Growth of Highly Ga-doped ZnO Transparent Conductor and Its Applications
Date Issued
2015
Date
2015
Author(s)
Yao, Yu-Feng
Abstract
In this dissertation, we first demonstrate the molecular beam epitaxy growth of highly-degenerate Ga-doped ZnO (GaZnO) nanoneedles (NNs) based on the vapor-liquid-solid (VLS) growth mode using Ag nanoparticles (NPs) as the growth catalyst. It is shown that when the growth substrate temperature is sufficiently high, a portion of an Ag NP can be melted for serving as the catalyst to precipitate GaZnO on the residual Ag NP and form a GaZnO NN. Record-low turn-on and threshold electric fields in the field emission test of the grown GaZnO NNs are observed. Also, a record-high field enhancement factor in field emission is calibrated. Such superior field emission performances are attributed to a few factors, including (1) the low work function and high conductivity of the grown GaZnO NNs due to highly degenerate Ga doping, (2) the sharp-pointed geometry of the vertically aligned GaZnO NNs, (3) the Ag doping in VLS precipitation of GaZnO for further reducing NN resistivity, and (4) the residual small Ag NP at the NN tip for making the tip even sharper and tip conductivity even higher. Then, the combined effects of a few mechanisms for emission efficiency enhancement produced in the overgrowth of the transparent conductor layer of Ga-doped ZnO (GaZnO) on a surface Ag-nanoparticle (NP) coated light-emitting diode (LED), including surface plasmon (SP) coupling, current spreading, light extraction, and contact resistivity reduction, are demonstrated. With a relatively higher GaZnO growth temperature (350 oC), melted Ag NPs can be used as catalyst for forming GaZnO NNs such that light extraction efficiency can be increased. Meanwhile, residual Ag NPs are buried in a simultaneously grown GaZnO layer for inducing SP coupling. With a relatively lower GaZnO growth temperature (250 oC), all the Ag NPs are preserved for generating a stronger SP coupling effect. By using a thin annealed GaZnO interlayer on p-GaN before Ag NP fabrication, the contact resistivity at the GaZnO/p-GaN interface and hence the overall device resistance can be reduced. Although the use of this interlayer blue-shifts the localized surface plasmon resonance peak of the fabricated Ag NPs from the quantum well emission wavelength of the current study (535 nm) such that the SP coupling effect becomes weaker, it is useful for enhancing the SP coupling effect in an LED with a shorter emission wavelength. Besides, an LED structure consisting of a p-GaN layer, a CdZnO/ZnO quantum-well (QW) structure, a high-temperature-grown ZnO layer, and a GaZnO layer is fabricated. Under forward bias, the device effectively emits green-yellow light from the QW structure at the rim of device mesa. Under reverse bias, electrons in the valence band of the p-GaN layer move into the conduction band of the GaZnO layer through a QW-state-assisted tunneling process to recombine with the injected holes in the GaZnO layer for emitting yellow-red and shallow ultraviolet lights over the whole mesa area. Also, carrier recombination in the p-GaN layer produces blue light. By properly designing the thickness of the high-temperature-grown ZnO layer, the emission intensity under forward bias can be controlled such that under alternating-current operation at 60 Hz, the spatial and spectral mixtures of the emitted lights of complementary colors under forward and reverse biases result in white light generation based on persistence of vision.
Subjects
ZnO
Ga-doped
Transparent Conductor
Type
thesis
File(s)![Thumbnail Image]()
Loading...
Name
ntu-104-F98941073-1.pdf
Size
23.32 KB
Format
Adobe PDF
Checksum
(MD5):4bf7228b12b8860914090c143180f3bc