Study On The Epitaxial Growth Of CdTe/PbTe Materials And Their Physical Properties

PbTe is a member of IV-VI group compound semiconductors with the traits of direct transition and narrow bandgap, and it emits photons in the mid-infrared (MIR) range at room temperature. Compared to its counterparts belonging to III-V goup, it features the absense of a degenerate heavy hole band and low Auger recombination rates, etc. Therefore, it finds a broad applications in the opto-electronic devices operating in the MIR range. Moreover, PbTe posseses the natural merits of a low thermal conductivity and a large Seebeck coefficient, making it an ideal material in thermoelectrics for waste heat harvesting and recycling into electrical power. CdTe/PbTe heterostructures are an important material system, of which the opto-electronic and thermoelectric properties have been extensively studied due to their nearly identical lattice parameters, totally different lattice structures (PbTe:NaCl type; CdTe:Zinc blende type), and pretty large bandgap difference etc..After a thorough investigation of the epitaxial growth of PbTe and CdTe thin films, this thesis focuses on the energy levels related to intrinsic defects in PbTe, and then further explores the optical and electrical properties of CdTe/PbTe(111) hetero-system. The innovative research results are as follows:1. Regular shape defects on the surface of PbTe thin films grown by molecular beam epitaxy (MBE) were studied by scanning electron microscope (SEM). Two types of regular shape defects were observed on Te-rich PbTe films grown at a substrate temperature T>235℃ with a beam flux ratio of Te to PbTe (Rf) to be 0.5 and at 280℃ with a Rf≥0.4, which include cuboids and triangular pyramids. The formation mechanism of the observed regular shape defects is interpreted as following:They are the outcome of fast growth rate along {100} crystal planes that have the lowest surface energy and the enclosure of the {100} crystal planes. The formation of the regular shape defects in the growth of PbTe needs appropriate substrate temperature and Te-rich ambience. However, when Rf is decreased low enough to make the films slightly Pb-rich, triangular pits that originate from the insufficient glide of the threading dislocations along the main <110>{100} glide system of PbTe in Cottrell atmosphere, will be the main feature on the film surface.2. We directly grew CdTe (111) on BaF2 substrates using molecular beam epitaxy (MBE). In situ characterization of reflection high energy electron diffraction (RHEED) revealed growth modes transition from 2D to 3D. XRD analysis results verified the single crystalline property of the as-grown films. Theoretical method was adopted to fit the measured near infrared transmission spectrum, revealing the CdTe energy gap of 1.511 eV at room temperature.3. Step-scan Fourier-transform infrared photoreflectance and modulated photoluminescence spectroscopy were used to characterize the optical transitions of the epitaxial PbTe thin film grown by molecular beam epitaxy on BaF2 (111) substrate in the vicinity of energy gap of lead telluride at 77 K. It is found that the intrinsic defect energy levels in the electronic structure are of resonant nature. The Te-vacancy energy level is located above the conduction band minimum by 29.1 meV. Another defect (Vx) energy level situated below valance band maximum by 18.1 meV is also revealed. Whether it is associated with the Pb vacancy is still not clear. It might also be related to the misfit dislocations stemming from the lattice mismatch between PbTe and BaF2 substrate. The experimental results support the theory prediction (N. J. Parada and G. W. Pratt, Jr., Phys. Rev.Lett.22,180 (1969), N. J. Parada, Phys. Rev. B 3,2042 (1971)) and are consistent with the reported Hall experimental results (G. Bauer, H. Burkhard, H. Heinrich, and A. Lopez-Otero, J. Appl. Phys.47,1721 (1976)).4. Efficient phonon blockage by forming a polar CdTe/PbTe heterojunction is explicitly observed by Raman scattering. The unique phonon screening effect can be interpreted by recent discovery of high-density two dimensional electrons (2DEG) at the polar CdTe/PbTe(111) interface which paves a way for design and fabrication of thermoelectric devices.5. We performed high resolution TEM characterization of the interfacial atomic coordination of the novel PbTe/CdTe (111) heterostructure and studied the electronic transport properties of the 2DEG at the polar interface. The mismatch in the bonding configuration at the interface offers a distinctly different mechanism of 2DEG formation from the known 2DEG systems. Quantum oscillations are observed both in the magneto-resistance Rxx and Hall resistance Rxy. Evidence of a possible π Berry phase was observed by analyzing the quantum oscillations, indicating the Dirac nature of the 2D electron system at the interface. Our results suggest PbTe/CdTe heterostructures being a new candidate of topological crystalline insulators.