Joo-Hyong Noh

Scanning tunneling microscopy/spectroscopy study of self-organized quantum dot structures formed in GaP/InP short-period superlattices

Self-organized quantum dot (QD) structures formed in (GaP) n (InP) m short-period superlattices (SLs) grown on GaAs (N11) substrates by gas source MBE (molecular beam epitaxy) are studied by scanning tunneling microscopy (STM)/scanning tunneling spectroscopy (STS). STM images show high density QD structures as bright areas. The dot size of these structures ranges from is nm to 25 nm with a dispersion of ±10% depending on the n and m, of the SLs. In the STS measurement, the voltage width for dI/dV = 0 varies along the lateral direction on the sample surface. This voltage width variation corresponds to the lateral variation of the band-gap energy and of the tunneling probability by the lateral composition modulation.

Influence of One Monolayer Thickness Variation in GaAs/AlGaAs Five-Layer Asymmetric Coupled Quantum Well upon Electrorefractive Index Change

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) for ultrafast and low-voltage optical modulators and switches. First, the influence of one monolayer (ML) thickness variation of a single layer in the GaAs/AlGaAs FACQW on the electrorefractive index change Δn is theoretically studied. The thickness variation of two thicker GaAs layers has a considerable influence on Δn of the FACQW, while the thickness variation of thin AlAs and AlGaAs barrier layers has a smaller influence on Δn. The ratio of the thicknesses of the two GaAs well layers significantly affects the Δn characteristics of the FACQW. The change Δn does not vary appreciably as long as the ratio is kept constant. Second, the influence of the statistical fluctuation of the layer thickness by 1 ML in all of the layers on the Δn characteristics of the FACQW is also discussed. Even when Δn decreases with the increase of the occurrence probability of a layer being thicker or thinner by 1 ML, the FACQW still has a much larger Δn than conventional rectangular quantum wells do.

Gas source molecular beam epitaxy growth of GaN-rich side of GaNP alloys and their observation by scanning tunneling microscopy

The GaN-rich side of a GaNP alloy exhibits a potentially large variation in band-gap energy with P content due to its large bowing. To study the phase separation observed in GaNP grown on a (0001) sapphire substrate with a P content larger than 0.015, GaNP layers are grown on (111)A substrates by electron cyclotron resonance molecular beam epitaxy (ECR-MBE). During the growth of GaNP layers, reflection high energy electron diffraction (RHEED) exhibits additional spotty patterns indicating phase separation as well as (1 x 1) streaks. Scanning tunneling microscopy (STM) images on the phase-separated samples show the bright clusters with about 3 nm in size, which correspond to the phase-separated GaP-rich region. I-V curves on the bright clusters are quite different from those on other areas indicating a lower band-gap energy.

Fabrication and Optical Characterization of Five-Layer Asymmetric Coupled Quantum Well (FACQW)

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well (QW) that is promising for ultrafast and ultralow-voltage optical modulators and switches. We succeeded in fabricating GaAs/AlGaAs FACQW with monolayer accuracy by the molecular beam epitaxy (MBE) method by monitoring reflection high-energy electron diffraction (RHEED) specular beam intensity oscillation. Photoabsorption current measurements of the FACQW sample showed good agreement with theoretical results, and a potential for much lower voltage operation. In addition, we studied the growth sequences of GaAs/AlGaAs QWs in the migration-enhanced epitaxy (MEE) method in order to fabricate the FACQW with steeper and flatter heterointerfaces. The sequence of supplying materials for Al0.3Ga0.7As growth, on which there is no report, was modified and optimized, and the QWs of higher quality were obtained at a growth temperature of 490°C using the optimized sequence. The results of photoluminescence measurements show that the MEE method modified as mentioned above is a promising growth technique for the fabrication of FACQWs of higher quality.

Anomalous Sharp Dip of Large Field-Induced Refractive Index Change in GaAs/AlGaAs Five-Layer Asymmetric Coupled Quantum Well

The five-layer asymmetric coupled quantum well (FACQW) is a new potential-tailored quantum well for ultrafast and low-voltage optical modulators and switches. Almost linear and large electrorefractive index change can be obtained in the transparency wavelength regions. In the GaAs/AlGaAs FACQW, an abrupt change in refractive index change Δn due to an applied electric field F occurs at a certain electric field range, which results in an anomalous sharp dip of Δn versus F. The physical origin and the elimination of the dip are discussed in detail. The abrupt change of refractive index is caused by significant changes of the wavefunction overlap integrals (and exciton binding energies) of transitions between the ground states for an electron (e1) and a heavy hole (hh1), and transitions between e1 and the first excited state for a heavy hole (hh2). The overlap changes are mainly due to shifts of the wavefunction distribution of hh1 and hh2, respectively. The dip can be eliminated by changing the position or Al content of the AlGaAs barrier layer in the FACQW. In addition, the larger negative index change in a modified FACQW structure is demonstrated.

Scanning Tunneling Microscopy Study on Self-Formation Process of Quantum Dot Structures by the Growth of GaP/InP Short-Period Superlattices on GaAs (311)A Substrate

Self-formation process of quantum dot (QD) structures in (GaP)1.5 (InP)1.88 short-period superlattices (SLs) grown on GaAs (311)A substrates by gas source MBE (molecular beam epitaxy) are studied by scanning tunneling microscopy (STM). STM images reveal high-density (1011–1012 cm-2) QD structures periodically aligned along the [233] and [011] directions. STM observations of the self-formed structures as a function of SL cycle number show that almost complete QD structures are formed only after the growth of 1 cycle of SL (1.5 monolayer GaP and 1.88 monolayer InP), although the periodic structures elongated only along the [011] direction are formed even after the growth of 0.5 cycle of SL (1.5 monolayer GaP). In the dI/dV vs. V measurement, the voltage width (ΔV) for the dI/dV=0 varies periodically along the lateral direction on the sample surface, and the amplitude of this periodic variation increases with the SL cycle number and saturates at 2 SL cycles.

Scanning Tunneling Microscopy/Scanning Tunneling Spectroscopy Observation of III–V Compound Semiconductor Nanostructures

InAs submonolayer samples grown on the vicinal (100) GaAs substrates and the self-organized quantum wire structures formed in the (GaP)2(InP)2 short-period superlattices (SLs) on the nominally (100) GaAs substrates by gas-source molecular beam epitaxy (GS-MBE) are studied by scanning tunneling microscopy (STM) and spectroscopy (STS). In the former nanostructures, the STM images show alternating bright stripes (InAs area) and dark stripes (GaAs area), and the bright stripe width increases with the amount of In supplied during growth. A difference is also observed in the I–V curve; the slope dI/ dV is different between the bright and dark areas and shows the same value in each area, although the onset voltage for the current rise is the same in all areas independent of InAs and GaAs areas. In the latter nanostructures, the STM images show alternating bright and dark stripes corresponding to the self-organized wire structures. In this case the onset voltage for the current rise as well as the slope dI/ dV varies depending on the lateral alloy composition modulation in the wire structures.

Improvement of Optical Properties of Multilayer Quantum Dots Self-Formed in GaP/InP Short-Period Superlattices on GaAs(311)A

Multilayer quantum dots (MQDs) structures are fabricated on a GaAs(311)A substrate by sandwiching the quantum dots (QDs) self-formed in (GaP)1.5(InP)1.88 short-period superlattices (SLs) with InGaP/InAlP SL layers instead of InGaP layers, as barrier and cladding layers. Narrower photoluminescence (PL) and electroluminescence (EL) linewidths and weaker temperature variations are observed for the modified MQDs compared with the previously reported best values for MQDs with InGaP barrier and cladding layers. PL and EL peak energies for the modified MQDs are higher than those for the previous MQDs. These results suggest the enhancement of carrier confinement by the use of InGaP/InAlP SL layers as barrier and cladding layers. The temperature dependence of EL intensity is also improved.

Improved Optical Properties of Strain-Induced Quantum Dots Self-Formed in GaP/InP Short-Period Superlattices.

Optical properties of multilayer quantum dots (MQDs) self-formed in the GaP/InP short-period superlattice (SL)/InGaP multilayer structures are investigated as a function of InGaP barrier thickness (B). Photoluminescence (PL) linewidth broadening with temperature is improved and tends to reduce by decreasing B. This is attributed to the vertical coupling effect between QDs and their vertical alignment. Temperature variation of PL properties shows the exciton behavior. At low temperatures, emissions from both bound exciton and free exciton appear under the weak excitation density condition. Integrated PL intensity is quite stable up to 120 K.

Growth Temperature Dependence of Self-Formation Process of Quantum Dot Structures in GaP/InP Short-Period Superlattices Grown on GaAs (311)A Substrate

Growth temperature dependence of the self-formation process of quantum dot (QD) structures in (GaP)1.5 (InP)1.88 short-period superlattices (SLs) grown on GaAs (311)A substrates is studied by scanning tunneling microscopy (STM). SLs are grown by gas-source molecular beam epitaxy (MBE) at 420–500°C. The STM image of the sample grown at 460°C reveals completely self-formed QD structures aligned along both [233] and [011] directions due to the strain-induced lateral composition modulation. On the other hand, both below (420°C) and above (480°C, 500°C) this temperature the self-formation process of QD structures is suppressed and only incomplete structures elongated along the [011] direction are formed, probably due to the suppressed or over-enhanced migration of group III atoms on the surface, respectively. Scanning tunneling spectroscopy (STS) measurements reveal that the amplitude of the lateral periodic variation of the band-gap energy in the self-formed structures also decreases both below and above the optimum growth temperature.