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

For many semiconductor light emitters, both carrier and optical confinement are needed to reduce threshold current and improve optical efficiency. In many cases, ion implantation [4,5] is employed to modify the material properties in planar devices, such as using O⁺ and H⁺ ion implantation to produce device isolation. It is believed that in future ZnO-based technology bombardment with energetic ions can be used for electrical isolation of closely spaced devices [6]. However, previous studies of ion irradiation on ZnO crystal targeted mostly on their electrical properties, there is no detail report on the optical properties of ion implant ZnO crystal. In this paper, we report the waveguide formation and properties in O⁺ implanted ZnO crystal. Ion implantation offers possibility of introducing any given impurity with accurate control of both the depth and lateral concentrations of the dopant, which defines the profile of the refractive index in the waveguide. To analyze the mechanics of waveguide formation, a theoretical model, describing the effects of various parameters on the refractive index change in ZnO, is introduced which includes the contribution from the molar polarization, molar volume, the spontaneous polarization and the photoelastic effect. It will be valuable and helpful for ZnO device design and application.

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 . 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°.

Table 1. Experiment Parameters for Samples Suffered O⁺ Irradiation

View Table

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 . 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.

 

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.

Download Full Size | PDF

Figure 2 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.

 

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.

Download Full Size | PDF

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 ). Figure 4 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.

 

Fig. 3 The schematic of the planar waveguide fabricaed by O⁺ ion implantation.

Download Full Size | PDF

 

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#.

Download Full Size | PDF

In order to analyze the mechanics of waveguide formation in ZnO crystal, a theoretical model applied for ferroelectric crystal is introduced to explain the properties of ZnO waveguide. Here, four different factors are assumed to be responsible for the refractive index change: spontaneous polarization, the photoelastic effect, molar polarization and molar volume. The ion implantation causes the crystal lattice damage, which would result in the change of these parameters. In a ZnO crystal, the refractive indices n_{ij ,0} suffer a refractive-index change due to spontaneous polarization (P_s), the quadratic electro-optic coefficients (g_ij), the second-order strain tensor (S_kl) and the fourth-order photoelastic tensor, which has the expression [7,8]:

According to the Lorentz–Lorenz equation, the relation among the average refractive index $n={(n_e{n}_o^2)}^{1/3}$, the molar polarization α_M and the molar volume V_M can be given [14]:

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

According to Eq. (5), we can obtain the refractive index profiles of n_e (related to TM-polarized light) and n_o (related to TE-polarized light) for different lattice damage ratios in z-cut ZnO, as seen in Fig. 5 . The calculations clearly demonstrate a same trend between n_TM and n_TE 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).

 

Fig. 5 Refractive indices of n_e and n_o versus the lattice damage in the ZnO crystal. The dashed lines represent the refractive indices of the virgin crystal.

Download Full Size | PDF

By introducing the approximation ${(1+△)}^{-1/2}\approx 1-1/2△$ into Eq. (5), we have:

 

Fig. 6 Extraordinary (a) and ordinary (b) refractive index variable profiles as a function of the lattice damage ratio in z-cut ZnO crystal.

Download Full Size | PDF

As is seen, the extraordinary refractive index change of n_e 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 n_o 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 n_o. 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 Δn_e or Δn_o 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] (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) and 7(b). The dashed lines represent the refractive indices of the virgin ZnO. For both ordinary refractive index n_o and extraordinary index n_e only negative refractive-index changes occurred.

 

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.

Download Full Size | PDF

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.