Adam C. Scofield

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.

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.

Direct-bandgap epitaxial core-multishell nanopillar photovoltaics featuring subwavelength optical concentrators.

Semiconductor nanomaterials have recently fueled numerous photonic scientific fields. Arrays of nanopillars (NPs) have been examined by the photovoltaic (PV) community as highly efficient solar absorbers, with potential material/cost reductions compared to planar architectures. Despite modeled predictions, experimental efficiencies are limited by surface recombination and poor light management, once integrated in a practical PV device. In this Letter, we correlate optoelectronic modeling with experimental results for direct-bandgap arrays of core-multishell GaAs NPs grown by selective area, catalyst-free epitaxy and capped by epitaxial window layers, with efficiencies of 7.43%. Electrically, improved open-circuit voltages are yet partly affected by residual surface state density after epitaxial passivation. Optically, dome-shaped indium-tin-oxide (ITO) top electrode functions as a two-dimensional (2-D) periodic array of subwavelength lenses that focus the local density of optical states within the NP active volume. These devices provide a path to high-efficiency NP-based PVs by synergistically controlling the heteroepitaxy and light management of the final structure.

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.

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.

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.

Composite axial/core-shell nanopillar light-emitting diodes at 1.3 μm

Selective-area growth of III-V nanopillars (NPs) is used to demonstrate near-infrared emitters that employ a composite axial/core-shell heterostructure. The axial p-i-n heterostructure allows growth of strain relaxed InGaAs inserts emitting at 1.3 μm. Radial growth of an InGaP shell provides in-situ surface passivation to reduce non-radiative recombination and space-charge limited transport due to mid-gap surface states. The resulting light-emitting diode is comparable to bulk devices with an ideality factor of η = 1.67 and reverse bias leakage of 12 nA at −5 V. This device performance makes the combination of axial current injection with in-situ passivation a promising approach to NP based emitters.

Enhanced InAs nanopillar electrical transport by in-situ passivation

We investigate the effects of in-situ passivation on the electrical transport of InAs nanopillars (NPs) grown on InAs (111)B substrates via selective-area epitaxy. Before passivation, the transport properties of InAs NPs, studied by single-NP field-effect transistors, are highly dependent on NP dimensions. With diameters ranging from 70 nm to 200 nm, we find significant differences in resistivity and extracted field-effect mobility (μeff). Growing a 6 nm InP shell for in-situ passivation significantly enhances these transport properties of InAs channel with diameter-independent μeff as high as 6900 cm2/V s. Such heterostructures have the potential as future high electron mobility transistors.

Improved room-temperature luminescence of core-shell InGaAs/GaAs nanopillars via lattice-matched passivation

Optical properties of GaAs/InGaAs/GaAs nanopillars (NPs) grown on GaAs(111)B were investigated. Employment of a mask-etching technique allowed for an accurate control over the geometry of NP arrays in terms of both their diameter and separation. This work describes both the steady-state and time-resolved photoluminescence of these structures as a function of the ensemble geometry, composition of the insert, and various shell compounds. The effects of the NP geometry on a parasitic radiative recombination channel, originating from an overgrown lateral sidewall layer, are discussed. Optical characterization reveals a profound influence of the core-shell lattice mismatch on the carrier lifetime and emission quenching at room temperature. When the lattice-matching conditions are satisfied, an efficient emission from the NP arrays at room temperature and below the band-gap of silicon is observed, clearly highlighting their potential application as emitters in optical interconnects integrated with silicon platf...

Axial diffusion barriers in near-infrared nanopillar LEDs

The growth of GaAs/GaAsP axial heterostructures is demonstrated and implemented as diffusion current barriers in nanopillar light-emitting diodes at near-infrared wavelengths. The nanopillar light-emitting diodes utilize an n-GaAs/i-InGaAs/p-GaAs axial heterostructure for current injection. Axial GaAsP segments are inserted into the n- and p-GaAs portions of the nanopillars surrounding the InGaAs emitter region, acting as diffusion barriers to provide enhanced carrier confinement. Detailed characterization of growth of the GaAsP inserts and electronic band-offset measurements are used to effectively implement the GaAsP inserts as diffusion barriers. The implementation of these barriers in nanopillar light-emitting diodes provides a 5-fold increase in output intensity, making this a promising approach to high-efficiency pillar-based emitters in the near-infrared wavelength range.