The structure of ultra thin rare earth silicides on silicon 100 111
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The structure of ultra-thin rare-earth silicides on silicon (100) & (111). Steve Tear, Department of Physics, University of York, York, UK. Introduction.

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The structure of ultra thin rare earth silicides on silicon 100 111

The structure of ultra-thin rare-earth silicides on silicon (100) & (111)

Steve Tear, Department of Physics, University of York, York, UK.


Introduction
Introduction

  • Interests are in surface and interface structure and, more recently, the correlation of the structure with electronic, magnetic and transport properties.

  • Detailed knowledge of the structure of surfaces, interfaces and nanostructures is a key component in the quantitative understanding of their chemical, electronic and magnetic properties.

  • Interfaces are one of the central features of spintronic devices and quality is important to their ultimate performance.

  • One of the key interfaces for hybrid spintronic devices is the ferromagnetic-semiconductor interface for spin injection.


Introduction1
Introduction

Rare earth silicides on silicon (100) & (111)

Results of structure analyses of ultra-thin films of silicides

Cooling during growth process may have beneficial effect for outcome of final structure: Mn on 2D RE silicide and Si on 2D RE silicide.


Techniques
Techniques

  • Experimental equipment at York (UHV):-

    • Low-energy electron diffraction (LEED)

    • Omicron scanning tunnelling microscope (STM)

  • STFC Daresbury:-

    • Medium-energy ion scattering (MEIS).

  • York JEOL Nanocentre:-

    • JEOL 2200FS double aberration-corrected scanning transmission electron microscope (AC-STEM)


Rare earth silicides
Rare earth silicides

Rare earth (lanthanides) metals

  • Divalent or trivalent: 4fn(5d6s)2 or 4fn(5d6s)3

  • Gd, Tb, Dy, Ho, Er, Tm, trivalent; Y

  • Sm trivalent /divalent.

  • Eu, Yb, Nd, divalent.

  • Majority of trivalent rare earths have hcp structure.

    Rare earth silicides

  • Disilicides (RESi2–X , x~0.3): hexagonal, tetragonal, or ortho.

  • On Si(111): hex. RE silicides(0001) lattice match (~few %)*

  • On Si(100): silicide nanowires are formed 10 nm x ~1 m, hexagonal, tetragonal, or orthorhombic?

* J Knapp and S Picraux. Appl. Phys. Lett. 48, 466(1986)


Meis principles of structure analysis
MEIS: principles of structure analysis

100keV H+

Ho

  • Double alignment:-

  • Shadowing — align incident ion beam to gain surface sensitivity

Si

Counts

Scattered ion energy

  • Blocking — detect angle-dependent scattered ion yield;

  • Dips in yield correspond to positions of surface atoms

Scattering angle


Meis si 111 1x1 re

<100>

Yield

4

5

5

0

5

5

6

0

6

5

S

c

a

t

t

e

r

i

n

g

a

n

g

l

e

/

d

e

g

r

e

e

s

MEIS: Si(111)(1x1)-RE

100keV H+

2D silicides: Si(111)(1x1)-RE

(RE = Y, Gd, Dy, Ho, Er, Tm)

Ho

1 ML of Ho deposited onto Si(111) held at 550°C for 10 mins.

Si

Scattered ion energy

Counts

Scattering angle

Wood T et al. Phys. Rev. B 72, 165407 (2005).


Meis si 111 1x1 re1

Si(111)1×1-Gd

Si(111)1×1-Y

Si(111)1×1-Er

Si(111)1×1-Dy

Si(111)1×1-Tm

Si(111)1×1-Ho

MEIS: Si(111)(1x1)-RE

Wood T et al. Phys. Rev. B 72, 165407 (2005).


Meis si 111 1x1 re2
MEIS: Si(111)(1x1)-RE

Bulk RE-Si2 bond length:-

3.060 Å (Gd) – 2.979 Å (Tm)

Structural parameters obtained from MEIS for the 2D RE silicides on Si(111). Errors have been obtained via the 2R-factor.

Wood T et al. Phys. Rev. B 72, 165407 (2005).


Strain in the 2D silicides

  • Compare effective c-axis strain with the a-axis strain in a bulk RE silicide distorted to fit Si(111).

  • Plotted as a function of lattice mismatch

  • The a-axis strain which results from the lattice mismatch is compensated for by strain in the c-axis which acts to maintain constant density in the surface silicide layer

Tm Er Ho DyY Gd


Growth on 2d re silicide
Growth on 2D RE silicide

Motivation

2D layer offers passivated silicon terminated (1x1) surface, stable up to ~600°C.

Template for further growth on a ideal silicon surface.

Bury delta-layer of rare earth in silicon?

Modification of metal-semiconductor interface – spintronics?

Experimental

Create 2D silicide: 1 ML of Ho onto Si(111) held at 550°C

Cool to ~ minus 100°C.

1. Si:- Deposit 6 ML of silicon. Post anneal at 400°C for 10 mins.

2. Mn:- Deposit 3 ML of manganese. Post anneal at 300°C for 1 min.


Si ho si 111
Si/Ho/Si(111)

A-type orientation of silicon overlayers on 2D Re-silicide.



Silicon ho si 111
Silicon / Ho / Si(111)

<110> <100>

Experimental MEIS data compared with simulations for a double bilayer of a-type Si above a 2D Ho layer.


Si ho si 1112
Si / Ho / Si(111)

Include 7x7 surface termination of added silicon layers in simulation

<110> <100>

Experimental MEIS data compared with simulations for a double bilayer of a-type Si above a 2D Ho layer and

added silicon surface terminated by 7x7.


STM: Mn on 2D RE silicide

50 nm x 50 nm

2 ML Mn on RT silicide

2ML Mn on cooled 2D RE silicide + 1 min anneal at 250C

4 ML Mn on cooled 2D RE silicide

+ 1 min 250°C anneal

300 nm x 300 nm

20 nm x 20 nm

M Reakes et al J Phys Cond. Matter, 21 265001 (2009)


Meis energy spectra
MEIS: energy spectra

Experimental spectra compared to simulations (smooth line):

Mn layer above 2D HoSi on Si

Mn layer below Si and above Ho

Mn layer above Ho on Si


Stm re silicide nanowires on si 100
STM: RE silicide nanowires on Si(100)

Sub-ML coverages of Ho deposited onto clean Si(001) substrates at between 500 – 600°C with no post anneal

c(2x2)

0.25 ML Ho , 700 x 700 nm2

0.3 ML Ho , 100 x 100 nm2

The NWs have widths in the range 1.5 – 5 nm and lengths ~300 nm or more

Strong reordering of step edges as Si atoms are donated to forming silicide

Tetragonal or Hexagonal NW silicide?

D. R. Bowler, J. Phys.: Cond. Matter 16, R721 (2004)

Chen et al., Appl. Phys. Lett. 76, 4004 (2000)


Meis data 6ml holmium on si 100 at 180 c 650 c vs 650 c
MEIS data6ML Holmium on Si(100) at -180°C/650°C vs 650°C

<110> geometry

<110> geometry

Cooled growth

+ 650°C anneal

650°C growth exptvshexagonalsimulation

650°C growth exptvstetragonal simulation (c-axis = 13.12 Å).


Summary
Summary

  • MEIS has enabled precise determination of strain in 2D RE silicides

  • Using MEIS has identified the hexagonal silicide grown on cooled Si(100) but tetragonal silicide when grown @ 650°C – implications for electronic properties of interface.

  • MEIS has provided some detailed information on 2D RE silicide as template for Mn and Si overlayer growth on cooled surface –buried delta-RE layer and modification of interface structure


Future work
Future work

  • MEIS: structure of initial interface formation: dependence

  • on initial surface structure, for example.

  • non-destructive c.f. AC-STEM

  • Structure of nanowires on surfaces

  • Full 2D energy loss & scattering angle analysis

  • SBH of metal – silicon vs metal – 2D RE silicide – silicide

  • Calculation of interface structures using CASTEP

  • Calculation of SBH using modified CASTEP


Acknowledgements
Acknowledgements

Dr Matt Probert (CASTEP)

Dr Phil Hasnip

Dr Paul Bailey (Daresbury)

Dr Tim Noakes (Daresbury)

Tim Wood

Ed Perkins

Mike Reakes

Charles Woffinden

Andrew Vick

Jeremy Mitchell

Dr Chris Eames

Dr Chris Bonet

York-JEOL Nanocentre



Haadf ac stem er silicide nw on si 100
HAADF AC-STEM: Er silicide NW on Si(100)

Hex

[01-10]

[0001]

[001]

1nm

[1-10]


Ac stem er silicide nw on si 100
AC-STEM: Er silicide NW on Si(100)

1nm

Tet

[001]

[100]

[001]

[1-10]


Interfaces overview
Interfaces Overview

Hexagonal

Tetragonal

Hexagonal nanowires

have stepped interfaces

Tetragonal nanowirestend to have regular defects along the interface



I/V between two islands

Initial fit for curve

Schottky barrier height=0.48eV



Conduction along Nanowire

  • Resistance of 54.5 kΩ


Sb nw to silicon substrate
SB: NW to silicon substrate

Initial fit for curve

Schottky barrier height=0.53eV

NW sample

transferred in vac

suitcase to reduce

oxidation


Schottky barrier

Vacuum

Schottky

Barrier ht

C.B.

Fermi

level

V.B.

Metal

n-type semiconductor

Schottky Barrier

Electrical properties of junction: Ohmic or Schottky barrier

I

0

V

metal +ve w.r.t.

semiconductor


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