Transport Properties of Amorphous Silicon / Silicon Oxide Heterostructures

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TRANSPORT PROPERTIES OF AMORPHOUS SILICON / SILICON OXIDE HETEROSTRUCTURES H. Steemers, J. Mort, I. Chen, F. Jansen, S. Grammatica and D. Kuhman Xerox Webster Research Center, Webster, NY 14588 ABSTRACT in glow-discharge deposited aThe transport of excess carriers Si:H/insulator heterostructures has been studied by time-of-flight and xerographic discharge techniques. Efficient injection of photocarriers from aSi:H into, and transport through, relatively thick SiOx:N:H has been achieved. 2 A mobility-lifetime product approaching i1"6 cm V"I is found for electrons in SiOx:N:H, and time resolved measurements indicate a room temperature mobility 1 4 2 of 5x10- 6 cm V'Is'1 at a field of 2x10 Vcm" , suggesting an electron lifetime of the order of 8.2 seconds. The results are contrasted with transport measurements on thermally grown Si0 2 on Si and a transport model involving hopping through defect states within the gap of SiOx:N:H is discussed. INTRODUCTION Semiconductor-insulator heterolayers are an important feature of many solid state electronic devices, as demonstrated by thermally grown silicon dioxide on silicon in metal-oxide-semiconductors which has been the object of extensive studies. One of the most widely used techniques in the study of the electronic structure and transport properties in wide band gap materials is that of internal photoemission [1]. In Si0 2 , for example, the band gap between extended states of - 8eV precludes significant intrinsic photoconductivity. Instead, the injection of photoexcited carriers in metals or photogenerated carriers in the contiguous crystalline silicon has been used to determine the electronic energy levels and subsequent charge transport in Si0 2 . In the case of the Si/SiO2 system, as illustrated in figure (1), the large mismatch in bandgaps of 1.1 eV versus - 8 eV, results in a large spectral shift in the onset of photocurrents from 1.1 eV to - 4.2 eV, corresponding to the energy difference between the valence band of the Si and the conduction band of SiO2 . Studies of the saturation of the photoinjection currents as a function of applied field [2], led to estimates of the electron drift mobility IL - 28 cm2vls"I, subsequently confirmed by Hall-effect [3], and time-of-flight [4] studies using pulsed X-ray irradiation. Despite this relatively high drift mobility, the uT value of electrons was estimated to be - 18- cm2 v"I [2], indicating a very short lifetime with respect to deep traps. Recently, studies of a-Si:H/SiOx:N:H heterostuctures have been described [5], and this work discusses the results of some transport studies on these structures. SiOx:N:H films are produced by the glow discharge of mixtures of silane and nitrous oxide and as such can be expected to differ significantly from Si0 2 produced by CVD or thermal oxidation. Highly insulating materials of wide bandgaps can be obtained if the N2 0/SiH4 gas ratios are made sufficiently high, but little detail is known of the charge transport properties of these materials particularly as a function of composition. F