1. Introduction

III-nitride materials have become important as efficient light sources for generating visible light, as indicated by their pervasive presence in solid state lighting [2,3]. These materials are also becoming increasingly important for coherent [4] and quantum light sources [5–7] operating at room temperature. Recent demonstration of topological lasing [8] and unidirectional single photon emission [9] in III-V systems that achieve high slope efficiency while operating in single mode also suggests such possibilities at visible and ultra-violet wavelength using III-nitrides. In such light sources, high emission rates and control of emission directionality are extremely desirable. Increased emission rates reduce lasing thresholds (and enhances photon counts for quantum light sources) while control of emission directionality improves light collection efficiency.

Photonic crystals (PhC) offer both of those desired properties and have been extensively used to modify light emission. For instance, PhCs have been broadly demonstrated in III-V semiconductor systems as lasers and single photon emitters. In III-nitride material systems there have also been demonstrations of incoherent light emission and lasing in PhC geometries featuring nanowire arrays [10–16]. Complementary structures fabricated from III-nitrides, namely two-dimensional hole array PhCs [17–27], have also been demonstrated. Compared to nanowire array PhCs, hole array PhCs are more suitable for making electrical contacts, thus lending themselves more easily to electrical injection. Furthermore, hole array PhCs can enable excellent light confinement within a thin active layer through optical isolation within a high refractive index substrate. Optical isolation can be achieved by having a low index cladding layer separating the substrate from the active layer or by completely removing the cladding to create an active layer membrane. Such optical isolation substantially enhances quality factors (Q-factors) and Purcell enhancement, thereby maximizing the benefits of photonic nanostructuring. PhC membranes have been extensively demonstrated in photonic devices made from narrower gap infrared III-V materials like GaAs and InP [28–35]. In such material systems, the active layers have high refractive index (n ~3.4) enabling excellent light confinement. Furthermore, sacrificial layers like Al_xGa_1-xAs can be grown to desired thickness which can be subsequently oxidized to a low refractive index Al_xO_y or etched away entirely, thereby providing isolation from the underlying substrate. Alternatively, easier substrate removal (e.g. InP) with wet etching enables flip-chip bonding to low-index substrates.

Unlike infrared III-V materials, achieving optical isolation in III-nitride hole array PhCs remains challenging. III-nitrides typically possess a lower refractive index (n ~2.4) resulting in reduced light confinement and thereby requiring larger separation from an underlying substrate for isolation. Moreover, a sacrificial material analogous to Al_xGa_1-xAs that can be grown to sufficient thickness and subsequently either etched away (or chemically modified) is also unavailable for the III-nitrides. Due to these challenges, a few different approaches have been explored to address the issue of optical isolation. Such approaches include growing III-nitride material on Si substrates coupled with a substrate dry etch [36], focused ion milling [37], or flip-chip bonding techniques. However, all such approaches pose different challenges and have their own limitations.

Another approach that has been studied for achieving optical isolation of active regions in III-nitrides is the photoelectrochemical etching (PEC) technique [18–20]. PEC etching is a bandgap selective wet-etch [1,38–40], wherein absorption of light with energy above the material electronic bandgap generates carriers aiding in the etch process. If a sacrificial material of smaller bandgap absorbs light of lower energies, it can be preferentially etched away while preserving any structures with larger bandgap. In previous approaches [18–20,40], optical stimulation was provided by high powered Xe lamps using unintentionally doped GaN film as a filter to etch sacrificial In_xGa_1-x N superlattice layers with low indium content (x~8%). However, a GaN filter still transmits some amount of higher energy light causing a non-trivial amount of etching within any higher bandgap materials.

In this work we demonstrate laser-assisted PEC etching of a sacrificial multi-quantum well (MQW) layer containing In_0.16Ga_0.84N for a PhC cavity structure using laser illumination. The In content in the sacrificial MQW InGaN layer used for PEC etching is substantially higher than previously reported. Moreover, we utilize a tunable picosecond (ps) laser source for illumination as reported in our previous works [1,38,39], instead of a filtered broadband source. Our tunable laser source allows for substantially better control of the PEC etch process due to the narrow linewidth of the laser whereby there is negligible illumination at higher energies. This fact helps minimize unintentional etching of higher bandgap material as described previously [40]. Use of a high intensity tunable laser also enables incorporation of higher In concentration in the sacrificial In_xGa_1-xN layers which consequently permits higher In concentration in the active QW regions. The higher active layer In content enables PhC devices with longer emission wavelengths.

2. Nanofabrication

Here, the epitaxial structure is grown using metal-organic chemical vapor deposition (MOCVD) (Fig. 1). The wafer consists of a nominally 4.7 μm thick n-GaN buffer layer grown on a sapphire template. The buffer layer growth is followed by a 150nm thick In_0.03Ga_0.97N underlayer and proceeds with a fifteen pair MQW structure, featuring 2.7 nm thick In_0.16Ga_0.84N QW layers and 8 nm thick n-GaN barriers, resulting in a total thickness of approximately 160nm. A MQW structure was chosen for the sacrificial region instead of a thick InGaN layer in order to enable higher In content while retaining good epitaxial morphology for the single quantum well (SQW) active layer. Peak emission associated with the MQW is centered near a wavelength of 460nm. The MQW growth is followed with a 65nm thick n-GaN layer. Next, a single 2.5 nm thick In_0.10Ga_0.90N QW layer is grown andcapped with 65 nm of n-GaN. Peak emission associated with the single quantum well (SQW) is centered near a wavelength of 410 nm. In future work, the SQW can be PEC etched to create a single quantum dot. With an appropriately designed topological PhC [41–43] in-plane unidirectional single photon emission is also possible [9].

 

Fig. 1 Schematic showing the MOCVD epitaxial growth structure. The inset shows the single quantum well (SQW) active layer and the multiple quantum well (MQW) sacrificial layer.

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To fabricate the photonic crystal device (Fig. 2(a)-2(e)) we begin with a plasma-enhanced deposition of SiO₂ to be used as a hard mask. Next, we spin polymethylmethacrylate (PMMA) resist onto our sample and pattern a series of holes using electron beam lithography (EBL). Following development, a F₃^- based reacive ion etch (RIE) transfers the pattern from the PMMA to the underlying SiO₂. Next, an inductively coupled plasma (ICP) Cl₂/BCl₃-based etch transfers the photonic crystal pattern to the III-nitride sample. The dry etch step is followed by a laser-assisted PEC wet etch. Figure 2(f) shows a schematic of the PEC etch setup used in this step. During the PEC etch process, the III-nitride sample is partially submerged in a solution of 0.2 M H₂SO₄. An In contact is bonded to the III-nitride sample while a Pt electrode is suspended in the electrolyte solution. A tunable, frequency doubled ps Ti:Sapphire laser with a relatively narrow-bandwidth (~1 nm) is used as the photoexcitation source. During the PEC wet-etch, H₂SO₄ accesses the MQW sacrificial region through holes in the patterned substrate (Fig. 2(e)), and then proceeds to etch laterally through the MQW layer. Since the PEC wet-etch is a bandgap selective etching mechanism, the laser wavelength was set at 450nm. The resulting laser energy is above the absorption edge of the In_0.16Ga_0.84N layer while still below that of the SQW (In_0.10Ga_0.90N) layer. Further, the narrow linewidth of the laser enables high PEC etch selectivity between the MQW and the SQW regions. Laser light absorption results in the photoexcitation of carriers needed for the wet chemical etching. A voltage is applied across the In contact and Pt electrode to attract photoexcited carriers towards the surface of the sample where holes promote surface oxidation required for etching.

 

Fig. 2 Device fabrication: (a) PECVD of SiO₂ (b) EBL patterning and development (c) Flourine based dry etch (d) Chlorine based dry etch and (e) PEC etching. Light green indicates GaN, dark green indicates SiO₂, blue indicates PMMA, a single black line represents a SQW structure and a series of black lines represents a MQW structure. (f) Schematic of the QSC-PEC etching setup. A tunable picosecond laser (blue) excites carriers in a III-nitride sample (green) that is partially submerged in an electrolyte solution (clear).(figure adapted from Fig. 1 of Ref [1])

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The PhC cavity pattern consists of a modified L1 cavity design (Fig. 3(a)) with lattice constants varying between a = 130nm-160nm. The cavity was designed using finite-difference time-domain method (Lumerical FDTD). The parameters associated with this cavity are r/a = 0.3 and h/a = 0.93, where r refers to the radius of the holes, h is the height of the slab and a refers to the lattice constant of the hexagonal lattice. The six holes surrounding the center defect are smaller (r/a = 0.25). The four holes along ‘y’ direction were shifted outward from their nominal lattice position, Δy = 0.15a while the two holes along x where not shifted (Δx = 0). This design exhibited a reasonably high Q factor (~3000) while providing a high electric field intensity distribution in the defect region (Fig. 3(b)) at a frequency of ~0.36 a/λ. Figure 3(c) shows an SEM image of structure fabricated in III-nitride using EBL pattering approach described in Fig. 2.

 

Fig. 3 (a) Schematic illustration of the shifted-hole L1 PhC cavity indicating vertical shifts (Δy) of top and bottom holes of the cavity and horizontal shifts (Δx) of the two side holes. (b) Simulated E-filed intensity profile of the cavity mode. (c) SEM of a fabricated device. The black scale bar represents a physical length of 500 nm.

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3. Optical characterization

Fabricated devices were optically characterized at room-temperature using an ultraviolet micro-photoluminescence (µ-PL) setup. Our excitation source was a quadrupled Nd:YAGlaser with a peak emission wavelength of 266 nm, a 10 kHz repetition rate and pulse lengths of ~500 ps. The laser’s peak power density was adjusted using a tunable neutral density filter. A 50X Mitutoyo deep-UV objective focused the laser to a spot size of nearly 5 µm onto the sample. The device was imaged using the same objective and directed into a 300 mm focal length spectrometer with an attached liquid nitrogen cooled CCD camera.

Figure 4 shows room-temperature photoluminescence (PL) taken from an unpatterned, unetched region (Fig. 4(a)) and an unpatterned, PEC-etched region (Fig. 4(b)) of the sample. The PL was collected after various PEC etch durations. As shown in Fig. 4(a), the unetched area produces broad emission peaks associated with the SQW and MQW structures (center emission wavelengths near 410 nm and 460 nm, respectively). As the PEC etch duration lengthens, no significant differences in emitted PL are observed in the unetched reference region. However, (Fig. 4(b)), illustrates a different result for the unpatterned, PEC-etched region. Before any etching takes place (t = 0 min.), two broad peaks are again observed near wavelengths of 410 nm and 460 nm. However, as the etch duration lengthens, the broad peak associated with the broad MQW emission reduces; the reduction in PL intensity results from the MQW structure being selectively etched away. Following 150 minutes of etching, emission from the MQW structure is almost unobservable, indicating the material has been removed. Despite significant etching of the MQW structure, emission associated with the SQW is uninhibited by the PEC etch or its corresponding durations. This result is a consequence of selecting the appropriate etch wavelength: carriers can be excited and contribute to surface oxidation in the MQW layers but not within the SQW. This preferential behavior is enabled by the narrow laser linewidth. Consequently, a membrane-like structure has been selectively formed through PEC etching. These results suggest an interesting procedure for selectively fabricating membrane structures in pre-fabricated III-nitride PhCs while preserving active layers of lower In composition(s). Also, by using higher In content in the sacrificial region, the active region can also contain higher In concentration, thereby enabling longer wavelength emission.

 

Fig. 4 Room-temperature PL of an (a) unetched reference region and (b) PEC etched reference region after various etch durations. The dashed line in each figure coincides with the wavelength of the laser excitation source.

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Figure 5 shows room-temperature μ-PL measurements taken from various sample regions following the complete PEC etch duration. PL is collected from an unetched reference region (red), etched reference region (purple) and etched PhC device region (black). The upper inset in the figure shows a bright-field optical micro-graph (OM) of the sample. Each measured region is indicated with a dot of the corresponding color. For the unetched reference region, we again observe broad PL centered near wavelengths of 410 nm and 460 nm, associated with peak SQW and MQW emission, respectively. However, for the etched reference region, the MQW peak is unobservable due to selective etching of the MQW structure. Finally, PL collected from a PhC device region reveals the presence of a strong resonance near a wavelength of 385 nm. This resonant peak is a cavity mode of the L1 PhC and was unobservable preceding PEC etching. Further, the broad emission associated with the embedded SQW is clearly visible in the spectrum. Thus, our fabrication strategy enables wide tuning of cavity resonances matched to InGaN quantum dot emitters, spanning from the near-UV into the far-blue regions of the visible spectrum. Such wide-range tunability would be wholly unobtainable in a membrane composed entirely of GaN. However, it should be noted that the associated cavity resonance is detuned from peak SQW emission, and thus detunedform our theoretical calculations. This discrepancy is likely due to fabrication imperfections resulting from hard mask degradation, as evidenced from asymmetric as well as reduced diameter holes in the fabricated structure (Fig. 3(c)). In the future, a thicker SiO₂ hard mask with better proximity correction in EBL pattering should help improve the spectral positioning of our cavity mode.

 

Fig. 5 (a) schematic of PL measurement of the PhC cavity (b) Room-temperature PL emission from various regions on a sample. Red represents an unetched reference region, purple represents an etched reference region and black repre-sents an etched device region. The inset shows an optical microscope image of the sample. Markers of corresponding color indicate each region. Scale bar in inset is 5 μm.

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4. Summary

We have demonstrated a top-down fabrication strategy for preferential membrane formation in a pre-patterned III-nitride photonic crystal (PhC) cavity. Making use of a bandgap selective PEC wet-etching technique, we were able to selectively remove a multiple quantum well (MQW) heterostructure without affecting a single quantum well (SQW) of lower In concentration. Room-temperature micro-photoluminescence (μ-PL) measurements confirmed the selective removal of the MQW structure and preservation of an embedded SQW. Moreover, PhC cavity resonances, unobserved before PEC etching, were present in the final fabricated devices. Our results indicate an interesting method for creating III-nitride membranes of specified thickness that contain QWs with large emission bandwidths. Further, the embedded QW could be used for subsequent quantum-size-controlled photoelectrochemical (QSC-PEC) etching, which has been shown to create quantum dots (QDs) of controlled size and placement [1,39]. Thus, by spectrally tuning the second PEC etch wavelength to an observed cavity resonance, it is possible to incorporate QDs into a pre-fabricated PhC cavity. We believe that our top-down fabrication approach offers much opportunity for the seamless, spectral and spatial coupling of QDs to high-Q photonic crystal cavities for the III-nitride material system.