A Quaternary ZnCdSeTe Nanotip Photodetector
The authors report the growth of needle-like high density quaternary Zn0.87Cd0.13Se0.98Te0.02nanotips on oxidized Si(100) substrate. It was found that average length and average diameter of the nanotips were 1.3 μm and 91 nm, respectively. It was also found that the as-grown ZnCdSeTe nanotips exhibit mixture of cubic zinc-blende and hexagonal wurtzite structures. Furthermore, it was found that the operation speeds of the fabricated ZnCdSeTe nanotip photodetector were fast with turn-on and turn-off time constants both less than 2 s.
KeywordsZnCdSeTe Nanotips MBE Photodetector
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 temperature. Two-dimensional (2-D) ZnSe-based epitaxial layers can also be grown directly on the closely lattice matched GaAs substrate. Other than the binary ZnSe, it is possible to grow strained and/or lattice matched ternary/quaternary epitaxial layers so as to achieve ZnSe-based heterostructures. Using these heterostructures, ZnSe-based light emitting diodes , laser diodes , and photodetectors  have all been demonstrated. Other than 2-D epitaxial films, one-dimensional (1-D) semiconductor nanostructures such as nanowires, nanorods, nanotips, and nanotubes have attracted great attention in recent years. With a significantly larger surface-to-volume ratio, 1-D semiconductor nanostructures exhibit unique optical and electronic properties that make them desirable for use in various novel devices. For example, it has been shown that 1-D ZnO nanowire photodetector exhibit extremely high photoconductive gain . Very recently, we also reported the fabrication of a 1-D ZnSe nanowire photodetector .
Similar to devices based on 2-D epitaxial films, heterostructure also plays an important role in 1-D nanostructured devices. For example, it is possible to couple the longitudinal confinement with radial confinement in heterostructure nanowires. This could provide more functionalities for heterostructure nanowires, as compared to homogeneous ones . To realize ZnSe-based heterostructure nanowires, it is necessary to form ZnSe-based ternary/quaternary nanowires. The growth of ternary ZnCdSe nanowires has already been demonstrated by Colli et al. . Compared to ternary ZnCdSe, the quaternary ZnCdSeTe provides an extra degree of freedom to control the bandgap energy and lattice constant. By controlling the composition ratio among Zn, Cd, Se, and Te carefully, we should be able to grow lattice matched ZnCdSeTe on ZnSe with adjustable bandgap energy . Furthermore, it has been reported that binding energy of Te bound excitons in related II–VI compound semiconductors is very large. Extremely strong room temperature photoluminescence (PL) and electroluminescence (EL) signals were also observed from localized excitons bound to Te atom (Te1 emission) and Ten (n ≥ 2) cluster (Ten cluster emission) in 2-D ZnSeTe films [9, 10, 11]. It has also been shown that the extrinsic self trapping of excitons (STE) formed in Te-related emission in ZnSeTe films could provide a much higher luminescence efficiency, as compared to free excitons [11, 12, 13, 14, 15, 16]. These observations suggest that Te-containing II–VI nanowires are potentially useful for various applications. If we can grow 1-D ZnCdSeTe nanowires successfully, we should also be able to achieve ZnSe-based heterostructure nanowires with improved optical properties. This will be important in realizing ZnSe-based nanodevices. In this work, we report the growth of quaternary Zn0.87Cd0.13Se0.98Te0.02 nanotips on oxidized Si(100) substrate. A ZnCdSeTe nanotip photodetector was also fabricated. Structural, physical, electrical, and optical properties of ZnCdSeTe and the fabricated photodetector will also be discussed.
The quaternary ZnCdSeTe nanotips used in this study were grown by a Riber 32P solid source molecular beam epitaxy (MBE) system using vapor–liquid–solid (VLS) mechanism with Au nanoparticles as the catalyst . The source materials for the MBE system were elemental Zn(6 N), Cd(6 N), Se(6 N), and Te(6 N). Prior to the growth of quaternary ZnCdSeTe 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 substrate was then rinsed in deionized water and dried with nitrogen flow. The substrate was then thermally oxidized to form a 150-nm-thick SiO2 film. A 0.6-nm-thick Au film was subsequently sputtered onto the oxidized substrate. The substrate was then loaded onto the preparation chamber and annealed at 280 °C for 10 min to form Au nanoparticles . Subsequently, the substrate was transferred into the growth chamber to grow the quaternary ZnCdSeTe nanotips at 280 °C for 1 h. During the growth, the beam equivalent pressures of Zn, Cd, Se, and Te were kept at 1.7 × 10− 7, 5.1 × 10− 8, 1.5 × 10− 6, and 2.5 × 10− 7 Torr, respectively. With careful calibration and precise control of growth parameters, we can thus control the composition ratio of our nanotips at Zn0.87Cd0.13Se0.98Te0.02.
Surface morphology of the sample was then characterized by a Hitachi S-4700I field-emission scanning electron microscope (FESEM) operated at 15 kV. A Philips FEI TECNAI G2high 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 quaternary ZnCdSeTe nanotips. The micro-Raman measurements were also performed using a 532 nm laser excitation. Photoluminescence (PL) properties of the nanotips were then measured at 20 K. The excitation source of the PL measured was a chopped continuous wave (CW) He–Cd laser operated at 325 nm. The luminescence signal emitted from the sample was then recorded by a lock-in amplifier.
Results and Discussion
Crystal structure and lattice constants of group-II–VI semiconductors
In summary, we report the growth of high density quaternary Zn0.87Cd0.13Se0.98Te0.02nanotips on oxidized Si(100) substrate using VLS mechanism by MBE with Au nanocatalyst. It was found that average length and average diameter of the nanotips were 1.3 μm and 91 nm, respectively. It was also found that the as-grown ZnCdSeTe nanotips exhibit mixture of cubic zinc-blende and hexagonal wurtzite structures. Furthermore, it was found that the operation speeds of the fabricated ZnCdSeTe nanotip photodetector were fast with turn-on and turn-off time constants both less than 2 s.
This work was supported in part by the Taiwan Semiconductor Manufacturing Company, Ltd. (TSMC), in part by the Center for Frontier Materials and Micro/Nano Science and Technology, National Cheng Kung University (NCKU), Taiwan (D97-2700), and in part by the Advanced Optoelectronic Technology Center, NCKU, under projects from the Ministry of Education. This work was also in part supported by Ministry of Economic Affairs (MOEA) and NSC 98-EC-17-A-09020769. The authors would also like National Taiwan University of Science and Technology for the assistance in TEM measurements.
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