We report observations of three distinct conduction mechanisms in n-type doped mixed-phase amorphous/nanocrystalline silicon thin films over a range of nanocrystallite concentrations and temperatures. As the temperature is varied from 470 to 10 K, we observe activated conduction, multiphonon hopping (MPH), and Mott variable range hopping (VRH) as the nanocrystal content is increased. The transition from MPH to Mott-VRH hopping around 50 K is tentatively ascribed to the freeze out of the phonon modes.

A detailed description of the microscopic nature of electronic conduction in mixed-phase silicon thin films near the amorphous/nanocrystalline transition is presented. A conduction model utilizing both the conductivity and the reduced activation energy data, involving the parallel contributions of three distinct conduction mechanisms, is shown to describe the data to a high accuracy, providing a clear link between measurement and theory in these complex materials.

The production of hydrogenated amorphous silicon films containing silicon nanocrystal-line inclusions (a/nc-Si:H) is demonstrated using a new deposition process. Crystalline Si nanoparticles around 5 nm in diameter are generated in a flow-through plasma reactor, and are introduced into a downstream capacitively-coupled plasma enhanced chemical vapor deposition reactor where the particles are “co-deposited” with the amorphous phase of the film. Transmis-sion electron microscopy confirms the presence of crystalline inclusions in these films, as well as providing confirmation that the crystalline particles are indeed produced in the flow-through re-actor and not in the capacitive plasma. Electrical measurements indicate an improvement in the dark conductivity of the intrinsic a/nc-Si:H films as the particle concentration is increased, sug-gesting that the particles have a doping effect on the films charge transport properties.

Studies of the electronic transport properties of n-type doped hydrogenated amorphous/nanocrystalline silicon (a/nc-Si:H) films deposited in a dual-plasma co-deposition reactor are described. For these doped a/nc-Si:H, the conductivity increases monotonically for increasing crystal fractions up to 60% and displays marked deviations from a simple thermally activated temperature dependence. Analysis of the temperature dependence of the activation energy for these films finds that the dark conductivity is best described by a power-law temperature dependence, σ = σ o (T/T o ) n where n = 1 -4, suggesting multiphonon hopping as the main transport mechanism. These results suggest that electronic transport in mixed-phase films occurs through the a-Si:H matrix at lower nanocrystal concentrations and shifts to hopping conduction between clusters of nanocrystals at higher nanocrystal densities.

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 a-Si: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.