Quantum-well Band Research Articles

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.

Cathodoluminescence as an effective probe of carrier transport and deep level defects in droop-mitigating InGaN/GaN quantum well heterostructures

Commercial InGaN/GaN light emitting diodes continue to suffer from efficiency droop at high current densities, and urgently require enhanced structural-optical toolsets for active region characterization. In our work, we measure delayed (tens of seconds) cathodoluminescence which is influenced by carrier transport and deep level defects. Further, we observe that drops in efficiency are not correlated with quantum well (QW) width fluctuations. To explain the rise dynamics, we propose a model involving filling of deep level defects and simultaneous reduction of built-in field within the multi-QW active region. These measurements yield insights into carrier transport, efficiency-reducing defects, and QW band structure.

Effect of electric field on the ratio between the rashba and dresselhaus parameters in III–V heterostructures

The size-quantized energy subbands and envelope wave functions for [001] quantum wells based on zinc-blende III–V semiconductors are numerically calculated using the eight-band Kane model and finite-difference discretization scheme in coordinate space. The effect of the quantum-well band parameters and external electric field oriented along the structure growth direction on the ratio between the Rashba and Dresselhaus spin–orbit coupling parameters is studied. It is demonstrated that at certain electric-field values the spin–orbit coupling parameters in GaAs/InGaAs structures can be equal, which ensures the condition for forming stable spin helices. In addition, it is established that the spin–orbit coupling linear in wave vector in symmetric GaAs/InGaAs wells can disappear under certain well widths and barrier chemical compositions.

High Quantum Yield Blue Emission from Lead-Free Inorganic Antimony Halide Perovskite Colloidal Quantum Dots.

Colloidal quantum dots (QDs) of lead halide perovskite have recently received great attention owing to their remarkable performances in optoelectronic applications. However, their wide applications are hindered from toxic lead element, which is not environment- and consumer-friendly. Herein, we utilized heterovalent substitution of divalent lead (Pb2+) with trivalent antimony (Sb3+) to synthesize stable and brightly luminescent Cs3Sb2Br9 QDs. The lead-free, full-inorganic QDs were fabricated by a modified ligand-assisted reprecipitation strategy. A photoluminescence quantum yield (PLQY) was determined to be 46% at 410 nm, which was superior to that of other reported halide perovskite QDs. The PL enhancement mechanism was unraveled by surface composition derived quantum-well band structure and their large exciton binding energy. The Br-rich surface and the observed 530 meV exciton binding energy were proposed to guarantee the efficient radiative recombination. In addition, we can also tune the inorganic perovskite QD (Cs3Sb2X9) emission wavelength from 370 to 560 nm via anion exchange reactions. The developed full-inorganic lead-free Sb-perovskite QDs with high PLQY and stable emission promise great potential for efficient emission candidates.

Performance improvement by enhancing the well-barrier hole burning in a quantum well semiconductor optical amplifier

In this paper, we demonstrated a novel physical mechanism based on the well-barrier hole burning enhancement in a quantum well (QW) semiconductor optical amplifier (SOA) to improve the operation performance. To completely characterize the physical mechanism, a complicated theoretical model by combining QW band structure calculation with SOA’s dynamic model was constructed, in which the carrier transport, interband effects and intraband effects were all taken into account. The simulated results showed optimizing the thickness of the separate confinement heterostructure (SCH) layer can effectively enhance the well-barrier hole burning, further enhance the nonlinear effects in SOA and reduce the carrier recovery time. At the optimal thickness, the SCH layer can store enough carrier numbers, and simultaneously the stored carriers can also be fast and effectively injected into the QWs.

Theoretical study of O- and Zn-face polarity effect on the optical properties of the conventional and staggered ZnO/Zn1 − xCdxO/ZnO quantum wells

In this work we present a comparative study of Zn-face and O-face polarity Zn1−xCdxO-based conventional and staggered quantum-well (QW) structures. The calculation of optical properties of QWs was performed by means of self-consistent Schrodinger–Poisson solver with consideration of polarization-induced effects. The conventional Zn-face and O-face QWs possess similar values of transition energy and an overlap of electron and hole wave functions. A change of the polarity from Zn-face to O-face for the conventional QWs influences only a shape of the conduction and valence band edge profile. It is revealed that the utilization of the staggered QWs leads to an improvement of the confinement characteristics. In addition, the O-face staggered QW structure has larger values of transition energy and overlap integral compared to the Zn-face staggered QW structure. O-terminated staggered QW structure is less dependent on the well thickness and has lower sensitivity to Cd content in embedded Zn1−xCdxO layer. Control of the material polarity and design of the staggered QWs provide cost-effective approach for engineering the QW band structures with enhanced QW performance. This enables constructing of the Zn1−xCdxO-based light emission diodes with improved radiative efficiency emitting, applicable for solid state lighting.

Experimental demonstration and devices optimization of NRZ-DPSK amplitude regeneration scheme based on SOAs.

We propose and experimentally demonstrate a novel scheme which can simultaneously realize wavelength-preserving and phase-preserving amplitude noise compression of a 40 Gb/s distorted non-return-to-zero differential-phase-shift keying (NRZ-DPSK) signal. In the scheme, two semiconductor optical amplifiers (SOAs) are exploited. The first one (SOA1) is used to generate the inverted signal based on SOA's transient cross-phase modulation (T-XPM) effect and the second one (SOA2) to regenerate the distorted NRZ-DPSK signal using SOA's cross-gain compression (XGC) effect. In the experiment, the bit error ratio (BER) measurements show that power penalties of constructive and destructive demodulation at BER of 10-9 are -1.75 and -1.01 dB, respectively. As the nonlinear effects and the requirements of the two SOAs are completely different, quantum-well (QW) structures has been separately optimized. A complicated theoretical model by combining QW band structure calculation with SOA's dynamic model is exploited to optimize the SOAs, in which both interband effect (carrier density variation) and intraband effect (carrier temperature variation) are taken into account. Regarding SOA1, we choose the tensile strained QW structure and large optical confinement factor to enhance the T-XPM effect. Regarding SOA2, the compressively strained QW structure is selected to reduce the impact of excess phase noise induced by amplitude fluctuations. Exploiting the optimized QW SOAs, better amplitude regeneration performance is demonstrated successfully through numerical simulation. The proposed scheme is intrinsically stable comparing with the interferometer structure and can be integrated on a chip, making it a practical candidate for all-optical amplitude regeneration of high-speed NRZ-DPSK signal.

Plasmaron excitations in p(2×2)-K/graphite

A new type of plasmarons formed by the compound of photoelectrons and acoustic surface plasmon excitations is investigated in the system p(2x2)-K/Graphite. The physics behind these types of plasmarons, e-plasmarons, is different from the ones recently found in graphene where the loss feature is argued to result from the photohole-plasmon interaction, h-plasmarons. Based on first principles methods we calculate the dispersion of e-plasmaron excitation rate which yields a broad feature below the parabolic quantum-well band with a peak about 0.4 eV below the quantum well band in the Gamma-point.

Determination of the quantum-well thickness of ZnO-ZnMgO core-shell cylindrical heterostructures by using interband optical transitions

ZnO hexagonal core nanorods with an average diameter of 55 nm were initially grown by using the hydrothermal method; then, ZnO-Zn1−x Mg x O quantum wells were deposited around the ZnO core nanorods by using the atomic layer deposition technique. To investigate the optical band structures, we measured the photoluminescence transitions of as-grown ZnO nanorods, the ZnO-ZnMgO coreshell and the ZnO-ZnMgO core-shell + quantum wells. To determine the thickness of quantum wells, we attempted to solve the self-consistent nonlinear Poisson-Schrodinger equation for two-dimensional hexagonal quantum-well band structures. Using the experimental and the calculated results, we were able to quantitatively determine the quantum-well thicknesses.

Quantum confinement in two dimensional layers of PbSe/ZnSe multiple quantum well structures

The structural and optical properties of thermally evaporated PbSe/ZnSe multiple quantum well (MQW) structures as a function of the PbSe quantum well (QW) layer thickness in the range between 2.5 and 10 nm have been investigated. An ordered periodicity in the MQW structure was confirmed by transmission electron microscopy. The discrete linear resonances in the absorption spectra and the corresponding blue shift observed with decreasing well layer thickness unambiguously reflect the quantum confinement effect. The effective QW band gap is calculated from the infinite well approximation and compared to the experimentally observed value. The QW emission is identified and discussed.

Extremely confined quantum-well states for (2×2)-K and (3×3)-K on graphite: First-principles calculations

We report on the atomic and electronic structure obtained by first-principles density functional theory calculations for a (2 x 2)-K monolayer as well as a (root 3 x root 3)-K monolayer on graphite represented by an 11-layer carbon slab. In both cases, the calculations predict that the K atoms reside above hollows [2.93 angstrom above the surface atomic layers for (2 x 2)-K]. The electronic structure is characterized by a partially occupied, free-electron-like overlayer quantum-well (QW)-state band (E(F) - 0.76 eV at (Gamma) over bar, 1.14m(e)) and one empty QW band (E(F) + 1.7 eV, 1.0m(e)), for (2 x 2)-K. The partially filled QW band, which has an energy and dispersion close to that which has been experimentally observed, provides examples of extremely confined states with nearly all charge (93%) deposited in the overlayer. In the substrate, the layer-confined sigma bands are rigidly downshifted, by 0.6 eV for the outermost carbon-atom layer and by 0.14 eV for the second layer. For the pi electrons, downshifted bands are split off from the ladder of closely spaced band energies, which approximate the continuum of states in a thick graphite sample. The splitting off can be regarded as the formation of a QW state, since the electrons are found mainly in the outermost layer of carbon atoms. The split-off band is not obtained via a downshift of the band with the lowest energy among the closely spaced states, since the band-edge states of the bare substrate have only a small amplitude in the outermost layer of carbon atoms. The high degree of confinement for both the K overlayer states and the C underlayer pi states should make the system of interest for studies of the excitations of two near-two-dimensional electron gases at short distance.

Recent results for single‐junction and tandem quantum well solar cells

AbstractThe band gap of the quantum well (QW) solar cell can be adapted to the incident spectral conditions by tailoring the QW depth. The single‐junction strain‐balanced quantum well solar cell (SB‐QWSC) has achieved an efficiency of 28.3%. The dominant loss mechanism at the high concentrator cell operating bias is due to radiative recombination, so a major route to further efficiency improvement requires a restriction of the optical losses. It has been found that (100) biaxial compressive strain suppresses a mode of radiative recombination in the plane of the QWs. As biaxial strain can only be engineered into a solar cell on the nanoscale, SB‐QWSCs are seen to have a fundamental efficiency advantage over equivalent bulk cells. Strain‐balanced quantum wells in multi‐junction solar cells can current match the sub‐cells without the introduction of dislocations. Calculations are shown which predict efficiency limits as a function of QW absorption and band gap for such cells. A dual‐junction InGaP/GaAs solar cell with QWs in the bottom sub‐cell has been grown and characterized. Laboratory and calculated efficiencies relative to control cells are presented for the reported cell and a modeled device, respectively. Copyright © 2011 John Wiley & Sons, Ltd.

Photoelectron driven acoustic surface plasmons inp(2×2)K/Be(0001):Ab initiocalculations

We report on low-energy dynamical surface response properties of the $p(2\ifmmode\times\else\texttimes\fi{}2)\text{K}/\text{Be}(0001)$ system calculated within a first-principles approach. It is shown that a partly occupied adsorbate-induced quantum-well band dramatically affects the dynamical properties of the beryllium surface. We demonstrate clear evidence that the observed anomalous features in photoemission spectra of this and a similar system can be explained by acoustic surface plasmon excitations.

Direct monitoring of the excited state population in biased SiGe valence band quantum wells by femtosecond resolved photocurrent experiments

The authors report a direct measurement of the optical phonon intersubband hole relaxation time in a SiGe heterostructure and a quantitative determination of hole relaxation under electrically active conditions. The results were obtained by femtosecond resolved pump-pump photocurrent experiments using a free electron laser (wavelength 7.9μm). Additionally, the intensity dependence of the nonlinear photocurrent response was measured. Both types of experiments were simulated using a density matrix description. With one parameter set, a consistent modeling was achieved confirming the significance of the extracted heavy hole relaxation times. For an intersublevel spacing of 160meV, a value of 550fs was obtained.

Photoconductive gain of SiGe/Si quantum well photodetectors

A SiGe/Si Multiple Quantum Wells (MQWs) structure is proposed for highly sensitive photodetection. A large photoconductive gain is obtained because of the unique SiGe/Si band structure, i.e., a large band offset in the valence band, but a small band offset in the conduction band. Such a structure allows the trapping of photogenerated holes inside the valance band quantum wells. Alternatively, photogenerated electrons appear in shallow quantum wells and have relatively high mobility. These give rise to a large photoconductive gain. The calculated photoconductive gain for this structure exceeds 2 × 107. Experimental results confirmed a high gain of the structure.

Quantum-well band structure effects on the emission polarization from a spin-polarized electron reservoir

Many spin-polarized-based devices utilize the optical polarization from semiconductor quantum wells (QWs) as a read out. Under ideal conditions at zero crystal momentum, 100% optical polarization is obtained from these QWs for fully polarized electrons. However, carrier populations typically extend over nonzero crystal momentum states, where band mixing results in nonideal optical polarization. We investigate a single InxGa1−xAs(x=0.2, and 0.08)QW in GaAs in a typical p-i-n spin injector structure, using eight band k.p theory including strain, electric field and quantum-confined Stark effects. By evaluating the carrier distribution and wave functions of the QW states, we find the resulting optical polarization is reduced to ∼60% at 10K, and further for high temperature and high fields. We show that under certain conditions we can flip the sign of the optical polarization, suggesting the possibility of an electric field controlled optical or spin polarization switch.

Controllable band structure in a periodic quantum well

Electron states in a two-dimensional (2D) infinitely deep quantum well, having periodically corrugated walls, are analyzed. It is shown that corrugation causes resonant interaction between the transverse modes of the electron wave function. The interaction results in the non-Bragg nature electron resonances, which give rise to opening of additional band gaps, besides usual Bragg gaps. The position of the non-Bragg and Bragg gaps in the quantum-well band structure depends on a period of corrugation and thickness of the quantum well. For example, for a change in average thickness or period, at some of its value, one of the 2D levels becomes resonant, and it causes the level splitting and the creation of a 2D subband with the negative effective mass. The values of the effective mass and gap width depend also on the phase shift between two periodic boundaries. These quantities vary from zero to a maximum value upon shifting of one periodic boundary with respect to another on the half period of corrugation. In the well having the congruent periodic boundaries, there are no Bragg gaps in the band structure. The lateral periodicity causes the anisotropic stripelike electron-density distribution in such a periodic quantum well.

Postgrowth control of the quantum-well band edge for the monolithic integration of widely tunable lasers and electroabsorption modulators

We describe a quantum-well intermixing process for the monolithic integration of various devices, each with a unique band edge. The process involves a single ion implant followed by multiple etch and anneal cycles. We have applied this method to design and fabricate widely tunable sampled-grating distributed Bragg reflector lasers with integrated electroabsorption modulators. The devices employ three unique band edges, and demonstrate exceptional tuning, gain, and absorption characteristics.

Many particle theory for luminescence in quantum wells

A Green's function theory is applied to the description of luminescence and absorption spectra of low dimensional semiconductors. Progress in the numerical solution of the Bethe Salpeter equation for coupled band quantum wells with a T-matrix structure for the polarisation function and carrier-carrier dephasing is given within an approach that satisfy the Kubo-Martin-Schwinger sum rule and eliminates typical artifacts in computed optical spectra.

Selective excitation and photoinduced bleaching of defects in InAlGaAs/GaAs high-power diode lasers

Mounting-induced defects in semiconductor quantum-well (QW) lasers are investigated by photocurrent spectroscopy. The defects are located in the laser waveguides and give rise to an absorption band below the QW band gap with a maximum absorption cross section of σ=2×10−15 cm2. We observe a nonlinear fully reversible photobleaching of the defects and a resulting increase of QW photocurrent upon continuous wave irradiation of the devices, demonstrating a direct interaction between quantum-confined carriers and a defect level.