Band Structure Of Quantum Wells Research Articles

Quantum confinement of carriers in the type-I quantum wells structure

Abstract Quantum confinement is recognized to be an inherent property in low-dimensional structures. Traditionally, it is believed that the carriers trapped within the well cannot escape due to the discrete energy levels. However, our previous research has revealed efficient carrier escape in low-dimensional structures, contradicting this conventional understanding. In this study, we review the energy band structure of quantum wells along the growth direction considering it as a superposition of the bulk material dispersion and quantization energy dispersion resulting from the quantum confinement across the whole Brillouin zone. By accounting for all wave vectors, we obtain a certain distribution of carrier energy at each quantized energy level, giving rise to the energy subbands. These results enable carriers to escape from the well under the influence of an electric field. Additionally, we have compiled a comprehensive summary of various energy band scenarios in quantum well structures relevant to carrier transport. Such a new interpretation holds significant value in deepening our comprehension of low-dimensional energy bands, discovering new physical phenomena, and designing novel devices with superior performance.

The strain, energy band and photoluminescence of GaAs0.92Sb0·08/Al0.3Ga0.7As multiple quantum wells grown on GaAs substrate

GaAsSb based materials have become the promising system for infrared semiconductor lasers and detectors. In this article, the strain, energy band structures and photoluminescence (PL) of GaAs0.92Sb0·08/Al0.3Ga0.7As strained quantum wells (QWs) grown with molecular beam epitaxy are systematically analyzed both theoretically and experimentally. The theoretical results are derived by Kane's model and k⋅p method, and the optical properties of a high-quality GaAs0.92Sb0·08/Al0.3Ga0.7As strained QWs sample are thoroughly investigated by excitation- and temperature-dependent PL measurements. The theoretical results show the strain has significant influence on the band structure of QWs. In experimental part, it is found that the light-hole exciton emission coexists with the heavy-hole exciton line in the temperature range of 50 K–300 K. However, the emission of localized excitons, which is caused by the nonuniformity of component in the GaAsSb well layer, takes over the light-hole exciton emission at lower temperatures (<50 K).

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.

Strain-optoelectronic coupling properties of externally deformed nanoribbons with embedded quantum well

Strain-engineering provides a critical mechanism for tuning the band structure of quantum wells (QWs). However, due to the inherent rigidity of the semiconductor materials, the strain induced by lattice-mismatch is restricted to be invariable, uniform, thus cannot be subject to direct mechanical loading processes. Therefore, the flexible nanoribbons (NRs) in a variety of configurations offer entirely new research perspectives in modulation of the optoelectronic characteristic of QWs by curvature-induced inhomogeneous strain. In this paper, we propose a strategy to fabricate the QW layer embedded in uniaxially wavy NRs. To thoroughly figure out the internal strain-optoelectronic coupling mechanism, we establish a simple and accurate calculation model for externally deformed QW NRs using the eight-band k·p perturbation method for the first time. Luttinger-Kohn-Pikus-Bir Hamiltonian (LKPBH) for strained semiconductor is adopted and solved by finite-difference method (FDM). Theoretical calculations reveal inclined band edges, which result in the quantum-confined stark effect (QCSE) in the bended QW NRs. Continuously and periodically varied band gap of the wavy QW is revealed with a blue-shift from peak to valley of the sample NR, which agrees well with the μ-photoluminescence measurements. Further adjustments of the external configuration are explored to study the impact of the larger deformed structure on the curvature and band gap of the wavy QW within the confinement of the fracture limit.

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.

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.

Spectroscopic method of strain analysis in semiconductor quantum-well devices

The modification of the electronic bandstructure in a semiconductor, quantum well due to an induced strain is well known. Recently, we have developed a generalized, spectroscopic-based technique for analyzing the strain condition within devices based on quantum wells. This approach couples experimental data describing interband transition energies within strained, quantum-well devices with a rigorous theoretical description of the quantum-well bandstructure. The theoretical formalism is described, and various important theoretical predictions necessary in the application of this method are given. The accuracy of the theoretical model used in this approach is critical to its success, and it is therefore necessary to ensure the validity of the theoretical formalism as applied to quantum wells under a variety of strain conditions. We show the good agreement between theory and experiment for a number of known strain conditions within quantum wells and quantum-well devices. This agreement indicates the validity of the theoretical formalism in the method being described, and the applicability of the method to a wide class of quantum-well based semiconductor devices. A key result in applying this method is the piecewise linearity of the change in interband transition energy with strain for the ranges of strain of interest. The method is then applied to the important case of packaging-induced strain in high-power diode lasers or “cm-bars” as they are sometimes known. Experimental results indicate that the method provides an excellent means of analyzing packaging-induced strain in cm-bars and similar devices.

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.

What limits the maximum output power of long-wavelength AlGaInAs/InP laser diodes?

We analyze the high-temperature continuous-wave performance of 1.3-/spl mu/m AlGaInAs/InP laser diodes grown by digital alloy molecular-beam epitaxy. Commercial laser software is utilized that self-consistently combines quantum-well bandstructure and gain calculations with two-dimensional simulations of carrier transport, wave guiding, and heat flow. Excellent agreement between simulation and measurements is obtained by careful adjustment of material parameters in the model. Joule heating is shown to be the main heat source; quantum-well recombination heat is almost compensated for by Thomson cooling. Auger recombination is the main carrier loss mechanism at lower injection current. Vertical electron escape into the p-doped InP cladding dominates at higher current and causes the thermal power roll-off. Self-heating and optical gain reduction are the triggering mechanisms behind the leakage escalation. Laser design variation is shown to allow for a significant increase in the maximum output power at high temperatures.

Thermal Annealing of InGaN/GaN Strained-Layer Quantum Well

Quantum well (QW) material engineering has attracted a considerable amount of interest from many people because of its ability to produce a number of optoelectronic devices. QW composition intermixing is a thermal induced interdiffusion of the constituent atoms through the hetero-interface. The intermixing process is an attractive way to achieve the modification of the QW band structure. It is known that the band structure is a fundamental determinant for such electronic and optical properties of materials as the optical gain, the refractive index and the absorption. During the process, the as-grown square-QW compositional profile is modified to a graded profile, thereby altering the confinement profile and the subband structure in the QW. The blue-shifting of the wavelength in the intermixed QW structure is found in this process.In recent years, III-nitride semiconductors have attracted much attention. This is mainly due to their large bandgap range from 1.89eV (wurtzite InN) to 3.44eV (wurtzite GaN). InGaN/GaN quantum well structures have been used to achieve high lumens blue and green light emitting diodes. Such structures also facilitate the production of full colour LED displays by complementing the colour spectrum of available LEDs.In this paper, the effects of thermal annealing on the strained-layer InGaN/GaN QW will be presented. The effects of intermixing on the confinement potential of InGaN/GaN QWs have been theoretically analysed, with sublattices interdiffusion as the basis. This process is described by Fick’s law, with constant diffusion coefficients in both the well and the barrier layers. The diffusion coefficients depend on the annealing temperature, time and the activation energy of constituent atoms. The optical properties of intermixed InGaN/GaN QW structure of different interdiffusion rates have been theoretically analyzed for applications of novel optical devices. The photoluminescence studies and the intermixed QW modeling have been used to understand the effects of intermixing.

Theoretical analysis of 630-nm band GaInP-AlGaInP strained quantum-well lasers considering continuum states

GaInP-AlGaInP strained quantum-well lasers with emission wavelength at 630-nm band are theoretically analyzed in detail and then optimized. The valence band structure of quantum wells is obtained by evaluating the 6/spl times/6 Luttinger-Kohn Hamiltonian including the coupling among the heavy hole, the light hole, and the spin-orbital spilt-off hole bands. The effect of optical transition from/to continuum states not confined to the quantum well is studied. It is found that the optical transition from/to the continuum states is serious as the band gap of the confining layers is close to the quasi-Fermi level separation, leading to considerable radiative current. This radiative current is undesirable since the corresponding optical transition does not contribute significantly to the threshold gain. The gain-radiative current characteristic is therefore poor for confining layers containing a low Al content. To avoid unreasonable gain/absorption, the non-Markovian convolution lineshape is used instead of the conventional Lorentzian lineshape. The leakage current is high for single quantum-well lasers with wide bandgap confining layers, it can be reduced by increasing the quantum-well number, the dopant concentration, and the band gap of cladding layers. The calculated threshold current agrees well with the observation. The band gap shrinkage due to the carrier-carrier interaction is considered to obtain an emission wavelength consistent with the experimental result.

Amplified spontaneous emission spectroscopy in strained quantum-well lasers

Amplified spontaneous emission spectroscopy is used to extract the gain and refractive index spectra systematically. We obtain the gain and differential gain spectra using the Hakki-Paoli method. The refractive index profile, the induced change in refractive index by an incremental current, and the linewidth enhancement factor are measured from the Fabry-Perot peaks and the current-induced peak shifts in the amplified spontaneous emission spectra. The measured optical gain and refractive index are then compared with our theoretical model for strained quantum-well lasers. We show that a complete theoretical model for calculating the electronic band structure, the optical constant, and the linewidth enhancement factor agrees very well with the experiment. Our approach demonstrates that amplified spontaneous emission spectroscopy can be a good diagnostic tool to characterize laser diodes, extract the optical gain and index profiles, and confirm material parameters such as the strained quantum-well band structure parameters for a semiconductor structure under carrier injection.

Microwave and millimeter wave generation using nonlinear optical mixing in asymmetric quantum wells

The possibility of microwave and millimeter wave generation using nonlinear difference-frequency mixing of optical lasers in an asymmetric quantum well (QW) is examined. In principle, this method imposes no limit on the frequency of generated microwave signals. It is shown that these structures are particularly suitable for enhancing the susceptibility of difference-frequency generation, χ(2)(Ω;ω1,−ω2); this originates from the ability to tune the fundamental photon energies ℏω1 and ℏω2 to valence-conduction subband transitions, while ℏΩ is tuned to intervalence and interconduction subband transitions. Under such conditions, the existence of a broad resonant enhancement region of χ(2), which is closely associated with the QW band structure is demonstrated. Enhancements of χ(2)xzx and χ(2)zxx exceeding 10 and 104 times over the bulk values are predicted, respectively.

Spin-flip-induced hole burning in GaAs quantum wells: Determination of the exciton Zeeman splitting.

A new method of four-wave-mixing spectroscopy in GaAs quantum wells reveals spectral hole burning due to spin relaxation of magnetoexcitons. The measurements resolve the Zeeman doublet of the lowest-energy heavy-hole exciton where the doublet splitting is much less than the exciton inhomogeneous width. The Zeeman splitting depends nonlinearly on the magnetic field and is small compared with that of bulk GaAs. The results reflect effects of the complex band structure of quantum wells. Information on exciton spin relaxation is also provided by the hole-burning measurements.

Band structure of quantum wells under crossed electric and magnetic fields.

We investigate the band structure in GaAs quantum wells in the presence of an electric field applied along the growth direction (z) and a magnetic field perpendicular to it. The strong coupling between heavy and light holes gives rise to a nonquadratic behavior of the hole states as a function of the magnetic field. The presence of the electric field introduces new transitions by breaking the z symmetry and by coupling with the magnetic field. The Kramers degeneracy is also lifted by the external fields. The results are discussed in terms of band-to-band transitions.