IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 10, NO. 3,MAY 2011 379 Growth and Photoelectric Properties of Twinned ZnSe 1-x Te x Nanotips S. J. Chang, Senior Member, IEEE, S. H. Chih, C. H. Hsiao, B. W. Lan, S. B. Wang, Y. C. Cheng, T. C. Li, and S. P. Chang Abstract—The authors report the growth of high density ZnSe 0 .9 Te 0 .1 nanotips by molecular beam epitaxy and the fab- rication of ZnSeTe nanotip photodetector. It was found that the as-grown ZnSe 0 .9 Te 0 .1 nanotips were twinned with alternative multidomains and mixture of cubic zinc-blende/hexagonal wurtzite phases. With 5-V applied bias, it was found that photocurrent to dark current contrast ratio of the fabricated photodetector was larger than 700. Index Terms—Molecular beam epitaxy (MBE), photodetector, wurtzite, zinc blende, ZnSeTe nanotips. I. INTRODUCTION I N RECENT years, 1-D semiconductor materials, such as nanowires and nanorods, have attracted much attention, ow- ing to their unique properties, such as large surface-to-volume ratio [1], [2]. Indeed, the growth of group IV, III–VI, and II–VI semiconductor-based 1-D nanostructures have all been demon- strated [3]–[5]. Among the II–VI semiconductors, 1-D zinc ox- ide (ZnO) nanostructures have been extensively studied [5]–[7]. In contrast, only few reports on 1-D zinc selenium (ZnSe) can be found in the literature [8]–[11]. ZnSe is an important II– VI semiconductor with wide direct bandgap energy of 2.67 eV and large exciton binding energy of 21 MeV at room temper- ature. With these properties, 1-D ZnSe nanostructures are po- tentially useful for various nanooptoelectronic devices. On the other hand, bandgap energy is one of the most important param- eters for semiconductor materials. Bandgap energy determines many important electronic and optical properties. For example, it determines the emission wavelength of semiconductor light Manuscript received June 4, 2009; revised October 22, 2009; accepted January 5, 2010. Date of publication January 15, 2010; date of current version May 11, 2011. This work was supported in part by the Taiwan Semiconductor Manufacturing Company, Ltd., in part by the Center for Frontier Materials and Micro/NanoScience and Technology, National Cheng Kung University, Taiwan, under Grant D97-2700, and in part by the Advanced Optoelectronic Technology Center, National Cheng Kung University, under projects from the Ministry of Education. The review of this paper was arranged by Associate Editor J. Rogers. S. J. Chang is with the Institute of Microelectronics and Department of Elec- trical Engineering, Center for Micro/Nano Science and Technology, Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 70101, Taiwan (e-mail: changsj@mail.ncku.edu.tw). S. H. Chih, C. H. Hsiao, B. W. Lan, S. B. Wang, and S. P. Chang are with the Institute of Microelectronics, National Cheng Kung University, Tainan 70101, Taiwan (e-mail: sam900738@gmail.com; g9413746@yuntech. edu.tw; EE1219@hotmail.com; g9518711@yuntech.edu.tw; q1895111@mail. ncku.edu.tw). Y. C. Cheng and T. C. Li are with the Materials and Electro-Optics Re- search Division, Chung Shan Institute of Science and Technology, Taoyuan 325, Taiwan (e-mail: yccheng99@mail2000.com.tw; tcku@mail.csist.org.tw). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2010.2040626 emitting diodes and cutoff wavelength of photodetectors. One possible way to change bandgap energy of a certain semicon- ductor material is to ally it with another semiconductor material. Indeed, we can easily tune the bandgap energy of a ternary alloy by changing its composition ratio. For ZnSe-based ternary al- loys, the growth of ZnMnSe [12], ZnCdSe [13], and ZnSSe [14] nanowires have all been demonstrated. ZnSeTe is also an inter- esting ternary compound. It has been reported that the binding energy of the Te-bound excitons is very large [15]. It has also been shown that the extrinsic self-trapping of excitons (STEs) formed in Te-related emission for ZnSe 1−x Te x layers can result in much higher luminescence efficiency [16]. To our knowledge, however, no report on the growth of 1-D ZnSeTe nanowires can be found in the literature. In this paper, we report the growth and characterization of ZnSeTe nanotips. A ZnSeTe nanotip photodetector was also fabricated. Physical, electrical, and op- tical properties of the fabricated device will also be discussed. II. EXPERIMENT The ZnSe 0. 9 Te 0. 1 nanotips used in this paper were grown by a Riber 32P solid source molecular beam epitaxy (MBE) system using vapor–liquid–solid (VLS) mechanism with an Au-based nanocatalyst. The source materials for the MBE system were elemental Zn (6N), Se (6N), and Te (6N). Prior to the growth of nanotips, we cleaned a Si(1 0 0) substrate by standard Radio Corporation of America (RCA) process. We then thermally ox- idize the substrate to form a 150-nm-thick SiO 2 film followed by sputtering a 0.6-nm-thick Au film onto the oxidized surface. The SiO 2 film acts as a catalyst diffusion barrier. The sample was then loaded into the preparation chamber and annealed at 280 ◦ C for 10 min to form Au nanoparticles. The substrate was subsequently transferred to the growth chamber to grow the ternary ZnSe 0. 9 Te 0. 1 nanotips at 280 ◦ C for 1 h. During the growth, we controlled the beam equivalent pressures (BEPs) so as to keep the beam flux rates of Zn, Se, and Te at 2.2×10 −7 , 1.9×10 −6 , and 5.7×10 −7 torr, respectively. With careful cali- bration and precise control of growth parameters, we can thus grow high quality ZnSe 0. 9 Te 0. 1 nanotips on oxidized Si(1 0 0) substrate. Surface morphology of the sample was then char- acterized by a Hitachi S-4700I field emission SEM (FESEM) operated at 15 kV. A Philips FEI TECNAI G 2 high-resolution transmission electron microscopy (HRTEM) operated at 200 kV and a Siemens D5000 X-Ray diffractometer (XRD) system were used to evaluate crystallographic and structural properties of the as-grown ternary ZnSeTe nanotips. Photoluminescence (PL) properties of the nanotips were also characterized by a continuous-wave (CW) He–Cd laser operated at 325 nm as the 1536-125X/$26.00 © 2010 IEEE