Karine Hestroffer

The structural properties of GaN/AlN core–shell nanocolumn heterostructures

The growth and structural properties of GaN/AlN core-shell nanowire heterostructures have been studied using a combination of resonant x-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy experiments. For a GaN core of 20 nm diameter on average surrounded by a homogeneous AlN shell, the built-in strain in GaN is found to agree with theoretical calculations performed using a valence force field model. It is then concluded that for an AlN thickness up to at least 12 nm both core and shell are in elastic equilibrium. However, in the case of an inhomogeneous growth of the AlN shell caused by the presence of steps on the sides of the GaN core, plastic relaxation is found to occur. Consistent with the presence of dislocations at the GaN/AlN interface, it is proposed that this plastic relaxation, especially efficient for AlN shell thickness above 3 nm, is promoted by the shear strain induced by the AlN inhomogeneity.

In situ study of self-assembled GaN nanowires nucleation on Si(111) by plasma-assisted molecular beam epitaxy

Nucleation of GaN nanowires grown by plasma-assisted molecular beam epitaxy is studied through a combination of two in situ tools: grazing incidence x-ray diffraction and reflection high energy electron diffraction. Growth on bare Si(111) and on AlN/Si(111) is compared. A significantly larger delay at nucleation is observed for nanowires grown on bare Si(111). The difference in the nucleation delay is correlated to a dissimilarity of chemical reactivity between Al and Ga with nitrided Si(111).

Polarity in GaN and ZnO: Theory, measurement, growth, and devices

The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade.

Ultraviolet Raman spectroscopy of GaN/AlN core-shell nanowires: Core, shell, and interface modes

The vibrational properties of GaN/AlN core-shell nanowire heterostructures with shell thicknesses ranging from 0.5 to 12 nm have been investigated by means of Raman spectroscopy with ultraviolet and visible excitation. The selection of an excitation line close to the GaN band gap allows the study of the strain state of the GaN core, while excitation at higher energies activates modes localized at the core-shell interface and the AlN shell, which are as well strain dependent. The results are compared with calculations of the vibrational modes based on the dielectric continuum model, including strain through the continuum elasticity theory.

Strain state of GaN nanodisks in AlN nanowires studied by medium energy ion spectroscopy

Medium energy ion spectroscopy experiments have been performed on an ensemble of nanowires deposited by molecular beam epitaxy on Si(111), taking advantage of their reduced in-plane mosaicity. In particular, the strain in nanometric GaN insertions embedded in AlN sections deposited on top of GaN nanowires has been determined. The measured strain is consistent with atomistic valence force field calculations. This opens the way for the structural study of a new range of discontinuous nanowire-based nanostructures by medium energy ion spectroscopy and to the determination of the strain profile of nanodisks in nanowires at the monolayer scale.

A polarity-driven nanometric luminescence asymmetry in AlN/GaN heterostructures

Group III Nitrides nanowires are well suited materials for the design of light emitting devices. The internal electric field created by spontaneaous and piezoelectric polarizations in these materials poses some difficulties, but also possible solutions, towards this goal. Here, we report on the high spatial asymmetry of the cathodoluminescence intensity across a GaN quantum well embedded in an AlN nanowire, when a 60 keV, 1 nm wide electron beam is scanned over this heterostructure. This asymmetry is remarkable between positions at different sides of the quantum well. We interpret this asymmetry as originating from the different drift directions of carriers due to the internal electric field. This interpretation is corroborated by the direct determination of the polarity with convergent beam electron diffraction. A precise knowledge of hole mobility and diffusion coefficients would allow an estimate of the electric field in the AlN segment of the nanowire.

A geometrical model for the description of the AlN shell morphology in GaN-AlN core-shell nanowires

A geometrical model based on the one formulated by Foxon et al. [J. Cryst. Growth 311, 3423 (2009)] is developed to describe the morphology of AlN shells in GaN-AlN core-shell nanowires grown by plasma-assisted molecular beam epitaxy. The shell aspect ratio is studied as a function of the atomic beam flux incidence angles and of the ratio between Al and N species. The comparison between experimental data and the developed geometrical model suggests the diffusion of about 55% of Al atoms from the side walls to the top surface.

Measurement of the autocorrelation function of a cathodoluminescence signal : characteristics and applications in nanosecond time resolved and nanometer spatially resolved experiment

In this presentation we show that the autocorrelation function of the cathodoluminescence signal (CL-g⁽²⁾(τ)) can be different from the photoluminescence PL-g⁽²⁾(τ) showing a huge nanosecond bunching effect g⁽²⁾(0) >> 1, allowing to retrieve emitters lifetime at nanometer scale.