Group Si Nano IV Nano Optoelectronics: Recent Developments based on Bottom-Up Approaches

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1 Group Si Nano IV Nano Optoelectronics: Recent Developments based on Bottom-Up Approaches The 6 th US-Korea Forums on Nanotechnology: Nanoelectronics and its Integration with Applications April Moon-Ho Jo Dept. of Materials Science and Engineering Pohang University of Science and Technology (POSTECH)

2 (Vapor) 2H 2 SiH 4 SiGe (Solid) GeH 4 (Liquid) Bottom-Up Nanowires for Integrated Nanosystems?

3 Nanowire Photonics/Optoelectronics/Photovoltaics 1. Unique size effects at the individual NW level 2. Large-area integrated NW arrays

4 Nano Device Materials & Physics Laboratory Nanowire Photonics/Optoelectronics/Plasmonics Nature Phys., Accepted (2009) Nano Lett., Accepted (2009) Appl. Phys. Lett., In Press (2009) Appl. Phys. Lett., (2008) Nano Lett (2006) 1.55μm 1.35μm [Near Infrared] [Visible] [Ultraviolet] SiO 2 /Si Nanowire Growth Submitted, (2009) Nano Lett (2008) Adv. Mater (2008) Chem. Mater (2008) Appl. Phys. Lett., 91, (2007) Adv. Mater., 19, 3637 (2007) Appl. Phys. Lett. 88, (2006) Nano Lett (2004) 0.209nm 20 nm 2 nm Nanowire Electronics Nano Lett (2008) Appl. Phys. Lett., 91, (2007) Nano Lett, (2006)

5 Nano Si Photonics Si Quantum Dot Photonics Photoluminescence in NIR to UV from Si quantum dots (QD) of various size * Exciton-Bohr radius of Si is ~5nm. (~18nm for Ge) Because of (possible) quantum confinements, Si QDs smaller than 5nm, can emit light from the near infrared throughout the visible with quantum efficiencies in excess of 10%. Radiative transition rates increase due to the confinement of e-h pairs. T.Y. Kim et al., Appl. Phys. Lett. 85, 5355 (2004) Lorenzo Pavesi and David J. Lockwood, Materials Today, Jan. 26, 2005

6 Nano Silicon Photovoltaics Multiple Exciton Generation in Si Quantum Dots M.C. Beard, Nano. Lett. 7, 2506 (2007) Multiple bound e-h pairs (excitons) can be generated in Si nanocrystals (9.5 nm) upon photon absorption of energy greater than twice the band gap. The exciton production quantum was found to be 2.6 excitons per absorbed photon at 3.4E g.

7 Si:Ge Nanowire Optoelectronics Si:Ge Nanowire Optoelectronics Si:Ge Nano Crystals: The model system for continuously varying lattices and energy band-gaps at the nanometer scale 1. Nanowires: Electrically driven Efficient Light-Emitting/Detecting Devices 2. Si:Ge Alloys: Tunable Energy upon Light-Matter Interaction

8 Growth of Single-Crystalline Si 1-x Ge x Nanowires Vapor-Liquid-Solid (VLS) Nanowire Growth Conventional VLS-CVD Nanowire Growth 2μm Catalyst-assisted CVD of group IV semiconductor nanowires: - Sources: SiH 4 and 5nm GeH 4 -Dopants: PH 3 and B 2 H 6 Chang-Beom Jin et al., Appl. Phys. Lett. 88, (2006) Jee-Eun Yang, et al., Nano Lett. 6, 2679 (2006) (Vapor) 2H 2 SiH 4, GeH 4 Si (Solid) (Liquid)

9 Band-Gap Modulation in Si 1-x Ge x Nanowires Si 1-x Ge x Nanowire Crystals: Optical Band-Edge Absorption Jee-Eun Yang et al., Nano Lett. 6, 2679 (2006) The optical band-edge of 0.68eV and 1.05eV for Ge and Si nanowires, and these values agree with the energy band-gaps of bulk Ge and Si crystals of 0.65eV and 1.12eV. The optical band-edge in various Si 1-x Ge x nanowires systematically shifts from that of Si nanowires to that of Ge nanowires with increasing Ge content. We observed strong blue-shift of optical band-edge for thinner nanowires whose diameter is smaller than 10nm. (Exciton- Bohr radius of 4.7nm for Si and 17.7nm for Ge)

10 Nano Optoelectronics Laboratory Spatially Resolved Optoelectronic Measurements 532 nm Laser Diffraction limit : Δ R = k λ N. A. = 360 nm, 650 nm k : technical constant (~0.61) N. A. : numerical aperture (0.5, 0.9) A new experimental setup based on a scanned laser confocal microscope allows combined measurements of spatially resolved electroluminescence and photoconductivity. The setup also allows the spectral measurements of electroluminescence and correlated photon counting. With the addition of an ultrafast laser, it should also allow time-resolved measurements.

11 Intra-Nanowire p-n diode Photocurrent in Si Nanowire p-n diode 100 D V sd = 5.0 V S n p V b 0 V sd = 2.5 V 2um V sd = 0.0 V hν e V sd = -2.5 V Drain Source V sd = -5.0 V h -300 na n p Cheol-Joo Kim et al., Nano Lett., Accepted (2009)

12 Raman Scattering in Si 1-x Ge x Semiconductors Confocal Raman Spectro-Microscopy Jee-Eun Yang et al., Appl. Phys. Lett., 92, (2008) (with Prof. Zee Hwan Kim, Korea Univ. )

13 Ge Nanowire Photodetector Cheol-Joo Kim et al., Nano Lett., Accepted (2009) Photodiode Laser 532 nm ΔG (S) Ge NW Si NW Objective Lens (N.A. =0.5) Intensity (W/cm 2 ) A V g V sd X Si NW or Ge NW Y PC Gain Ge NW Si NW XY piezo-scanner The PC for Ge NWs is more than two orders of magnitude higher than that of Si NWs. This PC enhancement in Ge NWs is even more pronounced at lower light intensity. - Ge NW can be an excellent candidate for polarization-sensitive nanoscale photodetectors especially in the visible range. - Ge NWs show extremely sensitive photoresponse especially at a low intensity regime, which is attributed to the internal gain mechanism, originating from the surface state filling Intensity (W/cm 2 )

14 Epitaxial NW Growth for Ordered Arrays Cheol-Joo Kim et al., Appl. Phys. Lett., In Press (2009) Kibum Kang et al., Adv. Mater (2008) Vertical Growth by Epitaxy on (111) Si Substrates 10 μm (a) (b) (c) Substrate Nanowire 20 μm 50 nm 2 nm

15 Epitaxial NW Growth for Ordered Arrays (I) PS Nanosphere Lithography Substrate: SiO2(100 nm)/si(111)-p Reactive ion etching Metal deposition & PS lift-off SiO2 Dry & Wet etching Growth of Si NWs Au deposition & Lift-off Si(111) SiO2 Metal PS Au

16 Templated-Assisted NW Growth for Ordered Arrays Si Nanowire Arrays from Au-Catalyst Patterns by Nanosphere Lithography

17 Si NW Li-Battery at POSTECH (1) Si nanowire vs. NiSi nanowire World-record Capacity and Efficiency (charging/discharging) up to 4,000 and 99 %! Capacity fading is still small and is maintained up to 80 % after 50 cycles! Anode Discharge e - LOAD Si NW Anode Charge Li Cathode Electrolytes Potentiostat/Galvanostat

18 NANO DEVICE MATERIALS ATERIALS & Physics LABL Jee-Eun Yang Hyun-Seung Lee Cheol-Joo Kim Kibum Kang