Review of defects of HgCdTe films grown by LPE

  Significance   The II-VI semiconductor material Hg_1−xCd_xTe (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 (Te_Hg), 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.