Germanium Quantum Dots Research Articles

Fast and efficient germanium quantum dot photodetector with an ultrathin active layer

An ultrathin layer (13 nm) of germanium (Ge) quantum dots embedded in a SiO2 matrix was deposited on a Ge substrate for photodetection in both the visible and near-infrared (IR). Operated at T = 150 K, the device exhibits higher than 105% internal quantum efficiency (IQE) at a reverse bias of −1.3 V under low light conditions (<30 nW) at both λ= 640 and 1550 nm. The transient response of 640 nm pulses stays below 15 ns for both rise and fall times; the IR response is only slightly slower. Our work demonstrates a high-performance broadband photodetector with high IQE and fast response in a simple silicon technology-compatible device structure.

Inserting Hydrogen into Germanium Quantum Dots

Inserting Hydrogen into Germanium Quantum Dots

(Invited) Low-Dimensional Nanomaterials: Synthesis to Applications

One of the greatest challenges besetting the development of Li ion battery technologies these days is the difficulty of fast charging, which is even more difficult to maintain a high energy density within a flexible and compact design. An energy dense battery within a small, confined space runs a higher risk of explosion due to volumetric expansions within the anode material, which is traditionally graphite. However, given the strong demand for electric vehicles and wearable electronics, it is crucial that we surmount this challenge. In a recent work, Yang and her team have designed an innovative nanoscale architecture comprising of a nitrogen-doped graphene shell covering high capacity germanium quantum dots (Ge-QDs) on a nitrogen-doped graphene scaffold. Such “yolk-shell” architecture can provide void space for the volumetric expansion of Ge-QDs as Li ions move in and out of the QDs. The specific reversible capacity is more than 1,220 mAh g-1 (4 times higher than commercial batteries) and a fast charging time 90S is achieved for this new anode material.

Germanium Quantum-Dot Array with Self-Aligned Electrodes for Quantum Electronic Devices.

Semiconductor-based quantum registers require scalable quantum-dots (QDs) to be accurately located in close proximity to and independently addressable by external electrodes. Si-based QD qubits have been realized in various lithographically-defined Si/SiGe heterostructures and validated only for milli-Kelvin temperature operation. QD qubits have recently been explored in germanium (Ge) materials systems that are envisaged to operate at higher temperatures, relax lithographic-fabrication requirements, and scale up to large quantum systems. We report the unique scalability and tunability of Ge spherical-shaped QDs that are controllably located, closely coupled between each another, and self-aligned with control electrodes, using a coordinated combination of lithographic patterning and self-assembled growth. The core experimental design is based on the thermal oxidation of poly-SiGe spacer islands located at each sidewall corner or included-angle location of Si3N4/Si-ridges with specially designed fanout structures. Multiple Ge QDs with good tunability in QD sizes and self-aligned electrodes were controllably achieved. Spherical-shaped Ge QDs are closely coupled to each other via coupling barriers of Si3N4 spacer layers/c-Si that are electrically tunable via self-aligned poly-Si or polycide electrodes. Our ability to place size-tunable spherical Ge QDs at any desired location, therefore, offers a large parameter space within which to design novel quantum electronic devices.

Enhancement the intensity of optical transitions in the germanium/silicon nanosystem with germanium quantum dots

It is shown that in a germanium/silicon nanosystem with germanium quantum dots, the hole leaving the germanium quantum dot causes the appearance of the hole energy level in the bandgap energy in a silicon matrix. The dependences of the energies of the ground state of a hole and an electron, as well as spatially indirect excitons on the radius of the germanium QD and on the depth of the potential well for holes in the germanium QD are obtained. It is found that as a result of a direct electron transition in real space between the electron level, located in the conduction band of the silicon matrix, and the hole level located in the bandgap of the silicon matrix, the radiative recombination intensity in the germanium/silicon nanosystem with germanium quantum dots increases significantly.

Synthesis of superlattice heterostructure of germanium quantum dots in silicon spacer layers and its application in photovoltaic solar cells

Superlattice heterostructure of dislocation-free Ge quantum dots buried in Si spacer layers has been synthesized using ultra high vacuum-chemical vapor deposition technique. The heterostructure has been used as active layer in Si solar cells based upon vertical p-i-n junction configuration. Experiment shows that such solar cells exhibit strong repose to infrared radiation in contrast with the negligible response of solar cells with active layers of pure Si having the same thickness. A solar cell with 300 nm thick superlattice is shown to have thermodynamic efficiency of about 11.2%. This is correlated with the rather high quality of the heterostructure used in the present study as judged from photoluminescence spectroscopy and electron microscopy as well as strain and dark current measurements. It is concluded that such type of heterostructure may provide a pathway to develop efficient and cost effective solar cells. • Superlattice heterostructure of Ge quantum dots on Si has been synthesized. • Various techniques have been used to characterize the nanostructures. • The heterostructure has been used as active layer in Si (p-i-n)-type solar cells. • The thermodynamic efficiency of respective solar cell is determined.

A four-qubit germanium quantum processor.

The prospect of building quantum circuits1,2 using advanced semiconductor manufacturing makes quantum dots an attractive platform for quantum information processing3,4. Extensive studies of various materials have led to demonstrations of two-qubit logic in gallium arsenide5, silicon6-12 and germanium13. However, interconnecting larger numbers of qubits in semiconductor devices has remained a challenge. Here we demonstrate a four-qubit quantum processor based on hole spins in germanium quantum dots. Furthermore, we define the quantum dots in a two-by-two array and obtain controllable coupling along both directions. Qubit logic is implemented all-electrically and the exchange interaction can be pulsed to freely program one-qubit, two-qubit, three-qubit and four-qubit operations, resulting in a compact and highly connected circuit. We execute a quantum logic circuit that generates a four-qubit Greenberger-Horne-Zeilinger state and we obtain coherent evolution by incorporating dynamical decoupling. These results are a step towards quantum error correction and quantum simulation using quantum dots.

Intensity of Radiative Recombination in the Germanium/Silicon Nanosystem with Germanium Quantum Dots

It is shown that in a germanium/silicon nanosystem with germanium quantum dots, the hole leaving the germanium quantum dot causes the appearance of the hole energy level in the bandgap energy in a silicon matrix. The dependences of the energies of the ground state of a hole and an electron are obtained as well as spatially indirect excitons on the radius of the germanium quantum dot and on the depth of the potential well for holes in the germanium quantum dot. It is found that as a result of a direct electron transition in real space between the electron level that is located in the conduction band of the silicon matrix and the hole level located in the bandgap of the silicon matrix, the radiative recombination intensity in the germanium/silicon nanosystem with germanium quantum dots increases significantly.

A two-dimensional array of single-hole quantum dots

Quantum dots fabricated using methods compatible with semiconductor manufacturing are promising for quantum information processing. In order to fully utilize the potential of this platform, scaling quantum dot arrays along two dimensions is a key step. Here, we demonstrate a two-dimensional quantum dot array where each quantum dot is tuned to single-charge occupancy, verified by simultaneous measurements using two integrated radio frequency charge sensors. We achieve this by using planar germanium quantum dots with low disorder and a small effective mass, allowing the incorporation of dedicated barrier gates to control the coupling of the quantum dots. We measure the hole charge filling spectrum and show that we can tune single-hole quantum dots from isolated quantum dots to strongly exchange coupled quantum dots. These results motivate the use of planar germanium quantum dots as building blocks for quantum simulation and computation.

Light–Matter Interactions between Germanium Nanocavities and Quantum Dots at Visible Wavelengths

All-dielectric cavities are developed as a promising platform for light–matter interactions at visible wavelengths instead of plasmonic components because of their unique electromagnetic field conf...

High-performance germanium quantum dot photodetectors: Response to continuous wave and pulsed excitation

Efficiency and response speed are key figures of merit for high-performance photodetectors, with high efficiency often obtained at the expense of slow photoresponse. Here, we report on germanium quantum dot photodetectors (Ge QD PDs) with a 25-nm-thick active layer that possesses both high internal quantum efficiency (IQE) and fast photoresponse, yet is still based on simple design and fabrication. We characterize these devices with continuous wave (CW) and pulsed excitation at room temperature as a function of incident power and applied bias. Under the reverse bias of –4 V, the IQE approaches ∼2000% over a broad spectral range (λ = 500–800 nm). The transient photoresponse speed to a 4.5 ns laser pulse at λ = 640 nm is under 20 ns. Furthermore, we observe an interesting phenomenon: by superimposing a weak CW HeNe laser beam (λ= 632.8 nm) on the laser pulse, we obtain an optically tunable photoresponse while retaining fast speed. This study elucidates the role of photocarrier generation, trapping, and hopping in the percolative Ge QD oxide matrix and helps explain the observed high gain and fast response speed. The demonstrated IQE and nanosecond response time render our devices suitable for low-light detection and imaging.

Coupling of Germanium Quantum Dots with Collective Sub-radiant Modes of Silicon Nanopillar Arrays.

In this paper, we demonstrate the infrared photoluminescence emission from Ge(Si) quantum dots coupled with collective Mie modes of silicon nanopillars. We show that the excitation of band edge dipolar modes of a linear nanopillar array results in strong reshaping of the photoluminescence spectra. Among other collective modes, the magnetic dipolar mode with the polarization along the array axis contributes the most to the emission spectrum, exhibiting an experimentally measured Q-factor of around 500 for an array of 11 pillars. The results belong to the first experimental evidence of light emission enhancement of quantum emitters applying collective Mie resonances in finite nanoresonators and therefore represent an important contribution to the new field of active all-dielectric meta-optics.

Effect of Nucleation Parameters of Ge Quantum Dots Grown over Silicon Oxide by LPCVD

Germanium quantum dots (Ge-QD) were grown by Low Pressure Chemical Vapor Deposition (LPCVD) on Si nucleus previously grown on 3 nm thick SiO2 ultra thin film. Samples were analyzed by atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). We report the analysis of the influence of the nucleation parameters on size and spatial distribution of Ge-QD. AFM images show a Ge-QD density of around 3.6x1010 cm-2, with an 11 nm mean size and 2.9 nm height. Finally, HRTEM investigation shows that the Ge-QD have a crystalline structure, i.e., they are nanocrystals.

The splitting of electron states in Ge/Si nanosystem with germanium quantum dots

It is shown that electron tunneling through a potential barrier that separates two quantum dots of germanium leads to the splitting of electron states localized over spherical interfaces (a quantum dot – a silicon matrix). The dependence of the splitting values of the electron levels on the parameters of the nanosystem (the radius a of germanium quantum dot, as well as the distance D between the surfaces of the quantum dots) is obtained. It is shown that, the splitting of electron levels in the quantum dot chain of germanium causes the appearance of a zone of localized electron states, which is located in the bandgap of silicon matrix. It is found that the motion of a charge-transport exciton along a chain of germanium quantum dots of germanium causes an increase in photoconductivity in the nanosystem.

Insight into the Effect of Ligands on the Optical Properties of Germanium Quantum Dots and Their Applications in Persistent Cell Imaging

Germanium quantum dots (GeQDs) show unique advantages in fluorescence applications due to their large quantum confinement effect and excellent biocompatibility. However, GeQDs are confronted with difficulty in accurately controlling the fluorescence emission. This defect brings challenges to understanding the fluorescence mechanism and limits the potential applications of GeQDs. In this paper, a series of GeQDs with the average diameter of about 2.6 nm modified with different ligands were synthesized by the chemical reduction method. The fluorescence emission of GeQDs can be changed from blue to yellow-green through adjusting the surface ligands. The influence of surface ligands on the fluorescence emission of GeQDs was thoroughly investigated by experimental and theoretical calculations. Furthermore, the synthesized GeQDs exhibit good biocompatibility and photostability and can act as high-performance fluorescence probes for long-term fluorescent bioimaging. This work provides a good and deep understanding of the fluorescence mechanism of GeQDs and will facilitate diverse promising applications of GeQDs in the near future.

Spatially Indirect Excitons’ Spectroscopy in Germanium Quantum Dots

Spatially Indirect Excitons’ Spectroscopy in Germanium Quantum Dots

Ge Quantum Dots Coated with Metal Shells (Al, Ta, and Ti) Embedded in Alumina Thin Films for Solar Energy Conversion

A method to enhance the optoelectronic properties of thin films containing three-dimensional ordered germanium quantum dots (QDs) coated with metal shell (Al, Ta, and Ti) in alumina matrix is prese...

Ge quantum dot lattices in alumina prepared by nitrogen assisted deposition: Structure and photoelectric conversion efficiency

Thin films comprising three dimensional germanium (Ge) quantum dot lattices formed by nitrogen (N) assisted magnetron sputtering deposition in alumina (Al2O3) matrix have been studied for light harvesting purposes. In order to expand the application of this material it is necessary to reduce germanium oxidation and to achieve stabilization of the germanium/alumina interface. Effects of tuning the N concentration, substrate temperature and Ge sputtering power during the films preparation are monitored. It is shown that the N presence not only reduces Ge oxidation during annealing but also affects the internal structure, size and arrangement of Ge quantum dots. Additionally, the deposition temperature and Ge sputtering power are used to tune the Ge quantum dot size, separation and the regularity of their positions. It is shown that the optical and electrical properties of the films, especially their photo-induced current, and quantum efficiency are strongly tunable by the deposition conditions. Moreover, a significant photo-response and effect of multiple exciton generation effect is observed. The materials presented could be used as a sensitive layer for photodetectors or photovoltaic light harvesting devices. The presented tools can be used for future fine tuning of the material to achieve the optimal quantum efficiency.

Spin Relaxation Benchmarks and Individual Qubit Addressability for Holes in Quantum Dots.

We investigate hole spin relaxation in the single- and multihole regime in a 2 × 2 germanium quantum dot array. We find spin relaxation times T1 as high as 32 and 1.2 ms for quantum dots with single- and five-hole occupations, respectively, setting benchmarks for spin relaxation times for hole quantum dots. Furthermore, we investigate qubit addressability and electric field sensitivity by measuring resonance frequency dependence of each qubit on gate voltages. We can tune the resonance frequency over a large range for both single and multihole qubits, while simultaneously finding that the resonance frequencies are only weakly dependent on neighboring gates. In particular, the five-hole qubit resonance frequency is more than 20 times as sensitive to its corresponding plunger gate. Excellent individual qubit tunability and long spin relaxation times make holes in germanium promising for addressable and high-fidelity spin qubits in dense two-dimensional quantum dot arrays for large-scale quantum information.

The Splitting of Electron States in Ge/Si Heterostructure with Germanium Quantum Dots

It is shown that electron tunneling through a potential barrier that separates two quantum dots (QDs) of germanium leads to the splitting of electron states localized over spherical interfaces (QD–silicon matrix). The dependence of the splitting values of the electron levels on the parameters of the nanosystem (the radius a of germanium QDs, as well as the distance D between the surfaces of the QDs) is obtained. It is shown that, the splitting of electron levels in the QD chain of germanium causes the appearance of a zone of localized electron states, which is located in the bandgap of silicon matrix. It is found that the motion of a charge‐transport exciton along a chain of QDs of germanium causes an increase in photoconductivity in the nanosystem.