光子学报  2019, Vol. 48 Issue (10): 1031002  DOI: 10.3788/gzxb20194810.1031002
0

引用本文  

张健, 唐吉龙, 亢玉彬, 等. GaSb衬底外延InAs薄膜及光学性质研究[J]. 光子学报, 2019, 48(10): 1031002. DOI: 10.3788/gzxb20194810.1031002.
ZHANG Jian, TANG Ji-long, KANG Yu-bin, et al. Epitaxial InAs Films on GaSb Substrate and Study on Optical Properties[J]. Acta Photonica Sinica, 2019, 48(10): 1031002. DOI: 10.3788/gzxb20194810.1031002.

基金项目

国家自然科学基金(Nos.61704011,61674021,11674038),高功率半导体激光器国家重点实验室基金,吉林省科技发展项目(Nos.20160519007JH,20160520117JH,20160101255JC,20170520118JH)

第一作者

张健(1995-), 男, 硕士研究生, 主要研究方向为Ⅲ-Ⅴ族半导体材料外延生长及表征.Email:zhang15764339456@163.com

通讯作者

唐吉龙(1977-), 男, 副教授, 博士后, 主要研究方向为半导体材料外延生长与器件.Email:jl_tangcust@163.com

文章历史

收稿日期:2019-05-24
录用日期:2019-07-23
GaSb衬底外延InAs薄膜及光学性质研究
张健 , 唐吉龙 , 亢玉彬 , 方铉 , 房丹 , 王登魁 , 林逢源 , 魏志鹏     
(长春理工大学 高功率半导体激光国家重点实验室, 长春 130022)
摘要:利用分子束外延技术在GaSb(100)衬底上先生长作为缓冲层以降低薄膜失配度的低Sb组分的三元合金InAsSb,再生长InAs薄膜.在整个生长过程中通过反射高能电子衍射仪进行实时原位监测.InAs薄膜生长过程中,电子衍射图案显示了清晰的再构线,其薄膜表面具有原子级平整度.利用原子力显微镜对InAs薄膜进行表征,结果显示较低Sb组分的InAsSb缓冲层上外延InAs薄膜的粗糙度比较高Sb组分的InAsSb缓冲层上外延InAs薄膜的粗糙度降低了约2.5倍.通过对不同Sb组分的三元合金InAsSb缓冲层上外延的InAs薄膜进行X射线衍射测试及对应的模拟,结果表明在较低Sb组分的InAsSb缓冲层上外延InAs薄膜的衍射峰半高峰宽较小,说明低Sb组分的InAsSb作为缓冲层可以降低InAs薄膜的内应力,提高InAs薄膜的结晶质量.利用光致发光光谱对高结晶质量的InAs薄膜进行发光特性研究,10 K下InAs的发光峰位约为0.418 eV,为自由激子发光.
关键词InAs薄膜    InAsSb缓冲层    晶格失配    晶体质量    发光特性    分子束外延技术    X射线衍射    
中图分类号:O484      文献标识码:A      
Epitaxial InAs Films on GaSb Substrate and Study on Optical Properties
ZHANG Jian , TANG Ji-long , KANG Yu-bin , FANG Xuan , FANG Dan , WANG Deng-kui , LIN Feng-yuan , WEI Zhi-peng     
(State Key Laboratory of High Power Semiconductor Laser, Changchun University of Science and Technology, Changchun 130022, China)
Foundation item: The National Natural Science Foundation of China (Nos.61704011, 61674021, 11674038), the National Key Laboratory of High Power Semiconductor Lasers, the Science and Technology Development Project of Jilin Province (Nos.20160519007JH, 20160520117JH, 20160101255JC, 20170520118JH)
Abstract: The ternary alloy InAsSb of the low Sb component as the buffer layer, which reduces the mismatch of the film, was grown on the GaSb (100) substrate by using the molecular beam epitaxy, and then regrown the InAs film. Real-time in situ monitoring by reflection high energy electron diffraction throughout the growth process. Following the growth of the InAs film, the electron diffraction pattern showed a clear reconstitution line, with the surface of the film having atomic flatness. Atomic Force Microscopy is used to characterize InAs films, and the results show that the roughness of epitaxial InAs film on the InAsSb buffer with lower Sb components is reduced by about 2.5 times than the roughness of epitaxial InAs film on the InAsSb buffer with higher Sb components. X-ray diffraction and simulation were performed for epitaxial InAs films on ternary alloy InAsSb buffer with different Sb composition. The results show that the full width half maximun of the diffraction peak of the epitaxial InAs film on the lower Sb composition of InAsSb buffer layer is smaller. It is indicated that the InAsSb with a low Sb component acting as a buffer layer can reduce the internal stress of the InAs film and can improve the crystal quality of the InAs film. The luminescence properties of InAs films with high crystal quality were studied by photoluminescence spectroscopy. The luminescence peak of InAs is about 0.418 eV at 10 K, which is free exciton luminescence.
Key words: InAs film    InAsSb buffer layer    Lattice mismatch    Crystal quality    Luminescence properties    Molecular beam epitaxy    X-ray diffraction    
OCIS Codes: 310.6860;310.6188;310.4925;300.6280
0 引言

InAs由于高的电子迁移率[1-2]、小的有效电子质量[3-4]和窄的直接带隙[5],一直以来都是中红外波段不可或缺的Ⅲ-Ⅴ族半导体材料.InAs薄膜材料既在探测器(InAs/GaSb)方面扮演着重要角色[6-8],也在InAs/GaInSb “W”型量子阱激光器结构中起着重要的作用[9-12],同时InAs材料具有很高的THz辐射效率,是THz发射器件中一种很有前途的材料[8, 13].因此对于制备高结晶质量的InAs薄膜材料是至关重要的.

InAs材料由于其较窄的带隙,可以与一些化合物,例如GaxIn1-xSb等材料形成断隙型结构[14-15],在此结构中电子和空穴分别被限制在InAs和GaxIn1-xSb材料中,导带和价带之间的跃迁都要比除InSb以外任何一种材料的带隙要小[14, 16],因此可以向更长波长方向扩展.Ⅱ类量子阱结构可以使得导带和价带之间的质量更趋向于平衡,有助于降低材料之间的俄歇复合[17-18].相比于GaAs材料,GaSb半导体材料与InAs有更小的晶格失配度[19-21].因此,本文在GaSb衬底上异质外延InAs薄膜,利用InAs1-xSbx作为缓冲层进一步降低薄膜的晶格失配,并对其结构及光学特性展开研究.

本文通过分子束外延(Molecular Beam Epitaxy,MBE)技术在GaSb(100)衬底上先外延生长InAs1-xSbx缓冲层,在此基础上外延生长InAs薄膜,通过反射高能电子衍射仪(Reflection High Energy Electron Diffraction,RHEED)进行实时原位监测,判断InAs薄膜的结晶质量.同时根据RHEED衍射图样得到对应的振荡曲线,进而得到InAs薄膜的生长速率,并利用原子力显微镜(Atomic Force Microscope,AFM)测试表征了薄膜的表面粗糙度.通过X射线衍射(X-Ray Diffraction,XRD)测试对InAs薄膜的结晶性质进行了研究,对比了相应的XRD的模拟结果.同时分析了在不同Sb组分的InAsSb缓冲层上生长InAs薄膜的XRD曲线,得到低Sb组分的InAsSb薄膜作缓冲层能够降低InAs薄膜的失配,进而提高薄膜的结晶质量.通过光致发光(Photoluminescence,PL)光谱对InAs薄膜的发光特性进行了研究.

1 实验

本文中InAs薄膜的外延生长是在超高真空(~2.2×10-8 Pa)MBE系统中实现的,使用的衬底为GaSb(100)基片.材料在生长之前对GaSb衬底进行除气处理.首先,将衬底送入进样室(loadlock)中,在200℃下加热处理2 h以除去基片上吸附的水汽;然后将衬底转移到预处理室(buffer)中,在400℃下进行预除气2 h;最后,将处理好的衬底转移到MBE生长室中,在生长室Ⅴ族元素气氛的保护下,高温650℃环境中再次进行30 min除气处理,至此对衬底的除气处理全部完成.在生长InAs薄膜之前,使用束流器(Beam Flux Monitor, BFM)对外延所需的In源、As源和Sb源的束流进行校准,确定生长所需的束流.用热偶标称温度对衬底温度进行校准后,开始生长InAs薄膜.首先,GaSb衬底在高温下进行脱氧处理,A样品与B样品所用的Sb束流压(Beam Equivalent Pressure, BEP)分别为3.6×10-6 Pa和2.0×10-6 Pa,随后将衬底温度降低至420℃,打开In和As源炉快门,As束流与In束流之比为4,先生长InAsSb缓冲层30 s,As束流为3.2×10-5 Pa,然后关闭Sb源快门,生长InAs薄膜1 h.利用RHEED进行实时原位监测并记录整个生长过程,对RHEED衍射图样和振荡曲线进行分析.利用AFM测得A样品与B样品的表面粗糙度,得到A样品中InAs薄膜的表面粗糙度约为B样品的2.5倍.采用德国布鲁克公司生产的D8 DISCOVER X射线衍射仪对生长的InAs薄膜进行XRD测试,同时对其进行XRD模拟.对比在不同Sb组分InAsSb缓冲层上生长的InAs薄膜的XRD曲线,分析得到低Sb组分InAsSb缓冲层降低了InAs薄膜中的应变,改善了InAs薄膜结晶质量.使用配备有低温冷却InSb探测器的HORIBA iHR550光谱仪对外延的InAs薄膜进行PL光谱测试,用波长655 nm半导体二极管激光器作为激发源,得到变温及变功率密度下InAs薄膜的发光曲线.

2 结果与讨论

InAs薄膜在GaSb衬底上外延生长过程中RHEED衍射图样如图 1所示.首先GaSb衬底在温度为650℃、Sb束流的保护下进行脱氧化层处理,如图 1(a)(b)所示.脱氧结束后GaSb衬底的表面处于富Sb的环境中,从图 1(b)中的衍射图样可以看到清晰再构线,这说明此时GaSb衬底已经完全进行了脱氧化层处理,表面光滑平整.随后将衬底温度降低至420℃,打开In和As源炉快门,As束流与In束流之比为4.首先引入的In原子和As原子与衬底表面的Sb原子结合,形成InAsSb三元合金,之后关闭Sb源快门,InAsSb缓冲层生长结束的RHEED衍射图样如图 1(d),可以看到对应的再构线.随后开始异质外延生长InAs薄膜,由于原子在衬底表面脱附、吸附和结合,因此在衬底表面形成三维岛状结构,表面平整度再次被破坏,表面再构发生改变[22],RHEED衍射图像中表现为以衍射斑点为主,如图 1(e).随着生长时间的延长,岛状结构逐渐粗化、相互接触,最后样品表面逐渐平整,衍射图样从亮斑转变为亮线,如图 1(f)所示.

图 1 生长过程中RHEED衍射图像 Fig.1 RHEED diffraction image during growth

为了确定InAs薄膜的生长速率,在420℃生长温度下获得RHEED衍射图样对应的振荡曲线,如图 2所示.通过RHHED振荡曲线可以计算出InAs薄膜的生长速率, 生长50 s出现了10个完整的振荡周期,InAs原子层(Monolayer, ML)的生长速率为0.2 ML·s-1.

图 2 InAs薄膜在最优生长条件下的RHEED振荡曲线 Fig.2 RHEED oscillation curveof InAs film under optimal growth conditions

为了得到在三元合金InAsSb缓冲层上外延生长的InAs薄膜的表面形貌信息,利用AFM对InAs薄膜进行表征,如图 3所示.结果显示,A样品与B样品的表面平均粗糙度(RMS)分别为1.701和0.691,前者约为后者的2.5倍.对比结果表明,利用较低Sb组分的InAsSb作为缓冲层,减小了在其上外延InAs薄膜的位错密度,降低了应变.

图 3 InAs薄膜的AFM图像 Fig.3 AFM image of InAs film

为了分析不同Sb组分的InAsSb缓冲层对薄膜结晶质量的影响,并确定异质外延生长的InAs薄膜的结晶信息,对A样品和B样品进行XRD测试,分别对应图 4(a)4(b)上端曲线.同时模拟了对应的XRD曲线作为对比,对应图 4(a)4(b)中的下侧曲线.所设计的InAs薄膜的厚度约为300 nm,A样品与B样品中InAs1-xSbx的Sb组分分别约为10%和6%,从图中可以看到模拟结果与测试结果匹配度较好,除了在30.27°处观察到GaSb衬底的衍射峰以外,在30.68°处显示出了InAs薄膜的衍射峰.A样品与B样品的InAs衍射峰的半峰宽分别约为0.08°和0.03°,相对于A样品,B样品较小的半峰宽说明了外延得到的InAs薄膜具有理想的结晶性能.对于A样品,还能观察到位于GaSb衬底峰左侧的肩峰,如图 4(a)插图所示,对应的峰位约为30.2°,对应InAsSb的衍射峰,而B样品如图 4(b)插图所示,并没有观察到明显的InAsSb的衍射峰.根据Vegard's law(韦德)公式可以得到InAs1-xSbx三元合金的晶格常数[23]

图 4 InAs薄膜的XRD测试结果及模拟曲线 Fig.4 XRD spectrum and simulation curves of InAs film
$ {{a}_{\ln \text{A}{{\text{s}}_{1-x}}{{\operatorname{Sb}}_{x}}}}=(1-x){{a}_{\ln \text{As}}}+x{{a}_{\ln \text{Sb}}} $ (1)

式中,InAs、InSb和GaSb对应的晶格常数分别为0.606 nm、0.648 nm和0.610 nm,因此A样品与B样品中InAs1-xSbx缓冲层的晶格常数分别为0.610 nm和0.608 nm.A样品中InAs1-xSbx缓冲层的晶格常数大于GaSb的晶格常数,而B样品中InAs1-xSbx缓冲层的晶格常数则介于InAs和GaSb之间.InAs外延层的应变ε定义为InAs1-xSbx缓冲层与InAs外延层的晶格常数的相对差异,由式(2)给出[24].

$ \varepsilon =\frac{{{a}_{\text{InAsSb }}}-{{a}_{\text{InAs }}}}{{{a}_{\text{InAs }}}}\times 100% $ (2)

计算得到A样品与B样品中InAs薄膜中存在的应变分别为0.695%和0.417%,因此在薄膜中存在的应力均为拉应力.而对于直接在GaSb上外延生长InAs薄膜而言,其所受应变也可由式(2)计算得到,对应的ε为0.622%.对于在GaSb衬底上外延生长的InAs薄膜而言,低Sb组分InAsSb缓冲层能够进一步降低InAs薄膜中的应变,提高InAs薄膜的结晶质量.

为更进一步研究InAs薄膜的发光性能,对上述两组样品中结晶质量较好的一组InAs薄膜样品(B样品)进行变温及变功率密度PL测试.图 5(a)是样品测得的变温PL光谱,温度变化范围在10~150 K,表明样品大致存在两种发光.随着温度的升高,发光峰位存在明显的红移.对变温光谱进行分峰拟合,如图 5(b)所示.根据温度依赖性公式[25]进行拟合

图 5 InAs薄膜的温度依赖PL光谱测试及峰位分析,激发功率密度为100 mW·cm-2 Fig.5 Temperature-dependent PL spectrum test and peak position analysis of InAs film, the excitation power density of 100 mW·cm-2
$ E_{\mathrm{g}}(T)=E_{\mathrm{g}}(0)-\left[\alpha T^{2} /(T+\beta)\right](\mathrm{meV}) $ (3)

结果表明,对于Peak 2,拟合结果中Eg(0)=418 meV,αβ的值分别为0.264和170 K,这符合Dixon等报道的InAs带隙的温度依赖性公式中参数αβ的数值范围[26].因此Peak 1和Peak 2分别对应于InAs薄膜中的束缚激子发光和自由激子发光.在低温10 K下,两种发光对应的峰位分别在0.413 eV和0.418 eV附近,后者即InAs在10 K下的带边发射,这与标准InAs低温10 K下的带隙Eg一致[27-28].

样品B在低温10 K下的变功率密度光谱曲线如图 6(a)所示,功率密度从100~200 mW·cm-2变化.观察到InAs薄膜的发光峰位随功率密度的变化无明显移动.对其分峰,并绘制了变功率密度PL光谱分峰后的积分强度随功率密度的变化曲线,如图 6(b)所示.利用式(4)进行拟合.

图 6 低温10 K下InAs薄膜的功率依赖性PL光谱测试及分析 Fig.6 Power-dependent PL spectrum test and analysis of InAs film at low temperature 10 K
$ \mathit{I=}\eta \mathit{I}_{0}^{\alpha } $ (4)

式中,I为发光峰的积分强度,η为常数,I0为激发源的积分强度(功率密度),指数α反应了发光峰的辐射复合机制.α<1时发光来源为杂质能级发光,1<α<2时发光来源为自由激子发光.结果显示Peak 1的α值为0.76,Peak 2的α值为1.2,这一结果说明前者为杂质能级发光,后者为自由激子发光.因此利用InAsSb缓冲层可以获得高结晶质量,发光较好的InAs薄膜,这对材料的外延生长及高性能器件制备有着重要意义.

3 结论

本文使用MBE技术在GaSb衬底上利用低Sb组分的InAsSb作为缓冲层,外延得到高结晶质量低应变的InAs薄膜.通过RHEED原位监测及其对应的振荡曲线确定了InAs薄膜的生长速率,RHEED衍射图样显示了InAs薄膜的原子级平整度.对比不同Sb组分的InAsSb缓冲层上外延生长的InAs薄膜的AFM,结果显示了较低Sb组分的InAsSb缓冲层上外延的InAs薄膜的表面粗糙度更小.两组XRD测试结果与模拟有很高的匹配度.通过计算分析,证实了Sb组分为6%的InAsSb缓冲层可以降低InAs薄膜的应变,较小的半峰宽表明了InAs薄膜具有良好的结晶质量.InAs薄膜的变温PL光谱显示了发光峰位随温度的升高而红移,在低温10 K下的发光峰位与标准InAs低温10 K下的带隙一致,约为0.418 eV.变功率密度PL光谱分峰后积分面积的拟合表明了InAs的自由激子发光.本工作利用InAsSb缓冲层成功的降低了GaSb衬底上外延InAs薄膜的应变,得到了高结晶质量的InAs薄膜.这对生长高结晶质量的外延材料有着指导作用,同时在光电子器件和电子器件的应用方面具有重要意义.

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