An Electron Microscopic Characterization of MBE-Grown ZnSe/CaAs and ZnSe/Ge Heterointerfaces
- PDF / 3,139,151 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 78 Downloads / 208 Views
AN ELECTRON MICROSCOPIC CHARACTERIZATION OF MBE-GROWN ZnSe/CaAs AND ZnSe/Ge HETEROINTERFACES S.B.
SANT*,
*Department
R.W.
SMITH* and G.C.
of Metallurgical
WEATHERLY**
Engineering,
Queen's
Ontario, Canada K7L 3N6 **Department of Metallurgy and Materials Science, Toronto, Ontario, Canada M5S IA4
University,
Kingston,
University of Toronto,
ABSTRACT Molecular beam epitaxy (MBE) grown ZnSe/GaAs and ZnSe/Ge heterointerfaces have been studied by transmission electron microscopy (TEM). Defect characterization of cross-sectional and planar specimens showed that ZnSe epitaxial films contain numerous twins that predominantly arise at the interface. Planar specimens of ZnSe/Ge were in-situ TEM annealed, for 5.5 hours at 873K . The twins are thermally very stable which would indicate that they arise during the growth process. The occurrence of these twins in the ZnSe film is explained by nucleation and growth of normal and twinned nuclei. Some of the ZnSe films grown on (10O)Ge substrates have low-angle boundaries indicating that the initial growth of the film is by the formation of islands.
INTRODUCTION ZnSe, a II-VI compound semiconductor with the zinc-blende structure and a direct energy band-gap of 2.7 eV at room temperature is a promising candidate for blue light emitting devices. Suitable substrates for the MBE growth of ZnSe are (lO0)GaAs and (10O)Ge because of their relatively small lattice mismatch with ZnSe, 0.25% and 0.17% respectively. TEM has been used to study the defects present in ZnSe films (1-6]. Although twins have been observed by a number of groups, to date, the mechanism of twin formation in epitaxial ZnSe films is rather unclear. Some stresses may have developed due to the lattice mismatch and thermal expansion coefficient difference between the film and the substrate and it is believed that in-situ TEM annealing of ZnSe/(100)Ge would provide some clues to a possible mechanism by studying the stability of the twins and other defects to this heating-cooling sequence. A comparison of in-situ TEM annealing of planar specimens and bulk vacuum annealing of ZnSe/(lO0)Ge as-grown film is reported elsewhere [5]. A model has been developed based on the free energy of formation of nuclei having the epitaxial relationship between the substrate and the film and those nuclei having a twin relationship [6]. It is known that twin orientations give rise to minima in the interfacial energy curves and thus it is likely that both the epitaxial and twin oriented nuclei are able to form.
EXPERIMENTAL PROCEDURE The growth of ZnSe films on (lO0)CaAs and (l0O)Ge substrates by MBE has been described earlier [7,8]. Cross-sectional and planar specimens tor TEM investigation were prepared by polishing, dimpling and ion milling as described previously (6]. The in-situ TEM annealing experiments have been described previously [5]. Both cross-sectional and planar specimens were studied using diffraction contrast in a Hitachi H-800 TEM operated at
Mat. Res. Soc. Symp. Proc. Vol. 198. 01990 Materials Research Society
43
Data Loading...
