Variation of Nitrogen Plasma Power for Observation of Structural-Dependent Magneto-Optical Birefringence
Chiung-Wu Su (蘇炯武)1*, Tzung-Han Tsai (蔡宗翰)1, Ruei-Yi Lee (李睿益)1
1Department of Electrophysics, National Chiayi University, Chiayi 60004, Taiwan
* presenting author:Chiung-Wu Su, email:cwsu@mail.ncyu.edu.tw
Gallium Nitride is a promising semiconductor material for blue light-emitting diode technology. The crystalline of epitaxial film revealing (002) and (103) two surface phases by grazing angle x-ray diffraction were found. The structure of the films reveals a competition in XRD peak intensity between two crystal phases. Three-stage processing of GaN films was surface nitridation, low-temperature deposition, and high-temperature deposition. All GaN films were deposited on c-sapphire substrates by plasma-assisted molecular beam epitaxy. The ion excitation power of N+ was controlled from 120 to 240 W to form the varied crystalline of GaN. The deposition of GaN films modifies the effective optical refractive indices of bulk sapphires in extraordinary refractive index. In order to observe the optical properties of interfaces and surfaces, the magneto-optical properties were systematically studied by magneto-optic Faraday effect (MOFE) and magneto-optic Kerr effect (MOKE) when a magnetic cobalt layer was capped on the top. We found the interface properties can be observed using two crossed high-extinction-ratio polarizers and angle-dependent magneto-optic hysteresis measurements. The optical and magnetic data are included within a complete cycle of hysteresis measurement. Shape and coercivity of hysteresis loops were varied due to the optical birefringence. A periodic reversal in hysteresis loop during the angle scan is dependent of effective optical thickness and refractive indices. The hysteresis flipping is a proof corresponding to the interface properties of GaN/c-sapphires and the magnetic layer. Different periodicities of the oscillation of magneto-optic intensity correspond to varied Co/GaN/sapphires. These optical parameters, i.e. ordinary and extraordinary refractive indices, inside the film structure can be fitted by an analytical formula. Through the angle-dependent magneto-optic measurements, optical parameters of ultrathin optical materials may be possibly explored by this nondestructive method. The study is mainly aimed to spin-optics in the future.


Keywords: magneto-optics, birefringence, optical crystal, magnetic films, plasma-assisted molecular beam epitaxy