3.1 Surface and structural properties
The FESEM analysis (Figure 1) reveals that the as-grown nanorods are vertically aligned and have an average length and diameter of 1.5 μm and 75 nm, respectively. It also shows that the morphology of treated ZnO NRs, even at longer exposures, remains as same as untreated structures. On the other hand, the as-grown ZnO NRs exhibited three XRD peaks diffracted at 2θ = 31.9, 34.55 and 36.4° (Figure 2), which belong to (100), (002) and (101) orientations respectively since the evaluated d-spacing values (0.2803, 02594, and 0.466 nm) exactly matched with the bulk hexagonal ZnO data (JCPDS: 36–1451) [16]-[18]. As compared to other diffraction peaks, (002) peak is more dominant, and therefore the as-grown ZnO NRs are preferentially oriented along <001 > direction, i.e. c-axis. While increasing plasma exposure time, the intensity of preferential peak gradually increased upto the exposure time of 15 min and above this, it slightly decreased. However, the crystallinity of ZnO NRs, i.e. the intensity ratio of I(002) and I(101) peaks, gradually increased with the increase of exposure time, as shown in Figure 3. Further, the evaluated full width at half maximum (FWHM) value of (002) peak decreased with the increase of exposure time, as shown in Figure 3. However, at higher exposure timings (>15 min), the plasma treated ZnO NR structures exhibited two new diffraction peaks at 30.5 (d = 0.293 nm) and 35.4° (d = 0.253 nm), which belong to Zn3N2 phase [19].
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 [20],[21]. 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
PL studies show that the as-grown ZnO NRs have a large number of defect states that are mainly attributed to the interstitials and vacancies of zinc and oxygen atoms since the emission intensity of the broad band (BB) peak centered at 590 nm is comparatively higher than the intensity of ultra-violet (UV) peak (Figure 4) centered at 390 nm. Upon increasing the nitrogen plasma exposure time, the UV peak intensity drastically increased and BB peak intensity considerably decreased, which can be clearly seen from Figure 5. The variation of emission intensity ratio of UV and BB peaks (i.e. IUV/IBB) with plasma exposure time, Figure 5, reveals that while increasing exposure time the quality of ZnO NRs gradually increased upto the exposure time of 15 min. Above this, a degradation in the emission quality of NRs started. These results indicate that the structures treated for 15 min span consist of high quality optical properties since IUV/IBB ratio of these structures is 11 times higher than that of as-grown nanorods. An improvement in the optical quality of 15 min treated ZnO NRs is attributed to the passivation of defects states present on the surface of ZnO NRs. At higher exposure timings, the regeneration of oxygen interstitials (Oi) through the replacement of oxygen in ZnO lattice by nitrogen atoms probably lead the density of defect states to higher values and causes for the overall degradation in quality of ZnO NRs. While increasing the plasma exposure time the structures exhibited slight red-shift in the UV peak position upto the exposure time of 15 min and at higher exposure timings, the UV peak position drastically shifted towards lower wavelengths (i.e. blue-shift), Figure 6. In general, the UV peak position, represents the near band edge emission (band gap, Eg), strongly depends on the crystalline quality of ZnO NRs. As observed in XRD studies, the crystalline quality of plasma treated ZnO NRs improved upto the exposure time of 15 min that probably leads the band gap of ZnO NRs to slightly lower values due to the passivation of defect states.
It is well know that in ZnO lattice matrix nitrogen impurities act as acceptors [22],[23]. In this view, various groups have adopted nitrogen as doping agent for the development of p-type ZnO films and also nanostructures [24]-[28]. 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 [29],[30]. In the present case, the interstitial incorporation of nitrogen atoms in place of oxygen vacancies (VO) [11],[13] 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 [31]. 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 untreated and 15 min plasma treated ZnO NRs grown on ITO substrates were coated with PEDOT:PSS using spin coater with a thickness of ~4 μm thick and allowed at room temperature for a few days. The schematic diagram and cross-section FESEM images of the device with the configuration of Glass/ITO/ZnO seed-layer/ZnO NRs/PEDOT:PSS are shown in Figure 7. Finally, a thin gold (Au) layer with a thickness of 100 nm was deposited by e-beam evaporation and the current (I) - voltage (V) characteristics were measured at room temperature (Figure 8). Here, the devices prepared with untreated (D1) and treated (D2) ZnO NRs exhibited slightly similar p-n junction diode characteristics between the applied bias voltages of ± 5 V. Comparatively D2 device exhibited better I-V properties than that of D1 device [32]. For example, the maximum current through D2 device at the forward bias voltage of 5 V is about 32.3 μA, whereas in D1 device the observed maximum current is nearly 10 times low (~3.13 μA). The knee voltage (Vd) of D1 is higher (4.16 V) than that of D2 device (3.3 V) and the rectification factor (If/Ir) at the bias voltage of 5 V is about 78 and 408. Further, the series resistance of D1 and D2 devices is found to be 3.4 × 105 and 5.3 × 104 Ω, respectively.
From ln(I) versus V plots (Figure 9), the diode quality factor (n) of the devices was calculated using the formula: n = (q/kT)(ΔV/Δln(I)), where q-charge of electron, k-Boltzmann constant and T-absolute temperature. At lower voltages (<1 V) the conduction mechanism in D1 as well as D2 devices (R1 region) dominated by diffusion of carriers since the observed diode quality factors (n) is greater than 2 [33]. Above this voltage, D1 exhibited only one region (R2), whereas D2 device exhibited three more regions (R2, R3 and R4). The ideality factor of these devices particularly at higher voltages (>2 V) is too high probably due to the presence of defects states (or space-charges) on the surface of ZnO NRs. This reveals that at higher voltages, the conduction through the devices strongly dominated by the space-charge limited current and/or its combination with series resistance. However, the space-charge limited current region (R2) for D2 device is low as compared to D1 device. A decrease in density of surface defects states in plasma treated ZnO NRs probably the cause for better device performance than the device developed with untreated ZnO NRs.