Resonant Tunneling Diode Structure Research Articles

Fabrication of a few-electron In 0.56Ga 0.44As vertical quantum dot with an Al 2O 3 gate insulator

For the study of spin-dependent transport in narrow-gap semiconductor nanostructures, we fabricated a few-electron quantum dot based on an In 0.56Ga 0.44As resonant tunneling diode structure with an Al 2O 3 gate insulator formed by atomic layer deposition and an air-bridge drain electrode. This gated quantum dot device allows us to control the number of electrons from zero to a few dozen without leakage problem. We describe the processing techniques and the characteristics of the low-temperature transport.

Suppression of Leakage Current of CdF2/CaF2 Resonant Tunneling Diode Structures Grown on Si(100) Substrates by Nanoarea Local Epitaxy

CdF2/CaF2 resonant tunneling diode (RTD) structures of 100 nm diameter were fabricated in SiO2 hole arrays formed on Si(100) substrates. RTD structures were grown by molecular beam epitaxy (MBE) in SiO2 hole arrays. After the growth of the initial CaF2 layer at a substrate temperature of 120 °C, in situ annealing at 500 °C was carried out to reduce the density of defects or pinholes in the barrier layer. In the measurement of current–voltage (I–V) characteristics, negative differential resistance (NDR) was clearly demonstrated at room temperature. The peak-to-valley current ratio (PVCR) was 106. The valley current was significantly suppressed. Results obtained here strongly indicates that the RTD structures proposed in this study are promising for high-quality NDR elements compatible with Si LSI technology based on Si(100) substrates.

Room temperature negative differential resistance of CdF2∕CaF2 double-barrier resonant tunneling diode structures grown on Si(100) substrates

The authors have demonstrated the crystal growth of CaF2∕CdF2∕CaF2 multilayered heterostructures on Si(100) substrates as double-barrier resonant tunneling diode structures by a low-temperature growth technique. Current-voltage characteristics were investigated and the authors observed negative differential resistance (NDR) characteristics at room temperature. The peak-to-valley current ratio was 2–8, and 13 at maximum, and peak current density was 80–90A∕cm2. The quantum-well layer thickness dependence of NDR peak voltages is also discussed on the basis of qualitative analytical model using the Esaki-Tsu formula.

Micro-patterned RTDs: Fabrication details and device performance

Resonant tunnelling diode (RTD) structures of complex design can be fabricated from diluted magnetic semiconductors (ZnSe-based material) grown by molecular beam epitaxy. The heterostructures contain one or more pairs of tunnel barriers such as for example (Zn,Be)Se/(Zn,Mn)Se/(Zn,Be)Se in the growth sequence. The fabrication process for a double RTD considered in this work has peculiarities and also common approaches distinctive to this material. Process optimization confirmed the possibility to obtain complex devices from this material interesting both from fundamental and practical points of view.

NEGF simulation of the RTD bistability

The simulation of I-V characteristics of Al0.3Ga0.7As-GaAs and AlAs-GaAs resonant tunneling diodes (RTD) is presented. The nonequilibrium Green function (NEGF) based 1D quantum transport simulator Wingreen is used in our case. The plateau region on the IV characteristics usually present only by the Wigner function equation (WFE) based simulation appeared now by the NEGF simulation of our AlAs-GaAs RTD and its shape is comparable with our experimental measurements. Analysis of our results from point of view of the scattering and geometrical parameters of the RTD structure is presented.

Capacitance–voltage characteristics and switching time of double barrier resonant tunneling diode fabricated with epi-Si and γ-Al2O3

Capacitance-voltage and conductance-voltage characteristics of resonant tunneling diodes fabricated with epitaxial Si/γ-Al2O3 heterostructure have been studied at room temperature. The capacitance-voltage characteristics of this structure show a large capacitance peak near the resonant tunneling bias. This capacitance peak is considered as quantum capacitance originated from the charge storage in the quantum well of the structure during tunneling process. Capacitance-voltage characteristics also were studied at different frequencies to understand the charge storage mechanism in the resonant tunneling diode structure. Resonant tunneling diodes with different barrier thicknesses were studied and tremendous improvement in the NDR characteristics was observed. A maximum peak-to-valley current ratio of 248 was obtained at room temperature. Using this capacitance value switching time and maximum operational frequency of the RTD structure were determined.

Fabrication of Fluoride Resonant Tunneling Diodes on V-Grooved Si(100) Substrates

Fluoride resonant tunneling diodes (RTDs) composed of CaF2/CdF2 heterolayers were fabricated in the V-grooved structures surrounded by (111) faces, which were preferred for the growth of the fluoride layers. The structures were formed by anisotropic etching with potassium hydroxide (KOH) or tetramethyl ammonium hydroxide (TMAH) on Si(100) substrates. In the case of KOH etching, RTDs with a high peak to valley current ratio (PVCR) over 105 were obtained. Although the yield of RTDs whose active region was grown directly on the etched surface was very low because of surface roughness, an improved RTD structure, in which the CaxCd1-xF2 separation layer was inserted under the active region, exhibited higher yield. In the case of TMAH etching, the yield was higher than for KOH etching for directly grown RTD structures due to the presence of a smoother etched surface. These results show that the new method proposed in this work is an effective process to fabricate fluoride RTDs on Si(100) substrates.

Characteristics of GaMnN based ferromagnetic resonant tunneling diode without external magnetic field

Achievement of a spin polarization without magnetic field through a GaMnN based ferromagnetic resonant tunneling diode (RTD) is proposed theoretically. The influences of the temperature and structure on the spin polarization current and differential conductance have been investigated. The clear spin splitting current can be observed in an optimal RTD structure without magnetic field even though the splitting energy of the ferromagnetic barrier is very small. Large spin polarization can be obtained and the polarization orientation can be varied by an applied bias voltage without external magnetic field.

Spin-dependent electron transport through a magnetic resonant tunneling diode

Electron transport properties in nanostructures can be modeled, for example, by using the semiclassical Wigner formalism or the quantum mechanical Green's functions formalism. We compare the performance and the results of these methods in the case of magnetic resonant-tunneling diodes. We have implemented the two methods within the self-consistent spin-density-functional theory. Our numerical implementation of the Wigner formalism is based on the finite-difference scheme whereas for the Green's function formalism the finite-element method is used. As a specific application, we consider the device studied by Slobodskyy et all. [Phys. Rev. Lett. 90, 246601 (2003)] and analyze their experimental results. The Wigner and Green's functions formalisms give similar electron densities and potentials but, surprisingly, the former method requires much more computer resources in order to obtain numerically accurate results for currents. Both of the formalisms can successfully be used to model magnetic resonant tunneling diode structures.

Building and testing submicrometer metallic (gold) air-bridges for nanotransport applications

The properties of submicrometer gold air-bridges are investigated for their suitability as elements of an electronic device. The fabrication procedure of a gated vertical quantum dot resonant tunneling diode structure implementing air-bridges is presented as a demonstration. Our investigations show that air-bridges produced by multiple acceleration voltage lithography exhibit excellent mechanical and electrical stability for nanotransport application.

Growth and characterization of InAlP/InGaAs double barrier RTDs

The material system InAlP/InGaAs is of interest especially due to the high conduction band discontinuity at the heterojunction. The composition of In 0.67Al 0.33P results in a bandgap of E G,InAlP=2.01 eV. In this work we present a double barrier In 0.67Al 0.33P/In 0.55Ga 0.45As/In 0.67Al 0.33P resonant tunnelling diode. The device function is very sensitive to interface effects like roughness, abruptness and homogeneity. The growth of this material system is challenging because of the large lattice mismatch of the Al containing material on an InP substrate. A fully non-gaseous source LP-MOVPE configuration is used for the growth. The resonant tunnelling diode structures are characterized by HRXRD measurements, which are compared to simulation result, and dc measurement. A high peak current density of S p=3×10 5 A/cm 2 at V Peak=1.3 V with a peak-to-valley current ratio PVR=1.2 has been achieved. Both, the symmetric behaviour of the dc-measurement and the X-ray results confirm a high quality of the highly strained InAlP barrier layers and abrupt interfaces to InGaAs.

Does Circulation in Individual Current States Survive in the Total Current Density?

We describe the two-dimensional simulation of a bent resonant tunneling diode structure which displays vortices in its total current density pattern over a range of applied bias. In contrast, a double gate n-MOSFET is shown where such circulation exists in individual subband states but does not survive in the total current density solution. Both devices are simulated assuming ballistic quantum transport in Si at 300 K.

Rashba Spin–Orbit Interaction and Its Applications to Spin-Interference Effect and Spin-Filter Device

The Rashba spin–orbit interaction in InGaAs quantum wells (QW) is studied using the weak antilocalization analysis as a function of the structural inversion asymmetry (SIA). We have observed a clear cross-over from positive to negative magnetoresistance near zero-magnetic field by controlling the degree of the SIA in the QWs. This is a strong evidence of a zero-field spin splitting that is induced by the Rashba effect. The spin-interference effect in a gate-controlled mesoscopic Aharonov–Bohm ring structure is investigated in the presence of Rashba spin–orbit interaction. The oscillatory behavior appearing in ensemble averaged Fourier spectrum of h/2e oscillations as a function of gate voltage is possibly because of the Aharonov–Casher type interference. We propose a spin-filter device based on the Rashba effect using a nonmagnetic resonant tunneling diode structure. Detailed calculation using InAIAs/InGaAs heterostructures shows that the spin-filtering efficiency exceeds 99.9%.

Growth of III/V resonant tunnelling diode on Si substrate with LP-MOVPE

We present in this work the first monolithic integration of a III/V resonant tunnelling diode (RTD) on an exactly (0 0 1)-oriented Si substrate. The crystalline quality of the InP starting layer and that of the RTD was determined by high-resolution X-ray diffractometry (HRXRD). A 2 μm InP starting layer on a Si substrate exhibited a FWHM HRXRD layer peak intensity of 90 arcsec. The layer stack on Si accurately matched the same layer stack grown on an InP substrate. This demonstrated the high potential of this approach. Room temperature DC measurements of RTD structures exhibited a negative differential resistance at a high peak current density of 6×10 4 A/cm 2.

Modelling of Polarization Charge-Induced Asymmetry of I-V Characteristics of AlN/GaN-Based Resonant Tunnelling Structures

Numerical simulations of AlN/GaN-based resonant tunnelling diode structures are presented, employing self-consistent real-time Green's functions. The simulated current–voltage characteristics show strong asymmetry effects due to polarization charges at the heterointerfaces in the double-barrier region.

Fabrication of Resonance Tunnel Diode by γ-Al2O3/Si Multiple Heterostructures

Fabrication of epitaxial γ-Al2O3(111)/Si(111) heterostructures with smooth surfaces for resonance tunnel diode structures is presented in this study. γ-Al2O3 layers were fabricated by molecular beam epitaxy (MBE) and Si layers were fabricated by a mini e-beam evaporator. Epitaxial growth and surface morphology of these layers were studied by reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM). The obtained root mean square (RMS) values of the surface roughness of these layers were 0.39 nm for γ-Al2O3 (3 nm) on Si(111)-substrate and 0.65 nm for Si (4 nm) on γ-Al2O3/Si(111)-substrate. Conduction band offset of the γ-Al2O3/Si heterostructure was studied by Fowler-Nodheim tunneling current measurement, and the conduction band offset (ΔEc) value of 2.2 eV was obtained. Then, resonant tunneling diode (RTD) structures with double and triple barriers were fabricated using this γ-Al2O3(111)/Si(111) heterostructure. Electrical properties of these RTDs were studied to find negative differential resistance (NDR). The NDR at room temperature was observed in both structures for the first time with a peak to valley (P/V) current ratio of 3.0 for the double barrier RTD, and 4.5 for the triple barrier RTD.

Fluoride Resonant Tunneling Diodes Co-integrated with Si-MOSFETs

Resonant tunneling diodes (RTDs) composed of heteroepitaxial CaF2/CdF2/CaF2 structure grown on Si substrates were co-integrated with Si-metal oxide semiconductor field effect transistors (MOSFETs), and test circuits of the static random access memory (SRAM) cell were fabricated. The RTD structures were grown in the opened diode holes on the oxide film, where a high dose of phosphorus ions (P+) had been implanted. Surface roughening of ion-implanted Si before the fluoride growth due to an increase of the ion dose was investigated, and the optimum dose was determined. Normal operations of both fluoride RTD and Si-MOSFET in the fabricated circuit were obtained. Bistable operation was also observed for the circuit in which two RTDs were connected in series.

Improved Molecular Beam Epitaxy for Fabricating AlGaN/GaN Heterojunction Devices

The polarity-control technology for Ga-polar GaN with rf plasma nitrogen-source molecular beam epitaxy (RF-MBE) was established. Inserting high-temperature grown AlN multiple intermediate layers (HT-AlN-MIL) into MBE-grown GaN at 750 °C suppressed the propagation of dislocations into upper GaN, and the electrical and optical properties were improved. The HT-AlN-MIL was deposited on GaN templates grown by metal organic chemical vapor deposition (MOCVD), followed by GaN grown by MBE. The dislocations with screw character in MBE-grown GaN were reduced by about two orders of magnitude compared to those of the bottom GaN template, which produced the step-like surface morphology for MBE-grown GaN. To demonstrate the capability of MBE in heterojunction control, GaN/AlN double barrier resonant tunneling diode (RTD) structures were fabricated to show the negative differential resistance with a peak-valley ratio of 3.1.

Critical layer thickness study in In 0.75Ga 0.25As/In 0.5Al 0.5As pseudomorphic resonant tunneling diode structure grown on GaAs substrates

We have studied critical layer thickness (CLT) in an In 0.75Ga 0.25As resonant tunneling diode structure grown on a GaAs substrate via an InAlAs step-graded buffer (SGB) with two types of inverse step(IS)-SGB. We have observed red or blue shift in photoluminescence spectra and CLT change depending on SGB condition. This change reflects from the difference of residual strain InAlAs SGBs adopted here.

Effects of Growth Temperature on Electrical Properties of InP-based Pseudomorphic Resonant Tunneling Diodes with Ultrathin Barriers Grown by Molecular Beam Epitaxy

The impact of growth temperature on the epitaxial layer structure and the negative differential resistance characteristics of pseudomorphic In0.53Ga0.47As/AlAs/InAs resonant tunneling diodes (RTDs) with the high peak current density of 5–10×104 A/cm2 grown by molecular beam epitaxy was studied. For RTDs with a nominally symmetrical structure and with an InAs sub-well layer thinner than an estimated critical thickness, strong asymmetry in current–voltage characteristics was observed at growth temperatures below the critical growth temperature of 410°C, while a slight reverse asymmetry was observed at higher temperatures. Examinations of the RTD structures by transmission electron microscopy and atomic force microscopy indicated that three-dimensional growth of InAs at lower temperatures degrades the top-AlAs barrier structure. These asymmetric characteristics are explained in terms of the barrier structure asymmetry caused by temperature-dependent growth kinetics by using a simplified current density calculation model.