Impact of precipitates in CdZnTe substrates on defects of HgCdTe film grown by molecular beam epitaxy

HgCdTe films with low defect density grown by molecular beam epitaxy(MBE) has been proved to be irreplaceable materials for fabricating high performance infrared focal plane arrays(IRFPAs) such as dual band detector, high operating temperature(HOT) detector and avalanche photodiode(APD) detector. CdZnTe is the best choice of substrates for epitaxy of HgCdTe owning to the perfectly matched lattice. However, the Cd-rich or Te-rich precipitates in CdZnTe substrates are hard to completely eliminate. These precipitates in the CdZnTe substrates induced the formation of precipitate-related defects at the HgCdTe layer, resulting in detrimental device performance, especially for detectors with small pixel size and high operability. To understand the origin of the precipitate-related defects at the HgCdTe layer, we investigated the influences of Cd-rich and Te-rich precipitates in CdZnTe substrates on dislocations and macro-defects in HgCdTe gown by MBE. Gridlines were produced by photolithographic and wet etching process to locate small defects in SEM according to the location in dark field microscopy. Results indicated that Cd-rich precipitate leads to formation of a dislocation cluster in HgCdTe film. Etch pit density(EPD) in dislocation cluster area is in the range of 2×10⁶ to 6×10⁶ cm^-2, about one to two orders of magnitude larger than EPD in a normal region. EPD in the cluster varies with depth of Cd-rich precipitates, and the relationship between them accords well with 1/h dislocation fall-off law, suggesting a minimum EPD of 5×10⁵ cm^-2 in dislocation cluster. In contrast, no dislocation cluster was found in HgCdTe film on CdZnTe with Te-rich precipitates, but Te-rich precipitates can lead to macro-defects, even under the optimum growth conditions. The typical macro-defect density resulted from Te-rich precipitates of state-of-the-art HgCdTe/CdZnTe is range from 100 to 500 cm^-2, which is considered limited by Te-rich precipitates with size of 100 to 1000 nm if the high-density macro-defects caused by particles or pollution residues on CdZnTe substrates are excluded.