Relation between Electronic Properties and Density of Crystalline Agglomerates in Microcrystalline Silicon
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0989-A07-01
Relation between Electronic Properties and Density of Crystalline Agglomerates in Microcrystalline Silicon Paula C.P. Bronsveld1, Arjan Verkerk1, Tomas Mates2, Antonin Fejfar2, Jatindra K. Rath1, and Ruud E.I. Schropp1 1 Faculty of Science, Utrecht University, Princetonplein 5, Utrecht, 3508 TA, Netherlands 2 Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnick· 10, Praha, 162 53, Czech Republic
ABSTRACT A series of silicon thin films was made by very high frequency plasma enhanced chemical vapour deposition (VHF PECVD) at substrate temperatures below 100 ∞C at different hydrogen to silane dilution ratios. The electronic properties of these layers were studied as a function of the surface crystalline fraction as determined accurately from a combination of microscope images at different length scales (gathered by using different types of microscopes). The results show that the electrical conductivity increases monotonously as a function of crystalline surface coverage and no discontinuity is observed at the percolation threshold. An increase in conductivity of four orders of magnitude for layers with a high crystalline content is observed after annealing at temperatures up to 170 ∞C. Combined with the information that oxygen is incorporated at Si-H surface bond sites, this suggests that gas adsorption effects might be dominantly responsible for the electronic properties of mixed phase silicon. INTRODUCTION It is often observed that hydrogenated silicon layers deposited just over the amorphous to microcrystalline transition edge consist of cone-shaped agglomerates of crystallites, embedded in amorphous material. According to the observations of Collins et al. [1], changing the crystallinity by changing the hydrogen to silane dilution ratio can influence both the incubation height and the density of these cones. At higher dilutions, the incubation layer diminishes and the nucleation density of crystalline cones becomes higher, leading to a surface covered with microcrystalline material and an incubation layer in the order of a few tens of nanometer. It is also observed that an increase in crystallinity in a (mixed phase) µc-Si:H layer causes an effective increase in the conductivity. For example, recent studies by Azulay et al. [2] that make use of conductive atomic force microscopy (C-AFM) to test the transport properties of the different phases, have shown that for layers with a very high crystalline content, as determined by Raman spectroscopy, a percolation network can be formed by the tissue surrounding the crystalline agglomerates. The formation of this network is associated with a large ìjumpî in conductivity at a crystalline volume fraction of ≈ 0.7. Another percolation threshold was found at a volume fraction of ≈ 0.3, associated with conduction through a regular percolation path formed by the more conducting crystallites embedded in a less conducting amorphous tissue. The low volume fraction threshold is most often reported for p-type or n-type doped layers (refs. in [2]
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