【作者】 查钢强；

【导师】 介万奇；

【作者基本信息】 西北工业大学 ， 材料学， 2007， 博士

【摘要】 CdZnTe晶体是迄今制造室温X射线及γ射线探测器以及HgCdTe等红外薄膜外延衬底最为理想的半导体材料。尽管对CdZnTe的研究由来已久，但在CdZnTe晶体的表面处理、金属与CdZnTe晶体接触及晶体缺陷行为研究上尚存在诸多难题。本文主要研究了CdZnTe晶体表面处理工艺，如机械抛光，化学抛光，钝化，及这些工艺对表面质量的影响。深入分析了CdZnTe晶体表面的基本物理性质，如表面的原子结构、电子结构和功函数。进而，采用MBE在CdZnTe衬底上沉积Au和Ag，研究了Au和Ag与CdZnTe之间的界面反应和肖特基接触势垒。此外，采用化学镀金法制备了Au的欧姆电极，研究了欧姆电极的界面结构以及欧姆接触形成的机理。最后，研究了CdZnTe中位错的一些基本性质。采用机械抛光获得的光亮CdZnTe晶体表面存在一定的损伤层，通过X射线摇摆曲线并结合化学腐蚀，测算出该损伤层的厚度约为13μm。采用Br-MeOH溶液腐蚀可以有效地去除损伤层，但Br-MeOH腐蚀后的晶片表面通常会形成的无定形的富Te层，进一步的化学机械抛光可以去除该富Te层。CdZnTe晶体表面损伤层，会造成较大的表面漏电流和一定的红外吸收。表面富Te层是一个高导电区，它会造成较大的表面漏电流，但基本不影响红外透过率。表面钝化处理可以有效地减少表面漏电流，但是钝化层（氧化层）会吸收红外光，导致红外透过率的下降。与常用的钝化液NH₄F+H₂O₂相比，采用Br₂水溶液进行钝化处理可以获得钝化效果更好的TeO₂钝化层，而不是通常得到的CdTeO₃层。在超高真空条件下采用Ar离子刻蚀并进行原位退火，获得了清洁有序的CdZnTe单晶表面。采用低能电子衍射（LEED）测量了不同表面的原子结构。在（110）面观察到（110）-（1×1）的原子结构，并通过光电子能谱验证了表面弛豫的存在。在（111）A和（111）B面上分别观察到（111）A-（1×1）和（111）B-（1×1）的完整表面原子结构以及（111）A-(3^1/2×3^1/2)R30°和（111）B-（2×2）的表面重构。（111）A面的重构是由Cd空位引起的，而（111）B面的重构是Te的顶戴原子形成的。通过同步辐射角分辨光电子能谱（SRARPES）计算出CdZnTe（110）面的悬挂键密度约为6.9×10¹⁴／cm²，即每一个表面原子含一个悬挂键。发生重构的（111）A-(3^1/2×3^1/2)R30°表面Cd空位增加，Cd空位引入了施主型的表面态，致使能带向下弯曲。发生重构的（111）B-（2×2）表面Te顶戴原子起着受主的作用，导致价带顶向上弯曲。采用光电子能谱技术测量CdZnTe不同表面的功函数，结果表明（111）A面的功函数大于（111）B面；吸附气体和Au会增加CdZnTe晶片的功函数；排列整齐有序的表面的功函数要低于存在损伤的表面；（111）A面发生重构后，Cd空位增加，能带向下弯曲，增加了功函数；而（111）B面发生重构时，Te顶戴原子吸附在晶体表面，形成表面偶极层，也会增大表面功函数。研究了Au与清洁CdZnTe不同晶面的接触势垒，其中（110）面的接触势垒最大，（111）B面次之，（111）A最小。这是由于（110）面的表面能最低，是一稳定面，难以发生界面反应。（111）A面的Te原子不稳定，容易与Au发生化学反应。通过紫外光电子能谱也证实了Au／CdZnTe（111）A的界面确实发生了界面反应。Ag与清洁CdZnTe存在比较明显的界面反应，导致接触势垒很低，约为0.2eV。而Ag与经化学腐蚀的CdZnTe晶片的接触势垒约为0.67eV，这是由于氧化层和富Te层阻挡了Ag与CdZnTe的反应，从而导致了较高的接触势垒。CdZnTe（111）A面发生重构后表面Cd空位增加，在沉积Au的过程中，Au会填补这些空位，作为Cd的替换原子与周围的三个Te原子结合。这样将有利于Au的扩散和界面反应的发生，因而会降低接触势垒。另外表面Cd空位引入的缺陷能级也会降低肖特基接触势垒。Au／CdZnTe（111）B-（2×2）的肖特基势垒低于Au／CdZnTe（111）B-（1×1）面。分析认为这是由于CdZnTe（111）B-（2×2）最外两层原子均是Te原子，Te与Au之间容易发生电子云重叠，形成化学键，即形成合金化的界面层，使富Te面容易形成低的肖特基势垒。对Au／CdZnTe在350℃下退火1小时，可以改善Au膜的有序性，并降低接触势垒约0.15eV。采用AuCl₃水溶液在CdZnTe表面分解制备了Au的欧姆电极，并采用XPS研究了化学镀金法制备的Au／CdZnTe的界面成分分布，确定了CdTeO₃界面层的存在。分析了Au层与CdZnTe形成欧姆接触的机理，提出了CdTeO₃界面层导致欧姆接触的新模型，该模型可以解释目前所有的实验结果。通过塑性变形在CdZnTe晶体分别引入了Cd（g）位错和Te（g）位错。由于位错内的悬键电子会吸收红外光，导致晶片红外透过率下降。变形后的晶片的漏电流显著增大，运用普尔-弗兰克效应解释了实验结果。采用I-V、PL谱和Hall等测量方法研究了位错的电学特性，结果表明，Cd（g）位错在CdZnTe晶体中起受主作用，而Te（g）位错在晶体中起施主作用。Cd（g）位错会会加大对空穴的散射，而导致空穴的迁移率下降，而Te（g）位错会加速空穴的漂移，使空穴的迁移率增加。更多还原

【Abstract】 CdZnTe is the most promising material for room temperature X-ray andgamma-ray detectors, as well as epitaxial substrate for HgCdTe and other infraredmaterials. Although the researches on CdZnTe lasted for over 20 years, there are stillmany unknows about the surface treatment, metal-CdZnTe contact properties anddislocations behaviors of CdZnTe crystal, which are investigated in this work.The surface treatments of CdZnTe crystals, such as mechanical polishing, chemicalpolishing and passivation, were first studied. The characters of CdZnTe surfaces withdifferent treatments, such as the surface atomic structure, electronic structure and workfunction were analyzed with variant advanced technologies. The interface behaviorbetween CdZnTe and metal contacts, including the interface chemical reaction, Schottkybarrier heights, and electroless ohmic contact were studied in detail. At last, thebehaviors and the effects of dislocations in CdZnTe single crystals were investigated.Mechanical polishing produces damaged layer on CdZnTe surface. By comparingthe weight lose and FWHM of X-ray rocking curves after different etching time, thethickness of the surface damaged layer is determined to be about 13μm. The damagedlayer results in high surface leakage current and IR absorption. Etching with Br-MeOHis an effective method to remove the surface damaged layer, and in return produces Teenriched surface. Te enriched layer results in high surface leakage current, but has littleeffect on IR transmittance. Te enriched layer could be deprived by chemo-mechanicalpolishing. Surface passivation can decrease the surface leakage current, but has greatnegative effect on IR transmittance.A new passivation solution, i.e. Br₂ aqueous solution was adopted to replace theconventional NH₄F+H₂O₂ solution, and produced the better passivation efficiency,because TeO₂ passivation layer formed in’stead of TeCdO₃.The atomic structure of the clean CdZnTe surface obtained by Ar⁺ etching andannealing in-situ in ultrahigh vacuum is observed by Low energy electron diffraction.The CdZnTe （110） surface was determined to be unreconstructed, and the surfacerelaxation was identified by X-ray photoemission spectroscopy. Both theunreconstructed （111）A-（1×1） surface and reconstructed （111）A-(3^1/2×3^1/2)R30°structure were found on the CdZnTe （111）A surface. On CdZnTe （111）B surface, theunreconstructed （111）B-（1×1） surface and （111）B-（2×2） reconstructed structure werefound. The reconstruction mechanisms are different for different surface.（111）A-(3^1/2×3^1/2)R30°reconstruction was induced by Cd vacancy, while （111）B-（2×2）was induced by Te adatoms on top of the ideal truncation. Angle resolved photoemission spectroscopy was used to characterize the surfacestate of the clean CdZnTe surface, by which the density of dangling-bonds on theCdZnTe （110） surface is determined to be 6.9×10¹⁴/cm², approximately one electron persurface atom. The valance band of （111）A-(3^1/2×3^1/2)R30°bends down compared to（111）A unreconstructed surface due to the donor surface state caused by the higher Cdvacancy at reconstruction surface. The valance band of （111）B-（2×2） bends up, ascribedto the acceptor of Te adatoms.The work function of CdZnTe surfaces was determined by ultravioletphotoelectron spectroscopy. The results suggested that the work function of CdZnTe（111）A surface is higher than that of （111）B. Gas and gold adsorption increases the workfunction of CdZnTe, while clean and ordered surface presents lower work function. Thework function of （111）A-(3^1/2×3^1/2)R30°reconstructed surface is higher than that ofunreconstructed one due to Cd vacancy on the reconstructed surface. The work functionof （111）B-（2×2） reconstructed surface is higher than that of （111）B-（1×1）, ascribed tothe surface dipole layer caused by Te adatoms.Schottky barrier height of Au/CdZnTe interface on CdZnTe （110） surface is thehighest, on （111）B surface is lower, and that on CdZnTe （111）A is the lowest. Theresults can be explained as follows. The （110） surface is a stabilized one with the lowestsurface energy, so it is not easy for chemical reaction between Au and CdZnTe to takeplace, and preserved the high Schottky barrier. （111）A surface is a polarized one, wherethe Te atoms are easy to react with gold to reduce the barrier. The reaction wasconfirmed by ultraviolet photoelectron spectroscopy. The interface reaction between Agand clean CdZnTe is more obvious, which results in a low Schottky barrier height ofabout 0.2 eV. While the Schottky barrier height between Ag and etched CdZnTe surfaceis as high as 0.67 eV, ascribed to the interception of the oxide and Te enriched layer,which stoped the chemical reaction between Ag and CdZnTe.On the reconstructed CdZnTe （111）A-(3^1/2×3^1/2)R30°surface, the Au atoms fill-upCd vacancies and bond with around Te atoms, which promotes the interface reactionand diffusion of the gold into the crystal. So the Schottky barrier height on Au/CdZnTe（111）A-(3^1/2×3^1/2)R30°surface is lower than that of unreconstructed one. The Schottkybarrier height on Au/CdZnTe （111） B-（2×2） surface is lower than that of Au/CdZnTe-（111）B-（1×1）, which is determined by the top two layer Te atoms on （111）B-（2×2）surface, where Te is easy to react with gold to form the alloyed interface layer anddecreases Schottky barrier height. Annealing at 350℃for one hour in vacuum couldimprove the ordering of gold film and decrease the barrier height to about 0.15 eV.Au ohmic electrode was also prepared by the reaction of gold chloride solution onCdZnTe surface. The component of the interface was studied with X-ray photoelectronspectroscopy, and CdTeO₃ interface layer was found. A new model was proposed to explain the ohmic contact mechanism, and explain experiment results of otherresearchers.Cd（g） and Te（g） dislocations were introduced into CdZnTe by the means ofbending deformation at an elevated temperature. The average IR transmittance ofCdZnTe was decreased apparently after deformation, ascribed to the polarizationabsorption of danling-bond electrons in the dislocations. The leakage current of CdZnTeafter deformation increased largely, and Poole-Frenkel effect should be responsible forthe increase. The effect of dislocations on the carriers density, was analyzed by I-Vcharacters, photoluminescence spectra and Hall effect. It was found that, Cd（g）dislocations act as acceptors, scatter hole and decrease the mobility of hole, while Te（g）dislocations act as donors, promote dispersion of hole and increase the mobility of hole.更多还原