AlGaN-based Laser Diodes Research Articles

AlGaN-based laser diodes with reduced Al composition in quantum barriers in the deep ultraviolet region

Subject of study. In AlGaN-based deep ultraviolet laser diodes, the quantum well affects the performance in the active region. Moreover, the composition of aluminum in quantum barriers in the quantum well matters to the performance of AlGaN-based deep ultraviolet laser diodes. It might be observed effectively by looking at performance indicators like the emitted power and optical confinement factor (band diagram, carrier concentration, and stimulated recombination). Method. Two deep ultraviolet laser diode devices with a nominated wavelength of 267.5 nm are simulated and compared in this paper using the Crosslight program LASTIP. The proposed deep ultraviolet laser diode device B with a reduced Al composition in the quantum barrier is used in the active region as opposed to the reference deep ultraviolet laser diode device A. Main results. This led to an improvement in emitted output power of 0.109 W with a reduced injection current of 0.02 A and an improvement in the optical confinement factor of 19%. Practical significance. The structure of the reference device is inspired by the experimental structure. The results of the proposed device are calculated and are evaluated based on parameters such as the improved conduction band barrier height, reduced valence band barrier height, and improved stimulated recombination rate. It outperforms the deep ultraviolet laser diode for the reference device.

Demonstration of ultraviolet-B AlGaN-based laser diode operation with a peak light output power of 150 mW by improving injection efficiency through polarization charge modulation

In this study, AlGaN-based ultraviolet-B band laser diodes with 150-mW peak output power in pulsed operation were demonstrated at room temperature. The oscillation wavelength, differential quantum efficiency, and slope efficiency of a laser diode were 300 nm, 3.6%, and 0.15 W/A, respectively. These results were obtained by increasing the injection efficiency and decreasing the positive fixed polarization charge formed at the interface between a p-side waveguide layer and an electron blocking layer when polarization doping is formed in a p-AlGaN cladding layer.

Key temperature-dependent characteristics of AlGaN-based UV-C laser diode and demonstration of room-temperature continuous-wave lasing

Although the pulsed operation of AlGaN-based laser diodes at UV-C wavelengths has been confirmed in the previous studies, continuous oscillation without cooling is difficult because of the high operating voltage. In this study, the temperature dependence of key parameters was investigated and their impact on achieving continuous-wave lasing was discussed. A reduction in the threshold voltage was achieved by tapering the sides of the laser diode mesa and reducing the lateral distance between the n- and p-electrodes. As a result, continuous-wave lasing at room temperature was demonstrated at a threshold current density of 4.2 kA/cm2 and a threshold voltage of 8.7 V.

AlGaN-based UV-B laser diode with a high optical confinement factor

To reduce the threshold current density (Jth) of ultraviolet (UV)-B AlGaN-based laser diodes, we investigated the critical parameters aiming to increase the injection efficiency ηi and the optical confinement factor Γ. Optimization of the thickness of the waveguide layer, the average Al content of the p-type AlGaN cladding layer, and the film thickness of the cladding layer demonstrated that the device characteristics can be improved. This optimization achieved a reduction in Jth to 13.3 kA cm−2 at a lasing wavelength of 300 nm, thus offering the lowest Jth value yet achieved for a UV-B laser diode.

Enhancing the Light-Extraction Efficiency of an AlGaN Nanowire Ultraviolet Light-Emitting Diode by Using Nitride/Air Distributed Bragg Reflector Nanogratings

The performance and efficiency of AlGaN ultraviolet light-emitting diodes have been limited by the extremely low light-extraction efficiency (LEE) due to the intrinsic material properties of AlGaN. Here, to enhance the LEE of the device, we demonstrate an AlGaN nanowire light-emitting diode (NW-LED) integrated with nitride/air distributed Bragg reflector (DBR) nanogratings. Compared to a control device (only mesa), the AlGaN NW-LED with the nitride/air DBR nanogratings exhibits enhancement in the light output power and external quantum efficiency (EQE) by a factor of ∼1.67. The higher light output power and EQE are attributed mainly to the multiple reflectances laterally for the transverse magnetic (TM)-polarized light and scattering introduced by the nanogratings. To further understand the LEE enhancement, the electrical field distribution, extraction ratio, and polar pattern of the AlGaN NW-LED with and without the nitride/air DBR nanogratings were analyzed using the finite-difference time-domain method. It was observed that the TM-field emission was confined and scattered upward, whereas the polar pattern was intensified for the AlGaN NW-LED with the nanogratings. Our approach to enhance the LEE via the nitride/air DBR nanogratings can provide a promising route for increasing the efficiency of AlGaN-based LEDs, also, to functioning as facet mirror for AlGaN-based laser diodes.

Radiative and nonradiative recombination in an ultraviolet GaN/AlGaN multiple-quantum-well laser diode

We have experimentally investigated the radiative and nonradiative recombination in a GaN/AlGaN multiple-quantum-well laser diode. The each carrier lifetime has been evaluated based on a rate equation analysis of light output-current characteristics of the laser diode. The estimated nonradiative carrier lifetime is 830 ps, and the Auger recombination is negligibly small at room temperature. At a threshold current density of 8 kA cm−2, the carrier density and the internal quantum efficiency are estimated to be 2.6×1019 cm−3 and 34%, respectively. These results are responsible for experimental and theoretical analysis of optical and electrical properties in AlGaN-based laser diodes.

AlGaN-based laser diodes for the short-wavelength ultraviolet region

We have demonstrated the room-temperature operation of GaN/AlGaN and indium-free AlGaN multiple-quantum-well (MQW) laser diodes under the pulsed-current mode. We have successfully grown low-dislocation-density AlGaN films with AlN mole fractions of 20 and 30% on sapphire substrates using the hetero-facet-controlled epitaxial lateral overgrowth (hetero-FACELO) method. GaN/AlGaN and AlGaN MQW laser diodes have been fabricated on the low-dislocation-density Al0.2Ga0.8N and Al0.3Ga0.7N films, respectively. The GaN/AlGaN MQW laser diodes lased at a peak wavelength ranging between 359.6 and 354.4 nm. A threshold current density of 8 kA cm−2, an output power as high as 80 mW and a differential external quantum efficiency (DEQE) of 17.4% have been achieved. The AlGaN MQW laser diodes lased at a peak wavelength down to 336.0 nm far beyond the GaN band gap. For the GaN/AlGaN MQW laser diodes, the modal gain coefficient and the optical internal loss are estimated to be 4.7±0.6 cm kA−1 and 10.6±2.7 cm−1, respectively. We have observed that the characteristic temperature T0 ranges from 132 to 89 K and DEQE shows an almost stable tendency with increase of temperature. A temperature coefficient of 0.049 nm K−1 is also found for the GaN/AlGaN MQW laser diode. The results for the AlGaN-based laser diodes grown on high-quality AlGaN films presented here will be essential for the future development of laser diodes emitting much shorter wavelengths.