Properties of Amorphous Silicon Germanium Films and Devices Prepared Using Chemical Annealing

Abstract

<p>Amorphous silicon germanium solar cells have been extensively used for many years due to the low cost, easy fabrication, tunable bandgap and special properties. However, it is found that the properties of a-SiGe:H materials is not good as a-Si:H, and this limits the application of a-SiGe:H solar cells and brings the stability concern. Recently, it has been shown that this instability is correlated with the presence of multiple bonded Si-H bonds (i.e., SiH2), and a technique, namely chemical annealing, was suggested to improve the quality and the stability of a-SiGe:H. Although a number of results have been reported to produce good a-SiGe:H films but, no chemical annealed devices with good quality were reported and no systemic study was ever done on it. .</p> <p>In this work, chemical annealed a-SiGe:H films and devices, and non chemical annealed films and devices were produced in very high frequency plasma enhanced CVD, and systematic experiments were carried out to study the role of chemical annealing in enhancing the quality of a-SiGe:H solar cells.</p> <p>It is found that the materials were grown using a layer-by-layer approach, where the growth of a thin film was followed by a chemical anneal in hydrogen or helium plasma. Multiple cycles were used to build up the total film thickness. The purpose of the anneal cycle was to subject the material to controlled ion bombardment so as to reduce void density and thereby lead to better microstructure and fewer clustered Si-H and Ge-H bonds. FTIR measurements showed that the films which were prepared using chemical annealing had fewer SiH2 and GeH2 bonds. Electrical measurements showed that the films subjected to chemical anneal had a higher photo/dark conductivity ratio, a smaller Urbach energy and higher electron mobility-lifetime products. p-i-n and n-i-n devices were prepared on stainless steel substrates. Measurements of quantum efficiency in p-i-n devices showed that the hole mobility-lifetime product was also improved when chemical annealing was used.</p>