Laser-beam-induced current mapping of spatial nonuniformities in molecular beam epitaxy As-grown HgCdTe

Abstract

The formation of dislocations and corresponding built-in electric fields in molecular beam epitaxy (MBE)-grown HgCdTe can have a major impact on the performance and yield of photodetectors fabricated from this material. This paper investigates the presence of such built-in electric fields arising from dislocation segregation in MBE as-grown HgCdTe, and their subsequent removal via a low-temperature Hg-saturated anneal. The electrical properties and surface morphology of an HgCdTe layer grown on a thin CdTe buffer layer are compared with those of an HgCdTe layer grown directly on the CdZnTe substrate. Laser-beam-induced current (LBIC) imaging is a nondestructive technique capable of mapping built-in electric fields present in a semiconductor material, which, in the present case, has been used to reveal dislocation distributions present in as-grown, unintentionally doped, MBE-grown Hg 0.71 Cd 0.29 Te. Two-dimensional scanning LBIC measurements at 160 K allow spatial mapping of electric fields across the HgCdTe wafer. Subsequent isothermal annealing of the wafer in an Hg atmosphere has been found to decrease the magnitude of the built-in electric fields to below the LBIC detection limit. However, of particular note, is that before and after annealing, crosshatch patterns can be seen using Nomarski microscopy, with the crosshatching being predominantly in the [011 -] direction and, to a lesser extent, in the [2 -31] and [2 -13] directions. Defect-decoration etching of the annealed wafer reveals dislocation banding parallel to the [011 -] direction, which closely resembles the contrast observed in the LBIC image of the wafer before annealing. These Nomarski and LBIC images are compared with those of a second wafer, which incorporates a 40-nm CdTe buffer layer. The second wafer does not show significant Nomarski or LBIC contrast, indicating a flat, electrically uniform as-grown layer. Variable magnetic-field Hall measurements at 77 K and quantitative mobility-spectrum analysis (QMSA) indicate predominately p-type conduction with a doping density of 2 ϫ 10 15 cm Ϫ3 in the as-grown layer. After Hg annealing at 240°C, no LBIC signals are observed at 160 K, and Hall measurements at 77 K indicate the presence of two n-type carriers, with a combined doping density of 2 ϫ 10 15 cm Ϫ3 . Double-crystal x-ray diffraction measurements show no evidence of twinned crystal volumes in the layers before or after annealing, or any change in the full-width at half-maximum (FWHM) (41 arcsec) of the (422) reflection. The similarity between the dislocation density distribution, as revealed by defect decoration, and the LBIC image suggests that Hg out-diffusion during growth is expedited in regions of high dislocation concentration, thus creating a nonuniform Hg vacancy-acceptor concentration. The as-grown acceptor concentration, in turn, modulates the hole concentration, creating p ϩ /p Ϫ junctions and built-in electric fields in the material. Low-temperature annealing in a saturated-Hg atmosphere does not remove the crosshatch patterns or...

High-resolution X-ray diffraction studies of molecular beam epitaxy-grown HgCdTe heterostructures and CdZnTe substrates

“…9 Double-layer structures consisted of higher-x capping layers approximately 2-µm thick grown on x ϳ 0.3 absorber layers approximately 10-µm thick. This type of structure provides sufficient layer thicknesses for compositional measurements of the cap and absorber by EDS and FTIR, respectively, and is also useful for subsequent photoconductor device fabrication on these layers.…”