Measurement of mobility in nanocrystalline semiconductor materials using space charge limited current
Stieler, Daniel (2005) Measurement of mobility in nanocrystalline semiconductor materials using space charge limited current. Masters thesis, Iowa State University.
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A new technique for measuring mobility of carriers in device-type structures is used to determine the mobility of both electrons and holes in nanocrystalline Si materials. The technique is based on space charge limited current. When the injected space charge exceeds the resident charge, the current is directly related to the mobility. Unlike Hall measurements, this technique allows for a determination of mobility in the direction of transport (vertical or growth direction) in device type structures, deposited on conducting substrates. The samples that were studied were fabricated primarily using a hot wire deposition technique from mixtures of silane and hydrogen. The crystallinity and grain size were systematically varied by changing the deposition chemistry and growth temperatures. Crystallinity was determined using Raman spectroscopy, and grain sizes were determined using x-ray diffraction techniques. In addition to hot wire samples, a few samples were also made using both ECR-PECVD and VHF-PECVD techniques. For measuring electron mobility, samples of the type n+nn+ were used. Care was taken to minimize series resistance due to contacts or thin oxide layers. Four point probe measurements were used to eliminate spurious series resistance. For measuring hole mobility, samples of the type p+pp+ were used. The electron mobility was found to vary between 1 and 5 cm2/V-s. In general, the mobility increased with grain size. The mobility also increased with measurement temperatures, indicating that transport across the grain boundaries was the factor limiting the mobility. Mobility also increased with thickness, because the material became more crystalline with increasing thickness. The largest mobility of holes measured was 2.7 cm2/V-s, which is on the same order as the mobility of electrons. A major advantage of the current technique for measuring mobility is that it can be used for both doped and undoped layers, unlike time of flight technique which can only be used for relatively lightly-doped layers. Another advantage is that we obtain values of mobility in the direction in which transport takes place in solar cell devices.
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