Microelectronic Engineering 162 (2016) 79–81
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Photoresponse characteristics of silicon carbide nanowires Kasif Teker ⁎ College of Engineering, Istanbul Sehir University, Istanbul, Turkey Mechanical Engineering, Frostburg State University, 101 Braddock Road, Frostburg, MD, USA
a r t i c l e
i n f o
Article history: Received 2 April 2016 Accepted 8 May 2016 Available online 14 May 2016 Keywords: SiC nanowires Photosensitivity UV-photoconductors Nanoscale UV-light sensors
a b s t r a c t This paper presents photoresponse characteristics of a CVD-grown single SiC nanowire. The SiCNW device exhibited signiﬁcant positive and fast photocurrent response to UV light exposure. The SiCNW device did not exhibit any persistent photocurrent after the illumination ended, and showed great reversibility and recovery in photoconductance. This suggests an enhanced surface recombination of photoexcited electron-hole pairs due to the complete depletion of the space charge layer of the SiC nanowire. Therefore, the SiCNW devices show great potential for very sensitive nanoscale UV light sensors and photosensing elements in optoelectronic devices. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting nanowires continue to attract great interest due to their unique physical, electrical, and optical properties for applications in optoelectronics and nanoelectronics. Silicon carbide (SiC), a wide bandgap semiconductor, has extensively been investigated  due to its excellent properties such as high breakdown electric ﬁeld, high electron drift velocity, high chemical stability at harsh environments, physical stability at high temperatures, and high thermal conductivity. Due to these excellent properties along with the advancements in device fabrication, SiC devices have emerged as great candidates in smart grid and electric vehicle applications for efﬁcient and reliable energy conversion and grid integration. Furthermore, SiC nanowires (SiCNWs) combine the unique properties of 1D materials with that of intrinsic SiC characteristics, and thereby offer great advantages in many engineering applications. For instance, SiCNWs show enormous application potentials in nanosensors, ﬁeld emission displays, nanoscale electronic devices, and optoelectronic devices [2,3]. Nanowires are very pertinent for various photosensing elements in many highly-integrated optoelectronic devices due to their high photosensitivity with respect to bulk materials. These superior characteristics are related to the existence of very high density of surface states due to large surface-to-volume ratio in nanowires. Thus, many nanostructures including GaN nanowires [4–7], ZnO nanowires , AlN nanowires , and WO3 nanowires  have been studied to manufacture high performance nanoscale photosensitive optoelectronic devices. Nevertheless, more research and improvements in fabrication and device design are ⁎ College of Engineering, Istanbul Sehir University, Istanbul, Turkey E-mail addresses: [email protected]
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http://dx.doi.org/10.1016/j.mee.2016.05.002 0167-9317/© 2015 Elsevier B.V. All rights reserved.
required for the realization of commercial nanoscale photosensitive devices. This paper investigates photoresponse characteristics of a single SiC nanowire to UV light, for the ﬁrst time to the author's knowledge. Photosensitivity investigation of the SiC nanowire has been conducted through a UV light source of 254 nm wavelength. The utilized device fabrication scheme is very compatible towards the scale-up manufacturing opportunities. Moreover, the mechanism comparisons about the photoresponse characteristics of various nanostructured materials is provided. Furthermore, an explanation about the fast photocurrent response is presented.
2. Experimental details SiC nanowires were grown using hexamethyldisilane as the source material with iron and Ni catalyst materials on SiO2/Si substrates in a hot-wall 25-mm horizontal LPCVD reactor. The growth runs have been carried out at temperatures between 1050 and 1100 °C under H2 as carrier gas. To investigate the photoresponse characteristics of the SiC nanowires, SiC nanowire based devices have been fabricated through the electrodes (10 nm Ti/90 nm Au) with a spacing about 3 μm onto SiO2/Si substrate. The photosensitivity measurements were conducted under room temperature conditions with UV illumination (Spectroline handheld E-series, 254 nm) and semiconductor parameter analyzer (Keithley 4200 SCS) attached to a probe station. The synthesized nanowires and fabricated devices have also been analyzed by scanning electron microscopy (SEM, JEOL JSM 6060 and JEOL 7600F SEM with Oxford Inca EDS), X-ray diffraction (XRD, Rigaku 300), and transmission electron microscopy (TEM, JEOL JEM 1011).
K. Teker / Microelectronic Engineering 162 (2016) 79–81
Fig. 1. SEM images of high density of SiC nanowires grown on SiO2/Si substrate with different catalysts: (a) Fe ﬁlm, (b) Ni nanoparticles.
3. Results and discussions Fig. 1 exhibits SEM images of dense SiC nanowires synthesized with Fe-ﬁlm and Ni-nanoparticles on SiO2/Si substrates. Nanowires have high aspect ratios and are relatively long with typical lengths of several tens of microns. The SiC nanowires are single crystals and have cubic zinc blende crystal structure, as determined by XRD and TEM. This practical and very efﬁcient synthesis method offers great opportunities for scale-up manufacturing. The growth process of nanowires occurs through vapor-liquid-solid (VLS) mechanism. The VLS mechanism is based on a catalytic liquid alloy, which can adsorb source vapor to supersaturation levels, and thereby nanowire growth takes place from these nucleated seeds at the liquid-solid interface. Next, SiC nanowire device has been fabricated through dielectrophoresis for integrating nanowires onto pre-patterned electrodes. The highly anisotropic shape of nanowires and large spontaneous polarization lead to alignment of these nanowires into the electrodes. First, nanowires were suspended in isopropyl alcohol. A 2 μL of nanowire solution is applied to the surface of the pre-fabricated substrate. Then, AC peak-to peak voltage of 5 V with a frequency of 1 kHz (Vpp = 5 V, f = 1 kHz) was applied to the electrodes via a function generator (HP33120A). The voltage was kept until the substrate was dried (~40–60 s). Then, the nanowire devices were annealed at 300 °C under argon gas about 5 min to improve the contact quality between
nanowires and the gold electrodes. Further, electrical measurements have been conducted through semiconductor parameter analyzer. Fig. 2a exhibits an SEM image of the SiC nanowire connecting the electrodes (the diameter of the nanowire is 38 nm). Fig. 2b shows the IV curve of SiCNW device, which indicates that SiC nanowire maintains its electrical properties through the integration process. The IV curve also indicates that the argon annealing process is very successful for ensuring good contact between nanowire and the metal electrodes. The photocurrent-time response of the device has been investigated under the 254 nm UV light at 2 V bias (Fig. 3). The UV light was on for about 30 s for each illumination cycle. The SiCNW device showed notable positive photocurrent response to the UV light exposure. This indicates that the UV exposure has been very successful at forming a large number of photogenerated carriers. Moreover, the photoresponse is fast such that the rise and decay times are within 3 s and 5 s, respectively. After the light exposure has ended, the photocurrent has decayed to the dark current value rapidly. The SiCNW device did not exhibit any persistent photocurrent and showed great reversibility and recovery in photoconductance. In fact, a good number of previous studies reported long and persistent photoconductivity on various nanostructured materials [4,5,7,8,10,11], which hindered the quick recovery of these nanodevices. Some earlier proposed claims for the persistent photoconductivity in nanodevices can be stated as: i) intrinsic structural defects and point defects and complexes, and ii) electron-hole separation near
Fig. 2. (a) SEM image of the integrated SiC nanowire between electrodes; (b) IV curve of the SiC nanowire device showing very good contact.
K. Teker / Microelectronic Engineering 162 (2016) 79–81
and fast photocurrent response to the 254 nm-UV light exposure. The SiCNW device did not exhibit any persistent photocurrent after the exposure ended, and showed great reversibility and recovery in photoconductance. This suggests an enhanced surface recombination of photoexcited electron-hole pairs due to the complete depletion of the space charge layer for the SiC nanowire. Therefore, the SiCNW devices show great potential for very sensitive nanoscale UV light sensors and nanophotonic devices.
Fig. 3. Photocurrent-time response of the SiC nanowire device under the 254 nm UV light (8 W) at 2 V bias (two 30-s exposures).
The author gratefully thanks to the Marie Curie FP7 Integration Grant within the 7th European Union Framework Programme (334256) and the U.S. Appalachian Regional Commission (ARC) (MD15854) for providing support for this research.
References the surface. The surface built-in potential causes separation of photoexcited electrons and holes, which accumulate at the surface. When the exposure ends, the charge separation makes the photoexcited electron-hole recombination difﬁcult. Electrons have to overcome the conduction band barrier at the surface to realize the surface recombination process. Therefore, a delayed recombination process emerges leading to persistent photocurrent event. In fact, in an earlier study on GaN nanowires , the photocurrent rise time was about 2000 s and the decay time was about a day. In contrast to the earlier reports about persistent photoconductivity observed at various nanostructured materials, the SiC nanowire device showed a quick decay after the UV light turned off. This suggests a very small barrier for surface electron-hole recombination, which is probably due to the complete depletion of the space charge layer, i.e., enhanced surface recombination of photoexcited electron-hole pairs. Owing to the fast photoresponse characteristics, the SiCNW devices are great candidates for very sensitive nanoscale UV light sensors and nanophotonic devices. Additionally, these results suggest that the SiC nanowires have very high crystal quality and the SiCNW-electrode contact is very good. 4. Conclusions Photoresponse characteristics of the CVD-grown single SiC nanowire has been investigated. The SiCNW device exhibited signiﬁcant positive
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