Physical structure of molecular-beam epitaxy growth defects in HgCdTe and their impact on two-color detector performance

Abstract

The flexible nature of molecular-beam epitaxy (MBE) growth is beneficial for HgCdTe infrared-detector design and allows for tailored growths at lower costs and larger focal-plane array (FPA) formats. Control of growth dynamics gives the MBE process a distinct advantage in the production of multicolor devices, although opportunities for device improvement still exist. Growth defects can inhibit pixel performance and reduce the operability in FPAs, so it is important to understand and evaluate their properties and impact on detector performance. The object of this paper is to understand and correlate the effects of macrodefects on two-color detector performance. We observed the location of single-crystal and polycrystalline regions on planar and cross-sectioned surfaces of two-color device structures when void defects were viewed by scanning electron microscopy (SEM). Compositional analysis via energy dispersive x-ray analysis (EDXA) of voids in the cross section showed elevated Te and reduced Hg when compared to defect-free growth areas. The second portion of this study examined the correlation of macrodefects with pixel operability and diode current-voltage (I–V) characteristics in mid-wavelength infrared (MWIR)/MWIR (M/M) and long wavelength infrared (LWIR)/LWIR (L/L) two-color devices. The probability of diode failure when a void is present is 98% for M/M and 100% for L/L. Voids in two-color detectors also impact diodes neighboring their location; the impact is higher for L/L detectors than M/M detectors. All void-containing diodes showed early breakdown in the I–V characteristics in one or both bands. High dislocation densities were observed surrounding voids; the high density spread further from the void for L/L detectors compared to M/M detectors.