Asymmetric Resonant Tunneling Research Articles

Modeling of the tandem III–V dilute nitride bulk or quantum engineered/silicon solar cells

Tandem silicon solar cells are designed in order to lower the levelized cost of solar electricity by increasing the cell’s efficiency to more than 30%. Dual junction thin film III–V/Si cell efficiencies are limited to 20% because of high dislocation densities (>106 cm−2) due to lattice mismatch between them. GaAsyP1−x−yNx has a lattice constant matched with silicon at y = 4.7*x − 0.1 and has an optimal bandgap i.e. between 1.7 and 1.9 eV for tandem III–V’s with Si. This work designs a p-i-n GaAsPN/GaP symmetric or asymmetric (resonant thermos tunneling) multi-quantum well (MQW) device for a top sub-cell of a dual junction silicon tandem to minimize the effect of the degraded minority carrier transport properties of bulk dilute nitride layers and presents the simulated solar cell properties using a realistic drift-diffusion approach. Using extracted experimental spectral responses as well as other solar cell parameters of various silicon solar cell like: HIT, PERC, IBC etc, the tandem efficiencies of designed p-i-n MQWs devices in conjunction with silicon solar cells have been evaluated. The tandem efficiency of an optimized p-i-n MQWs device under AM 1.5 G spectrum can exceed 33% when in conjunction with a 25.6% efficient HIT silicon solar cell.

Repeatable asymmetric resonant tunneling in AlGaN/GaN double barrier structures grown on sapphire

We report repeatable AlGaN/GaN resonant tunneling diodes (RTDs) grown on a sapphire substrate by metal organic chemical vapor deposition. The RTDs exhibit clear negative differential resistance at low temperature in both bias directions. Peak-to-valley current ratios of 1.4 and 1.08 and peak current densities of 6 kA/cm2 and 0.65 kA/cm2 are extracted at 6.5 K for forward and reverse bias, respectively. The polarization fields in III-nitrides are found to affect the diode electrical behaviors by modulating the symmetry of the two barriers and altering the thickness of the depletion region, which eventually triggers asymmetric resonant tunneling transport.

Insights on energy selective contacts for thermal energy harvesting using double resonant tunneling contacts and numerical modeling

Energy selective electrical contacts have been proposed as a way to approach ultimate efficiencies both for thermoelectric and photovoltaic devices as they allow a reduction of the entropy production during the energy conversion process. A self-consistent numerical model based on the transfer matrix approach in the effective mass and envelope function approximation has been developed to calculate the electronic properties of double resonant tunneling barriers used as energy selective contacts in hot carrier solar cells. It is found that the application of an external electric bias significantly degrades the electronic transmission of the structure, and thus the tunneling current in the current-voltage characteristic. This is due to a symmetry breaking which can be offset using finely tuned asymmetric double resonant tunneling barriers, leading to a full recovery of the tunneling current in our model. Moreover, we model the heterostructure using electrons temperature in the emitter higher than that of the lattice, providing insights on the interpretation of experimental devices functioning in hot carrier conditions, especially regarding the previously reported shift of the resonance peak (negative differential resistance), which we interpret as related to a shift in the hot electron distribution while the maximum remains at the conduction band edge of the emitter. Finally, experimental results are presented using asymmetric structure showing significantly improved resonant properties at room temperature with very sharp negative differential resistance.

Improving the performance of a far-infrared quantum-ring-based photodetector utilizing asymmetric multi-barrier resonant tunneling

In this paper, a novel structure for quantum ring inter-subband photodetectors (QRIP) is proposed to reduce its dark current. Some additional layers including asymmetric multi-barrier resonant tunneling (AMBRT) in absorption region layers are exploited to provide near unity tunneling probability for generated photocurrents and completely reject thermally generated electrons. AMBRT structure consists of three asymmetric AlGaAs barriers and two InGaAs wells which are designed for operation wavelength of generated photocurrents by absorption of 20μm. Simulation results show that AMBRT can considerably reduce the dark current compared to previously proposed resonant tunneling structure about three orders of magnitude. As a consequent, higher specific detectivity for AMBRT-QRIP is obtained in the order of ∼1011cmHz1/2/W at 100K.

Tuning of terahertz intrinsic oscillations in asymmetric triple-barrier resonant tunneling diodes

Intrinsic terahertz oscillations of the electronic current in the asymmetric triple-barrier resonant tunneling diode are investigated by means of the time-dependent Wigner-Poisson method. The current-voltage characteristics calculated for the nanodevice exhibits four separate bias voltage windows with the current oscillations, which are caused by two different mechanisms. One of these bias voltage windows, for which the electronic current oscillations stem from the negative feedback between the electronic current and the coupled quasi-bound states in the quantum wells embedded in the active region of the nanodevice, is considered in detail. It is demonstrated that the amplitude and frequency of the current oscillations in this bias voltage window depend on the coupling between the quasi-bound states formed in the quantum wells. Strength of this coupling is controlled by the thickness of the central barrier separating the quantum wells, which allows to tune the amplitude and frequency of the terahertz oscillations. Additionally, it was shown that the amplitude of the current oscillations can be tuned by shifting up or down the energy of the bottom of the wider quantum well. These properties suggest that the considered nanodevice may be of interest because of its possible application as tunable terahertz generator.

Optical and structural studies of dual wavelength InGaN/GaN tunnel-injection light emitting diodes grown by metalorganic chemical vapor deposition

InGaN/GaN multiple quantum well light emitting diodes with charge asymmetric resonance tunneling structure, which can emit dual color lights of blue/green or blue/yellow, were grown on sapphire by metalorganic chemical vapor deposition. Their optical and structural properties are studied by high-resolution X-ray diffraction, high-resolution transmission electron microscopy, temperature-dependent photoluminescence (PL) and photoluminescence excitation (PLE). PL peak shifts and PLE features are found to vary with the well-growth temperature. The luminescence mechanism is discussed in details, correlating to the In composition fluctuation, carrier localization and the quantum confined Stark effect.

Intrinsic current oscillations in an asymmetric triple-barrier resonant tunnelling diode

The electronic transport characteristics of an asymmetric triple-barrier resonant tunnelling diode are calculated by the time-dependent Wigner–Poisson method. The intrinsic current oscillations are found in two separate bias voltage ranges. The first one is located below the resonant current peak, and the second lies in the negative differential resistance region. We provide the explanation of the current density oscillations in these two separate bias voltage ranges based on the analysis of the self-consistent potential profiles and changes of electron density. We have shown that two different formation mechanisms are responsible for the current density oscillations in these two bias voltage ranges. In the bias voltage range below the resonant current peak in the current–voltage characteristics, the current density oscillations are caused by the coupling between quasi-bound states in the left and right quantum wells. On the other hand, the current density oscillations in the negative differential resistance region result from the coupling between quasi-bound states in the left quantum well and the quantum well formed in the region of the left contact.

Terahertz generation based on an optically pumped ballistic electron wave swing device

Terahertz (THz) signals can be generated by the periodic motion of ballistic electron bunches inside a quantum well cavity surrounded by two asymmetric resonant tunneling structure implemented in an InAlAs/InGaAs heterostructure. The electron bunches are produced in an adjacent active quantum well by optical excitation with a femtosecond laser pulse, and transferred into the cavity by resonant tunneling. The device contains also a superlattice that eliminates the electrons, which exit the cavity in the direction of the active quantum well. In consequence, the device output consists of a train of ultrashort pulses, of less than 100 fs, which preserve their coherence. Electrical signals up to few tens of THz can be obtained, depending on the resonant filter or antenna placed at the end of the device.

Light controlled spin polarization in asymmetric n-type resonant tunneling diode

The authors have observed a strong dependence of the circular polarization degree from the quantum well emission in an asymmetric n-type GaAs∕AlAs∕AlGaAs resonant tunneling diode on both the laser excitation intensity and the applied bias voltage. The sign of the circular polarization can be reversed by increasing the light excitation intensity when the structure is biased with voltages slightly larger than the first electron resonance. The variation of polarization is associated with a large density of photogenerated holes accumulated in the quantum well, which is enhanced due to the asymmetry of the structure.

Charge buildup effects in asymmetric p-type resonant tunneling diodes

We have investigated p-doped GaAs-AlAs resonant tunneling devices with asymmetric barriers under optical excitation. Transport and photoluminescence measurements were performed under identical bias conditions as a function of the light excitation intensity. We have observed the development of additional peaks, induced by illumination, between the main light- and heavy-hole resonances in the current-voltage characteristics (I(V)). We describe the behavior of these photo-induced peaks under a magnetic field parallel to the current. We propose that the observed properties are related to resonant tunneling of photoinduced electrons and associated excitonic effects.

Experimental demonstration of split side-gated resonant interband tunneling devices

We report a prototype side-gated asymmetric resonant interband tunneling device (RITD) fabricated with an AlSb∕InAs∕GaSb∕AlSb heterostructure for Rashba spin filter applications. This device features independent control gates along the sides of an RITD mesa structure that can be used to provide an electric field orthogonal to the resonant tunneling current. The distribution of lateral wave vectors for electrons approaching the RITD active layers obtains a nonzero average through the application of the orthogonal electric field, a requirement for Rashba spin filter operation. Measured current–voltage curves show a modulation of the tunneling current with a side gate bias, demonstrating lateral electric-field projection into the resonant tunneling mesa structure. The prototype device was fabricated to submicron dimensions utilizing a conformal and scalable processing scheme.

Rashba effect resonant tunneling spin filters

We propose an InAs/GaSb/AlSb-based asymmetric resonant interband tunneling diode (a-RITD) as a spin filter. The device exploits the Rashba effect to achieve spin polarization under zero magnetic field using nonmagnetic III-V semiconductor heterostructures. We discuss the basic principles of the interband tunneling spin filter, and present modeling results that demonstrate its advantage. We also propose an implementation procedure for realizing device structure.

Resonant interband tunneling spin filter

We propose an InAs/GaSb/AlSb-based asymmetric resonant interband tunneling diode as a spin filter. The interband design exploits large valence band spin–orbit interaction to provide strong spin selectivity, without suffering from fast hole spin relaxation. Spin filtering efficiency is also enhanced by the reduction of tunneling through quasibound states near the zone center, where spin spitting vanishes and spin selectivity is difficult. Our calculations show that, when coupled with an emitter or collector capable of lateral momentum selectivity, the asymmetric resonant interband tunneling diode can achieve significant spin filtering in conventional nonmagnetic semiconductor heterostructures under zero magnetic field.

InGaN/GaN tunnel-injection blue light-emitting diodes

A charge asymmetric resonance tunneling (CART) structure was applied to nitride-based blue light emitting diodes (LEDs) to enhance their output efficiency. It was found that with a 20-nm-thick In/sub 0.18/Ga/sub 0.82/N electron emitter layer, we could increase the LED output intensity from 28.3 minicandela (mcd) to 43.2 mcd (i.e., a 53% increase). However, a further increase in electron emitter layer thickness will reduce the intensity due to relaxation. It was also found that we could decrease the 20 mA forward voltage from 4.16 V to 3.58 V with a proper electron emitter layer.

High brightness green light emitting diodes with charge asymmetric resonance tunneling structure

In this work, we have applied the so called charge asymmetric resonance tunneling (CART) structure to nitride-based green light emitting diode (LED). From our CART LED, we observed an abrupt turn-on voltage near 2.2 V, and the forward voltage is around 3.2 V at 20 mA injection current. At 20 mA, the output power, and external quantum efficiency of the CART LED are about 4 mW, and 6.25%, respectively. The high brightness and efficiency green LED can be obtained by using the CART structure.

Modeling spin-dependent transport in InAs/GaSb/AISb resonant tunneling structures

The Rashba effect resonant tunneling diode is a candidate for achieving spin polarizing under zero magnetic field using only conventional non-magnetic III–V semiconductor heterostructures. We point out the challenges involved based on simple arguments, and offer strategies for overcoming these difficulties. We present modeling results that demonstrate the benefits of the InAs/GaSb/AlSb-based asymmetric resonant interband tunneling diode (a-RITD) for spin filtering applications.

High-efficiency InGaN-GaN MQW green light-emitting diodes with CART and DBR structures

Distributed Bragg reflector (DBR) and charge asymmetric resonance tunneling (CART) structures were applied to nitride-based green light-emitting diodes (LEDs) to enhance their output efficiency It was found that we can reduce the forward voltage at 20 mA from 3.7 to 3.2 V with the inclusion of CART structure. It was also found that the electroluminescence peak wavelength of the CART LED is less sensitive to the amount of injection current. The output power and external quantum efficiency of the CART LED with DBR structure measured at 20 mA can reach 7.2 mW and 11.25%, respectively.

Light Emitting Diode with Charge Asymmetric Resonance Tunneling

We suggest a system of two wells connected with Charge Asymmetric Resonance Tunneling (CART) as a basic element of light emitting diode (LED) structure for semiconductors with different masses of electrons and holes. The system consists of an emitter of electrons, an emitter of holes and an active layer. The hole emitter is coupled with the active in such a way that holes can be freely supplied into the active layer without a barrier. The electron emitter is coupled to the active layer via a barrier. The barrier design uses the charge asymmetric resonance tunneling phenomenon which allows to make the barrier transparent for electrons and blocking for holes. Advantages of this design are: the increased capture efficiency of the electrons into the active layer due to direct resonance tunneling of the electrons from the electron emitter on bound electron level in the active quantum well, the suppression of electron leakage into the hole emitter, the elimination of the parasitic light generated outside the active layer, and the electron emitter acts also as a good current spreading layer. First results of experimental investigation and theoretical modeling of the CART LED devices are reported.

Spin-polarized electronic current in resonant tunneling heterostructures

The spin-dependent tunneling phenomenon in symmetric and asymmetric resonant semiconductor heterostructures is employed in a theoretical study to investigate the output tunnel current polarization at zero magnetic field. A simple model of the resonant tunneling structures and a simple one-electron band approximation with spin-orbit interaction are used in this work. It is shown that asymmetry in the electron distribution at the electrode regions provides spin-polarized tunnel current. An approach to optimize this spin-dependent effect is explored theoretically. In asymmetric resonant tunneling structures, we estimate theoretically that the polarization can reach 40% with a moderate applied electric field.

Magnetic-field-induced bistability in resonant tunneling

We report an unusual magnetic-field-induced bistability in the current-voltage characteristic of an asymmetric double-barrier resonant tunneling structure. It is suggested that this bistability is the experimental manifestation of self-sustained current oscillations that have recently been predicted by Orellana, Anda, and Claro [Phys. Rev. Lett. 79, 1118 (1997)].