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This dissertation presents research findings on the three materials systems: lateral Si nanowires (SiNW), In2Se3/Bi2Se3 heterostructures and graphene. The first part of the thesis was focused on the growth and characterization of lateral SiNW. Lateral here refers to wires growing along the plane of substrate; vertical NW on the other hand grow out of the plane of substrate. It was found, using the Au-seeded vapor – liquid – solid technique, that epitaxial single-crystal SiNW can be grown laterally along Si(111) substrates that have been miscut toward [11− 2]. The ratio of lateral-to-vertical NW was found to increase as the miscut angle increased and as disilane pressure and substrate temperature decreased. Based on this observation, growth parameters were identified whereby all of the deposited Au seeds formed lateral NW. Furthermore, the nanofaceted substrate guided the growth via a mechanism that involved pinning of the trijunction at the liquid/solid interface of the growing nanowire.
Next, the growth of selenide heterostructures was explored. Specifically, molecular beam epitaxy was utilized to grow In2Se3 and Bi2Se3 films on h-BN, highly oriented pyrolytic graphite and Si(111) substrates. Growth optimizations of In2Se3 and Bi2Se3 films were carried out by systematically varying the growth parameters. While the growth of these films was demonstrated on h-BN and HOPG surface, the majority of the effort was focused on growth on Si(111). Atomically flat terraces that extended laterally for several hundred nm, which were separated by single quintuple layer high steps characterized surface of the best In2Se3 films grown on Si(111). These In2Se3 films were suitable for subsequent high quality epitaxy of Bi2Se3 .
The last part of this dissertation was focused on a recently initiated and ongoing study of graphene growth on liquid metal surfaces. The initial part of the study comprised a successful modification of an existing growth system to accommodate graphene synthesis and process development for reproducible graphene growth. Graphene was grown on Cu, Au and AuCu alloys at varioua conditions. Preliminary results showed triangular features on the liquid part of the Cu metal surface. For Au, and AuCu alloys, hexagonal features were noticed both on the solid and liquid parts.
Next, the growth of selenide heterostructures was explored. Specifically, molecular beam epitaxy was utilized to grow In2Se3 and Bi2Se3 films on h-BN, highly oriented pyrolytic graphite and Si(111) substrates. Growth optimizations of In2Se3 and Bi2Se3 films were carried out by systematically varying the growth parameters. While the growth of these films was demonstrated on h-BN and HOPG surface, the majority of the effort was focused on growth on Si(111). Atomically flat terraces that extended laterally for several hundred nm, which were separated by single quintuple layer high steps characterized surface of the best In2Se3 films grown on Si(111). These In2Se3 films were suitable for subsequent high quality epitaxy of Bi2Se3 .
The last part of this dissertation was focused on a recently initiated and ongoing study of graphene growth on liquid metal surfaces. The initial part of the study comprised a successful modification of an existing growth system to accommodate graphene synthesis and process development for reproducible graphene growth. Graphene was grown on Cu, Au and AuCu alloys at varioua conditions. Preliminary results showed triangular features on the liquid part of the Cu metal surface. For Au, and AuCu alloys, hexagonal features were noticed both on the solid and liquid parts.
ContributorsRathi, Somilkumar J (Author) / Drucker, Jeffery (Thesis advisor) / Smith, David (Committee member) / Chen, Tingyong (Committee member) / Arizona State University (Publisher)
Created2014
Description
This dissertation describes the characterization of optoelectronic and electronic materials being considered for next generation semiconductor devices, primarily using electron microscopy techniques. The research included refinement of growth parameters for optimizing material quality, and investigation of heterostructured interfaces. The results provide better understanding of the fundamental materials science and should lead to future improvements in device applications.
A microstructural study of tin selenide and tin manganese selenide thin films grown by molecular beam epitaxy (MBE) on GaAs (111)B substrates with different Se:Sn flux ratios and Mn concentrations was carried out. Low flux ratios lead to highly defective films, mostly consisting of SnSe, whereas higher flux ratios gave higher quality, single-phase SnSe2. The ternary (Sn,Mn)Se films evolved quasi-coherently, as the Mn concentration increased, from SnSe2 into a complex lattice, and then into MnSe with 3D rock-salt structure. These structural transformations should underlie the evolution of magnetic properties of this ternary system reported earlier in the literature.
II-VI/III-V compound semiconductor heterostructures have been characterized for growth in both single- and dual-chamber MBE systems. Three groups of lattice-matched materials have been investigated: i) 5.65Å materials based on GaAs, ii) 6.1Å materials based on InAs or GaSb, and iii) 6.5Å materials based on InSb. High quality II-VI materials grown on III-V substrates were demonstrated for ZnTe/GaSb and CdTe/InSb. III-V materials grown on II-VI buffer layers present additional challenges and were grown with varying degrees of success. InAsSb quantum wells in between ZnTe barriers were nearly defect-free, but showed 3D island growth. All other materials demonstrated flat interfaces, despite low growth temperature, but with stacking faults in the II-VI materials.
Femtosecond laser-induced defects (LIDs) in silicon solar cells were characterized using a variety of electron microscopy techniques. Scanning electron microscope (SEM) images showed that the intersections of laser lines, finger and busbar intersections, exhibited LIDs with the potential to shunt the contacts. SEM and transmission electron microscope (TEM) images correlated these LIDs with ablated c-Si and showed these defects to come in two sizes ~40nm and ~.5µm. The elemental profiles across defective and non-defective regions were found using energy dispersive x-ray spectroscopy.
A microstructural study of tin selenide and tin manganese selenide thin films grown by molecular beam epitaxy (MBE) on GaAs (111)B substrates with different Se:Sn flux ratios and Mn concentrations was carried out. Low flux ratios lead to highly defective films, mostly consisting of SnSe, whereas higher flux ratios gave higher quality, single-phase SnSe2. The ternary (Sn,Mn)Se films evolved quasi-coherently, as the Mn concentration increased, from SnSe2 into a complex lattice, and then into MnSe with 3D rock-salt structure. These structural transformations should underlie the evolution of magnetic properties of this ternary system reported earlier in the literature.
II-VI/III-V compound semiconductor heterostructures have been characterized for growth in both single- and dual-chamber MBE systems. Three groups of lattice-matched materials have been investigated: i) 5.65Å materials based on GaAs, ii) 6.1Å materials based on InAs or GaSb, and iii) 6.5Å materials based on InSb. High quality II-VI materials grown on III-V substrates were demonstrated for ZnTe/GaSb and CdTe/InSb. III-V materials grown on II-VI buffer layers present additional challenges and were grown with varying degrees of success. InAsSb quantum wells in between ZnTe barriers were nearly defect-free, but showed 3D island growth. All other materials demonstrated flat interfaces, despite low growth temperature, but with stacking faults in the II-VI materials.
Femtosecond laser-induced defects (LIDs) in silicon solar cells were characterized using a variety of electron microscopy techniques. Scanning electron microscope (SEM) images showed that the intersections of laser lines, finger and busbar intersections, exhibited LIDs with the potential to shunt the contacts. SEM and transmission electron microscope (TEM) images correlated these LIDs with ablated c-Si and showed these defects to come in two sizes ~40nm and ~.5µm. The elemental profiles across defective and non-defective regions were found using energy dispersive x-ray spectroscopy.
ContributorsTracy, Brian David (Author) / Smith, David J. (Thesis advisor) / Bennett, Peter A (Committee member) / Drucker, Jeffery (Committee member) / Mccartney, Martha R (Committee member) / Zhang, Yong-Hang (Committee member) / Arizona State University (Publisher)
Created2018
Description
The research of this dissertation involved quantitative characterization of electrostatic potential and charge distribution of semiconductor nanostructures using off-axis electron holography, as well as other electron microscopy techniques. The investigated nanostructures included Ge quantum dots, Ge/Si core/shell nanowires, and polytype heterostructures in ZnSe nanobelts. Hole densities were calculated for the first two systems, and the spontaneous polarization for wurtzite ZnSe was determined. Epitaxial Ge quantum dots (QDs) embedded in boron-doped silicon were studied. Reconstructed phase images showed extra phase shifts near the base of the QDs, which was attributed to hole accumulation in these regions. The resulting charge density was (0.03±0.003) holes
m3, which corresponded to about 30 holes localized to a pyramidal, 25-nm-wide Ge QD. This value was in reasonable agreement with the average number of holes confined to each Ge dot determined using a capacitance-voltage measurement. Hole accumulation in Ge/Si core/shell nanowires was observed and quantified using off-axis electron holography and other electron microscopy techniques. High-angle annular-dark-field scanning transmission electron microscopy images and electron holograms were obtained from specific nanowires. The intensities of the former were utilized to calculate the projected thicknesses for both the Ge core and the Si shell. The excess phase shifts measured by electron holography across the nanowires indicated the presence of holes inside the Ge cores. The hole density in the core regions was calculated to be (0.4±0.2)
m3 based on a simplified coaxial cylindrical model. Homogeneous zincblende/wurtzite heterostructure junctions in ZnSe nanobelts were studied. The observed electrostatic fields and charge accumulation were attributed to spontaneous polarization present in the wurtzite regions since the contributions from piezoelectric polarization were shown to be insignificant based on geometric phase analysis. The spontaneous polarization for the wurtzite ZnSe was calculated to be psp = -(0.0029±0.00013) C/m2, whereas a first principles' calculation gave psp = -0.0063 C/m2. The atomic arrangements and polarity continuity at the zincblende/wurtzite interface were determined through aberration-corrected high-angle annular-dark-field imaging, which revealed no polarity reversal across the interface. Overall, the successful outcomes of these studies confirmed the capability of off-axis electron holography to provide quantitative electrostatic information for nanostructured materials.
m3, which corresponded to about 30 holes localized to a pyramidal, 25-nm-wide Ge QD. This value was in reasonable agreement with the average number of holes confined to each Ge dot determined using a capacitance-voltage measurement. Hole accumulation in Ge/Si core/shell nanowires was observed and quantified using off-axis electron holography and other electron microscopy techniques. High-angle annular-dark-field scanning transmission electron microscopy images and electron holograms were obtained from specific nanowires. The intensities of the former were utilized to calculate the projected thicknesses for both the Ge core and the Si shell. The excess phase shifts measured by electron holography across the nanowires indicated the presence of holes inside the Ge cores. The hole density in the core regions was calculated to be (0.4±0.2)
m3 based on a simplified coaxial cylindrical model. Homogeneous zincblende/wurtzite heterostructure junctions in ZnSe nanobelts were studied. The observed electrostatic fields and charge accumulation were attributed to spontaneous polarization present in the wurtzite regions since the contributions from piezoelectric polarization were shown to be insignificant based on geometric phase analysis. The spontaneous polarization for the wurtzite ZnSe was calculated to be psp = -(0.0029±0.00013) C/m2, whereas a first principles' calculation gave psp = -0.0063 C/m2. The atomic arrangements and polarity continuity at the zincblende/wurtzite interface were determined through aberration-corrected high-angle annular-dark-field imaging, which revealed no polarity reversal across the interface. Overall, the successful outcomes of these studies confirmed the capability of off-axis electron holography to provide quantitative electrostatic information for nanostructured materials.
ContributorsLi, Luying (Author) / McCartney, Martha R. (Thesis advisor) / Smith, David J. (Thesis advisor) / Treacy, Michael J. (Committee member) / Shumway, John (Committee member) / Drucker, Jeffery (Committee member) / Arizona State University (Publisher)
Created2011