MOCVD Growth And Characterization Of Nonpolar GaN-based Materials

Gallium nitride (GaN) is a direct band-gap material with broad bandwkh, stable chemical property, and high melting point (2300?). It has been used in a wide range of application, including solid-state lighting, solar cells, sterilization, laser, etc. (0001)GaN materials were usually obtained grown on e-plane sapphire substrates. However, the piezoelectric polarization in the GaN-based materials can produce the QCES (quantum confined Stark effect). The adoption of the nonpolar GaN material can eliminate the built-in electric field, overcome the problem of spatial separation of electrons and holes, and improve internal quantum efficiency of GaN-based devices. In this paper, the nonpolar a-plane GaN epitaxial layers were successfully grown on r-plane sapphire substrates by metalorganic chemical vapour deposition (MOCVD). The properties of these epitaxial layers were also studied. Moreover, the nonpolar GaN-based LEDs with AIInGaN/GaN multiple quatum wells (MQWs) and A?nGaN/GaN superlattices (SLs) electron blocking layer (EBL) were theoretically studied, respectively. In addition, the GaAs-based terahertz modulator was investigated. The results showed that an uhrafast modulation speed of over 11 MHz was achieved.The major research contents and the results achieved in this study were listed as below:1. Using the MOCVD system, the nonpolar a-plane GaN epitaxial layers with the GaN nucleation layer were deposited. The two-step growth method was used to grow high quality a-plane GaN epitaxial layer on r-plane sapphire substrate. Then, the effects of growth pressure, TMG flow, and ?/? ratio on surface morphology, crystal quality, optical and electrical properties for a-plane GaN epitaxial layers were studied. Furthermore, Si-doped (11-20) a-plane GaN epitaxial layers on (1-102) r-plane sapphire substrates were also grown. The crystal quality, surface morphology, and electrical properties of Si-doped a-plane GaN films were characterized by high-resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM), photoluminescence (PL), and Hall effect measurements. The results showed that the crystal quality and surface morphology were degraded slightly with increasing Si doping level, while the mobility increased from 9.15 cm2/V·s to 66.90 cm2/V·s.2. The nonpolar a-plane GaN epitaxial layers with the AN nueleation layer were also grown in a MOCVD system. The high-low-high temperature growth process was used to improve the crystal quality. Ammonia (NH3) pulse-flow growth method and the AlGaN interlayer with Al component graded between GaN and AIN layers were also adopted. The result demonstrated that the FWHM value of X-ray rocking curve along m-axis direction was reduced by 28%.3. The conventional c-plane GaN-based LEDs was simulated by using the APSYS simulation software. The band structure, internal quantum efficiency (IQE), and electron leakage were numerically evaluated to analyze the reason for the performance degradation of the c-plane GaN-based LEDs. Subsequently, the nonpolar Al0.089In0.018Gao.893N/GaN quantum well was used to eliminate the polarization electric field and the lattice mismatch between GaN and AlGaN. Moreover, an A?nGaN/GaN superlattice (SL) EBL instead of a single A?nGaN EBL was adopted to improve the IQE and output power, minimize the electron leak current, and increase the hole injection efficiency.4. The GaAs-based terahertz modulator was investigated by computer simulation technology (CST) software. Firstly, by changing the size of split-ring resonators (SRRs), the resonance frequency of 0.34 THz for a terahertz modulator was obtained. Then, an innovative terahertz modulator with the capability of responding to the incident multiple-frequency terahertz waves was designed. In addition, the terahertz modulator was investigated. The density of two-dimensional electron (2DEG) inside the high electron mobility transistror (HEMT) was controlled by modulating vokage which changed the transmission of the incident terahertz waves. The device exhibited the capability of dynamical response to the incident terahertz waves under a fast time-varying voltage. The measurement result revealed that the ukrafast modulation speed was over 11 MHz.