Growth, Characterization, and Light-emitting Diode Application of CdZnO/ZnO Quantum Wells
Date Issued
2012
Date
2012
Author(s)
Ting, Shao-Ying
Abstract
In this dissertation, we first demonstrate the growths of CdZnO/ZnO quantum well (QW) samples on GaN and ZnO templates under different growth conditions with the plasma-assisted molecular beam epitaxy (PA-MBE). Temperature-dependent and excitation-power-dependent photoluminescence (PL) measurements are undertaken for illustrating their different emission characteristics. The crystal characteristics of the QWs structures are measured by high-resolution transmission electron (TEM), X-ray diffraction (XRD). It is found that the Cd incorporation on the ZnO template is lower, when compared with that on the GaN template, such that the O2 flow rate needs to be reduced for stoichiometric CdZnO/ZnO QW growth on the ZnO template. Besides the wurtzite (wt) CdZnO structure, the rock-salt (rs) CdZnO structure exists in the CdZnO well layers when the total Cd content is high. The rs structures may dominate over the wt structures in PL intensity when the total Cd content is high. In either group of samples on the GaN and ZnO templates, the emission efficiency first increases and then decreases with increasing total Cd content. The low emission efficiencies at low (high) Cd contents are attributed to the weaker quantum confinement (poorer crystal quality) of the QWs. The emission efficiencies of the QW samples on the GaN template are generally higher than those on the ZnO template. The strength of the quantum-confined Stark effect generally increases with increasing Cd content in either group of samples on the GaN and ZnO templates.
Besides, a CdZnO/n-ZnO multiple-QW lateral light-emitting diode (LED) grown with the QWs and n+-ZnO capping layer with MBE on p-GaN, which is grown with metalorganic chemical vapor deposition (MOCVD), is fabricated and characterized. On the epitaxial surface, there exist some V-shape pits, which correspond to threading dislocations beneath. With SiO2 nanoparticles filling in the pits, the turn-on voltage and device resistance of this device are around 4 V and 224 Ω, respectively. However, because of the weak carrier localization mechanism in the ZnO-based LED, its defect emission is quite strong and dominates the LED output at low injection current levels. The blue shift of LED output spectrum in applying a forward-biased voltage and the large blue-shift range in increasing injection current show the different behaviors of such a ZnO-based LED from those of a nitride LED.
In addition, we also demonstrate the fabrication procedures and characterization results of a vertical light-emitting diode (VLED) with the CdZnO/n-ZnO QWs and n+-ZnO capping layer grown with MBE and the p-GaN layer grown with MOCVD. Its performances are compared with those of a lateral LED based on the same epitaxial structure to show the significantly lower device resistance, larger spectral blue-shift range in increasing injection current, smaller leakage current, weaker output intensity saturation, and relatively lower defect emission in the VLED.
Subjects
ZnO
CdZnO
QW
LED
Type
thesis
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