Haeyeon Yang

Control of size and density of InAs/(Al,Ga)As self-organized islands

We report on the influence of the chemical composition of the (Al, Ga)As surface on the formation of strain induced three-dimensional (3D) InAs islands. The experiments have been carried out using a molecular beam epitaxy facility combined with a scanning tunneling microscope enabling in situ surface characterization. The evolution of the density and morphology of these islands is investigated as a function of the Al composition. The InAs deposition, substrate temperature, and annealing time effects on the island formation and morphology are studied. The morphologies of the (Al, Ga)As surface as well as that of the reconstructed InAs “wetting layer” are also described. Results indicate that there are major differences between the InAs/GaAs and the InAs/AlAs systems despite the same lattice mismatch. We observe these differences varying the aluminum content in the starting (Al, Ga)As surface. We show that control of the Al fraction leads to control of the size and density of the 3D islands. The control of ...

Formation of quantum wires and dots on InP(001) by As/P exchange

We report on the use of in situ scanning tunneling microscopy to study As/P exchange on InP(001) surfaces by molecular beam epitaxy. Results demonstrate that the exchange process can be controlled to selectively produce either quantum wires or quantum dots. 15 nm wide self-assembled nanowires are observed, and they are elongated along the dimer row direction of the InP(001)-2×4 surface with a length of over 1 μm and flat top 2×4 surfaces. In addition, when the nanowires are annealed with no arsenic overpressure, the surface reconstruction transforms from 2×4 to 4×2 and the nanowires transform into dots with a rectangular base and flat top.

Beam interactions with a blocker soliton in one-dimensional arrays

We investigate experimentally and numerically the interaction of a highly localized, single-channel discrete soliton (blocker) with a wide, tilted beam in a one-dimensional AlGaAs array. In agreement with theory the blocker is observed to discretely shift its position by multiple channels, depending on the intensity and relative phase of the tilted beam.

Activation energy for Ga diffusion on the GaAs(0 0 1)-(2×4) surface: an MBE-STM study

The pure migration of individual Ga atoms on the technologically important GaAs(0 0 1)-(2]4) reconstructed surface has been studied as a function of substrate temperature using a combined molecular beam epitaxy and scanning tunneling microscopy (STM) ultra-high vacuum, multi-chamber facility. We have successfully deposited 1 10 of a plane of Ga atoms onto a pristine GaAs surface under a constant As 4 beam equivalent pressure of 10~6 Torr, at various substrate temperatures. After deposition the substrate was quenched to room temperature and transferred to the surface analysis chamber for STM imaging. A plot of the number density of islands formed as a function of deposition temperature follows an Arrhenius relationship. Assuming either a pure one-dimensional di!usion model or a pure isotropic two-dimensional di!usion model, the activation energy for di!usion is 2.3 or 1.7 eV, respectively. ( 1999 Elsevier Science B.V. All rights reserved.

OBSERVATION OF ONE- AND TWO-DIMENSIONAL DISCRETE SURFACE SPATIAL SOLITONS

The recent theoretical predictions and experimental observations of discrete surface solitons propagating along the interface between a one- or two-dimensional continuous medium and a one- or two-dimensional waveguide array are reviewed. These discrete solitons were found in second order (periodically poled lithium niobate) and third order nonlinear media, including AlGaAs, photorefractive media and glass, respectively.

Power thresholds of families of discrete surface solitons

We have investigated both theoretically and experimentally the power threshold of discrete Kerr surface solitons at the interface between a discrete one-dimensional (1D) (waveguide array) and a continuous 1D (slab waveguide) AlGaAs medium. Decreasing power thresholds were predicted and measured for soliton trapping at sites with increasing distance from the boundary. The thresholds approached asymptotically the power required for a discrete soliton of equivalent width in an infinite lattice. The minimum threshold coincided with a minimum in the interchannel coupling strength.

Role of As4 in Ga diffusion on the GaAs(001)-(2×4) surface: A molecular beam epitaxy-scanning tunneling microscopy study

The role of As4 molecules in Ga diffusion on the GaAs(001)-(2×4) reconstructed surface has been studied using a combined molecular beam epitaxy and scanning tunneling microscopy multichamber facility. We deposited 10% of a plane of Ga atoms onto an otherwise pristine surface, while exposed to two separate As4 beam equivalent pressures of 10−5 and 10−6 Torr. The higher As4 flux resulted in the production of fewer and larger islands, indicating that increasing the As4 flux increases the total interrogation area available to the Ga atoms before forming islands.

Incoherent blocker soliton interactions in Kerr waveguide arrays

We have observed the incoherent interaction between a highly confined soliton and a wide, moving beam of different wavelength in an AlGaAs waveguide array. Digital switching of the blocker solitons to adjacent channels was observed.

Self-assembled InAs quantum wires on InP(001)

We report a study on self-assembled InAs quantum wires on a planar surface of InP(001), grown by molecular-beam epitaxy and examined by in situ scanning tunneling microscopy and photoluminescence (PL). The detailed morphology of the quantum wires including width and height distributions is presented. The quantum wires cover more than 95% of the surface area. The spectral range of the PL emission includes the technologically important 1.55 μm. In the PL emission, high optical anisotropy of more than 50% has been observed at 4.2 K and at room temperature indicating strong confinement.

Nonlinear beam interactions in 1D discrete Kerr systems.

The interaction between parallel beams in one-dimensional discrete Kerr systems has been investigated using arrays of coupled channel waveguides. The experiments were performed in AlGaAs waveguides at 1550 nm which corresponds to photon energies just below one half the semiconductors bandgap. The input intensity and relative input phase between the input beams was varied and the output intensity patterns were recorded. Observed was behavior ranging from a linear response, to soliton interactions between moderately and then strongly localized spatial solitons. Finally the influence of multiphoton absorption and asymmetric beam inputs on these interactions was investigated at very high intensities.