A few XPS spectra of Al metal on Co at different oxygen exposure doses (the numbers represent the total oxygen exposure expressed in langmuir and are valid for all the traces, from bottom to top). (a) and (b) regions. Note the shift of energy level, the broadening of , and the appearance of oxidized cobalt levels with increasing the oxygen exposure dose.
The BE (squares, left axis) and FWHM (circles, right axis) of energy level as a function of oxygen exposure for (from top to down) 6, 8, and Al on Co. The vertical dashed lines represent the oxygen exposures of the three samples ( Al) for which the pentacene energy level alignment was measured by UPS; see Sec. III C.
(a) Binding energy shift and (b) FWHM of (squares) and (circles) as a function of the initial Al layer thickness.
(a) UPS spectra of the optimum oxidized Al layers showing the top of valence band of AlOx (VB). (b) The BE of VB as a function of the oxidation of Co for the sample (see the text). (c) BE of VB of AlOx as a function of initial thickness of the Al layer, .
(a), (b) UPS spectra of pentacene deposited on Co in the region of secondary electron cutoff and Fermi level, respectively. (c), (d) UPS spectra in the case of insertion of an optimum oxidized Al layer between Co and pentacene. The dashed lines are a guide to the eyes and represent the FL and the evolution of HOMO level with increasing pentacene coverage.
(a) The variation of , the binding energy shift of the VL at the Co/AlOx interface, the VB of AlOx, , and for the three differently oxidized Al samples. The hole injection barrier decreases with the oxidation of the underlying Co. (b) The variation of and binding energy shifts of VL (at the Co/AlOx interface) and VB as a function of the initial Al layer thickness, . The hole injection barrier increases as a function of .
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