Measurements of the electronic current fluctuations of free-standing hydrogenated amorphous silicon nanoparticles are described. The nanoparticles are synthesized by high-density plasma chemical vapor deposition and are deposited onto conducting substrates. An insulating matrix, either silicon oxide or silicon nitride is then grown in order to electrically isolate the particles. Electronic measurements are performed in this transverse geometry, and underneath a top electrode of area 1mm x 1mm are typically 10,000 nanoparticles with an average diameter of 150 nm in parallel. 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, as in the case of bulk a-Si:H films. The variation of the correlation coefficients with frequency octave separation of the noise power fluctuations in bulk a-Si:H films indicates serial interactions between fluctuators. In contrast, the octave separation dependence of the correlation coefficients for the nanoparticles are very well described by an ensemble of fluctuators whose amplitudes are independently modulated in parallel.
Measurements of the second spectra that characterize the non-Gaussian statistical nature of conductance fluctuations are reported for a series of hydrogenated amorphous silicon thin films. The deposition conditions used to synthesize the films were systematically varied in order to observe the effect that differing amounts of disorder have on the noise statistics. One series of n-type films were deposited at varying substrate temperatures, another n-type series was grown at varying rf powers, and a third series of compensated films was synthesized with varying ratios of phosphine to diborane. None of these series shows any significant change in the non-Gaussian noise statistics as the long-range disorder and deposition properties are changed. Measurements of the second spectra for a film synthesized in an inductively coupled plasma thermal growth system, which yields nano-particles of ~ 150 nm in diameter, are also reported. These results are discussed in terms of models for the non-Gaussian noise properties in amorphous silicon.
The inherent non-uniformity of a Wideband Infrared Scene Projector (WISP) necessitates an analytical prediction of the contribution of the scene projector's non-uniformity to a test article's output image non-uniformity. A mathematical model has been developed to calculate this non-uniformity based upon a number of input parameters. The output image non-uniformity is dependent on both the non-uniformity of the scene projector and the test article, as well as a weighting factor that results from the relative contribution of the different emitters to the individual detector elements. It is through this weighting factor that parameters such as the sampling ratio, the optical blur of the emitters on the detector' focal plane array, the fill factor of the detector array, and the alignment of the emitters with respect to the detector elements affect the non-uniformity of the output image. Using this model, a theoretical limit for the maximum output image non-uniformity can be calculated for particular values of the scene projector's non-uniformity and the test article's non- uniformity. Realistic situations likely to be encountered during simulation testing were all found to be below the maximum. In order to make this model a useful tool for the laboratory environment, a computer program has been written that calculates the output image non-uniformity based on a given set of input parameters and a numerical approximation of the weighting factor.