On the route towards Si-based optical interconnects

We have developed and fabricated a prototype of optical interconnects on a Si substrate. The original device design includes an aluminum–porous silicon light-emitting diode connected with a photodetector by an alumina waveguide. In order to minimize optical losses, the waveguide has been realized by subsequent deposition of three aluminum layers among which the intermediate one was doped with a titanium large refractive index layer. This multilayer structure was then anodically oxidized to form an Al₂O₃/Al₂O₃+TiO₂/Al₂O₃ layered waveguide. In the integrated optoelectronic unit it provides up to 50% increase of the detector response with respect to a waveguide of pure Al₂O₃. Optical losses in the visible range have been estimated to be about 1 dB/cm. Another method for increasing the detector response through the use of a microcavity is discussed.     

Design and development of a novel high-transconductance pH-ISFET (ion-sensitive field-effect transistor)-based glucose biosensor

A glucose biosensor based on a high-transconductance ISFET transduction element with aspect ratio (channel width/length) of 400 has been developed. This biosensor is an N-channel enhancement mode device with interdigitated drain-source geometry, fabricated by the NMOS process, in which glucose oxidase (GOD) enzyme has been immobilized over the silicon dioxide-silicon nitride dual-dielectric gate. The device has been operated in the active mode by applying a gate voltage through Ag/AgCl reference electrode. Electrical characterization has been performed in terms of I-V characteristics like output characteristics and leakage current. The pH response characteristics have been measured and the pH sensitivity factor has been found to be?≥?50?mV/decade. Device characterization has also been performed by a signal conditioning circuit developed for direct readout of pH from the ISFET device. Temperature behaviour and drift phenomenon have been investigated.

The glucose response characteristics of the ISFET have been determined, without and with the glucose oxidase enzyme layer. Improvement of the glucose sensitivity by deposition of the enzyme layer has been studied and cross-sensitivity of the device towards urea has been examined. The advantage of the high transconductance was evident from the ability of the sensor to detect small glucose concentrations without the enzyme layer. The paper describes the design, fabrication and characterization of the sensor.     

Fabrication and electrical characterization of n-InSb on porous Si heterojunctions prepared by liquid phase epitaxy

Thin films of InSb were grown on p-type porous silicon (PSi) (1 1 1) substrates by liquid phase epitaxy (LPE) to obtain monocrystalline InSb epilayer on a PSi substrate for low cost device applications. The structural characterization of the devices was carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The X-ray diffraction measurements indicate that InSb monocrystalline epilayer was successfully grown onto PSi. The current-voltage (I-V) characteristics of n-InSb/p-PSi heterojunction devices were measured in the temperature range of 298-398 K. The measurements indicate that these heterojunctions have good rectifying characteristics. The estimated zero-bias barrier height φ_BO and the ideality factor η show strong temperature dependence. The conventional Richardson plot exhibits linear behavior in the entire temperature range indicating that the conduction seems to be predominantly due to thermionic emission mechanism. In addition, the capacitance-voltage characteristics are investigated at frequency of 1 MHz. The built-in potential of the heterojunction is determined after eliminating the effect of the capacitance effect of the interface state caused by the lattice mismatch.     

Structural and optical characterization of GaN grown on porous silicon substrate by MOVPE

GaN was grown on porous silicon (PS) substrates by Metalorganic Vapour Phase Epitaxy at temperature of 1050 °C. An additional AlN buffer layer is used between GaN and PS. The crystalline quality and surface morphology of GaN films were studied by X-ray diffraction and scanning electron microscope (SEM), respectively. Preferential growth of hexagonal GaN with 〈00.1〉 direction is observed and is clearly improved when the thickness of AlN buffer layer increases. Morphological changes in PS layer appearing after growth have been also discussed.GaN optical qualities were determined by photoluminescence at low and room temperature (RT).     

硅基纳米材料制备技术的新进展

近年,硅基低维材料物理与工艺的研究预示,硅基光电子学将是今后半导体光电子学的一个主要发展方向,而对硅基纳米材料的研究一直是本领域的一个研究热点,评述了近年硅基纳米材料在制备技术方面所取得的新进展,并展望了今后的发展方向。     

Research progress of Si-based germanium materials and devices

Si-based germanium is considered to be a promising platform for the integration of electronic and photonic devices due to its high carrier mobility, good optical properties, and compatibility with Si CMOS technology. However, some great challenges have to be confronted, such as: (1) the nature of indirect band gap of Ge; (2) the epitaxy of dislocation-free Ge layers on Si substrate; and (3) the immature technology for Ge devices. The aim of this paper is to give a review of the recent progress made in the field of epitaxy and optical properties of Ge heterostructures on Si substrate, as well as some key technologies on Ge devices. High crystal quality Ge epilayers, as well as Ge/SiGe multiple quantum wells with high Ge content, were successfully grown on Si substrate with a low-temperature Ge buffer layer. A local Ge condensation technique was proposed to prepare germanium-on-insulator (GOI) materials with high tensile strain for enhanced Ge direct band photoluminescence. The advances in formation of Ge n⁺p shallow junctions and the modulation of Schottky barrier height of metal/Ge contacts were a significant progress in Ge technology. Finally, the progress of Si-based Ge light emitters, photodetectors, and MOSFETs was briefly introduced. These results show that Si-based Ge heterostructure materials are promising for use in the next-generation of integrated circuits and optoelectronic circuits.     

Formation and characterization of porous silicon films obtained by catalyzed vapor-chemical etching

Porous silicon films obtained by the metal-assisted vapor-chemical etching technique have been characterized. For the film formation, epitaxial (100) N/P⁺, 1–5 Ω cm monocrystalline silicon wafers were used. The vapors of an alcoholic solution of H₂O₂/HF were drawn towards the silicon surface, which was previously covered with a thin layer of gold (~8 nm) for the catalytic etching. For the optical and morphological characterization of porous films, Raman spectroscopy, Ellipsometry, FTIR spectroscopy and SEM images were used. The films thickness kept a linear relationship with etching time. A porosity gradient from the surface towards the interface (65% to 12%) was observed in the films. A large amount of Si–H bonds as related to O–Si–O bonds were observed and the pore size depends on the HF concentration. Irregular morphology was found in films formed with 50% HF.     

A theoretical model of the formation morphologies of porous silicon

In this paper a qualitative theoretical model describing the mechanism and formation morphology of porous silicon is presented. The model is based on the diffusion limited aggregation models of Witten and Sanders. The validity of this model is verified by performing small scale computer simulations to construct various porous silicon structures. These structures are compared with the known properties of bulk silicon and morphologies of porous silicon. The postulates of the model are sufficiently rich to explain the relationship between pore density, pore diameter, porosity as well as crystallographic etch selectivity and electropolishing.     

Waveguide Prism Based on Porous Silicon

The fabrication of the oxidized porous silicon waveguide prism is reported by selectively electrochemical anodisation process. The direction changes of light beams in TE and TM polarization out of this waveguide prism were respectively measured,and the experimental results were analyzed.     

GaN growth on porous silicon by MOVPE

GaN films have been grown at 1050 °C on porous silicon (PS) substrates by metalorganic vapour phase epitaxy. The annealing phase of PS has been studied in temperature range from 300 to 1000 °C during 10 min under a mixture of ammonia (NH₃) and hydrogen (H₂). The PS samples were characterized after annealing by scanning electronic microscope (SEM). We observed that the annealing under the GaN growth conditions does not affect the porous structure.For the growth of the active GaN layer we used a thin AlN layer in order to improve wetting between GaN and PS/Si substrate. The growth of AlN and GaN films was controlled by laser-reflectometry. We estimated the porosity of PS samples from the evolution of the reflectivity signal during the AlN growth. The crystalline quality and surface morphology of GaN films were determined by X-ray diffraction and SEM, respectively. Preferential growth of hexagonal GaN with (0002) direction is observed and is clearly improved when the thickness of AlN layer increases. Epitaxial GaN layers were characterized by photoluminescence.     

Luminescent chemically-etched porous poly-crystalline silicon on insulating substrates

Polycrystalline silicon deposited on insulating substrates has been chemically-etched to form thin films of porous material exhibiting room temperature visible photoluminescence with emission wavelengths of around 650 nm. Material of 4000 ? thickness was quickly converted to porous silicon within 15 s of etching, with an etch rate of 1–1.5 μm/h. In contrast to anodization, chemical-etching parameters have little effect on modulating the resultant peak wavelength. Peak photoluminescence intensity was achieved 8–12 s of etching in 1:3:5 parts HF:HNO₃:H₂O at room temperature with ambient lighting. The chemical etching process and its etch characteristics have been discussed in relation to its suitability for large area thin film devices.     

Characterization of an Oxidized Porous Silicon Layer by Complex Process Using RTO and the Fabrication of CPW‐Type Stubs on an OPSL for RF Application

This paper proposes a 10‐µm thick oxide layer structure that can be used as a substrate for RF circuits. The structure has been fabricated using an anodic reaction and complex oxidation, which is a combined process of low‐temperature thermal oxidation (500 °C, for 1 hr at H₂O/O₂) and a rapid thermal oxidation (RTO) process (1050 °C, for 1 min). The electrical characteristics of the oxidized porous silicon layer (OPSL) were almost the same as those of standard thermal silicon dioxide. The leakage current density through the OPSL of 10 µm was about 10 to 50 nA/cm² in the range of 0 to 50 V. The average value of the breakdown field was about 3.9 MV/cm. From the X‐ray photo‐electron spectroscopy (XPS) analysis, surface and internal oxide films of OPSL prepared by a complex process were confirmed to be completely oxidized. The role of the RTO process was also important for the densification of the porous silicon layer (PSL) oxidized at a lower temperature. The measured working frequency of the coplanar waveguide (CPW) type short stub on an OPSL prepared by the complex oxidation process was 27.5 GHz, and the return loss was 4.2 dB, similar to that of the CPW‐type short stub on an OPSL prepared at a temperature of 1050 °C (1 hr at H2O/O2). Also, the measured working frequency of the CPW‐type open stub on an OPSL prepared by the complex oxidation process was 30.5 GHz, and the return was 15 dB at midband, similar to that of the CPW‐type open stub on an OPSL prepared at a temperature of 1050 °C (1 hr at H₂O/O₂).     

注入型多孔硅基发光二极管的研究进展

主要评述了Schottky结和PN结多孔硅基发光二极管的结构和性能及其研究进展。讨论了器件界面态对其性能的影响。最后分析了目前器件研究中存在的亟需解决的问题以及解决方法。     

Experimental characterization of electron-hole generation in silicon carbide

Thermal generation in wide bandgap semiconductors can be observed by monitoring the capacitance recovery transients of npn (or pnp) storage capacitors in which the middle layer is floating. In this article, we report a study of thermal generation in 4H and 6H silicon carbide (SiC). Three generation mechanisms are identified: bulk generation in the depletion regions of the pn junctions, surface generation at the periphery of the capacitors, and defect generation associated with imperfections in the material. All three generation mechanisms are thermally activated. Bulk generation and surface generation have activation energies of approximately half bandgap, while defect generation exhibits field-induced barrier lowering resulting in an apparent activation energy less than half bandgap. Because the generation rate is extremely low, most measurements are conducted at elevated temperatures (250-350°C). However, we also describe a long-term measurement at room temperature in which the 1/e recovery time appears to be in excess of 100 years.     

A mask-free method of patterned porous silicon formation by a localized electrical field

A simple and mask-free method is proposed for the fabrication of p-type patterned porous silicon (PS) using a localized electric field. The electric field is applied by the patterned electrodes (anode and cathode) which are horizontally placed underneath the sample. No masking-layer and related photo-lithography process are needed in this method. Besides, no metal electrodes and hence no metal-pollution in electrolyte have to be concerned in the formation of PS. The morphology of the PS prepared by this method is investigated. Strong visible photoluminescence emissions in the selected areas of PS are demonstrated on PS at about 650 nm.     

Origins of photoluminescence degradation in porous silicon under irradiation and the way of its elimination

Using of infrared (IR) and photoluminescence (PL) spectroscopy a comparative study of distinctions in composition and photoluminescence properties of porous silicon with different morphology was performed. Basing on the obtained experimental data and conventional theoretical models the main factors were found that have a negative effect on the intensity of PL in porous silicon and its degradation under the impact of directed irradiation in the visible range. With porous silicon as an example having the pores of 50–100 nm in size there was demonstrated a possibility for improving of these characteristics by its chemical treatment in polyacrylic acid.     

多孔硅的能带变化及光致发光的研究

多孔硅与硅材料相比，有明显的可见光致发光特性，其原理可用量子限制模型给予解释。文章通过对多孔硅的光致发光、光电子辐射、光电效应和X光吸收方面的实验的讨论，研究了退火对多孔硅导带和价带边缘的量子漂移的影响。对于大多数多孔硅，过高的退火温度可造成多孔硅的导带和价带边缘的量子漂移的改变。     

Experimental characterization of coaxial through silicon vias for 3D integration

Coaxial through silicon via (TSV) technology is gaining considerable interest as a 3D packaging solution due to its superior performance compared to the current existing TSV technology. By confining signal propagation within the coaxial TSV shield, signal attenuation from the lossy silicon substrate is eliminated, and unintentional signal coupling is avoided. In this paper, we propose and demonstrate a coaxial TSV 3D fabrication process. Next, the fabricated coaxial TSVs are characterized using s-parameters for high frequency analysis. The s-parameter data indicates the coaxial TSVs confine electromagnetic propagation by extracting the inductance and capacitance of the device. Lastly, we demonstrate the coaxial TSVs reduce signal attenuation and time delay by 35% and 25% respectively compared to the shield-less standard TSV technology. In addition, the coaxial interconnect significantly decreases electromagnetic coupling compared to traditional TSV architectures. The improved signal attenuation and high isolation of the coaxial TSV make it an excellent option for 3D packaging applications expanding into the millimeter wave regime.     

铁掺杂对不同导电类型硅材料电阻率的影响

采用电阻率为4.8Ω·cm的p型硅片和10Ω·cm的n型硅片,通过高温扩散法制备出了Fe掺杂的补偿硅材料.在室温避光条件下,测量样品电阻率p,并用XRD对扩散后的样品进行分析,研究了Fe掺杂对不同导电类型硅材料电阻率的影响.结果表明:相对于n型硅材料,深能级杂质Fe掺杂对p型硅材料电阻率的影响更大,其Fe掺杂p型硅材料电阻率远大于Fe掺杂n型硅材料;当p型硅表面Fe扩散源浓度为1.74× 10-5 mol/cm2时,在1 200℃下扩散1h后,材料具有最大电阻率7 246Ω· cm.     

Forming Antifouling Organic Multilayers on Porous Silicon Rugate Filters Towards In Vivo/Ex Vivo Biophotonic Devices

We describe the development and optimization of porous silicon photonic crystal surface chemistry towards implantable optical devices. Porous silicon rugate filters were prepared to obtain a narrow linewidth reflectivity peak in the near‐infrared (700–1000 nm) with low background reflectivity elsewhere. The morphology of the mesoporous structures (pore diameter < 50 nm) was such that only small proteins could infiltrate the pores whereas larger proteins were excluded. To provide stability in biological media, we established an approach to build organic multilayers containing hexa(ethylene oxide) moieties in porous silicon. The optical changes associated with organic derivatization were monitored concurrently with FTIR characterization. Furthermore, the antifouling capability of our chemical strategy is assessed and the penetration of different sized proteins into the structure was determined. The structural stability in biological environments was evaluated by incubation in human blood plasma over time while monitoring the optical signature of the photonic crystal.