Investigation of Growth of III-Nitride Alloys by Plasma-Assisted Molecular Beam Epitaxy

• Main Authors: Yang. Wei Chen, 楊偉臣
• Other Authors: Cheng. Keh Yung
• Format: Others
• Language: en_US
• Published: 2016
• Online Access: http://ndltd.ncl.edu.tw/handle/mzzfkv

博士 === 國立清華大學 === 電子工程研究所 === 105 === Over the past two decades, III-nitride materials system boasts outstanding properties of optoelectronic devices and high-power devices. However, many of these applications are currently unavailable using current III-nitride thin film growth techniques, including heterojunction bipolar transistors (HBTs) with a base layer of high hole concentration, vertical power devices with high breakdown voltages, green wavelength light-emitter diodes (LEDs). This is due to three mainly problems: (1) there is no lattice-matched substrate for III-nitride growth, creating high density defects in the grown material, (2) the issue of phase separation in InGaN alloys, which results in a fluctuated In composition and low In incorporation, and (3) high ionization energy of Mg leads to a low hole concentrations in GaN. In this thesis, III-nitride materials grown by plasma-assisted molecular beam epitaxy are investigated. The objectives of this study are to improve and solve several pressing issues associated with the growth of III-nitride materials; (1) An improved two-step growth method under high growth temperature is successfully developed to increase the In content of the InGaN/GaN SQW to ~30 % while maintaining a strong luminescence intensity in the green spectral range. The In composition in InGaN/GaN SQW grown under group-III-rich condition increases with increasing growth temperature following the growth mechanism of liquid phase epitaxy. (2) A nano-hole patterned GaN substrate template has been developed to reduce the threading dislocation (TD) density in GaN epilayers. The grown layers are analyzed by high resolution x-ray diffraction, photoluminescence, and cross-section transmission electron microscopy. It is confirmed that the TD density of GaN epilayers has been successfully reduced from 109 down to 107 cm2. (3) The influence of growth conditions on the incorporation and activation of Mg in GaN at high growth temperature is studied. It is found that the electrical activated Mg in GaN is highly sensitive both to the III/V flux ratio and Mg/Ga flux ratios. Although the highest hole concentration achieved under a low III/V flux ratio and a high Mg/Ga flux ratio reaches 7.5×1018 cm-3, the hole mobility is suffered due to the formation of defects by the excess Mg. Therefore, to further improve the electrical properties of Mg doped GaN, we found that a maximum Mg activation of ~5 % can be achieved at the optimized growth condition. The lowest resistivity of 0.56 Ω-cm is achieved, which is associated with a high hole mobility of 6.42 cm2/V-s and a moderately high hole concentration of 1.7×1018 cm-3. In addition, we show that modulated beam growth methods do not enhance Mg incorporation at high growth temperatures in contrast to those grown at a low temperature of 500 °C