Stimulated emission and lasing of GaN-based laser diodes (LDs) were reported at 1995[1] and 1996[2], right after the breakthrough of p-type doping[3−5], material quality[6] and the invention of high-brightness GaN-based LEDs[7, 8]. However, it took much longer time for GaN-based LDs to achieve high power, high wall plug efficiency, and long lifetime. Until 2019, Nichia reported blue LDs with these performances[9], which open wide applications with GaN-based blue LDs.
In the past 5 years, various organizations have reported GaN-based blue LDs with high output power. Osram reported 453 nm blue LDs with wall plug efficiency of 42% at 2.2 W and output power of 5 W at 3 A[10]. Institute of Semiconductors reported 442 nm blue LDs with output power of 6 W at 5 A[11]. Suzhou Institute of Nano-tech and Nano-bionics reported 442 nm blue LDs with output power of 7.5 W at 6 A[12]. Tsinghua University and Anhui GaN Semiconductor achieved a maximum output power of 15 W and the wall plug efficiency of 38% under pulse operation conditions[13]. Nichia reported 455 nm blue LDs with output power of 5.99 W, wall plug efficiency of 52.4% at 3 A, and lifetime more than 30 000 h[14], which is only reported with long lifetime.
Our group reported the first high power blue GaN-based LD and the first green GaN-based LD in China[15−17]. We then greatly improved green LD performance by using new structure with ITO cladding layer[18]. UCSB also reported GaN-based LDs using ITO as cladding layer[19], with the device performance only comparable to conventional ones, which could be caused by the problems of high contact resistance and ITO absorption. In this article, we report GaN-based blue laser diodes with ITO cladding layer with reduced contact resistance and absorption coefficient. Output power of 5 W and wall plug efficiency of 41% have achieved at operation current of 3 A. Moreover, lifetime over 20 000 h has been achieved for LDs aged at 60 °C case temperature.
Schematic LD layer structure is shown in Fig. 1, where hybrid GaN-based LD structure use ITO layer to replace part of conventional p-AlGaN cladding layer as a cladding layer and metal p-electrode, which has several advantages over conventional p-AlGaN cladding layer, such as much lower resistance, better optical confinement, lower internal loss, and no thermal budget on InGaN QWs[18]. Because the refractive index of ITO is much lower than that of the p-AlGaN cladding layer, it can provide more sufficient optical confinement. Meanwhile, the temperature to deposit ITO can be around 300 °C or even room temperature, therefore reduce the thermal budget imposed on InGaN QWs and thermal degradation of InGaN QWs. Moreover, the absorption coefficient of ITO is 2 orders of magnitude lower than that of metal, which means the internal loss of hybrid GaN-based LDs with ITO layer can be lower.
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We simulated and compared conventional blue LDs and hybrid blue LDs with ITO layer by the transfer matrix method[20]. The Internal loss as a function of p-AlGaN cladding layer thickness for conventional and hybrid LDs is shown in Fig. 2. In regard to the conventional LDs, when the p-AlGaN cladding layer thickness decreases, the internal loss has a quick increase which results from the significant increase of the absorption in metal p-electrode. As for the hybrid ITO LDs, the internal loss decreases at 300 nm, on the contrary, and then increases slightly. Our internal loss for hybrid LDs is almost one order of magnitude lower than that reported by UCSB group[19].
The LD epitaxial structure with p-AlGaN cladding layers of 300 nm was grown by metal−organic chemical vapor deposition (MOCVD) on c-plane GaN substrates. In order to achieve high power, high wall plug efficiency and long lifetime GaN-based blue LDs, we have optimized the crystalline quality of LD structure, the structure of the LDs, the manufacturing process, and the heat dissipation of p-down packaging[12, 21−24].
The blue LDs with a ridge width of 45 μm and a cavity length of 1200 μm were fabricated and the LD characteristics were measured under continuous conditions at room temperature. The typical power−current−voltage (P−I−V) curves for optimized hybrid GaN-based LDs with ITO cladding layer are shown in Fig. 3(a), the threshold current density is 0.7 kA/cm2, and the slope efficiency is 1.9 W/A. The output power of 5 W and wall plug efficiency of 41% have been achieved at operation current of 3 A. And the lasing wavelength is 450 nm as shown in Fig. 3(b).
We also measured the high-temperature characteristics of the optimized hybrid GaN-based LDs with ITO cladding layer. The P−I−V curves of blue LDs at 60 °C are shown in Fig. 4(a), the threshold current density increases to 1 kA/cm2, and the slope efficiency drops to 1.76 W/A. The output power is 4.3 W at the current of 3 A. Meanwhile, the aging test was performed at operation current of 3 A and 60 °C case temperature. The output power slightly decreases after more than 1000 h of aging, and the extrapolated high-temperature lifetime exceeded 20 000 h, which is shown in Fig. 4(b).
In summary, hybrid GaN-based blue LDs with ITO cladding layer were developed. Output power of 5 W and wall plug efficiency of 41% have been achieved at operation current of 3 A. Moreover, lifetime over 20 000 h has been achieved for LDs aged at 60 °C case temperature.
Acknowledgments
This work was supported by the Natural Science Foundation of Jiangsu Province (Grant. BK20232042).
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