Date

July 1999

Document Type

Dissertation

Degree Name

Ph.D.

Department

Dept. of Electrical and Computer Engineering

Institution

Oregon Graduate Institute of Science & Technology

Abstract

Thin-film transistors (TFTs) are a class of field-effect devices produced on insulating substrates rather than single crystal silicon. The channel of such devices is typically made of either polysilicon (poly-Si) or amorphous silicon (a-Si), and for this reason, the performance is inferior to their single crystal counterparts. However, TFTs find widespread use in display applications, where the substrate must be translucent. Because of this, and due to the proliferation of portable, flat-panel displays (FPDs), research on TFTs has increased rapidly over the last quarter century. As a result of the limitations of amorphous silicon devices, including low carrier mobility and on-current, interest in polysilicon TFTs has grown. Polysilicon films for use in TFTs can be prepared using a variety of methods, but excimer laser annealing (ELA) of an initially amorphous thin film has, to date, produced the best performing devices. In this thesis, the effect of various parameters on the qualities of laser annealed polysilicon and polysilicon TFTs was investigated. These included the laser annealing ambient, use of low temperature substrate heating during laser annealing, the effect of Si02 barrier layers, laser activation of implanted dopants, use of an anti-reflective coating, and multiple laser scans. By pursuing the three-fold effort of materials characterization, modeling and device characterization, the underlying features determining device performance were uncovered. In fundamental terms, three qualities influenced most directly the ultimate capabilities of laser annealed poly-Si TFTs: the grain size, the interface characteristics and the incorporation of impurities. By maximizing the grain size, the density of grain boundary defects was reduced. This improved the on-state and off-state qualities by presenting fewer trap states to fill and limiting the availability of intermediate energy states in the band-gap for carrier tunneling. Interface qualities, stemming from the choice of dielectric materials and possible interface damage, have been shown to affect the subthreshold characteristics. By studying the affect of various ambients on the ELA process, it was determined that incorporation of oxygen into the poly-Si thin film increased the number of scattering centers and also increased the trap state density by disturbing the Si matrix.

Identifier

doi:10.6083/M4WQ01SV

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