Microstructural Evolution near the InGaAs/GaAs Stranski-Krastanow Transformation Rosa Leon Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA Simon Fafard Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada K1A OR6 The layer-by-layer growth mode is observed for lattice- matched materials of identical crystal structure. Examples: Au on Ag and AlGaAs on GaAs Direct island growth is seen in materials with large lattice mismatch, high interfacial energy or different crystal structures. Examples: GaN on saphire and InAs on GaP This type of growth occurs for crystals of dissimilar lattice parameters but low interfacial energy, like Ge on Si and InAs on GaAs. After an initial layer-by-layer growth, islands form spontaneously, leaving a thin “wetting layer” underneath. Van de MerweVolmer-WeberStranski-Krastanow Stranski-Krastanow growth can be used to make small, uniform semiconductor Quantum Dots with defect free interfaces InGaAs GaAs We can study the evolution of Quantum Dot formation with a special growth technique that gives a graded deposition or “wedge” shaped growth: ABSTRACT The evolution of Stranski-Krastanow (S-K) quantum dot (QD) formation in a ternary (In 0.6 Ga 0.4 As/GaAs) was studied with graded structures grown via organometallic vapor phase epitaxy. Surface probe microscopy showed island evolution between 3.5 and 6.5 monolayers (ML) deposition. Island densities increased exponentially (over three decades with 0.2 ML deposition) before saturation ~ 4.7 ML. Photoluminescence (PL) of capped structures show that wetting layer (WL) PL energy does not shift beyond the onset of the S-K transition. PL intensities increased with QD concentration but not in proportion to QD density. After saturation, a sharp drop in PL intensity was observed, which we attribute to island coalescence and incoherent island formation. Excitation power dependence of the luminescence at different stages of QD evolution indicates a concentration dependence of optical saturation in self-forming InGaAs QDs. CONCLUSIONS This optical and structural study of QD evolution has shown that: Once the islands/dots start forming, any extra deposited InGaAs goes either into the pre- formed islands or into making new islands, and the wetting layer thickness stays constant. An abrupt drop in PL intensity is observed beyond saturation island densities. This drop in PL corresponds with a sudden increase in the concentration of incoherent islands. The integrated QD PL increases with QD concentration, but not proportionally, so the intensity per QD drops as the concentration of QDs increases. QD saturation behavior also changes with dot concentration. The WL/QD intensity ratio increases with increased laser excitation for lower QD concentrations. Evolution in island concentration over depositions between 4.0 and 6.5 ML. Arrows at different stages of growth indicate representative structures shown in figure 2. Deflection FM images of surface evolution for InGaAs/GaAs islands at the points indicated by arrows in figure 1. Each frame is 1 x 0.5 mm. Luminescence emission begins with a thin QW which progressively red shifts (becomes thicker) as InGaAs deposition is increased. In the next stage, the QD concentration rises until the threshold for QD PL detection. Once the QD PL peak increases in intensity, the WL peak diminishes rapidly. The evolution of WL to QD luminescence occurs over a broad range in QD concentrations but this corresponds to a very narrow range in InGaAs deposition: from 4.08 ML to 4.14 ML. PL emission intensities from QDs increases as their concentration increases, and that the WL emission is reduced. However, the energy of the weaker WL PL peak stays at the same value once the QD PL peak becomes detectable and grows. The WL thickness then does not increase (or decrease) with further InGaAs deposition once the QD start forming. At the next stage, which occurs over the next ~ 0.5 ML deposition, the PL QD emission does not change significantly. This stage corresponds to island saturation. In the last stage, the PL intensity drops in magnitude to roughly a third of its former intensity, and stays at this lower intensity over the next ~ 2 ML deposition. PL spectra and calibrated relative intensities in different regimes of QD formation. (a) WL PL shifts before QD formation, (b) evolution of PL spectra at low QD densities when both QD and WL peaks are simultaneously observed, (c) strongest PL integrated intensity near island saturation, and (d) weak PL emission seen after island saturation and coalescence. Relative integrated PL intensity with above bandgap (532nm) [open circles] and below bandgap (980nm) [solid diamonds] excitation as a function of InGaAs deposition. Inset shows relative integrated intensities from WL and QD luminescence as a function of QD concentration. Power dependence of PL signal showing emission from QD and WL states for different values of QD areal concentrations. (a) The power excitation ratios for the solid lines are 192/64/6.4/0.192 Watts/cm 2. Simulations adding 2 Gaussian curves centered at E i =1.135 and eV with 65 meV inhomogeneous broadening are shown separately in dashed lines. (b) Excitation power ratios 192/19.2/0.192 W/cm 2 and (c) power ratios 192/1.92 W/cm 2. In this study, we have examined the evolution of S-K island formation and determined the 2D to 3D transition in a ternary (In 0.6 Ga 0.4 As/GaAs) from wetting layer growth until the onset of island coalescence. Our results have shown an exponential increase in QD concentration with deposition, and a critical thickness of 4 Monolayers deposition. Correlating the structural and luminescence results allowed following the progression in photoluminescence (PL) evolution from initial WL growth to QD formation, island saturation and the effects of coalescence and incoherent island formation on QD PL emission. Investigation of these graded structures also showed some effects of varying QD concentration and dot-to-dot interaction, on the optical emission intensity and saturation behavior of InGaAs QDs. Related recent publications: Structural and radiative evolution of Quantum Dots near the InGaAs/GaAs Stranski-Krastanow transformation, R. Leon and S. Fafard, Phys Rev. B 58, R1726 (1998). Stable and metastable InGaAs/GaAs island shapes and surfactant-like suppression of the wetting transformation, R. Leon, C. Lobo, J. Zou, T. Romeo, and D. J. H. Cockayne, Phys. Rev. Lett. 81, 2486 (1998). Tunable Intersublevel Transitions in self-forming Semiconductor Quantum Dots, R. Leon, S. Fafard, P.G. Piva, S. Ruvimov and Z. Liliental-Weber, Phys. Rev. B 58, R4262, (1998). Transmission-electron microscopy study of the shape of buried InGaAs/GaAs quantum dots, X. Z. Liao, J. Zou, X. F. Duan, D. J. H. Cockayne, R. Leon, and C. Lobo, Phys Rev B 58, R4235 (1998). Different paths to tunability in III-V quantum dots, R. Leon, C. Lobo, A. Clark, R. Bozek, A. Wysmolek, A. Kurpierski, and M. Kaminska, J. Appl. Phys 84, 248(1998). Self-forming InAs/GaP quantum dots by direct island growth, R. Leon, C. Lobo, T. P. Chin, J. M. Woodall, S. Fafard, S. Ruvimov and Z. Liliental-Weber, Appl. Phys. Lett 72, 1356 (1998). The deposition scale was calibrated by measurements of the PL emission and corresponding shifts with distance from graded InGaAs/GaAs quantum wells grown under the same conditions as the graded QD samples but using a shorter deposition time. This established a growth rate in MLs as a function of distance from the edge of the MOCVD susceptor. The critical thickness for the 2D to 3D transition in the MOCVD growth of In 0.6 Ga 0.4 As occur after 4.0 ML deposition. The onset of island coalescence becomes apparent with the decrease in island concentration with further deposition.