J. D. Song

Observation of a Biexciton Wigner Molecule by Fractional Optical Aharonov-Bohm Oscillations in a Single Quantum Ring.

The Aharonov-Bohm effect in ring structures in the presence of electronic correlation and disorder is an open issue. We report novel oscillations of a strongly correlated exciton pair, similar to a Wigner molecule, in a single nanoquantum ring, where the emission energy changes abruptly at the transition magnetic field with a fractional oscillation period compared to that of the exciton, a so-called fractional optical Aharonov-Bohm oscillation. We have also observed modulated optical Aharonov-Bohm oscillations of an electron-hole pair and an anticrossing of the photoluminescence spectrum at the transition magnetic field, which are associated with disorder effects such as localization, built-in electric field, and impurities.

Influence of arsenic during indium deposition on the formation of the wetting layers of InAs quantum dots grown by migration enhanced epitaxy

We compared the structural and optical properties of InAs∕GaAs quantum dots grown by migration enhanced epitaxy, with and without arsenic, during indium deposition. The uniformity and size of the quantum dots are enhanced in a sample without arsenic. As a result, narrower and longer wavelength photoluminescence is observed in this sample. Furthermore, the thickness of the wetting layers is reduced by ∼20% in the sample without arsenic, and this result agrees well with the speculation that metallic indium has a smaller driving force for corrugating the InAs wetting layers before they are transformed from two-dimensional to three-dimensional layers. Additionally, the photoluminescence linewidth of the sample without arsenic is insensitive to the cryostat temperature due to two major factors: the reduced thickness of the wetting layers and the enhanced uniformity. In the sample with arsenic, however, the photoluminescence linewidth shows typical anomalies.

Nanometer-scale measurements of electronic states in InAs∕GaAs quantum dots

We have investigated the origins of electronic states in individual (uncoupled) quantum dots (QDs) and the surrounding wetting layers (WLs) using a combination of cross-sectional scanning tunneling microscopy (XSTM) and scanning tunneling spectroscopy (STS). XSTM images reveal uncoupled ellipse-shaped QDs with 18±5nm (9±3nm) major (minor) axes. Room temperature STS spectra reveal a gradient in the effective bandgap within the QDs with smallest values near the QD core and top surfaces. The variations in effective bandgap are apparently dominated by indium composition gradients, with minimal effects due to the QD shape and strain. Indium composition gradients also dominate the effective bandgap variations in the WL.

Effect of Modified Growth Method on the Structural and Optical Properties of InAs/GaAs Quantum Dots for Controlling Density

We introduce a thermal treatment, namely, a growth technique to form low-density quantum dots (QDs) with homogeneous InAs deposition, and study the structural and optical properties of InAs/GaAs QDs during the formation of dots by the thermal treatment. The structural and optical properties are studied by atomic force microscopy and photoluminescence (PL). We achieve a wide range of dot densities from 1010 to 106 per cm2 by adjusting the thermal treatment temperature. The uniformity of InAs dots improves as the thermal treatment temperature increases. Comparing PL spectra with those in the literature, we confirm that the dots in this work are not QDs but small quasi-three-dimensional (Q3D) clusters. Q3D clusters are left after the thermal treatment. This behavior of Q3D is different from that of the QDs reported in the literature. We conclude that three phenomena occur during the thermal treatment: 1) In segregation, 2) In re-evaporation, and 3) the intermixing of InAs. As a result, we conclude that the thermal treatment is a very useful method for controlling the dot density.

Enhanced open-circuit voltage of InAs/GaAs quantum dot solar cells by hydrogen plasma treatment

The authors describe performance enhancement in InAs/GaAs quantum dot solar cells (QDSCs) using hydrogen plasma treatment. Photoluminescence (PL) and time-resolved PL revealed clearly decreased defect levels in QDSCs and improved crystal quality after hydrogen passivation. As a result, the open-circuit voltage and efficiency of the hydrogen-treated QDSCs were largely increased about 70u2009mV and 10%, respectively.

Characteristics of thermal treated quantum dot infrared photodetector

We investigated the device performances for a post-growth thermally treated In0.5Ga0.5As/GaAs quantum-dot infrared detector (QDIP). Device characteristics, such as dark current, photoluminescence (PL), and photocurrent spectra, have been studied and compared for the as-grown and thermally treated QDIPs. After the thermal treatment with a SiO2 capping layer, the dark current was increased, the PL peak position was blue-shifted, and the detection wavelength was redshifted due to In/Ga interdiffusion in the quantum dot (QD) structure. Furthermore, the activation energies estimated from the integrated PL intensities agreed well with the peak positions of the photocurrent spectra.

Effect of Growth Interruption in Migration Enhanced Epitaxy on InAs/GaAs Quantum Dots

In this study, we investigated the effect of growth interruption time (tGI) during migration enhanced epitaxy (MEE) growth of self-assembled InAs/GaAs quantum dots (QDs) to control the density of the QDs without any substrate rotation stop during QD formation. By manipulating the growth factor (tGI), the control of QD density in the range of 3.4 ×109–3.5 ×1010 dots/cm2, as well as the QD shape, was demonstrated. We concluded that three phenomena occur during growth interruption: 1) In re-evaporation, 2) In segregation, and 3) the redistribution of InAs QDs. From photoluminescence (PL), it is found that the emission wavelength of samples increased from 967.7 to 1151.7 nm as tGI increased due to redistribution. In addition, we confirmed PL peak emissions from QDs, quasi-three-dimensional (Q3D) clusters, and wetting layer. As a result, the manipulation of tGI in the MEE method can control the density, uniformity, size, and wavelength of QDs.

Effect of rapid thermal annealing on the noise properties of InAs/GaAs quantum dot structures

Self-assembled InAs quantum dots (QDs) were grown by molecular beam epitaxy (MBE) on n+‐GaAs substrates, capped between 0.4μm thick n-type GaAs layers with electron concentration of 1×1016cm−3. The effect of rapid thermal annealing at 700°C for 60s on the noise properties of the structure has been investigated using Au∕n‐GaAs Schottky diodes as test devices. In the reference sample without containing QDs, the noise spectra show a generation-recombination (g-r) noise behavior due to a discrete energy level located about 0.51eV below the conduction band edge. This trap is ascribed to the M4 (or EL3) trap in GaAs MBE layers, related to a chemical impurity-native defect complex. In the structure with embedded QDs, the observed g-r noise spectra are due to a midgap trap level ascribed to the EL2 trap in GaAs, which is related to the InAs QDs dissolution due to the thermal treatment.

Current–voltage and noise characteristics of reverse-biased Au/n-GaAs Schottky diodes with embedded InAs quantum dots

Schottky contacts on n-type GaAs with embedded InAs quantum dots (QDs) were studied by current–voltage (I–V) and low-frequency noise measurements. For comparison, diodes not containing QDs were investigated as reference devices. A wide distribution of the ideality factor was observed, correlated with the level of the leakage current. Reverse I–V characteristics on the logarithmic scale indicate that the space-charge limited current dominates the carrier transport in these diodes. In all diodes, the reverse current noise spectra show 1/f behaviour, attributed to traps uniformly distributed in energy within the band-gap of the GaAs capping layer. Depth profiling measurements of the 1/f noise power spectral density demonstrate the impact of the QDs on these traps. In diodes containing QDs, in addition to the 1/f noise, a generation–recombination noise is found originating from a deep trap level localized in the vicinity of the QD plane.

Optical and structural properties of In0.5Ga0.5As quantum dots with different numbers of stacks grown by atomic layer molecular beam epitaxy: vertical realignment of weakly coupled quantum dots

We have investigated optical and structural properties of various In0.5Ga0.5As quantum dot (QD) structures as a function of stacking number, grown by atomic layer molecular beam epitaxy (ALE). We found that the excitation power and temperature dependence of well-separated two emissions from 10 stacked InGaAs QD samples are different from those of other QDs with the stacking numbers of 1, 3 and 5. Although the GaAs spacer thickness is ~35 nm at which the strain field penetration can be ignored for Stranski–Krastanov mode-grown InAs QDs, our ALE-grown QDs are influenced by the strain field penetration due to the larger size of ALE QDs. From transmission electron microscopic images, we observed that the ten stacked QD sample has a complex QD size distribution predominantly due to vertical size variation with stacking and that upper stacked (5–10) QD layers are vertically realigned due to the merged strain field penetration between laterally coupled QDs.