Shawn Mack

Terahertz electron-hole recollisions in GaAs/AlGaAs quantum wells: robustness to scattering by optical phonons and thermal fluctuations.

Electron-hole recollisions are induced by resonantly injecting excitons with a near-IR laser at frequency fNIR into quantum wells driven by a 10u2009u2009kV/cm field oscillating at fTHz=0.57u2009u2009THz. At T=12u2009u2009K, up to 18 sidebands are observed at frequencies fsideband=fNIR+2nfTHz, with -8≤2n≤28. Electrons and holes recollide with total kinetic energies up to 57 meV, well above the ELO=36u2009u2009meV threshold for longitudinal optical (LO) phonon emission. Sidebands with order up to 2n=22 persist up to room temperature. A simple model shows that LO phonon scattering suppresses but does not eliminate sidebands associated with kinetic energies above ELO.

Dynamical birefringence: Electron-hole recollisions as a probe of Berry curvature

The direct measurement of Berry phases is still a great challenge in condensed matter systems. The bottleneck has been the ability to adiabatically drive an electron coherently across a large portion of the Brillouin zone in a solid where the scattering is strong and complicated. We break through this bottleneck and show that high-order sideband generation (HSG) in semiconductors is intimately affected by Berry phases. Electron-hole recollisions and HSG occur when a near-band gap laser beam excites a semiconductor that is driven by sufficiently strong terahertz (THz)-frequency electric fields. We carried out experimental and theoretical studies of HSG from three GaAs/AlGaAs quantum wells. The observed HSG spectra contain sidebands up to the 90th order, to our knowledge the highest-order optical nonlinearity observed in solids. The highest-order sidebands are associated with electron-hole pairs driven coherently across roughly 10% of the Brillouin zone around the Gamma point. The principal experimental claim is a dynamical birefringence: the sidebands, when the order is high enough (> 20), are usually stronger when the exciting near-infrared (NIR) and the THz electric fields are polarized perpendicular than parallel; the sideband intensities depend on the angles between the THz field and the crystal axes in samples with sufficiently weak quenched disorder; and the sidebands exhibit significant ellipticity that increases with increasing sideband order, despite nearly linear excitation and driving fields. We explain dynamical birefringence by generalizing the three-step model for high order harmonic generation. The hole accumulates Berry phases due to variation of its internal state as the quasi-momentum changes under the THz field. Dynamical birefringence arises from quantum interference between time-reversed pairs of electron-hole recollision pathways.

Development of recessed contacts for mechanical stacking of GaSb solar cells

Transfer printing is a formidable technique to stack multijunction solar cells while avoiding existing epitaxial limitations. Thus, a larger portion of the solar spectrum can be harvested through careful design optimization. In this process, metal contacts are embedded within a highly conductive lateral conduction layer (LCL) on the lower subcells. A readily available low bandgap candidate for a multijunction stack includes a GaSb solar cell with a 1.0 eV Al0.2Ga0.8Sb LCL. Processing of a bare AlGaSb layer is difficult, as standard photolithography processes and wet etch chemistries leave the surface excessively roughened and inhibit good ohmic contact. Several dry etch recipes are investigated to etch the AlGaSb LCL while leaving a smooth surface. A recipe based on a gas mixture of BCl3/Ar was determined to be a viable candidate for dry etching of AlGaSb and used to fabricate a GaSb solar cell with recessed contacts in an AlGaSb LCL.

Growth and characterization of (110) InAs quantum well metamorphic heterostructures

An understanding of the growth of (110) quantum wells (QWs) is of great importance to spin systems due to the observed long spin relaxation times. In this article, we report on the metamorphic growth and characterization of high mobility undoped InAs (110) QWs on GaAs (110) substrates. A low-temperature nucleation layer reduces dislocation density, results in tilting of the subsequent buffer layer and increases the electron mobility of the QW structure. The mobility varies widely and systematically (4000–16u2009000u2009cm2/Vs at room temperature) with deposition temperature and layer thicknesses. Low-temperature transport measurements exhibit Shubnikov de-Haas oscillations and quantized plateaus in the quantum Hall regime.

Towards the ultimate multi-junction solar cell using transfer printing

Transfer printing is a uniquely enabling technology for the heterogeneous integration of III-V materials grown on dissimilar substrates. In this paper, we present experimental results for a mechanically stacked tandem cell using GaAs and GaSb-based materials capable of harvesting the entire solar spectrum with 44.5% efficiency. We also present the latest results toward developing an ultra-high performance heterogeneous cell, integrating materials grown on GaAs, InP and GaSb platforms.

High-order sideband generation in semiconductors: Beyond the three step model

Measurements of high-order sideband generation and absorption as a function of near-ir frequency in strong monochromatic sub-THz fields highlight opportunities to explore recollision physics in regimes that are difficult to access in atomic systems.

High-order sideband generation: Effect of optical polarization

Continuous optical excitation of excitons in quantum wells driven by intense, monochromatic terahertz fields leads to high-order sideband generation. With optical and terahertz fields perpendicular, sideband generation is enhanced, leading to 60th-order and higher processes.

Transfer-printing for the next generation of multi-junction solar cells

Transfer-printing is a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked III-V solar cells with low cost. In this paper, we present the latest results for microscale CPV cells grown on GaAs and InP substrates for ultra-high-efficiency, four-terminal, mechanically stacked architectures. We describe the latest findings from a combination of modeling, growth, characterization and processing of tunnel junctions, single junction and multijunction solar cells, with the ultimate goal of using transfer-printing to produce the first solar cell with 50% conversion efficiency.

Growth and Characterization of (110) InAs Quantum Well Heterostructures by Transmission Electron Microscopy and Electron Channeling Contrast Imaging

InAs quantum wells grown in the (110) direction are an important materials system in the field of spin systems. The major hurdle in realizing such devices is structural defects arising from the large mismatch, 7.2%, between the InAs film and its GaAs substrate. Thus, we embark on a study utilizing transmission electron microscopy (TEM) and electron channeling contrast imaging (ECCI) to understand the density and nature of these defects.

Realizing the next generation of CPV cells using transfer printing

Transfer-printing is an important, commercial technology for manufacturing state of the art CPV modules, and has emerged recently as a key enabling technology for the realization of ultra-high-efficiency, mechanically stacked III-V solar cells with low cost. This paper presents the latest results for microscale CPV cells grown on GaAs, InP and GaSb substrates for ultra-high-efficiency, four-terminal, mechanically stacked architectures. The latest findings from a combination of modeling, growth, processing and characterization of single and multijunction solar cells are described, and the roadmap to the long-term goal of using transfer-printing to produce the first solar cell with 50% conversion efficiency is outlined.