In situ TEM study of crystal-phase heterostructures in III-V nanowires
Starting from March 2022
Context of the project: Semiconductor nanowires (NWs) exhibit unique properties that make them potential building blocks for a variety of next generation devices such as biosensors, solar cells, transistors, quantum light sources and lasers. In order to take advantage of the physical properties of NWs, it is crucial to control their geometry, crystal structure and doping. This goal will ultimately be achieved by a deep understanding of the growth mechanisms. The most common growth technique is the vapor-liquid-solid (VLS) method, where a liquid metal droplet catalyzes the growth of a solid NW from gas phase precursors. In this growth mode, the droplet plays a fundamental role in determining the structure of the nanowire, and the remarkable range of structures
enabled by VLS can be thought of as the result of engineered changes to the droplet.
For example, growth of III-V semiconductor NWs using the VLS method can result in crystal structures different from their bulk phase . In GaAs NWs stable zincblende (ZB) phase coexists with metastable wurtzite (WZ) structure resulting in NWs having a mixed-phase structure. Remarkably, the valence and conduction bands of the two phases are misaligned so that small sections of one phase within the other effectively confine charge carriers.
Controlled switching between the two phases enables the synthesis of novel heterostructures, crystal-phase quantum dots (CPQD), with exceptional properties and potential applications in photonics [2,3] and quantum computing . In contrast to compositional heterojunctions, CPQDs have intrinsically abrupt interfaces and hence do not suffer from alloy intermixing at the interface, which hampers precise control of the electronic properties in compositional heterostructures.
Even though CPQDs were first discovered more than ten years ago, their technological application has been severely limited by the difficulty of needing precise control over their growth. In particular, the physics underlying the phase selection mechanism was poorly understood. Only recently, thanks to in situ transmission electron microscopy (TEM), we started to shed light on this mechanism. In situ TEM provides unparalleled imaging resolution and allows the capturing of the growth dynamics [5,6,7] and the effect of growth parameters in real- time. Using this technique, we demonstrated that the sole parameter determining the phase selection is thecontact angle between the droplet and the NW interface (Figures 1a and 1b) [8,9,10].
The aim of this project is to develop strategies for the control of the contact angle of GaAs NWs to achieve
unprecedented control over the crystal phase.
Master 2 internship: The successful applicant will actively participate in the in-situ experiments and will be in charge of the data analysis. The in situ experiments will be conducted using the MOCVD growth method in the NanoMAX TEM . The analysis will be carried out by developing dedicated image-processing algorithms able to extract relevant geometric parameters (i.e. droplet size, contact angle, crystal phase) from each image. The resulting data will be used to correlate the changes of contact angle to phase switching and develop a model to predict the crystal phase for different growth conditions.
This work could be extended to a Ph.D. funded by the ANR project ELEPHANT.
Candidate profile: Highly motivated candidates enrolled in a master's degree or equivalent, with a background in
physics, materials science or engineering. Prior knowledge in programming (possibly Python) would be highly
Application procedure: For additional information about the project and/or the recruitment process, please contact Federico PANCIERA (email@example.com). The candidate should include a CV.
1 Algra et al. Twinning superlattices in indium phosphide nanowires. Nature 2008, 456 (7220), 369-372. 2 Akopian et al. Crystal phase quantum dots. Nano Lett. 2010, 10 (4), 1198-1201. 3 Assali et al. Crystal phase quantum well emission with digital control. Nano Lett. 2017, 17 (10), 6062-6068. 4 Hastrup et al. All-optical charging and charge transport in quantum dots. Scientific Reports 2020, 10(1), 1-6. 5 Ross, F.M. Controlling Nanowire Structures through Real Time Growth Studies. Reports Prog. Phys. 2010, 73(11),
6 Panciera et al. Synthesis of nanostructures in nanowires using sequential catalyst reactions. Nature materials,
2015, 1498, 820-825. 7 Harmand et al. Atomic step flow on a nanofacet. Phys. Rev. Lett. 2018, 121, 166101. 8 Jacobsson, Panciera et al. Interface dynamics and crystal phase switching in GaAs nanowires. Nature, 2016,
531(7594), 317-322. 9 Panciera et al. Phase selection in self-chatalyzed GaAs nanowires. Nano Lett. 2020, 20(3), 1669-1675. 10 Panciera et al. Controlling nanowire growth through electric field-induced deformation of the catalyst droplet.
Nature communications, 2016, 7(1), 1-8.
Simulation of hot carriers by using ab-initio parameters for energy harvesting
Starting from October 2021
Nowadays, energy conversion devices are mainly designed by using macroscopic models (such as the drift-diffusion
and the heat Fourier’s formalisms) which assume local equilibrium and simplified material properties (energy dispersion, relaxation times...) that must be known a priori.
In the framework of future generations of energy converters that will most probably involve high energy carriers, out-of-equilibrium carrier distributions and complex band structures as well as strong quantum effects [1,2], these tools
reach their limit of accuracy .
Besides, these new routes of development will require to investigate a large panel of new materials  and nanostructures probably different from those which are well known and commonly used today in the microelectronics industry such as 2D materials (mono or
multilayers). In recent years, a spectacular progress in ab-initio DFT (density functional theory)-based description of the electronic and thermal transport has been achieved and this method can be used to investigate these recently studied materials.
The goal of this internship proposed in the COMputational electronICS group at the C2N is to developed transport
simulation of charge and heat by coupling existing simulation platform  with parameter calculated by using ab-initio methods. More precisely, in our home-made Monte Carlo simulator will be included the full band electron/phonon scattering rates, in particular electron/phonon coupling and impact ionization phenomenon (which is important for hot carriers in small gap materials like InAs) as well as interface transmission. Electrical thermal properties as well as the thermalization time of hot carriers in nanostructures
will be investigated and compared with the experiment results. The proposed modeling development will be carried out in close connection with the experimental activities performed by J. Chaste and his co-workers in the C2N.
This work is supported by the ANR project “Placho” and the Labex Nanosaclay via the MACACQU flagship. This
internship is expected to be continued in a Phd program.  D. Cakiroglu et al., Solar Energy Materials and Solar Cells 203, 110190 (2019).
 T. Abu Hamed et al., EPJ Photovolt. 9, 10 (2018).
 A. J. Nozik, Nature Energy 3, 170 (2018).
 I. Konovalov and V. Emelianov, Energy Sci Eng 5, 113 (2017).
 B. Davier et al., J. Phys.: Condens. Matter 30, 495902 (2018).