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Register a new userEffect of different buffer layers on the quality of InGaN layers grown on Si
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This work studies the effect of four different types of buffer layers on the structural and optical properties of InGaN layers grown on Si(111) substrates and their correlation with electrical characteristics. The vertical electrical conduction of n-InGaN/buffer-layer/p-Si heterostructures, with In composition near 46%, which theoretically produces an alignment of the bands, is analyzed. Droplet elimination by radical-beam irradiation was successfully applied to grow high quality InGaN films on Si substrates for the first time. Among several buffer choices, an AlN buffer layer with a thickness above 24 nm improves the structural and optical quality of the InGaN epilayer while keeping a top to bottom ohmic behavior. These results will allow fabricating double-junction InGaN/Si solar cells without the need of tunnel junctions between the two sub-cells, therefore simplifying the device design.
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The non-covalent functionalisation of graphene is an attractive strategy to alter the surface chemistry of graphene without damaging its superior electrical and mechanical properties. Using the facile method of aqueous-phase functionalisation on large-scale CVD-grown graphene, we investigated the formation of different packing densities in self-assembled monolayers (SAMs) of perylene bisimide derivatives and related this to the amount of substrate contamination. We were able to directly observe wet-chemically deposited SAMs in scanning tunnelling microscopy (STM) on transferred CVD graphene and revealed that the densely packed perylene ad-layers adsorb with the conjugated {pi}-system of the core perpendicular to the graphene substrate. This elucidation of the non-covalent functionalisation of graphene has major implications on controlling its surface chemistry and opens new pathways for adaptable functionalisation in ambient conditions and on the large scale.
Structural and superconducting properties of magnesium diboride thin films grown by pulsed laser deposition on zirconium diboride buffer layers were studied. We demonstrate that the ZrB2 layer is compatible with the MgB2 two step deposition process. Synchrotron radiation measurements, in particular anomalous diffraction measurements, allowed to separate MgB2 peaks from ZrB2 ones and revealed that both layers have a single in plane orientation with a sharp interface between them. Moreover, the buffer layer avoids oxygen contamination from the sapphire substrate. The critical temperature of this film is near 37.6 K and the upper critical field measured at Grenoble High Magnetic Field Laboratory up to 20.3 T is comparable with the highest ones reported in literature.
We have employed state-of-the-art cross-correlation noise spectroscopy to study carrier dynamics in silicon heterojunction solar cells, complimented by SENTARUS simulations of the same devices. These cells were composed of a light absorbing n-doped crystalline silicon layer contacted by passivating layers of i-a-Si:H and doped a-Si:H electrode layers. The method provided a two-orders-of-magnitude improved sensitivity and allowed to resolution of three additional contributions to noise in addition to 1/f noise. We have observed shot noise with Fano factor close to unity. We have also observed a peculiar generation-recombination term, which presents only under light illumination with energy above 2 eV and thus reflects light absorption and carrier trapping in the a-Si:H layers. A second, low-frequency generation-recombination term was detected at temperatures below 100 K. We argue that it appears because the process of charge carrier transfer across i-a-Si:H occurs via an intermediate defect limited by tunneling above about 100 K and a thermally assisted process below this temperature. We also discuss the spatial selectivity of noise spectroscopy, namely the tendency of the method to amplify noise contributions from the most resistive element of the cell. Indeed, in our case, all three terms are linked to the passivating i-a-Si:H layer.
Perovskite solar cells (PSCs) with transparent electrodes can be integrated with existing solar panels in tandem configurations to increase the power conversion efficiency. A critical layer in semi-transparent PSCs is the inorganic buffer layer, which protects the PSC against damage when the transparent electrode is sputtered on top. The development of n-i-p structured semi-transparent PSCs has been hampered by the lack of suitable p-type buffer layers. In this work we develop a p-type CuOx buffer layer, which can be grown uniformly over the perovskite device without damaging the perovskite or organic charge transport layers, can be grown using industrially scalable techniques and has high hole mobility (4.3 +/- 2 cm2 V-1 s-1), high transmittance (>95%), and a suitable ionisation potential for hole extraction (5.3 +/- 0.2 eV). Semi-transparent PSCs with efficiencies up to 16.7% are achieved using the CuOx buffer layer. Our work demonstrates a new approach to integrate PSCs into tandem configurations, as well as enable the development of other devices that need high quality p-type layers.
We report on total-energy electronic structure calculations in the density-functional theory performed for the ultra-thin atomic layers of Si on Ag(111) surfaces. We find several distinct stable silicene structures: $sqrt{3}timessqrt{3}$, $3times3$, $sqrt{7}timessqrt{7}$ with the thickness of Si increasing from monolayer to quad-layer. The structural bistability and tristability of the multilayer silicene structures on Ag surfaces are obtained, where the calculated transition barriers infer the occurrence of the flip-flop motion at low temperature. The calculated STM images agree well with the experimental observations. We also find the stable existence of $2times1$ $pi$-bonded chain and $7times7$ dimer-adatom-stacking fault Si(111)-surface structures on Ag(111), which clearly shows the crossover of silicene-silicon structures for the multilayer Si on Ag surfaces. We further find the absence of the Dirac states for multilayer silicene on Ag(111) due to the covalent interactions of silicene-Ag interface and Si-Si interlayer. Instead, we find a new state near Fermi level composed of $pi$ orbitals locating on the surface layer of $sqrt{3}timessqrt{3}$ multilayer silicene, which satisfies the hexagonal symmetry and exhibits the linear energy dispersion. By examining the electronic properties of $2times1$ $pi$-bonded chain structures, we find that the surface-related $pi$ states of multilayer Si structures are robust on Ag surfaces.
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