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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 14 — Jul. 2, 2012
  • pp: 15247–15252
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Effect of laser annealing on photoluminescence properties of Phosphorus implanted ZnO nanorods

Tetsuya Shimogaki, Kota Okazaki, Daisuke Nakamura, Mitsuhiro Higashihata, Tanemasa Asano, and Tatsuo Okada  »View Author Affiliations


Optics Express, Vol. 20, Issue 14, pp. 15247-15252 (2012)
http://dx.doi.org/10.1364/OE.20.015247


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Abstract

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.

© 2012 OSA

1. Introduction

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

1. R. Q. Guo, J. Nishimura, M. Matsumoto, D. Nakamura, and T. Okada, “Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition,” Appl. Phys., A Mater. Sci. Process. 93(4), 843–847 (2008).

3

3. E. S. P. Leong and S. F. Yu, “UV Random Lasing Action in p-SiC(4H)/i-ZnO–SiO2 Nanocomposite/n-ZnO:Al Heterojunction Diodes,” Adv. Mater. 18(13), 1685–1688 (2006).

]. More recently the laser action has been realized from the homo p-n junction by current injection [4

4. S. Chu, G. Wang, W. Zhou, Y. Lin, L. Chernyak, J. Zhao, J. Kong, L. Li, J. Ren, and J. Liu, “Electrically pumped waveguide lasing from ZnO nanowires,” Nat. Nanotechnol. 6(8), 506–510 (2011).

]. 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

5. K. Nakahara, S. Akasaka, H. Yuji, K. Tamura, T. Fujii, Y. Nishimoto, D. Takamizu, A. Sasaki, T. Tanabe, H. Takasu, H. Amaike, T. Onuma, S. F. Chichibu, A. Tsukazaki, A. Ohtomo, and M. Kawasaki, “Nitrogen doped MgxZn1-xO/ZnO single heterostructure ultravioletlight-emitting diodes on ZnO substrates,” Appl. Phys. Lett. 97(1), 013501 (2010).

10

10. E. S. Kumar, J. Chatterjee, N. Rama, N. DasGupta, and M. S. R. Rao, “A Co doping Route to Realize Low Resistive and Stable p-Type Conduction in (Li,Ni)-ZnO Thin Films Grown by Pulsed Laser Deposition,” ACS Appl. Mater. Interfaces 3(6), 1974–1979 (2011).

]. 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 [7

7. X. W. Sun, B. Ling, J. L. Zhao, S. T. Tan, Y. Yang, Y. Q. Shen, Z. L. Dong, and X. C. Li, “Ultraviolet emission from a ZnO rod homojunction light-emitting diode,” Appl. Phys. Lett. 95(13), 133124 (2009).

]. 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) [11

11. D. Nakamura, A. Kumeda, K. Toya, K. Okazaki, K. Kubo, K. Tsuta, M. Higashihata, and T. Okada, “Synthesis of Layer-Structured ZnO Nano-Crystals by Nanoparticle-Assisted Pulsed Laser Deposition,” Proc. SPIE 7940, 990–992 (2011).

]. 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)
Fig. 1 SEM images of ZnO NRs, (a) as-grown and(c) as-implanted. (45þ tilted and top view) (b) Simulation results of P+-distribution after ion implantation. (d) RT-PL spectra of as-grown and P+-implanted ZnO NRs.
. In NAPLD, a variety of ZnOnano-crystals can be synthesized by controlling the process conditions [12

12. K. Sakai, S. Oyama, K. Noguchi, A. Fukuyama, T. Ikari, and T. Okada, “Optical properties of nanostructured ZnO crystal synthesized by pulsed-laser ablation,” Physica E 40(7), 2489–2493 (2008).

]. 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. Results

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 [13

13. A. Janotti and C. G. V. d. Walle, “Oxygen vacancies in ZnO,” Appl. Phys. Lett. 87, 122102 (2005).

]. 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 [14

14. Z. Q. Chen, A. Kawasuso, Y. Xu, H. N. H. Yan, X. L. Yuan, T. Sekiguchi, R. Suzuki, and T. Ohdaira, “Production and recovery of defects in phosphorus-implanted ZnO,” J. Appl. Phys. 97, 013528 (2005).

].

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 [15

15. J. Maeng, S. Heo, G. Jo, M. Choe, S. Kim, H. Hwang, and T. Lee, “The effect of excimer laser annealing on ZnO nanowires and their field effect transistors,” Nanotechnology 20(9), 095203 (2009).

]. 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)
Fig. 2 (a) SEM images of ZnO NRs after laser annealing P+-implanted ZnO NRs followed by laser annealing at a fluence of 100mJ/cm2. (45þ tilted and top view)(b) PL spectra of P+-implanted ZnO NRs followed by laser annealing and thermal annealing.
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 [16

16. L. J. Brillson and Y. Lu, “ZnO Schottky barriers and Ohmic contacts,” J. Appl. Phys. 109(12), 121301 (2011).

].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.

Fig. 3.I-V characteristics of P+-implanted ZnO NRs followed by laser annealing at a fluence of 100mJ/cm2. The top left inset shows I-V characteristics at p-p and n-n areas. The bottom right inset shows a schematic of prove electrode.

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
Fig. 4 SEM images of ZnO NRs (a) as-grown ZnO NRs, laser annealed at a fluence of (b) 70mJ/cm2, (c) 100mJ/cm2 and (d) 150mJ/cm2. (45þ tilted and top view)(e) RT-PL spectra of ZnO NRs laser annealed at fluences of 70mJ/cm2, 100mJ/cm2 and 150mJ/cm2.
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 [17

17. Y. Zhao and Y. Jiang, “Effect of KrF excimer laser irradiation on the properties of ZnO thin films,” J. Appl. Phys. 103(11), 114903 (2008). [CrossRef]

]. It is a promising fact that production technology of ZnOhomo junction UV-LED will be better by optimizing fluence of laser annealing.

4. Summary

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.

Acknowledgments

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).

References and links

1.

R. Q. Guo, J. Nishimura, M. Matsumoto, D. Nakamura, and T. Okada, “Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition,” Appl. Phys., A Mater. Sci. Process. 93(4), 843–847 (2008).

2.

S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G. C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett. 84(17), 3241–3243 (2004).

3.

E. S. P. Leong and S. F. Yu, “UV Random Lasing Action in p-SiC(4H)/i-ZnO–SiO2 Nanocomposite/n-ZnO:Al Heterojunction Diodes,” Adv. Mater. 18(13), 1685–1688 (2006).

4.

S. Chu, G. Wang, W. Zhou, Y. Lin, L. Chernyak, J. Zhao, J. Kong, L. Li, J. Ren, and J. Liu, “Electrically pumped waveguide lasing from ZnO nanowires,” Nat. Nanotechnol. 6(8), 506–510 (2011).

5.

K. Nakahara, S. Akasaka, H. Yuji, K. Tamura, T. Fujii, Y. Nishimoto, D. Takamizu, A. Sasaki, T. Tanabe, H. Takasu, H. Amaike, T. Onuma, S. F. Chichibu, A. Tsukazaki, A. Ohtomo, and M. Kawasaki, “Nitrogen doped MgxZn1-xO/ZnO single heterostructure ultravioletlight-emitting diodes on ZnO substrates,” Appl. Phys. Lett. 97(1), 013501 (2010).

6.

M. T. Chen, M. P. Lu, Y. J. Wu, J. Song, C. Y. Lee, M. Y. Lu, Y. C. Chang, L. J. Chou, Z. L. Wang, and L. J. Chen, “Near UV LEDs Made with in Situ Doped p-n Homojunction ZnO Nanowire Arrays,” Nano Lett. 10(11), 4387–4393 (2010).

7.

X. W. Sun, B. Ling, J. L. Zhao, S. T. Tan, Y. Yang, Y. Q. Shen, Z. L. Dong, and X. C. Li, “Ultraviolet emission from a ZnO rod homojunction light-emitting diode,” Appl. Phys. Lett. 95(13), 133124 (2009).

8.

V. Vaithianathan, B. T. Lee, and S. S. Kim, “Pulsed-laser-deposited p-type ZnO films with phosphorus doping,” J. Appl. Phys. 98(4), 043519 (2005).

9.

T. Aoki, Y. Shimizu, A. Miyake, A. Nakamura, Y. Nakanishi, and Y. Hatanaka, “p-Type ZnO Layer Formation by Excimer Laser Doping,” Phys. Status Solidi B 229(2), 911–914 (2002).

10.

E. S. Kumar, J. Chatterjee, N. Rama, N. DasGupta, and M. S. R. Rao, “A Co doping Route to Realize Low Resistive and Stable p-Type Conduction in (Li,Ni)-ZnO Thin Films Grown by Pulsed Laser Deposition,” ACS Appl. Mater. Interfaces 3(6), 1974–1979 (2011).

11.

D. Nakamura, A. Kumeda, K. Toya, K. Okazaki, K. Kubo, K. Tsuta, M. Higashihata, and T. Okada, “Synthesis of Layer-Structured ZnO Nano-Crystals by Nanoparticle-Assisted Pulsed Laser Deposition,” Proc. SPIE 7940, 990–992 (2011).

12.

K. Sakai, S. Oyama, K. Noguchi, A. Fukuyama, T. Ikari, and T. Okada, “Optical properties of nanostructured ZnO crystal synthesized by pulsed-laser ablation,” Physica E 40(7), 2489–2493 (2008).

13.

A. Janotti and C. G. V. d. Walle, “Oxygen vacancies in ZnO,” Appl. Phys. Lett. 87, 122102 (2005).

14.

Z. Q. Chen, A. Kawasuso, Y. Xu, H. N. H. Yan, X. L. Yuan, T. Sekiguchi, R. Suzuki, and T. Ohdaira, “Production and recovery of defects in phosphorus-implanted ZnO,” J. Appl. Phys. 97, 013528 (2005).

15.

J. Maeng, S. Heo, G. Jo, M. Choe, S. Kim, H. Hwang, and T. Lee, “The effect of excimer laser annealing on ZnO nanowires and their field effect transistors,” Nanotechnology 20(9), 095203 (2009).

16.

L. J. Brillson and Y. Lu, “ZnO Schottky barriers and Ohmic contacts,” J. Appl. Phys. 109(12), 121301 (2011).

17.

Y. Zhao and Y. Jiang, “Effect of KrF excimer laser irradiation on the properties of ZnO thin films,” J. Appl. Phys. 103(11), 114903 (2008). [CrossRef]

OCIS Codes
(140.3390) Lasers and laser optics : Laser materials processing
(160.6000) Materials : Semiconductor materials
(160.4236) Materials : Nanomaterials
(220.4241) Optical design and fabrication : Nanostructure fabrication

ToC Category:
Laser Microfabrication

History
Original Manuscript: April 3, 2012
Revised Manuscript: June 3, 2012
Manuscript Accepted: June 7, 2012
Published: June 22, 2012

Citation
Tetsuya Shimogaki, Kota Okazaki, Daisuke Nakamura, Mitsuhiro Higashihata, Tanemasa Asano, and Tatsuo Okada, "Effect of laser annealing on photoluminescence properties of Phosphorus implanted ZnO nanorods," Opt. Express 20, 15247-15252 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-14-15247


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References

  1. R. Q. Guo, J. Nishimura, M. Matsumoto, D. Nakamura, and T. Okada, “Catalyst-free synthesis of vertically-aligned ZnO nanowires by nanoparticle-assisted pulsed laser deposition,” Appl. Phys., A Mater. Sci. Process.93(4), 843–847 (2008).
  2. S. F. Yu, C. Yuen, S. P. Lau, W. I. Park, and G. C. Yi, “Random laser action in ZnO nanorod arrays embedded in ZnO epilayers,” Appl. Phys. Lett.84(17), 3241–3243 (2004).
  3. E. S. P. Leong and S. F. Yu, “UV Random Lasing Action in p-SiC(4H)/i-ZnO–SiO2 Nanocomposite/n-ZnO:Al Heterojunction Diodes,” Adv. Mater. 18(13), 1685–1688 (2006).
  4. S. Chu, G. Wang, W. Zhou, Y. Lin, L. Chernyak, J. Zhao, J. Kong, L. Li, J. Ren, and J. Liu, “Electrically pumped waveguide lasing from ZnO nanowires,” Nat. Nanotechnol.6(8), 506–510 (2011).
  5. K. Nakahara, S. Akasaka, H. Yuji, K. Tamura, T. Fujii, Y. Nishimoto, D. Takamizu, A. Sasaki, T. Tanabe, H. Takasu, H. Amaike, T. Onuma, S. F. Chichibu, A. Tsukazaki, A. Ohtomo, and M. Kawasaki, “Nitrogen doped MgxZn1-xO/ZnO single heterostructure ultravioletlight-emitting diodes on ZnO substrates,” Appl. Phys. Lett.97(1), 013501 (2010).
  6. M. T. Chen, M. P. Lu, Y. J. Wu, J. Song, C. Y. Lee, M. Y. Lu, Y. C. Chang, L. J. Chou, Z. L. Wang, and L. J. Chen, “Near UV LEDs Made with in Situ Doped p-n Homojunction ZnO Nanowire Arrays,” Nano Lett.10(11), 4387–4393 (2010).
  7. X. W. Sun, B. Ling, J. L. Zhao, S. T. Tan, Y. Yang, Y. Q. Shen, Z. L. Dong, and X. C. Li, “Ultraviolet emission from a ZnO rod homojunction light-emitting diode,” Appl. Phys. Lett.95(13), 133124 (2009).
  8. V. Vaithianathan, B. T. Lee, and S. S. Kim, “Pulsed-laser-deposited p-type ZnO films with phosphorus doping,” J. Appl. Phys.98(4), 043519 (2005).
  9. T. Aoki, Y. Shimizu, A. Miyake, A. Nakamura, Y. Nakanishi, and Y. Hatanaka, “p-Type ZnO Layer Formation by Excimer Laser Doping,” Phys. Status Solidi B229(2), 911–914 (2002).
  10. E. S. Kumar, J. Chatterjee, N. Rama, N. DasGupta, and M. S. R. Rao, “A Co doping Route to Realize Low Resistive and Stable p-Type Conduction in (Li,Ni)-ZnO Thin Films Grown by Pulsed Laser Deposition,” ACS Appl. Mater. Interfaces3(6), 1974–1979 (2011).
  11. D. Nakamura, A. Kumeda, K. Toya, K. Okazaki, K. Kubo, K. Tsuta, M. Higashihata, and T. Okada, “Synthesis of Layer-Structured ZnO Nano-Crystals by Nanoparticle-Assisted Pulsed Laser Deposition,” Proc. SPIE7940, 990–992 (2011).
  12. K. Sakai, S. Oyama, K. Noguchi, A. Fukuyama, T. Ikari, and T. Okada, “Optical properties of nanostructured ZnO crystal synthesized by pulsed-laser ablation,” Physica E40(7), 2489–2493 (2008).
  13. A. Janotti and C. G. V. d. Walle, “Oxygen vacancies in ZnO,” Appl. Phys. Lett.87, 122102 (2005).
  14. Z. Q. Chen, A. Kawasuso, Y. Xu, H. N. H. Yan, X. L. Yuan, T. Sekiguchi, R. Suzuki, and T. Ohdaira, “Production and recovery of defects in phosphorus-implanted ZnO,” J. Appl. Phys.97, 013528 (2005).
  15. J. Maeng, S. Heo, G. Jo, M. Choe, S. Kim, H. Hwang, and T. Lee, “The effect of excimer laser annealing on ZnO nanowires and their field effect transistors,” Nanotechnology20(9), 095203 (2009).
  16. L. J. Brillson and Y. Lu, “ZnO Schottky barriers and Ohmic contacts,” J. Appl. Phys.109(12), 121301 (2011).
  17. Y. Zhao and Y. Jiang, “Effect of KrF excimer laser irradiation on the properties of ZnO thin films,” J. Appl. Phys.103(11), 114903 (2008). [CrossRef]

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