High resolution TEM imaging of CdTe [electronic resource]

HIGH RESOLUTION TRANSMISSION ELECTRON MICROSCOPY OF CdTe Tsutomu T. Yamashita, Ph.D. Stanford University, 2015 Reading Committee Member: Robert Sinclair Cadmium Telluride (CdTe) is an II-VI compound semiconductor which has become an important material used in solar cells with over 10 GW of installed capacity. It has other important applications as infrared sensors and radiation detectors when alloyed with other elements. Transmission electron microscopy (TEM) is a useful technique to study this type of materials on a small scale down to near atomic resolution in order to try to relate the structure of the material to other functional properties. In the first part of the present study, a Titan 80-300 TEM equipped with an aberration corrector is used to determine the conditions necessary to obtain atomic resolution in CdTe imaged in the [011] projection. The NCSI or Negative spherical aberration (Cs) Imaging technique was used to resolve the cadmium and tellurium atoms separated by 0.16 nm. Computed images are used to verify that imaging conditions result in clear atomic resolution. In the second part of the study, high resolution TEM is used to examine various types of dislocations found in CdTe. The dislocation types and the reactions between them are identified. Their motion and reactions were observed at near atomic resolution dynamically using a video camera attached to the TEM. For example, a motion of Shockley partial dislocations and diffusion driven climb of Frank dislocations were imaged in-situ in the microscope. Other effects such as surface reconstruction that occurs during TEM investigation were also recorded, also at near atomic resolution. The third part of the study was on high resolution TEM investigations of CdS/CdTe heterojunction interfaces. This structure is the basis for the great success that CdTe-based photovoltaics have had in recent years. CdS films were deposited on single crystal CdTe substrates by electron beam evaporation or by chemical vapor deposition. Different crystal orientations and surface treatments of the CdTe substrate result in a variety of CdS film structures, and different epitaxial relationships develop between CdS with hexagonal wurtzite structure and CdTe with cubic sphalerite structure. Photovoltaic measurements were made on some of the junctions, and some conclusions can be drawn from the microstructural characterization. The best photovoltaic performance was obtained for CdS film with hexagonal wurtzite structure forming an epitaxial film on CdTe {111} surface. The clean and abrupt interface between the CdS and CdTe lead to high open circuit voltage, Voc, and high short-circuit current, Jsc as well as excellent fill factor. The junction characteristics with light are surprisingly robust against a variety of microstructures of the CdS-CdTe heterojunction interface. This may be one of the reasons why this structure has had great commercial success in delivering a low cost solution in the photovoltaic arena. The fourth part of the study is on characterization of grown-in defects in tellurium-doped GaAs single crystals prepared by the liquid encapsulated Czochralski (LEC) method. Different sections of the boule were examined by X-ray topography, conventional and high resolution TEM methods in order to characterize the types of defects present in the boule. Outer regions of the boule have dislocations that are pinned by precipitates. Both helical and bowed dislocation lines were observed. Towards the inside of the boule, a high density (~1014 cm-3) of microdefects was found, and high resolution TEM analysis show that they are Frank loops containing extrinsic stacking faults. GaAs films grown on the (100) orientation LEC-GaAs substrates by molecular beam epitaxy (MBE) were examined in plan view and in cross-section by conventional and high resolution TEM to characterize the defects that can be generated from the substrate or from the interface between the substrate and the MBE GaAs. In addition, oval defects on MBE GaAs films were studied by scanning electron microscopy (SEM) and by conventional TEM to ascertain how they form. An anisotropic etch was used to determine that the long axis of the oval defects are always aligned along the < 110> direction on the (100) wafer, and the smooth side-wall of the oval defects are oriented close to the {111}Ga faces. The base of the oval defects contains dislocation tangles that originate from the GaAs substrate. The oval defects are typically 5 to 15 m in size, and they interfere with device structures put on top of the wafer. Additional work on MBE GaAs includes the characterization of whiskers that formed on the film surface during unusual growth conditions of the film. A high density of whiskers can grow when the substrate temperature is low (< 550oC) and the substrate is not cleaned completely prior to the MBE deposition. The whiskers can reach a length of 100 m or more when the GaAs film growth is targeted for 2 to 3 m in thickness. It was determined by X-ray diffraction and by TEM analysis that the whiskers have the hexagonal wurtzite crystal structure whereas GaAs itself has the cubic sphalerite structure. The lattice parameters were co = 0.653 nm and ao = 0.400 nm, which is consistent with the cubic GaAs structure with ao = 0.565 nm transforming to the hexagonal structure (e.g., (〖a 〗_o (cubic) )/√2 = 0.400 = ao (hexagonal). The growth direction is along the c-direction of the whisker. Additionally, the growth is always along the {111}Ga direction with respect to the (100) GaAs wafer, which is at an angle of 54.7o to the surface. There are many basal plane stacking faults inside the whisker. The whiskers are generally more resistive compared to the surrounding GaAs film as they are seen to charge-up during electron microscopy. The actual growth mechanism could not be determined, but it is not by the vapor-liquid-solid (VLS) growth mechanism as there were no liquid droplets observed at the tip of the whiskers and the surface is too rough for the VLS mechanism. High voltage TEM analysis of the base of the whiskers suggests that there may be a connection between the oval defects and the whisker growth, as both contain a high density of dislocations at the base, and they share similar morphology (at the base) and crystallographic orientations. The last part of the work is on the history of the development of thin film magnetic media for use in hard disk drives (HDD) from the early 1970s to the present. The important role that grain isolation plays in increasing the magnetic recording density from the adoption of the thin film media to the present is outlined. CoRe films were analyzed by conventional TEM and by Lorentz magnetic imaging to determine the relationship between the microstructure and the "ripple" patterns in Lorentz images of recorded magnetic bits. Various grain isolation schemes that were employed for CoPt-based films from high pressure sputtering to oxide addition and chromium segregation are shown and how they contributed to the development of sputtered thin film media as the dominant technology to be used in HDD. In conclusion, the conventional TEM and high resolution TEM techniques are very useful for determining the structure of materials and relating the observed microstructure and the defects present in the material to their functional properties. The TEM techniques will continue to play an important role in materials analysis and characterization in the future.

Keywords: Materials Science and Engineering.
Advisor: Sinclair Robert (Materials scientist) primary advisor. Thesis advisor
Nix William D.
Ponce Fernando