Search

We report a highly flexible strain sensor which exploits the piezoelectric properties of ultra-long gallium nitride (GaN) wires. Langmuir–Blodgett assembled wires are encapsulated in a dielectric material (parylene-C), which is sandwiched between two planar electrodes in a capacitor-like configuration. Through FEM simulations we show that encapsulating densely aligned conical wires in a properly designed dielectric layer can maximize the amplitude of the generated piezoelectric output potential. According to these considerations we designed and fabricated macroscopic flexible strain sensors (active area: 1.5 cm2). The sensor was actuated in three point configuration inducing curvature radii of less than 10 cm and has a typical force sensitivity of 30 mV N−1. [ABSTRACT FROM AUTHOR]

A catalyst-free method for growing self-assembled GaN wires on c-plane sapphire substrates by metal-organic vapour phase epitaxy is developed. This approach, based on in situ deposition of a thin SiNx layer ([?]2 nm), enables epitaxial growth of c-oriented wires with 200-1500 nm diameters and a large length/diameter ratio (>100) on c-plane sapphire substrate. Detailed study of the growth mechanisms shows that a combination of key parameters is necessary to obtain vertical growth. In particular, the duration of the SiNx deposition prior to the wire growth is critical for controlling the epitaxy with the substrate. The GaN seed nucleation time determines the mean size diameter and structural quality, and a high Si-dopant concentration promotes vertical growth. Such GaN wires exhibit UV-light emission centred at [?]350 nm and a weak yellow band ([?]550 nm) at low temperature. [ABSTRACT FROM AUTHOR]

We report a systematic experimental and theoretical investigation of core-shell InGaN/GaN single wire light-emitting diodes (LEDs) using electron beam induced current (EBIC) microscopy. The wires were grown by catalyst-free MOVPE and processed into single wire LEDs using electron beam lithography on dispersed wires. The influence of the acceleration voltage and of the applied bias on the EBIC maps was investigated. We show that the EBIC maps provide information both on the minority carrier effects (i.e. on the local p-n junction collection efficiency) and on the majority carrier effects (i.e. the transport efficiency from the excited region toward the contacts). Because of a finite core and shell resistance a non-negligible current redistribution into the p-n junction takes place during the majority carrier transport. A theoretical model for transport in a core-shell wire is developed, allowing to explain the dependence of the EBIC profiles on the experimental parameters (the electron beam acceleration voltage and the bias applied on the device) and on the structural parameters of the wire (core and shell resistance, shunt resistance, etc). Comparison between simulated and experimental profiles provides valuable information concerning the structure inhomogeneities and gives insight into the wire electrical parameters.

Large-area Si(001) nanopatterned surfaces obtained by etching dislocation line arrays have been used to drive the positioning of metallic islands. A method combining wafer bonding of (001) silicon on insulator layers and preferential chemical etching allows controlling the periodicity of square trench arrays in the 20-50 nm lateral periodicity range with an accuracy of less than 1 nm and a depth of about 4-5 nm. The interfacial area containing the dislocation line plane can be removed and a single crystal maintaining the morphological patterning can be obtained. It is shown that oxidized or deoxidized silicon nanopatterned surfaces can drive the positioning of Ni, Au and Ag islands for a 20 nm lateral periodicity and that a lateral long range order, directly transferred from the dislocation network, can be obtained in the Ni and Au cases.