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Semiconductor Nanowires

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Presentation on theme: "Semiconductor Nanowires"— Presentation transcript:

1 Semiconductor Nanowires
JASS 05 Yvonne Gawlina Technische Universität München April 2005 Yvonne Gawlina

2 Overview Introduction Synthesis of Nanowires - Pseudowires
- Free standing nanowires Properties of Nanowires Applications April 2005 Yvonne Gawlina

3 Introduction The “first nanotechnologists” worked in the middle ages: ® stained glass: nanoparticles of gold and silver in glass April 2005 Yvonne Gawlina Creation of Nanowire a new challenge in modern age!

4 “Pseudowires” Methods of manufacturing:
Pseudowires: wires which are enclosed in other material Methods of manufacturing: - lithography and etching ® top down - electrostatically induced wires - strain induced wires - growth on patterned surfaces - cleaved edge overgrowth April 2005 Yvonne Gawlina

5 “Pseudowires” Litography and etching Formation of 2d quantum well
Coating with resist Create pattern Etch, until wire remains Sometimes overgrown again to shield wire! April 2005 Yvonne Gawlina Disadvantages: optical and electrical dead layer because of defects due to etching

6 “Pseudowires” Electrostatically induced wires
- Creation of a Schottky contact (metal on semiconductor): - application of voltage raises/lowers the bands ® creation of “wires” for holes/electrons at certain voltages split gates: two slightly separated metal strips ® with voltage: potential minimum creates of wire of variable width disadvantage: potential minima not very deep ® only for low temperature April 2005 Yvonne Gawlina

7 “Pseudowires” Strain induced wires 2d quantum wire Carbon as stressor
Wires through strain April 2005 Yvonne Gawlina Disadvantage: very small potential ® only for low temperatures

8 “Pseudowires” V-groove nanowires
V-shape due to different etching directions Growth of barrier material Growth of wire material Growth of 2nd barrier material to sharpen groove again April 2005 Yvonne Gawlina wire

9 “Pseudowires” Cleaved edge overgrowth wire Growth of quantum well
rotation Growth of second quantum well April 2005 Yvonne Gawlina Disadvantage: low temperatures needed

10 Synthesis of Nanowires
Methods of Nanowire synthesis VLS (Vapour Liquid Solid) method Modification of VLS CVD (Chemical Vapour Deposition) LCG (Laser Ablation catalytic Growth) Low temperature VLS method FLS (Fluid Liquid Solid) mechanism SLS (Solution Liquid Solid) mechanism OAG (Oxide Assisted Growth) April 2005 Yvonne Gawlina

11 Vapour Liquid Solid method
Basics about phase diagrams Alloys have phase diagrams Lever rule: T liquid liquidus al + as = 1 liquid and solid solidus April 2005 Yvonne Gawlina mixed crystal gl gtot gs B A

12 Vapour Liquid Solid method
Eutectic: - coexistence of 3 phases lowest temperature where system is still totally liquid minimum of liquidus curve solid in solid + liquid phase consists of only one material Eutectic T A B solidus liquidus liquid Mixed crystal A + liquid B+ liquid April 2005 Yvonne Gawlina

13 Vapour Liquid Solid method
- mix of semiconductor and metal eutectic melting point of Semiconductor with metal lower - growth of one pure material ® metal as catalyst T A B l Mixed crystal A + l B+ l Growth procedure: reactant vapour reactant vapour reactant vapour reactant vapour April 2005 Yvonne Gawlina metal metal +Sc metal +Sc metal +Sc Sc Liquid catalytic nanocluster Nanowire nucleation Nanowire growth supersaturating

14 Vapour Liquid Solid method
Synthesis of multicomponent semiconductor, like binary III-V materials (GaAs, GaP,InAs, InP) ternary III-V materials (GaAs/P, InAs/P) binary II-VI materials ( ZnS, ZnSe, CdS, CdSe) binary Si Ge alloys Pseudobinary phase diagram GaAs T Au liquid Au + GaAs Au + liquid GaAs+ liquid April 2005 Yvonne Gawlina E.g. Au - GaAs pseudobinary phase diagram

15 Vapour Liquid Solid method
- critical diameter, so that the liquid catalyst clusters are stable in equilibrium a = surface free energy W = molar Volume R = gas constant T = absolute temperature C = concentration of semiconductor component in liquid alloy C¥ = equilibrium concentration Problem: in fluid at according temperature ® critical diameter about d = 0.2 mm April 2005 Yvonne Gawlina Goal: finding methods to get smaller metal clusters to start NW growth

16 Chemical Vapour Deposition
E.g. growth of GaN nanowires in CVD reactor - Ni catalyst on Si substrate with 0.5 M Ni(NO3)2·6H2O ® drying in oven - formation of Ni islands on Si substrate - Ga and GaN powder in inner reactor - Hydrogen in outer tube to minimise side reactions until 700 °C - Ammonia gas into inner reactor ® start of nanowire growth - Nitrogen gas during cooling phase April 2005 Yvonne Gawlina

17 Chemical Vapour Deposition
CVD reactor 1. Vertical tubular furnance 2. Gas inlet line 3. Ni-coated Si substrate 4. Gas outlet line 5. Outer reactor tube 6. Inner reactor tube April 2005 Yvonne Gawlina

18 Laser Ablation Catalytic Growth
nanometer sized cluster with laser ablation SC SC SC M hn M, SC SC M SC SC SC Laser ablation Vapour condenses in cluster Supersaturation until start of wire growth Transport from growth zone April 2005 Yvonne Gawlina

19 Laser Ablation Catalytic Growth
LCG reactor Cold finger Focus Tube furnace Laser Target in quartz tube April 2005 Yvonne Gawlina Gas: in Gas: out

20 Laser Ablation Catalytic Growth
Results with LCG: with Si: - uniform Diameter down to 3 nm. - Amorphous coating, consisting of SiO2 - Nanocluster at the end of the wire, consisting of metal and Si (e.g. FeSi2) - [111] growth direction Nanowire diameter depends on nanocluster catalyst diameter: April 2005 Yvonne Gawlina Nanocluster nm / / / /- 3.0 Nanowire nm / / / /- 2.7

21 Low temperature VLS method
- metal with low melting point (e.g. Ga ,In, Bi..) - eutectic with very low semiconductor content - silane decomposition by atomic hydrogen e.g. SiHx(g) + xH(g) ® Ga-Si(l) + xH2(g) Ga April 2005 Yvonne Gawlina

22 Low temperature VLS method
a = surface free energy W = molar Volume R = gas constant T = absolute temperature C = concentration of semiconductor component in liquid alloy C¥ = equilibrium concentration E.g. T = 400°C and 1% of Si ® d = 6nm usually with Si conc. of about % ® d= 0.2 mm April 2005 Yvonne Gawlina E.g. Ge with Ga forms eutectic at only 30 °C!

23 Fluid Liquid Solid mechanism
E.g growth of Si nanowires - alkanethiol coated Au nanocrystals (d = 6.7 +/- 2.6 nm) tethered on Si substrate - diphenysilane (C12H12Si) decomposes in supercritical cyclohexane (C6H12) Si Si Si Si Si Si Si Au Au Au SiO2 SiO2 SiO2 April 2005 Yvonne Gawlina Si Si Si

24 Fluid Liquid Solid mechanism
FLS reactor manipulation of NW: - metal seed density and size Diphenylsilane rate Temperature T small: few nanoparticles but nanowires curled - T high: straight nanowires but more nanoparticles April 2005 Yvonne Gawlina

25 Solid Liquid Solid mechanism
E.g. amorphous Si nanowires with SLS Si nanowires Si - Ni alloy Ni Si substrate Si Si Si Si Si Si Si Si Heat Heat Heat Ni coated Si substrate Heat ® diffusion of Si into Ni Supersaturating of Ni Growth of Si nanowires April 2005 Yvonne Gawlina

26 Oxide Assisted Growth - Oxides as catalyst instead of metal
- production of Ge nanowires, Si nanowires, carbon nanowires, silicon and SnO2 nanoribbons, Group III - V and II - VI compound semiconductor nanowires April 2005 Yvonne Gawlina Ge nanowires Silicon nanoribbons

27 Oxide Assisted Growth Process of OAG for Si:
- SiO2 powder added to Si (SiO2 needed throughout process) - ablation of powder - silicon sub-oxides form bonds with Si substrate - “dangling bonds” act as nuclei - Si takes places of oxide ® start of nanowire growth and outer layer of SiOx April 2005 Yvonne Gawlina

28 Oxide Assisted Growth - kinds of silicon oxide cluster:
+ oxygen rich cluster + silicon rich cluster + silicon monoxide like clusters (Si : O = 1:1) - highest reactivity in Si rich cluster - growth surpressed in certain directions yield [001] [110] April 2005 Yvonne Gawlina Triangle [110] Rough circle [110] Rough rectangle [112] Pentagon [001]

29 OAG, VLS and temperature
For Si: T = 1100 °C °C : d gets smaller with decreasing T, metal found in wire ® VLS mechanism with [111] as favoured growth direction T = 850 °C °C : no metal in wire ® OAG region, diameter not dependant on T T = 1100 °C : Coexistence of OAG and VLS April 2005 Yvonne Gawlina

30 Nanowires Summary of some single crystal nanowires synthesised
Minimum d in[nm] Ratio of components structure Material GaAs GaP GaAs0.6P.0.4 InP InAS InAs0.5P0.5 Zns ZnSe CdS CdSe Si1-xGex 3 3-5 4 4-6 Zinkblende Wurtzite Diamant 1.00 : 0.97 1.00 : 0.98 1.00 : 0.58 : 0.41 1.00 : 1.19 1.00 :0.51 : 0.51 1.00 : 1.08 1.00 : 1.01 1.00 : 1.04 1.00 : 0.99 Si1-xGex April 2005 Yvonne Gawlina

31 Properties of Nanowires
PL characterisation PL dependence on direction: parallel ® “on” perpendicular ® “off” - intensity uniform along wire - periodic cos2a dependence April 2005 Yvonne Gawlina

32 Properties of Nanowires
Size Dependant PL April 2005 Yvonne Gawlina - Shift to higher energies with decreasing diameter - Quantum confinement effects below d = 20nm - T - dependant shift

33 Properties of Nanowires
Size Dependant PL Theory: particle in an infinite cylinder Wave function: Energy shift April 2005 Yvonne Gawlina

34 Properties of NW Polarised excitation and emission
Polarisation rate: Most nanowires r = 0.96 Theory: infinite dielectric cylinder in vacuum and laser is constant Excitation emission April 2005 Yvonne Gawlina Solid line: parallel dashed line: perpendicular ® with e = 12.4 for InP ® r = 0.96

35 Properties of NW Polarised Photodetection photodetector
Conductance vs. power density: upper branch: light parallel polarised lower branch: light perpendicular polarised April 2005 Yvonne Gawlina Conductance vs. polarisation angle

36 Properties of NW Thermal conductivity Cv= specific heat
v = velocity of phonons l = mean free path Mean free path for phonons in solids in the nm range Alteration of phonon transport in nanowires: - more boundary scattering - changes in phonon dispersion relation - quantization of phonon transport April 2005 Yvonne Gawlina

37 Properties of Nanowires
Thermal conductivity Si NW thermal conductivity 2 orders of magnitude smaller than in bulk Si April 2005 Yvonne Gawlina Deviation from the Debye T3 law

38 Properties of NW Doping
- possible to dope nanowires, e.g silicon: boron doped ® p-type phosphor doped ® n-type April 2005 Yvonne Gawlina Lightly doped Heavily doped ® metallic ® many new exciting possibilities for application of nanowires

39 Applications - Nanowire heterostructures
+ axial heterostructures, e.g GaP-GaAs heterojunction + radial heterostructures, e.g. Si-Ge + Nanowire superlattices April 2005 Yvonne Gawlina

40 Applications - Sensors + pH sensors
+ gas sensors (e.g. Ammonium, Water) - Single mode optical wave guides Gas in Gas out April 2005 Yvonne Gawlina

41 Applications ® light from crossing point at forward bias)
- Nanophotonics + nanoLEDs (p and n type nanowires in crossed nanowire device ® light from crossing point at forward bias) - Nanoprobes + Tips for Atomic Force Microscopy - High temperature, high current superconductors - Lasers (electrically driven) - nanoFETs etc. April 2005 Yvonne Gawlina

42 Summary Synthesis Pseudowires Free standing Nanowires Properties PL
Thermal Doping Applications April 2005 Yvonne Gawlina

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