Joshua Shapiro

Patterned radial GaAs nanopillar solar cells.

Photovoltaic devices using GaAs nanopillar radial p-n junctions are demonstrated by means of catalyst-free selective-area metal-organic chemical vapor deposition. Dense, large-area, lithographically defined vertical arrays of nanowires with uniform spacing and dimensions allow for power conversion efficiencies for this material system of 2.54% (AM 1.5 G) and high rectification ratio of 213 (at ±1 V). The absence of metal catalyst contamination results in leakage currents of ∼236 nA at -1 V. High-resolution scanning photocurrent microscopy measurements reveal the independent functioning of each nanowire in the array with an individual peak photocurrent of ∼1 nA at 544 nm. External quantum efficiency shows that the photocarrier extraction highly depends on the degenerately doped transparent contact oxide. Two different top electrode schemes are adopted and characterized in terms of Hall, sheet resistance, and optical transmittance measurements.

Bottom-up Photonic Crystal Lasers

The directed growth of III-V nanopillars is used to demonstrate bottom-up photonic crystal lasers. Simultaneous formation of both the photonic band gap and active gain region is achieved via catalyst-free selective-area metal-organic chemical vapor deposition on masked GaAs substrates. The nanopillars implement a GaAs/InGaAs/GaAs axial double heterostructure for accurate, arbitrary placement of gain within the cavity and lateral InGaP shells to reduce surface recombination. The lasers operate single-mode at room temperature with low threshold peak power density of ∼625 W/cm2. Cavity resonance and lasing wavelength is lithographically defined by controlling pillar pitch and diameter to vary from 960 to 989 nm. We envision this bottom-up approach to pillar-based devices as a new platform for photonic systems integration.

Hybrid conjugated polymer solar cells using patterned GaAs nanopillars

In this work, we study hybrid solar cells based on poly(3-hexylthiophene)-coated GaAs nanopillars grown on a patterned GaAs substrate using selective-area metal organic chemical vapor deposition. The hybrid solar cells show extremely low leakage currents (I≅45 nA @−1V) under dark conditions and an open circuit voltage, short circuit current density, and fill factor of 0.2 V, 8.7 mA/cm2, and 32%, respectively, giving a power conversion efficiency of η=0.6% under AM 1.5 G illumination. Surface passivation of the GaAs results in further improvement, yielding η=1.44% under AM 1.5 G illumination. External quantum efficiency measurements of these polymer/inorganic solar cells are also presented.

InGaAs heterostructure formation in catalyst-free GaAs nanopillars by selective-area metal-organic vapor phase epitaxy

We investigate axial GaAs/InGaAs/GaAs heterostructures embedded in GaAsnanopillars via catalyst-free selective-area metal-organic chemical vapor deposition. Structural characterization by transmission electron microscopy with energy dispersive x-ray spectroscopy(EDS) indicates formation of axial In x Ga 1 − x As ( x ∼ 0.20 ) inserts with thicknesses from 36 to 220 nm with ±10% variation and graded Ga:In transitions controlled by In segregation. Using the heterointerfaces as markers, the vertical growth rate is determined to increase linearly during growth.Photoluminescence from 77 to 290 K and EDS suggest the presence of strain in the shortest inserts. This capability to control the formation of axial nanopillarheterostructures is crucial for optimized device integration.

Bottom-up Photonic Crystal Cavities Formed by Patterned III–V Nanopillars

We report the demonstration of photonic crystal lasers formed bottom-up by patterned III-V nanopillar (NP) arrays. In this work, we present a method whereby the photonic band gap region and active gain regions are formed simultaneously by selective-area metal-organic chemical vapor deposition. This approach allows us the ability to design device parameters lithographically. By accurate control of position and diameter of the NPs, high-Q cavities can be formed entirely with NPs. This particular model cavity supports a non-degenerate hexapole mode1 with a high overlap between the E-field and the center pillars. Design optimization by finite-difference time-domain simulations yields a cavity Q of ~5000.

Three-Dimensional Core–Shell Hybrid Solar Cells via Controlled in Situ Materials Engineering

Three-dimensional core-shell organic-inorganic hybrid solar cells with tunable properties are demonstrated via electropolymerization. Air-stable poly(3,4-ethylenedioxythiophene) (PEDOT) shells with controlled thicknesses are rapidly coated onto periodic GaAs nanopillar arrays conformally, preserving the vertical 3D structure. The properties of the organic layer can be readily tuned in situ, allowing for (1) the lowering of the highest occupied molecular orbital level (|ΔE| ∼ 0.28 eV), leading to the increase of open-circuit voltage (V(OC)), and (2) an improvement in PEDOT conductivity that results in enhanced short-circuit current densities (J(SC)). The incorporation of various anionic dopants in the polymer during the coating process also enables the tailoring of the polymer/semiconductor interface transport properties. Systematic tuning of the device properties results in a J(SC) of 13.6 mA cm(-2), V(OC) of 0.63 V, peak external quantum efficiency of 58.5%, leading to a power conversion efficiencies of 4.11%.

Thin 3D Multiplication Regions in Plasmonically Enhanced Nanopillar Avalanche Detectors

We demonstrate a nanopillar (NP) device structure for implementing plasmonically enhanced avalanche photodetector arrays with thin avalanche volumes (∼ 310 nm × 150 nm × 150 nm). A localized 3D electric field due to a core-shell PN junction in a NP acts as a multiplication region, while efficient light absorption takes place via surface plasmon polariton Bloch wave (SPP-BW) modes due to a self-aligned metal nanohole lattice. Avalanche gains of ∼216 at 730 nm at -12 V are obtained. We show through capacitance-voltage characterization, temperature-dependent breakdown measurements, and detailed device modeling that the avalanche region is on the order of the ionization path length, such that dead-space effects become significant. This work presents a clear path toward engineering dead space effects in thin 3D-confined multiplication regions for high performance avalanche detectors for applications in telecommunications, sensing and single photon detection.

Photoconductive gain in patterned nanopillar photodetector arrays

We report on the photoconductance characteristics of indium tin oxide (ITO)-GaAs photodetectors based on patterned nanopillar (NP) arrays grown by metal-organic chemical vapor deposition. The NPs are partially encapsulated by commercially available polymer to allow transparent ITO contact to exposed NP tips. Under illumination, the NP photodetectors demonstrate photoconductive gain in both forward and reverse bias. The mechanism for photoconductive gain is attributed to both the lowering of the Schottky barrier at the ITO-GaAs interface by photogenerated holes, and also the increase in the conduction volume of the NPs under illumination.

Temperature dependence of stacking faults in catalyst-free GaAs nanopillars

Impressive opto-electronic devices and transistors have recently been fabricated from GaAs nanopillars grown by catalyst-free selective-area epitaxy, but this growth technique has always resulted in high densities of stacking faults. A stacking fault occurs when atoms on the growing (111) surface occupy the sites of a hexagonal-close-pack (hcp) lattice instead of the normal face-centered-cubic (fcc) lattice sites. When stacking faults occur consecutively, the crystal structure is locally wurtzite instead of zinc-blende, and the resulting band offsets are known to negatively impact device performance. Here we present experimental and theoretical evidence that indicate stacking fault formation is related to the size of the critical nucleus, which is temperature dependent. The difference in energy between the hcp and fcc orientation of small nuclei is computed using density-function theory. The minimum energy difference of 0.22 eV is calculated for a nucleus with 21 atoms, so the population of nuclei in the hcp orientation is expected to decrease as the nucleus grows larger. The experiment shows that stacking fault occurrence is dramatically reduced from 22% to 3% by raising the growth temperature from 730 to 790 ° C. These data are interpreted using classical nucleation theory which dictates a larger critical nucleus at higher growth temperature.

3D Nanopillar optical antenna photodetectors

We demonstrate 3D surface plasmon photoresponse in nanopillar arrays resulting in enhanced responsivity due to both Localized Surface Plasmon Resonances (LSPRs) and Surface Plasmon Polariton Bloch Waves (SPP-BWs). The LSPRs are excited due to a partial gold shell coating the nanopillar which acts as a 3D Nanopillar Optical Antenna (NOA) in focusing light into the nanopillar. Angular photoresponse measurements show that SPP-BWs can be spectrally coincident with LSPRs to result in a x2 enhancement in responsivity at 1180 nm. Full-wave Finite Difference Time Domain (FDTD) simulations substantiate both the spatial and spectral coupling of the SPP-BW / LSPR for enhanced absorption and the nature of the LSPR. Geometrical control of the 3D NOA and the self-aligned metal hole lattice allows the hybridization of both localized and propagating surface plasmon modes for enhanced absorption. Hybridized plasmonic modes opens up new avenues in optical antenna design in nanoscale photodetectors.