A Silicon-Based Infra-Red Photodetector Exploiting Erbium-Doped Silicon Nanocrystals
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A Silicon-Based Infra-Red Photodetector Exploiting Erbium-Doped Silicon Nanocrystals Anthony J. Kenyon, Sukhvinder S. Bhamber, and Christopher W. Pitt Department of Electronic & Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, United Kingdom. ABSTRACT We have exploited the interaction between erbium ions and silicon nanoclusters to produce a photodetector for use in the spectral region around 1.5 µm. The device consists of an MOS structure in which the oxide layer has been implanted with both erbium and silicon and annealed to produce silicon nanocrystals around 3 nm in diameter. Upon illumination with a 1480 nm laser diode, the well-known interaction between the nanocrystals and the rare-earth ions results in a transfer of excitation from the erbium ion to nearby silicon nanocrystals. The resultant modification of the conductivity of the oxide layer enables a current to flow when a voltage is applied. INTRODUCTION It is now well established that there exists a strong interaction between silicon nanoclusters and rare-earth ions in silica matrices [1,2]. Exciton-mediated excitation of the rare-earth ions proceeds via absorption by the silicon nanocluster, generation of a carrier pair, and rapid and efficient excitation exchange to the rare-earth ion. The erbium-silicon nanocluster system has been of particular interest because of its technological importance, but the effect has been reported for other rare earths, including neodymium and terbium. Excitation of the silicon nanoclusters is achieved readily by above-band gap illumination; the transfer of excitation to nearby rare-earth ions quenches the characteristic emission from the silicon nanoclusters, and luminescence at 1.54 µm from Er3+ ions is seen [3]. Because of the large absorption cross-section of silicon nanoclusters at wavelengths in the visible region [4], the effective absorption crosssection of erbium ions can be increased by up to four orders of magnitude as a result of the interaction [3,5]. This, coupled with the broad-band absorption of the silicon nanoclusters, opens up the possibility of flashlamp-pumped erbium-doped gain elements [6,7]. Recent results have demonstrated that erbium-doped silicon exhibits a photoresponse in the 1.54 µm region due to backtransfer of excitation from the erbium ions to the silicon host [8]. The excitation mechanism of erbium in bulk silicon is complex, involving carrier generation or injection, trapping at an erbium-related trap level that lies 0.15 eV below the silicon conduction band, and Auger transfer [9]. Backtransfer of excitation from the erbium ion to the silicon host becomes significant for temperatures above 130 K, and can lead either to re-excitation of the Er3+ ion, or loss of excitation by nonradiative recombination of the carrier pair via phonon coupling. The 0.15 eV required to promote carriers from the trap centre to the conduction band is provided easily by phonons, hence the thermal activation of this mechanism, which is therefore that that most severely
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