http://doc.rero.ch/record/9704/files/Shah_A.V._-_Material_and_solar_cell_research_20080822.pdf

Material and solar cell research in microcrystalline silicon

Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry

Silicon heterojunction solar cells have high open-circuit voltages thanks to excellent passivation of the wafer surfaces by thin intrinsic amorphous silicon (a-Si:H) layers deposited by plasma-enhanced chemical vapor deposition. We show a dramatic improvement in passivation when H2 plasma treatments are used during film deposition. Although the bulk of the a-Si:H layers is slightly more disordered after H2 treatment, the hydrogenation of the wafer/film interface is nevertheless improved with as-deposited layers. Employing H2 treatments, 4 cm2 heterojunction solar cells were produced with industry-compatible processes, yielding open-circuit voltages up to 725 mV and aperture area efficiencies up to 21%.

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Improved amorphous/crystalline silicon interface passivation by hydrogen plasma treatment

Hydrogenated nanocrystalline silicon (nc-Si:H) films were deposited by using 13.56MHz plasma-enhanced chemical vapor deposition at 260°C by means of a silane (SiH4) plasma heavily diluted with hydrogen (H2). The high-quality nc-Si:H film showed an oxygen concentration (CO) of ∼1.5×1017at.∕cm3 and a dark conductivity (σd) of ∼10−6S∕cm, while the Raman crystalline volume fraction (Xc) was over 80%. Top-gate nc-Si:H thin-film transistors employing an optimized ∼100nm nc-Si:H channel layer exhibited a field-effect mobility (μFE) of ∼150cm2∕Vs, a threshold voltage (VT) of ∼2V, a subthreshold slope (S) of ∼0.25V∕dec, and an ON∕OFF current ratio of ∼106.

High-mobility nanocrystalline silicon thin-film transistors fabricated by plasma-enhanced chemical vapor deposition

A series of samples was deposited by very high frequency glow discharge in a plasma of silane diluted in hydrogen in concentrations SiH4/(SiH4+H2) varying from 100% to 1.25%. For silane concentrations below 8.4%, a phase transition between amorphous and microcrystalline silicon occurs. Microcrystalline silicon has been characterized by transmission electron microscopy (TEM) and x-ray diffraction. The medium-resolution TEM observations show that below the transition, the microstructure of microcrystalline silicon varies in a complex way, showing a large variety of different growth structures. For the sample close to the phase transition, one observes elongated nanocrystals of silicon embedded in an amorphous matrix followed at intermediate dilution by dendritic growth, and, finally, at very high dilution level, one observes columnar growth. X-ray diffraction data evidence a (220) crystallographic texture; the comparison of the grain sizes as evaluated from TEM observations and those determined using Scherr...

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Evolution of the microstructure in microcrystalline silicon prepared by very high frequency glow-discharge using hydrogen dilution

Hydrogen incorporation in silicon layers prepared by plasma‐enhanced chemical‐vapor deposition using silane dilution by hydrogen has been studied by infrared spectroscopy (IR) and elastic recoil detection analysis (ERDA). The large range of silane dilution investigated can be divided into an amorphous and a microcrystalline zone. These two zones are separated by a narrow transition zone at a dilution level of 7.5%; here, the structure of the material cannot be clearly identified. The films in/near the amorphous/microcrystalline transition zone show a considerably enhanced hydrogen incorporation. Moreover, comparison of IR and ERDA and film stress measurements suggests that these layers contain a substantial amount of molecular hydrogen probably trapped in microvoids. In this particular case the determination of the total H content by IR spectroscopy leads to substantial errors. At silane concentrations below 6%, the hydrogen content decreases sharply and the material becomes progressively microcrystalline...

/pdf/hydrogen-in-amorphous-and-microcrystalline-silicon-films-1t1x2it3gm.pdf

S. Mikhailov

Papers

Hydrogen in amorphous and microcrystalline silicon films prepared by hydrogen dilution