A novel RTD-HEMT-RTD structure based on simulations

The influences of interface roughness (IR) scattering on transmission coefficient and current-voltage ( I–V ) characteristics of double-barrier resonant tunneling diodes are investigated. Simulation results reveal that asymmetric interface roughness scattering on different interfaces of the heterostructure may result in asymmetric I–V characteristics. In addition, it is suggested that the splitting of subband energy levels inside the quantum well caused by IR with large terraces (≥15 nm) may be one of the causes leading to a steplike I–V curve in the negative differential resistance region.

Several techniques were proposed to achieve solid state spin filtering such as magnetic tunnel junctions comprised of half-metallic compounds or solid state Stern-Gerlach apparatus. Another alternative consists in using spin-dependent resonant tunneling through magnetically active quantum wells. Recent advances in molecular beam epitaxial growth made it possible to fabricate exotic heterostructures comprised of magnetic films or buried layers (ErAs, GaxMn1-xAs) integrated with conventional semiconductors (GaAs) and to explore quantum transport in these heterostructures. It is particularly interesting to study spin-dependent resonant tunneling in double-barrier resonant tunneling diodes (RTD) with magnetic elements such as GaAs/AlAs/ErAs/AlAs/ErAs/AlAs/GaAs, GaxMn1-xAs/AlAs/GaAs/AlAs/GaAs, and GaAs/AlAs/GaxMn1-xAs/AlAs/GaAs. We present the results of our theoretical studies and computer simulations of transmission coefficients and current-voltage characteristics of resonant tunneling diodes based on these double-barrier structures. Resonant tunneling of holes (GaxMn1-xAs-based RTDs) is considered. Our approach is based on k·p perturbation theory with exchange splitting effects taken into account. We analyze exchange splitting of different resonant channels as a function of magnetization as well as spin polarization of the transmitted current as a function of bias. We found that resonant tunneling I – V characteristics of the double-barrier magnetic hererostructures strongly depend on the doping level in the emitter as well as on the orientation of the magnetization.

The effect of impurities placed in the wells of double-barrier resonant tunneling diodes on the current-voltage characteristics was experimentally determined. Four different double-barrier structures were grown by molecular beam epitaxy with n-type, p-type, undoped, and highly compensated doping in the center of the well. Resonant tunneling devices of various sizes were fabricated, and measured at 77 K. Systematic shifts in the peak position and peak to valley ratios were observed for the different dopant profiles. The shifts in peak position are correctly predicted by a ballistic model which includes the effects of band bending due to ionized impurities in the well. The doped devices showed a systematic decrease in the peak to valley ratio which is not predicted by the ballistic model. By scaling our results, it is apparent that in most cases unintentional background impurities are not sufficient to significantly degrade the current-voltage characteristics of resonant tunneling diodes.

The effect of the indium concentration on the quality of the AlAs/InGaAs interfaces in resonant tunneling diodes is studied. It is shown by high-resolution transmission electron microscopy that the interface quality of the normal interface (AlAs on InGaAs) nearly achieves even at low growth temperatures TG=500 °C the quality of the corresponding AlAs/GaAs interface at the optimum substrate temperature. The improvement in the interface quality and the higher potential barrier in the InGaAs system leads to higher peak-to-valley ratios (PVRs) of the diode relative to that in the GaAs system. In addition, current–voltage characteristics of resonant tunneling diodes are calculated as an integral over the transmission coefficient and the supply function. Scattering effects are included by folding the transmission probability with a Lorentzian. The theoretical simulations give further insight into the physics of the diodes.

We demonstrate that a novel method of current-voltage (I-V) spectra measurement by scanning force microscopy (SFM) reveals local electrical characteristics of resonant tunneling diodes (RTDs) on a nanometer scale. Measured SFM I-V spectra of RTDs show negative differential resistance features, and the spatial resolution of this method was found to be 20 nm. Experimental evidence for the quantized nature of an SFM pointcontact was observed for the first time. High spatial resolution of this method was confirmed by a simple calculation for the area of current flow through RTD. Fine structure in the SFM I-V spectra was also observed.

This paper presents a fully analytical physics-based model for the current-voltage (I-V) characteristics of resonant tunneling diodes. We consider the whole electrical potential distribution in the structure, including the space charge regions in the emitter and collector layers. Additionally, we account for scattering mechanisms experienced by carriers during tunneling through the double barrier region as a function of the applied bias voltage. The model is validated with experimental and numerical data, yielding excellent agreement

A simple expression for the conductance-voltage characteristics of resonant tunneling diodes (RTD) is presented, which takes the asymmetry of the measured characteristics into account. It allows for a simple and accurate analysis of RTD-based oscillators to optimize them. Moreover, the dynamic capacitance of an RTD oscillator is calculated in order to describe its oscillation frequency. The measured oscillation power and frequency accurately can be described using the presented model and analysis without the need of a circuit simulator.

The secure identification of an object or electronic system is carried out through the provision of some unique internal or external characteristic. The most obvious examples of these include passwords and fingerprints that can identify a person or an electronic device, and holograms that can tag any given object to provide a check of its authenticity. Unfortunately, modern technology provides resources that enable the trust of these everyday techniques to be undermined. Identification schemes have been proposed to address these issues by extracting the identity of a system from its underlying physical structure, which is constructed such that the system is hard‐to‐ clone or predict. These systems are known as Unique Objects (UNOs) and Physically Unclonable Functions (PUFs). The aim of the work in this thesis is to create a novel type of UNO/PUF that utilises the atomic‐scale uniqueness of semiconductor devices by measuring a macroscopic quantum property of the system. The variations in these quantum properties are amplified by the existence of such atomic‐scale imperfections, meaning these devices would be the hardest possible system to clone, use the least resources and provide robust security. Such devices would be of great societal and political significance and would provide the biggest technological barrier between the good guys and the bad. Specifically, this work has introduced three distinct devices based on semiconducting systems that could provide atomic‐scale unique identification: • Electronically ‐ Fluctuations in the current‐voltage characteristics of Resonant Tunneling Diodes (RTDs) were found to provide a simple measurement of the underlying quantum state electronically. • Optically ‐ Macroscopic thin films of the two‐dimensional material, MoS2, were created by the Langmuir‐Blodgett technique for the first time and have laid the foundations for the formation of an optical analogue of an atomic‐PUF/UNO system. • Optoelectronically ‐ The Langmuir‐Blodgett technique’s flexibility was utilised to fabricate complex heterostructures that couple graphene to semiconducting nanoparticles. This system should provide an ideal system with efficient electronic and optical characteristics that would be useful in a range of applications, including unique identification.

Tunneling transport assisted by the Coulomb interaction of Wannier–Mott exciton is investigated within the framework of the sequential tunneling formalism. The exciton-assisted tunneling (EAT) probability is derived from Bardeen’s transfer Hamiltonian. The EAT-induced current-voltage (I-V) characteristics are evaluated using a set of three-particle rate equations for double-barrier and triple-barrier tunneling structures. We found that the EAT results in additional current at low bias voltages with respect to the resonant tunneling (RT) current. Their offset in the I-V spectra is associated to the exciton binding energy. The current intensity of the electron EAT is dependent on the quantum-well hole density as well as the exciton Bohr radius. Calculated I-V characteristics of electron tunneling via the heavy-hole excitonic states are presented and discussed for typical AlxGa1−xAs/GaAs nanostructures. In particular, the line shape of the EAT I-V spectra is shown to have a fingerprint distinguishable from that of the RT at low temperature.

We investigate the effects of impurity scattering on resonant transmission coefficients in GaAs/AlAs double-barrier structures. The coefficients are estimated from d2I/dV2-V characteristics of resonant tunneling diodes, using a recently introduced method. To our knowledge, this is the first time that resonance level broadening due to ionized impurity scattering has been shown experimentally.

The decoupled subbands model is widely used to approximate the two-dimensional Schrödinger equation. While computationally efficient, this simplified model loses accuracy in complex quantum transport simulations because it omits inter-subband interactions. We propose a novel transfer matrix method based on the multi-subband framework to solve the two-dimensional Schrödinger equation, which explicitly incorporates interactions among low-energy subbands. The transfer matrix construction leads to an overdetermined system, so we solve it using either least-squares fitting or singular value decomposition. We assess the accuracy of the proposed method with numerical experiments, showing that both approaches can achieve an error of $\mathcal{O}(\Delta x + \Delta y^2)$. To demonstrate practical utility, we simulate the current-voltage characteristics of resonant tunneling diodes, confirming the method's reliability for realistic quantum device simulations. \end{abstract}

We have investigated the resonant interband tunneling currents (IRIT) in InAs/AlSb/GaSb/AlSb/InAs double-barrier resonant interband tunneling diodes. We have prepared samples with various thicknesses of the GaSb well (Lw) ranging from 5 to 47 monolayers (ML) and various thicknesses of the AlSb barrier (Lb) ranging from 0 to 5 ML, and analyzed the dependence of IRIT on these thicknesses. As Lw increases, IRIT increases if Lw is smaller than a certain boundary value of Lw (Lwb), but IRIT decreases if Lw is larger than Lwb. We have found that this boundary value Lwb depends on Lb, that is, Lwb decreases from 23 to 10 ML when Lb changes from 0.5 to 5 ML. Furthermore, we have also found that, for constant Lw, IRIT increases as Lb increases to a certain boundary value of Lb, and IRIT decreases for further increase in Lb. These marked behaviors of Lwb and IRIT on Lb are characteristic of the type II material systems and cannot be explained by the conventional model applied to type I resonant tunneling diodes. To explain these characteristic behaviors, we propose a model where the resonance level in the valence band of the GaSb well moves up toward the valence-band edge with an increase in Lb.

We present a self-consistent and three-dimensional quantum simulation for nanowire resonant tunneling diodes (RTDs) based on the Wigner transport equation with spatially dependent effective masses (SDEM), which is discretized by a third-order upwind difference scheme for high-accuracy calculation. Our calculation shows that the current density/voltage (J-V ) characteristics of nanowire RTDs with a radius (R) <; 5 nm largely deviate from those of the one-dimensional RTDs. As R decreases below 5 nm, the peak-to-valley ratio (PVR) of the low-V negative differential resistance (NDR) decreases rapidly, and the high-V NDR peak shifts sensitively to lower V and becomes sharper. In this study, these results are shown to be closely related to the SDEM. According to our analysis, the SDEM along the transport direction contributes to the formation of an `effective double barrier' in the energy subbands that is higher than the double barrier expected by the band offset, and the SDEM along the confinement direction reduces the height of the effective double barrier. The performance of nanowire RTDs with R <; 5 nm was proven to be highly dependent on R because of the SDEM along the confinement direction.

Several designs for a high-speed static random access multivalued memory using the folding characteristics of multiple peak resonant tunneling diodes (RTDs) are presented. The different designs are described and studied by comparing their power consumption under different conditions of device parameters and the switching speed. It is shown that the proposed memory cell using a pair of multiple-peak RTDs yields the best result from the standpoint of size, power dissipation, and speed. >

The effect and influence of dry plasma etching processes of Si/SiGe using HBr for the formation of diode mesa structures has been investigated to minimise sidewall leakage current. To characterise sidewall damage electrically, Si-based resonant interband tunnel diodes (RITD) were processed and the completed RITDs compared by their peak-to-valley current ratio (PVCR) and valley current density (VCD), which are sensitive to defect related currents. Dry processed RITDs were compared to reference RITDs fabricated by wet chemical etching (HNO3:HF:H2O=100:1:100). The combination of HBr process gas and very low substrate bias power (10 W) for inductively coupled plasma reactive ion etching (ICP-RIE) yielded the better results. The resulting RITDs processed by ICP-RIE using HBr chemistry show high PVCR of 4.02 with VCD of 32 A/cm2 while wet etched RITDs show a PVCR of only 2.81 with VCD of 40 A/cm2. Hydrogen passivation during the HBr plasma process may play a role that overcomes the slightly higher surface roughness compared to wet etching.