Experimental Condensed Matter Physics Henry FenichelHolographic Imaging Howard JacksonSemiconductor Nano Young KimHi-TC/Strongly Cor. e - David MastNear-field Microwave Richard NewrockJosephson/1D Transport Phillippe DeBray Leigh SmithSemiconductor Spins/Nano Hans-Peter WagnerSemiconductor Nonlinear

Experimental Condensed Matter Physics in the Nanoscale Leigh M. Smith Howard Jackson Jan Yarrison-Rice Sebastian MackowskiAditi Sharma Kapila Hewaparakrama Nguyen Tuan Tak Gurung Amensisa Abdi Firoze HaqueAnthony Wilson

The year(s) of the nano

Reduced Dimensionality Based on Bimberg (1999) Bulk Quantum Well Quantum Wire Quantum Dot Energy   D (E) Energy   D (E) 11 33 22 44 Energy   D (E)  1,1  1,2  1,3 Energy   D (E) Confining the electron motion in at least one spatial dimension affects the energy levels and the density of states…

Nano-Photonics: Controlling the Electromagnetic Field

“Pocket Guide” to our Group Imaging “Developing new techniques for directly imaging small things” Spectroscopy “Using optical spectroscopy to look at the interactions and dynamics of the electronic and vibronic states in nanostructures”

Nano-Imaging: How to see things much smaller than the wavelength of light NSOM: Scanned nano-apertures Fixed Apertures Solid Immersion Lenses

VCSEL Structure Selectively oxidized layers Light Output Injection Current p-DBR n-DBR GaAs AlGaAs 8 nm QWs in 1 cavity Square mesas etched past active layers via RIE Lateral oxidation of high Al content layer forms the aperture 10 µm square aperture leads to transverse multimode structure

Experimental Setup He-Ne Beam Dither Piezo Contact pad Optical fiber to spectrometer Emission from VCSEL Spectrometer & CCD Y X Z Scanning Stage Subwavelength tip aperture (80~100 nm) for spatially resolved information Near field collection (<20 nm from surface) for a spatial picture of modes at surface Spectral resolution for transverse mode differentiation

1-0, 849.6nm 0-1, nm 0-0, 850nm 0-2, nm 2-0, nm Transverse modes at 5mA

Expect states with strong binding (confinement) to CdSe dots. Strain, alloying, and dot-layer morphology very important. Strain Driven Quantum Dot Growth ~ 50 nm ZnSe cap ~ 1  m ZnSe GaAs Substrate z - direction CdSe ZnSe ECEC  V E CdSe E ZnSe ZnSe

Atomic-Force Microscopy Observations “Pancake” in shape Somewhate uniform in size height ~ 2-4 nm, diameter ~ nm Distinguishable from surface variations Number density is about 1000  m -2 ! Characterization of CdSe SAQDs Observations Even at 1.5 ML, CdSe layer not uniform Variation in size both laterally and vertically Co-existence of 2-D platelets and 3-D islands Dots extend above and below the interface Scanning Tunneling Electron Microscopy Phys. Rev. Lett. 85, 1124 (2000). 1.5 ML 2.6 ML

Photoluminescence Spectroscopy  A laser excites electrons from the valence band into the conduction band, creating electron-hole pairs.  These electrons and holes recombine and emit a photon.  We measure the number of emitted photons (intensity) as a function of energy. CB VB E k ħ=Egħ=Eg CB VB ħ  = E g- E ex E ex Exciton band

Looking for single dots GaAs Substrate ZnSe buffer layer (~1  m) ZnSe capping layer (~50 nm) Apertures Al Pad SAQDs Laser Beam Fig. 1

From thousands to tens…

A new high-resolution imaging tool with 200 nm resolution Using a truncated solid immersion lens we can directly image up to a 5x5 micron region of a sample with 200 nm resolution. The excitation laser is de-focused to a 20 micron radius spot. The entrance slit is imaged onto the CCD camera so that each CCD image contains both x-position and wavelength information. Then the sample is scanned across the entrance slit in the y-direction, an image taken at each point. This results in a 100x100x2000 data- cube with x, y and energy along each axis. Such a high-spectral and spatial resolution image can be taken in less than 20 minutes with an appropriate sample. x y

CdTe Nonresonant Excitation xy Shown here are grey-scale and contour- plot images of a 3x3-micron region of the CdTe QDs selected over a limited (0.1 nm) spectral range The dots marked 1 and 2 exhibit single emission lines at and eV, while dot 3 exhibits a cluster of at least 3 dots within 500 nm (partially resolved): two with single emission lines (3a and 3b) and a doublet (3c) presumably from an assymetric dot. Spatial scans of dot 1 show 200 nm resolution along y and 350 nm resolution along x. x y

Position sensitivity of dots QD 1 (A and C) AB C QD 1 (A and B) By spectrally selecting particular emission peaks one can look at the emission profile of each peak. Note that peaks A and B collected near QD1 show clearly that they are separated by 200 nm along the y-direction and and 200 nm along the x-direction On the other hand, peaks A and C are aligned (both in size and position within less than 20 nm. Are A and C from the same dot (biexcitons perhaps), or are they two dots separated by less than 20 nm? x y

Position sensitivity (continued) QD 2 (C and D) D C In another example there are two emission lines (C and D above) emitted near QD 2. These two peaks are separated spatially by 75 nm along y 68 nm along x. x y

Nano-Spectroscopy: Using spectroscopy to look inside small things Polarized Photoluminescence Magneto-Photoluminescence Excitation Spectroscopy

From thousands to tens…

Some dots are different than others…. Symmetric Quantum Dot Asymmetric Quantum Dot

Magneto-PL

Photoluminescence Spectroscopy  A laser excites electrons from the valence band into the conduction band, creating electron-hole pairs  These electrons and holes recombine (annihilate) and emit a photon.  We measure the number of emitted photons (intensity) as a function of energy. CB VB Electronic bound state h  excitation h  emission E k PL Intensity Laser energy Continuum states

PLE spectra for single CdTe QDs  The sharp peaks of about 200  eV linewidth in the PL spectrum reflect quasi-zero dimensional densities of state of the quantum dots  Broad resonances in both PL and PLE spectra are related to LO phonon-assisted absoprtion  Intense and narrow lines in the PLE spectrum originate from direct excitation into an excited state  Excitation spectra vary from dot to dot in ensemble E LO 1 st LO 2 nd LO Laser 3 rd LO

QD and Electron-Phonon Coupling

Recent Publications ( )  “Exciton spin relaxation in CdTe/ZnTe self-assembled quantum dots,” S. Mackowski, T.A. Nguyen, T. Gurung, K. Hewaparkarama, H.E. Jackson, L.M. Smith, J. Wrobel, K. Fronc, J. Kossut, and G. Karczewski, submitted to Physical Review B.  “Optically-induced magnetization of CdMnTe self-assembled quantum dots,” S. Mackowski*, T. Gurung, T.A. Nguyen, H.E. Jackson, L.M. Smith, G. Karczewski and J. Kossut, submitted to Applied Physics Letters (2004).  “Optically controlled magnetization of zero-dimensional magnetic polarons in CdMnTe self-assembled quantum dots,” S. Mackowski, T. Gurung, T.A. Nguyen, H.E. Jackson, L.M. Smith, J. Kossut and G. Karczewski, to be published in physica status solidi (b) (March, 2004)  “Optical Studies of Spin Relaxation in CdTe Self-Assembled Quantum Dots,” S. Mackowski, T. Gurung, T.A. Nguyen, K.P. Hewaparakrama, H.E. Jackson, L.M. Smith, J. Wrobel, K. Fronc, J. Kossut, and G. Karczewski, to be published in physica status solidi (b) (March, 2004).  “Exciton-LO-phonon interaction in II-VI self-assembled quantum dots,” T.A. Nguyen, S. Mackowski, H.E. Jackson, L. M. Smith, G. Karczewski, and J. Kossut, M. Dobrowolska and J. Furdyna, to be published in physica status solidi (b) (March, 2004).  “Tuning the optical and magnetic properties of II-VI quantum dots by post-growth rapid thermal annealing,” T. Gurung, S. Mackowski*, H.E. Jackson, L.M. Smith, W. Heiss, J. Kossut and G. Karczewski, to be published in physica status solidi (b) (March, 2004).  S. Mackowski, L.M. Smith, H.E. Jackson, W. Heiss, J. Kossut, and G. Karczewski, “Optical properties of annealed CdTe self-assembled quantum dots”, Applied Physics Letters, 83, 254 (2003).  T.A. Nguyen, S. Mackowski, H.E. Jackson, L.M. Smith, M. Dobrowolska, J. Furdyna, K. Fronc, J. Wrobel, J. Kossut, G. Karczewski, “Resonant Spectroscopy of II-VI Self-Assembled Quantum Dots: Excited States and Exciton-LO Phonon Coupling”, submitted to Phys. Rev. B. (2003).  “Tuning the Properties of Magnetic CdMnTe Quantum Dots,” S. Mackowski, H.E. Jackson, L.M. Smith, W. Heiss, J. Kossut, and G. Karczewski, Applied Physics Letters, 83, 3575 (2003).  "Nano-photoluminescence of CdSe self-assembled quantum dots: experiments and models," R.A. Jones, Jan M. Yarrison-Rice, L.M. Smith, Howard E. Jackson, M. Dobrowolska, and J.K. Furdyna, Phys. Rev. B 68, (2003).  “Magneto-photoluminescence measurements of symmetric and asymmetric CdSe/ZnSe self-assembled quantum dots,” K.P. Hewaparakrama, N. Mukolobwiez, L.M. Smith, H.E. Jackson, S. Lee, M. Dobrowolska, J. K. Furdyna, in “Proceedings of the 26th International Conference on the Physics of Semiconductors, Edinburgh, 2002,” (World Scientific, 2003).  “Resonant and non-resonant PL and PLE spectra of CdSe/ZnSe and CdTe/ZnTe self-assembled quantum dots,” T.A. Nguyen, S. Mackowski, L.M. Robinson, H. Rho, H.E. Jackson, L. M. Smith, M. Dobrowolska, J.K. Furdyna, and G. Karczewski, in “Proceedings of the 26th International Conference on the Physics of Semiconductors, Edinburgh, 2002,” (World Scientific, 2003).  “Exciton Spin Relaxation in Quantum Dots Probed by Continuous-Wave Spectroscopy,” S. Mackowski, T. A. Nguyen, H. E. Jackson, L. M. Smith, J. Kossut, and G. Karczewski, Applied Physics Letters, 83, 5524 (2003).  “Optical Properties of Semimagnetic Quantum Dots,” S. Mackowski, T. A. Nguyen, H. E. Jackson, L. M. Smith, J. Kossut, and G. Karczewski, and W. Heiss, Quantum Confined Semiconductor Nanostructures. Symposium (Mater. Res. Soc. Symposium Proceedings Vol.737) (2003).  “Resonant photoluminescence and excitation spectroscopy of CdSe/ZnSe and CdTe/ZnTe self-assembled quantum dots,” T. A. Nguyen, S. Mackowski, H. Rho, H. E. Jackson, L. M. Smith, J. Wrobel, K. Fronc, J. Kossut, G. Karczewski, M. Dobrowolska and J. Furdyna, Quantum Confined Semiconductor Nanostructures. Symposium (Mater. Res. Soc. Symposium Proceedings Vol.737) 71-6 (2003).