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Different Electronic Materials

Different Electronic Materials. Semiconductors: Elemental (Si, Ge) & Compound (GaAs, GaN, ZnS, CdS, …) Insulators: SiO 2 , Al 2 O 3 , Si 3 N 4 , SiO x N y , ... Conductors: Al, Au, Cu, W, silicide, ...

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Different Electronic Materials

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  1. Different Electronic Materials • Semiconductors: Elemental (Si, Ge) & Compound (GaAs, GaN, ZnS, CdS, …) • Insulators: SiO2, Al2O3, Si3N4, SiOxNy, ... • Conductors: Al, Au, Cu, W, silicide, ... • Organic and polymer: liquid crystal, insulator, semiconductor, conductor, superconductor • Composite materials: multi-layer structures, nano-materials, photonic crystals, ... • More: magnetic, bio, …

  2. Insulators, Conductors, SemiconductorsInorganic Materials E E E conduction band conduction band empty - Band gap partially-filled band electron hole Band gap Forbidden region Eg < 5eV Eg > 5eV + valence band valence band filled Insulator Semiconductor Conductor Si: Eg = 1.1 eV Ge: Eg = 0.75 eV GaAs: Eg = 1.42 eV SiO2: Eg = 9 eV

  3. Electronic properties & device function of molecules • Electrons in molecule occupy discrete energy levels---molecular orbitals • Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are most important to electronic applications Bandgap of molecule: Eg = E(LUMO) - E(HOMO) • Organic moleculeswith carbon-based covalent bonds, with occupied bond states ( band) as HOMO and empty antibonding states (* band) as LUMO

  4. Lower energy by delocalization:  Benzene Biphenyl Conducting Polymers Polyacetylene: Eg ~ 1.7 eV  ~ 104 S cm-1 Polysulphur nitride (SN)n  ~ 103-106 S cm-1 Poly(phenylene-vinylene) (PPV) High luminescence efficiency

  5. Diodes and nonlinear devicesMolecule with D--A structure C16H33Q-3CNQ D  A Highly conductive zwitterionic D+--A- state at 1-2V forward bias Reverse conduction state D---A+ requires bias of 9V I-V curve of Al/4-ML C16H33Q-3CNQ LB film/Al structure

  6. Negative differential resistance (NDR): electronic structural change under applied bias, showing peak conductance 2’-amino-4-ethynylphenyl-4’ethynylphenyl-5’-nitro-1-benzennthiol Self-assembled layer between Au electrodes NDR peak-to-valley ratio ~ 1000

  7. Molecular FET and logic gates Molecular single-electron transistor: Could achieve switching frequency > 1 THz

  8. Assembly of molecule-based electronic devices “Alligator clips” of molecules: Attaching functional atoms S for effective contact to Au High conductance through leads but surface of body is insulating

  9. Self-assembled Molecular (SAM) Layers Carene on Si(100) Simulated STM images for (c) for (a) 0.1 ML 1-nitronaphthalene adsorbed on Au(111) at 65 K Ordered 2-D clusters

  10. Self-assembled patterns of trans-BCTBPP on Au(111) at 63 K Interlocking with CN groups

  11. Conventional Organic Electronic Devices Organic Thin Film Transistors (OTFT) Organic Light Emitting Diode (OLED) For large-area flat-panel displays, circuit on plastic sheet

  12. Printing: Soft-lithographic process in fabrication of organic electronic circuits

  13. Unique electronic & opto-electronic properties of nanostructures • DOS of reduced dimensionality (spectra lines are normally much narrower) • Spatial localization • Adjustable emission wavelength • Surface/interface states Effective bandgap blue-shifted, and adjustable by size-control

  14. Optical properties of quantum dot systems Excitons in bulk semiconductors An e-h pair bound by Coulomb potential H-atom like states of exciton in effective-mass approximation: M = me*+ mh*, ħK: CM momentum  = me*mh*/(me*+ mh*) reduced mass Bohr radius of the exciton: Bohr radius of electron or hole: (a0 = 0.529 Å) aB = ae + ah

  15. In GaAs (me*= 0.067m0, mhh*= 0.62m0, r = 13.2) Binding energy (n = 1): 4.7 meV, aB = 115 Å Generally, binding energy in meV range, Bohr radius 50-400 Å Excitons in QDs Bohr radius is comparable or even much larger than QD size R Weak-confinement regime: R >> aB, the picture of H atom-like exciton is still largely valid:

  16. Strong confinement regime (R << ae and ah): model of H atom-like exciton is not valid, confinement potential of QD is more important. Lowest energy e-h pair state {1s, 1s}:

  17. Production of uniform size spherical QDs Controlled nucleation & growth in supersaturated solution All clusters nucleate at basically same moment, QD size distribution < 15% QDs of certain average size are obtained by removing them out of solution after a specific growth period Further size-selective processing to narrow the distribution to  5%

  18. Similar nucleation and growth processes of QDs also occur in glass (mixture of SiO2 and other oxides) and polymer matrices Ion implantation into glass + annealing Mono-dispersed nanocrystals of many semiconductors, such as CdS, CdSe, CdTe, ZnO, CuCl, and Si, are fabricated this way Optimal performance of QDs for semiconductor laser active layers requires 3D ordered arrays of QDs with uniform size In wet chemical QDs fabrication: proper control of solvent composition and speed of separation

  19. In SK growth of QDs: strain-mediated intra- and inter-layer interactions between the QDs Aligned array of GaN QDs in AlN

  20. Passive optic devices with nanostructures:Photonic Crystal An optical medium with periodic dielectric parameter r that generates a bandgap in transmission spectrum

  21. Luminescence from Si-based nanostructures Luminescence efficiency of porous Si (PSi) and Si QDs embedded in SiO2 ~ 104 times higher than crystalline Si Fabrication of PSi: electrochemical etching in HF solution, positive voltage is applied to Si wafer (anodization) Sizes of porous holes: from nm to m, depending on the doping type and level

  22. Nano-finger model of PSi: from Si quantum wires to pure SiO2 finger with increasing oxidation Emission spectrum of PSi: from infrared to the whole visible range

  23. Remarkable increase in luminescence efficiency also observed in porous GaP, SiC Precise control of PSi properties not easy Si-based light emitting materials and devices Digital Display

  24. Atomic structures of carbon nanotubes Stable bulk crystal of carbon Graphite Layer structure: strong intra-layer atomic bonding, weak inter-layer bonding 3.4 Å 1.42 Å

  25. Enclosed structures: such as fullerene balls (e.g., C60, C70) or nanotubes are more stable than a small graphite sheet Trade-off: curving of the bonds raises strain energy, e.g., binding energy per C atom in C60 is ~ 0.7 eV less than in graphite MWNT, layer spacing ~ 3.4 Å SWNT

  26. Index of Single-wall Carbon Nanotubes (SWNT) Armchair (n, n) Zigzag (n, 0) General (m, n)

  27. Synthesis of CNTs by Laser vaporization: Pulsed laser ablation of compound target (1.2% at. Co-Ni + 98.8% C) High yield (~70%) of SWNT ropes

  28. Carbon arc discharge: ~500 Torr He, 20-25 V across 1-mm gap between 2 carbon rods Plasma T > 3000C, CNT bundles deposited on negative electrode With catalyst (Co, Ni, Fe, Y, Gd, Fe/Ni, Co/Ni, Co/Pt)  SWNTs Without catalyst  MWNTs

  29. Vapor-phase synthesis: similar to CVD Substrate at ~ 700-1500C decorated with catalyst (Co, Ni or Fe) particles, exposed to hydrocarbon (e.g. CH4, C6H6) and H2 Aligned CNTs grow continuously atop of catalyst particles Regular CNT arrays on catalyst pattern Useful for flat panel display

  30. Growth mechanisms of C nanotubes 1) C2 dimer addition model: C2 dimer inserted near pentagons at cap 2) Carbon addition at open ends: attach C2 at armchair sites and C3 at zigzag sites Functions of catalyst clusters: stabilizing terminators, cracking of hydrocarbons Fit the controlled CVD process, the open-end is terminated by a catalyst cluster

  31. Structural identification of nanotubes: with TEM, electron diffraction, STM HRTEM: number of shells, diameter STM: diameter, helicity of nanotube out-shell, electronic structure

  32. Electronic properties of SWNTs SWNTs: 1D crystal If m - n = 3qmetallic Otherwise semiconductor Zigzag, dt = 1.6nm =18, dt = 1.7nm Bandgap of semiconducting SWNTs: =21, dt = 1.5nm =11, dt = 1.8nm Armchair, dt = 1.4nm = 1.42 Å,  5.4 eV, overlap integral STM I-V spectroscopy

  33. Junctions between SWNTs:homojunctions, heterojunctions, Schottky junctions, but how to connect and dope? SWNT connections: insert pentagons and heptagons Natural SWNT Junctions

  34. Doping of semiconductor SWNTs N, K atoms  n-type; B atoms, oxygen  p-type SWNT CMOS inverter & its characteristics

  35. Other nanotubes and nanowires GaN nanowires BN nanotubes p-Si/n-GaN nanowire junction Si nanowires

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