The public trial lecture presented by Mohammadreza Nematollahi on 8th of October 2014 at Norwegian University of Science and Technology. The theoretical models and the experimental progress of highly mismatched alloys, as well as their optoelectronic applications are covered.

1. Highly mismatched alloys for optoelectronics
Mohammadreza Nematollahi
Trial Lecture
Department of Physics, NTNU
October 08, 2014

2. Outline
• Introduction
• Properties of well-matched alloys
• Highly mismatched alloys (HMAs)
• Band anticrossing (BAC) theory
• Synthesis of HMAs
• Applications of HMAs
• Summary
2

3. 3
Optoelectronics
The study and application of electronic devices that source,
detect and control light.
(from wikipedia)
Semiconductor is the cornerstone of optoelectronic devices.
Conduction band
Valence band
Bandgap energy
in eV
2.6 eV 2.3 eV 1.9 eV
Introduction

4. Optoelectronics
4
Space solar panels*
GaInP2/GaAs/Ge multijunction
30% AM0
*http://www.spectrolab.com
3D laser and light detection systems
1.06 - 1.5 μm Infrared Laser
InGaAsP/InP InGaAs/InP
Light emitting diodes (LEDs)
Introduction

5. Band gap engineering
5
III-V and II-VI Compound Semiconductors
http://woodall.ece.ucdavis.edu
Structural methods:
• Size (quantum confinement)
• Shape: Quantum dot, rod, …
• Compressive or tensile stress.
By alloying
Introduction

6. 6
Highly mismatched alloys (HMAs)
Large change in electronic
structure
Eg. Large change in band gap
Electronegativity,
Size and/or ionization energy
Like As rich GaAs(N)
Alloying results in:
Band anticrossing theory
Introduction

7. HMAs can expand the reachable wavelengths
7
http://woodall.ece.ucdavis.edu
Available from ultraviolet to short wavelength infrared.
They can be utilized in various devices:
• Diodes and lasers
• Detectors
• Photoelectrochemical cells
• Heterostructures
• Solar cells
Introduction

8. Applications in photovoltaics
8
Multi-junction solar cell
Intermediate band solar cell
• Each cell efficiently converts photons from a
narrow range of solar irradiation.
• Search for materials with optimum bandgap
and lattice parameter.
Highly mismatched alloys can potentially be
used in heterostructures, specifically in multi-
junction cells.
• A single material with one IB  3 energy gaps.
Highly mismatched alloys can potentially be used
as intermediate band materials.
With solar cells that only have one bandgap, we lose more than 50 % of solar power!
Eg1
Eg2
Eg3
Eg1 > Eg2 > Eg3
Introduction

9. Well-matched alloys
9

10. Well-matched alloys
Virtual crystal approximation (VCA)
II III IV V VI
Electronegativity
10
Fig: Hyung Soon Im et. al. J. Phys. Chem. C 118 4546–4552 (2014)
1
( ) (1 )x xA B C AC BC
g g gE x xE x E
  Other examples:
(Ga, In) As, (Ga, In) P, …

11. Well-matched alloys
II III IV V VI
Hiroyuki Okuyama et al. Phys. Rev. B 57 2257 (1998)
1
( ) (1 )x xA B C AC BC
g g gE x xE x E
   (1 )bx x 
1
( ) (1 )x xA B C AC BC
a x xa x a
  
Electronegativity
Vegard rule
11


12. Highly mismatched alloys
12

13. Highly mismatched alloys
13
II III IV V VI
Electronegativity
Fig with slight modification: J Wu, W Shan and W Walukiewicz, Semicond. Sci. Technol. 17 860–869 (2002)
b 16 eV
VCA

14. Band anticrossing model (BAC)
• Localized impurities in real space  Delocalized in k space.
• Randomly distributed and low concentration impurities.
• Highly localized N atoms and the extended states of the host
semiconductor matrix interacts and results in:
Restructuring of the conduction band
14
J Wu, W Shan and W Walukiewicz, Semicond. Sci. Technol. 17 860–869 (2002)

15. Anomaly - HMAs
15
J Wu, W Shan and W Walukiewicz, Semicond. Sci. Technol. 17 860–869 (2002)
• Redshift of bandedge (E_).
• An additional feature above the
bandgap (E+), blue shift of E+ .

16. Anomaly - Hydrostatic pressure effect
• With small amount of N in GaAs or GaInAs, the bandgap shifts to the blue under
hydrostatic pressure at a much slower rate than expected.
16
W. Shan et al Phys. Rev. B 82 1221 (1999)
W. Walukiewicz et al, Ch3, Ed. Ayse Erol, Springer series in
materials science 105 (2008)

17. Hydrostatic pressure effect for GaPN
17
Interaction constant:
With  : V = 3.05 eV
With X : V = 0.90 eV
The interactions of the N-localized states with the  band edge of GaP
at low pressures and with the X band edge at high pressures.
J Wu, W Shan and W Walukiewicz, Semicond. Sci. Technol. 17 860–869 (2002)
EN

18. Electron mobility
• The electron mobilities of GaAs and Ga1−yInyAs are reduced by
1–2 orders of magnitude upon the incorporation of N.
18
W. Walukiewicz et al, Ch3, Ed. Ayse Erol, Springer series in materials science 105 (2008)
At high concentration: Electron
scattering is due to the homogeneous
broadening in the anticrossing band.
At low concentration: Electron
scattering is due to the potential
fluctuations that occur from the
structural and compositional disorder.
Ga0.93In0.07N0.017As0.983:Si measured at room temperature

19. Localized impurity energy level
Impurity Interacts with
conduction band
• Above the CB minimum
GaAsN, CdTeO, ZnSeO
• Below the CB minimum
GaAsPN, ZnTeO, ZnMnOTe.
Impurity interacts with
valence band
• Above the VB maximum
GaNAs, GaPBi, AlAsBi
• Below the VB maximum
GaAsBi, GaAsSb, AlAsSb,
19
Zn0:88Mn0:12OxTe1x
K. M. Yu et al., Phys. Rev.
Lett., 91 246203 (2003).
Conduction band
anticrossing
Valence band
anticrossing
K. Alberi et al Phys Rev B 75, 045203 (2007)

20. Valence band anticrossing
20
I. Vurgaftman, J. R. Meyer, and L. R. Ram-Mohan, J Appl Phys 89, 5815 (2001)
K. Alberi et al Phys Rev B 75, 045203 (2007)
HH: Heavy hole
LH: Light hole
SO: Split-off

21. BAC for GaAsSb (whole range)
21
K. Alberi et al Phys Rev B 75, 045203 (2007)
As a function of x

22. GaNAs (whole composition range)
22
R. Broesler et al. Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE

23. Band anticrossing theory
and other models
23

24. Localized impurities
 Isoelectronic impurities.
 Large difference in electronegativity or size  The impurities
act as deep centers with localized potentials.
 Localized in real space  They are comprised of Bloch
functions originating from many bands in a wide region of k-
space.
 Not sensitive to the positions of the CB and VB edges.
 No significant shift in energy with a change in composition or pressure
is expected.
24

25. Band anticrossing model (BAC)
• Randomly distributed and low concentration impurities (not
interacting with each other)
25
J Wu, W Shan and W Walukiewicz, Semicond. Sci. Technol. 17 860–869 (2002)
• Two interacting energy levels
associated with:
 Localized impurity (eg. N) state.
 Highly delocalized band state of
the host.

26. Band anticrossing model (BAC)
26
J Wu, W Shan and W Walukiewicz, Semicond. Sci. Technol. 17 860–869 (2002)
J. Wu W. Walukiewicz and E. E. Haller B 65 233210 (2002)
Clear gap in DOS between E+ and E_
• Randomly distributed and low concentration impurities (not
interacting with each other)

27. Other models
• BAC theory consider only isolated states.
 Remarkable fit with experimental data.
 Random substitution of impurity atoms onto the atomic sites of a host
crystal creates a statistical distribution of isolated impurities, impurity
pairs, triplets, etc.
• Empirical psudopotential calculations.
 Pairs and clusters are considered.
• First principle local density approximation : the bandgap is not
determined accurately.
• BAC based on the Anderson impurity Hamiltonian formulated
in empirical tight binding theory.
27
Titus Sandu and W. P. Kirk, Phys Rev B 72, 073204 (2005)
P. R. C. Kent and Alex Zunger, Phys Rev Lett 86 2613 (2001)

28. Synthesis of HMAs
28

29. Synthesis of HMAs
• Dilute nitrides (and HMAs in general):
 Concentrations far beyond the thermodynamically allowed solubility.
 Metastable compounds.
• Growth methods
 Ion implantation and post processing.
 Radio frequency plasma assisted molecular beam epitaxy (MBE)
 Pulsed laser deposition (PLD)
 Metalorganic vapor phase epitaxy (MOVPE).
29

30. Ion implantation and post processing
Ion implantation
 Injection of atoms into host.
 Post implantation annealing is required to control the
phase.
Post-implantation processing
 Furnace annealing (FA)
 Rapid thermal annealing (RTA) (10 – 100 s)
 Pulsed laser melting (PLM) (less than micro second)
• Kinetically limited growth
30

31. Ion implantation & pulsed-laser melting (II-PLM)
31
GaAs
N
GaAs
GaAs1-xNx
Method is used for a group of III-N-V & II-O-VI highly mismatched alloys
K.M. Yu et al, Ch1, Ed. Ayse Erol, Springer series in materials science 105 (2008)

32. Comparison of II-RTA and II-PLM
32
The amount of N incorporated in the As sublattice (“active” N) for
GaNxAs1−x layers is calculated using the band anticrossing model.
K.M. Yu et al, Ch1, Ed. Ayse Erol, Springer series in materials science 105 (2008)
RTA: Rapid thermal annealing
PLM: Pulsed laser melting

33. Applications of HMAs
33

34. Applications
• Fermi level in the gap  bandgap reduction
 Heterostructures, eg. multi-junction solar cells
 LEDs and short wavelength IR lasers
 IR detectors
 Photoelectrochemical cells
• Fermi level within the band
 Intermediate band solar cell (IBSC)
34
Valence band

35. Nitrides in photoelectrochemical cells
35
Taken with some changes from: Michael Grätzel Nature 414, 338-344(2001)
It generates hydrogen (a chemical fuel) by photo-cleavage of water.
2h+ + H2O  ½ O2 + 2H+
2e- + 2H+ -> H2 (g)
2hn + H2O  H2(g) + ½ O2 (g)
• There are material challenges.
• Stable in solution (corrosion)
• Semiconductor band edges
should match oxygen and
hydrogen redox potentials
under dark.
• ZnO1-xSex has been studied.
Photo-anode
Semiconductor
Cathode
Metal
H+/H2
O2/H2O
Marie Annette Mayer PhD thesis “Band
structure engineering for solar energy
applications: ZnO1-xSex films and devices”
University of California, Berkeley

36. Amorphous GaNAs for multi-juction solar cells
36
R. Broesler et al. Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE & K. M. Yu. et al. Phys. Status Solidi C 8, No. 7–8, 2503–2505 (2011)
• For all range of solar spectrum.
• No lattice matching is needed.
• On cheep substrates.
• In1-xGaxN is also suggested for
multi-junction solar cells.

37. IBSC (Zn0.88Mn0.12OxTe1-x)
37
K. M. Yu, W.Walukiewicz, J.Wu, W. Shan, and coworkers
Phys Rev Lett 91 246403 (2003)
O ion implantation followed by PLM
GaN0.02As0.58P0.4
K. M. Yu et al. Appl Phys Lett. 88, 092110 (2006)

38. IBSC (ZnTe:O)
38
Grown by molecular beam epitaxy, and a rf plasma source for oxygen and
nitrogen incorporation.
Wang, Lin, and Phillips Appl. Phys. Lett. 95, 011103 (2009)

39. IBSC (GaN0.02As0.98 )
39
N. Lopez, L. A. Reichertz, K. M. Yu, K. Campman, and W. Walukiewicz, Phys. Rev. Lett. 106, 028701 (2011)
• Devices made by metalorganic chemical
vapor deposition (MOCVD)
• The n-type doping resulted in partial
occupation of the IB.
Blocked intermediate band (BIB)
(Isolated IB)
Unblocked intermediate band (UIB)
(As reference cell)
EG 2.0 eV
EH 1.1 eV

40. IBSC (GaN0.02As0.98 )
40
CB
VB
IB
2.0 eV
1.1 eV
N. Lopez, L. A. Reichertz, K. M. Yu, K. Campman, and W. Walukiewicz, Phys. Rev. Lett. 106, 028701 (2011)

41. Summary
• Highly mismatched alloys:
 Elements with very different electronegativity or size
 Unusual physical properties.
• Band anticrossing model can be used to quantitatively explain
concentration, pressure, and other properties of HMAs.
• Synthesis of HMAs can be difficult.
 Non-equilibrium and kinetic limited methods.
• HMAs can be applied in:
 Intermediate band solar cells.
 Heterostructures.
 Photoelectrochemical cells.
 Optoelectronic devices working in short wavelength infrared range.
41

42. 42
Thank You for your attention.