Double-barrier Quantum-well Structures Research Articles

Surface passivation in c-Si solar cells via a double-barrier quantum-well structure for ameliorated performance

Modern solar cell technology suffers low surface passivation, high recombination losses at the interface, and dopant diffusion losses that limit solar cell’s efficiency. Therefore, a double-barrier quantum-well (DBQW) structure-based surface passivation contact is introduced here to resolve these shortcomings. In this regard, DBQWs of different thicknesses with stacks of SiOx/ nc-SiOx (3, 5, 8, and 10 nm)/SiOx layers were explored as surface passivation layers and dopant diffusion barriers. Multiple characterizations were utilized to examine DBQW thickness-dependent properties, such as contact resistance, passivation, and recombination current density; the best result was for the 5-nm-thickness QW passivation layer. To justify the obtained result, theoretical calculations were carried out based on the experimental results, which suggested the resonance tunneling of charge carriers across the 5 nm DBQW structure. Furthermore, SIMS was explored to examine the diffusion barrier property of these QWs, which revealed that dopant diffusion was suppressed by the QW with the double SiOx layer. Finally, the nc-SiOx(n) /5-nm QW/c-Si surface passivation structure showed substantial enhancement in a lifetime (τeff) of 3560 μs, an implied open-circuit voltage (iVoc) of 735 mV, and reduced recombination current density (Jo) = 1.5 fA/cm2.

Resonant Tunneling Diode by Means of Compound Armchair Boron/Nitride and Graphene Nanoribbons

The band gap of armchair graphene nanoribbons (AGNRs) can be modulated by replacing the carbon atoms with boron/nitride (BN) atoms to produce the compound nanoribbons, while the width of ribbons remains constant. By introducing BN doping atoms in the proper positions along the ribbon length, a double-barrier quantum-well structure is constructed. Consequently, negative differential resistance properties can be obtained by a combination of armchair BN nanoribbons (ABNNRs) and AGNRs as the compound ABNxGyNRs, in which x and y denote the number of BN and C atoms in the ribbon width, respectively. The proposed resonant tunneling diode (RTD), called an armchair BN graphene nanoribbon resonant tunneling diode (ABNGNR-RTD), is investigated in three different platforms including W, S, and H shapes. The numerical tight-binding model along with non-equilibrium Green’s function formalism is taken into account to study the electronic properties of the proposed RTD. The performance of the ABNGNR-RTD is examined in terms of device characteristics such as peak-to-valley ratio (PVR) and power dissipation. Based on the presented results, the performance of H-shaped devices is better than those of the other two cases in terms of PVR and power dissipation. In addition, the electronic properties of ABNGNR-RTDs can be modified by varying the relative width of ABNNRs with respect to AGNRs.

Shape effects in graphene nanoribbon resonant tunneling diodes: A computational study

The possibility of using graphene nanoribbons (GNRs) as the material for resonant tunneling diodes (RTDs) was investigated using a device simulator based on the nonequilibrium Green’s function with the π-orbital tight-binding approach. The double-barrier quantum well (DBQW) requirements of a RTD can be implemented by adjusting the width of a GNR to derive a negative differential resistance (NDR). The implementation of such a device is demonstrated in this paper and its mechanism was also found to be robust regardless of the eventual shape of the GNR patterned. Furthermore, the effects of the shape of the patterned GNR and the operating temperature on the performance of the device were explored by looking at the real space current flux of the device and the temperature dependency of the peak-valley ratio (PVR), respectively. Although the different shapes of GNR RTDs had a similar DBQW structure, their PVRs were different due to their conduction mechanisms which were dependent on the different geometrical shapes of each case. Lastly, the effect of thermal broadening, and width/length dependence of the central GNR between two barriers on the device performance, was further investigated in order to provide insights into the device physics of GNR RTDs for future study on performance optimization.

Dilute nitride based double-barrier quantum wells for intersubband absorption at 1.31 and1.55 μm

A systematic investigation of intersubband optical absorptions in ${\text{In}}_{x}{\text{Ga}}_{1\ensuremath{-}x}{\text{As}}_{1\ensuremath{-}y}{\text{N}}_{y}/\text{AlAs}/{\text{Al}}_{z}{\text{Ga}}_{1\ensuremath{-}z}\text{As}$ double-barrier quantum well (DBQW) structures is reported. Electron subbands and the energy dependent effective potential for the envelope wave functions are calculated by means of the ten-band $k\ensuremath{\cdot}p$ scheme, which takes into account the effects of subband nonparabolicity and strain. From the calculations of the energy dispersions and wave functions of the respective states, we find that when the conduction-band offset is very large such that the first excited state lies above the localized nitrogen level, each of the ground and the first excited states is split into two levels. The intersubband absorptions in the DBQW structure have been studied by varying the well width and the compositions of the alloys. The strongest calculated absorption peak corresponding to the transition from the ground level to the higher excited level shows the same peculiarities. Finally, DBQW structures, which correspond to 1.31 and $1.55\text{ }\ensuremath{\mu}\text{m}$ intersubband absorptions, respectively, have been achieved.

Characteristics of electron transport through vertical double-barrier quantum-dot structures: Effects of symmetric and asymmetric variations of the lateral confinement potentials

We report on a theoretical study of the electron transport through laterally-confined, vertical double-barrier resonant-tunneling (DBRT) structures, defined as one-dimensional (1D)-0D-1D systems, with a tunable lateral confinement. The current and the differential conductance of the systems are calculated and the influence caused by varying the lateral confinement on the device characteristics is investigated. Three representative systems are studied. First of all, a 1D-0D-1D device, symmetric with respect to the current flow, with a variable lateral confinement in the double-barrier quantum-well (DBQW) region, is investigated. This device would in an experimental setup correspond to the structure in which a thin, lateral metallic gate is placed in the DBQW region. Subsequently, calculations are performed for two asymmetric 1D-0D-1D devices, in which the strongest, but varying, lateral confinement is placed either in the collector or in the emitter region. In experiments, these two devices would correspond to the situations where a lateral metallic gate is positioned below or on top of the DBQW structure. The calculations predict several phenomena for the device characteristics. It is shown that as the lateral confinement increases, in addition to those normally observed current onsets and pinch-offs that move toward higher bias voltages, several current onsets and pinch-offs move towards lower bias voltages. These negative shifts of the current onsets and pinch-offs with increasing of the lateral confinement have so far not been expected for gated DBRT devices. It is also found that the threshold voltages, at which the current onsets and pinch-offs appear, depend strongly on the strength and position of the lateral confinement and on the Fermi levels in the collector and the emitter. The models that explain these predictions are presented and discussed.

THz-frequency intrinsic oscillations in double-barrier quantum well systems

Traditional implementations of double-barrier quantum well structures (DBQWSs) have not been successful as oscillator sources at THz frequencies because they are utilized in an extrinsic (i.e., external charge exchange) manner. Indeed, the true failing of a “traditional” DBQWS-based oscillator is tied directly to the physical principles associated with its implementation. In this paper, greater insight into the physics of instabilities within nanoscale tunneling structures is revealed. Here, self-consistent, time-dependent Wigner–Poisson simulations demonstrate sustained THz-frequency current-oscillations in a DBQWS that arise without the benefit of external charging processes. More importantly, dependencies between the emitter-boundary structure and the DBQWS are identified that strongly influence the internal instability phenomenon. These new results offer potential methodologies for inducing and controlling intrinsic oscillations in DBQWSs.

Operation Principle of Resonant Tunneling THz Oscillator at Fixed Bias Voltages

Based on time-dependent numerical simulation of an double barrier quantum well structure, a time-dependent energy coupling model is presented to account for the operational principle of a new kind of resonant tunneling THz oscillators.

One-dimensional to one-dimensional electron resonant tunneling in a double asymmetric quantum-wire structure

An atomically precise double asymmetric quantum-wire structure was fabricated with the cleaved edge overgrowth method. A resonant tunneling diode for such a structure was successfully achieved. Its voltage–current characteristic was measured at 77 K. A sharp resonant tunneling current peak was observed and its peak-to-valley ratio is about 18:1, which is much larger than that of a double-barrier quantum-well structure, and it is ascribed to one-dimensional to one-dimensional electron resonant tunneling through the double asymmetric quantum-wire structure.

Competition between single-and multiparticle resonances in tunneling

A new mechanism is proposed for the anomalous tunneling transmittance of a double-barrier quantum-well structure doped with a transition-metal impurity and possessing an intrinsic two-dimensional continuum. The new tunneling channels are due to exponentially-narrow single-particle resonances arising near the edge of a 2D band during tunneling. They are temperature-independent and their contribution to the transmittance can exceed the contribution of the Kondo resonance even at temperatures T<T K .

Tunneling escape time of electrons from the quasibound Stark localized states in ultrathin barrier GaAs/AlAs superlattices.

Band-edge optical absorption spectra in two series of monoperiodic GaAs/AlAs superlattice diodes are investigated as a function of bias voltage by low-temperature photocurrent spectroscopy. In the superlattices the well width is fixed at 11 or 22 monolayers (ML) and only the ultrathin barrier thickness is varied between 2 and 6 ML. In the limit of high electric field conditions where the fundamental Stark ladder transition dominates, the linewidth of the heavy-hole excitonic peak is found to increase systematically with decreasing the barrier thickness. That is, the linewidth for the superlattice with thinner barriers is broader than that with the thick barrier superlattice, which is basically determined by the usual inhomogeneous broadening. This enhancement of the spectral linewidth by more than 10 meV is due to a lifetime broadening of the quasibound Stark localized states. This resonance energy broadening due to the rapid tunneling escape of electrons is used to determine the tunneling time by the uncertainty principle. The tunneling escape time thus determined as a function of barrier thickness shows good agreement with values deduced from the confined electron level broadening for the biased double-barrier quantum-well structures based on simple transfer matrix calculations.

Transport mechanism of Gamma - and X-band electrons in AlxGa1-xAs/AlAs/GaAs double-barrier quantum-well infrared photodetectors.

The effect of the \ensuremath{\Gamma}- and X-band electrons in the ${\mathrm{Al}}_{0.25}$${\mathrm{Ga}}_{0.75}$As/AlAs/GaAs double-barrier quantum well (DBQW) is investigated by a microscopic empirical pseudopotential calculation. The DBQW structure used in the calculation is designed as a 3--5-\ensuremath{\mu}m quantum-well infrared photodetector with an associated transition energy of 313 meV. DBQW tunneling transmission via \ensuremath{\Gamma}- and X-like states as a function of electron energy and applied voltage are described and compared to that in a single-barrier AlAs/GaAs quantum well. The dark current is simulated by the confined ground-state electron tunneling out of the well. We find that, at high-bias voltage, tunneling via X-like states increases the current by a few orders of magnitude. We have also varied the additional barrier thickness and found that for a very thin (20 \AA{}) additional barrier DBQW, the excited-state electrons are not blocked by the \ensuremath{\Gamma}-band barrier, and may give a high photocurrent without the assistance of the X band, although the dark current also increases. \textcopyright{} 1996 The American Physical Society.

Scattering theoretic approach to the electronic structure of double-barrier quantum-well heterostructure

The effects of aperiodicity, indirect bandgap, band non parabolicity multi-orbital and external bias on the AlAs/GaAs/AIAs double-barrier quantum-well structure in the [001] direction are considered explicitly by using the Green's function scattering theory incorporated with sp 3s ∗ band structures of constituent semiconductors. The calculated spectral local density of states simultaneously exhibits confined states originated from both quantum-well Γ-valley and barrier X-valley. Multi-orbital features of electron and hole states of the heterostructure, and the effect of the external bias on the local density of states are displayed. The calculated energies of confined quantum-well electron states and light-hole states are compared with the reported photoluminescence excitation experimental data.

Traversal time in an asymmetric double-barrier quantum-well structure

A study of traversal time for asymmetric double-barrier quantum-well structure (DBQWS) is presented. The calculations are self consistent in nature. The analysis shows that traversal time is affected more strongly by the width of the emitter barrier than that of the collector barrier of the structure in the presence of space charge. Bistability in traversal time is observed. Bistability becomes greater if the emitter barrier is made wider than the collector barrier and is explained in terms of space charge developed in the quantum well. However, a wider collector will dominate traversal time at zero bias, especially at eigen energy. >

Density of states for double-barrier quantum-well structures under the influence of external fields and phase-breaking scattering.

Density of states for double-barrier quantum-well structures under the influence of external fields and phase-breaking scattering.

Green's-function approach to the electronic structure of double-barrier quantum-well heterostructures.

The scattering-theoretic Green's-function method incorporated with band structures, which takes into account the indirect band gap, the band nonparabolicity, and the multiorbital and external-bias effect, is used to study AlAs/GaAs/AlAs double-barrier quantum-well structures in a [001] direction. The calculated spectral local density of states simultaneously exhibits confined states originating from both the quantum-well \ensuremath{\Gamma} valley and the barrier X valley. The calculated energies of confined quantum-well electron states and light-hole states are compared with the reported photoluminescence-excitation experimental data.

A GaAs/AlAs/AlGaAs and GaAs/AlGaAs stacked quantum well infrared photodetector for 3–5 and 8–14 μm detection

A new two-color n-type GaAs/AlAs/AlGaAs double barrier quantum well (DBQW) and bound-to-miniband GaAs/AlGaAs quantum well infrared photodetector (QWIP) with photovoltaic (PV) and photoconductive (PC) dual-mode operation in the 3–5 and 8–14 μm atmospheric spectral windows has been demonstrated in this work. It consists of a stack of the midwavelength infrared (MWIR) QWIP and the long-wavelength infrared (LWIR) QWIP. The PV detection scheme uses transition from the ground bound state to the first quasi-bound excited state for the MWIR-QWIP. The PC detection scheme has two different transitions, one identical to the PV mode detection scheme while the other uses transition from the ground bound state to the miniband state of the superlattice barrier LWIR-QWIP. The peak responsivity for the PV mode was found to be 17 mA/W at λp=4.1 μm and T=50 K with a bandwidth Δλ/λp=15%. The peak responsivities for the PC mode were found to be 25 mA/W at λp=4.1 μm, Vb=1.0 V, and 0.12 A/W at λp=11.6 μm, Vb=3.2 V, and at T=50 K, with a bandwidth Δλ/λp=18%. The PV responsivity was found to be 68% of the PC responsivity at λp=4.1 μm and T=50 K, demonstrating the ability for efficient PV mode operation at 3–5 μm by using the DBQW structure. This is the highest ratio reported for the spectral region of 3–5 μm wavelength with a DBQW structure.

Above-barrier resonant transitions in AlxGa1-xAs/AlAs/GaAs heterostructures.

In this work we demonstrate the existence of above-barrier quasibound states in ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As/AlAs/GaAs double-barrier quantum-well structures, by using both continuous-wave and time-resolved photoluminescence techniques in addition to reflectivity measurements. A series of clearly resolved resonances is found in the optical spectra. Their energy positions agree fairly well with the energies found for the quasibound states when analyzing the density of states of the heterostructure continuous spectrum. As shown by the analysis of the envelope wave function these resonances are localized in the ${\mathrm{Al}}_{0.3}$${\mathrm{Ga}}_{0.7}$As barrier regions and are found to induce both an increase of the interband absorption and a reduction of the carrier capture efficiency into the wells.

Disorder effects on tunneling through one-dimensional double-barrier quantum-well structures: A coherent-potential approximation.

We present a formalism for studying the disorder effects on electron tunneling through one-dimensional double-barrier quantum-well structures based on the coherent-potential approximation. This formalism enables us to calculate the configuration-averaged transmission coefficients in a nonperturbative way. It is shown that elastic scattering reduces and broadens the resonance peak, and also destroys its Lorentzian behavior. For sharp resonance structures, the transmission coefficient directly reflects the structures in the density of states within the well. The result obtained using this method agrees well with direct numerical simulation.

Noise in alloy-based resonant-tunneling structures.

The noise properties of semiconductor alloy-based resonant-tunneling double-barrier quantum-well structures are calculated within a nonequilibrium Green's-function formulation. Alloy scattering is treated in the coherent-potential approximation including vertex corrections. Different noise characteristics are found depending on applied bias, scattering strength, and the transparency of the barriers. A characteristic frequency for the noise current power spectrum S(ω) is given by the inverse response time

Thermal escape of carriers out of GaAs/AlxGa1-xAs quantum-well structures.

Nonradiative recombination processes in thin GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well (QW) and double-barrier quantum-well structures have been investigated by means of continuous-wave and time-resolved photoluminescence (PL) measurements. We find that, due to the temperature dependence of the radiative time constant, the Arrhenius plot of the PL intensity cannot be used for extracting the activation energy of the nonradiative channels. In fact it is the temperature dependence of the PL decay time ${\mathit{T}}_{\mathit{L}}$ which directly gives information on the thermal activation of the loss mechanism. The activation energies obtained from the Arrhenius plot of ${\mathit{T}}_{\mathit{L}}$ demonstrate that, at least in the case of thin wells, the main nonradiative mechanism is the thermal escape of the less-confined species of carriers.