Understanding atomic distributions on the order of nanometers is becoming ever more essential to solid-state
electronic device design. The local composition of any singular constituent can have a great effect on a host of materials
properties. Atom probe tomography is currently the only characterization technique that can provide direct physical
detection of ionic species of atoms. In this work, MOCVD grown GaMnN thin films are characterized utilizing the state
of the art local electrode atom probe (LEAPTM) to determine the atomic ordering of Mn in an effort to help understand
room-temperature ferromagnetic exchange mechanisms in wide-bandgap dilute magnetic semiconductors. The results
support prior magnetometry data that suggest paramagnetism results primarily from isolated Mn atoms. A predisposition
for the formation of dimers, trimers or clusters does not exist in the samples grown. The ultimate goal in determining the
physical arrangements of atoms and how they related to exchange mechanisms is to understand the structure-propertygrowth
condition relationships for the tailoring of specific MOCVD processes that will lead to the ability to selectively
control spintronic device functionalities.
One key challenge with the the use of III-nitride materials in solid state lighting devices is the use
on thermally and/or electrically insulating substrates. In order to transition the lift-off tehniques from laserbased
processing to more benign chemical techniques, it is essential to realize the integration of chemical
soluble layers into the LED-growth process. This work presents the comparison of physical and chemical
vapor deposition techniques used in the growth of ZnO, one such possible isostructural buffer layer and a
comparison of the materials properties of samples grown by low temperature chemical vapor deposition
and pulsed laser deposition. The quality of the films was highly dependent on the growth conditions and
substrate preparation. For room temperature depostion, amorphous ZnO films were observed, while
completely oriented films were observed for 750°C growth. Pulsed chemical vapor deposition of the films
at 175°C showed smoother polycrystalline films with c-axis texture. ZnO films were etched at room
temperature demonstrating the suitability for chemical liftoff.
GaN epilayers and AlGaN/GaN superlattice structures have been deposited on (0001) ZnO substrates by
metalorganic vapor phase epitaxy (MOVPE) using GaN and AlN buffer layers. The growth conditions were first
optimized on GaN templates using N2 as carrier gas at relatively low temperature (<800 °C), which is suitable for
GaN growth on a ZnO substrate. Experimental results show that high interfacial quality can be achieved in the
superlattice by using TMIn as a surfactant. The optimized growth conditions were subsequently transferred to ZnO
substrates. The influence of growth temperature on the material quality was studied. A proper growth temperature for
both GaN cover layer and AlGaN/GaN superlattice can improve the structural and optical properties of the structures
on ZnO. This improvement is verified using x-ray diffraction, atomic force microscopy and photoluminescence
characterizations. The growth temperature must be chosen with these two factors in mind, with too low a growth
temperature leading to poor quality material and too high a temperature causing reactions at the GaN/ZnO interface
that degrade quality. AlN buffer layers on ZnO were also studied to increase subsequent GaN epilayer quality.
Effects of buffer layer growth conditions on optical and structural quality were studied.
Recent theoretical predictions of ferromagnetic behavior in transition metal (TM)-doped ZnO have focused significant
attention on these materials for use as spintronic materials. Moreover, rare earth (RE) elements in wide bandgap
semiconductors would be useful not only in spintronics but also in optoelectronic applications. This work presents
results obtained from an investigation into the optical, magnetic, and structural properties of transition-metal (TM)-
doped ZnO and rare earth (RE) doped ZnO (TM = Mn, Co, Ni, and Fe; RE = Gd, Eu, and Tb) bulk crystals and thin
films. Properties of TM- and RE-doped ZnO bulk crystals and thin films were studied and compared in order to better
understand the nature of these dopant centers and their effects on the properties of the host crystal. Optical properties
confirm the incorporation of substitutional transition metal ions on cation sites. While most thin film samples show
ferromagnetic behavior, the magnetic response of the bulk crystals varies. This suggests that the magnetic behavior of
TM-doped ZnO is highly dependent on growth conditions, and growth conditions which favor the formation of grain
boundaries and interfaces may be more likely to result in ferromagnetic behavior. Origins of this ferromagnetic behavior
are still under investigation. Defect luminescence observed in the RE-doped samples suggests that these materials may
prove useful in optoelectonic applications as well.
Wide bandgap nitrides and oxides have been heralded as a possible platform for future semiconductor spintronics applications based on the inherent compatibility of these materials with existing semiconductors as well as theoretical predictions of room temperature ferromagnetism. Experimental reports of room temperature ferromagnetism in these materials are complicated by disparate crystalline quality and phase purity in these materials, as well as
conflicting theoretical predictions as to the nature of ferromagnetic behavior in this system. A complete understanding of these materials, and ultimately intelligent design of spintronic devices, will require an exploration of the relationship between the processing techniques, resulting transition metal atom configuration, defects, and electronic compensation as related to the structure, magnetic, and magneto-optical properties of this material. This work explores the growth and properties of Ga1-xMnxN films by metalorganic chemical vapor deposition on cplane sapphire substrates with varying thickness, Mn concentration, and alloying elements. Homogenous Mn incorporation throughout the films was verified with Secondary Ion Mass Spectroscopy (SIMS), and no macroscopic second phases were detected using X-ray diffraction (XRD). SQUID and vibrating sample magnetometry measurements showed an apparent room temperature ferromagnetic hysteresis, whose strength can be altered considerably through annealing and introduction of either Si or Mg during the growth process. Three sets of Raman modes appeared to be sensitive to Mn incorporation. The intensities of a broad band around 300cm-1 and sharper modes near 669cm-1 increased with increasing Mn concentration. The rise of the former is attributed to a decrease in long-range lattice ordering for higher Mn concentration. The second mode is due to nitrogen vacancy-related local vibrational modes of the GaN host lattice. Si co-doped Ga1-xMnxN results in shallow donor states in GaN suppress the formation of nitrogen vacancies by compensating the p-type deep level defects introduced by substitutional Mn. The formation of a Mn-related midgap impurity band is observed via optical transmission measurement in Ga1-xMnxN with strong magnetic signatures, but not for Si co-doped samples. Initial studies on light emitting diodes (LEDs) containing a Mn-doped active region have also been produced. Devices were fabricated with different Mn-doped active layer thicknesses, and I-V characteristics show that the devices become highly resistive as thickness of the Mn-doped active layer increases. The electroluminescence of these devices is dominated by a high suppressed band-edge recombination and a midgap defect-related emission, leading to an orange-colored but weakly emitting LED. These results suggest that traditional theoretical and device approaches akin to those realized in Ga1-xMnxN may be difficult to realize in Ga1-xMnxN, and exploitation of these materials will require further novel device approaches taking into account the nature of this material.
This work focuses on the development of materials and growth techniques suitable for future spintronic device applications. Metal-organic chemical vapor deposition (MOCVD) was used to grow high-quality epitaxial films of varying thickness and manganese doping levels by introducing bis-cyclopentadienyl as the manganese source. High-resolution X-ray diffraction indicates that no macroscopic second phases are formed during growth, and Mn containing films are similar in crystalline quality to undoped films Atomic force microscopy revealed a 2-dimensional MOCVD step-flow growth pattern in the Mn-incorporated samples. The mean surface roughnesses of optimally grown Ga1-xMnxN films were almost identical to that from the as-grown template layers, with no change in growth mechanism or morphology. Various annealing steps were applied to some of the samples to reduce compensating defects and to
investigate the effects of post processing on the structural, magnetic and opto-electronic properties. SQUID measurements showed an apparent ferromagnetic hysteresis behavior which persisted to room temperature. An optical absorption band around 1.5 eV was observed via transmission studies. This band is assigned to the internal Mn3+ transition between the 5E and the partially filled 5T2 levels of the 5D state. The broadening of the absorption band is
introduced by the high Mn concentration. Recharging of the Mn3+ to Mn2+ was found to effectively suppress these transitions resulting in a reduction of the magnetization. The structural quality, and the presence of Mn2+ ions were
confirmed by EPR spectroscopy, meanwhile no Mn-Mn interactions indicative of clustering were observed. The absence of doping-induced strain in Ga1-xMnxN was observed by Raman spectroscopy.
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