Single and double hole quantum dots in strained Ge/SiGe quantum wells

In the field of semiconductor quantum dot spin qubits, there is growing interest in leveraging the unique properties of hole-carrier systems and their intrinsically strong spin-orbit coupling to engineer novel spin qubits. In contrast to the weak spin-orbit interaction found in Si, strong spin-orbit coupling in Ge may allow purely electrical spin control using the electric field component of a microwave drive signal applied to a confinement gate, without the need for additional components such as microwave stripline antennas or micromagnets. A crucial aspect in the search for alternative semiconductor qubit host materials is the need to maintain the advantages of existing platforms such as Si: low disorder, long coherence times, extensibility, and semiconductor foundry compatibility. Recent advances in semiconductor heterostructure growth have made available high quality, undoped Ge/SiGe quantum wells that can serve as hosts for hole spin qubits. These quantum wells consist of a pure strained Ge layer flanked by Ge-rich SiGe layers above and below, grown by reduced pressure chemical vapor deposition. Isotopic enrichment is possible if required. The heavy hole charge carriers are characterized by a small effective mass m* ~ 0.08m 0 (about 2.4x lighter than for electrons in Si), high mobility μ ≥ 1E5 cm2/Vs, and highly anisotropic g-factors. The small effective mass should allow relatively large and easily-coupled quantum dots. The next-nearest light hole band is theoretically predicted to be on the order of ~ 100 meV below the heavy hole band. The p-orbital character of the hole wavefunction may provide an advantage in minimizing contact hyperfine interactions with nonzero-spin lattice nuclei. There is both theoretical and experimental evidence of a cubic Rashba-type spin-orbit interaction in this material, whose large strength is manifest as a spin-orbit length on the order of 1 micrometer. An additional advantage over Si is that the two-dimensional band structure of Ge quantum wells does not feature valley states, which can present difficulties for qubit operations. Here, we will describe our ongoing efforts to prepare spin qubits in this platform, including development of gated device architectures, device tuning protocols, and charge-sensing capabilities. Initial work focused on single-layer gate layouts, in which lithographically-defined single and double quantum dots were demonstrated at dilution refrigerator temperatures. Charging energies of ~ 1.7 meV were observed. These devices featured a broad and shallow potential landscape, and only a small number of holes could be confined to a quantum dot. Although adjacent quantum dots could be coupled to one another, this coupling was not easily tuned using this gate design. These limitations in the confinement and control of quantum dots motivated the subsequent adoption of multilayer gate-stack architectures, similar to what has been demonstrated in Si. Our three-layer devices feature gate electrodes designed for accumulation of holes, local depletion to form quantum dots, and selective isolation/screening of electric fields. Device structures feature Ti/Pt gates, ALD oxide dielectrics, and implanted ohmic contacts. Symmetrically constructed devices comprise two gated channels for quantum dot confinement, one to be used as the quantum dot under study and the other operated as a local charge sensor. By monitoring the conductance of the charge sensor as the quantum dot is tuned, we can remotely determine the dot’s occupation number, in principle down to the last hole. Iterative cycles of device fabrication and measurement, combined with numerical modeling, have allowed continuous improvements to the device design. Early three-layer device designs suffered from poor isolation between the upper and lower mirror-symmetric device channels. Numerical modeling showed (and measurements have confirmed) that an additional isolation gate could allow improved autonomy of the two channels while maintaining their electrostatic coupling needed for charge sensing. Ongoing, simulation-informed work to fine-tune the device geometry, as well as efforts toward a qubit demonstration, will be discussed. This work was funded, in part, by the Laboratory Directed Research and Development Program and performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the US Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in this article do not necessarily represent the views of the US Department of Energy or the United States Government. The work at NTU was supported by the Ministry of Science and Technology (107-2622-8-002-018-). Figure 1

We report on the magneto-optical study of spin polarized energetic fine structures for exciton complex in single CdSe quantum dot (QD) by using micro- photoluminescence (micro-PL) spectroscopy. The zero-field splitting of exciton luminescence peak arisen from the anisotropic exchange interaction of carriers in the QDs was observed. The g-factors for exciton and negatively-charged exciton, i.e. trion in a single QD were determined by fitting the magnetic field dependence of the corresponding PL peaks. By exciting the single QD with circularly polarized light of σ- and σ+ polarization, the spin-up and spin-down trions were selectively generated. The ratio, τ/τsf, of the exciton lifetime and the time constants for the spin-flipping process of trion in a single QD was estimated to be 0.13, which implies a long spin-lifetime in single CdSe QD.

We study the triplet-singlet relaxation in two-electron semiconductor quantum dots. Both single dots and vertically coupled double dots are discussed. In our work, the electron-electron Coulomb interaction, which plays an important role in the electronic structure, is included. The spin mixing is caused by spin-orbit coupling which is the key to the triplet-singlet relaxation. We show that the selection rule widely used in the literature is incorrect unless near the crossing and/or anticrossing point in single quantum dots. The triplet-singlet relaxation in double quantum dots can be markedly changed by varying barrier height, interdot distance, external magnetic field, and dot size.

We demonstrate that ultraclean single, double, and triple quantum dots (QDs) can be formed reliably in a carbon nanotube (CNT) by a straightforward fabrication technique. The QDs are electrostatically defined in the CNT by closely spaced metallic bottom gates deposited in trenches in SiO2 by sputter deposition of Re. The carbon nanotubes are then grown by chemical vapor deposition (CVD) across the trenches and contacted using conventional resist-based electron beam lithography. Unlike in previous work, the devices exhibit reproducibly the characteristics of ultraclean QDs behavior even after the subsequent electron beam lithography and chemical processing steps. We specifically demonstrate the high quality using CNT devices with two narrow bottom gates and one global back gate. Tunable by the gate voltages, the device can be operated in four different regimes: (i) fully p-type with ballistic transport between the outermost contacts (over a length of 700 nm), (ii) clean n-type single QD behavior where a QD can be induced by either the left or the right bottom gate, (iii) n-type double QD, and (iv) triple bipolar QD where the middle QD has opposite doping (p-type). Our simple fabrication scheme opens up a route to more complex devices based on ultraclean CNTs, since it allows for postgrowth processing.

Summary form only given. Direct observations of the dynamics of single excitonic quantum dots require the use of transient optical spectroscopy. Measurements of the lifetimes of single QD states, Rabi oscillations, biexciton formation and decay, and the development of quantum logic gates are examples of dynamical effects in quantum dots most easily studied through transient nonlinear spectroscopy, but are extremely challenging at the single dot level. We report here the first observation of transient differential transmission from single quantum dots as well as evidence of Rabi oscillations in single quantum dots.Under investigation are quantum dots naturally formed by interface fluctuations in a 42 /spl Aring/ GaAs/Al/sub 0.3/Ga/sub 0.7/As quantum well. Excitons in isolated QD's are probed through 500 nm apertures in an Al mask laid directly onto the sample's surface.

In recent years interest arose into the study of transport through quantum dots. The interest is driven by two reasons. First, small devices like quantum dots and charging effects [1] In such devices open the possibility for future applications in electronics as transistors, memory cells, sensors etc. Small devices are especially interesting in high frequency applications due to their small capacitances. Although different schemes for applications have been proposed, none of it is under development or even in production and it is not clear what will be of real use in the end. The second reason for the interest into these systems is a basic physics point-of-view. These systems are extremely interesting since single quantum dots can be seen as a sort of artificial atom. In this manner-of-speaking coupled quantum dots can be called artificial molecules. These quantum dots are similar to atoms or molecules since a certain number of electrons is confined in a potential. But, in contrast to real atoms, in these artificial systems the number of electrons, the size of the system and the strength of the confinement potential can easily be changed. Therefore, in these quantum dots parameters, such as the ratio between cyclotron frequency and confinement energy or coupling strength between two quantum dots etc, can be easily varied, which is not achievable in real atoms or molecules.

A detailed understanding of the population and coherence dynamics in optically driven individual emitters in solids and their signatures in ultrafast nonlinear-optical signals is of prime importance for their applications in future quantum and optical technologies. In a combined experimental and theoretical study on exciton complexes in single semiconductor quantum dots we reveal a detailed picture of the dynamics employing three-beam polarization-resolved four-wave mixing (FWM) micro-spectroscopy. The oscillatory dynamics of the FWM signals in the exciton-biexciton system is governed by the fine-structure splitting and the biexciton binding energy in an excellent quantitative agreement between measurement and analytical description. The analysis of the excitation conditions exhibits a dependence of the dynamics on the specific choice of polarization configuration, pulse areas and temporal ordering of driving fields. The interplay between the transitions in the four-level exciton system leads to rich evolution of coherence and population. Using two-dimensional FWM spectroscopy we elucidate the exciton-biexciton coupling and identify neutral and charged exciton complexes in a single quantum dot. Our investigations thus clearly reveal that FWM spectroscopy is a powerful tool to characterize spectral and dynamical properties of single quantum structures.

Summary form only given. The excitonic optical response of single quantum dots (QD) has recently been isolated by using high spatial resolution techniques. Photoluminescence (PL) magneto-optical studies of these confined systems have shown interesting physical phenomena such as Zeeman splittings, diamagnetic shifts and the Overhauser effect. Coherent nonlinear optical spectroscopy provides a powerful additional tool for the study of the interaction between magnetic substrates, as has been demonstrated for a single quantum dot. By directly probing the quantum dot resonances under the effect of a magnetic field we can obtain information about spin relaxation times and the presence of dark states. In this paper we show the effect of a magnetic field on single quantum dot resonances (i.e. Zeeman splitting and diamagnetic shift) and report the nonlinear optical response as a function of magnetic field.

A system of quantum dots based on Al x In1−xAs/Al y Ga1−yAs solid solutions is investigated. The use of Al x In1−xAs wide-gap solid solutions as the basis of quantum dots substantially extends the spectral emission range to the short-wavelength region, including the wavelength region near 770 nm, which is of interest for the development of aerospace systems of quantum cryptography. The optical characteristics of Al x In1−xAs single quantum dots grown by the Stranski–Krastanov mechanism were studied by cryogenic microphotoluminescence. The statistics of the emission of single quantum dot excitons was studied using a Hanbury Brown–Twiss interferometer. The pair photon correlation function indicates the sub-Poissonian nature of the emission statistics, which directly confirms the possibility of developing single-photon emitters based on Al x In1−xAs quantum dots. The fine structure of quantum dot exciton states was investigated at wavelengths near 770 nm. The splitting of the exciton states is found to be similar to the natural width of exciton lines, which is of great interest for the development of entangled photon pair emitters based on Al x In1−xAs quantum dots.

Introduction The advanced growth of semiconductor quantum dots (QDs) with high optical quality is one key to realizing novel devices in various research disciplines related to modern semiconductor technologies. Despite recent progress in the top-down lithographic fabrication of single semiconductor QD-like emitters [43], bottom-up fabrication methods are commonly applied for the realization of high quality light emitting QDs [7]. The exploitation of high-density QD arrays as an active material in laser diodes [17] and vertical cavity surface emitting lasers (VCSEL) resulted in a new class of devices featuring lower lasing thresholds and improved device performance (such as temperature stability) in comparison to devices with higher dimensional gain [2, 3]. Spintronics and quantum information processing are intensively studied fields in order to provide complementary or entirely novel routes to a future information technology [12, 5]. Single self-assembled quantum dots (QDs) grown in low-density arrays are promising candidates for realizing functional building blocks in these research fields as they allow to confine single charge carriers or spin carriers while providing a solid-state platform capable of electrical injection or manipulation. Extensive research has resulted in the realization of various devices relying on few or single QDs, for instance quantum light emitters such as single photon sources [23, 36], sources of pairs of entangled photons [1, 45], and fewto single-QD lasers [38]. Moreover, many milestone experiments have been performed with In(Ga)As QDs grown on GaAs, for instance the observation of the Purcell effect [10] or reaching the strong QD exciton–photon coupling regime [26], demonstration of the indistinguishability of photons emitted by a QD [29], or complete control of a single spin by ultrafast optical pulses [25].

We report the fabrication of single and double hole quantum dots using a double-layer-gate design on an undoped accumulation mode /GaAs heterostructure. Electrical transport measurements of a single quantum dot show varying addition energies and clear excited states. In addition, the two-level-gate architecture can also be configured into a double quantum dot with tunable inter-dot coupling.

The observation of photoluminescence spectra of self-assembled single InGaAs quantum dots at room temperature was performed under weak excitation conditions using a near-field scanning optical microscope. Operation in illumination-collection mode with a highly sensitive double-tapered optical fibre probe enabled detection of weak photoluminescence signals at room temperature with high efficiency and high spatial resolution. Each single quantum dot was imaged with a spatial resolution of about 250 nm, which corresponded to a quarter of the wavelength of the photoluminescence from quantum dots. The photoluminescence yields of individual quantum dots were widely distributed and were found to decrease with photoluminescence energy. This result serves as a clue to be pursued for better understanding of the thermal excitation of the carrier from confined states in quantum dots.

We demonstrate a straightforward way to obtain single well-isolated quantum dots emitting in the visible part of the spectrum and characterize the optical emission from single quantum dots using this method. Self-assembled InP quantum dots are grown using gas-source molecular-beam epitaxy over a wide range of InP deposition rates, using an ultralow growth rate of about 0.01 atomic monolayers/s, a quantum-dot density of 1 dot/μm2 is realized. The resulting isolated InP quantum dots embedded in an InGaP matrix are individually characterized without the need for lithographical patterning and masks on the substrate. Such low-density quantum dots show excitonic emission at around 670 nm with a linewidth limited by instrument resolution. This system is applicable as a single-photon source for applications such as quantum cryptography.

The photoluminescence correlation from a single CdSe nanocrystal under pulsed excitation is studied, and a single photon is realized at wavelength 655 nm at room temperature. The single colloidal CdSe quantum dot is prepared on a SiO2/silicon surface by a drop-and-drag technique. The long-term stability of the single-photon source is investigated; it is found that the antibunching effect weakens with excitation time, and the reason for the weakening is attributed to photobleaching. The lifetimes of photoluminescence from a single quantum dot are analyzed at different excitation times. By analyzing the probability distribution of on and off times of photoluminescence, the Auger assisted tunneling and Auger assisted photobleaching models are applied to explain the antibunching phenomenon.

We demonstrate optical single-electron spin initialization, storage, and readout in a single self-assembled InGaAs quantum dot using a spin memory device. Single-electron spin relaxation is monitored over time scales exceeding $\ensuremath{\ge}30\text{ }\ensuremath{\mu}\text{s}$, defined only by extrinsic experimental parameters such as the optical detection efficiency. The selective generation of a single electron in the dot is performed by resonant optical excitation and subsequent partial exciton ionization; the hole is removed from the dot while the electron remains stored. When subject to a magnetic field applied in Faraday geometry, we show how the spin of the electron can be prepared with a well-defined spin projection relative to the light propagation direction simply by controlling the voltage applied to the gate electrode. The spin is stored then in the dot before being read out using an optical implementation of spin to charge conversion, whereby the spin projection of the electron is mapped onto a more robust variable, the charge state of the dot. After spin to charge conversion, we show how the charge occupancy can be repeatedly and nonperturbatively measured by pumping a luminescence recycling transition. The approach is shown to provide a readout signal ${10}^{4}$ times stronger per spin when compared to previous methods. In combination with spin manipulation using the optical Stark effect or microwaves, our approach provides an ideal basis for probing spin coherence in single self-assembled quantum dots over long-time scales and the development of methods for coherent spin control.