Effect of gamma irradiation on the photoluminescence of porous silicon

The effect of gamma irradiation on the luminescence properties of porous silicon produced by the electrochemical technique is studied. Changes in the photoluminescence intensity between irradiation doses and over a period of several days after the last irradiation are recorded. The quenching of photoluminescence at low irradiation doses and recovery after further irradiation are registered. It is found that porous silicon is strongly oxidized after gamma irradiation and the oxidation process continues for several days after irradiation. It is conceived that the change in the photoluminescence spectra and intensity of porous silicon after gamma irradiation is caused by a change in the passivation type of the porous surface: instead of hydrogen passivation, more stable oxygen passivation is observed. To stabilize the photoluminescence spectra of porous silicon, the use of fullerenes is proposed. No considerable changes in the photoluminescence spectra during irradiation and up to 18 days after irradiation are detected in a porous silicon sample with a thermally deposited fullerene layer. It is shown that porous silicon samples with a deposited C₆₀ layer are stable to gamma irradiation and oxidation.     

Evaluation of the influence of an embedded porous silicon layer on the bulk lifetime of epitaxial layers and the interface recombination at the epitaxial layer/porous silicon interface

Porous silicon plays an important role in the concept of wafer‐equivalent epitaxial thin‐film solar cells. Although porous silicon is beneficial in terms of long‐wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron‐doped (10¹⁶/cm³) epitaxial layers grown on reorganised porous silicon as well as on pristine mono‐crystalline, Czochralski, p⁺ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100–115 µs) than that of layers on top of bare p⁺ substrate (32–50 µs). It is believed that proper surface closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p⁺ substrate interface (S ≅ 10³ cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface. Copyright © 2013 John Wiley & Sons, Ltd.     

多孔硅可见光发射研究

用常规电化学方法制备了发射高效可见光的多孔硅样品。对样品进行了光谱研究。用非化学方法从样品表面得到了粉末状荧光物质(非多孔硅膜研磨产物),光谱测定证实它的光致发光光谱与原多孔硅样品光致发光光谱相同,粉末进一步研磨后仍能发出同样的可见光。多孔硅样品还可以实现阴极射线激发发出同样的高效可见光,但易因电子束的轰击而发光强度较快减弱。用扫描电镜(SEM)对多孔硅样品的形貌、结构、荧光粉末的形状、尺寸、多孔硅样品阴极荧光发射区域进行了系统的研究。实验结果表明多孔硅样品一般可分为三层结构:表面层、多孔层和单晶硅衬底,样品荧光是来源于其表面层的。对样品表层组分的x射线光电子谱(XPS)分析表明,此时的多孔硅表层有大量非硅元素存在,如:C、O、F(没考虑H),硅元素的原子个数比只占30％～50％。用同样的电化学方法在单晶硅未抛光面上和多晶硅未抛光面上制得了均匀发射可见光样品。上述实验结果说明,多孔硅的高效可见光发射是来源于样品制备过程中在其表面层中形成的粉末状荧光物质。     

电化学刻蚀半导体的结构分析

结合多孔硅(Si)、多孔砷化镓(GaAs)以及多孔磷化铟(InP)的不同孔形貌,综合分析了元素半导体硅及Ⅲ-Ⅴ族化合物半导体GaAs、InP的刻蚀结构,系统地阐述了晶体结构在电化学刻蚀中的作用。化合物半导体由于存在晶格极化和各向异性,使得不同晶面的溶解速率或钝化速率不同,导致孔沿着溶解速率较大的方向生长,钝化速率较大的晶面成为孔壁,在一定程度上影响了孔的形状、大小及周期性排列等特征。用电流控制模型对不同孔的生长过程进行了较好的解释,进一步证明了晶体的结构特征对其产生的重要影响。     

发光多孔硅微结构及其发光起源探讨

用制备发光多孔硅样品的常规电化学方法，在未抛光多晶硅表面，成功地制备出了均匀地发射肉眼可分辨的可见光样品。样品的先致发光光谱得到了测定，证明是一种典型的多孔硅光致发光光谱。用扫描电镜对样品的表面形貌、截面结构进行了详细的分析，摄制出了发光多孔硅样品的完整多孔状微结构清晰照片。实验结果认为多孔硅样品可分成三层：表面层、多孔层、单晶硅衬底；而多晶硅表面上制备的发光样品只有两层结构：表面层、多晶硅衬底。文中认为多孔硅可见光发射应来自其表面层，而与层下的多孔层微结构无关。     

多孔硅发光—纳米硅量子海绵中的量子约束

多孔硅的发光是源自其最表面层,该层是非晶态的。本研究揭示其结构是随机分布在此表面的纳米尺度的硅。多孔硅的微结构好象量子海绵,没有观察到“线”状结构,是一种无序材料。多孔硅的发光极象是由于这种纳米硅原子簇中的量子限制。     

Specific features of the sol-gel formation and optical properties of 3d metal/porous silicon composites

The composition and optical properties of composite materials based on porous silicon with iron, cobalt, and nickel deposited by the sol-gel technique are studied. It is shown that the deposition of metal-oxide films onto the surface of porous silicon enables stabilization of the photoluminescence and an increase in its intensity, as well as the storage of hydrogen in the porous layer.     

Photosensitive heterostructures based on porous nanocrystalline silicon

The features of the manufacturing process, as well as the results of studies on the morphology, electrophysical, and photoelectric properties of photosensitive structures based on silicon containing siliconcarbide and porous silicon layers, are considered. A porous layer is created on the surface of a monocrystalline silicon substrate via electrolytic etching in fluorine-containing solutions. Wafers with a different surface microrelief such as a ground, polished, and textured one, has been used. The carbonization of the samples resulting in the formation of SiC/Si heterostructures has been carried out via gas transport endotaxy in a hydrogen flow using a vertical reactor with cold walls and a graphite container. The structure and composition of the manufactured SiC/Si heterostructures formed on different types of structured surfaces on polycrystalline and monocrystalline silicon, including the surface porous silicon layer, are investigated. It is shown that the process of endotaxy on all the types of surfaces leads to the formation of a single-crystal phase of silicon carbide by cubic modification. Using scanning and transmission electron microscopy, the morphology of the produced structures is investigated. Filiform entities with a different structure have been revealed on nonporous surfaces identified as silicon carbide, whereas the cylindrical or conical structures, whose nature is uncertain, have been observed on porous surfaces. The current-voltage and current-power curves are plotted for all types of manufactured structures, the general form of which indicates the presence of several potential barriers there. The photoelectric properties of the structures and the prospects of their use in photoelectric converters of solar cells are analyzed.     

The crucial role of singlet oxygen in the formation of photoluminescence from nanoporous silicon

It is shown that nanoporous silicon electrochemically fabricated on p-type silicon (1–4 Ω cm) in complete darkness exhibits nearly no photoluminescence in the visible spectral range. Photoluminescence properties appear upon irradiation of a porous silicon layer with white light in a water-alcohol solution of HF. The effect is accounted for by the action of the singlet form of molecular oxygen, generated when dissolved oxygen comes in contact with a porous silicon layer under illumination.     

Photosensitivity of porous silicon-layered III–VI semiconductors heterostructures

Rectifying heterojunctions with photosensitivity 1–5 V/W at T=300 K were obtained by forming optical contacts between free porous silicon and layered InSe and GaSe semiconductors. A wide-band photovoltaic effect was obtained when these heterostructures were illuminated on the free porous silicon plate side. The long-wavelength photosensitivity edge of these devices is determined by direct transitions in InSe or GaSe, respectively. It is concluded that heterojunctions based on free porous silicon plates can be used as wide-band phototransducers. Fiz. Tekh. Poluprovodn. 32, 353–355 (March 1998)     

激光加工形成硅基上的氧化低维纳米结构的PL发光

用激光辐照硅样品和硅锗合金样品能够形成多种氧化低维纳米结构,可以用激光与半导体相互作用产生的等离子体波模型来解释这些结构的形成机理.其中,在单晶硅上形成的网孔壁结构有很强的706nm波长的光致发光(PL)峰,在硅锗合金上形成的多孔状结构于波长为725nm处有极强的PL峰,在硅锗合金上形成的条形片状结构分别在波长为760nm和866nm处也有较强的PL峰;这些结构表面都覆盖有氧化硅层,在硅锗合金上的氧化硅层中镶嵌有纳米锗品团簇,并用相应的模型解释了这些氧化低维纳米结构的强光致发光效应.     

Effect of parabenzoquinone adsorption on the magnetic properties of nanostructured silicon

The effect of parabenzoquinone molecule adsorption on the magnetic properties of nanostructured silicon is studied. A mesoporous silicon sample with a specific surface area of ?? 800 m²/g was used as nanostructured silicon. It is found that parabenzoquinone molecule adsorption on a porous silicon surface results in the formation of a new nanocomposite material, i.e., a porous silicon matrix filled with clusters of exchange-coupled adsorbed molecules. The superparamagnetic behavior of this nanocomposite is detected. It is shown that the saturation magnetic moment remains unchanged in the temperature range from 100 to 360 K and does not degrade in air.     

Application research on the sensitivity of porous silicon

Applications based on sensitive property of porous silicon (PSi) were researched. As a kind of porous material, the feasibility of PSi as a getter material was studied. Five groups of samples with different parameters were prepared. The gas-sensing property of PSi was studied by the test system and suitable parameters of PSi were also discussed. Meanwhile a novel structure of humidity sensor, using porous silicon as humidity-sensitive material, based on MEMS process has been successfully designed. The humidity-sensing properties were studied by a test system. Because of the polysilicon layer deposited upon the PSi layer, the humidity sensor can realize a quick dehumidification by itself. To extend service life and reduce the effect of the environment, a passivation layer (Si₃N₄) was also deposited on the surface of electrodes. The result indicated the novel humidity sensor presented high sensitivity (1.1 pF/RH%), low hysteresis, low temperature coefficient (0.5%RH/℃) and high stability.     

Study of the processes of carbonization and oxidation of porous silicon by Raman and IR spectroscopy

Porous silicon layers were produced by electrochemical etching of single-crystal silicon wafers with the resistivity 10 Ω cm in the aqueous-alcohol solution of hydrofluoric acid. Raman spectroscopy and infrared absorption spectroscopy are used to study the processes of interaction of porous silicon with undiluted acetylene at low temperatures and the processes of oxidation of carbonized porous silicon by water vapors. It is established that, even at the temperature 550°C, the silicon-carbon bonds are formed at the pore surface and the graphite-like carbon condensate emerges. It is shown that the carbon condensate inhibits oxidation of porous silicon by water vapors and contributes to quenching of white photoluminescence in the oxidized carbonized porous silicon nanocomposite layer.     

多孔硅纵向孔隙率与残余应力分布的研究

利用电化学方法在p型重掺杂单晶硅(100)基体上制备了多孔硅薄膜,通过质量计算法得到多孔硅的孔隙率,并研究了多孔硅孔隙率随腐蚀深度变化的规律。利用显微拉曼光谱技术对多孔硅纵切面上的残余应力进行了测量,结果表明,多孔硅的孔隙率随腐蚀时间/深度的增加有先增加后减少的趋势;多孔硅纵向上存在拉伸残余应力,拉伸应力的分布与纵切面上孔隙率的分布成正比,先增大,再减小;到达多孔硅与基体硅的界面处时,拉伸应力减小为零,靠近硅一侧,转变为压应力;残余应力的最大值出现在临近多孔硅表面以下的区域。这主要与多孔硅制备过程中孔内HF酸浓度的降低和硅/电解液表面的电偶层有关。     

基于微机械的多孔硅牺牲层技术

多孔硅作为一种牺牲层材料 ,在表面硅微机械加工技术中有着重要的应用。文中综合讨论了三种不同的多孔硅牺牲层技术 ,并用后两种“在低掺杂衬底上的多孔硅牺牲层技术”,制作了良好的悬空微薄膜结构 ,同时对多孔硅表面的薄膜淀积 ,和制备过程中的掩膜材料等进行了分析 ,为利用多孔硅工艺制作各种 MEMS器件奠定了基础。     

The Phenomenology of Ion Implantation-Induced Blistering and Thin-Layer Splitting in Compound Semiconductors

Hydrogen and/or helium implantation-induced surface blistering and layer splitting in compound semiconductors such as InP, GaAs, GaN, AlN, and ZnO are discussed. The blistering phenomenon depends on many parameters such as the semiconductor material, ion fluence, ion energy, and implantation temperature. The optimum values of these parameters for compound semiconductors are presented. The blistering and splitting processes in silicon have been studied in detail, motivated by the fabrication of the widely used silicon-on-insulator wafers. Hence, a comparison of the blistering process in Si and compound semiconductors is also presented. This comparative study is technologically relevant since ion implantation-induced layer splitting combined with direct wafer bonding in principle allows the transfer of any type of semiconductor layer onto any foreign substrate of choice—the technique is known as the ion-cut or Smart-Cut™ method. For the aforementioned compound semiconductors, investigations regarding layer transfer using the ion-cut method are still in their infancy. We report feasibility studies of layer transfer by the ion-cut method for some of the most important and widely used compound semiconductors. The importance of characteristic values for successful wafer bonding such as wafer bow and surface flatness as well as roughness are discussed, and difficulties in achieving some of these values are pointed out.     

The effect of oxidation on the efficiency and spectrum of photoluminescence of porous silicon

The photoluminescence spectra of porous silicon and their temperature dependences and transformations on aging are studied. It is shown that the infrared band prevailing in the spectra of as-prepared samples is due to exciton recombination in silicon crystallites. On aging, a well-pronounced additional band is observed at shorter wavelengths of the spectra. It is assumed that this band is due to the recombination of carriers that are excited in silicon crystallites and recombine via some centers located in oxide. It is shown that the broad band commonly observable in oxidized porous silicon is a superposition of the above two bands. The dependences of the peak positions and integrated intensities of the bands on time and temperature are studied. The data on the distribution of oxide centers with depth in the porous layer are obtained.     

一维纳米硅光子晶体的制备与优化

采用双槽电化学腐蚀法制备了纳米多孔硅,主要研究了腐蚀时间和腐蚀电流对重掺杂p型(100)硅衬底上制备的多孔硅层有效光学厚度的影响,采用U-4100光谱仪、场发射扫描电子显微镜(FESEM)技术对所制备的多孔硅光子晶体的结构和有效光学厚度进行了分析表征。研究结果表明,通过合理地选择腐蚀时间和腐蚀电流,可以比较精确地制备特定有效光学厚度的多孔硅薄膜,此方法可广泛应用于纳米多孔硅光子晶体的制备中。     

Experimental testing of a random medium optical model of porous silicon for photovoltaic applications

We have developed a model for light propagation in porous silicon (PS) based on the theory of wave propagation in random media. The low porosity case is considered, with silicon being the host material assuming randomly distributed spherical voids as scattering particles. The specular and the diffuse part of the light could be determined and treated separately. The model is applied to the case in which porous silicon would be used as a diffuse back reflector in a thin‐film crystalline silicon solar cell realized in an ultrathin (1–3 μm) epitaxially grown Si layer on PS. Three layer structures (epi/PS/Si) have been fabricated by atmospheric pressure chemical vapor deposition (APCVD) of 150–1000 nm epitaxial silicon layers on silicon wafers of which 150–450 nm of the surface has been electrochemically etched. An excellent agreement is found between the experimentally measured reflection data in the 400–1000 nm wavelength range and those calculated using the proposed model. The values of the layer thickness agree, within a reasonable experimental error, with those obtained independently by cross sectional transmission electron microscopy (XTEM) analysis. This provides an experimental verification of the random medium approach to porous silicon in the low porosity case. The analysis shows that the epitaxial growth process has led to appreciable porosity decrease of an initially high porosity layer from about 60% to 20–30%. Copyright © 2001 John Wiley & Sons, Ltd.