Over 1 W record-peak-power operation of a 338 nm AlGaN multiple-quantum-well laser diode on a GaN substrate

Ultraviolet (UV) lasers are widely utilized in various applications such as laser microscopy, fluorescence spectroscopy, mass spectrometry, surface analysis, material processing, and laser lithography because of their high photon energy and the low diffraction limit of UV light. Conventional gas and solid-state UV lasers are large, heavy, inefficient, and inflexible in wavelength. UV semiconductor laser diodes (LDs) are expected to provide alternative solutions to the issues. III–nitride semiconductors, especially AlGaN alloy, are candidate materials for UV optical devices since their band gap energy can be varied from 3.4 eV for GaN to 6.0 eV for AlN. The development of AlGaN-based UV light-emitting diodes (LEDs)^1–4) were reported and the shortest emitting wavelength of 210 nm from AlN pin-LEDs was achieved in the deep-UV region.⁵⁾ Electron-beam-pumped UV light sources using an AlGaN layer were also reported in the 200 nm band.^6,7) From the point of view of lasers, an AlGaN nanowire laser under current injection was demonstrated in the 260 nm band at 77 K.⁸⁾ Very recently, the room-temperature (RT) operation of the AlGaN nanowire laser at 289 nm has been reported.⁹⁾ In contrast, AlGaN multiple-quantum-well (MQW) UV-LDs, which would be suitable for practical use, were limited at 325.8 nm at RT¹⁰⁾ although optically pumped stimulated emission below 300 nm from an AlGaN MQW structure was reported.^11–13)

AlGaN MQW UV-LDs were grown on foreign substrates such as sapphire¹⁴⁾ and SiC.¹⁵⁾ The UV-LDs were fabricated on AlGaN underlying layers using the epitaxial lateral overgrowth (ELO) method owing to the lattice mismatch between the AlGaN layers and the GaN/sapphire templates. The ELO technique not only reduces dislocation density but also suppresses crack formation.¹⁶⁾ Thus, high-quality AlGaN underlying layers were obtained, and the pulsed laser operation of UV-LDs on sapphire was achieved at lasing wavelengths between 336.0 and 361.6 nm at RT.^17–22) However, the reported peak output power of these UV-LDs were less than 100 mW and a higher peak power is expected to satisfy application requirements. The use of broad-area structures is an approach to realizing the higher peak power operation of UV-LDs. The UV-LDs fabricated on sapphire proved lateral conduction due to the insulating property of sapphire. Therefore, broad-area UV-LDs on sapphire are unsuitable since the lateral conduction leads to the ununiformity of current injection.

Recently, the fabrication of 356.6 nm AlGaN-based UV-LDs on bulk GaN substrates have been successfully demonstrated.²³⁾ Broad-area UV-LDs on GaN substrates have a strong potential to achieve higher-peak power operation because uniform current injection can be expected. In this paper, we demonstrate the broad-area AlGaN-based UV-LDs on conductive GaN substrates with a peak power of over 1 W in 338.6 nm under pulsed operation at RT, which is the highest peak power ever obtained for AlGaN-based UV-LDs.

Figure 1 shows a schematic illustration of a 338.6 nm broad-area UV-LD fabricated on a bulk GaN(0001) substrate. The epitaxial growth was carried out by metal–organic vapor phase epitaxy using trimethylgallium, trimethylaluminum, and ammonia as source materials. Silane and bis(cyclopentadienyl)magnesium were employed as n- and p-dopant sources, respectively. A high-quality and crack-free Al_0.3Ga_0.7N underlying layer was grown using the ELO technique, which means that inclined GaN seeds were grown on SiO₂-striped GaN(0001), and the Al_0.3Ga_0.7N layer was laterally grown on the GaN seeds. A laser structure was grown on the Al_0.3Ga_0.7N underlying layer and consisted of an n-Al_0.3Ga_0.7N cladding layer, an n-Al_0.16Ga_0.84N guiding layer, Al_0.06Ga_0.94N/Al_0.16Ga_0.84N MQWs, an Al_0.16Ga_0.84N guiding layer, a p-AlGaN electron blocking layer, a p-Al_0.3Ga_0.7N cladding layer, and a p-GaN contacting layer. The layers were designed for a lasing wavelength of 330 nm band. After the epitaxial growth, a broad-area structure with a ridge width of 50 µm was formed by dry etching and was covered with a SiO₂ insulating layer outside the top of ridge. A p-electrode was deposited on the p-GaN contacting layer, and the n-electrode was deposited on the back surface of the n-GaN substrate with a thickness of about 100 µm after back surface grinding and polishing. Mirror facets of laser cavities with a length of 600 µm were formed by cleaving along the $(1\bar{1}00)$ face. High-reflection and anti-reflection coatings were employed for the rear and front facets, respectively. The characteristics of UV-LDs were evaluated under identical pulsed current injection with a pulse duration of 4 ns and a repetition frequency of 5 kHz.

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Before the fabrication of the UV-LD structure, it is extremely important to obtain a high-quality Al_0.3Ga_0.7N underlying layer using the ELO method. The Al_0.3Ga_0.7N underlying layer was confirmed to be crack-free and was of sufficiently high quality for fabricating the AlGaN-based UV-LD. There were no cracks over the entire area of the UV-LD wafer.

Figure 2 shows a typical light output–current (L–I) characteristic of the broad-area UV-LD under the pulsed operation. The peak output power exhibited a clear nonlinear behavior around a threshold current of 7 A corresponding to a current density of 38.9 kA/cm², and linearly increased above the threshold. A peak power of 1 W was obtained at an operating current of about 10 A, which is the highest peak power ever obtained for AlGaN MQW UV-LDs. The slope efficiency and differential external quantum efficiency (DEQE) were estimated to be 0.31 W/A and 8.5%, respectively. The threshold current density and slope efficiency were inferior to those of the conventional InGaN-based LDs. It could result from the weak optical confinement due to the smaller difference in refractive index between the guiding layers and the cladding layers, and the electron overflow from the active region into the p-AlGaN cladding layer. The threshold current density was higher than that of 336 nm UV-LD on a sapphire substrate despite having almost the same crystal structure.¹⁹⁾ To realize a lower threshold current density, further optimization of the crystal structure and laser design for the broad-area UV-LD is required.

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Figure 3 shows the emission spectra of the device operated under pulsed injection currents of 4.0, 5.4, 7.3, and 12 A. At an injection current of 4 A below the threshold, a weak and broad spontaneous emission was observed at approximately 340 nm. The full-width at half maximum (FWHM) was 6.6 nm. With increasing injection current, the emission peak was blue-shifted and intensified because of the screening of an internal electric field in the AlGaN MQW,^24,25) and a strong, sharp lasing emission suddenly appeared above the threshold current. The lasing wavelength and FWHM were 338.6 and 0.5 nm at an injected current of 7.3 A, respectively. With further increase in injection current up to 12 A corresponding to a peak output power of about 1.5 W, the FWHM was slightly spread to 0.8 nm.

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Figure 4 shows the horizontal and vertical far-field patterns (FFPs) from the broad-area UV-LD on the GaN substrate at a current of 10.3 A which corresponds to a peak output power of about 1 W. The inset shows a pseudo color image of FFPs. Although unimodal FFPs were obtained for the UV-LDs with narrow stripes as reported in Ref. 23, the broadening of FFPs with multiple peaks were observed. The reason for this would be attributed to not only the multi transverse mode but also the filamentation processes due to the broad-area structure.^26,27)

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The temperature dependence of the device characteristics were evaluated between 20 and 60 °C. Figure 5 shows the temperature dependence of threshold current under the pulsed operation. The threshold current was increased from 7.0 to 9.9 A with increasing temperature. The characteristic temperature (T₀) of threshold current was calculated to be 119 K. This value is comparable to that of previously reported AlGaN-based UV-LDs on sapphire.²²⁾ Figure 6 shows the temperature dependence of peak lasing wavelength at a peak power of approximately 0.5 W. The emission peak wavelengths were red-shifted from 338.4 to 339.8 nm at a temperature change of 40 °C. The temperature coefficient of lasing wavelength was estimated to be 0.033 nm K⁻¹ from the slope of the fitted line, which is slightly smaller than the previously reported value of 0.049 nm K⁻¹ for AlGaN-based UV-LD on sapphire.²²⁾

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We have demonstrated the high-peak-power operation of AlGaN-based UV-LD fabricated on a bulk GaN(0001) substrate via the crack-free Al_0.3Ga_0.7N underlying layer using the ELO method. The broad-area and vertical conductive structure of UV-LD with a ridge width of 50 µm was employed to achieve higher peak power. Consequently, a peak output power of over 1 W was achieved at 338.6 nm under the pulsed operation with a pulse duration of 5 ns and a repetition frequency of 5 kHz at room temperature, which is the highest peak power ever obtained for AlGaN-based UV-LDs.