Abstruct-The conduction mechanism and the origins of the leakage current in undoped channel polycrystalline silicon thin-film transistors fabricated under a variety of processing conditions were investigated. Leakage currents below 1 nA at drain-source voltages of 40 V were achieved in both n-type and p-type devices. The effective channel electron and hole mobilities were 75 and 42 cm2/Vs, respectively. Measured stage delay times for CMOS ring oscillators as a function of supply voltage agreed well with theoretical calculations. The effective carrier mobility was shown to have a minimum at a gate voltage corresponding to the point at which all traps are filled. Both dark and photo-induced leakage currents were determined to be controlled by generation from the grain boundary traps. The voltage drop across individual gates in multi-gated structures was investigated as a function of gate voltage. The use of multiple gates at high drain-source potentials was found to decrease both dark and photo-induced leakage currents.
To investigate the effect of growth area on interface dislocation density in strained-layer epitaxy, we have fabricated 2-.um-high mesas of varying lateral dimensions and geometry in (001) GaAs substrates with dislocation densities of 1.5 X 10\ 10 4 , and 10 2 cm--2 • 3500-, 7000-, and 8250-A-thick In o (J5 Gao 95 As layers, corresponding to 5, to, and 11 times the experimental critical layer thickness as measured for large-area samples, were then deposited by molecularbeam epitaxy. For the 3500-A layers, the linear interface dislocation density, defined as the inverse of the average dislocation spacing, was reduced from greater than 5000 to less than 800 em -I for mesas as large as 100 pm. A pronounced difference in the linear interface dislocation densities along the two interface (110) directions indicates that a dislocations nucleate about twice as much as /3 dislocations. For samples grown on the highest dislocation density substrates, the linear interface-dislocation density was found to vary linearly with mesa width and to extrapolate to a zero linear interface-dislocation density for a mesa width of zero. This behavior excludes dislocation multiplication or the nucleation of surface half"loops as operative nucleation sources for misfit dislocations in these layers. Only nucleation sources that scale with area (termed fixed sources) are active. In specimens with lower substrate dislocation densities, the density of interface dislocations still varies linearly with mesa size, but the slope becomes independent of substrate dislocation density, indicating that surface inhomogeneities now act as the dominant source for misfit dislocations. Thus, in 3500-A-thick overlayers, substrate dislocations and substrate inhomogeneities are the active fixed nucleation sources. Since only fixed nucleation sources are active, a single strained layer wiH dramatically reduce the threading dislocation density in the epilayer. For the 7ooo-A layers, we observe a superlinear increase in linear interface-dislocation density with mesa size for mesas greater than 200,um, indicating that dislocation mUltiplication occurs in large mesas. For mesas less than 200 pm in width, linear interface-dislocation density decreases linearly with mesa size, but extrapolates to a nonzero linear interface-dislocation density for a mesa size of zero. This nonzero extrapolation suggests an additional active source which generates a dislocation density that cannot be decreased to zero by decreasing the mesa size. Cathodoluminescence eeL) images using radiative recombination indicate that the additional source is nucleation from the mesa edges. Despite a doubling in epilayer thickness from 3500 to 7000 A, the linear interface-dislocation density for mesas 100 [tm in width is stm very low, approximately 1500 cm --l. The 8250"A layers possess interface-dislocation densities too high to be accurately determined with CL. However, increases in CL intensity as mesa width is reduced indicate that the interface-dislocation density is decreasing and that growth...
We have eliminated interface defects from the mismatched Ino.()S GaO 95 As/ (001) GaAs interface by controlling the size of the growth area. 2-Jim-high pillars with different lateral shapes and dimensions were defined within the GaAs substrate before the molecular beam epitaxial growth of 3500 A of In o. 05 Gao,!" As, greater than four times the critical thickness. On the pillars, the linear density of misfit dislocations was reduced from > 5000 dislocations!cm for large (several hundred pm lateral dimensions) growth areas to nearly zero for 25 f.lm lateral dimensions. The dislocation density remains less than 800 dislocations/em for lateral dimensions up to 100 ,urn. We find that there is also a decrease in dislocation density in narrow channels between the pillars; therefore, the pillars also block the glide of misfit dislocations.
The influence of the presence or absence of HCl during gate oxidation at 1100 and 1150 °C on the electrical and structural properties of polycrystalline silicon thin-film transistors was investigated. Devices processed without HCl exhibited a lower leakage current, a larger current switching ratio, a 25%–55% increase in carrier mobility, and a 21%–30% lower grain-boundary trap density. Materials investigation showed that the improvement was not primarily due to an increase in grain size, which was about 1000 Å irrespective of processing conditions, but to a change in crystallite orientation. X-ray diffraction measurements showed an increased {110} texture in devices processed without HCl. In addition, in the latter devices, dopant diffusion under the gate from the source and drain contacts was reduced by 7.6% to 12%. A linear relationship was observed between the average grain-boundary trap density and the average lateral dopant diffusion length. These findings are explained by considering the point defect chemistry and its relationship to grain-boundary mobility. Removal of HCl from the oxidation ambient increases the concentration of silicon self-interstitials. The increased migration of these point defects to grain boundaries enhances their nonconservative motion and therefore increases grain-boundary mobility. This in turn, allows the boundary to more completely acquire a lower-energy configuration. This lower-energy configuration of the boundary contains a minimum of broken bonds and therefore minimizes both the grain-boundary electrical activity and the diffusion along grain boundaries.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.