Formation of Large Conduction Band Discontinuities of Heterointerfaces Using CdF 2 AND CaF 2 on Si(111)
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ABSTRACT We proposed use of a new CdF 2/CaF 2 heterointerface for the formation of large conduction band discontinuities to apply quantum effect devices fabricated on Si substrates. Resonant tunneling diodes using this heterointerface on Si were fabricated and negative differential resistance whose P/V current ratio of 24 at highest was observed at room temperature.
INTRODUCTION Quantum effect devices fabricated on Si substrate will be very attractive in future [1]. Abrupt heterointerfaces which have the large conduction band discontinuity (AEJ) are very desirable for them. Such heterointerfaces are very useful not only for the operation of electrical devices such as resonant tunneling devices but also for short wave length emission of intersubband laser [2]. A lot of different materials such as CaF 2 [3], Si0 2 [4-8], ZnS [9], ternaries based on Mg and Se, and SiGeC [10] were proposed for application in quantum effect devices.To date, most successful resonant tunneling diode (RTD) fabricated on Si substrates has been obtained by the III-V heterojunctions on Si [11]. However, in the III-V on Si case, a fairly thick buffer layer of III-V is necessary in order to obtain good crystallinity. Moreover, autodop ing between III-V and Si becomes problem in device fabrication process. Thus, materials and structures which do not have such problems and make it possible to realize quantum structures directly formed on Si surface are very desirable. We suggested a use of new CdF2/CaF 2 interface for the formation of large AEcs which is fabricated on Si. Some properties of CdF 2 and CaF 2 are summarized in Table 1 comparing with that of Si [12-15]. CdF2 and CaF 2 have the fluorite lattice structure nearly lattice matched to Si, so one can expect that they can be grown on Si epitaxially. Recently we realized epitaxial growth
t:Present address YAMAHA Corp., Iwata, Shizuoka 438-01, Japan 171 Mat. Res. Soc. Symp. Proc. Vol. 448 01997 Materials Research Society
CdF 2
Table 1 Material constants of CdF 2, CaF 2 and Si at room temperature. Materials
CdF 2
CaF 2
Si
Structure
Fluorite
Fluorite
Diamond
5.388
5.463
5.431
Mismatch with Si(%)
-0.8
0.6
-
Melting point (C)
1100
1360
1414
1.82x10"
2.5x t0.' 1.1
Lattice constant (A)
Thermal expansion coefat RT (deg"')
2. WxO1
Energy bandgap (eV)
8.0
12.11
Electron afTinity (eV)
4.1
0
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