Photoluminescence and electroluminescence from porous silicon

The properties and origins of the red, blue and infrared photoluminescence bands of porous silicon are reviewed and discussed in the light of the models that have been proposed to explain the experimental and theoretical results. The red band is due to quantum confinement possibly supplemented by surface states; the blue band is linked to the presence of silicon dioxide; the infrared band is correlated with dangling bonds and bandgap luminescence in large crystallites. The fabrication and characterization of light-emitting devices made of porous silicon are reported and discussed with respect to critical issues such as the device stability, efficiency, modulation speed, emission wavelength, and compatibility with microelectronic processing.     

The integration of nanoscale porous silicon light emitters: materials science, properties, and integration with electronic circuitry

The stability, efficiency, speed, and spectral range of light-emitting devices (LEDs) made of nanoscale porous silicon is improving. The first part of this paper discusses the preparation and properties of nanoscale silicon, with emphasis on porous silicon. In the second part, the properties of LEDs made of porous silicon are reviewed. In the third part, the integration of PSi LEDs with silicon microelectronic circuits is discussed. The paper ends with a short discussion of the prospects for realistic optoelectronic devices.     

Voltage-tunable photo- and electroluminescence of porous silicon

Experimental results showing two electrically induced phenomena, namely the voltage-tunable electroluminescence (VTEL) and the voltage-induced quenching of porous silicon photoluminescence (QPL) are given. In both cases, a spectral shift as large as 300 nm can be recorded for an external bias variation of only 0.5 V. This spectral shift is characterised by a blue-shift of the whole EL line in the case of the VTEL whereas it results from a progressive and selective quenching starting by the low-energy part of the luminescence line in the case of the QPL experiments. The origin of this spectral shift is discussed in relation with an electrically induced selective carrier injection into the silicon nanocrystallites accompanied with an enhancement of the non-radiative recombination taking place by an Auger relaxation process. Finally, it is shown that a partial oxidation of the porous silicon layer leads to a complete loss of the selectivity of these two phenomena. This result is qualitatively discussed by considering the voltage drop distribution between the substrate and the silicon nanocrystallites. The voltage drops are modified by the growth of the oxide layer on the nanocrystallite surface leading to a modification of the energy barriers at the crystallite boundaries.     

Self-affinity study of nanostructured porous silicon-crystalline silicon interfaces

Self-affinity was used to analyze the roughness at the porous silicon (PS)-crystalline Si (cSi) interfaces fabricated under different conditions. Using the variable bandwidth method, the self-affinity behavior was, qualitatively and quantitatively, analyzed from the cross-section micrographs of the PS samples obtained by field emission scanning electron microscope. The results show that correlation length is related with the average pore width. Roughness exponent is found to be correlated with the interface roughness. In addition, similar experimental roughness exponents were obtained for several interfaces grown by different methods, indicating the intrinsic fractal nature of the PS-cSi interfaces. The results were confirmed through the self-affinity analysis done on the atomic force microscopy profiles.     

Characterization of ITO/porous silicon LED structures

The electrical behavior and the electroluminescence (EL) obtained from n- and p-type ITO/porous silicon LEDs have been characterized simultaneously at different temperatures. Stability and aging in air were investigated, and means for avoiding their detrimental effects in the experiments are suggested. The dominating current carrying mechanism responsible for visible light emission in both substrate types has been identified to be Fowler–Nordheim tunneling. This emphasizes the contribution of embedded nanoparticles (quantum dots) rather than the role of nanowires in efficient EL.     

Surface wave photonic device based on porous silicon multilayers

Porous silicon is widely studied in the field of photonics due to its interesting optical properties. In this work, we present theoretical and first experimental studies of a new kind of porous silicon photonic device based on optical surface wave. A theoretical analysis of the device is presented using plane-wave approximation. The porous silicon multilayered structures are realized using electrochemical etching of p⁺-type silicon. Morphological and optical characterizations of the realized structures are reported.     

基于纳米硅结构的氮化硅基发光器件电致发光特性研究

利用等离子体增强化学气相沉积法制备了镶嵌于氮化硅的高密度纳米硅薄膜,并以此作为发光有源层构建基于p-Si/氮化硅基发光层/AZO结构发光二极管,在室温下观察到了电致可见发光.在此基础上,在器件p-Si空穴注入层与氮化硅基发光层之间加入纳米硅薄层作为空穴阻挡层,研究器件电致发光性质,实验结果表明器件的发光强度显著增强,并且发光效率较无纳米硅阻挡层的发光器件提高了80%以上.     

Room-temperature electroluminescence from erbium-doped amorphous hydrogenated silicon

We have studied electroluminescence (EL) in the amorphous silicon-based erbium-doped structures under reverse bias in the temperature range 77–300 K. The intensity of electroluminescence at the wavelength of 1.54 μm exhibits a maximum near the room temperature. The excitation of erbium ions occurs by an Auger process which involves the capture of conduction electrons by neutral dangling bonds (D⁰-centers) located close to erbium ions. The stationary current through the structure is kept by a reverse process of thermally activated tunnel emission of electrons from negatively charged dangling-bond defects (D⁻-centers) to the conduction band of the amorphous matrix. A theoretical model proposed explains consistently all of our experimental data.     

Efficient luminescence from porous silicon

Photoluminescence measurements are carried out on porous silicon layers. We show the enhancement and stabilization of the luminescence when depositing a silicon nitride layer on top of porous layers.We also demonstrate that direct- and remote-plasma nitridation are good ways to reduce the ageing effect of porous silicon layers due to a passivation of dangling bonds.     

Electroluminescence properties of light emitting devices based on silicon nanocrystals

We have fabricated MOS devices where the dielectric layer consists of a substoichiometric SiO_x (x<2) thin film, annealed at 1100°C for 1 h to induce the separation of the Si and SiO₂ phases, with the formation of silicon nanocrystals (nc) embedded in the insulating matrix. We have studied the electroluminescence (EL) properties of such devices as a function of the current density and of the temperature. We have evaluated the excitation cross section of Si nc under electrical pumping at room temperature and at low temperature (12 K). Moreover, we have used the experimental EL intensities and decay times to evaluate the radiative rate as a function of the temperature.     

Infrared induced visible emission from porous silicon: the mechanism of anodic oxidation

The visible luminescence caused by anodic oxidation of p-type porous silicon has been studied. It is shown that similar luminescence can be observed in n-type material by illumination with near-infrared light. Addition of a suitable reducing agent to the electrolyte solution can both suppress the oxidation of the porous layer and quench its luminescence. These results confirm a previously suggested mechanism, in which the capture of a valence band hole in a surface bond of the porous semiconductor gives rise to a surface state intermediate capable of thermally injecting an electron into the conduction band.     

Electroluminescence from GaN-polymer heterojunction

Inorganic and organic semiconductor devices are generally viewed as distinct and separate technologies. Herein we report a hybrid inorganic-organic light-emitting device employing the use of an air stable polymer, Poly (9,9-dioctylfluorene-alt-benzothiadiazole) as a p-type layer to create a heterojunction, avoiding the use of p-type GaN, which is difficult to grow, being prone to the complex and expensive fabrication techniques that characterises it. I-V characteristics of the GaN-polymer heterojunction fabricated by us exhibits excellent rectification. The luminescence onset voltage is typically about 8-10 V. The device emits yellowish white electroluminescence with CIE coordinates (0.42, 0.44).     

基于MEH-PPV/ZnSe纳米复合器件的电致发光特性的研究

以巯基乙酸作为稳定剂在水相中制备了ZnSe纳米晶,用X射线粉末衍射(XRD)和X射线光电子能谱(XPS)对其进行了表征。用表面活性剂将ZnSe纳米晶从水相中转移到有机相中,使其与聚合物MEH-PPV复合作为发光层,制备了多层电致发光器件Glass/ITO/MEH-PPV∶ZnSe/BCP/Alq₃。对ZnSe纳米晶和MEH-PPV薄膜的光致发光谱及其吸收光谱的比较表明ZnSe纳米晶和MEH-PPV之间存在着能量传递,这是导致纳米复合薄膜的光致发光光谱和电致发光光谱存在差异的原因之一。文章对其在光激发和载流子注入条件下的不同发光机制进行了讨论。通过对器件的光电特性进行研究,发现ZnSe纳米晶发光的比例随着外加电压的增加而增加,而且器件的I-V特性基本上符合二极管的特性。     

Irreversible quenching of luminescence in porous silicon

The influence of applied voltage on photoluminescence (PL) in porous silicon was studied. A strong PL band around 680 nm was observed when excited by a 300 nm ultraviolet light with no voltage applied, but upon increasing the bias voltage, a strong and progressive decrease of the PL intensity was observed leading finally to a complete quenching of the emitted light at 1.80 V. The peak position of the emission appears to be stable. This effect is completely irreversible, and the spectra depend on the increased voltage to the sample and corresponding temperature increase. Nonradiative recombination resulting from the thermal oxidation was suggested to be responsible for the quenching.     

Interference filters from porous silicon with laterally varying wavelength of reflection

Porous silicon reflection interference filters of Bragg type consists of up to 40 quarter wave layers with alternating high- and low-refraction index. The refraction index depends on the porosity of the silicon. The reflection wavelength can vary over a wide range and depends on the thickness and refraction index of the porous layers. A laterally continuously varying wavelength with linear profile of the filter can be achieved by manipulating the porosity and thickness of the silicon in the lateral direction. Our approach is to vary the Fermi level laterally by applying a potential parallel to the surface of the wafer. The slope of the Fermi level is easily controlled by the magnitude of the potential. The lateral current density and thus the porosity and thickness is related to the potential difference between the laterally varying Fermi level and the potential induced by the counter electrode. This relation is the well-known current–voltage characteristic of a Silicon hydrofluoric acid contact. The linearity of the etch profile across the wafer is demonstrated and the properties of preliminary reflection filters are shown.     

Eu-TTA复合体系荧光发光与电致发光性能研究

本文合成了Ｅｕ－ＴＴＡ复合体系了其性能,该材料有大的应用前景。     

Nanoscale morphology dependent hydrogen coverage of meso-porous silicon

Hydrogen coverage of the specific surface of meso-porous silicon nanostructures is studied by means of attenuated total reflection infrared spectroscopy. A strong correlation of silicon hydride bonds and of total amount of the adsorbed hydrogen to the nanoscale morphology parameters such as porosity, nanocrystallites dimensions and fractal-like specific surface is reported.     

Low-porosity porous silicon nanostructures on monocrystalline silicon solar cells

In this work we present a study of low-porosity porous silicon (PS) nanostructures stain etched on monocrystalline silicon solar cells. The PS layers reduce the reflectance, improve the diffusion of dopants by rapid thermal processes, and increase the homogeneity of the sheet resistance. Some samples were subjected to chemical oxidation in HNO₃ to reduce the porosity of the surface layer. After the diffusion process, deposition of a SiN_x antireflection layer, and screen printing of the samples, an efficiency of 15.5% is obtained for low-porosity PS solar cells, compared with an efficiency of 10.0% for standard PS cells and 14.9% for the reference Cz cells.     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.     

多孔硅表面钝化对其发光性能的影响

报道多孔硅（ＰＳ）的表面钝化对其光致发光（ＰＬ）和电致发光（ＥＬ）的影响,ＰＬ和ＥＬ谱表明,经钝化处理的ＰＳ的ＰＬ和ＥＬ强度明显增强,且发光峰位较大蓝移;存放实验表明,经钝化处理的ＰＳ的ＰＬ和ＥＬ发光强度和发光峰位具较好的稳定性;Ｉ～Ｖ曲线显示,经钝化处理的ＰＳ发光器件具有较低的劝电压,结结果表明：用钝化处理的方法是几ＰＬ和ＥＬ强度和稳定性及改善器件性能的有效途径。