The record in photovoltaic conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, to a large extent due to the expensive III-V substrates. As an alternative, a hybrid geometry has been proposed [LaPierre JAP 2011], which combines a 2D Si bottom cell with a III-V nanowire top cell in a tandem device. This approach, which may reach theoretical efficiencies of approx. 34%, requires smaller amounts of expensive III-V materials compared to conventional III-V tandem cells and benefits from the nanowire light trapping effects.
In this work, we report the fabrication and nanoscale characterization of two types of nanostructures for solar cells: radial GaAlAs and axial GaAsP p-n junction nanowires. Nanowires are grown by gallium-assisted molecular beam epitaxy using Be and Si as doping sources. The composition (probed by EDX and cathodoluminescence) was adjusted to tune the bandgap toward the optimal value for a III-V-on-Si tandem cell (approx. 1.7 eV). Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases are used to probe the electrical properties and the generation pattern of individual nanowires. For radial junction nanowires, EBIC mappings revealed a homogeneous collection of carriers on the entire nanowire length. For axial junction nanowires, the doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. The effect of an epitaxial GaP passivating shell on the optical and generation properties was assessed.
“Photonics Multiannual Strategic Roadmap 2014-2020” mentions flexible electronics, light sources, displays, sensors and solar cells as key emerging technologies with a high expected growth of the market share. Technologies based on organic semiconductors still suffer from a short lifetime and low efficacy as compared to their inorganic counterparts. To make a flexible device from inorganic semiconductors one should shrink the size of the active elements and to integrate them on mechanically-flexible substrates. This can be achieved using control-by-design nanowires.
In this work, we address the growth of nitride nanowires on novel substrates and the fabrication and characterization of flexible devices based on nitride nanowires. First, we will discuss the epitaxy of GaN nanowires on graphene-on-SiO2 substrates. We show that without any catalyst or intermediate layer, the nanowires grow on graphene with an excellent selectivity compared to the uncovered SiO2 surface. Taking advantage of this selectivity, we demonstrate that organized arrays of nanowires can be synthesized by structuring the graphene layer. Next, we will discuss the approach for nanowire lift-off, transfer into polymer-embedded membranes and flexible contacting. The realization and characterization of flexible light sources, photodetectors and piezogenerators will be presented.
Mode-locked vertical-extended-cavity-surface emitting lasers (ML-VECSEL) are promising candidates for the
generation of stable short pulses at multi-GHz rate. However, the poor thermal behavior of quaternary InP-based
semiconductor compounds often limits the performance of ML-VECSELs operating at 1.55 μm. In this work, we report
on a specific approach using downward heat sinking to optimize the heat dissipation out of the active region. VECSEL
chips with a low thermal resistance are fabricated using a hybrid metal-metamorphic GaAs/AlAs mirror and bonding to a
highly thermally conductive host substrate. We show that superior performance can be obtained with a CVD diamond
substrate, while electroplated copper host substrate can afford a flexible and low cost alternate approach for moderate
(~100 mW) output power. The VECSEL chip assembled with a 1.55μm fast InGaAs(Sb)N/GaAsN semiconductor
saturable absorber mirror (SESAM) produces nearly Fourier transform-limited mode-locked pulses at ~ 2 GHz repetition
frequency, and the RF linewidth of the free running laser is measured to be less than 1000 Hz. When the resonance and
group delay dispersion of the SESAM microcavity are tuned by selective etching of specific top phase layers, the modelocked
pulse width is reduced from several picoseconds to less than 1 ps.
In this paper we present our recent developments in control and manipulation of individual spins and photons in a single
nanowire quantum dot. Specific examples include demonstration of optical excitation of single spin states, charge
tunable quantum devices and single photon sources. We will also discuss our recent discovery of a new type of charge
confinement - crystal phase quantum dots. They are formed from the same material with different crystal structure, and
today can only be realized in nanowires.
The catalyst-assisted growth of semiconductor nanowires heterostructures offers a very flexible way to design and
fabricate single photon emitters. The nanowires can be positioned by organizing the catalyst prior to growth. Single
quantum dots can be formed in the core of single nanowires which can then be easily isolated and addressed to generate
single photons. Diameter and height of the dots can be controlled and their emission wavelength can be tuned at the
optical telecommunication wavelengths by the material composition. The final morphology of a wire can be shaped by
the radial/axial growth ratio, offering the possibility to form single mode optical waveguides with a tapered end for
efficient photon collection.
Real-time Monte Carlo Molecular Dynamics (MC-MD) simulation techniques have been developed to model the nucleation, the initial stages of growth, and thin film growth, during InP Molecular Beam Epitaxy (MBE) on InP. The simulation mode includes tetrahedral lattice coordination, species-species interactions out to third-nearest neighbor, heterogeneous photolysis of precursors molecules on vacuum UV, adspecies migration on the lattice, nucleation on conventional and charge activated centers, and desorption dynamic effects. An InP homoepitaxy system, permits the simulator validation against MBE experimental results; although the model and the corresponding simulator are easily applied to a variety of other problems. The amount of InP epitaxy as a function of time is obtained over surface are of 50 X 50 atomic sites. The result of the simulations demonstrate that model treatment is accurate and encompasses several improvements over previous treatments. The agreement between experimental and simulated roughness serves to build confidence in the use of Mc-MD for MBE studies.
Negative differential velocity is evidenced in semiconductor superlattices through several experimental approaches: temperature dependence of the current-voltage characteristics, frequency spectrum of the microwave S-parameters, and time-resolved photocurrent induced by a short optical pulse. In particular, new experimental data for GaInAs/AlInAs superlattices matched to InP are analyzed owing to classical models. They yield the miniband width dependence of the mobility, critical field and peak velocity which describe the electron velocity laws. The latter are in fair agreement with a semiclassical model based on numerical solutions of the Boltzmann equation, i.e., a rigorous extension of the simpler Esaki-Tsu model of miniband conduction. In the dynamical experiments, the temporal evolution of the electron distribution in the superlattice structure is represented in terms of propagating space charge waves, which can give rise to amplification and oscillation. Consequences of miniband conduction regarding maximum frequency and noise of superlattice-based oscillators are also examined.