Growth and Characteristics of Selenide Compounds by Molecular Beam Epitaxy

• Main Authors: Chia-Hsing Wu, 吳家興
• Other Authors: Chu-Shou Yang
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/03622307654698455672

博士 === 大同大學 === 光電工程研究所 === 103 === This dissertation is devoted to CuGaSe2 and GaSe thin films grown using molecular beam epitaxy. For CuGaSe2 thin films, the growth and physical properties were studied. CuGaSe2 were obtained with varying metallic ratio of (Cu/Cu+Ga) from 0.23 to 0.75. When the metallic ratio increases from 0.23 to 0.75, surface morphology of CuGaSe2 thin films implies the transformation of crystal structure from GaSe-like to CuGaSe2 (Cu-rich). The optical performances exhibit the optimum growth condition for CGSe2 is obtained. Following, this condition is used to grow CuGaSe2 thin films on molybdenum coated stainless steel(SS) substrate. Two kinds of stainless steel substrates (304SS、430SS ) were used. The different of stainless 430SS and 304SS is the content with/ without chromium (Cr), respectively. CuGaSe2 and Cu2-xSe phases were observed by Raman scattering measurement. The photoluminescence emission of donor-acceptor pair exhibits a red shift, which implies the different of content ratio in these samples. Temperature dependence PL was used to measure activation energy of CGS grown on two substrates. The activation energy (Ea) of CGS2 grown on 430SS and 304SS were estimated as 41 and 30 meV. The CuGaSe2 thin film growth on 430SS was better than 304SS substrate. The second parts focus on the growth of GaSe epitaxial single crystal thin films. The GaSe epilayer during growth were monitored by in situ reflection high-energy electron diffraction (RHEED). Streak RHEED patterns demonstrated a flat and crystalline sample surface Lattice constant in a-axis was approximately 0.375±0.012 nm, which was correspondedwith single-crystal GaSe. Furthermore, two kinds structure of GaSe thin films which were correlated with the m-axis and a-axis of hexagonal was observed by RHEED after 15 min. The single crystal GaSe was verified using X-ray diffraction and high-resolution cross-section transmission electron microscopy. The full width at half-maximum of peak (0002) which was defined in the XRD rocking-curve spectrum of GaSe epilayer was obtain around 207 arcsec. The epitaxial growth of GaSe demonstrated the feasibility of growing large-area ultrathin epilayers. The surface effect of selenium pretreated sapphire substrate (Se-sapphire) at low temperature (500℃) was investigated. GaSe thin films were deposited on Se-sapphire. The growth rate of GaSe/Se-sapphire was 5 times than GaSe/sapphire. It implies that Se terminated sapphire surface assisted GaSe deposition. Moreover, the improvement conductivity of GaSe was investigated by doping copper and zinc. The conductivity of intrinsic GaSe, GaSe:Cu, and GaSe:Zn were 10-7, 10-6, and 10-4 (Ω×cm)-1, respectively, by four point probe measurement. The reason is the incorporation of metal cations (Cu and Zn) increasing hole concentration. The crystal structure became worse when the concentration of dopant increased, especially in the doped copper series. In future work, based on these works, the ultra-thin film of the epitaxial will be demonstrated. Finally, the photocurrent in metal-semiconductor-metal (MSM) photo-detectors based on layered GaSe will be generated.