Review of defects of HgCdTe films grown by LPE
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摘要: 液相外延是碲镉汞(MCT)薄膜生长领域最成熟的一种方法,被众多红外探测器研究机构和生产商所采用。然而由于MCT材料自身属性和具体制备工艺的原因,液相外延生长过程中不可避免地会产生各种缺陷,从而降低红外探测器的性能。为了增加对液相外延MCT薄膜中缺陷的认识,并对具体的生长工艺提供指导性建议,基于已报道的文献总结了液相外延MCT薄膜中所存在一些缺陷的特征以及形成机理和消除方法。对各类缺陷的形成机理和消除方法进行探讨和评估,有助于提高MCT薄膜液相外延的水平,为制造高性能MCT探测器做好材料技术支撑。Abstract:
Significance The II-VI semiconductor material Hg1−xCdxTe (MCT) is widely used in the manufacture of infrared detectors due to its excellent infrared photoelectric properties at low temperatures. Liquid phase epitaxy (LPE) method is one of the most important techniques for the preparation of MCT materials because of its low cost and excellent process stability. However, due to the properties of MCT materials and the specific preparation technology, various defects will inevitably occur in the process of LPE growth, thus reducing the performance of infrared detectors. Therefore, it is necessary to study the types and causes of defects in detail and develop corresponding inhibition measures, so as to improve the growth technology of LPE and develop high-performance infrared detectors. Progress According to the literature reports, the defects of MCT films grown by LPE method mainly include point defects, dislocation, surface defects, growth ripples and terracing. Point defects mainly include Hg vacancy and anti-site defect of Te (TeHg), and the defect concentration is closely related to mercury pressure and temperature (Fig.1), which can be effectively controlled by selecting appropriate annealing conditions. Dislocation defects mainly include mismatched dislocation and threading dislocation. Due to the lattice mismatch between substrate material and epitaxial material, i.e. lattice constants are not equal, the epitaxial layer will be grown under strain conditions, resulting in the generation of mismatches. Mismatch dislocations appear on the surface of the film in the form of Crosshatch and Mosaic (maximum mismatch) (Fig.4). At the same time, the surface roughness of the film will increase (Fig.6). By adjusting Zn components of substrate, a good lattice match between substrate and film can be achieved to improve the quality of film. The threading dislocations extend from the substrate and are formed during the substrate growth process, which is mainly related to the impurities in the melt, the properties of the crucible wall and the stoichiometric deviation of the melt components, and often occurs around the precipitations. This type of dislocation can be effectively controlled by optimizing substrate growth process and designing subsequent appropriate annealing process. Surface defects mainly include crater defects, hill-like defects and surface crystalline defects, etc. The crater defects are mainly related to the quality of the substrate, so it is necessary to reduce the size and density of the precipitations of the substrate and avoid the introduction of surface impurities (Fig.9). The hill-like defects are mainly related to substrate debris at the growth interface and the precipitation generated in the growth process (Fig.10), so it is necessary to optimize the quality of substrate, reduce edge breakage, and improve the uniformity of mother liquor. The surface crystalline defects are mainly related to the epitaxial growth process (Fig.11), so it is necessary to optimize the epitaxial growth process and improve the composition uniformity of mother liquor. The crystal direction deviation of substrate is the main cause of terracing defects, and the deviation angle should be controlled within 0.2° (Fig.12). However, the melt back process of substrate, epitaxial temperature field and other growth parameters may also affect the generation of terracing defects. The growth of surface ripple is mainly related to the convection of melt, so it is necessary to comprehensively optimize the epitaxial growth parameters including temperature field, undercooling degree during growth, growth rate and the melting process of substrate (Fig.13). Conclusions and Prospects In order to increase the understanding of defects in MCT films grown by LPE and provide guidance for mass production, the characteristics, formation mechanism and elimination methods of the defects were summarized based on the reported literatures. The formation mechanism and elimination methods of various defects were discussed and evaluated, which was expected to improve the quality of the MCT films grown by liquid-phase epitaxy, and provide technical support for manufacturing high-performance MCT detectors. -
图 2 温度(a)和汞分压(470 ℃)(b)对本征点缺陷和杂质缺陷浓度的影响以及Li、Na、Cu元素在 MCT材料和CdTe衬底中的化学势与Cd分压的关系(c)[24]
Figure 2. The effects of temperature (a) and mercury partial pressure (470 ℃) (b) on defect concentration, and the relationship of relative chemical potential of Li, Na, and Cu in MCT and CdTe with Cd partial pressure (c)[24]
图 5 晶格失配与薄膜表面Crosshatch形貌(a)、(b)和摇摆曲线半峰宽(c)的关系[35],以及MCT和CZT的热膨胀系数(d),其中标记为实心和空心圆的数据由Skauli等测试所得[37],标记为1、2、3、4的曲线来自另外所报道的文献[38-43]
Figure 5. The relationship between lattice mismatch and Crosshatch (a), (b) as well as FWHM (c)[35],thermal expansion coefficient of MCT and CZT (d) as a function of composition, the data marked with solid and hollow circles were measured by Skauli et al[37], line 1, 2, 3, and 4 were from the reported literatures[38-43]
图 7 穿越位错的示意图(a)以及表征结果(b)[48],衬底表面的富Cd沉积相及位错(c)[50],起源于富Cd沉积相的MCT薄膜表面的位错(d)[51]
Figure 7. The schematic diagram (a) and characterizations of threading dislocations (b) [48], the Cd-rich precipitates and dislocations in CZT substrate (c) [50], the dislocations in MCT film deriving from the Cd-rich precipitates (d) [51]
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