Tetragonal Au-Ni (001) Solid Solutions Grown by MBE
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CODEPOSITED SAMPLES 4.10
Preparation Good quality Au(001) buffer layers (50 nm in thickness) having a lx5 surface reconstruction were obtained on a MgO(001) substrate via a thin (2 nm) Fe seed layer as has been described elsewhere [5]. AulxNix solid solution layers were then grown at room temperature by coevaporation from two e-beam evaporators. The fluxes were controlled by quartz microbalances previously calibrated using RHEED oscillations. Samples having 20, 30, 40, 50 and 60 at.% Ni and a thickness of approximately 50 nm were prepared in this way. RHEED intensity oscillations showed that the growth of the first layers was 2D in character (see the inset in fig. 1). The
,4.05
00
4.00
3.95 0
50
100 150 200 250 300 350
Thickness (ML) Fig. 1: The in-plane lattice parameter as a function of thickness for a Au60Ni 4 0 deposit. Inset: RHEED intensity as a function of the thickness in ML. 169
Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
diffraction rods in the RHEED pattern were recorded with a video camera and analyzed so as to give the evolution of the in-plane lattice parameter with the thickness of the deposit. An example is shown in fig. 1 for Au60Ni4O. X-Ray Measurements The in-plane (a//) and out-of-plane (aj) lattice parameters of the codeposited samples were measured using grazing incidence X-Ray diffraction (GIXD) as well as non-symmetric X-ray diffraction on the (113) line. The results, which are presented in table 1 and in fig. 2 (the white circles) show a strong tetragonality, since a// is always considerably larger than al. The tetragonality observed represents misfit which has not been fully accomodated by the relaxation of the AulxNix layer, and if the alloy is completely disordered, the resulting stresses in the layer are very large. However, if the alloy becomes ordered, the stresses in the layer may be reduced, lowering the elastic energy of the system.
The values of the in-plane lattice parameters determined by X-ray diffraction are the same as those found using RHEED measurements at the end of the growth of the sample (see fig. 1). Since RHEED measures the lattice parameter of material near the surface while GIXD measures the lattice parameter of the deposit as a whole, it is apparent that the relaxation of the AuI-xNix deposit is such that the region close to the buffer layer is relaxed to the same extent as the region near the surface. Table 1: Lattice Parameters Sample a// Au8 oNi 20 4.03 Au 70 Ni 30 4.00 Au 60 Ni 40 3.96 Au50 Ni 50 3.90 Au40 Ni 6o 3.80 Pure Au 4.08 AuNi-L10 [3] 3.83
4.1 Au 4.0 • 3.9
0
3.8 AuNi3 3
a_L_ 3.90 3.79 3.78 3.76 3.71 4.08 3.72
Au 3 Ni
AuNi 00
A
3.6
3.6
00 (
I
I
3.7
3.8
1
3.9
a, (A)
I
4.0
4.1
Fig. 2: a_L as a function of all for various Auj-xNix compositions (white circles). The black circles give results determined by other workers.
Electron Microscopy Plan view and [110] cross-sectional TEM specimens were prepared by ion milling and examined in a JEOL 4000EX transmission electron microscope operating at 400 kV.
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