Exceptionally High Radiative Efficiencies in Silicon
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EXCEPTIONALLY HIGH RADIATIVE EFFICIENCIES IN SILICON M.A. Green, A. Wang, J. Zhao, T. Trupke, E-C. Cho and J. Xia Centre of Excellence for Advanced Silicon Photovoltaics and Photonics University of New South Wales, Sydney, NSW, Australia, 2052 Telephone: (61-2-) 9385-4018; Facsimile: (61-2-) 9662-4240; Email: [email protected]
ABSTRACT Although silicon is known as a poor emitter of light, recent work has demonstrated silicon light emission efficiencies in the range normally associated with III-V semiconductors. The present paper explores the light emission efficiency potential of silicon as well as the scope for implementing silicon optical functions into high density integrated circuits. INTRODUCTION Silicon’s indirect bandgap decreases the probability for radiative processes in silicon since a momentum conserving phonon is required in the process, as shown in Figure 1. Also shown is one of the competing recombination processes through bulk defect levels. Since the corresponding defects are localised, momentum conservation is not required for transitions through such defect levels. Recombination rates, for similar levels of defects, are therefore expected to be similar for both indirect and direct bandgap material. The competing optical process is free carrier absorption, which generally requires phonon participation for both indirect and direct bandgap material.
Figure 1: Radiative transition in silicon together with competing defect recombination and free carrier absorption.
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On the other hand, silicon’s elemental nature and well-developed technology allows tighter control over defects and carrier concentration than possible with direct bandgap material. The present work shows that these advantages can be used to offset silicon’s apparent fundamental disadvantages to produce devices demonstrating reasonably high radiative efficiencies. Furthermore, quantum confinement is shown to have the potential to further improve such efficiencies, making silicon-on-insulator (SOI) technology of interest for exploring prospects for integrated silicon optoelectronic devices. ELECTROLUMINESCENCE The structure of Figure 2 has resulted in very efficient silicon light-emitting-diode (LED) performance [1]. The structure is based on earlier solar cell devices designed to reduce defect recombination both in the device bulk and at surfaces, as well as having low values of free carrier absorption due to the minimisation of the total volume of heavily doped areas.
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Figure 2: High-efficiency Si LED demonstrating “wall plug” efficiency above 1%. A key feature of these devices is the use of surface texture to improve device absorptance and hence to improve its light emission efficiency by a
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