High Efficiency Bulk Crystalline Silicon Light Emitting Diodes
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High Efficiency Bulk Crystalline Silicon Light Emitting Diodes Jianhua Zhao, Aihua Wang, Thorsten Trupke and Martin A. Green Centre for Photovoltaic Engineering, University of New South Wales, Sydney 2052, Australia ABSTRACT A high power conversion efficiency above 1% from a bulk crystalline silicon (c-Si) lightemitting diode (LED) has been demonstrated at near room temperature. These devices are based on normally weak one- and two-phonon assisted sub-bandgap light emission processes. Their improved performance results from device designs that take advantage of enhanced light absorption by a light trapping scheme which was developed for high efficiency silicon solar cells, and from reducing scope for parasitic non-radiative recombination within the diode. Each feature individually is shown to improve efficiency by a factor of ten, accounting for an improvement by factor of one hundred compared to baseline devices. Also demonstrated is a greatly improved band-edge light emission and detection using bulk c-Si diodes. A bulk c-Si LED is combined with a similar diode used as a detector that collects the light emitted with a high quantum collection efficiency of 33%, to produce a silicon to silicon optically coupled system that demonstrates 0.18% coupling quantum efficiency. The crystalline silicon LED demonstrates similarly high performance at very low power levels, where it has even higher power efficiency than a high efficiency GaAlAs LED. INTRODUCTION Traditionally, bulk c-Si does not emit light efficiently due to its indirect band gap structure. The band-edge phonon-assisted photoluminescence (PL) from bulk c-Si material has a low efficiency, with a measured internal quantum efficiency of 10-6 – 10-4 and an external quantum efficiency of 10-7 – 10-5 [1-3]. One recent paper has also reported an improved band-edge electroluminescence (EL) power efficiency up to 10-4 [4]. To improve the light emission efficiency of crystalline silicon, the recent research was mainly concentrated on quantum confinement effects [5, 6] in nanosized silicon crystals. One such device has demonstrated power efficiency up to 0.37% [7]. Active research interest in light emission from silicon is due to the fact that most integrated circuits are made on c-Si substrates. Efficient silicon light emitting diodes (LEDs) could allow “super-integration” of optical and electronic functions in high density silicon microelectronic circuits [8]. Integrating III-V materials with Si to form hybrid circuits is also an active research area [6]. However, the high dislocation densities and stability problems generated by the large lattice mismatch between GaAs and silicon create difficulties [6]. Integration of silicon light emitting diodes (LEDs) into silicon chips has enormous potential advantages over using separate, discrete LEDs or hybrid silicon chips incorporating LEDs made from other materials. Hirschman et al. has demonstrated integrated LEDs intended for displays emitting visible luminescence using oxidised porous silicon with an external powe
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