Visible Luminescence from Silicon: Quantum Confinement or Siloxene ?
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VISIBLE LUMINESCENCE FROM SILICON: QUANTUM CONFINEMENT OR SILOXENE ?
M.S. BRANDT, H.D. FUCHS, A. HOPNER, M. ROSENBAUER, M. STUTZMANN, J. WEBER, M. CARDONA, AND H. J. QUEISSER Max-Planck-Institut ffir Festk6rperforschung, Heisenbergstr. 1, D 7000 Stuttgart 80, Germany ABSTRACT The discovery of strong visible photoluminescence at room temperature from porous silicon has triggered new hope that light-emitting devices compatible with existing Sitechnology might become possible. We first review the luminescence behavior observed in silicon-based materials such as amorphous Si, microcrystalline Si, or SiO 2. We then critically discuss the present model for the luminescence from porous silicon based on quantum confinement in view of the growing experimental evidence for the importance of both hydrogen and oxygen to obtain efficient luminescence from this material. We propose an alternative explanation based on the presence of siloxene (Si 6 0 3 H6) in porous silicon which is corroborated by experimental results obtained with photoluminescence, Raman and IR spectroscopy. An important aspect is that siloxene can be prepared by methods different from anodic oxidation, and one particular technique will be described together with possible ways to tune the luminescence energy. INTRODUCTION The development of active optoelectronic devices compatible with Si technology has been an important challenge during the last decade. Undoped crystalline silicon (c-Si) with its indirect bandgap shows a weak luminescence at 1.1 eV due to excitonic recombination [1]. This luminescence can be electrically excited in a pn-diode when operated in breakdown at high reverse bias voltages, still the external quantum efficiency, 71, is well below 10-' [2]. Various possibilities to achieve stronger luminescence have been explored, and some of the results are given in Table 1 which summarizes the spectral positions and the approximate room temperature efficiencies of the observed luminescence. A possible approach has been the introduction of specific luminescence centers in crystalline silicon. This has been shown for carbon implanted samples which, after electron irradiation, exhibit a strong electroluminescence at 0.95 eV [7]. At 77 K, the luminescence intensity is comparable to that of N-doped GaP, also an indirect band gap material which is used for commercial fabrication of green light emitting diodes. Unfortunately, the luminescence in C-doped Si has not been observed at higher temperatures. In erbium implanted Si, the characteristic intraatomic transition of Er3+-ions is observed at ; 0.8 eV [8]. This luminescence, which would be suitable for silica-fiber-based optical communication systems, however, disappears at temperatures above 200 K.
Mat. Res. Soc. Symp. Proc. Vol. 262. 01992 Materials Research Society
850
Material
Mechanism/Preparation
Position (300 K) I
c-Si
exciton, electron-hole-liquid Auger luminescence centers C implanted Er implanted
1.1 eV 2.2 eV 0.49 - 1.15 eV 0.95 eV 0.8 eV
tIc-Si
rf sputtered in Si0 2-matrix post-hyd
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