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红外光电探测器是把入射的红外辐射转变成电信号输出的器件，其在夜视、光通讯、大气和质量检验光谱学、导弹制导和红外遥感等方面都有重要应用。想要获得高性能的红外光电探测器，选用合适的高质量的探测材料至关重要。目前用于红外光电探测器的材料主要分为传统半导体材料和新型二维半导体材料。传统红外光电探测器的材料代表有碲镉汞(HgCdTe)、锑化铟(InSb)、铟镓砷(InGaAs)、Ⅱ类超晶格(InAs/GaSb或InAs/InAsSb)以及量子阱(GaAs/InGaAs/AlGaAs等)。以碲镉汞为例，其具有禁带宽度可调、响应速度快、量子效率高和低功耗等优点，但它的材料均匀性难以控制、本征缺陷浓度高、生长成本高以及需低温制冷等。于是，一些新型二维材料凭借其各自的优点开始备受光电探测领域的关注，典型的有石墨烯(Graphene)、过渡金属硫化物(Transition metal sulfide)、钙钛矿(Perovskite)、黑磷(Black phosphorus)和拓扑绝缘体(Topological insulator，TI)等。石墨烯的优点是具有超宽的吸收光谱(紫外到太赫兹)、超高的电子迁移率、超快的光响应速度等；但是单层石墨烯光吸收少、载流子具有超快复合速率，导致器件光响应度较低[1-6]。黑磷材料性能优异，迁移率在室温下可与硅媲美、带隙随厚度可调（响应波段可从紫外、可见到近红外）以及开关比高；但黑磷材料本身不稳定，大面积、高质量的薄膜很难获得[7-9]。过渡金属硫化物以二硫化钼（MoS2）为例，其优点是拥有高开关比、有优异的机械性能可做柔性器件、有良好的光电性能以及带隙随厚度可调；但其迁移率低、响应速度慢、响应波段只能到近红外[10-15]。拓扑绝缘体，不同于普通概念的绝缘体和金属，其内部绝缘，而表面或界面允许电荷移动。拓扑绝缘体典型的代表材料有：第一代的HgTe/CdTe量子阱、第二代的碲化锑（Sb2Te3）、碲化铋（Bi2Te3）和硒化铋（Bi2Se3）等化合物。拓扑绝缘体具有无带隙的表面态和窄带隙体态以及较好的导电能力等优点，但其光吸收比很小（特别是在近红外区域），当入射光的激发能比较小时，其光电流比较弱。因此，这些新型光电探测材料虽然由于其各自新颖特性而被大量关注和研究，但是，它们也由于自身的原因而光电探测性能受到限制。
新型拓扑晶体绝缘体(Topological crystal insulator, TCI) 碲化锡铅（PbxSn1−xTe)、硒化锡铅（PbxSn1−xSe）和碲化锡（SnTe）属于第三代拓扑绝缘体，与受时间反演对称性保护的传统拓扑绝缘体不同，其有自己的特点：（1）拓扑表面态受晶体对称性（镜面对称）保护，高对称性晶面具有不受杂质散射的狄拉克电子态；（2）拥有多重表面态，且位于不同对称性表面上的狄拉克表面态性质不同；（3）电子性质可调，通过改变温度和组分可以打破对称性从而使拓扑表面态可调。基于以上性质，拓扑晶体绝缘体在低功耗的电子器件、自旋电子器件、红外探测器件及热电器件等领域均有很大应用潜力。而且，与同是拓扑晶体绝缘体的Pbx Sn1−x (Te, Se)相比，SnTe还具有元素组成简单、化学结构稳定单一从而更易合成的特点，从而备受研究者们的关注，其在热电器件（Thermoelectric device）[16]、场效应晶体管(Field effect transistor，FET)[17]、太阳能电池（Solar cell)的电极[18]、光电探测器(Photodetector)[19]、超导器件(Superconducting device)[20]以及铁电器件（Ferroelectric device）[21]等领域均有极大的应用价值，其中主要应用器件的结构如图1所示。
图 1 SnTe材料的应用
Figure 1. Applications of SnTe material
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材料制备方法的选择，主要考虑所选方法是否工艺简单、重复性好、稳定性好，是否适合大批量生产、高效节能，制备的材料是否具有较高的质量从而可以满足研究和实际应用的需要等。已被报道的SnTe材料的不同生长方法、材料形态及其特性如表1所示[23-41]，SnTe薄膜和SnTe纳米结构常用制备方法包括分子束外延法(Molecular beam epitaxy, MBE)、化学气相沉积法(Chemical vapor deposition，CVD)、物理气相沉积法(Physical Vapor Deposition，PVD)、液相合成法、熔融退火法以及直接合金法等。
表 1 碲化锡制备方法、材料形态及其特性
Table 1. Preparation methods, material morphology and properties of tin telluride
Preparation methods Morphological structure Features Year Ref. MBE SnTe-based films and superlattices The structure parameters of the PbTe/SnTe superlattice were
determined by the selected buffer layer material1997 [23] MBE Si (111) substrate/
SnTe thin filmThe electronic structure of the film was adjustable by
changing thickness and lead doping level2014 [24] MBE BaF2 (001) substrate/
SnTe filmBy increasing the growth temperature, the film has higher
mobility and lower carrier concentration2014 [25] MBE Sapphire substrate/Bi2Te3 buffer layer/SnTe film Dirac electrons on the SnTe (111) surface was gained by transmission measurements on a high quality film grown on the Bi2Te3 buffer layer 2014 [26] MBE
BaF2substrate/SnTe
filmBy optimizing the growth conditions and film thickness, the carrier concentration is reduced, which conduced to study the surface magnetic transport characteristics 2015 [27] MBE GaAs (111) A substrate/CdTe/
SnTe filmSingle-phase very low hole concentration of SnTe (111) can be
obtained by optimizing the growth temperature of SnTe and
CdTe layers and the growth rate of SnTe2016 [28] MBE Substrate/Bi2Te3 buffer layer/SnTe film An efficient photoconductive photodetector was prepared
based on 10 nm TCI SnTe2017 [29] CVD SnTe nanowire The exposed surface of SnTe micro-nano structure can be adjusted by experimental parameters such as temperature 2014 [30] CVD SnTe nanoribbon The controlled growth of crystal surface of SnTe nanocrystals {100} can be realized by Bi doping 2016 [31] CVD SnTe thin film/n-Si Nps heterojunction A photovoltaic photodetector was prepared based on
SnTe/Si Nps heterojunction2017 [32] PVD SnTe thin film/n-Si heterojunction A photovoltaic photodetector was prepared based on
SnTe/Si heterojunction2017 [33] PVD SnTe flake A field effect transistor photodetector was prepared based on
SnTe single crystal2018 [34] PVD SnTe thin film/n-Bi2Se3 heterojunction A photovoltaic photodetector was prepared based on
SnTe/Bi2Se3 heterojunction2020 [19] Hot wall epitaxy SnTe-based films and superlattices The prepared EuTe/SnTe SL showed a high mobility of 2720 cm2/(V·S) at room temperature. The Seebeck coefficients of SnTe/PbSe and SnTe/PbS SLs can be close to those of PbSe and PbS 2009 [35] Solution-phase synthesis SnTe quantum dot By changing the growth temperature, concentration of reaction mixture, etc., the average diameter of SnTe NCs was adjustable within 4.5-15 nm, and the band gap correspondingly is 0.8-0.38 eV. It can be
used in near-infrared photoelectric devices2007 [36] Solution-based synthesis SnTe nanostructure The shape/size controlled preparation of SnTe nanotubes, nanorods and nanowires promotes the application of colloidal infrared active nanomaterials in practical technologies 2015 [37] Vapor-liquid−
solid growthSnTe nanoplates SnTe nanoplate were prepared with large {100} or {111} surface areas, allowing selective study of the surface states on these surfaces.
The phase transition from rock salt structure to rhombic structure
was observed at low temperature2014 [38] Solid solution alloying Sn 1.03−x Mg x Te ingot Adjusting the SnTe electron band structure by Mg doping, the Seebeck coefficient was improved and the thermoelectric property was optimized 2014 [39] Microwave solvothermal method Se/Cd co-doped SnTe octahedral particles By using the strategy of co-doping selenium and cadmium, the energy band structure of SnTe was optimized to improve the
power factor and thermoelectric optimization value2017 [40] Alloying Ge doped SnTe alloy The local structure distortion and related ferroelectric instability of SnTe were adjusted by Ge doping, and the ultra-low lattice thermal conductivity was aquired to optimize the thermoelectric performance of SnTe 2019 [41] -
高质量薄膜材料的获得对于潜在的器件应用至关重要。分子束外延是指在超高真空下，源材料经过高温蒸发产生的分子束流经衬底表面吸附、迁移、成核以及外延生长单晶薄膜的方法。由于MBE方法生长环境洁净、衬底温度低、制备的薄膜晶体质量好以及可精确控制掺杂浓度和膜层组分等优点，很适宜用来制备原子级超薄层或多层异质结构的光电薄膜材料。
缓冲层材料的选择对MBE制备高质量的薄膜至关重要。在异质外延时，由于衬底和外延薄膜材料不同，二者存在晶格失配，引入缓冲层的目的是释放薄膜中的应力、减小位错失配，缓冲层的生长直接影响到后续外延薄膜的制备质量。早在1997年，波兰科学院物理研究所的J. SADOWS等人在BaF2 (111) 衬底及SnTe，Pb0.5Sn0.5Τe和PbTe不同缓冲层上制备了(50 Å SnTe)/(50 Å PbTe)超晶格，结果发现缓冲层材料的选择在很大程度上决定了整个结构的电学参数[23]。2014年，清华大学的Yan等人[24]利用MBE方法首次在Si (111)衬底上制备出了SnTe高质量薄膜，通过改变薄膜厚度和铅掺杂水平使薄膜的电子结构可调。由于Sn空位和Te取代Sn，非故意掺杂的SnTe是p型半导体材料，在小于1 µm的厚度时，薄膜往往是高度粒状和粗糙的，这会显著降低载流子迁移率，为解决该问题，很多研究之前是集中在SnTe薄膜的缓冲层使用和化学掺杂上，而研究者后来发现通过优化MBE制备SnTe薄膜时的工艺参数可有效降低载流子浓度和增加迁移率。2014年，美国东北大学物理系的B. A. Assaf等人[25]用MBE方法制备SnTe薄膜时发现提高生长温度不仅改善了薄膜的表面形貌和结晶质量，而且载流子浓度也随之降低，当载流子浓度为p=8×1019 cm−3时，霍尔迁移率可达760 cm2/(V·S)。2016年，Ryota Akiyam等人报道了用MBE方法在CdTe上沉积SnTe(111)层，通过优化SnTe、CdTe层的生长温度以及SnTe的生长速率，获得了仅在(111)方向生长的SnTe单相，表面平整度比在BaF2衬底上生长的有很大改善[28]。2017年，国防科技大学的 Jiang等人[24]报道了一种利用MBE方法制备晶圆规模的SnTe薄膜(5 mm×2 mm)的可控方法，即在生长SnTe膜之前，先在绝缘的钛酸锶(STO)(111)衬底上生长四、五层Bi2Te3薄膜(4 nm)以减少晶格的失配，再生长10 nm的SnTe薄膜[29]，并基于SnTe薄膜制备了高效光导型探测器，为优化这些器件用于宽带和灵敏的光电应用提供了指导。
MBE制备方法非常先进，但它也有自身地一些局限性：MBE设备昂贵、操作复杂、生长速率慢以及制备的晶圆规模的SnTe薄膜很难被转移到包括柔性衬底的其他衬底上，这限制了它的兼容性和实际应用，不利于实现产业化生产。
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化学气相沉积法是指利用含有构成薄膜元素的一种或几种气态或液态反应物（单质或化合物）的蒸气，在衬底表面上进行化学反应生成薄膜的方法。CVD方法的特点是设备简单易操作、制备的薄膜重复性和均匀性较好、沉积温度较低以及通过改变气相组成可实现薄膜化学成份可控等。
合理设计具有明确表面的半导体纳米晶体是实现下一代光电探测器、热电和自旋电子器件的关键，SnTe纳米晶体其表面晶面(surface facets)决定了它的表面状态。然而，大多数可用的SnTe纳米晶是由热力学稳定的{100}面组成的，生长具有{111}面的均匀纳米晶体具有挑战性。2014年中国科学院大学的Muhammad Safdar[30]等人用CVD方法获得了具有明显的高对称晶体表面的SnTe纳米线和微晶体，其微纳结构的暴露面可以通过改变CVD过程中的实验参数如沉积温度来调制，在不添加金催化剂的情况下，在高温沉积区域，SnTe为具有{100}面的立方块，而在较低的生长温度下，SnTe为具有{111}面的八面体。该研究为可控合成SnTe半导体微纳结构材料提供了指导。2016年，澳大利亚昆士兰大学的Yi-Chao Zou等人[31]在表面能计算的指导下，使用CVD制备了Bi掺杂SnTe纳米结构，其表面晶面通过Bi掺杂进行调制，实验结果得到了具有明显{111}表面的Bi掺杂SnTe纳米带，为今后发展晶面可控纳米结构提供了机会。2017年，苏州大学的Suhang Gu等人利用CVD方法首次在硅纳米柱表面生长了SnTe薄膜，形成的高质量SnTe/Si纳米柱（Nanopillars）异质结光伏型光电探测器可实现从紫外到近红外的宽谱探测，响应速度超快，探测率高[32]。
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物理气相沉积是指在真空条件下进行的物理气相反应生长方法，或是在低气体放电条件下使用固体材料作为源材料, 经过“蒸发或溅射”后, 以物理方法实现物质从源材料沉积到衬底上的薄膜的可控原子转移过程。PVD方法主要分为真空蒸发镀膜法、真空溅射镀膜法以及真空离子镀膜法。相比于溅射镀膜法，蒸发法具有的优点是：真空度较高、沉积速率较高以及制备的薄膜质量较高。而溅射镀膜法也有优点：通过调整工艺参数易实现膜厚可控、工艺重复性较好、制备多元合金薄膜时化学成分易控以及所沉积的薄膜对衬底的附着力较好等。
2017年，山东师范大学的张宏斌等人在没有使用任何催化剂的情况下，采用PVD方法在Si上制备了具有(111)面和(100)面的SnTe薄膜，该薄膜呈现立方岩盐晶体结构且无其他相的存在，其Sn/Te原子比为1∶1.06，说明薄膜中存在一定数量的Sn空位，导致p型载流子的输运。衬底温度对PVD制备SnTe纳米片的形貌有很大影响，为得到适于制备光电探测器的SnTe纳米片，需要对生长SnTe的温度进行探索[33]。2018年，中国科技大学的张凯等人采用PVD法，在Si/SiO2衬底上对生长SnTe的温度做了探索，发现随生长温度不同，制备出的样品其形貌差异很大。生长温度低于300 ℃时得到具有线状结构表面的SnTe，其表面不够平整，不利于光电器件的制备；而生长温度过高为600 ℃时得到的是紧密堆积方块状SnTe厚纳米片，同样不适于光电器件的制备及性能测试；而生长温度是480 ℃时, 所有SnTe纳米片都均匀分布在整个衬底上，表面干净平整，厚度大小均匀，可很好的满足不同尺寸光电器件的制备需求[34]。2020年，山东师范大学同一团队的张宏斌等人在SiO2衬底上采用一种离位两步PVD的生长策略在无催化剂的情况下制备了高质量的SnTe/Bi2Se3异质结构，在垂直异质结的区域获得了干净的界面，这种新型异质结构不仅利用了拓扑绝缘体对光的有效吸收，而且为研究p型和n型拓扑表面态之间的能带耦合效应提供了理想平台[19]。
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还有一些其他的方法比如液相合成法[36-40]、熔融退火法、直接合金法[41]等也用于制备SnTe薄膜及其纳米结构。但是，液相合成法制备的材料面积要么是太小以至于无法在其上进行器件制作，要么是晶体质量不好无法进行表面态的探索。而熔融退火法、直接合金法等制备的SnTe材料是多用于热电应用领域的探索，而未见用于光电器件方面的研究。
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SnTe材料的性质决定了它的应用，而性质又与材料结构紧密相关。如图2(a)所示，SnTe半导材料具有三种晶型结构，分别为菱方结构的α-SnTe[42]、盐岩结构（面心立方结构）的β-SnTe[22, 43]和斜方结构的γ-SnTe[42]，其具体的空间群和晶格参数如表2所示[44-45]。α-SnTe是低温相，在小于100 K时存在；β-SnTe在100 k以上存在，而在高压条件下（>18 kbar的压力），β-SnTe可沿[111]方向发生畸变转变为γ-SnTe。由于具有面心立方结构的β-SnTe在室温和大气压下是稳定的相，因此通常用于光电探测器件的是这种结构。
图 2 SnTe的结构及表征：(a) 不同结构相变；(b) 面心立方结构的SnTe布里渊区；(c) 面心立方结构的SnTe能带结构；(d) 随厚度或应变可调的带隙；(e) X射线光电子能谱；(f) X射线衍射图谱；(g) 高分辨透射电子显微镜图
Figure 2. Structure and characterization of SnTe: (a) Different phase transitions; (b) FCC Brillouin zone; (c) Band dispersion; (d) Strain- and layer-dependent band gap; (e) XPS spectra; (f) XRD pattern; (g) HRTEM image
表 2 SnTe不同相的晶体结构
Table 2. Crystal structure of different phases of SnTe
Phases Crystal structures Space groups Lattice parameters α-SnTe Rhombohedral structure R3m α = 89.895°, a = 6.325 Å （1Å = 0.1 nm）, β-SnTe Rock-salt Cubic structure Fm3m α = 90°, a = 6.3268 Å γ-SnTe Orthorhombic structure Pnma α = 90°, a = 11.95 Å, b = 4.37 Å, c = 4.48 Å SnTe是第一个被理论预测并已被实验证实的拓扑晶体绝缘体，具有高度对称的晶体结构，拥有无带隙的拓扑表面态和窄带隙体态，且无带隙的表面态仅仅存在于那些镜面对称的表面如(100)、(110)、(111)，其螺旋形的多重表面态和强健的拓扑保护特性在制备无能耗光电器件方面可应用，图2(b)和(c)分别是SnTe面心立方结构的布里渊区和能带结构[43]，它在面心立方布里渊区具有特殊的镜像对称性。SnTe是窄带隙半导体材料，在室温下其体带隙为0.18 eV，可制备从紫外光、可见光到中红外波段的宽谱光电探测器；并且通过改变SnTe薄膜厚度或所受应力使带隙可调，如图2(d)所示[46]。SnTe的结构通常采用X射线衍射（X-Ray diffraction，XRD）、高分辨透射电子显微镜（High resolution transmission electron microscope，HRTEM）以及光电子能谱（X-ray photoelectron spectroscopy，XPS)等手段来表征；非故意掺杂的SnTe由于Sn空位和Te取代Sn从而呈现出P型半导体，Sn:Te通常小于1，如图2(e)XPS谱所示[32]；图2（f）和(g)的XRD[32]和HRTEM图[30]表明了SnTe具有(111)、(100)等拓扑面。
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不同于只有一个表面态的拓扑绝缘体，拓扑晶体绝缘体SnTe拥有多重表面态，其4个拓扑表面态存在于每一个{100}、{110}和{111}表面[47]。研究者通过调整制备方法、生长条件以及采用施加应变或外加电场等手段，精确获得了特殊的拓扑表面态，制备出了具有不同晶体形状SnTe纳米晶体（纳米线、纳米带、纳米片等），图3(a)显示了SnTe微晶的晶体形状从覆盖{100}表面的立方微晶到覆盖{111}表面的八面体微晶的精确裁剪[49]。SnTe纳米线导电性随温度变化如图3(b)[30]，表明SnTe纳米线具有350 meV的热激活能，这种巨大的热激活能可以保护SnTe纳米线上的拓扑表面状态不受热激发的干扰，因此，SnTe纳米线在实现室温拓扑表面输运效应、纳米电子和自旋器件方面具有很大的潜力。图3(c)[31]是具有明显{111}表面的Bi掺杂SnTe纳米带的磁阻测试，在强磁场下表现出弱的反定位效应和线性磁阻，显示出其在自旋电子领域的巨大应用潜力。
图 3 SnTe的形貌、光学、电学及热电性质：(a) SnTe微晶；(b) 基于SnTe纳米线双端器件的I-V曲线；(c) 磁导Δρ/ρ0–B曲线；(d) 7.2 nm和14 nm直径的SnTe纳米晶体的红外吸收光谱；(e) SnTe薄膜的霍尔载流子浓度；(f) SnTe薄膜的霍尔迁移率；(h) SnTe热电材料的功率因数
Figure 3. Morphology, optical, electrical and thermoelectric properties of SnTe: (a) SnTe microcrystals; (b) Current−voltage curves; (c) Magneto-conductance Δρ/ρ0–B curves; (d) IR absorption spectra of 7.2 nm and 14 nm SnTe NCs; (e) Hall carrier density; (f) Hall mobility; (h) Power factor
SnTe有很强的铁电性[21]，使其在诸如非易失性高密度存储、纳米传感器和电子工业等领域有潜在应用；SnTe还具有超导性[20]。
此外，SnTe在3~5 µm范围内表现出高效的光探测性能，图3(d)[36]为SnTe纳米晶体的红外吸收光谱，通过改变生长温度、反应混合物浓度等，SnTe 纳米晶的平均直径在4.5~15 nm内可调，相应带隙为0.8~0.38 eV，在近红外、中红外光电探测中具有很大的应用潜力。
SnTe具有高的载流子浓度、高的空穴迁移率和极低的电阻率,其电学性能优异。室温下高的迁移率，可在超快响应的新型光电探测器领域有应用潜力；SnTe导电性好，在太阳能电池的背电极中已有应用[48]。并且，通过掺杂元素的选择、生长参数的调整以及膜厚的改变等其电学性能可调。图(e)和(g)[25]通过提高生长温度，使空穴载流子浓度从1.0×1021 cm−3降低了两个数量级到8×1018 cm−3，迁移率从60 cm2/(V·S)增加了一个数量级到 720 cm2/(V·S)。SnTe电阻率随温度的变化规律同金属的一致，是随温度升高电阻率增大，如图3(f)所示[27]。
SnTe因具有与PbTe、PbSe有着非常相似的电子能带结构，SnTe在室温以上是面心立方结构，是单相，结构稳定，在最佳工作温度范围内(600~920 K)SnTe材料结构无相变，极有潜力成为性能优异的P型中温热电材料。但是因为未掺杂的SnTe存在大量的Sn空位，是P型半导体，其载流子浓度高(1020~1021 cm−3)、泽贝克（Seebeck）系数低且热导率高，想要优化载流子浓度又比较困难，无量纲的热电优值（Thermoelectric figure of merit ZT, ZT值）小，很难实现工业应用。目前，大量研究都集中在通过共振掺杂、能带简并、载流子浓度优化和点缺陷工程等策略来优化其ZT值上，并且取得了相当不错的进展，图3(h)显示的是通过In和Cu共掺杂，使SnTe热电材料的功率因数在873 K达到较高的值29 µWcm−1K−1，从而ZT值达到较高的1.55[49]。
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SnTe由于其无带隙的表面态和窄带隙的体态，在制备高效、宽谱光电探测器领域具有应用。但是SnTe材料由于光激发载流子在通过带间和带内的声子激发散射而发生指数级的衰减，从而经常呈现出比较弱的光响应，所以，考虑在SnTe和其他半导体材料之间构筑异质结是一种增强光生载流子在SnTe中的分离和传输的有效方法。
苏州大学的顾苏杭等人[32]于2017年首次制备了SnTe/Si高质量异质结构光电探测器，图4(a)是其器件结构。图4(b)是在暗条件和808 nm激光照射下器件典型的I-V曲线，表明该器件在光照下有明显的光响应，特别是在负电压方向上。图4(c)是其器件的能带结构示意图及光电响应机理图，由于在SnTe/Si界面上的内建电势的存在，光生载流子的分离和传输得到极大的便利，制备的器件在零偏压下，有超快的响应速度 8 μs 和高的探测率 8.4 × 1012 Jones，响应率为0.128 A·W−1，并且从紫外到近红外（254~1550 nm）都有响应。SnTe/Si异质结在对近红外光的强响应，对开发低成本、高性能的通信波段近红外探测器有重要意义。但是，该SnTe/Si异质结构以可探测的波长范围为代价而提高了近红外光的吸收效率，因此在波长拓展方面还有很大的上升空间。
图 4 SnTe光伏型器件及其光电特性：(a) SnTe/Si异质结器件示意图；(b) 典型的电流-电压特性；(c)SnTe/Si异质结构的能带图；(d) SnTe/Si器件的截面图；(e) SnTe/Si器件的光电流开关行为；(f)外量子效率图；(g) SnTe/Bi2Se3异质结构示意图；(h) 异质结构横截面的HRTEM图像；(i) SnTe/Bi2Se3的狄拉克能带图
Figure 4. SnTe photovoltaic detector and its photoelectric properties: (a) Scheme of SnTe/Si heterostructure device; (b) Typical I–V characteristics; (c) Energy band diagram; (d) Cross section diagram of SnTe/Si devices; (e) Photocurrent switching behavior of the device; (f) EQE spectrum; (g) Schematic drawing of the SnTe/Bi2Se3 heterostructure; (h) HRTEM image of the heterostructure cross-section; (i) Dirac band diagrams of SnTe/Bi2Se3
同年，山东师范大学的张宏斌等人[33]制备了器件结构如图4(d)所示的高质量SnTe/Si垂直异质结构光伏探测器，其具有优良的二极管特性。图4(e)是在零偏置电压、1064 nm光照下器件的光电流开关比, 可高达8.0×106。图4(f)是SnTe/Si光伏器件在波长范围为300~1100 nm的外量子效率（EQE）光谱，制备的60 nm 厚度的SnTe薄膜作为异质结构光电探测器中的空穴输运层，有效地促进空穴输运至电极，减少了电子空穴复合，这些优点使SnTe/Si光电探测器拥有高的响应率为2.36 A·W−1以及高的探测率为1.54×1014 Jones，并且在近红外波长有一个大的带宽104 Hz （1064、1310 、1550 nm）, 这使得该光电探测器在新型器件应用中具有广阔前景。
2020年，同一团队的张宏斌等人[19]制备了高质量的SnTe/Bi2Se3异质结光电探测器，图4(g)是其结构示意图。制备的异质结具有显著的二极管特性，整流比高达700。n型/Bi2Se3和p型SnTe的狄拉克能带图如图4(i)所示，利用该异质结制备了近红外光电探测器，对1550 nm入射光的响应超快也很稳定，其响应率可达145.74 m A·W−1, 最大探测率为1.15×1010 Jones，响应速度为6.90 μs。这对低功耗、低成本的光电探测应用的研究具有重要意义，并在超快存储器件的应用方面有巨大的潜力。图4(h) 是异质结构横截面HRTEM图，表明在垂直异质结的区域获得了干净的界面，没有其他合金相的存在。
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SnTe材料为窄带隙半导体, 其光电探测波段可至中波红外(3~5 µm)，并且通过施加应变或电场可调制它的表面态，因此在红外探测中有很大的应用潜力。
2017年，国防科技大学的Jiang等人基于超薄SnTe单晶二维薄膜材料[29]制备了光导型探测器，图5(a)和(b)是其器件结构示意图，使用了Bi2Te3缓冲层以减少钛酸锶(SrTiO3)衬底与SnTe薄膜之间的晶格失配。该光导型器件从可见光到中红外范围(405~3800 nm)可实现快速、稳定的响应如图5(c)和(d)所示，其响应波段相较于基于SnTe薄膜的光伏型探测器[32]得到了进一步的拓展；光电流在2003 nm处对激光功率强度的依赖关系如5(e)所示, 其呈现的线性关系表明该器件具有较大的线性动态范围。这些结果表明SnTe薄膜材料在宽谱、高灵敏红外光电探测方面具有很大应用潜力。2020年，西澳大学的雷文团队[50]利用超薄SnTe纳米片制备了近红外波段响应的柔性探测器，图5(f)是其器件示意图, 原子力显微镜图如图5(g)所示，在云母衬底上生长的SnTe纳米片厚度仅3.6 nm；该器件性能优异，在室温、980 nm激光照射下，响应率为698 m A·W−1，探测率为3.89×108 Jones，外部量子效率为88.5%；图5(h) 是在980 nm激光照射下，显示光电流随激光功率强度的增加而增大；制备的柔性器件在不同弯曲角度下性能稳定，如图5(i)所示。这些结果有助于深入理解非层状材料的范德华生长机理，也证明SnTe纳米片在柔性近红外探测器领域有巨大潜力。
图 5 SnTe光导型器件及其光电特性: (a)光导型探测器的示意图；(b)光导型探测器的光学显微镜图；(c) 在405 nm光照下的光响应图；(d) 3.8 µm光照下的光响应图；(e)光电流随激光功率强度的变化关系；(f) 柔性SnTe近红外单个纳米片光电探测器原理图；(g) AFM图；(h)在980 nm激光照射下光电流随激光功率强度的变化关系；(i)不同弯曲角度的SnTe纳米片在980 nm激光照射下的光电流
Figure 5. SnTe photoconductive detector and its photoelectric properties: (a) Schematic diagram of the photodetector; (b) OM image of the detector; (c) Time-dependent photo-response with 405 nm; (d) Time-dependent photo-response with 3.8 µm; (e) The laser power intensity dependence of the photocurrent; (f) Schematic diagram of flexible SnTe NIR single nanoplate photodetectors; (g) Representative AFM image; (h) Dependence of laser intensity on the photocurrent under the illumination of a 980 nm laser; (i) Photocurrent of SnTe nanoplate photodetectors bending with different radii under the illumination of a 980 nm laser
场效应晶体管（FET）是通过控制输入回路的电场效应（栅极电压VGS）来控制输出回路电流(漏极电流ID)的一种半导体器件。其具有输入电阻高、温度稳定性好、噪声低、功耗低、动态范围大等特点，在大规模集成电路中被广泛应用。为实现基于SnTe红外探测器的集成化和小型化，可制备SnTe 场效应晶体管。
2018年，中国科学技术大学的张凯等人[34]基于高结晶度的SnTe单晶纳米片制备了器件结构如图6(a)所示的FET光电探测器，其中p型Si衬底同时也是作为背栅，图6(b)和(c)分别是该SnTe纳米片的扫描电镜(Scanning electron microscope，SEM)和原子力显微镜(Atomic force microscope，AFM)图。通过制作短的通道长度在中波红外4650 nm处获得了4.17 A·W−1的响应率，如图6(d)所示，且在254~4650 nm实现了超宽谱光电响应。首次直接在聚对苯二甲酸乙二醇酯（Polyethylene terephthalate，PET）衬底上制备了柔性SnTe光电探测器并进行了在不同弯曲角度（0°~90°）下光电流随时间变化的曲线测试，图6(e)和(f)表明其光电流保持稳定。该团队研究结果相较于其他基于SnTe薄膜的光伏型器件[32]和光导型器件[29]而言，把可探测的波长范围进一步拓展到了中波红外，实现了从紫外到中红外波段的超宽谱光电响应，器件的响应率高。为进一步研究SnTe纳米片的电学性能，2020年，西澳大学的雷文团队[50]以带有300 nm SiO2层的硅为衬底制备了SnTe FET，图6(g)是分别在暗条件和980 nm激光（强度为21.5 mW/cm2）照射下SnTe FET的转移特征曲线，显示其为n型通道且开关比为5.5, 计算得到的迁移率较低为0.228 cm2/(V·S)，主要是由于其超薄的厚度导致了电荷杂质的库仑散射；器件的1/f噪声对栅极电压（Vg）的依赖关系如图6(h)，这与传统半导体光电探测器不同，但与其他2D材料基探测器相似；图6(i)当栅极电压增加时（从−30 V到−2.5 V），SnTe FET的响应率得到了提高（从2.96 m A·W−1到723 m A·W−1），同时探测率也得到了提高（从2.4×106到5.3×108 Jones），表明通过改变电压可以轻易地调节SnTe FET近红外探测器的光响应，该器件的响应率已与近红外探测范围内目前使用的商用硅、锗光电探测器的响应率(0.5~0.85 A/W)比较接近，但探测率仍需优化。对SnTe FET光电探测器的这些研究结果表明，其在宽谱光电探测、柔性可穿戴器件、远程通信等领域均有应用潜力。
图 6 SnTe 场效应晶体管器件及其光电特性: (a) SnTe基光电探测器原理图；(b) SnTe纳米片的SEM图像；(c) SnTe纳米片的AFM图像；(d) 4650nm光照下的光响应图；(e) SnTe基柔性光电探测器的光响应测量；(f) 柔性器件的光电流图；(g) 在暗条件和980nm激光照射下SnTe探测器的转移特征曲线；(h) 典型的噪声-栅极电压图；(i) 依赖于栅极电压的响应率和探测率
Figure 6. SnTe FET photodetector and its photoelectric properties: (a) Diagrammatic drawing of the SnTe-based photodetector; (b) SEM image; (c) AFM image; (d) Time-dependent photo-response with 4650 nm; (e) Photoresponse measurements of the SnTe-based flexible photodetectors; (f) Time-dependent photocurrent of the flexible device; (g) Transfer curves of SnTe detectors; (h) Typical noise as a function of gate voltage; (i) Gate voltage dependent responsivity and detectivity
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由ⅣA族元素Sn和ⅥA族元素Te组成的Ⅳ-Ⅵ族化合物SnTe，属直接窄带隙半导体材料；其熔点为790 ℃，常温常压下是离子型晶体但有一定的共价键成分，材料稳定性好且易于制备；室温下迁移率高，并且其电学参数（迁移率和载流子浓度）可通过改变制备工艺参数或元素掺杂进行调节，可实现紫外、可见到红外波段的宽谱、快速、高效光电探测，因此，SnTe材料在光电探测器方面有极大的应用潜力。
SnTe目前的研究大多是集中在对其作为拓扑晶体绝缘体材料本身物理特性的研究上、对其薄膜和纳米晶体的制备研究上以及在热电应用方面采用各种策略来提高其热电优值上，而对SnTe在光电探测器领域中的研究相对很少，已见的研究报道得到的光电探测器件性能虽比较好，但仍有很大的提升空间。
目前SnTe作为光电器件仍有一些方面需进一步深入研究。在SnTe薄膜制备方面，获得高质量、大面积的SnTe薄膜仍是巨大的挑战，而长线阵和大规模焦平面阵列等多像素探测器的制备对此是有要求的。在SnTe光电器件的设计方面，已做过的研究有光伏型、光导型和光电场效应管，但光伏型器件尝试选用过的异质材料较少，只有与硅结合或与拓扑绝缘体硒化铋结合的，其响应波段只达到近红外和短波红外波段，器件性能有待优化；场效应晶体管的器件结构也比较单一，可对其进行不同器件结构的设计和研究。在SnTe光电器件的制作方面，目前仅仅是在实验室条件下尝试制备单元器件，很少使用到标准的半导体制备工艺流程和相关设备，器件制作方法有待标准化，器件性能可望进一步提高。在SnTe光电器件的测试方面，测试平台受限比如光源达不到更长红外波段的要求从而没法验证它在更长波段的响应等，需进一步优化测试平台。
Preparation, structure and properties of tin telluride and its research progress in infrared photodetection (Invited)
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摘要: Ⅳ-Ⅵ族碲化锡化合物是直接带隙半导体材料，在室温和大气压条件下具有稳定存在的面心立方结构。作为拓扑晶体绝缘体，碲化锡具有高度对称的晶型结构、螺旋形的多重表面态和强健的拓扑保护特性、无带隙的拓扑表面态和窄带隙体态、室温下高的迁移率等优异性能，在制备无能耗、宽谱（从紫外光、可见光到红外光）、超快响应的新型光电探测器领域有巨大潜力。文中从适宜应用于光电探测器件的角度出发，对碲化锡材料的制备方法、晶体结构、性质进行了阐述，对近年来碲化锡在红外光电探测领域的研究进展进行了总结，展望了其在光电探测领域的发展前景，并提出了碲化锡作为光电器件亟需深入研究的几个方面。Abstract: As Ⅳ-Ⅵ compound, tin telluride belongs to direct band gap semiconductor materials. Under the condition of room temperature and atmospheric pressure, tin telluride has a stable face-centered cubic crystal structure. Being a topological crystal insulator, tin telluride has a highly symmetrical crystal structure. Due to its helical multiple surface states and strong topological protection characteristics, tin telluride can be used to fabricate new electronic devices without energy consumption. Moreover, on account of its excellent properties such as band-gap free topological surface state and narrow band gap posture, it has great potential in the field of preparing new photodetectors with wide spectral response from ultraviolet, visible light to infrared. In addition, because of its high mobility at room temperature, tin telluride is expected to be used for high performance photoelectric detection with ultra-fast response speed. In this review, the preparation methods, crystal structures and properties of tin telluride materials were summarized from the point of view that they were suitable for photodetectors. And the research progress of tin telluride in infrared photoelectric detection in recent years was summarized. Then the development potential of tin telluride in the field of photodetectors was prospected, and several aspects that need to be further studied as photodetectors were also put forward.
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Key words:
- SnTe /
- material preparation /
- photoelectrical properties /
- photodetectors
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图 2 SnTe的结构及表征：(a) 不同结构相变；(b) 面心立方结构的SnTe布里渊区；(c) 面心立方结构的SnTe能带结构；(d) 随厚度或应变可调的带隙；(e) X射线光电子能谱；(f) X射线衍射图谱；(g) 高分辨透射电子显微镜图
Figure 2. Structure and characterization of SnTe: (a) Different phase transitions; (b) FCC Brillouin zone; (c) Band dispersion; (d) Strain- and layer-dependent band gap; (e) XPS spectra; (f) XRD pattern; (g) HRTEM image
图 3 SnTe的形貌、光学、电学及热电性质：(a) SnTe微晶；(b) 基于SnTe纳米线双端器件的I-V曲线；(c) 磁导Δρ/ρ0–B曲线；(d) 7.2 nm和14 nm直径的SnTe纳米晶体的红外吸收光谱；(e) SnTe薄膜的霍尔载流子浓度；(f) SnTe薄膜的霍尔迁移率；(h) SnTe热电材料的功率因数
Figure 3. Morphology, optical, electrical and thermoelectric properties of SnTe: (a) SnTe microcrystals; (b) Current−voltage curves; (c) Magneto-conductance Δρ/ρ0–B curves; (d) IR absorption spectra of 7.2 nm and 14 nm SnTe NCs; (e) Hall carrier density; (f) Hall mobility; (h) Power factor
图 4 SnTe光伏型器件及其光电特性：(a) SnTe/Si异质结器件示意图；(b) 典型的电流-电压特性；(c)SnTe/Si异质结构的能带图；(d) SnTe/Si器件的截面图；(e) SnTe/Si器件的光电流开关行为；(f)外量子效率图；(g) SnTe/Bi2Se3异质结构示意图；(h) 异质结构横截面的HRTEM图像；(i) SnTe/Bi2Se3的狄拉克能带图
Figure 4. SnTe photovoltaic detector and its photoelectric properties: (a) Scheme of SnTe/Si heterostructure device; (b) Typical I–V characteristics; (c) Energy band diagram; (d) Cross section diagram of SnTe/Si devices; (e) Photocurrent switching behavior of the device; (f) EQE spectrum; (g) Schematic drawing of the SnTe/Bi2Se3 heterostructure; (h) HRTEM image of the heterostructure cross-section; (i) Dirac band diagrams of SnTe/Bi2Se3
图 5 SnTe光导型器件及其光电特性: (a)光导型探测器的示意图；(b)光导型探测器的光学显微镜图；(c) 在405 nm光照下的光响应图；(d) 3.8 µm光照下的光响应图；(e)光电流随激光功率强度的变化关系；(f) 柔性SnTe近红外单个纳米片光电探测器原理图；(g) AFM图；(h)在980 nm激光照射下光电流随激光功率强度的变化关系；(i)不同弯曲角度的SnTe纳米片在980 nm激光照射下的光电流
Figure 5. SnTe photoconductive detector and its photoelectric properties: (a) Schematic diagram of the photodetector; (b) OM image of the detector; (c) Time-dependent photo-response with 405 nm; (d) Time-dependent photo-response with 3.8 µm; (e) The laser power intensity dependence of the photocurrent; (f) Schematic diagram of flexible SnTe NIR single nanoplate photodetectors; (g) Representative AFM image; (h) Dependence of laser intensity on the photocurrent under the illumination of a 980 nm laser; (i) Photocurrent of SnTe nanoplate photodetectors bending with different radii under the illumination of a 980 nm laser
图 6 SnTe 场效应晶体管器件及其光电特性: (a) SnTe基光电探测器原理图；(b) SnTe纳米片的SEM图像；(c) SnTe纳米片的AFM图像；(d) 4650nm光照下的光响应图；(e) SnTe基柔性光电探测器的光响应测量；(f) 柔性器件的光电流图；(g) 在暗条件和980nm激光照射下SnTe探测器的转移特征曲线；(h) 典型的噪声-栅极电压图；(i) 依赖于栅极电压的响应率和探测率
Figure 6. SnTe FET photodetector and its photoelectric properties: (a) Diagrammatic drawing of the SnTe-based photodetector; (b) SEM image; (c) AFM image; (d) Time-dependent photo-response with 4650 nm; (e) Photoresponse measurements of the SnTe-based flexible photodetectors; (f) Time-dependent photocurrent of the flexible device; (g) Transfer curves of SnTe detectors; (h) Typical noise as a function of gate voltage; (i) Gate voltage dependent responsivity and detectivity
表 1 碲化锡制备方法、材料形态及其特性
Table 1. Preparation methods, material morphology and properties of tin telluride
Preparation methods Morphological structure Features Year Ref. MBE SnTe-based films and superlattices The structure parameters of the PbTe/SnTe superlattice were
determined by the selected buffer layer material1997 [23] MBE Si (111) substrate/
SnTe thin filmThe electronic structure of the film was adjustable by
changing thickness and lead doping level2014 [24] MBE BaF2 (001) substrate/
SnTe filmBy increasing the growth temperature, the film has higher
mobility and lower carrier concentration2014 [25] MBE Sapphire substrate/Bi2Te3 buffer layer/SnTe film Dirac electrons on the SnTe (111) surface was gained by transmission measurements on a high quality film grown on the Bi2Te3 buffer layer 2014 [26] MBE
BaF2substrate/SnTe
filmBy optimizing the growth conditions and film thickness, the carrier concentration is reduced, which conduced to study the surface magnetic transport characteristics 2015 [27] MBE GaAs (111) A substrate/CdTe/
SnTe filmSingle-phase very low hole concentration of SnTe (111) can be
obtained by optimizing the growth temperature of SnTe and
CdTe layers and the growth rate of SnTe2016 [28] MBE Substrate/Bi2Te3 buffer layer/SnTe film An efficient photoconductive photodetector was prepared
based on 10 nm TCI SnTe2017 [29] CVD SnTe nanowire The exposed surface of SnTe micro-nano structure can be adjusted by experimental parameters such as temperature 2014 [30] CVD SnTe nanoribbon The controlled growth of crystal surface of SnTe nanocrystals {100} can be realized by Bi doping 2016 [31] CVD SnTe thin film/n-Si Nps heterojunction A photovoltaic photodetector was prepared based on
SnTe/Si Nps heterojunction2017 [32] PVD SnTe thin film/n-Si heterojunction A photovoltaic photodetector was prepared based on
SnTe/Si heterojunction2017 [33] PVD SnTe flake A field effect transistor photodetector was prepared based on
SnTe single crystal2018 [34] PVD SnTe thin film/n-Bi2Se3 heterojunction A photovoltaic photodetector was prepared based on
SnTe/Bi2Se3 heterojunction2020 [19] Hot wall epitaxy SnTe-based films and superlattices The prepared EuTe/SnTe SL showed a high mobility of 2720 cm2/(V·S) at room temperature. The Seebeck coefficients of SnTe/PbSe and SnTe/PbS SLs can be close to those of PbSe and PbS 2009 [35] Solution-phase synthesis SnTe quantum dot By changing the growth temperature, concentration of reaction mixture, etc., the average diameter of SnTe NCs was adjustable within 4.5-15 nm, and the band gap correspondingly is 0.8-0.38 eV. It can be
used in near-infrared photoelectric devices2007 [36] Solution-based synthesis SnTe nanostructure The shape/size controlled preparation of SnTe nanotubes, nanorods and nanowires promotes the application of colloidal infrared active nanomaterials in practical technologies 2015 [37] Vapor-liquid−
solid growthSnTe nanoplates SnTe nanoplate were prepared with large {100} or {111} surface areas, allowing selective study of the surface states on these surfaces.
The phase transition from rock salt structure to rhombic structure
was observed at low temperature2014 [38] Solid solution alloying Sn 1.03−x Mg x Te ingot Adjusting the SnTe electron band structure by Mg doping, the Seebeck coefficient was improved and the thermoelectric property was optimized 2014 [39] Microwave solvothermal method Se/Cd co-doped SnTe octahedral particles By using the strategy of co-doping selenium and cadmium, the energy band structure of SnTe was optimized to improve the
power factor and thermoelectric optimization value2017 [40] Alloying Ge doped SnTe alloy The local structure distortion and related ferroelectric instability of SnTe were adjusted by Ge doping, and the ultra-low lattice thermal conductivity was aquired to optimize the thermoelectric performance of SnTe 2019 [41] 
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表 2 SnTe不同相的晶体结构
Table 2. Crystal structure of different phases of SnTe
Phases Crystal structures Space groups Lattice parameters α-SnTe Rhombohedral structure R3m α = 89.895°, a = 6.325 Å （1Å = 0.1 nm）, β-SnTe Rock-salt Cubic structure Fm3m α = 90°, a = 6.3268 Å γ-SnTe Orthorhombic structure Pnma α = 90°, a = 11.95 Å, b = 4.37 Å, c = 4.48 Å
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