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Self-assembly of vertically aligned III–V semiconductor nanowires (NWs) on two-dimensional (2D) van der Waals (vdW) nanomaterials allows for integration of novel mixed-dimensional nanosystems with unique properties for optoelectronic and nanoelectronic device applications. Here, selective-area vdW epitaxy (SA-vdWE) of InAs NWs on isolated 2D molybdenum disulfide (MoS 2 ) domains is reported for the first time. The MOCVD growth parameter space ( i.e. , V/III ratio, growth temperature, and total molar flow rates of metalorganic and hydride precursors) is explored to achieve pattern-free positioning of single NWs on isolated multi-layer MoS 2 micro-plates with one-to-one NW-to-MoS 2 domain placement. The introduction of a pre-growth poly- l -lysine surface treatment is highlighted as a necessary step for mitigation of InAs nucleation along the edges of triangular MoS 2 domains and for NW growth along the interior region of 2D micro-plates. Analysis of NW crystal structures formed under the optimal SA-vdWE condition revealed a disordered combination of wurtzite and zinc-blend phases. A transformation of the NW sidewall faceting structure is observed, resulting from simultaneous radial overgrowth during axial NW synthesis. A common lattice arrangement between axially-grown InAs NW core segments and MoS 2 domains is described as the epitaxial basis for vertical NW growth. A model is proposed for a common InAs/MoS 2 sub-lattice structure, consisting of three multiples of the cubic InAs unit cell along the [21̄1̄] direction, commensurately aligned with a 14-fold multiple of the Mo–Mo (or S–S) spacing along the [101̄0] direction of MoS 2 hexagonal lattice. The SA-vdWE growth mode described here enables controlled hybrid integration of mixed-dimensional III–V-on-2D heterostructures as novel nanosystems for applications in optoelectronics, nanoelectronics, and quantum enabling technologies. 

ABSTRACT Rigorous coupled wave analysis (RCWA) simulation was used to model the absorption in periodic arrays of GaAs(0.73)P(0.27) nanowires (NWs) on Si substrates dependent upon the diameter (D), length (L), and spacing (center-to-center distance, or pitch, P) of the NWs. Based on this study, two resonant arrangements for a top NW array sub-cell having the highest limiting short-circuit current densities (J_sc) were found to be close to D = 150 nm, P = 250 nm and D = 300 nm, P = 500 nm, both featuring the same packing density of 0.28. Even though a configuration with thinner NWs exhibited the highest J_sc = 19.46 mA/cm^2, the array with D = 350 nm and P = 500 nm provided current matching with the underlying Si sub-cell with J_sc = 18.59 mA/cm^2. Addition of a rear-side In(0.81)Ga(0.19)As nanowire array with D = 800 nm and P = 1000 nm was found to be suitable for current matching with the front NW sub-cell and middle Si. However, with thinner and sparser In(0.81)Ga(0.19)As NWs with D = 700 nm and P = 1000 nm, the J_scof the bottom sub-cell was increased from 17.35 mA/cm^2 to 18.76 mA/cm^2 using a planar metallic back surface reflector, thus achieving a current matching with the top and middle cells. 

Heterogeneous self-assembly of III–V nanostructures on inert two-dimensional monolayer materials enables novel hybrid nanosystems with unique properties that can be exploited for low-cost and low-weight flexible optoelectronic and nanoelectronic device applications. Here, the pseudo-van der Waals epitaxy (vdWE) growth parameter space for heterogeneous integration of InAs nanowires (NWs) with continuous films of single layer graphene (SLG) via metalorganic chemical vapor deposition (MOCVD) is investigated. The length, diameter, and number density of NWs, as well as areal coverage of parasitic islands, are quantified as functions of key growth variables including growth temperature, V/III ratio, and total flow rate of metalorganic and hydride precursors. A compromise between self-assembly of high aspect ratio NWs comprising high number density arrays and simultaneous minimization of parasitic growth coverage is reached under a selected set of optimal growth conditions. Exploration of NW crystal structures formed under various growth conditions reveals that a characteristic polytypic and disordered lattice is invariant within the explored parameter space. A growth evolution study reveals a gradual reduction in both axial and radial growth rates within the explored timeframe for the optimal growth conditions, which is attributed to a supply-limited competitive growth regime. Two strategies are introduced for further growth optimization. Firstly, it is shown that the absence of a pre-growth in situ arsine surface treatment results in a reduction of parasitic island coverage by factor of ∼0.62, while NW aspect ratio and number densities are simultaneously enhanced. Secondly, the use of a two-step flow-modulated growth procedure allows for realization of dense fields of high aspect ratio InAs NWs. As a result of the applied studies and optimization of the growth parameter space, the highest reported axial growth rate of 840 nm min −1 and NW number density of ∼8.3 × 10 8 cm −2 for vdWE of high aspect ratio (>80) InAs NW arrays on graphitic surfaces are achieved. This work is intended to serve as a guide for vdWE of self-assembled III–V semiconductor NWs such as In-based ternary and quaternary alloys on functional two-dimensional monolayer materials, toward device applications in flexible optoelectronics and tandem-junction photovoltaics. 

Vertically-aligned, high aspect ratio In InAsyP1-y, InxAl1-xAs, and core-shell InAsP-InP nanowires (NWs) are grown directly on two-dimensional (2-D) monolayer graphene via seed-free pseudo-van der Waals epitaxy (vdWE), as reported here for the first time. Growth is achieved using metalorganic chemical vapor deposition (MOCVD). By altering growth temperature and molar flow ratio of precursors, the composition Of InAsyP1-y NWs can be tuned within the 1 ≤ y ≤ 0.8 range. Similarly, by tuning the group-III precursor flow rates, InxAl1-x As composition can be modified in the range. NW morphology and NW array number density variances are measured for different ternary compositions as functions of precursor flow rates and growth temperature.