Impact Of Quantum Confinement Research Articles

Real space characterization of nonlinear hall effect in confined directions

The nonlinear Hall effect (NLHE) is a phenomenon which could produce a transverse Hall voltage in a time-reversal-invariant material. Here, we report the real space characterizations of NLHE evaluated through quantum transport in TaIrTe4 nanoribbon without the explicit Berry curvature dipole (BCD) information. We first characterize the NLHE in both transverse confined directions in global-level measurement. The impact of quantum confinement in NLHE is evaluated by adjusting the width of nanoribbons. Then, the probing area is trimmed to the atomic scale to evaluate the local texture, where we discover its patterns differ among the probed neighboring atomic groups. The analysis of charge distribution reveals the connections between NLHE’s local patterns and its non-centrosymmetric nature, rendering nearly an order of Hall voltage enhancement through probe positioning. Our work paves the way to expand the range of NLHE study and unveil its physics in more versatile material systems.

Quantum confinement in optically active ultrathin (Cd,Mn)Te/(Cd,Mg)Te core/shell nanowires

The fabrication of ultrathin (Cd,Mn)Te nanowires with diameters reaching values below 10 nm is reported. The thinning procedure relies on a thermally induced reverse reaction process applied to (Cd,Mn)Te nanowires grown using the vapor-liquid-solid method. The impact of quantum confinement is manifested by a distinct blueshift of the excitonic emission from individual nanowires. Cathodoluminescence mapping reveals that the optical emission originates from relatively short nanowire segments, which is explained in terms of exciton localization on irregularities of nanowire core diameter. Magneto-optical study performed on several individual nanowires reveals a change from light hole to heavy hole character of the excitonic emission depending on the emission energy, i.e., on the size of the emitting object. The results are interpreted in terms of the influence of axial and radial quantum confinement on excitons in ultrathin nanowires.

Ultrafast Carrier Dynamics in Two-Dimensional Electron Gas-like K-Doped MoS2

Electronic interactions associated with atomic adsorbates on transition metal dichalcogenides such as MoS2 can induce massive electronic reconstruction that results in the formation of a new type of heavily doped transition metal dichalcogenide that exhibits characteristics of a two-dimensional electron gas. The impact of quantum confinement and reduced dimensionality on the carrier dynamics in such two-dimensional systems is at present not known, but is of paramount importance if they are to find application in optoelectronic devices. Here we show by a combination of angle-resolved photoemission and advanced x-ray spectroscopies that many-body interactions in reduced dimension drastically shorten carrier lifetimes to below 0.5 fs, and reveal how potassium intercalation in MoS2 forces orbital rehybridization to create a two-dimensional electron gas.

Synthesis and characterization of cadmium sulfide (CdS) thin film for solar cell applications grown by dip coating method

The CdS nanostructure thin films have been deposited on glass substrates the usage of dip coating method. The structural, optical and surface morphologies of CdS nanostructure thin film were investigated by XRD, SEM, TEM, EDAX, UV–Vis and PL spectra. The outcomes indicated the films were hexagonal shape and the grains are spherically formed and allotted uniformly over the complete surface of the glass substrate. The impact of quantum confinement and the shifts in optical band gap had been calculated. The results indicated CdS nanostructure thin film is appropriate for solar cell window layer application and the solar cell conversion efficiency was calculated.

Giant Fine Structure Splitting of the Bright Exciton in a Bulk MAPbBr3 Single Crystal.

Exciton fine structure splitting in semiconductors reflects the underlying symmetry of the crystal and quantum confinement. Because the latter factor strongly enhances the exchange interaction, most work has focused on nanostructures. Here, we report on the first observation of the bright exciton fine structure splitting in a bulk semiconductor crystal, where the impact of quantum confinement can be specifically excluded, giving access to the intrinsic properties of the material. Detailed investigation of the exciton photoluminescence and reflection spectra of a bulk methylammonium lead tribromide single crystal reveals a zero magnetic field splitting as large as ∼200 μeV. This result provides an important starting point for the discussion of the origin of the large bright exciton fine structure splitting observed in perovskite nanocrystals.

(Invited) Manipulation of the Energy Band Levels in Plasma-Produced Energy Materials

Progress in energy conversion and energy storage strongly rely on our ability to produce “new” and “old” materials with improved properties. Materials properties are always closely linked to the synthesis process and materials integration in application devices is also critical to the overall performance. Therefore, materials synthesis, properties and device integration are intimately interrelated [1]. In addition, the electronic energy band structure of materials largely determines their behaviour and impact the materials performance in application devices [2-4]. For instance, the bandgap of semiconductor is crucial for a large variety of applications in solar energy conversion. However, the full energy band diagram (EBD) requires accurate knowledge of a range of other critical parameters in addition to bandgap. These include the conduction and valence band edge location and Fermi level; together these are decisive for a very wide range of applications and the ability to manipulate the corresponding energy levels represent an attractive opportunity. Quantum confinement in nanoscale materials offers one possible avenue for the manipulation of the EBD whereby bandgap engineering, the ability to tailor the bandgap of nanoscale semiconductors, has inspired a considerable excitement at the visionary technological opportunities it presents. However, the impact of quantum confinement on the other energy band parameters has received little attention, in part due to measurement challenges as well as due to the higher sensitivity and precision required. Another potential avenue for EBD manipulation is also offered through doping, alloying as well as introducing “defects” in the crystal structure of materials. These, and in particular “defects”, are also as effective as quantum confinement strategies to modify energy levels to suit specific application. The synthesis of such materials, i.e. with strong quantum confinement and/or with engineered defects, has been explored and widely reported. However, many approaches have also revealed limitations closely bound to thermodynamic limit of solubility or crystal structure stability. Plasma-based synthesis however can break through the boundaries of these limitations and achieve composition that are beyond those expected by phase diagrams for instance. More specifically, atmospheric pressure microplasmas (APMs) are highly versatile and amenable to explore materials with untested composition or “exotic” structures and morphologies [5-7]. APMs can therefore contribute to progress materials science by pushing the boundaries beyond current synthetic capabilities. APM processes can be easily integrated in the application devices fabrication steps so that materials properties can be verified within the relevant application context [8-10]. Here we will report on various synthesis achievements with APMs demonstrating first the synthesis of both strongly quantum confined structures as well as materials with unexpected doping levels and chemical compositions [11-14]. We will then provide preliminary results on the ability of controlling defects in metal oxides. Corresponding changes in the EBD parameters will be evaluated, compared and discussed with a range of analytical techniques. These results will also offer the opportunity to discuss challenges in the measurement methods. Acknowledgments This work is supported by EPSRC (awards n. EP/R023638/1, EP/M024938/1, EP/M015211/1) References [1] Rocks C, Švrček V, Velusamy T, Macias-Montero M, Maguire P, Mariotti D: Nano Energy 50 (2018) 245 [2] Bera D, Qian L, Tseng TK, Holloway PH: Materials (Basel), 2010, 3, 2260. [3] Capasso F, Science 235 (1987), 172 [4] Smith AM, Nie S: Acc. Chem. Res. 43 (2010) 190 [5] Mariotti D, Belmonte T, Benedikt J, Velusamy T, Jain G, Švrček V: Plasma Processes and Polymers 13 (2016) 70 [6] Maguire P, Rutherford D, Macias-Montero M, Mahony C, Kelsey C, Tweedie M, Perez-Martin F, McQuaid H, Diver D, Mariotti D: Nano Letters 17 (2017) 1336 [7] Askari S, Mariotti D, Stehr JE, Benedikt J, Keraudy J, Helmersson U: Nano Letters 18 (2018) 5681 [8] Švrček V, Mariotti D, Shibata Y, Kondo M: Journal of Physics D: Applied Physics 43 (2010) 415402 [9] McKenna J, Patel J, Mitra S, Soin N, Švrček V, Mariotti D: The European Physical Journal-Applied Physics 56 (2011) 24020 [10]Švrček V, Kondo M, Kalia K, Mariotti D: Chemical Physics Letters 478 (2009) 224 [11]Ni C, Carolan D, Rocks C, Hui J, Fang Z, Padmanaban DB, Ni J, Xie D, Maguire P, Irvine JTS, Mariotti D: Green Chemistry 20 (2018) 2101 [12]Haq AU, Askari S, McLister A, Rawlinson S, Davis J, Chakrabarti S, Svrcek V, Maguire P, Papakonstantinou P, Mariotti D Nature Communications 10 (2019) 817 [13]Askari S, Haq AU, Macias-Montero M, Levchenko I, Yu F, Zhou W, Ostrikov K, Maguire P, Švrček V, Mariotti D: Nanoscale 8 (2016) 17141 [14]Askari S, Švrček V, Maguire P, Mariotti D: Advanced Materials 27 (2015) 8011

Impact of Quantum Confinement on Band-to-Band Tunneling of Line-Tunneling Type L-Shaped Tunnel Field-Effect Transistor

An L-shaped tunnel field-effect transistor (LTFET) employs an overlapped gate/channel/source architecture to maximize band-to-band tunneling (BTBT) area. The overlapped channel is very thin and suffers from the geometrical quantum confinement effect (QCE). The analysis of QCE in LTFET has been shown to significantly affect the BTBT in LTFET. As a consequence of QCE, conduction band in the overlapped channel becomes a discrete set of energy subbands, whereas the valence band in the source is continuous. The discretization of conduction band reduces the BTBT drain–source current ( ${I}_{\textsf {ds}}$ ) of LTFET. QCE also forces the lateral and parasitic component of BTBT to dominate for a significant portion of the gate-bias swing.

(Invited) Epitaxy of Direct Bandgap Group IV Si-Ge-Sn Alloys towards Heterostructure Light Emitters

Silicon (Si)-based group IV materials form the backbone of our digital society, being found in every smartphone and tablet. They, however, lack the possibility to efficiently emit and detect light, because of their indirect bandgap. A truly Si-compatible direct bandgap material allows merging of state-of-the-art CMOS technology on-chip with photonic components to tackle bottlenecks of modern electronics by radically reducing the required power consumption or increasing the available data bandwidth [1]. Alloying the group IV elements germanium (Ge) and tin (Sn) has turned out to be a viable solution, as a direct bandgap semiconductor – even suitable for lasing – is formed for relaxed epilayers with Sn incorporation above ∼9 at.% [2]. The value of the bandgap can further be controlled by adding Si into GeSn, which can be exploited for carrier confinement inside heterostructures [3]. In this contribution, we discuss the epitaxy of various Sn-based group IV alloys. Different heterostructure designs combine direct bandgap active layers, ternary barriers and a GeSn buffer technology. In Figure 1a, a TEM micrograph of a SiGeSn/GeSn/SiGeSn double heterostructure (DHS) is presented, overlapped with the elemental distribution taken by energy-dispersive X-Ray spectroscopy (EDX). A Sn incorporation of roughly 14.5 at.% ensures a direct bandgap inside the active region. Another approach facilitates direct bandgap GeSn multi quantum wells (MQW), sandwiched between SiGeSn barriers. Atom Probe Tomography (APT) allows to obtain precise elemental distribution within this complex heterostructure stack, as is demonstrated in Figure 1b. Techniques such as temperature-dependent photoluminescence are used to compare the optical properties of the grown heterostructures. Moreover, the impact of quantum confinement on MQW emission is revealed, backed up by bandstructure calculations. Optically pumped microdisk resonators are formed to demonstrate lasing from different types of heterostructures, as shown in Figure 1c. As depicted in the inset, the superiority of MQW structures over DHSs (and bulk GeSn layers) is demonstrated, leading to an order of magnitude reduced lasing thresholds at cryogenic temperatures. In the next step of integrated light emitters, carriers need to be injected electrically, rather than optically. We will show first experiments on epitaxially grown p-i-n heterostructure diodes. In Figure 1d, incorporation of phosphorus and boron for the formation of n- and p-doped carrier injection layers is shown by SIMS. We will further point towards possible improvements that may pave the way towards future energy efficient opto-electronic integrated circuits (OEICs). [1] L. Vivien, Computer technology: Silicon chips lighten up, Nature. 528 (2015) 483–484. [2] S. Wirths, R. Geiger et al., Lasing in direct-bandgap GeSn alloy grown on Si, Nat. Photonics. 9 (2015) 88–92. [3] N. von den Driesch et al., Advanced GeSn/SiGeSn Group IV Heterostructure Lasers, Adv. Sci. (2018). doi:10.1002/advs.201700955. Figure 1

Impact of quantum confinement on transport and the electrostatic driven performance of silicon nanowire transistors at the scaling limit

In this work we investigate the impact of quantum mechanical effects on the device performance of n-type silicon nanowire transistors (NWT) for possible future CMOS applications at the scaling limit. For the purpose of this paper, we created Si NWTs with two channel crystallographic orientations 〈110〉 and 〈100〉 and six different cross-section profiles. In the first part, we study the impact of quantum corrections on the gate capacitance and mobile charge in the channel. The mobile charge to gate capacitance ratio, which is an indicator of the intrinsic performance of the NWTs, is also investigated. The influence of the rotating of the NWTs cross-sectional geometry by 90° on charge distribution in the channel is also studied. We compare the correlation between the charge profile in the channel and cross-sectional dimension for circular transistor with four different cross-sections diameters: 5nm, 6nm, 7nm and 8nm. In the second part of this paper, we expand the computational study by including different gate lengths for some of the Si NWTs. As a result, we establish a correlation between the mobile charge distribution in the channel and the gate capacitance, drain-induced barrier lowering (DIBL) and the subthreshold slope (SS). All calculations are based on a quantum mechanical description of the mobile charge distribution in the channel. This description is based on the solution of the Schrödinger equation in NWT cross sections along the current path, which is mandatory for nanowires with such ultra-scale dimensions.

The impact of quantum confinement on the electrical characteristics of ultrathin-channel GeOI MOSFETs

The impact of quantum confinement on the electrical characteristics of ultrathin-channel GeOI n-MOSFETs is investigated on the basis of the density-gradient model in TCAD software. The effects of the channel thickness (Tch) and back-gate bias (Vbg) on the electrical characteristics of GeOI MOSFETs are examined, and the simulated results are compared with those using the conventional semi-classical model. It is shown that when Tch > 8 nm, the electron conduction path of the GeOI MOSFET is closer to the front-gate interface under the QC model than under the CL model, and vice versa when Tch < 8 nm. Thus the electrically controlled ability of the front gate of the devices is influenced by the quantum effect. In addition, the quantum-mechanical mechanism will enhance the drain-induced barrier lowering effect, increase the threshold voltage and decrease the on-state current; for a short channel length (≤ 30 nm), when Tch > 8 nm (or < 8 nm), the quantum-mechanical mechanism mainly impacts the subthreshold slope (or the threshold voltage). Due to the quantum-size effect, the off-state current can be suppressed as the channel thickness decreases.

Investigation of Backgate-Bias Dependence of Threshold-Voltage Sensitivity to Process and Temperature Variations for Ultra-Thin-Body Hetero-Channel MOSFETs

This paper investigates the impact of backgate bias (Vbg) on the sensitivity of threshold voltage (Vth) to process and temperature variations for ultra-thin-body (UTB) GeOI and InGaAs-OI MOSFETs. Our study indicates that the quantum-confinement effect significantly suppresses the Vbg dependence of the Vth sensitivity to process and temperature variations. Since Si, Ge, and InGaAs channels exhibit different degrees of quantum confinement, the impact of quantum confinement has to be considered when one-to-one comparisons among hetero-channel UTB devices regarding variability are made. Our study is crucial to the robustness of multi- Vth designs with advanced UTB technologies.

Impact of Quantum Confinement on Stress-Induced nMOSFET Threshold Voltage Shift

In this paper, we propose a comprehensive model to express nMOSFET threshold voltage shift induced by stress, ranging from a high tensile one to a high compressive one. Using this model, the quantum confinement effect, combined with large out-of-plane stress, is shown to play an important role to cause the threshold voltage shift as large as about 80 mV induced by high-film-stress contact etch-stop layer.

Impact of Quantum Confinement on Gate Threshold Voltage and Subthreshold Swings in Double-Gate Tunnel FETs

We investigate how the inclusion of quantum confinement in double-gate tunneling field-effect transistors (DG-TFETs) modifies the conventional behavior of electrical parameters of utmost importance in these devices, such as subthreshold swings (point and average) and the gate threshold voltage. We make use of a simple approach that allows us to incorporate a quantum-mechanical description in which the discreteness of subband energy levels causes a significant reduction in the band-to-band tunneling probabilities. The inclusion of quantum confinement along with a nonlocal band-to-band model for tunneling is shown to greatly affect the aforementioned parameters as key issues for the characterization of these novel devices.

Quantum Confinement-Controlled Exchange Coupling in Manganese(II)-Doped CdSe Two-Dimensional Quantum Well Nanoribbons

The impact of quantum confinement on the exchange interaction between charge carriers and magnetic dopants in semiconductor nanomaterials has been controversially discussed for more than a decade. We developed manganese-doped CdSe quantum well nanoribbons with a strong quantum confinement perpendicular to the c-axis, showing distinct heavy hole and light hole resonances up to 300 K. This allows a separate study of the s-d and the p-d exchange interactions all the way up to room temperature. Taking into account the optical selection rules and the statistical distribution of the nanoribbons orientation on the substrate, a remarkable change in particular of the s-d exchange constant with respect to bulk is indicated. Room-temperature studies revealed an unusually high effective g-factor up to ~13 encouraging the implementation of the DMS quantum well nanoribbons for (room temperature) spintronic applications.

Impact of Quantum Confinement on Backgate-Bias Modulated Threshold-Voltage and Subthreshold Characteristics for Ultra-Thin-Body GeOI MOSFETs

This paper investigates the impact of quantum confinement (QC) on the backgate-bias (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bg</sub> ) modulated subthreshold and threshold-voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ) characteristics of ultra-thin-body germanium-on-insulator (UTB GeOI) MOSFETs using an analytical solution of the Schrödinger equation verified with TCAD numerical simulation. Our study indicates that the QC effect reduces the sensitivity of the subthreshold swing to V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bg</sub> . In addition, the sensitivity of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> to V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bg</sub> can be enhanced by the QC effect particularly for electrostatically well-behaved UTB MOSFETs with triangular potential well. Aside from that, the sensitivity of V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> roll-off to V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bg</sub> is reduced by the QC effect. Since Ge and Si channels exhibit different degrees of QC due to different quantization effective mass, the impact of QC has to be considered when one-to-one comparisons between GeOI and SOI MOSFETs regarding the backgate-bias modulated threshold-voltage and sub-threshold characteristics are made. Our study may provide insights for multi-V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> device/circuit designs using advanced UTB GeOI technologies.

Impact of Quantum Confinement on Subthreshold Swing and Electrostatic Integrity of Ultra-Thin-Body GeOI and InGaAs-OI n-MOSFETs

This paper investigates the electrostatic integrity (EI) of ultra-thin-body (UTB) germanium-on-insulator (GeOI) and InGaAs-OI n-MOSFETs considering quantum confinement (QC) using a derived analytical solution of Schrodinger equation verified with TCAD numerical simulation. Although the electron conduction path of the high-mobility channel device can be far from the frontgate interface due to high channel permittivity, our study indicates that the quantum confinement effect can move the carrier centroid toward the frontgate and, therefore, improve the subthreshold swing (SS) of the UTB device. Since InGaAs, Ge, and Si channels exhibit different degrees of quantum confinement due to different quantization effective mass, the impact of quantum confinement has to be considered when one-to-one comparisons among UTB InGaAs-OI, GeOI, and SOI MOSFETs regarding the subthreshold swing and electrostatic integrity are made.

Impact of Quantum Confinement on Short-Channel Effects for Ultrathin-Body Germanium-on-Insulator MOSFETs

This letter investigates the impact of quantum confinement (QC) on the short-channel effect (SCE) of ultrathin-body (UTB) and thin-buried-oxide germanium-on-insulator (GeOI) MOSFETs using an analytical solution of Schrodinger equation verified with TCAD simulation. Our study indicates that, although the QC effect increases the threshold voltage (Vth) roll-off when the channel thickness (Tch) is larger than a critical value (Tch,crit), it may decrease the Vth roll-off of GeOI MOSFETs when the Tch is smaller than Tch,crit. Since Ge and Si channels exhibit different degrees of confinement and Tch,crit, the impact of QC must be considered when one-to-one comparisons between UTB GeOI and Si-on-insulator MOSFETs regarding the SCE are made.

Size Dependence of the Surface Plasmon Resonance Damping in Metal Nanospheres

The impact of quantum confinement on the width of the surface plasmon resonance of a metal nanoparticle is theoretically investigated in a model system formed by a silver nanosphere in different environments. Calculations are performed using the time-dependent local density approximation (TDLDA) for nanoparticle diameters up to about 11 nm, permitting precise quantification of the surface plasmon broadening due to size reduction. As expected, this is found to be inversely proportional to the particle diameter, but with an amplitude strongly depending on the environment (increasing by a factor of 4 when changing from vacuum to alumina). This is ascribed to the fact that damping is governed by the electronic surface spill-out (inherent in any quantum model) and thus strongly depends on the surface profile of the confining potential, that is, on the particle surface conditions.

Influence of strain and quantum confinement on the optical properties of InGaAs/GaAs V-groove quantum wires

We report on the impact of quantum confinement and strain effects on the optical properties of state-of-the-art, densely stacked, In0.15Ga0.85As/GaAs V-groove quantum wires. High uniformity and efficient carrier capture lead to narrow (6 meV) and intense emission from the wires. Large optical polarization anisotropy is obtained thanks to the combined effects of lateral quantum confinement and triaxial strain. Band filling in the fundamental subband occurs at a modest carrier density (∼9×105 cm−1), and is accompanied by a small spectral blueshift of the emission. Several sharp excitonic resonances associated with two dimensionally confined subbands of dominant heavy-hole character are observed in photoluminescence excitation spectroscopy, together with a remarkably small Stokes shift (3 meV). The subband separations (∼24 meV) are nearly independent of the wire thickness, as the nonuniform Indium composition across the structure is found to dominate the lateral confinement for thick wires. Such strained quantum wires are promising for the realization of advanced nanostructure devices.