High-efficiency erbium ion luminescence in silicon nanocrystal systems

The photoluminescence spectra and kinetics of both erbium-doped and undoped multilayer structures of quasi-ordered silicon nanocrystals in a silicon dioxide matrix were studied. It was shown that the optical excitation energy of silicon nanocrystals 2–3 nm in size can be practically completely transferred to Er³⁺ ions in the oxide surrounding the nanocrystals, with its subsequent radiation at 1.5 μm. Possible reasons for the high excitation efficiency of the Er³⁺ ions are discussed, and the conclusion is drawn that the F?rster mechanism is dominant in the energy transfer processes occurring in these structures. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 105–109. Original Russian Text Copyright ? 2004 by Kashkarov, Kamenev, Lisachenko, Shalygina, Timoshenko, Schmidt, Heitmann, Zacharias.     

Excitation mechanisms and localization sites of erbium-doped porous silicon

Porous silicon (PS) is doped with erbium by electrochemical anodisation. The penetration of erbium into the PS layer is confirmed by Rutherford backscattering spectroscopy (RBS) and energy dispersive X-ray (EDX) measurements. Efficient green and infrared emissions were observed at room temperature. The investigations are focused on the evolutions versus temperature and pump intensity of the green photoluminescence (PL) corresponding to the ⁴S_3/2 → ⁴I_15/2 transition. It was found that an erbium related level defect can be involved on the excitation and emission processes of erbium. Pump intensity dependent PL studies revealed that for the electrochemical incorporation, most of the Er³⁺ ions are localized inside the Si nanocrystallites and not in stoichiometric SiO₂. The optical cross-section is close to that of erbium in Si nanocrystallites.     

Erbium excitation in a SiO<Subscript>2</Subscript>: Si-nc matrix under pulsed pumping

The photoluminescence of Er³⁺ ions in a SiO₂ matrix containing silicon nanocrystals 3.5 nm in diameter is studied under resonant and nonresonant pulsed pumping with pulses 5 ns in duration. The effective erbium excitation cross section under pulsed pumping, σ_eff = 8.7 × 10^?17 cm₂, is close to that for nanocrystals. Comparison of the erbium photoluminescence intensity obtained for a SiO₂ matrix with and without nanocrystals made it possible to determine the absolute concentration of optically active nanocrystals capable of exciting erbium ions, the concentration of optically active erbium, and the average number of erbium ions excited by one nanocrystal. The study revealed that excitation transfer from one erbium ion to another is a relatively slow process, which accounts for the low efficiency of erbium ion excitation under pulsed pumping in a SiO₂ matrix containing silicon nanocrystals.     

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.     

Photoluminescence of Er<Superscript>3+</Superscript> ions in layers of quasi-ordered silicon nanocrystals in a silicon dioxide matrix

The spectra and kinetics of photoluminescence from multilayered structures of quasi-ordered silicon nanocrystals in a silica matrix were studied for undoped samples and samples doped with erbium. It was shown that the optical excitation energy of silicon nanocrystals could be effectively transferred to Er³⁺ ions, which was followed by luminescence at a wavelength of 1.5 µm. The effectiveness of energy transfer increased as the size of silicon nanocrystals decreased and the energy of exciting light quanta increased. The excitation of erbium luminescence in the structures was explained based on dipole-dipole interaction (the Förster mechanism) between excitons in silicon nanocrystals and Er³⁺ ions in silica surrounding them.     

Excitation of Er atoms by energy transfer from Si nanocrystallites embedded in SiO2 matrices fabricated by laser ablation

We have investigated excitation of Er³⁺ ions via energy transfer from Si nanocrystallites embedded in SiO₂ films. The Er-doped films were fabricated using a laser ablation technique. We found that a photoluminescence (PL) excitation spectra of Er³⁺ ions coincides with that of Si nanocrystallites. Thus, it is evident that Er³⁺ ions are excited via the luminescent singlet state in Si nanocrystallites. Furthermore, we obtained the results that support the energy transfer mechanism. PL intensity of Er³⁺ ions increases with Er concentration while that of Si nanocrystallites decrease inversely. PL intensity of Er³⁺ ions increases with temperature from cryogenic to room temperature under photo-excitation at power density higher than 110 mW/cm². The increase is characteristic of the luminescent state in Si nanocrystallites but not any state in Er³⁺ ions. PACS 61.72.Ww; 61.46.+w; 81.15Fg     

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⁺).     

Role of Bi ions for Er ions efficient 1.54 μm light emission in Er/Bi codoped SiO2 thin film prepared by sol-gel method

Er/Bi codoped SiO₂ thin films were prepared by sol-gel method and spin-on technology with subsequent annealing process. The bismuth silicate crystal phase appeared at low annealing temperature while vanished as annealing temperature exceeded 1000 °C, characterized by X-ray diffraction, and Rutherford backscattering measurements well explained the structure change of the films, which was due to the decrease of bismuth concentration. Fine structures of the Er³⁺-related 1.54 μm light emission (line width less than 7 nm) at room temperature was observed by photoluminescence (PL) measurement. The PL intensity at 1.54 μm reached maximum at 800 °C and decreased dramatically at 1000 °C. The PL dependent annealing temperature was studied and suggested a clear link with bismuth silicate phase. Excitation spectrum measurements further reveal the role of Bi³⁺ ions for Er³⁺ ions near infrared light emission. Through sol-gel method and thermal treatment, Bi³⁺ ions can provide a perfect environment for Er³⁺ ion light emission by forming Er-Bi-Si-O complex. Furthermore, energy transfer from Bi³⁺ ions to Er³⁺ ions is evidenced and found to be a more efficient way for Er³⁺ ions near infrared emission. This makes the Bi³⁺ ions doped material a promising application for future erbium-doped waveguide amplifier and infrared LED.     

Optical losses and gain in silicon-rich silica waveguides containing Er ions

Rib-loaded waveguides containing Er³⁺ coupled to Si-nc have been produced by magnetron sputtering and successive thermal annealing to investigate optical gain at 1535 nm. It has been shown that all Er ions are optically active, whereas the fraction that can be excited at high pump rates under non-resonant excitation is strongly limited by confined carrier absorption (CA), up-conversion processes, and mainly by the lack of coupling to the Si-nc. Er³⁺ absorption cross-section is found comparable to that of Er³⁺ in SiO₂, but a dependence with the effective refractive index has been found. Although the presence of Si-nc strongly improves the efficiency of Er³⁺ excitation, it introduces additional optical loss mechanisms, such as CA. These Si-nc losses affect the possibility of obtaining net optical gain. In the present study, they have been minimized by lowering the annealing time of the Er-doped Si-rich oxide. In pump-probe measurements it is shown that signal enhancement of the transmitted signal can be achieved at low pumping rate when the detrimental role of confined CA is attenuated by reducing the annealing time. A maximum signal enhancement of about 1.30 at 1535 nm was observed.     

Sensitisation of erbium emission by silicon nanocrystals-doped SnO2

SnO₂ thin films undoped and doped with antimony (Sb), erbium (Er) and Si nanocrystals (Si-nc) have been grown on silicon (Si) substrate using sol-gel method. Room-temperature photoluminescence (PL) measurement of undoped SnO₂, under excitation at 280 nm, shows only one broad emission at 395 nm, which is related to oxygen vacancies. The PL of Er³⁺ ions was found to be enhanced after doping SnO₂ with Sb and Si-nc. The excitation process of Er is studied and discussed. The calculation of cross-section suggests a sensitisation of Er PL by Si-nc.     

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.     

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.     

Visible luminescence properties of Er3+–Dy3+ codoped tellurite glasses

Er³⁺ and Dy³⁺ codoped tellurite glasses have been synthesized. Five emission bands in the PL spectrum under 325 nm pumping were observed. Three of them correspond to Er³⁺ and the other two correspond to Dy³⁺, respectively. The PL spectrum revealed that the intensity of Dy³⁺ characteristic emission was enhanced as Er³⁺ concentration increased while keeping Dy³⁺ concentration constant. Due to small mismatch between the energy level of Er³⁺:⁴F_7/2 and Dy³⁺:⁴F_9/2 resonant energy was possibly transferred between them. This process can give rise to an enhancement of the PL intensity of 484 and 574 nm from Dy³⁺. The PL spectra of these glasses cover the blue, green and red wavelength range and the intensities of those emission bands could be controlled by adjusting the concentration of relevant rare-earth ions. These glasses with the controllable CIE coordinates might be a potential candidate for the widely realistic application such as solid-state white lighting and multicolor display.     

Energy transfer from the host to Er<Superscript>3+</Superscript> dopants in semiconductor SnO<Subscript>2</Subscript> nanocrystals segregated in sol–gel silica glasses

Undoped and Er³⁺-doped glass–ceramics of composition (100−x)SiO₂–xSnO₂, with x = 5 or 10 and with 0.4 or 0.8 mol% of Er³⁺ ions, were synthesised by thermal treatment of precursor sol–gel glasses. Structural studies were developed by X-Ray Diffraction. Wide band gap SnO₂ semiconductor quantum-dots embedded in the insulator SiO₂ glass are obtained. The mean radius of the SnO₂ nanocrystals, ranging from 2 to 3.2 nm, is comparable to the exciton Bohr radius. The luminescence properties have been analysed as a function of sample composition and thermal treatment. The results show that Er³⁺ ions are partially partitioned into the nanocrystalline phase. An efficient UV excitation of the Er³⁺ ions by energy transfer from the SnO₂ nanocrystal host is observed. The Er³⁺ ions located in the SnO₂ nanocrystals are selectively excited by this energy transfer mechanism. On the other hand, emission from the Er³⁺ ions remaining in the silica glassy phase is obtained by direct excitation of these ions.     

溶胶-凝胶法Er2O3薄膜的结构和光学特性

针对稀土Er掺杂Si光源中Er离子掺杂浓度低的问题,采用溶胶-凝胶（Sol-gel）法在Si(100)和SiO2/Si(100)基片上旋涂法制备Er₂O₃光学薄膜,Er离子浓度与以前掺杂方法相比提高了2个数量级.900 ℃热处理获得单一立方结构的Er₂O₃薄膜材料.光致发光（PL）特性研究表明在654 nm波长的激光泵浦下,Er₂O₃薄膜材料获得了1.535 μm的发光峰,并具有较小的温度猝灭1/5.在SiO₂/Si(100)基体上制备的Er2O3薄膜材料的光致发光强度比Si(100)基体上制备的薄膜提高2-3倍.研究结果表明具有强光致发光特性的Er₂O₃薄膜是一种有前景的硅基光源和放大器材料.     

Er3+离子掺杂钡镓锗玻璃上转换发光机理研究

研究了Er³⁺离子掺杂钡镓锗玻璃的吸收光谱、拉曼光谱和上转换光谱.分析了Er³⁺离子在钡镓锗玻璃中的上转换发光机理.结果表明：玻璃的最大声子能量为828cm^-1，紫外截止波长为275nm.采用800nm和980nmLD激发玻璃样品，在室温下观察到强烈的上转换绿光和红光发射.随着Er³⁺离子浓度的增加，绿光发光强度先增加后减小，而红光发光强度呈单调递增趋势.能量分析表明：800nmLD激发产生的绿光主要源于Er³⁺离子4I_13/2能级的激发态吸收过程；红光发射主要源于Er³⁺离子4I_13/2能级与4I_11/2能级之间的能量转移过程.980nmLD激发产生的绿光主要源于Er³⁺离子4I_11/2能级之间的能量转移过程；而红光发射主要源于Er³⁺离子4I_13/2能级与4I_11/2能级之间的能量转移过程和4I_13/2能级的激发态吸收过程.通过量子效率分析，发现采用800nmLD激发Er³⁺离子掺杂浓度为1mol% 的样品时，上转换绿光发光效率最高. 关键词： 上转换发光机理 3+离子掺杂')" href="#">Er³⁺离子掺杂 钡镓锗玻璃     

Optically active Er3+ centers in Si/Si1 ? x Ge x structures under optical pumping

The Si/Si_{1 − x} Ge _x :Er/Si heterostructures, which are of interest for laser applications, have been investigated. The types of optically active Er³⁺ centers making a dominant contribution to the photoluminescence response of the structures studied are analyzed and their relationship with the parameters of the Si_{1 − x} Ge _x :Er heterolayer and the post-growth annealing conditions is shown. On the basis of the PL kinetic analysis of the structures with an isolated type of optically active Er³⁺ centers, it is concluded that population inversion of Er³⁺ ion states can be obtained under optical pumping and the effect of nonradiative recombination channels in Si_{1 − x} Ge _x :Er heterolayers on the excitation efficiency of Er centers and the conditions for population inversion is shown. Original Russian Text ? L.V. Krasilnikova, N.A. Baidakova, M.V. Stepikhova, Z.F. Krasilnik, V.Yu. Chalkov, V.G. Shengurov, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 103–108.     

The influence of SiO2 content on spectroscopic properties and laser emission efficiency of Yb3+-Er3+ co-doped calcium aluminosilicate glasses

We have studied the influence of SiO₂ content on the spectroscopic properties and laser emission efficiency of Yb³⁺-Er³⁺ co-doped calcium aluminosilicate glasses. An increase in SiO₂ content resulted in higher phonon energy, which reduced the up-conversion emission, enhanced the energy transfer efficiency up to 70 % from Yb³⁺ to Er³⁺, and enhanced the optical quality. All these results led to an increase from 20 to 30 % in the laser emission efficiency.     

Infrared electroluminescence from erbium-doped spark-processed silicon

The infrared (IR) electroluminescence (EL) of erbium-doped spark-processed silicon (sp-Si) was investigated. For this, a device was constructed which consisted of a silicon wafer on which an erbium layer was vapor deposited, followed by spark-processing and rapid thermal annealing for 15 min at 900 °C in air. The metallization consisted of a 200 nm Ag layer (above the spark-processed area) and a 50 nm thick Al film (on the “back side”), containing a window through which the light could escape. Maximal light emission occurred near 1.55 μm, that is, at a wavelength where commonly used fiber optical materials have their minimum in energy loss. The processing parameters for most efficient light emission were an Er thickness of 200-300 nm, a spark-processing time of about 30 s, an n-type Si wafer having a low (3-5 Ω cm) resistivity, an operating temperature near room temperature, and an operating voltage between 25 and 40 V under reverse bias. The results are interpreted by postulating an energy transfer from sp-Si to the Er³⁺ ions involving the first excited state ⁴I_13/2 to ground state ⁴I_15/2. Further, impact excitation and hot electrons that are accelerated into the erbium doped sp-Si by the applied field (100 kV/cm) are considered.     

Thermal and optical properties of Er-doped oxyfluorotellurite glasses

Er³⁺-doped oxyfluorotellurite glasses with four different concentrations of Er³⁺ ions have been prepared and investigated their thermal, optical absorption, excitation and luminescence properties. From the DSC spectra, glass transition and onset of crystallization temperatures have been found. Judd-Ofelt intensity parameters have been derived from the absorption spectrum and are in turn used to calculate radiative properties for the important luminescent levels of Er³⁺ ions. The calculated radiative properties are comparable to experimental values. The glasses show intense green and weak red emission under normal excitation with 451 nm. The decrease in emission intensities and lifetimes of the ⁴S_3/2 level with concentration of Er³⁺ ions has been explained as due to energy transfer processes between Er³⁺ ions. The stimulated emission cross-sections and quantum efficiencies of the green and infrared emissions have been determined. The results indicate that the glasses may be suitable for use as a laser medium in making solid-state green laser by normal pumping route and as laser medium and optical amplifier in the 1.5 μm region.