Nanostructure of a-Si:H and related alloys by small-angle scattering of neutrons and X-rays: Annual technical progress report: May 22, 1998 -- May 21, 1999

Abstract

The materials for analyses were supplied by NREL-supported device-making groups as well as by other groups with relevant expertise. Electron microprobe analyses of film compositions were carried out by Alice Mason of NREL. The co-P.I., David Marr, of the Chemical Engineering Department of the Colorado School of Mines, contributed to the research project via assistance in the preparation of the proposal to NIST for the small-angle neutron scattering experiments and via assistance in the design of the samples for these experiments. OBJECTIVES/APPROACHThe general objective of this research is to provide detailed microstructural information on the amorphous-silicon-based, thin-film materials under development for improved multijunction solar cells. Correlation of this microstructure with opto-electronic properties and device performance is an integral part of the research. The principal experimental techniques used are small-angle x-ray scattering (SAXS), small-angle neutron scattering (SANS), and conventional xray diffraction (XRD). These provide quantitative microstructural data on microvoid fractions, sizes, shapes, preferred orientations, hydrogen clustering, microcrystallinity, and medium-range order. An important task is to establish whether SANS can be used to determine the hydrogen nanostructure and any changes that might occur due to light soaking. Some experiments are conducted using anomalous SAXS (ASAXS) via special facilities in Germany. This method provides information on the Ge uniformity and nanostructure in a-SiGe:H alloys. Two types of material have been investigated during this first phase of the research in collaboration with the NREL Low-gap and Mid-gap/Metastability National Teams. Some of the films will be put in the light-soaked state prior to the SANS experiments so that SANS measurements followed by in-situ annealing and further SANS can be performed to look for any light-induced structural change.iv List of Tables INTRODUCTIONSeveral experiments have been completed during Phase I of this research project in collaboration with NREL and NREL-supported groups. The planned SANS studies have been delayed due to an unexpected shut-down of the research nuclear reactor at the NIST Center for Neutron Research in Gaithersburg, Maryland. However, the facility is again operational and SANS beam time has been awarded to us for three days beginning on August 9, 1999. A summary of the samples being prepared for this experiment will be presented below. The results and discussion of the in-house SAXS and XRD experiments will be broken into sections according to whether the materials are low-gap, mid-gap, and high-gap, consistent with the NREL team structure. We have not received any high-gap materials for study during Phase I.Several experiments were done with mid-gap materials. A systematic study of the mediumrange order (MRO) of PECVD and HWCVD a-Si:H films was done in collaboration with USSC, University of Oregon, and NREL. Discoveries were made on microcrystalline (µc)-Si:H formation in highly-hydro...