Photoluminescence of PbBr2, PbCl2 and β-PbF2 single crystals

Photoluminescence measurements on a PbBr₂ single crystal at 4.2 K revealed a violet and a yellow emission band, in addition to the UV, blue and red band which were already known. The violet and blue luminescence are ascribed to emission of an exciton bound by a defect. The UV emission is quenched at much lower temperature than reported previously, because of a more effective energy transfer from the exciton to defect centres. Luminescence results on PbCl₂ and β-PbF₂ are in fair agreement with those of other authors.     

CdS单晶中的绿色电致发光和光致发光

在50-290K温度范围内,研究了在正向电压激发下,用高纯CdS单晶制备的MIS发光二极管的电致发光光谱。在室温时,发射光谱分别由峰值为5135±25Å和5300±15Å的二个绿色谱带所组成,而在低温下,观测到有很多结构的光谱。在50K时,自由和束缚激子的发射实际上占有统治地位。文中提出,在室温时,谱峰为5135±25Å的谱带是与受导带自由电子散射的自由激子的发射有关;而谱峰为5300±15Å的谱带归结为与同时发射二个纵光学声子的自由激子的发射有关。比较了刚生长的高阻CdS单晶以及在熔融镉中热处理的低阻CdS单晶的光致发光光谱。在熔融镉中的热处理抑制了自由到束缚(HES)和束缚到束缚(LES)的边缘发射,也抑制了I2束缚激子谱线,但是增强了自由激子的发射,在电场激发下,也使自由激子的发射增强。     

Optical properties of Tm-doped GaSe single crystals

Optical properties of Tm-doped GaSe single crystals were investigated by measurements of optical absorption and photoluminescence. The single crystals were grown by the Bridgman technique. The X-ray diffraction analysis revealed that the single crystals were in the ε-type GaSe phase. The optical absorption spectra showed a sharp absorption peak at 582 nm near the band edge, which is due to direct free exciton. The temperature dependence of the energy of the exciton absorption peak was well fitted by the Varshni relation. In the photoluminescence spectrum at 10 K, we observed a very weak emission peak at 586 nm, a relatively strong emission peak centered at 613 nm, and several sharp and narrow emission peaks in the 790-840 nm region. The two emission peaks at 586 and 613 nm were associated with intrinsic emission lines due to direct free exciton and indirect bound exciton. The emission peaks in the 790-840 nm region, which were related to extrinsic emission, were assigned as due to the ³F₄→³H₆ transition of Tm³⁺ ions with a low symmetry of D₃ in the host lattice.     

Abnormal Energy Dependence of Photoluminescence Decay Time in InGaN Epilayer

We employ photoluminescence (PL) and time-resolved PL to study exciton localization effect in InGaN epilayers.By measuring the exciton decay time as a function of the monitored emission energy at different temperatures,we have found unusual behaviour of the energy dependence in the PL decay process. At low temperature, the measured PL decay time increases with the emission energy. It decreases with the emission energy at 200K, and remains nearly constant at the intermediate temperature of 12OK. We have studied the dot size effect on the radiative recombination time by calculating the temperature dependence of the exciton recombination lifetime in quantum dots, and have found that the observed behaviour can be well correlated to the exciton localization in quantum dots. This suggestion is further supported by steady state PL results.     

ZnO薄膜近带边紫外发光的研究

用ZnO陶瓷靶,采用脉冲激光沉积(PLD)技术在c-Al₂O₃衬底上制备了ZnO薄膜。通过不同温度下光致发光(PL)光谱的测量,对样品的紫外发光机理进行研究。 在较低温度(10 K)下的PL光谱中,观测到一个位于3.354 eV处的束缚激子(D⁰X)发射,随着温度的升高(~50 K),在D⁰X的高能侧观测到了自由激子的发射峰。在10 K温度下,3.309 eV处出现了一个较强的发光带A,此发光带强度随着温度升高先增大然后减小,并且一直延续到室温。重点讨论了此发光带的起源,并认为A带可归属于自由电子-受主之间的复合发射。     

Excitonic emission in highly excited lead iodine

Excitonic luminescence in PbI₂ has been investigated in the temperature range between 77 and 300°K under high excitation by a nitrogen laser. A single emission band is observed which, at low pumping, is centered at the free exciton energy. When the excitation intensity is increased, the band broadens and shifts towards lower energies. These facts are interpreted on the basis of an exciton-electron interaction.     

Luminescence of CuInS2: II. Exciton and near edge emission

The near-edge (exciton) emission of CuInS₂ is investigated for various material-compositions as a function of temperature. From these investigations the exciton ionization energy (20 meV) and the temperature dependence of the energy gap were determined. For the first time, recombination of the free exciton belonging to the deeper lying Γ₇ valence bands has been observed. Moreover, six different bound exciton emission lines and a donor to valence band transition were detected. These emissions could be assigned in terms of the defect-chemical model presented in Part I.     

纳米ZnO薄膜的激子光致发光特性

报道了纳米ZnO薄膜激子光致发光(PL)与温度的关系。首先利用低压金属有机化学气相沉积(LPMOCVD)技术生长ZnS薄膜,然后将ZnS薄膜在氧气中于800℃下热氧化2h获得纳米ZnO薄膜。X射线衍射(XRD)结果表明,纳米ZnO薄膜具有六角纤锌矿多晶结构且具有择优(002)取向。室温下观察到一束强的紫外(326eV)光致发光(PL)和很弱的深能级(DL)发射。根据激子峰的半高宽(FWHM)与温度的关系,确定了激子纵向光学声子(LO)的耦合强度(ГLO)。     

正向高密度脉冲电流激发下ZnSe MIS二极管中的自由激子发光

本文测量了在77K和正向脉冲电流密度为50～500mA/mm2的激发下,ZnSe MIS二极管的电致发光光谱.首次在高电流密度激发下的ZnSe晶体的电致发光光谱上,观测到自由激子与自由激子间(Ex-Ex)的散射.本中根据自由激子的动能分布,讨论了2LO声子协助的自由激子伴线的形状,发现当激发电流密度增高时,自由激子的有效温度大于晶格温度,这可归结为激子与激子间的非弹性散射.     

激子转移对GaP:N发光热猝灭过程的影响

本文研究了20K-120K范围内GaP:N束缚激子发光与温度的关系,从实验结果发现,各发光中心之间存在着激子转移.初步分析表明,采用带带激发方式,激子转移会导致深束缚中心的热激活能拟合值偏高,其发光热猝灭是由于空穴的热离化所造成的.     

Pressure dependence of the near-band-edge photoluminescence from ZnO microrods at low temperature

The temperature and pressure dependences of band-edge photoluminescence from ZnO microrods have been investigated. The energy separation between the free exciton (FX) and its first order phonon replica (FX-1LO) decreases at a rate of k_BT with increasing temperature. The intensity ratio of the FX-1LO to the bound exciton (BX) emission is found to decrease slightly with increasing pressure. All of the exciton emission peaks show a blue shift with increasing pressure. The pressure coefficient of the FX transition, longitudinal optical (LO) phonon energy, and binding energy of BX are estimated to be 21.4, 0.5, and 0.9 meV/GPa, respectively.     

High-energy frenkel cation exciton and specific features of its self-trapping in the CdI<Subscript>2</Subscript>-PbI<Subscript>2</Subscript> crystal system

It has been shown using atomic-force microscopy that the PbI₂ impurity is embedded in the CdI₂ crystal lattice in the form of nanocrystalline inclusions. The model of a high-energy cation exciton related to the ³ P ₂ state of a free Pb²⁺ ion has been considered for the impurity absorption (excitation) band at 3.23 eV. The resonance narrow photoluminescence bands with the split absorption band at 3.12 and 3.20 eV have been compared with the emission of a free Frenkel exciton. It has been demonstrated that, in the temperature range 25–45 K, there arises a self-trapped exciton state, and the main role in its formation is played by the bending vibrations of the CdI₂ crystal lattice. The potential barrier separating the self-trapped state from the free exciton is 23 meV. The photoluminescence band at 2.4 eV is assigned to the emission of the self-trapped high-energy cation exciton of PbI₂ in the CdI₂ crystal lattice.     

Mössbauer-type photoluminescence spectra of irradiated germamum

A Mössbauer-type structure has been observed in the 15°K spectra of germanium crystals irradiated at room temperature by Co⁶⁰ γ-rays and 700 keV electrons. The spectra consist of an intense band peaking at 0.688 eV accompanied by mainly TA-phonon-assisted lower energy sidebands. The transition producing the luminescence is believed to be the annihilation of an exciton bound to a defect center by forces of shorter range than coulombic attraction. The dependence of the new spectra upon irradiation temperature and certain sample characteristics indicate that the center contains both primary defect(s) and an unknown impurity.     

High-pressure spectroscopic studies of free and self-trapped excitons in alkali halides at low temperatures

Abstract

A brief survey of our studies of free and self-trapped excitons (FE and STE) in alkali halide crystals under hydrostatic pressure up to 12.5 kbar at 4.2–140 K is presented. Main attention is paid to the following effects observed: (1) the strong coupling of three energy levels of FE in CsI revealing itself as an exciton analog of pressure-scanned Fermi resonance; (2) emergence of a new emission band of STE in CsI under pressure; (3) a large pressure shift of the thermal quenching curve for STE emission in NaCl.     

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.     

Photoluminescence and absorption in sol-gel-derived ZnO films

Highly c-axis-oriented ZnO films were obtained on corning glass substrate by sol-gel technique. The characteristics of photoluminescence (PL) of ZnO, as well as the exciton absorption in the absorption (UV) spectra are closely related to the post-annealing treatment. The difference between PL peak position and the absorption edge, designated as Stokes shift, is found to decrease with the increase of annealing temperature. The minimum Stokes shift is about 150 meV. The decrease of Stokes shift is attributed to the decrease in carrier concentration in ZnO film with annealing. X-ray diffraction, surface morphology and refractive index results indicate an improvement in crystalline quality with annealing. Annealed films also exhibit a green emission centered at ∼520 nm with activation energy of 0.89 eV. The green emission is attributed to the electron transition from the bottom of the conduction band to the antisite oxygen O_Zn defect levels.     

Optical properties of Zn0.5Cd0.5Se thin films grown on InP by molecular beam epitaxy

We report photoluminescence (PL) and reflectivity measurements of Zn_0.5Cd_0.5Se epilayers grown by molecular beam epitaxy on InP substrates. The low-temperature PL spectra are dominated by asymmetric lines, which can be deconvoluted into two Gaussian peaks with a separation of ∼8 meV. The behavior of these peaks is studied as a function of excitation intensity and temperature, revealing that these are free exciton (FE) and bound exciton emission lines. Two lower energy emission lines are also observed and assigned to the first and second longitudinal optical phonon replicas of the FE emission. The temperature dependence of the intensity, line width, and energy of the dominant emission lines are described by an Arrhenius plot, a Bose-Einstein type relationship, Varshni's and Bose-Einstein equations, respectively.     

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.     

Photoluminescence study of Si doped and undoped Chalcopyrite CuGaSe2 thin films

Photoluminescence (PL) characteristics have been studied on undoped and Si-doped CuGaSe₂ single crystal thin films grown on GaAs (001) substrate by migration-enhanced epitaxy. Room temperature PL spectrum of an undoped layer clearly shows free excitonic emission bands related to the minimum band-edge and to the split-off valence band, but no discernible emission has been observed in the low energy area. At 4.2 K, the excitonic emission due to the split-off valence band disappears. Instead, two additional emissions appear at 1.68 and 1.715 eV which are attributed to the bound exciton and band-to-acceptor transition. The Si doping to CuGaSe₂ produces two additional PL bands around 1.61 and 1.64 eV. These PL bands are attributed to the donor acceptor pair emissions due to the doped Si impurity which probably occupies Cu or Ga sites and intrinsic Cu vacancy.     

高质量ZnO及BeZnO薄膜的发光性质

用分子束外延设备插入缓冲层在c面蓝宝石上生长得到高质量ZnO和BeZnO薄膜。XRD测试显示薄膜具有六方结构和c轴取向,并具有良好的晶体质量,其中ZnO薄膜的半高宽仅为108 arcsec,Be_xZn_1-xO薄膜的半高宽小于600 arcsec。对ZnO和BeZnO薄膜的拉曼光谱进行对比研究发现,随着Be元素的掺入,A₁(LO)、A₁(2LO)声子模频率往大波数方向移动,并且首次发现了与Be元素掺杂有关的局域振动模。利用变温光致发光光谱研究了薄膜的发光性质,结果显示ZnO薄膜室温光致发光只出现一个紫外发发光峰(378 nm),而低温光谱(80 K)则出现了很强的自由激子发光峰。随着温度的升高,束缚激子发光逐渐湮灭向自由激子发光转变,并且峰值位置红移。相对于ZnO薄膜,BeZnO薄膜的紫外发光主峰位置蓝移,并且由于Be元素的掺入导致薄膜晶体质量下降,在低温(80 K)光致发光光谱中没有出现强的自由激子发光峰。另外,在低温光致发光及拉曼光谱中,主峰位置在100~200 K之间有局部最大值,推测为由于合金晶格热膨胀系数失配而引起的应力效应。