Valence Band Mixing Effects Research Articles

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.

Theoretical analysis of influence of random alloy fluctuations on the optoelectronic properties of site-controlled (111)-oriented InGaAs/GaAs quantum dots

We use an $sp^3d^5s^* $ tight-binding model to investigate the electronic and optical properties of realistic site-controlled (111)-oriented InGaAs/GaAs quantum dots. Special attention is paid to the impact of random alloy fluctuations on key factors that determine the fine-structure splitting in these systems. Using a pure InAs/GaAs quantum dot as a reference system, we show that the combination of spin-orbit coupling and biaxial strain effects can lead to sizeable spin-splitting effects in these systems. Then, a realistic alloyed InGaAs/GaAs quantum dot with 25\% InAs content is studied. Our analysis reveals that the impact of random alloy fluctuations on the electronic and optical properties of (111)-oriented InGaAs/GaAs quantum dots reduces strongly as the lateral size of the dot increases and approaches realistic sizes. For instance the optical matrix element shows an almost vanishing anisotropy in the (111)-growth plane. Furthermore, conduction and valence band mixing effects in the system under consideration are strongly reduced compared to standard (100)-oriented InGaAs/GaAs systems. All these factors strongly indicate a reduced fine structure splitting in site-controlled (111)-oriented InGaAs/GaAs quantum dots. Thus, we conclude that quantum dots with realistic (50-80~nm) base length represent promising candidates for polarization entangled photon generation, consistent with recent experimental data.

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.

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.

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

In this work, AlGaN based quantum well laser has been designed with delta layer and characterized the designed laser structure. After the characterization, Genetic Algorithm process has been performed over optical gain to optimize it. In this design, AlN has been used as barrier material, Al0.8Ga0.2N has been utilized as active region material and GaN layer has been used as delta layer. For optimization, well width, delta layer width and barrier width have been taken as the variable for which genetic algorithm has performed the optimization process. Characterization of the designed laser shows good agreement with previously published works. Optimization of optical gain for both TE and TM gain also exhibits good improved result than previously published works. From characterization, it is found that the designed laser structure gives peak optical gain of 3589.5 cm−1 and peak spontaneous emission rate of 12.94×1028 s-1cm-3eV-1 at 246.89 nm for 5×1019 cm-3 carrier density. From optimization, it is obtained that optimum TE optical gain is found 6479.1 cm-1 for 11 Å of QW along with 7 Å delta layer at 279 nm wavelength and optimum TM gain is found found 116.95 cm-1 for 20 Å of QW along with 3 Å delta layer at 289.8 nm wavelength. In this work, electronic characterization has been performed with 6 band k.p Hamiltonian for valence subband calculation including valence band mixing effect, strain effect due to lattice mismatch, spontaneous and piezoelectric polarization. For conduction subband calculation, single band Hamiltonian has been solved with strain and polarization effect. Figure 1

Valence-band mixing effects in the upper-excited-state magneto-optical responses of colloidal Mn2+-doped CdSe quantum dots.

We present an experimental study of the magneto-optical activity of multiple excited excitonic states of manganese-doped CdSe quantum dots chemically prepared by the diffusion doping method. Giant excitonic Zeeman splittings of each of these excited states can be extracted for a series of quantum dot sizes and are found to depend on the radial quantum number of the hole envelope function involved in each transition. As seven out of eight transitions involve the same electron energy state, 1Se, the dominant hole character of each excitonic transition can be identified, making use of the fact that the g-factor of the pure heavy-hole component has a different sign compared to pure light hole or split-off components. Because the magnetic exchange interactions are sensitive to hole state mixing, the giant Zeeman splittings reported here provide clear experimental evidence of quantum-size-induced mixing among valence-band states in nanocrystals.

Dispersions of hole and electron g-factors in single InAs quantum rings evaluated using optically induced nuclear spin polarization

The electron and hole g-factors in individual InAs/GaAs quantum rings were evaluated using the bistable responses of the optically induced nuclear spin polarization. Although the dispersions of the hole and exciton g-factors were larger than that of the electron g-factor in the individual quantum rings, a strong correlation between the hole and exciton g-factors was clearly observed. Part of the dispersion of the measured hole g-factor was explained well by considering the effect of strain-induced valence band mixing.

A theoretical analysis of the optical absorption properties in one-dimensional InAs/GaAs quantum dot superlattices

We present theoretical investigations of miniband structures and optical properties of InAs/GaAs one-dimensional quantum dot superlattices (1D-QDSLs). The calculation is based on the multi-band k·p theory, including the conduction and valence band mixing effects, the strain effect, and the piezoelectric effect; all three effects have periodic boundary conditions. We find that both the electronic and optical properties of the 1D-QDSLs show unique states which are different from those of well known single quantum dots (QDs) or quantum wires. We predict that the optical absorption spectra of the 1D-QDSLs strongly depend on the inter-dot spacing because of the inter-dot carrier coupling and changing strain states, which strongly influence the conduction and valence band potentials. The inter-miniband transitions form the absorption bands. Those absorption bands can be tuned from almost continuous (closely stacked QD case) to spike-like shape (almost isolated QD case) by changing the inter-dot spacing. The polarization of the lowest absorption peak for the 1D-QDSLs changes from being parallel to the stacking direction to being perpendicular to the stacking direction as the inter-dot spacing increases. In the case of closely stacked QDs, in-plane anisotropy, especially [110] and [11¯0] directions also depend on the inter-dot spacing. Our findings and predictions will provide an additional degree of freedom for the design of QD-based optoelectronic devices.

Effects of valence-band mixing and strain on thermoelectric properties of p-type quantum wells

Thermoelectric properties of p-type quantum wells (QWs) with flat subbands are investigated theoretically. The effects due to valence-band mixing and strain in QWs are considered in the calculation of electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity, and figure of merit (ZT). It is found that the maximum ZT of QWs can be enhanced under strain. Such an enhancement of ZT results from the fact that the enhancement of S2 is stronger than the reduction of σ. The maximum values of ZT at room temperature can reach 0.55, 0.7, and 2.2 for GaAs, InAs, and InSb QWs with well width of 5 nm. For comparison, the ZT values of p-type silicon (Si) and germanium (Ge) QWs are also calculated. The optimal ZT values of Si and Ge QWs at room temperature can reach 0.21 and 0.3, respectively.

Investigation of optical properties of InGaN–InN–InGaN/GaN quantum-well in the green spectral regime

The optical properties of the InGaN/GaN quantum well with insertion of ultrathin InN layer is investigated by using the effective mass theory taking into account the valence band mixing effects. The total spontaneous emission radiation recombination rate can be optimized by modulating the position of InN layer in the InGaN QW. Meanwhile, it is observed that the difference of the spontaneous emission rate becomes smaller with increasing the sheet carrier density. Then, the influences of intermixing effect at the interface between InN and InGaN layers on the optical gain are analyzed. It shows the emission intensity is reduced as compared to the ideal QW structure while peak wavelength is red-shifted by ∼10 nm in the investigation range of L sn. Finally, the influence of partial strain relaxation on the lasing wavelength is discussed, which shows a blue shift of ∼27 nm in the case with residual strain of 50% in comparison to the no strain relaxation case.

Effect of indirect interband absorption in Ge/SiGe quantum wells

The effect of the indirect interband absorption both below and above the direct gap in Ge/SiGe quantum-well (QW) structures is theoretically investigated in comparison to the direct interband absorption. The theoretical formula for the indirect interband absorption is derived based on the second-order time-dependent perturbation theory. The direct interband absorption is calculated by summing the bound-state exciton contribution based on the variational method and the QW continuum-state contribution in the consideration of the valence band mixing effect and a Sommerfeld excitonic enhancement factor. The calculated indirect interband absorption parabolically increases below the direct bandgap and shows a stair-step-like gradual increase above the direct bandgap. The calculated optical absorption spectra at room temperature are compared with the experimental results measured by Kuo et al. [Nature 437, 1334 (2005)]. Although the indirect interband absorption is very small below the direct bandgap, the contribution of the gradually increasing indirect interband absorption is significant at the high transition energy above the direct bandgap.

Effect of Valence-Band Mixing on Density Oscillations in 2D Hole Systems

We have calculated the spatial profile of density modulations generated in two-dimensional (2D) hole gases in response to an impurity potential. Both short-range and Coulombic point impurities are considered. The density response of hole systems turns out to be generally different from that seen in conduction-electron systems. Our results point to the importance of valence-band-mixing effects, especially in the regime of higher hole sheet densities.

Wavelength selective switching in dilute nitrides multi quantum well photonic band gap waveguides

AbstractThe analysis of the gain behaviour in active GaInNAs–GaInAs Photonic Band Gap (PBG) ridge waveguides is reported. The analysis is performed using proprietary codes, based on the Bidirectional Beam Propagation Method with the Method of Lines, introducing the rate equations. The bandstructure of the active material is modelled using the k · p theory for the valence band mixing effects and the band anticrossing model for the conduction band. A wavelength selective switch configuration is proposed exploiting a periodic grating patterned on a ridge waveguide in which an active defect is inserted. The presence of the active defect allows the localization of states and the transmission of the optical signal within the PBG. The wavelength selective switching mechanism stems from the possibility of shifting the wavelength of the localized state by properly choosing the defect length and the injected current density. From the numerical analysis, the proposed device exhibits, at the switching wavelength λB = 1.289 µm, good values of the crosstalk CT = −11.55 dB, the contrast ratio CR = 14.29 dB, the modulation depth MD = 0.93, and the bandwidth Δλ = 0.8 nm.

Optimization of a GaN-based irregular multiple quantum well structure for a dichromatic white LED**Project supported by the National Natural Science Foundation of China (Grant No. 60777013), the Nature Science Foundationof Beijing, China (Grant No. 4082023), and the Excellent Doctoral Science and Technology Innovation Foundation of Beijing Jiaotong University, China (Grant No. 141063522).

GaN-based irregular multiple quantum well (IMQW) structures assembled two different types of QWs emitting complementary wavelengths for dichromatic white light-emitting diodes (LEDs) are optimized in order to obtain near white light emissions. The hole distributions and spontaneous emission spectra of the IMQW structures are analysed in detail by fully considering the effects of strain, well-coupling, valence band-mixing and polarization effect through employing a newly developed theoretical model from the k · p theory. Several structure parameters such as well material component, well width, layout of the wells and the thickness of barrier between different types of QWs are employed to analyse how these parameters together with the polarization effect influence the electronic and the optical properties of IMQW structure. Numerical results show that uniform hole distributions in different types of QWs are obtained when the number of the QWs emitting blue light is two, the number of the QWs emitting yellow light is one and the barrier between different types of QWs is 8nm in thickness. The near white light emission is realized using GaN-based IMQW structure with appropriate design parameters and injection level.

Barrier effects on the optoelectronic properties of GaN-based irregular multiple quantum wells for dichromatic white LEDs

The GaN-based irregular multiple quantum well (IMQW) structures for dichromatic white light-emitting diodes (LEDs) are assembled by two different types of QWs emitting complementary blue and yellow light. The electronic and optical properties of the designed GaN-based IMQW structures are investigated in details by fully considering the effects of strain, well-coupling, valence band-mixing and polarization effect by employing a newly modified theoretical model based on the k·p theory. The influences of the height and thickness of the barrier between blue QWs and yellow QWs together with the polarization effect on the optoelectronic properties of GaN-based IMQW structure are analyzed. Numerical results show that the ratio of the two color lights emmited from the IMQW structure for dichromatic white LED can be tuned by changing the height and thickness of the barrier between two types of QWs.

Theoretical investigation of optical polarization properties in Al-rich AlGaN quantum wells with various substrate orientations

The optical polarization properties of Al-rich AlGaN thin quantum wells on AlN substrates with various substrate orientations are numerically calculated using a 6×6 k⋅p Hamiltonian. The calculation results show that the predominant polarization direction abruptly switches from the in-plane direction to the c-axis direction at an Al composition of ∼0.76 in quantum wells on c-plane substrates. It is also shown that the polarization characteristics drastically change by small inclination of the substrates due to valence band mixing effects. It is found that the use of the vicinal substrates as well as semipolar and nonpolar substrates could be beneficial in improving optical device performance.

Design of Phosphor-Free Single-Chip White Light-Emitting Diodes Using InAlGaN Irregular Multiple Quantum Well Structures

A novel approach for the design of phosphor-free single-chip white light-emitting diodes (LEDs) is proposed by employing InAlGaN irregular multiple quantum well (IMQW) structures. The electronic and optical properties of the designed InAlGaN IMQWs are analyzed in detail by fully considering the effects of strain, well-coupling, valence band-mixing, and quasi-bound states using the effective-mass Hamiltonian deduced from k · p theory. For comparison, three different types of InAlGaN IMQW structures with ultra-wide band spontaneous emission spectra are analyzed, and the results show that phosphor-free single-chip white light LEDs with more than 200 nm emission band can be obtained using properly designed InAlGaN IMQW structures.

Valence-band mixing effects in the exciton capture and escape in quantum-well structures

We study exciton capture and escape in shallow quantum wells including the effects of valence subband mixing. We show that such a mixing process has significant impacts, yielding one order of magnitude smaller capture and escape times than the previous results, which do not include band mixing. These results can have impacts on the various functionalities of optoelectronic devices, such as their speed and leakage currents.

Giant optical anisotropy in R-plane GaN/AlGaN quantum wells caused by valence band mixing effect

Abstract This study investigates the optical anisotropy spectrum in the R-plane (i.e., the [ 10 1 ¯ 2 ] -oriented layer plane) of GaN/Al0.2Ga0.8N quantum wells of different widths. The optical matrix elements in the wurtzite quantum wells are calculated using the k ⋅ p finite difference scheme. The calculations show that the valence band mixing effect produces giant in-plane optical anisotropy in [ 10 1 ¯ 2 ] -oriented GaN/Al0.2Ga0.8N quantum wells with a narrow width. The nature of the in-plane optical anisotropy is found to be dependent on the well width. Specifically, it is found that the anisotropy changes from x ′ -polarization to y ′ -polarization as the well width increases.