1/f Noise in Mott Variable Range Hopping Conduction in p-type Amorphous Silicon
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1/f Noise in Mott Variable Range Hopping Conduction in p-type Amorphous Silicon V.C. Lopes, A.J. Syllaios, D. Whitfield, K. Shrestha, and C.L. Littler Department of Physics, University of North Texas, Denton, TX 76203 ABSTRACT We report on electrical conductivity and noise measurements made on p-type hydrogenated amorphous silicon (a-Si:H) thin films prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD). The temperature dependent electrical conductivity can be described by the Mott Variable Range Hopping mechanism. The noise at temperatures lower than ~ 400K is dominated by a 1/f component which follows the Hooge model and correlates with the Mott conductivity. At high temperatures there is an appreciable G-R noise component. INTRODUCTION Amorphous silicon is one of the base materials used for infrared bolometer applications. The key figures of merit are the temperature coefficient of resistance (TCR) and electrical noise. As a result, a basic understanding of the conduction and noise mechanisms are important for the development of this material technology. In this paper, we report on and correlate conductivity and noise measurements made on p-type hydrogenated amorphous silicon (a-Si:H) material in the temperature range from 200 K to 450 K. EXPERIMENTAL AND THEORY Films of p-type a-Si:H (boron doped) were deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) at substrate temperature 365oC. The a-Si:H films were prepared >H 2 @ and boron dopant ratio r >B@ . using different level of hydrogen dilution R H >SiH 4 @ >SiH 4 @ Test resistor devices were fabricated using standard photolithography techniques. The electrical conductivity was determined from standard I-V measurements. In the case of a-Si:H material, it was found that the temperature dependent electrical conductivity V(T) can be described by the Mott variable range hoping (VRH) conduction1 with p §T · ¨ 0 M ¸ Eq.(1) V T V OM e © T ¹ E Where p 1 4 and T0 M is the Mott characteristic temperature. Here [ is the decay kN ( E F )[ 3 length of the localized wave function for tunneling from one defect state to another, N ( E F ) is the density of states at the Fermi level and E is a constant.
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The total noise measured is comprised of 1/f noise, white noise, and potentially G-R 2 2 2 2 V1 / f V V V white G R . The 1/f noise can be described using the noise. Quantitatively, noise 'f 'f 'f 'f Hooge model2, with 2 2 2 V1 / f D H VBias D H VBias B Eq. (2) J J P f pV f 'f fJ
DH 2 VBias , P is the total number of carriers, p is the carrier density, V is the device pV volume, VBias is the device bias, and J is the frequency exponent, typically near a value of 1 (in D the range from 0.9 to 1.1)2. The normalized Hooge parameter is H , where D H is the Hooge p where B
coefficient. The G-R noise for a single trap is described by3,4 2 2 VG R AWVBias 2 'f 1 2SWf
Eq. (3)
nt . Here, nt is the trap density, V is the device Vp 2 volume, and p is the carrier density. The noise waveform can be analyzed for trapping by: where W is the trap time co
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