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Nitride semiconductors have wide applications in electronics and optoelectronics technologies. Understanding the nature of the optical recombination process and its effects on luminescence efficiency is important for the development of novel devices. This dissertation deals with the optical properties of nitride semiconductors, including GaN epitaxial layers and more complex heterostructures.

Nitride semiconductors have wide applications in electronics and optoelectronics technologies. Understanding the nature of the optical recombination process and its effects on luminescence efficiency is important for the development of novel devices. This dissertation deals with the optical properties of nitride semiconductors, including GaN epitaxial layers and more complex heterostructures. The emission characteristics are examined by cathodoluminescence spectroscopy and imaging, and are correlated with the structural and electrical properties studied by transmission electron microscopy and electron holography. Four major areas are covered in this dissertation, which are described next. The effect of strain on the emission characteristics in wurtzite GaN has been studied. The values of the residual strain in GaN epilayers with different dislocation densities are determined by x-ray diffraction, and the relationship between exciton emission energy and the in-plane residual strain is demonstrated. It shows that the emission energy increases withthe magnitude of the in-plane compressive strain. The temperature dependence of the emission characteristics in cubic GaN has been studied. It is observed that the exciton emission and donor-acceptor pair recombination behave differently with temperature. The donor-bound exciton binding energy has been measured to be 13 meV from the temperature dependence of the emission spectrum. It is also found that the ionization energies for both acceptors and donors are smaller in cubic compared with hexagonal structures, which should contribute to higher doping efficiencies. A comprehensive study on the structural and optical properties is presented for InGaN/GaN quantum wells emitting in the blue, green, and yellow regions of the electromagnetic spectrum. Transmission electron microscopy images indicate the presence of indium inhomogeneties which should be responsible for carrier localization. The temperature dependence of emission luminescence shows that the carrier localization effects become more significant with increasing emission wavelength. On the other hand, the effect of non-radiative recombination on luminescence efficiency also varies with the emission wavelength. The fast increase of the non-radiative recombination rate with temperature in the green emitting QWs contributes to the lower efficiency compared with the blue emitting QWs. The possible saturation of non-radiative recombination above 100 K may explain the unexpected high emission efficiency for the yellow emitting QWs Finally, the effects of InGaN underlayers on the electronic and optical properties of InGaN/GaN quantum wells emitting in visible spectral regions have been studied. A significant improvement of the emission efficiency is observed, which is associated with a blue shift in the emission energy, a reduced recombination lifetime, an increased spatial homogeneity in the luminescence, and a weaker internal field across the quantum wells. These are explained by a partial strain relaxation introduced by the InGaN underlayer, which is measured by reciprocal space mapping of the x-ray diffraction intensity.
ContributorsLi, Di (Author) / Ponce, Fernando (Thesis advisor) / Culbertson, Robert (Committee member) / Yu, Hongbin (Committee member) / Shumway, John (Committee member) / Menéndez, Jose (Committee member) / Arizona State University (Publisher)
Created2012
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Description
Water affinity and condensation on Si-based surfaces is investigated to address the problem of fogging on silicone intraocular lenses (IOL) during cataract surgery, using Si(100), silica (SiO2) and polydimethylsiloxane (PDMS) silicone (SiOC2H6)n. Condensation is described by two step nucleation and growth where roughness controls heterogeneous nucleation of droplets followed by

Water affinity and condensation on Si-based surfaces is investigated to address the problem of fogging on silicone intraocular lenses (IOL) during cataract surgery, using Si(100), silica (SiO2) and polydimethylsiloxane (PDMS) silicone (SiOC2H6)n. Condensation is described by two step nucleation and growth where roughness controls heterogeneous nucleation of droplets followed by Ostwald ripening. Wetting on hydrophilic surfaces consists of continuous aqueous films while hydrophobic surfaces exhibit fogging with discrete droplets. Si-based surfaces with wavelength above 200 nm exhibit fogging during condensation. Below 200 nm, surfaces are found to wet during condensation. Water affinity of Si-based surfaces is quantified via the surface free energy (SFE) using Sessile drop contact angle analysis, the Young-Dupré equation, and Van Oss theory. Topography is analyzed using tapping mode atomic force microscopy (TMAFM). Polymer adsorption and ion beam modification of materials (IBMM) can modify surface topography, composition, and SFE, and alter water affinity of the Si-based surfaces we studied. Wet adsorption of hydroxypropyl methylcellulose (HPMC) C32H60O19 with areal densities ranging from 1018 atom/cm2 to 1019 atom/cm2 characterized via Rutherford backscattering spectrometry (RBS), allows for the substrate to adopt the topography of the HPMC film and its hydrophilic properties. The HPMC surface composition maintains a bulk stoichiometric ratio as confirmed by 4.265 MeV 12C(α, α)12C and 3.045 MeV 16O(α, α)16O, and 2.8 MeV He++ elastic recoil detection (ERD) of hydrogen. Both PIXE and RBS methods give comparable areal density results of polymer films on Si(100), silica, and PDMS silicone substrates. The SFE and topography of PDMS silicone polymers used for IOLs can also be modified by IBMM. IBMM of HPMC cellulose occurs during IBA as well. Damage curves and ERD are shown to characterize surface desorption accurately during IBMM so that ion beam damage can be accounted for during analysis of polymer areal density and composition. IBMM of Si(100)-SiO2 ordered interfaces also induces changes of SFE, as ions disorder surface atoms. The SFE converges for all surfaces, hydrophobic and hydrophilic, as ions alter electrochemical properties of the surface via atomic and electronic displacements.
ContributorsXing, Qian (Author) / Herbots, Nicole (Thesis advisor) / Culbertson, Robert (Thesis advisor) / Chamberlin, Ralph (Committee member) / Treacy, Michael (Committee member) / Smith, David (Committee member) / Arizona State University (Publisher)
Created2011
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This dissertation covers my doctoral research on the cathodoluminescence (CL) study of the optical properties of III-niride semiconductors.

The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high

This dissertation covers my doctoral research on the cathodoluminescence (CL) study of the optical properties of III-niride semiconductors.

The first part of this thesis focuses on the optical properties of Mg-doped gallium nitride (GaN:Mg) epitaxial films. GaN is an emerging material for power electronics, especially for high power and high frequency applications. Compared to traditional Si-based devices, GaN-based devices offer superior breakdown properties, faster switching speed, and reduced system size. Some of the current device designs involve lateral p-n junctions which require selective-area doping. Dopant distribution in the selectively-doped regions is a critical issue that can impact the device performance. While most studies on Mg doping in GaN have been reported for epitaxial grown on flat c-plane substrates, questions arise regarding the Mg doping efficiency and uniformity in selectively-doped regions, where growth on surfaces etched away from the exact c-plane orientation is involved. Characterization of doping concentration distribution in lateral structures using secondary ion mass spectroscopy lacks the required spatial resolution. In this work, visualization of acceptor distribution in GaN:Mg epilayers grown by metalorganic chemical vapor deposition (MOCVD) was achieved at sub-micron scale using CL imaging. This was enabled by establishing a correlation among the luminescence characteristics, acceptor concentration, and electrical conductivity of GaN:Mg epilayers. Non-uniformity in acceptor distribution has been observed in epilayers grown on mesa structures and on miscut substrates. It is shown that non-basal-plane surfaces, such as mesa sidewalls and surface step clusters, promotes lateral growth along the GaN basal planes with a reduced Mg doping efficiency. The influence of surface morphology on the Mg doping efficiency in GaN has been studied.

The second part of this thesis focuses on the optical properties of InGaN for photovoltaic applications. The effects of thermal annealing and low energy electron beam irradiation (LEEBI) on the optical properties of MOCVD-grown In0.14Ga0.86N films were studied. A multi-fold increase in luminescence intensity was observed after 800 °C thermal annealing or LEEBI treatment. The mechanism leading to the luminescence intensity increase has been discussed. This study shows procedures that significantly improve the luminescence efficiency of InGaN, which is important for InGaN-based optoelectronic devices.
ContributorsLiu, Hanxiao (Author) / Ponce, Fernando A. (Thesis advisor) / Zhao, Yuji (Committee member) / Newman, Nathan (Committee member) / Fischer, Alec M (Committee member) / Arizona State University (Publisher)
Created2020