Epitaxial Formation of Rare Earth Silicides by Rapid Annealing
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ABSTRACT Rapid electron beam and lamp heating have been used to form thin epitaxial films of rare-earth silicides by reacting overlayers of the rare earths with (111) Si substrates. Of the metals Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, all but Gd are found to form epitaxial silicide layers by rapid solid-phase reaction, while silicides of Gd, Dy, Tm, Yb and Lu have been formed epitaxially by liquid phase reaction. For all but Er this is the first demonstration of epitaxial growth on Si. Details obtained from ion beam channeling analysis and transmission electron microscopy confirm the expected epitaxial structure and also show that the Si vacancies in the silicide form an ordered superlattice, rather than a random array as had been assumed before. INTRODUCTION Metal silicide films on Si substrates have been studied extensively over the last few years[1-4], primarily because of their wide application in the semiconductor industry as contacts and as low resistance interconnects. Epitaxial silicides are of particular interest for fundamental studies of the interface[3], as well as for possible unique applications. A number of metals had been shown to form epitaxial silicides, including PtSi, NiSi 2, CoSi 2 , Pd Si[1-3], and some of the refractory metals.[4] Recently, we reported ?hat several of the Rare Earth (RE) metals can be reacted with rapid e-beam heating to form epitaxial silicides as well.[5]
These included
silicides of Y, Tb, Ho, Er, Tm, Yb, and Lu, which form epitaxial layers by rapid solid-phase reaction, and silicides of Tm, Yb, Lu, Gd and Dy, which form epitaxially by liquid phase reaction.
The RE silicides are particularly interesting because they form the lowest known Schottky barrier heights (0- 0.3-0.4 eV) on n-type Si, suggesting applications in infrared detectors and as ohmic contacts.[6,7J
Fig. 1. Temperature history of the sample, surface of the Er/Si(ll)
1600
34 keV
1400
55 kW/cff2l 249 cm/s
L 1200
410o00 treated with a 55 kW/cm e-beam with an effective FWHM dwell time of 434 ps. 800 600 400 0.2
0.6
1.0
1.4
TIME (msec)
*This work performed at Sandia National Laboratories supported by the U.S. Department of Energy under contract number DE-ACO4-76DPOO789.
Mat. Res. Soc. Symp. Proc. Vol. 54. ' 1986 Materials Research Society
262
TABLE I.
Lattice constants[12,15], lattice match to (111) Si, and experimental ion channeling minimum yields for RESi 1 7 :
Mismatch to Si(111)
Tm
3.831 4.121
3.816 4.107
3.798 4.088
3.768 4.070
3.784 4.098
3.747 4.046
0.05
-0.36
-0.75
-1.22
-2.0
-1.59
-2.55
Tb
3.842 4.144
3.877 4.172
3.847 4.146
(%) 0.0
0.83
a (A) c (A)
Lu -
Er
Gd
Dy
Yb
Ho
Y
Observed Xmi e-beam •%)
lamp
26
--
85
--
81
35
60
71
78
(%) 69
--
..
90
78
44
95
-25
72
liquid (%) --
93
57
75
20
However, a difficulty with the RE silicides is that solid-phase furnace reactions between deposited thin films of these metals and single crystal Si typically result in a rough, pitted surface.[8,9J We have shown that electron beam heating wi
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