Alexandre Jaffré

Characterization of graphene oxide reduced through chemical and biological processes

The study of new materials for transparent electrodes or new heterojunctions made of 2D materials combinations is a very active research topic. Challenges to overcome are the modulation of the optoelectronic properties of such materials to achieve competitive photovoltaic devices. In this work, graphene oxide was reduced into graphene through different chemical (hydrazine, ultraviolet photocatalysis) and biological (microorganisms) processes. W e benchmarked the reduction efficiency by probing materials characteristics using various physical characterization techniques. X-ray photoelectron spectroscopy (XPS) analyses were carried out to observe the effectiveness of the reduction processes through the sp2/sp3 content. In addition, the homogeneity of the reduction is investigated on micrometer scale sample with micro Raman mapping and extraction of the ID/IG ratio. Conductive-probe atomic force microscopy (CP-AFM) was employed to investigate the longitudinal conductivity of the different samples. The results show that hydrazine based reduction remains the most efficient. However, the bacterial procedure demonstrated partial reconstruction of the carbon network and reduced the amount of oxygenated functional groups.

Excitation transfer in stacked quantum dot chains

Stacked InAs quantum dot chains (QDCs) on InGaAs/GaAs cross-hatch pattern (CHP) templates yield a rich emission spectrum with an unusual carrier transfer characteristic compared to conventional quantum dot (QD) stacks. The photoluminescent spectra of the controlled, single QDC layer comprise multiple peaks from the orthogonal QDCs, the free-standing QDs, the CHP, the wetting layers and the GaAs substrate. When the QDC layers are stacked, employing a 10 nm GaAs spacer between adjacent QDC layers, the PL spectra are dominated by the top-most stack, indicating that the QDC layers are nominally uncoupled. Under high excitation power densities when the high-energy peaks of the top stack are saturated, however, low-energy PL peaks from the bottom stacks emerge as a result of carrier transfers across the GaAs spacers. These unique PL signatures contrast with the state-filling effects in conventional, coupled QD stacks and serve as a means to quickly assess the presence of electronic coupling in stacks of dissimilar-sized nanostructures. S Online supplementary data available from stacks.iop.org/SST/30/055005/mmedia

Structural, Optoelectronic and Electrical Properties of GaAs Microcrystals Grown from (001) Si Nano-areas

An innovative approach is being investigated to develop III–V compounds on silicon (Si) substrates with the purpose to offer a technological alternative for the development of high efficiency solar cells ( ∼ 30 %). Until now, germanium (Ge) substrate has been the privileged material for the development of III–V multi-junctions (MJ) solar cells mainly dedicated to space applications. Ge offers several advantages, namely the lattice matching to Si and its use as a bottom cell in the MJ. However, the main drawback remains the cost of Ge substrates, which makes it inappropriate for terrestrial photovoltaic (PV) applications. New routes for high efficiency MJ solar cells are expected through the significant improvements of the selective area epitaxy (Li et al., J Appl Phys 103:106102, 2008; Deura et al., J Cryst Growth 310:4768–4771, 2008; Hsu et al., Appl Phys Lett 99:133115, 2011) allowing defect free III–V compounds to be grown on Si substrates patterned with dielectric films. In this work, Si nanoscale areas opened through a SiO2 layer ( < 1 nm) formed on (001) Si have been used to grow GaAs microcrystals by chemical beam epitaxy (CBE) in the temperature range 550–600 ∘C (Renard et al., Appl Phys Lett 102:191915, 2013). Structural, optoelectronic and electrical properties of GaAs microcrystals have been analyzed at room temperature by micro-Raman, photoluminescence and conductive probe atomic force microscopy (CP-AFM). The fine structure of crystals (facet orientations, crystal defects) has also been investigated by transmission electron microscopy (TEM). Linear polarized Raman spectroscopy performed on multiple microcrystals shows exclusively the TO mode which is typically expected for (110) GaAs plane orientations and/or heavily n-type Si-doped GaAs (Zardo et al., Phys Rev B 80:245324, 2009). TEM confirms that all facets are {110}, but unintentionally Si doping cannot be excluded. Indeed, PL measure-ments point out a red shift for the microcrystals for which nucleation seeds were created by silane exposure. CP-AFM imaging of GaAs microcrystals performed at + 1 and − 1 V, respectively, points out a current rectification behavior confirmed by local I–V measure-ments (Fig. 37.1). These results can be interpreted as a sign of the presence of a p-n junction, which agrees well with the p-type doping of Si substrates used in this study (1–5 Ωcm) and the unintentionally n-type doping of GaAs microcrystals suggested by PL measurements (Pavesi and Henini, Microelectron J 28:717–726, 1997).

Growth route toward III-V multispectral solar cells on silicon

To date, high efficiency multijunction solar cells have been developed on Ge or GaAs substrates for space applications, and terrestrial applications are hampered by high fabrication costs. In order to reduce this cost, we propose a breakthrough technique of III-V compound heteroepitaxy on Si substrates without generation of defects critical to PV applications. With this technique we expect to achieve perfect integration of heterogeneous Ga1-xInxAs micro-crystals on Si substrates. In this paper, we show that this is the case for x=0. GaAs crystals were grown by Epitaxial Lateral Overgrowth on Si (100) wafers covered with a thin SiO2 nanostructured layer. The cristallographic structure of these crystals is analysed by MEB and TEM imaging. Micro-Raman and Micro-Photomuminescence spectra of GaAs crystals grown with different conditions are compared with those of a reference GaAs wafer in order to have more insight on eventual local strains and their cristallinity. This work aims at developping building blocks to further develop a GaAs/Si tandem demonstrator with a potential conversion efficiency of 29.6% under AM1.5G spectrum without concentration, as inferred from our realistic modeling. This paper shows that Epitaxial Lateral Overgrowth has a very interesting potential to develop multijunction solar cells on silicon approaching the today 30.3% world record of a GaInP/GaAs tandem cell under the same illumination conditions, but on a costlier substrate than silicon.