Detection of the metastable Tll-type phase of PbSe in films grown bV electron beam evaporation: I. X-ray diffraction

摘要

PbSe crystallizes into the f.c.c. NaCl-type structure (O5h) at ambient conditions. It is known that PbSe, like the other lead salts PbTe and PbS, undergoes two high-pressure-induced structural phase transi-tions: to the orthorhombic Til-type phase (D162h) at pressures about 4.5 GPa and further to the CsCl-type (O1h) phase at about 16 GPa [1,2]. Thus PbSe is a material which possesses two high-pressure (HP) metastable crystal modifications. On the grounds of the structural investigations of PbTe and PbSe films grown by laser-assisted deposition (LAD) and elec-tron beam evaporation (EBE) it was shown that the metastable phases of these compounds can be grown in the form of films [3,4]. The mechanism of growth of the films was analysed in the framework of the general crystallization theory and it was concluded that the main reason for the formation of metastable phases is the high degree of supercooling (the low substrate temperature) and the non-equilibrium growth technique—LAD. Using LAD and varying the substrate temperature Ts in the range from 20 °C to 350 °C we succeeded in growing films containing the two metastable CsCl- and GeS-type phases of PbTe. However, in PbSe films grown at the same technological conditions only the CsCl-type metast-able phase of the compound was detected. The latter was related to the higher melting temperature of the stable f.c.c. PbSe phase as well as to the narrow range of substrate temperatures where the Tll-type PbSe phase could be grown. It was concluded that in LAD the fluctuations of the crystallization front temperature from pulse to pulse could be sufficient for the growth with the intermediate metastable Tll-type phase to be passed over [4]. Thus only the CsCl-type metastable phase was detected in PbSe films grown by LAD. Fluctuations in the crystalliza-tion front temperature are avoided in the non-equili-brium technique, EBE. In the EBE technique the evaporated material continuously reaches the sub-strate, leading in this way to a gradual rise of the crystallization front temperature. The technological conditions which are varied in this technique are substrate temperature Ts, substrate-to-target dis-tance L and electron beam power EB. It is found that the films stoichiometry reproduces the targel one when L is about 20 cm, EB is in the range between 100 W and 300 W and Ts is maintained al different temperatures between 150 °C and 350 °C [5].