Long-wavelength infrared (IR) III-V based devices have long been of interest for potential applications such as chemical sensing and large format IR imaging. Within the III-V family, only the InAs_1-xSb_x bulk alloy in composition range 0.45 ≤ x ≤ 0.8, offers the required bandgap energy (E_g) from 100 to 125 meV at an operating temperature of 80K and below [1]. Over this composition range however, the InAs_1-xSb_x lattice constant varies from 6.24 to 6.39 Å, where a lack of conventional substrates has restricted progress on the growth and study of this material system. To bridge this lattice-constant gap while maintaining relatively low defect densities, we employed a metamorphic step-graded InAs_1-xSb_x buffer on GaSb, enabling the study of low-E_g InAs_1-xSb_x as a function of growth conditions. Using this method, we investigated the effect of substrate temperature (T_sub) and group V to group III flux ratio (beam equivalent pressure, V/III) on Sb incorporation of the lowest-E_g cap layer [2]. We also used x-ray reciprocal space mapping (RSM) to examine the effect of growth conditions on strain and dislocation dynamics. Following these growth studies, we employed the metamorphic InAs_1-xSb_x in an InAs/InAsSb superlattice designed with a cutoff wavelength of 9 µm which leads to improved absorption compared with the lattice-matched counterpart.

We first grew, via molecular beam epitaxy, several InAs_1-xSb_x step-graded structures in which the Sb/(As+Sb) flux ratio was varied from 0.05 to 0.50 in 0.05 increments (see figure), under various T_sub and V/III. Nomarski imaging revealed smoother surfaces under a V/III=10, the highest ratio we attempted. At this higher V/III, we observed the cross-hatch morphology expected for metamorphic materials and found that the cross-hatch spacing changes, implying a change in dislocation dynamics, with T_sub. We then used photoluminescence (PL) to measure the Sb-content in the cap layer as well as compare intensities between samples. We found the highest Sb-incorporation to occur when T_sub=415 C and V/III=10, while the most intense samples used T_sub=415-430 C and V/III=10 [2].

Using RSM along [110] with (004) and (115) reflections, we identified the Sb composition in each layer. This allowed comparison of Sb-content as a function of Sb/(As+Sb) for various T_sub and V/III. The results suggest that V/III has little effect on Sb incorporation, in direct conflict with our previous PL results [2]. To understand the discrepancy between PL and RSM, we measured (004) RSM of the same three samples with the x-ray beam incident along [1-10], revealing extremely different strain relaxation compared to the [110] case (see figure). Asymmetric strain relaxation has been observed in other III-V graded buffer systems and has been explained by different dislocation formation energies and glide velocities along each direction resulting from the core structure of the dislocation being terminated with either a group-III or a group-V element [3]. Transmission electron microscopy is ongoing to further understand the dislocation dynamics in these samples.

Taking this all together allowed us to investigate the effect of substrate lattice-constant on strain-balanced InAs/InAsSb superlattices designed for 9 µm cutoff wavelength [4]. Theoretically, by using a larger substrate lattice-constant the superlattice design results in larger electron-hole wavefunction overlap, ultimately increasing photon absorption. Our experimental results confirm this theory, even in the presence of increased threading dislocations inherent to the required lattice-mismatch.

[1] I. Vurgaftman et al. JAP 89, 5815-5875 (2001).

[2] Tomasulo et al. J. Vac. Sci. and Technol. B 36, 02D108 (2018).

[3] France et al. J. Appl. Phys. 107, 103530 (2010); Gelczuk et al., J. Cryst. Growth 310, 3014 (2008).

[4] Affouda, Tomasulo et al., Appl. Phys. Lett. 110, 181107 (2017).