Analyses of Ballistic Electron Transport in Nanocrystalline Porous Silicon
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ANALYSES OF BALLISTIC ELECTRON TRANSPORT IN NANOCRYSTALLINE POROUS SILICON Akira Kojima, Xia Sheng and Nobuyoshi Koshida Tokyo Univ. of A&T, Dept of Electrical and Electronic Eng., Koganei, Tokyo 184-8588, Japan ABSTRACT The characteristics of ballistic electron transport in porous silicon (PS) are investigated in terms of the electron emission from PS diodes and the energy distribution of emitted electrons. The energy distributions show a behavior of ballistic electron emission that is quite different with the Maxwellian distribution. This is clearly observed at low temperatures below 150 K where the electrical conduction in PS is dominated by the tunneling mode. At 100 K, the peak position of distribution curve becomes more close to the energy corresponding to the energy gain expected from ballistic transport without any scattering losses. The simulated energy distribution suggests that the electrons having higher energies in a non-equilibrium state travel ballistically in PS via field-induced tunneling process. These results support that electrons can travel ballistically in nanocrystalline layer under a high electric field. The observed ballistic transport indicates the further technological potential of silicon nanocrystallites. 1. INTRODUCTION As a quantum-sized Si nanocrystalline system, PS has new physical properties to overcome the restriction on the physical properties of single crystalline Si (c-Si). Many useful photonic and electronic functions of PS are involved in the strong quantum effect. These are visible PL [1], visible EL [2], nonlinear optical response [3], nonvolatile memory [4], ballistic electron emission [5], and thermally induced ultrasound generation [6]. We reported previously that the PS diodes with controlled band structure show an efficient ballistic electron emission [7]. The emission characteristics, especially the energy distribution of electrons emitted from PS diodes, suggest that the electrons is accelerated in the PS layer under high electric field strength and become ballistic without suffering significant scattering losses. Also, it is shown that the electron drift length in PS layer reaches longer than 2 µm at the room temperature, from the analysis of transient photocurrent by TOF technique [8]. Regarding these experimental results, there is a possibility that electrons injected into the PS layer travel ballistically via multiple tunneling through Si nanocrystallites, and that kinetic energy gain of electrons reaches several to ten electron volts. To confirm this hypothesis, the temperature dependence of electron emission properties including energy distribution of emitted electrons and current-voltage characteristics are investigated. This provides an evidence for ballistic transport. Its technological significance is also discussed. 2. EXPERIMENTAL DETAILS 2.1. Fabrication of PS Diodes Experimental PS diodes were composed of thin Au film electrodes, structure controlled PS layers, n-type Si substrates, and back electrodes as shown in Fig. 1. F3.3.1
n+-Si
PS Au
Collector
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