The primary objective of this dissertation is provided some approaches to enhance the performance of AlGaInP and InGaN-GaN LED. It could using wafer-bonding technology to transfer AlGaInP or GaN thin film on the high conductivity substrate. Furthermore, InGaN-GaN LED with roughening both the p-GaN surface and the undoped-GaN surface by double transfer methods and applying a mirror coating to the sapphire substrate have also demonstrated.
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There are six chapters in this dissertation. In chapter 1, the background theory for light emitting diode and the mechanism of wafer bonding and the applications of III-V compound semiconductor by wafer bonding technology will be introduced.
In chapter 2, high-power LED fabricated on Cu and SiC substrates were investigated.
The AlGaInP LED structure was bonded to a Cu substrate by using indium-tin-oxide (ITO) as the diffusion barrier layer. It was found that Cu-substrate-bonded LED devices could be operated in a much higher injection forward current than that used in traditional GaAs-substrate LED and the degradation of the luminescence-output intensity was less than 5 % after 500 hours of life test as shown in section 2-2. In section 2-3, the AlGaInP LED with SiC substrate by wafer bonding was proposed. It was found that the luminous intensity and saturation current of SiC-substrate LED was much higher than that of GaAs-substrate LED which due to the reason of the high reflectivity mirror and high conductivity substrate.
In chapter 3, vertical InGaN-GaN LED epitaxial films were successfully fabricated on a 50mm Si substrate using glue bonding and laser lift-off technology. A high-temperature stable organic film, rather than a solder metal, was used as the bonding agent. It was found that the light output of the vertical InGaN LED chip exceeded that of the conventional sapphire-substrate LED by about 20% at an injection current of 20 mA. The vertical InGaN LED operated at a much higher injection forward current than were sapphire substrate LED.
Furthermore, the vertical InGaN LED remain highly reliable after 1000 h of testing.
In chapter 4, an InGaN-GaN light emitting diode (LED) with double roughened (p-GaN and undoped-GaN) surfaces was fabricated by surface-roughening, wafer-bonding and laser lift-off technologies. It was found that the frontside and backside luminance intensity were higher than that of the conventional LED. This is because the double roughened surfaces can provide photons multiple chances to escape from the LED surface, and redirect photons, which were originally emitted out of the escape cone, back into the escape cone as shown in
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section 4-2. Furthermore, the effect of the roughness of the undoped-GaN layer on the performance of double roughened LED was also investigated in section 4-3. It was found as the root mean square (rms) roughness of undoped-GaN layer increased, the output power and the view angle would be increased and decreased, respectively.
In chapter 5, an InGaN-GaN LED with a roughened undoped-GaN surface and a silver mirror on the sapphire substrate was successfully fabricated through a double transfer method.
It was found that, at an injection current of 20 mA, its luminance intensity and the output power was larger than conventional LED’s.
Finally, the summary of the overall results and the future work was shown in chapter 6.
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Fig. 1-1. The evolution of light emitting diodes.
[Quoted from Stringfellow et al.]
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Fig. 1-2. Band diagram illustrating non-radiative recombination (a) via a deep level (b) via an Auger process and (c) radiative recombination. [5]
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Fig. 1-3. Band structure near a semiconductor p-n junction: (a) homojunction under zero bias, (b) homojunction and (c) heterojunction under forward bias. [5]
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Fig. 1-4. The bandgap energy and corresponding wavelength versus lattice constant of AlGaInP and InGaN system at room temperature.
[5]
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Fig. 1-5. The schematic of a typical surface-emitting LED.
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Fig. 1-6. Schematic of surface waviness. (U. Gosele et al.) (a)
(b)
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Fig. 1-7. Saturation values of surface or bonding energy measured by thecrack opening method as a function of temperature after long-time heat treatments (up to 100 h) [10].
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Fig. 1-8. An infrared transmission image of a wafer with four devices showing bonding failure during fabrication of the device.
The fringes and dark regions indicate un-bonded areas. [8]
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Fig. 1-9. The bonding flowchart of AlGaInP LED with a GaP transparent substrate (TS). [18]
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Fig. 1-10. The LED chip of (a) GaAs substrate LED and (b) GaP transparent substrate LED. [32]
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TABLE I. Commonly used wafer-bonding techniques. [21]
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Chapter 2 High power AlGaInP light emitting diode with Cu and SiC substrate fabricated by wafer bonding
2-1 Introduction
The epitaxial growth techniques have significantly improved the brightness and efficiency of light-emitting diodes (LED). These improvements, along with higher reliability, lower costs, and the inherent properties of solid-state devices for low-voltage operation have enabled LED to be applied widely in solid-state lighting and displays. LED operating in the wavelength region ranging from red to green light have been employed by the AlGaInP alloy system grown on GaAs substrates [1]. However, its use in high-efficiency applications has been limited because of the substrate absorption and the internal reflection. Fortunately, the wafer bonding process can solve these problems. For instance, Hewlett Packard prepared the AlGaInP LED using a wafer-bonding technique that replaced the GaAs substrate with a GaP transparent substrate (TS) [2-4]. Another wafer-bonding approach was to bond an AlGaInP LED structure onto an Au/AuBe/SiO2/Si mirror substrate (MS) [5-7]. These studies have demonstrated high efficiency and reliability in LED performance. Nevertheless, despite improving luminous efficiency, LED sources are limited to use in low power applications due to the low thermal conductivity of the substrate. To achieve higher light output performance, it is necessary to drive the LED at a higher current. In this study, AlGaInP LED with wafer-bonded on Cu substrates and SiC substrate were proposed. The performances of the bonded LED were also been investigated.
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