Resonant Tunneling Diode Structure Research Articles

Monolithic Integration of Resonant Tunneling Diodes, Schottky Barrier Diodes and 0.1-µm-gate High Electron Mobility Transistors for High-Speed ICs

Monolithic integration technology for resonant tunneling diodes (RTDs), Schottky barrier diodes (SBDs), and 0.1-µm-gate high electron mobility transistors (HEMTs) was developed. The integration process was investigated to overcome the difficulty in fabricating both RTDs with a mesa height of 530 nm and HEMTs with a gate length of 0.1 µm. The HEMT and SBD structures were grown by metalorganic chemical vapor deposition (MOCVD) on a 2-inch InP substrate. The RTD structure was regrown by molecular beam epitaxy (MBE) on the top layer grown by MOCVD. The characteristics of the fabricated devices showed good uniformity. An analog-to-digital converter and a selector circuit fabricated with these devices operated properly. The presented integration technology is a promising way to provide novel circuits with RTDs, SBDs and 0.1-µm-gate HEMTs.

An Optoelectronic Logic Gate Monolithically Integrating Resonant Tunneling Diodes and a Uni-Traveling-Carrier Photodiode

InP-based InGaAs/AlAs/InAs resonant tunneling diodes (RTDs) and a uni-traveling-carrier photodiode (UTC-PD) are monolithically integrated to construct an optoelectronic logic gate. RTD structures are regrown by molecular beam epitaxy on the top of UTC-PD structures grown by metalorganic chemical vapor deposition. The characteristics of the regrown RTDs are almost the same as those of conventional RTDs directly grown on semi-insulating InP substrates. The UTC-PD provides a sufficient current drivability along with a high-speed operation demonstrated by the 3-dB bandwidth of 80 GHz, even at the low bias voltage corresponding to the peak voltage of the RTD. An optoelectronic logic gate using two RTDs and one UTC-PD was fabricated. This simple optoelectronic logic gate exhibited high-speed delayed flip-flop operation of 40 Gbit/s at a small power consumption of 7.75 mW. These results suggest that an optoelectronic logic gate using RTDs and a UTC-PD is suitable for the construction of ultrahigh-speed and low-power optoelectronic circuits.

Planar integration of a resonant-tunneling diode with pHEMT using a novel proton implantation technique

A novel technique of integrating resonant-tunneling diodes (RTDs) with pseudomorphic high-electron-mobility transistors (pHEMTs) is demonstrated. A proton was implanted through the pHEMT layers to convert the RTD structure underneath to a high-resistivity buffer without degrading the performance of the pHEMT. The cutoff frequency is 16 GHz for a 1.5-/spl mu/m-gate-length pHEMT on such an implanted buffer. Substituting the conventional deep mesa etch with ion implantation maintains a highly planar surface. Such a monolithically integrated RTD/pHEMT oscillator is described.

Monolithic integration of resonant tunneling diodes and heterojunction bipolar transistors on patterned InP substrates

We report the successful fabrication of coplanar, monolithic integrated circuits based on heterojunction bipolar transistors (HBTs) and resonant tunneling diodes (RTDs). Molecular beam epitaxy growth of the integrated device structure is performed on an InP wafer, which is patterned by photolithography and dry etching prior to growth. The substrate pattern consists of pedestals, which support RTDs after postgrowth device fabrication, and a lower level which ultimately supports HBTs. In situ sensors for layer thickness (photoemission oscillations) and substrate temperature (absorption-edge spectroscopy) play a critical role in controlling key growth and structural parameters for the HBT and RTD devices. Utilization of the photoemission oscillation sensor during deposition of the AlAs barrier layers of the RTD structure has enabled control of peak currents to a design tolerance of ±20% for devices with current densities in the mid-104 A/cm2 range. Implementation of proportional–integral–derivative control based on a transmission-mode absorption-edge spectroscopy sensor has enabled regulation of the substrate temperature to a precision of ±1 °C throughout deposition of the RTD/HBT structure.

Self‐Consistent Scattering Calculation of Resonant Tunneling Diode Characteristics

We perform a self‐consistent calculation of resonant tunneling diode (RTD) I‐V characteristics including optical phonon scattering. The self‐consistency is obtained by solving the Schrödinger equation and Poisson’s equation iteratively with the Thomas‐Fermi approximation used for the device contact regions. For evaluation of phonon‐assisted current density, the optical phonon scattering in the quantum well is modeled using the optical model potential. Electron transverse momentum is also incorporated. The calculated current and electron wavefunction illustrate the optical model and effects of the phonon scattering on the current transport. The I‐V characteristics we obtain from the model calculation are in good agreement with experimental results. This work manifests the importance of including self‐consistency, optical phonon scattering, and electron transverse momentum in modeling realistic RTD structures.

Bistability of Resonant Tunnel Diode Structure with InAs Quantum Dots

We have experimentally demonstrated the extrinsic and intrinsic bistability in resonant tunnel diode (RTD) structures with InAs dots. We have clarified the operating mechanism of the extrinsic bistability in double barrier resonant tunnel diodes (DBRTDs) with InAs dots and pointed out that the relatively long hot electron charging time (more than 20 ms) must be overcome in order to apply this phenomenon to high speed switching two-terminal devices. We have also demonstrated the intrinsic bistability in electron tunneling through a single dot for the first time and examined its physical mechanism.

Tunneling of electrons in conventional and half-metallic systems: Towards very large magnetoresistance

The tunnel magnetoresistance (TMR) is analyzed for ferromagnet-insulator-ferromagnet junctions, including half-metallic systems. Direct tunneling in a corrected standard model is compared with impurity-assisted and resonant TMR. Direct tunneling in iron group systems leads to about a 20% change in resistance, as observed experimentally. Impurity-assisted tunneling decreases the TMR down to 4% with Fe-based electrodes, and spin-flip scattering from defect states will further reduce this value. A resonant tunneling diode structure would give a TMR of about 8%. The model applies qualitatively to half-metallics with 100% spin polarization, where the change in resistance in the absence of spin-flips may be arbitrarily large. Even in the case of imperfect magnetic configurations the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO${}_{2}$/TiO${}_{2}$ and CrO${}_{2}$/RuO${}_{2}$, and a brief account of their peculiar band structures is presented.

High peak-current-density strained-layer In0.3Ga0.7As/Al0.8Ga0.2As resonant tunneling diodes grown by metal-organic chemical vapor deposition

Peak current densities two times higher than the best values reported for GaAs-based resonant tunneling diode (RTD) structures have been obtained from metal-organic chemical-vapor deposition (MOCVD)-grown deep-quantum-well strained-layer In0.3Ga0.7As/Al0.8Ga0.2As RTDs. By growing on nominally exact (100) +/−0.1° GaAs substrates, we have been able to obtain smooth interfaces between the strained-layer In0.3Ga0.7As quantum well and Al0.8Ga0.2As barriers, which, in turn, enabled us to benefit from resonant tunneling through the second resonant energy level of In0.3Ga0.7As/Al0.8Ga0.2As structures. Peak current densities in excess of 300 kA/cm2, and peak-to-valley current ratios as high as 3:1, at 300 K, have been obtained from structures with 14-Å-thick barriers and a 57-Å-thick well.

Modeling of three-peak current–voltage characteristics with two resonant tunneling diodes connected in series

The conditions for three-peak current–voltage (I–V) characteristics with two discrete resonant tunneling diodes (RTDs) connected in series are discussed and analyzed. With suitable design, one can obtain three-peak I–V characteristics with two series-connected RTDs, and some equations are derived to help the design. The effect of parasitic resistance on the I–V characteristics is investigated. A large-signal model of the RTD is developed by using the piecewise-linear approximation technique, and this model can be easily implemented in PSpice. The simulated I–V curve of the series circuit is in good agreement with the measured results. The results in this paper can be applied to the design of multipeak I–V curve with less RTD structure in comparison with the traditional device that stacks the same RTDs to obtain a multipeak I–V curve.

Nanoelectronic architecture for Boolean logic

A nanoelectronic implementation of Boolean logic circuits is described where logic functionality is realized through charge interactions between metallic dots self-assembled on the surface of a double-barrier resonant tunneling diode (RTD) structure. The primitive computational cell in this architecture consists of a number of dots with nearest neighbor (resistive) interconnections. Specific logic functionality is provided by appropriate rectifying connections between cells. We show how basic logic gates, leading to combinational and sequential circuits, can be realized in this architecture. Additionally, architectural issues including directionality, fault tolerance, and power dissipation are discussed. Estimates based on the current-voltage characteristics of RTD's and the capacitance and resistance values of the interdot connections indicate that static power dissipation as small as 0.1 nW/gate and switching delay as small as a few picoseconds can be expected. We also present a strategy for fabricating/synthesizing such systems using chemical self-organizing/self-assembly phenomena. The proposed synthesis procedure utilizes several chemical self-assembly techniques which have been demonstrated recently, including self-assembly of uniform arrays of close-packed metallic dots with nanometer diameters, controlled resistive linking of nearest neighbor dots with conjugated organic molecules and organic rectifiers.

The possibility of a very large magnetoresistance in half-metallic oxide systems

The tunnel magnetoresistance (TMR) is analyzed for ferromagnet-insulator-ferromagnet junctions, including novel half-metallic systems with 100% spin polarization. Direct tunneling is compared with the impurity-assisted and resonant TMR. Direct tunneling in iron-group systems leads to about a 20% change in resistance, as observed experimentally. Impurity-assisted tunneling decreases the TMR to 4% with Fe-based electrodes. A resonant tunnel diode structure would give a TMR of about 8%. The model applies qualitatively to half-metallics, where the change in resistance in the absence of spin flips may be arbitrarily large and even in the case of imperfect magnetic configurations the resistance change can be several thousand percent. Examples of half-metallic ferromagnetic systems are CrO2/TiO2 and CrO2/RuO2. A discussion of their properties is presented.

Determination of interface composition in III-V heterojunction devices (HBT and RTD) with atomic resolution using STEM techniques

In this work, we will present the determination of chemical composition across heterointerfaces in the layer stacks of devices such as heterojunction bipolar transistors (HBT) and resonant tunneling diodes (RTD) by scanning transmission electron microscopy (STEM). We used STEM-based atomic number ( Z) contrast imaging in order to analyze interface abruptness in HBT and RTD structures qualitatively with monolayer spatial resolution. Electron energy-loss spectroscopy (EELS) was used for the quantitative chemical analysis of individual ultrathin layers such as RTD barriers. The obtained structural data can be used to improve and refine device modeling and design, e.g., composition profiles can be converted into band structures or barrier widths and exact barrier shapes in RTDs can be measured directly. Examples for correlations between structural data and device properties will be given for InP-based HBT and RTD structures.

Spin Tunneling in Conducting Oxides

ABSTRACTDifferent tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Direct tunneling is compared with impurity-assisted, surface state assisted, and inelastic contributions to a tunneling magnetoresistance (TMR). Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close to experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.

Magnetism and Spin Tunneling in Nanostructures

ABSTRACTIn the present paper different tunneling mechanisms in conventional and half-metallic ferromagnetic tunnel junctions are analyzed within the same general method. Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR.The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials, which are nearly half-metallic, are discussed.

Thin film pseudomorphic AlAs/In/sub 0.53/Ga/sub 0.47/As/InAs resonant tunnelling diodes integrated onto Si substrates

We report high peak-to-valley current ratio (PVR) resonant tunneling diodes (RTDs) bonded to silicon. Pseudomorphic AlAs/In/sub 0.53/Ga/sub 0.47/As/InAs resonant tunneling diode structures grown on semi-insulating InP with peak-to-valley current ratios as high as 30 at 300 K have been separated from the growth substrate and bonded to silicon substrates coated with Si/sub 3/N/sub 4/, forming thin film devices. In addition, thin film multiple stack RTD structures have been bonded to silicon substrates. The I-V characteristics of both the single and multi-stacked thin film RTD's exhibit no signs of degradation after bonding to the host substrate. These results are the first successful demonstration of InP based electronics bonded to a silicon host substrate and enable the integration of RTDs with conventional silicon circuitry.

Co-integration of high speed InP-based HBTs and RTDs using chemical beam epitaxy

CBE is used to co-integrate InP-based heterojunction bipolar transistors (HBTs) and resonant tunneling diodes (RTDs) for the first time to reduce the complexity of conventional digital circuit technology. The integrated transistors have maximum dc and differential current gain of ∼ 53.6 and 122, respectively, with breakdown voltages V ceo ≈ 6 V and V cbo ≈ 9 V. For microwave performance, the maximum f T and f max are ∼ 77 (63) and ∼ 60 (53) GHz after (before) the onset of negative differential resistance. For digital circuit application, the HBT + RTD structure was used to build bistable mode low power monolithic integrated circuits, including a NAND gate, an inverted majority gate and a NOR gate.

A transfer length model for contact resistance of two-layer systems with arbitrary interlayer coupling under the contacts

An improved analytical solution to the two-layer transmission line model for determining contact resistances to semiconductor layers is presented. In contrast to previously published two-layered analyses, the present solution is valid for arbitrary strength linear coupling between the two conducting layers in the region under the contact. A comparison of limiting cases and a physical interpretation of the differences between predictions of this model and previous models are presented. The predicted resistance versus pad separation behaviour obtained from the present model and from a two-layer model which assumed weak interlayer coupling beneath the contacts are compared for identical physical structures. Experimental data for contact resistance measurements on a double-barrier resonant tunneling diode structure are presented. The measured data exhibit the nonlinear resistance versus pad spacing predicted by the model and show a dependence on current level which can be modelled as a variation of the interlayer coupling strength. The values of the contact resistance, interlayer coupling resistance and sheet resistance extracted from the measured data using the present model and the weakly coupled model are presented and compared with independently measured and calculated parameters.

InAs/AlSb/GaSb resonant interband tunneling diodes and Au-on-InAs/AlSb-superlattice Schottky diodes for logic circuits

Integrated resonant interband tunneling (RIT) and Schottky diode structures, based on the InAs/GaSb/AlSb heterostructure system, are demonstrated for the first time. The RIT diodes are advantageous for logic circuits due to the relatively low bias voltages (/spl sim/100 mV) required to attain peak current densities in the mid-10/sup 4/ A/cm/sup 2/ range. The use of n-type InAs/AlSb superlattices for the semiconducting side of Schottky barrier devices provides a means for tailoring the barrier height for a given circuit architecture. The monolithically integrated RIT/Schottky structure is suitable for fabrication of a complete diode logic family (AND, OR, XOR, INV).

Electron transfer between two coupled quantum wells in a resonant tunneling diode structure

Resonant tunneling effects between the quantum well of a double barrier heterostructure and the accumulation layer of the emitter region are investigated. Numerous singularities in the current-voltage characteristic of a GaAs Al 0.4Ga 0.6As diode, such as a negative differential conductance effect with a current contrast of 15:1, have been found and analyzed by means of self-consistent calculations of conduction band profile and tunneling probabilities. At relatively low bias, charge transfer can be interpreted as direct or scattering-assisted transitions with optical phonon emission, by considering the anti-crossing of the ground states or the energy conservation rules. At higher voltages, very fast tunneling processes at the time scale of relaxation mechanisms (through the second level of the accumulation layer) were demonstrated. Such a hot electron transition with direct evidence of negative differential conductance effect involves a non expected build-up of the resonance as a function of the symmetry of confining potentials in terms of transmission probabilities.

Intrinsic high-frequency characteristics of tunneling heterostructure devices.

A general numerical method has been developed to solve the periodic time-dependent Schr\odinger equation under a weak ac field where the quantum transmitting boundary method is employed to formulate boundary conditions of far-from-equilibrium open systems. Also derived are current components for ac small-signal analysis. We apply the method to a resonant tunneling diode (RTD) structure. Our calculations illustrate that the assumption of Lorentzian-like form of line shapes of the current functions is no longer valid at high frequencies. Thus a careful treatment to these integral functions is fundamental to obtain a physically reasonable result. Results of linear admittance, rectification coefficient, and second-harmonic generation coefficient are presented as a function of frequency and bias, at both positive differential resistance and negative-differential resistance (NDR) region. The calculations have shown that at high frequencies (several THz), the reactive feature of RTD, whether inductive or capacitive, depends on the bias and frequency. The capacitive feature, i.e., the positive imaginary part of the admittance, reaches maximum in the middle of the NDR region. This behavior can be ascribed to the confined electrons in the well. The characteristic of the transition from electron to optical behavior is observed when the frequency increases. The rectification coefficient and second-harmonic generation coefficient show a resonant enhancement at high frequencies. A comparison with the results obtained by the Wigner function is demonstrated. Different definitions of the ac reactive current component are discussed in order to clarify the confusion in the literature.