| GaN-based semiconductor is a very important material for microelectronics and optoelectronics applications, and, as well as SiC and diamond, called the third generation semiconductor materials, following first generation-Si and second generation-GaAs. Special properties ofⅢ-nitride materials, such as Wide bandgap, strong atom bond, high thermal conductivity, and chemical inertness, are attracting the applications in the fields of optoelectronics, high temperature, high power, and high frequency devices.Etching technology is a critical step for the GaN-based devices fabrication. But so far, there is still no effective wet etching technique for GaN-based materials due to its chemical inertness. Dry etching (ICP) is normally used to fabricate GaN-based devices. But this technique is not only expensive and low selectivity but also damages the GaN materials during process. Developing a new wet etching technique is very important for the reducing cost and damage of the GaN-based devices. Meanwhile, solid state lighting market exploring needs large area, vertical LEDs, which needs liftoff technique to.remove the substrates. In addition, nonpolar and semipolar GaN materials will push optoelectronics devices to longer wavelength and better performance. But the high defect density of nonpolar and semipolar GaN inhabits its development. In this dissertation, based on the problems mentioned above, we investigate the electrochemical etching GaN liftoff, fabrication of nanoporous GaN and large area, free standing GaN by electrochemical etching, semipolar (11-22) GaN MOCVD growth by two-step growth, and defect reduction of semipolar (11-22) GaN using a nanoporous GaN interlayer. The main conclusions are summarized below.We studied the conductivity based selective etching of GaN at room temperature. Three regions are identified including no etching region, porous GaN formation region, and the complete removal or electropolishing region with increasing conductivity or applied voltage, which provides flexibility for the fabrication of GaN devices. Based on this etching technique, we lifted off a large area (>1×1 mm2), crack free GaN thin film with a thichness of～1μm, which is large enough for large area LEDs (1×1mm2); a few photonic applications (microdisks and DBRs) and GaN beams and cantilever were demonstrated. The simplicity of EC etching and its compatibility with conventional GaN structures allow us to anticipate a variety of useful device applications in the future.Secondly, we fabricated nanoporous GaN by a simply electrochemical etching. Applied voltage and Si doping concentration of n-GaN are two very important parameters for the electrochemical etching; the properties of nanoporous GaN, such as pore size, porosity, pore density and etch rate, can be tuned by changing these two parameters. Pore size and porosity of nanoporous GaN increase with applied voltage increasing for the same Si-doped GaN, and pore density and etch rate increase first, and decrease when applied voltage is larger than a certain value. However, pore size of nanaoporous GaN decrease, and porosity, pore density and etch rate increase with the Si doping concentration of n-GaN increasing at the same applied voltage. we obtained 2 inch uniform nanoporous GaN sample. For the electrochemical etching mechanism, we think that the EC etching in GaN involves a two-step process of oxidation of GaN (into GaOx) and the dissolution of GaOx by the oxalic acid; the holes in GaN and the electrons in oxalic acid solution (OA) recombined at the interface between GaN and OA due to present of electrical field, some GaN are oxidized into GaOx, GaOx dissolved in the OA solution to form nanoporous structures. More works should be done about the electrochemical etching mechanisms in the future.Based on the study of nanoporous GaN, we present a new scheme in splitting and lifting-off GaN using nanoporous (NP) GaN medium by a simple and robust electrochemical (EC) etching process. This procedure can be considered an implementation of the "smart-cut" principle using nanoscale wet etching and is compatible to wafer-level scaling up. The NP GaN produced by the EC etching offers a new way to selectively weaken the mechanical strength of GaN, making it possible to split and separate epitaxial GaN layer. The use of the nanoetching leads to a flexible process in forming columnar pores during the initial vertical drilling, followed by localized isotropic etching deep in the layer to create lift-off. This procedure can be applied to almost all semiconductors but is especially pertinent to GaN with its given its chemical inertness. We demonstrate that large area (≥1 cm2), free-standing GaN layers, with a thickness from 0.5 to a few microns, can be separated in less than 20 minutes, and the mono-crystallinity of the lift-off GaN layers is well preserved by this process. After NP GaN liftoff, we can transfer it onto different substrates, such as glass, Si, and polydimethylsiloxane (PDMS).Another advantage of this liftoff technique is that the substrate can be reused after liftoff free standing GaN, which we demonstrated in this dissertation.Last, we investigated the growth of semipolar (11-22) GaN by two-step growth on m-plane sapphire, and using a nanoporous GaN interlayer to reduce the defect density of semipolar (11-22) GaN.A pure (11-22) GaN has been attained with a great reproducibility by adopting an appropriate nitridation for m-sapphire. A two-step growth approach is introduced to substantially improve (11-22) GaN quality evaluated by a comprehensive x-ray analysis. With the insertion of an islanding growth step, the FWHMs of the ori-axis (11-22) XRCs decreased by-55%, and the FWHM plots became less steep. The growth mode changes from 3D to 2D making the dislocation bending, interaction and annihilation.Semipolar (11-22) nanoporous GaN was fabricated by the electrochemical etching. The properties of semipolar (11-22) nanoporous GaN is very similar with the (0002) nanoporous GaN. Pore size and porosity of semipolar (11-22) nanaoporous GaN increase with the applied voltage increasing at the same Si doping concentration of n-GaN; etch rate increases first, then decreases when applied voltage is larger than a certain value. There is no plane selectivity for electrochemical etching comparing to semipolar (11-22) nanoporous GaN by PEC etching; nanoporous structure formation is only related to applied voltage and Si doping concentration of n-GaN.A new method is presented toward the reduction of microstructural defects during the growth of (1122) semipolar GaN on sapphire. The use of a nanoporous and monocrystalline interlayer, produced by a simple electrochemical (EC) etching process, facilitates homogeneous lateral growth. TEM results show that the partial dislocation density above the NP GaN interlayer is nearly reduced by a factor of two. The direct blocking of dislocations is the main mechanism of defect reduction of semipolar (11-22) GaN using a nanoporous interlayer. The InGaN/GaN quantum wells grown on NP GaN exhibit a three-time enhancement in photoluminescence (PL) intensity over that on planar semipolar GaN templates. The improvement in PL intensity is likely due to 1) improved material quality,2) the presence of NP layers for diffusive scattering underneath the MQWs, and 3) the reduction of index of refraction in the NP layer. The surface pit density of InGaN/GaN MQWs grown on nanoporous GaN layer reduced by a factor of 2 (from～1.42×108/cm2 to～7.8×107/cm2), which confirmed the defects reduction of semipolar GaN.
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