Persistent Photoconductivity Studies in Nanostructured ZnO UV Sensors
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The phenomenon of persistent photoconductivity is elusive and has not been addressed to an extent to attract attention both in micro and nanoscale devices due to unavailability of clear material systems and device configurations capable of providing comprehensive information. In this work, we have employed a nanostructured (nanowire diameter 30–65 nm and 5 μm in length) ZnO-based metal–semiconductor–metal photoconductor device in order to study the origin of persistent photoconductivity. The current–voltage measurements were carried with and without UV illumination under different oxygen levels. The photoresponse measurements indicated a persistent conductivity trend for depleted oxygen conditions. The persistent conductivity phenomenon is explained on the theoretical model that proposes the change of a neutral anion vacancy to a charged state.
KeywordsPersistent photoconductivity Semiconducting II–VI materials Zinc oxide UV sensor Nanoscale device
The synthesis methods and the use of nanostructures for various applications have been a very lucrative topic in the last decade . These efforts have lead to discoveries of unknown phenomena and/or new approaches to explain with precision the observed experimental and theoretical facts from the macro/micro world . When all is said and done, the issues in nano-sized devices (individual or arrays) and basic impediments in device operation have not been addressed largely due to not having a perception of end-user requirements, leaving the device’s operational bottlenecks unaddressed . This is true for two well-researched opto-electronic materials GaN-  and ZnO-based  devices like light-emitting diodes and photodetectors. In the case of GaN, more emphasis was given to high crystal quality growth, epitaxy, and understanding the Mg–H complex in determining the p-doping that eventually lead a lone scientist, S. Nakamura at Nichia Chemical Industries, Japan, to invent the first working solid state blue laser. In case of ZnO, the large part of the investment from university and industry arenas is still devoted to realizing the p-type doping along with some initial success from M. Kawasaki’s group at Tohoku University, Japan that recently demonstrated the first ZnO p–n homojunction light-emitting diode .
ZnO is emerging as a potential candidate due to its direct wide bandgap and its ability to tailor electronic, magnetic, and optical properties through doping and alloying. One significant property that has brought ZnO and its alloys with Mg to the forefront of a flurry of research activity is the large exciton binding energy (60 meV when compared to 25 meV for GaN) for use in UV lasers. ZnO has been widely reported as a visible-blind UV sensor  over a wide range of applications in military and non-military arenas  that includes missile plume detection for hostile missile tracking, flame sensors, UV source monitoring, and calibration. However, recent research in nanostructures of ZnO has proved that the reduced dimensions have the potential to provide more untapped properties if harnessed in a systematic manner. Many simple fabrication techniques , devices [10, 11], and applications  have been demonstrated and reproduced. ZnO nanoscale structures such as one-dimensional nanowires are attracting more attention because of their enormous potential as fundamental building blocks for nanoscale electronic  and photonic devices due to the enhanced sensitivity offered by quantum confinement effects . In this work, we address the prominent defect-related property (could be sum or individual defects due to non-crystallinity, surface charge imbalance, or substrate to film interface strains) that affects the electrical properties of the ensuing device. The phenomenon of persistent photoconductivity (PPC) is a situation in which a photo-induced current in the device continues to flow even after the exciting photon source is turned off. PPC is a major issue in device operation that became a topic of intense research interest during development of GaN [15, 16] and AlGaN  photodetectors. The motivation of the present work is to understand the origin of PPC in ZnO by employing a simple device configuration consisting of a metal–semiconductor–metal structure. PPC is very difficult to observe in bulk materials and needs to be measured at very low temperature, which in turn complicates the carrier transport mechanisms, thus limiting the ability to extract and interpret the exact cause of the problem . This phenomenon is observable in both macro and nanostructured films; however, the effects are more prominent in nanostructured materials due to singularity in their joint density of states, thus allowing a bulk phenomenon to be observable clearly even at room temperature.
ZnO nanowires were synthesized in a horizontal tube furnace that was programmed for a processing temperature of 800 °C with heating rate 10 °C min−1. The source material Zn (99.9%) in granular form was placed at the center of the furnace. Double side-polished Al2O3(0001) and Si (100) samples were used as substrates for optical characterization. In the initial stage, the furnace was flushed by Ar gas and was stabilized. When the furnace reached 420 °C, the Zn metal evaporated and O2gas was introduced with a combined Ar/O2gas mixture. The evaporated Zn metal formed ZnO nanostructures when the reactants achieved supersaturation and was deposited on substrates and also on the walls of the tube furnace. The process was carried out for 90 min and samples were removed after the furnace was cooled down to room temperature. ZnO nanostructures were characterized by environmental scanning electron microscope (E-SEM) (Electro Scan) and photo-luminescence (PL) at room temperature (Laser Science, Inc, Model VSL-337 ND-S, 337 nm, 6 mW and Ocean Optics SD5000 spectrometer) measurements to monitor the morphology and the bandgap. The X-ray photoelectron spectroscopy (XPS) measurements were performed using Kratos Axis 165 spectrometer at a vacuum of 4 × 10−10 Torr with non-monochromatic MgK α radiation. All binding energies were calibrated with respect to C 1s at 284.6 eV.
Results and Discussion
Dark, photocurrent and their ratios as a function of background oxygen pressure
7.6 × 102
1.5 × 10−8
1.3 × 10−7
4 × 10−1
1.0 × 10−7
8.2 × 10−7
8 × 10−2
8.2 × 10−8
2.8 × 10−6
The inverse correlation between the background oxygen pressure and the photocurrent decay time clearly demonstrates the effect of oxygen on the sensor performance. PPC is commonly attributed to the existence of defects, which are metastable between shallow and deep levels and dislocations in the materials. One such defect is the deep unknown center (DX, discussed in the following section), which forms when shallow donors convert into deep donors after a large lattice relaxation . When the background oxygen is depleted (4 × 10−1 and 8 × 10−2 Torr conditions), the ZnO lattice undergoes a dynamic equilibrium between the chemisorbed oxygen and the interstitial oxygen (anion) vacancies. Under these circumstances, the interstitial vacancies dominate the conduction process over the chemisorbed oxygen. The space charge regions then modulate the effective conduction cross-section of the device .
When the Zn–Zn inter-atomic distance is modified by inward and outward movement, Zn vacancies (VZn) are produced which are intrinsic acceptors. The appearance of a level at ~1.5 eV verifies the evolution of VZn when the background oxygen is reduced . The reaction kinetics of VO0 thus results in metastable configuration change, constituting the PPC in ZnO. The above explanation follows very well in the present investigation of PPC phenomenon observed under depleted oxygen conditions.
To conclude, the phenomenon of PPC is a defect-related issue that depends entirely on the oxygen atmosphere around a nano-ZnO device. There has been a major thrust in fabricating nanostructured ZnO devices for gas, piezo, light, and biosensor applications. Mostly, these applications require device to be a resistor type that is prone to change by virtue of ambient rather than the stimulants, thereby opening many research opportunities to passivate the device effectively. The technique and the approach described in this paper can be extended to observe similar effects prevalent in any bulk material systems.
Authors SSH and NVH acknowledge the constant support from Buck Sharpton, Vice Chancellor (Research), Daniel White, Director, Institute of Northern Engineering and acknowledge the financial support from the U.S. Defense Micro Electronic Activity (DMEA) and the U.S. Defense Advanced Research Projects Agency (DARPA) at University of Alaska, Fairbanks.