How can we answer these questions? Questions to Answer. When you grow A on B. How it s done. Experimental tools: Microscopy.

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1 Building Atomic Bridges Between Dissimilar Materials o o 1930 s Victrola Miniaturization 1960 s Tabletop Turntable 1990 s Walkman 2020 s AAAAAAAAAAA AAAAAAAAAAA AAAAAAAAAAA AAAAAAAAAAA AAAAAAAAAAA AABABBAAAABABB BBBBAAABBAABAB AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA Professor Marjorie Olmstead, Department of Physics olmstd@uw.edu Vacuum Tube L ~ 5 mm Transistor L ~ 5 μm Heterostructure Laser L ~ 5 nm 1 Olmstead Feb Olmstead Feb 2010 Flash Memory Inside your I-Pod Future Devices Interfaces Matter Silicon Based Nanoelectronics: o Join Silicon with things that do what Silicon doesn t do GMR Read Head Nature Materials 6, (2007) W ~ 100 nm 3 Olmstead Feb 2010 B ~ 30 nm Field Effect Transistors Intel Press Release on cnet.com 4 Olmstead Feb 2010 o Increase speed: Strain electron channel by adding Germanium Islands and Interdiffusion o Modulate Light: Add layers of compound materials Interface Compounds and Crystal Symmetry o Add Magnetic Effects Transition-metal doped oxides and semiconductors Unwanted Reactions and Phase Segregation

2 Questions to Answer How can we answer these questions? When you grow A on B Is there intermixing? How does B s structure influence A s? Does A form a flat film (laminar) or form islands? Does A have new properties? 5 Olmstead Feb 2010 Ultrahigh Vacuum o Atmospheric pressure: Surface atoms hit once each 10-9 s o 1 layer/second = 1 micron/hour o Work at ~ atmospheres, grow ~ few layers / minute Microscopy with sub-nm resolution 6 Olmstead Feb 2010 o Atomic spacing ~ nm Atom-specific structural information o Elemental distribution perpendicular to growth direction nm nm 0.14 nm Silicon Lattice How it s done Experimental tools: Microscopy T c Theory Th e im h e- Substrate T sub UHV Reality: Scanning tunneling microscopy (STM): o Electrons tunnel between tip and sample o Measure electronic state corrugation Si(100) 400 x 400 nm 2 Evaporation cell B049 Real space information No direct information of the elements Si(100) 40 x 40 nm 2 7 Olmstead Feb 2010 Synchrotron Source-Berkeley B009 8 Olmstead Feb 2010 Si(100)+ 1 ML Arsenic 400 x 400 nm 2

3 1s Binding Energy h 2s 2p VB Intra-Atomic Atomic Potential Chemical Bonding Other Excitations Extra-Atomic Electrostatic Potential Polarization Screening Inelastic Scattering 9 Olmstead Feb 2010 Experimental Tool: Photoemission Spectroscopy Vacuum Level inelastic scattering Kinetic Energy 2s Counts Si(111):GaSe Bilayer Si 2s Element-specific Environment-specific 80 Sub-monolayer Sensitivity Photon In 40 Si 2p Si 2p - plasmon Counts Se 3d Electron Out Se 3d Ga 3d VB A Few Intrinsic Factors in Heteroepitaxy Surface Structure -- Symmetry, Defects o GaSe vs Ga 2 Se 3 /Si: Substrate control of crystal structure Chemical Reaction Interface Compound Formation o TiO 2 /Si: Buffer layer inhibition of interface reaction Impurity Incorporation Solubility Limits, Phase segregation o Cr and Mn-doped Ga 2 Se 3 /Si: Concentration-dependent structure 10 Olmstead Feb 2010 Surface Symmetry Optoelectronics Heteroepitaxy Semiconductor bonding disrupted at surface (001) Square (111) Hexagon Energy Gap (ev) Optical Band Gap vs. Lattice Parameter GaSe, Ga 2 Se 3, Al 2 Se GaS -Ga 2 S 3 Ga 2 Se 3 -Ga 2 S 3 AlP GaP GaSe ZnS Al 2 Se 3 Si GaTe GaAs InP InSe Ga 2 Te 3 Ge ZnSe AlAs Hexagonal Plane Lattice Spacing (Å) 4.3 ZnTe GaSb InAs 4.4 Matched to Si Non-linear optics Direct band gap Anisotropic Cool growth physics Useful material?? Silicon Crystal (Diamond structure) 12 Olmstead Feb Olmstead Feb 2010

4 Gallium Selenide Crystal Structure Si(111): Passivate Dangling Bonds a hex = 0.97a Si-111 a cube = 1.01a Si-cube (001) Si(111)7x7 AlSe or As-Si(111) H-Si(111) GaSe-Si(111) (111) Ga H As Si Se Silicon 13 Olmstead Feb 2010 Lines Planes Flexible Bonding Configuration: Vacancies and Lone Pairs 14 Olmstead Feb 2010 Layered GaSe Cubic Ga 2 Se 3 Wurtzite Al 2 Se 3 GaSe Bilayer on Si(111)7x7 GaSe Nucleation and Growth on Si(111) 500 x 500 nm 2 Si(111) +1 HBL GaSe 2.7 HBL (1 HBL QL) 500x500nm, 2V, 0.1nA, height range 0.8nm + GaSe at ~ 525 C 500x500nm, 3V, 0.1nA, height range 0.7nm 1.1 HBL GaSe 6.5 HBL (1 HBL + 2.7QL) 10x10nm, 2.2V, 0.2nA, height range 0.1nm 15 Olmstead Feb 2010 Si(111)7x7 1x1 smooth structure Si(111)1x1 4x4nm, 1.7V, 0.2nA, height range 0.03nm PRB Feb Olmstead Feb HBL = 1 HBL QL Surface height [nm] nm GaSe 2 QL Islands 1 0 Si substrate steps Scan distance [nm] T. Ohta Thesis (UW) 2004.

5 1.1QL on bilayer (3.2HBL coverage) Nucleation on Si(111):GaSe Hexagonal, Layered GaSe Triangular Islands 1 QL high (0.8 nm) Carpet on steps over substrate steps Ga Se Si Change Symmetry: Si(001) vs. Si(111) (001) Square Ga 2 Se 3 Linear Square vs. Hexagonal 2 DB/atom vs. 1 DB/atom Triangular 2.7QL on bilayer (6.5HBL coverage) GaSe (111) Hexagon [111] Scale bar 100 nm 17 Olmstead Feb [112] Silicon Crystal (Diamond structure) 18 Olmstead Feb 2010 bare Si(100) Growth on Si(001) Zinc-blende Ga 2 Se 3 (2.5CBL) Nanorod Nucleation and Growth Large scale 500x500 nm 2 Scale Bar 25 nm Bare Si(001) BL Dimer Rows alternate direction on successive terraces + 1 ML As BL Quasi-rectangular islands, 4-8 nm wide x nm long 100x100nm 2, -2V, 0.1nA 100x100nm 2, 5.4V, 0.09nA 3.2HBL As dimer rows also alternate. See occasional islands and trenches +1/3 BL Ga x Se y BL Nanorods narrow, sides straighten 19 Olmstead Feb 2010 on Si(111) 20 Olmstead Feb 2010 Ga x Se y islands perpendicular to As dimer rows Sharp, narrow nanoridges. Shape stable with further growth

6 Ga 2 Se 3 Nanoridge Structure Growth Ga Se Si Al 2 Se 3 on Si(001):As Bulk Al 2 Se 3 is hexagonal o Can we induce cubic? o Does intermixing still occur? o Do vacancies align for nanoridges? -5.4 V, 0.09 na, 20x20nm 2 1 Ga-Se bilayer high Corrugation = Ga-Ga distance Rods perp. to As dimer rows Lateral shift between layers 21 Olmstead Feb 2010 T. Ohta et al, PRL Olmstead Feb 2010 Force cubic? Hexagonal matching possible Helices C. Lu thesis Al 2 Se 3 Growth on Si(001):As Interface Structure and Orientation 1.0CBL 2.0CBL Initial growth o Cubic symmetry o Incipient nanoridges Al 2 Se 3 ridges form As dimer rows 50x50nm 2 2.5CBL 15x20nm 2 3.2CBL Ga 2 Se 3 3.3CBL Al 2 Se 3 50x50nm 2 derivative Ga 2 Se 3 ridges form As dimer rows Thin: Al 2 Se 3 nanoridges sharper than Ga 2 Se 3 Thicker: Al 2 Se 3 disorders, Ga 2 Se 3 orders 23 Olmstead Feb CBL Ga 2 Se 3 15x20nm 2 24 Olmstead Feb 2010

7 Why? Different Reactivity Ga 2 Se 3 : As interdiffuses into Ga 2 Se 3 Al 2 Se 3 : As stays bonded to Si A Few Intrinsic Factors in Heteroepitaxy Surface Structure Symmetry, Defects 3d 2p Silicon Intensity Si-As Bond Si 2p Al 2 Se 3 Arsenic Intensity As 3d Si(100):As + Al 2 Se 3 + Ga 2 Se 3 Al 2 Se 3 Chemical Reaction Interface Compound Formation o TiO 2 /Si: Buffer layer inhibition of interface reaction Impurity Incorporation Solubility Limits, Phase segregation 25 Olmstead Feb 2010 Ga 2 Se Ga 2 Se o Cr and Mn-doped Ga 2 Se 3 /Si: Concentration-dependent structure 26 Olmstead Feb 2010 Chemical Reactions: Good or Bad? Bad o Wrong properties for desired application o Passivate to the point nothing wets the surface Good o Unzip surface reconstruction o Satisfy electron counting at interface o Special properties of unique material TiO 2 : Heteroepitaxial Oxide on Silicon High K dielectric for transistor as area shrinks V Source Gate OXIDE Q V = C = K oa d Drain Ferromagnetic semiconductor for spin-transistor Rashbah effect: Voltage rotates spin 27 Olmstead Feb Olmstead Feb 2010

8 TiO 2 Problem: Unwanted SiO 2 Low dielectric constant layer in series o Large capacitance Amorphous interface o scatters spin-polarization TiO 2 SiO x Si Kaspar JAP 97, (2005) Scanning Tunneling Microscopy Scale bar 25 nm As termination Ga 2 Se 3 Buffer TiO 2 Solution: Nanoscale Buffer Layer TiO 2 TiO 2 As Si Ga 2 Se 3 As Si Co:TiO nm thick 500x500 nm 2 Co:TiO 2 4 nm thick 500x500 nm 2 29 Olmstead Feb Olmstead Feb 2010 Co:TiO 2 /Ga 2 Se 3 /As/Si(001) Photoemission - Si Chemistry doublet from spin Element specific Chemical environment 0.8 Si 2p 0.6 Si 2p vacuum level photoelectron Intensity (Arb. Units) SiO x Intensity (Arb. Units) h 3d 3p 3s nm nm2 2.8 V, 0.09 na 2.9 V, 0.1 na nm V, 0.1 na nm 2 2 V, 0.12 na 2p 2s 1s oxide Si Lend valence electrons to Oxygen -- Remaining electrons are more tightly bound buffer layer 31 Olmstead Feb Olmstead Feb 2010

9 A Few Intrinsic Factors in Heteroepitaxy Surface Structure -- Symmetry, Defects o GaSe vs Ga 2 Se 3 /Si: Substrate control of crystal structure A Few Intrinsic Factors in Heteroepitaxy Surface Structure Symmetry, Defects Chemical Reaction Interface Compound Formation o TiO 2 /Si: Buffer layer inhibition of interface reaction Impurity Incorporation Solubility Limits, Phase segregation o Cr and Mn-doped Ga 2 Se 3 /Si: Concentration-dependent structure Chemical Reaction Interface Compound Formation Impurity Incorporation Solubility Limits TiO 2 o Cr and Mn-doped Ga 2 Se 3 /Si: Phase segregation 33 Olmstead Feb Olmstead Feb 2010 Can we Make the Buffer Layer Magnetic? Ferromagnetism in Cr-doped Ga 2 Se 3 Doping with transition metals o Mn-doped GaAs Ferromagnetic below 100 C Adding Chromium to Ga 2 Se 3 makes it a ferromagnet at room temperature o Cr-doped GaN or TiO 2 Ferromagnetic at room temperature Needs defects to work impurity phases? Cr-doped Pure Si(100):As 35 Olmstead Feb Olmstead Feb 2010 E.N.Yitamben et al., submitted to PRL 2010

10 Surface morphology (solubility limit?) Mn doping leads to MnSe 3% 6% 9% 17% Increasing Cr Concentration 100 nm Pure Si(100):As Cr-doped Pure Si(100):As Low Cr Incorporates into Nanorods Higher Cr Nucleates Islands Islands are Cr-rich New Compound? 8 % Mn Segregates to pure MnSe islands on Ga 2 Se 3 substrate 37 Olmstead Feb Olmstead Feb 2010 T. Lovejoy, et al APL 95 (2009) A Few Intrinsic Factors in Heteroepitaxy Surface Structure -- Symmetry, Defects 3.2HBL Building Atomic Bridges Between Dissimilar Materials o o Chemical Reaction Interface Compound Formation Impurity Incorporation Phase Segregation TiO 2 AAAAAAAAAAA AAAAAAAAAAA AAAAAAAAAAA AAAAAAAAAAA AAAAAAAAAAA AABABBAAAABABB BBBBAAABBAABAB AAAAAAAAAAAAAAA AAAAAAAAAAAAAAA 39 Olmstead Feb 2010 Professor Marjorie Olmstead, Department of Physics olmstd@uw.edu 9 % Mn 9 % Cr 40 Olmstead Feb 2010