A Significant Change in Refractive Index of Nanocrystalline Porous Silicon Induced by Carrier Injection
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A Significant Change in Refractive Index of Nanocrystalline Porous Silicon Induced by Carrier Injection Y. Toriumi, M. Takahashi, and N. Koshida Department of Electrical and Electronic Engineering, Faculty of Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan ABSTRACT To verify the availability of porous silicon (PS) device for application to optical switching, the photonic response of PS for carrier injection has been studied for a microcavity configuration. The observed reflectance spectral shift in the PS microcavity with increasing diode current is analyzed under the assumption that the refractive index of nanocrystalline silicon is increased by carrier injection and the subsequent accumulation possibly in localized states. This is consistent with the experimental results obtained from dynamic response measurements: there are fast and slow components in the refractive index change. It is also demonstrated that under the pulsed operation, the PS microcavity operates as a current-induced nonlinear optical switching device. INTRODUCTION Since room-temperature visible photoluminescence (PL) [1] and electroluminescense (EL) [2] from porous silicon (PS) were reported, many studies have been conducted towards PSbased optoelectronic integration. Because PS consists of a great number of Si nanocrystallites, visible light-emission from PS has been discussed in relation to quantum confinement effects. In addition to luminescence, PS might also be a promising material for optical devices. For example, usefulness of PS as microcavities [3] and optical waveguides [4] were demonstrated. The optical nonlinearity in PS have also been reported [5-8] which show a significant large photoinduced absorption effect. In this work, a significant current-induced change in the reflectance spectra of PS is presented using the microcavity configuration. This change is explained as a result of the refractive index change. The experimental results show that the refractive index change is caused by carrier injection into silicon nanocrystallites and subsequent carrier accumulation into localized states. The characteristics of current-induced refractive index change of PS are discussed by theoretical and experimental analyses. EXPERIMENT The PS Fabry-Pérot resonator (FPR) diode is composed of a transparent ITO top contact, a PS FPR, p-type Si substrate and an ohmic back contact. The resonating layer, consisting of an active layer and two distributed Bragg reflectors (DBRs), was prepared by anodizing p-type (100) Si wafers (0.1-0.5 cm) in a mixture of HF (55wt.%):ethanol=1:1 at 0 °C. To fabricate the DBRs, the anodization current density was modulated between 20 mA/cm2 and 250 mA/cm2 for periods of 4.56 s and 0.885 s, respectively, in order to obtain the same optical thickness as a quarter of the resonant wavelength with different refractive indices. The corresponding refractive indices are 2.60 and 1.54, respectively. The anodization current density and the time to form the F8.3.1
active PS layer were
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