Double Periodicity Formation in EuTe/PbTe Superlattices
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3 Mat. Res. Soc. Symp. Proc. Vol. 399 ©1996 Materials Research Society
RESULTS AND DISCUSSION We have previously reported that EuTe(x)/PbTe(y) SLs grown on the (11l) surface of BaF 2 substrates show double periodicity modulation for alloys grown at 543K [1]. The 1/2 order peaks indicate the existence of the double periodicity modulation in the originally designed nominal periodicity. We have observed 1/2 order peaks in the x-ray spectra obtained from samples with nominal periodicity of x=2-7 and y=3x. No 1/2 order satellite peaks were observed for samples with x/y = 3/24, 2/20 and 5/90, i.e., for samples with small ratios of x/y. The x-ray diffraction results of EuTe (x) / PbTe (y) short period superlattice samples are summarized in Table 1.This indicates that the thickness ratio of EuTe and PbTe layers affects the formation of the double periodicity. The x-ray diffraction spectrum in the vicinity of the Bragg (222) reflection shown in Fig. I is an example of the double periodicity observed in a EuTe/PbTe SL as seen by the 1/2 order satellite peaks in addition to the integral peaks. The nominal periodicity of this sample contains x=3.5 and y=9 monolayers. Figure 2 shows a ( 1 1 2) high-resolution lattice image from this sample. This image obviously exhibits double periodicity of the SL with x=7 and y=17 monolayers (right hand side of the figure) in addition to the nominal periodicity of x=3.5 and y=9 (left hand side of the figure). The wavelengths of nominal and double periodicities are knominal = 4.7 nm and Xdouble = 9.3 nm, respectively.
Table 1. Existence of the nominal and double periodicities in EuTe(x)/ PbTe(y) superlattices; O and x denote that the periodicity is observed and not observed, respectively. x/y nominal
periodicity
1/3
2/6
0
0(
0000000
X
0
0
double
periodicity
3/9 3.5/9 4/12 5/15 6/18 7/21
0
3/24 2/20 5/90
0
0 0
0
0
0
X
X
X
It is expected that the state with double periodicity is more stable than that with nominal periodicity because the total interfacial energy of double periodicity formation is lower than that of the nominal periodicity. The films were grown at the substrate temperature of Ts=543K and this temperature was maintained for 150 min. until the film growth was completed. Therefore, in order to reduce the total interfacial area interdiffusion can take place between the EuTe and PbTe layers during the growth especially in the region close to the buffer layer. This result indicates that the double periodicity SL must be a more stable state than the solid solution. The stability of PbizEuzTe alloys (z=0.5) grown on BaF2 substrates has been studied by L. Salamanca-Riba et. al[5]. The PblzEuzTe solid solution becomes stable as a result of the strain energy induced between the film and the PbTe buffer layer. The estimated critical temperature for decomposition is in the range of 600K < Tc < 960K[5]. In the case of EuTe(x)/PbTe(y) SL with the nominal periodicity of x=3.5 and y=9 the average composition is z=0.28 in the PblzEuzTe system. At the growth temperature Ts=54
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