Seigo Tarucha

Electron Transport in Quantum Dots

The ongoing miniaturization of solid state devices often leads to the question: “How small can we make resistors, transistors, etc., without changing the way they work?” The question can be asked a different way, however: “How small do we have to make devices in order to get fundamentally new properties?” By “new properties” we particularly mean those that arise from quantum mechanics or the quantization of charge in units of eeffects that are only important in small systems such as atoms. “What kind of small electronic devices do we have in mind?” Any sort of clustering of atoms that can be connected to source and drain contacts and whose properties can be regulated with a gate electrode. Practically, the clustering of atoms may be a molecule, a small grain of metallic atoms, or an electronic device that is made with modern chip fabrication techniques. It turns out that such seemingly different structures have quite similar transport properties and that one can explain their physics within one relatively simple framework. In this paper we investigate the physics of electron transport through such small systems.

Microwave spectroscopy of a quantum-dot molecule

Quantum dots are small conductive regions in a semiconductor, containing a variable number of electrons (from one to a thousand) that occupy well-defined, discrete quantum states—for which reason they are often referred to as artificial atoms. Connecting them to current and voltage contacts allows the discrete energy spectra to be probed by charge-transport measurements. Two quantum dots can be connected to form an ‘artificial molecule’. Depending on the strength of the inter-dot coupling (which supports quantum-mechanical tunnelling of electrons between the dots), the two dots can form ‘ionic’ (refs 2–;6) or ‘covalent’ bonds. In the former case, the electrons are localized on individual dots, while in the latter, the electrons are delocalized over both dots. The covalent binding leads to bonding and antibonding states, whose energy difference is proportional to the degree of tunnelling. Here we report a transition from ionic bonding to covalent bonding in a quantum-dot ‘artificial molecule’ that is probed by microwave excitations. Our results demonstrate controllable quantum coherence in single-electron devices, an essential requirement for practical applications of quantum-dot circuitry.

Reduction of quantized conductance at low temperatures observed in 2 to 10 μm-long quantum wires

Abstract We observe a small but definite decrease in quantized conductance at low temperatures in 2 to 10 μm-long single-mode quantum wires. The temperature dependence is stronger in the longer wires. The conductance of the 2 μm-long wire is nearly constant below a critical temperature, at which the thermal length becomes longer than the wire length, while that of the 5 and 10 μm-long wires decreases fairly monotonically with decreasing temperature. We will discuss the effect of mutual Coulomb interaction that is responsible for the change in quantized conductance observed as a function of temperature and wire length.

Effect of Well Size Fluctuation on Photoluminescence Spectrum of AlAs–GaAs Superlattices

Photoluminescence spectra for undoped superlattices exhibited smaller half widths compared with those obtained in GaAs bulk crystals. However, both the spectral width and shape were found to be very sensitive to the well size. When the well size is greater than about 80 A, the spectrum at 77 K showed smaller linewidth, which agrees with theoretical results, whereas when the well size is smaller than this, the spectral width increased with decreasing well size. In some cases, additional emission bands appeared in the lower energy side of the main emission peak. These phenomena were interpreted in terms of well size fluctuations.

Monolithic integration of a laser diode and an optical waveguide modulator having a GaAs/AlGaAs quantum well double heterostructure

This letter describes the monolithic integration of a laser diode and an optical waveguide modulator that have the same GaAs/AlGaAs quantum well double heterostructure grown on the same substrate. The intensity of the light emitted from the laser diode is modulated by the modulator, utilizing the effect of an electric field on exciton absorption. The absorption loss coefficient of the modulator waveguide with no applied voltage was estimated to be 60 cm−1. This value is much smaller than the previously reported corresponding value of a veguide having a conventional GaAs double heterostructure. The modulation depth achieved was 7 dB for a driving voltage of 2.3 V; the cut‐off modulation frequency was 0.88 GHz. A prospect for improving these characteristics is also presented.

Compositional disordering of GaAs-AlxGa1−xAs superlattice by Ga focused ion beam implantation and its application to submicron structure fabrication

Using a focused ion beam technology, Ga ion is implanted into a GaAs-AlxGa1-xAs superlattice epitaxial wafer. The compositional disordering of the superlattice occurs during an annealing after ion-implantation. Interdiffusion coefficient of Ga and Al is measured from the photoluminescence peak energy shift as a function of annealing time and the result shows that it is as large as that for Si ion implanted superlattice. By line-and-space scan of the Ga ion beam, a submicron periodic structure of the superlattice and the mixed crystal is fabricated over the epi-wafer and examined by low temperature cathodo-luminescence topography.

Exciton binding energy in GaAs quantum wells deduced from magneto-optical absorption measurement

Abstract Magneto-optical absorption spectra due to exciton states and Landau-levels were measured in GaAs/AlAs multi-quantum-wells. By extrapolating the photon energies of the absorption peaks to zero magnetic field, the lowest state (1S) heavy hole exciton binding energy, EB h (1S), was obtained as a function of well size Lz in the range 58 A ⩽ L z ⩽252 A . The Lz dependence of EBh showed the change of the exciton character from three-dimensional to two-dimensional with decreasing Lz. The diamagnetic shift observed for the heavy hole exciton peak was larger than that for the light hole exciton peak, showing the anisotropic nature of the Luttinger-Kohn Hamiltonian. The diamagnetic shift of the heavy hole exciton peak became smaller as Lz was decreased, suggesting the enhancement of the two-dimensional exciton character.

Carrier-Induced Energy-Gap Shrinkage in Current-Injection GaAs/AlGaAs MQW Heterostructures

Recombination radiation below the lowest confined particle transition energy observed in GaAs/AlGaAs multi-quantum-well heterostructure lasers was studied in comparison with that in conventional GaAs double-heterostructure lasers. The spontaneous emission shows lower-energy-side broadening in proportion to the 1/2.6 power of the injected carrier density for both the MQW structures and the conventional DHs. This carrier density dependence suggests that the observed low-energy-side broadening of the spontaneous emission is due to carrier-induced energy-gap shrinkage. The energy gap shrinkage in the MQW structures is considered theoretically by modifying the well-known bulk theory for gap shrinkage and taking the effective mass anisotropy into account. This theory is found to account satisfactorily for the observed lower-energy-side broadening. The results suggests that lasing below the lowest confined particle transition energy in MQW laser diodes is due to gap shrinkage rather than to LO-phonon participation as reported by other workers.

Si and Sn Doping in AlxGa1-xAs Grown by MBE

Electrical properties of Si and Sn doped AlxGa1-xAs epitaxial films grown by MBE have been studied in the wide range of AlAs mole fraction. With a constant doping level, decrease in the carrier concentration was observed around the direct-indirect band crossover point, for Si-doped and Sn-doped AlxGa1-xAs. Abrupt increase in donor ionization energy ED occurred in Si-doped AlxGa1-xAs films. These characteristics are similar to those reported for Te and Sn doped AlxGa1-xAs grown by LPE.

Optical Absorption Characteristics of GaAs–AlGaAs Multi-Quantum-Well Heterostructure Waveguides

Optical waveguide absorption characteristics of MBE grown GaAs–AlGaAs MQW doubleheterostructure wafers are studied by measuring edge luminescence under photoexcitation, and are compared with those of conventional GaAs DH wafers. The edge luminescence spectra show a double peak structure which is well explained by waveguide self-absorption, rather than by L0-phonon participation as has been suggested by other workers. The optical absorption loss of the unexcited MQW waveguide at its lasing wavelength is found to be much smaller than that for the conventional DH waveguide.