Reversible Ordering of a-Si 1-x Gex by the Combined Effect of Light and Temperature
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A4.39.1
Reversible Ordering of a-Si1-xGex by the Combined Effect of Light and Temperature P. Martín 1, A. Torres 1, J. Jiménez 1, A. Rodríguez 2, J. Sangrador 2, T. Rodríguez 2 1 Dpto. Física de la Materia Condensada, E.T.S.I.I., U. de Valladolid, Valladolid, Spain. 2 Dpto. Tecnología Electrónica, E.T.S.I.T., Universidad Politécnica de Madrid, Madrid, Spain.
ABSTRACT Amorphous Si and a-Si1-xGex (0 ≤ x ≤ 0.38) films illuminated by above bandgap light, and heated between 110 and 180 ºC, undergoes an ordering transition, which is reversed to the amorphous phase by either heating up above 180 ºC or cooling down below 110 ºC. This structural change is investigated by Raman spectroscopy. The changes observed in the Raman spectra reveal the formation of small ordered clusters (only a few lattice parameters in diameter). The ordered state was not observed by the only effect of either light or temperature, suggesting that specific temperature and light excitation conditions are necessary to generate and support the ordered state, that appears as a nano-crystalline structure.
INTRODUCTION Amorphous Si and SiGe thin films are interesting for diverse applications [1-3]. Amorphous films are sensitive to light, which can induce structural changes involving either crystallization [4] or order relaxation [5]. Amorphous Si exhibits different metastable states under illumination [6, 7], which influence its optical and electrical properties. The understanding of these structural changes is important for the applications of these films and their reliability in applications where the films are under light exposure. A big deal of attention has been paid to the metastable state induced by light soaking in aSi:H. The most studied effect is known as Staebler-Wronski [6], where mid-gap states associated with the formation of dangling bonds, at relatively low concentrations (1016-1017 cm-3) are formed. Other metastable effect reported in the literature consists of the structural transformation concerning a large number of atomic sites, which manifested as partially ordered Si [8]. These transformations were found to disappear by thermal annealing below 200 ºC. We present herein, a new light and temperature induced, reversible ordered state in amorphous Si and SiGe alloy films deposited by Low Pressure Chemical Vapor Deposition (LPCVD). The characteristics of this reversible state are different from those of the metastable effects previously reported in the literature [6, 8]. Small crystalline grains, a few nanometers in diameter, are observed for light intensities much smaller than the threshold for laser crystallization, and temperatures much lower that the ones needed for Solid Phase Crystallization (SPC). The ordered state reverts to amorphous, and vice versa, in a repetitive way once these conditions are off and on.
A4.39.2
EXPERIMENT Amorphous Si1-xGex films were deposited in an LPCVD reactor, under a constant flow of Si2H6 and GeH4 gas mixture. The substrates were silicon wafers with a 200 nm thick thermally grown oxide layer on b
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