Quantum Effect Devices Research Articles

Preparation of Ordered Anodic Porous Alumina with Single-Nanometer-Order-Size Holes by Atomic Layer Deposition.

Ordered anodic porous alumina with controlled-size holes on the order of a single-nanometer scale was obtained by the atomic layer deposition (ALD) of Al2O3 or TiO2. The thin metal oxide layers of uniform thickness were formed successfully on the inner wall of the hole of the ordered anodic porous alumina with high aspect ratios by ALD. The hole diameter of the ordered anodic porous alumina could be controlled precisely by adjusting the number of cycles of ALD. The obtained anodic porous alumina with an ordered hole arrangement of reduced holes will be applied to various application fields requiring uniform-sized holes on the order of a single-nanometer scale, such as the starting material for preparing various types of quantum effect devices.

Structural and electrical properties of ZnO/Zn0.85Mg0.15O thin film prepared by pulsed laser deposition

The electrical transport properties are the key factors to determine the performance of ZnO-based quantum effect device. ZnMgO is a typical material to regulate the band of ZnO. In order to investigate the electrical properties of the interface of ZnO/Zn[Formula: see text]Mg[Formula: see text]O films, three kinds of ZnO/Zn[Formula: see text]Mg[Formula: see text]O films have been fabricated with different thickness. After comparing the structural and electrical properties of the samples, we found that the independent Zn[Formula: see text]Mg[Formula: see text]O hexagonal wurtzite structure (002) peak can be detected in XRD spectra. Hall-effect test data confirmed that the two-dimensional electron gas (2DEG) became lower because of the decrease of thickness of Zn[Formula: see text]Mg[Formula: see text]O films, increase of impurity scattering and lattice structure distortion caused by the increase of Mg content.

Epitaxial lanthanide oxide thin films on Si for high-k gate dielectric application: Growth optimization and defect passivation

Abstract

Entropy, Electric Field and Heavily Doped Nanowires

In this paper an attempt is made to study, the entropy in the presence of intense electric field in nanowires (NWs) of heavily doped (HD) III–V and optoelectronic materials on the basis of newly formulated electron dispersion relations within the frame work of k → ⋅ p → formalism. It is found taking HD NWs of InSb, InAs, Hg(1– x) Cd (x)Te and In(1– x) Ga( x) As( y) P(1– y) lattice matched to InP as examples III–V, ternary and quaternary compounds that the entropy increases with increasing electron concentration per unit length and decreasing film thickness in different spiky manners, since the coincidence of Fermi energy with the sub-band energy leads to the step functional dependence of the density state function and this fact is being reflected in the quantized variations of the entropy with the said variables. The entropy increases with increasing electric field and decreasing alloy composition respectively. The numerical values of entropy with all the physical variables are totally band structure dependent for all the cases. The most striking features are that the presence of poles in the dispersion relation of the materials in the absence of band tails creates the complex energy spectra in the corresponding opto-electronic HD NWs and the effective electron mass exists within the band gap which is impossible without the concept of band tailing. The well-known classical result of entropy for non-degenerate bulk semiconductors having parabolic energy bands has been obtained as a special case of our generalized formulation and thus confirming the compatibility test. The content of this paper finds four important applications in the field of quantum effect devices of nanoscience and nanotechnology.

(Invited) Epitaxial Oxides on Silicon for CMOS and Beyond

A very promising way to realize advanced future devices is using single-crystalline, closely lattice matched oxides, which will be grown on the substrate of choice. The ability to integrate crystalline metal oxide dielectric layers into silicon structures can open the way for a variety of novel applications which enhances the functionality and flexibility ranging from high-K replacements in future MOS devices to oxide/silicon/oxide heterostructures for nanoelectronic application in quantum-effect devices. We present results for crystalline gadolinium oxides on silicon in the cubic bixbyite structure grown by solid source molecular beam epitaxy. This material has a large band gap of about 6 eV and nearly symmetrical band offsets as well as a low lattice mismatch of about -0.4 % to Si. Layers grown by an optimized process display a sufficiently high-K value to achieve equivalent oxide thickness values below 1 nm, combined with ultra-low leakage current densities, good reliability, and high electrical breakdown voltage. A variety of MOS devices has been fabricated based on these layers. The dielectric properties of such oxides are sensitive to small variations in structure and symmetry. It is known that thin layers of crystalline rare earth oxides can exhibit significant larger dielectric constants compared to bulk materials. For example, thin crystalline Gd2O3 films epitaxially grown on silicon exhibit dielectric constants up to 25 although the known bulk value is only around 13. The reason for that “enhancement effect” is not fully understood yet. As model systems, we chose Gd2O3 and Nd2O3 having very similar bulk dielectric constants and band gaps. The crystalline structures are also identical. On the other hand, the lattice spacing in Nd2O3 is larger while that of Gd2O3 smaller than the lattice spacing in silicon; i.e. one layer should be under compressive strain and the other under tensile strain. First, we will report on the dependence of the dielectric constant on layer thickness for epitaxial Gd2O3 on Si(111). The K-value strongly decreases with increasing layer thickness and reaches the bulk value at around 8 nm. Controlling the oxide composition in ternary (Gd1-xNdx)2O3 thin films enables us to tune the lattice mismatch to silicon, and thus the strain-induced variation in the dielectric constants of the layer from 13 (close to the bulk value) up to 20. Finally, we will show that solely tetragonal distortion of the cubic lattice is not sufficient to explain the huge lattice-mismatch induced enhancement in K-values. Thus, we will explain these effects by more severe strain induced structural phase deformations. Further, dielectric properties of epitaxial oxide thin films grown on Si have been found to improve significantly by incorporation of suitable dopants. We observe substantial reduction of the leakage current density in nitrogen-doped Gd2O3 layers. To achieve optimum electrical properties from such doped oxides it is important to understand the correlation between doping and the electronic structure of the material. X-ray photoelectron spectroscopy investigations revealed band gap narrowing in epitaxial Gd2O3 due to nitrogen doping, which leads to reduction in the valence band offset to Si. The observed reduction of the leakage current densities in the these layers with increasing nitrogen content suggests that nitrogen doping can be an effective route to eliminate the adverse effects of the oxygen vacancy induced defects in the oxide layers. We will finally demonstrate different approaches to grow Si nanostructures embedded into crystalline rare earth oxides. By efficiently exploiting the growth kinetics one could create nanostructures exhibiting various dimensions, ranging from three dimensionally confined quantum dots to the quantum wells, where the particles are confined in one of the dimensions. Double-barrier structures comprising epitaxial insulator as barriers and Si as quantum-well are attractive candidate for resonant tunneling devices. Embedded Si quantum dots (with average sizes below 5 nm) exhibit excellent charge storage capacity with competent retention and endurance characteristics suitable for nonvolatile memory device applications.

Sudden restoration of the band ordering associated with the ferromagnetic phase transition in a semiconductor.

The band ordering of semiconductors is an important factor in determining the mobility and coherence of the wave function of carriers, and is thus a key factor in device performance. However, in heavily doped semiconductors, the impurities substantially disturb the band ordering, leading to significant degradation in performance. Here, we present the unexpected finding that the band ordering is suddenly restored in Mn-doped GaAs ((Ga,Mn)As) when the Mn concentration slightly exceeds ∼0.7% despite the extremely high doping concentration; this phenomenon is very difficult to predict from the general behaviour of doped semiconductors. This phenomenon occurs with a ferromagnetic phase transition, which is considered to have a crucial role in generating a well-ordered band structure. Our findings offer possibilities for ultra-high-speed quantum-effect spin devices based on semiconductors.

Energy Quantization in Solution‐Processed Layers of Indium Oxide and Their Application in Resonant Tunneling Diodes

The formation of quantized energy states in ultrathin layers of indium oxide (In2O3) grown via spin coating and thermally annealed at 200 °C in air is studied. Optical absorption measurements reveal a characteristic widening of the optical band gap with reducing In2O3 layer thickness from ≈43 to ≈3 nm in agreement with theoretical predictions for an infinite quantum well. Through sequential deposition of In2O3 and gallium oxide (Ga­2O3) layers, superlattice‐like structures with controlled dimensionality and spatially varying conduction band characteristics are demonstrated. This simple method is then explored for the fabrication of functional double‐barrier resonant tunneling diodes. Nanoscale current mapping analysis using conductive atomic force microscopy reveals that resonant tunneling is not uniform but localized in specific regions of the apparent device area. The latter observation is attributed to variation in the layer(s) thickness of the In2O3 quantum well and/or the Ga2O3 barrier layers. Despite the nonidealities, the tremendous potential of solution‐processable oxide semiconductors for the development of quantum effect devices that have so far been demonstrated only via sophisticated growth techniques is demonstrated.

Ring structures formed inside voids in SiO2 layer on Si(100) during thermal decomposition

Ring structures inside voids in the SiO2 layer on a Si(100) substrate, which are concentrically formed by repeating thermal annealing in vacuum, have been investigated by scanning electron microscopy and atomic force microscopy. We demonstrate that slight exposure of the surface to volatile organic compounds during a cooling process significantly affects the formation of the ring structures. This result clearly shows that the key to ring-structure formation is surface adsorption of carbon atoms, which probably suppresses surface migration of silicon atoms. Our research provides a novel technique for the fabrication of nanostructured semiconductors for such applications as quantum effect devices.

Einstein's Photoemission from Quantum Confined Superlattices.

This paper is dedicated to the 83th Birthday of Late Professor B. R. Nag, D.Sc., formerly Head of the Departments of Radio Physics and Electronics and Electronic Science of the University of Calcutta, a firm believer of the concept of theoretical minimum of Landau and an internationally well known semiconductor physicist, to whom the second author remains ever grateful as a student and research worker from 1974-2004. In this paper, an attempt is made to study, the Einstein's photoemission (EP) from III-V, II-VI, IV-VI, HgTe/CdTe and strained layer quantum well heavily doped superlattices (QWHDSLs) with graded interfaces in the presence of quantizing magnetic field on the basis of newly formulated electron dispersion relations within the frame work of k · p formalism. The EP from III-V, II-VI, IV-VI, HgTe/CdTe and strained layer quantum wells of heavily doped effective mass superlattices respectively has been presented under magnetic quantization. Besides the said emissions, from the quantum dots of the aforementioned heavily doped SLs have further investigated for the purpose of comparison and complete investigation in the context of EP from quantum confined superlattices. Using appropriate SLs, it appears that the EP increases with increasing surface electron concentration and decreasing film thickness in spiky manners, which are the characteristic features of such quantized hetero structures. Under magnetic quantization, the EP oscillates with inverse quantizing magnetic field due to Shuvnikov-de Haas effect. The EP increases with increasing photo energy in a step-like manner and the numerical values of EP with all the physical variables are totally band structure dependent for all the cases. The most striking features are that the presence of poles in the dispersion relation of the materials in the absence of band tails create the complex energy spectra in the corresponding HD constituent materials of such quantum confined superlattices and effective electron mass exists within the band gap which is impossible without the concept of band tails. The well-known result of EP for bulk semiconductors having parabolic energy bands can be obtained as a special case of our generalized formulation and thus confirming the compatibility test. The content of this paper finds four important applications and we have suggested the methods of experimental determinations of important transport quantities in the field of quantum effect devices of nanoscience and nanotechnology.

(Invited) Neutral Beam Technology – Defect-free Nanofabrication for Novel Nano-materials and Nano-devices

In the fabrication of semiconductor devices, reactive plasmas arewidely used in key processes suchasmicrofabrication, surface modificationand film deposition, and there are now demands for processingprecision at the atomic layer level, and for deposition accuracy thatallows the control of structures at the molecular level. However, inultra-miniature nanoscale devices that will become the mainstream inthe future, the use of plasma processes can cause serious problemssuch as abnormal etching and breakdown of insulation films by theaccumulation of ions or electrons emitted from the plasma , also the formation of surface defects (dangling bond) of overa few tens nm in depth by exposure to ultraviolet (UV) emissions fromthe plasma. In particular, since nano-scale devices have a larger surfacearea compared with the bulk material, plasma processes can havea large influence on the electrical and optical properties of devices dueto process-induced defects caused by ultraviolet exposure,since future nano-devices will require size control of three-dimensionalstructures at the atomic layer level, it will be absolutely essential tocontrol surface chemical reactions with high precision and selectivity at the atomic layer level. To achieve charge-free and UV photon irradiation damage-free processes, we have developed a new neutral beam generation system based on my discovery that neutral beams can be efficiently generated from the acceleration of negative ions produced in pulsed plasmas. Using the neutral beam processing, we successfully demonstrated sub-50nm damage-free gate electrode etching, damage-free Si channel etching for 45 nm fin-FETs, ultra-thin gate dielectric film formation for 32 nm fin-FETs, damage-free low dielectric film deposition for 22 nm FETs, and low-damage surface modification of carbon materials (including nanotubes, graphenes and organic moleculers) for future nanodevices. More recently we have investigated processing technologies based on the combination of biotechnology with neutral-beam-based nano-processes, i.e., bio-nano processes, for future nanoelectronics devices and successfully achieved the fabrication of sub-10-nm-diameter and high density Si, Ge, GaAs, InGaAs and Graphene nanodisk (nanodot) array structures. The quantum effects of these nano-scaled structures were shown to manifest themselves at room temperature due to the damage-free surfaces made possible by the neutral beam processes. Now, by using these nanodisk structures, we are actively developing “Novel Quantum Effect Devices”, such as quantum dot solar cell for high energy conversion efficiency of more than 45 % and quantum dot laser. This paper introduces the neutron beam generation technique developed by S. Samukawa [1], and discusses its application to nanoprocessing and nanodevices that have recently been vigorously pursued. We conclude our developed neutral beam process is indispensable to progress atomic layer pure surface reactions for atomic layer process such as etching (ALE) and deposition (ALD).We are actively developing ultra-low-damage nanofabrication techniques using neutral beam technology that tap into the essential nature of nano-materials and nano-structures, and developing innovative nanodevices. S. Samukawa, ECS Journal of Solid State Science and Technology, 4 (6) N5089 (2015).

A Sigma-Delta Domain Lowpass Wave Filter

This paper presents a sigma-delta (SD) domain lowpass wave filter modeled after an analog distributed parameter filter (ADPF). The SD domain filter operating on SD modulated signals is built with two kinds of units, shift registers on which two-level-quantized waves propagate and sorter-based units that simulate reflection, transmission, and superposition of waves propagating on the transmission lines of the ADPF. The SD domain lowpass filter has the following advantages: First, design techniques for ADPFs can be applied directly to the SD domain filter. Secondly, the SD domain filter consists mainly of shift registers and sorting networks. Because the structures of the registers and the networks are spatially uniform and their internal connections are local, the filter architecture will mitigate difficulties in physical design and fabrication of filter circuits using nanometer-scale quantum effect devices. We confirmed by computer simulation that the designed SD domain filter possessed the same frequency response characteristics as the theoretical characteristics of the reference ADPF. Although internal units of SD domain filters generate SD modulation noise, it does not change to aliasing noise in the signal band because the operations of the units are linear. Therefore, the quantization noise contained in the outputs of SD domain filters is shaped like SD modulation noise.

Formation and Characterization of Strained Si1-XGex Films Epitaxially Grown on Si(100) by Low-Energy ECR Ar Plasma CVD without Substrate Heating

For quantum-effect nano heterostructures of group IV semiconductors, formation and characterization of Si1-xGex films epitaxially grown on Si(100) by using low-energy electron-cyclotron-resonance (ECR) Ar plasma chemical vapor deposition (CVD) without substrate heating were investigated. Epitaxial Si0.50Ge0.50 films with smooth surface can be obtained on Si(100), where the thickness is almost proportional to deposition time and an incubation period is scarcely observed. For a 12 nm-thick epitaxial Si0.50Ge0.50 film on Si(100), its lattice constant in the direction perpendicular to the interface shows that the lattice is highly strained compared to a case of strain-relaxed Si0.50Ge0.50. This indicates that our ECR plasma CVD process has a potential to achieve highly strained heterostructures with suppressed strain relaxation and thermal interdiffusion at the heterointerface which are necessary for quantum-effect devices.

Sorter-Based Arithmetic Circuits for Sigma-Delta Domain Signal Processing—Part II: Multiplication and Algebraic Functions

We construct arithmetic modules for signal processing with sigma-delta modulated signal form which has advantage in signal quality over other pulsed signal forms. In the second part of this paper, multi-input multipliers are presented first. Secondly, dividers and square root function modules with the multiplier on their internal feedback path are constructed. Combined use of the multipliers, dividers, and the square root functions creates various algebraic functions including polynomial and rational functions. Only two bit-manipulations, bit-permutation with sorting networks and bit-reversal with NOT gates, have built up all the algebraic operations on any form of SD modulated signals. These modules, together with transcendental functions presented in the first part of this paper, organize an extensive module library for the sigma-delta domain signal processing. The multiplier output contains noise components which originate from quantization. The noise power can decrease in exchange for circuit complexity. A time-division multiplexing technique based on N-tone sigma-delta modulation is applied to the multipliers for reducing the complexity. Signal processing circuits built of nanometer-scale quantum effect devices must be equipped with fault tolerance of transient device error. By computer simulation of a multiplier built of single-electron tunneling devices, we found that the multiplier decreased its output SNDR from 43 to 27 dB at an OSR of 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> as the device error rate increased from 0 to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . However, the multiplier was never functionally failed during the simulation.

Design and Modelling of Different SRAM'S Based on CNTFET 32NM Technology

Carbon nanotube field-effect transistor (CNTFET) refers to a field-effect transistor that utilizes a single carbon nanotube or an array of carbon nanotubes as the channel material instead of bulk silicon in the traditional MOSFET structure. Since it was first demonstrated in 1998, there have been tremendous developments in CNTFETs, which promise for an alternative material to replace silicon in future electronics. Carbon nanotubes are promising materials for the nano-scale electron devices such as nanotube FETs for ultra-high density integrated circuits and quantum-effect devices for novel intelligent circuits, which are expected to bring a breakthrough in the present silicon technology. A Static Random Access Memory (SRAM) is designed to plug two needs: i) The SRAM provides as cache memory, communicating between central processing unit and Dynamic Random Access Memory (DRAM). ii) The SRAM technology act as driving force for low power application since SRAM is portable compared to DRAM, and SRAM doesn't require any refresh current. On the basis of acquired knowledge, we present different SRAM's designed for the conventional CNTFET. HSPICE simulations of this circuit using Stanford CNTFET model shows a great improvement in power saving.

Near room-temperature formation of a skyrmion crystal in thin-films of the helimagnet FeGe

The skyrmion, a vortex-like spin-swirling object, is anticipated to play a vital role in quantum magneto-transport processes such as the quantum Hall and topological Hall effects. The existence of the magnetic skyrmion crystal (SkX) state was recently verified experimentally for MnSi and Fe(0.5)Co(0.5)Si by means of small-angle neutron scattering and Lorentz transmission electron microscopy. However, to enable the application of such a SkX for spintronic function, materials problems such as a low crystallization temperature and low stability of SkX have to be overcome. Here we report the formation of SkX close to room temperature in thin-films of the helimagnet FeGe. In addition to the magnetic twin structure, we found a magnetic chirality inversion of the SkX across lattice twin boundaries. Furthermore, for thin crystal plates with thicknesses much smaller than the SkX lattice constant (as) the two-dimensional SkX is quite stable over a wide range of temperatures and magnetic fields, whereas for quasi-three-dimensional films with thicknesses over as the SkX is relatively unstable and observed only around the helical transition temperature. The room-temperature stable SkX state as promised by this study will pave a new path to designing quantum-effect devices based on the controllable skyrmion dynamics.

Room-Temperature Resonant Tunneling Diode with High-Ge-Fraction Strained Si1-xGex and Nanometer-Order Ultrathin Si

For the purpose of heterointegration of Si-based group IV semiconductor quantum effect devices into Si large-scale integrated circuit, formation of atomically flat heterointerfaces in quantum heterostructure by lowering Si barrier growth temperature was investigated in order to improve negative differential conductance (NDC) characteristics of high-Ge-fraction strained Si1-xGex/Si hole resonant tunneling diode. It was found that roughness generation at heterointerfaces is drastically suppressed by utilizing, Si barriers with nanometer order thickness deposited using Si2H6 reaction at a lower temperature of 400oC instead of SiH4 reaction at 500oC after the Si0.42Ge0.58 growth. NDC characteristics show that difference between peak and valley currents is effectively enhanced at 11-295K by using Si2H6 at 400oC, compared with that using SiH4 at 500oC. Thermionic-emission dominant characteristics at higher temperatures above 100 K indicates a possibility that introduction of larger barrier height (i.e., larger band discontinuity) enhances the NDC at room temperature by suppression of thermionic-emission current.

Encapsulated solid phase epitaxy of a Ge quantum well embedded in an epitaxial rareearth oxide

An efficient method based on molecular beam epitaxy has beendeveloped to integrate an epitaxial Ge quantum well buried into a singlecrystalline rare earth oxide. The monolithic heterostructure comprised ofGd2O3–Ge–Gd2O3 grown on an Si substrate exhibits excellent crystalline quality with atomically sharpinterfaces. This heterostructure with unique structural quality could be used for novelnanoelectronic applications in quantum-effect devices such as nanoscale transistors with ahigh mobility channel, resonant tunneling diode/transistors, etc. A phenomenologicalmodel has been proposed to explain the epitaxial growth process of the Ge layerunder oxide encapsulation using a solid source molecular beam epitaxy technique.

A Novel High-Speed Multiplexing IC Based on Resonant Tunneling Diodes

A new multiplexing IC based on the resonant tunneling diode (RTD) is proposed. The unique negative differential resistance characteristics arising from quantum effects of the RTD enable us to develop a new functional low-power digital circuit. The proposed multiplexing IC consists of two current-mode-logic monostable-bistable transition logic elements (CML-MOBILEs) based on the RTD and a low-power selector circuit block. The proposed circuit has been fabricated by using an InP RTD/ heterojunction bipolar transistor monolithic microwave integrated circuit technology. The multiplexing operation of the fabricated quantum effect IC has been confirmed up to 45 Gb/s for the first time as a monolithic technology based on the quantum effect devices. The dc power consumption is only 23 mW, which is found to be one-fourth of the current state-of-the-art conventional transistor-based multiplexing IC.

Einstein relation in n-i-p-i and microstructures of nonlinear optical, optoelectronic and related materials: Simplified theory, relative comparison and suggestions for an experimental determination

We investigate the Einstein relation for the diffusivity-mobility ratio (DMR) for n-i-p-i and the microstructures of nonlinear optical compounds on the basis of a newly formulated electron dispersion law. The corresponding results for III-V, ternary and quaternary materials form a special case of our generalized analysis. The respective DMRs for II-VI, IV-VI and stressed materials have been studied. It has been found that taking CdGeAs 2, Cd 3As 2, InAs, InSb, Hg 1− x Cd x Te, In 1− x Ga x As y P 1− y lattices matched to InP, CdS, PbTe, PbSnTe and Pb 1− x Sn x Se and stressed InSb as examples that the DMR increases with increasing electron concentration in various manners with different numerical magnitudes which reflect the different signatures of the n-i-p-i systems and the corresponding microstructures. We have suggested an experimental method of determining the DMR in this case and the present simplified analysis is in agreement with the suggested relationship. In addition, our results find three applications in the field of quantum effect devices.

Two-Dimensional Si-Nanodisk Array Fabricated Using Bio-Nano-Process and Neutral Beam Etching for Realistic Quantum Effect Devices

A high-density, large-area, and uniform two-dimensional (2D) Si-nanodisk array was successfully fabricated using the bio-nano-process, advanced etching techniques, including a treatment using nitrogen trifluoride and hydrogen radical (NF3 treatment) and a damage-free chlorine neutral beam (NB). By using the surface oxide formed by neutral beam oxidation (NBO) for the preparation of a 2D nanometer-sized iron core array as an etching mask, a well-ordered 2D Si-nanodisk array was obtained owing to the dangling bonds of the surface oxide. By changing the NF3 treatment time without changing the quantum effect of each nanodisk, we could control the gap between adjacent nanodisks. A device with two electrodes was fabricated to investigate the electron transport in a 2D Si-nanodisk array. Current fluctuation and time-dependent currents were clearly observed owing to the charging-discharging of the nanodisks adjacent to the current percolation path. The new structure may have great potential for future novel quantum effect devices.