Sequential Resonant Tunneling Research Articles

Nonlinear Charge Transport and Excitable Phenomena in Semiconductor Superlattices.

Semiconductor superlattices are periodic nanostructures consisting of epitaxially grown quantum wells and barriers. For thick barriers, the quantum wells are weakly coupled and the main transport mechanism is a sequential resonant tunneling of electrons between wells. We review quantum transport in these materials, and the rate equations for electron densities, currents, and the self-consistent electric potential or field. Depending on superlattice configuration, doping density, temperature, voltage bias, and other parameters, superlattices behave as excitable systems, and can respond to abrupt dc bias changes by large transients involving charge density waves before arriving at a stable stationary state. For other parameters, the superlattices may have self-sustained oscillations of the current through them. These oscillations are due to repeated triggering and recycling of charge density waves, and can be periodic in time, quasiperiodic, and chaotic. Modifying the superlattice configuration, it is possible to attain robust chaos due to wave dynamics. External noise of appropriate strength can generate time-periodic current oscillations when the superlattice is in a stable stationary state without noise, which is called the coherence resonance. In turn, these oscillations can resonate with a periodic signal in the presence of sufficient noise, thereby displaying a stochastic resonance. These properties can be exploited to design and build many devices. Here, we describe detectors of weak signals by using coherence and stochastic resonance and fast generators of true random sequences useful for safe communications and storage.

Optical gain reduction caused by nonrelevant subbands in narrow-period terahertz quantum cascade laser designs

The recent designs of terahertz quantum cascade lasers usually employ the short periodic length and also the tall barriers for high-temperature operation. In this work, the effect of high-energy lying non-relevant subbands is studied based on nonequilibrium Green’s function formalisms model, demonstrating those subbands are probable to play a minor role on the population inversion, but play a major role on the optical gain at high temperatures. The phenomenon can be ascribed to the appearance of leakages crossing neighboring periods via sequential resonant tunneling, and those leakages are inherently created by the specific features of the two-well configuration in this design that the phonon well should be wide enough for performing the phonon scattering to depopulate the lower-laser subband. The narrower periodic length design can strengthen this inter-period leakage. A parasitic absorption between the first high-lying nonrelevant subbands from two laser wells can closely overlap the gain shape and thus significantly reduce the peak gain.

Transport Properties of Se/As2Se3 Nanolayer Superlattice Fabricated Using Rotational Evaporation

AbstractSince their proposal by Esaki, superlattices have been observed to have fascinating features such as quantum size effects, negative differential resistance, and sequential resonant tunneling. However, the technology threshold for fabricating superlattices is high, requiring methods like molecular beam epitaxy (MBE) and atomic layer deposition (ALD), among others, even for amorphous materials. Thus, the desirable features from superlattices have not been extensively utilized. It is shown that superlattices of Se and As2Se3 (superlattice‐Se), fabricated using rotational evaporation, exhibit sequential tunneling, a typical superlattice feature. From current–voltage measurements of the superlattice deposited on n‐type Si, oscillations in the characteristics are observed. Using models of reverse‐biased Schottky barriers, the observations are explained as tunneling in sequence from superlattice minibands. The superlattice‐Se also shows carrier blocking, with a resistivity of the order of 1012 Ω cm in dark conditions at room temperature, despite the low resistivity (≈10 Ω cm) of the n‐type Si substrate. When the Si is illuminated, the device shows higher detectivity for weaker signals compared to higher illumination. The ease of fabrication, and the blocking and amplifying capabilities make superlattice‐Se an interesting “add‐on” structure to improve conventional photodetector materials such as Si or Ge, which have issues with dark currents at room temperature.

Fast Detection of a Weak Signal by a Stochastic Resonance Induced by a Coherence Resonance in an Excitable GaAs/Al_{0.45}Ga_{0.55}As Superlattice.

The effect of a coherence resonance is observed experimentally in a GaAs/Al_{0.45}Ga_{0.55}As superlattice under dc bias at room temperature, which is driven by noise. For an applied voltage, for which no current self-oscillations are observed, regular current self-oscillations with a frequency of about 82MHz are induced by exceeding a certain noise amplitude. In addition, a novel kind of a stochastic resonance is identified, which is triggered by the coherence resonance. This stochastic resonance appears when the device is driven by an external ac signal with a frequency, which is relatively close to that of the regular current self-oscillations at the coherence resonance. The intrinsic oscillation mode in the coherence resonance is found to be phase locked by an extremely weak ac signal. It is demonstrated that an excitable superlattice device can be used for the fast detection of weak signals submerged in noise. These results are very well reproduced by results using numerical simulations based on a sequential resonant tunneling model of nonlinear electron transport in semiconductor superlattices.

Parameter dependence of high-frequency nonlinear oscillations and intrinsic chaos in short GaAs/(Al, Ga)As superlattices.

We explore the design parameter space of short (5-25 period), n-doped, Ga/(Al,Ga)As semiconductor superlattices (SSLs) in the sequential resonant tunneling regime. We consider SSLs at cool (77 K) and warm (295 K) temperatures, simulating the electronic response to variations in (a) the number of SSL periods, (b) the contact conductivity, and (c) the strength of disorder (aperiodicities). Our analysis shows that the chaotic dynamical phases exist on a number of sub-manifolds of codimension zero within the design parameter space. This result provides an encouraging guide towards the experimental observation of high-frequency intrinsic dynamical chaos in shorter SSLs.

Modeling of mid-infrared quantum cascade lasers: The role of temperature and operating field strength on the laser performance

In this paper a self-consistent numerical approach to study the temperature and bias dependent characteristics of mid-infrared (mid-IR) quantum cascade lasers (QCLs) is presented which integrates a number of quantum mechanical models. The field-dependent laser parameters including the nonradiative scattering times, the detuning and energy levels, the escape activation energy, the backfilling excitation energy and dipole moment of the optical transition are calculated for a wide range of applied electric fields by a self-consistent solution of Schrodinger–Poisson equations. A detailed analysis of performance of the obtained structure is carried out within a self-consistent solution of the subband population rate equations coupled with carrier coherent transport equations through the sequential resonant tunneling, by taking into account the temperature and bias dependency of the relevant parameters. Furthermore, the heat transfer equation is included in order to calculate the carrier temperature inside the active region levels. This leads to a compact predictive model to analyze the temperature and electric field dependent characteristics of the mid-IR QCLs such as the light-current (L–I), electric field-current (F–I) and core temperature-electric field (T–F) curves. For a typical mid-IR QCL, a good agreement was found between the simulated temperature-dependent L–I characteristic and experimental data, which confirms validity of the model. It is found that the main characteristics of the device such as output power and turn-on delay time are degraded by interplay between the temperature and Stark effects.

Resonant-tunneling structure of quantum wells in the p-i-n photovoltaic element

The method for efficient separation of photoexcited carriers, based on the resonant tunneling phenomenon in the quantum well structure placed into the i-region of the p-i-n photovoltaic element, is proposed. The parameters of quantum well structures based on the GaAs/Ga1−xInxAs system, implementing the mode of sequential resonant tunneling in the electric field of the GaAs p-i-n junction, is calculated. A microscopic model of resonant-tunneling transport in such structures is constructed, and the kinetic tunneling times are calculated depending on well and barrier parameters. The possibility of achieving sufficiently short (<∼10 ps) tunneling times and, hence, quite efficient removal of photoelectrons and photoholes from quantum wells at a proper choice of barrier powers is shown.

Resonant thermotunneling design for high-performance single-junction quantum-well solar cells

In a material system displaying a negligible valence band offset, which enables the smooth transport of holes, we show that the conduction band (CB) confinement energies and barrier thicknesses can be designed to favor a sequential thermionic promotion and resonant tunneling of electrons to the CB continuum resulting in an overall faster carrier collection. Using 1 eV dilute nitride semiconductor quantum wells that are embedded in conventional GaAs solar cells, we present practical energy-level engineering designs that significantly facilitate the collection of all photogenerated carriers within several picoseconds (10 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{-12}$</tex></formula> s) from deep quantum wells rather than several nanoseconds, as it is the case for conventional designs. A preliminary evaluation of a GaAs/GaAsN multiquantum well device that incorporates such thermotunneling design indicates potential for significant efficiency improvement over a conventional GaAs solar cell, thus surpassing the Shockley–Queisser efficiency limit for a single-junction device.

Intersubband population inversion and stimulated terahertz transitions between Landau levels in resonant tunneling multiple quantum well structures

It is shown that a population inversion can be achieved in the system of Landau levels in cascade quantum well structures in a magnetic field under a condition of sequential resonant tunneling. This mechanism allows a wide range tuning of the emitted terahertz frequency by the magnetic field strength variation.

Size-induced metal-to-semiconductor transition and room temperature sequential resonant tunneling in La0.5Sr0.5CoO3−δ quantum dots embedded in La0.5Sr0.5CoO3−δ nanotubes

The sequential resonant tunneling in La0.5Sr0.5CoO3−δ (LSCO) semiconductor quantum dots (QDs) arrays were observed by current-voltage measurements under room temperature. A series of spikelike current peaks are found, which are resulted from the accumulation and depletion of electrons tunneling through the QDs embedded in insulated barriers. Temperature dependent onset-voltage blueshift and number variation in current peaks are also observed. These results confirm the size-induced metal-to-semiconductor transition in LSCO.

Tunneling effects in short period strained AlN/GaN superlattices

AbstractWe study electronic conduction of short period strained GaN/AlN superlattices. The mechanisms of current transport perpendicular to the layers were studied, with an emphasis on the elastic and inelastic tunneling through the AlN barriers. Electron transport as a function of temperature is examined in I‐V, G‐V (first derivative dI/dV). The current and the conductance show nonlinear oscillatory character. The observed characteristics are interpreted as sequential resonant tunneling through AlN barriers and reveal negative differential conductivity (NDC). (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Sequential resonant tunneling in quantum cascade lasers

A model of sequential resonant tunneling transport among two-dimensional subbands that takes into account explicitly elastic scattering is investigated. It is compared to transport measurements performed on quantum cascade lasers, where resonant tunneling processes are known to be dominating. Excellent agreement is found between experiment and theory over a large range of current, temperature, and device structures.

The self-sustained current oscillation and the dynamics in superlattices under the action of electric and magnetic fields

Self-sustained time-dependent current oscillations have been found in weakly coupled GaAs∕AlAs superlattices when the sequential resonant tunneling between adjacent quantum wells is the main electron transport mechanism. The oscillation regime was tunable by varying the doping densities and applied dc voltages. Based on the discrete sequential tunneling model, we theoretically studied the magnetic field dependence of the oscillation. The magnetic field B seems to be favorable for the formation of the static electric-field domains and to depress the current oscillation. Thus, the oscillation regime will be narrowed as the magnetic field strength increases. Driven by a transverse external microwave excitation, the system shows interesting nonlinear behaviors like quasiperiodicity, frequency locking, and periodicity.

Vertical transport in GaAs/ Ga 1− yAl yAs barrier structures containing quantum wells: Current–temperature characteristics

Vertical transport in GaAs/ Ga 1− y Al y As barrier structures was investigated using current–temperature ( I – T ) measurements in the dark. The samples studied had 500 Å thick Ga 1− y Al y As (0≤ y ≤0.26 ) linearly graded barriers between the n +-GaAs contacts and the Ga 0.74Al 0.26As central barrier containing N w (=0, 4, 7 and 10) n-doped GaAs quantum wells of width 35 Å. The thickness of Ga 0.74Al 0.26As barrier between the wells was 310, 135 and 77 Å for the samples with N w = 4 , 7 and 10, respectively. The vertical current was measured as a function of temperature (3.5–295 K) at applied voltages in the range 0.01–0.7 V. The total barrier height ( Δ U m ) and the space charge density in the sample with N w = 0 , and the effective barrier height ( Δ E ) for the samples with N w = 4 , 7 and 10 were determined by analysing the experimental I – T characteristics using drift–diffusion theory for thermal current. The variation of Δ U m and Δ E with applied voltage was discussed in connection with various dark current mechanisms in barrier structures, including thermionic emission, Fowler–Nordheim tunnelling and sequential resonant tunnelling.

Analytical expression of leakage current through double quantum dots in Pauli spin blockade

We theoretically examine a small current through double quantum dots with Pauli spin blockade, in which electrons cannot be transported without spin–flip or higher-order tunnel processes (cotunneling). Analytical expressions for the current are given in both cases of sequential tunneling and resonant tunneling between the quantum dots and leads. In the former case, the cotunneling weakens the spin blockade and increases a coherent component of the current. When the cotunneling can be disregarded, effective magnetic fields created by nuclear spins (Overhauser fields) can be detected by the observation of the current. In the case of resonant tunneling with Zeeman splitting in an external magnetic field ( ∼ 1 T ) , small Overhauser fields ( ∼ 5 mT ) could be detected by the fine tuning of the resonant condition.

Electronic characteristics of n-type nanocrystalline/p-type crystalline silicon heterostructure

A heterojunction of ultrathin phosphorus-doped hydrogenated nanocrystalline silicon with p-type crystalline silicon was fabricated to investigate carrier information and conduction behaviour. The temperature-dependent carrier information of holes and electrons as well as the two-dimensional electron gas were identified. The forward conduction can be ascribed to tunnelling-aided carrier emission recombination while the reverse transport can be divided into different mechanisms within different reverse bias ranges. The reverse current can be assigned to minority carrier tunnelling through the space charge region within the backward bias voltage from 0 to about −10 V. With decreasing negative applied voltage from −10 V to around −22 V, the reverse current can be attributed to the tunnelling-assisted Poole–Frenkel emission. With further reduction of the reverse bias voltage from −22 V to about −38 V under 100 K, the reverse current staircases can be allocated to sequential resonant tunnelling; otherwise it can be due to the thermic effect at higher temperature. The main origin of short transient time for the fabricated device can be due to the tunnelling mechanism.

HYBRID NANOMATERIALS FOR MULTI-SPECTRAL INFRARED PHOTODETECTION

Quantum dot infrared photodetectors (QDIPs) using quantum dots (QDs) grown by strained-layer epitaxy have demonstrated low dark current, multi-spectral response, high operating temperature, and infrared (IR) imaging. However, achieving near room-temperature, multi-spectral operation is a challenge due to randomness in QD properties. The ability to control dopant incorporation is important since charge carrier occupation influences dark current and IR spectral response. In this work, dopant incorporation is investigated in two classes of QDs; epitaxial InAs/GaAs QDs and CdSe colloidal QDs (CQDs) embedded in MEH-PPV conducting polymers. The long-term goal of this work is to combine these hybrid nanomaterials in a single device heterostructure to enable multi-spectral IR photodetection. Two important results towards this goal are discussed. First, by temperature-dependent dark current-voltage and polarization-dependent Fourier transform IR spectroscopy measurements in InAs/GaAs QDIPs featuring different doping schemes, we have provided experimental evidence for the important contribution of thermally-activated, defect-assisted, sequential resonant tunneling. Second, the enhanced quantum confinement and electron localization in the conduction band of CdSe / MEH-PPV nanocomposites enable intraband transitions in the mid-IR at room temperature. Further, by controlling the semiconductor substrate material, doping type, and doping level on which these nanocomposites are deposited, the intraband IR response can be tuned.

Doping effect on dark currents in In0.5Ga0.5As∕GaAs quantum dot infrared photodetectors grown by metal-organic chemical vapor deposition

Stacked self-assembled In0.5Ga0.5As∕GaAs quantum dot infrared photodetectors grown by low-pressure metal-organic chemical vapor deposition, with and without silicon dopants in the quantum dot layers, are investigated. The increase of dark currents observed at higher doping levels is attributed to higher defect density leading to stronger sequential resonant tunneling and to lowering of the operating temperature of the device.

Conduction Mechanisms and Low-Frequency Electrical Noise Studies in pin InGaAs/InAlAs Strained MQW Photodiodes

The current-voltage (I-V) characteristics as a function of temperature of different strained multiple-quantum-well pin InGaAs/InAlAs photodiodes were investigated in the dark from 15 K to 300 K. Analysis of the slope variation of the I-V curves as a function of temperature, under forward bias, indicate a conduction mechanism by tunneling effect assisted by recombination centers. For temperatures below around 100 K, as the voltage increases, a negative resistance appears, followed by oscillations suggesting a sequential resonant tunneling between electronic states of the quantum wells. Low frequency electrical noise measurements performed at 300 K between 1 and 10/sup 5/ Hz confirm the existence of recombination centers.

A microscopic model of sequential resonant tunneling transport through weakly coupled superlattices

A microscopic model is developed for resonant tunneling transport in weakly coupled semiconductor superlattices in a constant external electric field. The model takes into account multiple subbands and electric-field dependence of scattering by acoustic and optical phonons, charged impurities, and interface roughness. The model is used as a basis for computing the resonant-tunneling profiles for structures with small size-quantization energies. The computed results are in good agreement with experiment. In structures of this type, an important role is played by electric-field dependence of scattering processes and the threshold behavior of elastic processes is strongly manifested. A substantial asymmetry is predicted not only for the first tunneling resonance, but also for higher order resonant tunneling processes.