Selective MOCVD Growth of ZnO Nanotips

50 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 2, NO. 1, MARCH 2003 Selective MOCVD Growth of ZnO Nanotips Sriram Muthukumar, Haifeng Sheng, Jian Zhong, Student Member, IEEE, Zheng Zhang, Nuri William Emanetoglu, Associate Member, IEEE, and Yicheng Lu Abstract ZnO is a wide bandgap semiconductor with a direct bandgap of 3.32eV at room temperature. It is a candidate material for ultraviolet LED and laser. ZnO has an exciton binding energy of 60 mev, much higher than that of GaN. It is found to be significantly more radiation hard than Si, GaAs, and GaN, which is critical against wearing out during field emission. Furthermore, ZnO can also be made as transparent and highly conductive, or piezoelectric. ZnO nanotips can be grown at relatively low temperatures, giving ZnO a unique advantage over the other nanostructures of wide bandgap semiconductors, such as GaN and SiC. In the present work, we report the selective growth of ZnO nanotips on various substrates using metalorganic chemical vapor deposition. ZnO nanotips grown on various substrates are single crystalline, n-type conductive and show good optical properties. The average size of the base of the nanotips is 40 nm. The room temperature photoluminescence peak is very intense and sharp with a full-width-half-maximum of 120 mev. These nanotips have potential applications in field emission devices, near-field microscopy, and UV photonics. Index Terms Fabrication, nanotechnology, semiconductor growth. I. INTRODUCTION NANOTIPS are of strong interest for applications such as field emission and near-field microscopy. Nano- and microtips have been demonstrated in Si using anisotropic wet chemical etching [1]. A nanotip Al Ga As GaAs vertical cavity surface emitting laser (VCSEL), integrated with a photodetector, has also been demonstrated for near-field microscopy [2]. One of the major technologies competing for the flat screen market is the field emission display. This is similar to a conventional cathode-ray tube, except that electrons are emitted from the cathode that consists of thousands of conductive micro-tips, when a high electric field is formed between the tips and the anodes. A wide bandgap semiconductor with a lower work function presents a smaller barrier height for the electrons to overcome. In the traditional micro-tip field-emission devices, the wearing out of the tip due to radiation damage is a major reliability issue. Therefore, a wide bandgap semiconductor material would be preferred for field-emission. There have been reports on SiC [3], and GaN [4], [5] nanowires. However, such nanowires show random orientation and dimensions. For practical device applications, it is desired to have a highly oriented nanotip array that is built on a patterned area. Recently, there have been a few reports on Manuscript received August 23, 2002; revised November 17, 2002. This work was supported by the National Science Foundation (NSF) under Grant CCR- 0103096 and Grant ECS-0088549. The authors are with the School of Engineering, Rutgers University, Piscataway, NJ 08854 USA (e-mail: ylu@ece.rutgers.edu). Digital Object Identifier 10.1109/TNANO.2003.809120 the fabrication of self-assembled ZnO nanowires [6] [9]. ZnO is a wide bandgap semiconductor with a high excitonic binding energy (60 mev), and hence can facilitate low-threshold stimulated emission at room temperature. This low-threshold is further enhanced in low-dimensional compound semiconductors due to carrier confinement. ZnO is also found to be significantly more radiation hard than Si, GaAs, and GaN [10]. Nanowires of ZnO, Si, SiC, and GaN have been grown using various other methods such as vapor-phase transport process [9], chemical vapor deposition [3], direct gas reaction [4] etc.. In these methods, the growth temperatures were in the range of 900 C and above. In contrast to these growth techniques, ZnO nanotips can be grown using metalorganic chemical vapor deposition (MOCVD) at relatively low temperatures [6] [8]. MOCVD growth technology also offers large area uniformity and ease of integration with mainstream semiconductor processes. In this work, we report the selective growth of ZnO nanotips on various substrates using MOCVD. ZnO nanotips are self-assembled and have uniform size and orientation. electron microscopy, X-ray diffraction, photoluminescence, and spectrophotometer techniques were used to characterize the structure, morphology and optical properties of the ZnO nanotips. II. EXPERIMENTAL ZnO nanotip growth was carried out in a vertical flow MOCVD reactor. Diethylzinc (DEZn) and oxygen were used as the Zn metalorganic source and oxidizer, respectively. Film deposition was carried out at a substrate temperature in the range of 300 C 500 C. The reactor design is described elsewhere [11]. X-ray diffraction measurements were carried out using a Bruker D8 Discover diffractometer using Cu K with an angular resolution of 0.005. Leo-Zeiss field emission scanning electron microscope (FESEM) was used to characterize the morphology of the films and a Topcon 002B transmission electron microscope was used to do detailed structural characterizations. The room temperature photoluminescence (PL) spectrum was conducted using a 325 nm CW He Cd laser as the excitation source. The wavelength resolution is 0.5 nm. III. RESULTS AND DISCUSSIONS Fig. 1 is a FESEM image of ZnO nanotips grown on various substrates, including: (a) c-plane Al O, (b) epitaxial GaN film grown on c-al O, (c) fused silica, and (d) thermally grown SiO Si. The growth conditions were the same for all the substrates. ZnO has a wurtzite structure with a close lattice match to GaN. ZnO also satisfies the epitaxial relationship 1536-125X/03$17.00 2003 IEEE

MUTHUKUMAR et al.: SELECTIVE MOCVD GROWTH OF ZnO NANOTIPS 51 Fig. 1. FESEM images of columnar growth of ZnO on (a) c 0 Al O, (b) epi GaN, (c) fused silica, and (d) SiO =Si substrates. Fig. 2. Field Emission Scanning Electron Microscope image of ZnO nanotips on (100) Si. The inset shows the planar view of the surface of the ZnO nanotips. TABLE I CRYSTAL STRUCTURE AND LATTICE PARAMETERS OF ZnO, GaN, AND Al O with sapphire. Table I lists the crystal structure parameters of ZnO, GaN and Al O. The epitaxial relationship between ZnO and c-sapphire is ZnO Al O and ZnO Al O, while the epitaxial relationship between ZnO and a-sapphire is ZnO Al O and ZnO Al O. Therefore, ZnO on these substrates grows with the c-axis perpendicular to the plane. Very dense and smooth epitaxial films of ZnO have been grown on various orientations of sapphire and GaN. Under certain growth conditions columnar growth can be obtained on these substrates. Alternatively, when ZnO grows on fused silica or on amorphous SiO thermally grown on Si, it forms the columnar structure. ZnO nanotips growth is also observed on Si as shown in Fig. 2. In the case of columnar growth on various substrates ZnO nanotips are all preferably oriented along the c-axis and have a base diameter of 40 nm and terminate with a very sharp nanoscale tip. The crystalline orientation of the ZnO nanotips was determined using XRD measurements as shown in Fig. 3. Shown in Fig. 4(a) is a dark field transmission electron microscopy (TEM) image of a single ZnO nanotip and in Fig. 4(b) is an electron diffraction image obtained from the single ZnO Nanotip aligned to the zone axis. Defects in single crystal materials are better characterized in dark field Fig. 3. XRD analysis of ZnO nanotips grown on Silicon substrate. The preferred orientation of the nanotips is along the c-axis. imaging mode. The dark field TEM image of a single ZnO nanotip shows very few defects. The indexed diffraction pattern further confirms the single crystal quality of the ZnO nanotips. The columnar growth is a result of a high growth rate along the c-axis of ZnO. ZnO is a polar semiconductor, with (0001) planes being Zn-terminated and being O-terminated. These two crystallographic planes have opposite polarity, hence have different surface relaxation energies, resulting in a high growth rate along the c-axis. Therefore, by controlling the ZnO growth parameters, ZnO nanotips with c-axis perpendicular to the substrate and with a high aspect ratio can be grown on these substrates. The inset of Fig. 5 shows the optical transmission spectrum of ZnO nanotips grown on fused silica substrate measured at room temperature by an UV-Visible spectrophotometer. The transmission spectrum indicates that the cutoff wavelength of ZnO nanotips is around 370nm material. It can also be seen that the transmission over 82% is achieved in the transparency region with a sharp absorption edge. The fringes in the transparency

52 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 2, NO. 1, MARCH 2003 Fig. 4. (a) Dark field TEM image of a single ZnO nanotip. The arrow points to a single defect in the column; and (b) electron diffraction image obtained from the single ZnO Nanotip aligned along the [2 110] zone axis. The spots marked by x are those due to forbidden reflections. Fig. 5. Room temperature photoluminescence spectra of ZnO nanotips on amorphous SiO deposited on r-sapphire. Inset shows a transmission spectra of ZnO nanotips grown on fused silica substrate measured at room temperature. region of the spectrum are mainly due to the interference effect. Fig. 5 also shows the room temperature PL spectrum of ZnO nanotips grown on SiO /r-sapphire. The amorphous SiO layer was deposited on the r-sapphire substrate using plasma-enhanced chemical vapor deposition (PECVD) and is amorphous. A strong PL peak is observed at 3.32eV (373.5nm), whose intensity is ten times stronger than those obtained on ZnO epilayers. This peak results from the free-exciton recombination that is prominent in ZnO nanotips. The full-width-half-maximum (FWHM) of PL is measured to be 120 mev ( 13nm) for the ZnO nanotips grown over an amorphous SiO layer on r-sapphire. The intense and sharp intrinsic PL emission peak confirms the good optical property of the ZnO nanotips. It also complements the structural analysis from the TEM measurement that the ZnO nanotips are of single crystal quality. A weaker Fig. 6. FESEM image of ZnO film on r-al O grown using MOCVD. The surface morphology is flat. Inset is a TEM image showing the epitaxial relationship between ZnO and r-al O. emission peak around 2.8 ev is also observed. Several groups have reported the observation of deep level induced green-band emission in ZnO epilayers grown on sapphire [12]. Similar observations have been reported in ZnO nanostructures on c-sapphire substrates [13]. The detailed mechanism for this emission is currently under investigation. In contrast to the columnar growth, the ZnO film grown on r-al O under the same growth conditions results in a flat film with a smooth morphology. Fig. 6 is a FESEM image of a ZnO film grown on r-al O. The ZnO film shows a flat surface with the epitaxial relationship ZnO Al O, and ZnO Al O [13]. Hence, the c-axis of ZnO lies in the growth plane. This is different from the ZnO films grown on c-sapphire and a-sapphire substrates. The FWHM -rocking curve was measured to be 0.25 for the ZnO film grown on r-al O using MOCVD. The significant difference in the growth of ZnO film on r-sapphire substrates and silicon or SiO has been used to obtain selective growth of ZnO nanotips on patterned silicon-on-sapphire (SOS) substrates. The patterning of the SOS substrates was realized by first depositing a thin SiO film on the SOS substrate using low-pressure chemical vapor deposition (LPCVD), which serves as a mask for etching the silicon film. Then, a KOH solution and buffered oxide etchant (BOE) were used to selectively etch silicon and SiO, respectively. Fig. 7 shows a ZnO grown on patterned SOS substrate. The ZnO nanotips are only observed on the exposed silicon top (100) surface and the sidewall (111) surface as KOH anisotropically etches (100) Si producing sidewalls oriented along the direction. The growth of ZnO nanotips on the sidewalls of the silicon islands can be avoided by using dry etching methods, such as inductively coupled plasma (ICP) or reactive ion etching (RIE) that give a vertical etching profile. Similar selective growth was also obtained for patterned amorphous SiO deposited on r-sapphire substrates. The as-grown ZnO nanotips using MOCVD show n-type conductivity. The resistivity of the ZnO epilayer grown on the sapphire of the SOS substrate was measured using four-point probe method.

MUTHUKUMAR et al.: SELECTIVE MOCVD GROWTH OF ZnO NANOTIPS 53 Fig. 7. FESEM image of selective growth of ZnO nanotips grown on silicon-on-sapphire (SOS) substrate. A resistivity of 3.4 cm was obtained for the ZnO epilayer. The carrier concentration was evaluated to be cm correspondingly. [7] S. Muthukumar, N. W. Emanetoglu, J. Zhong, S. Feng, and Y. Lu, ZnO nanoscale materials: technology and applications, in Proc. 15th Annu. Symp. Laboratory for Surface Modification. New Brunswick, NJ, Mar. 9, 2001. [8] Y. Lu, S. Muthukumar, and N. W. Emanetoglu, Feasibility studies on ZnO nanostructures and their device applications, in Picture/Poster Presentation, NSF Nanoscale Science and Engineering Forum. Washington, DC, Sept. 13, 2001. [9] J. C. Johnson, H. Yan, R. D. Schaller, L. H. Haber, R. J. Saykally, and P. Yang, Single nanowire lasers, J. Phys. Chem., vol. B, no. 46, Nov. 2001. [10] D. C. Look, D. C. Reynolds, J. W. Hemsky, R. L. Jones, and J. R. Sizelove, Production and annealing of electron irradiation damage in ZnO, Appl. Phys. Lett., vol. 75, no. 6, p. 811, Aug. 1999. [11] C. R. Gorla, N. W. Emanetoglu, S. Liang, W. E. Mayo, M. Wraback, H. Shen, and Y. Lu, Structural, optical, and surface acoustic wave properties of epitaxial ZnO films grown on (01-12) sapphire by metalorganic chemical vapor deposition, J. Appl. Phys., vol. 85, no. 5, p. 2595, Mar. 1999. [12] D. C. Reynolds, D. C. Look, B. Jogai, and H. Morkoc, Similarities in the bandedge and deep-center photoluminescence mechanisms of ZnO and GaN, Solid State Comm., vol. 101, no. 9, p. 643, 1997. [13] W. I. Park, D. H. Kim, S. W. Jung, and G. Yi, Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nanorods, Appl. Phys. Lett., vol. 80, no. 22, p. 4232, June 2002. IV. CONCLUSION Self-assembled ZnO nanotips have been grown on various substrates using MOCVD. The nanotips are of uniform size and orientation. These ZnO nanotips are of single crystal quality, show n-type conductivity and have good optical properties. Selective growth of ZnO nanotips has been realized on patterned (100) silicon on r-sapphire (SOS), and amorphous SiO on r-sapphire substrates. Such self-assembled ZnO nanotips are promising for applications in field emission devices, near-field microscopy, and UV optoelectronics. Sriram Muthukumar received the B.Tech degree in Metallurgical Engineering from Indian Institute of Technology, Chennai, India, in 1997 and the M.S. degree in ceramics and materials engineering from Rutgers University, New Brunswick, NJ, where he is currently working toward the Ph.D. degree with Prof. Y. Lu. His research interests include structural, optical and electrical characterization of MOCVD grown ZnO and MgxZnl x0 thin films and growth and fabrication of ZnO based nanoscale structures and exploring the feasibility of these nanostructures toward novel device applications. ACKNOWLEDGMENT The authors thank Dr. H. M. Ng of Lucent Technologies for assistance in the PL measurements and Prof. F. Cosandey of Rutgers University for assistance in TEM measurements. REFERENCES [1] V. V. Poborchii, T. Tada, and T. Kanayama, Optical properties of arrays of Si nanopillars on the (100) surface of crystalline Si, Physica E, vol. 7, p. 545, 2000. [2] S. Khalfallah, C. Gorecki, J. Podlecku, M. Nishioka, H. Kawakatsu, and Y. Arakawa, Wet-etching fabrication of multilayer GaAlAs/GaAs microtips for scanning near-field microscopy, in Appl. Phys. A. Materials Science and Processing: Springer-Verlag, June 2000. [3] K. W. Wong, X. T. Zhou, F. C. K. Au, H. L. Lai, C. S. Lee, and S. T. Lee, Field-emission characteristics of SiC nanowires prepared by chemical-vapor deposition, Appl. Phys. Lett., vol. 75, no. 19, p. 2918, Nov. 8, 1999. [4] J. Y. Li, X. L. Chen, Z. Y. Qiao, Y. G. Cao, M. He, and T. Xu, Synthesis of aligned gallium nitride nanowire quasiarrays, in Appl. Phys. A. Materials Science and Processing: Springer-Verlag, Aug. 2000. [5] G. S. Cheng, L. D. Zhang, Y. Zhu, G. T. Fei, L. Li, C. M. Mo, and Y. Q. Mao, Large-scale synthesis of single crystalline gallium nitride nanowires, Appl. Phys. Lett., vol. 75, no. 16, p. 2455, Oct. 1999. [6] S. Muthukumar, C. R. Gorla, N. W. Emanetoglu, S. Liang, and Y. Lu, Control of morphology and orientation of ZnO thin films grown on SiO =Si substrates, J. Crys. Growth, vol. 225, p. 197, May 2001. Haifeng Sheng received the B.S. and M.S. degrees in electrical engineering from Tsinghua University, Beijing, China, in 1996 and 1999, respectively. He is currently working toward the Ph.D. degree in the Department of Electrical and Computer Engineering at Rutgers University, New Brunswick, NJ. His research interests are Schottky and ohmic contacts to ZnO, and ZnO based photodetectors and devices. Mr. Sheng received the Outstanding Student Paper Award at the 2001 U.S. Workshop on the Physics and Chemistry of II-VI Materials. Jian Zhong (S 00) received the B.S. and M.S. degrees in electronic engineering from Tsinghua University, Tsinghua, China, in 1992 and 1995, respectively. Currently, she is working toward the Ph.D. degree at Rutgers University, New Brunswick, NJ. Her research is focused on fabrication and characterization of ZnO based UV optical devices and sensors.

54 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 2, NO. 1, MARCH 2003 Zheng Zhang received the B.S. degree in information science and electronic engineering from Zhejiang University, Hangzhou, China. She is currently pursuing the Ph.D. degree in the Department of Electrical and Computer Engineering at Rutgers University, New Brunswick, NJ. Her research interests are ZnO-based SAW devices and nanostructured devices for sensors. Nuri William Emanetoglu (S 97 A O2) was born in Istanbul, Turkey, in 1973. He received the B.S. degree in electronics and communications engineering from Istanbul Technical University, Istanbul, Turkey, in 1995 and the M.S. degree in electrical and computer engineering from Rutgers University, New Brunswick, NJ, in 1998, where he is currently working toward the Ph.D. degree. He has published eleven journal articles and eleven conference proceedings. His research interests include modeling, fabrication and characterization of solid state devices based on acoustic, optical and electronic interaction; and their applications to communications systems and sensor technologies. Mr. Emanetoglu received the 2nd Best Student Award at the ACCG/East-97 Conference of the American Association for Crystal Growth, and Best Student Paper Award in Surface Acoustic Waves, at the IEEE 2001 International Ultrasonics Symposium, Atlanta, GA, in October 2001. Yicheng Lu received the B.S. degree in applied physics from Jiao Tong University, Shanghai, China in 1982 and the Ph.D. in electrical engineering from the University of Colorado, Boulder, in 1988. In 1988, he joined the faculty of Rutgers University, New Brunswick, NJ, where he is currently a Professor in the Department of Electrical and Computer Engineering, and a Graduate Faculty Member in the Department of Ceramics and Materials Engineering. His early research was involved metal semiconductor contacts, and rapid thermal processing for electronic materials. Since he joined Rutgers University, his research includes vacuum microelectronics, piezoelectric thin films and devices, wide bandgap semiconductors (ZnO and GaN), and integrated RF passive devices. His recent research has been focused on ZnO based materials, nanostructures, and multifunctional devices. He has published over 130 refereed articles, 160 conference presentations and invited talks, and five U.S. patents. Dr. Lu received the 1993 Warren I. Susman Award for Excellence in Teaching, which is the highest teaching award at Rutgers, the Rutgers University Board of Trustees Research Fellowship Award for Scholarly Excellence in 1994, the IEEE Outstanding Student Counselor and Advisor Award in 1995, and the Rutgers University Scholar-Teacher Award in 2002.