Prof. Dan Rich Department of Physics Ben-Gurion University of the Negev, Beer-Sheva, Israel

Transcription

1 Enhancing the optical properties of semiconductor nanostructures with metal films and surface plasmons Prof. Dan Rich Department of Physics, Beer-Sheva, Israel Collaborators: Yevgeni Estrin (Ph.D. student, BGU) Samples were provided by: Dr. Andrey V. Kretinin (Weimann Institute of Science, Israel) Dr. Hadas Shtriman (Weimann Institute of Science, Israel) Dr. Stacia Keller (Univ. California, Santa Barbara, USA) Prof. Steve DenBaars (Univ. California, Santa Barbara, USA) Prof. Amir Sa ar (Hebrew University, Israel) Physics Fete 07

2 An important focus in nanostructures is the design of plasmonic metal/semiconductor composite structures to obtain novel optical device properties for applications in biological labeling/sensing, light-emitting diodes, lasers, and solar cells. Outline of Tal. Growth of GaAs/AlAs/GaAs core-shell nanowires (NWs), InGaN/GaN QWs, and Si nano-crystals (SiNCs) (i.e., quantum dots (QDs)). Deposition of thin metal films: Gold, Silver and Aluminum films on the three material systems with dimensionalities of (i) QWs, (ii) NWs, and (iii) QDs 3. Spatially, Spectrally, and temporally-resolved cathodoluminescence (CL): e- beam probe and easy penetration of thin metal films 4. Assess coupling of excitons (e-h pairs) to surface plasmon polaritons (SPPs) 5. Measure changes in the spontaneous emission rate (SER) of excitons in the nanowires and quantify the Purcell Enhancement Factor (F p )

3 Semiconductor Physics (a) (b) e - e - h + h + Exciton (e-h pair) GaAs, GaN, InGaN Silicon, Germanium

4 The face centered cubic (fcc) Brillouin Zone Relevant Semiconductors: Si, Ge, GaAs and many III-V compounds Special symmetry points: X=(0,,0)/a, L=(/,/,/) /a, K=(3/4,3/4,0)/a, W=(/,,0) /a, U=(/4,,/4)/a

5 Band structure properties Direct band gap GaAs, InSb Indirect band gap Si, Ge

6 Basics of Surface Plasmon Polaritons (SPPs) / 3/ / i i x SP x x x L c i c i dielectric () metal () (Propagation length) (E-field decay length) SPPs are oscillations in the charge density that propagate along the interface between a metal and dielectric. They exhibit a complex wavevector from solutions to Maxwell s equations:

7 0 Examination of the dielectric constants for both the metal ( ) and dielectric ( ) layers and calculation of the dispersion relation ( x ) for surface plasmons. sp n p ( ) - p x c sp ) For small, - and x (/c)( ) / = /v d ) At = sp, - (from below) and x + o Surface plasmon dispersion is below the light line. In general, the phase velocity of SPPs (v ph = /) is less than that of light in the dielectric adjacent to a metal film. x

8 ) Exciton (e-h pair) to SPP coupling (quantum well, wire or dot) ) SPP conversion to a photon Model of the exciton-spp coupling mechanism. (K. Oamoto at al. APL 87, 070 (005)) Scattering occurs by the surface/interface roughness or grain boundaries of the polycrystalline metal film. Momentum change associated with the scattering enables a matching with the vs light dispersion relation. Roughness is a superposition of many gratings with different G. (a) (b) SP light n G light sin SPP n G Light Line a SPP Plasmon DOS: SPP dispersion relation The decoupling of SPPs into light is described by the inematic equation: Re[ SP ()] n G = (/c)sin(). D v g ( ) v g 0

9 Challenges and Motivation for increasing the SER in quantum heterostructures / nanostructures. In x Ga -x N/GaN QWs used for LEDs / Lasers Growth on c-plane (000) Sapphire : Conventional MOCVD techniques lead to a high threading dislocation density (~ cm - ) and V-pit defects, owing to the large lattice mismatch between wurtite (hexagonal) GaN and Sapphire. Large pieo-electric fields (~0 6 V/cm) lead to a reduced oscillator strength. Thus, the non-radiative recombination rate (R NR = / NR ) will compete with the radiatiatve rate (R R = / R ). For metal-coated QWs, coupling between excitons and SPPs may lead to a larger R R for improved IQE for opto-electronics and overcoming green gap in LED performance. E Large strain (~ %), high pieoelectric E-field In 0. Ga 0.9 N QW GaN e () h h () E GaN E c E v

10 Challenges and Motivation for increasing the SER in quantum heterostructures / nanostructures. III-V core-shell nanowires possess a large surface recombination velocity (S). For bare GaAs wires, NR =d/s 0 - s for a NW diameter (d) of 30 nm and S = 0 6 cm/s. For GaN wires, S ~ 0 4 cm/s, but the pieoelectric fields can limit the IQE, as seen for the QWs. 3. Si Nanocrystals (SiNCs) Bul Si is an indirect bandgap material which leads to long e-h lifetimes (~ ms) due to the need for momentum conservation (phonons); limits usefulness in light emitters. However, for small SiNCs (D ~ 3 nm), Momentum conservation is partially relaxed due to Heisenberg uncertainty principle: ~ /D. Quantum confinement leads to quasi-direct transition. Coupling of excitons to SPPs for metal-coated nanostructures for larger QE.

11 MBE Growth of GaAs nanowires: Vapor-Liquid-Solid (VLS) self-assisted catalyst-free growth Gallium droplets are formed on the surface of the wafer which, on reaching a critical sie, become supersaturated and begin nanowire growth. The As fluxes feeding the (Ga, As) droplet: ρ d - direct beam ρ r - re-emitted by environment ρ e - evaporation ρ n - solid nucleation and growth by empty red arrows. Only ρ d is directional, due to As 4 beam incident at angle α. Predictive modeling of self-catalyed III-V nanowire growth, F. Glas et. al., Phys. Rev. B 88, (03). Structural Phase Control in Self-Catalyed Growth of GaAs Nanowires on Si(), P. Krogstrup, R. Popovit-Biro, E. Johnson, M. Hannibal Madsen, J. Nygård and H. Shtriman. Nano Letters 0, 4475 (00).

12 Nanowire growth: MBE growth of GaAs/AlAs/GaAs core-shell nanowires using the selfassisted VLS method Substrate: Si() with native oxide layer Water removal at ~00 o C, outgassing at ~600 o C Growth: initiated by condensation of Ga, T G 640 o C, and a V/III (As 4 /Ga) ratio of ~00 Uniform ~6 nm AlAs shell and ~ nm GaAs capping layer, T G 50 C Wire growth direction:, hexagonal shape with {0} facets Uniform diameter of ~00 nm, ~ m length, aspect ratio ~00, without tapering, and a pure inc-blende structure (TEM). TEM SEM

13 Nanowires were harvested and randomly dispersed onto heavily doped p-type Si wafers coated with ~0 nm-thic Al (to minimie charging in CL). GaAs nanowire metal deposition Polar angle () Orientation angle () We used = 0 (normal incidence). Au and Al evaporation thicness (t 0 ) ~0 nm Flux of metal atoms. Nanowire = 0, any. (Top view). Nanowire > 0, = 0. Si() p + -doped Metal beam t = t 0 cos GaAs core shell Metal AlAs (Side view) GaAs Tapered thicness or crescent shape in cross-section

14 Energy What is Cathodoluminescence (CL)? Electron Beam Interaction with a Semiconductor ~0 ev Electron Source e - R e e - e - e - Surface states h DOS h h h E D E A E g E C E V E F p-type semiconducto r Surface Distance

15 Cathodoluminescence (CL) system SEM Vacuum Sy stem JEOL 590 Advantage of e-beam over laser excitation for metal covered nanostructures: h e-beam Detector Three-axis Manipulator Condens er Lens Scan Coils SEM Imaging & Control External Y X Electron Beam Ellipsoidal Mirror GaAs e-h pair core generation shell volume Metal AlAs C ontroller Coherent Optical Fiber Bundle M onochrom ator or Spectrograph Loc-in Amp/Photon Counting Unit Sam ple Polarier CO M ADC/ Counter Computer Im aging and Spectros copy Liquid H e Stage DAC Y DAC X Schematic diagram of optical collection system and data acquisition setup in our CL system.

16 Time-resolved CL using time-correlated single photon counting Instrumentation Bloc-diagram Method of delayed coincidence in an Inverted single photon counting mode

17 Temperature-dependent CL spectroscopy and time-resolved CL GaAs NBE emission for e-beam excitation of a single nanowire (broadening, red-shift, and reduced intensity as T increases). Carrier lifetimes () from slope of CL transients (single exponential fits). Deposition of Gold reduces at all temperatures relative to bare and Aluminum films.

18 Temperature dependence of R (T) from (T) and the Purcell Factor (F p ) We have extracted R (T) from (T) and I (T). Radiative efficiency: ( T ) ( T ) ( T ) R I( T ) I I 0 is the saturation CL intensity at low temp. 0 F P () = R(bare) / R(metal) GaAs.50 ev at T = 50 K Au/GaAs: SP.804 ev Al/GaAs: SP.700 ev (From vs dispersion calculation) Therefore, we expect F p (Au) > F p (Al)

19 Time-resolved CL and CL imaging of InGaN/GaN QWs with metal films for exciton-to-spp coupling MOCVD-grown In x Ga -x /GaN MQW Samples Metal films: Ag, Au, and Al, thicness of 0 nm Calculation of Energy states and wavefunctions for confined electrons and holes: E-field and confinement potential are different for SQW and CR regions.

20 Time-resolved CL of bare and metal-coated InGaN/GaN QWs MOCVD-grown In x Ga -x /GaN MQW Samples Metal film: Ag, Au, and Al, thicness of 0 nm Defocused e-beam area: ~500 m

21 Temperature Dependence of the Purcell Factor (F P ) for the InGaN/GaN SQW S4 F P (T) = R(bare) / R(metal) The Ag film yields the largest F p, followed by Al and Au. Losses in the metal liely cause decrease in F p as temp. increases. S4 4 QWs in the CR S8 8 QWs in the CR

22 Enhancement in the excitonic spontaneous emission rates for Si nanocrystal (SiNC) multi-layers covered with thin films of Au, Ag, and Al The radiative decay rates of SiNCs are relatively small (lifetimes,, of ~0-6 to 0-3 s) compared to III-V NCs ( ~ 0-9 s). Compatibility with current Si microelectronics technology, Si LED would lead to optical interconnection applications in various stages of integrated-circuit fabrication. SiNCs were grown using plasma enhanced chemical vapor deposition (PECVD). Amorphous silicon (a-si) and SiO thin films were deposited on top of commercial Si(00) wafers. The as-grown multi-layer sample was annealed at 50C for hour in a N flow chamber: a-si SiNCs

23 The wavelength dependence of the lifetime ( ) for Au and Ag films on the SiNC sample. Plots of vs are superimposed on plots of the CL spectra for the bare and metal-covered sample. The results are shown for temperatures of 55 and 50 K. The peas of the lifetime plot are shifted ~0 nm (~30 mev) towards larger for temperatures of 55 and 50 K, consistent with a reduced oscillator strength for larger SiNCs within the ensemble. The maximum ratio (r) of the lifetimes, for which r = (bare) / (metal), is ~.4 and.0 at T = 55 K for the Au and Ag covered samples, respectively. Related to the energy difference between SiNC emission and sp.

24 The calculated vs SPP dispersion relations for metal/sio/sinc The method is the same as used for metal/gan/ingan with different (, T) for SiO p( T) (, T) CP(, T) CP(, T) For Au and Ag, as before. i ( T) Numerical solutions to the dispersion relation: d d air x m m air air c m air, m d d m x m m c m, air air d m m exp x d c m h 0

25 3 4 ) ( ), ( ' '), ( 4 ) ( ), ( d u E c F D D E p i = to 3 represent the vacuum, metal, and GaAs regions ) ( exp exp exp ), ( t t c F p For a dielectric in the transparency region (i.e., () << () ) the energy density of the electric field in the layer is given by ) ( ) ( 8 ), ( E u E However, for metals with dissipation, the expression for the energy density is ) ( 8 ), ( E u E is the damping constant from the Drude model. from Fermi s Golden rule; Phys. Rev. B 60, 564 (999).

26 The calculated Average Purcell Factors F p metal/sio /SiNC F p is the average of F p over the 0-period SiO /SiNC structure. The calculations were performed for two different periodicities (P) of 3 and nm. F p is remarably consistent with the experimental results for r = (bare) / (metal), which are ~.4 and.0 at T = 55 K for the Au and Ag films.

27 Summary The coupling of excitons to surface plasmon polaritons (SPPs) in metal films on GaAs/AlAs/GaAs core-shell nanowires, InGaN/GaN QWs, and Si Nano-crystals was probed using timeresolved cathodoluminescence (CL). Excitons were generated in the metal-coated nanostructures and QWs by injecting a pulsed high-energy electron beam through the thin metal films. The average Purcell enhancement factor, F P, was obtained by the direct measurement of the changes in the integrated NBE CL emission intensities and the temperature-dependent lifetimes. A model was presented for F P for the three material systems, which taes into account the effects of ohmic losses of the metals and changes in the dielectric properties due to the temperature dependence of (i) the intraband behavior in the Drude model and (ii) the interband critical point transition energies which involve the d-bands ofau and Ag.

28 Additional information is found in our papers on this subject:. Y. Estrin, D. H. Rich, A. V. Kretinin, and H. Shtriman, Nano Letters 3 (4), pp (03).. Y. Estrin, D. H. Rich, S. Keller, and S. P. DenBaars, Journal of Applied Physics 7, 04305, pp. -4, (05). 3. Y. Estrin, D. H. Rich, S. Keller, and S. P. DenBaars, Journal of Physics: Condensed Matter 7, 6580, pp. -3, (05). 4. Y. Estrin, D. H. Rich, N. Roenfeld, N. Arad-Vos, A. Ron, and A. Sa ar, Nanotechnology 6, (05).