Temperature Dependence of 1/f Noise and Electrical Conductivity Measurements on p-type a-Si:H Devices
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Temperature Dependence of 1/f Noise and Electrical Conductivity Measurements on p-type a-Si:H Devices V. C. Lopes, E. Hanson, D. Whitfield, K. Shrestha, C. L. Littler, and A. J. Syllaios University of North Texas, 1155 Union Circle #311427, Denton, TX 76203-5017 ABSTRACT Noise and electrical conductivity measurements were made at temperatures ranging from approximately 270°K to 320°K on devices fabricated on as grown Boron doped p-type a-Si:H films. The room temperature 1/f noise was found to be proportional to the bias voltage and inversely proportional to the square root of the device area. As a result, the 1/f noise can be described by Hooge’s empirical expression [1]. The 1/f noise was found to be independent of temperature in the range investigated even though the device conductivity changed by a factor of approximately 4 over this range. Conductivity temperature measurements exhibit a T-0.25 dependence, indicative of conduction via localized states in the valence band tail [2,3]. In addition, multiple authors have analyzed hole mobility in a-Si:H and find that the hole mobility depends on the scattering of mobile holes by localized states in the valence band tail [4-7]. We conclude that the a-Si:H carrier concentration does not change appreciably with temperature, and thus, the resistance change in this temperature range is due to the temperature dependence of the hole mobility. Our results are applicable to a basic understanding of noise and conductivity requirements for a-Si:H materials used for microbolometer ambient temperature infrared detection. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) is a base material for microelectromechanical systems (MEMS) uncooled infrared imaging focal plane array systems using microbolometer technology [8]. Infrared detection is based on changes in the electrical conductivity with the figure of merit being the temperature coefficient of resistance (TCR). However, electrical 1/f noise may limit detectivity of this material technology. The 1/f noise is attributed to charge number (McWhorter model) or mobility fluctuations (Hooge model) in the conductivity [1]. In a-Si:H material, 1/f noise has been correlated to hydrogen content and microcrystallinity [9], and generation-recombination noise [10]. In addition, authors have related 1/f noise with variable range hopping [11,12]. In this paper, we report on electrical conductivity and 1/f noise measurements made on a-Si:H material. EXPERIMENTAL Amorphous silicon thin films were grown by the plasma enhanced chemical vapor deposition (PECVD) technique. Samples were p-type by Boron doping. Measurements were made on two samples: S-30 (H2 dilution of SiH4 16:1, BCl3 to SiH4 ratio 0.30, and thickness 505 Å), and S-192 (H2 dilution of SiH4 55:1, BCl3 to SiH4 ratio 0.32, and thickness 1060 Å). XPS measurements show a total concentration of ~ 10% atomic concentration of the highest BCl3 to SiH4. Test structures were fabricated using standard photolithographic techniques.
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The total measured noise is comprised of the noise gen
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