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Excellent enhancement in the device performance of nitrogen plasma treated ZnO nanorods based diodes
© Reddy et al.; licensee Springer. 2014
Received: 31 March 2014
Accepted: 7 August 2014
Published: 2 October 2014
Impact of the plasma exposure time on the physical properties of homo-epitaxial ZnO nanorods (NRs) and their devices was investigated. Here, ZnO NRs were synthesized by chemical solution method on glass substrates and treated under high intensity nitrogen plasma at different exposure timings. The as-grown as well as treated ZnO NRs exhibited hexagonal crystal structure and (001) as a preferential orientation. While increasing the plasma exposure time from 1 to 15 min, the structural and optical quality of ZnO NRs gradually improved and above this exposure time, both the properties degraded. The devices fabricated with 15 min plasma treated ZnO NRs showed excellent diode performance than the untreated nanostructures based devices. The diodes developed with treated ZnO NRs showed a low turn-on voltage (3.3 V) than the devices developed with untreated NRs.
In recent years, the development of optoelectronic, electronic and bio devices on flexible substrates has received tremendous interest due to their unique advantages over the rigid-substrates ,. Zinc oxide (ZnO) is a wide band gap (~3.37 eV) semiconductor compound and exhibits high exciton binding energy (~60 meV, which is 2.5 times higher than that of room temperature thermal energy), good thermal and chemical stability ,. In this aspect, different type of ZnO nanostructures including nanosheets (NSs), nanorods (NRs), nanotubes (NTs), nanoflowers (NFs), nanobelts (NBs), nanowires (NWs) and nano-cages (NCs) etc. have been synthesized using different physical and chemical methods, and also adopted for various device applications ,. Among various synthetic methods, low temperature chemical solution method(s) have been widely applied for the synthesis of ZnO nanostructures due to their possible control over the growth parameters, scalability and versatility. ZnO nanostructures developed with chemical solution approach consist of pure phase and good stoichiometry since the growth occurs at atomic levels. However, these ZnO nanostructures suffer with poor structural and optical quality due to the presence of interstitial and surface defect states .
The plasma treatment is a versatile method for the processing of materials like surface cleaner, adhesion promotion, surface energy controlling and enhancement in bio-compatibility and device performance . For example, oxygen radio frequency plasma treated multi-wall carbon nanotubes (CNT) exhibited improved uniformity in the distribution of surface defects and thereby dispersion of metallic nanoparticles . Excellent electrochemical contacts between carbon nano tubes and enzymes have obtained by treating the as-grown CNTs by nitrogen plasma due to the flip of surface from hydrophobic to hydrophilic . Similarly, there has been a various number of reports on plasma treated ZnO materials reported elsewhere -. By keeping in mind about the future multifunctional applications of ZnO nanostructures as flexible device and existing data, we made an attempt to produce the best quality of ZnO NRs by plasma treatment since it is safe and environmental friendly than the other traditional methods. In this present study we have adopted nitrogen (N2) plasma to treat the hydrothermally synthesized ZnO NRs since the ionic radius of nitrogen is close to O and it also acts as deep acceptor and good compensator ,. The impact of nitrogen plasma treatment on the surface morphology, crystal structures and optical properties of ZnO NRs reported and discussed. Further, p-n junction diodes prepared with treated and untreated ZnO NRs and estimated their device performance at room temperature.
Vertically aligned ZnO NRs were grown by chemical solution technique at a growth temperature of ~70°C. In this process, zinc nitrate hexahydrate and hexamethylenetetramine (from Sigma Aldrich) analytical grade chemicals were used as reagents without further purification. The ZnO NRs were grown on ZnO layer (or seed layer) coated (~50 nm) glass (bare and indium tin oxide coated; 7–8 Ω/sq resistance) substrates. A detailed description about the synthesis of ZnO NRs has been reported elsewhere . The as-grown ZnO NRs structures were treated by nitrogen (4 N pure nitrogen gas with a flow rate of 100 sccm) plasma in the plasma-enhanced chemical vapor deposition system for different time spans, 0–20 min. Here, the plasma was created at a fixed gas pressure of 200 mTorr with a power of 100 W . However, upon treatment we have noticed a gradual raise in temperature from 25 to 120°C. Finally, the p-n junction diodes were fabricated by spin coating of the poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) polymer on ZnO NRs grown on ITO/glass substrates.
The structure and morphology of the structures were examined with X-ray diffractometer (XRD, Philips X’Pert Pro) and field emission scanning electron microscopy (FESEM, JSM-840 A). The photoluminescence (PL) measurements were carried out at room temperature with 325 nm He-Cd laser in the wavelength rage of 350–1000 nm. The electrical properties of the p-n diodes were studied at room temperature by using probe station attached with semiconductor parameter analyzer.
3. Results and discussion
3.1 Surface and structural properties
A considerable improvement in the crystalline quality of thermo chemically grown ZnO NRs with the treatment of high intensity plasma can be explained using the existing literature. In general, ZnO NRs grown by thermo chemical method usually consist of different surface defect states due to the presence of water and hydroxyl ions, and non-reacted Zn and O ions as interstitial defect states ,. These defect states probably act as amorphous centers and as results, the as-grown structures exhibit slightly poor crystallinity. Upon plasma treatment, these surface defect states probably released due to the bombardment of energetic nitrogen ions, which also induce the re-crystallization of Zn and O ions. Further, the diffusion of Ni ions into the core-lattice of ZnO probably neutralizes the defect states present in the ZnO NRs. As result, the overall crystallinity of ZnO NRs enhanced with the increase of nitrogen plasma treatment. On the other hand, a possible reason for the formation of Zn3N2 phase could be unintentional raise in temperature during plasma treatment since nitrogen ions can easily replace the oxygen atoms at temperatures higher than 110°C. Therefore, these analyses clearly emphasized that the structure and phase purity of ZnO NRs remains as same as the untreated nanostructures upto the plasma exposure time of 15 min, and the crystalline quality of 15 min treated ZnO NRs improved nearly by three times than that of untreated ones.
3.2 Optical emission properties
It is well know that in ZnO lattice matrix nitrogen impurities act as acceptors ,. In this view, various groups have adopted nitrogen as doping agent for the development of p-type ZnO films and also nanostructures -. In general, upon increasing plasma exposure time, the amount of nitrogen implantation or absorption in ZnO NRs increases. The incorporation of nitrogen into ZnO NR structures probably occurs in two ways: interstitial and substitutional doping. As interstitial doping, the nitrogen atoms neutralize the defect states present on the surface of ZnO materials, whereas in substitutional doping, nitrogen impurities generate interstitial defects (OI) by replacing oxygen atoms. Usually, the electrical conductivity of ZnO primarily dominated by electrons generated from oxygen vacancies and zinc interstitial atoms ,. In the present case, the interstitial incorporation of nitrogen atoms in place of oxygen vacancies (VO) , diminish the existing defects states due to passivation, and leads the density of carriers to lower values. Thus, the defects related BB peak intensity strongly reduced. Further, the decrease of carrier density leads band gap of ZnO NRs to lower values since Eg α ni 2/3. At higher exposure timings, there are two possible reasons for the formation of Zn3N2 phase: i) replacement of oxygen atoms in Zn-O lattice by entering nitrogen ions as substitutional impurity thereby release of oxygen atoms as interstitials, and/or ii) nitrification of zinc interstitials (Zni) under moderate temperatures . The newly formed Zn-N phase and/or regenerated oxygen interstitials probably leads the band gap of ZnO NR structures to slightly higher values. Therefore, the structures exposed to nitrogen plasma for 15 min duration consist of better crystallinity as well as optical quality and thus, these structures are adopted for the development of p-n junction diodes.
3.3 Devices properties
The ZnO NRs were synthesized using simple and low temperature chemical solution method on ZnO seeded glass and ITO substrates. The as-grown structures were exposed to high intensity nitrogen plasma and studied their physical properties. Finally, the p-n junction diodes were fabricated using as-grown and 15 min plasma treated ZnO NRs and studied the device performance. The observed results are summarized below.
The as-grown ZnO NRs have hexagonal crystal structure and are preferentially oriented along <001 > direction. Upon exposure to nitrogen plasma, the morphology and crystal structure of ZnO NRs remained as same upto the exposure time of 15 min. The ZnO NRs exposed to plasma for 15 min duration have better crystalline and optical quality than the as-deposited and other treated nanostructures. The devices fabricated with 15 min treated ZnO NRs exhibited excellent p-n junction diode properties at room temperature. Therefore, the overall observations emphasized that the quality of ZnO NRs grown even on flexible substrates can be improved to a large extent without disturbing their morphology as well as crystal structure by treating them under nitrogen plasma for the duration of 15 min.
This work was supported by National Leading Research Laboratory program (2013064831) through the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning. N.K. Reddy wishes to acknowledge European Commission Research Executive Agency for the sanction of Marie Curie Actions - International Incoming Fellowship (No: PIIF-GA-2012-331003: NRforHF) and CSIR for the sanction of SRA fellowship under the scheme of Scientist’s pool (No: 13(8525-A) 2011-Pool). M. Devika wishes to acknowledge UGC for the sanction of Dr. D.S. Kothari Postdoctoral fellowship No: F. 4-2/2006(BSR)/13-703/2012(BSR).
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