Zinc oxide (ZnO) as an extremely bright emitter is an attractive material for photonic devices. However, devices made of epitaxially grown ZnO are difficult to fabricate due to the lack of selective etching processes. Here, we demonstrate that by a low-temperature growth process on pre-patterned silicon dioxide (SiO2) microdisks (MDs) high quality ZnO resonators are created. The devices exhibit whispering gallery modes (WGMs) over the blue-green part of the visible spectrum with quality factors exceeding Q = 3500, which are among the highest values reported in this material system so far. By deposition of SiO2 capping layers we find an enhanced coupling of the spontaneous emission from the active medium into the MDs, observed by sharp WGMs up to a radial quantum number of N = 3.
© 2013 Optical Society of America
Photonic resonators play an important role as building blocks in integrated photonic circuits and in the field of quantum information technology. In comparison to resonators based on Fabry-Pérot interferences, circularly shaped microdisks (MDs) exhibit low losses, allowing to trap light on much longer timescales, resulting in higher quality factors, lower lasing thresholds and less demand of gain material. Therefore, MDs are ideal not only for fundamental research, e.g., in nonlinear optics, but also for micro-laser devices  with small spectral bandwidths and high efficiency. Furthermore, their usability as sensitive chemical detectors has been demonstrated . These reports range from single-molecule analysis  to medical and biological applications like virus detection . For a variety of applications in the visible (VIS) spectral range transparent materials with large band gaps are essential. Zinc oxide (ZnO) is a versatile material for electronic, optoelectronic and photonic devices in the ultraviolet (UV) spectral range due to its high emission efficiency [5, 6]. Intrinsic point defects in ZnO give rise to luminescence over a broad range in the VIS spectrum. This enables tunable resonant emission in the VIS spectral region with a single material system. Several reports have demonstrated “bottom-up” grown disk-like hexagonal flakes [7, 8], as well as other resonant structures like needles  or self-assembled hexagonal microresonators [10, 11]. Recently, other examples for photonic devices based on self-organized ZnO devices have been reported, including lasing in ZnO nanosheets  and polariton confinement in ZnO nanocylinders .
However, up to date no fully suspended photonic devices based on single crystalline heterostructures have been demonstrated in the ZnO material system. This is due to the lack of selective dry or wet etching processes that are difficult to achieve in this material system [14, 15]. Therefore, Liu and associates  proposed the fabrication of ZnO MDs on pre-patterned silicon substrates by metal-organic vapor phase deposition (MOCVD) and demonstrated UV lasing in this system. Nevertheless, silicon (Si) and silica (SiO2) are due to the different lattice constants and thermal expansion coefficients not an ideal choice as a substrate for epitaxial growth of ZnO. Instead of two-dimensional growth, at lower growth temperatures typically the formation of polycrystalline films with small grains is observed. In spite of these fundamental challenges, silicon remains an interesting substrate for photonic ZnO-based devices as the Si planar technology is highly developed and the highest quality factors for photonic resonators so far have been achieved in the SiO2/Si system . Several groups report on the successful growth directly on Si templates  or by using buffer layers [19, 20]. To overcome the problems with silicon substrates for ZnO growth, we have recently developed a low-temperature molecular beam epitaxy (MBE) growth process that leads to a very smooth, closed but polycrystalline ZnO layer of granular material on Si(111) [21, 22] and SiO2 . The luminescence of these layers is dominated by an emission over a broad range of the visible spectrum centered at around 2.95 eV. Thus, devices with sharp resonances over the visible spectral range can be fabricated from LT-ZnO.
Here, we report on the structure and optical resonances of ZnO/SiO2-based photonic devices. The structural and optical properties of low-temperature (LT) grown ZnO/SiO2 MDs and the effects of a capping layer deposition on the devices are analyzed. The optical properties of the samples are investigated by confocal micro-photoluminescence (µPL), the structural properties by atomic force microscopy (AFM), scanning electron microscopy (SEM) and X-ray diffraction (XRD).
Commercial Si(111) substrates are used as a starting material after a chemical cleaning procedure. In the first step, a 90 nm thick SiO2 layer is grown using thermal oxidation in a tube furnace at 1050° C under dry oxygen (O2) atmosphere. Using electron beam lithography (EBL), circles with diameters in the µm range are patterned into a positive resist on top of the oxide layer. Thereafter, 50 nm of aluminum (Al) are evaporated as a hard-mask layer. The EBL-defined patterns are transferred into the Al layer using a lift-off process. Subsequently, the structures are etched into the Si/SiO2 material by anisotropic etching in a dry reactive plasma. First the SiO2 layer is removed completely in a CHF3 based plasma. Afterwards, the structures are extended ~500 nm deep into the Si layer by a C4F8/SF6 process. SiO2 MDs on Si posts were then formed in a 80° C hot 1:1:4 C3H8O:KOH:H2O solution. As the KOH etching of silicon in the  equivalent directions is almost 1000 times faster than in the  direction, the post of the disk remains around 500 nm high. The finished SiO2 MDs are cleaned in a H2SO4:H2O2 solution at 80° C for 20 min followed by a rinse of water before the ZnO overgrowth. The ZnO films are grown in a vertical plasma-assisted MBE system, where oxygen radicals (O) are supplied via a radio-frequency plasma source (ν = 13.56 MHz, P = 300 W), while zinc (Zn) is provided by thermal evaporation using a double-zone effusion cell with a tantalum inset. Before growth, a pure oxygen plasma is used for additional cleaning and oxygen termination of the surface. At a substrate temperature of 150° C, ~55 nm ZnO were grown on the pre-patterned samples at an oxygen flow rate of 0.5 sccm and a Zn beam equivalent pressure of 1.4 × 10−7 Torr. Some of the samples have been capped by an additional SiO2 layer in order to study the coupling of LT-ZnO into a SiO2 membrane. This capping layer was deposited in a plasma enhanced chemical vapor deposition (PECVD) system using N2O and SiH4 at 300° C.
For the micro-photoluminescence experiments, single MDs have been excited using the 325 nm laser line of a continuous wave HeCd laser. The numerical aperture of the microscope objective was NA = 0.55, yielding a diffraction-limited focus diameter of d ≈0.7 μm with an excitation power density of 6.5 W/cm2. A spectral resolution of 0.09 nm is achieved in a Czerny-Turner monochromator with a focal length of 500 mm. The dispersed light was detected with a UV-enhanced charge-coupled device (CCD).
3. Results and discussion
The structural properties of the ~55 nm thin, LT grown ZnO films on thermal SiO2 are analyzed using AFM to obtain the surface morphology, while the crystal quality is investigated by cross-sectional SEM and XRD measurements.
As shown in Fig. 1(a), the surface of the ZnO film is smooth over a wide range of 10 × 10 μm2 revealing a RMS roughness of 0.5 nm. However, the ZnO film is polycrystalline and consists of many small grains as clearly visible in the cross-section SEM image in Fig. 1(b). Anyway, only the dominant ZnO(0002) XRD reflex is observed in a symmetric full range ω/2θ scan at around 34.4° in Fig. 1(c), while the FWHM of the corresponding rocking curve is 5.8° and therefore very broad. This indicates that the grains are highly oriented along the c-axis but slightly tilted against each other. Furthermore, an in-plane rotation of the single grains is observed by transmission electron microscopy (TEM, data not shown here). On the pre-patterned SiO2 MDs these films grow with almost vertical edges and a smooth and flat surface shape, although the granular structure is visible at the edges. Beneath the disks no ZnO is deposited due to the highly directional material flux, neither at the shaded Si(111) surface, nor at the bottom of the disk [see Fig. 2(a)]. After deposition of an additional SiO2 layer using PECVD, the surface exhibits a similar roughness than the LT-ZnO film while the edges appear smoother and slightly more tilted as visible in the SEM image in Fig. 2(b).
The SiO2 capping layer strongly affects the optical properties of the ZnO/SiO2 MDs, which are investigated by micro-photoluminescence (μPL) measurements. Figure 3 shows the room temperature μPL spectra of a 4 μm MD before (blue) and after the capping with 30 nm SiO2 (red). Both spectra are characterized by a broad emission in the visible spectral range centered at around 2.95 eV.
The observed blue-green emission from the LT-ZnO layer is very different from the well-known photoluminescence of single crystalline ZnO. The energy of the maximum intensity is clearly below the near band edge transition of 3.37 eV  known from ZnO, which is also present in the spectra as a weak shoulder. It also does not fit to the typical yellow-green emission in ZnO that is attributed to intrinsic vacancies at around 2.5 eV [25, 26]. Other intrinsic defects like interstitial Zni or Oi and antisite defects ZnO or OZn, according to recent theoretical calculations [27, 28], do not have transition energies that could explain the blue-violet emission, too. As described above, the LT-grown ZnO film is polycrystalline with very small grain diameters of just a few nanometers. Therefore, we assign the recombination band at 2.95 eV to electron interface traps as suggested by Cordaro et al. . These recombinations are located at the grain boundaries, which represent a double Schottky-barrier and cause a depletion zone of some nanometers on both sides.
Figure 3 also shows that the additional capping with SiO2 leads to a strongly enhanced emission from the disk. At the same time, the resonance peaks of the whispering gallery modes (WGMs) become much more prominent in comparison to the spontaneous emission background. The increased overall emission intensity can be attributed to the lowering of the refractive index contrast by the PECVD SiO2 index of nPECVD,SiO2 = 1.46 from around nZnO ≈1.9 of the ZnO layer to nAir ≈1.0 for the outside environment. For the low-index SiO2 capping layer devices, the escape cone for light extraction that is limited by Brewster’s angle is increased (θExtraction,SiO2 = 68.8°, θExtraction,ZnO = 55.6°). The enhancement of the resonances is caused by the light emission from the center of the sandwich structure and the associated improved guiding and lower scattering losses at the interfaces. Note, the active region is embedded in-between the two SiO2 layers, leading to an improved spatial overlap of the field components with the ZnO active medium in the center. Thus, the coupling to the resonator modes is significantly improved, as shown later by numerical simulations. Due to the fact that the ZnO layer is sandwiched between the two non-conducting SiO2 layers, TE-like polarized WGMs are expected to dominate over TM-like modes. However, as the system is not fully 2D confined, the presence of TM-like polarized modes cannot be fully excluded.
In Fig. 4 the WGMs are further investigated by high resolution μPL measurements. A clear substructure of the different modes is observed illustrated by four different colors and corresponding spectral position markers underneath. The most prominent modes shown in red belong to the radial quantum number of N = 1 and are distributed over the entire investigated spectral region. These are followed by less intense WGMs with higher radial mode numbers N = 2 (green) and N = 3 (blue), localized closer to the center of the disk, where the light is absorbed by the remaining Si post. WGMs with the highest intensity are located on the low energy side of the emission curve due to the increased self-absorption at higher energies. Despite that, a fourth type of modes (orange) with very weak intensities is observed only at the high energy side, whose origin remains unclear. It should be noted that for N = 4 modes the radiative loss is higher than for modes of lower radial order, thus leading to lower quality factors for theses modes. The mode spacing versus mode energy is shown in the lower part of Fig. 4 for the four mode types. With increasing energy, the spacing between the N = 1 modes (red triangles) strongly decreases from around 54 meV at 2.4 eV to 39 meV at 3.0 eV. A similar behavior is also observed for the three other WGM types, indicating the dispersion of the underlying materials. As the refractive index of SiO2 is almost constant in this spectral region, the LT-ZnO layer is assumed to be the origin of the observed dispersion. Close to the main electronic resonance of the LT-ZnO at ELT-ZnO≈2.95 eV both the real and the imaginary part of the refractive index are increasing due to the electronic polarization which interacts with the optical fields.
The strong dispersion is also observed when analyzing the mode spacing at different energies as a function of the MD diameter. In Fig. 5(a) the size dependence of the mode spacing is shown for different disk diameters between 4.0 and 5.0 μm. The mode spacing of a microdisk resonator can be approximated using the model of a dielectric cylinder with perfectly conducting walls. In this approximation, the confined field components inside the cylinder are described by Bessel functions, and one obtains for the mode spacing ΔE = hc/2πnR, where R is the radius of the dielectric cylinder and n is the refractive index. Thus, one expects a decreasing mode spacing with an increasing effective refractive index. This behavior is clearly visible in Fig. 5(a), where for all investigated microdisk diameters the mode spacing decreases towards higher energies when approaching the resonance energy E = 2.95 eV. Following the same approximation, a reciprocal relation between the mode spacing and the microdisk diameter/radius is expected. This relation is also found in the experimentally observed mode spacings, which are fitted in Fig. 5(a) using a 1/d-relation. While the real part of the refractive index is related to the mode spacing, the imaginary part – the extinction coefficient – is an important measure for losses due to re-absorption for a given optical mode. Thus, high-quality modes with low extinction/absorption are expected only on the low-energy wing of the PL peak, where the self-absorption due to electronic excitations is weak. Figure 5(b) shows the FWHM of a WGM fitted using a Lorentzian line shape at around 2.15 eV revealing a quality factor of Q = 3530, which is among the highest values reported in the ZnO material system so far. It should be noted that the quality factor after capping is mostly limited due to the absorption of the LT-ZnO band edge, which is also visible from the lower quality factors observed on the high-energy side of the emission.
The effect of the SiO2 capping layer on the mode confinement and the spectral response of the microdisks has been simulated using the finite-differences time domain (FDTD) method. The simulated devices consisted of a bottom SiO2 layer (nSiO2 = 1.46) with a thickness of 90 nm, a LT-ZnO layer (nZnO = 1.9) with a thickness of 50 nm, and – for the capped devices – a top layer of SiO2 with a thickness of 50 nm. The devices had a diameter of 5 µm. As boundary conditions, perfectly matched layers (PML) were employed in all three Cartesian directions, with 2 µm free space (vacuum) added between the edges of the device and the PML boundaries. For both types of device, the simulation was carried out in two steps. First, an ultrashort spectrally broad pulse (fmin = 666.2 THz, fmax = 856.5 THz corresponding to λmax = 450 nm and λmin = 350 nm) was excited inside the device using a current source oriented in the device plane (xz-plane), so that TE-like modes were excited with Hy ≠ 0. The field components were probed inside the device far away from the excitation spot. The spectral response was obtained using a discrete Fourier transform from the time-dependent field components. In the next step, a distinct mode was selected and excited using a narrow excitation (Δf = 0.6 THz). For this mode, the full spatial distribution of the field components was obtained from the time-domain data.
The results are shown in Fig. 6. The |Hy(E)| spectra show the results of the broad pulse excitation simulation for the uncapped and the capped device. Moreover, the Hy-fields are plotted for the xz-plane through the center of the LT-ZnO layer and for a plane parallel to the y-axis for a selected mode.
For both types of devices a series of equidistant sharp resonances is found in the |Hy(E)| spectra. The peaks are caused by the WGMs localized in the periphery of the devices. In the simulations the resonances with a radial quantum number N = 1 dominate the spectra due to the position of the excitation source in the simulation, which was located close to the edge of the device so that these modes were preferably excited. With regard to the mode spacing, small differences between the capped device (ΔE = 64.1 meV) and the uncapped device (ΔE = 69.3 meV) are found. These can be explained with the better overlap of the field distribution with high-index material for the capped device, suggesting an improved confinement. This is in agreement with the simple approximation used above, from which the relation ΔE ~1/neff can be derived. The analysis of the field distribution supports these results. In both cases, the field maximum is located inside the LT-ZnO film. For the uncapped device, however, a large fraction of the field is not localized and thus subject to scattering and radiative loss. The fact that the capped device provides a much better confinement is also visible in the broad pulse computations, where an identical pulse has been used for both devices. In the case of the uncapped device, the field energy is found to decay with a rate of ~1 dB/ps (Q = 10 329), while the capped device only exhibits an energy decay rate of 0.069 dB/ps (Q = 12 800). This demonstrates that the confinement is significantly improved by the capping layer.
Smooth, highly c-oriented, polycrystalline ZnO layers were grown on pre-patterned thermal SiO2 MDs at low substrate temperatures using MBE. The devices show strongly confined whispering gallery modes (WGM) over the blue-green part of the visible spectrum. It has been shown that the optical confinement could be significantly improved by the deposition of a SiO2 capping layer subsequent to the MBE overgrowth. Numerical simulations were carried out that confirmed that the confinement is significantly improved in the capped devices. High quality factors up to Q = 3530 were measured on the low energy wing of the spontaneous emission, exceeding the highest values reported in the ZnO material system so far for “top-down” fabricated photonic devices.
This work was supported by the BMBF via grant no. 03X5509 and the Deutsche Forschungsgemeinschaft (DFG) via the Research Training Group GRK 1464 “Micro- and Nanostructures for Optoelectronics and Photonics”.
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