Irradiation Induced Doping of Topological Insulators – IRIDOTI

Irradiation experiments were carried out on two platforms: heavy ion beam at GANIL Caen and low temperature electron irradiation on SIRIUS platform at LSI, Ecole Polytechnique. Two runs, one with 900 MeV ions of U238 and Pb208 were performed at December 2013 and Mars 2014. In the first run we exposed several samples issued from the same crystal of Bi2Te3 to the different doses from 1e9 to 5e11 impacts/cm2. In the second run, we focalized on the doses granting the sample close to neutrality point. Low temperature electron irradiation was performed in April 2014. Samples of Bi2Te3 exposed to the doses exceeding that of conductivity type conversion were produced and used in further experiments, at LSI and CUNY (USA). Basic experiments used to characterize effect of irradiation were magneto transport measurements. At LSI in the temperature range from 4.2°K and in fields up to 3T, at CUNY from 1.3°K and up to 14T. In addition CUNY platform allows rotation of the sample and separation bulk from surface contributions in the conductivity.
Annealing experiments were performed at LSI, in-situ after electron irradiation at 20K and ex-situ above room temperature.
Growth of the nanowires of topological insulators by electrodeposition in porous membrane (Al2O3) was performed at both CUNY and LSI sites. Transmission electron microscopy (TEM) imaging of nanowires (after dissolution of membrane) was performed at LSI.
ARPES spectroscopy was realized by LPS team on Elletra synchrotron facility (Trieste) on samples prepared and characterized by LSI team.

Determination of the doping effect of electron and heavy ion irradiation induced defects was the first achievement of project. Precise estimate of the rates of carrier concentration increment per irradiation dose allowed determination of doses requested for conductivity type inversion. This was a first step to define protocol how to reach insulating sample by two-step procedure: irradiation followed by annealing. The illustration of efficiency of such method is presented on the graph below. The second parameter to be defined was migration energy of defects involved in annealing. This was done for above room temperature stage of Bi2Te3, and energy 0.78 eV what corresponds to that expected of vacancy migration. In the vicinity of the neutrality condition (maximum resistivity) conductivity by surface channels become relevant and characteristic linear magneto resistance was observed. Fit to the Hikami-Larkin-Nagaoka formula allows determination of phase coherence length and number of surface conduction channels.
Conditions of template growth of topological insulators in form of nanowires were established and samples of Bi2Te3 and Sb2Te3 of different diameters were synthetized. We discovered unique possibility to change type of material during growth and form nanometer size p-n junctions within single nanowire. Another hybrid structures, superconductor-topological insulators were grown by electro deposition. Those structures exhibit proximity induced superconducting state on TI side.
Major discovery of our consortium was singular spin response of Dirac fermions, (Nature Materials, 13, 580 (2014)). Spatial fluctuations of the local charge on the surface of TI are the direct consequence of this finding. This give a direction for novel ARPES experiment with focalized probing beam, realized by LPS partner of the project.

Depression of carrier concentration by controlled irradiation induced doping was proven as efficient method to reveal surface states mediated conduction in Bi2Te3. Samples available for in-depth experiments in low temperatures and high magnetic field as well as other experiments (ARPES, microwave conductivity and optics) were produced. Novel structures (p-n junctions superconductor-TI hybrids) were fabricated by electrodeposition method. Those achievements open new perspectives and generated broad interest in the community. Preliminary experiments on other categories of TI's (PbSxSe1-x) prove that the same scheme may be used in larger context.
In the next step we will enlarge our spectrum of investigated materials to Bi2Se3 doped with Ca and Mn. We expect higher migration energy of vacancies in this material and better stability of damage. Simulation of irradiation damage in Bi2Te3 let us suppose that lower electron energy induced damage (below 1.5MeV) may have inverse, acceptor like doping effect what will open another possibility to manipulate Fermi energy. PbSxSe1-x family is of particular interest because we can turn from trivial to topological insulator within the same compound by changing composition. This will allow us to nail specific properties of topologically protected states.

[1] «Compensation of intrinsic charge carriers in topological insulators using high energy electron beams«,
Lukas Zhao, Haiming Deng, Jeff Secor, Marcin Konczykowski, Andrzej Hruban, and Lia Krusin-Elbaum
Bulletin of the American Physical Society, Volume 59, Number 1, J41.00007 (March 2014).
[2] «Topological insulator nanowires and nanowire hetero-junctions«
Haiming Deng, Lukas Zhao, Travis Wade, Marcin Konczykowski, and Lia Krusin-Elbaum
Bulletin of the American Physical Society, Volume 59, Number 1, D41.00004 (March 2014).

[3] «Singular robust room-temperature spin response from topological Dirac fermions«
Lukas Zhao, Haiming Deng, Inna Korzhovska, Zhiyi Chen, Marcin Konczykowski, Andrzej Hruban, Vadim Oganesyan and Lia Krusin-Elbaum
Nature Materials, 13, 580 (2014)

In the last few years there has been an explosive development in material science. It began with the theoretical prediction of a new class of three dimensional (3D) topological insulators (TIs) which are fully gapped in the bulk, but with unusual gapless ‘protected’ 2D Dirac surface states. The protection arises from the linear energy-momentum dispersion, with the surface states near the Fermi surface residing on a single “massless” Dirac cone with locked spins. If realized, these systems could be the Holy Grail in the fields of spintronics and fault-tolerant quantum computing. However, access to this 2D quantum matter is a challenge, due to the difficulty of separating surface contribution from the non-zero conductivity of the bulk. In approaches taken thus far, such as nanostructured synthesis/growth, doping, compositional tuning, or band-gap engineering via device gating, complete suppression of the bulk conduction in TIs has not yet been realized.
We propose a new approach, which consists of using controlled disorder to create stable charged point defects in the bulk of topological insulators by particle irradiation in order to compensate for the “intrinsic” charged defects and to achieve a fully insulating bulk. Using swift (<3 MeV range) electron or proton beams we will create simple, vacancy and interstitial type defects that will enable us to (a) tune the bulk carrier density, thereby tuning the Fermi level across the Dirac point, and (b) to reduce bulk conductivity by creating Anderson localization. The first objective will give rise to charge compensation in a bulk TI, while the second will enable us to test recent theoretical predictions of Quantized Anomalous Hall Effect (QAHE). Identification of bulk and surface contribution in irradiation-doped samples will be obtained by combination of electronic transport in high magnetic field by Shubnikov-de Haas oscillations (SdH) and Angular Resolved Photoemision Spectroscopy (ARPES). Definition of the route for fabrication of TIs with suppressed bulk conductivity is the primary goal of the project.
The second goal is determination of the effect of disorder on the surface conducting states of TI’s. With the proper choice of irradiation dose, the bulk resistivity may be increased by many orders of magnitude when the chemical potential reaches the Dirac point. Using this technique we expect to further test a recent prediction of a Topological Anderson Insulator – a nontrivial quantum phase with quantized conductance obtained by introducing disorder in a metal with strong spin-orbit interaction.
The third goal of the project is fabrication by proton implantation of p-n-p or n-p-n structures below the surface of TI’s. Such structures, if realized and properly contacted, are the prototype of a tunable spintronic device that may open a path for novel applications.
This proposal is an international collaboration between two groups at Ecole Polytechnique in Palaiseau, and at Orsay University, France, with unique expertise in swift particle irradiation techniques and femtosecond ARPES with the consortium lead by condensed matter physics group of the City College of New York (CCNY) -CUNY. This collaboration combines the complementary technical strengths of material science with particle beam technology to control and tune key electronic properties of the newly discovered functional class of materials.