Study On Epitaxial Growth And Properties Of IV-VI Semiconductor

IV-VI group semiconductors (such as PbTe, PbSe) are in possession of interesting intrinsic physical natures:They have symmetric band structures with narrow direct band gaps at the L point in the Brillouin zone, positive temperature coefficients of the band gaps ((dE8)/(dT)>0). The absence of the heavy hole band and low non-radiative Auger recombination rates which are more than one order of magnitude lower in comparison withⅢ-ⅤandⅡ-Ⅵmaterials, are other unique properties. These characteristics make them as promising materials for the development of mid-infrared optoelectronic devices that could be utilized in atmospheric pollution monitoring, toxic gas analysis systems, and human breath analysis in medical diagnostics and military. Therefore, the study on the epitaxial growth and the physical properties ofⅣ-Ⅵsemiconductor is of great importance. In the thesis, we have studied the growth of new PbTe/CdZnTe heterostructures and low dimensional structures as well as their optical and electrical properties. Molecular beam epitaxy (MBE) has been used for the growth of PbTe layers and PbSe quantum dots on the BaF2(111) substrates. In addition the PbTe/CdZnTe heterostructures have also been grown by MBE. The characterization of these materials were performed by atomic force microscopy (AFM), high resolution x-ray diffraction (HRXRD), high resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS) and photoluminescence (PL). The micro-structural and optoelectronic properties of the MBE grown PbSe quantum dots and PbTe/CdZnTe heterostructures were characterized by above technologies. The main conclusions are as follows:(1) Growth spirals with monolayer steps on PbTe surface are obtained by optimization of the growth parameters, such as using high Te/PbTe beam flux ratio (Gr≥0.33). It is found that the crystal qualities of PbTe grown on BaF2 substrates are improved and the defect densities are reduced as the growth temperatures increased from the 375°to 525°. The mid-infrared photoluminescence of PbTe/BaF2(111) were observed at different temperatures. (2) The self-assembled PbSe quantum dots were grown on the PbTe(111) buffer layer surfaces by molecular beam epitaxy. The shapes and sizes of PbSe quantum dots as a function of PbSe coverage were characterized using atomic force microscopy. It suggests that the high elastic anisotropy in the PbSe is main factor to impact the size and distribution of PbSe quantum dots. In addition, the stripe and two dimensional square PbTe templates were fabricated by electron beam lithography and wetting etching on the epitaxial PbTe layer.(3) Successful epitaxial growth of PbTe/Cd0.96Zn0.04Te (111) heterostructure using molecular beam epitaxy is described. The thermal misfit strain relaxation in epitaxial PbTe grown on Cd0.96Zn0.04Te (111) substrate has been studied by consideration on the effects of the growth temperature and the cooling rate. It is shown that the surface is dominated by terraces with triangular shape and anomalous spiral steps, which originate from the thermal misfit relaxation. Post-growth treatment with a slower cooling rate improves the crystalline quality of PbTe grown on Cdo.96Zn0.04Te (111). We found that the lattice fringes of the (200) plane in the PbTe rotated of 70.6°with respect to the (200) plane of Cd0.96Zn0.04Te at the interface.(4) The PbTe/CdTe (111) heterostructures were synthesized through molecular beam epitaxy and x-ray photoelectron spectroscopy has been used to measure the valence band offset at the PbTe/CdTe (111) heterostructure interface. A type-Ⅰband alignment with a valence band offset ofΔEv= 0.135±0.05 eV and conduction band offset ofΔEC=1.145 eV±0.05 eV is concluded. The piezoelectric field and band bending effects on the experimental results have been taken into consideration. Within the experimental error the directly obtained valence band offset is in agreement with theoretical prediction by the inclusion of spin-orbit interaction. In addition the origin of the nearly zero valence band offset in the PbTe/CdTe (111) heterostructure is analyzed.(5) Non-intentionally doped PbSe crystalline films were grown on insulating BaF2(111) by molecular beam epitaxy. Five scattering mechanisms limiting hole mobilities were theoretically analyzed, i.e. polar optical modes scattering, impurity scattering, acoustic mode scattering, piezoelectric acoustic modes scattering and non-polar optical modes scattering. The calculations show that in the temperatures between 77 and 295 K the scattering of polar optical phonon modes dominate the impact on the observed hole mobility in the epitaxial PbSe films.