Indrasen Bhattacharya

Illumination Angle Insensitive Single Indium Phosphide Tapered Nanopillar Solar Cell.

Low cost, high efficiency photovoltaic can help accelerate the adoption of solar energy. Using tapered indium phosphide nanopillars grown on a silicon substrate, we demonstrate a single nanopillar photovoltaic exhibiting illumination angle insensitive response. The photovoltaic employs a novel regrown core-shell p-i-n junction to improve device performance by eliminating shunt current paths, resulting in a high VOC of 0.534 V and a power conversion efficiency of 19.6%. Enhanced broadband light absorption is also demonstrated over a wide spectral range of 400-800 nm.

Ultrahigh Responsivity-Bandwidth Product in a Compact InP Nanopillar Phototransistor Directly Grown on Silicon.

Highly sensitive and fast photodetectors can enable low power, high bandwidth on-chip optical interconnects for silicon integrated electronics. III-V compound semiconductor direct-bandgap materials with high absorption coefficients are particularly promising for photodetection in energy-efficient optical links because of the potential to scale down the absorber size, and the resulting capacitance and dark current, while maintaining high quantum efficiency. We demonstrate a compact bipolar junction phototransistor with a high current gain (53.6), bandwidth (7 GHz) and responsivity (9.5 A/W) using a single crystalline indium phosphide nanopillar directly grown on a silicon substrate. Transistor gain is obtained at sub-picowatt optical power and collector bias close to the CMOS line voltage. The quantum efficiency-bandwidth product of 105 GHz is the highest for photodetectors on silicon. The bipolar junction phototransistor combines the receiver front end circuit and absorber into a monolithic integrated device, eliminating the wire capacitance between the detector and first amplifier stage.

Nanopillar quantum well lasers directly grown on silicon and emitting at silicon-transparent wavelengths

Future expansion of computing capabilities relies on a reduction of energy consumption in silicon-based integrated circuits. A promising solution is to replace electrical wires with optical connections, for which a key component is a nanolaser that coherently emits into silicon-based waveguides to route information across a chip, in place of bulky off-chip devices. We report room temperature, sub-μm2 footprint, quantum-well-in-nanopillar lasers grown directly on silicon and silicon-on-insulator (SOI) substrates that emit within the silicon-transparent wavelength range under optical excitation. The laser wavelength is controlled by changing the InGaAs quantum well thickness and alloy composition, quite independent of lattice mismatch with the InP barrier, a unique property of the 3D core-shell growth mode. We achieve excellent luminescence yield and low continuous wave transparency power due to the well-passivated InGaAs/InP interfaces. These sub-μm2 footprint long-wavelength lasers could enable optoelectronic integration and photon routing with silicon waveguides on the technologically relevant SOI platform.

Room-temperature Fabry-Perot resonances in suspended InGaAs/InP quantum-well nanopillars on a silicon substrate.

We present a new platform based on suspended III-V semiconductor nanopillars for direct integration of optoelectronic devices on a silicon substrate. Nanopillars grown in core-shell mode with InGaAs/InP quantum wells can support long-wavelength Fabry-Pérot resonances at room temperature with this novel configuration. Experimental results are demonstrated at a silicon-transparent wavelength of 1460 nm, facilitating integration with silicon platform.

Bright LEDs using position-controlled MOCVD growth of InP nanopillar array on a silicon substrate

We demonstrate catalyst-free metal organic chemical vapor deposition (MOCVD) growth of position-controlled InP nanopillars on Si substrates for the first time. The nanopillars were grown at a low growth temperature of 460°C, compatible with CMOS back-end integration and resulting arrays of nanopillars show high yield of ~97%. Nanopillars grow vertically, in single-phase, wurtzite crystalline form. The taper angle of the nanopillars can be engineered by tuning growth parameters. Site-controlled nanopillars show excellent optical properties- narrow linewidth (~50 meV), long lifetimes ~4 ns, and high internal quantum efficiency. Core-shell p-n junction devices were grown on n-Si by introducing dopants. InGaAs/InP quantum wells were incorporated in the active region of the diode heterostructure to obtain silicon transparent electroluminescence (λ ~ 1500nm) from the nanopillar LED. Position controlled InP-based diodes on silicon have tremendous implications for on-chip emitters and detectors in Si photonics links.

InP nanowire avalanche photodiode and bipolar junction phototransistor integrated on silicon substrate

Using high quality, single crystalline InP nanowire grown on silicon substrate, we demonstrates sensitive avalanche photodiode with 26.6 A/W and bipolar junction phototransistor with 4 A/W integrated onto silicon substrate. The avalanche photodiode has unique radial p-n junction that allows it to reach a high avalanche gain of 100 at a low bias of 1 V. The bipolar junction phototransistor integrates a sensitive photodiode with a receiver circuit, creating a compact, monolithic receiver circuit for optical interconnect application. These devices are promising in bringing low energy, high bandwidth optical interconnects to silicon electronics.

Computed axial lithography: volumetric 3D printing of arbitrary geometries (Conference Presentation)

Lower-dimensional photopolymerization based additive manufacturing techniques have several drawbacks that currently limit the applicability and scope of 3D printing, including: topological constraints, the requirement for numerous complex support structures that later need to be removed, long print times for complex geometries, relative motion between the liquid resin and printed part, as well as debilitating mechanical weakness and anisotropy resulting from the inherently layered structure of the parts. We propose and demonstrate a novel volumetric 3D printing technique based on one of the most ubiquitous computational imaging methods in the field: computed axial tomography. Computed axial lithography (CAL) is a vat photopolymerization technique that exposes the entire resin volume by projecting images from a multiplicity of angles. The technique is a physical implementation of the filtered back projection algorithm for tomographic reconstruction. We use constrained non-convex optimization in order to generate images that are projected into the resin in order to sculpt a 3-dimensional energy dose that cures the desired arbitrary geometry. This eliminates the requirement for supports and enables complex and nested structures that were previously challenging or impossible to print. Further, the process is layer-less and does not involve any relative motion between the resin and the printed part, which has positive implications for mechanically isotropic part strength. We demonstrate support-less printing of complex geometries containing 10^8-10^9 voxels in 2-4 minutes, orders of magnitude faster than comparable techniques.

Efficient electroluminescence from III/V quantum-well-in-nanopillar light emitting diodes directly grown on silicon

We have grown and fabricated InGaAs active region, InP clad nanopillar diodes at deterministic positions on silicon using selective area epitaxy. Room temperature radiative dominant electroluminescence at 1460 nm wavelength is reported from these devices.

Room-temperature InGaAs/InP quantum-well-in-nanopillar laser directly grown on silicon

We demonstrate room-temperature transparency and optically pumped lasing with an InGaAs quantum well active region integrated in an InP nanoresonator cavity grown monolithically on silicon. As-grown silicon transparent lasers will enable on-chip optical interconnects.

III-V nanopillar phototransistor directly grown on silicon

We demonstrate InP nanopillar bipolar junction phototransistors monolithically integrated on a Silicon substrate. With a responsivity of 4 A/W and bandwidth of 7.5 GHz, these receivers indicate a route towards efficient on-chip optical interconnects.