Coulombic bound states in semiconductor quantum wells

Recent theoretical works on Coulombic bound states in semiconductor quantum wells (Q.W.) are reviewed. Due to carrier confinement along the growth axis the bound impurity or exciton states display enhanced binding energies over the bulk values. The presence of free carriers in modulation-doped quantum wells decreases the impurity binding energies. However the quasi-bidimensionality of the carrier motion prevents a complete vanishing of impurity bound states. The photoluminescence line of high-quality quantum well is often Stokes-shifted with respect to the absorption or excitation spectra. This Stokes shift can be correlated with interface defects in a qualitative fashion.     

Evidence for exciton binding at Ni impurity sites in ZnSe

A broad charge transfer band is observed in the photoluminescence excitation (PLE) spectrum of the 2.5 μ Ni²⁺ luminescence in ZnSe : Ni. This band lies above the highest energy d-d excitation bands and exhibits a ZPL at 1.8163 eV and LO(#38;0lambda;) phonon replicas at higher energy. In contrast, PLE spectra of Co²⁺ luminescence in ZnSe:Co contain only d-d excitation bands. The charge transfer band in ZnSe:Ni is interpreted as evidence for bound exciton formation at the Ni site. The recombination energy of this exciton is transferred efficiently to the excited d-band states of the Ni ion, leading to characteristic Ni²⁺d-d luminescence.     

Excitonic polaron and phonon assisted photoluminescence of ZnO nanowires

The coupling strength of the radiative transition of hexagonal ZnO nanowires to the longitudinal optic (LO) phonon polarization field is deduced from temperature dependent photoluminescence spectra. An excitonic polaron formation is discussed to explain why the interaction of free excitons with LO phonons in ZnO nanowires is much stronger than that of bound excitons with LO phonons. The strong exciton-phonon coupling in ZnO nanowires affects not only the Haung-Ray S factor but also the FXA-1LO phonon energy spacing, which can be explained by the excitonic polaron formation.     

Band-edge emission in CuI single crystals

The photoluminescence spectra of CuI single crystals have been studied at T = 4.2 K and at various excitation levels. The emission band of donor-acceptor pairs (DAP) with a maximum at about 4200 Å has been shown to possess a complex structure. Theoretical analyses and exciton spectroscopy data make it possible to calculate the ionization energies for the donors and acceptors participating in the formation of DAP, which are equal to E_D = = 0.045?0.065 eV and E_A = 0.155?0.170 eV, respectively. The fine structure of emission due to the annihilation of excitons bound on acceptor pairs (band maximum 4075 Å) has been detected and calculated. The energy of the longitudinal optical phonon participating in the exciton-phonon interaction (LO ? 18.7 meV) has been determined.     

Spectroscopy of charge transfer states in Mg<Subscript>1 – <Emphasis Type="Italic">x</Emphasis> </Subscript>Ni<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>O

Photoluminescence and photoluminescence excitation spectra of solid solution Mg_1–xNi_xO (x = 0.008) have been analyzed. The contributions of charge transfer electronic states and nonradiative Auger relaxation to the formation of the photoluminescence spectrum are discussed.     

Correlated exciton relaxation in Poly(3-hexylthiophene)

Two-color 3 pulse photon echo peak shift (2C-3PEPS) measurements on poly(3-hexylthiophene) (3PHT) demonstrate that spectral regions in the photoluminescence remain correlated with the excitation, despite large differences in energy (>0.5 eV). The observations are explained in terms of exciton-phonon coupling that is dominated by only two motions: one high frequency bond stretch and a low frequency torsional motion. Numerical simulations of the 2C-3PEPS are shown to be consistent with the experimental observations. The results demonstrate that initial intramolecular exciton relaxation in P3HT is not primarily a stochastic process, but is driven by strong, selective exciton-phonon coupling to torsional motions.     

纳米Gd2O3中两种格位Eu3+的电荷迁移态激发跃迁

观测了粒径分别为15,23,135m的立方相Gd2O3：Eu^3 的选择激发光谱、发射光谱和激发光谱。受强量子限域效应的影响,纳米Gd2O3：Eu^3 的激发光谱的强度表现出对颗粒尺寸的明显依赖性。用Jorgensen公式计算电荷迁移带的位置,与实验测得激发光谱中位置相一致。通过电荷迁移带不同位置的选择激发光谱可以分辨出立方相Gd2O3：Eu^3 中C2和S6格位Eu^3 的发光,从选择激发的发射光谱和激发光谱结果计算出C2和S6格位电荷迁移带的激发光谱,与实验结果相符合。     

Selective excitation of self-assembled quantum dots by using shaped pulse

We carried out optical selective excitation of individual self-assembled quantum dots by using phase-modulated pulses. Based on scattered photoluminescence excitation resonances in individual QDs, the excitation pulses modulated in the spectral region allows for addressing individual ground states emission. The photoluminescence spectra including several QDs showed intensity changes according to both the modulation energies and phases. The results also suggested the individual control of selective QDs even in collective excitation.     

Optical properties of neodymium incorporated in porous silicon

We report the optical properties of Nd-incorporated porous silicon. Photoluminescence and photoluminescence excitation measurements have been performed. Room temperature emission spectra from dried or annealed samples have been studied. While steady-state photoluminescence results indicate porous silicon light absorption by the Nd, the photoluminescence excitation shows a deficiency of energy transfer between porous silicon and Nd ions.     

铽配合物与PVK混合体系的发光研究

研究了铽配合物[Tb(m-MBA)₃phen]₂·2H₂O与PVK共掺杂体系的激发光谱和光致发光谱, 发现两者之间存在着能量转移,由于PVK的发射光谱和铽配合物的激发光谱重叠很小,两者之间发生Frester能量传递的几率很小,铽的发光主要是由于铽配合物的配体俘获了电子-空穴对使配体受到激发而导致的,在电致发光中, PVK的发光完全被抑制, 与光致发光的表现不同, 这是由于两种发光(光致和电致)机理不同造成的。     

Alloying induced shift between excitation and luminescence of the nitrogen bound exciton in GaPxAs1−x alloys

Photoluminescence and luminescence excitation spectra have been performed on epitaxial layers of nitrogen doped GaP_xAs_1?x alloys (x > 0.85). The main luminescence excitation band A appears above the photoluminescence band N_x. The composition dependence of this energy shift suggests an alloying energy shift. The origin of this new effect would be the thermalization of the bound exciton population to the lower energy states of the A absorption band which reflects the density of states broadening due to As-P disorder around N atoms     

Temperature and excitation dependence of stimulated emission and spontaneous emission in InGaN epilayer

Temperature and excitation dependent photoluminescence(PL) of InGaN epilayer grown on c-plane Ga N/sapphire template by molecular beam epitaxy(MBE) has been systematically investigated. The emission spectra of the sample consisted of strong multiple peaks associated with one stimulated emission(SE) located at 430 nm and two spontaneous emissions(SPE) centered at about 450 nm and 480 nm, indicating the co-existence of shallow and deep localized states.The peak energy of SE exhibiting weak s-shaped variation with increasing temperature revealed the localization effect of excitons. Moreover, an abnormal increase of the SPE intensity with increasing temperature was also observed, which indicated that the carrier transfer between the shallow and deeper localized states exists. Temperature dependent time-resolved PL(TRPL) demonstrated the carrier transfer processes among the localized states. In addition, a slow thermalization of hot carriers was observed in InGaN film by using TRPL and transient differential reflectivity, which is attributed to the phonon bottleneck effect induced by indium aggregation.     

Photoluminescence of tetrahedral quantum-dot quantum wells

Taking into account the tetrahedral shape of a quantum-dot quantum well (QDQW) when describing excitonic states, phonon modes, and the exciton-phonon interaction in the structure, we obtain within a nonadiabatic approach a quantitative interpretation of the photoluminescence spectrum of a single CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW is bright, in contrast to the dark ground state for a spherical QDQW. The position of the phonon peaks in the photoluminescence spectrum is attributed to interface optical phonons. We also show that the experimental value of the Huang-Rhys parameter can be obtained only within the nonadiabatic theory of phonon-assisted transitions.     

用MOCVD技术在Al2O3衬底上外延GaN的光致发光研究

本文通过变温和变激发强度的光致发光研究了用ＭＯＣＶＤ在Ａｌ２Ｏ３上生长ＧａＮ单晶薄膜的带边发射，通过分峰拟合得到Ａ，Ｂ，Ｃ，Ｄ四个谱峰，其中半峰宽分别为１３．８ｍｅＶ，１０．８ｍｅＶ，１５．６ｍｅＶ，和５０ｍｅＶ。Ａ对应自由激子谱，Ｂ，Ｃ为两种束缚激子的跃迁，Ｄ与氧杂质谱有关。     

Photoluminescence investigation of ZnO:P nanoneedle arrays on InP substrate by pulsed laser deposition

Phosphorus-doped ZnO nanoneedle arrays were prepared by phosphorus diffusion from InP substrate using a pulsed laser deposition (PLD) technique. The optical properties of ZnO nanoneedle were investigated by photoluminescence (PL) spectroscopy. Low-temperature photoluminescence spectrum measurements exhibited five acceptor-related emission peaks. The excitation intensity and temperature dependent photoluminescence spectra confirmed that the emission peaks corresponded to neutral-acceptor bound exciton, free electron to acceptor, donor-acceptor pairs, and their first and second photon replicas transitions. Acceptor-binding energy was determined to be 135-167 meV, which agrees well with the best-fitting result of the temperature dependent photoluminescence measurements and is reasonable in terms of theoretic prediction in ZnO.     

Low-temperature photoluminescence of CoO excited by synchrotron radiation

We report the first observation of low-temperature luminescence of CoO crystals under synchrotron irradiation. At 8 K, the photoluminescence of CoO is characterized by smaller bandwidth and higher intensity relative to the corresponding photoluminescence band of NiO. The photoluminescence excitation spectra of CoO and NiO are similar. Position of the band related to charge transfer from oxygen ions to 3d-shell of cobalt ions is determined. The excitation energy is found to be 3.5 eV.     

Two-photon absorption from single InGaN/GaN quantum dots

We present a study of the time-integrated and time-resolved photoluminescence properties of single-InGaN/GaN quantum dots (QDs) using two-photon spectroscopy. Two samples containing QDs produced by different growth techniques are examined. We find that two-photon excitation results in the suppression of the emission from the underlying quantum well to which the QDs are coupled and yet relatively strong QD emission is observed. This effect is explained in terms of the enhancement of two-photon absorption in QDs due to the full confinement of carriers. Furthermore, evidence of the presence of excited states is revealed from the two-photon photoluminescence excitation spectra presented in the study.     

Temperature dependence of photoluminescence of semiconductor quantum dots upon indirect excitation in a SiO2 dielectric matrix

The processes of excitation and relaxation of confined excitons in semiconductor quantum dots upon indirect high-energy excitation have been considered. The temperature behavior of photoluminescence of quantum dots in a SiO₂ dielectric matrix has been described using a model accounting for the process of population of quantum-dot triplet states upon excitation transfer through mobile excitons of the matrix. Analytical expressions that take into account the two-stage and three-stage schemes of relaxation transitions have been obtained. The applicability of these expressions for analyzing fluorescence properties of semiconductor quantum dots has been demonstrated using the example of silicon and carbon nanoparticles in the thin-film SiO₂ matrix. It has been shown that the complex character of the temperature dependences of the photoluminescence upon indirect excitation can be an indication of a multistage relaxation of excited states of the matrix and quantum dots. The model concepts developed in this study allow one to predict the form of temperature dependences of the photoluminescence for different schemes of indirect excitation of quantum dots.

     

Photoluminescence spectroscopy of carbon nanotube bundles: evidence for exciton energy transfer

We investigate photoluminescence of nanotube bundles. Their spectra are explained by exciton energy transfer between adjacent tubes, whereby excitation of large gap tubes induces emission from smaller gap ones. The consequent relaxation rate is faster than nonradiative recombination, leading to enhanced photoluminescence of acceptor tubes.