Nanotechnology and Nanoelectronics

1. Nanotechnology and Nanoelectronics Presented by S. Mohajerzadeh Department of Electrical and Computer Engineering University of Tehran

2. Nano-size formation • Growth from liquid phase, aqueous solutions • Growth from gas phase, CVD, CVS • Growth from vapor phase, PVD, MBE • Sol-Gel solution, nano-particle formation • Hydrogenation assisted growth

3. Nano-rods • Standard patterning is used to form vertical “walls”. • A conformal depositing technique (like CVD) is used to cover the walls as well as the top surfaces, • A plasma (reactive) etching mechanism is utilized to remove the layer from top to the bottom, • The remaining side-walls are nano-metric in size and micro-metric in separation and length. • Silicon-nitride is a suitable choice. Polysilicon is the sacrificial layer.

4. Nano-particles • Gold nano-particles are formed as clusters on already patterned structures. • Original patterns are micro-metric in size, • After removal of the sacrificial layer the gold-containing clusters are exposed • Further annealing is needed to form nano-particles.

5. Electro-chemical reactors • Formation of porous silicon from bulk crystalline silicon, • Usually P-type silicon gives nano-metric pores, whereas n-type silicon yields sub-micrometer structures, • Light emitting devices made by this material. • Use of light is necessary to promote the formation of holes and to form porous silicon.

6. Electro-Chemical processes • Wet etching by HF/ethanol solution • Lack of capability of integration with Si technology • The use of light during the etching promotes the etching in vertical direction, leading to nano-rod structures.

7. Liquid growth • Need to a non-soluble product in liquid reaction, • Colloids grow and agglomerate to form larger islands, • To avoid overgrowth of particles, surfactants are needed, • They cover the surface of colloids and avoid their attachments • Nano-clays are a good example

8. Liquid phase growth • Chemical solution deposition, • First step deposition is like drying, • Burning, annealing, chem. reaction leads to densification of the layer, • Final layer is formed by extended heat treatment which forms a crystalline layer.

9. Sol-Gel processes • Use of aloxides such as M(C2H5O)n where “M” is a metal like aluminum. • Hydrolysis, strong reaction with water, • Forming a Sol(ution). • Subsequent Gel formation by agglomeration of colloidal particles in Sol. • Nano-metric features, de-hydrogenation leads to porous layers • Suitable for gas sensors.

10. CVS approach • Chemical vapor Synthesis, deposition from gas phase • Gas converts into solid (nano-metric) particles in the gas phase. • A proper collection is needed. • Suitable for metallic nano-particles, Al, Ag, Au

11. CVS/CVD reactions • Particle formation in gas phase, • Deposition of nano-size/micro-size particle on the surface, • Not a surface-catalyzed reaction, • Not suitable for layer-deposition • TiO2 formation from TiCl4 and oxygen

12. º400C دما: 5/6W/cm2چگالي توان: Nano-particles

13. Nano-size agglomeration • TEM image (learn later) • Dark field image shows the co-oriented nano-grains • Bright field image (top) shows the overall view of the grain

14. High resolution images

15. Ion Implantation: quantum dots • Ion implantation of Si nano-grains into SiO2 or Si3N4 layers • Heavy cost, high doses of silicon ions • Optoelectronic devices: thick insulating layer? • Compatible with silicon technology • Suitable for MOSFET devices

16. Nano-germanium layers

17. Vacuum effect

18. Layer growth, epitaxy • Layer growth, • Epitaxy is a favorite growth approach in semiconductors • Kinks have two-sides and are favorable for the growth. • Medium growth rate • Wafers are cut with an angle (4degree) to enhance this type of growth.

19. Growth methods • In homo-epitaxy the layer on top is essentially the same as the substrate, • In hetero-epitaxy layers are different. • Three dimensional growth is possible, • Stress relaxation leads to defects in layers

20. Growth vs. Nucleation • There are two stages of the film formation: the initial formation of the little islands or Nucleation and the aggregation of such islands to form a continuous layer (growth). • dni/dt= (Rads + Rdet + 2R1) – (Revap + Rcap +2R’1) • n1: concentration of individual adatoms, n2: pairs of adatoms, n3: clusters of size three, … • Rads: rate of adsorption of individual adatoms, • Rdet: detachment of atoms from larger clusters, • Revap: rate of evaporation, • Rcap: rate of capture by larger clusters, • R1 : rate of breaking of pairs and R’1: rate of formation of pairs. • dn2/dt: rate of changing the pairs, dn3/dt: triplet atoms ,…

21. Growth vs. Nucleation • At the second stage, that is the aggregation of small islands, the Gibbs free energy rules. • We have three cases, the energy of the free-surface (gsur-vac), energy of the covered surface with the layer (gsur-lay) and the energy stored in the layer with respect to vacuum (glay-vac). • g=gsur-vac (1-ε) + (gsur-lay + glay-vac) ε • Here “ε” is the percentage of coverage. • If gsur-vac > (gsur-lay + glay-vac)  increase in “fraction” leads to a reduction in the overall energy, so it is favorable (Frank-vander-Merve mode) • If gsur-vac < (gsur-lay + glay-vac)  increase in “fraction” results in an increment in the overall energy, not favorable (Volmer-Weber growth).

22. Molecular Beam Epitaxy • A fragile, yet valuable equipment!, • Many stages of pumping, • Liquid nitrogen consumptions, 200liters per day!! • Very expensive running • Delicate layers are grown with excellent control on their periodicity and quality

23. MBE apparatus • High accuracy for layer definition, • High quality layers, • Low temperature growth, • Monitoring the growth, • Expensive facility • Extensive use of LN2 • Mostly for compound SC, quantum devices

24. Growth of epi-layer, CVD

25. MOCVD technique • Use of metal-organics like phenyl-phosphine • Poisonous materials, at ppm level • Use of bubbler, • Medium vacuum needed, • Lack of monitoring during the growth.

26. MOCVD apparatus

27. Furnaces, • Applications in high temperature oxide growth, • CVD of oxide, nitride layers, • Batch process, temperature dependence • Hot wall vs. cold wall

28. Chemical Vapor Deposition • Growth from a gas phase, • Conformal deposition • Chance of epitaxial alignment, • Deposition limited by T or rate • High quality films, • Less sharp films • Excellent for HBT’s

29. Layer-by-layer growth • Extreme control on incoming gases, • Essentially A CVD method, • Surface adhesion of first incoming layer • Introduction of oxygen to complete the oxide layer • Formation of layers in an atomic scale. • Atomic Layer Deposition

30. Clean room!

31. Electron Beam evaporators • Versatile and easy to use, • Low or medium quality layers, • Mostly for metals and coatings

32. Sputtering

33. Growing hard-to-deposit films, Evaporating high melting point metals, Excellent adhesion, Stoichiometry is preserved Various cases are possible, Poor quality semiconductor deposition Sputtering units

34. Sputtering, DC or RF

35. Reactive Ion Etching

36. Laser ablation

37. Comparison

38. Langmuir-Blodgett growth • Coating with a solution with hydrophobic and hydrophilic surfaces, • The hydrophilic surface sticks to the surface of the substrate forming a thin nano-layer • Very difficult to form, suitable for enzymes, antibodies, ligands, biosensors