10:00AM, A1 *Invited
"Semiconductor Nanocrystallites as Isolated Quantum Dots and in Complex Structures:" M. BAWENDI, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA
Nanometer size semiconductor crystallites show a striking evolution of electronic properties with size. These particles (quantum dots) are large enough to exhibit a crystalline core, but small enough that solid state electronic band structure is not yet developed. We describe a synthetic methodology which produces samples of CdSe quantum dots which are passivated by trioctylphosphine oxide (TOPO) ligands and which have diameters tunable from ~15 to 100 Angstroms with distributions <5%. We probe their electronic structure using a variety of techniques. This reveals a number of discrete electronic transitions which we can follow as a function of size and which we assign using simple "particle-in-a-sphere"concepts. We use single dot fluorescence at low temperature to show that the fluorescing state has a narrow width (~0.15 meV), confirming the "artificial atom" label often used for these systems. We then turn to structures of dots and describe the manipulation of the dots into close packed glassy and ordered arrays, essentially "crystals" of quantum dots. We demonstrate control over dot-dot distance, dot size and structure (from glassy to perfectly ordered). We observe and study interdot energy transfer (from small to large dot). We demonstrate electroluminescence from CdSe quantum dots in combination with polymer films of PVK.
10:40AM, A2 *Invited
"Visible to Near-Infrared Luminescence from Quantum Dots Induced by Self-Organized InP Stressors:" M. SOPANEN, H. Lipsanen, J. Tulkki, Optoelectronics Laboratory, Helsinki University of Technology, Otakaari 1, FIN-02150 Espoo, Finland; J. Ahopelto, VTT Electronics, Otakaari 7B, FIN-02150 Espoo, Finland
Fabrication of quantum dots (QD's) from quantum wells (QW's) by self-organized stressors [1] opens up new possibilities to tailor the luminescence wavelength. We have shown that high-quality quantum dots can be fabricated from InGaAs/GaAs quantum wells emitting close to 1 um at low temperature [2]. Furthermore, the excited states seen in luminescence spectra have been verified using theoretical calculations [3]. Here we show that shorter emission wavelengths are obtained by changing the quantum well material to GaAs/AlGaAs or to GaInP/AlGaInP.
The samples were fabricated in a single growth run using metalorganic vapor phase epitaxy on (100) GaAs substrates. The structure contained a single QW having a typically 5 nm-thick upper barrier. The inclusion of a thin GaAs or InGaP layer on barriers containing aluminum was found to be crucial for the successful fabrication of strain-induced QD's. The islands were very homogeneous in size, especially on the GaAs layer.
The low-temperature luminescence wavelength of InGaAs/GaAs QD's varied between 0.9 to 1.1 um. The GaAs/AlGaAs QD's fabricated using a thin GaAs surface layer prior to InP deposition showed sharp QD luminescence closer to the visible part of the spectrum. The wavelength of the AlGaInP/GaInP QD's was tuned in wide range from deep red to orange by varying the strain and thickness of the QE. For all materials the FWHM of the PL peaks from QD's and QW's were about the same, which shows that the size fluctuation of the stressor islands is negligible.
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[1] M. Sopanen, H. Lipsanen and J. Ahopelto, Appl. Phys. Lett. 66, 2364 (1995).
[2] H. Lipsanen, M. Sopanen and J. Ahopelto, Phys. Rev. B 51, R13868 (1995).
[3] J. Tulkki and A. Heinämäki, Phys. Rev. B 52, 8239 (1995).
11:20AM, A3 *Invited
"Photoluminescence Studies of Excited States in a Single InP Quantum Dot:" D. HESSMAN, P. Castrillo, M.-E. Pistol, N. Carlsson, W. Seifert, I. Maximov, L. Samuelson, Dept. Solid State Physics, University of Lund, Box 118, S-22100 Lund, Sweden
In this work we present photoluminescence (PL) studies of individuals InP quantum dots embedded in a Ga0.5In0.5P matrix. The carrier population, carrier dynamics and relaxation processes have been studied by power dependent PL, photoluminescence excitation (PLE), and time resolved PL. Evidence of (a) lack of relaxation between the lowest energy states and (b) state filling in individual dots are obtained.
The GaInP/InP structures have been grown by metal-organic vapor phase epitaxy on GaAs (001) substrates. Coherent InP dots, about 40x50x13 nm3 in size, were formed by the Stranski-Krasanow growth mechanism. Details of growth procedure and structural characterization are reported in Refs. [1,2]. In order to perform single dot spectroscopy we have defined micro sized areas (0.01- 1 mm2) using either dry etching or metal masking. For window sizes comparable to the estimated dot separation, the emission, if present, typically shows three peaks separated by about 20 meV. The reproducibility from window to window suggests that we observe the spectrum of an individual dot. The relative intensities of the peaks hardly change when the power is decreased. The presence of multiple peaks from a single dot therefore implies a carrier relaxation slower than the radiative recombination. The peak linewidth is around 2 meV.
We have also recorded PLE spectra, showing sharp resonances on an increasing background with onset at the highest energy PL peak. Similarities between PLE spectra detected at the different PL peaks indicate that the emitting states are connected to common excited states, proving that, in fact, all of them correspond to the same dot. Moreover, no transfer between the emitting states is observed, confirming that the relaxation between the lower states is negligible.
The PL is strongly nonlinear for power densities higher than 5 W/cm2. At this power we start observing additional emission peaks at similar energies as the PLE resonances. These peaks are therefore related to excited states that start to be populated as a consequence of the filling of the lower energy states. We also observe a broadening of the peaks with increasing excitation power which is probably related to carrier-carrier interaction. Almost all spectral features of the single dot are smeared out at power densities higher than 150 W/cm2.
The previous observations have been complemented by measuring time-resolved PL in single quantum dots. The emission peaks observed in low power PL have very similar decay times, about 1 ns. This again confirms that the relaxation time is longer than the radiative decay time.
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[1] Carlsson et al., Appl. Phys. Lett 65, 3093 (1994)
[2] Georgsson et al., Appl. Phys. Lett 67, 2940 (1995).
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