The authors report the growth of ZnSe/ZnSeTe superlattice nanotips on oxidized Si(100) substrate. It was found the nanotips exhibit mixture of cubic zinc-blende and hexagonal wurtzite structures. It was also found that photoluminescence intensities observed from the ZnSe/ZnSeTe superlattice nanotips were much larger than that observed from the homogeneous ZnSeTe nanotips. Furthermore, it was found that activation energies for the ZnSe/ZnSeTe superlattice nanotips with well widths of 16, 20, and 24 nm were 76, 46, and 19 meV, respectively.
KeywordsZnSe/ZnSeTe superlattice nanotip Photoluminescence Activation energies
Wide bandgap ZnSe-based II–VI materials are attractive materials that can be used in various optoelectronic applications. Using two-dimensional (2D) epitaxial layers, ZnSe-based light emitting diodes, laser diodes, and photodetectors have also been demonstrated [1–3]. Other than 2D epitaxial films, it is also possible to prepare one-dimensional (1D) ZnSe nanowires. With a large surface-to-volume ratio, 1D systems including nanowires and nanorods have attracted great interest in recent years. It is generally recognized that 1D materials are the ideal building blocks for novel nano-scaled optoelectronic devices. 1D ZnSe nanowires can be prepared by pulse laser deposition , metalorganic chemical vapor deposition , phase vapor growth , and molecular-beam epitaxy (MBE) . Among these methods, MBE can be used to grow samples in high vacuum, which is important when preparing crystalline materials. Indeed, device quality ZnSe epitaxial layers reported in the literature were all prepared by MBE [8–10]. With the ability to precisely control growth parameters and to accurately monitor growth process, MBE should be an ideal tool to grow nano-structured materials.
For conventional 2D devices, heterostructure plays a very important role. Similarly, the concept of heterostructure can be applied to 1D nanowires/tips. Indeed, GaN/InGaN, InAs/InP, Si/SiGe, and ZnMgO/ZnO heterostructure nanowires/tips have all been demonstrated [11–14]. It is also possible to prepared nanowires/tips with superlattice structure. For these superlattice nanowires/tips, the longitudinal confinement could couple with radial confinement. This should provide more functionalities for the superlattice nanowires/tips . To realize ZnSe-based superlattice nanowires/tips, it is necessary to form ternary nanowires/tips. Very recently, we reported the growth of ternary ZnSeTe nanotips by MBE and the fabrication of a ZnSeTe nanotip-based photodetector . ZnSeTe is a ternary material with interesting optical properties. It has been shown that strong luminescence signal could be observed from localized excitons bound to Te atom (Te1 emission) and Te n (n ≥ 2) cluster (Te n cluster emission) in 2D ZnSeTe epilayer [17, 18]. In this study, we report the growth of ZnSe/ZnSeTe superlattice nanotips by MBE on oxidized Si(100) substrates. Physical and optical properties of the ZnSe/ZnSeTe superlattice nanotips will be discussed.
The ZnSe/ZnSeTe superlattice nanotips used in this study were grown by a Riber 32P solid source MBE system on oxidized Si(100) substrate using vapor–liquid–solid (VLS) mechanism with an Au-based nano-catalyst. The source materials for the MBE system were elemental Zn (6N), Se (6N) and Te (6N). Prior to the growth of the nanotips, a Si(100) substrate was first immersed in boiled acetone for 10 min, in boiled isopropyl alcohol for 10 min, and in hydrofluoric acid solution for 30 s. The chemically cleaned Si substrate was thermally oxidized to form a 150-nm-thick SiO2 film. This SiO2 film acts as a catalyst diffusion barrier . A 0.6-nm-thick Au film was then deposited onto the SiO2 layer by sputtering. The sample was then loaded onto the preparation chamber and annealed at 280°C to transfer Au film into Au nano-particles . Subsequently, the substrate was transferred into the growth chamber to grow the ZnSe/ZnSeTe superlattice nanotips at 280°C for 1 h.
After the growth, surface morphologies of the samples were characterized by a Hitachi S-4700I field-emission scanning electron microscope (FESEM) operated at 15 kV. A Philips FEI TECNAI G2 high resolution transmission electron microscopy (HRTEM) operated at 200 kV and a Siemens D5000 X-ray Diffraction (XRD) system were applied to analyze crystallographic and structural properties of these superlattice nanotips. To characterize optical properties, photoluminescence (PL) measurements were performed by a continuous wave (CW) He–Cd laser operated at 325 nm as the excitation source. The luminescence signals generated from the samples were then recorded by a lock-in amplifier at 20 to 100 K.
Results and Discussion
In summary, we reported the growth of ZnSe/ZnSeTe superlattice nanotips on oxidized Si(100) substrate by MBE using VLS mechanism with an Au-based nanocatalyst. It was found that the ZnSe/ZnSeTe superlattice nanotips exhibit mixture of cubic zinc-blende and hexagonal wurtzite structures. It was also found that PL intensities observed from the ZnSe/ZnSeTe superlattice nanotips were significantly larger than that observed from the homogeneous ZnSeTe nanotips.
This study was supported in part by the Center for Frontier Materials and Micro/Nano Science and Technology and in part by the Advanced Optoelectronic Technology Center, National Cheng Kung University, under projects from the Ministry of Education. This study was also in part supported by Ministry of Economic Affairs (MOEA) and NSC 98-EC-17-A-09020769. The authors would like to thank the Bureau of Energy, Ministry of Economic Affairs of R.O.C. for financially supporting this research under Contract No. 98-D0204-6 and the LED Lighting and Research Center, NCKU for the assistance regarding measurements.
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