Continuous‐Wave Operation of 457 nm InGaN Laser Diodes with Etched Facet Mirrors for On‐Chip Photonics

The success of silicon photonics is sparking widespread interest in photonic integrated circuits at visible light wavelengths using SiN and other waveguiding platforms. Compact active circuits desire the heterogeneous integration of GaN‐based laser diodes. Herein, the optimization of smooth and vertical facets is reported using a combination of inductively coupled plasma etching followed by wet etching with a tetramethylammonium hydroxide‐based solution. Facet quality for concave‐, flat‐, and convex‐shaped structures surrounding the mirror is compared. Convex‐shaped structures result in the highest facet quality due to the evolution of the crystal plane‐dependent etching. 2 μm‐wide ridge waveguide, etched facet Fabry–Pérot cavity lasers with length of 1.5 mm emitting at 457 nm are realized using the optimized process. The lasers deliver up to 28 mW of optical power at 250 mA under continuous wave with slope efficiency of 0.26 W A−1 and lasing threshold current density of 4.6 kA cm−2.


Introduction
[3][4] Low-loss waveguides in the visible region can be realized on silicon nitride (SiN), aluminum nitride (AlN), and lithium niobate (LN) platforms but these do not have monolithic active devices such as lasers, detectors, or modulators.The light must be delivered from an external source using end-fire or grating coupling.[7] However, the most compact solution is heterogeneous integration of the laser on the PIC.10][11][12][13][14] GaN-based etched facet lasers use inductively coupled plasma (ICP) etching, which results in nonvertical facets and a large surface roughness and potential etch damage induced by highenergy ions leading to increased laser thresholds and mirror losses.[16][17] In this work, we explain the mechanism by which the wet etching proceeds such that an optimized defect-free facet can be formed in the multilayer AlInGaN laser structure.We then apply the process and monitor the reduction in lasing threshold with etch time and characterize the resulting lasers.

Evolution of the Dry-Etched Laser Facet with Wet Etching
A conventional laser structure containing p-and n-doped AlGaN, InGaN, and GaN was grown on c-plane GaN substrates (experimental section).The facet verticality and smoothness was optimized using ICP with different gases (Cl 2 , Ar, N 2 , BCl 3 ) and different etching conditions (bias power, ICP power, gas flow rate, pressure).These optimizations resulted in the highest facet angle of 88°, elimination of trenches adjacent to the facet, and low hexagonal defect/pillar density when utilizing Cl 2 /BCl 3 chemistry (Experimental Section) and dielectric masks where rectangular patterns were aligned to the m-and a-crystal planes.After the ICP step, the etched surfaces are sloped and with surface corrugations transferred from the resist patterns formed by photolithography process (Figure 1a).These features are detrimental for low-threshold lasing.To address these issues, the exposed surfaces are wet etched in dilute KOH or TMAH present in conventional photoresist developers.Note that the wet etching of m-and a-planes exhibits different behaviors with the former tending to form a flat and smooth surface while the latter tends to stop on adjacent m-planes resulting in zigzag features (see Figure 1a,b and S1, Supporting Information).First, we investigated the evolution of the dry-etched laser facet morphology during wet etch in AZ400K developer, which contains 2.4% KOH, at 90 °C in the dark for different etching times of 0, 20, 45, and 75 min.Figure 1a,b shows the evolution from two different viewing angles using scanning electron microscopy (SEM).During wet etching, the facet profile exhibits different behaviors with time in the vertical and lateral directions, as shown in Figure 1c.In the vertical direction, the etching initially forms multiple ministeps on the facet, that is, actual m-planes, due to the extremely slow etch rate on these m-planes.As the etching time increases, these ministeps are further etched and then merged with each other, resulting in major steps on the surface (see Figure S3, Supporting Information) with each being exactly at 90°to the c-plane surface.On the other hand, as shown in Figure 1b, etching along the horizontal direction finds the actual m-planes by forming multiple sliced planes.Note that although the facet was nominally aligned to m-plane during the process, a tiny misalignment between the facet plane and the actual m-plane is inevitable.The wet etching finds the actual m-plane at end of the facets (corner) first and then extends inward.This lateral etching eliminates both the steps in the vertical direction and the sliced planes in the horizontal direction, forming an extended m-plane with a very smooth surface.Furthermore, it has been observed that the wet etch on the InGaN laser facets can be affected by background light.As shown in Figure S3, Supporting Information, where the same experiment was done under general lighting (the fluorescent light from the wet bench), the active region shows triangular holes after wet etch, while in Figure 1a,b the active region is intact without light.Such unintentional photoassisted etching can be attributed to the absorption of the residual ultraviolet and visible light by the InGaN layers in the laser structure. [18]Note that the a-plane oriented perpendicularly to the m-plane etch to form triangular facets composed of two m-planes at AE60°orientation.

Laser Facet Design and Wet Etch Evolution
As discussed earlier, to obtain vertical and smooth InGaN laser facets via the wet etch, it is important to enable the lateral etching mechanism in order to eliminate the steps and sliced planes from the facet.From the practical point of view, it is also important to minimize the wet etching time to avoid parasitic damage on the devices. [10,19]Therefore, to investigate how the geometry of the facets is affected by the wet etch process, etched-facet laser diodes with three types of facet geometries (flat, concave, and convex) were designed, fabricated, and exposed to the same wet etch.All laser facets were aligned to the wafer flat, that is, a nominal m-plane.As shown in Figure 2d,g,j, the flat facet is 160 μm wide with the ridge waveguide located near the center of the facet, while the concave (or convex) facet features a 15 μmwide protruding (or recessed) area.Wet etching was carried out simultaneously, at the end of device fabrication, for all lasers to polish the dry-etched facets, using 2.45% TMAH at 90 °C in the dark.Note that a TMAH-based solution is used here as the etchant instead of the KOH-based solution due to its much reduced parasitic etch on the passivation layer, that is, SiO 2 . [10]The wet etching mechanism on the facets was found to be very similar between TMAH and KOH based on our studies (Figure S2, Supporting Information).When the c-plane is exposed due to etching of the SiO 2 passivation layer, the intersection of the c-plane and m-plane becomes a source of additional etching steps on the vertical facets leading to deterioration of its verticality, increased rate of lateral etching (by stripping m-facet vertically layer by layer), and a risk of the formation of new step bunching.The facet morphology of the lasers was examined by both SEM and focused ion beam (FIB) cut after 240 min of wet etching with the results shown in Figure 2. The as-dry-etched facets with the flat geometry show corrugations on the surface with a sloped profile (Figure 2b,c), while, after wet etching, the facet has a smoother surface with vertical steps (Figure 2e,f ), due to the undeveloped lateral etch.As the center of the flat facets is far from the edges, it would take a long time to fully form the m-plane at the central part.The concave geometry results in two m-planes in the corners oriented at 120°to each other, which lock out the lateral etching toward the center of facet, leading to remaining vertical steps, which etch extremely slowly (Figure 2h,i).Finally, the convex geometry exhibits lateral etching from the corners, which results in a single crystallographically defined vertical and smooth m-plane facet without steps (Figure 2k,l).As this lateral expansion of single m-plane region is a slow process, the design of facets should make the convex part narrow to reduce the required wet etch time avoiding unintended damage elsewhere in the structure.It is noteworthy that the passivating SiO 2 layers survive after the long wet etch by the TMAH-based solution due to their slow etch rate.One observation is the lateral etched a-plane facets.Using a 120°angle in the device structure can mitigate this issue.
The schematic geometry of the convex-shaped etched facet laser diode with top p-and n-metal contacts, and the optical microscope image of a convex-shaped etched facet laser diode with 2 μm ridge width with 1.5 mm cavity length is shown in Figure 3.The schematic crystal structure of GaN including m-and a-planes and their alignment to the laser diodes is also included in Figure 3b.

Laser Characterization
The light intensity with pulsed (10 kHz, 300 ns, 0.3% duty cycle) Current injection (L-I) was measured for the devices directly on the chip after different wet etch times.All the samples exhibit the same lasing wavelength %457 nm.The device without wet etch polishing only showed partial superluminescence behavior, which can be attributed to the poor facet reflection.The threshold current shows a clear decrease with increasing wet etching time from 60 to 240 min (Figure 4a).After the first 60 and 120 min of etching, the threshold current of the devices decreased to 350 (11.7 kA cm À2 ) and 250 mA (8.3 kA cm À2 ), respectively.
After 240 min of etching, the threshold current decreased to 190 mA (6.3 kA cm À2 ).Note that the cavity length of the lasers was reduced by %3 μm after 240 min etching on the m-planes, which is a negligible reduction in the cavity length of 1.5 mm.The convex, concave, and flat designs were compared after 60 min of etching (Figure 4b) with the convex design having the lowest threshold current as expected from the earlier explanations.Following smoothing of the facets, a high-reflective (HR) mirror coating was selectively deposited on one facet.Laser bars were then cleaved and submounted to collect the majority of the light from the uncoated facet.The light-current-voltage (L-I-V) characteristic from the laser with a convex-shaped mirror (b) (a)   geometry and HR mirror coating under pulsed and continuous wave (CW) injection current is shown in Figure 4c.The laser with 2 μm ridge width and 1.5 mm cavity length had a threshold current of 120 mA (4 kA cm À2 ) and 140 mA (4.6 kA cm À2 ) at 20 °C under pulse and CW conditions, respectively.The operating voltage at the threshold current was 7.1 V. Slope efficiencies were measured as 0.69 W A À1 in pulsed mode and 0.26 W A À1 in CW mode.The increase in threshold current and decrease in slope efficiency observed in the CW data, as compared to the pulsed data, stem from the increase in junction temperature (T j ).Lasing central wavelength was 455.3 nm, under pulse, and 456.7 nm in CW conditions, and the 1.4 nm shift in peak wavelength is due to self-heating during the operation (Figure 4d).The linewidth of the lasing spectrum was limited by the resolution of the spectrometer, which is %1 nm.Emission spectra were measured over the temperature range in 10 °C steps at drive current of 160 mA (5.3 kA cm À2 ) above the threshold for that particular temperature under pulse conditions and the central wavelength shift with temperature was found to be 38 pm K À1 by fitting, which translates to a 60 °C rise in the junction temperature for the CW threshold current (Figure 4e). Figure 4f shows the far-field interference pattern under CW operation when the applied current was 1.1(I th ).2]

Conclusion
We have systematically investigated the evolution of the cavity mirror in an InGaN laser structure with different geometry considerations using a combination of ICP and dilute TMAH crystallographic by selective wet etching.We found that the facets reveal different etch behaviors in the vertical direction and along the horizontal direction of nearly m-plane facets.The former usually leads to steps due to the facet slope and difference in the epilayer compositions, while the latter stops on the actual m-plane to forms smooth and vertical facets.A convex facet geometry near the facet with narrow-reflective part is therefore suggested for InGaN lasers, to facilitate their wet etching and faster and more reliably achieve vertical and smooth mirrors without undesired steps.InGaN lasers fabricated with this approach operated at 457 nm with 28 mW optical power under CW conditions at room temperature.Lasing threshold measurements indicates that the mirror losses play a key role in the device performance of ICP plasma-defined cavities.Such losses can be significantly reduced using the additional wet etching process to dry etched mirrors, which leads to CW lasing and significant reduction of the lasing threshold.

Experimental Section
Laser Structure: The epitaxial structure was grown by metal-organic chemical vapor deposition on the free-standing GaN substrate.The active region consisted of an InGaN/GaN two-quantum-well gain region designed to emit at 450 nm surrounded by InGaN waveguiding layers.A 20 nm p-type AlGaN electron blocking layer was grown just above the active region.The n-and p-type AlGaN waveguide cladding layers with % 6% Al content were 1 μm and 800 nm respectively.A 20 nm p-type InGaN contact layer was used (Figure 5a,b).
Device Fabrication: Ridge waveguides, 2 μm wide aligned perpendicular to the m-plane, were defined by ICP etching with Cl 2 .Ohmic contacts to p-GaN were formed using electron-beam-evaporated Pd (20 nm)/Ni (30 nm)/Au (100 nm) metal stack which underwent thermal annealing in air at T = 500 °C for 10 min.The n-recesses were etched by ICP using Cl 2 and then the contact metal to the n-GaN Ti (20 nm)/Pt (30 nm)/Au (200 nm) was deposited using electron beam evaporation.Magnetronsputtered SiO 2 (600 nm) was patterned with photoresist and etched with CF 4 and acted as the mask for facet etching and as a passivation layer.The facets were etched by ICP using Cl 2 /BCl 3 (49/1 sccm), radio-frequency power 220 W, ICP power 800 W, temperature 20 °C, and pressure 15 mTorr.The etching rate was 265 nm min.Bond pads were deposited as Ti (30 nm)/Au (100 nm) using electron beam evaporator (Figure 5c).After facet polishing using 240 min wet etch, HR coating composed of SiO 2 /Al/SiO 2 (70/100/70 nm) was deposited on one facet using lift-off lithography and sputtering.
Wet Etching Facets: Wet chemical etching with MF319 developer from Microposit (contains 2.45% TMAH) was performed at the end of device fabrication.The temperature of the solution was kept at 90 °C and encapsulated with aluminum foil in the dark.
Measurement and Characterization: Electrical and optical properties of fabricated laser diodes were measured under CW and pulsed operation at temperatures between 20 and 80 °C using Keithley 2510 temperature controller (TEC).The pulse duration was 300 ns, and the frequency was 10 kHz, resulting in a duty ratio of 0.3% under pulse operation.Light output was measured using a Thorlabs S12 °C power meter by fixing the distance between samples and power meter.A Keithley 2400 sourcemeter was used for CW measurements.The spectra were measured using Ocean Insight HR4Pro spectrometer, with resolution of 1 nm.SEM and FIB-SEM images were taken using Zeiss Supra 40 and Tescan Solaris tools.

Figure 1 .
Figure 1.The evolution of a) the corner and b) the front side of facets at different times with dilute KOH (AZ400K).The lateral etching progress with time is shown in (b).c) Schematics of the wet etch mechanism.Dashed lines show actual m-planes and angles α and θ refer to the dry etch slope and misalignment between m-plane and facet plane, respectively.

Figure 2 .
Figure 2. a) Plan view of fabricated etched facet laser.b) Facet profile after dry etching.c) FIB cross-section view of dry-etched facet.d) Plan view of flat facet design, g) concave facet design, and j) convex facet design.e) Facet profile after 240 min TMAH etch of flat facet design, h) concave facet design, and k) convex facet design.f ) FIB cross section after 240 min TMAH etch for flat facet design, i) concave facet design, and l) convex facet design.

Figure 3 .
Figure 3. a) 3D illustration of convex-shaped etched facet cavity mirror laser diode.b) Optical microscope image of the fabricated etched facet single emitter and its alignment to the crystal planes including m-and a-planes of GaN.

Figure 4 .
Figure 4. a) Pulsed L-I with wet etching time.b) Pulsed L-I with differently shaped facet structures after 60 min TMAH treatment.c) L-I-V characteristic in CW and pulse conditions with HR mirror.d) Spectrum of laser diode in (c) under pulsed and CW conditions slightly above the threshold (1.1I th ).e) Temperature-dependent peak wavelength shift in pulse conditions.f ) Far-field interference pattern of etched facet laser under CW conditions.Devices with 15 μm-wide convex-shaped facet geometry were used in (a) and (c-f ).

Figure 5 .
Figure 5. a) Cross-section schematic of laser structure.b) High-resolution SEM image of etched structure.c) Fabrication steps of etched facet laser diodes.