MBE Growth and Optical Properties of ZnSeO
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MBE Growth and Optical Properties of ZnSeO Y. Nabetani, T. Mukawa, Y. Ito, T. Kato, and T. Matsumoto Department of Electrical Engineering, University of Yamanashi Takeda 4-3-11, Kofu 400-8511, JAPAN ABSTRACT ZnSeO alloy was successfully grown by molecular beam epitaxy on GaAs substrate using RF plasma. The crystal structure of epitaxial ZnSeO alloy was zincblende. No phase separation was observed by in-situ reflection high energy electron diffraction and X-ray diffraction measurements. O composition was estimated by lattice constant assuming Vegard’s law. Photoluminescence intensity was larger than that of ZnSe. The peak energy shifted toward lower energies by increasing O composition. The band gap energy determined by photoluminescence excitation decreased with increasing O composition. The bowing parameter was obtained as high as 8.2eV. This large band gap bowing widens controllable energy range of II-VI semiconductor. INTRODUCTION II-VI compound semiconductors ZnO and ZnSe have wide band gaps, 3.37 and 2.70eV at room temperature, respectively, and are known as materials for optical devices operated at ultra violet to blue region. These compounds have common cation, but anion species of O and Se are different. The electronegativity of O is 3.5eV and is much larger than that of Se ( 2.4eV )[1]. Therefore O atom attracts electrons when a part of Se atoms are replaced with O in ZnSe. This situation is similar to N atom in GaAs and in GaP. In the case of III-V-N alloy, it is widely known that large electronegativity of N induces large band gap bowing[2,3]. If large band gap bowing is realized in II-VI semiconductors, a wide range of wavelength region used by optical devices can be covered, since II-VI semiconductors have large band gaps. The wide controllability of band gap is attractive for band gap engineering. Moreover, exciton binding energy, which is larger in II-VI semiconductors than in III-V, can be made much larger by fabricating quantum structures with advantage of large band offsets. This enhancement leads to nonlinear optical phenomena operating at room temperature. In this study, we have grown ZnSeO alloy and investigated the structural and optical properties. The band gap energy was found to decrease with increasing O composition in ZnSeO. EXPERIMENT ZnSeO alloy was grown by molecular beam epitaxy ( MBE ). Elemental Zn and Se were used for solid sources. Oxygen was supplied by flowing high-pure O2 gas through an RF plasma cell operated at a power of 50W. O2 flow rate was varied in the range between 0 and 0.02ccm using a mass flow controller. Since our approach is O introduction into ZnSe, (001) oriented GaAs, which is almost lattice-matched to ZnSe ( lattice-mismatch ~ 0.2% ), was used as a substrate. The growth temperature was 350°C. At first, a 100nm-thick ZnSe buffer layer was grown on GaAs under VI/II beam flux ratio less than unity, in order to avoid oxidation of the GaAs surface. Then, Zn, Se and RF excited oxygen were simultaneously supplied to grow ZnSeO alloy. The thickness of epitaxial
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