张玉岐 左致远 阚强 赵佳
Common failure modes and mechanisms in oxide vertical cavity surface emitting lasers
ZHANG Yu-qi, ZUO Zhi-yuan, KAN Qiang, ZHAO Jia
引用本文:
张玉岐，左致远，阚强，赵佳. 氧化型垂直腔面发射激光器的常见失效模式和机理分析[J]. 中国光学, 2022, 15(2): 1-23. doi: 10.37188/CO.EN.2021-0012
ZHANG Yu-qi, ZUO Zhi-yuan, KAN Qiang, ZHAO Jia. Common failure modes and mechanisms in oxide vertical cavity surface emitting lasers[J]. Chinese Optics, 2022, 15(2): 1-23. doi: 10.37188/CO.EN.2021-0012
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第 15 卷 第 2 期中国光学Vol. 15 No. 2 2022年3月Chinese Optics Mar. 2022文章编号 2095-1531（2022）02-0001-23
Common failure modes and mechanisms in oxide vertical cavity
surface emitting lasers
ZHANG Yu-qi1,2，ZUO Zhi-yuan1，KAN Qiang3，ZHAO Jia1,4 *
（1. Key Laboratory of Laser & Infrared System, Shandong University, Qingdao 266237, China；
2. Xiamen San 'An Integrated Circuit Co., LTD, Xiamen 361000, China；
3. Institute of Semiconductors, University of Chinese Academy of Sciences, Beijing 100083, China；
4. School of Information Science and Engineering, Shandong University, Qingdao 266237, China）
光栅单元* Corresponding author，E-mail: zhaojia@
Abstract: Oxide Vertical Cavity Surface Emitting Lasers(VCSELs) are widely used in high-speed optical communications. The reliability of VCSELs is a very important index which requiring a high lifetime and low failure rate in the application process. Understanding the root causes and mechanisms of VCSEL fails is necessary and helpful to improve device reliability. In this paper, we s
交通事故现场图ummarize and analyze the most com-mon failure modes, causes and mechanisms observed in oxide VCSELs from three main aspects of design, manufacturing and application, and some appropriate measures and suggestions are applied to prevent or im-prove them. Moreover, the three dominating factors leading to the failure of VCSELs including oxide layer stress, ESD and humidity corrosion are introduced in more detail. This article can be used as a good VCSEL failure analysis library for chip development and production researchers. At finish, we will simply introduce the VCSEL failure cases encountered in the actual accelerated aging verification process for more references. Key words: VCSEL; oxide; failure modes and mechanisms; reliability
收稿日期：2021-11-22；修订日期：xxxx-xx-xx
基金项目：国家重点研发计划纳米专项课题(No. 2018YFA0209001)；国家重点研发计划课题(No. 2018YFA 0209002，No. 2018YFB2200700) Supported by the Nano Special Project of National Key Research and Development Program(No.
2018YFA0209001); National Key Research and Development Project(No. 2018YFA0209002, No.
2018YFB2200700)
氧化型垂直腔面发射激光器的
常见失效模式和机理分析
张玉岐1,2，左致远1，阚 强3，赵 佳1,4 *
（1. 山东大学 激光与红外系统集成技术教育部重点实验室，山东青岛 266237；
2. 厦门市三安集成电路有限公司，福建厦门 361000；
3. 中国科学院半导体研究所，北京 100083；
4. 山东大学 信息科学与工程学院，山东青岛 266237）
摘要：氧化型垂直腔面发射激光器（VCSEL）在高速光通信中有着广泛的应用，其中应用过程中的可靠性是一个非常重要的指标，要求有高寿命和低失效率。为了更好的了解VCSEL在应用过程中的失效模式和机理，提升器件的可靠性，本文从器件设计、加工制造和应用过程等3个环节总结分析了氧化型VCSEL的常见失效模式、产生原因和机理，并提出了适当的改善措施和建议。其中，对氧化应力、ESD和湿气腐蚀问题这三个主要失效因素进行了更为详细的分析。基于以上对业界研究工作的总结和分析，最后对实际工作中遇到的VCSEL失效案例进行简单的介绍，为VCSEL学者、研发设计、制造和使用人员提供一个较为全面的失效分析案例库。
关键词：VCSEL；氧化物；失效模式和机理；可靠性
中图分类号：TN365 文献标志码：A doi：10.37188/CO.EN.2021-0012
1 Introduction
Vertical Cavity Surface Emitting Lasers (VC-SELs) are one of the most popular types of semicon-ductor lasers, and have a wide application range in the industry. It has the advantages of low threshold, low power consumption, easy fabrication, high rate and low cost, and has become the core light source for short-reach fiber optics data communications and optical sensing[1–4]. Most of them use GaAs/Al-GaAs materials operating at 850 nm.
Reliability, as a key indicator of long term field use of semiconductor lasers, is very important for applications and is the core problem of device de-velopment, design, fabricate and application[5-6]. VC-SELs have a field failure rate of <10 ppm/year over the past decade, thanks to the improvement of design capacity and technological level, and the ad-option of extensive preventive measures and screen-ing measures[7-8]. However, a large number of VC-SELs have been used in the field[9], when these fail-ures are clustered in large data communication cen-ters with thousands of links, the system failure rate can still involve multiple unplanned failures per year. Moreover, in data communic
ation applica-tions, the service life of devices is generally long, usually more than 10 years[10], which puts forward higher requirements for the reliability of VCSEL devices. Without proper device design, manufactur-ing and usage these failure modes lead to high fail-ure rates in oxide VCSELs. For the oxide VCSELs based on GaAs/AlGaAs materials, due to the inher-ent reasons of device material and structure design, there will be potential reliability problems, that has received great attention from the industry[11-13].
Therefore, we focus on the most widely used commercial oxide VCSELs[9] made from GaAs/ AlGaAs materials in this paper. Expounding the common causes of VCSEL failure from three as-pects of device design, manufacturing and external factors, the failure phenomena and mechanisms are summarized and analyzed, and puts forward some improvement suggestions and preventive measures. Through this, it can provide reference for device
2中国光学第 15 卷
R&D and production personnel when they en-counter the same or similar situations, quickly and effectively understand the root cause of the prob-lems, and provide appropriate improvement meas-ures, which is helpful to improve the reliability of the devices.
gmr传感器2 VCSEL Structure
Oxide VCSELs structure is composed of sub-strate, top and bottom distributed Bragg mirror (DBR), quantum wells (also called active layer), ox-ide layer and positive and negative contact. The light output direction is perpendicular to the wafer surface, and most emit light at 850 nm. A schematic drawing of the oxide VCSEL structure is shown in Fig.1. The substrate is usually n-type doped GaAs material. The epi layers are usually grown by metal-organic chemical vapor deposition (MOCVD) or molecular beam epitaxy(MBE) on the substrate. The DBRs are multiple pairs of high and low refractive index materials (Al x Ga 1-x As materials) grown altern-ately, where the high-index layer contains the low aluminum content x typically 0.8−1.0. The low-in-dex of layer has a high aluminum content x typic-ally 0.15−0.20 for 850 nm VCSELs.
Oxide layer
Bottom DBR
n-contact
p-contact
Top DBR Active layer
Fig. 1 Schematic of the oxide VCSEL structure [16]
图 1 氧化型VCSEL 的结构示意图
[16]
The quantum wells (usually 3−5 wells) are composed of GaAs/AlGaAs or AlGaAs/InGaAs ma-terials. The oxide layer is AlGaAs with higher Al concentration or AlAs materials that is located in the p-DBR above the quantum wells, thickness gener-ally 20−30 nm. It is necessary to etch a mesa struc-ture and then perform selective wet oxidation to
form oxide aperture. This oxide aperture is used to confine current and light. Thus, the threshold cur-rent and optical loss can be effectively reduced, the power conversion efficiency of VCSEL is im-proved, and the performance of VCSEL is greatly improved [14-15].
3 Common failure modes and analysis
3.1 VCSEL Design
3.1.1 Materials System
For any semiconductor laser, the choice of ma-terials system is key, especially the material of the quantum wells used to make the device, and the bar-rier that makes contact with the quantum well. The widely used commercial oxide VCSELs are pre-pared by GaAs/AlGaAs epitaxial materials at present, due to GaAs and AlGaAs materials have a matched lattice and have a large refractive index dif-ference, so the device has a small DBR thickness to get a high reflectivity and excellent performance,etc. But it is found that dislocation is easily formed in GaAs/AlGaAs compound semiconductor materi-als [17]. This may be due to the band gap energy of the material, the bonding force, the size of the crystal atoms, point defects generation energy and migra-tion energy and deep levels, etc.
Table 1 shows a degree of dislocation forma-tion in some different III-V compound semiconduct-or materials. From this table, it is seen that for al-most all of these materials, the larger band gap en-ergy is more likely to form dislocation, such as GaAs and GaP are prone to dislocations, while In-GaAsP materials are not. This mechanism is the lar-ger the band gap energy is, the more energy can be provided for the formation of defects, so the defects are easier to generation and migration. However, ac-cording to this theory, InP with relatively wide bandgap should also be pr
one to dislocation, it’s not in fact. So bandgap energy alone is not sufficient to explain. There are other theories that need further investigation [18-19].
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ZHANG Yu-qi, et al. : Common failure modes and mechanisms in Oxide Vertical…
3
Tab. 1 A summary of easiness formation of dislocation loops in some III-V compound semiconductors[20]表 1 一些III-V族化合物半导体产生位错的难易程度[20]
Material
Band gap
energy/eV@300K
Formation of dislocation
loops
GaAs 1.42Yes
AlGaAs 1.42~2.15Yes
GaP 2.27Yes
GaAsP 1.42~2.27Yes
InP 1.34No
InGaAsP on InP0.75~1.34No
InGaP on GaAs 1.42~1.91Yes
InGaAsP on
GaAs
1.42~1.76Yes
Another dominant factor is the reconstruction energy between GaAs atoms is relatively weak, so the
activation energy of dislocation motion in GaAs is low, and the recombination is easily broken or af-fected by impurities, so dislocation defects are eas-ily formed[20]. Besides, other dislocation formation factors include the magnitudes of generation energy and migration energy for point defects and deep levels which are related to such defects as dangling bonds and native point defects, and non-radiative re-combination rates at the deep levels, etc[20].
A relatively ideal semiconductor laser materi-als system is that active materials are prone to dislo-cation defects but have sufficient compressive stress to prevent dislocation growth[21]. As shown in Fig.2, dark line defect(DLD) was successfully stopped in GaAs-based lasers by inducing compression strain through introducing 5%−7% indium in quantum wells with a relative suitable thickness. The propor-tion of indium element has a suitable range, too little will not prevent DLD growth, such as “No pinning”region in the Fig.2. However, introducing too much indium can lead to lattice mismatch and generate DLD, as shown above the solid line[19]. Kirkby et al[22] also showed that indium can prevent disloca-tion generation and migration even in the absence of compressive strain because the large atomic radius of indium can harden the lattice. Therefore, it can be considered to add moderate indium into VCSEL quantum wells to introduce compressive strain and lattice hardening to slow down dislocation forma-tion and migration.
100
10交换机面板
1
20%40%
No DLD growth大肠杆菌培养
N
o
p
i
n
n
i
n
g
Beyond critical thickness
(misfit dislocations formed)
Indium composition in InAlGaAs
Q
u
a
n
t
u
m
w
e
l
l
t
诱捕黄鳝h
i
c
k
n
e
s
s
/
n
m
Fig. 2 The effect of indium content on laser reliability[19]图 2 铟和应变对激光可靠性的影响[19]
3.1.2 Structure Design
(1) Oxide layer
The oxide layer problem is a major source of VCSEL failures and is difficult to avoid. As we know, in
oxide VCSELs, the oxide layer (Al x O y) is completed by the vapor oxidation of AlAs or Al-GaAs layer containing a very small amount of Ga by the mesa. After AlAs oxidation to form Al2O3, the volume will shrink by about 20%[23-24], thus cre-ates greater stress in the oxide layer, especially at the tip. At the same, Al x O y and DBR semiconductor lattice mismatch, have a weak binding force, and the thermal mismatch in coefficient of thermal expan-sion(CTE) at the interface. Improper control of ox-idation process can easily form delamination or crack between the two interfaces, which is the main source of dislocation defects.
Herrick et al. investigated the origin of disloca-tions in GaAs-based VCSEL[25]. The DLD in the device originate from the oxide tip as shown below in Fig.3 based on the actual DLD transmission elec-tron microscopy(TEM) images. According to TEM work done by others with DLD networks in the act-ive region of the device[26], it is suggested that the DLD may move down from the tip of the oxide lay-er to the active region below. The direction of propagation may be driven downward by current and form a DLD network as the line dislocations cross the active region[27].
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