Mixed arsenide-antimonide (AsSb) materials have many potential applications including photovoltaics and infrared detection as they possess direct bandgaps (E g ) ranging from ~0.5 eV to ~1.8 eV. However, synthesis of these materials is fairly immature and challenging. In this presentation, we will explore the molecular beam epitaxy (MBE) of InAlAsSb lattice-matched to InP and metamorphic InAsSb covering both ends of this E g range. As the widest-E g lattice-matched to InP, a high-efficiency, wide-E g InAlAsSb solar cell would be beneficial toward maximizing efficiency of an all-lattice-matched triple-junction solar cell. However, both the immaturity and mixed group-V nature of this alloy pose significant challenges, requiring in depth investigation.
Initial attempts at MBE of InAlAsSb resulted in anomalously low
photoluminescence emission energies, compared with energies extracted from variable angle spectroscopic ellipsometry. To further investigate the cause of this discrepancy, we performed a systematic study of the substrate temperature and V/III of In 0.26 Al 0.74 As 0.88 Sb 0.22 (expected E g =1.64 eV) and will report the results in this presentation. At the other end of the E g range, we have metamorphic InAsSb, which possesses the lowest E g of all the conventional III-V materials, making it attractive for mid-to-long wavelength IR applications.
However, since the composition possessing the lowest E g is not lattice-matched to existing substrates, compositional graded buffers are required to slowly grade the lattice-constant from that of the substrate to that of the desired composition in order to maintain a low defect density. Here we will present metamorphic step-graded InAs 1-x Sb x buffers on GaSb, enabling the study of Sb incorporation as a function of growth conditions over a range of x (~0.1-0.6). We also present investigation of the optimal growth conditions, as well as evaluation of material characteristics at the desired low-E g alloy fractions.
