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Light absorption enhancement in extremely confined Ge nanostructures

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Invited talk at EMRS Spring conference, Lille, France.
How and to which extent quantum confinement affects the light absorption bandgap and efficiency in germanium quantum dots embedded in silica.

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Light absorption enhancement in extremely confined Ge nanostructures

  1. 1. Light absorption enhancement in extremely confined Ge nanostructures S. Mirabella R.Raciti,S.Cosentino,E.Barbagiovanni, M.Miritello,A.Terrasi A.M.Mio,G.Nicotra,C.Spinella R.Bahariqushchi,A.Aydinli
  2. 2. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Cullis et al. - J. Appl. Phys., 82, 909 (1997) Park et al., Phys. Rev. Lett. 86, 1355 (2001) E. Barbagiovanni et al. - J. Appl. Phys., 111, 034307 (2012) 5 10 15 0.5 1.0 1.5 2.0 2.5 3.0 Energygap[eV] Size [nm] Si QD in SiO2 Si QWin SiO2 Ge QD in SiO2 The quantum chance F. Priolo et al., Nature Nanotechnology 9, 19 (2014) ( ) 2* 22 2 )( Lm bulkENSE gg π += Si SiO2
  3. 3. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Ge versus Si QDs 1 2 3 4 5 103 104 105 106 Absorptioncoefficient[cm-1 ] Energy [eV] Silicon Germanium Si QDs Ge QDs • Higher absorption coefficient • Larger size range for QCE • Bandgap tuning within solar spectrum
  4. 4. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Quantum effect on Eg E. Barbagiovanni et al. J. Appl. Phys., 111, 034307 (2012) ( ) 2* 22 2 )( Lm bulkENSE gg π +=Quantum confinement effect: does only size matter ? Optical bandgap depends on several factors: how to model the QCE on Eg ? other effects on absorption ? 0 200 400 600 800 1000 1200 2,1 2,4 2,7 3,0 MSsamples: 43-S 46-S PECVDsamples: 43-C 46-C 46-SL EOPT g [eV] Temperature [°C] Si QDs S. Mirabella et al. J. Appl. Phys., 106, 103505 (2009) Si QDs - calculation P. Hapala et al., Phys. Rev. B, 87, 195420 (2013)
  5. 5. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Quantum effect on absorption SiGe/Ge/SiGe QW - calculation Kuo and Li,, Phys. Rev. B, 79, 245328 (2009) herr −>herr −<herr −<< Absorption efficiency depends on oscillator strength (Os) but … no clear experimental evidence! Quantum confinement excitonic effect on absorption efficiency ? ( ) ( )∫ ∀ −−⋅⋅= k BZ vcs EE dk O nc e ωδ πωρµ π ωσ 3 2 2 0 22 )2( 24
  6. 6. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 OUTLINE Light absorption in Ge quantum dots • Extraction of optical properties (Eg and BTauc) • Interface effects on Eg • confining potential • Absorptionenhancement • … towards extreme confinement • Conclusions
  7. 7. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 OUTLINE Light absorption in Ge quantum dots • Extraction of optical properties (Eg and BTauc) • Interface effects on Eg • confining potential • Absorption enhancement • … towards extreme confinement • Conclusions
  8. 8. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Tauc model 8 Jan Tauc (1922-2010) • Parabolic v.b. and c.b. approximation • Eg opt  energy difference (Ef-Ei) • BTauc  ̴ absorption efficiency J.Tauc, Amorphous and Liquid Semiconductors, Plenum Press, London and New York Tauc plot ( ) ( )2opt g Tauc E B −⋅= ω ω ωα   ( )opt gTauc EB −= ωωα  S. Cosentino, S. Mirabella et al., Nanoscale Res. Lett. 8, 128 (2013)
  9. 9. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Single Ge quantum well 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 5 10 15 20 25 30 125 4 8 12 16 20 B[×10-1 (eV×nm)-1 ] (b) (a) Energygap[eV] EG Fit, Eg =Eg-Bulk +A/L2 A= 4.35 [eV×nm2 ] OS [×10-4 nm-2 ] Quantum well thickness [nm] OS in Ge QW (theory, Kuo PRB2009) 0.3 0.6 0.9 1.2 1.5 B (measured) Tauc approach to extract in Ge NS: - optical bandgap (Eg) - absorption efficiency (BTauc) BTauc is proportional to the oscillator strength S. Cosentino, S. Mirabella et al., Nanoscale Res. Lett. 8, 128 (2013)
  10. 10. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 OUTLINE Light absorption in Ge quantum dots • Extraction of optical properties (Eg and BTauc) • Interface effects on Eg • confining potential • Absorption enhancement • … towards extreme confinement • Conclusions
  11. 11. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Ge QDs synthesis SiGeO film PECVD or sputter (deposition 250°C: 8 - 20% Ge) (600-800°C annealing in N2) Ge QDs (2-8 nm) embedded in SiO2 5.0x1021 1.0x1022 1.5x1022 0 2 4 6 8 10 PECVD sputter QDsize[nm] Ge concentration [cm-3 ]
  12. 12. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 RBS sigeo90_asdep_ran(x2y7).RBS Simulated Channel 600550500450400350300250200 Counts 2.000 1.900 1.800 1.700 1.600 1.500 1.400 1.300 1.200 1.100 1.000 900 800 700 600 500 400 300 200 100 0 600 700 800 900 1000 1100 1200 1300 1400 1500 1600 Energy [keV] Si SiGeO 2 MeV He+ O Si Ge • Ge QDs density (~ 1018 cm-3) and spacing (1-3 nm) • Light absorption analysis 5.0x1021 1.0x1022 1.5x1022 0 1 2 3 4 PECVD sputter QD-QDdistance[nm] Ge concentration [cm-3 ]
  13. 13. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Light absorption cross section absorption (α)  absorption cross section (σ): photon absorption probability per Ge dose D t ασ = • Red shift with decreasing QD size • Greater shift in PECVD w.r.t. sputter 1 2 3 4 5 10-19 10-18 10-17 PECVD QDs (3.5 nm) PECVD QDs (4.4 nm) Sputter QDs (3 nm) Sputter QDs (4 nm) Absorptioncrosssection[cm2 ] Energy [eV]
  14. 14. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Optical bandgap variation 2 4 6 8 10 1.0 1.5 2.0 2.5 3.0 QD size [nm] PECVD Sputter a-Ge bulk OpticalBandgap[eV] 2 4 6 8 10 1.0 1.5 2.0 2.5 3.0 QD size [nm] PECVD Sputter a-Ge bulk EMA OpticalBandgap[eV] • Eg modulation is dependent on synthesis technique ( ) 2* 22 2 )( Lm bulkENSE gg π += QD Is the interface playing a role ? • Why different Eg modulation ? • How to model the QCE ? • Eg modulation is dependent on synthesis technique • EMA cannot account for none of the two
  15. 15. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 2 nm Ge QD TEM analysis Z contrast profiling reveals systematically thinner interfaces in PECVD samples 2 1)( 1)( 0 QDdiameter exf xx ≥Γ      += − Γ − − 3.5 nm QD
  16. 16. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 EELS-STEM analysis 2 nm • Chemical analysis of QD surrounding • Different oxide contribution 0.2 0.4 0.6 0.8 1.0 5 10 15 20 25 30 35 40 45 50 55 60 0.2 0.4 0.6 0.8 1.0 GeO SiO2 PECVD Intensity[a.u.] Core Interface Matrix Sputter Ge Intensity[a.u.] Energy [eV] 5 nm QD 5 nm QD JEOL ARM200CF www.beyondnano.it Probe size: 0.2x0.2 nm
  17. 17. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 EELS-STEM analysis 0.0 0.2 0.4 0.6 0.8 1.0 5 10 15 20 25 30 35 40 45 50 55 60 0.0 0.2 0.4 0.6 0.8 1.0 Sputter Intensity[a.u.] PECVD Intensity[a.u.] Energy [eV] EELScore QD Fit interband transition Ge Ge QDvol. plasmon SiO2 vol. plasmon Ge-Ge M4,5 band Ge-OM4,5 band AGe-O AGe-Ge AGe-pl )( plGeGeGe OGe OGe AA A F −− − − + = FGe-O ~ 16 % for sputter FGe-O ~ 8 % for PECVD S. Cosentino, S. Mirabella et al., Nanoscale (2015) submitted STEM: e-beam probe a cylinder of ~ 40 Ge atoms, 3 of which at surfaces • Significant Ge-O surface contribution • Greater Ge-O contribution in sputter samples e-beam
  18. 18. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Ge/SiO2 V0,e 2.8 eV V0,h 4.5 eV Interface role SiO2 SiO2Ge QD VB CB VB CB Potential well for e- Potential well for h+ V0,e V0,h Ideal case Real case SiO2 SiO2Ge QD GeO2 Ge/SiO2 Ge/GeO2 V0,e 2.8 eV 1.2 eV V0,h 4.5 eV 3.6 eV
  19. 19. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Interface effect on bandgap 2 4 6 8 10 1.0 1.5 2.0 2.5 3.0 QD size [nm] PECVD Sputter a-Ge bulk OpticalBandgap[eV] 2 4 6 8 10 1.0 1.5 2.0 2.5 3.0 QD size [nm] PECVD Sputter a-Ge bulk EMA OpticalBandgap[eV] 2 4 6 8 10 1.0 1.5 2.0 2.5 3.0 QD size [nm] PECVD Sputter a-Ge bulk EMA SPDEMPECVD SPDEMSputter OpticalBandgap[eV] E. G. Barbagiovanni, et al., J. Appl. Phys. (2012), 111, 034307 E. G. Barbagiovanni, et al. Physica E, (2014), 63, 14–20 E. G. Barbagiovanni, S. Mirabella et al., J. Appl. Phys. (2015) accepted S. Cosentino, S. Mirabella et al., Nanoscale (2015) submitted Ge/SiO2 Ge/GeO2 PECVD Sputter V0,e 2.8 eV 1.2 eV 1.1 eV 0.9 eV V0,h 4.5 eV 3.6 eV 3.3 eV 2.8 eV ( ) ( )         + ⋅ += * , , * , , 2 3 hc hc ec ecbulk gg m V m V DD EDE µ  SPDEM model well accounts for the Eg variation
  20. 20. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015Paper of Eric on arxiv? Interface effects S. Cosentino, S. Mirabella et al., Nanoscale (2015) submitted • GeO2 act as the confining potential • A thinner and GeO poor interface gives larger QCE • … what about absorption efficiency ? 2 4 6 8 10 5.0x10-18 1.0x10-17 1.5x10-17 Absorption Efficiency Ge QDs PECVD Ge QDs Sputter B* Tauc [eV-1 ×cm2 ] QD size [nm] 2X increase Light absorption in Ge quantum dots in SiO2
  21. 21. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 OUTLINE Light absorption in Ge quantum dots • Extraction of optical properties (Eg and BTauc) • Interface effects on Eg • confining potential • Absorptionenhancement • … towards extreme confinement • Conclusions
  22. 22. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Multilayer approach Sample t [nm] d [nm] N ML-2 2 20 15 ML-4 4.5 20 4 SL-330 330 - 1 Multilayer approach for: • narrowing size distribution (Zacharias APL2002) • increasing average distance among QDs Fixed SiO2 barrier thickness: 20 nm N SiGeO layers, from 4 to 15 Comparison with a single layer (330 nm) d: SiO2 barrier t: SiGeO layer
  23. 23. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Absorption coefficient 1 2 3 4 5 6 103 104 105 106 Absorptioncoefficient[cm-1 ] Energy[eV] c-Ge bulk SL-330 1 2 3 4 5 6 103 104 105 106 Absorptioncoefficient[cm-1 ] Energy[eV] c-Ge bulk SL-330 ML-2 ML-4 • Multilayered samples show similar absorption onset to single layer • … but much higher absorption efficiency!
  24. 24. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 RBS and TEM analysis Sample t [nm] d [nm] N Ge % in SiGeO QD size ML-2 2 20 15 10.6 2 ML-4 4.5 20 4 8.9 1.7 SL-330 330 - 1 10.0 2.9 450 500 550 0 50 100 150 200 250 300 He+ backscattered from Ge atoms RBSyield[counts] Channel ML-2 ML-4 2 MeV He+ beam 165° backscattering angle
  25. 25. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Light absorption Ge QD in MLs 10-17 10-16 σ[cm2 ] 2 3 4 5 6 0 1 2 ML-2 ML-4 SL-330 (σhν)1/2 [10-9 cmxeV1/2 ] Energy[eV] • Similar optical bandgap • Strong increase of absorption efficiency • Independent modulation of Eg and B* • ML configuration allows for absorption increase 10 X increase! Same Eg R. Raciti et al. Poster CP2 #44 Today 14-16
  26. 26. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 Multiple screening effect Si QDs - ε2 calculation in one Si QD R. Guerra et al., Phys. Rev. B, 84, 075342 (2011) Local Field Effects Lower screening of e.m. radiation by induced polarization (local field effects) 2 4 6 8 10 0.0 5.0x10-17 1.0x10-16 1.5x10-16 PECVD Sputter ML-PECVD Ge bulk BTauc [eV-1 ×cm2 ] QD size [nm] 15X increase
  27. 27. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 OUTLINE Light absorption in Ge quantum dots • Extraction of optical properties (Eg and BTauc) • Interface effects on Eg • confining potential • Absorptionenhancement • … towards extreme confinement • Conclusions
  28. 28. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 CONCLUSIONS Based on: S. Mirabella et al. JAP 106, 103505 (2009) E. Barbagiovanni et al. JAP 111, 034307 (2012) S. Cosentino et al. NRL 8, 128 (2013) S. Mirabella et al. APL 102, 193105 (2013) E. Barbagiovanni et al. PE 63, 14 (2014) S. Cosentino et al. JAP 115, 043103 (2014) S. Cosentino et al. SOLMAT 135, 22 (2015) E. Barbagiovanni et al. JAP (2015) accepted S. Cosentino et al. Nanoscale (2015) submitted SiO2 SiO2Ge QD GeO2 2 4 6 8 10 1.0 1.5 2.0 2.5 3.0 QD size [nm] PECVD Sputter a-Ge bulk EMA SPDEMPECVD SPDEMSputter OpticalBandgap[eV] 2 3 4 5 6 0 1 2 (σhν)1/2 [10-9 cmxeV1/2 ] Energy[eV] 10 X increase! Optical bandgap in Ge QDs in SiO2 • variation with size • interface drives confinement • SPDEM model Absorption efficiency in Ge QDs in SiO2 • large increase in multilayer (reduced screening)
  29. 29. International School for Materials for Energy and Sustainability (ISMES IV) July 13 – 20, 2015 Colorado School of Mines • Golden, Colorado USA Topics • Global Overview • Energy Overview • Critical Materials for Energy • Energy Analysis • Unconventional Materials for Energy • Nuclear Energy Including Fusion • Gas/Oil/Coal and Fracking • Solar Energy - PV • Wind Energy • Geothermal • Thermoelectricity/Piezoelectricity • Building Technology Future • … and more Lecturers Sponsors • Harry Atwater :: Caltech • Sally Benson :: Stanford • David Cahen :: Weizman Institute • George Crabtree :: Argonne Labs • David Ginley :: NREL • Sossina Haile :: Caltech • Carolyn Koh :: Colorado School of Mines • … and more Information http://csmspace.com/events/ismes
  30. 30. www.matis.imm.cnr.it mirabella@ct.infn.it EMRS Spring meeting, 13/05/2015 THANK YOU!

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