Optical properties of GaN nanowires grown on chemical vapor deposited-graphene.

Optical properties of GaN nanowires (NWs) grown on chemical vapor deposited-graphene transferred on an amorphous support are reported. The growth temperature was optimized to achieve a high NW density with a perfect selectivity with respect to a SiO2 surface. The growth temperature window was found to be rather narrow (815°C ± 5°C). Steady-state and time-resolved photoluminescence from GaN NWs grown on graphene was compared with the results for GaN NWs grown on conventional substrates within the same molecular beam epitaxy reactor showing a comparable optical quality for different substrates. Growth at temperatures above 820 °C led to a strong NW density reduction accompanied with a diameter narrowing. This morphology change leads to a spectral blueshift of the donor-bound exciton emission line due to either surface stress or dielectric confinement. Graphene multi-layered micro-domains were explored as a way to arrange GaN NWs in a hollow hexagonal pattern. The NWs grown on these domains show a luminescence spectral linewidth as low as 0.28 meV (close to the set-up resolution limit).

Piezo-generator integrating a vertical array of GaN nanowires.

We demonstrate the first piezo-generator integrating a vertical array of GaN nanowires (NWs). We perform a systematic multi-scale analysis, going from single wire properties to macroscopic device fabrication and characterization, which allows us to establish for GaN NWs the relationship between the material properties and the piezo-generation, and to propose an efficient piezo-generator design. The piezo-conversion of individual MBE-grown p-doped GaN NWs in a dense array is assessed by atomic force microscopy (AFM) equipped with a Resiscope module yielding an average output voltage of 228 ± 120 mV and a maximum value of 350 mV generated per NW. In the case of p-doped GaN NWs, the piezo-generation is achieved when a positive piezo-potential is created inside the nanostructures, i.e. when the NWs are submitted to compressive deformation. The understanding of the piezo-generation mechanism in our GaN NWs, gained from AFM analyses, is applied to design a piezo-generator operated under compressive strain. The device consists of NW arrays of several square millimeters in size embedded into spin-on glass with a Schottky contact for rectification and collection of piezo-generated carriers. The generator delivers a maximum power density of ∼12.7 mW cm(-3). This value sets the new state of the art for piezo-generators based on GaN NWs and more generally on nitride NWs, and offers promising prospects for the use of GaN NWs as high-efficiency ultra-compact energy harvesters.

Self-induced growth of vertical GaN nanowires on silica.

We study the self-induced growth of GaN nanowires on silica. Although the amorphous structure of this substrate offers no possibility of an epitaxial relationship, the nanowires are remarkably aligned with the substrate normal whereas, as expected, their in-plane orientation is random. Their structural and optical characteristics are compared to those of GaN nanowires grown on standard crystalline Si (111) substrates. The polarity inversion domains are much less frequent, if not totally absent, in the nanowires grown on silica, which we find to be N-polar. This work demonstrates that high-quality vertical GaN nanowires can be elaborated without resorting to bulk crystalline substrates.