Michael K. Yakes

Nanoscale Tunable Reduction of Graphene Oxide for Graphene Electronics

Writing Conductive Lines with Hot Tips The interface within devices between conductors, semiconductors, and insulators is usually created by stacking patterned layers of different materials. For flexible electronics, it can be advantageous to avoid this architectural constraint. Graphene oxide, formed by chemical exfoliation of graphite, can be reduced to a more conductive form using chemical reductants. Wei et al. (p. 1373) now show that layers of graphene oxide can also be reduced using a hot atomic force microscope tip to create materials comparable to those of organic conductors. This process can create patterned regions (down to 12 nanometers in width) that differ in conductivity by up to four orders of magnitude. Conducting regions can be drawn on graphene oxide sheets with a heated atomic force microscope tip. The reduced form of graphene oxide (GO) is an attractive alternative to graphene for producing large-scale flexible conductors and for creating devices that require an electronic gap. We report on a means to tune the topographical and electrical properties of reduced GO (rGO) with nanoscopic resolution by local thermal reduction of GO with a heated atomic force microscope tip. The rGO regions are up to four orders of magnitude more conductive than pristine GO. No sign of tip wear or sample tearing was observed. Variably conductive nanoribbons with dimensions down to 12 nanometers could be produced in oxidized epitaxial graphene films in a single step that is clean, rapid, and reliable.

Conductance Anisotropy in Epitaxial Graphene Sheets Generated by Substrate Interactions

We present the first microscopic transport study of epitaxial graphene on SiC using an ultrahigh vacuum four-probe scanning tunneling microscope. Anisotropic conductivity is observed that is caused by the interaction between the graphene and the underlying substrate. These results can be explained by a model where charge buildup at the step edges leads to local scattering of charge carriers. This highlights the importance of considering substrate effects in proposed devices that utilize nanoscale patterning of graphene on electrically isolated substrates.

Double quantum-well tunnel junctions with high peak tunnel currents and low absorption for InP multi-junction solar cells

Lattice matched InAlGaAs tunnel junctions with a 1.18 eV bandgap have been grown for a triple-junction solar cell on InP. By including two InGaAs quantum wells in the structure, a peak tunnel current density of 113 A/cm2 was observed, 45 times greater than the baseline bulk InAlGaAs tunnel junction. The differential resistance of the quantum well device is 7.52 × 10−4 Ω cm2, a 15-fold improvement over the baseline device. The transmission loss to the bottom cell is estimated to be approximately 1.7% and a network simulation demonstrates that quantum well tunnel junctions play a key role in improving performance at high sun-concentrations.

Design of an achievable, all lattice-matched multijunction solar cell using InGaAlAsSb

A design for a realistically achievable, multijunction solar cell based on all lattice-matched materials with >50% projected efficiencies under concentration is presented. Using quaternary materials such as InAlAsSb and InGaAlAs at stochiometries lattice-matched to InP substrates, direct bandgaps ranging from 0.74eV up to ∼1.8eV, ideal for solar energy conversion, can be achieved. In addition, multi-quantum well structures are used to reduce the band-gap further to <0.7 eV. A triple-junction (3J) solar cell using these materials is described, and in-depth modeling results are presented showing realistically achievable efficiencies of AM1.5D 500X of η ∼ 53% and AM0 1 Sun of η∼ 37%.

Simulation of novel InAlAsSb solar cells

This work uses simulations to predict the performance of InAlAsSb solar cells for use as the top cell of triple junction cells lattice matched to InP. The InP-based material system has the potential to achieve extremely high efficiencies due the availability of lattice matched materials close to the ideal bandgaps for solar energy conversion. The band-parameters, optical properties and minority carrier transport properties are modeled based on literature data for the InAlAsSb quaternary, and an analytical drift-diffusion model is used to realistically predict the solar cell performance.

Three-Dimensional Control of Self- Assembled Quantum Dot Configurations

We demonstrate techniques for growing three-dimensional quantum dot configurations using molecular beam epitaxy on faceted template islands. Molecular beam shadowing leads to new geometries through selective nucleation of the dots on the template edges. Strain-induced stacking converts the planar configurations into three-dimensional structures. The resulting dot morphologies and their configurational uniformity are studied using cross sectional scanning tunneling microscopy and atomic force microscopy. Combining photoluminescence measurements with structural characterization allows interpretation of the ensemble photoluminescence spectrum. Bright spectra for the three-dimensional structures suggest an improved method for combining lithographic nucleation sites with self-assembled dot growth. These techniques can be applied to lithographic templates to fabricate complex quantum dot networks.

Leveraging Crystal Anisotropy for Deterministic Growth of InAs Quantum Dots with Narrow Optical Linewidths

Crystal growth anisotropy in molecular beam epitaxy usually prevents deterministic nucleation of individual quantum dots when a thick GaAs buffer is grown over a nanopatterned substrate. Here, we demonstrate how this anisotropy can actually be used to mold nucleation sites for single dots on a much thicker buffer than has been achieved by conventional techniques. This approach greatly suppresses the problem of defect-induced line broadening for single quantum dots in a charge-tunable device, giving state-of-the-art optical linewidths for a system widely studied as a spin qubit for quantum information.

Hole spin mixing in InAs Quantum Dot Molecules

Holes confined in single InAs quantum dots have recently emerged as a promising system for the storage or manipulation of quantum information. These holes are often assumed to have only heavy-hole character and further assumed to have no mixing between orthogonal heavy hole spin projections (in the absence of a transverse magnetic field). The same assumption has been applied to InAs quantum dot molecules formed by two stacked InAs quantum dots that are coupled by coherent tunneling of the hole between the two dots. We present experimental evidence of the existence of a hole spin mixing term obtained with magneto-photoluminescence spectroscopy on such InAs quantum dot molecules. We use a Luttinger spinor model to explain the physical origin of this hole spin mixing term: misalignment of the dots along the stacking direction breaks the angular symmetry and allows mixing through the light-hole component of the spinor. We discuss how this novel spin mixing mechanism may offer new spin manipulation opportunities that are unique to holes.

Enhanced Hot-Carrier Effects in InAlAs/InGaAs Quantum Wells

Hot-carrier solar cells require absorber materials with restricted carrier thermalization pathways, in order to slow the rate of heat energy dissipation from the carrier population to the lattice, relative to the rate of carrier extraction. Absorber suitability can be characterized in terms of carrier thermalization coefficient (Q). Materials with lower Q generate steady-state hot-carrier populations at lower levels of incident solar power and, therefore, are better able to perform as hot-carrier absorbers. In this study, we evaluate Q = 2.5±0.2 W·K-1 · cm-2 for a In0.52 AlAs/In0.53 GaAs single-quantum-well(QW) heterostructure using photoluminescence spectroscopy. This is the lowest experimentally determined Q value for any material system studied to date. Hot-carrier solar cell simulations, using this material as an absorber yield efficiency ~39% at 2000X, which corresponds to a >5% enhancement over an equivalent single-junction thermal equilibrium device.

Drift-diffusion modeling of InP-based triple junction solar cells.

In this work, we use an analytical drift-diffusion model, coupled with detailed carrier transport and minority carrier lifetime estimates, to make realistic predictions of the conversion efficiency of InP-based triple junction cells. We evaluate the possible strategies for overcoming the problematic top cell for the triple junction, and make comparisons of the more realistic charge transport model with incumbent technologies grown on Ge or GaAs substrates.