Photoluminescence studies in sol-gel derived Zno films

The photoluminescence (PL) properties of high quality ZnO thin films grown on Si (1 0 0) substrates using spin coating technique are investigated as a function of temperature in the range 10-300 K. The PL spectra shows dominant donor bound excitonic emission along with free exciton related emission in the UV region. The corresponding activation energy of thermal quenching is found to be . The parameters that describe the temperature dependent red shift of the band-edge transition energy are evaluated using different models. The broadening of the PL peak due to increase in temperature is mainly attributed to the exciton-LO phonon coupling.     

Dependence of photoluminescence from a-Si nanoparticles on the annealing time and exciting wavelength

Thin films of SiO_x having thickness of 0.2 μm and oxygen content x=1.5 or 1.7 are prepared by thermal evaporation of SiO in vacuum. Then some samples are furnace annealed for various times (in the range ) at 770 and 970 K and some others are rapid thermal annealed at 970 K for 30 and 60 s. Photoluminescence (PL) measurements are carried out at room temperature using the 442 nm line of a He-Cd laser and the 488 nm of an Ar laser for excitation. The effect of the annealing conditions and wavelength of the exciting light on the shape of the PL from these films is explored. The deconvolution of the PL spectra measured with the 442 nm line from samples annealed at 770 K for reveals two distinct PL bands peaked at around 2.3 and 2.5 eV, which do not shift appreciably with increasing annealing time. In addition, at longer annealing times, a weak third band is resolved centred in the range 2.0-2.1 eV. It exists in the spectra of all samples annealed at 970 K being more prominent in the samples with x=1.5. The intensity of this band shows different dependences on the annealing time in the films with different initial composition. The results obtained are discussed in terms of radiative recombination via defect states in the SiO_x matrix (the 2.5 eV band) or at the a-Si-SiO_x interface (the 2.3 eV band). The band centred in the 2.0-2.1 eV range is related to recombination in amorphous silicon nanoparticles grown upon annealing.     

Influence of the incorporation of metals on the optical properties of MCM-41

We studied the optical properties of mesoporous aluminosilica incorporated with metal nanoparticles, which can be used to efficiently enhance catalytic activity for CO oxidation. Analysis of the PL spectra indicated that the incorporation of Al greatly enhanced the generation of oxygen related defects. When Au nanoparticles were deposited onto Al-MCM-41, both the PL and PLE spectra suggested that the metal nanoparticles were strongly attracted by the electrostatic force induced by the charged oxygen defects. Electron spin resonance measurement provided further evidence, registering a spike in the intensity following incorporation of the metal particles, as the strong interaction between the F+ center and the metal particles induced electron transfer to the trapped O₂ molecule. These discoveries shed further light on the perplexing mechanism by which the Si/Al ratio and metal particles can give rise to dramatic enhancement of the catalytic activity for CO oxidation.     

Studies on CdS nanoparticles dispersed in silica matrix prepared by sol-gel technique

SiO₂/CdS-nanoparticle composite films (SiO₂:CdS=85:15, 80:20, 75:25 and 70:30) were prepared by the sol-gel route. The films were characterized by studying microstructural (XRD and TEM) and optical (transmittance and photoluminescence) properties. Band gaps of these films annealed at different temperatures (373-473 K) for different times (10-120 min) indicated that the signature of nanocrystallinity is retained throughout the range of our experimental conditions. A thermal diffusion process controlled growth in the crystallite size with increasing annealing time and temperature. The average radii of the nanoparticles varied as the cube root of the annealing time but showed exponential dependence on the inverse of annealing temperature. Photoluminescence (PL) studies of the composite films indicated excitonic transitions. Theoretical analysis of the line shapes of the PL peaks recorded at 300 K and 80 K could be accounted for by the combined effects of size distribution and phonon broadening. It was observed that the deformation potential (E _d) effectively controlled the line shapes of the PL measurements. Received 24 May 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Enhanced emission of Er from alternately Er doped Si-rich Al2O3 multilayer film with Si nanocrystals as broadband sensitizers

Alternately Er doped Si-rich Al₂O₃ (Er:SRA) multilayer film, consisting of alternate Er-Si-codoped Al₂O₃ (Er:Si:Al₂O₃) and Si-doped Al₂O₃ (Si:Al₂O₃) sublayers, has been synthesized by co-sputtering from separated Er, Si, and Al₂O₃ targets. The dependence of Er³⁺ related photoluminescence (PL) properties on annealing temperatures over 700-1100 °C was studied. The maximum intensity of Er³⁺ PL, about 10 times higher than that of the monolayer film, was obtained from the multilayer film annealed at 950 °C. The enhancement of Er³⁺ PL intensity is attributed to the energy transfer from the silicon nanocrystals in the Si:Al₂O₃ sublayers to the neighboring Er³⁺ ions in the Er:Si:Al₂O₃ sublayers. The PL intensity exhibits a nonmonotonic temperature dependence: with increasing temperature, the integrated intensity almost remains constant from 14 to 50 K, then reaches maximum at 225 K, and slightly increases again at higher temperatures. Meanwhile, the PL integrated intensity at room temperature is about 30% higher than that at 14 K.     

Optical study of ZnP2 nanoparticles in zeolite Na-X

Nanoparticles of the II-V semiconductor (ZnP₂) were prepared and investigated. ZnP₂ nanoparticles were incorporated into zeolite Na-X matrix. Absorption, diffuse reflection (DR) and photoluminescence (PL) spectra of ZnP₂ nanoparticles were measured at the temperature of 77 K. Five bands B₁-B₅ are observed in both the DR and PL spectra demonstrating the blue shift from the line of free exciton in bulk crystal. We attribute the B₁-B₅ bands to five stable nanoparticles with size less than the size of zeolite Na-X supercage. We observed Stokes shift of the PL bands with respect to the absorption bands. This dependence of this Stokes shift on the particle size is nonmonotonic.     

Excitation of Intra-4f Shell Luminescence of Rare Earth Ions (Er3+ and Yb3+) by the Energy Transfer from Si Nanocrystals

Photoluminescence (PL) properties of SiO₂ films containing Si nanocrystals (nc-Si) and Er³⁺ (Yb³⁺) were studied. PL peaks attributable to the recombination of electron–hole pairs in nc-Si (1.5eV) and the intra-4f shell transition of Er³⁺ (0.81eV) (Yb³⁺ (1.26eV)) were observed simultaneously at room temperature. Correlation of the two peaks was studied as a function of nanocrystalline size. It was found that the intensity of the Er³⁺-related (Yb³⁺-related) peak increases drastically as the size of nc-Si decreases. Temperature dependence of PL spectra was studied. In the case of Yb-doped samples, temperature quenching of the PL became small as the size decreased, while in the case of Er-doped samples, no remarkable temperature dependence was observed. Two major features of the quantum size effects of nc-Si, i.e., the band-gap widening and the increase in the PL efficiency with decreasing the size, are thought to contribute to the improvement of room temperature PL efficiency of Er³⁺ (Yb³⁺).     

A new photoluminescence emission peak of ZnO-SiO2 nanocomposites and its energy transfer to Eu ions

This work reports a new photoluminescence (PL) emission peak at about 402 nm from amorphous ZnO nanoparticles in a silica matrix, and the energy transfer from it to Eu³⁺ ions. The amorphous ZnO-SiO₂ nanocomposites were prepared by the sol-gel method, which is verified by X-ray diffraction (XRD) profiles and FT-IR spectra. The luminescence emission spectra are fitted by four Gauss profiles, two of which at longer wavelength are due to the defects of the material and the others to amorphous ZnO nanoparticles and the Zn-O-Si interface state. With the reduction of Zn/Si ratio and diethanolamine, the relative intensities of visible emission decrease. The weak visible emission is due to the reduction of defects after calcined at high temperature. The new energy state at the Zn-O-Si interface results in strong emission at about 402 nm. When Eu³⁺ ions are co-doped, weak energy transfer from ZnO-SiO₂ nanocomposites to Eu³⁺ emission are observed in the excitation spectra.     

Temperature behaviour of optical properties of Si+-implanted SiO2

Silicon nanocrystals were prepared by Si⁺-ion implantation and subsequent annealing of SiO₂ films thermally grown on a c-Si wafer. Different implantation energies (20-150 keV) and doses - cm ^-2 ) were used in order to achieve flat implantation profiles (through the thickness of about 100 nm) with a peak concentration of Si atoms of 5, 7, 10 and 15 atomic%. The presence of Si nanocrystals was verified by transmission electron microscopy. The samples exhibit strong visible/IR photoluminescence (PL) with decay time of the order of tens of μs at room temperature. The changes of PL in the range 70-300 K can be well explained by the exciton singlet-triplet splitting model. We show that all PL characteristics (efficiency, dynamics, temperature dependence, excitation spectra) of our Si⁺-implanted SiO₂ films bear close resemblance to those of a light-emitting porous Si and therefore we suppose similar PL origin in both materials. Received 1st September 1998 and Received in final form 7 September 1999     

Photoluminescence spectra of rare earth doped CaGa2S4 single crystals

Six kind CaGa₂S₄ single crystals doped with different rare earth (RE) elements are grown by the horizontal Bridgman method, and their photoluminescence (PL) spectra are measured in the temperature range from 10 to 300 K. The PL spectra of Ce or Eu doped crystals have broad line shapes due to the phonon assisted 4f-5d transitions. On the other hand, those of Pr³⁺, Tb³⁺, Er³⁺ or Tm³⁺ doped samples show narrow ones owing to the 4f-4f transitions. The assignments of the electronic levels are made in reference to the reported data of RE 4f multiplets observed in same materials.     

Photoluminescence in amorphous (PbLa)TiO3 thin films deposited on different substrates

Pb_1−XLa_XTiO₃ thin films, (X=0.0; 13 and 0.27 mol%) were prepared by the polymeric precursor method. Thin films were deposited on Pt/Ti/SiO₂/Si (1 1 1), Si (1 0 0) and glass substrates by spin coating, and annealed in the 200-300°C range in an O₂ atmosphere. X-ray diffraction, scanning electron microscopy and atomic force microscopy were used for the microstructural characterization of the thin films. Photoluminescence (PL) at room temperature has been observed in thin films of (PbLa)TiO₃. The films deposited on Pt/Ti/SiO₂/Si substrates present PL intensity greater than those deposited on glass and silicon substrates. The intensity of PL in these thin films was found to be dependent on the thermal treatment and lanthanum molar concentration.     

Correlation between optical properties and Si nanocrystal formation of Si-rich Si oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiO_x, 0 < x < 2) films by thermal annealing in high vacuum. The SiO_x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N₂O/SiH₄) flow ratios. The as-deposited films show increased Si concentration with decreasing N₂O/SiH₄ flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N₂O/SiH₄ reaction. After thermal annealing at temperatures above 1000 °C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiO_x films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from ∼1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model.     

Infrared radiation of zinc selenide single crystals

Long-wave photoluminescence (PL) spectra of both as-grown and Au-doped n-ZnSe single crystals are studied in the temperature range from 81 to 300 K. A narrow band of infrared (IR) radiation centered at 878 nm (1.411 eV) manifests itself in the low-temperature PL spectrum. It is established that this band intensity first increases and then decreases with increasing concentration of doping impurity. With increasing excitation radiation intensity, spectral position of the IR PL band is unchanged and its intensity increases under the linear law. With increasing excitation radiation wavelength, the IR PL band intensity increases, it becomes narrower and shifts towards long wavelengths. It is shown that the observed IR radiation is caused by recombination of free electrons with holes localized on associative acceptors in the ZnSe:Zn:Au crystals or in the undoped crystals.     

The effects of substrate temperature on the structure, morphology and photoluminescence properties of pulsed laser deposited SrAl2O4:Eu,Dy thin films

In this study, SrAl₂O₄:Eu²⁺,Dy³⁺ thin film phosphors were deposited on Si (1 0 0) substrates using the pulsed laser deposition (PLD) technique. The films were deposited at different substrate temperatures in the range of 40-700 °C. The structure, morphology and topography of the films were determined by using X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) data was collected in air at room temperature using a 325 nm He-Cd laser as an excitation source. The PL spectra of all the films were characterized by green phosphorescent photoluminescence at ∼530 nm. This emission was attributed to 4f⁶5d¹→4f⁷ transition of Eu²⁺. The highest PL intensity was observed from the films deposited at a substrate temperature of 400 °C. The effects of varying substrate temperature on the PL intensity were discussed.     

Epitaxial growth and optical characterization of compound semiconductor β-FeSi2 prepared by pulsed laser deposition

β-FeSi₂ thin films were prepared on FZ n-Si (1 1 1) substrates by pulsed laser deposition (PLD). The structural properties and crystallographic orientation of the films were investigated by X-ray diffraction (XRD) analysis. This indicates that β-FeSi₂/Si (2 0 2/2 2 0) and the single-crystalline β-FeSi₂ can be prepared using PLD. In photoluminescence (PL) measurements at 8 K detected by Ge detector, the PL spectra of the samples annealed at 900 °C for 1, 5, 8 and 20 h showed that the PL intensity of the A-band peak increased depending on annealing time in comparison with those of as-deposited samples. The intrinsic PL intensity of the A-band peak at 0.808 eV of the β-FeSi₂ from the 20-h-annealed sample was investigated for the first time by the PLD method detected by an InGaAs detector. This result has been confirmed by temperature dependence and excitation power density of the 20-h-annealed sample with the comparison of other defect-related band peaks of the sample. Cross-sectional scanning electron microscopy (SEM) observation was also performed and the thickness of the thin films was found to be at 75 nm for 20-h-annealed. The thermal diffusion for the epitaxial growth of β−FeSi₂/Si was observed when the compositional ratio of Fe to Si was around Fe:Si=1:2 for 20-h-annealed carried out by energy dispersive X-ray spectroscopy (EDX). We discussed high crystal quality of the epitaxial growth and optical characterization of β-FeSi₂ achieved after annealing at 900 °C for 20 h.     

衬底温度对PLD法生长的Mg0.05Zn0.95O薄膜结构和发光特性的影响

用脉冲激光沉积(PLD)法在不同温度的Si(111)衬底上成功制备了c轴择优取向的Mg_005Zn_095O薄膜.通过X射线衍射(XRD)和光致发光谱(PL)研究了衬底温度对Mg_005Zn_095O薄膜结构和发光特性的影响,探讨了薄膜的结晶质量与发光特性之间的关系.结果表明,在衬底温度为450℃时生长的Mg_005Zn_095O薄膜具有很好的c轴取向和较强的光致发光峰.室温下分别用激发波长为240,300和325nm的氙灯作为激发光源得到不同样品的PL谱,分析表明紫外发光峰和紫峰来源于自由激子的复合辐射且发光强度与薄膜的结晶质量密切相关,蓝绿发光峰与氧空位有关.此外,探讨了衬底温度影响紫外光致发光峰红移和蓝移的可能机理. 关键词： 005Zn_095O薄膜')" href="#">Mg_005Zn_095O薄膜 PLD 衬底温度 光致发光     

Photoluminescence scanning on InAs/InGaAs quantum dot structures

The photoluminescence spectra of InAs quantum dots (QDs) embedded into four types of In_xGa_1−xAs/GaAs (x = 0.10, 0.15, 0.20 and 0.25) multi quantum well MBE structures have been investigated at 300 K in dependence on the QD position on the wafer. PL mapping was performed with 325 nm HeCd laser (35 mW) focused down to 200 μm (110 W/cm²) as the excitation source. The structures with x = 0.15 In/Ga composition in the In_xGa_1−xAs capping layer exhibited the maximum photoluminescence intensity. Strong inhomogeneity of the PL intensity is observed by mapping samples with the In/Ga composition of x ≥ 0.20-0.25. The reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300 K as follows from the quantum dot PL mapping. The mechanism of this effect has been analyzed. PL peak shifts versus capping layer composition are discussed as well.     

Room temperature photoluminescence mechanism of SiOx film after annealing at different temperatures

SiO_x films were deposited on Si(1 0 0) substrates by evaporation of SiO powder. The samples were annealed from room-temperature (RT) to 1100 °C. After the samples were cooled down to RT, photoluminescence (PL) spectra from these samples were measured. It was found that when the annealing temperature Ta is not higher than 1000 °C, there is a PL centered at 620 nm, and with Ta increasing the intensity increases at first and then decreases when Ta is higher than 500 °C. When Ta is no less than 1000 °C another PL peak located at 720 nm appears. Combined with Raman and XRD spectra, we confirm that the latter PL is from Si nanocrystals that start to form when Ta is higher than 1000 °C. PL spectra for Ta<900 °C were discussed in detail and was attributed to defects in the matrix rather than from Si clusters.     

Si-based nanoscale SiO2 islands and light-emitting source array

A SiO₂ nanoscale island array was fabricated on a Si substrate by using anodic porous alumina as a mask. Transmission electron microscopy observation and the atomic force microscopy pattern show that the arrangement of SiO₂ islands has a quasi-hexagonal symmetry. Ge ions with a dose of 1×10¹⁷ cm^-2 were subsequently implanted into the SiO₂ island array to form Ge-related light-emitting centers. The photoluminescence (PL) spectra of as-implanted and annealed samples show three PL bands at 370, 400 and 415 nm. Their intensities reach maximums in the sample with an annealing temperature of 700 °C. Spectral analysis suggests that the 370 and 415 nm PL bands arise from Ge-Ge and Ge-Si defect centers, while the 400 nm PL is related to GeO color centers in the SiO₂ islands. The existence of these PL bands indicates the formation of a Si-based nanoscale light source array. PACS 78.55.Mb; 42.72.Bj; 68.65.+g