Conductance Fluctuations and Thermal Equilibration Effects in Hydrogenated Amorphous Germanium

Abstract

Conductance fluctuations of undoped hydrogenated amorphous germanium (a-Ge:H) were measured and have power spectra that vary with inverse frequency (1/f) that are characterized by non-Gaussian statistics. This non-Gaussian aspect of the 1/f noise is reflected in (1) histograms of the noise power per octave that are not described by Gaussian distributions, (2) strong correlations of the noise power in frequency-space and (3) power-law second spectra. In particular, histograms of the 1/f noise power per octave for a-Ge:H are well described by a log-normal distribution. The correlation coefficients across frequencies are non-zero and larger than expected for independently modulated fluctuators and grow with averaging time with a logarithmic time-dependence. In contrast, the 1/f noise for polycrystalline germanium (pc-Ge), and free-standing nanocrystalline zinc oxide (ZnO) thin films are found to display Gaussian statistics. These results are discussed in terms of a model of filamentary conduction, where the conductance of the current microchannels is modulated by hydrogen motion governed by hierarchical kinetics. The non-Gaussian 1/f noise in a-Ge:H is very similar to the noise previously measured in hydrogenated amorphous silicon (a-Si:H), for which the above filamentary conduction method was first proposed. In addition, a hydrogen glass model was also successfully utilized to describe data in a-Si:H where stretched exponential relaxation (SER) of the electrical conductivity following cooling from a high temperature anneal was observed. The similarities in the 1/f noise of a-Ge:H and a-Si:H motivated the measurement of current relaxation in a-Ge:H. However, unlike the a-Si:H SER decay of the conductivity, in a-Ge:H a time-dependent increase of the conductivity at constant temperature was observed. The stretched exponential power-law exponent and time constant are similar to that observed in a-Si:H, but only for higher temperatures just below the thermal equilibration temperature. That is, while the hydrogen glass model applies for elevated temperatures in a-Ge:H, it does not hold for all temperatures.