正向电压激发下CdS MIS发光二极管中的自由激子发光

在44~77K温度范围内.在正向电压激发下的Cds MIS二极管中,观测到了发射0、1或2个纵光学(O、1LO或2LO)声子的自由激子的辐射衰减.根据激子的动能分布,讨论了1LO和2LO声子协助的伴线的形状和温度依赖.这里激子的有效温度等于晶格温度.     

Free exciton emission in forward-biased CdS MIS diodes

The radiative decay of free excitons with the emission of 0, 1, or 2LO phonons has been observed in electroluminescence in Cds MIS diodes operated in forward bias at temperatures in the range 40 – 77K. The line shapes and temperature dependence of the ILO and 2LO phonon assisted replicas are discussed in terms of the distribution of kinetic energy of the excitons with an effective temperature equal to that of the lattice.     

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.     

Temperature-dependent luminescence dynamics for ZnO thin films

The paper presents the photoluminescence investigation of zinc oxide thin films. A high quality ZnO films fabricated by dip-coating (sol–gel) method were grown on quartz wafers. The films with different thickness (number of layers) were annealed at different temperatures after the preparation process. It was found that high quality, transparent ZnO thin films could be produced on quartz substrates at relatively low annealing temperature (450–550  $^{\circ }\mathrm{C}$ ). The dependence of the ZnO thin film quality was studied by X-ray diffraction and atomic force microscopy techniques. Optical properties were investigated by classic and time-resolved photoluminescence (TRPL) measurements. Photoluminescence spectra allowed us to estimate energy of the free excitons, bond excitons and their longitudinal optical (LO) phonon replicas as a function of the annealing temperature. An innovative TRPL technique let us precisely measure the decay time of the free- and bond excitons’ in the real time. TRPL measurements as a function of temperature reveal a biexponential decay behavior with typical free/bound exciton decay constants of 970/5310 ps for the as-grown sample and 1380/5980 ps after annealing process. Presented spectra confirm high structural and optical quality of investigated films. We proved that the thermal treatment improve both optical and structural quality and extend the photoluminescence’s lifetimes. The obtained experimental results are important for identification of exciton’s peaks and their LO phonon replicas for the investigated ZnO films.     

Fabrication and low-temperature photoluminescence spectra of Mn-doped ZnO nanowires

Mn-doped ZnO nanowires have been fabricated through a high temperature vapor-solid deposition process. The low-temperature photoluminescence spectra of the samples show that there are multipeak emissions at the ultraviolet (UV) region (about 3.4?C3.0?eV). The excitonic and phonon-assisted transitions in Mn-doped ZnO nanowires were investigated. The results show that there is an obvious oscillatory structure emission at the UV region under low temperature from 12?C125?K. The oscillatory structure has an energy periodicity about 70?meV and the oscillatory structure is mainly attributed to longitudinal optical (LO) phonon replicas of free excitons?(FX). The multipeak emissions at 12?K are attributed to a donor-bound exciton (DBX, 3.3617?eV), 1LO-phonon replicas of a free exciton (FX-1LO, 3.3105?eV), 2LO-phonon replicas of a free exciton (FX-2LO, 3.2396?eV), and 3LO-phonon replicas of a free exciton (FX-3LO, 3.1692?eV), respectively. The intensity of UV emission and the efficiency of emission from the Mn-doped ZnO nanowires are improved.     

Temperature dependent photoluminescence processes in ZnO thin films grown on sapphire by pulsed laser deposition

Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300 K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons (B_X) and their phonon replicas. With increasing temperature, free exciton (F_X) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B_X peak with line width of ∼6 meV at 10 K exhibited thermal activation energy of ∼17 meV, consistent with the exciton-defect binding energy. The separation between the F_X and B_X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B_X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F_X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films.     

The optical properties of Be,Mg and Zn-diffused gallium phosphide

Interest in the Ga-site acceptors Be and Mg was stimulated by the possibility that they might produce efficient luminescence on association with O, analogous to the well-known red Zn-O luminescence in GaP but at higher transition energy. Attention was directed to diffusion doping by Be and Mg of GaP O-doped during growth because the reactivity of Be and Mg with O renders double doping during crystal growth very difficult. Structured green donor-acceptor pair spectra were observed at 1.6°K from many Be-diffused crystals, yielding an accurate measure of (E_A)_BE, 50 ± 1 meV. Moderately efficient orange-red luminescence was also observed below ∼ 100 °K from these crystals, but the intensity of this luminescence decreased rapidly to negligible levels by ∼ 200°K. This luminescence also contains sharp structure at 1.6°K, of a form characteristic of the decay of excitons bound to complex centres. Many sharp phonon replicas occur, involving local modes as well as characteristic GaP modes. One set of no-phonon lines, at least, near 2.19 eV, shows zero-field splitting, luminescence decay times and behaviour in magnetic and external strain fields characteristic of exciton decay at a centre with <100>; or <111>-type symmetry axes, containing no extra electronic particles. The exciton state is split by 2.4 meV by J-J coupling, and the axial field of the centre splits the hole states by ∼ 1.0 meV. These bound excitons are specifically characteristics of diffused GaP and appear analogous to bound excitons observed below 2.12 eV in Zn-diffused GaP. It is probable that the relevant centres contain diffusion components such as Be or Zn interstitials and improbable that O_P is involved. By contrast, weak orange bound exciton luminescence observed in Mg-diffused GaP does involve O, presumably as O_P. No analysis of the magneto-optical behaviour of this Mg-related bound exciton was possible in our crystals, so its symmetry axis was not established. It is possible that this is the Mg_Ga-O_P bound exciton. If so, the two-fold reductions in the exciton localisation energy from ∼ 0.32 eV to ∼ 0.15 eV and in the mass of the Ga-site substituent has produced dramatic changes in the form of the phonon cooperation between the Zn-O and “Mg-O” excitons. The “Mg-O” exciton luminescence is not dominant in our crystals, even at low temperature. The exciton state is again split by a local crystal field as well as by J-J coupling, but here the former splitting is predominant; 2_∈0 = 3.9 meV, Δ = 0.60 meV.     

Ultrafast optical response in polydiacetylenes and polythiophenes

Ultrafast optical response in the films of poly(3-dodecylthiophene) (P3DT) and blue-and red-phase polydiacetylenes (PDA-4BCMU) has been investigated by femtosecond absorption and picosecond luminescence spectroscopies. Several nonlinear optical processes, i.e., hole burning, Raman gain, inverse Raman scattering, and induced-frequency shift, have been observed. The relaxation processes from photoexcited free excitons to self-trapped excitons (STEs) has been observed. The time constant is estimated as 140±40 fs in the blue-phase PDA-4BCMU and 100±50 fs in P3DT. The generated unthermalized STEs thermalize with the time constant of about 1 ps. The STEs in the blue-phase PDA-4BCMU decay exponentially with lifetime of 1.6±0.1 ps at 290 K and 2.1±0.2 ps at 10 K. The decay curves in the red-phase PDA-4BCMU and P3DT are not single exponential but can be fitted to biexponential functions with time constants of slightly shorter than 1 ps and about 5 ps. These two decay time constants correspond to relaxations to the ground state, respectively, from the free exciton and unthermalized STE and from the thermalized STE.     

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.     

CdTe/CdZnTe多量子阱激子复合动力学性质的研究

报道了分子束外延制备的高质量CdTe/Cd_0.64Zn_0.36Te多量子阱结构的光学性质,由变温光致发光光谱讨论了随温度升高辐射线展宽和辐射复合效率降低的机理.在变密度激发的皮秒时间分辨光谱中,发现不同激发密度下发光衰减时间不同,并研究了它的机理.在高激发密度下观测到n=2的重空穴激子发光. 关键词：     

GaN激子跃迁的时间分辨光谱学研究

用时间分辨光谱学方法研究低压有机金属化学汽相沉积生长的ＧａＮ中自由，束缚激子（ＢＸ）的跃迁，讨论了这些跃迁的光致发光谱，复合寿命及其与温度的关系，给出了中性施主束缚激子和自由激子（ＦＸ）的辐射复合寿命分别为０．１２ｎｓ和０．４ｎｓ。     

Picosecond excitonic luminescence in ZnO and other wide-gap semiconductors

Radiative lifetimes of free-atom transitions, scaled by ω³ for comparison at 368 nm, are not faster than about 6.9 ns. BaF₂ core-valence luminescence, scaled in the same way from 220 to 368 nm, corresponds to 4.1 ns. In contrast, excitonic transitions in wide-gap semiconductors display subnanosecond radiative lifetimes, and in particular ZnO has radiative lifetimes measured at 50–300 ps for D^o,X and 400–900 ps for free excitons. The “giant oscillator strength” corresponding to these lifetimes can be explained by theories developed initially for defect-bound excitons, then quantum wells, and nanoparticles. An exciton is a coherent array of N dipoles, where N is the number of sites covered by coherent translational motion of the exciton. This is not essentially a phenomenon of multiple excitons, but applies as well to single-exciton decay. It differs in that regard from the more familiar Dicke giant dipole of N coherently excited atoms lacking translational periodicity. The phenomenon suggests possibilities for achieving ultrafast scintillators and high light yield.     

Photoluminescence of indium iodide

Polarized photoluminescence spectra of orthorhombic InI are measured from 1.7 K to 100 K. Strong edge luminescence is observed at 2.02 eV at LHeT and assigned to be due to free and bound exciton recombinations from its temperature dependence. Low energy bands are ascribed tentatively to LO (13.8 meV) and 2LO phonon replicas of the edge luminescence and donor-acceptor pair recombinations.     

Magnetoluminescence studies of excitonic scattering processes in high purity GaAs

The broad emission line at about 1.512 eV, which dominates the emission spectra of pure GaAs at intermediate excitation levels (1–300 kW/cm²) and low temperatures (<40 K) is investigated in magnetic fields up to magnetic flux densities of 10 T. The shift of the emission maximum in the magnetic field is exactly the same as recently reported for the free exciton. This demonstrates that at high excitation levels exciton-electron scattering is the dominant mechanism for the radiative decay of free excitons.     

Resonant exciton trapping at impurity states in silver bromide

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(Γ), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd²⁺ and Ca²⁺, respectively.     

在正向电压激发下ZnSe MIS二极管的自由激子发光

研究了最低温度为20~30K时,在正向电压激发下ZnSe MIS二极管的激子发光光谱,在这一温度下,二极管有可能通过足以产生发光的电流.对于利用通常气相技术生长的高纯晶体所制备的二极管,其电致发光几乎完全由Γ8→Γ6自由激子发光的1LO和2LO声子伴线所组成.根据Gross等人的半经典理论,讨论了两个谱带的形状.结果是谱带的宽度和不对称性归结为激子服从Maxwell-Boltzmann分布,其有效激子温度接近于晶格温度.     

Photoluminescence characterization of vertically aligned ZnO microrods

A detailed optical characterization of vertically aligned ZnO microrods (μRs) grown using a Ni-based catalyst was carried out by excitation-power- and temperature-dependent photoluminescence (PL) measurements. Low-temperature PL spectra of ZnO μRs are dominated by near-band-edge (NBE) emission consisted of a series of sharp lines typical for the bulk ZnO. Starting from the higher energy free exciton (FX) emission feature, the majority of them can be explained by radiative recombination of excitons bound to neutral donors (D⁰X), defect bound exciton (DBX), two-electron satellites emission, free-to-bound, i.e. free electrons to the neutral acceptors (eA⁰) transition, as well as their longitudinal-optical phonon replicas. An additional excitonic line located in between the FX and D⁰X lines, denoted as the surface excitons (SX) for ZnO μRs is observed. The intensity of the SX line is found to be smaller than that of the nanosized counterpart and has been attributed to the surface–volume ratio effects. The excitation-power-dependent results of FX line at low and high power regimes show quite close values corresponding to, respectively, p=2 and p=1 limits of the theoretical power law expression I～L^p and larger deviations for the D⁰X, SX and DBX lines. The temperature-dependent measurements confirmed the presence of eA⁰ line showing kT/2 influence to the position of eA⁰ emission line in comparison with FX. FX emissions persist up to 300 K and together with the dominant eA⁰ emission govern the line shape of the NBE emission range, while D⁰X and SX lines are quenched completely at 150 K.     

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 studies of ZnO thin films on R-plane sapphire substrates grown by sol–gel method

Zinc oxide (ZnO) thin films on R-plane sapphire substrates were grown by the sol–gel spin-coating method. The optical properties of the ZnO thin films were investigated using photoluminescence. In the UV range, the asymmetric near-band-edge emission was observed at 300 K, which consisted of two emissions at 3.338 and 3.279 eV. Eight peaks at 3.418, 3.402, 3.360, 3.288, 3.216, 3.145, 3.074, and 3.004 eV, which respectively correspond to the free exciton (FX), bound exciton, transverse optical (TO) phonon replica of FX recombination, and first-order longitudinal optical phonon replica of FX and the TO (1LO+TO), 2LO+TO, 3LO+TO, 4LO+TO, and 5LO+TO, were obtained at 12 K. From the temperature-dependent PL, it was found that the emission peaks at 3.338 and 3.279 eV corresponded to the FX and TO, respectively. The activation energy of the FX and TO emission peaks was found to be about 39.3 and 28.9 meV, respectively. The values of the fitting parameters of Varshni's empirical equation were α=4×10^?3 eV/K and β=4.9×10³ K, and the S factor of the ZnO thin films was 0.658. With increasing temperature, the exciton radiative lifetime of the FX and TO emissions increased. The temperature-dependent variation of the exciton radiative lifetime for the TO emission was slightly higher than that for the FX emission.     

Thermal dissociation of excitons in a type-I GaAs/AlAs superlattice studied by time-resolved photoluminescence measurements

In order to investigate thermal properties of excitons, we have performed time-resolved photoluminescence (PL) measurements for a type-I (GaAs)₁₅/(AlAs)₁₅ superlattice. At low temperatures, PL signals show ordinal exponential time decay, whereas at high temperatures, the PL shows power decay. Such change of PL signals is understood by considering thermal dissociation of exciton into account. At low temperatures, recombination of bound excitons generates PL which shows exponential decay. At temperatures higher than the exciton binding energy, correlation between electrons and holes disappears. Recombination of free excitons causes PL which decays by a power function. By means of the least-squares fitting, we evaluate the portion of bound excitons, recombination time of bound and free excitons as functions of temperatures.