ФИЗИКО- ХИМИЧЕСКИЕ ОСНОВЫ НАНОТЕХНОЛОГИИ Профессор Н.Г. Рамбиди
4. Квантовые колодцы, квантовые нити, к вантовые ямы
Плотность электронных состояний в твердом теле
Электрон в одномерной яме
Плотность электронных состояний в твердом теле
Квантовые колодцы
Quantum Wells 10 nm thick 10 cm Nanoscale Thin Film e - moves freely e - confined Quantum Well Enclosed region of negative energy Electrons confined Can exist in one, two, three dimensions Common example is a square well – sharpest boundary Discrete energies of electrons Narrow gap semiconductor between wide gap semiconductors Surfaces states may be considered like quantum well states
Quantum Well Applications Multi-spectral long wavelength quantum well infrared photodetectors: infrared radiation photoionize trapped carriers in quantum wells; for medical applications, locating hot spots in fires, observing volcanoes Quantum well lasers: quantum confinement effects increase luminescence efficiency Quantum well LEDs Quantum well Hall (magnetic) sensors JPLs QWIP detects minute differences associated with blood flow changes Quantum well laser mounted on the head of a pin:
Superlattices Alternating layers of thin films additional periodicity causes interesting effects Alternating layers of thin films additional periodicity causes interesting effects When l, λ > b phonon wave and particle effects When l, λ > b phonon wave and particle effects Electron transport well understood Electron transport well understood Limited understanding of phonon transport Limited understanding of phonon transport Are layers thin enough for electrons/phonons to tunnel? Is coherence maintained for mini-band formation? Si 0.76 Ge 0.24 / Si 0.84 Ge 0.16 superlattice: In a crystal, atomic periodicity leads to band formation In a superlattice, engineered periodicity leads to minibands Electron tunneling b
Superlattice Applications Magnetic superlattices for magneto-optical recording: large perpendicular anisotropies and enhancement of Kerr rotations provide unique properties Giant magnetoresistance Superlattice field effect transistors Thermoelectric materials Metallic superlattice for GMR Commercially available superlattice thermoelectric device from RTI Superlattice used in thermoelectrics:
Квантовые нити
Квантовые ямы
Квантовые ямы – миниатюрные устройства, которые содержат немного свободных электронов Квантовые ямы – миниатюрные устройства, которые содержат немного свободных электронов Типичные размеры лежат в области от нанометров до нескольких микрометров Типичные размеры лежат в области от нанометров до нескольких микрометров
Квантовые ямы В квантовой яме могут быть от одного до нескольких тысяч электронов В квантовой яме могут быть от одного до нескольких тысяч электронов Размеры и форма ямы и число электронов можно точно контролировать Размеры и форма ямы и число электронов можно точно контролировать
Квантовые ямы Так же, как и в атоме, энергетические уровни в квантовых ямах дискретны Так же, как и в атоме, энергетические уровни в квантовых ямах дискретны Структура уровней сходна с уровнями 3D потенциальной ямы Структура уровней сходна с уровнями 3D потенциальной ямы В квантовой яме свойства могут существенно измениться если удалить даже один электрон В квантовой яме свойства могут существенно измениться если удалить даже один электрон
Квантовые ямы В отличие от атомов квантовые ямы легко присоединять к электродам и создавать на их основе различные устройства В отличие от атомов квантовые ямы легко присоединять к электродам и создавать на их основе различные устройства
Semiconductor Band Gaps Energy states in an atom correspond to bands in a semiconductor Energy states in an atom correspond to bands in a semiconductor In between the valence and conduction bands, there are no states where an electron can exist In between the valence and conduction bands, there are no states where an electron can exist Electron-hole pairs (EHPs) can form by thermal or photo excitation Electron-hole pairs (EHPs) can form by thermal or photo excitation Electrons in the conduction band are free to conduct electricity Electrons in the conduction band are free to conduct electricity Different semiconductors have different band gaps Different semiconductors have different band gaps ECEC EVEV EGEG
The Energy Levels of Quantum Dots The Quantum Dot band gap is smaller than the surrounding material, so electrons will tend to fall into the dot to reach a lower-energy configuration The Quantum Dot band gap is smaller than the surrounding material, so electrons will tend to fall into the dot to reach a lower-energy configuration Because the Quantum Dots are so small (20-30 nm), quantum mechanics govern how an electron will behave in the dot Because the Quantum Dots are so small (20-30 nm), quantum mechanics govern how an electron will behave in the dot ECEC EVEV EGEG e-e- E electron hole
The Quantum Dot Confinement in all three dimensions E x, E y, and E z are quantized (discrete) Confinement in all three dimensions E x, E y, and E z are quantized (discrete) Higher probability of recombination means greater radiative emission Higher probability of recombination means greater radiative emission Electronic Structure of InAs Pyramidal Quantum Dots : E e = E z + E x + E y with all E discrete
Molecular Beam Epitaxy (MBE) Substrate wafers transferred to high vacuum growth chamber (red arrow) Substrate wafers transferred to high vacuum growth chamber (red arrow) Elements kept in K-Cells at high temp Elements kept in K-Cells at high temp Shutters over cells open to release vaporized elements, which deposit on sample Shutters over cells open to release vaporized elements, which deposit on sample Adapted from: Farrow, R.F.C., ed. Molecular Beam Epitaxy: Applications to Key Materials. Noyes Publications, Park Ridge, NJ, 1995.
Molecular Beam Epitaxy (MBE)
More About MBE The temperature of each K-Cell controls the rate of deposition of that element (Ga, In, Al, etc.) The temperature of each K-Cell controls the rate of deposition of that element (Ga, In, Al, etc.) As and P can also be flowed into chamber As and P can also be flowed into chamber Precise control over temperatures and shutters allows very thin layers to be grown (~1 ML/sec) Precise control over temperatures and shutters allows very thin layers to be grown (~1 ML/sec) RHEED patterns indicate surface morphology RHEED patterns indicate surface morphology
Fabrication of Wells Lattice matched AlGaAs grown on GaAs substrate Lattice matched AlGaAs grown on GaAs substrate Thin layer of GaAs (~10 nm) Thin layer of GaAs (~10 nm) Another layer of AlGaAs to finish the well Another layer of AlGaAs to finish the well d2d2 d1d1 z d 1 = d 2
Fabrication of Dots Thick layer of GaAs Thick layer of GaAs Begin growing InAs (greater lattice constant) Begin growing InAs (greater lattice constant) Crystal strain forces dot formation Crystal strain forces dot formation Cap dots with layer of GaAs Cap dots with layer of GaAs d2d2 d1d1 z d1d1
Epitaxy: Patterned Growth Growth on patterned substrates Growth on patterned substrates Grow QDs in pyramid- shaped recesses Grow QDs in pyramid- shaped recesses Recesses formed by selective ion etching Recesses formed by selective ion etching Disadvantage: density of QDs limited by mask pattern Disadvantage: density of QDs limited by mask pattern T. Fukui et al. GaAs tetrahedral quantum dot structures fabricated using selective area metal organic chemical vapor deposition. Appl. Phys. Lett. May, 1991
Epitaxy: Self-Organized Growth Self-organized QDs through epitaxial growth strains Self-organized QDs through epitaxial growth strains Stranski-Krastanov growth mode (use MBE, MOCVD) Stranski-Krastanov growth mode (use MBE, MOCVD) Islands formed on wetting layer due to lattice mismatch (size ~10s nm) Islands formed on wetting layer due to lattice mismatch (size ~10s nm) Disadvantage: size and shape fluctuations, ordering Disadvantage: size and shape fluctuations, ordering Control island initiation Control island initiation Induce local strain, grow on dislocation, vary growth conditions, combine with patterning Induce local strain, grow on dislocation, vary growth conditions, combine with patterning AFM images of islands epitaxiall grown on GaAs substrate. (a)InAs islands randomly nucleate. (b)Random distribution of InxGa1xAs ring-shaped islands. (c)A 2D lattice of InAs islands on a GaAs substrate. P. Petroff, A. Lorke, and A. Imamoglu. Epitaxially self-assembled quantum dots. Physics Today, May 2001.
QCA: Physics Basics Cell - Empty Containing electron Tunnel – allows electrons to move between dots Quantum Dot Using cells w/2 electrons. ? Possible configurations? ? Only 2, since electrons repel each other. Low energy state 0 1
QCA: Physics Basics Adjacent cells electrons also repel each other. Consumes/generates no energy or
QCA: Wires Adjacent cells in low-energy state One cell fixed to some value Electrons move into low-energy state. Value propogates.
QCA: Wires Same when rotated to vertical.
QCA: Wires Same idea when cells rotated 45º. Note complementation!
QCA: Wire Crossings Such wires cross w/o interference.
QCA: Wire Crossings Same value
QCA: Gates Fixed inputs Output What function computed? ? ? Majority. I.e., most common input value. abcmaj
Clocked Molecular Quantum-dot Cellular Automata Molecular quantum-dot proposed by Molecular quantum-dot proposed by Lent – Isakcsen Lent – IsakcsenAllyl Alkyl Alkyl
Clocked Molecular Quantum-dot Cellular Automata Allyl groups serve as dots with his red-ox centre that can be achieved cy halls Allyl groups serve as dots with his red-ox centre that can be achieved cy halls Alkyl groups serve as tunnel barrier that halls can pass through Alkyl groups serve as tunnel barrier that halls can pass through