摘要
提出了采用金属有机化学气相沉积(MOCVD)生长无Ga且应力平衡的InAsP/InAsSb超晶格,并探索了其作为红外吸收材料的可行性。首先采用k·p理论计算了InAsP/InAsSb超晶格的带隙,发现其波长调节范围可以从中波红外到长波红外。然后通过MOCVD技术在InAs衬底上生长了InAs0.8P0.2/InAs0.7Sb0.3超晶格。XRD测试结果表明,InAs衬底峰与超晶格零级卫星峰的失配仅61",即基本实现应力平衡;AFM测试材料表面形貌显示5 μm×5 μm范围内均方根粗糙度为0.4 nm;低温PL光谱显示较强的发光,峰位于3.3 μm的中波红外波段,接近设计值。这些结果表明采用MOCVD生长应力平衡的InAsP/InAsSb超晶格作为红外探测材料具有较好的可行性和实用性。
InAs/GaSb Ⅱ类超晶格作为制备第三代红外探测器最具影响力的材料之一,广泛应用到导弹制导、夜视、安检以及医疗等军事和民事领域。该材料体系具备如下的优点:(1)波长调节范围
1 000 ns的少子寿
S.P. Svensson等对InAs/GaSb超晶格载流子寿命较短的问题提出假设:可能是超晶格中InAs或者GaSb相关的缺陷限制了少子寿命,之后通过实验证实这是和Ga元素有关的点缺陷形成的带间能级有很大关系,因为这些带间能级是作为肖克莱-瑞德复合的主要复合中
InAs/InAsSb超晶格属于Ⅱ型能带对齐。为了拓展探测波长在设计生长该材料时通常采用较厚的InAs层,因为长波InAs/InAsSb超晶格对应着较大的Sb组分,InAs与InAsSb的晶格失配会随Sb组分增大迅速增加,结果材料质量也迅速下降。而为了平衡应力生长较厚的InAs层会使得InAs与InAsSb层电子与空穴波函数交叠程度减少,降低了材料吸收系数。所以Ariyawansa等人提出为了平衡应力而增加Ga元素生长的InGaAs/InAsSb超晶
另一方面,目前分子束外延(MBE)是生长InAs/GaSb超晶格及其它含锑半导体光电子材料和器件的主要生长技术,然而金属有机化学气相沉积(Metal Organic Chemical Vapor Deposition, MOCVD)技术也以其独特的优势长期垄断产业界,例如MOCVD生长技术除了具备易于控制反应物流量和材料生长条件接近热力学平衡的优点,更重要的是高产能低成本,非常适用于产业
基于目前在InAs/InAsSb超晶格红外探测器的研究现状,本文提出采用MOCVD生长新型“无Ga”应力平衡的InAsP/InAsSb超晶格。InAsP/InAsSb超晶格与InGaAs/InAsSb超晶格类似,均是InAs/InAsSb超晶格的衍生物,但是前者与上述的其他超晶格材料相比又具有如下的优势:(1)因为相较于InAs/InAsSb超晶格,每个生长周期的InAs层增加了P元素所以该材料平衡应力方面更加灵活,同时适用于能带工程,可以实现Ⅰ型、零型导带带阶和Ⅱ型带边对齐,工作波段可从中波红外到长波红外;(2)InAsP/InAsSb超晶格属于“无Ga”材料,不存在前述的与Ga相关的点缺陷等复合中心,可能具有较长的肖克莱-瑞德复合寿命;(3)MOCVD生长过程中仅切换P和Sb源即可实现周期性生长InAsP和InAsSb层,同时无需复杂的界面层调整晶格匹配,简化生长过程。
InAsP/InAsSb材料体系早在20多年前就有用于发光材料的报道。1997年美国 Sandia 国家实验室 Biefeld 等人利用MOCVD生长了基于InAsP/InAsSb超晶格激光器,并实现了3.8 μm的红外激
本文研究了在InAs衬底上通过MOCVD外延生长的InAsP/InAsSb超晶格并对其材料质量进行表征。X射线衍射仪(X-ray Diffractometer,XRD)和原子力显微镜(Atomic Force Microscope,AFM)分别用于测试材料的应变和表面形貌,除此之外采用光致发光谱(Photoluminescence,PL)用于表征材料的光学特性。
首先采用基于Kane方程的8带k·p模
, | (1) |
其中、和分别表示InAs、InAsP和InAsSb的晶格常数,和分别表示InAsP和InAsSb的厚度。

图1 k·p法计算的不同厚度InAs0.8P0.2/InAs0.7Sb0.3 超晶格材料截止波长
Fig. 1 The cutoff wavelength of InAs0.8P0.2/InAs0.7Sb0.3 superlattices with different thickness calculated by k·p method
对于不同组分的三元合金可以通过Vegard 定律计算其晶格常数,以和为例,
, | (2) |
, | (3) |
于是
, | (4) |
计算结果绘制与InAs衬底晶格匹配的应力平衡线,如
采用一台Aixtron CCS MOCVD生长系统在2 inch的InAs(100)衬底生长了150个周期的(1.5 nm)InAs0.7Sb0.3 /(5 nm)InAs0.8P0.2 超晶格材料。使用的三族源为三甲基铟(TMIn),五族源为三甲基锑(TMSb),砷烷(AsH3)和磷烷(PH3),以氢气(H2)作为载气。经调试反应腔压力设置为100 mbar。生长温度的设定同时兼顾了InAsP和InAsSb的生长,温度过低PH3难以裂解,温度过高InAsSb可能出现分解,综合考虑后生长温度最终设置为540 ℃。每个周期InAsP的Ⅴ/Ⅲ比和生长速率分别为300和0.2 nm/s,InAsSb的Ⅴ/Ⅲ比和生长速率分别为6.5和0.25 nm/s,其中InAsSb的V/III比是优化后确保不出现锑偏析现象。设置好相应参数后进行材料生长。
实验中的高分辨X射线衍射仪型号为Brucker D8 DISCOVER, X射线源为Cu Kα1,波长为0.154 05 nm,入射光狭缝水平宽度为1 mm,测试电压为40 kV,电流为40 mA。实验中在ω-2θ模式下扫描样品以获得晶格常数和材料组分等信息。原子力显微镜型号为Bruker Dimension ICON,垂直分辨率和水平分辨率分别为0.03 nm和0.1 nm,在轻敲模式下对材料5 μm×5 μm范围内的表面形貌进行观测以获取粗糙度、形貌特征等信息。用于获悉超晶格材料有效带宽的PL采用的是变条件集成红外调制光致发光谱实验系统。整个测试装置由傅立叶红外变换光谱仪(FTIR)、激光器、低温装置以及引导光束等部件组成。所有的PL光谱均采用步进式扫描调制技术。激光器的激发光源波长为532 nm,激发功率为100 mW。探测器为HgCdTe探测器,配备液氮装置可以测试的温度范围为77∼300 K。
为了获得应力平衡的三元超晶格,首先生长了20个周期的InAsSb/InAs 超晶格和InAsP/InAs超晶格测试结构用于调试InAsSb层的Sb组分和InAsP层的P组分。因为若InAsSb/InAs超晶格中InAs厚度已知,则InAsSb的厚度和组分只有唯一解。

图2 不同超晶格的XRD ω/2θ 曲线(a)InAsSb/InAs超晶格,(b)InAsP/InAs超晶格
Fig. 2 XRD ω/2θ curves of different superlattices (a) InAsSb/InAs superlattices, (b) InAsP/InAs superlattices
总厚度为0.975 µm的InAsP/InAsSb超晶格生长完成后表面干净平整,其XRD曲线如

图3 InAs衬底上的InAsP/InAsSb超晶格XRD 曲线
Fig. 3 XRD ω/2θ curve of InAsP/InAsSb superlattices on InAs substrate

图4 不同超晶格的AFM图像(a)InAsP/InAs超晶格,(b)InAsSb/InAs超晶格,(c)InAsP/InAsSb超晶格
Fig. 4 AFM image of differernt superlattices (a)InAsP/InAs superlattices, (b) InAsSb/InAs superlattices, (c) InAsP/InAsSb superlattices
采用FTIR实验系统对材料进行了变温PL光谱测试。测试范围选择2.1∼4.2 μm,如

图5 InAsP/InAsSb 超晶格材料的PL光谱 (a) 77 K(插图为高斯拟合光谱),(b) 77∼150 K
Fig. 5 PL spectra of InAsP/InAsSb superlattices (a) 77 K(the inset shows the Gaussian fitting of the spectrum), (b) 77∼150 K
就半高宽而言,与采用MBE生长的截止波长3.64 μm的In0.94Ga0.06As/InAs0.91Sb0.09超晶
变温PL光谱如
基于k·p理论8带模型设计了用于红外吸收的应力平衡InAs0.8P0.2/InAs0.7Sb0.3超晶格,并采用MOCVD生长了InAs/InAsP、InAs/InAsSb以及InAsP/InAsSb超晶格。精确设计的InAsP/InAsSb超晶格XRD零级峰与衬底峰的晶格失配仅61",正负一级峰的FWHM分别为75"和87";在5 µm×5 μm范围内AFM表面方均根粗糙度为0.4 nm,表明超晶格材料具有较好的结构质量和表面平整度。低温PL光谱显示材料峰位于3.3 μm中波红外波段,和设计值接近。XRD、AFM和PL的测试结果表明,采用MOCVD生长的InAsP/InAsSb超晶格初步显示了其作为红外吸收材料的可行性,为红外探测器材料家族增加一名成员。
References
Huang Y, Ryou J H, Dupuis R D, et al. Epitaxial growth and characterization of InAs/GaSb and InAs/InAsSb type-II superlattices on GaSb substrates by metalorganic chemical vapor deposition for long wavelength infrared photodetectors [J]. Journal of Crystal Growth, 2011, 314(1):92-96. [百度学术]
Zhu H, Liu J, Zhu H, et al. High operating temperature InAs/GaSb superlattice based mid wavelength infrared photodetectors grown by MOCVD [J]. Photonics, 2021, 8(12):564. [百度学术]
Chen Y, Liu J, Zhao Y, et al. MOCVD growth of InAs/GaSb type-II superlattices on InAs substrates for short wavelength infrared detection [J]. Infrared Physics & Technology, 2020, 105:103209. [百度学术]
Yang W, Ma W, Zhang Y, et al. High structural quality of type II InAs/GaSb superlattices for very long wavelength infrared detection by interface control [J]. IEEE Journal of Quantum Electronics, 2012, 48(4):512-515. [百度学术]
Zhu H, Hao X, Teng Y, et al. Long-wavelength InAs/GaSb superlattice detectors with low dark current density grown by MOCVD [J]. IEEE Photonics Technology Letters, 2021, 33(9):429-432. [百度学术]
Grein C H, Garland J, Flatté M E. Strained and unstrained layer superlattices for infrared detection [J]. Journal of Electronic Materials, 2009, 38(8):1800-1804. [百度学术]
Binh-Minh N, Guanxi C, Minh-Anh H, et al. Growth and characterization of long-wavelength infrared type-II superlattice photodiodes on a 3-in GaSb wafer [J]. IEEE Journal of Quantum Electronics, 2011, 47(5):686-690. [百度学术]
Ting Z Y, Soibel A, Hglund L, et al. Type-II superlattice infrared detectors [J]. Semiconductors and Semimetals, 2011, 84:1-57. [百度学术]
Donetsky D, Belenky G, Svensson S, et al. Minority carrier lifetime in type-2 InAs–GaSb strained-layer superlattices and bulk HgCdTe materials [J]. Applied Physics Letters, 2010, 97(5):052108 [百度学术]
Donetsky D, Svensson S P, Vorobjev L E, et al. Carrier lifetime measurements in short-period InAs/GaSb strained-layer superlattice structures [J]. Applied Physics Letters, 2009, 95(21):1897-1243. [百度学术]
Svensson S P, Donetsky D, Wang D, et al. Growth of type II strained layer superlattice, bulk InAs and GaSb materials for minority lifetime characterization [J]. Journal of Crystal Growth, 2011, 334(1):103-107. [百度学术]
Lackner D, Pitts O J, Steger M, et al. Strain balanced InAs/InAsSb superlattice structures with optical emission to 10 μm [J]. Applied Physics Letters, 2009, 95(8):091101. [百度学术]
Steenbergen E H, Connelly B C, Metcalfe G D, et al. Significantly improved minority carrier lifetime observed in a long-wavelength infrared III-V type-II superlattice comprised of InAs/InAsSb [J]. Applied Physics Letters, 2011, 99(25):251110. [百度学术]
Soibel A, Ting D Z, Rafol S B, et al. Mid-wavelength infrared InAsSb/InAs nBn detectors and FPAs with very low dark current density [J]. Applied Physics Letters, 2019, 114(16):161103. [百度学术]
Ariyawansa G, Reyner C J, Steenbergen E H, et al. InGaAs/InAsSb strained layer superlattices for mid-wave infrared detectors [J]. Applied Physics Letters, 2016, 108(2):022106. [百度学术]
Ariyawansa G, Reyner C J, Duran J M, et al. Unipolar infrared detectors based on InGaAs/InAsSb ternary superlattices [J]. Applied Physics Letters, 2016, 109(2):021112. [百度学术]
Zhu H, Chen Y, Zhao Y, et al. Growth and characterization of InGaAs/InAsSb superlattices by metal-organic chemical vapor deposition for mid-wavelength infrared photodetectors [J]. Superlattices and Microstructures, 2020, 146:106655. [百度学术]
Huang Y, Xiong M, Wu Q, et al. High-performance mid-wavelength InAs/GaSb superlattice infrared detectors grown by production-scale metalorganic chemical vapor deposition [J]. IEEE Journal of Quantum Electronics, 2017, PP(5):1-1. [百度学术]
Li X, Zhao Y, Wu Q H, et al. Exploring the optimum growth conditions for InAs/GaSb and GaAs/GaSb superlattices on InAs substrates by metalorganic chemical vapor deposition [J]. Journal of Crystal Growth, 2018, 502(15):71-75. [百度学术]
Zhu H, Zhu H, Liu J F, et al. Short wavelength infrared InPSb/InAs superlattice photodiode grown by metalorganic chemical vapor deposition [J]. Physica Scripta, 2022, 97(3):035002. [百度学术]
Hao X, Teng Y, Zhao Y, et al. Demonstration of a dual-band InAs/GaSb type-II superlattice infrared detector based on a single heterojunction diode [J]. IEEE Journal of Quantum Electronics, 2020, 56(2):1-6. [百度学术]
Kurtz S R, Allerman A A, Biefeld R M. Midinfrared lasers and light-emitting diodes with InAsSb/InAsP strained-layer superlattice active regions [J]. Applied Physics Letters, 1997, 70(24):3188-3190. [百度学术]
Biefeld R M, Allerman A A, Kurtz S R, et al. Progress in the growth of mid-infrared InAsSb emitters by metal-organic chemical vapor deposition [J]. Journal of Crystal Growth, 1998, 195(1-4):356-362. [百度学术]
Yu Z, Nicolaie J, Bertru N, et al. Sb surfactant mediated growth of InAs/AlAs0.56Sb0.44 strained quantum well for intersubband absorption at 1.55 μm [J]. Applied Physics Letters, 2015, 106(8):263. [百度学术]
Machowska-Podsiadlo E, Bugajski M. Eight-band k·p calculations of the electronic states in InAs/GaSb superlattices[C]. 2016 18th International Conference on Transparent Optical Networks , 2016:1-4. [百度学术]
Vurgaftman I, Meyer J R, Ram-Mohan L R. Band parameters for III–V compound semiconductors and their alloys [J]. J Appl Phys, 2001, 89(11):5815-5875. [百度学术]
Kurtz S R, Biefeld R M, Dawson L R, et al. Midwave (4 μm) infrared lasers and light‐emitting diodes with biaxially compressed InAsSb active regions [J]. Applied Physics Letters, 1994, 64(7):812-814. [百度学术]
Wu J, Xu Z, Chen J, et al. Temperature-dependent photoluminescence of the InAs-based and GaSb-based type-II superlattices [J]. Infrared Physics & Technology, 2018, 92:18-23. [百度学术]
Jie G, Peng Z, Sun W, et al. InAs/GaSb superlattices for photodetection in short wavelength infrared range [J]. Infrared Physics & Technology, 2009, 52(4):124-126. [百度学术]
Cardona M, Meyer T A, Thewalt M L W. Temperature dependence of the energy gap of semiconductors in the low-temperature limit [J]. Physical Review Letters, 2004, 92(19):196403. [百度学术]