Tunneling In Superlattices Research Articles

Demonstration of T2SLs InAs/InAsSb Based Interband Cascade Detector Supported by Immersion Lens for LWIR

This paper presents the performance of an interband cascade long-wavelength infrared detector designed for high operating temperatures supported by immersion lenses. The device is based on the “Ga-free” InAs/InAsSb type-II superlattice with highly doped p+/n+ superlattice tunneling junctions connecting adjacent stages. Detectivity of the multi-junction heterostructure detector exceeding 1010 cm Hz1/2/W was estimated at wavelength λ ~ 9 µm and T = 210 K and ~ 3 × 108 cm Hz1/2/W for T = 300 K, achieving a tenfold improvement in detectivity in comparison to a device without an immersion lens and 30-fold improvement in detectivity in comparison to the single-stage device.

Modeling of the effects of non-uniform electric field on resonant tunneling in superlattices

Modeling of the effects of non-uniform electric field on resonant tunneling in superlattices

Magnetoconductance modulations due to interlayer tunneling in radial superlattices.

Radial superlattices are nanostructured materials obtained by rolling up thin solid films into spiral-like tubular structures. The formation of these "high-order" superlattices from two-dimensional crystals or ultrathin films is expected to result in a transition of transport characteristics from two-dimensional to one-dimensional. Here, we show that a transport hallmark of radial superlattices is the appearance of magnetoconductance modulations in the presence of externally applied axial magnetic fields. This phenomenon critically relies on electronic interlayer tunneling processes that activate an unconventional Aharonov-Bohm-like effect. Using a combination of density functional theory calculations and low-energy continuum models, we determine the electronic states of a paradigmatic single-material radial superlattice - a two-winding carbon nanoscroll - and indeed show momentum-dependent oscillations of the magnetic states in the axial configuration, which we demonstrate to be entirely due to hopping between the two windings of the spiral-shaped scroll.

Collective excitations of dipolar gases based on local tunneling in superlattices

The collective dynamics of a dipolar fermionic quantum gas confined in a one-dimensional double-well superlattice is explored. The fermionic gas resides in a paramagnetic-like ground state in the weak interaction regime, upon which a new type of collective dynamics is found when applying a local perturbation. This dynamics is composed of the local tunneling of fermions in separate supercells, and is a pure quantum effect, with no classical counterpart. Due to the presence of the dipolar interactions the local tunneling transports through the entire superlattice, giving rise to a collective dynamics. A well-defined momentum-energy dispersion relation is identified in the ab-initio simulations demonstrating the phonon-like behavior. The phonon-like characteristic is also confirmed by an analytical description of the dynamics within a semiclassical picture.

Structurally Driven Enhancement of Resonant Tunneling and Nanomechanical Properties in Diamond-like Carbon Superlattices.

We report nitrogen-induced enhanced electron tunnel transport and improved nanomechanical properties in band gap-modulated nitrogen doped DLC (N-DLC) quantum superlattice (QSL) structures. The electrical characteristics of such superlattice devices revealed negative differential resistance (NDR) behavior. The interpretation of these measurements is supported by 1D tight binding calculations of disordered superlattice structures (chains), which include bond alternation in sp(3)-hybridized regions. Tandem theoretical and experimental analysis shows improved tunnel transport, which can be ascribed to nitrogen-driven structural modification of the N-DLC QSL structures, especially the increased sp(2) clustering that provides additional conduction paths throughout the network. The introduction of nitrogen also improved the nanomechanical properties, resulting in enhanced elastic recovery, hardness, and elastic modulus, which is unusual but is most likely due to the onset of cross-linking of the network. Moreover, the materials' stress of N-DLC QSL structures was reduced with the nitrogen doping. In general, the combination of enhanced electron tunnel transport and nanomechanical properties in N-DLC QSL structures/devices can open a platform for the development of a new class of cost-effective and mechanically robust advanced electronic devices for a wide range of applications.

Tunable wavevector and spin filtering in graphene induced by resonant tunneling

The resonant tunneling in graphene superlattices with an exchange field is studied theoretically. The results show that a resonant tunneling occurs in the transmission gap by virtue of the transverse wave vector. The position, width, and number of resonant tunneling can be effectively manipulated by adjusting the barrier strength, barrier width, and well width, respectively, which indicates the remarkable wavevector filtering behavior. This resonant effect together with the exchange splitting can be utilized to design an efficient spin filter. It is also found that the energy spectrum in the bound region displays bandlike distribution due to the coupling of eigenstates.

Tunneling and current characteristics of two-miniband superlattice

Electron tunneling in semiconductor superlattices with asymmetric unit cells is studied in the two-miniband approximation. It is shown that the probability of interminiband tunneling through a superlattice period is independent of the initial quasi-momentum and can take minimum values in weak electric fields under conditions of the “anticrossing” of Wannier-Stark ladders. In the same fields, currents are at maximum. The dependences of the probability of interminiband tunneling and current on the field direction are predicted. The role of the diagonal elements of the coordinate operator is determined.

Vertical Transport in GaN/AlGaN Resonant Tunneling Diodes and Superlattices

Using the transfer matrix formalism, we have theoretically studied the vertical ballistic transport in GaN/AlGaN resonant tunneling diodes (RTDs) and superlattices with a small number of periods. We have calculated the transmission probability versus the longitudinal electron energy (T–E) and the current density–voltage (J–V) characteristics. Calculations of both T–E and J–V characteristics have been performed for different Al contents in the barriers. The asymmetry effects due to the internal electric field in the barriers are discussed. Applied to the RTD structure, our calculations demonstrate: (i) the increase of the peak-to-valley ratio of the negative differential resistance (NDR) with increasing Al content in the barriers, (ii) the dependence of the J–V resonance values on the current direction, and (iii) the asymmetry of the NDR with respect to the current direction due to the huge internal electric field in the structure. In the case of multiple quantum well structure (MQWS), the calculation results confirm the same trends as in the RTD case when the Al content is varied. In spite of the fact that it is more difficult to analyze the results in the case of MQWS, the obtained calculations demonstrate the applicability of the used model and of the numerical method to study GaN/AlGaN devices based on quantum well (QW) heterostructures. Furthermore, a design of an optimized 7QW structure operating symmetrically whatever the direction of the applied voltage is presented.

Zener tunneling in semiconductor superlattices

Characteristics of miniband tunneling and Wannier–Stark levels in semiconductorsuperlattices are studied as regards their dependence on the symmetry of the unit cells andthe type of miniband structure. We modify the k ⋅ p method into a k ⋅ v form and on this basis generalize the Zener formula for the inter-band tunneling inhomogeneous semiconductors to the case of inter-miniband tunneling in superlattices,account being taken of the inhomogeneity of the electron effective mass. The correspondingsum rule for the effective masses in such structures is obtained. We develop a unifiedmatrix approach for the calculation of the inter-miniband tunneling and Wannier–Starklevels in the case of an arbitrary number of minibands. We study the electric fielddependence of the probability of inter-miniband tunneling for an electron transferredthrough the Brillouin minizone only once. The peculiarities of the inter-minibandtransitions for the case where this transfer is repeated are also examined for various unitcells and miniband structures of the superlattice. In addition, we discuss mechanisms andspecific features of the resonant Zener tunneling and its manifestations in electrontransport.

Interband tunneling and wannier—stark states in superlattices with complex primitive cell

Interband electron tunneling and Wannier-Stark st ates in superlattices with a complex primitive cell were investigated. The method developed in this study allows an arbitrary number of minibands to be considered.

Nonresonant and resonant Zener tunneling

The Zener tunneling rate is derived by quantum-mechanical perturbation theory, using the Kane functions as base functions. This result is able to describe both nonresonant and resonant tunneling and is in line with recent experimental findings on Zener tunneling in superlattices. In contrast, the result published in the literature, which is the state of art up until now, does not adequately describe resonant Zener tunneling and is not an improvement over the semiclassical theory. The reason for that is a misconception about the initial condition.

Highly reliable nitride-based LEDs with SPS+ITO upper contacts

Nitride-based blue light emitting diodes (LEDs) with an n/sup +/-short period superlattice (SPS) tunnel contact layer and an indium tin oxide (ITO) transparent contact were fabricated. Compared with conventional nitride-based LEDs with Ni/Au upper contacts, it was found that we could achieve a 60% increase in electroluminescence (EL) intensity by using ITO upper contacts. However, it was also found that the lifetime of ITO LEDs were much shorter. Furthermore, it was found that we could achieve a longer lifetime and a smaller reverse leakage current (I/sub R/) by the deposition of a SiO/sub 2/ layer on top of the ITO LEDs.

Shot noise in negative-differential-conductance devices

We have compared the shot-noise properties at T=4.2 K of a double-barrier resonant-tunneling diode and a superlattice tunnel diode, both of which exhibit negative differential-conductance (NDC) in their current–voltage characteristics. While the noise spectral density of the former device was greatly enhanced over the Poissonian value of 2eI in the NDC region, that of the latter device remained 2eI. This result implies that charge accumulation, not system instability, is responsible for shot-noise enhancement in NDC devices.

Nitride-based light emitting diodes with Si-doped In/sub 0.23/Ga/sub 0.77/N/GaN short period superlattice tunneling contact layer

The electrical properties of Si-doped n/sup +/-In/sub 0.23/Ga/sub 0.77/N/GaN SPS structure were investigated and compared with conventional Mg-doped GaN contact layer. Temperature dependent Hall measurement showed that such a SPS structure exhibits a high sheet electron concentration. It was found that we could reduce the 20 mA LED forward voltage from 3.78 V to 2.94 V and also reduce the series resistance of the LED from 41 /spl Omega/ to 10 /spl Omega/ by introducing such an n/sup +/-InGaN/GaN SPS top contact. It was also found that we could improve the LED lifetime by such a SPS structure.

Evolution and tunneling time of electron wave packets through a superlattice

We present a formalism to study the evolution of a wave packet inside and outside a scattering superlattice. Time series of specific Gaussian packets, centered at an arbitrary energy ${E}_{o},$ exhibit interesting back-scattering, trapping, and transmission effects. These effects depend on whether the energy ${E}_{o}$ is in a gap, coincides with a resonance, or is an arbitrary point in a band of the superlattice. The time evolution depends strongly on the transmission coefficient and on the superlattice tunneling time. We show that because of the reduced wave-packet distortion and the high intensity difference between the back-scattered and the transmitted wave, for ${E}_{o}$ in a gap, it is convenient to look at the back-scattered wave packet to study the superlattice tunneling time.

Relationship of resonant energies and Bloch wave vectors in resonant tunneling in superlattices

A relationship among the resonance energies in superlattices of different numbers of well and barrier layers has been established. It has been shown that the resonance wave vectors can be related to each other in such systems. The calculated dependence of resonance energies and resonance wave vectors on the mole fraction, and of the thickness of barrier layers of superlattices, throws new light on the quantum effects of electron transport through the superlattices.

Zener tunneling in superlattices in a magnetic field

We present a study of the Zener effect in the optical absorption of strongly coupled superlattices with both a magnetic and an electric field in growth direction. The in-plane continuum of electron states is discretized due to Landau quantization, which allows to directly observe the transition from discrete to continuum states due to Zener tunneling in a true 1D system.

Tunneling-induced global population inversion in biased multiband superlattices

The redistribution of carriers due to resonant tunneling in superlattices with three subbands is studied within the density-matrix approach. It is predicted that a global intrinsic population inversion may appear due to resonant tunneling into a third, wide subband, which is quickly depopulated. An analytical criterion is derived and discussed for the occurrence of such an inversion.

Field-dependent tunneling and population inversion in multiband semiconductor superlattices

The quasiclassical description of high-field effects in one-band superlattices is extended to multiband systems. Field-dependent tunneling is taken into account within a density-matrix approach. Analytical results are derived for the field dependence of the carrier redistribution in two- and three-band semiconductor superlattices. Some implications for the design of unipolar mid-infrared lasers are discussed.

Dynamics of electric field domain walls in semiconductor superlattices

A self-consistent microscopic model of sequential tunneling in superlattices is employed to investigate self-sustained current oscillations. Current spikes – high-frequency modulation due to well-to-well hopping of charge monopole domain walls – are naturally reproduced. Moreover, as the contact doping shrinks, the recycling and motion of charge dipoles resulting in a lower-frequency oscillatory mode is predicted. For low contact doping, this mode dominates and monopole oscillations disappear. At intermediate doping, hysteresis between both the oscillatory modes may be possible.