Spectrally resolved current losses in Cu(In,Ga)Se2 thin-film solar cells

Quantum efficiency measurements are among the fundamental characterization techniques for solar cells; probably the most important after current-voltage analysis of the diode characteristics. Through spectrally resolving the current yield of a photovoltaic device, quantum efficiency gives a closer view on the short-circuit current – one of the basic parameters of a solar cell – and loss mechanisms restricting it. It helps understanding the physics of current generation, recombination and carrier diffusion mechanisms. Hence, quantum efficiency is a valuable tool for scientists in this field. In the scope of this report, a quantum efficiency system was drafted and set up at the Université de Nantes to complement the characterizational possibilities of the research groups in Cu(In,Ga)Se2 thin-film photovoltaics and electrochemical dye-sensitized solar cells. Fundamentals on Cu(In,Ga)Se2 solar cells and their characterization, as well as details on the system setup and an evaluation of its performance are presented. An application of the system in a study on the correlation of Cu(In,Ga)Se2 absorber morphology and device performance resulting from varied duration of the Cu-rich interval during isothermal three-stage co-evaporation of the absorber exemplifies the application of this measurement setup

Top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation

We demonstrate the top-down fabrication of ordered arrays of GaN nanowires by selective area sublimation of pre-patterned GaN(0001) layers grown by hydride vapor phase epitaxy on Al$_{2}$O$_{3}$. Arrays with nanowire diameters and spacings ranging from 50 to 90 nm and 0.1 to 0.7 $\mu$m, respectively, are simultaneously produced under identical conditions. The sublimation process, carried out under high vacuum conditions, is analyzed \emph{in situ} by reflection high-energy electron diffraction and line-of-sight quadrupole mass spectromety. During the sublimation process, the GaN(0001) surface vanishes, giving way to the formation of semi-polar $\lbrace1\bar{1}03\rbrace$ facets which decompose congruently following an Arrhenius temperature dependence with an activation energy of ($3.54 \pm 0.07$) eV and an exponential prefactor of $1.58\times10^{31}$ atoms cm$^{-2}$ s$^{-1}$. The analysis of the samples by low-temperature cathodoluminescence spectroscopy reveals that, in contrast to dry etching, the sublimation process does not introduce nonradiative recombination centers at the nanowire sidewalls. This technique is suitable for the top-down fabrication of a variety of ordered nanostructures, and could possibly be extended to other material systems with similar crystallographic properties such as ZnO.Comment: This is the accepted manuscript version of an article that appeared in Nanoscale Advances. The CC BY-NC 3.0 license applies, see http://creativecommons.org/licenses/by-nc/3.0

Growth mechanisms in molecular beam epitaxy for GaN-(In,Ga)N core-shell nanowires emitting in the green spectral range

Using molecular beam epitaxy, we demonstrate the growth of (In,Ga)N shells emitting in the green spectral range around very thin (35 nm diameter) GaN core nanowires. These GaN nanowires are obtained by self-assembled growth on TiN. We present a qualitative shell growth model accounting for both the three-dimensional nature of the nanostructures as well as the directionality of the atomic fluxes. This model allows us, on the one hand, to optimise the conditions for high and homogeneous In incorporation and, on the other hand, to explain the influence of changes in the growth conditions on the sample morphology and In content. Specifically, the impact of the V/III and In/Ga flux ratios, the rotation speed and the rotation direction are investigated. Notably, with In acting as surfactant, the ternary (In,Ga)N shells are much more homogeneous in thickness along the NW length than their binary GaN counterparts

Lattice parameters of Scx_{\boldsymbol{\mathsf{x}}}Al1−x_{\boldsymbol{\mathsf{1-x}}}N layers grown on GaN(0001) by plasma-assisted molecular beam epitaxy

An accurate knowledge of the lattice parameters of the new nitride Sc$_\textit{x}$Al$_\textit{1-x}$N is essential for understanding the elastic and piezoelectric properties of this compound as well as for the ability to engineer its strain state in heterostructures. Using high-resolution x-ray diffractometry, we determine the lattice parameters of 100-nm-thick undoped Sc$_\textit{x}$Al$_\textit{1-x}$N layers grown on GaN(0001) templates by plasma-assisted molecular beam epitaxy. The Sc content $\textit{x}$ of the layers is measured independently by both x-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy and ranges from 0 to 0.25. The in-plane lattice parameter of the layers linearly increases with increasing $\textit{x}$, while their out-of-plane lattice parameter remains constant. Layers with $\textit{x}$ $\approx$ 0.09 are found to be lattice matched to GaN, resulting in a smooth surface and a structural perfection equivalent to that of the GaN underlayer. In addition, a two-dimensional electron gas is induced at the Sc$_\textit{x}$Al$_\textit{1-x}$N/GaN heterointerface, with the highest sheet electron density and mobility observed for lattice-matched conditions