Fachgebiet 3D-Nanostrukturierung, Institut für Physik Techniken der Oberflächenphysik (Techniques of Surface Physics) 2. VL im WS14/15, 28.11.2015 Prof. Yong Lei & Stefan Boesemann (& Liying Liang) Fachgebiet 3D-Nanostrukturierung, Institut für Physik Contact: yong.lei@tu-ilmenau.de stefan.boesemann@tu-ilmenau.de; liying.liang@tu-ilmenau.de Office: Gebäude V 202, Unterpörlitzer Straße 38 (tel: 3748) www.tu-ilmenau.de/nanostruk Vorlesung: Mittwochs (G), 9 – 10:30, C 108 Übung: Mittwochs (U), 9 – 10:30, C 108

Outline for today 0. Introduction and motivation Chemical Vapor Deposition Physical Vapor Deposition Atomic Layer Deposition

Well-controlled pore-opening process to the barrier layer of UTAMs realizing pore-opening and surface nanostructures within the quantum-sized range An alumina barrier layer between the pore bottom and the aluminum foil of as-prepared PAMs. It has a hemispherical and scalloped geometry. Using acidic etching solutions, the barrier layer can be thinned and finally removed.

Nanodots (top view, Pd) Nanoholes (top view, Si)

Tuning of the shapes and sizes of UTAM-prepared nanostructures To control the structural parameters (shape, size and spacing) is very important Controllable sizes and shapes: The pore diameters of the UTAMs can be adjusted from about 10 to 400 nm to yield nanoparticles of corresponding size. Nanometer-sized discs, hemispheres, hemi-ellipsoids, and conics (by changing the aspect ratio of the pores of the UTAMs, and the amount of material deposited through the UTAMs).

After 15 min etching in 5wt% H3PO4 solution (30 oC) The pore diameter is about 5 nm (c) After 18 min etching in 5 wt% H3PO4 solution (30 oC) The pore diameter is about 10 nm (d) After 24 min etching in 5 wt% H3PO4 solution (30 oC) The pore diameter is about 17 nm (e) After 30 min etching in 5 wt% H3PO4 solution (30 oC) The pore diameter is about 22 nm (f)

Highly ordered nano-hemisphere arrays Highly ordered nano-hemisphere arrays. Pore diameter, cell size and thickness of the UTAM are about 80, 105, and 240 nm, respectively. The aspect ratio of the apertures of the UTAM is about 1:3. The average height and base diameter of the nano-hemispheres are approximately 35-40 and 75 nm, respectively.

Metallic nanotube arrays (by ALD) Cover almost all inner surface of AAO

Positive electrode SnO2/MnO2 NT array o-SnO2/MnO2 etching MnO2 a) ALD SnO2 b) Au evaporation etching MnO2 c-SnO2/MnO2 Grote, Lei, et al., Journal of Power Sources, 2014, 256, 37-42. Grote, Lei, et al., Applied Physics Letters, 2014, 104.

The core material: Nanotube opening Partial etching and mechanical removal

Binary nanowire arrays realized by electrodeposition via template TiO2/Ag TiO2/Au TiO2/Ni

The diameter of PS spheres can be controlled within 200 nm - 4.5 μm Surface patterning using polystyrene (PS) sphere template The diameter of PS spheres can be controlled within 200 nm - 4.5 μm Fabrication of Ag Nanoshell Arrays S. Yang, Y. Lei, et al., Adv. Funct. Mater., 2010, 20, 2527

Synthesised by Polystyrene Spheres 3D Ordered Macro-mesoporous Mo:BiVO4 Synthesised by Polystyrene Spheres Adjustable template with interconnected area Suitable infiltration with high infiltration fraction Controllable dual pore diameter in resulting architectures Applicable to various attractive materials PS template BiVO4 Mo:BiVO4

Methoden zur Herstellung von Oberflächen Strukturen CVD Chemical Vapor Depsosition PVD Physical Vapor Deposition, Sputter coating ALD Atmoic Layer deposition Electrochemical deposition Spin coating Template assisted Lithography EB-Lithography, Photolithography Reactive-ion etching Printing Technology Molecular Beam Epitaxy …

I. Chemical Vapor Deposition CVD

Types of CVD CVD: Chemical Vapor Deposition PE-CVD: Plasma Enhanced CVD MP-CVD: Microwave plasma-assisted CVD RPE-CVD: Remote plasma-enhanced CVD MO-CVD: Metal Organic CVD AA-CVD: Aerosol assisted CVD DL-ICVD: Direct liquid injection CVD AP-CVD: Atmospheric pressure CVD LP-CVD: Low-pressure CVD UHV-CVD: Ultrahigh vacuum CVD Different types of CVDs for different materials, precursors, growthrates, substrates…

Chemical vapor deposition Chemical vapor deposition (CVD) is a reaction process to synthesize CNTs from different precursors. Compared to arc-discharge and laser ablation methods, the main advantages of this process are: (1) Production to industrial level (approximately a pound per day synthesized by Carbon Nanotechnology Inc., Houston, Texas). (2) Control over growth of desirable (diameter, length and position) CNTs which is more important for electronic applications. Dissociation of precursors at the catalytic particle surface is the key for CNT growth. Chemical vapor deposition (CVD) can be sorted depending on the energy sources: plasma-enhanced CVD (PECVD), thermal CVD, etc. Thermal - CVD SOURCE: http://www.azonano.com/images/Article_Images/ImageForArticle_3423(1).jpg When a conventional heat source (e.g., a furnace) is used, the technique is called thermal CVD. It consists of a quartz tube inserted into a tube furnace and has a gas inlet on one side and a gas outlet on the other side. The sample is placed onto a quartz boat inside the tube. Example Carbon Nanotubes: Hydrocarbons or CO are used as precursor. A typical growth process involves: 1st: purge reactor with inert gas; 2nd: gas flow is switched for specified growth period; 3rd: gas flow is switched back to inert gas while the reactor cools down. For growth on substrates, catalysts need to be applied on substrate before loading it inside reactor. Typical temperatures for catalytic CVD in CNT growth are in the range of 800–1500 K.

Reaction Process in CVD Mass transport of the reactant Gas-phase reactions Mass transport to the surface Adsorption on the surface Surface reactions Surface migration Incorporation of film constituents, island formation Desorption of by-products Mass transport of by-products

Reaction Process in CVD

a) Epitaxial Growth The term epitaxy describes an ordered crystalline growth on a monocrystalline substrate. Because the substrate acts as a seed crystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate Homoepitaxy: a crystalline film is grown on a substrate or film of the same material. This technique can grow more purified films than the substrate, can fabricate layers with different doping levels and layers of different isotopes. Heteroepitaxy: a crystalline film is grown on a substrate or film, but the materials are different from each other. This technique is used to grow e.g. GaN on Sapphire or AlGaInP on GaAs is different from other thin film deposition methods which deposit polycrystalline or amorphous films, even on single - crystal substrates This technology is often applied to growing crystalline films of materials of which single crystals cannot be obtained and to fabricating integrated crystalline layers of different materials

Epitaxial Growth Homoepitaxy of Si on a Si substrate SiCl4(g)+2H2(g = Si(s)+4HCl(g) at approx. 1000-1200 °C

b) Vapor-Liquid-Solid (VLS) growth Catalytic nanodots on substrate (e.g. UTAM technique) Equilibrium vapor pressure of the catalyst must be small so that the droplet does not vaporize Catalyst must be inert

Nanostructures by CVD 1D Zinc oxide (ZnO) nanowires and nanorods fabricated by CVD. diameters from 20 to 300 nm Length 20 µm Chang et al. Chem. Mater., Vol. 16, No. 24, 2004

Further Examples of CVD Dielectrics: silicon dioxide, silicon nitride… Metal: tungsten, copper, titanium, aluminium … Semiconductors: epitaxial silicon, germanium … Nitrides: TiN, TaN Many other nanostructures ,such as nanobelts, nanotube, SnO2 nanoboxes…. SnO2 nanoboxe Carbon Nanotube (SEM) Carbon Nanotube (TEM)

CVD Advantages: high growth rates possible can deposit materials which are hard to evaporate good reproducibility can grow epitaxial films Disadvantages high temperatures complex processes toxic and corrosive gasses

II. Physical Vapor Deposition Thermal evaporation Electron beam evaporation Sputtering

Physical Vapot Deposition - PVD Condensed Phase (mostly solid e.g. Au) Gas Phase (usually solid) evaporation condensation transport

Thermal evaporation holder Resistance heated evaporation sources Alumina crucible with wired basekt used in Ilmenau

Thermal evaporation Simple and in widespread use Common evaporation materials: - Au, Ag, Al, Sn, Cr, Sb, Ge, In, Mg, Ga … - CdS, PbS, Cdse … Use W, Ta or Mo filaments to heat evaporation source Typical filament currents are 200-300 Amperes Typical deposition rates are 1-20 Angstrom/sec. Can only achieve temperatures of about 1800°C

Electron beam evaporation electron beam heated evaporation source thermal evaporation mass transport condensation and layer growth If particles collide with each other or with other gas particles during the transportation they can lose a part of their energy which is required for the later layer growth. To reduce these energy losses the process pressure has an importan influence because it limits the mean free path (λ) essentially. λ can vary from 68 nm at atmospheric pressure to 105 km in ultra-high vacuum

Electron beam evaporation More complex, but extremely versatile Achieves temperatures up to 3000 °C Typical emission voltage is 8 – 10 kV Evaporation crucibles in a copper hearth Typical deposition rates 0.2-100 Angstrom/second Common evaporation sources - all materials accommodated by the thermal evaporation - Ni, Pt, Ir, Rh, Ti, V, Zr, W, Ta, Mo Al2O3, SiO, SiO2, SnO2, TiO2, ZrO2 E-beam evaporation in Ilmenau

Sputtering The substrate is placed in a vacuum chamber with the source material, named a target, and an inert gas (such as argon) is introduced at low pressure. A gas plasma is struck using an RF power source, causing the gas to become ionized. The ions are accelerated towards the surface of the target, causing atoms of the source material to break off from the target in vapor form and condense on all surfaces including the substrate. SOURCE: http://www.tcbonding.com/images/sput_diagram.gif

PVD advantages disadvantages Low substrate temperature Conformal film Relatively fast process Comparatively low cost Excellent thickness control No stoichiometric films By-products incorporated Cracking Peeling No high aspect ratio materials

Evaluation of layer thickness – oscillating crystal The thickness of a layer fabricated by thermal or electron beam evaporation can be measured continuously during the experiment by an oscillating crystal. The measuring method is based on the frequency shift of the oscillating crystal, which is caused by the material being evaporated onto the crystal. Thereby the resonance frequency is decreased with increasing material being deposited. used crystals

Evaluation of layer thickness – oscillating crystal 1. Δ𝑓 𝑓0 =− Δ𝑑 𝑑0 =− Δ𝑚 ρ𝑞∗𝐴∗𝑑0 f0= frequency of the cristal d0= thickness of the cristal ρq= density of the cristal A= area of the cristal Δm= mass of deposited layer Under consideration of: 2. φ= Δ𝑚 𝐴 and 3. 𝑁=𝑓0∗𝑑0 φ = mass coverage N = frequency constant This yields the frequency shift: 4. Δ𝑓=− 𝑓0∗φ 𝑑0∗ρ𝑞 =− 𝑓02 𝑁∗ρ𝑞 φ This is only valid, if Δ𝑚≪mq; mq= mass of crystal

Evaluation of layer thickness – oscillating crystal 3.1 FrequenzmeBmethode Die ideale Dickenscherschwingung einer Quarzplatte (Abb. 3), deren Dicke da sehr vie1 kleiner als die Ausdehnung in xund z-Richtung ist, stellt eine in y- Richtung stehende Welle dar mit der Schallgeschwindigkeit Cg = 3340 rn/s fur den AT-Schnitt, (= 5000 m/s fur BT). In der Grundschwingung ist die Dicke der Platte dQ genau eine halbe Wellen- Iange und die Frequenz fQ = co/dQ wird: fQ = C& . dg = NAT/dQ (2) NAT = c0/2 heiOt Frequenzkonstante, ihr Wert ist: NAT = 1670 m/s = 1670 kHz . rnm Je kleiner die Dicke do ist, umso groBer wird die Frequenz der Scherschwingung. Bei 1 MHz betragt die Dicke 1,67 mm, bei 6 MHz etwa 0,28 mm, bei 10 MHz etwa 0,167 mm. VergroOert man bei einem 6-MHz-Quarz die Dicke da nur um 1 nm (z. B. durch Aufdarnpfen weniger Atomlagen Si02), so andert sich die Frequenz des Quarzes deutlich meObar; sie sinkt um 20 Hz, wie man aus (2) leicht berechnet. Natiirlich verandert auch eine aufgebrachte dunne Fremdschicht die Frequenz der Quarzschwingung. Dies ha Sauerbrey [3] 1959 erkannt und fur die Schichtdickenmessung benutzt. An der Ober- und Unterseite des Quarzes befindet sich jeweils ein Schwingungsbauch; die Eigenschwingungsfrequenz wird daher in der Nahe der schwingenden Grenzflache nur durch die Massentragheit und nicht so sehr durch die elastischen Eigenschaften beeinflu0t. Eine dunne Fremdschicht wirkt also in erster Naherung so wie eine Quarzschicht gleicher Masse. So ergibt sich fur das Verhaltnis der Massen einer dunnen Schicht und des Quarzes: wobei fc die (verminderte) Frequenz des beschichteten (coated) Quarzes bedeutet. Gleichung (3) gilt unabhangig vom Material der Massen ms und mo. Da die Massen proportional zur Dichte und zur Dicke sind, berechnet man aus Gleichung (3) die Frequenzanderung (4) proportional zur Massenbelegung eS. ds bzw. Schichtdicke ds. Fur einen 6 MHz-Quarz, (eo = 2,65 g/cm3), berechnet man z.B. bei einer Massenbelegung von g/cm2 eine Frequenzanderung von 0,7 Hz, was etwa die MeOgrenze darstellt. Bei einer Nikkelschicht bedeutet das eine Schichtdikke von = 0.01 nm, also ein Zwanzigstel einer Atomlage. Die Bestimrnung der Massenbelegung durch Messung der Frequenzanderung des Schwingquarzes unter Benutzung der Gleichung (4) wird als ,,Frequenzme0methode" bezeichnet. Sie wird in Schichtdickenmonitoren der ersten Generation bis etwa 1970 verwendet. Die lineare Abnahme der Frequenz mit steigender Schichtdicke gilt nur bei kleinen Dicken. Bereits bei einer relativen Frequenzanderung (fQ-fc)/fQ < 2 YO ergeben sich Fehler der gemessenen Schichtdicke uber 2 %; d. h. fur 5 MHzoder 6 MHz-Quarze bedeutet das eine Gesamtfrequenzanderung von weniger als 100 KHz; danach ist die .Lebensdauer" des Quarzes erreicht; d. h. der Zusammenhang zwischen Frequenzanderung und Schichtdicke ist nicht mehr linear. Evaluation of layer thickness – oscillating crystal The layer thickness d=Δd can now beeing calculated, if the layer density ρs is known. From: 5. 𝑑= Δ𝑚 ρs∗𝐴 under consideration of 2. and 4. follows 𝑑=− 𝑁∗ρq∗Δ𝑓 𝑓02∗ρs

Frequency shift for different materials Layer thickness Frequency shift

III. Atmoic Layer Deposition Introduced with a name of Atomic Layer Epitaxy in 1974 by Dr. T. Suntola (Picosun Board Member) Mr. Sven Lindfors (Picosun CTO) and the early ALD reactor in 1978 Picosun ALD in Ilmenau

Principles of ALD ALD is a chemical gas phase thin film deposition method based on alternate, saturative, surface reaction

The ALD process „window“

Factors affecting ALD surface reactions Growth rate in ALD is typically 1Å/cycle or less. Cycle time varies Higher growth rates indicate in most cases the CVD growth ALD surface reactions can be affected by Reactivity of the precursor Reaction mode (ligand exchange, dissociation, agglomeration) Reactivity of the ligand removal agent at the selected temperature Number of the reactive sites Reaction mode (monofunctional, bifunctional) Size of the precursor, i.e. steric hindrance

Reviews about ALD mechanisms ‘Atomic layer deposition: an overview’, Chemical Reviews 110, 111 (2010) ‘Surface chemistry of atomic layer deposition: a case study for the TMA/water process’, Journal of Applied Physics 97, 121301 (2005) ‘Atomic layer deposition chemistry: recent developments and futrure challenges’, Angewandte Chemie, international edition 42, 5548 (2003) ‘Atomic layer deposition: from precursors to thin film structures’, Thin Solid Films 409, 138 (2002)

Key advantages of ALD Surface controlled (self-limiting) thin film ~100% conformal Precise thickness control Excellent uniformity Pinhole-free films Repeatable process Low process temperature Graded or mixed layers/nanolaminates High aspect ratio materials

Multiple Materials ‘Atomic layer deposition of transition metals’, Nature Materials 2, 749 (2003)

Ultrathin layer for high k gate As the size of electrical devices is scaled down continuously, it is said that the gate thickness need to be down to 1.0 nm in the near future. At this point, ALD is the only promising technique. (ZrO2, HfO2) ‘Self-aligned ballistic molecular transistors and electrically parrallel nanotube arrays’, Nano Letters 4, 1319 (2004) ‘Parralel core-shell metal-dielectric-semiconductor germanium nanowires for high-current surround-gate FETs’, Nano Letters 6, 2785 (2006) And other articles about high K gate oxides.

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