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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 8 — Apr. 11, 2011
  • pp: 7139–7146
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Optical confinement achieved in ZnO crystal by O+ ions implantation: analysis of waveguide formation and properties

Xianbing Ming, Fei Lu, Jiaojian Yin, Ming Chen, Shaomei Zhang, Xiuhong Liu, Zhenhua Qin, and Yujie Ma  »View Author Affiliations


Optics Express, Vol. 19, Issue 8, pp. 7139-7146 (2011)
http://dx.doi.org/10.1364/OE.19.007139


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Abstract

Optical confinement in ZnO crystal was observed by O+ implantation with different MeV energies and doses. Planar optical waveguides were formed in the as-implanted ZnO samples. The optical properties of the planar waveguide were investigated by the prism-coupling and the end-face coupling methods at the wavelength of 633 nm. The crystal lattice damage in the guiding region caused by the O+ ions implantation was analyzed by the Rutherford backscattering/Channeling technique, results show that even high dose at 2 × 1015 ions/cm2 can hardly produce defect in near surface of ZnO. A theoretical model is developed to explain the principle of waveguide formation in ZnO crystal and the refractive index profile in the implanted waveguide was reconstructed accordingly. The experimental result and analysis are significant for application of ZnO crystal, especially for the design of ZnO light emitter devices.

© 2011 OSA

1. Introduction

ZnO semiconductor has attracted a great deal of attention recently, the interest in this material is fueled and fanned by its versatile applications in optoelectronics owing to the combination of outstanding performance of various physical properties [1

1. C. Klingshirn, “ZnO: From basics towards applications,” Phys. Status Solidi, B Basic Res. 244(9), 3027–3073 (2007). [CrossRef]

]. It has been shown that some properties of ZnO are similar with its major rival GaN [2

2. C. Liu, F. Yun, and H. Morkoç, “Ferromagnetism of ZnO and GaN: a review,” J. Mater. Sci. Mater. Electron. 16(9), 555–597 (2005). [CrossRef]

], another wide-gap semiconductor which is widely used for electronic and optoelectronic applications. However, ZnO has some advantages over GaN among which are large high-quality ZnO bulk single crystals and a more radiation resistance which makes ZnO-based devices more attractive to be used in a radiation environment [3

3. D. C. Look, D. C. Reynolds, J. W. Hemsky, R. L. Jones, and J. R. Sizelove, “Production and annealing of electron irradiation damage in ZnO,” Appl. Phys. Lett. 75(6), 811–813 (1999). [CrossRef]

]. Moreover, the large exciton binding energy affords a stable exciton state for optical applications even at room temperature. Achievement of optical waveguides in ZnO material makes it possible to extend its application in light emitter devices and in integrated optics in an attempt to control the propagation of light and to enhance the optical efficiency.

2. Experimental details

Commercial Z-cut ZnO single crystals with the dimensions 10 × 5 × 0.5 mm3 were optically polished and cleaned before implantation. The ZnO was provided by Shanghai Daheng Optics and Fine Mechannics Co., Ltd. The energies of O+ ions were from 2 to 6 MeV, and the doses of them were from 5 × 1014 to 2 × 1015 ions/cm2. The detailed parameters in experiments are list in Table 1

Table 1. Experiment Parameters for Samples Suffered O+ Irradiation

table-icon
View This Table
. The ion implantation was performed at room temperature by a 1.7 MV tandem accelerator at Peking University. The samples were tilted by 7° off the incident beam direction in order to minimize the channeling effect during the implantation. The propagating modes of the samples were measured by a conventional m-line technique, using the prism coupling method with a Model 2010 Prism Coupler (Metricon 2010, USA). The near-field intensity profile of the planar waveguide was obtained by the end-face coupling arrangement. The He–Ne beam at 633 nm acts as a light beam in all measurements. The Rutherford backscattering/Channeling (RBS/Channeling) measurements were performed using a 2.1 MeV He+ beam generated by a 1.7 MV tandem accelerator at Shandong University. The backscattering of He+ particles was detected with a surface barrier detector at a scattering angle of 165°.

3. Results and discussion

The RBS/Channeling technique is extensively used in the investigation of the material damage. The RBS/Channeling spectra of the ZnO crystals after implantation by MeV O+ ion at different doses are indicated in Fig. 1
Fig. 1 RBS/Channeling spectra of MeV O+ ions implanted into the ZnO crystal. The random and channel spectra of the virgin ZnO crystal are also presented.
. The virgin and random spectra are also measured from the virgin ZnO crystal for comparison. It can be found that even for the fluence as high as 2 × 1015 ions/cm2, the amount of damage created by the implantation can hardly be observed. This result confirms that ZnO is very resistive to high-energy O+ ions radiation.

Figure 2
Fig. 2 Relative intensity of TE polarized light reflected from the prism versus the effective refractive index of the incident light in the ZnO waveguide formed by O+ implantation.
shows the measured relative intensity of the transverse electric (TE) polarized light reflected from the prism versus the effective refractive index of the incident light in the ZnO waveguide formed by O+ ion implantation. When the TE light was coupled into the waveguide, a lack of the reflected light would result in a dip in intensity, which may correspond to a waveguide mode. As one can see, all the samples have guiding modes. The first sharp mode TE0 means a good confinement of the light, which corresponds to the real waveguide mode. In sample 4#, corresponding to the implantation energy up to 6 MeV, although three dips are detected, the broader ones usually represent poor optical confinement; they are so called substrate modes. The optical confinement of first mode also degrades in this sample, which may due to the high transmission loss in waveguide, because light transmission experiences more absorption and scattering from point defects in a thicker waveguide, introduction of point defects is a typical result of implantation. The transverse magnetic (TM) polarized modes in the O+ ion-implanted ZnO waveguides were also measured in our experiments, and similar results have been obtained.

Light propagation property in waveguide is also investigated by using end-face coupling. The end-face coupling was performed with He-Ne laser at wavelength 633 nm. The input and output facets, with cross section dimensions of 5 × 0.5 mm2, were polished to allow for light to couple into and out of the sample (see Fig. 3
Fig. 3 The schematic of the planar waveguide fabricaed by O+ ion implantation.
). Figure 4
Fig. 4 The near field optical intensity profiles of the ZnO planar waveguides formed by O+ implantation (a)-(b) The 2D and 3D distributions for sample 1#; (c)-(d) The 2D and 3D distributions for sample 3#.
shows the near-field intensity distribution of the TE polarized light through sample 1# and sample 3# waveguides in two-dimensional (2D) and three-dimensional (3D) conditions. They show that the light can be confined to the ZnO waveguide area (between the ZnO surface and the optical barrier). The present data show that optical confinement can be achieved through O+ ion implantation under our experimental conditions. However, optical confinement becomes worse when the depth of index barrier increases, such as the cases in sample 2# and 4#. Except for the propagation loss from absorption and scattering, more light penetrates the index barrier into substrate, (see Figs. 4(c) and 4(d)). The result in Fig. 4 shows reasonable agreement with the result in Fig. 2. This situation may be improved by increasing the implantation dose of O+ ions or by performing suitable post-implant annealing.

SklVkV=VVk, Ps,k2=(1k)Ps2, gij,k=(1k)gij and pijl,k=(1k)pijl
(3)

According to the Lorentz–Lorenz equation, the relation among the average refractive index n=(neno2)1/3, the molar polarization αM and the molar volume VM can be given [14

14. H. Åhlfeldt, J. Webjörn, P. A. Thomas, and S. J. Teat, “Strutural and optical properties of annealed proton-exchanged waveguides in z-cut LiTaO3,” J. Appl. Phys. 77(9), 4467–4476 (1995). [CrossRef]

]:

(n21)/(n2+2)=α/VM
(4)

When combining Eqs. (1)(4), we have the following expression:

1nij2=1[nij,0n(M)ij]2+(1k)2gijPs2+(1k)kpijlVV
(5)

According to Eq. (5), we can obtain the refractive index profiles of ne (related to TM-polarized light) and no (related to TE-polarized light) for different lattice damage ratios in z-cut ZnO, as seen in Fig. 5
Fig. 5 Refractive indices of ne and no versus the lattice damage in the ZnO crystal. The dashed lines represent the refractive indices of the virgin crystal.
. The calculations clearly demonstrate a same trend between nTM and nTE in their variations with the lattice damage ratio. The both continually decline with increasing the lattice damage ratio. This simulation results show good agreement with our experimental results shown in Fig. 1 and Fig. 2. In the near surface region of ZnO the damage can hardly be detected (see Fig. 1), and the measured surface effect refractive is almost the same as that of virgin sample (the indicated inflexion by arrow, shown in Fig. 2).

As is seen, the extraordinary refractive index change of ne is mainly dominated by the term Δn(M), indicating the significant effects of molar polarization and molar volume on the extraordinary index. In comparison with Δn(P) and Δn(M), the effect of Δn(S), indicating to strain-induced photoelastic effect during O+ implantation, can be ignored due to its value is close to zero especially when the damage ratio is close to zero or close to 100%. Even around a medium damage level, e.g. damage ratio is close to 0.5, the contribution of Δn(S) is only account for 10% of the total index change. For the ordinary refractive index change no shown in Fig. 6(b), at the low damage ratio the contribution of Δn(P), originating from the effect of spontaneous polarization, is the dominant factor in determining no. Increasing the damage ratio, the effect of Δn(P) is weakening gradually, while the contribution of Δn(M) is becoming the dominant factor. The relationship between Δne or Δno and lattice damage shows a similar law that, increase as the damage ratio.

The damage ratio and profiles of ZnO after O+ implantation cannot be obtained directly in our experiment, because the depth of damage layer is far beyond the scope of detectable depth of RBS/Channeling. We use the SRIM’2003 [15

15. J. F. Ziegler, “SRIM-2003,” Nucl. Instrum. Methods Phys. Res. B 219–220, 1027–1036 (2004). [CrossRef]

] (the stopping and ranges of ions in matter) computer code to simulate the 4 MeV O+-implantation processes and to assume the normalized vacancy profile as the damage profile caused by O+ ion implantation. By referencing the damage level caused by heavy ion implanted ZnO crystal, we can evaluate approximately the damage level in ZnO caused by O+ implantation at dose of 2 × 1015 ions/cm2. By applying Eq. (5), the possible refractive-index profiles in the O+-implantation ZnO waveguides of sample 3# are calculated and shown in Figs. 7(a)
Fig. 7 (a)-(b)Calculated refractive-index profiles of sample 3# ZnO waveguides. (c) Calculated electric field distribution of TE modes (electric field strength of TE modes versus depth below surface) of sample 3# waveguide.
and 7(b). The dashed lines represent the refractive indices of the virgin ZnO. For both ordinary refractive index no and extraordinary index ne only negative refractive-index changes occurred.

By using the ordinary refractive index profile in Fig. 7(b), we calculate the electric field intensity distribution of the TE mode for ZnO waveguide of sample 3# by finite-difference beam propagation method and the result is shown in Fig. 7(c). It can be observed that the TE0 mode is relatively well confined within the region of waveguide, but TE1 field has most extended into the substrate, and too much energy leakage will make the corresponding mode TE1 far from practical use. The calculations have reasonable agreement with the experiment in Fig. 2(c) and Figs. 3(c) and 3(d). In the present case, a deeper index-barrier is helpful for better optical confinement in the single-mode waveguide. Anyway, the results provide the basis for preparation of single-mode waveguide.

4. Conclusion

In summary, planar optical waveguides were formed in ZnO single crystals by MeV O ions implantation with doses from 5 × 1014 to 2 × 1015 ions/cm2. The propagating modes were observed by the prism coupling technique and end-face coupling setup with He-Ne beam at 633 nm. Analyses show that at least one mode can be well confined within waveguide. The RBS/Channeling measurements show that O+ implantation did not causes obvious lattice damage at near surface of ZnO crytal, which indicates that ZnO is very resistive to high-energy radiation. A theoretical model is presented to explain the refractive-index changes in the ion-implanted ZnO, and the refractive index profile in waveguide is reconstructed accordingly. The experimental and theoretical research is meaningful to the application of ZnO crystal in the field of optoelectronics devices, especially in design of more efficient light emitting devices.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 10735070).

References and links

1.

C. Klingshirn, “ZnO: From basics towards applications,” Phys. Status Solidi, B Basic Res. 244(9), 3027–3073 (2007). [CrossRef]

2.

C. Liu, F. Yun, and H. Morkoç, “Ferromagnetism of ZnO and GaN: a review,” J. Mater. Sci. Mater. Electron. 16(9), 555–597 (2005). [CrossRef]

3.

D. C. Look, D. C. Reynolds, J. W. Hemsky, R. L. Jones, and J. R. Sizelove, “Production and annealing of electron irradiation damage in ZnO,” Appl. Phys. Lett. 75(6), 811–813 (1999). [CrossRef]

4.

D. Fluck, P. Günter, R. Irmscher, and Ch. Buchal, “Optical strip waveguides in KNbO3 formed by He ion implantation,” Appl. Phys. Lett. 59(25), 3213–3215 (1991). [CrossRef]

5.

P. D. Townsend, P. J. Chandler, and L. Zhang, Optical Effects of Ion Implantation (Cambridge University Press, 1994).

6.

S. O. Kucheyev, C. Jagadish, J. S. Williams, P. N. K. Deenapanray, M. Yano, K. Koike, S. Sasa, M. Inoue, and K. Ogata, “Implant isolation of ZnO,” J. Appl. Phys. 93(5), 2972–2976 (2003). [CrossRef]

7.

M. DiDomenico Jr and S. H. Wemple, “Oxygen-octahedra ferroelectics. I. Theory of electro-optical and nonlinear optical effects,” J. Appl. Phys. 40(2), 720–734 (1969). [CrossRef]

8.

S. H. Wemple and M. DiDomenico Jr., “Theory the elasto-optic effect in nonmetallic crystals,” Phys. Rev. B 1(1), 193–202 (1970). [CrossRef]

9.

Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, and H. Morkoç, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98(4), 041301 (2005). [CrossRef]

10.

M. J. Weber, Handbook of Optical Materials (Academic, CRC Press, 2003)

11.

M. C. Gupta and J. Ballato, The Handbook of Photonics (Academic, CRC Press, 2006), Chap. 6.

12.

Y. Jiang, K. M. Wang, X. L. Wang, F. Chen, C. L. Jia, L. Wang, Y. Jiao, and F. Lu, “Model of refractive-index changes in lithium niobate waveguides fabricated by ion implantation,” Phys. Rev. B 75(19), 195101 (2007). [CrossRef]

13.

J. J. Yin, F. Lu, X. B. Ming, Y. J. Ma, and M. B. Huang, “Theoretical modeling of refractive index in ion implanted LiNbO3 waveguides,” J. Appl. Phys. 108(3), 033105 (2010). [CrossRef]

14.

H. Åhlfeldt, J. Webjörn, P. A. Thomas, and S. J. Teat, “Strutural and optical properties of annealed proton-exchanged waveguides in z-cut LiTaO3,” J. Appl. Phys. 77(9), 4467–4476 (1995). [CrossRef]

15.

J. F. Ziegler, “SRIM-2003,” Nucl. Instrum. Methods Phys. Res. B 219–220, 1027–1036 (2004). [CrossRef]

OCIS Codes
(130.3130) Integrated optics : Integrated optics materials
(160.3130) Materials : Integrated optics materials
(230.7390) Optical devices : Waveguides, planar

ToC Category:
Integrated Optics

History
Original Manuscript: January 27, 2011
Revised Manuscript: March 5, 2011
Manuscript Accepted: March 9, 2011
Published: March 30, 2011

Citation
Xianbing Ming, Fei Lu, Jiaojian Yin, Ming Chen, Shaomei Zhang, Xiuhong Liu, Zhenhua Qin, and Yujie Ma, "Optical confinement achieved in ZnO crystal by O+ ions implantation: analysis of waveguide formation and properties," Opt. Express 19, 7139-7146 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-8-7139


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References

  1. C. Klingshirn, “ZnO: From basics towards applications,” Phys. Status Solidi, B Basic Res. 244(9), 3027–3073 (2007). [CrossRef]
  2. C. Liu, F. Yun, and H. Morkoç, “Ferromagnetism of ZnO and GaN: a review,” J. Mater. Sci. Mater. Electron. 16(9), 555–597 (2005). [CrossRef]
  3. D. C. Look, D. C. Reynolds, J. W. Hemsky, R. L. Jones, and J. R. Sizelove, “Production and annealing of electron irradiation damage in ZnO,” Appl. Phys. Lett. 75(6), 811–813 (1999). [CrossRef]
  4. D. Fluck, P. Günter, R. Irmscher, and Ch. Buchal, “Optical strip waveguides in KNbO3 formed by He ion implantation,” Appl. Phys. Lett. 59(25), 3213–3215 (1991). [CrossRef]
  5. P. D. Townsend, P. J. Chandler, and L. Zhang, Optical Effects of Ion Implantation (Cambridge University Press, 1994).
  6. S. O. Kucheyev, C. Jagadish, J. S. Williams, P. N. K. Deenapanray, M. Yano, K. Koike, S. Sasa, M. Inoue, and K. Ogata, “Implant isolation of ZnO,” J. Appl. Phys. 93(5), 2972–2976 (2003). [CrossRef]
  7. M. DiDomenico and S. H. Wemple, “Oxygen-octahedra ferroelectics. I. Theory of electro-optical and nonlinear optical effects,” J. Appl. Phys. 40(2), 720–734 (1969). [CrossRef]
  8. S. H. Wemple and M. DiDomenico., “Theory the elasto-optic effect in nonmetallic crystals,” Phys. Rev. B 1(1), 193–202 (1970). [CrossRef]
  9. Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, and H. Morkoç, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98(4), 041301 (2005). [CrossRef]
  10. M. J. Weber, Handbook of Optical Materials (Academic, CRC Press, 2003)
  11. M. C. Gupta and J. Ballato, The Handbook of Photonics (Academic, CRC Press, 2006), Chap. 6.
  12. Y. Jiang, K. M. Wang, X. L. Wang, F. Chen, C. L. Jia, L. Wang, Y. Jiao, and F. Lu, “Model of refractive-index changes in lithium niobate waveguides fabricated by ion implantation,” Phys. Rev. B 75(19), 195101 (2007). [CrossRef]
  13. J. J. Yin, F. Lu, X. B. Ming, Y. J. Ma, and M. B. Huang, “Theoretical modeling of refractive index in ion implanted LiNbO3 waveguides,” J. Appl. Phys. 108(3), 033105 (2010). [CrossRef]
  14. H. Åhlfeldt, J. Webjörn, P. A. Thomas, and S. J. Teat, “Strutural and optical properties of annealed proton-exchanged waveguides in z-cut LiTaO3,” J. Appl. Phys. 77(9), 4467–4476 (1995). [CrossRef]
  15. J. F. Ziegler, “SRIM-2003,” Nucl. Instrum. Methods Phys. Res. B 219–220, 1027–1036 (2004). [CrossRef]

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