Nanotechnology and Nanoelectronics Presented by S. Mohajerzadeh Department of Electrical and Computer Engineering University of Tehran
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
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.
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.
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.
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.
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
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.
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.
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
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
º400C دما: 5/6W/cm2چگالي توان: Nano-particles
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
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
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.
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
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 ,…
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).
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
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
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.
Furnaces, • Applications in high temperature oxide growth, • CVD of oxide, nitride layers, • Batch process, temperature dependence • Hot wall vs. cold wall
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
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
Electron Beam evaporators • Versatile and easy to use, • Low or medium quality layers, • Mostly for metals and coatings
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
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