Investigation of Nonpolar GaN-based Materials and Devices

• Main Authors: Sheng-HanSu, 蘇聖涵
• Other Authors: Yan-Kuin Su
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/35183195475779809948

碩士 === 國立成功大學 === 奈米科技暨微系統工程研究所 === 100 === The main goal of this thesis is to investigate and improve the crystalline quality, surface characteristics, and devices performance for nonpolar a-plane GaN film grown on r-plane sapphire. The direct-growth conditions of a-plane GaN on r-plane sapphire by metal organic chemical vapor deposition (MOCVD) have been optimized in a two-step process with step-stage multiple quantum wells (MQWs) structures. In this work, a set of step-stage multiple quantum wells (MQWs) is inserted between underlying GaN and overlying highly Indium composite MQWs to investigate it’s effectively influence on the optical properties of active region. The step-stage MQWs were deposited by varying the growth temperature, which are 790 under fixed precursor flow rate. Optical properties were investigated by means of temperature-dependent photoluminescence (TD-PL). The optical polarization ratio, activation energy, and the smile-like curve in full width at half maximum (FWHM) of PL were analyzed in detail. The XRD and temperature-dependent PL data shows the crystal quality is improved when increasing the pair of the step-stage MQW. The activation energy calculated by temperature-dependent PL was improved to 149.9 meV in the sample with 830 growth temperature due to less Indium fluctuation and localized states. Then, as our group has investigated the one-sidewall-seed epitaxial lateral overgrowth (OSELOG) technique, we adapted the step-stage MQWs active region with the OSELOG technique to achieve higher device performance. The nonpolar a-plane GaN-based photodetectors (PDs) with step-stage MQW and OSELOG technique have been demonstrated in this thesis. We also studied the direction effect between the basel stacking faults (BSFs) in a-plane GaN and the metal electrodes. First, the performance of a-plane GaN-based MSM-PDs operated under 1 V represent high photo-to-dark current ratio (~103), higher measured responsivity (~1 A/W), and high UV-to-visible rejection ratio (~103). Additionally, the performance of a-plane MSM-PD has greater internal gain effect when the direction of BSFs was parallel to metal electrodes. That means the nonpolar a-plane GaN-related materials have potential to fabricate high performance polarization-sensitive PD (PSPD). Also, we demonstrated the a-plane GaN-based high electron mobility transistors (HEMTs) and co-worked with GaN device technology, RWTH Aachen University, Germany. Although the leakage drain current in these transistors was at a very high level a 370 mA/mm, transistor operation could be demonstrated. According to these results, we also investigated and optimized the growth condition in a-plane GaN films. Hence, in the last part, we improved the crystal quality by three ways such as different buffer layer with high/low growth temperatures, the in situ etching methods, and modulating V/III ratio or growth pressure. First, we have grown AlN as the buffer layer between r-plane sapphire substrate and GaN film due to the lattice mismatch. And the low and high growth temperatures of AlN are both presented which are 600 and 1150 , respectively and the low temperature AlN has shown the better crystal quality and morphology than the high temperature one which were determined by HR-XRD and AFM. The FWHM value along m-axis of XRD was improved from 1564 to 1119 arcsec. Additionally, the surface crack in the high temperature AlN is so serious that may influence the device performance. Since, we adapted the low growth temperature AlN as the buffer layer. Second, the in situ etching process is used to improve the crystal quality. We used hydrogen as our etching gas in MOCVD with etching times and etching pressure are both investigated. When etching times increased from 1 to 2, the crystal quality is obviously improved by determining the FWHM of XRD represented as 1023 arcsec to 917.8 arcsec. However, the surface morphology is opposite with XRD results. Considering the devices performance, we decided to etch the GaN template with one time. Finally, the role of V/III ratio is also important to crystal quality. The growth rates along m- and c-axis are more uniform at a high V/III ratio so that we can achieve smooth and nearly pit-free a-plane GaN film. Under our optimized condition, the roughness determined by AFM is reduced from 22.11 nm to 4.3 nm. According to aforementioned, these results show great potential of a-plane GaN materials for the fabrication high performance optoelectronic and electronic devices in the future.