A. Bhaduri

Structural and optoelectronic properties of silicon germanium alloy thin films deposited by pulsed radio frequency plasma enhanced chemical vapor deposition

Hydrogenated amorphous silicon germanium (a-SiGe:H) alloy films are still under study to improve their incorporation into tandem solar cells. In this paper we have investigated the quality of films deposited in a radio frequency powered plasma enhanced chemical vapor deposition unit. Two series of samples were prepared either from a mixture of silane and germane diluted into hydrogen or from the same mixture to which a small amount of argon was added. The applied rf power at 13.56 MHz was either continuous or modulated by a square wave pulse at a frequency of 1356 kHz. Different films were deposited in the two series controlling the time the plasma was “on.” The ratio of the on time to the total period of the modulation, the duty cycle, was varied between 100% (continuous mode) and 50%. The plasma during deposition as well as structural and optoelectronic properties of the resulting films was studied with several techniques. We found an optimum in the transport properties for a duty cycle of 75%: an ambip...

Deposition of ultraviolet photoconductive films of amorphous hydrogenated carbon

The structural, optical, and transport properties of hydrogenated amorphous carbon films deposited on the grounded electrode under different dilutions (pure CH4, CH4+H2, and CH4+Ar) in radio frequency powered plasma enhanced chemical vapor deposition process have been studied. Though all the samples deposited were observed to be polymer-like carbon, they have different optical and transport properties. While some of the samples were highly conductive under dark (∼10−1Scm−1 at room temperature), others exhibited very low (<10−12Scm−1) dark conductivity and were photoconductive (∼10−9–10−10Scm−1) only under ultraviolet (UV) light (wavelength λ<250nm). There appears to be a close link between the transport properties and optical absorption. Based on optical absorption, we propose two possible models for the distribution of the density of states involving the π and σ states. The evolution of these states and of the deep defect absorption has a profound impact on the transport properties and particularly on th...

Aluminum recycling from reactor walls: A source of contamination in a-Si:H thin films

In this article, the authors investigate the contamination of hydrogenated amorphous silicon thin films with aluminum recycled from the walls and electrodes of the deposition reactor. Thin films of hydrogenated amorphous silicon were prepared under various conditions by a standard radio frequency plasma enhanced chemical vapor deposition process in two reactors, the chambers of which were constructed of either aluminum or stainless steel. The authors have studied the electronic properties of these thin films and have found that when using an aluminum reactor chamber, the layers are contaminated with aluminum recycled from the chamber walls and electrode. This phenomenon is observed almost independently of the deposition conditions. The authors show that this contamination results in slightly p-doped films and could be detrimental to the deposition of device grade films. The authors also propose a simple way to control and eventually suppress this contamination.

Powder evolution at low powers in silane-argon discharge

Powder formation in a 13.56-MHz radio frequency (rf) capacitive glow discharge plasma of silane-argon mixture has been studied by in situ laser light-scattering measurements. The rf power density (Prf) was varied from 18to53mW∕cm2. At high Prf the light scattering occurs all along the discharge and extends even beyond the exit end of the electrodes toward the pumping system. With decreasing Prf the maximum intensity of the light scattering decreases and the scattering zone shrinks and moves toward the exit end. With Prf≈20mW∕cm2 a very bright scattering zone only a few centimeters wide appears located at the electrodes outlet. The powders studied by transmission electron microscopy did not show a drastic decrease of their sizes with Prf though clear coagulation of small particles is observed at high Prf. In this paper we have tried to link the laser light-scattering evolution with Prf to various parameters such as the microstructure factor, the deposition rate, the electron mobility×lifetime product, the ...