Charge transport in nanocrystalline germanium/hydrogenated amorphous silicon mixed-phase thin films
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Charge transport in nanocrystalline germanium/hydrogenated amorphous silicon mixedphase thin films Kent E. Bodurtha and J. Kakalios School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455 ABSTRACT Mixed phase thin films consisting of hydrogenated amorphous silicon (a-Si:H) in which germanium nanocrystals (nc-Ge) are embedded have been synthesized using a dual-chamber codeposition system. Raman spectroscopy and x-ray diffraction measurements confirm the presence of 4 - 4.5 nm diameter nc-Ge homogenously embedded within the a-Si:H matrix. The conductivity and thermopower are studied as the germanium crystal fraction XGe is systematically increased. For XGe < 10%, the thermopower is n-type (as in undoped a-Si:H) while for XGe > 25% p-type transport is observed. For films with 10 < XGe < 25% the thermopower shifts from p-type to n-type as the temperature is increased. This transition is faster than expected from a standard two-channel model for charge transport. INTRODUCTION There has recently been great interest in the electronic properties of mixed-phase thin films consisting of semiconducting nanocrystals embedded within an amorphous semiconductor matrix [1,2]. These materials combine the large-area advantages of amorphous semiconductors with the superior optoelectronic properties of crystals. The charge transport properties of these composite materials can depend sensitively on the nanocrystal concentration [3]. We report here a unique transition in the conduction mechanism in intrinsic mixed-phase thin films of hydrogenated amorphous silicon (a-Si:H) in which nanocrystalline germanium (nc-Ge) particles have been homogenously embedded. The conductivity and thermoelectric effect in the nc-Ge/aSi:H films are studied as the germanium crystal fraction XGe is systematically varied from 0% to 75%. A transition from conduction through the a-Si:H matrix to through the nc-Ge phase is reflected in measurements of the thermopower, which finds a change from n-type (for XGe < 10%) to p-type transport (for XGe > 25%) as the nc-Ge concentration is increased. For germanium crystal fractions corresponding to the n- to p-type transition (10 < XGe < 25%), the thermopower is n-type above 400K and p-type near room temperature. The transition from n-type to p-type thermopower is inconsistent with a simple two-channel mode of charge transport. MATERIALS PREPARATION The nc-Ge/a-Si:H films described here were synthesized in a dual-chamber co-deposition plasma enhanced chemical vapor deposition (PECVD) system that enables the growth of a wide variety of mixed-phase thin film materials, described in detail previously [4]. Nanocrystalline particles are synthesized in an upstream flow-through tube plasma reactor [5], and are then injected into a second capacitively-coupled plasma (CCP) deposition system in which the surrounding matrix material is grown. The synthesis of the nanocrystals is completely decoupled from the deposition of the host semiconductor matrix, so that the growth conditions can be independently optimiz
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