Large area uniformity of plasma grown hydrogenated nanocrystalline silicon and its application in TFTs

This work investigates the effect of RF power density (100-444 mW/cm(2)) on the structural, optical and electrical properties of the hydrogenated nanocrystalline silicon (nc-Si:H) thin films grown by plasma enhanced chemical vapor deposition (PECVD) technique. Moreover, RF power effect on the large area uniformity was analyzed by comparing the properties of the films deposited at the center and near the edge of the PECVD electrode. Film characterization was performed by Fourier transform infrared and UV-visible spectroscopies, X-ray diffraction and current-voltage measurements. The films at the center of the electrode show variations in their properties with RF power, while the ones near the edge exhibit almost no dependence on RF power. With increasing RF power, the film structure at the center of the electrode transforms from amorphous to nanocrystalline, the dark resistivity (rho) decreases from similar to 1 x 10(8) Omega cm to similar to 1 x 10(5) Omega cm, while the Urbach tail (E(0)) expands from similar to 50 meV up to similar to 200 meV. For all powers, the films near the edge of the electrode, apart from their nanocrystalline structure, have higher crystalline volume fraction, microstructure factor and E(0), and lower rho and hydrogen content (C(H)) compared to the films at the center. The influences of the power density on the uniformity of the film properties are discussed in the frame of nc-Si:H film growth models. At the highest available RF power density (444 mW/cm(2)), nc-Si:H films show a slight deterioration in their properties and uniformity, revealing an optimum power density of similar to 300 mW/cm(2), where the lowest rho and C(H) together with high uniformity is reached. Bottom-gate thin film transistor (TFT) with the nc-Si:H channel layer deposited at this optimum power exhibits lower threshold voltage, better electrical stability and slightly higher mobility compared to the TFT with the channel grown at 100 mW/cm(2). (C) 2010 Elsevier B.V. All rights reserved.