Observation of quantum confinement in ZnO nanorods fabricated using a two-temperature growth method

We observed a quantum confinement effect in vertically well-aligned ultrafine ZnO nanorods using polarized excitation photoluminescence measurements. Room-temperature and low-temperature photoluminescence spectra revealed that free excitons were confined in the nanorods. The magnitude of the energy shift due to the quantum confinement in the ultrafine ZnO nanorods was 6 meV at room temperature, which corresponded to the luminescence from ZnO nanorods 12.8 nm in diameter. The diameter estimated from the spectra was comparable to the value measured from SEM images.     

Temperature dependence of luminescence intensity under Bose condensation of interwell excitons

Photoluminescence of interwell excitons in GaAs/AlGaAs double quantum wells (n-i-n heterostructure) containing large-scale random potential fluctuations in the planes of heteroboundaries is studied. The properties of excitons, in which a photoexcited electron and a hole are spatially separated in neighboring quantum wells, were investigated upon variation of the power density of off-resonance laser excitation and temperature (1.5–4.2 K), both under lateral (in the heteroboundary plane) confinement of the excitation region to a few micrometers and without such a limitation (directly from the region of laser-induced photoexcitation focused to a spot not exceeding 30 μ. Under low pumping (with a power smaller than a microwatt), interwell excitons are strongly localized due to small-scale random potential fluctuations and the corresponding photoluminescence line is nonhomogeneously broadened to 2.5–3.0 meV. With increasing pumping power, the narrow line of delocalized excitons with a width of approximately 1 meV emerges in a threshold manner (the intensity of this line increases superlinearly near the threshold with increasing pumping). For a fixed pumping, the intensity of this line decreases linearly upon heating until it completely vanishes from the spectrum. The observed effect is attributed to Bose condensation in a quasi-two-dimensional system of interwell excitons. Within the proposed model, we show that the linear mode in the behavior of the luminescence intensity until its disappearance in the continuum of the photoluminescence spectrum upon a change in temperature is observed only for the condensed part of interwell excitons. At the same time, the luminescence of the above-the-condensate part of excitons is almost insensitive to temperature variations in the temperature range studied.     

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：     

Interwell excitons in GaAs/AlGaAs double quantum wells and their collective properties

Luminescence spectra of interwell excitons in GaAs/AlGaAs double quantum wells with electric-field-tilted bands (n-i-n) structures were studied. In these structures the electron and the hole in the interwell exciton are spatially separated between neighboring quantum wells by a narrow AlAs barrier. Under resonant excitation by circularly polarized light the luminescence line of the interwell excitons exhibited appreciable narrowing as their concentration increased and the degree of circular polarization of the photoluminescence increased substantially. Under resonant excitation by linearly polarized light the alignment of the interwell excitons increased as a threshold process with increasing optical pumping. By analyzing time-resolved spectra and the kinetics of the photoluminescence intensity under pulsed excitation it was established that under these conditions the rate of radiative recombination increases substantially. The observed effect occurs at below-critical temperatures and is interpreted in terms of the collective behavior of the interwell excitons. Studies of the luminescence spectra in a magnetic field showed that the collective exciton phase is dielectric and in this phase the interwell excitons retain their individual properties.     

Effect of heteroboundary spreading on the properties of exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

Exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with different diffusion spreading of interfaces are studied by optical spectroscopy methods. It is shown that the emission spectrum of quantum wells at low temperatures is determined by free excitons and bound excitons on neutral donors. The nonlinear dependence of the stationary photoluminescence intensity on the excitation power density and the biexponential luminescence decay are explained by the neutralization of charged defects upon photoexcitation of heterostructures. With the stationary illumination on, durable (about 40 min) reversible changes in the reflection coefficient near the exciton resonances of quantum wells are observed. It is shown that, along with the shift of exciton levels, the spreading of heteroboundaries leads to three effects: an increase in the excitonphonon interaction, an increase in the energy shift between the emission lines of free and bound excitons, and a decrease in the decay time of exciton luminescence in a broad temperature range. The main reasons for these effects are discussed.     

Mechanism of quantum dot luminescence excitation within implanted SiO2:Si:C films

Results of the investigation of photoluminescence (PL) mechanisms for silicon dioxide films implanted with ions of silicon (100 keV; 7 × 10(16) cm(-2)) and carbon (50 keV; 7 × 10(15)-1.5 × 10(17) cm(-2)) are presented. The spectral, kinetic and thermal activation properties of the quantum dots (Si, C and SiC) formed by a subsequent annealing were studied by means of time-resolved luminescence spectroscopy under selective synchrotron radiation excitation. Independent quantum dot PL excitation channels involving energy transfer from the SiO(2) matrix point defects and excitons were discovered. A resonant mechanism of the energy transfer from the matrix point defects (E' and ODC) is shown to provide the fastest PL decay of nanosecond order. The critical distances (6-9 nm) of energy transport between the bulk defects and nanoclusters were determined in terms of the Inokuti-Hirayama model. An exchange interaction mechanism is realized between the surface defects (E(s)'-centres) and the luminescent nanoparticles. The peculiarities of an anomalous PL temperature dependence are explained in terms of a nonradiative energy transfer from the matrix excitons. It is established that resonant transfer to the luminescence centre triplet state is realized in the case of self-trapped excitons. In contrast, the PL excitation via free excitons includes the stages of energy transfer to the singlet state, thermally activated singlet-triplet conversion and radiative recombination.     

Peculiarities of photoluminescence of erbium in silicon structures prepared by the sublimation molecular-beam epitaxy method

The photoluminescence of semiconducting structures Si: Er: O/Si grown by the molecular-beam epitaxy method is studied. The dependences of Er photoluminescence intensity on the intensity of pumping are measured at the liquid helium temperature. An analysis of the experimental results on the basis of the exciton model of excitation of Er ions in a crystalline silicon matrix reveals the significant role played by an alternative channel of free-exciton trapping (apart from the donor energy levels of erbium-oxygen complexes), as well as that played by the nonradiative channel in the recombination of excitons, bound to erbium donors, without the excitation of erbium. The ratio of the concentration of optically active centers of erbium luminescence to the total concentration of introduced erbium is estimated.     

The kinetics of exciton photoluminescence in mercuric iodide

Time-resolved photoluminescence (TRPL) of red mercuric iodide single crystal is measured at low temperatures and its two-photon luminescence is measured at room temperature. Sharp near band-gap luminescence is observed around 530 nm and was ascribed to radiative annihilation of free and bound excitons; the phonon replica of exciton luminescence are found between 533 and 540 nm at low temperatures. TRPL experiment reveals that near band-gap luminescence comprises fast and slow decay components and shows the different relaxation processes between free and bound exciton annihilation. Luminescence of bound excitons steeply lowers with increasing temperature and disappears about 40 K. A luminescence tail band is observed around 540 nm that is ascribed to defects in the anion sublattice. The temporal behavior of the tail band is described by rate equations very well. A broad luminescent band appears at 630 nm. The decay curves suggest that the luminescence is ascribed to the radiative recombination of donor-acceptor pairs and there are two kinds of mechanisms to control the decay. At room temperature, a luminescent band appears at the band-gap region, which shows the band-gap at room temperature is about 2.125 eV.     

Exciton-induced defect formation in KJ-T1 crystal and the defect participation in thallium luminescence

Abstract

The interaction of excitons with lattice defects - Tl ions in the activated KJ crystal is studied at low temperatures. Luminescence of Tl under the excitation in the 1s exciton absorption band (5.85 eV) is examined. The participation of the localized excitons created and trapped near Tl ions during that process is shown. Those localized excitons decay into lattice defect pairs which take part directly in the excitation and luminescence of Tl and cause a broadening of Tl luminescence bands.     

Analytical temperature dependence of the photoluminescence of semiconductor quantum dots

The excitation and relaxation of spatially confined excitons in semiconductor quantum dots have been considered. The temperature dependence of the luminescence of quantum dots in dielectric matrices is described by the model taking into account the singlet-triplet intercombination conversion of spatially confined excitons. The analytical expression describing the temperature dependence of photoluminescence is derived and the physical meaning of the constants involved in this expression is determined. The applicability of the expression to the analysis of the luminescent properties of the quantum dots is demonstrated by the example of silicon nanoclusters in a thin-film SiO₂ matrix.     

Radiative recombination of excitons in semiconductors at high excitation intensities

The shape of the exciton luminescence band of the gaseous phase of free excitons in crystal Si is investigated under high level excitation. Only the exciton-exciton interaction is considered and the influence of collisions of excitons with phonons and electrons is not taken into account. The theoretical shape of the exciton luminescence band qualitatively agrees with experiment. The process of the radiative Auger-recombination of two indirect excitons without the participation of phonons is studied in Ge and Si. The expression for the frequency and temperature dependence of the probability of radiation is obtained. The band has the asymmetrical Gaussian form with the steep short-wave tail. The predicted luminescence band in Ge is shifted at the long-wavelength side from the well-known exciton luminescence bands with participation of the phonons. The selection rules for the probability amplitude of the process under consideration are obtained.     

Exciton-electron interaction in quantum wells with a two dimensional electron gas of low density

II–VI quantum-well structures containing a 2DEG of low density have been investigated by means of polarized photoluminescence, photoluminescence excitation and reflectivity in external magnetic fields up to 20 T. The spin splittings of the exciton X and the negatively charged exciton X ⁻ are measured as a function of the magnetic field strength. The behavior of the magnetic-field-induced polarization degree of the luminescence line related to X ⁻ demonstrates the formation process of negatively charged excitons from excitons and free carriers polarized by the external magnetic field. We have determined the binding energies of the trion formed either with the heavy-hole or the light-hole exciton. The optically detected magnetic resonance (ODMR) technique was applied for the first time to study the optical transition processes in a nanosecond timescale. The electron ODMR was observed with the detection on either the direct exciton or the negatively charged exciton X. Further evidence for the interaction of excitons with the electrons of the two-dimensional gas are demonstrated by a combined exciton-cyclotron resonance line observed in reflectivity and luminescence excitation, shake-up processes observed in photoluminescence, as well as inelastic and spin-dependent scattering processes. Fiz. Tverd. Tela (St. Petersburg) 41, 831–836 (May 1999) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Spectral behavior of the emission around 3.31 eV (A-line) from ZnO nanocrystals

The emission at around 3.31 eV (A-line) from three types of ZnO nanocrystals with different particle sizes (10-1000 nm) was studied. The photoluminescence (PL) measurements were performed under different excitation densities and at different temperatures. The A-line emission exhibited a strong dependence on temperature and excitation power density. With increasing excitation density and temperature overlapping of the closely spaced first longitudinal optical (LO) phonon replica of free excitons by the A-line emission was observed.     

Identification of F+ centers in hafnia and zirconia nanopowders

Luminescence properties of undoped hafnia and zirconia nanopowders prepared by solution combustion synthesis were investigated under photo- and electron-beam excitation in 10–400 K temperature range. Along with the main luminescence band revealed in investigated materials at low temperatures at 4.2–4.3 eV and ascribed to the emission of self-trapped excitons, there are luminescence bands due to defects and impurities introduced during sample preparation. At room temperature the latter emissions dominate in the luminescence spectra as the intrinsic self-trapped exciton emission is quenched. Analysis of decay kinetics of defect centers allowed identification of F⁺ centers emission at 2.8 eV with lifetimes ∼3–6 ns in hafnia and zirconia under intra-center excitation.     

Primary photoexcitations and the origin of the photocurrent in rubrene single crystals

By simultaneously measuring the excitation spectra of transient luminescence and transient photoconductivity after picosecond pulsed excitation in rubrene single crystals, we show that free excitons are photoexcited starting at photon energies above 2.0 eV. We observe a competition between photoexcitation of free excitons and photoexcitation into vibronic states that subsequently decays into free carriers, while molecular excitons are instead formed predominantly through the free exciton. At photon energies below 2.25 eV, free charge carriers are created only through a long-lived intermediate state with a lifetime of up to 0.1 ms and no free carriers appear during the exciton lifetime.     

Low temperature photoluminescence study of Ga As defect states

Low temperature(77 K)photoluminescence measurements have been performed on different GaAs substrates to evaluate the GaAs crystal quality.Several defect-related luminescence peaks have been observed,including 1.452 eV,1.476 eV,1.326 eV peaks deriving from 78 meV Ga_(As) antisite defects,and 1.372 eV,1.289 eV peaks resulting from As vacancy related defects.Changes in photoluminescence emission intensity and emission energy as a function of temperature and excitation power lead to the identification of the defect states.The luminescence mechanisms of the defect states were studied by photoluminescence spectroscopy and the growth quality of GaAs crystal was evaluated.     

Collective state in a bose gas of interacting interwell excitons

Experiments associated with direct observations of a collective state in a gas of interacting interwell excitons in GaAs/AlGaAs double quantum wells are discussed. The structures constitute Schottky photodiodes. In a metallic gate, circular windows of various sizes (diameters of 2 to 20 μm) are etched by means of electronic-beam lithography. Through these windows, the photoluminescence of interwell and intrawell excitons is excited and detected. A microscopic device allows the observation of the spatial structure of luminescence with a resolution of 1 μm through the windows of a sample placed in superfluid helium. Using optical interference filters, the spatial structure of the luminescence is analyzed selectively in the spectrum for interwell and intrawell excitons under the same experimental conditions. It is found that the photoluminescence of interwell excitons under certain conditions exhibits an axisymmetric spatial structure: along the perimeter of the windows through which the photoluminescence is observed, a regular ring pattern of equidistant bright spots of the luminescence of interwell excitons appears. This structure appears only above the photoexcitation power threshold and the number of equidistant bright spots in the ring increases with the pumping power. At high pumping powers, the structure of distinct periodic luminescence spots is smeared. At a fixed pumping power, the phenomenon exhibits explicit critical temperature dependence: the structure of regularly located luminescence spots is smeared at T > 4 K. Axisymmetric spatial configurations of equidistant luminescence spots are observed in windows of the diameters 2, 5, and 10 μm. For intrawell excitons, the spatial structure of luminescence is not observed under similar experimental conditions: the luminescence of intrawell excitons is spatially uniform in all the windows under investigation. The effect is a result of the collective behavior of interacting interwell excitons.     

Photoluminescence and time-resolved photoluminescence of star-shaped ZnO nanostructures

Optical properties of star-shaped ZnO nanostructures were studied. The temperature-dependent photoluminescence (PL) was examined up to fourth-order longitudinal optical (LO) phonon assisted emissions of free excitons and confirmed that the nature of the room temperature PL in ZnO is 1-LO phonon assisted emission of free excitons. Low threshold ultraviolet stimulated emissions (SE) were obtained for our powder samples at room temperature. Picosecond time-resolved PL measurements detected a bi-exponential decay behavior which is strongly dependent on the excitation intensity: the slow decay term decreased faster than the fast decay term as the excitation intensity increased and the emission decays were dominated by the fast one. We also found that the emission decays decreased super-linearly before the appearance of the SE. This behavior may be used to deduce the threshold of SE or lasing.     

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。     

Quenching by electric fields of the luminescence of As2Se3 single crystals

High electric fields reduce the luminescence of As₂Se₃ at 77 K and increase simultaneously the photoconductivity. Comparison of both effects points to excitons as the source for emission of light whereas free carriers contributing to photoconductance recombine non radiatively. The field affects free excitons and their thermalization but does not ionize excitons trapped in radiative centres. At low excitation energy, in the range of indirect transitions, the field quenching of luminescence obeys a Poole-Frenkel behaviour. Such interpretation yields an exciton binding energy of 50 meV. The quantum efficiency of luminescence decreases to higher photon energy. Part of this decrease is related to an increase of the photoconductivity.