arXiv subjects

T. Auzelle

Publications and source records attributed to T. Auzelle.

5 recordsLinked to original sources

arXiv2024

AlN Nanowire Based Vertically Integrated Piezoelectric Nanogenerators

In this study, detailed analysis of the direct piezo-response of AlN nanowire-based vertically integrated nanogenerators (VINGs) is undertaken as a function of mechanical excitation frequency. We show that the piezo-charge, piezo-voltage, and impedance measured at the same position of the devices can be directly correlated through an equivalent circuit model, in the whole frequency range of investigation. Our presented results are utilized to determine the performance figures of merit (FoM) of nanowire-based VINGs, namely the piezoelectric voltage constant (g) for sensing, and the product d g for energy harvesting, where d is the piezoelectric charge constant. By comparison of these metrics with those of freestanding single crystal GaN and quartz substrates, as well as sputtered AlN thin films, we suggest that the nanowires can outperform their rigid counterparts in terms of mechanical sensing and energy generation. This work provides experimental guidelines for understanding the direct piezo-characteristics of VINGs and facilitates a quantitative comparison between nanostructured piezoelectric devices fabricated using different materials or architectures.

physics.app-ph↗

arXiv2021

Enhanced radiative efficiency in GaN nanowires grown on sputtered TiN$_{\boldsymbol{x}}$: effects of surface electric fields

GaN nanowires grown by molecular beam epitaxy generally suffer from dominant nonradiative recombination, which is believed to originate from point defects. To suppress the formation of these defects, we explore the synthesis of GaN nanowires at temperatures up to 915 $°C$ enabled by the use of thermally stable TiN$_x$/Al$_2$O$_3$ substrates. These samples exhibit indeed bound exciton decay times approaching those measured for state-of-the-art bulk GaN. However, the decay time is not correlated with the growth temperature, but rather with the nanowire diameter. The inverse dependence of the decay time on diameter suggests that the nonradiative process in GaN nanowires is not controlled by the defect density, but by the field ionization of excitons in the radial electric field caused by surface band bending. We propose a unified mechanism accounting for nonradiative recombination in GaN nanowires of arbitrary diameter.

physics.app-ph↗

arXiv2020

Toward Quantitative Measurements of Piezoelectricity in III-N Semiconductors Nanowires

Piezoelectric semiconductor III-Nitride nanostructures have received increasing interest as an alternative material for energy harvesters, sensors, and self-sustainable electronics, demanding well-clarification of their piezoelectric behavior. Despite the feasibility of piezoresponse force microscopy (PFM) to resolve piezo-responses at the nanoscale, several difficulties arise when the measurements are performed on low piezo-coefficient materials due to various artifacts. This work shows that semi-quantitative PFM on low piezo-coefficient III-Nitrides can be achieved in high-aspect-ratio nanostructures such as nanowires or nanorods. For conventional bulks and thin films, accurate determination of their piezoresponses is limited because of clamping and bending effects which can occur simultaneously during PFM measurements. While the clamping effect only reduces the piezoresponse amplitude, the bending motion either increases or decreases this amplitude and can also rotate the phase by 180°. Improved electric field distribution in nanowires minimizes both artifacts, allowing correct determinations of crystal polarities and piezo-coefficients. In contrast to the reports in the literature, we do not observe giant piezoelectricity in III-N nanowires with a diameter in the range of 30-80 nm. This work provides an access to fundamental parameters for developing III-N based piezoelectric nano-devices.

cond-mat.mtrl-sci↗

arXiv2020

External control of GaN band bending using phosphonate self-assembled monolayers

We report on the optoelectronic properties of GaN$(0001)$ and $(1\bar{1}00)$ surfaces after their functionalization with phosphonic acid derivatives. To analyze the possible correlation between the acid's electronegativity and the GaN surface band bending, two types of phosphonic acids, n-octylphosphonic acid (OPA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA), are grafted on oxidized GaN$(0001)$ and GaN$(1\bar{1}00)$ layers as well as on GaN nanowires. The resulting hybrid inorganic/organic heterostructures are investigated by X-ray photoemission and photoluminescence spectroscopy. The GaN work function is changed significantly by the grafting of phosphonic acids, evidencing the formation of dense self-assembled monolayers. Regardless of the GaN surface orientation, both types of phosphonic acids significantly impact the GaN surface band bending. A dependence on the acids' electronegativity is, however, only observed for the oxidized GaN$(1\bar{1}00)$ surface, indicating a relatively low density of surface states and a favorable band alignment between the surface oxide and acids' electronic states. Regarding the optical properties, the covalent bonding of PFOPA and OPA on oxidized GaN layers and nanowires significantly affect their external quantum efficiency, especially in the nanowire case due to the large surface-to-volume ratio. The variation in the external quantum efficiency is related to the modication of both the internal electric fields and surface states. These results demonstrate the potential of phosphonate chemistry for the surface functionalization of GaN, which could be exploited for selective sensing applications.

physics.app-ph↗

arXiv2019

Absence of quantum-confined Stark effect in GaN quantum disks embedded in (Al,Ga)N nanowires grown by molecular beam epitaxy

Several of the key issues of planar (Al,Ga)N-based deep-ultraviolet light emitting diodes could potentially be overcome by utilizing nanowire heterostructures, exhibiting high structural perfection and improved light extraction. Here, we study the spontaneous emission of GaN/(Al,Ga)N nanowire ensembles grown on Si(111) by plasma-assisted molecular beam epitaxy. The nanowires contain single GaN quantum disks embedded in long (Al,Ga)N nanowire segments essential for efficient light extraction. These quantum disks are found to exhibit intense emission at unexpectedly high energies, namely, significantly above the GaN bandgap, and almost independent of the disk thickness. An in-depth investigation of the actual structure and composition of the nanowires reveals a spontaneously formed Al gradient both along and across the nanowire, resulting in a complex core/shell structure with an Al deficient core and an Al rich shell with continuously varying Al content along the entire length of the (Al,Ga)N segment. This compositional change along the nanowire growth axis induces a polarization doping of the shell that results in a degenerate electron gas in the disk, thus screening the built-in electric fields. The high carrier density not only results in the unexpectedly high transition energies, but also in radiative lifetimes depending only weakly on temperature, leading to a comparatively high internal quantum efficiency of the GaN quantum disks up to room temperature.

cond-mat.mes-hall↗