We measure the gap density of states and the Fermi level position in thin-film transistors based on pentacene and dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) films grown on various surfaces using Kelvin probe force microscopy. It is found that the density of states in the gap of pentacene is extremely sensitive to the underlying interface and governs the Fermi level energy in the gap. The density of gap states in pentacene films grown on bare silicon dioxide (SiO(2)) was found to be larger by 1 order of magnitude compared to that in pentacene grown on SiO(2) treated with hexamethyldisilazane and larger by 2 orders of magnitude compared to that of pentacene grown on aluminum oxide (AlO(x)) treated with a self-assembled monolayer (SAM) of n-tetradecylphosphonic acid (HC(14)-PA). When DNTT was grown on HC(14)-PA-SAM-treated AlO(x), the gap density of states was even smaller, so that the Fermi level pinning was significantly reduced. The correlation between the measured gap density of states and the transistor performance is demonstrated and discussed.

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We present a detailed experimental and theoretical study of the charging of silicon nitride and silicon oxide thin films following focused ion beam irradiation. The samples were irradiated using 30keV Ga+ ions at different ion doses and their consequent work function changes were measured by Kelvin probe force microcopy. The surface potential of both samples increased following the ion irradiation up to a critical ion dose, and then moderately decreased. The dependence of the sample surface potential on the irradiated ion dose is analyzed by taking into account all the main factors affecting charging in dielectric thin films: electron-hole generation by the incident fast ions, secondary ion-electron emission, sputtering of surface atoms, electron-hole recombination, electron recombination with the incident stopped ions, hole leakage current to the Si substrate, and various charge trapping processes. It was found that the much larger surface potential induced in Si3N4 in comparison to SiO2 is associated with the different resistance to the Ga+ ion bombardment. Under equal ion irradiation dose, a larger concentration of shallow traps is created in SiO2 than in Si3N4. This leads to an increased hole capture in shallow traps versus deep traps, and a consequent decrease in the surface potential.

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