Outline of This Dissertation

In this dissertation, the wafer bonding technique is used for enhancing efficiency of III-V compound semiconductor. All of investigations are based on wafer bonding technique, including metal bonding, glue bonding, and twice bonding. The device conventional substrate was removed and replaced with a new substrate, such as a high thermal conductively silicon substrate or a transparent sapphire substrate. After exchanging the conventional substrate, the III-V devices could achieve higher efficiency and characteristic, such as light extraction efficiency enhancement, high temperature insensibility and stability, suitable for high current operating, and stable performances in high temperature ambience.

In chapter 2, the high performances 650 nm resonant-cavity light-emitting diodes (RCLEDs) were demonstrated via high quality epitaxy and excellent chip process. The RCLEDs were designed with different light-output aperture sizes 84, 60, and 40 µm, for different applications of plastic optical fibers. In order to solve the RCLEDs performance in high temperature operation, the metal bonding RCLEDs (MBRCLEDs) with high performances, high temperature insensitively, and high reliability have been successfully fabricated on Si substrates by twice wafer bonding techniques. By wafer bonding technique, the RCLEDs, growing on conventional GaAs substrate, were replaced with a high thermal conductively silicon substrate. MBRCLEDs are that based on RCLED epi-layer structure and

changed GaAs substrate. Therefore, the MBRCLEDs maintained original RCLEDs performances and improve devices characteristic under high temperature ambiance especially.

The junction temperature variations of the MBRCLEDs were also relatively smaller as compared with the RCLEDs. Furthermore, the stable beam profile, high reliability over 1000 hours and clearly eye diagram in high temperature operation. These excellent performances of the MBRCLEDs devices should be suitable for high temperature ambiance, high current injection and high data communication applications.

In chapter 3, several methods to enhance AlGaInP LEDs performances were demonstrated. The AlGaInP LEDs performances enhancement key points in this study is to replace the absorbing GaAs substrate with a transparent sapphire substrate or high thermal conductively silicon substrate, and then fabricates surface textured for reducing total internal reflection effect. The LEDs efficiency enhancements are based on wafer bonding techniques of glue bonding and metal bonding process, and the surface roughness, geometric structure and flip-chip techniques were also applied for enhancing AlGaInP LEDs performances. The AlGaInP LEDs with a transparent sapphire substrate were fabricated by glue bonding method.

This transparent sapphire substrate is a geometric shaping sidewall (GSS-LEDs) structure by chemical wet etching processes. The GSS-LEDs surface has a nano-roughened texture by natural mask and chemical wet etching processes in order to improve light extraction efficiency. It was demonstrated that the GSS-LEDs structure could not only reduce the TIR effect but increase more probabilities of output light escaping from the transparent substrate due to the oblique sidewall structure. The GSS-LEDs are still not suitable for high current injection or high temperature ambience operating especially in short wavelength of AlGaInP system material. For this reason, a novel flip-chip AlGaInP-LEDs structure which has a thick geometric sapphire substrate (GSSFC-LEDs) window layer were demonstrated using glue

output power, longer reliability and higher wall-plug efficiency as compared with GSS-LEDs, conventional flip-chip LEDs (CFC-LEDs), and conventional glue bonding LEDs (GB-LEDs).

Finally, the AlGaInP epi-layers were bonded to a high thermal conductive Si substrate via metal bonding technique due to the sapphire is not a perfect substrate in devices, and achieved excellent light extraction efficiency using micro- and nano-scale surface textured technique.

This structure, having micro-bowls and nano-rods texture on surface has the highest output power as compared with micro-bowls only and plane surface devices.

In chapter 4, the InGaN/ GaN LEDs efficiency was enhanced by metal bonding, laser lift-off, and surface roughness technique. The conventional InGaN/ GaN LEDs with non-conductive and poor thermal conductivity property of sapphire substrate were replaced with Si substrate. Firstly, a novel and simple structure of single electrode pad (SEP-LEDs) in GaN-based LEDs are demonstrated. The concept was come from laser diode process of face coating. The ITO film was deposited on SEP-LEDs sidewall, and then a conductive current path was formed from chip sidewall to sapphire backside by ITO film. The SEP-LEDs has high performances than conventional lateral LEDs (CL-LEDs) although both substrates are sapphire. It is attributed that SEP-LEDs has more light output area and uniform current spreading properties. However, the SEP-LEDs structure can not be applied in future LED applications of high output power, high efficiency under higher current density, temperature insensitivity operating, and high reliability property after all. For this reason, the InGaN/ GaN vertical thin film LEDs (VTF-LEDs) structure is an important tread toward future LED lighting application. Many techniques and experiments are implanted in VTF-LEDs for improving VTF-LEDs performances and enhancing efficiency, such as high thermal stable Al-Ag alloy metal for reflector and p-ohmic contact, laser double cut before LLO process, proper chemical treatment and heavily doped in n-GaN layer for solving N-face surface of n-GaN issue, and optimized KOH chemical for surface random rough. By above mentioned, it

is demonstrated that the VTF-LEDs with lower forward voltage, uniform current spreading, high efficiency under the same current density, less wavelength variation in high current and high temperature ambience operating, higher maximum junction, and excellent thermal impendence performances as compared with CL-LEDs.

REFERENCE

[1] H. Wada; T. Takamori; and T. Kamijoh, “Room-temperature photo-pumped operation of 1.58-µm vertical-cavity lasers fabricated on Si substrates using wafer bonding”, IEEE Photon. Technol. Lett., vol. 8, pp. 1426–1428, Nov. 1996.

[2] H. Wada; T. Kamijoh, “Room-temperature CW operation of InGaAsP lasers on Si fabricated by wafer bonding”, IEEE Photon. Technol. Lett., vol. 8, pp. 173–175, Feb.

1996.

[3] Long Chen, Po Dong, and Michal Lipson, “High performance germanium photodetectors integrated on submicron silicon waveguides by low temperature wafer bonding”, Optics Express, Vol. 16, Issue 15, pp. 11513-11518, 2008.

[4] A. Georgakilas , M. Alexe , G. Deligeorgis , D. Cengher , M. Androulidaki , S. Gallis , Z.

Hatzopoulos and G. Halkias “III-V material and device aspects for the monolithic integration of GaAs devices on Si using GaAs/Si low temperature wafer bonding,” CAS 2001 Proc. Piscataway, NJ: IEEE, pp. 239, 2001.

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CHAPTER 2

High-Performance AlGaInP-Based Resonant-Cavity Light-Emitting Diodes in Wafer Bonding Technology

2.1 Introduction

In recent years, low-cost and short-distance-network optical devices based on the polymethyl methacrylate (PMMA) plastic optical fiber (POF) have been widely used for data communication, medium, automotive industry, and industrial sensors etc... The new development of these optical devices were focused on such factors as high power, high efficiency, high modulation speed, high coupling efficiency, reliability, low-cost fabrication, and high temperature insensitivity. On the basis of these requirements, the AlGaInP-based red 650 nm resonant-cavity light-emitting diodes (RCLEDs), according to these requirements, have been developed. A typical RCLED structure consists of an active layer for light emitting placed in a Fabry-Perot (F-P) resonator [1]. The F-P mirrors used are typically quarter-wavelength (λ/4) thickness semiconductors, dielectric distributed Bragg reflectors (DBRs) [2], or higher reflective metal films [3]. The RCLEDs structures are different from the conventional LEDs in some properties, such as optical profiles of directionality, narrower spectral bandwidth, higher quantum efficiency, higher output intensity, and suitable for optical communication of light sources [4]-[7]. The RCLEDs structure is similar to that of vertical-cavity surface-emitting lasers (VCSELs). Both of them have a high reflectivity (>98%) n-DBR as a function of bottom reflector, but the amount of top p-DBR pairs in RCLEDs structure is less than that in VCSELs. Actually, red visible VCSELs could not be operated in high-temperature environments such as in automobile parts or higher-driving-current modules. The VCSELs devices are sensitive to temperature variations

temperature sensitivity could be controlled by modifying the detuning cavity wavelength [9].

The detuning wavelength (λdetuning) means that the different wavelengths of the F-P resonator and the quantum well:

λ_detuning = λ_FP – λ_QW, (2-1)

where λFP and λQW are the wavelengths of the F-P resonator and quantum well, respectively.

The most important function of optimum detuning structure wavelength is that the devices temperature insensitively. In short-distance optical communication applications, POF offers several advantages over glass fiber such as low cost, higher number aperture (NA) for high coupling efficiency, decreasing fiber weight, visible light source for safety, and fiber flexibility. In order to meet these demands for high-performance POF communication, selected a light source to achieve high efficiency, reliability, suitable modulation bandwidth, operating-temperature range extensity, and higher coupling efficiency is very important.

Therefore, the visible red 650 nm RCLEDs is more ideal light source than conventional LEDs and VCSELs for the standard step-index polymethyl methacrylate (PMMA) based fibers due to the low-loss band (0.2 dB/m) at 650 nm [10]. The attenuation in plastic fiber is shown in figure 2.1. The preferred communication window of plastic fibers is at 650 nm, where the loss is of the order of 0.1-0.2dB per meter. At even shorter wavelengths, the attenuation in plastic fibers decreases.

In this chapter, we will discuss the high performance of 650 nm RCLEDs devices under special epi-structure and chip process firstly. And then we will introduce the high efficiency and temperature insensitive RCLEDs which were produced using twice wafer bonding technique. Finally, we will present all performances not only electrical and optical properties but also devices performances of applying on POF data communication.