Topological Insulator Thin Films: Growth and Characterisation

John Sandford O’Neill

Overview

• Introduction– Topological Insulators (TIs)– UHV Experimental Techniques– TI Thin Films on I07

• Method and Results– Bi - Te - Cu(110) – Sn - InSb(111)A

• Summary and Future Work

Topological Insulators• Recently-discovered

insulating materials with conductive surface states

• Surface states are topologically protected – they cannot be destroyed by defects

• Spin-dependent current flow– Spintronics– Quantum Computing

Topological Insulators• Topologically

protected surface states require strong spin-orbit coupling– Heavy metals– HgTe, Bi2Te3,

Bi2Se3, Sb2Te3, stanene - Sn

Ultra-High Vacuum (UHV) Experimental Techniques

• Film Growth:– Molecular Beam Epitaxy (MBE)

• Characterisation:– Low-Energy Electron Diffraction (LEED)– Auger Electron Spectroscopy (AES)– Scanning Tunnelling Microscopy (STM)– X-Ray Photoelectron Spectroscopy (XPS)– X-Ray Reflectivity (XRR)

Molecular Beam Epitaxy (MBE)• Used to deposit

epitaxial thin films• Knudsen effusion cells

used to sublimate solid materials

• Requires ultra-high vacuum (<10-9mbar)

Low-Energy Electron Diffraction (LEED)

Si(111) 7x7

• Principal technique for surface structure determination

• Low energy electrons used to achieve surface specificity

• Θ 1/a∝• Requires comparison

of LEED pattern before and after film growth

Auger Electron Spectroscopy (AES)

• The Auger electron kinetic energy spectrum is dependent on energy levels in the atom →elemental identification

Scanning Tunnelling Microscopy (STM)

• Images the surface topography to sub-nanometer resolution

• Provides information about growth mode of thin films

• Individual atoms can be imaged

X-Ray Photoelectron Spectroscopy

X-Ray Reflectivity (XRR)

• Specular reflection: angle of incidence = angle of reflection

• Provides information about the layered structure

X-Rays

Detector

TI Thin Films on I07

• Structural studies of topological insulators

• Correlate with electronic structure experiments

BaF2(111)

Bi2Te3

Bi-bilayer

Te

1 2 3 4 5 6

1000

10000

100000

1000000

inte

nsity

(a.u

.)

L

BaF2 + Bi2Te3 + Bi-bilayer + Bi-bilayerBaF2 + Bi2Te3 + Bi-bilayer BaF2 + Bi2Te3 BaF2

TI Thin Films on I07

STM

LEED/Auger

X-Ray Source

Knudsen cells

UHV Chamber EH2, I07

Bi - Te - Cu(110) • Surface cleaning:

– Ar+ ion sputtering 1.5 keV 20 mins

– Annealing to 550oC 20 mins

• Auger spectroscopy– Difficult to completely

remove Oxygen and Carbon contaminants from the surface

• Te growth– Did not stick to the substrate

• Bi growth– Very slow

Cu(110)

TeBi-bilayer

CarbonOxygen

α-Sn - InSb(111)A• Surface cleaning

– Ar+ ion sputtering 0.5 keV 20 mins

– Annealing to 400oC 1 hour

• LEED– 3x3 pattern?

• Auger– Analyser not sensitive at

higher energies required to detect In and Sb peaks

InSb(111)A

α-Sn

InSb(111)A LEED: 35eV

α-Sn - InSb(111)ASTM images:

“clean” InSb(111)A

α-Sn - InSb(111)ASTM images:

“clean” InSb(111)A

α-Sn - InSb(111)A

Carbon

Mo

Unknown

Mo

In

Sb

α-Sn - InSb(111)A

Mo

Mo

InSb

Carbon

InSb

Sb In

α-Sn - InSb(111)A

Mo

Mo

InSb

Carbon

InSb

Sb In

α-Sn - InSb(111)A• Tin growth:

– Sn Knudsen cell temp: 880oC – InSb temp: 200oC – XPS data taken at 10 minute intervals

InSb(111)A

α-Sn

α-Sn - InSb(111)A

In

Sb

Sn

α-Sn - InSb(111)A

In

Sb

Sn

α-Sn - InSb(111)A

Mo

Mo

In

Sb

Carbon

InSb

Sb In

Sn

SnSn

α-Sn - InSb(111)A

After 70 minute Sn deposition After deposition and annealing to 300oC

Summary

Future Work

• Improve Bi growth– Evaporator faulty?– Alignment in chamber?

• Repeat Sn growth on InSb(111)A– Find conditions for ordered monolayer of α-Sn– Try different substrate temperatures– Try other substrates

• InSb(111)B• InSb(100)

Acknowledgements

• Pilar Ferrer-Escorihuela• Matt Forster• Chris Nicklin• Jonathan Rawle• Adam Warne• Tom Arnold

Any Questions?