Structural and optical properties of Si/SiO2 multi-quantum wells

Structural and optical properties of Si/SiO₂ multi-quantum wells (MQW) were investigated by means of Raman scattering and photoluminescence (PL) spectroscopy. The MQW structures were fabricated on a quartz substrate by remote plasma enhanced chemical vapour deposition (RPECVD) of alternating amorphous Si and SiO₂ layers. After layer deposition the samples were subjected to heat treatments, i.e. rapid thermal annealing (RTA) and furnace annealing. Distinct PL signatures of confined carriers evidenced formation of Si-nanocrystals (nc-Si) in annealed samples. Analyses of Raman spectra also show presence of nc-Si phase along with amorphous-Si (a-Si) phase in the samples. The strong influence of the annealing parameters on the formation of nc-Si phase suggests broad possibilities in engineering MQW with various optical properties. Interestingly, conversion of the a-Si phase to the nc-Si phase saturates after certain time of furnace annealing. On the other hand, thinner Si layers showed a disproportionately lower crystalline volume fraction. From the obtained results we could assume that an interface strain prevents full crystallization of the Si layers and that the strain is larger for thinner Si layers. The anomalous dependence of nc-Si Raman scattering peak position on deposited layer thickness observed in our experiments also supports the above assumption.     

Infrared photoluminescence from GeSi nanocrystals embedded in a germanium–silicate matrix

We investigate the structural and optical properties of GeO/SiO₂ multilayers obtained by evaporation of GeO₂ and SiO₂ powders under ultrahigh vacuum conditions on Si(001) substrates. Both Raman and infrared absorption spectroscopy measurements indicate the formation of GeSi nanocrystals after postgrowth annealing at 800°C. High-resolution transmission electron microscopy characterizations show that the average size of the nanocrystals is about 5 nm. For samples containing GeSi nanocrystals, photoluminescence is observed at 14 K in the spectral range 1500–1600 nm. The temperature dependence of the photoluminescence is studied.     

掺铒Si/Al2O3多层结构中结晶形态对1.54 μm发光的影响

利用脉冲激光沉积的方法制备掺铒 Si/Al₂O₃多层结构薄膜,获得了由纳米结构的Si作为感光剂增强的Er³⁺在1.54 μm高效发光.利用拉曼散射、高分辨透射电镜和光致发光测量研究了在不同退火温度下（600—1000 ℃）纳米结构Si层的结晶形态变化,及对Er³⁺在1.54 μm的发光的影响特征.研究发现最佳发光是在退火温度600—700 ℃.在这个条件下纳米Si的尺寸和密度,Si和Er的作用距离以及Er^{3+ 关键词： 铒 纳米硅 能量转移 氧化铝}     

Ar irradiation of Si nanocrystal-doped SiO2: Evolution of photoluminescence

We report the evolution of photoluminescence (PL) of Si nanocrystals (nc-Si) embedded in a matrix of SiO₂ during Ar⁺ ion bombardment. The integrated intensity of nc-Si PL falls down drastically before the Ar⁺ ion fluence of 10¹⁵ ions cm⁻², and then decreases slowly with the increasing ion fluence. At the meantime, the PL peak position blueshifts steadily before the fluence of 10¹⁵ ions cm⁻², and then changes in an oscillatory manner. Also it is found that the nc-Si PL of the Ar⁺-irradiated sample can be partly recovered after annealing at 800 °C in nitrogen, but can be almost totally recovered after annealing in oxygen. The results confirm that the ion irradiation-induced defects are made up of oxygen vacancies, which absorb light strongly. The oscillatory peak shift of nc-Si can be related to a size-distance distribution of nc-Si in SiO₂.     

Erbium ion luminescence of silicon nanocrystal layers in a silicon dioxide matrix measured under strong optical excitation

The photoluminescence (PL) spectra and kinetics of erbium-doped layers of silicon nanocrystals dispersed in a silicon dioxide matrix (nc-Si/SiO₂) are studied. It was found that optical excitation of nc-Si can be transferred with a high efficiency to Er³⁺ ions present in the surrounding oxide. The efficiency of energy transfer increases with increasing pumping photon energy and intensity. The process of Er³⁺ excitation is shown to compete successfully with nonradiative recombination in the nc-Si/SiO₂ structures. The Er³⁺ PL lifetime was found to decrease under intense optical pumping, which implies the establishment of inverse population in the Er³⁺ system. The results obtained demonstrate the very high potential of erbium-doped nc-Si/SiO₂ structures when used as active media for optical amplifiers and light-emitting devices operating at a wavelength of 1.5 μm.     

Quantitative analysis of the phonon confinement effect in arbitrarily shaped Si nanocrystals decorated on Si nanowires and its correlation with the photoluminescence spectrum

Arrays of single‐crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) are grown by a metal‐assisted chemical etching process using silver (Ag) as the noble metal catalyst. The metal‐assisted chemical etching‐grown Si NWs exhibit strong photoluminescence (PL) emission in the visible and near infrared region at room temperature. Quantum confinement of carriers in the Si NCs is believed to be primarily responsible for the observed PL emission. Raman spectra of the Si NCs decorated on Si NWs exhibit a red shift and an asymmetric broadening of first‐order Raman peak as well as the other multi‐phonon modes when compared with that of the bulk Si. Quantitative analysis of confinement of phonons in the Si NCs is shown to account for the measured Raman peak shift and asymmetric broadening. To eliminate the laser heating effect on the phonon modes of the Si NWs/NCs, the Raman measurement was performed at extremely low laser power. Both the PL and Raman spectral analysis show a log‐normal distribution for the Si NCs, and our transmission electron microscopy results are fully consistent with the results of PL and Raman analyses. We calculate the size distribution of these Si NCs in terms of mean diameter (D₀) and skewness (σ) by correlating the PL spectra and Raman spectra of the as‐grown Si NCs decorated on Si NWs. Copyright © 2015 John Wiley & Sons, Ltd.     

Fabrication and Characterization of Si Nanocrystals Synthesized by Electron Beam Evaporation of Si and SiO2 Mixture

Silicon nanocrystals synthesized by electron beam （e-beam） evaporation of Si and SiO2 mixture are studied. Rutherford backscattering spectrometry of the as-deposited Si-rich silicon dioxide or oxide （SRO） thin film shows that after evaporation, the Si and SiO2 concentration is well kept, indicating that the e-beam evaporation is suitable for evaporating mixtures of Si and SiO2. The SRO thin films are annealed at different temperatures for two hours to synthesize silicon nanoerystals. For the sample annealed at 1050℃, silicon nanoerystals with different sizes and the mean diameter of 4.5 nm are evidently observed by high resolution transmission electron microscopy （HRTEM）. Then the Raman scattering and photoluminescence spectra arising from silicon nanocrystals are further confirmed the above results.     

Immobilization of luminescent nanosilicon in a microfine polytetrafluoroethylene matrix by means of supercritical carbon dioxide

With the use of supercritical carbon dioxide (SC-CO₂), the matrix immobilization of photoluminescent silicon nanocrystals (nc-Si) in polytetrafluoroethylene microparticles (mp-PTFE) is performed, which leads to the formation of mp-PTFE/nc-Si photoluminescent nanocomposite containing ∼10³–10⁴ nc-Si particles per mp-PTFE particle (1–2 μm in size). This approach is based on the effect of polymer swelling in SC-CO₂, efficient SC-CO₂-assisted transport of nanoparticles into the internal free volume of the polymer, and contraction of the nanocomposite after the release of CO₂, an effect that prevents the subsequent agglutination of nanoparticles. Particles of nc-Si photoluminescent in the visible spectrum were synthesized from silicon suboxide powder (SiO _x , x ≈ 1) heated at various temperatures within 25–950°C and then etched in concentrated hydrofluoric acid. The hydrosilylation procedure was used to graft 1-octadecene molecules to the surface of nc-Si particles. As a result, the photoluminescence intensity of nc-Si increased substantially. According to TEM images and small angle X-ray scattering data, the maximum size of nc-Si particles did not exceed 5 nm and 7 nm, respectively, and the core of these nanoparticles consisted of crystalline silicon. The structure and spectral properties of the initial nc-Si particles and synthesized mp-PTFE/nc-Si photoluminescent nanocomposite microparticles were studied.     

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.     

Investigation on preparation and physical properties of nanocrystalline Si/SiO2 superlattices for Si-based light-emitting devices

Structures containing silicon nanocrystals (nc-Si) are very promising for Si-based light-emitting devices. Using a technology compatible with that of silicon, a broader wavelength range of the emitted photoluminescence (PL) was obtained with nc-Si/SiO₂ multilayer structures. The main characteristic of these structures is that both layers are light emitters. In this study we report results on a series of nc-Si/SiO₂ multilayer periods deposited on 200 nm thermal oxide SiO₂/Si substrate. Each period contains around 10 nm silicon thin films obtained by low-pressure chemical vapour deposition at T=625°C and 100 nmSiO₂ obtained by atmospheric pressure chemical vapour deposition T=400°C. Optical and microstructural properties of the multilayer structures have been studied by spectroscopic ellipsometry (using the Bruggemann effective medium approximation model for multilayer and multicomponent films), FTIR and UV–visible reflectance spectroscopy. IR spectroscopy revealed the presence of SiO_x structural entities in each nc-Si/SiO₂ interface. Investigation of the PL spectra (using continuous wave-CW 325 nm and pulsed 266 nm laser excitation) has shown several peaks at 1.7, 2, 2.3, 2.7, 3.2 and 3.7 eV, associated with the PL centres in SiO₂, nc-Si and Si–SiO₂ interface. Their contribution to the PL spectra depends on the number of layers in the stack.     

Effect of coalescence and of the character of the initial oxide on the photoluminescence of ion-synthesized Si nanocrystals in SiO<Subscript>2</Subscript>

The photoluminescence intensity (PLI) related to Si nanocrystals in a SiO₂: nc-Si system synthesized by ion implantation is studied experimentally and theoretically as a function of the Si+ ion dose at various annealing temperatures T_ann (1000–1200°C). The dose corresponding to the maximum PLI is found to decrease with increasing T_ann. These data are explained in terms of a model taking into account the coalescence of neighboring nanocrystals and the dependence of the probability of radiative recombination of quantum dots on their size. It is found that, when silicon oxide is grown in a wet atmosphere, the photoluminescence spectrum contains an additional band (near 850 nm), which is related to shells around the nanocrystals. This band weakens abrupily after high-temperature annealing in an oxidizing atmosphere (air).     

Photoluminescence of GeO<Subscript>2</Subscript> films containing germanium nanocrystals

Germanium nanocrystals were formed in a GeO₂ film during the process of germanium monoxide gas-phase deposition onto a sapphire substrate and studied by photoluminescence (PL) and Raman scattering spectroscopy. A PL peak in this heterosystem was observed in the visible region at room temperature. The sizes of Ge nanocrystals were estimated from the position of a Raman peak corresponding to scattering by localized optical phonons in germanium. The PL peak position calculated with allowance for the electron and hole size quantization in Ge nanocrystals coincides well with the experimentally observed position of this peak.     

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.     

用硅离子注入方法制备的纳米硅的拉曼散射研究

在直角散射配置下测量了纳米硅样品的拉曼散射谱及其退火温度的关系。结果表明,在８００℃以下退火的样品只观察到单晶硅衬底的光学声子模,在９００℃以上退火,才观察到纳米硅的特征拉曼散射峰。在１２００℃下退火后,纳米硅的特征拉曼散射峰消失,观察到类似于非晶硅的光学声子特征峰,可能表示纳米硅不能承受这样的高温热退火。这些结果进一步证实了光致发光谱的结果。     

Vibrational and Light-Emitting Properties of Si/Si1−xSnx Heterostructures

Multilayer heterostructures Si/Si_1−xSn_x grown by molecular beam epitaxy on Si (001) substrates have been studied by Raman and photoluminescence spectroscopy. Raman spectra exhibit peaks corresponding to the vibrations of Si-Sn and Sn-Sn bonds; the vibrations of Sn-Sn bonds imply the presence of tin nanocrystals in heterostructures. Two photoluminescence bands at 0.75 eV (1650 nm) and 0.65 eV (1900 nm) have been observed in heterostructures at low temperatures. The former band can be attributed to optical transitions in quantum wells in the type II heterostructure Si/Si_1−xSn_x. The latter band can be associated with excitons localized in tin nanocrystals.

     

Raman analysis of amorphous silicon ruthenium thin films embedded with nanocrystals

We report the Raman analysis of both as‐deposited and annealed amorphous silicon ruthenium thin films embedded with nanocrystals. In the Raman spectra of as‐deposited films, variations of TO peak indicate a short‐range disorder of a‐Si network with an increase of Ru concentration. The substitutional Ru atoms lower the concentration of Si―Si bonds and suppress the intensity of TO peak, but have less effect on TA, LA and LO peaks. In the Raman spectra of annealed films, characteristic parameters confirm the upgrade of a‐Si network at a low annealing temperature and the emergence of both ruthenium silicide and silicon nanocrystals at 700 °C. Although ruthenium silicide nanocrystals present no Raman peaks in the Raman spectra of as‐deposited samples, the non‐linear variations of intensity ratios I_{LA + LO}/I_TO and I_TA/I_TO still suggest their existence, and these nanocrystals are subsequently verified by high‐resolution transmission electron microscopy. Copyright © 2015 John Wiley & Sons, Ltd.     

Substrate-induced stress in silicon nanocrystal/SiO2 multilayer structure

A Raman frequency upshift of nc-Si phonon mode is observed at room temperature, which is attributed to a strong compressive stress in Si nanocrystals. The 10-period amorphous-Si(3 nm)/amorphous-SiO₂ (3 nm) layers are deposited by high vacuum radio-frequency magnetron sputtering on quartz and sapphire substrates at different temperatures. The samples are then annealed in N₂ atmosphere at 1100 ℃ for 1 h for Si crystallization. It is demonstrated that the presence of a supporting substrate at the high grown temperature can induce different types of stresses in the Si nanocrystal layers. The strain is attributed to the difference in thermal expansion coefficient between the substrate and the Si/SiO₂ SL film. Such a substrate-induced stress indicates a new method to tune the optical and the electronic properties of Si nanocrystals for strained engineering.     

Photoluminescence and Raman spectra of SnO<Subscript>x</Subscript> nanostructures doped with Sm ions

The photoluminescence (PL) and Raman scattering of SnO_x nanoparticles deposited from vapor phase have been studied. The PL spectra are characterized by a two-band structure. The high-energy band in the range from 300 to 350 nm is due to the exciton pair annihilation and may characterize the band gap of SnO_x nanocrystals as a function of their diameter. In the red spectral region (from 600 to 700 nm), a luminescence band due to defects in nanocrystals manifests itself. The existence of defects in SnO_x nanostructures is confirmed by Raman spectroscopy. Doping of SnO_x nanoparticles with rare earth (samarium) atoms leads to the appearance of strong luminescence lines in the red region of the PL spectrum.     

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 and multiphonon resonant Raman scattering in Ni-and Co-doped Zn1−x MnxTe crystals

Low-temperature photoluminescence, exciton reflection, and multiphonon resonant Raman scattering spectra of Ni-and Co-doped Zn_1−x Mn_xTe crystals were investigated. Intense emission occurs in a broad spectral region (1100–17 000 cm⁻¹) in the crystals containing Ni atoms. It is caused by intracenter transitions involving Mn²⁺ ions and transitions between the conduction band and a level of the doubly charged acceptor. The features of the exciton photoluminescence and multiphonon resonant Raman scattering involving longitudinal-optical (LO) phonons at various temperatures are investigated. The insignificant efficiency of the localization of excitons on potential fluctuations in the Zn_1−x Mn_xTe:Co crystals is established. A temperature-induced increase in the intensity of the 5LO multiphonon resonant Raman scattering line due to the approach of the conditions for resonance between this line and the ground exciton state is observed in these crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 616–621 (April 1998)