Structural and optical properties of non-polar ZnO/Zn0.81Mg0.19O multiple quantum wells grown on r -plane sapphire substrates by plasma-assisted molecular beam epitaxy

This work investigates the structural and optical properties of non-polar ZnO/Zn_0.81Mg_0.19O multiple quantum wells (MQWs), which have been prepared on $r$ -plane sapphire substrates by plasma-assisted molecular beam epitaxy (MBE). The MQWs are ( $11\bar{2}0$ ) oriented ( $a$ -plane) as identified by the X-ray diffraction pattern. Structural properties are anisotropic and surfaces of MQWs show stripes running along the ZnO $c$ -axis direction. Sharp interfaces between the well layers and barrier layers can be clearly resolved by the secondary ion mass spectroscopy (SIMS) analysis. The room-temperature photoluminescence (PL) resulting from the well regions exhibits a significant blueshift with respect to ZnO single layer. Exciton emission in the ZnO QW is resolved into two components in the temperature dependence of the PL spectra. Two types of excitons are responsible for this feature. The excitons trapped by the potential minima dominate at low temperature, and the excitons localized in the “free exciton states” dominate at relatively high temperature. An activation energy of 7.3 meV for quenching of the exciton emission is in good agreement with the transition of the two types of excitons.     

Formation of excitons from free electron–hole pairs due to acoustic phonon interactions in quantum wells

A study of the process of exciton formation due to acoustic phonon interaction in quantum wells (QWs) is presented. Considering that excitons are formed from photoexcited free electron–hole pairs, we have derived the rate of such formation as a function of density and temperature of charge carriers and wavevectorK_|| of the center-of-mass motion of exciton, and finally applied our theory to GaAs/AlGaAs QWs. We have found that the formation of an exciton due to acoustic phonon emission is more efficient at relatively large values ofK_|| (hot excitons) whereas that due to longitudinal optical (LO) phonon emission is more efficient at relatively small values of K_||.     

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.     

ZnSe0.93Te0.07-ZnSe多量子阱的光泵受激发射

宽带Ⅱ-Ⅵ族化合物半导体在短波长可见光范围内优越的发光特性使其在器件的研制方面有着广阔的前景.ZnSe基量子阱和超晶格结构,尤其是ZnCdSe-ZnSe量子阱已经在器件的制备方面取得了可喜的进展,因而人们研究的比较多.     

m面蓝宝石上ZnO/ZnMgO多量子阱的制备及发光特性研究

利用等离子体辅助分子束外延设备(P-MBE)在m面的蓝宝石(m-Al₂O₃)衬底上制备了ZnO/Zn_0.85Mg_0.15O多量子阱.反射式高能电子衍射谱(RHEED)图样的原位观察表明,多量子阱结构是以二维模式生长的.从光致发光谱中可以看到ZnO/Zn_0.85Mg_0.15O多量子阱在室温仍具有明显的量子限域效应.在290 K时阱宽为3 nm的ZnO/Zn_{0.85关键词： 等离子体辅助分子束外延 ZnO多量子阱 光致发光}     

Negatively charged excitons and the optical properties of modulation-doped quantum wells

Optical and magneto-optical properties are studied for II-VI semiconductor multiple quantum wells (MQWs) doped with donors in the barriers to give electron concentrations of 2 10¹⁰ to 6 10¹¹ cm^-2 in the well layers. Following on from the recent identification of negatively charged excitons X^- (two electrons bound to one hole) in CdTe/Cd_1-xZn_xTe MQWs, this paper presents more specifically the circular polarisation of the luminescence associated with X^- and with the normal exciton X (one electron and one hole) in this type of structure. Very similar magneto-optical properties are observed for modulation doped Zn_0.9Cd_0.1Se/ZnSe MQWs, and X^- is identified in these wells with a binding energy as large as 7 meV for the second electron at 50Å, well-width.     

Spontaneous light emission from fractional-dimensional excitons

We apply the concept of fractional-dimensional excitons to study the process of light emission in quantum confined systems. We focus on a single parameter , known as the degree of dimensionality and which is related to the exciton coherence volume. We compute rates of light emission due to free excitons in GaAs/Al_xGa_1-xAs quantum wells as functions of and link them to experimental observations. The rates are compared with those of quantum well excitons embedded in a microcavity.     

The optical properties of high quality ZnO/ZnMgO quantum wells on Si(111) substrates

The ZnO/Zn_0.85Mg_0.15O multiple quantum wells (MQWs) were fabricated on Si(111) substrates by plasma-assisted molecular beam epitaxy (P-MBE) using ZnMgO as buffer layers. The RHEED images indicated that the MQWs were of high quality. The free exciton (FE) emission line originated from the well region and its phonon replicas (FE-1LO, FE-2LO and FE-3LO) were observed in the 86 K photoluminescence (PL) spectrum. Blueshift of the FE line in the MQW sample as compared to that in the ZnO bulk was found at temperatures as high as room temperature. Time-resolved PL study on the FE line showed a fast lifetime of 140 ps. The high quality of the MQW structure was revealed by the observation of the quantum-confinement-induced blueshift and shortened lifetime of the FE emission.     

ZnCdSe/ZnSe多量子阱的生长和激子光学性质的研究

用分子束外延在GaAs衬底上生长了ZnCdSe/ZnSe多量子阱结构.利用X射线衍射(XRD)、变温度PL光谱和ps发光衰减等研究了ZnCdSe/ZnSe多量子阱结构和激子复合特性.由变温PL光谱讨论了随温度升高辐射线宽展宽和辐射复合效率降低的机理.     

Photoluminescence dynamics due to exciton and free carrier transport in GaAs/AlAs superlattices

Photogenerated carrier transfer is investigated in a set of three GaAs/AlAs short-period superlattices (SPSs) with different barrier thicknesses by steady-state and time-resolved photoluminescence (PL) spectroscopy at 15–20 K as a function of excitation power. The tunneling transport of carriers is evaluated by detecting excitonic PL signals from an embedded GaAs single quantum well (SQW) in the middle of the SPS layer. We find that, as the barrier thickness is decreased, the PL intensity ratio of SQW/SPS increases systematically due to enhanced tunneling efficiencies of both electrons and holes. However, the PL intensity ratio significantly increases with decreases in the excitation power by more than two orders of the magnitude. We attribute the enhanced PL intensity of SQW relative to the SPS to the faster transport of electrons that can recombine with residual holes to form excitons in SQW. The PL dynamics of SQW and SPS thus shows unique density-dependent PL intensity and time behaviors due to variations in relative amounts of excitons and free carriers to be transported into the SQW layer.     

Exciton–phonon interaction in Al_(0.4)Ga_(0.6)N/Al_(0.53)Ga_(0.47)N multiple quantum wells

The exciton-phonon interaction in Al_(0.4)Ga_(0.6)N/Al_(0.53)Ga_(0.47)N multiple quantum wells(MQWs) is studied by deepultraviolet time-integrated and time-resolved photoluminescence(PL).Up to four longitudinal-optical(LO) phonon replicas of exciton recombination are observed,indicating the strong coupling of excitons with LO phonons in the MQWs.Moreover,the exciton-phonon coupling strength in the MQWs is quantified by the Huang-Rhys factor,and it keeps almost constant in a temperature range from 10 K to 120 K.This result can be explained in terms of effects of fluctuations in the well thickness in the MQWs and the temperature on the exciton-phonon interaction.     

ZnCdse-ZnSe多量子阱中的激子发光

在77-300K温度下研究了Zn_1-xCd_xSe-ZnSe多量子阱(MQWs)的光致发光特性.首次在77K,Ar离子激光器的457.9nm激发下,在Zn_0.68Cd_0.32Se-ZnseMQWs中观测到5个发光带,其中三个发光带被归因于不同的激子发射:即n=1重空穴(HH)激子;n=l轻-重空穴(LH)激子和n=IHH激子同时发射两个纵光学声子的复合发光,并且n=1HH激子发光可延续至室温.     

Binding energies of wannier excitons in GaAs-Ga1−xAlxAs quantum well structures

Binding energies of Wannier excitons in a quantum well structure consisting of a single slab of GaAs sandwiched between two semi-infinite slabs of Ga_1?xAl_xAs are calculated using a variational approach. Due to reduction in symmetry along the axis of growth of these quantum well structures and the presence of band discontinuities at the interfaces, the degeneracy of the valence band of GaAs is removed leading to two exciton systems, namely, the heavy hole exciton and the light hole exciton. The variations of the binding energies of these two excitons as a function of the size of the GaAs quantum wells for various values of the heights of the potential barrier are calculated and their behavior is discussed.     

Bosonic stimulation of cold 1s excitons into a harmonic potential minimum in Cu2O

The density distribution of cold exciton clouds generated into a strain-induced potential well by two-photon excitation in Cu₂O is studied at 2 K. We find that an anomalous spike, which can be interpreted as accumulation of the excitons into the ground state, emerges at the potential minimum. The accumulation can be due to stimulated scattering of cold excitons, mediated by acoustic phonon emission. The possibility of the formation of the thermodynamic Bose-Einstein condensate of paraexcitons is discussed.     

Stimulated light backscattering from exciton Bose condensate

A new effect—light backscattering from exciton Bose-condensate—is considered. This effect is connected with the photoinduced coherent recombination of two excitons in the condensate with the production of two photons with opposite momenta. The effect of two-exciton coherent recombination leads also to the appearance of the second-order coherence in exciton luminescence connected with squeezing between photon states with opposite momenta. The estimations given for Cu₂O and GaAs excitons show that the effect of stimulated light backscattering can be detected experimentally. Moreover, in the system of 2D excitons in coupled quantum wells, the effect of stimulated anomalous light transmission must also take place. Anologous effects can also take place in systems of Bose-condensed atoms in excited (but metastable) states.     

The temperature dependence of the photoluminescence from overgrown GaAs/Al0.3Ga0.7As quantum dots and wires

Photoluminescence measurements on GaAs/Al_0.3Ga_0.7As quantum dots and wires fabricated using electron bears lithography and reactive ion etching are reported both before and after regrowth with a layer of Al_0.4Ga_0.6As. Dots exhibit little change in luminescence efficiency from the bulk with a reduction in diameter either before or after regrowth. Surface recombination therefore appears to be suppressed. In wires, however, luminescence intensity is very sensitive to wire width, decreasing rapidly with this parameter, but recovers and becomes independent of size after overgrowth. The temperature dependence of the photoluminescence from the dots and wires showed that dots and wires less than 150nm in width luminesced to higher temperatures than the larger diameter structures and dots liminesced to higher temperatures than wires of comparable width. This suggests that there is a finite coherence area effect which increases the radiative lifetimes of excitons in the quantum structures due to the geometric constraint, in the lateral direction in the wires and in all three directions in the dots. Below 20K bound exciton luminescence dominates in the dots but not in the wires. In wires it is still possible for the excitons to diffuse to nonradiative sites within the exciton lifetime. Regrowth at 750°C causes migration of aluminium into the quantum well and causes the shape of the well to become parabolic resulting shifts in the exciton emission to shorter wavelengths, making it difficult to separate the effect of processing from those due to quantum confinement.     

Role of localized excitons in the stimulated emission in ultra-thin CdSe/ZnSe/ZnSSe single quantum-well structures

Photo-pumped lasing properties have been investigated in a CdSe/ZnSe/ZnSSE single quantum wells (SQWs) with the well-layer thickness (L_W) of 1, 2 and 3 monolayer (ML). At 20 K, the laser threshold for the SQW withL_W = 1 ML was the lowest in spite of the smallest active layer thickness. The carrier (exciton) sheet density at the threshold (n)_thwas estimated to be as low as 7 × 10¹⁰cm⁻², which is well below Mott's screening density. Time-resolved photoluminescence has revealed that the localized biexciton band, whose peak energy agrees with the lasing peak, appeared on the low-energy side of the main PL peak at this level of carrier concentrations. Theoretical calculation has also shown that the localized biexciton recombination has to be taken into account for the lasing process. On the contrary, then_thvalues of the SQWs with 2 and 3 ML are 1 order of magnitude larger than that of the SQW with 1 ML. This may be due to the smaller oscillator strength of both localized excitons and localized biexcitons because of the larger inhomogeneous broadening, resulting in an increased carrier density for achieving optical gain sufficient to overcome the reflection losses.     

Barrier-disorder induced exciton relaxation via LO-phonons in GaAs/Al x Ga1−x As multiple quantum wells

Hot exciton relaxation is observed in GaAs/Al _x Ga_1–x As multiple quantum wells. The photolumnescence excitation spectra of the localized exciton emission at low temperatures and excitation densities are composed of narrow equidistant peaks exactly separated by the GaAs LO-phonon energy (36 meV). The relaxation mechanism via LO-phonons is found to be important for localized excitons in multiple quantum wells with GaAs layer thicknesses of about 50 Å, where pronounced alloy fluctuations in the barriers provide a strong additional lateral potential which suppresses the dissociation of hot excitons.     

半导体量子阱中自由激子稳态荧光特征

在不同晶格温度和不同激发光强度下，测量了四元系GaInAsSb/GaAlAsSb单量子阱中自由激子的荧光光谱，导出了稳态光谱测量条件下自由激子荧光强度与激发光强度和晶格温度的一般性公式.计算结果表明，激子相对占有数引起的温度和密度效应会影响激子发光的强度关系.根据本文的简单模型，线性比例系数I/I₀实际上综合地反映了量子阱中自由激子的荧光效率，而从激子荧光强度的Arrhenius图的最佳拟合中不仅可以得到激子的束缚能和激活能，而且还能估计出量子阱材料的本底浓度和散射时间常数. 关键词：     

Quantum Monte Carlo simulation of exciton–exciton scattering in a GaAs/AlGaAs quantum well

We present a computer simulation of exciton–exciton scattering in a quantum well. Specifically, we use quantum Monte Carlo techniques to study the bound and continuum states of two excitons in a 10 nm wide GaAs/Al_0.3Ga_0.7As quantum well. From these bound and continuum states we extract the momentum-dependent phase shifts for s-wave scattering. A surprising finding of this work is that a commonly studied effective-mass model for excitons in a 10 nm quantum well actually supports two bound biexciton states. The second, weakly bound state may dramatically enhance exciton–exciton interactions. We also fit our results to a hard-disk model and indicate directions for future work.