Real-Time Studies of The Electrocrystallization of Nanoscale Ceramic Superlattices
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and tailoring of orientation and composition of the films. By pulsing the potential, abrupt interfaces are expected, but it is difficult to control the layer thickness. For galvanostatic control the thickness of the layers can be closely controlled, however, a concentration gradient can be present throughout the layers.7 Understanding the interfaces in superlattice films is essential for optimizing device structures. The abruptness of the interface or composition gradient throughout the layer can affect the electron transport properties and bandgap characteristics of the superlattice. Feenstra and coworkers have shown by STM that when GaSb is grown on InAs the interface is abrupt, but InAs grown on GaSb has Sb grading into the InAs overlayer. 8 Our earlier work using STM on the Pb-TI-O system showed that the apparent height profiles of cross sections of superlattices grown under potential control were more square than those in
superlattices grown under current control. 9 The composition of the Pb-TI-O conducting oxide superlattices can be systematically varied with either the potential or current. The end members of the series, T120 3 and PbO 2 , are n-type degenerate semiconductors with bandgaps of 1.4 and 1.8 eV, respectively. 10 In this study, we have utilized an in, situ technique in which the composition and interfaces of the Pb-TI-O superlattices can be examined during the growth process.11 123 Mat. Res. Soc. Symp. Proc. Vol. 382 ©1995 Materials Research Society
RESULTS AND DISCUSSION Potential and current-time waveforms for a superlattice grown between 230 and 70 mV vs SCE are shown in figure 1. The modulation wavelength is 10 nm for the superlattice. The 70 mV layer is 4 nm thick and the 230 mV layer is 6 nm thick. The applied potential waveform is square (figure IA). Figure lB shows the resulting current-time transient of the applied potential pulse. The region from I to 2 shows the initial double layer charging current, followed by a rising portion of the curve attributed to two dimensional growth. This time period lasts for 0.14 s and 0.6 nm of material is deposited. A plot of the region between 0.14 and I s (region 3 to 4) shows a t-1/ 2 dependence of the current with time, consistent with diffusion limitations of the current. Lu 250 S200A E cu
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Time (sec) Figure 1. Applied potential-time waveform (A) and the resulting current-time transient (B) for the electrodeposition of a Pb-TI-0 superlattice.
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When the diffusion coefficient of both TI(I) and Pb(II) in their individual solutions is measured, the diffiusion coefficient for TI is 2.1 x 10-6 cm 2/s and for Pb is 3.8 x 10-7 cm 2 /s. A Cottrell plot of the mixed Pb-TI solution gives a diffusion coefficient of 2.0 x 10-6 cm 2/s, which corresponds to the measured individual TI(I) diffusion coefficient. Therefore, the thallium species is diffusion controlled for the 230 mV pulse. This
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