Lecture 14 Nucleation And Growth Of Thin Films And Nanostructures

Lecture 14Nucleation and growth of thin films andnanostructures14.1 Thermodynamics and kinetics of thin film growth14.2 Defects in Films; Amorphous, polycrystalline and epitaxial films14.3 Vacuum film deposition techniques14.3.1 Physical Vapour Deposition (PVD)14.3.2 Epitaxy and Molecular Beam Epitaxy (MBE)14.3.3 Chemical Vapour Deposition (CVD)14.3.4 Atomic Layer Deposition (ALD)14.4 Nanomaterials growth approaches: top-down and bottom-upReferences:1. Zangwill, Chapter 162. Luth, p.89-1143. C.T. Campbell, Surf. Sci. Reports 27 (1997) 1-1114. Kolasinski, Chapter 7Lecture 14114.1 Thermodynamics and kinetics of thin film growthWhat is a “thin film”?How thin films are different from the bulk materials?Thin films may be: Lower in density (compared to bulk analog) Under stress Different defect structures from bulk Ultra-thin films ( 10-20nm): quasi two dimensional Strongly influenced by surface and interface effectsSteps in thin film growth Separation of particles from source (heating, high voltage) Transport Condensation on substrateLecture 1421

Steps in film formation1. Thermal accommodation2. Binding (physisorption and chemisorption)3. Surface diffusion (typically larger than bulkdiffusion)4. Nucleation5. Island growth6. Coalescence7. Continued growthNucleation and growth occurs on defects(or sites with higher bonding energy)Lecture 143Three different growth modes1. Island growth (Volmer – Weber)3D islands formation; film atoms more strongly bound to each other than tosubstrate and/ or slow diffusion2. Layer-by-layer growth (Frank – van der Merwe)generally the highest crystalline quality; film atoms more strongly bound tosubstrate than to each other and/or fast diffusion3. Stranski – Krastanov (mixed growth)initially layer-by-layer, then 2D islandsLecture 1442

Thin film growth is not an equilibrium process!1. Thermodynamics (Gibbs Free energy and phase diagram): can the sold phasebe formed at the given temperature?2. Kinetics (deposition rate and diffusion rate)Artificial superlattice is the best example of manipulating kinetics andthermodynamicsLecture 14514.2 Defects in FilmsCan be divided according to their geometry and shape 0-D or point defects 1-D or line defects (dislocations) 2-D and 3D (grain boundaries, crystal twins, twists, stacking faults, voidsand Lecture 1463

3D defectsCrystal twinsGrain boundary is not random, but have a symmetry (ex.:mirror)Crystal twinStacking faultsfcc: ABCABC ABCABABCABC Stacking faultVoids the absence of a number of atoms to form internalsurfaces; similar to microcracks (broken bonds at thesurface)Based on crystallinity:amorphous; polycrystalline and epitaxal (single crystal)Lecture 14714.3 Vacuum film deposition techniques1. Physical Vapour Deposition (PVD)Evaporation: thermal and electron-beam assistedSputtering: RF and DC MagnetronPulsed Laser Deposition (PLD)2. Molecular Beam Epitaxy (MBE)3. Chemical Vapour Deposition (CVD)Plasma-Enhanced CVD (PE-CVD)Atomic Layer Deposition (ALD) Need good vacuum for thin film growth!Lecture 1484

14.3.1 Physical Vapour Deposition (PVD)Thermal Evaporation for non-refractory materialsE-beam evaporation for refractory materialshttp://www.mdc-vacuum.comLecture 14http://www.mcallister.com/vacuum.html – seea largerLecture14 version attached in Appendix9105

Sputtering Deposition DC for conducting materials RF for insulating materialsMagnetron sputtering is most popular due to high rate and low operation pressureLecture 1411Pulsed Laser Deposition (PLD) Good for multielemental materials (P 1 Torr)Lecture 14126

14.3.2 Molecular Beam Epitaxy (MBE)Molecular Beam Epitaxy(p 10-8Torr)1. Elemental Superlattices: GiantMagneto-Resistiance (GMR)Devices2. Binary III-V Superlattices3. Complex Oxide SuperlatticesLecture 1413EpitaxyEpitaxy (“arrangement on”) refers loosely to control of the orientation of thegrowing phase by the crystal structure of the substratehomoepitaxy: host and growing phase are the same materialheteroepitaxy: host and growing phase are differentOrientation and StrainThere exist orientational relations between dissimilar crystal lattices in contact(e.g., fcc (111)/bcc (110); fcc (100) /rocksalt (100)NishiyamaWassermanLecture 14Kurdjimov-SachsZangwill, Ch.16 147

Epitaxial energyNW: Θ 0o, row-matching parallel to [001] bccKS: Θ 5.26o, rotational epitaxyEpitaxial energy at interface calculatedusing Lennard-Jones pairwise 6-12 potentialNote minima for0oand 5oE is indep. of r a/b for other anglesDefinition of misfit: f a baHeterointerface between 2 diff. crystals:the lattice mismatch is adjusted byedge dislocations or strainLecture 14Zangwill, Ch.16 15Strained vs DislocationsThe type of interface (strained vs dislocations) depends on the thickness of the film andlattice mismatch, f.The energy stored in an interface between epitaxial film and substrate is calculated fromthe relative contributions of elastic strain (deformation of the lattice of the film) andformation of edge dislocationsLeft: film thickness const.Right: misfit const.Often, pseudomorphic growth is found for the first monolayer or so in metals onmetals (i.e., overlayer adopts atomic arrangements of substrate)As film thickness , complexities develop .Lecture 14168

Structural phase diagramWe can illustrate the complexity of growth in the case of fcc(111)/bcc(110)inteface in plot of r vs λabcoupling strenth within filmλ interlayer coupling strenthr Lecture 1417Superlattices grown by MBEComplex oxides are not that complex: many of them are basedon the ABO3 cubic perovskite structureEx.:: SrTiO3, LaTiO3, LaMnO3, LaAlO3, Favorable to atomically smooth layered heterostructuresSrTiO3/BaTiO3/CaTiO3M. Warusawithana, J. Zuo, H. Chenand J. N. EcksteinABO3A: M2 (Ca, Sr, Ba, La)B: M4 (Ti, Zr, Mn)LaTiO3/SrTiO3 (PLD)A. Ohtomo, H. Y. Hwang, Nature 419, 378 (2002)Lecture 14189

Epitaxial oxide material integrated with Si1. Sc2O3/Si(111)Epitaxial vs amorphous or polycrystalline2. SrTiO3/Si(001)SrTiO35nm?Si Epitaxial structures may afford controllableinterfaces (no dangling bonds ) Demonstration of interface stability andidentification of potential stability problemsunder oxidizing/reducing conditionsLecture 14SrTiOSi1914.3.3 Chemical Vapour Deposition (CVD)Precursors are needed!CH4 (g) SWNT H2 (g) 700oC, Fe, Ni catalystsSiH4(g) Si 2 H2 (g)Si(OC2H5)4 (g) SiO2(s) (C2H5)2O (g)Lecture 142010

14.3.4 Atomic Layer Deposition1. MCl4 exposureH2O2. PurgeHCl3. H2O exposure4. Purge MO2 MLSiMCl4(ads, surf) 2 H2O(g) MO2(s) 4HCl(g)M(N(CH3)(C2H5))4(ads, surf) O3(g) MO2(s) Surface saturation controlled process Excellent film quality and step coverageLecture 142114.4 Nanomaterials growth methodsTwo approachesBottom-upTop-downPatterning in bulk materials bycombination ofStructure is assembled from well-definedchemically or physically synthesizedbuilding blocksLithographySelf-assemblyEtchingSelective growthDeposition- can be applied for variety of materials- limited by lithography resolution,selectivity of etching, etc.- require accurate control and tunablechemical composition, structure, size andmorphology of building blocks- in principle limited only by atomicdimensionsLecture 142211

Mechanical Methods (Mechanosynthesis)Low cost fabrication: ball milling or shaker millingKinetic energy from a rotating or vibrating canister is imparted to hard sphericalball bearings (under controlled atmosphere)(1) Compaction and rearrangement of particles(2) First elastic and then severe plastic deformation ofthe sample material formation of defects anddislocations(3) Particle fracture and fragmentation with continuoussize reduction formation of nanograined materialK IC Yσ F πaσF 1YK ICγE aaσF – stress level, when crack propagation leads tofracture; γ - surface energy of the particle; a - lengthof a crack-material with defects with a wide distribution of sizeLecture 1423High-Energy Methods: Discharge Plasma MethodApplication of high energy electric current (monochromatic radiation – laser ablation)Can be used for fullerenes and C nanotubesProcess depend on:-Pressure of He, process temperature,applied currentfinal product requires extensive purificationLecture 142412

Structure of the carbon nanotubesarmchairzig-zagchiralLecture 1425Carbon NanotubesThe structure can be specified by vector (n, m)which defines how the graphene sheet is rolled upA nanotube with the indices (6,3): the sheet is rolled up sothat the atom (0,0) is superimposed on the one labeled (6,3)m 0 for all zig-zag tubes, while n m for all armchair tubesLecture 142613

Chemical Fabrication MethodsAnodizing (and electropolishing)Insulating porous oxide layer is created on a conductive metal anode in electrolytic solution2Al0(s) 2 Al3 6eAnodic reactionOxide-electrolyte interface 2 Al3 3H2O 2 Al2O3 6H Cathodic reaction6H 6e 3 H2 (g)Overall oxide formation reaction:2Al0(s) 3H2O Al2O3 3 H2Porous Al2O3 membranes can be considered asultimate template or mask materialby J.LiuLecture 1427Lithographic MethodsLecture 142814

Top-bottom: High-Aspect Aspect-Ratio Si Structuresnanotextured Si surfacedense silicon pillar arrayLecture 1429Bottom-up: vapor-liquid-solid growth Metal particle catalyzed the decomposition of agaseous species containing the semiconductorcomponents, e.g. Ge, or Ga and As Metal catalyst particles absorb species,becoming saturated with them at eutectic point(relatively low temperature) When semiconductor reaches supersaturation,it precipitates out of the eutectic Metal prepared and deposited/grown on surface Metal droplet size determines eventual wirediameterVLS growth of Ge NWsw/AuLecture 143015

Cartoon of growthMetalliccatalystnanoclusterEutectic (AuGe alloy)NanowireTemperature isnucleation beginscontrolled to keep it in when the liquidthe liquid statebecome saturatedwith GeNanowire growthcontinues as longas Au-Ge alloy staysliquid and Geconcentration is highenoughLecture 1431Preferred crystallographic orientationProposed explanation: For small diameter VLS nanowire, the surfaceenergy minimization of the Si or Ge cap influencesthe Si NW nucleus structure and the growthdirection during NW nucleation event Alternatively, Au/Si interface decides growthdirection, 111 is favored for the lowest-freeenergy (111) solid – liquid interface.For 110 growth aixs, the solidliquid interface is still (111), butsurface energetics may drive thenucleation of a second (111)plane to enable 110 growth,which yields the lowest energysolid/vacuum interfacesLecture 143216

Electrochemical step decoration- minimization surface energy of the step- metal oxide electrochemical deposition reduction (H2)- metal electrochemical depositionLecture 1433Designed Synthesis of Hierarchical StructuresThe evolution of nanowire structural and compositional complexityenabled today by controlled synthesis(a) from homogeneous materials(b) axial and radial heterostructures(c) branched heterostructuresThe colors indicate regions with distinct chemical composition and/or dopingLecture 143417

Organization and AssemblyNW and N Dots materials produced under syntheticconditions optimized for their growth can be organizedinto arrays by several techniques(1) electric - field – directed (highly anisotropic structuresand large polarization)(2) fluidic - flow – directed (passing a suspension of NWsthrough microfluidic channel structure)(3) Langmuir–Blodgett (ordered monolayer is formed onwater and transferred to a substrate)(4) isothermal heating evaporation inducedself-assembly (IHEISA) of spheres on a planar substrate(5) patterned chemical assembly or imprintS.Wong, V.Kitaev, G.A. Ozin,JACS 125 (2003) 15589Lecture 1435Imprint based patterning of metal nanoparticlesLecture 143618

Island growth 6. Coalescence 7. Continued growth Nucleation and growth occurs on defects (or sites with higher bonding energy) Lecture 14 4 Three different growth modes 1. Island growth (Volmer - Weber) 3D islands formation; film atoms more strongly bound to each other than to substrate and/ or slow diffusion 2. Layer-by-layer growth (Frank .