Sb-incorporation in MBE-grown metamorphic InAsSb for long-wavelength infrared applications

Abstract

InAs1-xSbx is a unique semiconductor as it possesses the lowest bandgap (Eg) of the conventional III–V materials, yielding emission out to nearly 15 μm at room temperature. As such, it is well-suited as the absorber material in long-wavelength infrared (IR) applications such as chemical sensing and large format IR imaging. However, the compositions at the longest wavelengths are significantly lattice-mismatched to conventional substrates. Overcoming this lattice-mismatch requires using thick graded buffers, and to date, little work has focused on the effect of growth conditions on the Sb-incorporation or optical quality of the longest-wavelength InAs1-xSbx materials. Here, the authors investigate the molecular beam epitaxy (MBE) growth of this potential absorber material by using solid source MBE to grow InAs1-xSbx on step-graded buffers on GaSb substrates. The authors have achieved cap-layer compositions as high as x ≈ 0.55 under various substrate temperatures (Tsub) and V/III beam equivalent pressure ratios. By using InAs1-xSbx as the grading material as well as the cap material, the authors can measure Sb-content as a function of growth conditions over many compositions. The author's results show good agreement between x and Eg compared with previous reports, obtaining T = 8 K photoluminescence (PL) emission approaching 11 μm. However, the authors also found that Sb-incorporation falls off for x > 0.2 and worsens with increasing Tsub, suggesting that Sb-incorporation rates increase with reduced adatom mobility. Additionally, the PL of the samples show improved intensity when either Tsub or V/III were increased, while cross-sectional transmission electron microscopy revealed a significant decrease in threading dislocation density with increased V/III. Ultimately, the authors found that under the proper growth conditions, optically active InAs1-xSbx with x over 0.55 can be attained, and with an optimized grading structure, could be a low-cost alternative to HgCdTe for long-wavelength infrared optoelectronics.