Thin Films 1 SYNTHESIS OF THIN FILMS THIN FILMS ARE VITAL IN MODERN TECHNOLOGY: Protective coatings Optical coatings Filters, mirrors, lenses Microelectronic devices Optoelectronic devices Photonic devices Electrode surfaces Photoelectric devices, photovoltaics, solar cells Xerography Photography Lithography Catalyst surfaces Information storage, magnetic, magneto-optical, optical memories Thin Films 2 SYNTHESIS OF THIN FILMS Crystalline, Amorphous, Microcrystalline Monolayer, multilayer, superlattice, junctions Free-standing, supported Epitaxial (commensurate), incommensurate Thin Films 3 Free-standing films Fe(CO)5 + O2 amorph. Fe2O3 Prepared on as a film on a NaCl crystal support Dissolution of NaCl in water = Free-standing film Thin Films 4 SYNTHESIS OF THIN FILMS FILM PROPERTIES DEPEND ON NUMEROUS CONSIDERATIONS: Thickness Surface : volume ratio Structure, surface versus bulk, surface reconstruction, surface roughness Hydrophobicity, hydrophilicy (Si-OH vs. Si-H) Composition Texture: single crystal, microcrystalline, domains, orientation Form: supported or unsupported, nature of substrate Thin Films 5 SYNTHESIS OF THIN FILMS The TSK Model of a Surface * Terrace * Step * Kink * Vacancy * Adatom * Island Thin Films 6 Thin Films 7 SYNTHESIS OF THIN FILMS Surface diffusion coefficient D D = a2 ks a ... effective hopping distance between sites ks ... site-to-site hopping rate of an adatom ks = A exp(-Vs/kbT) Vs ... potential-energy barrier to hopping from site to site T ... substrate temperature Higher temperature = Faster diffusion Thin Films 8 Si(111) Surface Silicon "diamond lattice" structure a = 5.463 Si(111) = a set of atomic planes One plane outlined with red Thin Films 9 Si(111) Surface A top view of the atomic arrangement for the (111) plane orange = the top layer green = deeper layers Thin Films 10 Thin Films 11 7x7 reconstruction When (111) surface of Si is heated to high temperatures under the Ultra-High Vacuum conditions the surface atoms rearrange to a more energetically stable configuration called 7x7 reconstruction. STM image of Si(111) surface Thin Films 12 7x7 reconstruction Thin Films 13 7x7 reconstruction 3D representation of the 7x7 STM image The image area is 18x8 nm2, the height of the "bumps" is only about 0.04 nm Thin Films 14 Si(100) Surface ideal reconstructed Thin Films 15 STM images of the silicon-silicon dimers imaged with (a) Vsample = -2.0 V and (b) Vsample = 2.3 V. The filled and empty states of these highly ordered dimers can be probed by biasing the surface in the opposite directions. The dimensions of the figure are 2.3 nm x 7.7nm. Thin Films 16 H-terminated Si(100) Surface Thin Films 17 SYNTHESIS OF THIN FILMS MAIN METHODS OF SYNTHESIZING THIN FILMS: CHEMICAL, ELECTROCHEMICAL, PHYSICAL Cathodic deposition, Anodic deposition, Electroless deposition Thermal oxidation, nitridation Chemical vapor deposition (CVD) Metal organic chemical vapor deposition (MOCVD) Molecular beam epitaxy, supersonic cluster beams, aerosol deposition Liquid phase epitaxy Self-assembly, surface anchoring, SAM Discharge techniques, RF, microwave Laser ablation Cathode sputtering, vacuum evaporation Thin Films 18 SYNTHESIS OF THIN FILMS CATHODIC DEPOSITION Two electrodes, dipped into electrolyte solution External potential applied Metal deposition onto the cathode as thin film Anode metal slowly dissolves ELECTROLESS DEPOSITION Spontaneous, No applied potential Depends on electrochemical potential difference between electrode and solution redox active species to be deposited Both methods limited to metallic films on conducting substrates ANODIC DEPOSITION Deposition of oxide films, such as alumina, titania Deposition of conducting polymer films by oxidative polymerization of monomer, such as thiophene, pyrolle, aniline Oxide films formed from metallic electrode in aqueous salts or acids Thin Films 19 SYNTHESIS OF THIN FILMS Example: Anodic oxidation of aluminum in oxalic or phosphoric acid Pt|H3PO4, H2O|Al Al Al3+ + 3e- anode PO4 3- +2e- PO3 3+ O2- cathode 2Al3+ + 3O2- -Al2O3 (annealing) -Al2O3 overall electrochemistry: 2Al + 3PO4 3- Al2O3 + PO3 3The applied potential controls the oxide thickness and the rate at which it forms, oxide anions from solution have to diffuse through an Al2O3 layer of growing thickness on the reacting Al substrate, to attain an equilibrium thickness of the alumina film Thin Films 20 SYNTHESIS OF THIN FILMS Self-organizing process observed, whereby a regular array of size tunable hcp pores form and permeate orthogonally through the alumina film Exceptionally useful process for creating controlled porosity membranes, photonic gap materials, template for synthesizing semiconductor nanostructures, host for synthesizing and organizing aligned carbon nanotubes for high intensity electron emission displays, and last but not least, fuel cell electrode materials Thin Films 21 AFM picture of porous alumina film Thin Films 22 AFM picture of porous alumina film Thin Films 23 Porous Alumina Films Thin Films 24 SYNTHESIS OF THIN FILMS THERMAL OXIDATION Oxides, metal exposed to a glow discharge Al + O2 (RT) Al2O3, thickness 3-4 nm Similar method applicable to other metals, Ti, V, W, Zr etc Nitrides, exceptionally hard, high temperature protective coating Ti + NH3 TiN Al + NH3 AlN Thin Films 25 SYNTHESIS OF THIN FILMS CHEMICAL VAPOR DEPOSITION Pyrolysis, photolysis, chemical reaction, discharges, RF, microwave Epitaxial films, correct matching to substrate lattice EXAMPLES OF CVD CH4 + H2 (RF, MW) C, diamond films Et4Si (thermal, air) SiO2 SiCl4 or SiH4 (thermal, H2) a-HSi SiH4 + PH3 (RF) n-Si Si2H6 + B2H6 (RF) p-Si Thin Films 26 Thin Films 27 SYNTHESIS OF THIN FILMS SiH3SiH2SiH2PH2 (RF) n-Si Me3Ga (laser photolysis, heating) Ga Me3Ga + AsH3 + H2 GaAs + CH4 Si (laser evaporation, supersonic jet) Sin + (size selected cluster deposition) Si Thin Films 28 SYNTHESIS OF THIN FILMS METAL ORGANIC CHEMICAL VAPOR DEPOSITION, MOCVD Invented by Mansevit in 1968 Recognized high volatility of metal organic compounds as sources for semiconductor thin film preparations MOCVD PRECURSORS, SINGLE SOURCE MATERIALS Me3Ga, Me3Al, Et3In NH3, PH3, AsH3 H2S, H2Se Me2Te, Me2Hg, Me2Zn, Me4Pb, Et2Cd All toxic materials ­ a problem of safe disposal of toxic waste Example - IR detectors: Me2Cd + Me2Hg + Me2Te (H2, 500o C) CdxHg1-xTe Thin Films 29 SYNTHESIS OF THIN FILMS Specially designed MOCVD reactors Controlled flow of precursors to single crystal heated substrate Most reactions occur in range 400-1300o C Complications of diffusion at interfaces, disruption of atomically flat epitaxial surfaces/interfaces occurs during deposition Photolytic processes (photoepitaxy) help to bring the deposition temperatures to more reasonable temperatures REQUIREMENTS OF MOCVD PRECURSORS RT stable, no polymerization, decomposition Easy handling, simple storage Not too reactive Vaporization without decomposition at modest T < 100o C Low rate of homogeneous pyrolysis (gas phase) wrt heterogeneous decomposition (surface) HOMO : HETERO rates ~ 1 : 1000 Thin Films 30 Heterogeneous reaction on substrate Greater than on other hot surfaces in reactor Not on supports, vessel etc Ready chemisorption of precursor on substrate Detailed surface and gas phase studies of structure of adsorbed species, reactive intermediates, kineticss, vital for quantifying film nucleation and growth processes Electronic and optical films synthesized in this way Semiconductors, metals, silicides, nitrides, oxides, mixed oxides (e.g., high Tc superconductors) SYNTHESIS OF THIN FILMS Thin Films 31 CRITICAL PARAMETERS IN MATERIALS PREPARATION FOR SYNTHESIS OF THIN FILMS Composition control Variety of materials to be deposited Good film uniformity over large areas to be covered, > 100 cm2 Precise reproducibility Growth rate, thickness control, 2-2000 nm layer thickness Precise control of film thickness = accurate control of deposition, film growth rate Crystal quality, epitaxy High degree of film perfection Defects degrade device performance Purity of precursors: usually less than 10-9 impurity levels Stringent demands on starting material purity Challenge for chemistry, purifying and analyzing at the ppb level Demands exceptionally clean growth system otherwise defeats the object of controlled doping of films for device applications Interface widths Abrupt changes of composition and dopant concentration required, quantum confined structures 30-40 sequential layers often needed Alternating composition and graded composition films 0.5-50 nm thickness required with atomic level precision All of the above has been more-or-less perfected in the electronics and optics industries Thin Films 32 SYNTHESIS OF THIN FILMS TECHNIQUES USED TO GROW SEMICONDUCTOR FILMS AND MULTILAYERED FILMS MOCVD Liquid phase epitaxy Chemical vapor transport Molecular beam epitaxy Laser ablation Thin Films 33 PHYSICAL METHODS FOR PREPARING THIN FILMS CATHODE SPUTTERING Bell jar equipment 10-1 to 10-2 torr of Ar, Kr, Xe Glow discharge created Positively charged rare gas ions Accelerated in a high voltage to cathode target High energy ions collide with cathode Sputter material from cathode Deposits on substrate opposite cathode to form thin film Multi-target sputtering creates composite or multilayer films THERMAL VACUUM EVAPORATION High vacuum bell jar > 10-6 torr Heating e-beam, resistive, laser Gaseous material deposited on substrate Thin films nucleate and grow Containers must be chemically inert: W, Ta, Nb, Pt, BN, Al2O3, ZrO2, Graphite Substrates include insulators, metals, glass, alkali halides, silicon Sources include metals, alloys, semiconductors, insulators, inorganic salts Thin Films 34 SYNTHESIS OF THIN FILMS MOLECULAR BEAM EPITAXY Million dollar thin film machine, ideal for preparing high quality artificial semiconductor quantum superlattices, ferroelectrics, superconductors Ultrahigh vacuum system >10-12 torr Elemental or compound sources in shutter controlled Knudsen effusion cells Ar+ ion gun for cleaning substrate surface or depth profiling sample using Auger analyzer High energy electron diffraction for surface structure analysis Mass spectrometer for control and detection of vapor species e-gun for heating the substrate Thin Films 35 SYNTHESIS OF THIN FILMS Thin Films 36 SYNTHESIS OF THIN FILMS PHOTOEPITAXY Making atomically perfect thin films under milder and more controlled conditions, Mullin and Tunnicliffe 1984 Et2Te + Hg (pool) + H2 (h , 200 o C) HgTe + 2C2H6 MOCVD preparation requires 500 o C using Me2Te + Me2Hg Advantages of photoepitaxy Lower temperature operation Multilayer formation Less damage of layers Lower interlayer diffusion Easy to fabricate abrupt boundaries Less defects, strain, irregularities at interfaces CdTe can be deposited onto GaAs at 200-250 o C even with a 14% lattice mismatch GaAs is susceptible to damage under MOCVD conditions 650-750 o C Thin Films 37 SYNTHESIS OF THIN FILMS EXTENSIONS OF PHOTOLYTIC METHODS - LASER WRITING AND LASER ETCHING Laser writing: Substrate GaAs Me3Al or Me2Zn adsorbed layer or gas phase Focussed UV laser on film Photodissociation of organometallic precursor: Me3Al Al + C2H6 Creates sub-micron lines of Al or Zn Thin Films 38 SYNTHESIS OF THIN FILMS Laser photoetching: GaAs substrate Gaseous or adsorbed layer of CH3Br Focussed UV laser Creates reactive Br atoms CH3Br(g) (h ) CH3(g) + Br(g) Br(g) + GaAs(s) GaAs...Brn(ad) GaAs...Brn(ad) GaBrn(g) + AsBrn(g) Adsorbed reactive surface Br atoms erode surface regions irradiated with laser Vaporization of volatile gallium and arsenic bromides from surface creates sub-micron etched line Thin Films 39 Pulsed Laser Ablation Thin Films 40 Pulsed Laser Ablation (a) Initial absorption of laser radiation (indicated by long arrows), melting and vaporization begin (shaded area indicates melted material, short arrows indicate motion of solid­liquid interface) (b) Melt front propagates into the solid, vaporization continues and laser-plume interactions start to become important (c) Absorption of incident laser radiation by the plume, and plasma formation (d) Melt front recedes leading to eventual re-solidification. Thin Films 41 Porous Si SEM of a porous silicon, the pores extending from the surface of the Si(100) wafer down into the bulk, etched from n-type Si(100) (P-doped, 0.75-0.95 ,cm) at 77.2 mA cm-2 for 1 min with a 1:1 solution of 49% HF(aq)/ EtOH. Scale bar is 10 m. Thin Films 42 Porous Si HF:EtOH = 1:2.5 j = 10 mA/cm2 t = 30 min Thin Films 43 Chemistry on Si Surface Thin Films 44 Porous Si 500 600 700 800 900 0 1 2 3 4 2.4 2.2 2 1.8 1.6 1.4 PLIntensity[lin.u.] Wavelength[nm] Energy[eV] T = 295 K exc = 457.9 nm Luminiscence of p-Si Thin Films 45 Thin Films 46 Chemistry on Si Surface Thin Films 47 Hydrosilylation Thin Films 48 Chemistry on Si Surface Thin Films 49 Carbaanion LiR, RMgX Thin Films 50 Thin Films 51 2+2 Cycloaddition Thin Films 52 2+2 Cycloaddition Thin Films 53 DA 4+2 Thin Films 54 Secondary Chemistry Thin Films 55 Secondary Chemistry Thin Films 56 Secondary Chemistry Thin Films 57 Secondary Chemistry Thin Films 58 SELF-ASSEMBLED MONOLAYERS Self-assembly: spontaneous organization of molecules into stable, structurally well-defined aggregates Self-assembled monolayers (SAM): two-dimensional ordered assemblies of long hydrocarbon chains anchored through chemical bonds to surfaces of solid inorganic substrates Alkanethiolates on gold and alkylsiloxanes on silicon dioxide belong the most notoriously studied SAM systems Thin Films 59 Metal surfaces Au, Ag, Cu, Pt, Hg, Fe,... react with Thiols M + RSH M-S-R + 1/2 H2 Disulfides 2 M + RSSR 2 M-S-R Sulfides M + RSR M-S-R Same products formed in all three reactions: thiolates. RSH are more soluble and react 103 faster with Au than RSSR. SELF-ASSEMBLED MONOLAYERS R S R S R S R S R S R S R S R S R S R S R S R S Metal Surface Substrates: gold polycrystalline films on Si(SiO2), glass, mica. Thickness 5-300 nm, sputtering, evaporation Atomically flat Thin Films 60 Tilt Twist Precession Thin Films 61 Thin Films 62 Thin Films 63 SELF-ASSEMBLED MONOLAYERS Thermodynamics Au does not form surface oxide layer Reaction driving force: Au-S bond energy 160-185 kJ mol-1 van der Waals attraction between alkyl chains 6-8 kJ mol-1 per CH2 In t BuSH and n-C18SH competition reaction, the linear alkyl thiol binds 300 ­ 700 times better. Surface coverage 1014 molecules per cm2 C16 chain length 2.2 nm, 32-40° tilted, all-trans Chemical stability: Cu/C18SH sustains HNO3 Thermal stability: Au/RSH loses sulfur at 170-230 °C Thin Films 64 SELF-ASSEMBLED MONOLAYERS Binding modes on Au(111) On-top sites Hollow sites ­ threefold, more stable by 25 kJ mol-1 Bridging sites ­ the most stable!! (QM calculations) Au ­ S ­ C = 180°, sp Au ­ S ­ C = 104°, sp3 , more stable by 1.7 kJ mol-1 barrier to interconversion 10.5 kJ mol-1 Thin Films 65 SELF-ASSEMBLED MONOLAYERS Au(111) Hexagonal array of S, S....S distance 4.97 , interchain distance in crystalline paraffins 4.65 , tilt angles 25 - 30° to reestablish alkyl chain contacts, hollow site binding, 21.4 2 per molecule Ag(111) Hexagonal array of S, S....S distance 4.41 , on-top site binding, more tightly packed alkyl chains, no tilt Thin Films 66 SELF-ASSEMBLED MONOLAYERS Kinetics Au(111) + RSH reactions proceed in two steps: 1. First step, fast (minutes), diffusion controlled Langmuir adsorption, concentration dependent (1 mM 1 min, 1 M 100 min) 2. Second step, slow (hours), disordered film orders to a 2D crystal, surface crystallization, defect healing, trapped solvent expulsion. Mechanisms: alkyl chain flipping, RSlateral diffusion, equilibrium with dissolved RSH, Au atom diffusion, Au in solution. Better crystallinity of films in polar solvents: MeOH, EtOH,... Thin Films 67 SELF-ASSEMBLED MONOLAYERS Surface chemical derivatization HS ­ (CH2)n ­ X X = CH3, CF3, OH, NH2, SH, COOH, COOR, CN, CH=CH2, CCH, Cl, Br, OCH3, SO3H, SiMe3, ferrocenyl, .... Microfabrication Self-assembly, at thermodynamic minima, rejects defects, high degree of perfection. Dimension in the range 1 nm to 1000 m, too large for chemical synthesis, too small for microlithography. High efficiency, spontaneous. Thin Films 68 SiO2 Surfaces SiO2 Surfaces native oxide on Si silicagel Chemical derivatization methods are based on the reactivity of the surface hydroxyl groups with various reagents isolated vicinal geminal Thin Films 69 [O3Si]OH stands for the siliceous surface 1. Grafting Reactions with trifunctional reagents, such as alkyltrichlorosilanes and trialkoxyalkylsilanes, lead to the three-fold attachment of the SiR groups. 3 [O3Si]OH + Cl3SiR {[O3Si]O}3SiR + 3 HCl 3 [O3Si]OH + (MeO)3SiR {[O3Si]O}3SiR + 3 MeOH SiO2 Surfaces Thin Films 70 2. Chlorination/Displacement Method The first step is the replacement of the Si-OH groups by more reactive Si-Cl bonds by chlorination. [O3Si]OH + SOCl2 [O3Si]Cl + HCl + SO2 [O3Si]OH + CCl4 [O3Si]Cl + COCl2 + HCl In the subsequent step, the surface is treated with a Grignard or organolithium reagent with the formation of strong Si-C bonds. [O3Si]Cl + RMgCl [O3Si]R + MgCl2 [O3Si]Cl + RLi [O3Si]R + LiCl 400 °C SiO2 Surfaces Thin Films 71 3. Post Modification Method The organic groups (R) covalently anchored to the siliceous surface by the two previous methods can be subsequently chemically modified. The most extensively developed is chemistry of 3aminopropyl(trimethoxy)silane. A large number of chemical transformations of the amino moiety to other functional groups are known. 4. Hybrid sol-gel method (co-condensation) A thin layer of a hybrid (organically modified) silica gel can be deposited on the silica surface from a solution of TEOS and (MeO)3SiR by controlled hydrolysis and condensation. (MeO)3SiR + (MeO)4Si + 7 H2O [O3Si]R + 7 MeOH SiO2 Surfaces Thin Films 72 5. Organometallic modification method Organometallic reagents, such as metal alkyls, halides, amides, and alkoxides can be used to deposit a monolayer of metal complexes on the surface (MLn stands for an organometallic group, M for a metal, L for a ligand, R" for a short alkyl chain, X for halogen). [O3Si]OH + R"-MLn R"H + [O3Si]O-MLn [O3Si]OH + X-MLn HX + [O3Si]O-MLn [O3Si]OH + Me2N-MLn Me2NH + [O3Si]O-MLn [O3Si]OH + R"O-MLn R"OH + [O3Si]O-MLn SiO2 Surfaces Thin Films 73 These organometallic moieties can serve as attachment points for further modification with long chain alcohols, thiols, carboxylic acids, phosphates, and diketonates. [O3Si]O-MLn + HOR [O3Si]O-MLn-1-OR + HL [O3Si]O-MLn + HOOCR [O3Si]O-MLn-1-OOCR + HL SiO2 Surfaces Thin Films 74 Thin Films 75 Manipulations with SAM Thin Films 76 Manipulations with SAM Thin Films 77 Manipulations with SAM Thin Films 78 Manipulations with SAM Thin Films 79 Manipulations with SAM