Analysis of leakage currents in MOCVD grown GaInAsSb based photodetectors operating at 2 µm

This paper reports the fabrication and characterization of GaInAsSb photodetectors operating at 2 μm. At room temperature, the performance of these photodiodes under reverse bias conditions is limited by the surface leakage. A model has been developed to separate the bulk (diffusion and generation-recombination (g-r)) and the surface leakage contributions toward the total leakage current. By fitting this model to the experimental data, values of material parameters such as minority carrier diffusion length and lifetime have been estimated. The highest R₀A of 55 Ω-cm² has been obtained with a responsivity of 0.44 A/W at 2 μm.     

Optical and electrical properties of porous silicon layer formed on the textured surface by electrochemical etching

Porous silicon(PS) layers were formed on textured crystalline silicon by electrochemical etching in HF-based electrolyte.Optical and electrical properties of the TMAH textured surfaces with PS formation are studied. Moreover,the influences of the initial structures and the anodizing time on the optical and electrical properties of the surfaces after PS formation are investigated.The results show that the TMAH textured surfaces with PS formation present a dramatic decrease in reflectance.The longer the anodizing time is,the lower the reflectance.Moreover,an initial surface with bigger pyramids achieved lower reflectance in a short wavelength range.A minimum reflectance of 3.86%at 460 nm is achieved for a short anodizing time of 2 min.Furthermore,the reflectance spectrum of the sample,which was etched in 3 vol.%TMAH for 25 min and then anodized for 20 min,is extremely flat and lies between 3.67%and 6.15%in the wavelength range from 400 to 1040 nm.In addition,for a short anodizing time,a slight increase in the effective carrier lifetime is observed.Our results indicate that PS layers formed on a TMAH textured surface for a short anodization treatment can be used as both broadband antireflection coatings and passivation layers for the application in solar cells.     

Optical and electrical properties of porous silicon layer formed on the textured surface by electrochemical etching

Porous silicon (PS) layers were formed on textured crystalline silicon by electrochemical etching in HF-based electrolyte. Optical and electrical properties of the TMAH textured surfaces with PS formation are studied. Moreover, the influences of the initial structures and the anodizing time on the optical and electrical properties of the surfaces after PS formation are investigated. The results show that the TMAH textured surfaces with PS formation present a dramatic decrease of reflectance. The longer is the anodizing time, the lower is the reflectance. Moreover, an initial surface with bigger pyramids achieved lower reflectance in short wavelength range. A minimum reflectance of 3.86 % at 460 nm is achieved for a short anodizing time of 2 min. Furthermore, the reflectance spectrum of the sample, which was etched in 3 vol.% TMAH for 25 min and then anodized for 20 min, is extremely flat and lies between 3.67% and 6.15% in the wavelength range from 400 to 1040 nm. In addition, for a short anodizing time, a slight increase in the effective carrier lifetime is observed. Our results indicate that PS layers formed on a TMAH textured surface for a short anodization treatment can be used as both broadband antireflection coatings and passivation layers for the application in solar cells.     

Effects of post-annealing treatment on the structure and photoluminescence properties of CdS/PS nanocomposites prepared by sol-gel method

CdS nanocrystals have been successfully grown on porous silicon (PS) by sol-gel method. The plan-view field emission scanning electron microscopy (FESEM) shows that the pore size of PS is smaller than 5 μm in diameter and the agglomerates of CdS are broadly distributed on the surface of PS substrate. With the increase of annealing time, the CdS nanoparticles grow in both length and diameter along the preferred orientation. The cross-sectional FESEM images of ZnO/PS show that CdS nanocrystals are uniformly penetrated into all PS layers and adhere to them very well. photoluminescence (PL) spectra demonstrate that the intensity of PL peak located at about 425 nm has almost no change after the annealing time increases. The range of emission wavelength of CdS/PS is from 425 nm to 455 nm and the PL intensity is decreasing with the annealing temperature increasing from 100 °C to 200 °C.     

Efficient dye-sensitized solar cells from mesoporous zinc oxide nanostructures sensitized by N719 dye

Dye-sensitized solar cells (DSCs) have attracted a great deal of attention due to their low-cost and high power conversion efficiencies.They usually utilize an interconnected nanoparticle layer of TiO2 as the electron transport medium.From the fundamental point of view,faster mobility of electrons in ZnO is expected to contribute to better performance in DSCs than TiO2,though the actual practical situation is quite the opposite.In this research,we addressed this problem by first applying a dense layer of ZnO on FTO followed by a mesoporous layer of interconnected ZnO nanoparticle layer,both were prepared by spray pyrolysis technique.The best cell shows a power conversion efficiency of 5.2％ when the mesoporous layer thickness is 14μm and the concentration of the N719 dye in dye coating solution is 0.3 mM,while a cell without a dense layer shows 4.2％ under identical conditions,The surface concentration of dye adsorbed in the cell with a dense layer and that without a dense layer are 5.00 × 10-7 and 3.34 × 10-7 mol/cm2,respectively.The cell with the dense layer has an electron lifetime of54.81 ms whereas that without the dense layer is 11.08 ms.As such,the presence of the dense layer improves DSC characteristics ofZnO-based DSCs.     

Structure and photoluminescence properties of InN films grown on porous silicon by MOCVD

In this work, indium nitride (InN) films were successfully grown on porous silicon (PS) using metal oxide chemical vapor deposition (MOCVD) method. Room temperature photoluminescence (PL) and field emission scanning electron microscopy (FESEM) analyses are performed to investigate the optical, structural and morphological properties of the InN/PS nanocomposites. FESEM images show that the pore size of InN/PS nanocomposites is usually less than 4 μm in diameter, and the overall thickness is approximately 40 μm. The InN nanoparticles penetrate uniformly into PS layer and adhere to them very well. Nitrogen (N) and indium (In) can be detected by energy dispersive spectrometer (EDS). An important gradual decrease of the PL intensity for PS occurs with the increase of oxidation time, and the PL intensity of PS is quenched after 24 h oxidization. However, there is a strong PL intensity of InN/PS nanocomposites at 430 nm (2.88 eV), which means that PS substrate can influence the structural and optical properties of the InN, and the grown InN on PS substrate has good optical quality.     

Characterization of GaN Buffer Layers and Its Epitaxial Layers Grown by MOCVD

Low-pressure MOCVD has been used to investigate the properties of low-temperature buffer layer depodition conditions and their influence on the properties of high-temperature GaN epilayers grown subsequently.It is found that the surface morphology of the as-grown buffer layer after thermal annealing at 1030℃and 1050℃ depends strongly on the thickness of the buffer layer.In particular when a thick buffer layer is used, large trapezoidal nuclei are formed after annealing.     

Design of a Silicon Based High Speed Plasmonic Modulator

In this paper, we propose a silicon-based high-speed plasmonic modulator. The modulator has a double-layer structure with a 16 μm long metal-dielectric-metal plasmonic waveguide at the upper layer and two silicon single-mode waveguides at the bottom layer. The upper-layer plasmonic waveguide acts as a phase shifter and has a dielectric slot that is 30 nm wide. Two taper structures that have gradually varied widths are introduced at the bottom layer to convert the photonic mode into plasmonic-slot mode with improved coupling efficiency. For a modulator with two 1 μm-long mode couplers, simulation shows that there is an insertion loss of less than 11 dB and a half-wave voltage of 3.65 V. The modulation bandwidth of the proposed modulator can be more than 100 GHz without the carrier effect being a limiting factor in silicon. The fabrication process is also discussed, and the proposed design is shown to be feasible with a hybrid of CMOS and polymer technology.     

Dark current generating mechanisms in short wavelength infrared photovoltaic detectors

The current-voltage characteristics and quantum efficiencies of double layer planar heterostructure photodiodes were investigated. Results are reported on devices with cutoff wavelengths of 1.8, 2.4, and 3.3 μm. For these respective devices, the dominant currents for temperatures >250,>200,>150K are diffusion currents limited by shallow Shockley-Hall-Read (SHR) processes. The remarkable result is that the electrical and optoelectronic properties of these devices of diverse cut-off wavelength can be explained by simple models using independently measured layer parameters such as the minority carrier lifetimes. For all three cases, the analysis suggests that the same shallow (SHR) centers located at 78% of the energy gap are causing the observed effects. These traps located in then-type base of the device are not influenced by the magnitude of n-type doping and this observation was used to significantly improve the performance of the devices and validate the predictive capability of the models used in the analysis. The shallow centers appear to be process induced rather than grown-in. This assertion is based on the observation that changes in the annealing process led to an order of magnitude improvement in the minority carrier lifetime.     

Effect of temperature on the intensity and carrier lifetime of an AlGaAs based red light emitting diode

The influence of temperature on the intensity of light emitted by as well as the carrier life time of a standard AlGaAs based light emitting diode has been investigated in the temperature range from 345 to 136 K. The open-circuit voltage decay(OCVD) technique has been used for measured the carrier lifetime. Our experimental results reveal a 16% average increase in intensity and a 163.482-19.765 ns variation in carrier lifetime in the above temperature range. Further, theoretical and experimental analysis show that for negligible carrier density the intensity is inversely proportional to carrier lifetime for this sample.     

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.     

Vapor Sensing Theoretical Study on Optical Microcavities

When the organic vapors absorbed to the surface of porous silicon（PS）, capillary condensation takes place due to the porous structure of the PS layer, accordingly resulting in the effective refractive index changing. For PS multi-layer microcavities, the different resonant peaks shift in the reflectivity spectrum of porous silicon microcavities（PSMs）. The optical sensing model is set up by applying Bruggeman effective medium approximation theory, capillary condensation process and transfer matrix theoretically analytical method of one-dimensional photonic crystals. At the same time, comprehensively researched on are the sensing characteristics of PSMs which are exposed to give concentration organic vapors. At last, made is the theoretical simulation for sensing model of the PSMs in case of saturation by using computer numerical calculation, and found is the linearity relation between the refractive index of organic solvent and the peak- shift. At the same time deduced is the peak-shift as a function of the concentration of ethanol vapors.     

A compact dual-band bandpass filter based on porous silicon dual-microcavity of one-dimensional photonic crystal

We demonstrate a laser-diode(LD)pumped actively Q-switched laser with Nd:Sc0.2Y0.8Si O5(Nd:SYSO)crystal for the first time.A stable actively Q-switched laser is obtained at dual wavelengths of 1 075.2 nm and 1 078.2 nm.The maximum average output power of 720 m W is obtained at the repetition rate of 15 k Hz under the pump power of8.7 W.The minimum pulse width of 58 ns is obtained at the repetition rate of 5 k Hz under the pump power of 8.7 W,corresponding to the peak power of 1.9 k W and the pulse energy of 112μJ.     

多孔硅吸收光谱和反射光谱

测定了多孔硅的吸收光谱和反射光谱，结果发现其吸收边对应于可见光区域。同单昌硅要比，其吸收边发生了蓝移，并且吸收强烈。由多孔硅反射谱曲一，利用Ｋ－Ｋ关系对其光学常数进行了简单的计算和分析。     

多孔硅吸收光谱的研究

利用大电流剥离方法制备了多孔硅薄膜层，测定了其吸收光谱。结果发现其吸收边对应于可见光区域，同单晶硅吸收光谱相比，其吸收边发生了蓝移，并且吸收强烈。这说明多孔硅的能带结构较硅发生了改变，表现为一种新的能带结构特征。     

电解液浓度对双槽法腐蚀多孔硅的影响

设计了一种电化学双槽腐蚀装置,研究了不同电解液的浓度对制备多孔硅形貌和光学特性的影响.研究结果表明采用此装置制备的多孔硅孔洞均匀性好,并且随NaCl电解液浓度的增加,制备的多孔硅孔径呈减小的趋势,发光强度有所增强.     

Diamond Like Carbon Film Deposited on Porous Silicon as Passivation Coating

By depositing diamond like carbon(DLC)film with radio frequency plasma chemical vapor deposition(RFPCVD)method,a new surface passivation technique for photoluminescence porous silicon(PS)has been studied.The surface microstructure and photoelectric properties of both porous silicon and DLC coated PS have been analyzed by using AFM,FTIR and PL spectrometers.The results show the DLC film with dense and homogenous nanometer grains can be deposited on the PS used as passivation coating as it can terminate oxide reaction on the surface of the PS.Furthermore,certain ratio of hydrogen existed in the DLC film can be improved to form hydride species on the DLC/PS interface as the centers of the luminescence so that the DLC coating is of benefit not only to the passivation of the PS but also to the improvement of its luminescent intensity.     

Porous silicon as an intermediate buffer layer for GaN growth on (100) Si

We report preliminary results on the growth of GaN on (100) Si substrate using porous silicon (PS) as an intermediate buffer layer. The growth was in situ monitored by laser beam reflectivity. Analysis of the evolution of the reflectivity signal indicates a change from relatively flat surface to rough one as the growth temperature (T_g) is increased. At a temperature of about 1050°C, the growth rate is very low and the reflected signal intensity is constant. When the growth temperature is varied, no drastic change of the porosity of the intermediate layer was detected. Scanning electron microscope (SEM) observations of the GaN/SP/Si structure revealed a good surface coverage at 500°C. When T_g increases, the structure morphology changes to columnar like structure at 600°C, and well-developed little crystallites with no preferential orientation appear at 800°C. These observations agree well with the X-ray diffraction (XRD) analysis. A preferential hexagonal growth is obtained at low growth temperature, while cubic phase begin to appear at elevated temperatures.     

Full Field Temperature Measurement of Specular Wafers During Rapid Thermal Processing

Many of the processes involved in the creation of semiconductor devices involve high-temperature processing of silicon wafers. The benefits of reduced thermal budget and faster cycle time make rapid thermal processing (RTP) a possible key technology for semiconductor manufacturing. However, the problem of nonuniform wafer temperature has prevented it from further spread among the industry. The first step in developing controls to maintain a uniform wafer temperature is accurate temperature measurement during processing. In this paper, a system was developed to exploit the specular reflectivity of silicon wafers and obtain a measurement of the wafer temperature profile. The spectral reflectivity is determined by measuring the intensity of an incident beam and the beam reflected from the wafer surface. With this measured reflectivity value the spectral-directional wafer emissivity was determined using Kirchhoff's law. The obtained emissivity then was used to calculate the wafer temperature profile from an image obtained with an infrared camera. An experimental study of the transmittance of an undoped silicon calibration wafer at an elevated temperature is also discussed     

多晶黑硅表面微结构对电池效率的影响

采用Ag离子辅助化学刻蚀法制备了多晶黑硅薄片,使用NaOH溶液处理多晶黑硅表面,增大其表面纳米孔直径,使SiNx薄膜能够均匀覆盖整个黑硅表面,提高黑硅的钝化效果,进而提高多晶黑硅电池光电转化效率.通过反射谱仪、扫描电子显微镜(SEM)、太阳电池测试系统等测试和表征不同扩孔时间对多晶黑硅各方面性能的影响.结果表明:未被NaOH扩孔处理的多晶黑硅的反射率最低,为5.03％,多晶黑硅太阳电池的光电转化效率为16.51％.当多晶黑硅被NaOH腐蚀40 s时,反射率为10.01％,电池的效率为18.00％,比普通多晶硅太阳电池的效率高2.19％,比未被扩孔处理的多晶黑硅太阳电池的效率高1.49％.