Ultraviolet (UV) and deep-UV light emitters are promising for various applications including bioagent detection, water and air purification, dermatology, high-density optical storage, and lithography. However, to achieve shorter UV laser diodes (LDs), especially for the LDs with lasing wavelength less than 360 nm, requires high AlN mole fraction AlGaN cladding layer (CL) and waveguide (WG) layers, which usually leads to generate cracks in AlGaN epilayer due to lack of lattice-matched substrates. Meanwhile, due to high resistivity of high AlN mole fraction Mg doped AlGaN layers, only few groups have reported GaN-based LDs with emission wavelength shorter than 360 nm[1−8], and up to now, there is no room temperature continuous-wave (CW) operation UV LDs with a lasing wavelength shorter than 360 nm ever reported. Previously, we have reported a UV LD with lasing wavelength of 366 nm[9]. In this paper, a higher AlN mole fraction of AlGaN WG layers is employed to shorten the LD emission wavelength to less than 360 nm. A lasing wavelength of 357.9 nm is achieved. Firstly, two GaN-based UV LD structures with different AlN mole fraction of AlGaN WG layers were grown on c-plane GaN substrate by metal organic chemical vapor deposition (MOCVD), TMGa/TEGa, TMAl, TMIn and NH3 were used as Ga, Al, In and N sources, respectively. The schematic diagram of the structure of GaN-based UV LDs is shown in Fig. 1(a). It consists of a 1-μm thick n-type GaN layer, a 500 nm n-type Al0.07Ga0.93N cladding layer (CL) with n-doping concentration of 3 × 1018 cm–3, a 20 nm n-type Al0.25Ga0.75N hole blocking layer (HBL), a AlGaN lower WG (LWG) layer, an unintentional doped GaN/Al0.07Ga0.93N multiple quantum well (MQW) active region, a AlGaN upper WG (UWG) layer, a 20-nm p-type Al0.3Ga0.7N electron blocking layer (EBL), a 500-nm p-type Al0.07Ga0.93N CL layer, and a 40 nm p-GaN contact layer. After the epitaxial growth, a 10 μm-wide ridge stripe along the <1-100> direction was formed by dry etching on the epitaxial layers. The cavity of the laser diode with a length of 600 μm was fabricated by cleaving the epitaxial film and substrate together along the {1-100} plane. A Ni/Au contact was evaporated onto the p-type GaN layer and a Ti/Al contact was evaporated onto the n-type GaN layer. Then the output power versus current (P–I) curves were recorded at room temperature using a calibrated Si detector. The RT output power of one fabricated UV LDs, LD1, which is with about 4.5% AlN mole fraction AlGaN WG layers, as a function of injected current is measured under pulsed operation condition and shown in Fig. 1(b). It is found that the output power of LD1 is nonlinear with the injection current and it has an abrupt increase at the injection current of 1.5 A, corresponding to 25 kA/cm2. This threshold current value of lasing is similar to the reported ones by other groups[3, 4, 10–12]. The output power is about 11 mW at an injection current of 1.7 A. The inset shows the photo of lasing LD1 and the far field pattern of laser beam in blue color formed on the white paper screen. The leakage modes in the laser beam are not observed because the UV light should be absorbed by thick GaN layers existing in both n side and p side. Fig. 1(c) shows the electroluminescence spectrum of UV LD under pulsed operation condition at an injection around the threshold current of 1550 mA. It can be seen that the peak wavelength is 357.9 nm, and the full width at half maximum (FWHM) is about 0.3 nm which is obtained by Gaussian fitting to the emission peak. Another prepared UV LD, LD2, has a lower AlN mole fraction of AlGaN WG layer, which is approximately 3.5% for both lower and upper WG layers. The front and rear cleaved facets of laser diode were uncoated. The P–I curve and stimulation emission spectrum of LD2 are presented in Fig. 1(d). Compared with LD1, the wavelength of stimulation emission increases about 5 to 362.6 nm. Such an increase is attributed to the increased absorption of high energy UV light in the AlGaN WG layers with lower AlN mole fraction. In addition, it is found that in comparison with LD1, the optical characteristics of LD2 are improved obviously. The threshold current of LD2 is 760 mA, corresponding to 12.7 kA/cm2. The output power is 258 mW at the injection current of 1.9 A and the slope efficiency is about 0.23 W/A. The higher threshold current density of LD1 compared with LD2 may be attributed to the weaker optical confinement factor, which is due to a smaller difference in the AlN mole fraction of AlGaN CL and WG layers for LD1. It leads to a large percentage of light penetrating into CL or even to GaN layers. Therefore, the absorption loss of LD1 is larger than that of LD2. However, it is noted that during the growth of Al Correspondence to: D G Zhao, dgzhao@red.semi.ac.cn Received 13 DECEMBER 2021. ©2022 Chinese Institute of Electronics SHORT COMMUNICATION Journal of Semiconductors (2022) 43, 010501 doi: 10.1088/1674-4926/43/1/010501

中文翻译：

---

357.9 nm GaN/AlGaN 多量子阱紫外激光二极管

紫外 (UV) 和深紫外光发射器有望用于各种应用，包括生物制剂检测、水和空气净化、皮肤病学、高密度光存储和光刻。然而，为了实现更短的紫外激光二极管（LD），特别是对于激光波长小于 360 nm 的 LD，需要高 AlN 摩尔分数的 AlGaN 包覆层（CL）和波导（WG）层，这通常会导致 AlGaN 产生裂纹由于缺乏晶格匹配的衬底，外延层。同时，由于高 AlN 摩尔分数 Mg 掺杂的 AlGaN 层的高电阻率，只有少数研究小组报道了发射波长小于 360 nm [1-8] 的 GaN 基 LD，到目前为止，还没有室温连续 -激光波长小于 360 nm 的波 (CW) 操作 UV LD 曾有报道。之前，我们已经报道了一种激光波长为 366 nm [9] 的 UV LD。在本文中，采用较高的 AlN 摩尔分数的 AlGaN WG 层将 LD 发射波长缩短至小于 360 nm。实现了 357.9 nm 的激光波长。首先，通过金属有机化学气相沉积 (MOCVD) 在 c 面 GaN 衬底上生长两种具有不同 AlN 摩尔分数的 AlGaN WG 层的 GaN 基 UV LD 结构，以 TMGa/TEGa、TMAl、TMIn 和 NH3 作为 Ga，分别为 Al、In 和 N 源。GaN基UV LD的结构示意图如图1(a)所示。它由 1 μm 厚的 n 型 GaN 层、500 nm n 型 Al0.07Ga0.93N 覆层 (CL)（n 掺杂浓度为 3 × 1018 cm–3）、20 nm n 型 Al0 .25Ga0.75N 空穴阻挡层 (HBL)、AlGaN 下 WG (LWG) 层、无意掺杂的 GaN/Al0.07Ga0。93N 多量子阱 (MQW) 有源区、AlGaN 上 WG (UWG) 层、20-nm p 型 Al0.3Ga0.7N 电子阻挡层 (EBL)、500-nm p 型 Al0.07Ga0.93N CL 层和 40 nm p-GaN 接触层。外延生长后，通过干法刻蚀在外延层上形成沿<1-100>方向的10μm宽的脊条。通过沿{1-100}平面将外延膜和衬底切割在一起，制造了长度为600μm的激光二极管的腔体。Ni/Au接触被蒸发到p型GaN层上并且Ti/Al接触被蒸发到n型GaN层上。然后使用校准的 Si 检测器在室温下记录输出功率与电流 (P-I) 曲线。一个制造的 UV LD LD1 的 RT 输出功率，它具有约 4.5% 的 AlN 摩尔分数 AlGaN WG 层，在脉冲操作条件下测量注入电流的函数，如图1（b）所示。发现 LD1 的输出功率与注入电流呈非线性关系，在注入电流为 1.5 A 时急剧增加，对应 25 kA/cm2。这个激光的阈值电流值与其他组报道的相似[3, 4, 10-12]。在注入电流为 1.7 A 时，输出功率约为 11 mW。插图显示了激光 LD1 的照片和在白纸屏幕上形成的蓝色激光束的远场图案。没有观察到激光束中的泄漏模式，因为紫外光应该被存在于 n 侧和 p 侧的厚 GaN 层吸收。如图。图 1(c) 显示了在脉冲操作条件下，在 1550 mA 的阈值电流附近注入时 UV LD 的电致发光光谱。可以看出，峰值波长为 357.9 nm，半峰全宽（FWHM）约为 0.3 nm，通过对发射峰的高斯拟合得到。另一种制备的 UV LD，LD2，具有较低的 AlGaN WG 层的 AlN 摩尔分数，对于下部和上部 WG 层约为 3.5%。激光二极管的前后切割面没有涂层。LD2 的 P-I 曲线和刺激发射光谱如图 1(d) 所示。与LD1相比，刺激发射的波长增加了约5至362.6 nm。这种增加归因于具有较低AlN摩尔分数的AlGaN WG层中高能UV光的吸收增加。此外，发现与LD1相比，LD2的光学特性明显提高。LD2 的阈值电流为 760 mA，对应 12.7 kA/cm2。注入电流为 1.9 A 时输出功率为 258 mW，斜率效率约为 0.23 W/A。与 LD2 相比，LD1 的阈值电流密度较高可能是由于光限制因子较弱，这是由于 LD1 的 AlGaN CL 和 WG 层的 AlN 摩尔分数差异较小。它导致很大比例的光穿透到 CL 甚至 GaN 层。因此，LD1 的吸收损耗大于 LD2。但是，需要注意的是，在 Al 成长过程中，发给：DG Zhao，dgzhao@red.semi.ac.cn 的信件于 2021 年 12 月 13 日收到。