An improved AFM cross-sectional method for piezoelectric nanostructures properties investigation: application to GaN nanowires.

We present an improved atomic force microscopy (AFM) method to study the piezoelectric properties of nanostructures. An AFM tip is used to deform a free-standing piezoelectric nanowire. The deflection of the nanowire induces an electric potential via the piezoelectric effect, which is measured by the AFM coating tip. During the manipulation, the applied force, the forcing location and the nanowire's deflection are precisely known and under strict control. We show the measurements carried out on intrinsic GaN and n-doped GaN-AlN-GaN nanowires by using our method. The measured electric potential, as high as 200 mV for n-doped GaN-AlN-GaN nanowire and 150 mV for intrinsic GaN nanowire, have been obtained, these values are higher than theoretical calculations. Our investigation method is exceptionally useful to thoroughly examine and completely understand the piezoelectric phenomena of nanostructures. Our experimental observations intuitively reveal the great potential of piezoelectric nanostructures for converting mechanical energy into electricity. The piezoelectric properties of nanostructures, which are demonstrated in detail in this paper, represent a promising approach to fabricating cost-effective nano-generators and highly sensitive self-powered NEMS sensors.

Nanometer scale spectral imaging of quantum emitters in nanowires and its correlation to their atomically resolved structure.

We report the spectral imaging in the UV to visible range with nanometer scale resolution of closely packed GaN/AlN quantum disks in individual nanowires using an improved custom-made cathodoluminescence system. We demonstrate the possibility to measure full spectral features of individual quantum emitters as small as 1 nm and separated from each other by only a few nanometers and the ability to correlate their optical properties to their size, measured with atomic resolution. The direct correlation between the quantum disk size and emission wavelength provides evidence of the quantum confined Stark effect leading to an emission below the bulk GaN band gap for disks thicker than 2.6 nm. With the help of simulations, we show that the internal electric field in the studied quantum disks is smaller than what is expected in the quantum well case. We show evidence of a clear dispersion of the emission wavelengths of different quantum disks of identical size but different positions along the wire. This dispersion is systematically correlated to a change of the diameter of the AlN shell coating the wire and is thus attributed to the related strain variations along the wire. The present work opens the way both to fundamental studies of quantum confinement in closely packed quantum emitters and to characterizations of optoelectronic devices presenting carrier localization on the nanometer scale.

Investigation of the electronic transport in GaN nanowires containing GaN/AlN quantum discs.

We report the investigation of electronic transport in GaN nanowires containing GaN/AlN quantum discs (QDiscs). The nanowires were grown by plasma-assisted molecular beam epitaxy and contacted by electron-beam lithography. Three nanowire samples containing QDiscs are analyzed and compared to a reference binary n-i-n GaN nanowire sample. The current-voltage measurements on single nanowires show that if the QDiscs are covered with a lateral GaN shell, the current mainly flows through the shell close to the lateral surface and the wire conductivity is extremely sensitive to the environmental conditions. On the contrary, if no GaN shell is present, the current flows through the QDisc region and a reproducible negative differential resistance related to electron tunneling through the QDiscs can be observed for temperatures up to 250 K. The demonstration of the resonant tunneling in GaN/AlN superlattices is of major importance for the development of nitride-based far-infrared quantum cascade lasers operating at high temperature.

Exciton and biexciton luminescence from single GaN/AlN quantum dots in nanowires.

We present a microphotoluminescence study of single GaN/AlN quantum dots embedded in single nanowires. At low excitation power, single exciton lines with full width at half-maximum as narrow as 1 meV are observed. The study of the excitation power dependence of the emission allows us to identify the biexciton transitions with binding energies ranging from 20 to 40 meV.