Presentationer av deltagarna Sal F Generellt om kursen/utbildningen. Exempel på nanofenomen runt oss

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1 Upplägg och planering för NanoIntro 13; Lars Samuelson Måndag 2/9: Presentationer av deltagarna Sal F Generellt om kursen/utbildningen. Exempel på nanofenomen runt oss Onsdag 4/9: Viktiga grunder: energistruktur, atomer-molekyler-kristaller Sal F Metaller-halvledare-isolatorer. Bandgap hos halvledare (& isolatorer) Fredag 6/9: Nanofysik: kvantfysik & unika fenomen på nanoskalan Sal F Partikel-våg dualitet, konstgjorda atomer, tunnlingsfenomen Måndag 16/9: Materialvetenskap/teknik syntes på nanoskalan, funktionella material Sal F Epitaxi, nanomaterial, sveptunnel- och atomkraftmikroskop mm mm Tisdagen 17/9: Nanoelektronik och -optik, Nano-energi Sal F Transistorer, lysdioder, solceller mm Onsdagen 18/9: Sal F Övning & Frågestund

2 Micro/Nanoelectronics och technology for the fabrication of integrated circuits and advanced heterostructure devices Epitaxy Epitaxy allows layer-by-layer deposition of monocrystalline materials. Basis for fabrication of lowdimensional structures: quantum wells (QWs), quantum wires (QWRs)& quantum dots (QDs). Extremely good (ML) control of thicknesses Types of epitaxy: liquid phase epitaxy (LPE) vapor phase epitaxy (VPE, MOVPE) molecular beam epitaxy (MBE, also CBE) MBE: frequently used for III-V materials The MOVPE, MBE and CBE methods are used widely 1- RHEED screen for nanostructure/low-dimensional structure growth. 2-effusion oven shutters 3-effusion cells for several elements Example of MOVPE process (T 600 C): 4-cryo shrouds 5-RHEED electron gun TMGa + AsH > GaAs +by-products 6-main shutter TMIn + PH > InP +by-products 7-substrate

3 4 3,5 3 2,5 2 1,5 1 0,5 0 Band gaps of different semiconductors AlN (6.2 ev) SiC GaN BP UV InN IR GaP ZnSe AlP AlAs CdS GaAs Si 4 4,5 5 5,5 6 6,5 7 Lattice parameter (Å) InP Ge PbS AlSb ZnTe CdSe GaSb InAs PbSe SnTe HgTe CdTe InSb III IV V

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5 För lysdioder (LEDs) i UV - blått - grönt dominerar idag AlGaInN

6 B-A-B A-B-A A-B-A-B-A So - how do you do epitaxial growth and how do you form heterostructures? Supplied source atoms/molecules Desorption of excess molecules Growing crystal surface First layer of AlGaAs grown on a substrate of GaAs

7 So - how do you do epitaxial growth and how do you form heterostructures? Supplied source atoms/molecules Desorption of excess molecules Growing crystal surface Thin layer of GaAs First layer of AlGaAs grown on a substrate of GaAs So - how do you do epitaxial growth and how do you form heterostructures? Supplied source atoms/molecules Desorption of excess molecules Growing crystal surface Thin layer of GaAs First layer of AlGaAs grown on a substrate of GaAs

8 So - how do you do epitaxial growth and how do you form heterostructures? Supplied source atoms/molecules Desorption of excess molecules Growing crystal surface Top layer of AlGaAs Thin QW of GaAs First layer of AlGaAs grown on a substrate of GaAs So - how do you do epitaxial growth and how do you form heterostructures? Supplied source atoms/molecules Desorption of excess molecules Growing crystal surface Top layer of AlGaAs Thin QW of GaAs First layer of AlGaAs grown on a substrate of GaAs

9 So - how do you do epitaxial growth and how do you form heterostructures? Top layer of AlGaAs Thin QW of GaAs First layer of AlGaAs grown on a substrate of GaAs So - how do you do epitaxial growth and how do you form heterostructures? Top layer of AlGaAs Thin QW of GaAs First layer of AlGaAs grown on a substrate of GaAs

10 So - how do you do epitaxial growth and how do you form heterostructures? Conduction band (for electrons) Quantized energy levels Energy Valence band (for holes) Growth direction Heterostructures to confine carriers in quantum wells, wires and dots Conduction band (for electrons) Quantized energy levels Energy Valence band (for holes) Growth direction

11 Heterostructures to confine carriers in quantum wells, wires and dots Conduction band (for electrons) Quantized energy levels Energy Valence band (for holes) Growth direction Conduction band (for electrons) Quantized energy levels Energy Valence band (for holes) Growth direction

12 Heterostructures to create tunnel barriers IN CLASSICAL PHYSICS: The electron approaches what acts like a barrier which stops (blocks) the transport Energy Conduction band (for electrons) Valence band (for holes) Growth direction Heterostructures to create tunnel barriers Energy IN QUANTUM PHYSICS: If the barrier is thin the wavefunction can appear on the other side: the electron passes through by a quantum mechanical process, called TUNNELING Conduction band (for electrons) Valence band (for holes) Growth direction

13 Jämförelse mellan en riktig atom och en artificiell atom Väteatom Kvantprick 13,6 ev elektronens ljus tillåtna energinivåer ljus ~ 0,3 ev proton proton What s a Quantum Dot Like? InP dots grown on GaInP/GaAs [110] AFM 10nm [110] 10nm Contains ~10000 atoms K. Georgsson et al., Appl. Phys. Lett. 67, 2981 (1995).

14 Aerosol particles of III-V semiconductors Formation of ultrafine group-iii aerosol particles Size selection Adding group-v precursor Formation of III-V semiconductor nanocrystals K. Deppert et al., J. Aerosol Sci. 29 (1998) 737 Deppert and Samuelson, Appl. Phys. Lett. 68 (1996) 1409 Semiconductor nanoparticles InP GaAs 10 nm 10 nm K. Deppert et al., J. Aerosol Sci. 29 (1998) 737

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17 TOP-DOWN fabrication of 1D devices A top-down approach to making one-dimensional quantum devices. like resonant tunneling via quantum dots. Method pioneered by Randall and Reed at Texas instruments in the late 1980s. However, rather unsatisfactory device properties due to fabrication induced damage and poor lateral control.

18 Comparison between top-down & bottom-up fabrication of complex structures Alternative No. 1: TOP-DOWN fabrication Start with a block of wood and carve a small wooden mini-tree with trunk and branches. Alternative No. 2: BOTTOM-UP fabrication Plant a seed and control bottom-up growth of a perfectly functioning Bonsai tree.

19 A forest of nanotrees with multiply seeded trunks, branches and leaves, with the entire tree being single-crystalline and monolithic. Each level of branches is seeded by Au aerosol nanoparticles, allowing control of: diameter length composition including formation of heterostructures inside branches or at branchleaf interfaces. Kimberly Dick et al.

20 Top view Side view Au aerosol particles deposited on <111>Boriented nanowires (low density)

21 Kimberly Dick et al.

22 New initiative for substrate-free NW-growth AEROTAXY: a revolutionary new way to grow NWs Traditional NW growth (111) Generally accepted notion: NWs grow guided by the substrate on top of which the NW nucleates. The crystalline structure & orientation then governs the structure & orientation of the resulting NWs! AEROTAXY growth w/o substrate Would it be possible (a thought exp!) to initiate NW growth directly from a catalytic gold-particle, which we somehow hold in a nano-tweezer? If so, new possibilities could emerge for production of semiconductors w/o the need for expensive substrates, for areas like solar cells, LEDs, Batteries etc.. New initiative for substrate-free NW-growth AEROTAXY: a revolutionary new way to grow NWs - The growth rate is extremely high, >1µm/s, which is up to 1000 times faster than for normal epitaxial growth! From HRTEM+FFT we can say: - The NWs are perfect ZB, and virtually defect free - Growth direction is <111>B. 50nm Au seedst g = 525 C

23 Heurlin et al., Continuous gas-phase synthesis of nanowires with tunable properties, NATURE 492, 90, 6th Dec. 2012

24 Aerotaxy - Present status In our presently operated Gen 3.0 we produce perfectly straight and untapered GaAs nanowires of length 2-4µm 1"µm" 0.5"µm"

25 Lund Nano Lab First floor Cleanroom class ISO 5 class 100 Semiconductor processing 3 individual anti-vibration platforms Second floor Cleanroom class ISO 7 class 10,000 Semiconductor growth Connected with Berzelius Laboratory

26 Silicon-On-Insulator (SOI) SEM 200mm/300mm wafers w/technician

27 Lithographic techniques Lithography: from Greek writing on stone Lithography: pattern transfer into recording media (resist) and its subsequent transfer to a desired device structure (metallization, etching, ion implantation). Different types: 1. Optical lithography: contact, proximity and projection printing 2. X-ray lithography 3. Electron beam lithography 4. Ion beam lithography 5. Imprint lithography EBL exposure strategy Dedicated EBL system or modified SEM Exposure: sequential writing, pixel-bypixel (exceptions: shaped beam, cell projection lithography, SCALPEL systems) What is needed: 1. Source of e-beam 2. Pattern generator 3. Alignment system

28 R esist Su bstrate Pattern transfer after lithography: lifte - Exposure Developed backward leaning profile in resist (key requirement!) metal (a) (b) (c) metal etching contacts ion impl. etc evaporation of metal dissolution of resist (acetone) Nanoimprint lithography (NIL) New lithographic technique, S. Chou et al (1995) NIL processing: 1. Deposition of a polymer onto substrate. 2. Physical contact between stamp and substrate. Application of pressure (50-80 bar) and heating above T g of the polymer. 3. Cooling and release of stamp from the substrate. 4. Oxygen plasma ashing to remove resist residues on substrate.

29 Site control & morphology of NW growth induced by Au patterns Many applications require a high degree of control over site and morphology Site control - how ideal can we make it? We can determine the growth site by controlling the site of the seed particle. Bare wafer EBL opens up apertures in the resist Metallization, 1-50 nm Au Lift-off defines gold nanoparticle seeds Transferred to growth system, Au particles alloy with the substrate NW growth begins when precursors are introduced CONFERENCES + EXPO For details of EBL- and NIL-defined nanowire arrays, see for instance: T. Mårtensson et al., Fabrication of individually seeded nanowire arrays by VLS growth, Nanotechnology 14, 1255 (2003) T. Mårtensson et al., Nanowire arrays defined by nanoimprint lithography, Nano Letters 4, 699 (2004) InP NW array grown by Thomas Mårtensson using MOVPE