wavelength. With a highly photoconductive a-SiC:H buffer layer the collection efficiency of the bottom cell below a certain wavelength has been found to be maximum at a positive bias voltage corresponding to the maximum power point of the cell. Enhancement of the bottom cell collection efficiency at short wavelength on application of forward bias has been explained by injection of photogenerated carriers in the a-SiC:H layer into the bottom cell i-layer.

Electronic and optical properties of boron doped hydrogenated amorphous silicon thin films

Abstract The electronic and optical properties of p-type a-Si:H films prepared by the rf glow discharge decomposition of a mixture of silane and diborane gases have been studied. The films have been prepared under different conditions which include variation of volume ration of B 2 H 6 and SiH 4 and rf power. The properties of compensated a-Si:H films prepared with very small boron doping have also been studied. The effects of mixing Ar with SiH 4 + B 2 H 6 mixture have been investigated. The properties actually studied include (1) dark conductivity, (2) steady state photoconductivity, (3) spectral response, (4) optical absorption and band gap. Attempts have been made to analyse the data to yield information about the transport mechanism, and the existence of hole and electron traps.