有源区Be掺杂对1.3 μm InAs量子点激光器性能的影响
1 引言
自组织量子点具有与原子近似的分立能级和类δ函数的态密度,表现出独特的光学和电学特性,近年来受到人们的广泛关注[1-3]。量子点激光器具有低阈值电流密度、高特征温度、高增益分布、大调制带宽和长寿命等优点,展现出比量子阱激光器更加优越的性能[4-10]。与量子阱相比,量子点对位错的敏感度也大大降低。位错在穿透量子阱或块状材料时,容易产生大量的非辐射复合中心。然而,对于量子点而言,穿透位错只能破坏几个量子点而不影响其他量子点,因此基于量子点器件的性能不会显著降低[11-13]。InAs/GaAs量子点激光器是目前研究最为广泛的半导体量子点发光器件之一,它可以实现不含磷化物、氮化物和锑化物的1.3 μm重要光纤通信窗口的激射,在光通讯应用中具有很大潜力。InAs和GaAs两种单晶的晶格失配为7.2%,因此在GaAs上生长InAs易于实现 Stranski-Krastanov(SK)生长模式[14],并且InAs量子点材料与GaAs材料体系的兼容性好。自1994年第一个基于应变自组装生长模式的 InAs/GaAs量子点激光器研制成功以来[15],在器件性能和工作波长范围的研究都取得了长足的进步。近年来,InAs量子点在单光子领域的应用也取得了重要进展[16]。
利用分子束外延(MBE)技术在GaAs上直接生长InAs量子点很难把量子点发光波长拓展至1.3 μm[17]。有源区采用InAs DWELL(dot-in-well)结构减小了量子点周围的势垒和应力分布,可以将InAs量子点的发光波长拓展至1.3 μm。InAs量子点激光器的性能受到很多方面的影响,因此对生长条件的优化是很重要的一环。目前对不同的生长温度、量子点厚度、有源区层数、In生长速率等已经有了很多的研究[18-20],但是对有源区掺杂的研究还较少。有源区掺杂对激光器性能有很大影响,对其进行细致研究很有必要。
本文利用分子束外延技术在GaAs衬底上生长了不同的InAs DWELL量子点激光器结构。研究有源区Be掺杂对激光器性能的影响,结果表明对有源区进行Be掺杂可以有效降低InAs量子点激光器的阈值电流密度,提升激光器的输出功率,增加激光器的温度稳定性。
1 实验
使用DCA固态源III-V分子束外延系统在4英寸n+ GaAs(100)衬底上生长了InAs量子点激光器结构,激光器结构示意图如
图 1. GaAs基InAs量子点激光器结构示意图
Fig. 1. The schematic diagram of GaAs based InAs quantum dot laser structure
GaAs基InAs量子点激光器的工艺采用标准的光刻和刻蚀工艺。刻蚀至有源区上层的GaAs波导层制作脊条,然后沉积200 nm的SiNx。继续在脊条上的上电极GaAs接触层用反应离子刻蚀(RIE)去掉SiNx开好窗口,沉积Ti/Pt/Au形成P型欧姆接触,然后将GaAs衬底减薄至150 μm,沉积GeAu/Ni/Au后退火形成N型欧姆结接触。
2 结果与讨论
GaAs基未掺杂和Be掺杂InAs量子点激光器材料的光致荧光谱(PL)测试结果如
图 3. GaAs基未掺杂和Be掺杂InAs量子点激光器的PL光谱,插图:InAs量子点2×2 µm2 AFM图像
Fig. 3. The PL spectra of GaAs-based undoped and Be-doped InAs quantum dot lasers,Inset:a 2×2 µm2 AFM image of InAs quantum dots
为了检验生长的量子点结构的质量,我们在生长量子点激光器材料时生长了InAs量子点结构测试片,测试片在生长 InAs DWELL结构后,在表面沉积了一层InAs 量子点用于原子力显微镜(AFM)的测试。有源区Be掺杂对表面的量子点影响不大,量子点形貌与大小没有明显区别,
典型GaAs基未掺杂和Be掺杂InAs量子点激光器的IVP曲线如
图 4. 连续工作模式下GaAs基InAs量子点激光器的IVP特性曲线:(a)未掺杂,(b)Be掺杂
Fig. 4. The IVP characteristic curves of GaAs-based InAs quantum dot laser in CW mode:(a)undoped,(b)Be-doped
对腔长为4 mm,脊条宽度分别为10 μm和50 μm的两种不同的激光器进行了最大输出功率的测试,测试结果如
图 5. 连续工作模式下不同条宽GaAs基InAs量子点激光器的IP特性曲线:(a)10 μm,(b)50 μm
Fig. 5. The IP characteristic curves of GaAs-based InAs quantum dot laser with different ridge widths in CW mode:(a)10 μm,(b)50 μm
InAs量子点激光器具有良好的温度稳定性,在温度20 ℃下激光器发射光谱随注入电流的变化如
图 6. 20 ℃时GaAs基InAs量子点激光器发射光谱随注入电流的变化:(a)未掺杂,(b)Be掺杂
Fig. 6. The emission spectra of GaAs-based InAs QD laser with different injection currents at the temperature of 20 ℃:(a)undoped,(b)Be-doped
在注入电流为350 mA时量子点激光器的发射光谱随温度的变化如
图 7. 注入电流350 mA时GaAs基InAs量子点激光器发射光谱随温度的变化:(a)未掺杂,(b)Be掺杂
Fig. 7. The emission spectra of GaAs-based InAs QD laser with different temperatures at the same injection current of 350 mA:(a)undoped,(b)Be-doped
激光器的温度稳定性是衡量激光器性能的重要指标之一。半导体激光器的阈值电流与温度成指数关系[24]:
图 8. GaAs基InAs量子点激光器特征温度的变化曲线
Fig. 8. The characteristic temperature curves of GaAs-based InAs quantum dot laser
3 结论
利用分子束外延技术在GaAs(100)衬底上成功制备了1.3 μm波段的 InAs/GaAs量子点激光器。在室温连续工作模式下,对未掺杂和Be掺杂InAs量子点激光器的输出性能、光谱性能和温度稳定性进行了测试分析与比较。与未掺杂的激光器相比,Be掺杂可以有效地降低激光器的阈值电流密度到100 A/cm2,增大激光器的输出功率到183 mW,最高工作温度可以提高到130 ℃,激光器具有更好的温度稳定性。这对InAs量子点激光器在高速光纤通信领域的应用具有重要意义。
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杜安天, 曹春芳, 韩实现, 王海龙, 龚谦. 有源区Be掺杂对1.3 μm InAs量子点激光器性能的影响[J]. 红外与毫米波学报, 2023, 42(4): 450. An-Tian DU, Chun-Fang CAO, Shi-Xian HAN, Hai-Long WANG, Qian GONG. Effect of Be doping in active regions on the performance of 1.3 μm InAs quantum dot lasers[J]. Journal of Infrared and Millimeter Waves, 2023, 42(4): 450.