Conductance Fluctuations in Amorphous Silicon Nanoparticles
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Conductance Fluctuations in Amorphous Silicon Nanoparticles T. J. Belich1, Z. Shen2, C. Blackwell1, S. A. Campbell2 and J. Kakalios1 1
School of Physics and Astronomy, The University of Minnesota, Minneapolis, MN 55455, USA Dept. of Electrical and Computer Engineering, The University of Minnesota, Minneapolis, MN 55455, USA 2
ABSTRACT Hydrogenated amorphous silicon nanoparticles with an average diameter of 150 nm have been synthesized by high-density plasma chemical vapor deposition. The particles are deposited onto a conducting substrate and are then surrounded by an insulating matrix, electrically isolating the particles. Electrical contact is made to the top of each nanoparticle; the current-voltage characteristics of the nanoparticles indicate that transport is space-charge limited through the aSi:H. The spectral density of the current fluctuations in the a-Si:H nanoparticles is well described by a 1/f frequency dependence for frequency f. However, the octave separation dependence of the correlation coefficients of the noise power for the nanoparticles are very well described by an ensemble of fluctuators whose amplitudes are independently modulated in parallel, rather than the serial kinetics typically observed in bulk a-Si:H. INTRODUCTION Mixed-phase thin films consisting of nanocrystallites within an amorphous matrix have recently attracted considerable attention for a variety of applications. Silicon nanocrystals embedded in a silicon oxide matrix have been developed as non-volatile memory elements,1 and for efficient electron emitters,2 while silicon nanoparticles in a silicon nitride matrix have been fabricated for electroluminescent devices.3 Hydrogenated amorphous silicon thin films containing silicon nanocrystals (a/nc-Si:H) demonstrate high solar conversion efficiencies coupled with high deposition rates.4,5 These films are typically synthesized in a capacitively coupled Plasma Enhanced Chemical Vapor Deposition (PECVD) system, where the reactive silane gas is heavily diluted with hydrogen and the gas chamber pressure is above 500 mTorr.6 Silicon nanoparticles have also been synthesized in an Inductively Coupled Plasma system, where the average nanoparticle diameter is controlled by the gas pressure and plasma on time.7 If the silane is not heavily diluted with hydrogen gas, the resulting nanoparticles are amorphous rather than polycrystalline, raising the possibility of investigating the electronic properties of hydrogenated amorphous silicon (a-Si:H) nanoparticles.8 In this paper we describe current-noise measurements of free-standing a-Si:H nanoparticles within an insulating matrix, which allows the investigation of reduced sample geometry on the non-Gaussian conductance fluctuations observed in continuous thin film a-Si:H. MATERIALS PREPARATION The amorphous silicon nanoparticles are synthesized in an Inductively Coupled High Density PECVD system described previously.7 A computer was used to control the RF (13.56 MHz) power and to turn the plasma on and off remotely. The particle size is
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