The effect of the nanosecond laser annealing on the photoluminescence (PL) property of phosphorus ions (P+) implanted ZnOnanorods (NRs) has been investigated. The nanosecond laser annealing was performed with the third harmonic of a Q-switched Nd:YAG laser (355nm, 10ns/pulse) at a fluence of 100mJ/cm2. It turned out that nanosecond laser annealing is more effective in the recovery of the PL property compared with the thermal annealing using an electric furnace. As the results, the I-V characteristics of the p-n homojunctions along ZnO NRs showed rectifying property with a threshold voltage of approximately 6V.
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Zinc oxide (ZnO) is one of the most promising semiconductor materials for optoelectronic applications in particular ultra violet light emitting diode (UV-LED). ZnO has a wide band-gap of approximately 3.37eV at room temperature and a large exciton binding energy of approximately 60meV that is larger than the thermal energy of 26meV at room temperature. It can ensure an efficient exciton emission in UV spectral region. Furthermore, ZnO is rich in the natural abundant and a biologically-safe material. In addition, since ZnO has a strong tendency for self-organized growth, a variety of ZnO single-crystal nanostructures have been synthesized. The nanostructured ZnO crystals, including nanorods, nanosheets, nanowires etc., are quite attractive as building blocks for light emitting devices like laser and LED, because of their high crystallinity and light confinement properties. Recently several groups have reported the laser action from electrically-pumped ZnOnano-crystals [1–3]. More recently the laser action has been realized from the homo p-n junction by current injection . However, the construction of homo p-n junction is still problematic, and especially the method for the realization of the stable p-type ZnO has not been well established. Many groups have been working for the realization of p-type nanostructured ZnO crystals by using different growth methods, including carbo-thermal method, pulsed-laser deposition, hydro-thermal method and so on, with different acceptor dopants like nitrogen, phosphorous, antimony, or co-doping of lithium and nickel and so on [5–10]. Among them, ion implantation of acceptor dopants into ZnOnano-crystals is a straightforward method. Sun et al. have reported the p-type conduction in phosphorus ion (P+)-implanted ZnO nanowires and the construction of homo p-n junction . In the case of ion implantation, the activation process of acceptors is essential for the recovery of the optical property, since the ion implantation severely degrades the emission capability of the crystals by the introduction of large amount of defects.
In this study, we have investigated the effect of the nanosecond laser annealing on the recovery of the photoluminescence property of the P+-implanted ZnOnanorods (NRs), comparing with the thermal annealing using an electric furnace. The effects of laser annealing on the electrical property and on the morphological changes of ZnO NRs are also described.
2. Sample preparation
ZnO NRs used in this study were synthesized by the nanoparticles assisted pulsed laser deposition (NAPLD) . A sintered ZnO target with a purity of 99.99% was placed in a vacuum chamber filled with O2 gas at 665Pa, and was ablated for 20min by the third harmonic of a Q-switched Nd:YAG laser (Quanta-Ray, GCR-290, Spectra Physics) at 355nm with a repetition rate of 10Hz and a fluence of approximately 1.5J/cm2. ZnO NRs were grown on a pre-annealed (1000þC, for 120min) c-plane sapphire substrate (10mm × 10mm) placed on a SiC plate heated at 750þC in the chamber and the target-substrate distance was set to be 25mm. The flow rate of O2 gas was adjusted at 20cm3/min.
A scanning electron microscope (SEM: VE-7800S, KEYENCE) image of the as-grown ZnO NRs was shown in Fig. 1(a) . In NAPLD, a variety of ZnOnano-crystals can be synthesized by controlling the process conditions . In this study, we have chosen the densely packed NRs structure as shown in Fig. 1(a), in order to irradiate the ion beam only onto the top surface of the sample. It can be seen that vertically-aligned ZnO NRs are grown at high-density with a typical diameter of approximately 500nm. The length of ZnO NRs is estimated at approximately 1µm.
3.1 P+-ion implantation and its effects on morphology and photoluminescence
A half area of as-grown ZnO NRs substrate were implanted with P+ by an ion implantation equipment (UR-200, ULVAC) at an acceleration voltage of 50keVwith a dosage of 1.0×1014 ions/cm2. The implantation angle was 7þ, which is the minimum injection angle of the present ion implantation equipment, to reduce channeling effects. The ion accelerating voltage of 50keV was chosen, based on the simulation using the TRIM (the Transport of Ions in Matter) code. The simulated depth profiles of the implanted 2 million P+ at different ion accelerating voltages are shown in Fig. 1(b). The P+-implanted depth at a peak was evaluated to be about 50nm for an acceleration voltage of 50keV.Fig. 1(c) shows a SEM image of P+-implanted ZnO NRs. Compared with Fig. 1(a), little difference can be found in the morphology between before and after ion implantation.
Figure 1(d) shows the room temperature photoluminescence (RT-PL) spectra measured with a spectrometer (C10027-01, Hamamatsu Photonics K.K.) under the excitation at 325nm using a He-Cd laser (IK3301 R-G, KIMMON KOHA Co., Ltd.). The as-grown ZnO NRs have a sharp near band edge (NBE) peak and a broad peak located in a spectral region of green-red, which is attributed to the oxygen vacancy defects and results in the n-type conduction . P+-implanted ZnO NRs, on the other hand, have no photoluminescence throughout all wavelengths region between 350nm and 950nm. This is the indication that many defects are created in the crystal structures by implantation .
3.2 Ns-laser annealing and its effects on morphology and photoluminescence
Next, the nanosecond laser annealing was performed at a fluence of 100mJ/cm2 with the third harmonic of the Q-switched Nd:YAG laser in air . The laser beam squared 5mm on a side was scanned over the surface of the P+-implanted region with a single irradiation condition for each spot. Figure 2(a) shows a SEM image of the P+-implanted ZnO NRs followed by the laser annealing. It turned out that the tips of ZnO NRs were melted by laser irradiation, as can be seen in Fig. 2(a). This phenomenon is understood by the fact that the electric field is enhanced at the tips of ZnO NRs by the near field effect.
The photoluminescence spectrum of the laser-annealed ZnO NRs is shown in Fig. 2(b), where that of the sample thermally annealed at 700þC for 10min in air is also shown by the red curve, as a reference. In the case of the laser annealing, only NBE emission was observed without visible emission, indicating the donor vacancies are compensated. In the case of the thermal annealing, on the other hand, a weak NBE and a strong visible emission are observed. The ZnO layer damaged by ion implantation is melted and recrystallized by laser annealing process without diffusion of implanted ions, resulting in the full compensation of the damaged layer. In the case of the thermal annealing, on the other hand, the damaged layer can be recovered by thermal diffusion process, but the fact that only the visible fluorescence is observed after thermal annealing indicates that the thermal annealing is not sufficient for the compensation of the defects induced by ion implantation. Thus, dramatic improvement of optical properties has been achieved by the laser annealing, compared with the thermal annealing.
3.3 I-V characteristics of the P+-implanted ZnO NRs followed by ns-laser annealing
The electrical characteristics of the P+-implanted ZnO NRs followed by the laser annealing were evaluated by forming a junction on the sample. Metal-semiconductor Ohmic contact was realized by using a tungsten probe and gold-deposited probe .The present lateral arrangement is just for the I-V characteristics measurement of the p-n junctions by means of microprobes. In a LED fabrication, the vertical p-n junctions over full surface are more practical. Fig. 3 shows the I-V characteristics between the P+-implanted ZnO NRs followed by the laser annealing and un-implanted NRs, as shown in the lower right inset of Fig. 3. A good rectifying characteristic was observed in Fig. 3, where the positive voltage means that the P+-implanted ZnO NRs followed by the laser annealing was positively biased. The threshold voltage was approximately 6V and the breakdown voltage in the reverse bias was approximately −25V. Since as-grown ZnO NRs have n-type conduction, the rectifying characteristic indicates that the p-type conduction is realized in the P+-implanted ZnO NRs followed by the laser annealing. When the I-V characteristics were measured between two different areas only on un-implanted NRs or on the P+-implanted ZnO NRs followed by the laser annealing, the linear Ohmic characteristics are obtained, as shown in another upper left inset in Fig. 3.
The rectifying characteristic has been monitored for 3months after the fabrication of the sample. Until now, little degradation has been observed. Based on these results, we concluded that the nanosecond laser annealing is quite effective to activate the P+-implanted acceptors. In the next stage, I-V characteristics will be taken from vertical p-n junction.
3.4 Ns-laser annealing of ZnO NRs at different fluences
The influence of laser annealing on the morphology of ZnO NRs was investigated at different irradiation fluences. Figures 4 show the SEM images of (a) as-grown ZnO NRs, those of laser-annealed ZnO NRs with a fluence of (b) 70mJ/cm2, (c) 100mJ/cm2, and (d) 150mJ/cm2, respectively. Since it is possible to control the length of the melting layer along ZnO NRs, as shown in Figs. 4(a)-(d), there may be the best combination between the acceleration voltage in the ion implantation process and the fluence in the laser annealing process.
The PL spectra from the laser-annealed samples at different fluences are also shown in Fig. 4(e), where the symbols on the lines are just for easy to see. As the fluence increased, the tips of ZnO NRs were melted more. Besides, as shown in Fig. 4(e), NBE emission in PL is grown stronger as the fluence increase, but decrease at 150mJ/cm2. On the other hand, the ratio of UV peak to Green peak in as-grown, annealed at 70mJ/cm2, 100mJ/cm2 and 150mJ/cm2 are decreasing 19%, 5%, 2.5% and 1.5%, respectively. It is considered that the cause of UV peak decrease at 150mJ/cm2 is reduction of ZnO crystalline quality due to ablation effect by high-energy laser irradiation . It is a promising fact that production technology of ZnOhomo junction UV-LED will be better by optimizing fluence of laser annealing.
In summary, we have investigated the effect of the nanosecond laser annealing on the recovery of the photoluminescence property of the P+-implanted ZnO NRs, comparing with the thermal annealing using an electric furnace. Dramatic improvement of optical properties has been achieved by the nanosecond laser annealing, compared with the thermal annealing. As the result, the fabrication of the p-n homo junction along ZnO NRs has been demonstrated using P+ ion implantation and nanosecond laser annealing. Our approach has possibilities for realization of simple and low-cost methods to fabricate ZnO homo junction UV-LED.
The authors would like to thank Mr. Takayuki Takao and Mr. Mannam Ramanjaneyulu for their supports in the experiment. This work was supported in part by the Program under Special Coordination Funds for Promoting Science and Technology from Japan Science and Technology Agency (JST) and a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (JSPS, No. 24656053).
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