UV-ENHANCED OXYGEN SENSING OF ZINC OXIDE NANOWIRES

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Violet)enhanced oxygen sensing using ZnO (zinc oxide) nanowires has been successfully demonstrated with improved sensitivity.

1 UVENHANCED OXYGEN SENSING OF ZINC OXIDE NANOWIRES Lei Luo, Brian D. Sosnowchik, and Liwei Lin Berkeley Sensor and Actuator Center, Department of Mechanical Engineering University of California at Berkeley 97 Cory Hall, Berkeley, California, USA ABSTRACT UV (Ultra Violet)enhanced oxygen sensing using ZnO (zinc oxide) nanowires has been successfully demonstrated with improved sensitivity. These singlecrystalline nanowires are grown by the VLS (vaporliquidsolid) process using inductive heating and selfassembled between two MEMS microstructures by the technique of local vapor deposition. Under 36nm UV illumination, the asfabricated device shows up to 00% improvement in oxygen sensitivity as compared with sensing data from the same device without using UV light. The transient responses of ZnO nanowires under UV and oxygen environment are also investigated and a photocurrent decay process is observed and characterized. As such, this UV enhancement sensing scheme could be applicable for nanowirebased gas and chemical sensors. 1. INTRODUCTION Surfaceadsorbed species can affect the underlying charge carrier concentration and change the conductance of semiconducting metal oxides. For example, as one of the IIVI compound metal oxides, ZnO thin films and nanomaterials, including nanowires, have been intensively studied for chemical sensing applications [1]. Particularly, the large surface areatovolume ratio and singlecrystalline nanowires offer great potential to improve the sensitivity and stability of gas sensors [67]. Previously, the oxygen sensing capabilities of ZnO nanowires have been reported based on nanowires assembled and attached to the substrate [9]. These devices are typically constructed via the pickandplace methods with electronbeam lithography. This architecture reduces the exposed nanowire surface area, thereby limiting the maximum sensitivity. On the other hand, UV illumination can also influence the carrier density so the conductance of ZnO nanowires [1011]. However, the effects of oxygen sensing under UV illumination have not been fully explored yet. In this work, a local synthesis [113] and selfassembly [1] process via inductive heating has been employed for fast and direct ZnO nanowire integration. Synthesized nanowires are suspended between two silicon microelectrodes fabricated by a silicononinsulator (SOI) process, to utilize the whole nanowire surface area. Furthermore, oxygen sensing using the asfabricated ZnO nanowire device with and without 36nm UV illumination under different oxygen pressure environments is conducted to investigate the oxygen adsorption and desorption processes and to study the sensing mechanism.. WORKING PRINCIPLES ZnO is naturally doped as ntype material due to the oxygen vacancies and extra zinc interstitial atoms in the lattice during the synthesis process [1], and the same characteristics have also been observed for ZnO nanowires [1]. Figure 1 shows the schematic diagram of a ZnO nanowire without and with UV illumination for oxygen sensing. Without UV, oxygen molecules are adsorbed on the surface and capture free electrons in ZnO nanowire as: O e O (1) ( g) + ( ad ) A lowconductivity depletion layer is formed resulting in band bending near the surface for both the conduction and valence bands. This is illustrated as dash lines in the energy band diagram in Fig.. The increase in energy barrier height, Φ 1, and reduction in overall conductance of the nanowire are the direct results. Higher oxygen concentration can further bend the bands and lower the conductance. Under UV illumination with photon energy higher than the bandgap, E g, of ZnO at 3.3eV (wavelength of less than 30 nm), electronhole pairs are generated to reduce the depletion layer so higher output current and conductance is observed. Holes migrate to the surface along the slope produced by the bending band and discharge oxygen ions as: h + + O O () ( ad ) ( g) Consequently, oxygen is photodesorbed from the surface reducing the bendbending effects (Φ,UV < Φ 1 ) as illustrated as solid lines in Fig.. However, it is well known that the conductance of ZnO nanowires under UV illumination will increase without the existence of oxygen. The interesting issues to be studied are the effects of oxygen sensing under UV. Oxygen hν ZnO nanowire Depletion layer h + + e Current Current Thinner depletion layer Figure 1: Schematic diagram of a ZnO nanowire without (top) and with (bottom) UV illumination for oxygen sensing. Without UV, oxygen adsorbed onto the nanowire surface forms a depletion layer. Under UV illumination, oxygen is photodesorbed to reduce the depletion layer thickness and increase the conductance of the nanowire.

E E F E C E V Depletion layer hγ>e g + Φ,UV < Figure : ZnO band diagram under UV illumination. The depletion layer and barrier height decrease with UV (Φ,UV < Φ 1 ) due to the discharge of oxygen ion.

With the onchip local vapor transport scheme, generated Zn vapor is confined by the backside opening of the growth chip, which is fabricated by a SOI process.

Briefly, the standard setup in our process include: growth chip with backside opening holes of various sizes; 0 30 Ågold catalyst layer deposited on the front surface; and 10 min processing time.

Data were collected using HP 1B semiconductor parameter analyzer and HP 301A digital multimeter. A 36nm UV lamp is used as its wavelength is shorter than the cutoff λ max for ZnO [16].

2 E E F E C E V Depletion layer hγ>e g + Φ,UV < Figure : ZnO band diagram under UV illumination. The depletion layer and barrier height decrease with UV (Φ,UV < Φ 1 ) due to the discharge of oxygen ion. 3. DEVICE FABRICATION AND TESTS The synthesis and assembly process have been described previously [1]. With the onchip local vapor transport scheme, generated Zn vapor is confined by the backside opening of the growth chip, which is fabricated by a SOI process. Nanowires are grown locally and selfassembled between two microbridges as shown in Fig. 3. Briefly, the standard setup in our process include: growth chip with backside opening holes of various sizes; 0 30 Ågold catalyst layer deposited on the front surface; and 10 min processing time. Φ 1 Microelectrodes ZnO nanowire along the <0001> direction. Experimental measurements were performed in a vacuum chamber with oxygen and nitrogen supplies. Data were collected using HP 1B semiconductor parameter analyzer and HP 301A digital multimeter. A 36nm UV lamp is used as its wavelength is shorter than the cutoff λ max for ZnO [16]. The penetration depth of UV light at this wavelength into ZnO was measured as ~0nm [17] and it is comparable to the diameter of synthesized ZnO nanowires. Therefore, good light absorption is expected during the UV experiments. Furthermore, during IV experiments under UV illumination, UV light has been applied to the device for at least 60 seconds before the IV responses are recorded to assure that the photochemical response has reached the steady state. (a) ZnO NWs (b) Microelectrodes Buried Oxide Backside substrate (c) Figure 3: Schematic diagram of the selfassembled ZnO nanowires on SOI microelectrodes fabricated via the rapid and local synthesis process by inductive heating. Figure (a) illustrates the SEM image of the fabricated device with nanowire length up to ten microns and average diameter of 0 60 nm. There are about 10 nanowires with measured resistance vales of ~ MΩ in nitrogen. Figure (b) shows the hexagonal crystal crosssectional shape of the nanowires grown in SiC with the same synthesis procedure. Further highresolution TEM (HRTEM) characterization in Fig. (c) shows the lattice plane distance of ~.6Å, indicating ZnO nanowire is highly crystalline, and grows Figure : (a) SEM image of the asfabricated ZnO nanowire device. (b) Hexagonal crystal crosssectional shape of the nanowires. (c) HRTEM image of the singlecrystalline structure, with lattice plane distance of ~.6Å.

3 . RESULTS AND DISCUSSIONS Figures (a) and (b) are IV measurements of a ZnO nanowire device at various oxygen pressures from ~10 3 to 10 Pa without and with UV illumination at room temperature, respectively (all the testing pressures are the absolute pressure values). The effects of oxygen and enhanced oxygen sensitivity are observed qualitatively. First, when the gas is changed from nitrogen to oxygen, current output or conductance is reduced as the result of oxygen adsorption. For instance, under 7V, the current of the device decreases from 1.7 µa in nitrogen to 1.7 µa in oxygen ( Pa) without UV illumination. Furthermore, increasing oxygen pressure causes further reduction in nanowire conductance without or with UV illumination. This is expected as the depletion layer thickness is further increased with more adsorption of oxygen molecules on nanowires. Finally, enhanced sensitivity to oxygen is observed with UV illumination. For example, about 0% reduction in current is observed when oxygen pressure is at Pa under UV illumination, compared with only 10% current reduction without UV (a) No UV 1 N Pa Pa Pa.7 10 Pa (b) UV Voltage (V) 1 N Pa Pa Pa.7 10 Pa 0 6 Voltage (V) Figure : IV measurements of the asfabricated device under Pa nitrogen and various oxygen pressures at room temperature (a) without, and (b) with UV illumination. Figure 6 quantitatively summarizes the enhanced oxygen sensitivity under UV illumination, presenting an improvement up to 00% in terms of sensitivity with oxygen pressure of Pa. The sensitivity is defined as: S 100( G (%) N G = O) () GN where G N and G O are the conductances of nanowires in N and, respectively. Here, we use the inert nitrogen gas as the baseline reference. We believe that the desorption process of oxygen under UV is enhanced to have this effect and further investigations are underway. Sensitivity (%) Oxygen pressure (Pa) Figure 6: Enhanced oxygen sensitivity under UV illumination UV No UV Pa.6 10 Pa Time (sec) Figure 7: Photocurrent vs. time characterizations at different oxygen pressures with V bi = 7V. Figure 7 shows the photocurrent vs. time plot at two different oxygen pressures, and the decay of current follows the relationship [119]: I( t) = I0 exp( t ) () τ d where I 0 is the photocurrent before turning off the UV light, and τ d is the decay time constant. One may identify two decay time constants. The initial fast drop, or short τ d1, is due to the recombination of electrons and holes, which have sufficient energy to overcome the surface barrier height. The secondstage, slower decay response, or long τ d, could come from slow reactions of surface oxygen readsorption and diffusion. When oxygen molecules are readsorbed onto the nanowire surface, carrier concentration reduces. Since available binding sites on the surface decrease, oxygen molecules need to diffuse further into nanowires for

4 available sites. From experimental results, the time constants τ d1 and τ d are estimated to be around 10 s and 100 s, and 6. s and 0 s at oxygen pressures of Pa and.6 10 Pa, respectively. The decay of photocurrent is faster under higher oxygen pressure, because more oxygen is available for readsorption and diffusion to capture electrons from nanowires. The performance of the sensor could be further influenced by a number of factors, including the existence of a thin native silicon dioxide layer at the interfacial contacts between the nanowire and silicon microelectrode; contact resistance; or material defects of nanowires. Future investigations, such as device annealing and contact formation, could be performed to further improve the sensitivity of the sensor.. CONCLUSION The rapid, localized synthesis and assembly process is used to construct ZnO nanowire device for the demonstration of UVenhanced oxygen sensing. These singlecrystalline nanowires are grown via the VLS process using inductive heating and selfassembled between two MEMS microstructures. IV characteristics of an asfabricated device show up to 00% improved oxygen sensitivity under 36nm UV illumination as compared with the same device without UV. The twostage decay of photocurrent with respect to time is also identified and analyzed. The firststate fast decay is associated with recombination electron and holes while the second slow decay is dominated by the readsorption and diffusion of oxygen into the ZnO nanowires. The enhanced oxygen sensitivity under UV illumination could open up a new class of photochemical sensors while the analyses and characterizations could be useful for other wide bandgap materials such as GaN. ACKNOWLEDGEMENTS The authors would like to acknowledge the UC Berkeley Microfabrication Laboratory in which fabrication of test specimen was performed; and Dr. Takashi Ikuno in the Physics Department for taking TEM images. REFERENCES [1] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A New Detector for Gaseous Components Using Semiconductive Thin Films, Anal. Chem., vol. 3, pp , 196. [] H. Nanto, T. Minami, and S. Takata, ZincOxide ThinFilm Ammonia Gas Sensors with High Sensitivity and Excellent Selectivity, J. Appl. Phys., vol. 60, pp., 196. [3] R. Martins, E. Fortunato, P. Nunes, I. Ferreira, A. Marques, M. Bender, N. Katsarakis, V. Cimalla, and G. Kiriakidis, ZnO Oxide as An Ozone Sensor, J. Appl. Phys., vol. 96, pp , 00. [] Q. Wan, Q. H. Li, Y. J. Chen, T. H. Wang, X. L. He, J. P. Li, and C. L. Lin, Fabrication and Ethanol Sensing Characteristics of ZnO Nanowire Gas Sensors, Appl. Phys. Lett., vol., pp , 00. [] X. Chu, D. Jiang, A.B. Djurišic, and H. L. 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