Characterization and Analysis of Interface Characteristics of InAs/GaSb Type-II Superlattice Materials
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摘要: 本文系统地介绍了国内外研究机构对超晶格界面进行研究时采用的测试分析手段。其中,通过拉曼光谱、高分辨率透射电子显微镜(HRTEM)、扫描隧道显微镜(STM)、二次离子质谱(SIMS)、X射线光电子能谱(XPS)等测试方法可以对InAs/GaSb II类超晶格材料界面类型、界面粗糙度、陡峭性等特性进行测试分析,从而评估超晶格界面质量。光致发光谱(PL谱)、高分辨率X射线衍射(HRXRD)、霍尔测试、吸收光谱等测试方法则可以研究超晶格界面质量对超晶格材料能带、晶体质量、光学性质的影响。
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关键词:
- InAs/GaSb II类超晶格 /
- InSb-like界面 /
- GaAs-like界面 /
- 陡峭性
Abstract: This article systematically introduces the testing and analysis methods used by domestic and foreign research institutions to study the superlattice interface. To evaluate the quality of the superlattice interface, the InAs/GaSb type-II superlattice interface type, interface roughness, abruptness, and other characteristics can be tested and analyzed using Raman spectroscopy, high-resolution transmission electron microscopy, a scanning tunneling microscope, secondary ion mass spectroscopy, and X-ray photoelectron spectroscopy. Photoluminescence spectroscopy, high-resolution X-ray diffraction, Hall measurements, and absorption spectroscopy can be used to study the effect of the superlattice interface quality on the energy band, crystal quality, and optical properties of superlattice materials. -
0. 引言
二元材料InAs和GaSb交替生长的InAs/GaSb超晶格(superlattice, SL)体系,由于其特殊的II型离隙型能带排列[1-3],具有能带结构设计灵活(有效带隙可以从0到250 meV连续可调,实现探测波长3~30 μm范围的变化)、有效抑制俄歇复合、量子效率高等优点,使得其在红外探测领域得到广泛的关注,成为国内外众多研究机构的研究焦点[4-7]。因为其具有与碲镉汞材料相当的光电性能,与碲镉汞材料一起成为第三代红外探测器的首选材料[8]。
由于InAs/GaSb超晶格材料中InAs的晶格常数比GaSb的小0.6%,且高质量的超晶格结构生长在GaSb衬底上,所以在每个周期中需插入适当的界面层(interfaces,IFs)以实现应力补偿,这意味着在每一个超晶格探测结构中将存在大量的界面层,从而界面结构与其质量将会对外延的整个超晶格材料的质量产生重要的影响。因为InAs和GaSb的III、V族原子均不相同,所以InAs/GaSb超晶格界面会形成两种不同的界面类型:GaAs界面和InSb界面。而实际材料生长时界面不可能是完全的GaAs或InSb界面,所以这两种界面通常被称为GaAs-like界面和InSb-like界面。这两种不同的界面类型,不论是对超晶格的材料质量还是器件性能的影响都是不同的。一般认为,InSb-like界面类型要优于GaAs-like界面,使用GaAs-like界面的超晶格,界面更为粗糙,而界面粗糙度会引入缺陷态能级,导致相应超晶格材料光学质量下降[9-11]。
为了实现高性能红外探测器,必须制备出高质量的InAs/GaSb超晶格材料,而获得高质量的超晶格材料的关键在于获得平整陡峭的界面以及选择合适的界面平衡超晶格和衬底的应变。为此,通过各种测试分析手段对超晶格界面质量的表征,研究超晶格界面对超晶格材料晶体质量、能带结构、光电性质的影响就成了从事二类超晶格材料生长研究人员最关心的问题[12]。
本文详细介绍了国内外研究机构利用光致荧光光谱(photoluminescence spectroscopy,PL谱)、高分辨X射线衍射仪(high-resolution X-ray diffraction,HRXRD)、拉曼光谱、二次离子质谱(secondary ion mass spectrometry,SIMS)、高分辨透射电子显微镜(high-resolution transmission electron microscopy,HRTEM)、扫描隧道显微镜(scanning tunneling microscope,STM)、X射线光电子能谱(X-ray photoelectron spectroscopy,XPS)、霍尔测试及吸收光谱等测试表征技术对超晶格界面特性的研究,讨论了InAs/GaSb超晶格材料的界面类型的判定、界面陡峭性及粗糙度的测试分析手段,分析了界面质量对超晶格材料能带结构、晶体质量及光学性质影响的研究结果,粗略地说明了获得高质量的具有陡峭界面的超晶格材料的特征及性质。
1. 界面特性的表征分析
1.1 界面类型的判定及陡峭性(粗糙度)的表征分析
由于GaSb衬底上的InAs/GaSb超晶格处于拉伸应变,在SL层中引入界面层,从而实现应变补偿。超晶格材料所采用的不同类型界面,对超晶格的材料质量和器件性能的影响都是不同的。InAs/GaSb超晶格的光电导数据表明InSb比GaAs界面具有在更低能量下的吸收边缘。B. R. Bonnet和P. M. Thibado等人[13-14]报道称采用InSb界面比采用GaAs界面的超晶格质量更好,同时也允许厚结构的超晶格材料生长,其总有源区厚度能达到几微米,以获得高光学吸收效率[15]。
据相关研究机构报道,对超晶格材料界面类型的判定及界面陡峭性、粗糙度的表征分析方法主要有拉曼光谱、HRTEM、STM、SIMS和XPS等测试技术。
拉曼光谱中,通过分析光谱中光学声子振动模式来确定界面类型。因不同的界面类型(InSb或GaAs)具有相异的光学声子振动模式,从超晶格材料的拉曼光谱中可以通过直观地观测峰位的不同位置来进行界面类型判定[16]。拉曼光谱可以显示出良好分辨的InSb-like或GaAs-like界面模式,具体取决于所生长超晶格材料界面的预期类型,如图 1[16]所示。
图 1 在(0 0 1)GaAs上生长的InAs或GaSb缓冲层具有InSb或GaAs IF的InAs/GaSb SL的室温拉曼光谱[16]Figure 1. Room-temperature Raman spectrum of InAs/GaSb SLs with InSb- or GaAs-like IFs grown on(0 0 1)GaAs using either InAs or GaSb buffer layers通过横截面高分辨透射电子显微镜可以精确确定界面类型及特征。通过测量晶格条纹距离,可以清楚地观察到界面构造In-Sb或Ga-As之间的差异,另外,基于键长的明显差异进行分析也可以确定界面的类型。如图 2HRTEM所示,除了可以清晰地分辨出在InAs和GaSb层之间插入的界面层(界面类型和厚度)的明确存在之外,也显示出超晶格的横向均匀性,整个超晶格结构具有平滑和陡峭的界面[17]。
图 2 在GaSb(0 0 1)上生长的InAs/GaSb超晶格的HRTEM图像: (a)沿着[1 1 0]区轴向的InAs/InSb/GaSb SL的横截面HRTEM图像;(b)SL的(0 0 2)晶格条纹图像[17]Figure 2. HRTEM image of InAs/GaSb superlattice grown on GaSb(0 0 1): (a)Cross-sectional HRTEM images of the InAs/InSb/GaSb SL taken along the [1 1 0] zone axis;(b) Corresponding(0 0 2)lattice fringe image of the SL文献[18]中通过对HRTEM图 3分析表明:①GaAs-like的界面比InSb-like的界面更粗糙。②在InAs缓冲层上生长的InAs/GaSb SL的界面粗糙度大于在GaSb缓冲层上生长的相同SL的界面粗糙度。作者认为界面粗糙度的这种差异可能是由于处于压缩应变状态下材料界面粗糙度会大于处于拉伸应变状态下的界面状态。③在GaSb衬底上生长的GaAs和InSb键合的InAs/GaSb SL界面区域存在局部陡峭,提出界面粗糙化的主要机制是成岛化的趋势。
因与InAs/GaSb外延相关的同价杂质表现出独特的特征,通过扫描隧道显微镜图可以清晰精确地分辨出界面互混,表面和体内As与Sb原子的交换反应,V族元素的掺入情况等[19-20]。图 4(a)是不同As浸润时间情况下的InAs/GaSb超晶格界面STM测试结果,从下到上As2浸润时间依次增加。下图是没有As2浸润情况,界面呈现为InAs-like;中图为4 s浸润时间下,界面为GaAs-like;而上图为15 s浸润时间下,GaAs-like界面变厚,并且与InAs混在一起。结果表明将GaSb层暴露于As-soak下会导致界面质量变差,不利于InAs/GaSb超晶格材料的晶体质量。图 4(b)的STM结果中,GaSb层中随机分布的黑点是As掺入GaSb的结果,InAs层中类似的白点是从GaSb界面偏析进入InAs的Sb原子,作者通过统计得出大约有3%的Sb进入到InAs层中。
图 4 InAs/GaSb超晶格样品的XSTM图像: (a) InAs/GaSb超晶格样品在(110)横截面的InAs-on-GaSb和GaSb-on-InAs界面的阴离子亚晶格的原子分辨率STM图像[19];(b) Pb(As)= 5.3×10-6 Torr下生长的InAs/GaSb超晶格的XSTM(cross-sectionalscanning tunneling microscopy)图像[20]Figure 4. XSTM images(anion sublattice) of InAs/GaSb SL samples: (a) STM images(anion sublattice) of InAs/GaSb SL samples in(1 1 0) cross section. As2-soak times are 0 s(bottom), 4 s(middle), and 15 s(top), respectively;(b) XSTM image of a GaSb/ InAs superlattice grown with Pb=5.3×10-6 Torr revealing the anion sublattice(60 nm×60 nm). Inset: Close-up(7 nm×7 nm) of an InAs/GaSb interface where a segment of GaAs interfacial bonds are observed(dark area indicated by the arrow)通过二次离子质谱剖析V族元素在外延层中的深度分布来判定界面的粗糙度。在InAs-on-GaSb界面,来自下层GaSb的Sb可以掺入到InAs层中。但是与表面Sb相比,InAs层内Sb的存在量可以忽略不计[21-22](图 5所示)。通过SIMS分析表明InAs-on-GaSb界面存在Sb的偏析效应和In的偏析效应,GaSb-on-InAs界面不存在As的偏析情况,Sb-for-As交换在热力学上不如As-for-Sb交换有利。
图 5 GaSb层上生长的InAs层的SIMS测试: (a) GaSb层上生长的InAs层的SIMS深度剖面[21];(b) 通过SIMS测量在不同衬底温度下生长的两个InAs on GaSb界面的Sb分布[22]Figure 5. SIMS of InAs layer growing on GaSb layer: (a) SIMS depth profile of an InAs layer buried in GaSb;(b) Sb profile measured by SIMS for two InAs-grown-on-GaSb interfaces at substrate temperatures of 475℃(dashed line) and 380℃(solid line)通过对多个InAs/GaSb超晶格样品(相同的生长条件)的界面进行多次X射线光电子能谱测量,利用V族元素与III族元素的峰强度比值的范围可以确定界面的互混情况及陡峭性。图 6和表 1是文献[22]的研究结果,其中InAs/GaSb异质结构元素峰强度比的范围远大于GaSb/InAs异质结构。GaSb/InAs样品范围较小的峰强度比意味着界面组分变化很小,具有陡峭的界面。而InAs/GaSb样品峰强度比的范围较大,表明界面组分变化较大,说明其具有扩展的界面。另外,作者为了确定界面形成的过程中是否存在As对Sb的替换过程,通过XPS对As暴露下的GaSb表面进行了研究。图 7[22]中上图(a)显示As 3d与Ga 3d峰强度比随着As-soak时间的增加有一个明显的上升过程。另外,图 7中下图(b)随着As-soak时间的增加,Sb 4d对Ga 3d的峰强度比下降。这些证据都证实了As暴露过程中As-for-Sb交换反应的存在。在一定的逃逸深度范围下,对As 3d与Ga 3d峰强度比进行分析,表明不仅在表面存在As-for-Sb反应,在GaSb体内同样存在交换反应。
图 6 GaSb/InAs和InAs/GaSb异质结构的代表性的XPS[22]Figure 6. Representative XPS scans of GaSb/InAs and InAs/ GaSb scans used heterostructures. Measured variation in XPS peak intensity ratios, for both InAs/GaSb and GaSb/InAs growth orders表 1 GaSb/InAs和InAs/GaSb界面的同一侧上的元素之间的XPS峰强度比率[22]Table 1. XPS peak intensity ratios between elements primarily on the same side of the interface of GaSb/InAs and InAs/GaSbStructure Measured range of XPS peak area ratios Sb 4d/Ga 3d As 3d/In 4d GaSb/InAs
4g: 7s1.20-1.31 0.63-0.86 InAs/GaSb
12g: 17s0.55-1.24 0.81-1.37 Note: The notation xg: ys specifies x growths and y XPS scans used to obtain a given set of values 图 7 GaSb表面的As2暴露的XPS峰强度比[22]Figure 7. XPS peak intensity ratios for As2 exposures of GaSb surfaces1.2 界面对超晶格能带的影响表征分析
M. S. Daly等人研究发现,采用InSb-like界面的超晶格材料的能带交叠要比采用GaAs-like界面的大30±10 meV,但GaAs-like界面会降低超晶格微带中电子与空穴的波函数交叠程度,其超晶格带隙Eg相对较大,不利于保证材料的光吸收[23-25]。在超晶格中,InSb的价带顶位于GaSb价带顶之上,这使得空穴有趋向于InSb层聚集的特性,同时InSb层的插入拓宽了空穴势阱的宽度,导致超晶格的空穴微带上移,因此随着InSb界面厚度的增加,其超晶格材料对应的探测波长逐渐增加。GaAs界面对超晶格材料对应的探测波长的影响分析也类似。
对于界面对超晶格能带的影响,B. R. Bennnett[26]和D. H. Chow[27]等人分别利用测量光电导和不同温度下载流子浓度的方法,分析计算了InAs/GaInSb超晶格材料的禁带宽度,其结果表明具有GaAs-like界面的超晶格的禁带宽度较InSb-like界面的要高20~50 meV[28]。更直观的测试表征技术为PL谱和XPS。
因外延超晶格材料的界面类型(InSb-like或GaAs-like界面)及厚度等参数的不同,光致荧光光谱(PL谱)峰位会随之发生位移。如图 8[29],随着InSb界面厚度的增加,超晶格PL谱的峰位逐渐红移。而图 9[30]则是随着GaAs界面厚度增加,PL谱峰位逐渐蓝移。与上述分析相一致。
图 8 T=80 K,10MLs-InAs/nInSb ML-InSb/10MLs-GaSb SLs的PL谱[29]Figure 8. PL spectra of the 10MLs-InAs/nInSb ML-InSb/10MLs GaSb SLs collected at T=80 K. Inset: interface bandege alignment diagram at scale taking into account the inserted InSb layer at the InAs/GaSb heterointerface图 9 TPL=10 K,在InAs-on-GaSb界面为GaAs-like界面的超晶格材料的PL谱[30]Figure 9. TPL=10 K PL spectra for SLs structures grown at different As-soak times at InAs-on-GaSb interfaces(GaAs-like interfaces)据文献[21]报道,(Ga,In)Sb上的InAs的能带偏移也可由XPS确定。通过将实验得到的价带态密度(valence-band density of states,VBDOS)拟合到实验上扩展的理论VBDOS,获得了InAs和GaSb体材料的In 4d和Ga 3d电子态到价带顶的结合能差。利用高斯-洛伦兹函数拟合XPS峰值确定了InAs/GaSb结构中In 4d和Ga 3d核心电子态之间的能量差,得到能带偏移为0.62±0.1 eV。
界面类型对价带偏移的影响现在还没有具体结论。Meyer等研究人员[31]的实验结果表明GaAs型界面的价带偏移较InSb型界面高14±5 meV。而通过对XPS测试结果分析,Wang等研究人员[22, 32]认为价带偏移不受界面组分的影响,而与生长次序有关,即在GaSb上生长InAs形成的价带偏移较在InAs上生长GaSb要高。相关文献报道的造成这种实验结果差异的主要原因可能是材料的生长方式和应变状态造成的[28]。
1.3 界面对超晶格材料晶体质量、光学性质的影响
根据上述界面特性会对超晶格材料的晶体质量和光学性质造成影响,相应地对超晶格材料的测试表征方法有HRXRD、霍尔测试及吸收光谱。具体分析如下:
通过高分辨率X射线衍射失配度的计算来判定应力补偿效果,也可通过观察围绕卫星峰0级的Pendell Kosung fringes,表明高的超晶格晶体质量,Pendell Kosung fringes的情况可以评定InSb界面的(临界)厚度[33],超晶格卫星峰的相对强度也会受到As/Sb交换反应的影响。
霍尔测试结果参数中平面载流子迁移率是量化超晶格层质量和IF粗糙度散射(IRS)程度的有用工具。如表 2所示,不同IF类型时,载流子浓度和迁移率的变化,证明了IF质量对IF快门序列的敏感性[34]。
表 2 (InAs + IF)/(GaSb + IF) SLs插入InSb-like或/和GaAs-like界面的数据汇总[34]Table 2. Data summary of(InAs+IF)/(GaSb+IF) SLs: the InSb-like or/and GaAs-like IFs were inserted between the layers and their values were estimated from shutter timeIF type Period/Å Strain/% Rs/(Ω/sq) ns/(×1011cm−2) μ/(cm2/Vs) 0/0 44.8 −0.13 8563 1.8 4104 InSb/GaAs 45.4 −0.08 3409 2.8 6579 InSb/InSb 45.5 +0.10 7020 2.0 4343 GaAs/GaAs 45.2 −0.30 7675 1.5 5548 GaAs/InSb 45.1 −0.15 45690 52.2 26 Note: The Rs, ns, and μ represent the 10 K resistivity, hole density, and in-plane hole mobility 室温(room temperature,RT)吸收系数(吸收光谱)是从样品的透射光谱与去除外延层之后衬底的透射光谱之间的比较推导出的。引入界面层的超晶格材料完成结构特征研究之后,通过分析吸收光谱中多种高能跃迁参与的特征来表征证明高质量的超晶格材料光学性质。
有意引入InSb界面层的超晶格材料除了由于基态电子(C1)和重空穴(VH1)微带之间的吸收导致的C1-VH1跃迁外,从吸收光谱中也能够获得其他几种高能跃迁的参与,如图 10所示。根据对InAs/GaSb超晶格的精确计算,作者提出几种高能跃迁可能是C1-VL1跃迁(约300 meV)和C1-VH2跃迁(约430 meV),从而将最低电子微带C1与激发空穴子带相关联(分别为第一轻空穴VL1和第二重空穴VH2子带)[17, 23, 35]。
图 10 InAs/InSb/GaSb(10/1/10 MLs)超晶格的RT吸收光谱[17]Figure 10. RT measured absorption coefficient of strain compensated InAs/InSb/GaSb(10/1/10MLs)SLs2. 总结
对于InAs/GaSb超晶格来说,界面控制极为重要。为了对超晶格界面详细研究,反馈外延生长工艺,对界面质量进行测试表征分析就显得非常重要。本文详细介绍了拉曼光谱、SIMS、HRTEM、STM、XPS、PL谱、HRXRD、霍尔测试、吸收光谱等测试表征方法对InAs/GaSb超晶格界面特性的研究,如界面类型的判定、界面陡峭性、粗糙度及界面反应(如界面互混、As/Sb交换反应等)等,分析了界面质量对超晶格材料能带结构、晶体质量、光学性质的影响。
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图 1 在(0 0 1)GaAs上生长的InAs或GaSb缓冲层具有InSb或GaAs IF的InAs/GaSb SL的室温拉曼光谱[16]
Figure 1. Room-temperature Raman spectrum of InAs/GaSb SLs with InSb- or GaAs-like IFs grown on(0 0 1)GaAs using either InAs or GaSb buffer layers
图 2 在GaSb(0 0 1)上生长的InAs/GaSb超晶格的HRTEM图像: (a)沿着[1 1 0]区轴向的InAs/InSb/GaSb SL的横截面HRTEM图像;(b)SL的(0 0 2)晶格条纹图像[17]
Figure 2. HRTEM image of InAs/GaSb superlattice grown on GaSb(0 0 1): (a)Cross-sectional HRTEM images of the InAs/InSb/GaSb SL taken along the [1 1 0] zone axis;(b) Corresponding(0 0 2)lattice fringe image of the SL
图 4 InAs/GaSb超晶格样品的XSTM图像: (a) InAs/GaSb超晶格样品在(110)横截面的InAs-on-GaSb和GaSb-on-InAs界面的阴离子亚晶格的原子分辨率STM图像[19];(b) Pb(As)= 5.3×10-6 Torr下生长的InAs/GaSb超晶格的XSTM(cross-sectionalscanning tunneling microscopy)图像[20]
Figure 4. XSTM images(anion sublattice) of InAs/GaSb SL samples: (a) STM images(anion sublattice) of InAs/GaSb SL samples in(1 1 0) cross section. As2-soak times are 0 s(bottom), 4 s(middle), and 15 s(top), respectively;(b) XSTM image of a GaSb/ InAs superlattice grown with Pb=5.3×10-6 Torr revealing the anion sublattice(60 nm×60 nm). Inset: Close-up(7 nm×7 nm) of an InAs/GaSb interface where a segment of GaAs interfacial bonds are observed(dark area indicated by the arrow)
图 5 GaSb层上生长的InAs层的SIMS测试: (a) GaSb层上生长的InAs层的SIMS深度剖面[21];(b) 通过SIMS测量在不同衬底温度下生长的两个InAs on GaSb界面的Sb分布[22]
Figure 5. SIMS of InAs layer growing on GaSb layer: (a) SIMS depth profile of an InAs layer buried in GaSb;(b) Sb profile measured by SIMS for two InAs-grown-on-GaSb interfaces at substrate temperatures of 475℃(dashed line) and 380℃(solid line)
图 6 GaSb/InAs和InAs/GaSb异质结构的代表性的XPS[22]
Figure 6. Representative XPS scans of GaSb/InAs and InAs/ GaSb scans used heterostructures. Measured variation in XPS peak intensity ratios, for both InAs/GaSb and GaSb/InAs growth orders
图 7 GaSb表面的As2暴露的XPS峰强度比[22]
Figure 7. XPS peak intensity ratios for As2 exposures of GaSb surfaces
图 8 T=80 K,10MLs-InAs/nInSb ML-InSb/10MLs-GaSb SLs的PL谱[29]
Figure 8. PL spectra of the 10MLs-InAs/nInSb ML-InSb/10MLs GaSb SLs collected at T=80 K. Inset: interface bandege alignment diagram at scale taking into account the inserted InSb layer at the InAs/GaSb heterointerface
图 9 TPL=10 K,在InAs-on-GaSb界面为GaAs-like界面的超晶格材料的PL谱[30]
Figure 9. TPL=10 K PL spectra for SLs structures grown at different As-soak times at InAs-on-GaSb interfaces(GaAs-like interfaces)
图 10 InAs/InSb/GaSb(10/1/10 MLs)超晶格的RT吸收光谱[17]
Figure 10. RT measured absorption coefficient of strain compensated InAs/InSb/GaSb(10/1/10MLs)SLs
表 1 GaSb/InAs和InAs/GaSb界面的同一侧上的元素之间的XPS峰强度比率[22]
Table 1 XPS peak intensity ratios between elements primarily on the same side of the interface of GaSb/InAs and InAs/GaSb
Structure Measured range of XPS peak area ratios Sb 4d/Ga 3d As 3d/In 4d GaSb/InAs
4g: 7s1.20-1.31 0.63-0.86 InAs/GaSb
12g: 17s0.55-1.24 0.81-1.37 Note: The notation xg: ys specifies x growths and y XPS scans used to obtain a given set of values 表 2 (InAs + IF)/(GaSb + IF) SLs插入InSb-like或/和GaAs-like界面的数据汇总[34]
Table 2 Data summary of(InAs+IF)/(GaSb+IF) SLs: the InSb-like or/and GaAs-like IFs were inserted between the layers and their values were estimated from shutter time
IF type Period/Å Strain/% Rs/(Ω/sq) ns/(×1011cm−2) μ/(cm2/Vs) 0/0 44.8 −0.13 8563 1.8 4104 InSb/GaAs 45.4 −0.08 3409 2.8 6579 InSb/InSb 45.5 +0.10 7020 2.0 4343 GaAs/GaAs 45.2 −0.30 7675 1.5 5548 GaAs/InSb 45.1 −0.15 45690 52.2 26 Note: The Rs, ns, and μ represent the 10 K resistivity, hole density, and in-plane hole mobility -
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