Summary Abstract: Lowenergy electron transmission through Cu/Ni quantum wellsQiGao Zhu, Yunong Yang, Ellen D. Williams, and Robert L. Park Citation: Journal of Vacuum Science & Technology A 5, 2065 (1987); doi: 10.1116/1.574920 View online: http://dx.doi.org/10.1116/1.574920 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/5/4?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Summary Abstract: Lowenergy electron microscopy J. Vac. Sci. Technol. A 6, 573 (1988); 10.1116/1.575165 Summary Abstract: Lowenergy electron diffraction studies of incommensurate xenon layers on Cu(110) J. Vac. Sci. Technol. A 5, 711 (1987); 10.1116/1.574278 Summary Abstract: Surface segregation of Cu–Ni alloys J. Vac. Sci. Technol. A 3, 818 (1985); 10.1116/1.573319 Summary Abstract: Quantum size effect in electron transmission through Cu and Ag films on W(110) J. Vac. Sci. Technol. A 1, 1062 (1983); 10.1116/1.572042 Summary Abstract: Auger study of interdiffusion in Cu/Ni thin films J. Vac. Sci. Technol. 20, 466 (1982); 10.1116/1.571335

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Summary Abstract: Low-energy electron transmission through Cu/Ni quantum wellsS

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Qi-Gao Zhu, Yunong Yang, Ellen D. Williams. and Robert L. Park Department 0/ Physics and Astronomy, University o/Maryland, College Park, Maryland 20742

(Received 19 September 1986; accepted 14 October 1986)

Taking advantage of the difference in energy between the lower band edges (Xl point) of the a, bands ofCuand Ni along the [001] direction, a Ni/Cu/Ni sandwich structure can form a quantum well with energy between 7.3 eV (Xl point for Cu) and 9.5 eV (Xl point for Ni) above the Fermi level. Low-energy electron transmission has been measured as a function of incident energy on both single and multi quantum wells formed by alternate evaporation ofNi and Cu thin films on the (001) Ni substrate.

For a single quantum wen, the dependence of the trans­mission spectra on the thickness of the deposited Ni layer which forms the outer barrier ofthe well has been measured for different widths of the well (i.e., Cu thickness). A typical

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Vacuum: Ni : Cu: Ni (001)

9,3eV

8 10 12 14 i6 18

INCIDENT ENERGY (eV)

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FIG.!. Experimental plots of the negative second derivative of the transmis­sion current vs electron energy for different thickness of the Ni layer depos­ited on a 7-A Cu/Ni (001) substrate, The arrows on the horizontal axis mark the bottom and the top of the quantum well (7,3 and 9.5 eV above E F)' The broken lines indicate the thickness dependence of the QSE oscilla­tions. The insert shows the schematic of the potential diagram of the single quantum well.

series of spectra for a well width of 7 A is shown in Fig. 1 in the energy range above the bottom of the well. Within the energy range of the well, the structures are essentially stabi­lized, i.e., independent of the thickness of the barrier. We believe that these stabilized structures originate from the res­onant states. I The dependence of the resonant energy on the width of the well is in good agreement with calculated values of the lowest bound state in the well in accordance with the characteristic of the resonant states for a sufficiently thick (-1-2 A) barrier.1 Thus we have first observed resonant states in transmission spectra through a vacuum/Ni/Cu/Ni quantum well entity. On the other hand, the spectra exhibit pronounced thickness dependence above the well. These fea­tures are attributed to the ordinary quantum size effect (QSE) oscillations.2•3

For multi quantum wells, transmission spectra with successive deposition of unit cells have been measured. Here a unit cell is defined as a pair of deposited Ni and Cu layers and in our case, each layer has the same thickness. In Fig. 2 are plotted the transmission spectra for different numbers of unit cells (the thickness of each unit cell is 14 A). It is appar­ent that the structures are stabilized with respect to increas-

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FIG. 2. Transmission spectra for successive deposition of unit cells. The definition of the unit cell is shown in the insert. The broken lines show the stabilization of the structures.

2065 J. Vac. Sci. Technol. A 5 (4), Jul/Aug 1987 0734-21011871042065-02$01.00 @ 1987 American Vacuum Society 2065

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2066 Zhu et sl.: Summary Abstract: Low-energy electron transmission through quantum wells 2066

ing thickness above three unit cells. The energy positions associated with these structures are in good agreement with calculated values of the lower band edges of the allowed bands in such a Ni/Cu superlattice.4 This suggests that the system with only a few periodic unit cells has already re­vealed the superlattice regime in the band structure. Three allowed bands are identified in the energy range of 7.3 to 18 eV above EF • Similar spectra for a unit cell thickness of7 A

J. Vac. Sci. Technol. A, Vol. 5, No.4, Jul/Aug 1987

have been measured. Two allowed bands are observed in the same energy range, again in good agreement with the calcu­lated band edges.

oj Work supported by NSF Grant No. DMR-85-04163. lI. Bartos and S. G. Davison, Solid State Commun. 56, 1,69 (1985). 2B. T. Jonker, N. C. Bartelt, and R. L. Park, Surf. Sci. 127,183 (1983). 3H. Iwasaki, B. T. Jonker, and R. L. Park, Phys. Rev. B 32,643 (1985). 4Q._G. Zhu, Y. Yang, E. D. Williams, and R. L. Park (in preparation).

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