Report

XI. Reflection high energy electron diffraction Surface reconstruction studied by RHEED --- used in MBE (molecular beam epitaxy) Schematic of a RHEED setup The distance from sample to screen, L, and the energy of the electron beam must be known to derive the lattice spacing of the crystal. Streaky RHEED pattern (1) Streaky pattern t L tan( 2 ) 2d hk sin a 2 2 sin a small h k2 t sin t L tan( 2 ) 2 L 2L a t 2 h2 k 2 2L a L h 2 k 2 t Higher accuracy at longer L since t is longer. (2) Sensitivity of RHEED (2-1) Coherence zone The electron beam can be treated an in-phase source only within a coherence zone. The finite coherence zone is due to both finite convergence and finite energy spread of the electron beam. (a) Energy spread ∆E of incident electrons gives time incoherent 2k 2 2 2kk E E 2m 2m E 2k kE k E k 2E k 2k sin s t || Resolved parallel to surface, for small s kE kE k 2 sin s s 2E E t || (b) Spread in arrival angle over 2s gives spatial incoherence, k||s k sin s ( k sin s ) k 2 s (c) Combine uncertainties and define coherence zone diameter X t 2 s 2 Xk|| 2 k|| ( k|| ) ( k|| ) k|| ( k||t ) 2 ( k||s ) 2 ( kE s / E ) 2 ( 2k s ) 2 k|| 2k s 1 ( E / 2 E ) 2 2 2 X k|| 2k s 1 ( E / 2 E ) 2 2 s 1 ( E / 2 E ) 2 -Typical RHEED, E = 100 keV, E = 0.5 eV, and s = 10-5 rad X 200 nm! -Typical LEED, E = 50-200 eV, E = 0.5 eV, and s = 10-2 rad X 5-10 nm! RHEED＆LEED are sensitive to the order of surface atoms inside the area defined by coherence zone (area within the radius of X)! (2-2) Island growth 3. Surface reconstruction of GaAs(001)2x4 http://newton.ex.ac.uk/research/qsystems/people/jenkins/smg/ gaas/gaas_001_beta2_2x4_top.gif RHEED patterns of As-stabized GaAs(2x4) (30 KeV) (a) Along [ 1 1 0] (b) Along [1 1 0] Explanation: (1) Reciprocal lattice structure of GaAs(100)2x4 (2) RHEED patterns along two perpendicular directions 4. RHEED oscillation (adapted from Wiki) With RHEED oscillation, the epitaxial growth of a film within a precision less than a monolayer can be achieved. Mechanisms of RHEED intensity oscillations during growth of a monolayer. 5. A/B tilted superlattice ( Petroff)