A. Zrenner

Coherent properties of a two-level system based on a quantum-dot photodiode.

Present-day information technology is based mainly on incoherent processes in conventional semiconductor devices. To realize concepts for future quantum information technologies, which are based on coherent phenomena, a new type of ‘hardware’ is required. Semiconductor quantum dots are promising candidates for the basic device units for quantum information processing. One approach is to exploit optical excitations (excitons) in quantum dots. It has already been demonstrated that coherent manipulation between two excitonic energy levels—via so-called Rabi oscillations—can be achieved in single quantum dots by applying electromagnetic fields. Here we make use of this effect by placing an InGaAs quantum dot in a photodiode, which essentially connects it to an electric circuit. We demonstrate that coherent optical excitations in the quantum-dot two-level system can be converted into deterministic photocurrents. For optical excitation with so-called π-pulses, which completely invert the two-level system, the current is given by I = fe, where f is the repetition frequency of the experiment and e is the elementary charge. We find that this device can function as an optically triggered single-electron turnstile.

Growth and characterization of self-assembled Ge-rich islands on Si

Ge-rich islands have been grown on Si (100) substrates by molecular beam epitaxy. Their density and size distribution are analysed by atomic force microscopy as a function of growth temperature, growth rate and Ge coverage. Overgrown islands have been studied by transmission electron microscopy, Raman scattering and photoluminescence. The first results of photocurrent spectroscopy on Si/Ge/Si pin diodes show the expected shift of the energy gap. Based on these results, novel device applications of Ge-rich islands in Si are proposed.

Electrical detection of optically induced charge storage in self-assembled InAs quantum dots

Spectrally resolved photoresistance investigations of charge storage effects in self-assembled InAs quantum dots (QDs) are reported. Resonant optical excitation of the QDs produces a strong increase of the lateral resistance of a spatially separated electron channel (ΔR) which reflects the stored charge density. This photoresponse is persistent for many hours at 145 K and can be controllably reversed electrically. Pronounced oscillations observed in the spectral variation ΔR are shown to reflect the excitation spectrum of the QD ensemble showing resonances that arise from both direct and phonon-assisted absorption processes.

Normal-incident intersubband photocurrent spectroscopy on InAs/GaAs quantum dots

We report on intersubband photocurrent spectroscopy of self-assembled InAs/GaAs quantum dots (QDs) both in normal incidence and in multipass waveguide geometry. The bound-to-continuum transition energy in the conduction band lies in the 200–500 meV spectral range. Polarization dependent photocurrent spectroscopy shows that the intersubband transitions in the InAs-QDs are nearly independent of the polarization of the incoming radiation.

Subband physics for a “realistic” δ-doping layer

Abstract We consider the electronic level structure of Si-doping layers in MBE-grown GaAs for the case of a finite spreading of the dopant. From oscillatory magnetotransport data it is shown that so-called δ-doping layers described in the literature to date do not represent a system of donr ions confined in a single atomic layer.

Optical excitations of a self-assembled artificial ion

By use of magnetophotoluminescence spectroscopy, we demonstrate bias-controlled single-electron charging of a single quantum dot. Neutral, single, and double charged excitons are identified in the optical spectra. At high magnetic fields one Zeeman component of the single charged exciton is found to be quenched, which is attributed to the competing effects of tunneling and spin-flip processes. Our experimental data are in good agreement with theoretical model calculations for situations where the spatial extent of the hole wave functions is smaller as compared to the electron wave functions. Semiconductor quantum dots ~QD’s ! are often referred to as artificial atoms. Different levels of neutral occupancies in QD’s have been obtained in the last years by power dependent optical excitation. The associated few-particle states were intensively studied by multiexciton photoluminescence ~PL! spectroscopy and corresponding theoretical investigations. 1‐7 Occupancies with different numbers of electrons and holes result in charged exciton complexes. In analogy to QD’s with neutral occupancy—artificial atoms— charged exciton complexes may be considered as artificial ions. In the field of low-dimensional semiconductors charged excitons were first observed in quantum-well structures. 8 In QD’s, charged excitons were studied in inhomogeneously broadened ensembles by PL ~Ref. 9! as well as in interband transmission experiments, 10 and recently also in single, optically tunable QD’s, 11 as well as in electrically tunable quantum rings by PL. 12 Few-particle theory predicts binding energies for charged QD excitons on the order of some meV. 11,13 This allows for the controlled manipulation of energetically well-separated few-particle states under the action of an external gate electrode. Discrete and stable numbers of extra charges are thereby possible via the Coulomb blockade mechanism. In future experiments and possible applications, the resonant optical absorption and emission of such systems is expected to be tunable between discrete and characteristic energies. Moreover such few-particle systems are expected to exhibit an interesting variety of spin configurations, which can be controlled by an external magnetic field, gate-induced occupancy, and spin-selective optical excitation. In the present paper we present, for the first time to our knowledge, experimental and theoretical results on the gate-controlled charging of a single InxGa12xAs QD with an increasing number of electrons probed by magneto-PL. For controlled charging of individual QD’s a special electric-field tunable n-i structure has been grown by molecular-beam epitaxy. In0.5Ga0.5As QD’s are embedded in an i-GaAs region 40 nm above an n-doped GaAs layer (5 310 18 cm 23 ) which acts as a back contact. The growth of the QD’s is followed by 270-nm i-GaAs, a 40-nm-thick Al0.3Ga0.7As blocking layer, and a 10-nm i-GaAs cap layer. As a Schottky gate we use a 5-nm-thick semitransparent Ti layer. The samples were processed as photodiodes combined with electron-beam-structured shadow masks with apertures ranging from 200 to 800 nm. Schematic overviews of the sample and the band diagram are shown in Figs. 1~a! and 1~b!. The occupation of the QD with electrons can be controlled by an external bias voltage VB on the Schottky gate with respect to the back contact. For increasing VB the band flattens, and the electron levels of the QD are shifted below the Fermi energy of the n-GaAs back contact, which results in successive single-electron charging of the QD. In our experiments excitons are generated optically at low rate and form charged excitons together with the VB induced extra electrons in the QD. We used a HeNe laser ~632.8 nm! for

Photocurrent and photoluminescence of a single self-assembled quantum dot in electric fields

We have fabricated single-quantum-dot photodiodes by embedding InGaAs quantum dots in the intrinsic region of an n-i-Schottky diode combined with near-field shadow masks. As a function of the bias voltage, we study one and the same quantum dot in the two complementary regimes of photocurrent and photoluminescence. The Stark shift of the exciton ground state continues monotonically in both regimes, confirming nicely the observation of the same quantum dot in photoluminescence and photocurrent. In the limit of high electric fields, we observe a broadening of the photocurrent linewidth from which we determine a strongly reduced exciton lifetime of below 1 ps.

Quantum-dot infrared photodetector with lateral carrier transport

In this letter, we present a normal-incident quantum-dot infrared photodetector. The detection principle is based on intersubband transition between the p states and the wetting-layer subband in the conduction band of self-assembled In(Ga)As/GaAs quantum dots. Carrier transport takes place in a channel next to the quantum-dot layers. The photoresponse is peaked at λ=6.65 μm (186 meV) and reaches a maximum value of over 11 A/W at T=30 K with a wavelength resolution of 12%. Detector response time τ is determined to be about 0.8 ms. Temperature and frequency dependence of the detector structure are discussed.

Saturation of the free-electron concentration in delta -doped GaAs: the DX centre in two dimensions

The saturation of the free-electron concentration at high doping densities in delta -doped GaAs is studied by Shubnikov-de Haas measurements under hydrostatic pressure. Saturation occurs as soon as the depth of the Hartree potential is large enough to make the energy of the DX centre line up with the Fermi energy. The ultimate limit of the low-temperature equilibrium free-electron concentration in truly delta -doped GaAs is estimated to be 5.5*1012 cm-2 for Si donors. For realistic samples grown at Ts=500 degrees C the electron concentration is saturated at 7*1012 cm-2 due to the occupation of DX centres in a doping sheet, of finite thickness ( approximately=30 AA). For samples grown at higher temperatures (Ts=600 degrees C) autocompensation can occur prior to the population of the DX centre. Increases in mobility with increasing pressure observed in the Shubnikov-de Haas measurements for the i=0 sub-band provide additional evidence for a movement of the donors away from the original doping plane.

Lateral intersubband photocurrent spectroscopy on InAs/GaAs quantum dots

We report on lateral intersubband photocurrent spectroscopy on self-assembled InAs/GaAs quantum dots in normal incidence. The in-plane polarized intersubband transition between the first exited states and wetting layer in the conduction band is peaked at 180 meV with a spectral linewidth of 20 meV. In-plane polarization dependent measurements show that the energy separation between the (100) and (010) states caused by lateral confinement is about 3.5 meV. A comparison of photoluminescence and vertical and lateral photocurrent experiments leads to a consistent picture of the energy levels in the conduction and valence band.