We crystallize amorphous silicon films by a frequency doubled Nd:YVO 4 laser with a pulse energy of 18.5 µJ and a repetition frequency of 20 kHz. A sequential lateral solidification process yields polycrystalline silicon with grains longer than the channel of thin-film transistors. The resulting electron field effect mobility of 410 cm 2 V −1 s −1 shows the superiority of our process compared with excimer laser crystallization. A calculation results in a possible throughput of 35 cm 2 s −1 for our laser crystallization process if one used a laser with a pulse energy of 1.25 mJ and a repetition frequency of 100 kHz.

High-mobility low-temperature ( 600 • C) unhydrogenated in situ doped polysilicon thin film transistors (TFTs) are made. Polysilicon layers are grown by a low pressure chemical vapour deposition (LPCVD) technique and crystallized in a vacuum by thermal annealing. The source and drain regions are in situ doped. The gate insulator is made of an atmospheric pressure chemical vapour deposition (APCVD) silicon dioxide. Hydrogen passivation is not performed on the transistors. One type of transistor is made of two polysilicon layers, the other one is fabricated from a single polysilicon layer. The electrical properties are better for transistors made of a single polysilicon layer: a low threshold voltage (1.2 V), a subthreshold slope S = 0.7 V/dec, a high field effect mobility (≈100 cm 2 V −1 s −1 ) and an on/off-state current ratio higher than 10 7 for a drain voltage V ds = 1 V. At low drain voltage, for both transistors, the off-state current results from a pure thermal emission of trapped carriers. However, at high drain voltage, the electrical behaviour is different: in the case of single polysilicon TFTs, the current obeys the field-assisted (Poole-Frenkel) thermal emission model of trapped carriers while for TFTs made of two polysilicon layers, the higher off-state current results from a field-enhanced thermal emission.

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