Ji-Won Oh

A light-emitting diode fabricated from horse-heart cytochrome c

We have fabricated a light-emitting diode from horse-heart cytochrome c and measured the electro-luminescence (EL) spectra. The spectra exhibit broad peaks around 530 and 690 nm, and a weak shoulder around 410 nm. The EL spectra are completely different from the photo-luminescence spectra previously reported. The appearance of the 690 nm emission band suggests the charge-transfer between the iron and the axial methionine ligand plays a crucial role in the electrical conduction in the cytochrome c film.

Low and anisotropic barrier energy for adatom migration on a GaAs (110) surface studied by first-principles calculations

We determined potential-energy surfaces for Ga and As adatoms on a GaAs (110) surface by first-principles calculations in order to understand the epitaxial growth mechanism. We found small migration barrier energies for Ga and As, which explain the long atom-migration length suggested by experiments. We also found that Ga migration is one dimensional and As migration is two dimensional, and that, for both Ga and As adatoms, the sites near As of the topmost layer are stable while those near Ga are unstable.

Step-edge kinetics driving the formation of atomically flat (110) GaAs surfaces

Atomically flat (110) GaAs surfaces fabricated by the cleaved-edge overgrowth method and high-temperature growth-interrupt annealing are characterized at the atomic scale. We observe atomically flat (110) surfaces extending over areas more than 100 μm in size. Moreover, deposition of slightly less or more than integral monolayers (MLs) causes the appearance of characteristic step-edge shapes such as 1-ML-deep pits, or 2-to-3-ML-high isolated islands. Statistical analysis on the size and shape distribution of the 1-ML-deep pits supports a simple model based on the stability of Ga and As atoms on step edges with different bonding configurations, and reveals driving force to form an atomically flat (110) surface.

Control of MBE surface step-edge kinetics to make an atomically smooth quantum well

Summary form only given. On an atomic scale, all quantum wells grown by conventional MBE have rough barrier-well interfaces with step-edges, pits, or islands one or more monolayers (MLs) high. Cleaved-edge overgrowth is MBE on the atomically flat [110] surface exposed by an in situ cleave so that there exists no roughness on this bottom interface. The top surface of the overgrown [110] film, however, generally has large roughness due to the required GaAs [110] growth conditions. Here we control the molecular step-edge kinetics on the upper interface by growing on the smooth [110] cleave an integral number of GaAs MLs and then initiating a growth-interrupt-anneal. We characterize the structures using AFM and photoluminescence.

Novel Electroluminescence Properties of Thin Films Using Soluble Metallophthalocyanine Salts

We have prepared organic thin films of soluble dicyano(phthalocyaninato)iron salt, [(n-C7H15)4N][FeIII(Pc)(CN)2], and fabricated the electroluminescence (EL) devices having the Metal-Insulator-Metal structure. The device exhibits emission for both positive and negative bias voltages. We observed two bands in the EL spectra. One band was assigned to the Q band of the phthalocyanine molecule. The other, which shifts with changing the applied voltage, could correspond to the metal-to-ligand charge transfer (MLCT) band and/or the ligand-to-metal charge transfer (LMCT) band.

Micro-photoluminescence characterization of local electronic states in a (110) GaAs quantum well fabricated by cleaved-edge overgrowth

Local electronic states due to characteristic surface islands and pits formed on an atomically flat (110) GaAs interface of a quantum well fabricated by a cleaved-edge-overgrowth method with high-temperature growth-interrupt annealing are characterized by micro-photoluminescence (PL) imaging and spectroscopy. With the overall observation of the 6-nm-thick (110) quantum well with the area of 6.8-μm width and 3-mm length, we found local PL signals due to isolated 2- or 3-monolayer (ML) islands, isolated 1-ML pits, and 1-ML islands and pits formed along the cleavage atomic-step lines, and gradual change of the states across the full range of the sample.