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1 M. Casalino [1][2], L. Sirleto [1], L. Moretti [2], I. Rendina [1] [1] Istituto per la Microelettronica e Microsistemi (IMM), Consiglio Nazionale delle Ricerche (CNR), Sezione di Napoli -Via P. Castellino 111, 80131 (Italia). [2] Università degli studi Mediterranea di Reggio Calabria, Località Feo di Vito, 89060 Reggio Calabria (Italia). Silicon photodetector working at 1.55 m and operating at room temperature

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2 What makes a good detector? High bandwidth High efficiency Low voltage operation Low dark current CMOS compatibility Si Ge Silicon is not adapted for detection at 850nm, 1300nm, 1550nm. The growth of Germanium on silicon is still a challenge in terms of cost and complexity

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3 Proposed Device We carryed out the calculation of: QuantumEfficiency Bandwidth Dark current The bottom mirror could be a DBR fabricated using repeatedly a silicon-on-insulator process (SOI). On top of the DBR could be grown a n-Si layer and then deposited Schottky metal (Au in our simulations) which would be both the absorbing layer and top mirror of microcavity.

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4 Internal photoemission Advantages: - The design of device is completely compatible with ULSI silicon technology. - Fast devices Disadvantages: - The Internal Photoemission effect is very weak! Internal photoemission is the optical excitation of electrons in the Schottky metal to an energy above the Schottky barrier and transport of these electrons to the conduction band of the semiconductor (n-type in figure). The Internal photoemissio theory has been developed by Fowler R. H. Fowler, Physical Review, 1931, vol. 38, pp.45-56. V. E. Vickers, Applied Optics, vol. 10, No. 9, 1971, 2190-2192

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5 Absorptance calculation The calculation is based on the Transfer Matrix Method (TMM) M TOT the matrix M of the whole system. M A the matrix calculated from interface between 2 and 3 layers to the final plane. M B the matrix calculated from interface between 3 and 4 layers to the final plane. Bragg 1 Air N=1 N 2 N 3 N 6 N 5 Metal - λ/2-cavity - Substrate E 2,F E 2,B E 4,F E 4,B E inc E T E R N 4 Air N 7 =1 Dielectric Coating Air M. A. Muriel, A. Carballar, IEEE Photonic Technology Letters, (2005), vol. 9, N. 7, pp.955-957 Maximum absorptance is obtained for metal thickness of d=30nm, semiconductor thickness of 422nm and dielectric coating thickness of 390nm. Metal thickness[μm] Optimum absorptance

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6 Device quantum efficiency Refl. Bottom Mirror=0.99 Refl. Top Mirror=0.92 d Si3N4 =390nm d Au =30nm d Si =420nm |V Bias |=[0 - 40 V] N D =10 16 cm -3 Bragg: d Si =340nm d SiO2 =270nm Wavelength[um] QUANTUM EFFICIENCY[%] Inverse Bias applied[V] Efficiency in resonant case at 1.55μm

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7 Intrinsic device bandwidth The intrinsic limit of the device is: where: - v t is the silicon drift velocity - L is the λ/2-cavity thickness The device is very fast, being the metal the absorbing layer, the semiconductor can be made very thin. By improving the inverse voltage a 0.1 GHz bandwidth-efficiency product was obtained. Inverse voltage applied [V] Bandwidth-Efficiency product [GHz]

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8 Device dark current Dark current density [A/cm 2 ] Inverse voltage applied [V] Dark current is due to the thermal process and tunnelling process. For slightly doped silicon (<10 17 cm -3 ) and T300K the tunnelling current density, for a Au-Si barrier, can be neglected. - A * is the Richardson constant. - η c is the barrier escape probability. - ΔФ B is the potential barrier lowering. Device current density is given by: S. M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, New York, 2nd ed., 1981 The high potential barrier allows to work at room temperature obtaining a dark current density of 5.5μA/cm 2.

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9 Conclusions In this communication, the design of a Si resonant cavity enhanced Schottky photodetector, based on the internal photoemission effect, operating at room temperature and working at 1.55 micron, is reported. Using Au-Si as Shottky barrier all the device performance were calculated in term of efficiency, dark current density and bandwidth. The device is intrinsically very fast and its efficiency can be enhanced by improving the inverse applied voltage untill to obtain an efficiency-bandwidth product of 0.1GHz. We are confident that it is possible to improve the performances of device considering high Q-value optical microcavities (disk resonators) in silicon waveguide.

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