Physics and Material Science of Semiconductor Nanostructures

Transcription

1 Physics and Material Science of Semiconductor Nanostructures PHYS 570P Prof. Oana Malis Course website: 1

2 Introduction to semiconductor nanostructures Review of semiconductors Classification of semiconductors Low-dimensional semiconductors: from 3D to 2D, 1D and 0D Applications of semiconductor nanostructures Ref. Ihn Chapter 1

3 Phenomenology Metal, Insulator, and Semiconductor Resistivity (Ohm.cm) Conductor Semiconductor Insulator (Cu, Ag..) (Si, GaAs..) (SiO2,..) ~ ~ ~ 10 metal SC ins

4 Semiconductors Conductivity/Resistivity Definition Metals Semimetals

5 Phenomenology cont. Temperature dependence of resistivity and absorption Metal Metal Semiconductor Semiconductor

6 Band Diagram of Solids Single atom Solid 3s conduction band N 2p 2s 1s Valence band Energy position 6N 2N 2N

7 Metal, Insulator, and Semiconductor R metal R SC R ins metal insulator semiconductor Conduction Band (CB) Energy gap (Eg) Valence Band (VB) T>0 doping

8 Semiconductors: Bandgap Definition Semiconductor ~ A small bandgap insulator Strictly speaking, it must also be capable of being doped. Typical Bandgaps Semiconductors: 0 ~ E g ~ 3 ev Metals & Semimetals: E g = 0 ev Insulators: E g 3 ev Exceptions AlN, with E g = ~ 6 ev, is usually an insulator, but it can be doped & used as a semiconductor! Also, sometimes there is confusing terminology like GaAs: E g = 1.5 ev is sometimes called semi-insulating!

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10 Classification of semiconductors

11 Ternary and quaternary semiconductors On semiconductor technology, the concept of randomly mixing two or more semiconductors has two main objectives: Altering the gap energy to a previously determined value (e.g. laser/detectors) ex. HgCdTe IR detectors; ex. InGaAsP lasers ex. AlGaAs laser layer confinement; ex. InGaN; AlGaN Creating a material with an adequate lattice constant that matches the available substrates e.g. In 0.53 Ga 0.47 As matches InP Jan 2006

12 Ternary and quaternary semiconductors are alloys E.g. Solid Solution of type A x B 1-x C x atom elements A and (1-x) atoms of element B, randomly distributed over one of the sublattices (e.g. In the one of group III); Element C occupies the other sub lattice (e.g. Group V); x varies between 0 and 1 E.g.. Al x Ga 1-x As, GaAs 1-x P x,in x Ga 1-x N, Al x Ga 1-x N InSb

13 Alloys When two semiconductors A and B are mixed using a proper growth technique, the following alloy information should be obtained: The lattice crystalline structure: on most semiconductors the two (or more) alloy components have the same crystalline structure in a way that the final alloy has the same structure. For materials having the same structure, the lattice constant obeys the: Vergard law Jan 2006

14 In the case of direct gap semiconductors, the gap energies are also linearly weighted in accordance to: Bowing (C) pictures the deviation from the truly random behavior E g, liga xeg, A ( 1 x) Eg, B E g ( ev ) hc e ( nm) Energy gap of Ga 1-x In x As E g (InAs)=0.4 ev E g (GaAs)=1.4 ev Jan 2006 Eg(Ga 1-x In x As) For x=0.48, Eg=0.8 ev (1.5 m) lasers.: excellent for optical fiber communications

15 Doping Intrinsic Semiconductor n p n i Extrinsic Semiconductor + Si:As Donor impurities provide extra electrons to conduction (type n) e - Acceptor impurities provide excess holes to conduction (type p) B Si:B e +

16 Donors and acceptors At 0 K, the energy level is filled. Little thermal energy is needed in order to excite these electrons up to the CB. So, above K, electrons are virtually donated to the CB. Likewise, acceptor levels can be thermally occupied with VB electrons, therefore generating holes. 16 Jan 2006

17 Other materials that are semiconductors

18 Many interesting semiconductor materials: Have crystal lattice structures Diamond or Zincblende In these structures, each atom is tetrahedrally coordinated with four (4) nearest-neighbors. The bonding between neighbors is (mostly) sp 3 hybrid bonding (strongly covalent). There are 2 atoms/unit cell (repeated to form an infinite solid).

19 Zincblende (ZnS) Lattice Zincblende Lattice The Cubic Unit Cell.

20 The Zincblende (ZnS) Lattice Zincblende Lattice: The Cubic Unit Cell. If all atoms are the same, it becomes the Diamond Lattice! Zincblende Lattice: A Tetrahedral Bonding Configuration

21 Zincblende & Diamond Lattices Diamond Lattice The Cubic Unit Cell Zincblende Lattice The Cubic Unit Cell Semiconductor Physicists & Engineers need to know these structures!

22 Other semiconductor materials of interest: have crystal lattice structures Wurtzite Structure This is similar to the Zincblende structure, but it has hexagonal symmetry instead of cubic. In these structures, each atom is tetrahedrally coordinated with four (4) nearest-neighbors. The bonding between neighbors is (mostly) sp 3 hybrid bonding (strongly covalent). There are 2 atoms/unit cell (repeated to form an infinite solid).

23 Wurtzite Lattice Semiconductor Physicists & Engineers need to know these structures!

24 History of semiconductor technology Ge transistor LSI Quantum corral Carbon nanotube Point contact L. L. Sohn, Nature 394(1998)131

25 Low-Dimensional Systems Quantum Well (quasi-2d) <<100nm, in usual. Quantum Wire (quasi-1d) Quantum Dot (quasi-0d)

26 Formation of nanostructures Gate-defined dot 1 m~100nm Pillar dot 1 m~100nm etching Self-assembled dots ~10nm

27 Semiconductor Heterostructures* * 2000 Nobel prize in physics A B Confinement potential

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29 Quantum Structures & Density of States Bulk (3D) DOS Energy Quantum well (2D) Quantum wire (1D) Quantum dot (0D) DOS DOS DOS Energy Energy Energy

30 Quantum Phenomena and Quantum Devices with Semiconductor Nanostructures ENERGY Quantization FLUX Quantization CHARGE Quantization Low-Dimen. Elect. Band Modulation Resonant Tunneling Quantum Hall Effect Ballistic Resistance Optical Bistability Elect. Interference -Aharanov-Bohm effect -universal conduct. fluctuation Ballistic Transport Single Elect. Effect -electron charging -electron tunneling Current Standard Capacitance Standard - HEMT / MODFET - QWIP - Quantum Hall Effect - QWL/QWR/QD Laser - Quantum Interference Dev. - Elect. Wave Device - Ballistic Device - SET Transistor - Single Electron Devices

31 Photoluminescence (PL) from Quantum Wells

32 Photoluminescence (PL) from (parabolic) Quantum Well R.C. Miller, et al. Phys. Rev. B 29, 3740 ( 84) Also see sec. 4.3 in Davies textbook 40meV

33 PL from Ensemble of Quantum Dots Artificial atoms!!! ~20nm Sylvain Raymond and cowokers, NRC, Canada

34 PL from Single Quantum Dot 20meV ~20nm Robin Williams and cowokers, at NRC, Canada

35 Current transport through a classical resistance w I I + V _ Ohm's law Conductance (G) V I GV G W L W

36 Quantum Point Contact B.J. van Wees, PRL 60, 848(1988). (see also J.H. Davies Fig.5.22/p186)

37 Quantum Point Contact Vg I ~250nm Vg + _ V G(2e : metal (gate) : two-dimensional electron gas / h) h: Planck s constant von Klitzing's resistance R K h 2 e W Vg *see also quantum Hall effect (Nobel prizes in 85, 98)

38 Quantum Point Contact (metal) Quantized conductance through individual rows of suspended gold atoms H. OHNISHI, et al., Nature 395, p780 ( 98) F of metal: ( F, M F, SC 10 ) 0 ~ 10 1 nm ~0.9nm

39 Coulomb Blockade in Quantum Dot (Q.D.) Quantum dot J. Weis, et al. Phys. Rev. Lett. 71, (1993) Vg I Vg G single electron transistor (SET) S D S D G G (a review article about Q.D.: S.M. Reimann and M. Manninen, Review of Modern Physics, 74,1283 (2000)) Vg

40 Coulomb Blockade spectrum of a Single Nanocrystal U. Banin, Y. Cao,D. Katz, and O. Millo, Nature vol.400, 542 (1999) InAs NC