2, NanoPower Research Laboratory, Rochester Institute of Technology, Rochester, New York, United States
3, Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, Ohio, United States
4, Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Monolithic integration of high crystal quality III-V semiconductors with two-dimensional (2-D) monolayer nanosheets can be achieved via growth of high aspect ratio nanowire (NW) structures using the pseudo-van der Waals epitaxy (vdWE) approach. In this regime, growth of dislocation-free NWs is accommodated by weak vdW forces between a 2-D nanomaterial, with no surface dangling bonds, and the III-V epilayer. However, the monolayer substrate offers limited possibilities for arrangement of the overlaying crystal based on the lattice constant of the III-V compound, the finite number of atomic positions on the 2-D surface, and the limitations imposed by the binding energy of adatoms at available residence sites. Here, we present self-assembly of vertically-aligned, high aspect ratio InAsyP1-y, InxAl1-xAs, and core-shell InAsP-InP NW arrays on single layer graphene (SLG) via vdWE using metalorganic chemical vapor deposition (MOCVD). By altering growth temperature (TG) and molar flow ratio of precursors (ρ), compositional and morphological tuning of hybrid ternary III-V-on-2-D nanomaterials systems is realized. For InAsyP1-y NWs, the hydride precursor molar flow ratio (ρPH3) is varied from 0 to 0.98 at growth temperatures of 650 °C and 700 °C. The morphology and number density of NWs, as well as parasitic island coverage, are observed to be independent of ρPH3. Although growth at TG = 700 °C enables higher solid-phase P-incorporation, increasing temperature also adversely affects NW number density. To overcome this challenge, heterostructured InAsP/InAs NWs are grown in a two-temperature growth regime with InAs bases formed at TG-InAs = 650 °C, followed by InAsP segments formed at elevated TG-InAsP ≥ 700 °C. This approach permits growth of high number density NWs with InAsyP1-y segments having composition y ~ 0.80. Higher P-content segments are grown as shell layers on existing InAsP core segments. Next, the dependences of InxAl1-xAs NW morphology, verticality, and number density on growth temperature and composition are investigated. At constant TG = 600 °C, composition dependences are studied by varying the metalorganic precursor molar flow ratio (ρTMAl) between 0 and 0.75. Similarly, at constant ρTMAl = 0.50, temperature dependences are studied in the 600 °C ≤ TG ≤ 700 °C range. Optimal conditions are presented for uniform-diameter and vertical InAlAs NWs at TG = 650 °C and ρTMAl = 0.50. A growth mechanism for vdWE of ternary InAsP and InAlAs NWs is proposed to relate the observed trends for NW number density and morphology to lattice coordination and binding energy of growth species on SLG. We anticipate the use of such hybrid III-V-on-2-D nanosystems in low-cost and flexible optoelectronic device applications.