垂直腔面发射半导体激光器氧化优化研究【增强内容出版】
Vertical cavity surface-emitting lasers (VCSELs) have advantages such as a single longitudinal mode, a low threshold, and ease of two-dimensional integration. VCSELs have been widely used in data transmission, optical communication, and three-dimensional sensing. Oxidation is the most common process for oxide-confined VCSELs. AlGaAs materials with high Al contents are oxidized via wet oxidation to form oxide apertures of aluminum oxide, and the structures of oxide apertures with different shapes and sizes have different effects on the optoelectronic characteristics of VCSELs. However, during the actual oxidation of the AlGaAs oxide confinement layer, the shape and size of the oxide aperture do not satisfy expectations because of various factors, which adversely affect the performance of the device in terms of the excitation mode, threshold current, and divergence angle. In this study, the dry etching and wet oxidation processes of VCSELs are experimentally investigated, and an optimized process scheme for oxidation pretreatment that combines dry etching and (NH4)2S passivation is developed. An (NH4)2S solution is used to passivate the table structure after dry etching, which achieves a stable oxidation rate and improves the quality of the oxide aperture shapes, further improving the optoelectronic characteristics of VCSELs and extending the applications of VCSELs in optoelectronics.
In this study, an (NH4)2S solution is used. Prior to oxidation, a cleaned VCSEL is passivated in a (NH4)2S (sulfur mass fraction >8%) solution in a heated water bath. After oxidation, the surface and sidewall microstructures of the VCSEL are observed using scanning electron microscopy (SEM). The shapes and sizes of the oxidation apertures of the VCSELs are observed separately using a microscope, and the oxidation rates of the oxidation apertures are determined. Based on this, the photoelectric properties of the unpassivated and passivated VCSELs are comparatively analyzed.
After wet oxidation, the layered structure of the unpassivated VCSEL undergoes fracturing and separation, and the VCSEL structure undergoes distortion [Fig.3(a)]. However, the passivation-pretreated VCSEL exhibits less significant fracture and delamination and good sidewall integrity [Fig.3(b)]. The passivated VCSEL [Figs.4(a1) and (a2)] has smoother oxide hole edges and more regular oxide aperture shapes than the unpassivated VCSEL [Figs.4(b1) and (b2)]. With an increase in the oxidation depth, the oxidation aperture of the passivated VCSEL has a somewhat diamond shape [Fig.4(a3)], whereas that of the unpassivated VCSEL has an irregular pentagonal shape [Fig.4(b3)]. The oxidation rate of the unpassivated VCSEL always exceeds that of the passivated VCSEL (Fig.5). The test results (Fig.6) show that the saturated output power of the passivated VCSEL is stable at 6.16 mW, whereas that of the unpassivated VCSEL varies between 5.18 mW and 6.14 mW. Moreover, the slope efficiency of the unpassivated VCSEL fluctuates within 0.40?0.42 W/A, and the slope efficiency of the passivated VCSEL is improved by 5% and stabilizes at 0.44 W/A. In conclusion, the passivated VCSEL exhibits improved device performance consistency, whereas the unpassivated VCSEL exhibits unstable device performance. Variability in the performance of both devices exists. In addition, the threshold currents of both VCSELs are close to 0.80 mA, but the threshold currents of the passivated VCSEL decrease to 0.72 mA. As shown in Fig.7(a), the side-mode rejection ratio of the passivated VCSEL reaches up to 36 dB at a driving current of 1 mA, whereas that of the unpassivated VCSEL is 22 dB, with the appearance of two excitation modes. When the current reaches eight times the threshold, the passivated VCSEL excites two modes, and a third mode gradually starts to appear but still manages to maintain a few mode outputs [Fig.7(b)]; in comparison, the unpassivated VCSEL appears with four or more modes [Fig.7(c)].
In this study, the effect of a preoxidation pretreatment process scheme that combines dry etching and (NH4)2S passivation on the sidewall integrity and oxide aperture of a VCSEL is investigated. The (NH4)2S passivation technology can effectively remove nontarget products, such as oxides, on the sidewall of the stage and minimize device delamination and fracturing during oxidation, improving the sidewall integrity and sample quality. The oxidation rate of the high-alumina component AlGaAs layer on the sidewall is more uniform and stable, and the oxide aperture shape is regular. Based on this, the passivation process is applied to prepare oxide-confined VCSELs with a 5-μm-diameter oxide aperture. Comparison experiments show that the maximum slope efficiency and threshold current characteristics of the VCSEL prepared by this process improve, and the device performance consistency is enhanced. The side-mode rejection ratio of the passivated VCSEL can reach 36 dB at a driving current of 1 mA in a single-mode excitation state. This study shows that the proposed oxide-optimized process scheme based on dry etching and (NH4)2S passivation is beneficial for the preparation of oxide aperture structures with regular shapes and good follow-through, which improves the structural stability of the device and the device performance of oxide-confined VCSELs.
1 引言
垂直腔面发射激光器(VCSEL)具备单纵模、低阈值、易于二维集成等优点[1-2],已在数据传输[3]、偏振开关[4]、光通信[5]、3D传感[6]、泵浦源[7]等领域中得到广泛的应用。氧化工艺是氧化限制型VCSEL最常用的制备技术[8],氧化后所形成的Al2O3在器件上形成电流注入限制窗口,从而显著改善器件的阈值特性和光束质量[9]。但是,湿法氧化过程中存在诸多难以解决的困难,很难实现氧化孔径形状和大小的精确制备,氧化后器件的外延结构易发生畸变等问题。
为了改善湿法氧化工艺的可控性和稳定性,国内外学者对湿法氧化的机理以及氧化的影响因素等进行了大量的研究。2018年,李颖等[10]通过自制的红外光源显微镜和电荷耦合器件(CCD)观测系统对氧化过程进行了实时的动态监测,显著提高了氧化孔形状和大小的制备精度。2020年,陈磊等[11]对湿法氧化过程的热稳定性进行了细致研究,探究了氧化温度对氧化速率、氧化孔形状的影响,通过对氧化温控曲线进行优化,提高了氧化孔形状和大小的可控性。2022年,Fabbro等[12]利用扫描透射电子显微镜(TEM)对AlAs层在氧化过程中形成的缺陷进行了细致的研究,提出了一种原位TEM分析的失效分析方法,有助于开发高度可靠的VCSEL。同年,张玉岐等[13]深入研究了导致VCSEL氧化区域位错缺陷形成的主要因素,氧化过程中氧化层的体积收缩以及氧化应力将会导致层结构出现分层或者裂纹现象,严重影响VCSEL的可靠性。
本文对VCSEL的干法刻蚀和湿法氧化工艺进行了实验研究,提出了一种干法刻蚀和硫化铵钝化相结合的氧化预处理优化工艺方案:采用(NH4)2S溶液对干法刻蚀后的台面结构进行钝化处理,实现了稳定的氧化速率,改善了氧化孔形状质量。利用扫描电子显微镜(SEM)观察钝化氧化后样品的表面和侧壁微观结构,验证了台面表面和侧壁层结构无明显畸变现象,各层结构的线条分明、清晰整齐,氧化后器件的侧壁层结构稳定可靠。对两种不同工艺制备的VCSEL光电特性进行显微镜观察和测试分析,结果表明:新工艺方案制备的VCSEL氧化孔形状更为规则;器件结构更加稳定,器件斜率效率提高了5%;在1 mA的驱动电流下,激光器的边模抑制比(SMSR)为36 dB,处于单模激射状态。随着电流的增大,钝化后的器件依旧能保持少模激射状态。
2 实验
利用金属有机化学气相淀积(MOCVD)技术在n型GaAs衬底上生长出VCSEL外延结构,
本文提出了一种干法刻蚀和硫化铵钝化相结合的氧化前预处理工艺方案,在传统的电感耦合等离子体(ICP)干法刻蚀工艺之后,增加了湿法钝化工艺,之后再进行湿法氧化,从而制备出形状规则的氧化孔结构,具体工艺流程如
图 2. 基于硫化铵湿法钝化的VCSEL的氧化孔制备工艺流程图
Fig. 2. Production process of oxide aperture for VCSEL based on (NH4)2S wet passivation
首先将VCSEL外延片洗净,通过等离子体增强化学气相沉积(PECVD)法蒸镀SiO2薄膜;利用光刻技术和湿法腐蚀技术将掩模层图形转移到SiO2薄膜上;采用电感耦合等离子增强反应刻蚀机对VCSEL上的DBR进行干法刻蚀,以确保Al0.98Ga0.02As氧化限制层暴露出来,便于后续进行氧化孔的湿法氧化制备;氧化前,将清洗完成的样品放进(NH4)2S溶液(硫的质量分数>8%)进行水浴加热钝化。为了分析硫化铵溶液的钝化作用对器件侧壁结构的影响,讨论了硫化铵钝化对氧化速率、氧化孔形状的改善作用。在420 ℃炉温、96 ℃水浴温度、1.1 mL/min氧化载气N2流量的工艺条件下,将未经过硫化铵钝化处理的样品1和已经过硫化铵钝化处理的样品2同炉次放入氧化装置进行氧化孔的湿法氧化制备;通过磁控溅射工艺制备正面(P面)电极,并减薄器件衬底的厚度,制备背面(N面)电极;对器件进行退火处理,使得正面电极和背面电极能够形成良好的欧姆接触[14],最终制备得到VCSEL。
3 实验结果与讨论
3.1 样品结构稳定性的改善
经过刻蚀的GaAs台面与外界接触的表面以及侧壁处的半导体材料的化学性质极其活跃[15],这些界面态暴露在空气中会被潮解和氧化形成GaAs氧化物[16],这种不易挥发的非目标产物易残留在晶格中,导致器件的p-DBR在氧化过程中出现分层现象[11]。DBR是由多层高Al组分和低Al组分的GaAs材料交叠形成的,这些层结构在湿法氧化过程中也会被水汽氧化,被氧化的层结构的氧化应力会累积,从而破坏层结构稳定性。同时,各层累积而成的应力甚至会改变有源区的带隙,从而影响器件的性能,所以除去非目标产物是成功制备高性能激光器的关键。因此,在刻蚀之后、氧化之前,对器件进行合适的钝化处理,避免这类非目标产物的生成。结合上述氧化机理,采用硫化铵对刻蚀后的台面进行湿法钝化预处理。在钝化过程中,(NH4)2S首先水解生成NH4+和S2-,之后进一步反应产生HS-,器件侧壁及表面的镓砷氧化物在碱性(NH4)2S溶液中浸泡时发生溶解,GaAs生成的氧化物被去除[17],并进一步形成镓的硫化物和砷的硫化物钝化层[18]。镓的硫化物有着较好的稳定性,可抑制侧壁与环境的反应,提供了一个保护层,有效地抑制了GaAs表面活性,减小了界面态密度,实现了GaAs的钝化。
增加上述钝化过程,一方面,可以有效清除As2O3、As2O5、Ga2O3等非目标反应产物,减少这些非目标产物在晶格结构中的残留。另一方面,硫化铵钝化作用形成的硫化物在侧壁上形成一层隔离层,减缓了湿法氧化作用对DBR层结构的氧化速率,有利于层结构的氧化应力的释放,从而改善台面结构的完整性。质量良好的侧壁层结构有利于高Al组分层Al0.98Ga0.02As在湿法氧化过程中获得均匀稳定的氧化速率,从而制备出形状规则的氧化孔结构。
图 3. 不同氧化工艺条件下样品侧壁的SEM照片。(a)样品1,传统氧化;(b)样品2,钝化后氧化
Fig. 3. SEM pictures of sample sidewalls under different oxidation process conditions. (a) Sample 1,traditional oxidation; (b) sample 2, oxidation after passivation
3.2 硫化铵钝化处理对氧化孔形状的影响
GaAs氧化物在氧化过程中对层结构的破坏影响了规则氧化孔的制备,ICP刻蚀工艺不可避免对晶格完整性造成破坏形成损伤,导致氧化时难以形成规则的形状。在AlGaAs氧化限制层的实际氧化过程中,多种因素导致所制备的氧化孔形状和大小不符合预期,这对器件的激射模式[20]、阈值电流[21]、发散角[22]等产生了严重影响。因此,对于特定高Al组分的VCSEL,如何稳定可控地制备形状规则的氧化孔是器件制备过程中亟须解决的关键问题。
在氧化前,利用硫化铵钝化处理刻蚀后的台面。
3.3 硫化铵溶液对高Al组分AlGaAs层氧化速率的影响
将未经过硫化铵钝化处理的样品1和已经过硫化铵钝化处理的样品2同炉次放入氧化炉,炉温设置为420 ℃,鼓泡器中的去离子水温度设置为95 ℃,氧化载气N2流量设置为1.1 mL/min,控制上述条件不变,氧化时间分别设置为5.0、10.0、17.5、24.0、35.5、43.5 min。实验结束后先利用GaAs腐蚀液除去p-DBR,再通过显微镜观测氧化深度。
实验结果还表明,样品1在各个时间段的氧化速率都大于样品2。这是因为经过硫化铵钝化处理的样品侧壁的高Al组分层出现了一层薄的Ga和As的硫化物,所以水汽扩散进入高Al组分层的量少且扩散速度慢,这种薄的Ga和As的硫化物有效解决了氧化速率过大的问题,改善了氧化反应的可控性。缓慢且均匀的氧化速率有助于应力释放,提高了器件结构的稳定性。
3.4 器件性能测试
将上述制备的孔径为5 μm的氧化孔结构作为光电限制层,应用于940 nm垂直腔面发射激光器。首先,在室温(25 ℃)下分别对经过硫化铵钝化处理的和未经过钝化处理的VCSEL进行光电输出特性测试。
图 6. 室温(25 ℃)下氧化孔径为5 μm的940 nm VCSEL的P‐I‐V测试曲线
Fig. 6. P-I-V test curves of 940 nm VCSEL with oxide aperture size of 5 μm at room temperature (25 ℃)
室温下对这两种不同工艺制备的VCSEL进行了光谱测试,测用设备为光谱仪,其分辨率可达到0.02 nm。如
图 7. 室温(25 ℃)下VCSEL的光谱测试图。(a)当驱动电流为1 mA时,两种工艺条件下VCSEL的光谱对比图;(b)当电流达到8倍阈值时,未钝化的VCSEL的光谱图;(c)当电流达到8倍阈值时,钝化后的VCSEL的光谱图
Fig. 7. Measured spectra of VCSEL at room temperature (25 ℃). (a) Comparison of spectra of VCSEL under two process conditions when driving current is 1 mA; (b) spectrum of VCSEL before passivation when current is 8Ith; (c) spectrum of VCSEL after passivation when current is 8Ith
4 结论
通过理论及实验研究了干法刻蚀与硫化铵钝化相结合的氧化前预处理工艺方案对VCSEL侧壁完整性以及氧化孔制备的影响。实验结果表明:硫化铵钝化技术可以有效地去除台面侧壁的氧化物等非目标产物,减少氧化工艺过程中的器件分层和断裂现象,使得侧壁完整性更好,样品品质有所提高;侧壁的高铝组分AlGaAs层的氧化速度更加均匀稳定,氧化孔形状规则。将该工艺方案用于制备氧化孔直径为5 μm的氧化限制型垂直腔面发射激光器,对比实验结果表明,利用该工艺方案制备的器件的最大斜率效率和阈值电流特性均有所改善,器件性能一致性更好。在1 mA的驱动电流下,激光器的边模抑制比可达36 dB,处于单模激射状态。因此,该基于干法刻蚀和硫化铵钝化的氧化优化工艺方案有助于提高器件结构的稳定性,改善氧化限制型垂直腔面发射激光器的器件性能,并为使用干法刻蚀技术和湿法氧化技术制备形状规则、跟随性良好的GaAs基VCSEL氧化孔结构提供了参考。
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Article Outline
陈中标, 崔碧峰, 郑翔瑞, 杨春鹏, 闫博昭, 王晴, 高欣雨. 垂直腔面发射半导体激光器氧化优化研究[J]. 中国激光, 2024, 51(8): 0801003. Zhongbiao Chen, Bifeng Cui, Xiangrui Zheng, Chunpeng Yang, Bozhao Yan, Qing Wang, Xinyu Gao. Optimization of Oxidation for Vertical Cavity Surface Emitting Semiconductor Lasers[J]. Chinese Journal of Lasers, 2024, 51(8): 0801003.