Strong Green Light Emission from Low-Temperature Grown a-SiNx:H Film after Different Oxidation Routes

Room-temperature deposited amorphous silicon nitride （a-SiNx ：H） films exhibit intense green light emission after post-treated by plasma oxidation, thermal oxidation and natural oxidation, respectively. All the photoluminescence （PL） spectra are peaked at around 500nm, independent of oxidation method and excitation wavelength. Compared with the PL results from oxidized a-Si：H and as-deposited a-SiNx：H samples, it is indicated that not only oxygen but also nitrogen is of an important role in enhancing light emission from the oxidized a-SiNx：H. Combining the PL results with the analyses of the bonding configurations as well as chemical compositions of the films, the strong green light emission is suggested to be from radiative recombination in luminescent centres related to N Si-O bonds.     

Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride

Amorphous silicon quantum dots (a-Si QDs) were grown in a silicon nitride film by plasma enhanced chemical vapor deposition. Transmission electron micrographs clearly demonstrated that a-Si QDs were formed in the silicon nitride. Photoluminescence and optical absorption energy measurement of a-Si QDs with various sizes revealed that tuning of the photoluminescence emission from 2.0 to 2.76 eV is possible by controlling the size of the a-Si QD. Analysis also showed that the photoluminescence peak energy E was related to the size of the a-Si QD, a (nm) by E(eV) = 1.56+2.40/a(2), which is a clear evidence for the quantum confinement effect in a-Si QDs.     

Photoluminescence analyses of hydrogenated amorphous silicon nitride thin films

Silicon-rich hydrogenated amorphous silicon nitride (a-SiN_x:H) films were grown by plasma enhanced chemical vapor deposition (PECVD) with different r=NH₃/SiH₄ gas flow ratios. The optical absorption characteristics were analyzed by Fourier transform infrared (FTIR) and UV-visible transmittance spectroscopies. The recombination properties were investigated via photoluminescence (PL) measurements. As r was increased from 2 to 9, the PL emission color could be adjusted from red to blue with the emission intensity high enough to be perceived by naked eye at room temperature. The behaviors of the PL peak energy and the PL band broadness with respect to the optical constants were discussed in the frame of electron-phonon coupling and band tail recombination models. A semiquantitative analysis supported the band tail recombination model, where the recombination was found to be favored when the carriers thermalize to an energy level at which the band tail density of states (DOS) reduces to some fraction of the relevant band edge DOS. For the PL efficiency comparison of the samples with different nitrogen contents, the PL intensity was corrected for the absorbed intensity fraction of the incident PL excitation source. The resulted correlation between the PL efficiency and the subgap absorption tail width further supported the band tail recombination model.     

Wet-chemical passivation of atomically flat and structured silicon substrates for solar cell application

Special sequences of wet-chemical oxidation and etching steps were optimised with respect to the etching behaviour of differently oriented silicon to prepare very smooth silicon interfaces with excellent electronic properties on mono- and poly-crystalline substrates. Surface photovoltage (SPV) and photoluminescence (PL) measurements, atomic force microscopy (AFM) and scanning electron microscopy (SEM) investigations were utilised to develop wet-chemical smoothing procedures for atomically flat and structured surfaces, respectively. Hydrogen-termination as well as passivation by wet-chemical oxides were used to inhibit surface contamination and native oxidation during the technological processing. Compared to conventional pre-treatments, significantly lower micro-roughness and densities of surface states were achieved on mono-crystalline Si(100), on evenly distributed atomic steps, such as on vicinal Si(111), on silicon wafers with randomly distributed upside pyramids, and on poly-crystalline EFG (Edge-defined Film-fed-Growth) silicon substrates.The recombination loss at a-Si:H/c-Si interfaces prepared on c-Si substrates with randomly distributed upside pyramids was markedly reduced by an optimised wet-chemical smoothing procedure, as determined by PL measurements. For amorphous-crystalline hetero-junction solar cells (ZnO/a-Si:H(n)/c-Si(p)/Al) with textured c-Si substrates the smoothening procedure results in a significant increase of short circuit current I_sc, fill factor and efficiency η. The scatter in the cell parameters for measurements on different cells is much narrower, as compared to conventional pre-treatments, indicating more well-defined and reproducible surface conditions prior to a-Si:H emitter deposition and/or a higher stability of the c-Si surface against variations in the a-Si:H deposition conditions.     

Influence of successive electron and laser irradiation on the photoluminescence of porous silicon

The influence of electron irradiation on the light-emitting properties of p-and n-type porous silicon prepared by electrochemical etching is investigated. The dose and energy dependences of the electron-stimulated quenching of the photoluminescence (PL) are determined. It is shown that electron treatment of a porous silicon surface followed by prolonged storage in air can be used to stabilize the PL. The excitation of photoluminescence by a UV laser acting on sections of porous silicon samples subjected to preliminary electron treatment is discovered for the first time. The influence of the electron energy and the power of the laser beam on this process is investigated. The results presented are attributed to variation in the number of radiative recombination centers as a result of the dissociation and restoration of hydrogen-containing groups on the pore surface. Zh. Tekh. Fiz. 68, 58–63 (March 1998)     

镶嵌在氢化氮化硅中纳米非晶硅粒子光吸收的模拟

采用量子限制效应模型对镶嵌有纳米非晶硅粒子的氢化氮化硅薄膜的光吸收进行了理论模拟,探讨了由吸收谱分析给出该结构薄膜光学参数的方法,并通过对不同氮含量样品的讨论给出了量子限制效应和纳米硅粒子表面的结构无序对薄膜光吸收特性的影响规律。分析结果表明,随氮含量的增加,薄膜有效光学带隙增大,该结果与薄膜中纳米硅粒子平均尺寸的减小引起的量子限制效应的增强相关,而小粒度纳米硅粒子比例增加所引入的较高微观结构无序度和较多缺陷将会导致薄膜低能吸收区吸收系数增加。     

Photoluminescence of porous silicon coated by SILD method with LaF3 nanolayers

The method of lanthanum fluoride passivating layer synthesis in the matrix of porous silicon by successive ionic layer deposition was elaborated and optimized. Luminescence and FTIR of obtained structures demonstrate the crucial role of the chemical composition of silicon nanocrystallite surface in the formation of radiative recombination channels and in the stability of porous silicon photoluminescence. The combination of high optical transparency of LaF₃ layers and low recombination losses in silicon covered with such layers allows to recommend the lanthanum fluoride film as an effective passivating coating for silicon optoelectronics devices.     

Effect of surface Si-Si dimers on photoluminescence of silicon nanocrystals in the silicon dioxide matrix

The effect of surface states of silicon nanocrystals embedded in silicon dioxide on the photoluminescent properties of the nanocrystals is reported. We have investigated the time-resolved and stationary photoluminescence of silicon nanocrystals in the matrix of silicon dioxide in the visible and infrared spectral ranges at 77 and 300 K. The structures containing silicon nanocrystals were prepared by the high-temperature annealing of multilayer SiO _x /SiO₂ films. The understanding of the experimental results on photoluminescence is underlain by a model of autolocalized states arising on surface Si-Si dimers. The emission of autocatalized excitons is found for the first time, and the energy level of the autolocalized states is determined. The effect of these states on the mechanism of the excitation and the photoluminescence properties of nanocrystals is discussed for a wide range of their dimensions. It is reliably shown that the cause of the known blue boundary of photoluminescence of silicon nanocrystals in the silicon dioxide matrix is the capture of free excitons on autolocalized surface states.     

Photoluminescence properties of europium-doped porous silicon nanocomposites

We present new results concerning the photoluminescence properties of europium (Eu³⁺) incorporated in porous silicon (PS) matrix. Eu³⁺ ions were embedded in the matrix by simple impregnation of PS layers in chloride solution of europium. Complete and uniform penetration of Eu³⁺ into the pores is proved from RBS study.The PL spectrum shows the existence of several peaks superposed to the PL band of PS. These peaks are related to level transitions in Eu³⁺. The effect of the ray excitation on the PL shows that energy transfer is not the principal route for radiative recombination.A systematic study of the PL versus annealing temperature was performed. It was found that the optimised PL spectrum is found after annealing at 1000°C. Low-temperature study of the PL shows an important increase of the intensity and a broadness of the peaks due to the appearance of a second crystallographic site.     

Photoluminescence origin of lightly doped silicon nanowires treated with acid vapor etching

In this work, we carry out a comprehensive photoluminescence (PL) study to elucidate the origin of light emission from porous silicon nanowires (pSiNWs). SiNWs were first grooved in lightly doped Si wafer by silver assisted chemical etching, and then treated with an acid vapor emanating from HF/HNO₃ aqueous solution heated at 60 °C. Scanning and transmission electron microscopies were used to investigate the effect of the acid vapor etching on morphological properties of SiNWs. The as prepared pSiNWs exhibited a strong room temperature PL emission centered at 1.93 eV. An increase of the PL intensity was observed with the increase of HNO₃ in the acid solution. By varying the laser excitation density from 60 to 300 W/cm², we shed the light on the radiative recombination modes occurring within the Si nanocrystals (SiNCs) generated along the pSiNWs. We study as well the temperature-dependent PL of the pSiNWs in the range 10 to 300 K. Based on both laser excitation density and temperature-dependent PL, we propose a multilevel transition scheme resuming the PL origin taking into account the size distribution, shape and surface states of the SiNCs trimming the wire sidewalls.     

ECR-CVD制备的非晶SiOxNy薄膜的光致蓝光发射

使用90%N2稀释的SiH4与O2作为前驱气体，利用微波电子回旋共振等离子体化学气相沉积（ECR CVD）方法制备了非晶氮氧化硅薄膜（a-SiOxNy）.红外吸收光谱的结果表明，a SiOxNy薄膜主要由Si O Si和Si N键的两相结构组成，在存在氧流量的情形下，薄膜主要成分是SiOx相，而在无氧流量的情形下，薄膜则主要是SiNx相.使用565eV的紫外光激发，发现SiOxNy薄膜出现了位于460nm的光致蓝光主峰，且其发光强度随着氧流量的降低而显著增强.根据缺陷态发光中心和SiNx蓝光发射能隙态模 关键词： ECR CVD 红外吸收光谱 非晶氮化硅薄膜 光致发光     

Optical properties of p-type ZnO doped by lithium and nitrogen

A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by RF-magnetron sputtering and post annealing techniques with c-Al₂O₃ as substrate. Its transmittance was measured to be above 95%. Three dominant emission bands were observed at 3.311, 3.219 and 3.346 eV, respectively, in the 80 K photoluminescence (PL) spectrum of the p-type ZnO:(Li, N), and are attributed to radiative electron transition from conduction band to a Li_Zn-N complex acceptor level (eFA), radiative recombination of a donor-acceptor pair and recombination of the Li_Zn-N complex acceptor bound exciton, respectively, based on temperature-dependent and excitation intensity-dependent PL measurement results. The Li_Zn-N complex acceptor level was estimated to be about 126 meV above the valence band by fitting the eFA data obtained in the temperature-dependent PL spectra.     

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.     

nc-Ge/SiN_x多层膜的光致发光特性

通过将a-Ge∶H/a-SiN_x多层膜进行氧化,制备了nc-Ge/SiN_x多层膜。观察到了室温下的强烈可见光发射,发光波长为500nm。通过分析,排除了与量子限制效应有关的光发射机制,也排除了与Si和N相关的缺陷产生的复合机制,认为该发光源于氧化后的a-SiN_x介质层中带尾态之间的辐射复合,最有效的激发能量约为介质层的带隙。     

Mechanism of quantum dot luminescence excitation within implanted SiO2:Si:C films

Results of the investigation of photoluminescence (PL) mechanisms for silicon dioxide films implanted with ions of silicon (100 keV; 7 × 10(16) cm(-2)) and carbon (50 keV; 7 × 10(15)-1.5 × 10(17) cm(-2)) are presented. The spectral, kinetic and thermal activation properties of the quantum dots (Si, C and SiC) formed by a subsequent annealing were studied by means of time-resolved luminescence spectroscopy under selective synchrotron radiation excitation. Independent quantum dot PL excitation channels involving energy transfer from the SiO(2) matrix point defects and excitons were discovered. A resonant mechanism of the energy transfer from the matrix point defects (E' and ODC) is shown to provide the fastest PL decay of nanosecond order. The critical distances (6-9 nm) of energy transport between the bulk defects and nanoclusters were determined in terms of the Inokuti-Hirayama model. An exchange interaction mechanism is realized between the surface defects (E(s)'-centres) and the luminescent nanoparticles. The peculiarities of an anomalous PL temperature dependence are explained in terms of a nonradiative energy transfer from the matrix excitons. It is established that resonant transfer to the luminescence centre triplet state is realized in the case of self-trapped excitons. In contrast, the PL excitation via free excitons includes the stages of energy transfer to the singlet state, thermally activated singlet-triplet conversion and radiative recombination.     

Efficient light emission from crystalline and amorphous silicon nanostructures

Optical and electronic properties of crystalline silicon (c-Si) and amorphous silicon (a-Si) nanostructures are reviewed. The photoluminescence (PL) peak energies of c-Si and a-Si nanostructures are blueshifted from those of bulk c-Si and a-Si. The temperature dependence of the PL intensity is drastically improved in c-Si and a-Si nanostructures, and efficient luminescence from c-Si and a-Si nanostructures is observed at room temperature. The quantum confinement, spatial confinement, and surface effects on luminescence properties are summarized and the PL mechanism of silicon nanostructures is discussed.     

Influence of the temperature on the photoluminescence of silicon clusters embedded in a silicon oxide matrix

Amorphous silicon oxide thin films were prepared by co-evaporation of Si and SiO in ultra-high vacuum. Different compositions were obtained by changing the evaporation rate of silicon. After thermal annealing treatments, the dissociation of the silicon oxide in pure silicon and silicon dioxide leads to the formation of silicon clusters embedded in a silicon oxide matrix. Thus the samples were annealed to different temperatures up to 950°C. Depending on the annealing temperature and on the composition, different cluster sizes were obtained. The photoluminescence (PL) energy depends on the cluster size and a large range of wavelengths is obtained from 500 to 750 nm. The PL, attributed to a confinement effect of the electron–hole pairs in the silicon particles, is studied as a function of the temperature. It is demonstrated that the continuous decrease of PL intensity with the temperature from 77 to 500 K depends on the structure of the samples. For samples with well-separated clusters, the PL decreases rapidly with the temperature. For samples containing clusters separated by a small distance, the PL weakly depends on the temperature. No shift of the energy is observed. The results are discussed by taking into account the competition between the radiative recombination in the silicon clusters and the non-radiative escape of the carriers via a hopping mechanism.     

Visible photoluminescence from silicon nanoclusters embedded in silicon nitride films prepared by remote plasma-enhanced chemical vapor deposition

The photoluminescence (PL) of silicon nanoclusters embedded in silicon nitride films grown by remote plasma-enhanced chemical vapor deposition at 200 °C, using mixtures of SiCl₄/H₂/Ar/NH₃ is investigated. It was found that the color and the intensity of the PL of the as-grown samples depend on the H₂ flow rate, and there is an optimum flow for which a maximum luminescence is obtained. A strong improvement of the PL intensity and change in color was obtained with annealing treatments in the range of 500–1000 °C. The changes in the composition, structure and optical properties of the films, as a function of H₂ flow rate and thermal treatments, were studied by means of Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, ellipsometry and ultraviolet–visible transmission measurements. We conclude that the PL can be attributed to quantum confinement effect in silicon nanoclusters embedded in silicon nitride matrix, which is improved when a better passivation of the nanoclusters surface is obtained.