Mapping Atomic Structure at Epitaxial Interfaces

Transcription

1 Mapping Atomic Structure at Epitaxial Interfaces Roy Clarke, University of Michigan, Ann Arbor, MI Opportunities for interface science at the ERL ERL X-ray Science Workshop: Almost Impossible Materials Science, June 16, 2006

2 Outline At nanoscale, interfaces are everything Possible now: -3D maps of static thin-film/interface structure with atomic resolution -Some dynamic information, including strain, phonons, domain walls Opportunities for ERL: - ultrafast imaging at atomic scale - tracking growth, deposition processes - local probe of nano-structures DeCamp, Reis et al. Nature, PRL

3 Epitaxial Nanostructures Van der Merwe layer-by-layer growth Stransky-Krastanov mixed growth Volmer-Weber island growth Quantum wells, Superlattices, Tunneling devices Quantum dots, self-assembly Surface nanopatterning

4 Epitaxial Nanostructures (example: 6.1Å system ) No-common-atom superlattices NRL XSTM Results InAs-on-GaSb:...Sb-Ga-Sb-Ga-As In-As-In-As... GaSb InAs GaSb-on-InAs: ( inverted interface ): As-In-As-In-Sb-Ga-Sb-Ga... Band-gap tunability 0.2eV to 1.3 ev for IR applications but difficult to control growth Nosho, Barvosa-Carter, Yang, Bennett,Whitman Surf. Sci. 465, 361 (2000).

5 Band-gap tuning InAs layer width (Å) Haugen et al. J. Appl. Phys. 96 (2004) Type II band alignment InAs GaSb

6 Epitaxy and 2D periodicity 2D periodic structure coherent with the substrate (aka epitaxial film ) FILM SUBSTRATE Reciprocal space H If film and substrate are coherent then get interference fringes along the rods: Coherent Bragg Rod Analysis (COBRA) L Bragg Rods measure I(q) along each rod

7 -31L Bragg Rod Profile for sample 811 (InAs on GaSb) Log x-ray Intensity Weak fringes contain info on film and interface structure (APS, undulator eam line ~ 2hrs/rod eed ~ 10 rods) L (reciprocal lattice units) - large dynamic range: 14 orders!

8 total Scattering factor T(q)= S(q) + U(q) Fourier transform of T(q) => electron density reference (substrate) In the complex plane Unknown (film) know only length of T(q) from intensity U(q) T(q) Approximation: U(q)= U(q+Δq) S(q) T(q+Δq) T(q) S(q+Δq) COBRA METHOD S(q) Sowwan et al. Phys. Rev. B 66, (2002).

9 COBRA MAP of Group III (In,Ga) plane InAs GaSb Angstroms 9ML of GaSb on InAs very coherent interface surface roughness ~ 3ML

10 Composition profiles determined from COBRA electron density maps Assumes quaternary system: GaSb GaSb As 4 InAs(4) As As4 2 Ga m In 1-m Sb n As 1-n InAs o: m * : n InAs(2) GaSb Monolayer number

11 GaSb on InAs lattice spacing InAs GaSb Lattice spacing (A) indicates GaAs - like interface Gr.V, site1 5.9 Gr.V, site2 Position(A) Gr.III, site Gr.III, site Theory (Determined from Gaussian fit to electron density peaks)

12 Ferroelectric nanostructure TEM of MBE Superlattices Jiang et al., APL 74, 2851 (1999).

13 Ferroelectrics: Superlattice- Strain Effect bulk BaTiO 3 strained BaTiO HRTEM image [(BaTiO 3 ) 6 /(SrTiO 3 ) ] 5 20

14 COBRA MAP OF FERROELECTRIC INTERFACE SrTiO 3 PbTiO 3 a P Sr O(1) Ti Pb O(2) E b Sample made by MOCVD, Stephenson group in-situ facility, APS

15 COBRA results on ultrathin film of PbTiO 3 Fong, Cionca, Yacoby, Stephenson, Eastman, Fuoss, Streiffer, Thompson, Clarke, Pindak, Stern, PRB 2005 Interface polarization reversal?

16 Towards the EFL Smaller and faster nm : fs High rep-rate and tunability are great features!!

17 ERL characteristics for in-situ materials dynamics studies USING EVERY X-RAY PHOTON kw average power (fiber) lasers on the horizon (mj/pulse at 1MHz)

18 Ultrafast Laser Deposition 2 Substrate Holder: temperature: liquid N 2 to 1000 C rotation & translation Load-Lock Lock for Substrate Transfer Plasma Plume Imaging & Spectroscopy Xe lamp e - Electrostatic Analyzer (charge state & energy) Laser: 100 fs, Hz Multi-Target Selector (4 targets with rotation) Vacuum: 10-9 Torr N 2 backfill: 10-4 Torr RHEED: crystal phase Ellipsometry: thickness & n,k Discharge Ring: +500 to V

19 Co nano-disks on Si ultrafast laser deposition 810nm, 50 fs, 1mJ/pulse 1 khz Number C_35kx 3.5 x 3.5 micron area 40mW SIZE DISTRIBUTION Diameter (nm) 200nm spin vortex state 35kx

20 Probing the initial stages of ultrafast ablation Timescale of ERL matches simulation Laser pump x-ray (diffraction/xafs) at high rep rates

21 Detectors! Investment lags source development Need fast, gated area detectors badly! - efficient direct detection PADs - x2 from cross-scan, scan, more from // channels - n sec gating dynamic range - multiple read architecture - modular -..

22 Summary ERL source could enable dynamic imaging of atomic/spin motions associated with collective phenomena (e.g. fast switching with laser impulse) deposition processes also, esp. for nanostructures combination of fast rep-rate and short pulse matches well to laser pump development (high average power fiber lasers)

23 film x 0 Substrate Crystal with buried film Real space Reciprocal space Electron density U substrate Film S(q) U(q) This choice of origin satisfies the COBRA approximation: slowly varying U(q)

24 MBE Growth and In-situ Analysis Integrated MBE/RHEED/STM/MOS capabilities: RHEED: Realtime surface structure characterization (annealing) STM: surface morphology, roughness, coarsening MOSS: curvature mapping gives mismatch strain information.

25 Typical Growth Conditions GaSb on InAs T sub = 450C GaSb 9ML MBE (30sec) Ga/Sb 1ML/1ML MEE (3s/2s) Sb 1ML MEE (2s) In 1ML MEE (2s) InAs 0.2 micron Combined MBE growth chamber and STM chamber. Mirecki Millunchick Lab, University of Michigan In-situ: k-space Associates KSA 400 and MOSS

26 Sample 814 GaSb on InAs Group III, site 2 Group III electron density In = 49 Ga = Exp ratio ~ 1.6 Ideal = Accumulation of As at interface Position(point#) Group V, site Group V As = 33 Sb = 51 Exp ratio ~ 1 Ideal =