J. Yang, J.C. Kong, L. Zhao, C.Z. Yang, Y.H. Li, S.L. Wang, W. Zhao, G.Q. Zhao, P.Y. Li, J. Zhao, R.B. Ji
Kunming Institute of Physics, Kunming 650223, China
## Abstract
HgCdTe films grown by molecular beam epitaxy (MBE) are essential for creating high-performance infrared focal plane arrays (IRFPAs) like dual band detectors, high operating temperature (HOT) detectors, and avalanche photodiode (APD) detectors. CdZnTe is recognized as the optimal substrates for growing high crystal quality HgCdTe due to its lattice matching, which is adjusted by selecting the Zn mole fraction. However, defects like grain boundaries, twins, inclusions, dislocations, and sub-grain boundaries that often occur in CdZnTe substrates can significantly reduce the crystal quality of HgCdTe. HgCdTe grown on CdZnTe with these defects usually no longer suit for high quality device fabrication. Although the impact from various defects have been extensively studied in HgCdTe grown on CdZnTe, but the sub-grain boundaries is still not yet involved. In our study, to better understand the effect of sub-grain boundaries in CdZnTe on HgCdTe films, CdZnTe substrates with and without sub-grain boundary were selected to growth MWIR HgCdTe films under optimized condition. Results indicated that etch pit density (EPD) of CdZnTe substrates with sub-grain boundary is approximately 3–5 times higher than boundary free areas. But HgCdTe grown on these substrates bears more than an order of magnitude increase in dislocation proliferation compared to the boundary free region. Dislocation generation resulting from dislocation penetration from substrates, mismatch between HgCdTe and CdZnTe, and sub-grain boundaries were investigated and compared. The intense dislocation proliferation in HgCdTe may be mainly due to twins formed during the initial growth stage of HgCdTe films, rather than dislocation penetration from substrates, as revealed by reflection high-energy electron diffraction (RHEED) pattern monitoring. Therefore, screening for sub-grain boundaries in CdZnTe before HgCdTe growth is necessary.
## Introduction
Over the past decades, HgCdTe has emerged as the most critical and widely used material in infrared detectors, owing to its direct and adjustable bandgap, high absorption coefficient, and high quantum efficiency [1], [2]. Despite its advantages, dislocations in HgCdTe films can negatively impact the performance of infrared detectors by leading to a decline in quantum efficiency, an increase in dark current, and noise. To fabricate long-wavelength and high-performance devices, it is crucial to control the dislocation density within 5 × 105 cm−2 [3]. Dislocations can arise from several factors, such as penetration from the CdZnTe substrate, lattice mismatch between CdZnTe and HgCdTe, and twins or grain boundaries in the CdZnTe substrate [4], [5]. We have conducted extensive research on the twins, grain boundaries, Zn mole fraction, and dislocations in CdZnTe, and has successfully controlled them through routine (2 1 1) CdZnTe substrate characterization and screening. The dislocation density of (2 1 1) CdZnTe substrates at Kunming Institute of Physics ranges from 103–104 cm−2 and Zn mole fraction can reach ± 0.1 % across the entire wafer, size measuring up to 70 mm × 75 mm, due to the directional growth of high-quality CdZnTe crystal ingot. The related results will be published in a separate article soon.
The impact of sub-grain boundaries in CdZnTe substrates on MBE HgCdTe is not well documented in literature. Sub-grain boundaries are formed in regions of CdZnTe crystal with high thermal stresses, mainly arising from the growth or annealing processes of the CdZnTe crystal [6], [7], [8]. Dislocations in sub-grain boundaries can take the form of low-density walls or high-density, almost polycrystalline material, and appear to span the entire thickness of conventional CdZnTe substrates [8], [9], [10], [11]. Typically, CdZnTe substrates with sub-grain boundaries exhibit double or multi-peaks in X-ray diffraction (XRD) rocking curves, but are hardly distinguishable in our routine white beam x-ray diffraction topography measurement when the tilt angle of sub-grain boundaries is small. Since sub-grain boundaries may have a significant impact on the quality of epitaxial films, it is essential to conduct a thorough evaluation and gain an understanding of the mechanism of dislocation formation.
This paper presents our findings from comparing XRD rocking curve and EPD between regions with and without sub-grain boundaries in both CdZnTe substrates and MBE HgCdTe films. Additionally, we compared the dislocation density of HgCdTe resulting from HgCdTe-CdZnTe mismatch and sub-grain boundaries. Using RHEED monitoring, we also propose a possible mechanism for dislocation multiplication related to sub-grain boundaries.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/effect-of-sub-grain-boundaries-in-cdznte-on-hgcdte-film-grown-by-molecular-beam-epitaxy.html