Structural and Electronic properties of PbTerocksaltCdTezinc blende interfaces

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Structural and Electronic properties of
PbTe(rocksalt)/CdTe(zinc blende) interfaces
Roman Leitsmann, F. Bechstedt
Institut für Festkörpertheorie und -optik
Friedrich-Schiller-Universität Jena
H. Groiss, F. Schäffler, W. Heiss,
Institut für Halbleiter- und Festkörperphysik
Johannes Kepler Universität Linz
K. Koike, H. Harada, and M. Yano
Osaka Institute of Technology
Sponsored by
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Motivation

• Experiment

• Quantum-Dots are formed by an annealing process
  
• W. Heiss et al. Appl. Phys. Lett. 88, 192109(
  2006)
• they exhibit (110), (100), and (111) facets
• they show intense mid-infrared luminescence
• high potential for future applications like
• mid-infrared quantumdot-laser
• devices in medical diagnostics
• mid-infrared spectroscopy

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Challenge

• Theory

• computational demanding
• shallow Cd d-states
• relativistic effects
• exhibit a different lattice structure (PbTe-rs
  CdTe-zb)
• new kind of interfaces crystal structure
  mismatch
• PbTe and CdTe are nearly ionic crystals
• artificial electric fields are induced in the
  slab-approx.

PbTe(rocksalt)
CdTe(zincblende)
plane av.potential
position along 100
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Band offset

• Alignment of electrostatic potentials

Two step procedure
PbTe
CdTe
?VBM
VBM(CdTe)
VBM(PbTe)
?(PbTe-CdTe)

• bulk calculation of VBM w.r.t. the
  plane-averaged potential
• interface calculation of ?(PbTe-CdTe) ? ?VBM

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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
• residuum-minimization method for the electronic
  relaxation
• conjugate-gradient-algorithm for the ionic
  relaxation

• Interface calculations
• slabs with 28 or 24 double layers

100
111
110
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Results

• Bulk PbTe

Well-known problems with PbTe
Experimental-gap 0.19 eV (L-point) P. Dziawa
et al. phys.stat.sol. 2,1167(2005)

• too large LDA-gap 0.58 eV
• negative LDASO-gap -0.12 eV
• reasonable HSESO-gap 0.15 eV

LDA LDASO HSESO
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Results

• Bulk PbTe

Well-known problems with PbTe
Experimental-gap 0.19 eV (L-point) P. Dziawa
et al. phys.stat.sol. 2,1167(2005)

• too large LDA-gap 0.58 eV
• negative LDASO-gap -0.12 eV
• reasonable HSESO-gap 0.15 eV

LDA LDASO HSESO
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Results

• Bulk PbTe

Well-known problems with PbTe
Experimental-gap 0.19 eV (L-point) P. Dziawa
et al. phys.stat.sol. 2,1167(2005)

• too large LDA-gap 0.58 eV
• negative LDASO-gap -0.12 eV
• reasonable HSESO-gap 0.15 eV
• ? but HSE is not suitable for interface
  calculations !

LDA LDASO HSESO
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Results

• Bulk PbTe

Well-known problems with PbTe
Experimental-gap 0.19 eV (L-point) P. Dziawa
et al. phys.stat.sol. 2,1167(2005)

• too large LDA-gap 0.58 eV
• negative LDASO-gap -0.12 eV
• reasonable HSESO-gap 0.15 eV
• ? but HSE is not suitable for interface
  calculations !

? qualitative HSE and LDA differ only near the
L-point
LDA LDASO HSESO
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Results

• Structural properties

Structural properties
(110) PbTe/CdTe interface
(100) PbTe/CdTe interface
Leitsmann et al. PRB 74, 085309 (2006), New J.
Phys. submitted (2006)
Te-terminated
Cd-terminated
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Results

• Structural properties, (110) interface

green- 110 direction red - 001 direction
experimental data
lateral offset
Leitsmann et al. PRB 74, 085309 (2006), New J.
Phys. submitted (2006)
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Results

• Band offset

Type- I heterostructure
LDA
LDASO
? results are used to correctly align the
interface bandstructures
Experimental-gap PbTe 0.19 eV L-point
CdTe 1.6 eV ?-point P. Dziawa et al.
phys.stat.sol. 2,1167(2005)
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Results

• PbTe/CdTe (110)

PbTe/CdTe(110) interface LDA-bandstructure
PbTe/CdTe(110) interface relativistic-bandstructur
e
? practically no indications for interface states
in the fundamental gap region
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Results PbTe-dot

• Wulff construction

ECS of a PbTe-quantum dot in CdTe-matrix
PbTe/CdTe interface energies
Green - 110-facets (0.20 J/m2) Red -
100-facets (0.23 J/m2) Blue - 111-facets
(0.18 J/m2) uncertainty of 0.04 J/m2
Wulff construction
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Results PbTe-dot
Wulff construction
Leitsmann et al. PRB 74, 085309 (2006), New J.
Phys. submitted (2006)
gt good agreement with experimental observations
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Summary

• compensate artificial boundary effects at polar
  interfaces
• interface geometries ? excellent agreement with
  experiment (e.g. lateral offset, interface
  separation)
• interface energies ? equilibrium shape of PbTe
  quantum dots
• band offsets of PbTe/CdTe interfaces ? type I
  heterostructure
• interface bandstructure of PbTe/CdTe(110) ?
  almost no interface states inside the fundamental
  gap

Outlook

• description of free-standing or embedded (in
  CdTe-matrix)
• PbTe-dots
• electronic properties of quantum-dot systems
  (dependent on size, shape, etc.)

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Thank you for your attention.
Sponsored by
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Additional information

• Dipole correction

We compensate the artificial dipole potential ?
with a ramp-shaped potential
plane averaged potential
Calculate interface energies by total energy
differences
Energy corrections due to artificial dipole
potential ?
position along 100