Dissertations, Theses, and Capstone Projects

Date of Degree

10-2014

Document Type

Dissertation

Degree Name

Ph.D.

Program

Physics

Advisor

Mim L. Nakarmi

Subject Categories

Mechanics of Materials | Physics

Keywords

Activation Energy, Aluminum Nitride, Lutetium Ferrite, Photoluminescence Spectroscopy, Transmission Electron Microscopy, Zinc Oxide

Abstract

Structural, optical and electrical properties of zinc oxide (ZnO), aluminum nitride (AlN), and lutetium ferrite (LuFe2O4) have been investigated. Temperature dependent Hall Effect measurements were performed between 80 and 800 K for phosphorus (P) and arsenic (As) doped ZnO thin films grown on c-plane sapphire substrate by RF magnetron sputtering. These samples exhibited n-type conductivity throughout the temperature range with carrier concentration of 3.85 × 10 16 cm-3 and 3.65 × 10 17 cm-3 at room temperature for P-doped and As-doped ZnO films, respectively. The Arrhenius plots of free electron concentration of those doped samples showed double thermal activation processes with a small activation energy of about 0.04 eV due to shallow donors and a large activation energy of about 0.8 eV due to deep donors. The deep donor level could be related to oxygen vacancy. For undoped ZnO layer, growth condition was optimized to use as low background electron buffer layer. Hall Effect measurements showed that the resistivity and background electron concentration of the films decreases as the substrate temperature increases. The film deposited at 900 oC has more than two orders less background electron concentration than that deposited at 300 °C. Based on photoluminescence and Transmission Electron Microscopy (TEM) analysis, the ZnO grown under this condition is formed to be a greatly reduced density of stacking faults.

Transmission electron microscopy (TEM) was employed to investigate dislocations in aluminum nitride (AlN) epilayers grown on sapphire substrate using three-step growth method by metal organic chemical vapor deposition (MOCVD). AlN epilayers grown by this method have smooth surfaces, narrow width of X-ray rocking curves, and strong band edge photoluminescence (PL) emissions with low impurity emissions. Transmission electron microscopy revealed that most of the threading dislocations are annihilated within 300 nm. Stacking faults are greatly reduced in the epilayers grown by this method resulting in very low screw type threading dislocation density. Dominant threading dislocations in the AlN epilayers are edge type originated from misfit dislocations (MD).

The electro-optical and temperature-dependent electrical-transport properties of LuFe2 O4 (LFO) thin films have been investigated. The LFO thin films at 78 K showed the electro-optical effects of size up to 5% near the Fe2+ d to d on-site electronic transition. In the three-dimensional charge-ordered state of LFO, we observed hysteresis in dc voltage-current measurements and nonlinear voltage-current relationship in transient response of voltage under current pulses. The electro-optical and electrical properties of LFO thin films are interpreted in terms of the field-induced changes of the charge-ordered state mediated by the spin-charge-lattice coupling effect. We also discuss possible mechanisms of the complex electrical properties and electro-optical effects in conjunction with the Maxwell-Wagner effects.

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