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VISIBLE LUMINESCENCE FROM POROUS SILICON AND SILOXENE: RECENT RESULTS H.D. FUCHS, M. ROSENBAUER, M.S. BRANDT, S. ERNST, S. FINKBEINER, M. STUTZMANN, K. SYASSEN, J. WEBER, H.J. QUEISSER, AND M. CARDONA Max-Planck-Institut fur Festk~rperforschung, Heisenbergstr. 1, D-7000 Stuttgart, Germany ABSTRACT The optical properties of porous Si (p-Si) are compared to those of siloxene and its derivatives in order to gain more insight into the mechanism of the luminescence observed in p-Si. We report new results of photoluminescence (PL), photoluminescence excitation (PLE), time-dependent and pressure-dependent photoluminescence, and optically detected magnetic resonance (ODMR). Important information about the structural, electronic, and microscopic nature of the two classes of materials are deduced from these experiments. Annealed siloxene and p-Si show very similar properties, suggesting that siloxene-related structures, e.g. electrically isolated Sis-rings, might be responsible for the luminescence in p-Si. The Si-planes in as-prepared siloxene, with their green luminescence, are metastable and are readily oxidized into red-luminescent siloxene configurations. INTRODUCTION The strong, visible room-temperature luminescence of porous silicon (p-Si) has been reproduced by many groups around the world and has attracted considerable interest, as reflected in the present conference proceedings. There is an increasing effort to utilize the pronounced luminescence properties of p-Si in a variety of technological applications. However, the physical mechanism of the luminescence still remains a matter of controversy. The different models proposed in the past include quantum confinement in small silicon crystallites or wires,[1, 2] hydrogen passivated silicon surfaces,[3] amorphous silicon,[4] and siloxene, Si6O3H6[5, 6]. A year ago at the 1991 MRS Fall Meeting, we have proposed siloxene and its derivatives to be the origin of the luminescence in p-Si.[5] In this paper we follow up on this hypothesis and present a more detailed study of the optical properties of siloxene compared to those of p-Si. We investigate, in particular, effects of annealing on siloxene. In all experiments reported here, annealed siloxene is similar to p-Si, whereas as-prepared siloxene exhibits properties different from p-Si. SAMPLE PREPARATION A variety of p-Si samples were investigated in this work. Some of the samples originated from other groups (# 1,2, and 4), others (# 3 and 5) were prepared by us according to the procedure described in the literature.[7] Siloxene has been synthesized from CaSi 2 powder as described previously.[5, 6] Recently, we used an alternative method to grow a luminescent film of siloxene on a crystalline silicon substrate. This procedure is based on the chemical transformation of a calcium disilicide film deposited on the Si wafer.[8] This approach is promissing because of its possible compatibility with existing silicon technology. Prior to the measurements reported here, all samples were characterized by Raman and Fourier-transfo