We report observations of lateral mode confinement by a tapering nanoporous-GaN layer in the n-side cladding of a blue-emitting InGaN laser diode grown on the semipolar plane of bulk GaN. Little additional confinement occurred in the transverse direction, and the nanoporous layer did not serve as a current aperture. Nanoporous-GaN, with Si-doping of 8x1018 cm−3 and 20% porosity had a bulk resistivity of 3 Ω-cm and a thermal conductivity of 4 W/m-K, in general agreement with data reported on c-plane VCSEL structures. An excess modal loss of 19 cm−1 was found.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Group III-N edge emitting laser diodes (EELDs) are generally grown on the basal c-plane of bulk GaN substrates, and utilize some combination of InGaN, GaN and AlGaN to form the active region, waveguide core and cladding layers. Lasers fabricated on semipolar substrates are expected to provide higher gain, although the early onset of strain relaxation via glide of misfit dislocations [1–7] limits the composition and thickness of AlGaN cladding layer. Intentionally relaxed interfaces  and limited area epitaxy  have been somewhat successful, and recently nanoporous-GaN (NP-GaN) formed by electrochemical etching has been demonstrated as an alternative to n-AlGaN cladding in c-plane lasers , albeit only optically pumped. We report electrical operation of semipolar EELDs with NP-GaN in the lower cladding, and observation of lateral mode confinement by a tapering porosification front. Such lateral waveguiding, combined with a current blocking layer such as formed by ion implantation or a buried tunnel junction , would allow an extended p-contact area  and a planar structure for improved thermal management in a p-down or dual-side packaging design .
Two EELD samples were fabricated from pieces of the same grown wafer, one as a control and the other incorporating NP-GaN cladding, with final devices structures shown in Fig. 1. The epitaxial layers were grown on a bulk GaN substrate provided by Mitsubishi Chemical Corporation on the plane with metalorganic chemical vapor deposition (MOCVD) at atmospheric pressure. The initial n-GaN buffer layer was 400nm thick doped with Si at 8x1018 cm−3, followed by 800 nm thick of 2x1018 cm−3 Si-doped n-GaN, both at 1180°C. Next was a 65 nm thick In0.06Ga0.94N waveguide doped with Si at 2x1018 cm−3 grown at 965°C, a 20 nm thick unintentionally doped (UID) GaN, and a 2 period multiple quantum well (MQW) active region with 3 nm thick In0.18Ga0.82N QWs and 7 nm thick GaN barrier layers grown at 890°C, ending on a barrier. A 20 nm thick p-Al0.28Ga0.72N electron blocking layer (EBL) was grown at 1000°C, doped with Mg targeting 3x1019 cm−3, followed by a 65 nm p-In0.06Ga0.94N waveguide layer, grown at 965°C with 1x1019 cm−3 of Mg doping. A final 270 nm thick p-GaN layer was then grown targeting 2x1018 cm−3 for the first 250 nm and 1x1020 cm−3 for the final 20 nm, grown at 1000°C.
One sample was held as a baseline while the other was made porous through a wet electrochemical (EC) etch. First, a trench 3 μm deep and 30 μm wide was formed parallel to the surface projection of the c-axis by standard lithography and Cl2 reactive ion etching (RIE). A backside Ti/Al/Ni/Au 1.5/50/100/300 nm metal stack was deposited by e-beam evaporation as the anode for the EC etch. The sample was held by electrically conducting forceps in a room temperature bath of 0.3 M oxalic acid at an applied voltage of 6.5 V for 1.5 hours, with a submerged Pt wire. The forceps were elevated above the solution to avoid corrosion and contamination. Inspection by optical microscopy confirmed a lateral etch depth of about 17 μm. Following this, the sample was cleaned in 1:3 HNO3:HCl aqua regia at 225°C bath to strip any remaining backside metal and then buffered HF to remove the SiO2 RIE mask.
The porosified and baseline samples were then processed together into ridge lasers with thin p-GaN cladding and indium tin oxide (ITO) anode contacts and facets etched by chemically assisted ion beam etching (CAIBE). Backside Ti/Al/Ni/Au contacts were re-deposited as the final step in the fabrication process. Details may be found elsewhere . The lasers were positioned 3μm from the trench used for porosification. The EELDs were tested under pulsed electrical injection, with a pulse width of 500 ns and a duty cycle of 5%. For temperature measurements, a 1 µs pulse width was kept constant while decreasing the period to reach duty cycles from 1% to 94% while peak wavelength was measured.
The EC etch was not adequately selective to the intended 8x1018 cm−3 Si-doped layer. Inspection of the SEM micrograph in Fig. 2 indicates the 2x1018 cm−3 Si-doped n-GaN layer was also etched. In this device, the leading front of porous GaN slopes down toward the n+ layer, leaving 400 nm of unetched n-GaN below the InGaN waveguide where the lasing mode appears. This unintended problem was fortuitous because it allowed separate examination of the lateral and transverse optical and electrical confinement. For vertical confinement across the entire ridge, the EC etch selectivity must be improved to the target layer. This can be done with higher doping of the target layer, as achieved in the VCSEL DBRs of , or with a second mesa etch using sidewall protection of the first mesa to shield the waveguide, as has been done with photo-electrochemical etching of lateral current and optical apertures .
By image analysis, the apparent porosity ranged between 15 and 25%. No difference was discernable between highly doped and moderately doped layers, although such difference must exist to explain the tapered etch profile. To obtain more accurate porosities would require methods such as gravimetric analysis or tomography through successive focused ion beam etching , coupled with ellipsometric measurement of reliable test samples, worthwhile but far beyond the scope of this work. Accordingly, the simple waveguide simulations below are based on these estimates, recognizing their limited accuracy. The porosity affected the uniformity of the CAIBE facet etch, but the visible roughness was far below the optical mode and did not impact performance in the area of the lasing mode.
3. Results and discussion
Nearfield images of the facet of 900 μm x 15 μm EELDs during operation are shown in Fig. 3. Below threshold, the spontaneous emission was uniform across the full width of the facets of both the baseline and the NP-GaN structures. Above threshold, emission in the NP-GaN EELD was limited to about one fourth of the width of the baseline mode, confined laterally near one edge of the ridge.
The optical mode was simulated with a commercial finite difference mode solver  in Fig. 4, assuming a step-wise taper of the porosification front extending over 5 μm, as indicated in Fig. 2(a). A refractive index of 2.47 was assumed for the n-GaN and 2.25 of the NP-GaN as predicted by the volume average theory (VAT) model . The large refractive index step between the solid and porous layers provided tight optical confinement in lateral dimensions, with a 1/e2 mode size of 3.4 μm, in fair agreement with the image of Fig. 3(d).
3.1 Electrical properties
Light-Current-Voltage (LIV) curves comparing 900 μm x 15 μm EELDs of the baseline against the NP structure are shown in Fig. 5. With a peak wavelength emission o f 430 nm, the baseline EELD had a threshold current density of 2.6 kA/cm2 and a slope efficiency of 0.85 W/A, while the NP EELD had a threshold current density of 4.8 kA/cm2 and a slope efficiency of 0.14 W/A. At 1 A (7.4 kA/cm2) the excess voltage for the NP device was 2.8 V across a 1.2 µm thick NP layer giving an electrical resistivity of 3.2 Ω-cm. This is somewhat higher than the 0.9 Ω-cm reported in . This difference can be attributed to an order of magnitude lower Si doping in the remaining n-GaN.
The uniform spontaneous emission below the full width of the 15 μm ridge, as seen in Fig. 3(b), indicates that current injection was not restricted by the tapered porosification front seen in Fig. 2. We discount the possibility of light scatter from a narrower injected region because there is no sign of such scattering in the above-threshold image of Fig. 2(d). This is further supported by comparing the estimated spreading resistance in the thin p-GaN layer, to the vertical resistance of the n-type NP-GaN layer: 150 Ω for holes traveling from the center of the ridge to the center of the lasing mode, compared to 4 Ω vertically through the NP-GaN. Nor can ambipolar diffusion in the thin quantum wells support lateral current at lasing levels. The result is poor injection efficiency, discussed next.
3.2 Optical properties and lasing characteristics
The low differential efficiency of the EELD with NP-GaN cladding must be attributed to a combination of low injection efficiency, high internal loss relative to the mirror loss, or scattering loss at the facets. From the cavity length dependence of the differential efficiency, the injection efficiency of the uniformly injected baseline EELD was found to be 63%. We could not make this measurement directly on the tapered NP-GaN laser, because few devices reached threshold. Assuming carriers injected outside of the area of the lasing mode were wasted, as described above, we scale the injection efficiency by the reduced mode width to an estimated 16%. From the measured differential efficiency and calculated mirror loss of 6.5 cm−1, we then derive a high internal loss of 35 cm−1. By comparison, the internal loss of the baseline EELD was 16 cm−1. We neglect scattering at the facet because the mode overlap with the porosified portion of the facet is very small.
The simulations of Fig. 4 also show that the impact on the vertical mode width was minimal because the tapered edge of the porosification front was still far below the mode, and the mode was otherwise confined by the n-side InGaN waveguide layer. Other devices fabricated without the InGaN waveguide layers, intended to test designs utilizing a remote EBL , suffered from a vertically displaced optical mode due to the inadvertent etching of the low doped InGaN, and those devices could not reach lasing threshold. The mode overlap with the etched material is only 0.01%, and given the effective material loss of 200 cm−1 deduced from the VCSEL DBR data of , or similar low loss reported in , it is unlikely that direct scattering from the NP-GaN layer contributed much to the modal loss. The NP-GaN also did not cause a significant change in the mode overlap with the ITO or contact metal compared to the baseline structure. We do note that scattering at a rough interface can be comparable to bulk scattering , increasing as the square of the pore size, and thus difficult to estimate given the uncertainty in the interface morphology. We also considered inadvertent EC etching of the exposed InGaN quantum wells. Micro-PL did not show a difference between areas near an etched edge and areas remote from the edge, but we cannot rule out some shallow porosification. Scattering from similar roughness due to intentional photo-electrochemical etching of the active region was found to produce significant internal loss , and we can only speculate this accounts for some of the excess loss measured here.
3.3 Thermal properties
The lasing wavelength was measured as a function of dissipated electrical power, IV –Pout, adjusted for duty cycle. From the measured temperature dependence of wavelength, 0.03 nm/K, the temperature dependence on power dissipation was 44 K/W for the NP-GaN EELD and 27 K/W for the baseline EELD. The CW thermal performance was simulated with a 2D finite element solver  that included estimated heat generation in each layer according to the local resistivity, 1 Ω-cm for p-GaN, 5x10−3 Ω-cm for n-GaN, 3.2 Ω-cm for NP-GaN, and an anode contact layer operating at 5x10−5 Ω-cm2. The thermal conductivity of the Group III-N alloys was taken from , adjusted for temperature according to (T/300K)-0.43 . The interface between the Au-metalized rough substrate sitting without bonding on a copper heat sink held at 20°C was simulated with a thermal conductivity of 12 W/m-K. The predicted temperature rise of the active region at 1A (7.4kA/cm2) agreed well with the data for both the baseline and NP-GaN lasers when the thermal conductivity of the NP-GaN layer was set to 4 W/m-K, in general agreement with reported values which range between 1 and 15 W/m-K . A more accurate calculation must wait on reliable temperature-dependent data for the contact and bulk semiconductor layers in the actual device. This is because thermal effects cause a larger than expected variation in the experimental wafer-to-wafer contact resistance.
We have shown that NP-GaN can provide lateral optical confinement in addition to vertical confinement in electrically injected InGaN lasers, with lower excess loss than has been reported with photoelectrochemically etched optical confinement. We did not achieve loss low enough to be useful in high efficiency EELDs, but higher doping for smaller pore size might allow this. A separate current blocking structure is needed, and the thermal path must be considered. The ability to taper the porosification front by way of graded doping profiles may be of use to further tailor the mode confinement or loss.
U.S. Department of Energy (DE-AR0000671); National Science Foundation (DMR05- 20415). Solid State Lighting and Energy Electronics Center, University of California Santa Barbara (100010947)
This work was supported in part by the Solid State Lighting and Energy Electronics Center (SSLEEC) at UCSB. The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. A portion of this work was done in the UCSB nanofabrication facility, part of the National Science Foundation (NSF) funded NNIN. This work made use of MRL Central Facilities supported by the MRSEC Program of NSF.
The authors declare that there are no conflicts of interest related to this article
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