Validation of Vegard’s Law for Lattice Matching InxAl1-xN to GaN & The MOCVD Growth of AlxGa1-xN/AlN for Deep UV LEDs

The first section of this thesis presents an experimental investigation for validating Vegard’s Law for InxAl1-xN on GaN. Vegard’s law is useful for correlating materials composition and band gap to the a-lattice parameter, however it is primarily utilized for materials synthesized from powders and deviations for InAlN have been reported in the literature. Coherent InxAl1-xN (x = 0.15 to x = 0.28) films were grown by metalorganic chemical vapor deposition on GaN templates to investigate if the films obey Vegard’s Law by comparing the film stress-thickness product from wafer curvature before and after InxAl1-xN deposition. The In composition and film thickness were verified using atom probe tomography and high resolution x-ray diffraction, respectively. Ex-situ curvature measurements were performed to analyze the curvature before and after the InxAl1-xN deposition. At ~In0.18Al0.82N, no change in curvature was observed following InAlN deposition; confirming that the films obey Vegard’s law and that ~In0.18Al0.82N is lattice matched to GaN. The relaxed a0- and c0- lattice parameters of InxAl1-xN were experimentally determined and are in agreement with lattice parameters predicted by Vegard’s law.The second section of this thesis will present the growth and characterization of AlGaN/AlN for deep UV LEDs. Growth studies on AlN buffer and n-AlGaN will be discussed. First, we explore the dependence of the interlayer growth temperature on the AlN crystal quality, defect density, and surface morphology. The crystal quality was characterized using omega rocking curve scans and the threading dislocation density was determined by plan view transmission electron microscopy. The growth resulted in a threading dislocation density (TDD) of 7 x 108 cm-2, indicating a significant reduction in the dislocation density of AlN in comparison to direct growth of AlN on SiC (~1010-1011 cm-2). Atomic force microscopy images demonstrate a clear step-terrace morphology that is consistent with step flow growth at high temperature. Reducing the interlayer growth temperature increases the TD inclination and thus enhances TD-TD interactions. The TDD is decreased via fusion and annihilation reactions. Following that, we will discuss the growth studies conducted for growing AlGaN. First we will discuss finding the optimal conditions for growing smooth UID-AlGaN for a variety of alloy compositions as the UV LED will need AlGaN with Al% between 40-65%. Then the growth and electrical characterization of n-AlGaN will be discussed. After processing, the electron mobility, resistivity, and carrier concentration of n-Al0.43Ga0.57N was 96 cm2/Vs, 15 mΩ-cm, and 4 x 1018 cm-3, respectively

Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and Technology, SST-105156.R1 2018

Growth of highly conductive Al-rich AlGaN:Si with low group-III vacancy concentration

Publisher Copyright: © 2021 Author(s).The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during growth. A minimum resistivity of 7.5 m? cm was obtained for n-type Al0.65Ga0.35N, which is among the lowest reported resistivity for this composition.& nbsp;(c)& nbsp;2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/).& nbsp;Peer reviewe

Processing and properties of Na0.5Bi0.5TiO3 piezoelectric ceramics modified with La, Mn AND Fe

Although a great deal of work has been done to understand defect chemistry in "soft" and "hard" PZT-based materials, there is little understanding of how defect chemistry influences the properties of lead-free piezoelectric materials. This paper reports a systematic investigation of doping on the ferroelectric and piezoelectric properties in Na0.5Bi0.5TiO3 (NBT)-based ceramics. NBT-based ceramics have been synthesized by traditional solid state routes using several different dopants including lanthanum, manganese, and iron in 1 mol%. The addition of iron and manganese lead to an increase in the coercive field (Ec), a decrease in the piezoelectric coefficient (d33), and an increase in the thermal depoling temperature (Tdepole), similar to the behavior of "hard" PZT. Lanthanum, on the other hand, leads to a decrease in the Ec, an increase in d33, and a decrease in Tdepole similar to that seen in "soft" PZT

Structure and properties of La-modified Na0.5Bi0.5TiO3 at ambient and elevated temperatures

The crystal structure and property changes of sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) piezoelectric ceramics are reported as a function of La modification (0.5–2.0 at. %) and increasing temperature using high resolution x-ray diffraction, permittivity, depolarization, and polarization and strain hysteresis measurements. La substitution is found to decrease the depolarization temperature of NBT (e.g., 1.5 at. % La substitution lowers the depolarization temperature by 60 °C relative to the unmodified composition) with little impact on the room temperature polarization and strain hysteresis. The room temperature structures of the various NBT compositions were modeled using a mixture of the monoclinic Cc space group and the cubic Pmmathm phase, where the Pmmathm phase is used to model local regions in the material which do not obey the long range Cc space group. With increasing La substitution, the lattice parameter distortions associated with the Cc phase approached that of the prototypical cubic unit cell and the fraction of the Pmmathm phase increased. The relationship between these crystallographic changes and the depolarization behavior of La-modified NBT is discussed