Fabrication and simulation of 1540-nm transmission by 532-nm excitation in photonic crystal of Er:ZnO film

1 Introduction

Erbium (Er)-doped semiconductors are promising candidates for use in new optoelectronic devices because Er-related 1540 nm emission is within a wavelength range of a minimum loss for silica optical fibers [1], [2]. It has been reported that oxygen co-doping and using wide band gap materials as the host material increased the Er luminescence intensity greatly. Therefore, as a host oxide semiconductor, ZnO has been intensively studied in recent years because it meets the condition of being oxide and has a wide band gap of about 3.3 eV [3], [4], [5], [6]. However, compared to another wide band gap semiconductor GaN, which is currently the dominant material operating in the blue and UV spectral range in optoelectronic applications, luminescence intensity from Er-doped ZnO is still far from being satisfactory. One of the main reasons is attributed to the local structure around Er in ZnO, which is different from RE (rare earth) in III–V semiconductors. So many researches on improving the luminescence efficiency of Er³⁺ ions in ZnO is focused on the studies of luminescence mechanism, such as to encourage the intra-4f transition of Er by modifying the local surrounding in ZnO [7], [8], [9]. To effectively enhance the Er-related 1540-nm emission, another essential consideration is to improve luminescence excitation by means of a smart structure on Er:ZnO film, for example, by combining waveguide with photonic crystal structure to confine the light transmission. Waveguide based on photonic crystal is the most promising structure used to confine light and improve transmission efficiency. Photonic crystals (PCs) are periodic dielectric structures, which possess photonic band gap (PBG). Propagation is forbidden if the frequency of the electromagnetic wave falls in the PBG. PCs have attracted much attention because of their potential applications in various optoelectronic devices and integrated optical circuits [10], [11], [12]. PC waveguide can be implemented on the size of wavelength scale, making it very suitable for the design of integrated optical device. Compared with the traditional waveguide, the PC waveguide has greater design freedom. Its advantages over other conventional waveguides include the miniature-sized photonic circuits and slow light effects [13], [14]. Negligible bending loss waveguide with very small bend radius can be fabricated in the PC waveguide, and it could be expected to generate a wide bandwidth slow light at any wavelength at room temperature. The small volume of PC waveguide also makes it suitable for the integration with existing optical communication components [15], [16].

In the present work, we report a two-dimension (2-D) PC fabricated on Er:ZnO film waveguide. According to our calculation, a photonic band gap (PBG) at wavelength from 1510 to 1570 nm can be achieved. Based on this PC, a minimized Er:ZnO light emitter or transmission device can be expected. By selecting appropriate parameters and letting photoluminescence (PL) frequency of Er just fall within PBG of this PC, Er-related 1540 nm emission (excited by other wavelengths, e.g. 532 nm) can propagate along the only path confined by the introduced line defects in this PC.

2 Design

With the aim of restricting the propagation of Er-related 1540 nm (excited by other wavelengths, e.g. 532 nm) along a certain direction in the PC structure, a planar waveguide was first deposited on a sapphire substrate by radio frequency (RF) magnetron sputtering technology [17], [18]. The film thickness of about 600 nm could restrict the single mode at 1540 nm in the waveguide and guarantee that the air holes in the film exhibit a nearly cylindrical shape by etching. Then, a 2-D PC structure with a line defect was fabricated by focused ion beam (FIB) etching to restrict the 1540 nm propagation. A hexagonal lattice PC composed of air holes with lattice constant a=597 nm and hole radius r=0.25a is constructed in the planar waveguide. The values of lattice constant and hole radius are determined according to the TE band structure, in which 1540 nm will fall in the PBG. The width of the defect is determined by actual application, in this paper, three rows of air holes are left unpunched to introduce line defects in the above PC. As the transmission of 1540 nm light is prohibited in the PC structure, it could only propagate along the line defect path.

3 Fabrication and characterization of the film

Er:ZnO thin films are deposited on Al₂O₃ crystal substrates (0001) using RF magnetron sputtering technique. A ZnO target was prepared with co-doped 0.6 wt% Er₂O₃. The deposition was carried out in a vacuum chamber evacuated to a pressure of 10⁻⁴ Pa and backfilled with an oxygen and argon mixed gas atmosphere (Ar/O₂=2/5). The chamber pressure was maintained at a constant value of 0.5 Pa. Al₂O₃ substrates were rotated at a speed of 20 rpm during deposition, and substrate temperature was kept at 400°C. After deposition, the film thickness was measured to be about 600 nm. Some samples were post-annealed at 800°C in air ambient for half hour. The properties of as-deposited and annealed thin films were characterized by X-ray diffraction (XRD), atomic force microscope (AFM), prism coupling device, and PL.

AFM is used to examine the surface morphology and roughness of the films. Figure 1A and B shows the 2-D and 3-D AFM images for the samples of as-deposited and post-annealed at 800°C, respectively. The AFM image shows a structure with many small grains in the as-deposited samples, and the surface roughness (Ra) is about 2.88 nm. After the annealing treatment at 800°C, better surface smoothness with Ra ~2 nm is obtained. The columnar structure in an annealed sample is associated with the (002) ZnO textured growth. The XRD patterns of the films grown at 400°C and post-annealed at 800°C are compared in Figure 2. Compared to previous works on growth of ZnO [19], [20], all the samples show only one peak at about 34° corresponding to the diffraction from the (002) plane of ZnO, indicating a preferred c-axis orientation of wurtzite ZnO structure. The crystalline property of ZnO is improved after the annealing treatment, and the full width at half maximum (FWHM) of the diffraction peak shrinks from 0.81° for the as-deposited film to 0.33° for the annealed samples. During annealing, poly-crystalline ZnO grows as long hexagonal rods along the c-axis, resulting in columnar grains that are perpendicular to the substrate. The diffraction from these (002) preferential orientation grains exhibits a narrow peak shape and high intensity.

Figure 1:

(A) 2-D and 3-D AFM images of the film grown at 400°C. (B) 2-D and 3-D AFM images of the film post-annealed at 800°C.

Figure 2:

XRD patterns of the as-deposited and post-annealed films.

Prism coupling measurement is carried out to investigate the waveguide characteristic of the films. Reflectivity spectra are excited at a wavelength of transverse electric wave (TE)-polarized 1540 nm. When photons tunnel across the air gap into the film and a guided mode is stimulated, a sharp drop (corresponding to a dark mode) will appear in the reflectivity spectra. Figure 3A shows the results of prism coupling in the as-deposited film. As a reference, the results of samples at TE-polarized 633 nm excitation are also given. Under 1540 nm excitation, only one sharp decline is observed in the as-deposited film. It indicates that the Er:ZnO film could serve as a single-mode planar waveguide at this wavelength. For the post-annealed film in Figure 3B, film retains a single-mode waveguide under 1540 nm, but the waveguide property becomes worse. This may relate to the light scattering and leakage loss in a thin film. Because annealing treatment promotes the restructure of film, more interfaces are introduced when columnar grains grow. Meanwhile, as the ZnO film becomes denser, the thickness of the film will be thinner.

Figure 3:

Relative intensity of the TE-polarized light at 633 nm and 1540 nm for (A) as-deposited film and (B) post-annealed film.

The luminescence properties of Er:ZnO films are investigated by PL spectrum at room temperature. A semiconductor laser operating at 532 nm is used as a pump source to excite the Er:ZnO films, and the pump power is 600 mW. A lock-in amplifier and a thermo-electrically cooled InGaAs detector are used to work in the infrared region to detect the PL signal from Er:ZnO films. The PL spectra from both samples are shown in Figure 4. A peak centered at 1540 nm could be observed from both films, which originates from the transition between the first excited state (⁴I_13/2) and the ground state (⁴I_15/2) of the Er³⁺ ions. Compared with the PL from the as-deposited film, the PL intensity of a post-annealed film decreases obviously. This anti-correlation of PL intensity and annealing temperature has been observed and reported in previous researches [21], [22], [23]. It could be attributed to the modification of the local symmetry and structure around Er³⁺. It has been found that ZnO crystal with a lower symmetry is more suitable for the Er intra-4f transitions. Another reason may come from defect-related quenching effects. Deep levels can be introduced or formed in ZnO during annealing at high temperature. These defects may extract pump energy and quenching Er-related PL. Prism coupling measurement has proved that when the as-deposited film is stimulated by a 532 nm laser, it could serve as a single-mode waveguide for light at 1540 nm. As the as-deposited film exhibits a better PL and waveguide property, Er:ZnO PC is fabricated in this film.

Figure 4:

PL spectra of the as-deposited and post-annealed films.

4 Fabrication and evaluation of the photonic crystal

The focused ion beam (FIB) etching is used to form a PC structure in the Er:ZnO film. Etching is implemented by focusing gallium ions onto the sample with an acceleration voltage of 30 kV. The air holes are etched by a beam current of 700 pA. A hexagonal lattice PC composed of air holes with lattice constant a=597 nm and hole radius r=0.25a is constructed in the planar waveguide, with three rows of air holes unpunched to introduce line defects in the above PC. The schematic diagram and scanning electron microscope (SEM) image of the fabricated PC are shown in Figure 5. The cross-section of etched holes shows a conical type, which may arise from the redeposition effect during FIB milling and the reflection of Ga ions from the sidewall. However, the part of air hole in the Er:ZnO film exhibits a nearly cylindrical shape.

Figure 5:

(A) Schematic diagram of the PC structure on the film, a core Er:ZnO layer (green) is deposited on Al₂O₃ substrate (blue), and a is the lattice constant. (B) The SEM image of the PC with hexagonal lattice of air holes fabricated by FIB etching. The lattice constant and diameter of holes are set to be 597 nm and 149 nm, respectively. (C) The cross-section of the air holes.

In order to evaluate the propagation of 1540 nm, simulation of the PC band structure and the steady-state field distribution are carried out based on the plane-wave expansion technique and the finite-difference time-domain (FDTD) method, respectively [24], [25], [26], [27]. A 2-D hexagonal array was laid out in the x-z plane, and the band structure and steady-state field distributions were calculated by setting the polarization to “TE”. In the simulation of the 3-D PC, the lattice of the rods was set with finite vertical extent in the third dimension by a multilayer structure. The band structure could be generated after setting the final simulation parameters according to the PC design. The modes can be classified as either even or odd in the 3-D structure; they parallel the meaning of the TE and TM modes that are found in 1-D and 2-D structures.

By approximating the air hole to be a cylindrical structure, the TE band structure is simulated in Figure 6A. According to the numerical relation as “f=a/λ” (f, a, and λ represent frequency, lattice constant, and wavelength, respectively), it can be found that a PBG ranging from wavelength 1510 nm to 1570 nm, with 1540 nm just falling in the PBG, could be obtained in this PC. It represents that if 1540 nm luminescence is excited in the Er:ZnO film, it cannot propagate in this PC. However, this prohibition can be removed by introducing a line defect, which is shown in Figure 5A. When TE-polarized Gaussian beam at 1540 nm is launched at this PC, the steady-state field distribution is shown in Figure 6B. It can be found that the propagation of 1540 nm is well restricted within the line defect path in the PC structure. This result means that when the Er:ZnO film is stimulated by a short wavelength source, part of the Er-related 1540 nm luminescence and directional transmission can be achieved at the same time in this Er:ZnO PC structure.

Figure 6:

(A) TE band structure of the 2-D Er:ZnO PC, a PBG (red shadow region) appears ranging from 1510 nm to 1570 nm. (B) The steady-state field distribution for 1540 nm. (C) Band structure of the Er:ZnO PC.

The results of Figure 6A and B are based on an assumption that the PC structure is an ideal 2-D PC, which is arranged periodically in a 2-D space and is infinite in the third dimension. However, in actual situation, waveguide slabs with a periodic set of holes or a lattice of rods have finite thickness. If Er:ZnO PC is taken as a 3-D problem, the index contrast between film and substrate should be taken into consideration. In this case, the calculated result based on the plane wave expansion method is shown in Figure 6C. It can be found that a complete PBG does not exist in such structure, neither even nor odd modes. Significant light attenuation is caused due to the small index contrast between film and substrate. To circumvent this problem, an air bridge structure can be constituted in this film component, a cross-sectional schematic illustration of such air-bridge PC is shown in Figure 7A. A thin SiO₂ film can be first deposited on the Al₂O₃ substrate before the deposition of the Er:ZnO film. Then, an air bridge can be obtained in this sandwich structure by removing SiO₂ using chemical selective etching. The final PC structure can be achieved in this suspended Er:ZnO film by FIB punching. The band structures are shown in Figure 7B and C. It can be seen that a complete PBG exists in such PC for both even and odd modes. If the parameters of photonic crystals are properly designed to let 1540 nm fall within PBG, the light transmission will be completely confined in the line defect of this PC. This part of work will be carried out in the future.

Figure 7:

(A) Cross-section of the air bridge PC in Er:ZnO film deposited on SiO₂-Al₂O₃ substrate, which can be obtained by removing SiO₂ using chemical selective etching in the sandwich structure. Band structures of the air bridge Er:ZnO PC. (B) Even mode. (C) Odd mode.

5 Conclusions

In conclusion, a PC structure in an Er:ZnO thin film is fabricated by RF magnetron sputtering technology and FIB etching. It is found that the film grown at 400°C has good film quality and supports single waveguide transmission mode at 1540 nm, which is applicable for making a PC structure. Simulation results show that a PBG at a wavelength ranging from 1510 nm to 1570 nm can be obtained in the Er:ZnO PC. By introducing a line defect in this PC, the propagation of 1540 nm could be well confined by the line defect path. Because of a simple geometric structure and a clear operating principle, this kind of Er:ZnO PC is expected to be applicable in the future optical communications and optical integration.