Characteristics of polar and non-polar GaN-based films and light-emitting diodes

• Main Authors: Wei-Tsai Liao, 廖偉材
• Other Authors: Jyh-Rong Gong
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/67875869478802547985

博士 === 逢甲大學 === 材料科學所 === 95 === Polar AlxGa1-xN (0≦x≦0.5) films and AlxGa1-xN/GaN hetero- structures were grown on (0001) and (11-20) sapphire substrates at elevated temperatures by alternate supply of trimethylgallium (TMG) with (or without) trimethylaluminum (TMA) in group III flow and ammonia (NH3) in group V stream. The optical characteristics of GaN films deposited on (11-20) sapphire substrates were found to be comparable to those of GaN films grown on (0001) sapphire substrates under the same growth conditions. It appears that an increment of V/III ratio allows to improve the morphological and optical properties of a GaN film deposited on the (11-20) sapphire substrate. The best quality GaN films were achieved at a V/III ratio of 10400 with a quenched yellow luminescence and an enhanced room temperature near band edge photoluminescence (PL) emission having a linewidth of ~ 120 meV. Besides GaN-related thin films, we also report a comparative study on the performance of InGaN/AlGaN multiple quantum well light-emitting diodes (LEDs) fabricated on (0001) and (11-20) sapphire substrates by atmospheric pressure metalorganic vapor phase epitaxy (APMOVPE), respectively. It was found that the LEDs grown on (11-20) sapphire substrates exhibited enhanced electroluminescence intensity, decreased double crystal x-ray diffraction linewidth, reduced etching pit density, and smaller ideality factor compared to those deposited on (0001) sapphire substrates. The improved LED characteristics are attributed to threading dislocation density decrement inside the LEDs due to the reduced mismatch between LED structure and (11-20) sapphire substrate. Non-polar (11-20)-oriented GaN films were grown on (1-102) sapphire substrates at elevated temperatures using low-temperature(LT)-AlN buffer layers by alternate supply of TMG and NH3 in an inductively heated quartz reactor operated at atmospheric pressure. The structural, morphological and optical characteristics of the non-polar GaN films were investigated using x-ray diffractometry, transmission electron microscopy (TEM), field emission scanning electron microscopy, and PL spectroscopy. It was found that the non-polar GaN films were mono-crystalline in nature and the surface morphologies of the films were greatly improved by introducing LT-AlN buffer layers having certain thicknesses. Various types of threading dislocations (TDs) were identified by TEM analyses. It appears that the population of TDs having ±1/3<11-20>GaN type Burgers vectors is higher than that of the pure edge TDs with ±[0001]GaN type Burgers vectors . An optimized LT-AlN buffer layer thickness was found to enhance the near bandedge PL emission and quench the yellow luminescence of the non-polar GaN film.