Adjacent Quantum Wells Research Articles

Precise band gap engineering using double barrier InGaN/GaN superlattices

We consider InGaN based short period superlattices with two different quantum barriers. The superlattices are grown along c-direction of the wurtzite structure. Ab-initio calculations of the band structure are performed for sets of such “double barrier” superlattices.It has been shown that the “double barrier” superlattices offer the possibility to a significant increase of the grid in the band gap tuning in a given wide range of emission energies.The role of wave functions hybridization of the adjacent quantum wells and quantum barriers as well as a large built-in electric field is discussed. Both effects are crucial for the band gap engineering in the InGaN related spectral range. In parallel, photoluminescence measurements are carried out on sets of specially designed samples grown by MOVPE. The calculated band gaps are compared to the emission energies, confirming all observed trends.

Auger-Assisted Electron Transfer between Adjacent Quantum Wells in Two-Dimensional Layered Perovskites

Two-dimensional (2D) layered perovskites are naturally formed multiple quantum-well (QW) materials, holding great promise for applications in many optoelectronic devices. However, the further use of 2D layered perovskites in some devices is limited by the lack of QW-to-QW carrier transport/transfer due to the energy barrier formed by the insulating ligands between QWs. Herein, we report an Auger-assisted electron transfer between adjacent QWs in (CmH2m+1NH3)2PbI4 2D perovskites particularly with m = 12 and 18, where the electron energy barrier (Eb) is similar to the QW band gap energy (Eg). This Auger-assisted QW-to-QW electron transfer mechanism is established by the observation of a long-lived and derivative-like transient absorption feature, which is a signature of the quantum confined Stark effect induced by the electron-hole separation (thus an internal electric field) between different QW layers. Our finding provides a new guideline to design 2D perovskites with an optically tunable QW-to-QW charge transport property, advancing their applications in optoelectronics and optical modulations.

Critical role of parallel momentum in quantum well state couplings in multi-stacked nanofilms: An angle resolved photoemission study

We use angle resolved photoemission spectroscopy to investigate the coupling of electron quantum well states (QWSs) in epitaxial thin Pb and Ag films. More specifically, we investigate the Ag/Si, Pb/Si, and Pb/Ag/Si systems. We found that the parallel momentum plays a very profound role in determining how two adjacent quantum wells are coupled electronically across the interface. We revealed that in the Pb/Ag bimetallic system, there exist two distinctly different regimes in the energy vs momentum (E vs k) space. In one regime, the electronic states in Ag and Pb are strongly coupled, resulting in a new set of QWSs for the bi-metallic system. In the other regime, the electronic states in individual metallic layers are retained in their respective regions, as if they are totally decoupled. This result is corroborated by calculations using density functional theory. We further unravel the underlying mechanism associated with the electron refraction and total internal reflection across the interface.

Direct Probe of Room-Temperature Quantum-Tunneling Processes in Type-II Heterostructures Using Terahertz Emission Spectroscopy

Charge-carrier-transport phenomena on nanoscopic length and ultrashort timescales are of great interest for a multitude of ultrafast dynamic processes occurring in chemical reactions as well as modern devices such as solar cells or transistors. However, the investigation of such ultrashort current pulses is very challenging and mostly based on indirect methods such as spectroscopic changes induced by the charge transport. Here we monitor the short current pulse generated by the dissipative tunneling of charge carriers through a barrier between two adjacent quantum wells via its radiated legacy in the terahertz frequency range. By examining quantum structures with intermediate barriers of different thickness, we demonstrate that the spectrum of the emitted radiation is a direct measurement of the charge-transfer dynamics associated with the tunneling process. Our findings indicate that these incoherent tunnel currents do not start instantaneously but build up on a timescale of approximately 170 fs.

Design of Hybrid Plasmonic Multi-Quantum-Well Electro-Reflective Modulators Towards <100 fJ/bit Photonic Links

Realization of on-board and inter-chip optical interconnects requires a photonic data link with power consumption well below their electrical counterparts (i.e., 50 Gb/s requires 2–4 pJ/bit/channel. External reverse-biased modulators could drastically reduce this power consumption. Here we design ultralow power GaAs/AlGaAs multi quantum well electro-reflective modulators operating at 1 V for facile integration with polymer “optical bridges”, utilizing coupled quantum confined Stark effect between adjacent quantum wells and optical coupling to hybrid surface plasmon-slab modes for significantly enhanced extinction ratio and spectral bandwidth. Distinctive from conventional electro-optical or electro-absorption modulators, this new design synergistically leverages ultra-large changes in both refractive index (|Δn|∼0.05) and absorption coefficient (Δα∼104 cm−1), achieving 35-50 dB extinction ratio at 1 V reverse bias with a low insertion loss of 1–3 dB, an incident angle tolerance of ∼5°, and a spectral bandwidth of 7–10 nm. The modulator power consumption is ∼1.9 fJ/bit without the need of thermal tuning, and the RC-limited bandwidth well exceeds 100 GHz. This new modulator enables high bandwidth and ultralow power optical interconnect networks at >100 Gb/s/channel and <100 fJ/bit/channel compatible with ever-scaling CMOS technologies.

Quantum spin transport to semiconductor quantum dots through superlattice

Spin transport properties from the GaAs/AlGaAs superlattice (SL) to InGaAs quantum dots (QDs) are studied by circularly polarized time-resolved photoluminescence spectroscopy of QD excited states with the selective excitation of SL miniband states. For the SL with a thinner barrier, we observe an effective carrier transport in SL owing to the stronger overlap of wavefunctions of adjacent quantum wells and a simultaneous highly efficient carrier injection into QDs. Moreover, the SL with a thinner barrier demonstrates a quantum spin transport to QDs maintaining high spin polarization during the transport process.

Carrier diffusion as a measure of carrier/exciton transfer rate in InAs/InGaAsP/InP hybrid quantum dot–quantum well structures emitting at telecom spectral range

We investigate the diffusion of photo-generated carriers (excitons) in hybrid two dimensional–zero dimensional tunnel injection structures, based on strongly elongated InAs quantum dots (called quantum dashes, QDashes) of various heights, designed for emission at around 1.5 μm, separated by a 3.5 nm wide barrier from an 8 nm wide In0.64Ga0.36As0.78P0.22 quantum well (QW). By measuring the spectrally filtered real space images of the photoluminescence patterns with high resolution, we probe the spatial extent of the emission from QDashes. Deconvolution with the exciting light spot shape allows us to extract the carrier/exciton diffusion lengths. For the non-resonant excitation case, the diffusion length depends strongly on excitation power, pointing at carrier interactions and phonons as its main driving mechanisms. For the case of excitation resonant with absorption in the adjacent QW, the diffusion length does not depend on excitation power for low excitation levels since the generated carriers do not have sufficient excess kinetic energy. It is also found that the diffusion length depends on the quantum-mechanical coupling strength between QW and QDashes, controlled by changing the dash size. It influences the energy difference between the QDash ground state of the system and the quantum well levels, which affects the tunneling rates. When that QW–QDash level separation decreases, the probability of capturing excitons generated in the QW by QDashes increases, which is reflected by the decreased diffusion length from approx. 5 down to 3 μm.

Current density dependence of transition energy in blue InGaN/GaN MQW LEDs

AbstractIn this paper we report on the transition energy in InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) under various injection current density from 2 A/cm2 to 200 A/cm2. Various effects including strain, quantum confined Stark effect (QCSE), screening effect by carriers, bandgap renormalization, Stokes‐like shift, band‐filling effect, and quantum levels in triangular quantum wells, are considered quantitatively and analyzed comprehensively. By comparing these effects altogether, we found that when the In‐content in quantum wells is fixed, the transition energy is mainly determined by QCSE and quantum level energy, between which QCSE overweighs the other. The transition energy shift with current density is also mainly governed by the screening effect of QCSE, with detailed competition between band tailing and filling. Additional effect, the coupling between adjacent quantum wells, is investigated in this paper. The coupling between quantum wells with various barrier thicknesses is compared. By calculating the wavefunctions self‐consistently, it is found that for InGaN/GaN MQWs with 5 nm quantum barriers (QBs), the coupling of electron wavefunctions leads to about 2 meV difference in transition energy. While for the MQWs with 3 nm QBs, the influence of electron wave‐function overlap on transition energy is 12.9 meV, which is more significant than that of 5 nm QB case. However, for hole wavefunctions, the coupling effect is too small to be considered, which is mainly due to the much larger effective mass. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Study of the coupling of ultra-thin CdSe double quantum wells

The degree of coupling between the electronic states of adjacent quantum wells (QWs) will depend on the sample structure and material properties of the QW and the barrier. The design of heterostructures containing multiple QWs requires the precise knowledge of the minimum barrier thickness to uncouple them. Due to the finite confinement in ultra-thin QWs (UTQWs, thickness of a few monolayers), the electron and hole wave functions present a large penetration within the barriers; the penetration length may exceed many times the thickness of the UTQW. Following the evolution of the electron and hole states as a function of the barrier thickness of a symmetric double quantum well system, we have determined the thickness of the ZnSe barrier required to uncouple CdSe UTQWs. We found that ZnSe barriers with thickness in the range from 70 to 30 monolayers (ML) are necessary to uncouple symmetric double CdSe UTQWs with thickness in the range from 1 to 5 ML, respectively. Therefore, the thinner the QWs the thicker the barrier required to uncouple them. The coupling/uncoupling of asymmetric double UTQWs (2 and 3 ML CdSe) system was experimentally verified by the analysis of the low temperature excitonic spectra. The sample with a 5nm ZnSe barrier indicates the presence of coupling because we observe only one peak corresponding to the lowest energy transition; for the case of the uncoupled asymmetric UTQWs, with a 10nm ZnSe barrier, the spectrum presents two peaks, one for each independent (single) UTQW.

Dependence of electron dynamics on magnetic fields in semiconductor superlattices

Numerical simulation results are presented for a drift-diffusion rate equation model which describes electronic transport due to sequential tunneling between adjacent quantum wells in weakly coupled semiconductor superlattices (SLs). The electron dynamics is dependent on the external magnetic field perpendicular to the electron motion direction, and a detailed explanation is given. Using different parameters, the system shows different dynamic behaviors, and three distinct phenomena are observed and controlled by increasing magnetic field. (i) For a lower doping density, the system state transfers from stable state to oscillationary state. (ii) An opposite result is obtained to that in the case (i) for an intermediate value of the doping density, and the state changes from oscillationary to stationary. (iii) The state varies between oscillationary and stationary when doping density is large. Then, a detailed theoretical analysis is given to explain these surprise phenomena. The distribution of the electric-field domain along the SLs is plotted. We find the structure of the domain is almost uniform for a lower doping density, and no domain occurs in the SLs. By adding an external ac signal, complex nonlinear behaviors are observed from the Poincaré map and the corresponding phase diagrams when the driving frequency changes.

Controlled Interaction of Surface Quantum-Well Electronic States

We report on the construction of well-defined surface quantum well arrangements by combining self-assembly protocols and molecular manipulation procedures. After the controlled removal of individual porphyrin molecules from dense-packed arrays on Ag(111), the surface state electrons are confined at the bare silver patches. These act as quantum wells that show well-defined unoccupied bound surface states. Scanning tunneling spectroscopy and complementary boundary element method calculations are performed to characterize the interaction between the bound states of adjacent quantum wells and reveal a hybridization of wave functions resulting in bonding and antibonding states. The interwell coupling can be tuned by the deliberate choice of the molecules acting as potential barriers. The fabrication method is shown to be ideally suited to engineer specific configurations as one-dimensional chains or two-dimensional artificial molecules.

Nonresonant radiative exciton transfer by near field between quantum wells

We experimentally observed an increase in the intensity of photoluminescence from a wider quantum well (QW) when an exciton transition was induced in the neighboring narrower QW separated from the former one by a tunneling-nontransparent AlGaAs barrier. The dependence of the efficiency of the near-field radiative transfer of excitons on the distance between QWs was studied in heterostructures without coincidence of exciton resonances in the adjacent QWs. Theoretical results were qualitatively consistent with the available experimental data.

High-speed spin channels in a variably spaced multibarrier structure

We have studied the spin–dependent transport properties in a variably spaced multibarrier structure (VSMS) formed by layers of two zinc blende semiconductors with a relatively strong Dresselhaus spin−orbit interaction (SOI). The structure is designed such that, in the absence of SOI, the ground-state energies in adjacent quantum wells are resonantly aligned by an external electric field applied perpendicular to the layers, forming then an electron miniband. In the energy range where this spin degenerate miniband is localized, the transmission coefficient shows a resonant structure which splits into two transmission spectra corresponding to spin-up and spin down states in the presence of SOI. In the energy range where these spectra exhibit overlapping, the polarization efficiency is, in general, an oscillating function of energy. These oscillations tend to disappear as the overlap between the spin−splitting transmission spectra decreases. We have identified two energy ranges where the spin−split transmission spectra do not overlap and the polarization efficiency is essentially 100%, indicating that the VSMSs may be explored as spin filtering devices even for unpolarized injection. It was also shown that the appropriate choice and control of the barrier sizes are of significant importance for the possible development of spin filters based on VSMSs.

Collective modes of a bilayer double parabolic quantum well spin polarized electron gas

AbstractWe investigate the possible electromagnetically induced collective excitation modes in a bilayer quasi‐two‐dimensional structure build from two adjacent parabolic quantum wells (PQW). We consider both charge and spin excitation modes within and beyond the random phase approximation method. To account for higher order effects we used exchange and correlation spin‐dependent local field correction factors. We analyzed intra‐ and inter‐subband excitations and provide analytic results for their excitation frequencies in the long wavelength limit. In particular, we analyze the spin polarization dependence of the lowest frequency spin density mode and show that its relevant frequency is in the hundred GHz region. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Control of polariton scattering in resonant-tunneling double-quantum-well semiconductor microcavities

Electronic control of ultrafast optical amplification is achieved in specially designed semiconductor microcavities with double quantum well. The gain from parametric scattering of polaritons is selectively quenched by tuning the intracavity electric field to turn on and off interwell resonant tunneling. A $&gt;90\mathrm{%}$ reduction in optical gain is observed with only 100 mV change in applied bias. The strong exciton-photon coupling regime leads here to competition between the rate of Rabi coupling and of electronic tunneling between adjacent quantum wells.

Experimental Observation of Sequential Tunneling Transport in GaN/AlGaN Coupled Quantum Wells Grown on a Free-Standing GaN Substrate

AbstractA GaN/AlGaN multiple-quantum-well structure based on an asymmetric triple-quantum-well repeat unit was grown by molecular beam epitaxy, and its vertical electrical transport characteristics were investigated as a function of temperature. To minimize the density of dislocations and other structural defects providing leakage current paths, homoepitaxial growth on a free-standing GaN substrate was employed. The measured vertical-transport current-voltage characteristics were found to be highly nonlinear, especially at low temperatures, consistent with sequential tunneling through the ground-state subbands of weakly coupled adjacent quantum wells. Furthermore, different turn-on voltages were measured depending on the polarity of the applied bias, in accordance with the asymmetric subband structure of the sample repeat units.

Bistability operation of photocurrent due to wave‐function coupling of Wannier‐Stark localization states in a GaAs/AlAs superlattice

AbstractWe have investigated effects of the resonant wave‐function coupling between Wannier‐Stark localization states on photocurrent properties in a GaAs (6.8 nm)/Al0.1Ga0.9As (4.0 nm) superlattice. The resonant coupling, which causes an anticrossing behavior of the relevant transition energies, were clearly detected by electroreflectance spectroscopy and analyzed by calculating the Wannier‐Stark localization states as a function of electric field strength. It is found that the photocurrent‐voltage characteristics exhibit a peak structure, which results in a negative differential resistance, owing to a change of the optical transition probability under the resonant‐coupling condition between the first and second quantized electron states in adjacent quantum wells. Utilizing this negative differential resistance property, we have demonstrated a photocurrent‐bistability operation of a self‐electro‐optic effect device. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High-field transport in semiconductor superlattices for interacting Wannier–Stark levels

We developed a microscopic theory of electron transport in superlattices within the Wannier–Stark approach by including the interaction associated with Zener tunneling between the energy levels pertaining to adjacent quantum wells. By using a Monte Carlo technique we have simulated the hopping motion associated with absorption and emission of polar optical phonons and determined the main transport parameters for the case of a GaAs/GaAlAs structure at room temperature. Interaction between the levels is found to be responsible for a systematic increase of the level energy with respect to the bottom of the quantum well at electric fields above about 20 kV/cm. When compared with the non-interacting case, at the highest fields the average carrier energy evidences a consistent increase, which leads to a significant softening of the negative slope of both the drift velocity and diffusivity versus electric field behavior.

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.

Periodic Transient Inversion at Intersubband Laser Transitions in Quantum Wells in an External Electric Field

A method of the periodic transient inversion of an intersubband laser transition in a quantum well (QW) with the repetition rate < 1 GHz and the inversion lifetime of ~ 1 ps is suggested. The method is based on the rapid population of the upper transition level via carrier resonant tunneling from an adjacent QW under not an adiabatic (as in previous studies of the problem), but a quick biased electric field change. It allows one to speed up this process significantly (by an order of magnitude) employing the same fields, and thereby, reach a peak inversion of 4 times 1010 cm-2 at room temperature for a transition corresponding to lambda lsim 10 mum. This makes it possible to achieve the fivefold power amplification of picosecond mid-infrared pulses using three consequent synchronized QW heterostructures (the cascade scheme). The possibility of operating at room temperature renders this method attractive for various applications.