Influence of Plasma Chemistry on the Properties of Amorphous (Si,Ge) Alloy Devices

Abstract

We report on the growth and properties of a-(Si,Ge):H films and p-i-n solar cell devices prepared using a remote, low pressure ECR plasma deposition technique. The films and devices were prepared using either He or H 2 as the diluent gas. The plasma conditions were controlled so as to induce significant ion bombardment during growth. We find that there is a dramatic influence of plasma chemistry on the growth and properties of a-(Si,Ge):H films and devices. In particular, with hydrogen as the diluent gas, changing the pressure in the reactor dramatically changes both the Germanium incorporation in the film, and the electronic properties. Lower pressures lead to less Ge being incorporated, and higher mobility-lifetime product for holes for a given Tauc gap, as well as better p-i-n devices. In contrast, changing the pressure when He is the diluent gas does not produce such large changes. We speculate that the changes in device and film properties are due to the influence of ion bombardment on growth chemistry, and that both efficient energy and momentum transfer to the growing surface are necessary to achieve the best devices. The differences between He and hydrogen may simply be due to the fact that He plasma is much more energetic than a comparable hydrogen plasma, and there is more efficient momentum transfer when He is used as compared to when hydrogen is used. We have also produced very good single junction a-(Si,Ge) devices using the ECR technique.

Properties of a-(Si,Ge) films and devices deposited at high growth rates

“…Urbach energies for our films were slightly higher than those reported. It also seems that less Ge is incorporated at lower pressure as reported previously by Dalal et al [35].…”

Low bandgap α-(Si,Ge):H and α-(Ge):H devices

“…The growth chemistry is very important for the quality of the material [35][36][37][38][39][40][41]. A good growth mechanism will result in a better material.…”

“…If we use high hydrogen dilution, then reaction 1 and 4 are predominant and XH3 will be the major radical. Besides, hydrogen has other effects during the growth: passivate the dangling bonds, etch the surface hydrogen, break ihe weaker Si-Si and Ge-Ge bonds (etching during growth) etc [35][36][37][38][39][40][41].…”

“…It has been shown in Haroon's thesis [27] that low pressure will lead to better quality of the material because of higher ion bombardment. S. Haroon and V. Dalai [36][37][38] showed that the a-(Si,Ge):H films and devices grown at low pressure under hydrogen plasma had better quality. Optical and electronic properties , including bandgap, Urbach energy and moblity-lifetime for both electrons and holes were found to be greatly improved at low pressure.…”

Hydrogenated microcrystalline silicon germanium material and photovoltaic devices prepared using ECR PECVD

“…[ [46][47][48][49]. It is generally believed that the deposition process can be described in four steps: Since they are several different radicals formed in the plasma, the growth subsurface will be rough and forms many voids and dangling bonds [50].…”