Valence Band Mixing Research Articles

Physical origins of optical anisotropy in quantum-confined semiconductors: The roles of valence band mixing, transition broadening, and state filling

We investigate experimentally and theoretically the impact of valence band mixing and spectrum of confined states on the polarization of light emitted from or absorbed by GaAs/AlGaAs semiconductor quantum dots and quantum wires with tailored heterostructure potential. In particular, such nanostructures with parabolic-profile confinement potentials, realized by organometallic vapor phase epitaxy inside pyramidal pits, served as model systems for the study. Different degrees of linear polarization (DOLP) of emitted light, depending on the confinement potential profile, the specific excitonic transition, and the level of excitation, are observed. A theoretical model shows that, besides the impact of valence band mixing, the overlap of conduction and valence band wavefunctions as well as state occupation probability and broadening of transitions determine the DOLP. The conclusions are useful for the design of quantum light emitters with controlled polarization properties.

Light Hole Excitons in Strain-Coupled Bilayer Quantum Dots with Small Fine-Structure Splitting

In this work, we measure polarization-resolved photoluminescence spectra from excitonic complexes in tens of single InAs/GaAs quantum dots (QDs) at the telecom O-band with strain-coupled bilayer structure. QDs often show fine-structure splitting (FSS) ~100 μeV in uniform anisotropy and valence-band mixing of heavy holes (HH) and light holes (LH); the biaxial strain also induces LH excitons with small FSS (especially XX, <5 μeV, 70% of QDs); delocalized LH reduces the Coulomb interaction between holes Vhh and enhances population on LH excitons XX, XX11, X11+ and XX21+.

Selection rules for the excitation of quantum dots by spatially structured light beams: Application to the reconstruction of higher excited exciton wave functions

Spatially structured light fields applied to semiconductor quantum dots yield fundamentally different absorption spectra than homogeneous beams. In this paper, we theoretically discuss the resulting spectra for different light beams using a cylindrical multipole expansion. For the description of the quantum dots we employ a model based on the effective mass approximation including Coulomb and valence band mixing. The combination of a single spatially structured light beam and state mixing allows all exciton states in the quantum dot to become optically addressable. Furthermore, we demonstrate that the beams can be tailored such that single states are selectively excited, without the need of spectral separation. Using this selectivity, we propose a method to measure the exciton wave function of the quantum dot eigenstate. The measurement goes beyond electron density measurements by revealing the spatial phase information of the exciton wave function. Thereby polarization sensitive measurements are generalized by including the infinitely large spatial degree of freedom.

Dielectric Confinement and Excitonic Effects in Two-Dimensional Nanoplatelets.

Quasi-two-dimensional (2D) semiconductor nanoplatelets manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision. We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and density of excited states in a series of CdSe nanoplatelets. The tunneling spectra, measured in the double-barrier tunnel junction configuration, reveal the effect of quantum confinement on the band gap taking place mainly through a blue-shift of the conduction band edge, along with a signature of 2D electronic structure intermixed with finite lateral-size and/or defects effects. The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations. The calculations also point to strong valence band mixing between the light- and split-off hole levels. Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. Possible explanations for this, including an interplay of nanoplatelet charging, dielectric confinement, and difference in exciton binding energy for light and heavy holes, are analyzed and discussed.

Electronic and optical properties of highly boron-doped epitaxial Ge/AlAs(001) heterostructures

The impact of elemental boron (B) doping on the structural, optical, and magnetotransport properties of epitaxial Ge/AlAs/GaAs(001) heterostructures, grown by solid-source molecular beam epitaxy, was comprehensively investigated. Cross-sectional transmission electron microscopy analysis revealed atomically abrupt Ge:B/AlAs and AlAs/GaAs heterointerfaces and a lack of observable long-range defect formation or B segregation in the epitaxial Ge:B layer. Spectral broadening observed in the measured temperature-dependent photoluminescence spectra suggested valence band mixing during recombination, implying a splitting of the valence band heavy- and light-hole degeneracy due to residual strain resulting from substitutional B incorporation in the Ge epilayer. Temperature-dependent magnetotransport analysis of the B-doped Ge thin films exhibited the tell-tale signature of antilocalization, indicating observable spin–orbit interaction in the Ge:B system. Moreover, the temperature- and magnetic field-dependent magnetotransport results indicate the presence of single-carrier, p-type conduction in the Ge:B film, further affirming the successful incorporation and activation of B at a high concentration (∼4 × 1019 cm−3) and elimination of parallel conduction via the large-bandgap AlAs buffer. Together, these results provide insights into the effects of heavy doping (via elemental solid-source doping) on Ge-based heterostructures and their feasibility in future electronic and photonic applications.

Valence Band Mixing in GaAs/AlGaAs Quantum Wells Adjacent to Self-Assembled InAlAs Antidots

Optical properties of GaAs/AlGaAs quantum wells (QWs) in the vicinity of InAlAs quantum dots (QDs) were studied and compared with a theoretical model to clarify how the QD strain affects the electronic states in the nearby QW. In0.4Al0.6As QDs are embedded at the top of the QWs; the QD layer acts as a source of strain as well as an energy barrier. Photoluminescence excitation (PLE) measurements showed that the QD formation leads to the increase in the ratio Ie-lh/Ie-hh of the PLE intensities for the light hole (lh) and the heavy hole (hh), indicating the presence of the valence band mixing. We also theoretically calculated the hh-lh mixing in the QW due to the nearby QD strain and evaluated the PLE ratio Ie-lh/Ie-hh.

Band Structure and Optical Gain of InGaAs/GaAsBi Type-II Quantum Wells Modeled by the k · p Model**Supported by the National Basic Research Program of China under Grant No 2014CB643902, the Key Program of Natural Science Foundation of China under Grant No 61334004, the National Natural Science Foundation of China under Grant No 61404152, and the Strategic Priority Research Program of

Optical gains of type-II InGaAs/GaAsBi quantum wells (QWs) with W, N, and M shapes are analyzed theoretically for near-infrared laser applications. The bandgap and wave functions are calculated using the self-consistent k · p Hamiltonian, taking into account valence band mixing and the strain effect. Our calculations show that the M-shaped type-II QWs are a promising structure for making 1.3 μm lasers at room temperature because they can easily be used to obtain 1.3 μm for photoluminescence with a proper thickness and have large wave-function overlap for high optical gain.

Magnetic field induced mixing of light hole excitonic states in (Cd, Mn)Te/(Cd, Mg)Te core/shell nanowires

A detailed magneto-photoluminescence study of individual (Cd, Mn)Te/(Cd, Mg)Te core/shell nanowires grown by molecular beam epitaxy is performed. First of all, an enhancement of the Zeeman splitting due to sp-d exchange interaction between band carriers and Mn-spins is evidenced in these nanostructures. Then, it is found that the value of this splitting depends strongly on the magnetic field direction with respect to the nanowire axis. The largest splitting is observed when the magnetic field is applied perpendicular and the smallest when it is applied parallel to the nanowire axis. This effect is explained in terms of magnetic field induced valence band mixing and evidences the light hole character of the excitonic emission. The values of the light and heavy hole splitting are determined for several individual nanowires based on the comparison of experimental results to theoretical calculations.

Design and analysis of InN − In0.25Ga0.75N single quantum well laser for short distance communication wavelength

A single quantum well semiconductor laser based on wurtzite-nitride is designed and analyzed for short distance communication wavelength (at around 1300 nm). The laser structure has 12 Å well layer of InN, 15 Å barrier layer of In0.25Ga0.75N, and 54 Å separate confinement heterostructure layer of GaN. To calculate the electronic characteristics of the structure, a self-consistent method is used where Hamiltonian with effective mass approximation is solved for conduction band while six-bands Hamiltonian matrix with k · p formalism including the polarization effect, valence-band mixing effect, and strain effect is solved for valence band. The interband optical transition elements, optical gain, differential gain, radiative current density, spontaneous emission rate, and threshold characteristics have been calculated. The wave function overlap integral is found to be 45.93% for TE-polarized structure. Also, the spontaneous emission rate is found to be 6.57 × 1027 s − 1 cm − 3 eV − 1 at 1288.21 nm with the carrier density of 5 × 1019 cm − 3. Furthermore, the radiative current density and the radiative recombination rate are found to be 121.92 A cm − 2 and 6.35 × 1027 s − 1 cm − 3, respectively, while the TE-polarized optical gain of the structure is 3872.1 cm − 1 at 1301.7 nm.

Multi-excitonic emission from Stranski-Krastanov GaN/AlN quantum dots inside a nanoscale tip

Single-dot time-resolved micro-photoluminescence spectroscopy and correlated electron tomography (ET) have been performed on self-assembled GaN/AlN quantum dots isolated within a field-emission nanoscale tip by focused ion beam (FIB). Despite the effect of the FIB, the system conserves the capability of emitting light through multi-excitonic complexes. The optical spectroscopy data have then been correlated with the electronic structure and lifetime parameters that could be extracted using the structural parameters obtained by ET via a 6 band k.p model. A biexciton-exciton cascade could be identified and thoroughly analysed. The biexciton-exciton states exhibit a non-negligible polarization component along the [0001] polar crystal axis, indicating a significant valence band mixing, while the relationship between exciton energy and biexciton binding energy is consistent with a hybrid character of the biexciton.

Optical polarization switching in semipolar (202¯1) InGaN multiple quantum wells induced by strain engineering

Semipolar (202¯1) InGaN/AlGaN multiple quantum wells (MQWs) with a low In content (<0.05) were coherently grown on a partially relaxed AlxGa1−xN (0 < XAl < 0.4) layer. As XAl increased from 0.15 to 0.38, the degree of optical polarization measured by photoluminescence at 13 K changed from 0.49 to −0.24, where (−) indicates the change in the dominant polarization direction from [112¯0] to [1014¯]. Regardless of XAl and thickness, no polarization switching was observed if AlxGa1−xN was fully strained to the GaN substrate. Band structure modeling based on the k⋅p method implies that the polarization switching is associated with strong valence band mixing induced by the one-dimensional strain relaxation in AlGaN, which leads to a significantly stronger compressive strain in MQWs along [1014¯] relative to [112¯0]. These results demonstrate that the optical polarization switching in semipolar MQWs can be controlled by engineering the strain state of the underlying layer.

Estimation of electron spin polarization from circularly polarized photoluminescence in strained quantum wells

A general method based on the valence band mixing is proposed to establish a correlation between electron spin polarization and the degree of circular polarization of luminescence in a strained quantum well (QW). Key issues that are critical for establishing such a correlation are highlighted by invoking 4 band k.p calculations in a strained QW system. The proposed method takes care of the complex valence band mixing in strained QWs by simultaneously analyzing the heavy hole (e-hh) and light hole (e-lh) related luminescence transitions along the quantization direction. It is shown that the ratio of the degree of circular polarizations measured for the e-hh and e-lh features is a sensitive probe to estimate the values of band mixing. The method is reasonably supported by carefully conducted experiments on a tensile strained GaAsP/AlGaAs QW sample in the temperature range of 10 to 150 K. The proposed method shall be helpful for understanding the spin injection/transfer across the interfaces of QW, especially for the cases where heavy hole-light hole coupling is expected to be strong, and both the features are experimentally observed. It might lead to new directions in the field of semiconductor spin-optoelectronics in QWs.

Analysis of polarized light generation in anisotropic strained quantum dots

A theoretical analysis of the optical characteristics and polarization properties of quantum dot laser which uses interband transitions in shape asymmetric InGaAs/GaAs QDs is presented. We use multi-band effective mass k.p model with the strain effect taken into account to compute the electronic structure of asymmetric QDs. In-plane shape asymmetry in QDs is accounted through the anisotropic parabolic potential. The optical gain spectrum is calculated using the density matrix approach. We have solved coupled set of rate equations numerically using fourth-order Runge Kutta method to analyze the dynamic characteristics of InGaAs/GaAs quantum dot laser. The effect of shape anisotropy, material composition, carrier densities and strain on light emission polarization ratio has been studied. The dependence of gain on polarization direction has been explained in terms of the valence band mixing. The polarization is found to increase almost linearly with anisotropy parameter and decreases with the material composition. Furthermore, it is demonstrated that anisotropy and strain can be used to control the polarization. We found that the output has preferential polarization properties depending on the QD structure and composition.

Spontaneous brightening of dark excitons in GaAs/AlGaAs quantum dots near a cleaved facet

Dark excitons (DEs) confined in epitaxial quantum dots (QDs) are interesting because of their long lifetime compared to bright excitons (BEs). For the same reason they are usually difficult to access in optical experiments. Here we report on the observation of vertically polarized light emission from DEs confined in high-quality epitaxial GaAs/AlGaAs QDs located in proximity of a cleaved facet of the QD specimen. Calculations based on the eight-band k\ifmmode\cdot\else\textperiodcentered\fi{}p method and configuration interaction allow us to attribute the brightening of the DE to the anisotropic strain present at the sample edge, which breaks the symmetry of the system and enhances valence-band mixing. The mechanism of DE brightening is discussed in detail by inspecting both the Bloch and envelope wave functions of the involved hole states. In addition, by investigating experimentally and theoretically QDs with different sizes, we find that the energy separation between DE and BEs tends to decrease with increasing QD height. Finally, the presence of a cleaved facet is found also to enhance the BE fine structure splitting. This work provides a simple method to optically probe dark excitonic states in QDs and shows that the properties of QDs can be significantly affected by the presence of nearby edges.

Characterization of InN-In0.25Ga0.75N Quantum Well Laser with In0.4Al0.6N Layers for 1300 nm Band

ABSTRACTA wurtzite-strained nitride Quantum Well Laser has been characterized for short distance communication wavelength. InN and In0.25Ga0.75N have been chosen as well material and barrier material respectively with In0.4Al0.6N SCH layers at the end of barrier layers to improve the carrier and photon confinement within the active region. This structure shows less compressive strain (7.33%) with respect to previously proposed structure which makes the structure more suitable for fabrication. To obtain the electronic band structure, self-consistent method with k.p formalism has been performed where valence band mixing effect, strain effect and spontaneous and piezoelectric polarization effect has been included. From the electronic characteristics, the optical properties have been performed with numerical model. From the optical properties, the structure has been found as TE polarized with C1-HH1, C1-LH1, C2-HH1 and C2-LH1 dominating transition elements. From the performance of the numerical model, 4731.98 cm−1 optical gain for TE polarization at 1315.5 nm emission wavelength and 8.017×1027 cm−3s−1eV−1 spontaneous emission rate at 1301.7nm wavelength have been found for 12Å well width, 17Å barrier width and 52Å SCH layer width at 5×1019 cm−3 carrier density. The obtained properties have been shown a good agreement with previously published works.

Symmetry properties of n-doped (Cd,Mn)Te quantum well photoluminescence spectra: An exemplary evidence for anisotropy-induced valence-band mixing

The symmetry properties of photoluminescence spectra of an n-doped (001)-Cd0.99Mn0.01Te quantum well provide a simple and intuitive demonstration of the valence-band mixing induced by spontaneous in-plane anisotropy of the sample. The spectra were taken with linear-polarization resolution in an in-plane external magnetic field. They consisted of several features which are characteristic for a two-dimensional electron gas. Because the quantum well layer contained “magnetic” ions of Mn, the spectra showed pronounced polarization-dependent modifications induced by the magnetic field. With the field applied, a 90° rotation of the sample about the surface normal axis resulted in a clearly different spectrum, meaning that the nominally equivalent [110] and [11¯0] in-plane directions in the sample are not equivalent in fact. But, remarkably, the additional 90° rotations of both the polarizer and the analyzer restored the initial spectrum. This combined invariance regarding simultaneous 90° rotation of the sample and reversal of the polarization configuration was known earlier for spin-flip Raman spectra only. Our present observations are interpreted in terms of an intermixing of valence subbands leading to a pseudoisotropic g-factor of the ground-state holes.

Electrons, holes, and excitons in GaAs polytype quantum dots

Single and multi-band k⋅p Hamiltonians for GaAs crystal phase quantum dots are used to assess ongoing experimental activity on the role of such factors as quantum confinement, spontaneous polarization, valence band mixing, and exciton Coulomb interaction. Spontaneous polarization is found to be a dominating term. Together with the control of dot thickness [Vainorius et al., Nano Lett. 15, 2652 (2015)], it enables wide exciton wavelength and lifetime tunability. Several new phenomena are predicted for small diameter dots [Loitsch et al., Adv. Mater. 27, 2195 (2015)], including non-heavy hole ground state, strong hole spin admixture, and a type-II to type-I exciton transition, which can be used to improve the absorption strength and reduce the radiative lifetime of GaAs polytypes.

Type-II quantum wells with tensile-strained GaAsSb layers for interband cascade lasers with tailored valence band mixing

Optical properties of modified type II W-shaped quantum wells have been investigated with the aim to be utilized in interband cascade lasers. The results show that introducing a tensely strained GaAsSb layer, instead of a commonly used compressively strained GaInSb, allows employing the active transition involving valence band states with a significant admixture of the light holes. Theoretical predictions of multiband k·p theory have been experimentally verified by using photoluminescence and polarization dependent photoreflectance measurements. These results open a pathway for practical realization of mid-infrared lasing devices with uncommon polarization properties including, for instance, polarization-independent midinfrared light emitters.

Characterization of InN - In0.25Ga0.75N Quantum Well Laser Structure for 1330 nm Wavelength

A nitride based wurtzite-strained QW laser with 12Å InN well layer, 15Å In0.25Ga0.75N barrier layer and GaN SCH layer has been designed and characterized at 1330nm wavelength. To determine the electronic properties, a self-consistent method with 6-bands k.p formalism considering valence-band mixing effect, strain and polarization effect followed by Poisson's equation has been used. The interband momentum matrix elements, optical gain, spontaneous emission rate, and radiative current density have been calculated to analyze the optical properties of the laser. Due to the strain effect, the wave-function overlap integral was obtained as 43.27%. The structure is TE polarized with C1-HH1 and C1-LH1 dominating transitions, while the spontaneous emission rate per energy interval per unit volume was 7.21×1027 s-1cm-3eV-1 at 1329.55nm. Furthermore, the radiative recombination rate and the radiative current density were 7.77×1029 s-1cm-3 and 149.19 Acm-2, respectively. The optical gain of the structure is 5261.52cm-1 at 1336.7nm for TE-polarization.

Optical Gain Optimization of Al0.8Ga0.2N-Delta-GaN Quantum Well Laser in Ultraviolet Spectra Using Genetic Algorithm

An AlGaN based quantum well laser with GaN delta layer and AlN barrier layers has been designed and characterized for obtaining electronic and optical characteristics. The band structure was calculated with self-consistent 6-band k.p formalism including valence-band mixing effect, strain and polarization effect. A Genetic Algorithm based optimization has been performed to optimize the optical gain taking well width, delta width and barrier width into consideration. The Optimization of both TE and TM polarized optical gain exhibits good improvement than previously published work. The peak optical gain as obtained in analysis is 3589.5 cm−1 while the peak spontaneous emission rate is 12.94×1028 s-1cm-3eV-1 at 246.89nm for 5×1019cm-3 carrier density. The optimum TE optical gain is found to be 6479.1 cm-1 for 11Å QW with 7Å delta layer at 279nm wavelength and optimum TM gain is found to be 116.95 cm-1 for 20Å QW with 3Å delta layer at 289.8nm wavelength.