Stability and structure of freestanding IIIV nanorods: An ab initio investigation

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Stability and structure of free-standing III-V
nanorods An ab initio investigation
Roman Leitsmann and Friedhelm Bechstedt
Institut für Festkörpertheorie- und optik
Friedrich-Schiller-Universität Jena
Sponsored by
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Motivation

• Application

Eric Mankin mankin_at_usc.edu
Nanowire Memory Cells
In2O3 - Nanowire -gt possible data storage rates
of 40 Gbits/cm2 (USC / NASA Ames Research Center.)
National cancer Institute
Nanowire Sensors
NASA
Logical circuits
Nanowire properties are very sensible to certain
environmental conditions
3-D nanowire-based logic gate (inverter) -gt
push down CMOS gate length to 1 nm
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Motivation

• Growth

most growth-techniques is common
Experimental observations

• growth axis 111/0001
• hexagonal cross-section
• side facets 112/1100
• or 110/1120
• depending on growth conditions
• stacking faults
• rotational twin layers
• structural change zb -gt wz

• gaseous environment
• high temperatures

we expect

• surface passivation
• equilibrium rod-structure

GaAs
InP
Appl. Phys. Lett. 79, 3335 (2001)
Nanotech. 16, 2903 (2005)
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Aim of this study

• Theory

Aim of this study

• ab initio description of III-V nanorods
• investigate the zb -gt wz transition of III-V
  nanorods
• for different rod-structures
• for different environmental conditions
• extrapolation to thick (gt 5 nm) nanorods

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Modelling

• Rod types

Theoretical model

• free standing symmetric (C3v) III-V
  semiconductor nanorods
• no influence of the substrate
• total energy considerations
• no kinetic effects
• consider two different rod diameter
• infinitely thick rods are modelled by free
  surfaces

Structural rod models

• hexagonal cross-section
• III-V pair bond parallel to the rod axis in the
  centre of the rod
• consider two polytypes zincblende (zb) and
  wurtzite (wz)
• consider two types of side facets
• type I (112) zb or (1100) wz
• type II (110) zb or (1120) wz

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Modelling

• Type I

zb

zincblende
wurtzite
Pseudo-hydrogen
dangling bonds
wz
4/3
3/2
dangling bond densities
4v2/(a02 v3)
3v6/(2a02)
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Modelling

• Type II

zb

Pseudo-hydrogen
zincblende
wurtzite
wz
dangling bonds
1
1
dangling bond densities
2v2/a02
2v2/a02
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Numerical Details

• DFT-LDA ground state calculations (VASP 4.6.20)
• projector augmented wave (PAW) pseudopotentials
• 200 eV plane wave-cutoff
• preconditioned residuum-minimization method for
  the
• electronic relaxation
• conjugate-gradient-algorithm for the ionic
  relaxation

• Rod-Surface calculations
• vacuum region 2 nm (rod) and 1.5 nm (surf)
• supercell length 12 (rod) 6-8 (surf) atomic
  (double,triple)layers
• BZ-sampling Monkhorst-Pack k-points

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Results

• Rod-surface energy InAs

? Very fast convergence to free surface energies
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Results

• Extrapolation

Extrapolation to thick nanorods

• free surface energies
• dangling bond densities of the corresponding
  diameter
• difference between zb and wz (25 meV per pair)
  has to
• be taken into account

gt for very thick rods only zb structure is stable
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Results

• Phase-diagram

InAs 5 nm Nanorod
clean type II (zb,wz)
pass. type II (zb,wz)
pass. type I (wz)
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Thank you for your attention.

• Summary
• investigated III-V nanorods in wz and zb
  structure
• with ab initio methods
• achieved an extrapolation to thick nanorods
• investigated influence of nanorod-passivation,
• i.e. the influence of the environment
• could explain the occurrence of wz and zb
  nanorods

Sponsored by
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Results

• III-V Rods

other III-V semiconductors
Type I, pass
? same convergence behaviour
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Modelling

• Formation energies

General procedure

• rod-structure is ideal (bulk-like) - no
  relaxation !!
• passivation with pseudo-hydrogen relaxation of
  H-positions
• calculation of the rod-surface energies

Free rod-surface energy ?
? environmental influence
? diameter dependence
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Results

• Dangling bonds

• fast convergence
• different for type I
• equal for type II

Type I, wz
Type I, zb
Type II, wz,zb
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Results

• Rod structure

Structural properties
(clean rods)

• starting rod-structure is ideal (bulk-like)
• relaxation of c-lattice constant (for diag 2,
  free)
• ?c cideal - c0 0.036 nm 1.7 (zb)
• ?c 0.059 nm 2.8 (wz)
• relaxation of a-lattice constant at c0 (for diag
  2, free)
• ?a aideal - a0 -0.001 nm 0.06 (zb)
• ?a -0.001 nm 0.06 (wz)

gt negative deviation of c/a ratio