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Highly mismatched alloys for optoelectronics

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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.

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Highly mismatched alloys for optoelectronics

  1. 1. Highly mismatched alloys for optoelectronics Mohammadreza Nematollahi Trial Lecture Department of Physics, NTNU October 08, 2014
  2. 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. 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. 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. 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. 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. 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. 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. 9. Well-matched alloys 9
  10. 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. 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. 12. Highly mismatched alloys 12
  13. 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. 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. 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. 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. 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. 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. 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. 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. 21. BAC for GaAsSb (whole range) 21 K. Alberi et al Phys Rev B 75, 045203 (2007) As a function of x
  22. 22. GaNAs (whole composition range) 22 R. Broesler et al. Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE
  23. 23. Band anticrossing theory and other models 23
  24. 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. 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. 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. 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. 28. Synthesis of HMAs 28
  29. 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. 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. 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. 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. 33. Applications of HMAs 33
  34. 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. 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. 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. 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. 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. 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. 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. 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. 42 Thank You for your attention.

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