Si Quantum Dot Structures Research Articles

Design of high-performance entangling logic in silicon quantum dot systems with Bayesian optimization

Device engineering based on computer-aided simulations is essential to make silicon (Si) quantum bits (qubits) be competitive to commercial platforms based on superconductors and trapped ions. Combining device simulations with the Bayesian optimization (BO), here we propose a systematic design approach that is quite useful to procure fast and precise entangling operations of qubits encoded to electron spins in electrode-driven Si quantum dot (QD) systems. For a target problem of the controlled-X (CNOT) logic operation, we employ BO with the Gaussian process regression to evolve design factors of a Si double QD system to the ones that are optimal in terms of speed and fidelity of a CNOT logic driven by a single microwave pulse. The design framework not only clearly contributes to cost-efficient securing of solutions that enhance performance of the target quantum operation, but can be extended to implement more complicated logics with Si QD structures in experimentally unprecedented ways.

Electron Field Emission from Multiply-Stacked Si Quantum Dots Structures with Graphene Top-Electrode

Introduction Electron field emission from Si-based nanocrystals has been stimulated considerable interest because electron emission devices are expected to be used for various applications [1, 2]. In our previous work, we have demonstrated electron field emission from multiple-stacked Si-quantum-dots (Si-QDs) structures with ultrathin Au top electrodes under biases of 6V and over, in which electric field concentrates on the upper dot layers [3]. To reduce electron scattering in top electrodes, we fabricated the multiple stacked Si-QDs structures with single layer graphene top electrodes and evaluated their electron emission properties. Experimental After conventional wet chemical cleaning steps, n-type Si(100) wafers were oxidized at 1000 °C in dry O2 to form ~3.0 nm thick SiO2. After that, Si-QDs with an areal dot density as high as ~4.7×1011 cm−2 and an average dot size of ~4.5 nm were formed on SiO2 by low pressure chemical vapor deposition (LPCVD) using pure SiH4, followed by thermal oxidation to cover the surface of the dots with ~1.5 nm-thick SiO2. By repeating such a process sequence, 11-fold stacked Si-QDs layers were formed. Single layer graphene was formed on a Cu foil by solid-source LPCVD using Poly-styrene. After spin coating poly methyl methacrylate (PMMA) on the graphene, the Cu film and residual Fe were removed by dipping in the Fe(NO3)3·9H2O solution and nitric acid solution (HNO3: H2O = 1:5), respectively. After that, PMMA/graphene was transferred on the multiple stacked Si-QDs layers. Subsequently, the PMMA was removed by dipping in acetone and IPA. We also fabricated ~10 nm-thick Au films as top electrodes on the 11-fold stacked Si-QDs by electron beam (EB) evaporation as a reference. Finally, Al films were formed as bottom electrodes by thermal evaporation. Results and Discussion When forward bias (|V s|) exceeds ~6.0 V, electron emission current (I e) from the multiple stacked Si-QDs with the graphene top electrode appears. In addition, with an increase in the |V s|, the I e was exponentially increased as in the case of the Au top-electrodes. It is interesting to note that the I e for the sample with the graphene top electrode was larger than that for the sample with the Au top electrode although the sample current (I s) was suppressed by using graphene as the top electrode (Fig.1). This result implies reduction of electron tunneling current, which can be interpreted in terms of density-of-state difference between the graphene and Au. In addition, an effective contact area of the graphene might be smaller than that of a designed value. We also summarized emission efficiencies as a function of |V s|, as shown Fig. 2. Obviously, in both cases, with an increase in the |V s|, emission efficiencies were increased exponentially. Notice that, by using the graphene top electrode, the emission efficiency was increased by a factor of ~100 compared with that of the sample with the Au top electrode under the same |V s|. This result indicates that, by using a single layer graphene as a top electrode, electron emission efficiency can be enhanced by the reduction of electron scattering within the top electrode. Conclusion The graphene top electrode plays a role in high efficiency electron emission from the stacked structures of Si-QDs due to the reduction of electron scattering within the top electrode. Acknowledgment This work was supported in part by Grant-in-Aid for Scientific Research (A) No. 19H00762 of MEXT Japan. The formation of the graphene top electrode was supported by associate professor Karita Gorap at Nagoya Institute of Technology.

Electron Field Emission from Multiply-Stacked Si Quantum Dots Structures with Graphene Top-Electrode

We have fabricated a diode with a multiple stacked Si-QDs structure and a single layer graphene top electrode, and studied their electron field emission properties. Electron emission current from the diode was observed by the application of forward biases of 6.0 V and over, which is ~1.0 V lower than that from a diode with a Au top electrode. In addition, we found that electron emission efficiency was increased by a factor of ~ 100 by replacing the top electrode from Au to graphene. This result is explained by the reduction of electron scattering at the top electrode. The results lead to the development of high efficiency planar-type electron emission devices.

Evaluation of the potential distribution in a multiple stacked Si quantum dots structure by hard X-ray photoelectron spectroscopy

Six-fold stacked Si quantum dots (Si-QDs) structures with ultrathin SiO2 interlayers were formed on ultrathin SiO2 layer/n-Si substrates by low-pressure chemical vapor deposition using a SiH4 gas and their vertical electric potential distributions were evaluated using hard X-ray photoelectron spectroscopy under a DC bias application to the semitransparent electrodes formed on the stacked Si-QDs structure. The Si1s photoelectron spectra due to Si–Si bonding units can be deconvoluted into seven components corresponding to the six Si-QDs layers and Si substrate. Obviously, the energy shift between the components for two adjacent dots layers becomes larger towards the upper side of the stacked dots structure. This result indicates that the electric field concentrates on the upper side and is consistent with the proposed model that can explain the ballistic electron emission characteristics from multiple stacked Si-QDs structures.

Electron localization in self-assembled Si quantum dots grown on Ge(111)

Electron localization in a Si/Ge heterosystem with Si quantum dots (QDs) was studied by transport and electron spin resonance (ESR) measurements. For Si QD structures grown on Ge(111) substrates, the ESR signal with a g-factor and ESR line width Oe was observed and attributed to the electrons localized in QDs. The g-factor value was explained taking into account the energy band modification due to both strain and quantum confinement. The transport behavior confirms the efficient electron localization in QDs for a Si/Ge(111) system. A strong Ge–Si intermixing in QD structures grown on Ge(001) is assumed to be the main reason for an unobserved ESR signal from the QDs.

(Keynote) Devices Architectures and Technology for Quantum Computing

There have been many reports on quantum computing hardware using various quantum two-level systems. In principle, any quantum two-level system can be used as a basic unit of information in quantum computing, which is called quantum bit (qubit). Many ways for implementing qubits have been studied to date, for example, trapped ions, cavity quantum electrodynamics, single defect centers, single photons, nuclear spin resonances, superconductor circuits, and semiconductor quantum dots (QDs). Among these, solid-state system has an advantage of scalability, which will be required for practical applications of quantum computers. Especially an electron spin in a QD is a well-defined quantum two-level system characterized by a spin-up and a spin-down state, which is one of the most promising candidates for a qubit, because of its long coherence time. Electron spin-based qubits have been studied using QDs in GaAs, which has a relatively small electron effective mass, m* = 0.067 m 0, the QD size required to obtain quantum effects is relatively large (~100 nm) and can be readily obtained using electron beam lithography. However, further research into quantum operations has found difficulties related to decoherence. The main source of the decoherence is hyperfine interaction with the nuclear spins of the Ga and As atoms.To reduce this hyperfine interaction, the use of Si and SiGe has been investigated, since these materials have isotopes with zero nuclear spin. It is also possible to completely remove the hyperfine interaction through controlling the isotopic composition of the host. Si system is advantageous for future applications because of its potential for compatibility with conventional CMOS devices. However, it is necessary to fabricate much smaller QDs in Si in order to obtain quantum effects because of the larger electron effective mass in this material (m* = 0.19 m 0), and so a highly sophisticated fabrication technique is required.To properly apply qubits based on electron spins, it is necessary to observe spin-related tunneling phenomena and to reduce the electron number in individual QDs to only a few, so as to ensure energetically well-defined spin states. To date, single-electron regimes in single QDs and Pauli-spin blockades (PSBs) in coupled QDs have been observed in Si/SiGe heterostructures. Single-electron regimes in Si QDs have also been observed in MOS structures using a double top-gate design, MOS fine-gate structures and nanowire structures. Thus far, the majority of these Si QD devices have had gate-defined QD structures in which confinement potentials are generated by electric fields with fine top gates. In contrast, the physically-defined Si QD devices described below do not require gates to create confinement potentials for the QDs. These devices thus represent a technological simplification owing to the reduced number of gates and may be advantageous with regard to the integration of qubit technology, since common fabrication processes normally used to produce Si MOS technologies may be applied to make the physically-defined Si QDs. Recently, our group was able to obtain high-quality physically-defined Si QD structures by applying fabrication techniques such as electron beam lithography, oxidation and dry-etching. The resulting devices represented physically-defined coupled Si QDs with MOS structures incorporating a top gate for inducing inversion carriers and side gates for controlling potentials in the QDs. We observed spin-related tunneling phenomena [1] and reduced the electron number in the QDs to the few-electron regime [2,3]. Charge detection of change in number of carriers in the QDs, one by one, has also been successfully demonstrated [4]. These achievements are the important steps for realizing quantum computing devices,[1] G. Yamahata, T. Kodera, H. O. H. Churchill, K. Uchida, C. M. Marcus, and S. Oda, Phys. Rev. B 86, 115322 (2012). [2] K. Horibe, T. Kodera, T. Kambara, K. Uchida, and S. Oda, J. Appl. Phys. 111, 093715 (2012). [3] K. Horibe, T. Kodera, and S. Oda, Appl. Phys. Lett. 106, 083111 (2015). [4] K. Horibe, T. Kodera, and S. Oda, Appl. Phys. Lett. 106, 053119 (2015).

Si Quantum Dot Structures and Some Aspects of Applications

Papers in the Appendix were published in electronic format as Volume 1534

Si quantum dot structures and their applications

This paper presents briefly the history of emission study in Si quantum dots (QDs) in the last two decades. Stable light emission of Si QDs and NCs was observed in the spectral ranges: blue, green, orange, red and infrared. These PL bands were attributed to the exciton recombination in Si QDs, to the carrier recombination through defects inside of Si NCs or via oxide related defects at the Si/SiOx interface. The analysis of recombination transitions and the different ways of the emission stimulation in Si QD structures, related to the element variation for the passivation of surface dangling bonds, as well as the plasmon induced emission and rare earth impurity activation, have been presented.The different applications of Si QD structures in quantum electronics, such as: Si QD light emitting diodes, Si QD single union and tandem solar cells, Si QD memory structures, Si QD based one electron devices and double QD structures for spintronics, have been discussed as well. Note the significant worldwide interest directed toward the silicon-based light emission for integrated optoelectronics is related to the complementary metal-oxide semiconductor compatibility and the possibility to be monolithically integrated with very large scale integrated (VLSI) circuits. The different features of poly-, micro- and nanocrystalline silicon for solar cells, that is a mixture of both amorphous and crystalline phases, such as the silicon NCs or QDs embedded in a α-Si:H matrix, as well as the thin film 2-cell or 3-cell tandem solar cells based on Si QD structures have been discussed as well. Silicon NC based structures for non-volatile memory purposes, the recent studies of Si QD base single electron devices and the single electron occupation of QDs as an important component to the measurement and manipulation of spins in quantum information processing have been analyzed as well.

Effect of Surface Plasmon Resonance on the Photoluminescence from Si Quantum Dot Structures for Third Generation Solar Cell Applications

ABSTRACTLow dimensional structures like quantum dots (QDs) offers the the ability to tune the absorption properties of standard semiconductor materials. However, QDs are relatively weak light absorbers and hence may benefit significantly from coupling with plasmonic modes in nearby metal structures. In the case of a Si QD absorber layer for photovoltaic applications, enhanced absorption would lead to improved power conversion efficiency. Silver metal nanoparticles (MNPs) were deposited on Si QD structures using the self-assembly method of evaporation and annealing. Room temperature photoluminescence (PL) measurements were used to study the surface plasmon (SP) enhanced emission from the samples. The results were compared to conventional metal back reflectors. Enhanced surface plasmon coupled emission (SPCE) from Si QDs in the vicinity of silver metal nanoparticles (MNPs) is observed with a good correlation between the enhancement and the resonance excitation. Quenching was observed from the same emitter layers placed in close proximity to thin flat silver reflector layers, indicating the importance of the spacer layer between a metal layer and the quantum dots in optimising enhancement. The results have implications for the design of SP-enhanced QD solar cells.

Ultrafast carrier dynamics of Si quantum dots embedded in SiN matrix

Femtosecond spectrally resolved two-color three-pulse nonlinear spectroscopy is used to study the dynamics and coherence properties of excited carriers in Si quantum dot structures embedded in silicon nitride. A very short dephasing time of <180fs at room temperature is observed. Ultrashort population relaxation times of ∼400fs and 6–10ps are measured and discussed in the context of the different contributions from transverse optical and transverse acoustic phonon-assisted transitions.

Silicon nanostructures for third generation photovoltaic solar cells

The concept of third generation photovoltaics is to significantly increase device efficiencies whilst still using thin film processes and abundant non-toxic materials. This can be achieved by circumventing the Shockley–Queisser limit for single band gap devices, using multiple energy threshold approaches. At the University of NSW, as part of our work on Third Generation devices, we are using the energy confinement of silicon based quantum dot nanostructures to engineer wide band gap materials to be used as upper cell elements in Si based tandem cells. HRTEM data shows Si nanocrystal formation in oxide and nitride matrixes with a controlled nanocrystal size, grown by layered reactive sputtering and layered PECVD. Photoluminescence evidence for quantum confinement in the Si quantum dots in oxide agrees with the calculated increase in PL energy with reduction in dot size. Resistivity measurements with temperature give tentative proof of conduction and we are investigating junction formation in these materials. We are also using similar Si quantum dot structures in double barrier resonant tunneling structures for use in hot carrier solar cell contacts. These must collect carriers over a limited energy range. Negative differential resistance has been observed in room temperature I– V on these samples, a necessary proof of concept for selective energy filter contacts.

Transport in split-gate silicon quantum dots

We report on the transport properties of novel Si quantum dot structures with controllable electron number through both top and side gates. Quantum dots were fabricated by a split-gate technique within a standard MOSFET process. Four-terminal dc electrical measurements were performed at 4.2 K in a liquid helium cryostat. Strong oscillations in the conductance through the dot are observed as a function of both the top gate bias and of the plunger bias. An overall monotonic and quasi-periodic movement of the peak conductance is observed which is believed to be associated with the bare level structure of the electronic states in the dot coupled with the Coulomb charging energy. Crossing behavior is observed as well, suggestive of either many-body effects or symmetry breaking of the dot states by the applied bias.

Electrical characteristics of SiO 2/crystalline Si quantum dots/SiO 2 double-barrier diode

We report the fabrication, structure characteristics and vertical transport properties of SiO 2/crystalline Si quantum dots/SiO 2 double-barrier diodes in this paper. a-Si:H films of 20 nm thick were deposited using a high hydrogen dilution silane plasma enhanced chemical vapor deposition system; then the films are thermally crystallized and simultaneously oxidized. Cross-section and high resolution transmission electron microscopy photographs indicate the formation of SiO 2 barrier layers and crystalline Si quantum dots structures. The vertical electrical characteristics of these diodes exhibit a series of unique conductance maxima at room temperature. The real voltage spacing between adjacent maxima (∼1 V) is in good agreement with the calculated results based on the model of Coulomb blockade effect.