Internship

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  [1].  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  [4].  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 [9]. 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
appreciated.
 
Application  procedure:  For  additional  information  about  the  project  and/or  the  recruitment  process,  please contact Federico PANCIERA (federico.panciera@c2n.upsaclay.fr). The candidate should include a CV.

References
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),
114501.
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.

Internship

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 [3].
Besides, these new routes of development will require to investigate a large panel of new materials [4] 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 [5] 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. [1] D. Cakiroglu et al., Solar Energy Materials and Solar Cells 203, 110190 (2019).
[2] T. Abu Hamed et al., EPJ Photovolt. 9, 10 (2018).
[3] A. J. Nozik, Nature Energy 3, 170 (2018).
[4] I. Konovalov and V. Emelianov, Energy Sci Eng 5, 113 (2017).
[5] B. Davier et al., J. Phys.: Condens. Matter 30, 495902 (2018).

followed PhD