Observation of Asymmetric Nanoscale Optical Cavity in GaAs Nanosheets

GaAs nanosheets with no twin defects, stacking faults, or dislocations are excellent candidates for optoelectrical applications. Their outstanding optical behavior and twin free structure make them superior to traditionally studied GaAs nanowires. While many research groups have reported optically resonant cavities (i.e., Fabry–Perot) in 1D nanowires, here, we report an optical cavity resonance in GaAs nanosheets consisting of complex 2D asymmetric modes, which are fundamentally different from one-dimensional cavities. These resonant modes are detected experimentally using photoluminescence (PL) spectroscopy, which exhibits a series of peaks or “fringes” superimposed on the bulk GaAs photoluminescence spectrum. Finite-difference time-domain (FDTD) simulations confirm these experimental findings and provide a detailed picture of these complex resonant modes. Here, the complex modes of this cavity are formed by the three nonparallel edges of the GaAs nanosheets. Due to the asymmetrical nature of the nanosheets, the mode profiles are largely unintuitive. We also find that by changing the substrate from Si/SiO₂ to Au, we enhance the resonance fringes as well as the overall optical emission by 5× at room temperature. Our FDTD simulation results confirm that this enhancement is caused by the local field enhancement of the Au substrate and indicate that the thickness of the nanosheets plays an important role in the formation and enhancement of fringes

Twin-Free GaAs Nanosheets by Selective Area Growth: Implications for Defect-Free Nanostructures

Highly perfect, twin-free GaAs nanosheets grown on (111)B surfaces by selective area growth (SAG) are demonstrated. In contrast to GaAs nanowires grown by (SAG) in which rotational twins and stacking faults are almost universally observed, twin formation is either suppressed or eliminated within properly oriented nanosheets are grown under a range of growth conditions. A morphology transition in the nanosheets due to twinning results in surface energy reduction, which may also explain the high twin-defect density that occurs within some III–V semiconductor nanostructures, such as GaAs nanowires. Calculations suggest that the surface energy is significantly reduced by the formation of {111}-plane bounded tetrahedra after the morphology transition of nanowire structures. By contrast, owing to the formation of two vertical {11̅0} planes which comprise the majority of the total surface energy of nanosheet structures, the energy reduction effect due to the morphology transition is not as dramatic as that for nanowire structures. Furthermore, the surface energy reduction effect is mitigated in longer nanosheets which, in turn, suppresses twinning

Tandem Solar Cells Using GaAs Nanowires on Si: Design, Fabrication, and Observation of Voltage Addition

Multijunction solar cells provide us a viable approach to achieve efficiencies higher than the Shockley–Queisser limit. Due to their unique optical, electrical, and crystallographic features, semiconductor nanowires are good candidates to achieve monolithic integration of solar cell materials that are not lattice-matched. Here, we report the first realization of nanowire-on-Si tandem cells with the observation of voltage addition of the GaAs nanowire top cell and the Si bottom cell with an open circuit voltage of 0.956 V and an efficiency of 11.4%. Our simulation showed that the current-matching condition plays an important role in the overall efficiency. Furthermore, we characterized GaAs nanowire arrays grown on lattice-mismatched Si substrates and estimated the carrier density using photoluminescence. A low-resistance connecting junction was obtained using n⁺-GaAs/p⁺-Si heterojunction. Finally, we demonstrated tandem solar cells based on top GaAs nanowire array solar cells grown on bottom planar Si solar cells. The reported nanowire-on-Si tandem cell opens up great opportunities for high-efficiency, low-cost multijunction solar cells

Toward Optimized Light Utilization in Nanowire Arrays Using Scalable Nanosphere Lithography and Selected Area Growth

Vertically aligned, catalyst-free semiconducting nanowires hold great potential for photovoltaic applications, in which achieving scalable synthesis and optimized optical absorption simultaneously is critical. Here, we report combining nanosphere lithography (NSL) and selected area metal–organic chemical vapor deposition (SA-MOCVD) for the first time for scalable synthesis of vertically aligned gallium arsenide nanowire arrays, and surprisingly, we show that such nanowire arrays with patterning defects due to NSL can be as good as highly ordered nanowire arrays in terms of optical absorption and reflection. Wafer-scale patterning for nanowire synthesis was done using a polystyrene nanosphere template as a mask. Nanowires grown from substrates patterned by NSL show similar structural features to those patterned using electron beam lithography (EBL). Reflection of photons from the NSL-patterned nanowire array was used as a measure of the effect of defects present in the structure. Experimentally, we show that GaAs nanowires as short as 130 nm show reflection of <10% over the visible range of the solar spectrum. Our results indicate that a highly ordered nanowire structure is not necessary: despite the “defects” present in NSL-patterned nanowire arrays, their optical performance is similar to “defect-free” structures patterned by more costly, time-consuming EBL methods. Our scalable approach for synthesis of vertical semiconducting nanowires can have application in high-throughput and low-cost optoelectronic devices, including solar cells

GaAs Nanowire Array Solar Cells with Axial p–i–n Junctions

Because of unique structural, optical, and electrical properties, solar cells based on semiconductor nanowires are a rapidly evolving scientific enterprise. Various approaches employing III–V nanowires have emerged, among which GaAs, especially, is under intense research and development. Most reported GaAs nanowire solar cells form p–n junctions in the radial direction; however, nanowires using axial junction may enable the attainment of high open circuit voltage (<i>V</i>_oc) and integration into multijunction solar cells. Here, we report GaAs nanowire solar cells with axial p–i–n junctions that achieve 7.58% efficiency. Simulations show that axial junctions are more tolerant to doping variation than radial junctions and lead to higher <i>V</i>_oc under certain conditions. We further study the effect of wire diameter and junction depth using electrical characterization and cathodoluminescence. The results show that large diameter and shallow junctions are essential for a high extraction efficiency. Our approach opens up great opportunity for future low-cost, high-efficiency photovoltaics

Electrical and Optical Characterization of Surface Passivation in GaAs Nanowires

We report a systematic study of carrier dynamics in Al_<i>x</i>Ga_1–<i>x</i>As-passivated GaAs nanowires. With passivation, the minority carrier diffusion length (<i>L</i>_diff) increases from 30 to 180 nm, as measured by electron beam induced current (EBIC) mapping, and the photoluminescence (PL) lifetime increases from sub-60 ps to 1.3 ns. A 48-fold enhancement in the continuous-wave PL intensity is observed on the same individual nanowire with and without the Al_<i>x</i>Ga_1–<i>x</i>As passivation layer, indicating a significant reduction in surface recombination. These results indicate that, in passivated nanowires, the minority carrier lifetime is not limited by twin stacking faults. From the PL lifetime and minority carrier diffusion length, we estimate the surface recombination velocity (SRV) to range from 1.7 × 10³ to 1.1 × 10⁴ cm·s^–1, and the minority carrier mobility μ is estimated to lie in the range from 10.3 to 67.5 cm² V^–1 s^–1 for the passivated nanowires