ФИЗИКО-ХИМИЧЕСКИЕ ОСНОВЫ НАНОТЕХНОЛОГИИ

1. ФИЗИКО-ХИМИЧЕСКИЕОСНОВЫНАНОТЕХНОЛОГИИФИЗИКО-ХИМИЧЕСКИЕОСНОВЫНАНОТЕХНОЛОГИИ Профессор Н.Г. Рамбиди

2. 4. Квантовые колодцы, квантовые нити, квантовые ямы

3. Плотность электронных состояний в твердом теле

4. Электрон в одномерной яме

5. Электрон в одномерной яме

6. Электрон в одномерной яме

7. Электрон в одномерной яме

8. Плотность электронных состояний в твердом теле

9. Плотность электронных состояний в твердом теле

10. Плотность электронных состояний в твердом теле

11. Плотность электронных состояний в твердом теле

12. Плотность электронных состояний в твердом теле

13. Плотность электронных состояний в твердом теле

14. Квантовые колодцы

15. Quantum Wells • 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 10 cm e- moves freely 10 cm e- confined 10 nm thick Quantum Well Nanoscale Thin Film

16. 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 JPL’s QWIP detects minute differences associated with blood flow changes Quantum well laser mounted on the head of a pin: http://wwwrsphysse.anu.edu.au/admin/pgbrochure/quantum.html http://www.jpl.nasa.gov/technology/shrinking/tiny_tech.html

17. b Electron tunneling Superlattices • Alternating layers of thin films additional periodicity causes interesting effects • When l , λ> b → phonon wave and particle effects • Electron transport well understood • Limited understanding of phonon transport Are layers thin enough for electrons/phonons to tunnel? Is coherence maintained for mini-band formation? Si0.76Ge0.24 / Si0.84Ge0.16 superlattice: In a crystal, atomic periodicity leads to band formation In a superlattice, engineered periodicity leads to minibands

18. 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: http://www.nuee.nagova-u.ac.jp/labs http://www.rti.org/pubs/anser.pdf

19. Квантовые нити

20. Квантовые нити

21. Квантовые нити

22. Квантовые нити

23. Квантовые нити

24. Квантовые нити

25. Квантовые нити

26. Квантовые нити

27. Квантовые нити

28. Квантовые нити

29. Квантовые нити

30. Квантовые ямы

31. Квантовые ямы • Квантовые ямы – миниатюрные устройства, которые содержат немного свободных электронов • Типичные размеры лежат в области от нанометров до нескольких микрометров

32. Квантовые ямы • В квантовой яме могут быть от одного до нескольких тысяч электронов • Размеры и форма ямы и число электронов можно точно контролировать

33. Квантовые ямы • Так же, как и в атоме, энергетические уровни в квантовых ямах дискретны • Структура уровней сходна с уровнями 3D потенциальной ямы • В квантовой яме свойства могут существенно измениться если удалить даже один электрон

34. Квантовые ямы • В отличие от атомов квантовые ямы легко присоединять к электродам и создавать на их основе различные устройства

35. Semiconductor Band Gaps • 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 • Electron-hole pairs (EHPs) can form by thermal or photo excitation • Electrons in the conduction band are free to conduct electricity • Different semiconductors have different band gaps EC EG EV

36. 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 • Because the Quantum Dots are so small (20-30 nm), quantum mechanics govern how an electron will behave in the dot E electron EC EG e- EV hole

37. The Quantum Dot • Confinement in all three dimensions  Ex, Ey, and Ez are quantized (discrete) • Higher probability of recombination means greater radiative emission “Electronic Structure of InAs Pyramidal Quantum Dots”: http://www.sst.nrel.gov/research/InAs.html Ee = Ez + Ex + Ey with all E discrete

38. Molecular Beam Epitaxy (MBE) • Substrate wafers transferred to high vacuum growth chamber (red arrow) • Elements kept in K-Cells at high temp • 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.

39. Molecular Beam Epitaxy (MBE)

40. More About MBE • 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 • Precise control over temperatures and shutters allows very thin layers to be grown (~1 ML/sec) • RHEED patterns indicate surface morphology

41. Fabrication of Wells • Lattice matched AlGaAs grown on GaAs substrate • Thin layer of GaAs (~10 nm) • Another layer of AlGaAs to finish the well z d2 d1 d1= d2

42. Fabrication of Dots • Thick layer of GaAs • Begin growing InAs (greater lattice constant) • Crystal strain forces dot formation • Cap dots with layer of GaAs d1 d2 z d1

43. Epitaxy: Patterned Growth • Growth on patterned substrates • Grow QDs in pyramid-shaped recesses • Recesses formed by selective ion etching • 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

44. Epitaxy: Self-Organized Growth • Self-organized QDs through epitaxial growth strains • Stranski-Krastanov growth mode (use MBE, MOCVD) • Islands formed on wetting layer due to lattice mismatch (size ~10s nm) • Disadvantage: size and shape fluctuations, ordering • Control island initiation • Induce local strain, grow on dislocation, vary growth conditions, combine with patterning • AFM images of islands epitaxiall grown on GaAs substrate. • InAs islands randomly nucleate. • Random distribution of InxGa1­xAs ring-shaped islands. • 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.

45. Quantum Dot Empty Cell - Containing electron Tunnel – allows electrons to move between dots “0” Only 2, since electrons repel each other. “Low energy state” - - - - “1” QCA: Physics Basics Using cells w/2 electrons. ? Possible configurations? ?

46. or - - - - - - - - - - - - QCA: Physics Basics Adjacent cells’ electrons also repel each other. Consumes/generates no energy.

47. One cell “fixed” to some value. Electrons move into low-energy state.  Value propogates. - - - - - - - - - - - - - - - - - - - - - - - - - - - - QCA: Wires Adjacent cells in low-energy state

48. - - - - - - QCA: Wires Same when rotated to vertical.

49. Note complementation! - - - - - - - - - - - - QCA: Wires Same idea when cells rotated 45º.

50. - - - - - - - - - - - - - - QCA: Wire Crossings Such wires cross w/o interference.