Tunneling Conductivity in Thermally Oxidized Porous Silicon
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Tunneling Conductivity in Thermally Oxidized Porous Silicon Dmitrii G. Yarkin, Leonid A. Balagurov, Andrei F. Orlov, and Igor P. Zvyagin Institute of Rare Metals, B.Tolmachevsky 5, Moscow, 109017, Russia ABSTRACT Metal/PS/c-Si structures with porous silicon (PS) layers of 55-75% porosity were fabricated on moderately doped p- and n-type c-Si substrates and were thermally oxidized at 400-960°C. The effect of oxidation on the photoluminescence and the transport of charge carriers in these structures were studied. We demonstrated that trap-filled space charge limited current (SCLC) is the dominant transport mechanism at large forward bias. The analysis of the current – voltage characteristics in the SCLC region allowed us to determine the oxidation dependence of the effective thickness of the trap-rich tissue isthmuses, in which space charge is mostly accumulated. The exponential dependence of the ohmic conductance on the thickness of SiOX tissue is explained by tunneling of carriers through potential barriers formed by the tissue surrounding silicon crystallites. INTRODUCTION High activity aimed at the fabrication of light-emitting materials containing small silicon particles has began from the observation of visible photoluminescence (PL) from porous silicon (PS) [1]. Industrial-quality PS - based light-emitting diodes have not been fabricated, since high quantum efficiency combined with long-term stability of electroluminescence is not achieved. At present nanoscale material research turns to materials without pores consisting in particular of Si nanocrystals embedded in the oxide host [2]. It could be expected that these materials are similar to oxidized PS at least in their electrical properties because electrical properties of oxidized PS seem to be derivative of a-SiO2. Really, Fowler-Northeim [3], Poole-Frenkel [4] and SCLC [5] conduction mechanisms were observed in oxidized PS. In what follows we describe one more (tunneling) conduction mechanism.
EXPERIMENTAL DETAILS PS layers with 55-75% porosity and 1 – 2 µm thickness were fabricated by electrochemical etching of 10 Ωcm boron doped p-type c-Si (p-Si) and 4.5 Ωcm phosphorus doped n-type c-Si (n-Si) substrates. After etching, the samples were rinsed in ethanol and dried. The samples were thermally oxidized during 25 minutes in 1:10 oxygen: argon gas mixture with oxidation temperatures (TOX) in the range 400 – 960°C. Nonoxidized samples were kept for two weeks in air to make them partially natively oxidized (in what follows, this is referred to as the “initial state” or TOX =20°C). Relative concentrations of oxygen atoms (NO/NSi) in PS layer were determined from the weight measurements. Figure 1(a) shows that significant oxidation occurs at above 700 °C.
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