R. R. LaPierre

GaAs Core−Shell Nanowires for Photovoltaic Applications

We report the use of Te as an n-type dopant in GaAs core-shell p-n junction nanowires for use in photovoltaic devices. Te produced significant change in the morphology of GaAs nanowires grown by the vapor-liquid-solid process in a molecular beam epitaxy system. The increase in radial growth of nanowires due to the surfactant effect of Te had a significant impact on the operating characteristics of photovoltaic devices. A decrease in solar cell efficiency occurred when the Te-doped GaAs growth duration was increased.

Sulfur passivation and contact methods for GaAs nanowire solar cells

The effect of sulfur passivation on core-shell p-n junction GaAs nanowire (NW) solar cells has been investigated. Devices of two types were investigated, consisting of indium tin oxide contact dots or opaque Au finger electrodes. Lateral carrier transport from the NWs to the contact fingers was achieved via a p-doped GaAs surface conduction layer. NWs between the opaque contact fingers had sidewall surfaces exposed for passivation by sulfur. The relative cell efficiency increased by 19% upon passivation. The contribution of the thin film grown between the NWs to the total cell efficiency was estimated by removing the NWs using a sonication procedure. Mechanisms of carrier transport and photovoltaic effects are discussed on the basis of spatially resolved laser scanning measurements.

Numerical model of current-voltage characteristics and efficiency of GaAs nanowire solar cells

Numerical simulation of current-voltage (J-V) characteristics of III-V nanowire core-shell p-n junction diodes under illuminated conditions is presented with an emphasis on optimizing the nanowire design for photoconversion efficiency. Surface recombination and depletion effects are found to play a dominant role in the J-V characteristics. The impact of surface charge density, surface recombination velocity, doping concentration, and nanowire geometry are investigated. Investigation of contacting methodology indicated that solar cell efficiency is degraded with electrical contacts on the sidewalls of the nanowire due to Fermi level pinning at the metal/semiconductor interface. On the other hand, contacts on the top of nanowires with sidewall passivation provide solar cell performance close to the detailed balance efficiency limit of ∼30%. Elimination of the thin film between nanowires produces a smaller dark current and improved cell performance.

Theoretical conversion efficiency of a two-junction III-V nanowire on Si solar cell

The continuity and Poisson equations are solved numerically to obtain J-V characteristics and photoconversion efficiency of a two-junction solar cell. The cell consists of a top junction comprised of nanowires with bandgap of 1.7 eV grown on a bottom junction comprised of a Si substrate. The lattice relaxation possible in nanowires permits lattice-mismatched III-V material growth on Si, thereby achieving the optimum bandgaps in a two-junction cell. The model indicates a limiting efficiency of 42.3% under a concentration of 500 Suns (AM1.5 D spectrum). This limiting efficiency is similar to that calculated for the planar lattice-matched triple-junction Ge/InGaAs/InGaP cell. Methods of fabricating the nanowire/Si cell are discussed including requirements for nanowire sidewall surface passivation. The model indicated that passivation of the nanowire sidewall surfaces that produces a surface recombination velocity of 3000 cm·s−1 and surface trap density of 1012 cm−2 should be sufficient to yield high efficien...

Analytical description of the metal-assisted growth of III―V nanowires: Axial and radial growths

The growth of III–V nanowires from metal seed particles is described in an analytical manner within the framework of a material conservation model. Direct impingement of growth species on the particle, coupled to their diffusion from the sidewall and the substrate surface, are considered in the derivation of expressions for the time evolution of both axial and radial growths. Two regimes are distinguished: the structure originally grows in a purely axial manner until its length exceeds the diffusion length of adatoms incoming from the substrate, at which point sidewall nucleation is triggered, resulting in a shell expanding radially in the lower part of the wire. Factors that take into account the nonunity probability of inclusion of group III adatoms in the axially growing crystal are introduced. Moreover, a step-mediated growth is included to describe the axial evolution of the shell. The numerical values of the various parameters were assessed by fitting the model to experimental data on the morphology...

Spinodal-like decomposition of InGaAsP(100) InP grown by gas source molecular beam epitaxy

Abstract Epitaxial layers of In 1− x Ga x As y P 1− y have been grown lattice-matched to (100) InP substrates over a wide alloy range using gas source molecular beam epitaxy (GSMBE). Transmission electron microscopy (TEM), photoluminescence (PL), and X-ray diffraction have been used to characterize the spinodal-like decomposition of the InGaAsP layers that originates in the diffusion of adatoms at the growing surface. TEM analysis indicates that the dimensions of the spinodal structure are typically 100 A in the [011] direction, 1000 A in the [011] direction, and 700 A in the [100] growth direction. Compositional fluctuations as high as 2.3% in group III and V components have been derived from the PL data. Kinetic limitations of the decomposition have been studied in terms of growth temperature, total group V flux, and crystallographic direction. Decreasing the growth temperature and increasing the group V flux limits the diffusion length of the adatoms before incorporation takes place, and has been shown to decrease the decomposition of the layer during growth.

A GaAs nanowire/P3HT hybrid photovoltaic device

Hybrid solar cells with an energy conversion efficiency of 1.04% were fabricated by spin-coating poly(3-hexylthiophene) (P3HT) polymer onto vertically aligned n-type GaAs nanowire arrays synthesized by molecular beam epitaxy. Experimental parameters such as the P3HT solvent mixing ratio of chlorobenzene to dichlorobenzene and nanowire surface etching were studied with a view to improving the energy conversion efficiency.

Self-directed growth of AlGaAs core-shell nanowires for visible light applications.

Al0.37Ga0.63As nanowires (NWs) were grown in a molecular beam epitaxy system on GaAs(111)B substrates. Micro-photoluminescence measurements and energy dispersive X-ray spectroscopy indicated a core--shell structure and Al composition gradient along the NW axis, producing a potential minimum for carrier confinement. The core--shell structure formed during growth as a consequence of the different Al and Ga adatom diffusion lengths.

GaP/GaAsP/GaP core?multishell nanowire heterostructures on (111) silicon

GaP/GaAsP/GaP segmented nanowires were grown by gas source molecular beam epitaxy on silicon (111) substrates. The nanowires were grown by the vapour?liquid?solid process using Au nanoparticles. Transmission electron microscopy and energy dispersive x-ray spectroscopy indicated that the wires had wurtzite crystal structure with a core?multishell heterostructure. Stacking faults along the wire were removed after the growth-interrupted interfaces, indicating the potential for defect-free nanowires.

Dependence of InGaP nanowire morphology and structure on molecular beam epitaxy growth conditions.

InGaP nanowires (NWs) were grown by the Au-assisted method in a gas source molecular beam epitaxy system. The dependence of InGaP composition, morphology and stacking fault density was studied with respect to group III and V impingement rate and size of the Au particle. Compositional analysis showed that the NWs had an In-rich core and a Ga-rich shell structure. The In incorporation within the NW became limited as the Au seed particle size diminished or the group III and V flux decreased. The NWs had wurtzite (WZ) crystal structure with zinc blende (ZB) segments (stacking faults). The density of the stacking faults decreased as the group III flux decreased and the group V flux increased.