Debabrata Das

Control of the crystallite size and passivation of defects in porous silicon by a novel method

Abstract Porous silicon films were prepared by lateral anodization of crystalline silicon in HF based solutions at different current densities. At an optimum current density, passivation of the defects by an appropriate post-anodization treatment results in the significant enhancement in the photoluminescence (PL) efficiency. However, above the optimum current level, a phase is obtained which shows significant broadening of the PL spectrum indicating the quantum wire size distribution. The degraded PL intensity in the treated samples is higher as compared to that for the as-anodized samples. Infrared vibrational studies indicate that this enhancement is due to the H-passivation of defects in the Si-pore interface, though the presence of hydrogen-terminated silicon clusters cannot be ignored. Capacitance–voltage studies concur well with the photoluminescence and infrared results.

Role of buffer layer at the p/i interface on the stabilized efficiency of a-Si solar cells

Abstract Light induced degradation of single junction and double junction a-Si:H solar cells has been studied. Cells with and without buffer layers at the p/i interfaces have been fabricated. It is found that light induced degradation is faster in the cells with buffer layers. Defect density increases faster and degraded efficiency with respect to the initial efficiency decreases at a higher rate for the cells with buffer layers. Spectral response study for double junction cells shows that collection efficiency decreases for the bottom cell only. So it is found that the absolute stabilized efficiency is highest for a double junction cell with buffer layer at the top cell only.

Improvement in the optoelectronic properties of a-SiO:H films

Hydrogenated amorphous silicon oxide (a-SiO:H) films prepared by rf plasma enhanced chemical vapour deposition (PECVD) method have recently proved their potential as a photovoltaic material for the fabrication of high efficiency multijunction amorphous silicon solar cells. If deposited under proper conditions, it may be a better wide band gap material than the normally used a-SiC : H. In this paper we report the improvements achieved over the previously reported results. The films have been characterized in detail in terms of their optoelectronic properties, structural characteristics, defect density and light induced degradation.

Efficient Boron Incorporation in Hydrogenated Amorphous Silicon Films by a Novel Combination of RF Glow Discharge Technique and Heated Filament

Boron doped hydrogenated amorphous silicon films were prepared by a novel method involving a combination of plasma chemical vapour deposition (PCVD) and a heated filament. The films were characterized by electrical conductivity measurements. A combination of Raman scattering experiment and infrared (IR) vibrational spectroscopy were performed to study the structural characteristics of the a-Si:H films. Higher conductivities were obtained for the films prepared under the heated filament in comparison to those for the films prepared away from it. It was inferred that growth conditions mainly governed by BH3 radicals led to highly conducting films. For conditions conducive to the production of more atomic hydrogen, SiH3 and BH2 radicals were expected to govern the growth and put the boron atoms into the inactive three-fold coordination.

Optoelectronic and Structural Properties of Good Quality Hydrogenated Amorphous Silicon Carbide Films Deposited by Hot Wire Assisted RF Plasma Deposition Technique

Hydrogenated amorphous silicon carbide (a-SiC:H) films were deposited by using a combination of radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) and heated filament techniques with the objective of improving the quality of the films due to the possible beneficial effect of the latter technique. The atomic hydrogen produced via electron (emitted from the filament) impact dissociation of the process gases plays a significant role in improving the properties of the film such as the structure and bonding configuration. The electrons emitted from the hot filament also help in dissociation of methane molecules into different types of radicals. From the characterization of the films thus produced it is seen that by the combination of the two methods of deposition under optimised condition carbon is incorporated more as a Si–C bond which is structurally better. These results in better opto-electronic properties at high band gap of a-SiC:H which also shows lower light induced degradation than those of the films produced by only using the RF PECVD method.

Study of effects of interelectrode spacing and preheating of source gases on hydrogenated amorphous silicon films prepared at high growth rates

The effect of interelectrode spacing on the properties of hydrogenated amorphous silicon (a‐Si:H) films grown at high radio‐frequency (rf) power density by rf plasma enhanced chemical vapor deposition method, with control of dusty plasma conditions by heating both the electrodes, was investigated. The formation of precursors responsible for gas phase polymerization itself was sought to be controlled by preheating of the source gas mixture. Optimization of the interelectrode spacing for film characteristics was carried out for this novel deposition technique combining cathode heating and preheating of the source gases. The films were characterized by infrared vibrational spectroscopy, absorption and reflection measurements in the visible and near infrared regions, measurements of dark and photo‐conductivity (with light induced degradation), and electron spin resonance spectroscopy.

Improved quality a-SiC : H films deposited by a combination of heated filament and rf plasma deposition technique

Abstract Hydrogenated amorphous silicon carbide (a-Sic : H) film was deposited by using a combination of radio frequency plasma-enhanced chemical-vapour deposition (rf-PECVD) and heated filament techniques with an aim of benefiting from the advantages of both. The atomic hydrogen, produced via electron (emitted from the filament) impact dissociation of hydrogen and silane plays a significant role in improving the film properties. Also at the hot coil, efficient dissociation of methane molecules takes place. Thus, more carbon atoms are incorporated into the films in the favourable configurations and hence increasing the optical gap of the films. The deposition parameters were optimised to obtain wide band gap highly photosensitive films. The films were evaluated by dark and photoconductivity measurements (with light-induced degradation), absorption measurements in the visible region, Fourier-transform infrared spectroscopic measurements X-ray photoelectron spectroscopy measurements and dual-beam photoconductivity measurements.

A simple modification of the magnetron sputtering method for the deposition of boron-doped hydrogenated microcrystalline silicon films with enhanced doping efficiency

Abstract Boron-doped hydrogenated microcrystalline silicon films were prepared by the r.f. magnetron sputtering technique with a conventional set-up as well as by a modified set-up. The modification was in the form of immersion of the substrate in the field of an additional U-magnet. Both in the conventional as well as in the modified set-up, the thresholds of chamber pressure for microcrystalline formation were investigated. The modification with the U-magnet resulted in the enhancement of the microcrystalline phase in the entire range of chamber pressure studied and, at an optimum pressure range, highly conducting films were obtained. At relatively high chamber pressure, boron crystal segregation was observed with concomitant deterioration in the crystalline structure of the films. The films were characterized by conductivity measurements, Hall effect and thermoelectric power measurements, optical absorption measurements, X-ray diffractometry, infrared vibrational spectroscopy, Raman spectroscopy and transmission electron microscopy.

Deposition of boron doped a-Si:H films by a novel combination of rf glow discharge technique and filament heating: Enhancement of doping efficiency

Abstract A novel variation of plasma enhanced chemical vapour deposition in the form of insertion of a heated filament in the plasma was applied to a SiH 4 +1% B 2 H 6 (in H 2 )+H 2 gas mixture for the deposition of boron doped hydrogenated amorphous silicon thin films. Current levels giving filament temperatures below and above the thermionic emission level of the filament material were used to separate the effect of thermal dissociation of B 2 H 6 from that of the production of atomic hydrogen in combination with B 2 H 6 dissociation. Control runs without filament heating but accounting for rise in substrate temperature because of the heated coil were also carried out. The films were characterised by electrical and optical measurements, infrared vibrational spectroscopy and secondary ion mass spectroscopy. A significant enhancement in the doping efficiency of boron was obtained by this technique and thus was found not to be an artifact of increase in substrate temperature. Nor could it be attributed to the effect of atomic hydrogen only.

Influence of Chamber Pressure on Optoelectronic and Structural Properties of Boron-Doped Hydrogenated Silicon Films Prepared by RF Magnetron Sputtering

Results on the characterisation of boron-doped hydrogenated silicon films prepared by rf magnetron sputtering technique are presented. The effects of chamber pressure on the structure and electronic properties of the films were investigated. The films were characterised by conductivity and thermoelectric power measurements, optical absorption and reflectance (specular and diffuse) measurements, X-ray diffraction analysis, infrared vibrational spectroscopy, Raman spectroscopy and scanning electron microscopy. The structural characteristics were correlated with the conductivity and thermoelectric power. With increase in pressure the growth pattern changed from an amorphous one, with poor structural and optoelectronic properties, to one which was crystalline, with improved structural ordering. In the intermediate range of pressure growth was mainly expected to be governed by BH 3 radicals. With further increase in pressure, segregation of boron atoms with concomitant deterioration in crystalline structure was observed. The features were however distinct from those of the amorphous films obtained at low pressures probably due to the increased role of atomic hydrogen at higher pressures.