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We report our work on fabricating lithographically aligned patterned backgates on thin (50–60 μm) III-V semiconductor samples using single sided mask aligners only. Along with this we also present a way to photograph both sides of a thin patterned chip using inexpensive infrared light emitting diodes and an inexpensive (consumer) digital camera. A robust method of contacting both sides of a sample using an ultrasonic bonder is described. In addition we present a mathematical model to analyze the variation in the electrochemical potential through the doped layers and heterojunctions that are normally present in most GaAs based devices. We utilize the technique and the estimates from our model to fabricate an electron-hole bilayer device in which each layer is separately contacted and has tunable densities. The electron and hole layers are separated by barriers either 25 or 15 nm wide. In both cases, the densities can be matched by using appropriate bias voltages. [ABSTRACT FROM AUTHOR]

We present measurements of Coulomb drag in an ambipolar GaAs/AlGaAs double quantum well structure that can be configured as both an electron-hole bilayer and a hole-hole bilayer, with an insulating barrier of only 10 nm between the two quantum wells. Coulomb drag resistivity is a direct measure of the strength of interlayer particle-particle interactions. We explore the strongly interacting regime of low carrier densities (2D interaction parameter rs up to 14). Our ambipolar device design allows a comparison between the effects of the attractive electron-hole and repulsive hole-hole interactions and also shows the effects of the different effective masses of electrons and holes in GaAs. [ABSTRACT FROM AUTHOR]

Fundamental to integrated photonic quantum computing is an on-chip method for routing and modulating quantum light emission. We demonstrate a hybrid integration platform consisting of arbitrarily designed waveguide circuits and single-photon sources. InAs quantum dots (QD) embedded in GaAs are bonded to a SiON waveguide chip such that the QD emission is coupled to the waveguide mode. The waveguides are SiON core embedded in a SiO2 cladding. A tuneable Mach Zehnder interferometer (MZI) modulates the emission between two output ports and can act as a path-encoded qubit preparation device. The single-photon nature of the emission was verified using the on-chip MZI as a beamsplitter in a Hanbury Brown and Twiss measurement. [ABSTRACT FROM AUTHOR]

The interference of photons emitted by dissimilar sources is an essential requirement for a wide range of photonic quantum information applications. Many of these applications are in quantum communications and need to operate at standard telecommunication wavelengths to minimize the impact of photon losses and be compatible with existing infrastructure. Here, we demonstrate for the first time the quantum interference of telecom-wavelength photons from an InAs/GaAs quantum dot single-photon source and a laser; an important step towards such applications. The results are in good agreement with a theoretical model, indicating a high degree of indistinguishability for the interfering photons. [ABSTRACT FROM AUTHOR]

We report an ambipolar device fabricated in undoped GaAs/AlGaAs quantum wells (widths 10 and 25 nm) with front and backgates that allow almost two orders of magnitude in density to be accessed in the same device (7×109cm-2 to 5×1011cm-2). By changing the well width, the relative electron and hole mobilities can be tuned, approaching similar velocities. We describe an approach to fully characterize the quantum well, including the impurity backgrounds and both the upper and lower interfaces, making use of the ability to control the carrier density and the position of the wavefunction independently over a wide range. [ABSTRACT FROM AUTHOR]

Abstract: Discontinuous yield during nanoindentation is observed on a wide range of materials and has generally been associated with materials having a low dislocation density before deformation. The dislocation density of In x Ga1− x As films grown on GaAs can be varied by changing the atom fraction of In and thereby the lattice mismatch between substrate and film. Using TEM and by varying the dislocation density by means of indentations with different spacings, the probability of a discontinuous yield event in InGaAs was found to correlate with the initial dislocation density estimated from the misfit. [Copyright &y& Elsevier]

Abstract: We report on the use of a commercially available band-edge detection system for substrate temperature monitoring of gallium arsenide substrates. The extension of the technique to the cases where strong absorption by either the substrate or substrate holder might normally preclude the use of such systems due to poor signal levels is discussed. For indium-mounted wafers, a background subtraction/removal is applied which allows unambiguous determination of the band edge across the full temperature range. An alternative method of operation of the instrument as a highly configurable pyrometer allows measurements to be made on highly conducting p-type substrates where free carrier absorption swamps the band edge. [Copyright &y& Elsevier]

Abstract: In order to achieve reproducible layer thicknesses and compositions in samples which may be grown several weeks apart it is critical that the growth conditions are kept constant throughout this time. With this in mind the more in situ tools which are available to the grower, the better the control over critical parameters will be. In this paper we show that the interference effect which is seen when growing layers of differing refractive index on top of one another can be extended from the GaAs/AlAs materials system to include compounds containing Indium up to compositions near 55%. [Copyright &y& Elsevier]

We present a method of forming and controlling large arrays of gate-defined quantum devices. The method uses an on-chip, multiplexed charge-locking system and helps to overcome the restraints imposed by the number of wires available in cryostat measurement systems. The device architecture that we describe here utilises a multiplexer-type scheme to lock charge onto gate electrodes. The design allows access to and control of gates whose total number exceeds that of the available electrical contacts and enables the formation, modulation and measurement of large arrays of quantum devices. We fabricate such devices on n-type GaAs/AlGaAs substrates and investigate the stability of the charge locked on to the gates. Proof-of-concept is shown by measurement of the Coulomb blockade peaks of a single quantum dot formed by a floating gate in the device. The floating gate is seen to drift by approximately one Coulomb oscillation per hour. [ABSTRACT FROM AUTHOR]

We have developed a method of fabricating very shallow, gateable, undoped two-dimensional electron gases (2DEG) and making very low resistivity contacts to these. We studied the evolution of mobility as a function of the depth of the 2DEG (from 310 to 30 nm). We extract quantitative information about the background impurity concentrations in GaAs and AlGaAs, the interface roughness, and the charge in the surface states. Surface charge sets an intrinsic limit to the mobility of very shallow 2DEGs. It is probably impossible to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to accommodate a spacer layer. [ABSTRACT FROM AUTHOR]

The authors report on the experimental demonstration of a GaAs/AlGaAs-based quantum cellular automata cell fabricated using electron beam lithographically defined gates. These surface metallic gates form a pair of double quantum dots, as well as a pair of quantum point contacts (QPCs) that act as noninvasive voltage probes. Measurements at cryogenic temperatures show that an electron transfer in the input dots induces the relocation of a single electron in the output dots. Using the QPCs they were also able to determine the operating limits of the cell. [ABSTRACT FROM AUTHOR]

The authors have fabricated pillar microcavity samples with Bragg mirrors consisting of alternate layers of GaAs and aluminum oxide. Compared to the more widely studied GaAs/AlAs micropillars, these mirrors can achieve higher reflectivities with fewer layer repeats and reduce the mode volume. The authors have studied a number of samples containing a low density of InGaAs/GaAs self-assembled quantum dots in a cavity and here report observation of a threefold enhancement in the radiative lifetime of a quantum dot exciton state due to the Purcell effect. [ABSTRACT FROM AUTHOR]

Electron and hole sublevels in quantum dots (QDs) are experimentally determined using the excitation-power dependence of photoluminescence spectrum for a modulation-doped QD structure. The sublevel spacing between n=1 and n=2 electron states can be obtained from the photoluminescence (PL) spectrum under very low excitation and the spacing between n=1 and n=2 hole states can be obtained by comparing the PL spectrum under high excitation with the one under low excitation. The proposed method should give useful information in the design of QD devices, as well as for the verification of theoretical calculations of QD energy levels. [ABSTRACT FROM AUTHOR]

The performance of InAs/GaAs quantum dot solar cells was investigated up to an optical concentration of 500-suns. A high temperature spacer layer between successive layers of quantum dots was used to reduce the degradation in the open circuit voltage relative to a control device without quantum dots. This improvement is explained using optical data while structural imaging of quantum dot stacks confirm that the devices are not limited by strain. The evolution of the open circuit voltage as a function of number of suns concentration was observed to be nearly ideal when compared with a high performance single junction GaAs solar cell. Analysis of Suns- V oc measurements reveal diode ideality factors as low as 1.16 which is indicative of a low concentration of defects in the devices. [ABSTRACT FROM AUTHOR]

Abstract: We use quantum wires fabricated on undoped GaAs/AlGaAs heterostructures in which the average impurity separation is greater than the device size to compare the behaviour of the zero-bias anomaly against predictions from Kondo and spin polarisation models. Cleanliness and reproducibility are significantly improved in our undoped devices when compared to our doped devices. [Copyright &y& Elsevier]

Using a recent time-of-flight measurement technique with 1 ps time resolution and electron-energy spectroscopy, we develop a method to measure the longitudinal-optical-phonon emission rate of hot electrons traveling along a depleted edge of a quantum Hall bar. Comparison to a single-particle model implies the scattering mechanism involves a two-step process via an intra-Landau-level transition. We show that this can be suppressed by control of the edge potential profile, and a scattering length > 1 mm can be achieved, allowing the use of this system for scalable single-electron device applications. [ABSTRACT FROM AUTHOR]

Thermal dissociation of free and acceptor-bound quasi-two-dimensional positive trions is investigated by measuring the temperature dependence of the integrated emission intensity in magnetic fields up to 17 T in high quality GaAs/AlxGa1-xAs quantum wells. Three distinct dissociation processes are observed for the well-resolved hole cyclotron replicas ("shake-up") of positive trions bound to neutral acceptors in the spin-doublet state (SU-A0X+d). To demonstrate that the hole involved in the shake-up process is not bound by the Coulomb interaction to the charged A0X+ complex, we calculate the valence Landau levels using the Luttinger model beyond the axial approximation. The calculated value of the hole cyclotron energy agrees well with the experimental data for the energy separation of the A0X+ and SU-A0X+ lines, determined from the emission spectra. At low temperatures, below 6 K, the dominant dissociation results in a free hole and an exciton bound to the neutral acceptor in the spin-singlet or -triplet state, (A0X+d→A0Xs+h or A0Xt+h). At higher temperatures, above 9 K, the dissociation into the free positive trion and the neutral acceptor (A0X+d→A0+X+) predominates. From the temperature evolution of the integrated emission of the free trion lines (X+) we evaluate the transition energy between the two triplet trion states, the dark one (X+td) and the bright one (X+tb). The ionization energies of all detected dissociation processes are compared with the spectral positions of the relevant radiative recombination lines from which excellent quantitative agreement is achieved. [ABSTRACT FROM AUTHOR]