Si Quantum Research Articles

Rational Doping Strategy to Build the First Solution‐Processed p‐n Homojunction Architecture toward Silicon Quantum Dot Photodetectors

Semiconductor p‐n homojunction is a requisite building block of operating transistors and diodes which make up the modern electronic circuits and optoelectronic applications. However, it has been so far limited to bulk form of single crystals such as silicon (Si) or gallium arsenide. Herein, a brand‐new method of constructing p‐n homojunction architectures that breaks through the limitation is presented. Colloidal inks of p‐type and n‐type Si quantum dots (QDs) are synthesized by thermal disproportionation of (HSiO1.5)n doped with boron or phosphorus, followed by surface ligand engineering. Analysis combining UV photoelectron spectroscopy, electron spin resonance, and current–voltage characteristics confirms that an orthogonal solvent trick makes clean interfaces between n‐type and p‐type SiQD layers without disruption on film formation. The forward and reverse current–voltage characteristics of the diode, along with various spectroscopic characterizations, demonstrate the formation of the first p‐n homojunction of SiQDs. The self‐powered photodiode provides a tunable response specific to the wavelength.

Emission cells with quantum dots on silicon chip prepared by using fs pulsed laser

Emission efficiency of bulk-silicon is very low due to its indirect-gap of energy band. However, it is interesting that the enhanced emission has been observed in the micro-cavities array fabricated by using femtosecond (fs) pulsed laser, in which the stimulated emission characteristics occur after annealing for suitable time in the photo-luminescence (PL) measurement at room temperature. The results of experiment and calculation demonstrated that the enhanced emission may be originated from the Si quantum dots embedded in the micro-cavities prepared by fs pulsed laser. Here, the direct-gap of energy band appears after annealing due to the Heisenberg principle related to ⊿k–1/⊿x in quantum system of nanostructures. The PL intensity obviously increases on the Si quantum dots growing with annealing for better crystallization, in which the external quantum efficiency is higher than 40 % near 760 nm. A new kind of emission source of the micro-cavities array in visible wavelength has been built on silicon wafer, in which the Si quantum dots play a main role for enhancement of emission. It should have a good application in optical integrated chip based on silicon, such as emission cells built on Si chip.

Single-Step Nonthermal Plasma Synthesis of Water-Soluble and Near-Infrared-Emitting Si Quantum Dots for Bioimaging Applications

Single-Step Nonthermal Plasma Synthesis of Water-Soluble and Near-Infrared-Emitting Si Quantum Dots for Bioimaging Applications

Study of dot size effect on electron emission from Si-QDs multiple-stacked structures

We have fabricated diodes with different sized Si quantum dots (QDs) by precisely controlled low-pressure chemical vapor deposition using a pure SiH4 gas and studied the effect of dot size on field electron emission properties of their multiple‒stacked structures. At an applied bias of ∼9 V, the emission current of ∼4.0 nm height dot‒stacks is two orders of magnitude higher than that of ∼5.9 nm height dot‒stacks. These results can be interpreted in terms of an increase in the number of electrons with higher kinetic energy due to the increase in discrete energy levels associated with the reduction in the dot size, which suppresses electron scattering within the dot, and the electric field concentration resulting from the decrease in the curvature of the dot.

Transport and electrical properties of Si and Ge quantum dots embedded in oxide layers of MOS structures for optoelectronic applications

Transport and electrical properties of Si and Ge quantum dots embedded in oxide layers of MOS structures for optoelectronic applications

Transient characteristics of carrier transport in an isotopically enriched 28Si/SiGe undoped heterostructure

An isotopically enriched 28Si/SiGe undoped heterostructure is a promising platform for Si-based qubits due to the long coherence time by reducing 29Si isotopes with non-zero nuclear spins. Carriers in the buried Si quantum well (QW) of the Si/SiGe heterostructures could tunnel to the oxide/Si interface, increasing charge noise and leading to charge instability. In this work, we investigate the tunneling effects on carrier distribution and transport properties in an isotopically enriched 28Si/SiGe undoped heterostructure and its transient characteristics by controlling the hold time of gate biasing under different drain biases. By holding the gate bias for a longer time, the drain is reduced since more carriers in the buried QW tunnel to the oxide interface. Furthermore, under a larger drain bias, more carriers can move across the Si/SiGe heterojunction to the oxide interface and are trapped, resulting in a stronger current reduction, which is explained by a lucky electron model.

Mechanism of quantum yield enhancement in Si quantum dots by high-pressure water vapor annealing from single-dot studies

High-pressure water vapor annealing (HWA) was recently demonstrated as a method that can substantially improve the photoluminescence quantum yield (PLQY) of silicon quantum dots (Si QDs) with the oxide shell. In this Letter, the mechanism of this enhancement is studied optically on a single-dot level. HWA treatment is performed on Si QDs formed on a silicon-on-insulator wafer, and their photoluminescence (PL) properties were examined before and after the treatment. Our experiments show a 2.5 time enhancement in the average blinking duty cycle of Si QDs after 2.6 MPa HWA treatment without changing the average ON-state PL intensity. This observation proves the carrier trapping process is suppressed on the HWA-built Si/SiO2 interface. We also discussed the mechanism behind the PLQY enhancement of HWA-treated Si QDs by comparing single-dot-level data to reported ensemble PL Si QDs results. HWA treatment is found to mainly brighten “grey” (not 100% efficient) QDs, a mechanism different from changing dark (non-emitting) to bright (100% efficient) Si QDs by ligand passivation.

Epitaxial Group-IV Quantum Dots as Room-Temperature Light Emitters

Next-generation Si-based applications ranging from short-range (quantum) data transfer to sensing could highly benefit from potential progress in group-IV-based telecom light emitters that can be readily implemented with Si Photonics components [1]. However, since Si is known to be an indirect band gap semiconductor, light emission is limited and inhibits the integration of efficient emitters, such as, e.g. light-emitting diodes and lasers on Si Photonic platforms.In this presentation, we stress that a particular type of epitaxial Ge on Si quantum dots allows for overcoming the indirect bandgap limitations of Si and Ge and, thus, enables pronounced light emission even at room temperature and above [2]. These nanostructures consist of ~2 nm high Ge-rich quantum dots on Si(001) substrate epitaxially created via strain-driven formation. However, in contrast to commonly used nanostructures, we intentionally incorporate point defects into these nanostructures during their growth through in-situ low-energy Ge ion implantation [3-4]. Therefore, the point defects can be confined within the 2 nm thick Ge layer while the surrounding Si matrix remains perfectly crystalline. We will discuss the superior light-emission properties of these so-called defect-enhanced quantum dots (DEQDs) and present the current theoretical understanding of the defect’s effects on the band structure. The latter leads to the enhanced light emission from DEQDs and the opening of direct recombination channels in k-space [5,6].In addition to signs for optically pumped lasing from DEQDs embedded in microdisk resonators, implementing these emitters into the i-region of p-i-n diodes is remarkably straightforward since the matrix material is crystalline Si [7,8]. To showcase the superior properties of these zero-dimensional light emitters, we highlight light emission from DEQD light-emitting diodes with exceptional temperature stability up to 100°C [9]. This behavior starkly contrasts conventional Si/Ge light emitters, for which the light emission is typically confined to cryogenic temperatures.For optimizing the light emission from this interlaced defect/nanostructure complex [10], we elaborate on the testing of crucial parameters related to the temperature stability of the DEQDs [8,11], the scalability of light emission with increasing the density of the DEQDs [12] and passivation of detrimental defects in the Si matrix [13].[1] I. A. Fischer, et al., APL Photonics 7, 050901 (2022).[2 ] M. Brehm, Silicon Photonics IV, 67-103, Silicon Photonics IV: Innovative Frontiers, edited by David J. Lockwood and Lorenzo Pavesi, Springer series Topics in Applied Physics (2021).[3] M. Grydlik, et al., Nano Lett. 16, 6802–6807 (2016).[4] M. Grydlik, et al., ACS Photonics 3, 298–303 (2016).[5] F. Murphy-Armando, et al., Physical Review B 103 (8), 085310 (2021).[6] M. Brehm, et al., to be published[7] H. Groiss et al., Semicond. Sci. Technol. 32, 02LT01 (2017).[8] L. Spindlberger et al., Crystals 10, 351 (2020).[9] P. Rauter et al., ACS Photonics 5, 431-438 (2018).[10] M. Brehm and M. Grydlik, Nanotechnology 28, 392001 (2017).[11] L. Spindlberger et al., Physica Status Solidi (a) 216, 1900307 (2019).[12] H. Groiss et al., Semicond. Sci. Technol. 32, 02LT01 (2017).[13] L. Spindlberger et al., Appl. Phys. Lett. 118, 083104 (2021).

Design and Simulation of 4-State SRAMs Using 4-State Quantum Dot Gate (QDG) FETs

This paper describes fabrication of Quantum Dot Gate n-FETs using SiOx-cladded Si quantum dot self-assembled on the tunnel gate oxide. Experimental I-V characteristics exhibiting 4-states are presented. Simulation is presented for the operation of viable 4-state SRAMs using QDG-FETs.

Fabrication and Characterization of Four-State Inverter Utilizing Quantum Dot Gate Field-Effect Transistors (QDGFETS)

This paper presents the experimental results of nMOS quantum dot gate field-effect transistor (QDG-FET) based four-state inverter fabricated and tested with Si/SiO2 and Ge/GeO2 quantum dots. The site-specific self-assembly of SiOx-cladded Si and GeOx-cladded Ge quantum dot layers in the gate region implements both the driver and load FETs in enhancement nMOS inverters. A four-state inverter will allow the reduction of FET count in logic block in microprocessors.

SI/GE Quantum Dot Channel FETs for Multi-Bit Computing

This paper presents quantum dot channel (QDC) FETs in quantum wire and coupled quantum dot configurations for cryogenic operation with multi-state operation. It also describes gate-all-around (GAA) quantum dot channel (QDC) FETs that exhibit potential multi-state characteristics at room temperature. FETs with cladded Si and Ge quantum dot layers as a transport channel have been fabricated. The formation of a quantum dot superlattice (QDSL) when SiOx-cladded Si and/or GeOx-cladded Ge quantum dots (QD) are assembled results in mini-energy sub-bands in the conduction and valence band. The intra-mini-energy band transitions results in significant changes in the drain current when gate and/or drain voltages are varied. This novel feature provides a pathway for 16-/32-state logic in CMOS-X configuration. The gate-defined Si quantum dot FETs, comprising of tunnel barrier coupled, have been reported for quantum computing at cryogenic temperatures.

Electrical characterization of a single-crystalline Si quantum well formed by thermal oxidation of ultrathin silicon-on-insulator film (Al/SiO2:c-Si QW/n-Si) for optoelectronic applications

Electrical characterization of a single-crystalline Si quantum well formed by thermal oxidation of ultrathin silicon-on-insulator film (Al/SiO2:c-Si QW/n-Si) for optoelectronic applications

Hydrogen-like impurities in Si and Ge quantum dots in connection with 5 nm and beyond metal-oxide-semiconductor technologies

The industrial drive towards smaller semiconductor devices is at less than 10 nano-meters. In the next few years a three-dimensional quantum confined device structure is expected. We have studied the electronic structure of hydrogen-like donor impurities in Si and Ge quantum dots with varied shape of the confinement potential. These calculations are carried out within the Density-Functional-Effective-Mass Theory (DF-EMT). We have paid particular attention to the electron-electron interaction energy in a negative charge state of the donor impurity. The effect of electron correlation on the binding energy has been explored. In addition to these, we show that the second order correction to the ground state energy obtained through perturbative approach agrees well with the DF-EMT based numerical results, down to very small sizes of the quantum dots when the depth of the confinement is moderate or small. A non-monotonic shallow-deep transition of the binding energy of neutral and charged donors of hydrogen-like impurities is expected to occur with the reduction in the size of the quantum dot. A similar effect is expected for hydrogen-like acceptor impurities as well. Furthermore, the optical gap also shows non-monotonic shallow-deep behaviour with the quantum dot size reduction. Deepening of shallow donor or acceptor level implies that the carriers will freeze out at small sizes of the dot. We believe this is quite important in the context of metal-oxide-semiconductor (MOS) devices beyond 5 nm technologies, based on Si, Ge or a combination (SiGe). We also point out possible variabilities in the binding energy of donors and acceptors if there are variations in the size of host quantum dots in a large assembly of devices in future integrated circuits. This will lead to the variabilities in the characteristics from device to device. These results should be incorporated in the Technology Computer Aided Design (TCAD) simulation of less than or equal to 5 nm MOS devices.

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.

SiQDs-mediated highly efficient hole transfer for photocatalytic overall water splitting over Cu-doped ZnIn2S4

Sluggish hole transfer and the resulting pronounced charge recombination hinder the efficiency of photocatalytic overall water splitting. Herein, we present Si quantum dots (SiQDs) as hole transporter to construct SiQDs/Cu-doped ZnIn2S4 heterojunction photocatalysts (Cu0.48SZ) for overall water splitting. Under visible light illumination, Cu0.48SZ exhibits highly stable overall water-splitting activity of 210 μmol g−1 h−1 H2 and 97 μmol g−1 h−1 O2 with an apparent quantum yield (AQY) of 2.57 % at 420 nm. Results demonstrate that the synergy of SiQDs and Cu doping enhances carrier-separation efficiency and reduce surface kinetic barriers, leading to high photocatalytic performance. The presence of SiQDs effectively accelerates the transfer of photoexcited holes and avoids the self-oxidation of ZnIn2S4, favoring water oxidation. The Cu doping increases the electron density around the S atoms of ZnIn2S4 and thus promotes water reduction. This work extends the application of SiQDS as efficient hole transporter for photocatalytic overall water splitting.

Room Temperature Quantum Control of N‐Donor Electrons at Si/SiO2Interface

AbstractThe manuscript theoretically discusses various important aspects for donor atom based single qubit operations in silicon (Si) quantum computer architecture at room temperature using a single nitrogen (N) deep‐donor close to the Si/SiO2interface. Quantitative investigation of room temperature single electron shuttling between a single N‐donor atom and the interface is the focus of attention under the influence of externally applied electric and magnetic field. To apprehend the realistic experimental configurations, central cell correction along with effective mass approach is adopted throughout the study. Furthermore, a detailed discussion currently explores the significant time scales implicated in the process and their suitability for experimental purposes. Theoretical estimates are also provided for all the external fields required to successfully achieve coherent single electron shuttling and their stable maintenance at the interface as required. The results presented in this work offer practical guidance for quantum electron control using N‐donor atoms in Si at room temperature.

All-silicon multidimensionally-encoded optical physical unclonable functions for integrated circuit anti-counterfeiting

Integrated circuit anti-counterfeiting based on optical physical unclonable functions (PUFs) plays a crucial role in guaranteeing secure identification and authentication for Internet of Things (IoT) devices. While considerable efforts have been devoted to exploring optical PUFs, two critical challenges remain: incompatibility with the complementary metal-oxide-semiconductor (CMOS) technology and limited information entropy. Here, we demonstrate all-silicon multidimensionally-encoded optical PUFs fabricated by integrating silicon (Si) metasurface and erbium-doped Si quantum dots (Er-Si QDs) with a CMOS-compatible procedure. Five in-situ optical responses have been manifested within a single pixel, rendering an ultrahigh information entropy of 2.32 bits/pixel. The position-dependent optical responses originate from the position-dependent radiation field and Purcell effect. Our evaluation highlights their potential in IoT security through advanced metrics like bit uniformity, similarity, intra- and inter-Hamming distance, false-acceptance and rejection rates, and encoding capacity. We finally demonstrate the implementation of efficient lightweight mutual authentication protocols for IoT applications by using the all-Si multidimensionally-encoded optical PUFs.

Self-assembling mechanism of Si-QDs on thermally grown SiO2

The self-assembling formation of Si quantum dots (Si-QDs) on as-grown SiO2 layers was shown by controlling the early stages of low-pressure chemical vapor deposition of SiH4. The QD height and radius distributions assessed by atomic force microscopy and scanning electron microscopy images revealed that the Si-QDs become hemispherical due to them being rate-limited by aggregation, which reduces the surface energy at substrate temperatures above ∼580 °C. Moreover, at temperatures below ∼580 °C, semi-ellipsoidal shaped Si-QDs are formed because the precursor supply is a dominant factor.

Low-temperature high-yield fabrication of colloidal Si quantum dots with in situ tunability for luminescence band from red to green

Low-temperature high-yield fabrication of colloidal Si quantum dots with <i>in situ</i> tunability for luminescence band from red to green

Noise properties in the Coulomb blockade region of FinFETs

Fin FETs (FinFETs) are a promising candidate for the platform of Si quantum computers. The noise properties of commercial FinFETs were experimentally investigated at temperatures below 10 K. The drain current showed Coulomb oscillation, indicating that the FinFET channel became a single quantum dot. Moreover, the noise in the drain current was analyzed, and the basic properties of commercial FinFETs in the low-temperature region were discussed.