Electrical properties and degradation kinetics of compensated hydrogenated microcrystalline silicon deposited by very high-frequency-glow discharge

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Abstract
Microcrystalline silicon (μc-Si:H) layers deposited by the very high-frequency-glow discharge technique at a radio-frequency excitation of 70 MHz are observed to be basically slightly 〈n〉 type. By doping (so-called ‘‘microdoping’’) with boron in the gas phase volume part per million (vppm) range, compensated material could be obtained. The influence of this doping on the electronic transport properties is documented. A pronounced onset of the boron incorporation into the films measured by secondary-ion-mass spectrometry is observed around 3 vppm (B2H6/SiH4), together with marked changes in the electrical properties. The compensated film obtained for a microdoping of about 1 vppm shows the lowest dark conductivity [3×10−8 (Ω cm)−1], the highest activation energy (517 meV), and, finally, the highest photoconductive gain of 6×103 (photo/dark current ratio). Depending on the value of the activation energy (the critical value is ≊0.2 eV), two different transport models are identified, corresponding to ‘‘Meyer–Neldel’’ or ‘‘anti-Meyer–Neldel’’ behavior. As for light-induced degradation, the compensated film exhibits better stability than undoped films. Finally, the use of slightly boron doped μc-Si:H as photovoltaically active material will be discussed.