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We report here the direct observation of charge accumulation in GaAs/AlGaAs multiple quantum well (MQW) solar cells by employing cross-sectional Kelvin probe force microscopy (KPFM). This sample is characterized by thin barrier layers that enable miniband formation. The contact potential difference, or potential between the tip and the semiconductor sample, was measured along the p–i–n junction. We observed, under illuminated conditions, a change in the potential gradient, or bending, at a position of the MQW layer, but not in the reference sample without quantum well. This clearly shows that charge is accumulated in the MQW region. We also found that electron accumulation in the MQW layer and the density measured on the surface is about 1 × 1011 cm−2. Our experimental results show that KPFM is a powerful way of understanding the device physics of nanostructure-based solar cells.

The near-infrared photoresponses of an n-AlGaAs/GaAs heterojunction field-effect transistor (FET) were investigated for the irradiation of two lights: (A) a laser beam with the energy above the Schottky-barrier which uniformly illuminates the gate region and (B) a laser beam with the energy above the GaAs bandgap which locally illuminates the ungate region. We measured a lateral photocurrent in the two dimensional electron gas (2DEG) channel at the n-AlGaAs/GaAs heterojunction and found that the FET acts as an optical AND element; the lateral photocurrent is generated only when both the light A and B simultaneously illuminate the FET. The lateral current flows from left to right when the left side of the FET is illuminated with the light B, while the right side irradiation leads to the current from right to left. The experimental findings are well explained by a theory based on the current-continuity equation, where the lateral current in the 2DEG channel is driven by an asymmetric electron transfer resul...

Quantum energy teleportation (QET) is a proposed protocol related to the quantum vacuum. The edge channels in a quantum Hall system is well suited for the experimental verification of QET. For this purpose, we examine a charge density wave excited and detected by capacitively coupled front gate electrodes. We observe the waveform of the charge density wave, which is proportional to the time derivative of the applied square voltage wave. Further, we study the transmission and reflection behaviors of the charge density wave by applying a voltage to another front gate electrode to control the path of the edge state. We show that the threshold voltages where the dominant direction is switched in either transmission or reflection for dense and sparse waves are different from the threshold voltage where the current stops flowing in an equilibrium state., 4 pages, 4 figures

We experimentally and theoretically study the effects of interface roughness and phonon scattering on intersubband absorption linewidth in a modulation-doped GaAs/AlAs quantum well. Quantitative comparisons between experimental results and theoretical calculations make it clear that interface roughness scattering is the dominant scattering mechanism for absorption linewidth in the temperature range below 300 K. Even at room temperature, phonon scattering processes contribute little to linewidth, while polar-optical phonon scattering limits electron mobility.

We observe lateral currents induced in an n-AlGaAs/GaAs heterojunction channel of Hall bar geometry, when an asymmetric position of the Schottky metal gate is locally irradiated by a near-infrared laser beam. When the left side of the Schottky gate is illuminated with the laser, the lateral current flows from left to right in the two dimensional electron gas (2DEG) channel. In contrast, the right side illumination leads to the current from right to left. The magnitude of the lateral current is almost linearly dependent on the beam position, the current reaching its maximum for the beam at the edge of the Schottky gate. The experimental findings are well explained by a theory based on the current-continuity equation, where the lateral current in the 2DEG channel is driven by the photocurrent which vertically flows from the 2DEG to the Schottky gate.

The resonant tunneling of electrons through a 20 nm scale InAs quantum dot bound by a pair of very thin AlAs barriers is studied. A well-resolved composite peak resulting from the ground 1s states was observed at 4.2 K in current–voltage characteristics. By investigating the effects of inplane magnetic fields, the shape of the wave function and the spatial extent of the first two electronic states are clarified.

By employing a long growth interruption (GI) in molecular beam epitaxy, we have successfully determined the concentration of oxygen incorporated in GaAs/AlAs interfaces and AlAs layers by secondary ion mass spectrometry. The concentration of oxygen atoms incorporated on AlAs surfaces during GI is found to be proportional to the period of GI when the incoming fluxes of residual oxygen‐related species reach steady‐state values. The net incorporation rate of oxygen on the AlAs surface is found to be constant for a wide range of substrate temperatures from 540 to 620 °C, indicating that the oxygen desorption is negligible.

We have fabricated GaAs/AlGaAs quantum well (QW) solar cells in which 3 nm-thick QWs and indirect-gap Al0.78Ga0.22As barriers are embedded, and we studied extraction processes of photogenerated carriers in this QW system. The photocurrent under 700 nm light illumination at voltages close to the open-circuit voltage shows only a small reduction, indicating that the carrier recombination inside QWs is largely suppressed. We attribute this result to an efficient extraction of electrons from the QWs through the X-valley of AlGaAs. The insertion of QWs is shown to be effective in extending the absorption wavelengths and in enhancing the photocurrent. The use of indirect-gap materials as barriers is found to enhance carrier extraction processes, and result in an improved performance of QW solar cells.

We have studied the correlation length (Λ) of interface roughness in GaAs/AlAs quantum wells (QWs) prepared by molecular beam epitaxy (MBE). It is found that the mobility of two‐dimensional electrons in very thin selectively doped GaAs/AlAs QW structures is substantially enhanced when the bottom AlAs barrier layer is prepared by alternate beam MBE and/or by the use of superlattice buffer layer below the QW. By measuring the electron concentration dependence of mobility and comparing with the theory of interface roughness scattering, we have found that Λ of the bottom (GaAs‐on‐AlAs) interface of the QW gets as large as 200–300 A, when prepared by the modified growth technique, which is about three times as large as that (∼70 A) by conventional MBE.

We study the current-voltage characteristics of a GaAs/AlGaAs quantum well solar cell (QWSC) at different temperatures. The photocurrent of the QWSC decreases from the short-circuit level with increasing forward bias, resulting in a low fill factor of the cell. The photocurrent reduction is attributed to the carrier confinement in the QW, which is investigated in detail by changing the temperature and excitation wavelength. We observe two reduction steps in the photocurrent at low temperature, and find that the different hole and electron tunneling rates are responsible for the two reductions.

We observe photocurrents induced in an n-AlGaAs/GaAs heterojunction of Hall bar geometry, when its edge region is irradiated by unpolarized near-infrared laser beam at normal incidence. The magnitude and direction of the photocurrent depend sensitively on the beam position, the current reaching its maximum for the beam centered at the edge of the Hall bar. The experimental findings are well explained by a theory based on the Boltzmann kinetic equation, in which the photocurrent is ascribed to the anisotropic momentum distribution of photo-excited electrons at the channel edge. By comparing the measured photocurrents with the theory, the recombination time of the photo-excited electrons is also evaluated.

We form self-assembled GaSb type-II nanorods on a vicinal (111)B GaAs substrate by molecular beam epitaxy and study their optical anisotropy. The GaSb nanorods are elongated and aligned along the [−1 0 1] direction, where the average length, width, and height are about 84, 30, and 2.5 nm. In polarized photoluminescence (PL) measurements, the peak of the GaSb nanorods is observed at about 1.1 eV, where the PL intensity is largest for the [−1 0 1] polarization and smallest for the polarization perpendicular to it. The degree of polarization is more than 20% and depends on the recombination energy. By comparing with a theoretical model based on 4 × 4 Luttinger-Kohn Hamiltonian, we find that the experimental results are explained by considering the Sb/As inter-diffusion and the nanorod height distribution.

In this letter we have performed a structural analysis at the atomic scale of GaAs/AlGaAs quantum dots grown by droplet epitaxy. The shape, composition, and strain of the quantum dots and the AlGaAs matrix are investigated. We show that the GaAs quantum dots have a Gaussian shape and that minor intermixing of Al with the GaAs quantum dot takes place. A wetting layer with a thickness of less than one bilayer was observed. © 2010 American Institute of Physics. U7 - Export Date: 2 August 2010 U7 - Source: Scopus U7 - Art. No.: 062101

We study the optical properties of GaSb/GaAs type-ІІ quantum dots (QDs) on a GaAs substrate grown by droplet epitaxy. Ga droplets are formed on GaAs and then exposed to Sb flux to be clad by large granular crystals of Sb. Then the sample was annealed at 380 °C to enhance the reaction of Ga droplets with Sb and to evaporate the excess granular layer. In photoluminescence (PL) measurements, the peaks of the QDs and wetting layer (WL) are observed. The PL intensity of the QDs is much stronger than that of the WL, where the ratio IQD/IWL of the integral intensities is about 13.3. The PL peaks shift toward higher energies with increasing excitation energy, suggesting that the band lineups exhibit a type-ІІ staggered alignment. In addition, we investigate the temperature dependences of the PL peak energy and intensity.

We study experimentally and theoretically the influence of interface roughness on the mobility of two‐dimensional electrons in modulation‐doped AlAs/GaAs quantum wells. It is shown that interface roughness scattering is the dominant scattering mechanism in thin quantum wells with a well thickness Lw