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Description
HgCdTe is the dominant material currently in use for infrared (IR) focal-plane-array (FPA) technology. In this dissertation, transmission electron microscopy (TEM) was used for the characterization of epitaxial HgCdTe epilayers and HgCdTe-based devices. The microstructure of CdTe surface passivation layers deposited either by hot-wall epitaxy (HWE) or molecular beam epitaxy (MBE) on HgCdTe heterostructures was evaluated. The as-deposited CdTe passivation layers were polycrystalline and columnar. The CdTe grains were larger and more irregular when deposited by HWE, whereas those deposited by MBE were generally well-textured with mostly vertical grain boundaries. Observations and measurements using several TEM techniques showed that the CdTe/HgCdTe interface became considerably more abrupt after annealing, and the crystallinity of the CdTe layer was also improved. The microstructure and compositional profiles of CdTe(211)B/ZnTe/Si(211) heterostructures grown by MBE was investigated. Many inclined {111}-type stacking faults were present throughout the thin ZnTe layer, terminating near the point of initiation of CdTe growth. A rotation angle of about 3.5° was observed between lattice planes of the Si substrate and the final CdTe epilayer. Lattice parameter measurement and elemental profiles indicated that some local intermixing of Zn and Cd had taken place. The average widths of the ZnTe layer and the (Cd, Zn)Te transition region were found to be roughly 6.5 nm and 3.5 nm, respectively. Initial observations of CdTe(211)B/GaAs(211) heterostructures indicated much reduced defect densities near the vicinity of the substrate and within the CdTe epilayers. HgCdTe epilayers grown on CdTe(211)B/GaAs(211) composite substrate were generally of high quality, despite the presence of precipitates at the HgCdTe/CdTe interface. The microstructure of HgCdSe thin films grown by MBE on ZnTe/Si(112) and GaSb(112) substrates were investigated. The quality of the HgCdSe growth was dependent on the growth temperature and materials flux, independent of the substrate. The materials grown at 100°C were generally of high quality, while those grown at 140°C had {111}-type stacking defects and high dislocation densities. For epitaxial growth of HgCdSe on GaSb substrates, better preparation of the GaSb buffer layer will be essential in order to ensure that high-quality HgCdSe can be grown.
ContributorsZhao, Wenfeng (Author) / Smith, David J. (Thesis advisor) / McCartney, Martha (Committee member) / Carpenter, Ray (Committee member) / Bennett, Peter (Committee member) / Treacy, Michael J. (Committee member) / Arizona State University (Publisher)
Created2011
Description
Raman scattering from Ge-Si core-shell nanowires is investigated theoretically and experimentally. A theoretical model that makes it possible to extract quantitative strain information from the measured Raman spectra is presented for the first time. Geometrical and elastic simplifications are introduced to keep the model analytical, which facilitates comparison with experimental results. In particular, the nanowires are assumed to be cylindrical, and their elastic constants isotropic. The simple analytical model is subsequently validated by performing numerical calculations using realistic nanowire geometries and cubic, anisotropic elastic constants. The comparison confirms that the analytic model is an excellent approximation that greatly facilitates quantitative Raman work, with expected errors in the strain determination that do not exceed 10%. Experimental Raman spectra of a variety of core-shell nanowires are presented, and the strain in the nanowires is assessed using the models described above. It is found that all structures present a significant degree of strain relaxation relative to ideal, fully strained Ge-Si core-shell structures. The analytical models are modified to quantify this strain relaxation.
ContributorsSingh, Rachna (Author) / Menéndez, Jose (Thesis advisor) / Drucker, Jeffery (Committee member) / Ponce, Fernando (Committee member) / Tsen, Kong-Thon (Committee member) / Bennett, Peter (Committee member) / Arizona State University (Publisher)
Created2011
Description
This research focuses on the stress and structure evolution observed in-situ during the earliest stages of thin film growth in Cu on Au(111)-reconstruction. For the research, an ultra high vacuum-scanning tunneling microscopy (UHV-STM) system was modified to have the additional capabilities of in-situ deposition and in-situ stress evolution monitoring. The design and fabrication processes for the modifications are explained in detail. The deposition source enabled imaging during the deposition of Cu thin films, while also being columnar enough to avoid negatively impacting the function of the microscope. It was found that the stress-induced changes in piezo voltage occurred over a substantially longer time scale and larger piezo scale than used during imaging, allowing for the deconvolution of the two sources of piezo voltage change. The intrinsic stress evolution observed at the onset of Cu growth was tensile in character and reached a maximum of 0.19 N/m at approximately 0.8ML, with an average tensile slope of 1.0GPa. As the film thickness increased beyond 0.8 ML, the stress became less tensile as the observation of disordered stripe and trigon patterns of misfit dislocations began to appear. The transport of atoms from the surface of enlarged Cu islands into the strained layer played an important role in this stage, because they effectively reduce the activation barrier for the formation of the observed surface structures. A rich array of structures were observed in the work presented here including stripe, disordered stripe and trigon patterns co-existing in a single Cu layer. Heteroepitaxial systems in existing literature showed a uniform structure in the single layer. The non-uniform structures in the single layer of this work may be attributed to the room temperature Cu growth, which can kinetically limit uniform pattern formation. The development of the UHV-STM system with additional capabilities for this work is expected to contribute to research for the stress and structure relationships of many other heteroepitaxial systems.
ContributorsNah, Jungwoo (Author) / Friesen, Cody (Thesis advisor) / Sieradzki, Karl (Committee member) / Bennett, Peter (Committee member) / Arizona State University (Publisher)
Created2012
Description
A theoretical study of a three-dimensional (3D) N/S interface with arbitrary spin
polarization and interface geometry is presented. The 3D model gives the same intrinsic
spin polarization and superconducting gap dependence as the 1D model. This
demonstrates that the 1D model can be use to t 3D data.
Using this model, a Heusler alloy is investigated. Andreev reflection measurements
show that the spin polarization is 80% in samples sputtered on unheated MgO(100)
substrates and annealed at high temperatures. However, the spin polarization is
considerably smaller in samples deposited on heated substrates.
Ferromagnetic FexSix alloys have been proposed as potential spin injectors into
silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is
utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35
alloys. The amorphous phase has a significantly higher spin polarization than that of
the crystalline phase.
In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to
measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3.
Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors
can be used in ARS. Based on the behavior of the differential conductance for highly
spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc
is not a triplet superconductor.
Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been
utilized as a characteristic feature to reveal many novel physics. Complexities at a
normal metal/superconducting interface often cause nonessential ZBA-like features,
which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA,
which is due to the contact resistance, cannot be suppressed by a highly spin-polarized
current while a nonessential ZBA cannot be affected the contact resistance.
Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured
at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only
do the GMR structures have a large enhancement of resistance, but by applying an
external magnetic eld it is shown that, unlike most materials, the spin polarization
can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe.
polarization and interface geometry is presented. The 3D model gives the same intrinsic
spin polarization and superconducting gap dependence as the 1D model. This
demonstrates that the 1D model can be use to t 3D data.
Using this model, a Heusler alloy is investigated. Andreev reflection measurements
show that the spin polarization is 80% in samples sputtered on unheated MgO(100)
substrates and annealed at high temperatures. However, the spin polarization is
considerably smaller in samples deposited on heated substrates.
Ferromagnetic FexSix alloys have been proposed as potential spin injectors into
silicon with a substantial spin polarization. Andreev Reflection Spectroscopy (ARS) is
utilized to determine the spin polarization of both amorphous and crystalline Fe65Si35
alloys. The amorphous phase has a significantly higher spin polarization than that of
the crystalline phase.
In this thesis, (1111) Fe SmO0:82F0:18FeAs and Pb superconductors are used to
measure the spin polarization of a highly spin-polarized material, La0:67Sr0:33MnO3.
Both materials yield the same intrinsic spin polarization, therefore, Fe-superconductors
can be used in ARS. Based on the behavior of the differential conductance for highly
spin polarized LSMO and small polarization of Au, it can be concluded that the Fe-Sc
is not a triplet superconductor.
Zero bias anomaly (ZBA), in point contact Andreev reflection (PCAR), has been
utilized as a characteristic feature to reveal many novel physics. Complexities at a
normal metal/superconducting interface often cause nonessential ZBA-like features,
which may be mistaken as ZBA. In this work, it is shown that an extrinsic ZBA,
which is due to the contact resistance, cannot be suppressed by a highly spin-polarized
current while a nonessential ZBA cannot be affected the contact resistance.
Finally, Cu/Cu multilayer GMR structures were fabricated and the GMR% measured
at 300 K and 4.5 K gave responses of 63% and 115% respectively. Not only
do the GMR structures have a large enhancement of resistance, but by applying an
external magnetic eld it is shown that, unlike most materials, the spin polarization
can be tuned to values of 0.386 to 0.415 from H = 0 kOe to H = 15 kOe.
ContributorsGifford, Jessica Anna (Author) / Chen, Tingyong (Thesis advisor) / Bennett, Peter (Committee member) / Nemanich, Robert (Committee member) / Tsen, Kong-Thon (Committee member) / Arizona State University (Publisher)
Created2015
Description
An electrical current with high spin polarization is desirable for the performance of novel spintronics devices, such as magnetic tunnel junction and giant magnetoresistance devices. The generation of spin polarized current can be from ferromagnetic materials or triplet superconductors.
Anomalous Hall effect (AHE) is an effective way to study the properties of magnetic structures. The scattering of electrons by the magnetic moments affects the change of resistance, which can be used to detect the magnetization. In this dissertation, AHE is used to study the perpendicular magnetic anisotropy (PMA) structures, including Co/Pt and Ta/CoFeB/MgO.
Domain walls exist in all ferromagnetic materials. This dissertation studies the domain wall movement in the Ta/CoFeB/MgO structure. A single domain is observed by measuring the anomalous Hall effect. On the other hand, a zero Hall step is successfully observed in a single layer of magnetic material for the first time, which can be used to fabricate advanced domain wall spintronics devices.
Besides the normal ferromagnetic material, the generation of spin polarized current in superconductor is also important for Spintronics. The electrons in superconductors form Cooper pairs. In this dissertation, Andreev Reflection Spectroscopy (ARS) is used to study the spin configuration in Cooper pairs.
Generally, ferromagnetism and superconductivity can not co-exist. In this dissertation, the Bi/Ni bilayer structure has been studied with ARS, and the measurement results show a triplet superconductivity below 4K. The appearance of superconductivity is believed to be attributed to the Bi-Ni interface, and the triplet Cooper pair makes it a promising candidate in superconducting spintronics.
Besides, a Bi3Ni single crystal is also studied with ARS. The measurements show a singlet superconductivity in this material, which further proves the importance of the Bi/Ni interface to achieve triplet superconductivity.
Finally, ARS is also used to study NbSe2 monolayer, a 2D superconductor. The monolayer is verified by the measurements of critical temperature and critical field, which are different from the values of multilayer or bulk. Andreev reflection results show that NbSe2 monolayer is a singlet superconductor and there is no node exist in the superconducting gap for a in plane magnetic field up to 58 kOe.
Anomalous Hall effect (AHE) is an effective way to study the properties of magnetic structures. The scattering of electrons by the magnetic moments affects the change of resistance, which can be used to detect the magnetization. In this dissertation, AHE is used to study the perpendicular magnetic anisotropy (PMA) structures, including Co/Pt and Ta/CoFeB/MgO.
Domain walls exist in all ferromagnetic materials. This dissertation studies the domain wall movement in the Ta/CoFeB/MgO structure. A single domain is observed by measuring the anomalous Hall effect. On the other hand, a zero Hall step is successfully observed in a single layer of magnetic material for the first time, which can be used to fabricate advanced domain wall spintronics devices.
Besides the normal ferromagnetic material, the generation of spin polarized current in superconductor is also important for Spintronics. The electrons in superconductors form Cooper pairs. In this dissertation, Andreev Reflection Spectroscopy (ARS) is used to study the spin configuration in Cooper pairs.
Generally, ferromagnetism and superconductivity can not co-exist. In this dissertation, the Bi/Ni bilayer structure has been studied with ARS, and the measurement results show a triplet superconductivity below 4K. The appearance of superconductivity is believed to be attributed to the Bi-Ni interface, and the triplet Cooper pair makes it a promising candidate in superconducting spintronics.
Besides, a Bi3Ni single crystal is also studied with ARS. The measurements show a singlet superconductivity in this material, which further proves the importance of the Bi/Ni interface to achieve triplet superconductivity.
Finally, ARS is also used to study NbSe2 monolayer, a 2D superconductor. The monolayer is verified by the measurements of critical temperature and critical field, which are different from the values of multilayer or bulk. Andreev reflection results show that NbSe2 monolayer is a singlet superconductor and there is no node exist in the superconducting gap for a in plane magnetic field up to 58 kOe.
ContributorsZhao, Gejian (Author) / Chen, Tingyong (Thesis advisor) / Bennett, Peter (Committee member) / Nemanich, Robert (Committee member) / Qing, Quan (Committee member) / Arizona State University (Publisher)
Created2018
Description
In this dissertation I studied the anomalous Hall effect in MgO/Permalloy/Nonmagnetic Metal(NM) based structure, spin polarized current in YIG/Pt based thin films and the origin of the perpendicular magnetic anisotropy(PMA) in the Ru/Co/Ru based structures.
The anomalous Hall effect is the observation of a nonzero voltage difference across a magnetic material transverse to the current that flows through the material and the external magnetic field. Unlike the ordinary Hall effect which is observed in nonmagnetic metals, the anomalous Hall effect is only observed in magnetic materials and is orders of magnitude larger than the ordinary Hall effect. Unlike quantum anomalous Hall effect which only works in low temperature and extremely large magnetic field, anomalous Hall effect can be measured at room temperature under a relatively small magnetic field. This allows the anomalous Hall effect to have great potential applications in spintronics and be a good characterization tool for ferromagnetic materials especially materials that have perpendicular magnetic anisotropy(PMA).
In my research, it is observed that a polarity change of the Hall resistance in the MgO/Permalloy/NM structure can be obtained when certain nonmagnetic metal is used as the capping layer while no polarity change is observed when some other metal is used as the capping layer. This allows us to tune the polarity of the anomalous Hall effect by changing the thickness of a component of the structure. My conclusion is that an intrinsic mechanism from Berry curvature plays an important role in the sign of anomalous Hall resistivity in the MgO/Py/HM structures. Surface and interfacial scattering also make substantial contribution to the measured Hall resistivity.
Spin polarization(P) is one of the key concepts in spintronics and is defined as the difference in the spin up and spin down electron population near the Fermi level of a conductor. It has great applications in the spintronics field such as the creation of spin transfer torques, magnetic tunnel junction(MTJ), spintronic logic devices.
In my research, spin polarization is measured on platinum layers grown on a YIG layer. Platinum is a nonmagnetic metal with strong spin orbit coupling which intrinsically has zero spin polarization. Nontrivial spin polarization measured by ARS is observed in the Pt layer when it is grown on YIG ferromagnetic insulator. This result is contrary to the zero spin polarization in the Pt layer when it is grown directly on SiO2 substrate. Magnetic proximity effect and spin current pumping from YIG into Pt is proposed as the reason of the nontrivial spin polarization induced in Pt. An even higher spin polarization in the Pt layer is observed when an ultrathin NiO layer or Cu layer is inserted between Pt and YIG which blocks the proximity effect. The spin polarization in the NiO inserted sample shows temperature dependence. This demonstrates that the spin current transmission is further enhanced in ultrathin NiO layers through magnon and spin fluctuations.
Perpendicular Magnetic Anisotropy(PMA) has important applications in spintronics and magnetic storage. In the last chapter, I study the origin of PMA in one of the structures that shows PMA: Ru/Co/Ru. By measuring the ARS curve while changing the magnetic field orientation, the origin of the PMA in this structure is determined to be the strain induced by lattice mismatch.
The anomalous Hall effect is the observation of a nonzero voltage difference across a magnetic material transverse to the current that flows through the material and the external magnetic field. Unlike the ordinary Hall effect which is observed in nonmagnetic metals, the anomalous Hall effect is only observed in magnetic materials and is orders of magnitude larger than the ordinary Hall effect. Unlike quantum anomalous Hall effect which only works in low temperature and extremely large magnetic field, anomalous Hall effect can be measured at room temperature under a relatively small magnetic field. This allows the anomalous Hall effect to have great potential applications in spintronics and be a good characterization tool for ferromagnetic materials especially materials that have perpendicular magnetic anisotropy(PMA).
In my research, it is observed that a polarity change of the Hall resistance in the MgO/Permalloy/NM structure can be obtained when certain nonmagnetic metal is used as the capping layer while no polarity change is observed when some other metal is used as the capping layer. This allows us to tune the polarity of the anomalous Hall effect by changing the thickness of a component of the structure. My conclusion is that an intrinsic mechanism from Berry curvature plays an important role in the sign of anomalous Hall resistivity in the MgO/Py/HM structures. Surface and interfacial scattering also make substantial contribution to the measured Hall resistivity.
Spin polarization(P) is one of the key concepts in spintronics and is defined as the difference in the spin up and spin down electron population near the Fermi level of a conductor. It has great applications in the spintronics field such as the creation of spin transfer torques, magnetic tunnel junction(MTJ), spintronic logic devices.
In my research, spin polarization is measured on platinum layers grown on a YIG layer. Platinum is a nonmagnetic metal with strong spin orbit coupling which intrinsically has zero spin polarization. Nontrivial spin polarization measured by ARS is observed in the Pt layer when it is grown on YIG ferromagnetic insulator. This result is contrary to the zero spin polarization in the Pt layer when it is grown directly on SiO2 substrate. Magnetic proximity effect and spin current pumping from YIG into Pt is proposed as the reason of the nontrivial spin polarization induced in Pt. An even higher spin polarization in the Pt layer is observed when an ultrathin NiO layer or Cu layer is inserted between Pt and YIG which blocks the proximity effect. The spin polarization in the NiO inserted sample shows temperature dependence. This demonstrates that the spin current transmission is further enhanced in ultrathin NiO layers through magnon and spin fluctuations.
Perpendicular Magnetic Anisotropy(PMA) has important applications in spintronics and magnetic storage. In the last chapter, I study the origin of PMA in one of the structures that shows PMA: Ru/Co/Ru. By measuring the ARS curve while changing the magnetic field orientation, the origin of the PMA in this structure is determined to be the strain induced by lattice mismatch.
ContributorsLi, Bochao (Author) / Chen, Tingyong (Committee member) / Bennett, Peter (Committee member) / Nemanich, Robert (Committee member) / Qing, Quan (Committee member) / Arizona State University (Publisher)
Created2018
Description
The energy band gap of a semiconductor material critically influences the operating wavelength of an optoelectronic device. Realization of any desired band gap, or even spatially graded band gaps, is important for applications such as lasers, light-emitting diodes (LEDs), solar cells, and detectors. Compared to thin films, nanowires offer greater flexibility for achieving a variety of alloy compositions. Furthermore, the nanowire geometry permits simultaneous incorporation of a wide range of compositions on a single substrate. Such controllable alloy composition variation can be realized either within an individual nanowire or between distinct nanowires across a substrate. This dissertation explores the control of spatial composition variation in ternary alloy nanowires. Nanowires were grown by the vapor-liquid-solid (VLS) mechanism using chemical vapor deposition (CVD). The gas-phase supersaturation was considered in order to optimize the deposition morphology. Composition and structure were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). Optical properties were investigated through photoluminescence (PL) measurements. The chalcogenides selected as alloy endpoints were lead sulfide (PbS), cadmium sulfide (CdS), and cadmium selenide (CdSe). Three growth modes of PbS were identified, which included contributions from spontaneously generated catalyst. The resulting wires were found capable of lasing with wavelengths over 4000 nm, representing the longest known wavelength from a sub-wavelength wire. For CdxPb1-xS nanowires, it was established that the cooling process significantly affects the alloy composition and structure. Quenching was critical to retain metastable alloys with x up to 0.14, representing a new composition in nanowire form. Alternatively, gradual cooling caused phase segregation, which created heterostructures with light emission in both the visible and mid-infrared regimes. The CdSSe alloy system was fully explored for spatial composition variation. CdSxSe1-x nanowires were grown with composition variation across the substrate. Subsequent contact printing preserved the designed composition gradient and led to the demonstration of a variable wavelength photodetector device. CdSSe axial heterostructure nanowires were also achieved. The growth process involved many variables, including a deliberate and controllable change in substrate temperature. As a result, both red and green light emission was detected from single nanowires.
ContributorsNichols, Patricia (Author) / Ning, Cun-Zheng (Thesis advisor) / Carpenter, Ray (Committee member) / Bennett, Peter (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2012