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HYDROGEN STABILITY IN HYDROGENATED AMORPHOUS SILICON-BASED ALLOYS W. BEYER, J. HERION, H. WAGNER AND U. ZASTROW Institut fur Schicht- und Ionentechnik (ISI-PV), D-5170 Jilich, Germany

Forschungszentrum Jilich,

ABSTRACT The thermal stability of hydrogen in amorphous silicon-based alloy films was studied by deuterium/hydrogen interdiffusion and hydrogen effusion experiments. Depending on the film structure, hydrogen stability is limited by hydrogen surface desorption or hydrogen diffusion. The hydrogen surface desorption energy is found to decrease with rising germanium content and to increase with rising nitrogen and carbon content. At T = 400*C, hydrogen diffusion is found to proceed in the germanium subnetwork for a-SiGe alloys and in the silicon subnetwork for a-SiN and a-SiC alloys.

INTRODUCTION The thermal stability of hydrogen in amorphous semiconductors is known to be limited by surface desorption or diffusion processes depending on the microstructure of the material. While hydrogen diffusion and desorption are well documented for hydrogenated amorphous silicon (a-Si:H) [1,2,3] and germanium (a-Ge:H) (4], data for hydrogenated amorphous silicon-based alloys are still quite limited. Here we report on results of hydrogen stability measurements on a-Si-Ge, a-Si-N and a-Si-C alloys, which are all of interest for the application in tandem solar cells and other electronic devices. We employ hydrogen evolution and secondary ion mass spectroscopy (SIMS) measurements to determine the hydrogen diffusion coefficient. Hydrogen evolution is also used to study hydrogen desorption.

EXPERIMENTAL The alloy films investigated were prepared by rf plasma deposition in a capacitive (diode) reactor employing undiluted and hydrogen (deuterium)diluted SiH4 , SiD4 , GeH4 , GeD4 , NH3 , ND3 and CH4 gases at a rf frequency of 13.56 MHz, an rf power of 10 W for the a-Ge-Si alloys and 25 W for a-Si-N and a-Si-C alloys, a pressure of about 1 mbar and a flow of 0.5 - 10 sccm. The deposition rate was 1-5 R/s and the typical film thickness was ltm. The film composition was measured by electron probe microanalysis and hydrogen bonding to the host material was studied by infrared absorption. The hydrogen (deuterium) diffusion coefficient was evaluated from SIMS hydrogen and deuterium interdiffusion profiles in annealed sandwich structures of hydrogenated and deuterated material. For the SIMS experiment an oxygen (02+) sputtering beam at an energy of 9 key was used. For the hydrogen evolution, the samples were heated in a turbomolecular-pumped vacuum at a heating rate of 20K/min and desorbing gases were measured using a quadrupole mass analyzer. For compact amorphous material, hydrogen evolution (effusion) experiments yield information about the hydrogen out-diffusion while for films with an interconnected void network hydrogen surface desorption kinetics can be evaluated [5]. In this paper we restrict ourselves to results from undoped material. We noticed, however, that upon doping of a-Si based alloys similar effects as in doped a-Si:H oc