Absorption cross-sections and lifetimes as a function of size in Si nanocrystals embedded in SiO2

The photoluminescence (PL) emission yield of Si nanocrystals embedded in SiO₂ depends on their size and on Si–SiO₂ interface passivation. In this work we aim at clarifying the relative importance of both contributions by studying lifetimes and absorption cross-sections as a function of size, for samples with and without passivation in forming gas. We find that while the PL lifetime increases steadily (quasi-linear dependence), the radiative lifetime increases exponentially with the nanocrystal size. Thus, as expected, radiative oscillator strengths are much smaller for large nanocrystals, but this reduction is partially compensated by a less effective quenching at interfacial non-radiative states. The absorption cross-section per nanocrystal rises as the nanocrystal size decreases, for all excitation wavelengths, implying that the variation of oscillator strength dominates over the reduction of the density of states. Passivation processes do not affect the emission mechanism and increase the emission yield while reducing the density of non-radiative recombination centers at the Si–SiO₂ interface (P_b centers).     

The influence of the annealing conditions on the photoluminescence of ion-implanted SiO2:Si nanosystem at additional phosphorus implantation

The influence of P ion doping on the photoluminescence (PL) of the system of nanocrystals in SiO₂ matrix (SiO₂:Si) both without annealing and after annealing at various temperatures (provided before and after additional P implantation) is investigated. The Si and P implantation was carried out with ion energies of 150 keV and doses Φ_Si=10¹⁷ cm⁻² and Φ_P=(0.1–300)×10¹⁴ cm⁻² (current density j3 μAcm⁻²). The system after Si implantation was formed at 1000°C and 1100°C (2 h). For the case of SiO₂:Si system as-implanted by P, the intensity of PL was drastically quenched, but partially retained. As for the step-by-step annealing (at progressively increased temperatures) carried out after P implantation, the sign and degree of doping effect change with annealing temperature. The possible mechanisms of these features are discussed.     

A combined approach to fabricating Si nanocrystals with high photoluminescence intensity

This work demonstrates that by combining three methods with different mechanisms to enhance the photoluminescence (PL) intensity of Si nanocrystals embedded in SiO₂ (or Si-nc:SiO₂), a promising material for developing Si light sources, a very high PL intensity can be achieved. A 30-layered sample of Si-nc:SiO₂/SiO₂ was prepared by alternatively evaporating SiO and SiO₂ onto a Si(1 0 0) substrate followed by thermal annealing at 1100 °C. This multilayered sample possessed a fairly high PL efficiency of 14% as measured by Greenham's method, which was 44 times that of a single-layered one for the same amount of excess Si content. Based on this multilayered sample, treatments of CeF₃ doping and hydrogen passivation were subsequently applied, and a high PL intensity which was 167 times that of a single-layered one for the same amount of excess Si content was achieved.     

Hydrogenation effect on enhancement of photoluminescence of Er and Si nanocrystallites in Er-doped SiO2 synthesized by laser ablation

Significant enhancement of photoluminescence (PL) was attained for Er ions and Si nanocrystallites (nc-Si) in SiO₂ films by two kinds of hydrogenation, using H₂ molecules or H atoms. Er-doped SiO₂ films containing Er impurities and a high density of nc-Si were fabricated by laser ablation of Er films deposited on Si substrate in an O₂ gas atmosphere, followed by annealing at high temperatures in flowing Ar gas. Hydrogenation at 300–500 °C was found to effectively increase the PL intensity of Er ions as well as that of nc-Si. In particular, the hydrogen atom treatment dramatically shortens the hydrogenation time for the enhancement of Er PL compared to the hydrogen molecule treatment. Spectra of electron spin resonance showed a decrease in residual defects, namely, P_b-type defects located at the interfaces between nc-Si and SiO₂ by hydrogenation. These results clearly show the effectiveness of hydrogen passivation for Si nanostructures; i.e., the increase in the Er PL and nc-Si PL due to hydrogen passivation of the nonradiative recombination centers located at the interfaces between nc-Si and SiO₂. PACS 78.67.Bf; 71.20.Eh; 76.30.Mi; 81.15.Fg     

Size control of Si nanocrystals by two-step rapid thermal annealing of sputtered Si-rich oxide/SiO2 superlattice

Controllable size of silicon (Si) nanocrystals can be achieved by a two-step rapid thermal annealing technique consisting of rapid annealing at 1000°C in nitrogen ambient and rapid oxidation at 600–800°C of a radio frequency magnetron co-sputtered Si-rich oxide/SiO₂ superlattice structure. The photoluminescence (PL) spectra related to Si nanocrystals were observed in the visible range (600–900 nm). After rapid oxidation, the size of the nanocrystals was reduced and the quality of the Si nanocrystal/SiO₂ interface was improved, resulting in a blue shift and an increase of the PL peak intensity. Finally, annealing in air increases the PL intensity further.     

Structural and photoluminescence properties of silicon nanocrystals embedded in SiC matrix prepared by magnetron sputtering

Si nanocrystals (NCs) embedded in an SiC matrix were prepared by the deposition of Si-rich Si_1?xC_x/SiC nanomultilayer films using magnetron sputtering, subsequently followed by thermal annealing in the range of 800～1200 °C. As the annealing temperature increases to 1000 °C, Si NCs begin to form and SiC NCs also start to emerge at the annealing temperature of 1200 °C. With the increase of annealing temperature, two photoluminescence (PL) peaks have an obvious redshift. The intensity of the low-energy PL peak around 669～742 nm gradually lowers, however the intensity of high-energy PL peak around 601～632 nm enhances. The low-energy PL peak might attribute to dangling bonds in amorphous Si (a-Si) sublayers, and the redshift of this peak might be related to the passivation of Si dangling bonds. Whereas the origin of the high-energy PL peak may be the emergence of Si NCs, the redshift of this peak correlates with the change in the size of Si NCs.     

Annealing effect on the optical properties in Si delta-doped Al0.24Ga0.76As/In0.15Ga0.85As/GaAs pseudomorphic high electron mobility transistor structures

The optical properties of Si delta-doped Al_0.24Ga_0.76As/In_0.15Ga_0.85As/GaAs pseudomorphic high electron mobility transistor structure (PHEMTs) are estimated after the process of rapid thermal annealing (RTA) in the temperature range 500–750°C. After layer intermixing and decrease of 2DEG densities of PHEMTs just occurs around the annealing temperature of 650°C, the 12H transition peak at 1.354 eV above the annealing temperature of 650°C is newly observed from the photoluminescence (PL) and photoreflectance (PR) spectra. From the results of PL and PR measurements in the annealed PHEMTs, it is found that remarkable modification of band profile in InGaAs QW occur at annealing temperature above 650°C.     

镶嵌在氧化硅薄膜中纳米晶硅的可见光荧光谱与发光机制的研究

采用对非晶氧化硅薄膜退火处理方法，获得纳米晶硅与氧化硅的镶嵌结构．室温下观察到峰位为2.40eV光致发光．系统地研究了不同退火温度对薄膜的Raman谱、光荧光谱及光电子谱的影响．结果表明，荧光谱可分成两个不随温度变化的峰位为1.86和2.30eV的发光带．Si2p能级光电子谱表明与发光强度一样Si⁴⁺强度随退火温度增加而增加．Si平均晶粒大小为4.1—8.0nm，不能用量子限制模型解释蓝绿光的发射．纳米晶硅与SiO₂界面或SiO₂中与氧有关的缺陷可能是蓝绿光发射的主要原因 关键词：     

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.     

Annealing temperature dependence of Raman scattering in Si/SiO2 superlattice prepared by magnetron sputtering

Si/SiO₂ superlattices were prepared by magnetron sputtering, and the deposition temperature and annealing temperature had a great influence on the superlattice structure. In terms of SEM images, the mean size of Si nanocrystals annealed at 1100 °C is larger than that of nanocrystals annealed at 850 °C. It was found that the films deposited at room temperature are amorphous. With increasing deposition temperature, the amorphous and crystalline phases coexist. With increasing annealing temperature, the Raman intensity of the peak near 470 cm⁻¹ decreases, and the intensity of that at 520 cm⁻¹ increases. Also, on increasing the annealing temperature, the Raman peak near 520 cm⁻¹ shifts and narrows, and asymmetry emerges. A spherical cluster is used to model the nanocrystals in Si/SiO₂ superlattices, and the observed Raman spectra are analyzed by combining the effects of confinement on the phonon frequencies. Raman spectra from a variety of nanocrystalline silicon structures were successfully explained in terms of the phonon confinement effect. The fitted results agreed well with the experimental observations from SEM images.     

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.     

Structural and photoluminescence characteristics of ZnO films by room temperature sputtering and rapid thermal annealing process

ZnO thin films deposited on SiO₂/Si substrates at room temperature by sputtering technology were annealed with a rapid thermal annealing process at various temperatures from 200 °C to 900 °C. The physical and optical properties of the ZnO films were investigated by X-ray diffraction, scanning electron microscopy and room-temperature photoluminescence (PL). The surface structures of the thin films showed great variations with increased annealing temperature. The PL spectrum illustrated that a stronger UV emission intensity appeared at an annealing temperature of 500 °C. On the other hand, visible-light emission could be obtained when the ZnO films were annealed above 500 °C and reached a maximum intensity at 900 °C. The possible mechanisms for visible-light emission are discussed. PACS 81.15.Cd; 81.40.Ef; 78.55.-m; 78.55.Et     

Luminescent silicon nanocrystal dots fabricated by SiCl4/H2 RF plasma-enhanced chemical vapor deposition

Luminescent nanocrystalline Si dots were fabricated directly on thermally grown SiO₂ at 120°C by conventional RF plasma-enhanced chemical vapor deposition using tetrachlorosilane, SiCl₄ and H₂. As-deposited Si dot exhibits photoluminescence (PL) in the visible region, consisting of two broad bands corresponding to photon energies of 1.38 and 1.48 eV. Storage in air enhances PL and shifts the PL peak energy to higher wavelengths for dots of diameter less than 10 nm. Fourier transform attenuated total reflection absorption spectroscopy (FTIR-ATR) study reveals that the spontaneous oxidation proceeds until saturation after 70 h at dot sizes of 3–5 nm. The relationship between PL intensity, blueshift of PL peak energy, and surface termination species during oxidation indicates that these changes are attributed to the increased density of radiative centers at the Si nanocrystal dot/SiO₂ interface and enhancement of the quantum confinement effect.     

Relative enhancement of photoluminescence intensity of passivated silicon nanocrystals in silicon dioxide matrix

Photoluminescence (PL) intensity of passivated silicon nanocrystals (Si NCs) embeded in an SiO₂ matrix is compared with that of unpassivated ones. We investigate the relative enhancement of PL intensity (I_R) as a function of annealing temperature and implanted Si ion dose. The I_R increases simultaneously with the annealing temperature. This demonstrates an increase in the number of dangling bonds (DBs) with the degree of Si crystallization via varying the annealing temperature. The increase in I_R with implanted Si ion dose is also observed. We believe that the near-field interaction between DBs and neighboring Si NCs is an additional factor that reduces the PL efficiency of unpassivated Si NCs.     

The effect of Si surface nitridation on the interfacial structure and electrical properties of (La2O3)0.5(SiO2)0.5 high-k gate dielectric films

Thermal stability, interfacial structures and electrical properties of amorphous (La₂O₃)_0.5(SiO₂)_0.5 (LSO) films deposited by using pulsed laser deposition (PLD) on Si (1 0 0) and NH₃ nitrided Si (1 0 0) substrates were comparatively investigated. The LSO films keep the amorphous state up to a high annealing temperature of 900 °C. HRTEM observations and XPS analyses showed that the surface nitridation of silicon wafer using NH₃ can result in the formation of the passivation layer, which effectively suppresses the excessive growth of the interfacial layer between LSO film and silicon wafer after high-temperature annealing process. The Pt/LSO/nitrided Si capacitors annealed at high temperature exhibit smaller CET and EOT, a less flatband voltage shift, a negligible hysteresis loop, a smaller equivalent dielectric charge density, and a much lower gate leakage current density as compared with that of the Pt/LSO/Si capacitors without Si surface nitridation.     

Study on morphology and photoluminescence of Au/SiO2 nanocomposite films

Au/SiO₂ nanocomposite films were fabricated on Si (111) substrates by radio frequency (RF) magnetron sputtering technique and annealing at different temperature for 20 min (mode A) and at 1000 °C for different annealing time (mode B). The nanocomposite films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). SEM results demonstrate that the size of Au crystallites in mode A first increases and then decreases, on increasing annealing temperature, according to the results of XRD spectra. Analysis of PL spectra in mode B shows that the intensity of the emission peak at 440 nm and 523 nm early increases and late decreases, with increasing annealing time at 1000 °C. The origin of the emission peak at around 440 nm was related to the size and quantity of Au particles and one of the emission peak at around 523 nm was related to the nanostructure of films in agreement with SEM imagines. Experimental results indicated that morphology, microstructure and luminescence of Au/SiO₂ nanocomposite films showed close affinity with annealing temperature and annealing time.     

The influence of substrate on the properties of Er2O3 films grown by magnetron sputtering

The structural properties and the room temperature luminescence of Er₂O₃ thin films deposited by RF magnetron sputtering have been studied. Films characterized by good morphological properties have been obtained by using a SiO₂ interlayer between the film and the Si substrate. The evolution of the properties of the Er₂O₃ films due to rapid thermal annealing processes in O₂ ambient performed at temperatures in the range 800-1200 °C has been investigated in details. The existence of well-defined annealing conditions (temperature of 1100 °C or higher) allowing to avoid the occurrence of extensive chemical reactions with the oxidized substrate has been demonstrated and an increase of the photoluminescence (PL) intensity by about a factor of 40 with respect to the as deposited material has been observed. The enhanced efficiency of the photon emission process has been correlated with the longer lifetime of the PL signal. The same annealing processes are less effective when Er₂O₃ is deposited on Si. In this latter case interfacial reactions and pit formation occur, leading to a material characterized by stronger non-radiative phenomena that limit the PL efficiency.     

Structure and photoluminescence properties of Er3+-doped TiO2-SiO2 powders prepared by sol-gel method

Er 3+-doped TiO 2-SiO 2 powders are prepared by the sol-gel method,and they are characterized by high resolution transmission electron microscopy (HR-TEM),X-ray diffraction (XRD) spectra,and Raman spectra of the samples.It is shown that the TiO 2 nanocrystals are surrounded by an SiO 2 glass matrix.The photoluminescence (PL) spectra are recorded at room temperature.A strong green luminescence and less intense red emission are observed in the samples when they are excited at 325 nm.The intensity of the emission,which is related to the defect states,is strongest at the annealing temperature of 800 C.The PL intensity of Er 3+ ions increases with increasing Ti/Si ratio due to energy transfer between nano-TiO 2 particles and Er 3+ ions.     

Formation of epitaxial SiC nanocrystals

Epitaxial 3C-SiC grains are formed at 1190 °C in the top region of silicon, when Si wafers coated by SiO₂ are annealed in CO atmosphere. The formed SiC grains are 40-50 nm high and 100 nm wide in cross-section and contain only few defects. Main advantage of the method is that the final structure is free of voids.The above method is further developed for the generation of SiC nanocrystals, embedded in SiO₂ on Si, and aligned parallel with the interface. The nanometer-sized SiC grains were grown into SiO₂ close to the Si/SiO₂ interface by a two-step annealing of oxide covered Si: first in a CO, than in a pure O₂ atmosphere. The first (carbonization) step created epitaxial SiC crystallites grown into the Si surface, while the second (oxidation) step moved the interface beyond them. Conventional and high resolution cross-sectional electron microscopy showed pyramidal Si protrusions at the Si/SiO₂ interface under the grains. The size of the grains, as well as their distance from the Si/SiO₂ interface (peak of pyramids) can be controlled by the annealing process parameters. The process can be repeated and SiC nanocrystals (oriented in the same way) can be produced in a multilevel structure.     

Blue–violet photoluminescence from colloidal suspension of nanocrystalline silicon in silicon oxide matrix

We report room temperature visible photoluminescence (PL), detectable by the unaided eye, from colloidal suspension of silicon nanocrystals (nc-Si) prepared by mechanical milling followed by chemical oxidation. The PL bands for samples prepared from Si wafer and Si powder peak at 3.11 and 2.93 eV respectively, under UV excitation, and exhibit a very fast (～ns) PL decay. Invasive oxidation during chemical treatment reduces the size of the nc-Si domains distributed within the amorphous SiO₂ matrix. It is proposed that defects at the interface between nc-Si and amorphous SiO₂ act as the potential emission centers. The origin of blue–violet PL is discussed in relation to the oxide related surface states, non-stoichiometric suboxides, surface species and other defect related states.