MBE growth of ZnSe/MgCdS and ZnCdS/MgCdS superlattices for UV‐A sensors

Abstract

ZnSe/MgCdS and ZnCdS/MgCdS superlattices were grown on semi insulating (001) oriented GaAs substrates by molecular beam epitaxy (MBE). The crystal quality and optical properties were examined for both superlattices. The observed photoluminescence (PL) peak intensity of ZnCdS/MgCdS was much stronger than that of ZnSe/MgCdS while ZnSe/MgCdS showed sharper luminescence line width. The PL peak energy position was compared with the theoretical value derived from the Kronig-Penny model. The degradation of crystal quality was observed by increasing Mg content in MgCdS layer and drastic PL property degradation was observed when the Mg content in the layer exceeded a certain value. Two superlattices were applied to the UV-A sensor of the metal-semiconductor-metal (MSM) configuration. Both photodetectors exhibited a high sensitivity in the UV-A region with a sharp cut-off in the visible wavelength region; the ON-OFF ratio of sensor was about 10 3 for both structures. 1 Introduction Recently, problems associated with the exposure to the UV-A ray have been reconsidered. The realization of solid state visible blind UV-A sensors was studied using wide bandgap II-VI compound epitaxial layers [1][2][3]. The growth of ZnMgCdS quaternary alloy and ZnCdS/MgCdS superlattices, that exhibit a room-temperature band-gap corresponding to the UV-A region (about 3 eV) have been studied, and fabrication of UV-A sensors were performed [4,5]. ZnCdS/MgCdS superlattices that utilize quantum effects showed improved optical properties compared with ZnMgCdS thin films. In this study, ZnSe/MgCdS superlattices were explored for another UV-A sensor material. Estimating from the common cation-anion rule [6], only electrons could be confined in the conduction band quantum well for ZnCdS/MgCdS superlattices whereas both electrons and holes would be confined for ZnSe/MgCdS superlattice in quantum wells which were formed in both conduction band and valence bands. Consequently, ZnSe/ MgCdS superlattices would exhibit improved optical properties, such as sharper and intense photoluminescence (PL) compared with those from ZnCdS/MgCdS. ZnCdS/MgCdS superlattices, on the other hand, would exhibit higher sensitivity for light than ZnSe/MgCdS superlattices because the photo-absorption layer was consisted from S compounds. Since the effective bandgap would depend on periodic structures of superlattices, namely band offsets and effective masses of carriers, the design of the superlattice could include various structures even though the focused bandgap energy was identical. In order to obtain the high carrier confinement effect, bandgap energy of the MgCdS barrier layer should be high. High Mg content is required to expand the bandgap of MgCdS, then the degrada-

(211) oriented ZnTe growth on m‐plane sapphire by MBE

“…1 Introduction II-VI compound semiconductors have attracted much attention for use in various practical devices, including solar cells [1,2], light emitting diode [3,4], UV sensors [5][6][7], electro-optic (EO) devices [8][9][10] and terahertz device applications [11][12][13]. In particular, zinc telluride has a superior permeability and a comparatively high EO effect [14][15][16][17][18] is expected.…”