1. Introduction

GaN-based photonic devices are of great interest for optoelectronic applications in the visible range [1–6]. Freestanding GaN membranes are often required to guarantee a high refractive index contrast between GaN and its cladding layer. Because of the difficulties in sapphire etching, freestanding GaN-based photonic devices are fabricated with limited airgap on GaN-on-sapphire substrate by means of selective photoelectrochemical etching of sacrificial layers [7–9]. On the other hand, GaN-on-silicon material system can offer a flexible way to obtain freestanding GaN membrane by removing silicon substrate. By the utilization of the top-down undercutting technique, Choi et al. reported pivoted GaN microdisks on a GaN/Si platform [10]. Vico Triviño et al. demonstrated the achievement of freestanding GaN photonic crystal nanocavity with embedded InGaN/GaN quantum wells on silicon substrate [11]. They also illustrated fully suspended GaN wire waveguides and photonic crystal membranes on silicon substrate through the intricate process [12]. Sergent et al. developed layer transfer method to realize the freestanding GaN/Al dots on silicon substrate [13]. Although the undercutting technique is an efficient way to produce freestanding GaN-based structures, it is difficult to manufacture GaN membrane with a controllable thickness, especially for the realization of single layer GaN photonic device for short wavelength application.

Furthermore, from the material growth point of view, thick buffer layers are necessary to manage the lattice mismatch and thermal expansion coefficient difference between GaN and silicon. Guided-mode resonance (GMR) is strongly dependent on both wavelength and membrane thickness in the case of surface-normal incidence [14]. Increasing the membrane thickness will seriously degrade GMR [15] and gradually strengthen the optical interferences caused by the multiple reflections at the different medium interfaces [16, 17]. Since the GaN membrane thickness is an essential factor to influence the resonant performances, thin GaN membrane is needed to form GMR grating in the visible range.

To circumvent the challenging issue, we propose here a double-side process to obtain subwavelength GaN grating on freestanding GaN membrane. Subwavelength grating structure is achieved by ion beam etching (IBE) of GaN. The silicon substrate is removed from the backside through deep reactive ion etching (DRIE) and thus, backside thinning is invited to manufacture freestanding membrane by reactive ion etching (RIE). Enhanced surface-normal emission is experimentally demonstrated in photoluminescence (PL) measurement due to the strong coupling between the emitted light and GaN grating.

2. Device fabrication

The proposed subwavelength GaN gratings are implemented on one commercial GaN-on-silicon substrate. Figure 1 illustrates the schematic fabrication process of freestanding GaN gratings. Nanoscale gratings were first defined in PMMA resist using electron beam lithography (steps a-b). The grating patterns were then transferred to GaN film with a grating height of ~130nm by IBE (steps c), which uses an energetic and highly directional ion source to anisotropic etch GaN with high resolution. After removing the residual resist, the processed patterns were protected by thick photoresist. The silicon substrate underneath the grating region was patterned from the backside by photolithography and etched down to buffer layer by DRIE (steps d-e), which made the grating suspend in space. Subsequently, GaN membrane was thinned from the backside by RIE, and freestanding GaN gratings were finally generated by removing the residual photoresist (steps f-g).

 

Fig. 1 Schematic fabrication process of subwavelength GaN membrane grating.

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3. Experimental results and discussion

Subwavelength GaN gratings are freely suspended with air as the low refractive index materials on top and bottom. Figure 2(a) illustrates one micrograph image of subwavelength GaN gratings, which are realized on a freestanding GaN membrane with a diameter of 120μm, and consist of 60-period gratings. The freestanding GaN membrane can sustain the residual stress to support the grating. Actually, backside thinning of GaN is a challenging problem. Deflection and fracture issues are managed by optimizing DRIE of silicon and backside thinning technique. A variety of GaN gratings are achieved with different parameters and shapes, as illustrated in Figs. 2(b)–2(d). The main grating parameters include the refractive index n_GaN of GaN film, grating period Λ, grating thickness t_g, grating width w and total GaN membrane thickness t_m. The filling factor η is defined as the ratio of the grating width with respect to the grating period, i.e., η = w/Λ.

 

Fig. 2 (a) micrograph image of subwavelength GaN grating; (b) circular grating with Λ = 400nm and η = 0.6; (c) circular grating with Λ = 400nm and η = 0.5; (d) linear grating with Λ = 500nm and η = 0.6.

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The PL measurement of the freestanding GaN gratings was performed using a microzone confocal Raman spectroscope (HORIBA Jobin Yvon, LabRam HR 800) equipped with a color charge-coupled device camera. The 325nm He-Cd laser (Kimmon, ik3301R-G) was used as the excitation source with the laser spot size of 3μm. Figure 2(a) shows the normalized room temperature PL spectra. The GaN exhibited distinct PL peak around 364.5nm. Compared to the excitation of GaN on the GaN-on-silicon material, the PL peak for freestanding GaN membrane has a clear red-shift to 367.3nm due to the change in the stress state and a slight enhancement in the PL intensity. A large number of optical modes are excited in GaN membrane. Each mode has a different electromagnetic field distribution and wavelength. The inset of Fig. 3(a) illustrates broad PL spectra observed in the longer wavelength region, which is caused by the excitation of defects. These surface-normal emissions are used to investigate the interaction between the excited light and GaN grating structures [18, 19]. Although these emissions are very weak, they can efficiently couple to the gratings when the phase-matching conditions are satisfied. For the circular grating with Λ = 400nm and η = 0.7, a clear PL peak is observed around 560.9nm in comparison with the PL spectra of freestanding GaN membrane, and the improvement is evident for the PL peak around 367.3nm, as demonstrated in the inset of Fig. 3(b). With the introduction of subwavelength GaN gratings, guided-mode resonances are achieved when the phase-matching conditions are fulfilled. The emitted light couples to the GaN grating to enhance the PL intensity, and the PL peak is thus formed. The PL peak around 560.9nm may be attributed to the coupling between the emitted light and subwavelength GaN gratings. Figure 3(c) shows the PL spectra versus the filling factor, where the PL peak has a blue shift as the filling factor decreases. Figure 3(d) shows the measured contour plots as functions of wavelength and incident angle for freestanding GaN circular grating with Λ = 400nm and η = 0.6. The incident light is coupled to circular GaN grating and guided-mode resonances occur, which may be responsible for the extraordinary PL peak.

 

Fig. 3 (a) PL spectra for GaN membrane; (b) PL spectra for subwavelength GaN grating; (c) PL spectra versus filling factor for circular grating; (d) reflectivity spectra for circular grating with Λ = 400nm and η = 0.6.

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Figure 4(a) illustrates the measured PL spectra for the linear grating with Λ = 500nm. Similar coupling phenomenon is observed. Since the PL spectra are measured without polarization, in other words, the emissions have both a transverse electric (TE) polarization component and a transverse magnetic (TM) polarization component, where TE and TM polarizations are related to incident beams with E-field polarization parallel and perpendicular to the grating lines, respectively. The PL spectra are an intricate mixture of TE and TM polarizations. Decreasing the filling factor η from 0.7 to 0.6 will result in the blue-shifts of the PL peaks from 585.7nm, 554nm, 541.2nm to 575.3nm, 552nm, and 540.5nm, respectively. These results indicate that surface-normal emission can efficiently couple to GMR gratings, the coupling is sensitive to the parameters and shapes of gratings. Single layer GaN grating can form an optical resonator to facilitate surface-normal emission coupling with the integration of GaN-based photon emitters [20]. Figure 4(b) shows the reflectivity spectra versus the filling factor. The incident wave has a TM-polarized, surface-normal incidence. The blue-shifts are clearly observed as the filling factor decreases and the coupling strength is drastically degraded for η = 0.5, which corresponds well to the PL results. Figure 4(c) illustrates the polarization-dependent reflectivity spectra for linear GaN grating with Λ = 500nm and η = 0.7. The strongest resonances occur at 819nm with a reflectivity of 0.876 for TM polarization and at 635nm with a reflectivity of 0.955 for TE polarization. Particularly, there are some distinct resonances for TE and TM polarizations in the range of 500nm~600nm. The average reflectivity is calculated for comparison. These resonances may be responsible for the extraordinary PL peaks shown in Fig. 4(a). Energy will gradually start building up within these resonances to give rise to surface-normal emission coupling when the grating is excited in PL measurement. The excited light can couple to these resonant modes to enhance the PL intensities.

 

Fig. 4 (a) PL spectra versus filling factor for linear grating with Λ = 500nm. (b) reflectivity spectra versus the filling factor; (c) measured reflectivity spectra versus polarization; (d) Calculated reflectivity spectra versus the thickness of GaN membrane.

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In reality, the backside thinning is not uniform, the actual parameters of the membrane vary and different membranes would have different thicknesses. The influence of the membrane thickness on GMR GaN grating is investigated using rigorous coupled-wave analysis (RCWA). For simplicity, the same parameters for membrane and grating are used with the same refractive index n_GaN = 2.45 in numerical simulation, which is different from the actual element. Figure 4(d) shows the calculated thickness-dependent reflectivity for linear GaN grating, where the parameters are η = 0.7, Λ = 500nm, and t_g = 130nm for all three cases. The normal incidence is TM-polarized. Multimode resonances existing in thick GaN membrane grating are superimposed on the top of interference fringes. Since the refractive index changes immediately at the GaN/air interfaces, the light reflected from both the upper and bottom GaN/Air surfaces will interface. Strong reflectivity modulations are observed in the measured reflection spectra, which are attributed to the optical interferences of the multiple reflections at the different interfaces. The reflectivity interference fringes strongly depends the thickness of GaN film. As the thickness of GaN membrane is increased, the number of interference fringes within the wavelength range increases. There is also a corresponding increase in the number of resonances, and the wavelength separation between the resonances decreases. Decreasing GaN membrane thickness leads to a blue-shift in resonance location and an enhancement in the resonance strength. Moreover, a thinner membrane offers a less loss GaN membrane and broadens the reflectivity interference fringe. Obviously, ultrathin GaN membrane is required for the implementation of single layer optical resonator in the visible range. Moreover, freestanding GaN membrane also offers a feasible way to eliminate the interference fringes by depositing an antireflection coating from the backside [21, 22].

4. Conclusions

In conclusion, we have presented the fabrication of subwavelength GaN membrane grating by means of backside thinning technique. Subwavelength GaN grating behaves as an optical resonator and facilitates surface-normal emission coupling, which are sensitive to the parameters and shapes of gratings. Both numerical simulation and reflectivity measurement are in consistent with the PL experimental results. This work opens a promising way to embed GaN-based photon emitter inside subwavelength grating, and form a surface emitting device with a single layer GaN grating.