Production of hydrogenated nanocrystalline silicon based thin film transistor

Aliyeva, Tamila
The instability under bias voltage stress and low mobility of hydrogenated amorphous silicon (a-Si:H) thin film transistor (TFT), produced by plasma enhanced chemical vapor deposition (PECVD) technique, are the main problems impeding the implementation of active matrix arrays for light emitting diode display panels and their peripheral circuitry. Replacing a-Si:H by hydrogenated nanocrystalline silicon film (nc-Si:H) seems a solution due to its higher mobility and better stability. Therefore nc-Si:H TFT was produced and investigated in this thesis. All TFT layers (doped nc-Si:H, intrinsic nc-Si:H and insulator films) were produced separately, characterized by optical (UV-visible and FTIR spectroscopies, XRD) and electrical (current-voltage, I-V) methods, and optimized for TFT application. Afterwards the non self-aligned bottom-gate TFT structure was fabricated by the photolithographic method using 2-mask set. The n+ nc-Si:H films, used for TFT drain/source ohmic contacts, were produced at high H2 dilution and at several RF power densities (PRF). The change of their lateral resistivity (rho) was measured by reducing the film thickness via reactive ion etching. The rho values rise below a critical film thickness, indicating the presence of the disordered and less conductive incubation layer. The optimum PRF for the lowest incubation layer was determined. Among the deposition parameters only increased NH3/SiH4 flow rate ratio improved the insulating properties of the amorphous silicon nitride (a-SiNx:H) films, chosen as the TFT gate dielectric. The electrical characteristics of two TFTs with a-SiNx:H having low leakage current, fabricated at different NH3/SiH4 ratios (~19 and ~28) were compared and discussed. The properties (such as crystallinity, large area uniformity, etc.) of the nc-Si:H film as TFT channel layer, were found to depend on PRF. For the films deposited at the center of the PECVD electrode the change from an amorphous dominant structure to a nanocrystalline phase took place with increasing PRF, whereas those at the edge had always nanocrystalline nature, independent of PRF. The two different TFTs produced at the center of the electrode with a-Si:H and nc-Si:H grown at low and high PRF, respectively, were compared through their I-V characteristics and electrical stability under the gate bias voltage stress. Finally, nc-Si:H TFT structure, produced and optimized in this work, was analyzed through gate-insulator-drain/source capacitor by capacitance-voltage (C-V) measurements within 106-10-2 Hz frequency (F) range. The inversion regime was detected at low F without any external charge injection. Besides, ac hopping conductivity in the nc-Si:H bulk was extracted from the fitting results of the C-F curves.


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Citation Formats
T. Aliyeva, “Production of hydrogenated nanocrystalline silicon based thin film transistor,” Ph.D. - Doctoral Program, Middle East Technical University, 2010.