Abstract:The effects of dislocations on very-long-wavelength infrared (VLWIR) HgCdTe photodiodes (cutoff wavelength >14 lm at 40 K) have been determined experimentally and analyzed. The photodiodes are in the back-illuminated configuration, fabricated from HgCdTe p-on-n double-layer heterostructure (DLHJ) films grown at BAE Systems by liquid phase epitaxy (LPE) onto lattice-matched (111) CdZnTe substrates. Arrays were hybridized to silicon ROICs to form focal plane arrays (FPAs). After characterization for dark current… Show more
“…[1][2][3] However, their use in long-wavelength focal-plane arrays is limited by large dislocation densities, which are two orders of magnitude greater than for HgCdTe grown on lattice-matched CdZnTe substrates. 4,5 The increased dislocation density leads to higher dark currents in devices and poor performance in focal-plane arrays.…”
In our previous study of ex situ thermal cycle annealing (TCA) of molecular beam epitaxy (MBE)-grown mercury cadmium telluride (HgCdTe) on CdTe/ Si(211) composite substrates we showed consistent dislocation density reduction to $1 9 10 6 cm À2 . In this work, we have extended our study to understand the effects of TCA at lower temperatures and fewer cycles than studied previously. By examining TCA performed at the lower end of the temperature spectrum (as low as 385°C), we are able to show an exponential correlation between etch pit density (EPD) and temperature. Varying the number of cycles also shows a similar exponential correlation with EPD. These results suggest that these are the two major factors driving dislocation annihilation and/or coalescence. In this paper, we discuss the theoretical mechanism behind dislocation reduction, both at the surface and throughout the bulk of the HgCdTe layer.
“…[1][2][3] However, their use in long-wavelength focal-plane arrays is limited by large dislocation densities, which are two orders of magnitude greater than for HgCdTe grown on lattice-matched CdZnTe substrates. 4,5 The increased dislocation density leads to higher dark currents in devices and poor performance in focal-plane arrays.…”
In our previous study of ex situ thermal cycle annealing (TCA) of molecular beam epitaxy (MBE)-grown mercury cadmium telluride (HgCdTe) on CdTe/ Si(211) composite substrates we showed consistent dislocation density reduction to $1 9 10 6 cm À2 . In this work, we have extended our study to understand the effects of TCA at lower temperatures and fewer cycles than studied previously. By examining TCA performed at the lower end of the temperature spectrum (as low as 385°C), we are able to show an exponential correlation between etch pit density (EPD) and temperature. Varying the number of cycles also shows a similar exponential correlation with EPD. These results suggest that these are the two major factors driving dislocation annihilation and/or coalescence. In this paper, we discuss the theoretical mechanism behind dislocation reduction, both at the surface and throughout the bulk of the HgCdTe layer.
“…[1][2][3][4][5][6][7][8] However, the goal of advancing this material technology to the long-wavelength (LWIR) regime has been hampered by the one to two orders of magnitude increase in dislocation density with respect to HgCdTe grown on lattice-matched bulk CdZnTe substrates. This deficiency in dislocation density has been shown to limit lifetimes, 9,10 lower mobility of carriers, 11 lead to larger dark currents in longwavelength detectors, 12,13 and ultimately reduce focal-plane array operability. 9,14 Current growth methods of HgCdTe/Si result in a dislocation density of mid 10 6 cm À2 to low 10 7 cm À2 .…”
Current growth methods of HgCdTe/Cd(Se)Te/Si by molecular-beam epitaxy (MBE) result in a dislocation density of mid 10 6 cm À2 to low 10 7 cm À2 . Although the exact mechanism is unknown, it is well accepted that this high level of dislocation density leads to poorer long-wavelength infrared (LWIR) focal-plane array (FPA) performance, especially in terms of operability. We have conducted a detailed study of ex situ cycle annealing of HgCdTe/ Cd(Se)Te/Si material in order to reduce the total number of dislocations present in as-grown material. We have successfully and consistently shown a reduction of one half to one full order of magnitude in the number of dislocations as counted by etch pit density (EPD) methods. Additionally, we have observed a corresponding decrease in x-ray full-width at half-maximum (FWHM) of ex situ annealed HgCdTe/Si layers. Among all parameters studied, the total number of annealing cycles seems to have the greatest impact on dislocation reduction. Currently, we have obtained numerous HgCdTe/Si layers which have EPD values measuring $1 9 10 6 cm À2 after completion of thermal cycle annealing. Preliminary Hall measurements indicate that electrical characteristics of the material can be maintained.
“…22 These defects play a significant role in limiting HgCdTe device performance, especially in the long wavelength infrared (LWIR) and very long wavelength infrared (VLWIR) regimes. [23][24][25][26][27] Significant efforts have been mounted by many research and development groups to reduce the dislocation density in the CdTe epilayers, yet it remains in the low 10 5 cm À2 at best. 28 The use of an As monolayer on the Si substrate and a ZnTe buffer layer before the CdTe growth has resulted in some improvements.…”
We simulate in three dimensions molecular beam epitaxial (MBE) growth of CdTe/ZnTe/Si using classical molecular dynamics. Atomic interactions are simulated with Stillinger-Weber potentials, whose parameters are obtained by fitting to experimental data or density function theory-calculated distortion energies of the component crystals. The effects of substrate temperature and atomic species flux ratios on epilayer morphology are investigated. The agreement between simulations and experiments suggests that this model has reasonable ability to predict the microstructures of CdTe/ZnTe/Si grown by MBE.
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