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OF SILICON OXYNITRIDE

FILMS

T. FISCHER, T. MUSCHIK, R. SCHWARZ, D. KOVALEV, AND F. KOCH Technical University of Munich, Physik-Department E16, D-85747 Garching, Germany ABSTRACT Silicon oxynitride films, deposited by chemical vapour deposition using a high He dilution are investigated with respect to their photoluminescence properties. At low temperatures the PL spectra, which show broad maxima around 2 eV, consist of two contributions with different decay times: a ns-fast and a ms-slow component, the slow one disappearing above 80 K. To account for these findings and for absorption measurements, a model is proposed to describe the structure of the samples consisting of inclusions of Si rich regions in an a-SiONy:H matrix with a common molecular aggregate as luminescence center. The different PL lifetimes supposedly result from different feeding mechanisms. Such a behaviour may be present in oxidized porous Si as well. INTRODUCTION Two major sources of motivation could presently be seen for investigations of the light emission from amorphous silicon compounds: firstly, the search for suitable materials in the Si thin film optoelectronics technology and secondly the still ongoing discussion about an understanding of the various luminescence bands in porous silicon (PS). Basic research on the photoluminescence (PL) of alloys with C, N, and 0 dates back to about 15 years and can be found for example in the studies of Sussmann and Ogden for C [1], of Street and Knights for 0 [2], and of all three elements in that of Carius et al. [3]. An a-SiN:H LED, has been presented by Boonkosum et al. [4].

The implication of such work to the study of the luminescence mechanism in PS arises from the fact that many authors attribute the fast, blue luminescence band of PS to radiative recombination of carriers in bulk SiO. [5] or via some specific states of the Si-SiO, interface [6, 7], or to a not further specified interaction of Si inclusions with SiO [8, 9]. To discuss a possible occurrence of Si domains in chemical vapour deposited (CVD) SiO, on the basis of PL data is the major intent of this paper. Similar systems have already been investigated in published work of Bustarret et al. [10] and Zacharias et al. [11] which do not emphasize the fast PL bands considered here. METHODS OF SAMPLE PREPARATION AND MEASUREMENTS The samples have been deposited in a conventional capacitively coupled rf plasma CVD reactor with a grounded substrate electrode. For the oxynitride depositions we chose the flows of the source gases SiH 4 , N2 0 and He to be 3.5, 7, and 50 sccm, respectively. The pressure has been kept at 0.5 mbar, the substrate temperature at 270°C and the rf power density at 300 mW/cm 2 resulting in a deposition rate of 7 A/sec on synthetic quartz substrates. Forming gas anneals of 1/2 h have been performed at the deposition temperature, which should only affect H bonding and limit further structural change. Continuous wave PL characterization was done with an Ar+ laser at 3.5 eV excitation energy. For the time resolved measurem