Chemovoltaic effect for renewable liquid and vapor fuels on semiconductor surfaces.

The chemovoltaic effect - generation of electronic excitation by exergonic redox reactions - has been observed on metallic surfaces of Schottky junctions and is proving to be pivotal in explaining in detail the momentum conservation relations of chemically active collisions. As shown in this work, it can hold keys for direct chemical energy harvesting by semiconductor solar cells. To study the possibilities of chemovoltaic energy conversion by semiconductors, we have modeled and designed an 'electrolyte-free fuel cell' formed by a GaAs diode that can host electrochemical fuel oxidation and oxidant reduction reactions on its conduction and valence bands and as a result convert renewable chemical energy (as well as light) into electricity. The experimental results show that exposing the surface of a suitably designed solar cell to methanol liquid or vapor in the presence of oxygen or hydrogen peroxide leads to the generation of electrical power.

Strain-Engineered Multilayer Epitaxial Lift-Off for Cost-Efficient III-V Photovoltaics and Optoelectronics.

The efficient removal of epitaxially grown materials from their host substrate has a pivotal role in reducing the cost and material consumption of III-V solar cells and in making flexible thin-film devices. A multilayer epitaxial lift-off process is demonstrated that is scalable in both film size and in the number of released films. The process utilizes in-built, individually engineered epitaxial strain in each film to tailor the bending without the need for external layers to induce strain. Even without external support layers, the films retain good integrity after the lift-off, as evidenced by photoluminescence measurements. The films can be further processed into devices, demonstrated here with the fabrication of cm-scale solar cells using a three-layer lift-off process. Based on the included cost analysis, the solar cells are fabricated with a facile two-step process from the as-released films. The scalable multilayer lift-off process is highly cost-efficient and enables a 4-to-6-fold reduction in the cost with respect to the single-layer epitaxial lift-off process. The results are therefore significant for III-V photovoltaics and any other technologies that rely on thin-film III-V semiconductors.

Electron Injection in Metal Assisted Chemical Etching as a Fundamental Mechanism for Electroless Electricity Generation.

Metal-assisted chemical etching (MACE) is a widely applied process for fabricating Si nanostructures. As an electroless process, it does not require a counter electrode, and it is usually considered that only holes in the Si valence band contribute to the process. In this work, a charge carrier collecting p-n junction structure coated with silver nanoparticles is used to demonstrate that also electrons in the conduction band play a fundamental role in MACE, and enable an electroless chemical energy conversion process that was not previously reported. The studied structures generate electricity at a power density of 0.43 mW/cm2 during MACE. This necessitates reformulating the microscopic electrochemical description of the Si-metal-oxidant nanosystems to separately account for electron and hole injections into the conduction and valence band of Si. Our work provides new insight into the fundamentals of MACE and demonstrates a radically new route to chemical energy conversion by solar cell-inspired devices.

Diffusion-Driven Charge Transport in Light Emitting Devices.

Almost all modern inorganic light-emitting diode (LED) designs are based on double heterojunctions (DHJs) whose structure and current injection principle have remained essentially unchanged for decades. Although highly efficient devices based on the DHJ design have been developed and commercialized for energy-efficient general lighting, the conventional DHJ design requires burying the active region (AR) inside a pn-junction. This has hindered the development of emitters utilizing nanostructured ARs located close to device surfaces such as nanowires or surface quantum wells. Modern DHJ III-N LEDs also exhibit resistive losses that arise from the DHJ device geometry. The recently introduced diffusion-driven charge transport (DDCT) emitter design offers a novel way to transport charge carriers to unconventionally placed ARs. In a DDCT device, the AR is located apart from the pn-junction and the charge carriers are injected into the AR by bipolar diffusion. This device design allows the integration of surface ARs to semiconductor LEDs and offers a promising method to reduce resistive losses in high power devices. In this work, we present a review of the recent progress in gallium nitride (GaN) based DDCT devices, and an outlook of potential DDCT has for opto- and microelectronics.

Interference-exact radiative transfer equation.

The Purcell effect, i.e., the modification of the spontaneous emission rate by optical interference, profoundly affects the light-matter coupling in optical resonators. Fully describing the optical absorption, emission, and interference of light hence conventionally requires combining the full Maxwell's equations with stochastic or quantum optical source terms accounting for the quantum nature of light. We show that both the nonlocal wave and local particle features associated with interference and emission of propagating fields in stratified geometries can be fully captured by local damping and scattering coefficients derived from the recently introduced quantized fluctuational electrodynamics (QFED) framework. In addition to describing the nonlocal optical interference processes as local directionally resolved effects, this allows reformulating the well known and widely used radiative transfer equation (RTE) as a physically transparent interference-exact model that extends the useful range of computationally efficient and quantum optically accurate interference-aware optical models from simple structures to full optical devices.

Optimization of Current Injection in AlGaInP Core-Shell Nanowire Light-Emitting Diodes.

Core-shell nanowires offer great potential to enhance the efficiency of light-emitting diodes (LEDs) and expand the attainable wavelength range of LEDs over the whole visible spectrum. Additionally, nanowire (NW) LEDs can offer both improved light extraction and emission enhancement if the diameter of the wires is not larger than half the emission wavelength (λ/2). However, AlGaInP nanowire LEDs have so far failed to match the high efficiencies of traditional planar technologies, and the parameters limiting the efficiency remain unidentified. In this work, we show by experimental and theoretical studies that the small nanowire dimensions required for efficient light extraction and emission enhancement facilitate significant loss currents, which result in a low efficiency in radial NW LEDs in particular. To this end, we fabricate AlGaInP core-shell nanowire LEDs where the nanowire diameter is roughly equal to λ/2, and we find that both a large loss current and a large contact resistance are present in the samples. To investigate the significant loss current observed in the experiments in more detail, we carry out device simulations accounting for the full 3D nanowire geometry. According to the simulations, the low efficiency of radial AlGaInP nanowire LEDs can be explained by a substantial hole leakage to the outer barrier layer due to the small layer thicknesses and the close proximity of the shell contact. Using further simulations, we propose modifications to the epitaxial structure to eliminate such leakage currents and to increase the efficiency to near unity without sacrificing the λ/2 upper limit of the nanowire diameter. To gain a better insight of the device physics, we introduce an optical output measurement technique to estimate an ideality factor that is only dependent on the quasi-Fermi level separation in the LED. The results show ideality factors in the range of 1-2 around the maximum LED efficiency even in the presence of a very large voltage loss, indicating that the technique is especially attractive for measuring nanowire LEDs at an early stage of development before electrical contacts have been optimized. The presented results and characterization techniques form a basis of how to simultaneously optimize the electrical and optical efficiency of core-shell nanowire LEDs, paving the way to nanowire light emitters that make true use of larger-than-unity Purcell factors and the consequently enhanced spontaneous emission.

Aluminum-Induced photoluminescence red shifts in core-shell GaAs/Al(x)Ga(1-x)As nanowires.

We report a new phenomenon related to Al-induced carrier confinement at the interface in core-shell GaAs/Al(x)Ga(1-x)As nanowires grown using metal-organic vapor phase epitaxy with Au as catalyst. All Al(x)Ga(1-x)As shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to ~66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron-hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell-core-shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.