Optical properties of GaNAs/GaAs triple quantum well structures

Optical properties of the GaNAs/GaAs triple quantum well structures were characterized by using photoreflectance and photoluminescence spectroscopy at different temperatures. The excitonic interband transitions of the triple quantum well systems were observed in the spectral range above hν=E_g(GaN_xAs_1−x). A matrix transfer algorithm was used to match the GaN_xAs_1−x/GaAs boundary conditions and calculate the triple quantum well subband energies numerically for theoretical comparison. The internal electric field in the system was extracted from Franz-Keldysh oscillations in the photoreflectance spectra. The influences of the annealing treatment on the transition energy and the internal electric field are also analyzed.     

Optical properties of GaInNAsSb/GaAs/GaAs1−xNx (x ≈ 10%) saturable absorber quantum wells

We study the effect of the GaAsN narrow QWs on the optical properties of the GaInNAsSb/GaAs QWs using photoluminescence spectroscopy. A drastic effect of the N-rich layers on the QW photoluminesecnec (PL) intensity was observed with a strong influence of the spacer thickness. In the PL spectra a broad band caused by excitonic transitions related with N-related clusters in GaAs barriers is found. Based on calculations from experimental data, we have identified the low QW peak energy to the E₁-H₁ transition using the shear deformation potentials report Δp/p = 0.24.     

Contactless electroreflectance of GaNyAs1−y/GaAs multi quantum wells: The conduction band offset and electron effective mass issues

Interband transitions in GaN_yAs_1−y/GaAs multi quantum well (MQW) samples with y=0.012 and 0.023 have been studied by contactless electroreflectance spectroscopy (CER). Optical transitions related to absorption in the GaAs barriers and in the GaN_yAs_1−y/GaAs QWs have been observed and analyzed. The GaAs related transition exhibits clear Franz-Keldysh oscillations with the period corresponding to the built-in electric field of 14 and 17 kV/cm for samples with y=0.012 and 0.023, respectively. The portion of the CER spectrum related to absorption in the GaN_yAs_1−y/GaAs QW exhibits two clear resonances which are attributed to optical transitions between the ground and excited states confined in the QWs. The resonance attributed to the ground state transition is associated with absorption between the first light- and heavy-hole subbands and the first electron subband (11L and 11H) while the resonance attributed to the excited state transition is associated with absorption between the second heavy-hole subband and the second electron subband (22H). The energies of the 11H and 22H transitions have been matched with those obtained from theoretical calculations performed within the effective mass approximation. Thus, the GaN_yAs_1−y/GaAs QWs are type-I structures with a conduction band offset, Q_C, between 70 and 80%. Moreover, the incorporation of N atoms into GaAs is found to cause a significant increase in the electron effective mass. The determined values of electron effective mass for GaN_yAs_1−y/GaAs QW with y=0.012 and 0.023 are 0.105m₀ and 0.115m₀, respectively.     

Gain and threshold properties of InGaAsN/GaAsN material system for 1.3-μm semiconductor lasers

The gain properties and valence subbands of InGaAsN/GaAsN quantum-well structures are numerically investigated with a self-consistent LASTIP simulation program. The simulation results show that the InGaAsN/GaAsN has lower transparency carrier density than the conventional InGaAsP/InP material system for 1.3-μm semiconductor lasers. The material gain and radiative current density of InGaAsN/GaAsN with different compressive strains in quantum well and tensile strains in barrier are also studied. The material gain and radiative current density as functions of strain in quantum well and barrier are determined. The simulation results suggest that the laser performance and Auger recombination rate of the 1.3-μm InGaAsN semiconductor laser may be markedly improved when the traditional GaAs barriers are replaced with the AlGaAs graded barriers.     

Interband transitions and electronic properties of InAs/GaAs quantum dots embedded in AlxGa1−xAs/GaAs modulation-doped heterostructures

Reflection high-energy electron diffraction, atomic force microscopy, transmission electron microscopy, and double-crystal X-ray curves showed that high-quality InAs quantum dot (QD) arrays inserted into GaAs barriers were embedded in an Al_0.3Ga_0.7As/GaAs heterostructure. The temperature-dependent photoluminescence (PL) spectra of the InAs/GaAs QDs showed that the exciton peak corresponding interband transition from the ground electronic subband to the ground heavy-hole subband (E₁-HH₁) was dominantly observed and that the peak position and the full width at half maximum corresponding to the interband transitions of the PL spectrum were dependent on the temperature. The activation energy of the electrons confined in the InAs/GaAs QDs was 115 meV. The electronic subband energy and the energy wave function of the Al_0.3Ga_0.7As/GaAs heterostructures were calculated by using a self-consistent method. The electronic subband energies in the InAs/GaAs QDs were calculated by using a three-dimensional spatial plane wave method, and the value of the calculated (E₁-HH₁) transition in the InAs/GaAs QDs was in reasonable agreement with that obtained from the PL measurement.     

Effect of strain and nonparabolicity on interband transition energies of InAs/GaAs coupled double quantum dots

Strained potential profiles and electronic subband energies of InAs/GaAs coupled double quantum dots (DQDs) were calculated by using a three-dimensional finite-difference method (FDM) taking into account shape-based strain and nonparabolic effects. The interband transition energies from the ground electronic subband to the ground heavy-hole band (E₁-HH₁) in the InAs/GaAs DQDs, as determined from the FDM calculations taking into account strain and nonparabolic effects, were in reasonable agreement with the experimental peaks corresponding to the (E₁-HH₁) interband transition energies at several temperatures, as determined from the temperature-dependent photoluminescence spectra.     

Optical gain spectra of unstrained graded GaAs/AlxGa1 − xAs quantum well laser

We have calculated the optical gain spectra in unstrained graded GaAs/Al_xGa_1 − xAs single quantum well lasers as a function of the energy of the radiation, the quantum well width and the interface thickness. The optical gain spectra were calculated using the density matrix approach (Luttinger–Kohn method), considering the parabolic band model (conduction band), all subband mixing between the heavy and light holes (valence band), and the transversal electrical light polarization. Our results show that the optical peak gain is sensitive to the width and the graded profile of the interfaces, and is blue-shifted as a function of the interface width.     

GaAsN/GaAs量子阱在1 MeV电子束辐照下的退化规律

为研究GaAsN/GaAs量子阱在电子束辐照下的退化规律与机制,对GaAsN/GaAs量子阱进行了不同注量(1×1015,1×1016 e/cm2)1 MeV电子束辐照和辐照后不同温度退火(650,750,850℃)试验,并结合Mulassis仿真和GaAs能带模型图对其分析讨论。结果表明,随着电子注量的增加,GaAsN/GaAs量子阱光学性能急剧降低,注量为1×1015 e/cm2和1×1016 e/cm2的电子束辐照后,GaAsN/GaAs量子阱PL强度分别衰减为初始值的85%和29%。GaAsN/GaAs量子阱电子辐照后650℃退火5 min,样品PL强度恢复到初始值,材料带隙没有发生变化。GaAsN/GaAs量子阱辐照后750℃和850℃各退火5 min后,样品PL强度随退火温度的升高不断减小,同时N原子外扩散使得样品带隙发生约4 nm蓝移。退火温度升高没有造成带隙更大的蓝移,这是由于进一步的温度升高产生了新的N—As间隙缺陷,抑制了N原子外扩散,同时导致GaAsN/GaAs量子阱光学性能退化。     

(CdSe)1(ZnSe)3/ZnSe短周期超晶格多量子阱的共振Ramam谱

在(CdSe)₁(ZnSe)₃/ZnSe短周期超晶格多量子阱中,根据一维线性链模型计算的结果与实验结果的比较表明,我们在不同的共振条件下分别观察到了来自多量子阱的阱中和垒中ZnSe限制纵光学声子模的Raman散射。与GaAs/AlAs量子阱的偏振选择定则不同,在共振条件下,我们在两种偏振配置下都观察到了阱中ZnSe限制模LO₁,并认为这种不同可能来源于样品特殊的电子子带结构和光学声子行为。 关键词：     

Interband optical absorption in a strained CdxZn1−xO/ZnO quantum dot

Numerical calculations of the excitonic absorption spectra in a strained Cd_xZn_1−xO/ZnO quantum dot are investigated for various Cd contents. We calculate the quantized energies of the exciton as a function of dot radius for various confinement potentials and thereby the interband emission energy is computed considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption as a function of photon energy for different dot radii is discussed. Decrease of exciton binding energy and the corresponding optical band gap with the Cd concentration imply that the confinement of carriers decreases with composition x. The main results show that the confined energies and the transition energies between the excited levels are significant for smaller dots. Non-linearity band gap with the increase in Cd content is observed for smaller dots in the strong confinement region and the magnitude of the absorption spectra increases for the transitions between the higher excited levels.     

Microstructural and optical properties of multiple closely stacked InAs/GaAs self-assembled quantum dot arrays

The microstructural and the optical properties of multiple closely stacked InAs/GaAs quantum dot (QD) arrays were investigated by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The AFM and the TEM images showed that high-quality vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The PL peak position corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band (E₁-HH₁) of the InAs/GaAs QDs shifted to higher energy with increasing GaAs spacer thickness. The activation energy of the electrons confined in the InAs QDs increased with decreasing with GaAs spacer thickness due to the coupling effect. The present results can help to improve the understanding of the microstructural and the optical in multiple closely stafcked InAs/GaAs QD arrays.     

Exciton binding energy and the optical absorption coefficient in a strained CdxZn1−xO/ZnO quantum dot

Numerical calculations of the excitonic absorption spectra in a strained Cd_xZn_1?xO/ZnO quantum dot are investigated for various Cd contents. We calculate the quantized energies of the exciton as a function of dot radius for various confinement potentials and thereby the interband emission energy is computed considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption as a function of photon energy for different dot radii is discussed. Decrease of exciton binding energy and the corresponding optical band gap with the Cd concentration imply that the confinement of carriers decreases with composition x. The main results show that the confined energies and the transition energies between the excited levels are significant for smaller dots. Non-linearity band gap with the increase in Cd content is observed for smaller dots in the strong confinement region and the magnitude of the absorption spectra increases for the transitions between the higher excited levels.     

Derivation of the optical constants of spin coated CeO2-TiO2-ZrO2 thin films prepared by sol-gel route

Ternary thin films of cerium titanium zirconium mixed oxide were prepared by the sol-gel process and deposited by a spin coating technique at different spin speeds (1000-4000 rpm). Ceric ammonium nitrate, Ce(NO₃)₆(NH₄)₂, titanium butoxide, Ti[O(CH₂)₃CH₃]₄, and zirconium propoxide, Zr(OCH₂CH₂CH₃)₄, were used as starting materials. Differential calorimetric analysis (DSC) and thermogravimetric analysis (TGA) were carried out on the CeO₂-TiO₂-ZrO₂ gel to study the decomposition and phase transition of the gel. For molecular, structural, elemental, and morphological characterization of the films, Fourier Transform Infrared (FTIR) spectral analysis, X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), cross-sectional scanning electron microscopy (SEM), and atomic force microscopy (AFM) were carried out. All the ternary oxide thin films were amorphous. The optical constants (refractive index, extinction coefficient, band gap) and thickness of the films were determined in the 350-1000 nm wavelength range by using an nkd spectrophotometer. The refractive index, extinction coefficient, and thickness of the films were changed by varying the spin speed. The oscillator and dispersion energies were obtained using the Wemple-DiDomenico dispersion relationship. The optical band gap is independent of the spin speed and has a value of about E_g≈2.82±0.04 eV for indirect transition.     

The optical properties of ZnO/ZnMgO single quantum well grown by P-MBE

In this paper, ZnO/Zn_0.9Mg_0.1O single quantum well (SQW) structures were fabricated on c-plane sapphire (Al₂O₃) substrate by plasma-assisted molecular beam epitaxy (P-MBE). The photoluminescence (PL) peak of the SQW shifted from 3.31 to 3.37 eV as the well layer thickness was decreased from 6 to 2 nm. The spectral linewidth increases with temperature due to the scattering of excitons with acoustic and optical phonons. The transition energy of the localized exciton in the ZnO/Mg_0.1Zn_0.9O SQW with well width of 3 nm was found to be about 3.407 eV at 80 K, consistent with theoretical calculation. The first subband energies in the conduction and valence band were calculated to be 49 and 11 meV, respectively.     

Ellipsometric characterization of PbI2 thin film on glass

The optical properties of polycrystalline lead iodide thin film grown on Corning glass substrate have been investigated by spectroscopic ellipsometry. A structural model is proposed to account for the optical constants of the film and its thickness. The optical properties of the PbI₂ layer were modeled using a modified Cauchy dispersion formula. The optical band gap E_g has been calculated based on the absorption coefficient (α) data above the band edge and from the incident photon energy at the maximum index of refraction. The band gap was also measured directly from the plot of the first derivative of the experimental transmission data with respect to the light wavelength around the transition band edge. The band gap was found to be in the range of 2.385±0.010 eV which agrees with the reported experimental values. Urbach's energy tail was observed in the absorption trend below the band edge and was found to be related to Urbach's energy of 0.08 eV.     

Valence-band structure and optical absorption inp-type GaAs–Al0.3Ga0.7As multi-quantum-well infrared photodetectors under an electric field

Hole structure of a GaAs–Al_0.3Ga_0.7Asp-type multiple quantum well (MQW) subjected to an electric field parallel to the growth axis is studied using the envelope-function approximation and taking into account the valence subband mixing. The system considered in this work consists of five GaAs wells and six thick Al_0.3Ga_0.7As barriers. The valence subband structure and the optical-absorption coefficient are calculated as functions of the electric-field strength for various doping levels. The subband structure is shown to be nonparabolic and anisotropic in the plane of the layers with a four-fold symmetry. The spin splitting due to the lack of specular symmetry of quantum wells is a growing function of the electric-field strength. The calculated optical absorption is in good agreement with the experimental spectra.     

Optical absorption spectrum of Cu2O-CaO-P2O5 glasses

Homogeneous CaO-P₂O₅ and Cu₂O-CaO-P₂O₅ glasses were prepared using a melt-quenched method under controlled conditions. The binary glasses were found to be colourless and transparent while the ternary glasses changed from light green to dark green as the Cu₂O content increased. From the absorption edge studies, the values of the optical band gap, E_opt and Urbach energy, ΔE were evaluated. The position of the absorption edge and hence the optical band gap were found to depend on the glass composition. Analysis of the optical band gap shows that for the binary glasses, the value increases as the content of CaO decreases, while for the ternary glasses, the value of the optical band gap increases as the content of the Cu₂O decreases. The density of the glasses was also measured and was found to increase with the increase in CaO and Cu₂O contents.     

Electronic excitations in hole-doped La1−xSrxMnO3 studied by resonant inelastic X-ray scattering

We report a resonant inelastic X-ray scattering (RIXS) study on perovskite manganese oxides La_1−xSr_xMnO₃ (x=0, 0.2, and 0.4) at Mn K-absorption edge. Hole-doping effect on the electronic excitations in the strongly correlated electron systems is elucidated by comparing with undoped LaMnO₃. The scattering spectra of metallic La_0.6Sr_0.4MnO₃ show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the spectral weight shifts toward lower energies. Though the peak position of the excitations shows weak dispersion in momentum dependence, RIXS intensity changes as a function of the scattering angle (2θ), which is related to the anisotropy. Furthermore, anisotropic temperature dependence is observed in La_0.8Sr_0.2MnO₃ which shows a metal-insulator transition associated with a ferromagnetic transition. We consider that the anisotropy in the RIXS spectra is possibly attributed to the correlation of the orbital degrees of freedom. The anisotropy is large in LaMnO₃ with long-range orbital order, while it is small but finite in hole-doped La_1−xSr_xMnO₃ which indicates persistence of short-range orbital correlation.     

Investigation of various optical transitions in GaAs/Al0.3Ga0.7As double quantum ring grown by droplet epitaxy

This work examines the optical transitions of a GaAs double quantum ring (DQR) embedded in Al_0.3Ga_0.7As matrix by photoreflectance spectroscopy (PR). The GaAs DQR was grown by droplet epitaxy (DE). The optical properties of the DQR were investigated by excitation‐intensity and temperature‐dependent PR. The various optical transitions were observed in PR spectra, whereas the photoluminescence (PL) spectrum shows only the DQR and GaAs band emissions. The various optical transitions were identified for the GaAs near‐band‐edge transition, surface confined state (SCS), DQR confined state, wetting layer (WL), spin–orbital split (E_GaAs + Δ_o), and AlGaAs band transition. PR spectroscopy can identify various optical transitions that are invisible in PL. The PR results show that the GaAs/AlGaAs DQR has complex electronic structures due to the various interfaces resulting from DE.     

Impurity photoconductivity in strained <Emphasis Type="Italic">p</Emphasis>-InGaAs/GaAsP heterostructures

The impurity-photoconductivity spectrum is observed for strained quantum wells of the p-InGaAs/GaAs solid solution at T= 4.2 K. In addition to the broad photoconductivity band attributed to the transitions from the acceptor ground state to the continuum of the first size-quantization subband, the spectrum exhibits a peak due to the transitions from the ground state to the excited localized acceptor state, a band corresponding to the transitions to the resonance states associated with the second heavy-hole size-quantization subband, and a narrow photoconductivity peak (Fano resonance) in the spectral range corresponding to the optical-phonon energy.