Hiroyuki Sakaki

Multidimensional quantum well laser and temperature dependence of its threshold current

A new type of semiconductor laser is studied, in which injected carriers in the active region are quantum mechanically confined in two or three dimensions (2D or 3D). Effects of such confinements on the lasing characteristics are analyzed. Most important, the threshold current of such laser is predicted to be far less temperature sensitive than that of conventional lasers, reflecting the reduced dimensionality of electronic state. In the case of 3D‐QW laser, the temperature dependence is virtually eliminated. An experiment on 2D quantum well lasers is performed by placing a conventional laser in a strong magnetic field (30 T) and has demonstrated the predicted increase of T0 value from 144 to 313 °C.

Scattering Suppression and High-Mobility Effect of Size-Quantized Electrons in Ultrafine Semiconductor Wire Structures

Transport properties of electrons confined in ultrafine wire structures are studied theoretically. The scattering probability of such size-quantized electrons is calculated for Coulomb potential and is shown to be suppressed drastically because of the one-dimensional nature of the electronic motion in the wire. Mobilities are estimated to be well beyond 106 cm2/Vs for a properly-designed GaAs wire at low temperatures. The feasibility of preparing such ultrafine structures with and without ultrafine lithography is discussed.

Interface roughness scattering in GaAs/AlAs quantum wells

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<60 A, where electron mobilities are proportional to L6w, reaching 2×103 cm2/V s at Lw∼55 A. From detailed comparison between theory and experiment, it is determined that the ‘‘GaAs‐on‐AlAs’’ interface grown by molecular beam epitaxy has a roughness with the height of 3–5 A and a lateral size of 50–70 A.

Trapping of photogenerated carriers by InAs quantum dots and persistent photoconductivity in novel GaAs/n-AlGaAs field-effect transistor structures

The trapping of photogenerated carriers by embedded InAs quantum dots (QDs) has been studied at 77 K in novel GaAs/n-AlGaAs structures. It is found that the concentration Ns of two dimensional electrons at a given gate voltage Vg is persistently increased by light illumination, because of the trapping of holes by QDs. By the interplay of the gate voltage and photocarrier generation, a distinct hysteresis is observed in the Ns-Vg characteristics. A drastic change of electron mobility by a factor of 19 is achieved by light illumination. The applications of this device for a novel light-controllable floating dot memory is suggested.

Field‐effect transistors using alkyl substituted oligothiophenes

Field‐effect transistors (FETs) have been prepared using thin films of alkyl substituted oligothiophenes. These compounds bring about a significant increase in the source‐drain channel current when compared to the conventional nonsubstituted oligothiophenes. The increased channel current mostly results from the enhanced carrier mobility of the material. We report that the FETs are readily made by a single routine process of casting or evaporation.

One Atomic Layer Heterointerface Fluctuations in GaAs-AlAs Quantum Well Structures and Their Suppression by Insertion of Smoothing Period in Molecular Beam Epitaxy

Photoluminescence spectra of GaAs-AlAs quantum wells are studied to evaluate the flatness of heterointerfaces prepared by molecular beam epitaxy. We examine the correlation of the interface roughness with the measured intensity oscillations of reflective high energy electron diffraction (RHEED) during the growth. The crystal surface is found to roughen after the growth of 10 or more atomic layers. The growth interruption of 10–100 seconds prior to the interface formation is effective in achieving an atomically flat interface, leading to sharp photoluminescence with the linewidth <30 A at 77 K even when the quantum well width is reduced to 40 A.

Intersubband absorption linewidth in GaAs quantum wells due to scattering by interface roughness, phonons, alloy disorder, and impurities

We calculate the intersubband absorption linewidth 2Γop in quantum wells (QWs) due to scattering by interface roughness, LO phonons, LA phonons, alloy disorder, and ionized impurities, and compare it with the transport energy broadening 2Γtr=2ℏ/τtr, which corresponds to the transport relaxation time τtr related to the electron mobility μ. Numerical calculations for GaAs QWs clarify the different contributions of each individual scattering mechanism to the absorption linewidth 2Γop and transport broadening 2Γtr. Interface roughness scattering contributes about an order of magnitude more to the linewidth 2Γop than to the transport broadening 2Γtr, because the contribution from the intrasubband scattering in the first excited subband is much larger than that in the ground subband. On the other hand, LO phonon scattering (at room temperature) and ionized impurity scattering contribute much less to the linewidth 2Γop than to the transport broadening 2Γtr. LA phonon scattering makes comparable contributions to ...

Room Temperature Observation of Differential Negative Resistance in an AlAs/GaAs/AlAs Resonant Tunneling Diode

A resonant tunneling diode having AlAs/GaAs/AlAs double barrier structure is designed to enhance the resonant tunneling current component and to suppress the excess current component which is believed to dominate the transport at high temperatures. Based on this design, a diode structure with optimized parameters is prepared by careful molecular beam epitaxial growth, in which the dopant incorporation into the well layer and the interface asperities are minimized. The AlAs/GaAs/AlAs diodes thus fabricated were found to exhibit differential negative resistance at room temperature for the first time.

Formation of GaAs ridge quantum wire structures by molecular beam epitaxy on patterned substrates

A ridge quantum wire structure has been successfully fabricated on a patterned (001) GaAs substrate by first growing a (111)B facet structure with a very sharp ridge and then depositing a thin GaAs quantum well on its top. Electron microscope study has shown that a GaAs wire with the effective lateral width of 17–18 nm is formed at the ridge top. Photoluminescence and cathodoluminescence measurements indicate that one of the luminescence lines comes from the wire region at the ridge and its blue shift (∼60 meV) agrees with the quantum confined energy calculated for the observed wire structure.

Atomistic models of interface structures of GaAs-AlxGa1−xAs (x = 0.2−1) quantum wells grown by interrupted and uninterrupted MBE

Atomic-scale models of heterointerface structures of GaAs-AlxGa1−xAs quantum wells (QWs) are presented. For QWs with x > 0.5, top (AlGaAs-on-GaAs) interfaces formed by continuous MBE have the roughness of one atomic layer height whose atomic step interval is 200 A, resulting in a well-known photoluminescence (PL) broadening from QWs. This roughness can be smoothed by growth interruption for tens of second prior to the formation of top interfaces, leading to a drastic sharpening of the PL spectra by a factor of 4–5. In contrast, bottom (GaAs-on-AlGaAs) interfaces have atomic steps of 40 A in width irrespective of growth interruption; this step interval is so small as compared with excition size that they act as a “pseudo-smooth” interface. For QWs with x < 0.3, however, the bottom interfaces become similar to the top interface, and can be smoothed by growth interruption.