光致电化学法提高垂直结构发光二极管出光效率的研究

研究了在垂直结构发光二极管(VLED)器件中, 光致电化学法(PEC)刻蚀N极性n-GaN的速率受不同刻蚀条件(刻蚀浓度、刻蚀时间和光照强度)的影响. 并选择N极性n-GaN表面含有较理想六角金字塔结构(侧壁角为31°)的样品制成器件, 研究PEC刻蚀对VLED的欧姆接触和光电性能的影响. 结果表明, 与未粗化样品相比, PEC刻蚀后的样品接触电阻率明显降低, 形成更好的欧姆接触; 其电学特性有较好的改善, 光输出功率有明显提高, 在20 mA电流下光输出功率增强了86.1%. 对不同金字塔侧壁角度的光提取效率用时域有限差分法(FDTD)模拟, 结果显示光提取效率在侧壁角度为20°– 40°有显著提高, 在23.6° (GaN-空气界面的全反射角)时达到最大.     

Comparison of laser-induced etching behavior of III–V compound semiconductors

Laser-induced maskless etching of III–V compound semiconductors (InSb, GaAs, and InP) in a KOH aqueous solution by irradiation with a focused argon-ion laser has been investigated to obtain high etching rates and aspect ratios of etched grooves. The etching rate at low laser power was found to depend on the carrier density of the sample and its type. With the increase of the laser power, the etching reaction becomes primarily a thermochemical reaction. High etching rates and aspect ratios have been achieved with a single scan of the laser beam. The damage induced by laser wet etching is less than that by laser dry etching, and the damage at the etched side wall is less than that at the etched bottom. Grooves with locally controlled depth and slab structures have been fabricated for application.     

Growth of ZnO Nanostructures on Porous Silicon and Oxidized Porous Silicon Substrates

We have investigated an oxidation of substrate effect on structural morphology of zinc oxide (ZnO) rods. ZnO rods are grown on porous silicon (PS) and on thermally oxidized porous silicon substrates by carbothermal reduction of ZnO powder through chemical vapour transport and condensation. Porous silicon is fabricated by electrochemical etching of silicon in hydrofluoric acid solution. The effects of substrates on morphology and structure of ZnO nanostructures have been studied. The morphology of substrates is studied by atomic force microscopy in contact mode. The texture coefficient of each sample is calculated from X-ray diffraction data that demonstrate random orientation of ZnO rods on oxidized porous silicon substrate. The morphology of structures is investigated by scanning electron microscopy that confirms the surface roughness tends to increase the growth rate of ZnO rods on oxidized PS compared with porous silicon substrate. A green emission has been observed in ZnO structures grown on oxidized PS substrates by photoluminescence measurements.     

Self-assembled MnN superstructure

Self-assembled MnN nanoislands have been prepared on Cu(001) substrate. The nanoislands show a square shape and a well-defined size. They are regularly arrayed with a periodicity of (3.5+/-0.1) nanometer and form a two-dimensional square superstructure. The MnN island superstructure is stabilized by a short-range mechanism. A structural model has been proposed to explain the self-assembly and the high quality of the superstructure.     

Preparation and characterization of MgO stepped surfaces

We present a study of the preparation procedure for stepped MgO surfaces which can be used as templates for the deposition of metallic nanostructures. A cleaved sample of MgO(0 0 1) was mechanically polished to reach the desired miscut angle along the [1 1 0] direction. Then a thermal annealing was performed. The effect of an intermediate chemical etching has been also studied. The surface was analyzed by means of contact AFM in air, LEED and XPS in UHV. The role of the chemical etching and the dependence of the final morphology on the annealing time and temperature were investigated. The influence of the miscut angle on the final surface topography is also briefly discussed.     

Exchange bias in patterned FeMn/NiFe bilayers

Electron beam lithography and ion beam etching have been used to pattern a wire-like array in FeMn/NiFe bilayers. The variation of hysteresis loops with the etching depth in FeMn layer has been presented, and it has been found that with increasing etching depth the coercivity increases and M–H loops show an asymmetric kink. Detailed studies of the magnetic behaviors of the asymmetric kink in the patterned sample with 3.5 nm thick FeMn layer have been performed, and a magnetization component perpendicular to the wire direction has been observed.     

Surface modification of medical PVC by remote oxygen plasma

Surface modification of medical PVC by remote oxygen plasma has been studied: surface structure and performance of treated material are analyzed by weight loss rate, contact angle measurement and XPS. With the increase of sample distance from the plasma source, weight loss rate drops, electrons and ions and etching action are restrained; and C=O groups are found in the treated PVC surface. The surface wettability of PVC film can be improved by remote low-temperature plasma and the radical reaction is intensified.     

等离子体刻蚀工艺中的光学发射光谱仪数据的模型研究(英文)

原位光学发射光谱仪(OES)已经成为等离子体刻蚀工艺控制过程中的一种非常有潜力的在线传感器系统。采用光谱仪实时采集高密度等离子体刻蚀机中的OES光谱数据,利用BP神经网络算法对特定波长的OES数据进行分析及建模,以便对等离子体刻蚀工艺过程进行反馈控制。由刻蚀产物SiCl发出的405nm谱线被选为特征谱线来确定刻蚀工艺过程的终点。     

Etching nano-holes in silicon carbide using catalytic platinum nano-particles

The catalytic reaction of platinum during a hydrogen etching process has been used to perform controlled vertical nanopatterning of silicon carbide substrates. A first set of experiments was performed with platinum powder randomly distributed on the SiC surface. Subsequent hydrogen etching in a hot wall reactor caused local atomic hydrogen production at the catalyst resulting in local SiC etching and hole formation. Secondly, a highly regular and monosized distribution of Pt was obtained by sputter deposition of Pt through an Au membrane serving as a contact mask. After the lift-off of the mask, the hydrogen etching revealed the onset of well-controlled vertical patterned holes on the SiC surface. PACS 61.46.+w; 68.37.-d; 81.65.Cf     

Fabrication of regular mesoscopic networks of GaAs wires

A novel and simple approach to nanolithography has been developed as a technique for fabricating two-dimensional periodic mesoscopic (submicrometre) networks of semiconductor wires with a feature size down to about 20 nm. The etching mask is made by coating the material's surface with an initially structured polymer network . This is obtained by self-organized patterning in a complex liquid (nitrocellulose solution). Reactive ion etching is used for GaAs surface patterning. This technique can find potential applications in many areas of science and technology which use low-dimensional and mesoscopic devices.     

Electrical properties of the CdTe back contact: A new chemically etching process based on nitric acid/acetic acid mixtures

The performance of the back contact is one of the major issues of CdTe solar cell research. Standard nitric-phosphoric (NP) acid chemical etching before metallization is widely used to improve contact formation. However, previous studies of this traditional etching method indicated a blocking Schottky barrier at the back contact, and a roll-over phenomenon was found in the J-V curves of the CdTe solar cells. In this work, a new etching solution, i.e. a nitric-acetic (NA) acid was employed. The etching rate was slow and a Te-rich layer was formed on the surface, which was less than 1 nm. The CdTe solar cell with this new etching method showed no roll-over phenomenon and displayed a good ohmic back contact performance. XPS analysis demonstrated that the back contact barrier height was close to those of CdTe with standard NP etching. A possible mechanism was presented for the improvement of back contact properties.     

Optical diffraction by two-dimensional photonic structures with hexagonal symmetry

Photonic structures with hexagonal symmetry have been prepared by the additive technology of two-photon laser lithography, and their optical properties have been investigated. The structure of the samples has been examined using scanning electron microscopy. The calculations have been performed for the optical diffraction in the Born approximation of the scattering theory for structures with a limited number of scatterers. The images formed in the monochromatic light on a flat screen located behind the sample have been calculated. The diffraction patterns on the screen have C_6v symmetry and consist of three straight lines intersecting at an angle of 120° and hyperbolas, the number of which is a multiple of six. An important feature of these diffraction patterns is the superstructure, i.e., the partition of straight lines and hyperbolas into individual diffraction reflections, the number of which is determined by the number of scatterers of a particular sample. The results of the experimental investigation of the diffraction patterns completely coincide with the calculated data, including the number and arrangement of the superstructure reflections.     

Observations of fission-tracks in zircons by atomic force microscope

The fission-track (FT) method is a dating technique based on the observation of damage (tracks) by spontaneous fission of ²³⁸U left in a mineral. The date is calculated from the track density and the uranium concentration in the mineral. This is possible because the number of tracks is a function of uranium concentration and time since the start of track accumulation. Usually, the number of tracks is counted under an optical microscope after etching (chemical expansion of a track). However, as FT density per unit area rises, it becomes difficult to count the number of tracks. This is due to the fact that FTs overlap one another and are unable to be readily distinguished. This research examines the potential of atomic force microscope (AFM) for FT dating using zircons, which are likely to show higher FT density than other minerals due to their high U concentrations.To obtain an AFM image for a sample prepared for FT dating, removing the static electricity of the sample is essential to avoid an unexpected movement of the cantilever. A grain should be wider than about 30 μm to bring the cantilever on the mineral surface. Polishing with a fine grained compound is very important. There is not much difference in sharpness between images by AC mode (scanning with vibrating cantilever at a constant cycle) and Contact mode (scanning with the cantilever always in close contact with the surface). To confirm how tracks can be identified with the AFM, an AFM image was compared with an image obtained with the optical microscope. When change in the number of tracks and their shapes were observed through stepwise etching, the track expanded as the etching time increased. In addition, the etching rate was slower for large tracks than those for small tracks. This implied that the AFM can be used to observe etching of zircons with different degrees of nuclear fission damage. A track that could not be seen with the optical microscope due to insufficient etching could be observed by AFM methods, indicating the possibility of FT dating with high track densities using AFM after relatively short etching periods.     

Submicron-resolution ablation with a KrF excimer laser beam patterned with a projection lens

A mask projection system working with KrF laser radiation is described. This system produces microscopic laser beams able to pattern gratings on the surface of various materials in the micrometer range by direct etching. Models have been developed to simulate the beam intensity profile on the sample. Various polymers have been irradiated, and their experimental profiles are compared with theoretical ones. Different ablation behaviors have been evidenced. Interesting consequences of thermal effects at this submicron scale are reported.     

Absolute coverage of C2H2 and C2H4 on Pt(111)

Quantitative XPS measurements have been performed in order to determine the absolute coverage of acetylene and ethylene adsorbed on Pt(111) showing a 2 × 2 LEED pattern. This LEED pattern has so far been attributed to a 2 × 2 superstructure with a coverage of 0.25. A quantitative evaluation of the C(1s) peak intensities for these adsorbed layers in comparison with adsorbed CO shows that the coverage is 0.5 instead of 0.25. Therefore the 2 × 2 LEED pattern should be assigned to a 2 × 1 superstructure in three domains rather than a 2 × 2 superstructure.     

Surface roughness analysis of hydrophilic SiO2/TiO2/glass nano bilayers by the level crossing approach

The effect of etching time on the statistical properties of hydrophilic surfaces of SiO₂/TiO₂/glass nano bilayers has been studied using atomic force microscopy (AFM) and a stochastic approach based on a level crossing analysis. We have created rough surfaces of the hydrophilic SiO₂/TiO₂ nano bilayer system by using 26% potassium hydroxide (KOH) solution. Measuring the average apparent contact angle allowed us to assess the degree of hydrophilicity, and the optimum condition was determined to be 10 min etching time. A level crossing analysis based on AFM images provided deeper insight into the microscopic details of the surface topography. With different etching times, it has been shown that the average frequency of visiting a height with positive slope behaves in a Gaussian manner for heights near the mean value and obeys a power law for heights far away from the mean value. Finally, by applying the generalized total number of crossings with positive slope, it was found that the both high heights and deep valleys of the surface have a great effect on the hydrophilic degree of the SiO₂/TiO₂/glass nano bilayer investigated system.     

垂直非结构模型应用于换热网络优化

节点非结构模型优化换热网络时会出现换热器交叉排列的情况,分析发现在相同传热负荷下交叉结构比垂直结构换热单元总传热面积增大,同时明显降低算法的计算效率.基于此,建立垂直非结构模型.与节点非结构模型相比,垂直非结构模型降低了优化过程中的计算复杂度,并且能够消除交叉结构带来的不利影响,有效提升了优化的效率及求解精度.应用两个算例对基于垂直非结构模型的强制进化随机游走算法（RWCE）进行优化性能分析,取得了优于文献的结果.     

Fabrication of flower-shaped Bi2O3 superstructure by a facile template-free process

A novel flower-shaped Bi₂O₃ superstructure has been successfully synthesized by calcination of the precursor, which was prepared via a citric acid assisted hydrothermal process. The precursor and Bi₂O₃ were characterized with respect to morphology, crystal structure and elemental chemical state by field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). It was shown that both the precursor and Bi₂O₃ flower-shaped superstructure were constructed of numerous nanosheets while the nanosheets consisted of a great deal of nanoparticles. Furthermore, key factors for the formation of the superstructures have been proposed; a mechanism for the growth of the superstructure has been presented based on the FESEM investigation of different growth stages.     

Photoreflectance study of plasma-etched semi-insulating GaAs substrates treated

Semi-insulating GaAs(100) substrates subjected to ion plasma etching in different regimes have been investigated by photoreflectance spectroscopy. The photoreflectance spectra of the processed samples exhibit Franz-Keldysh oscillations, which indicate a decrease in the defect density in the surface region. On the basis of the experimental data and the results of simulation of the photoreflectance spectra, optimal regimes of sample etching have been found.     

虹吸动态化学腐蚀法制备近场光学显微镜光纤探针的研究

基于虹吸原理 ,设计了一种动态化学腐蚀法的简易装置 ,用于制备近场光学显微镜光纤探针。在一般化学腐蚀法的基础上 ,通过改变虹吸管中水的流向和流速来有效地控制探针锥角和锥长 ,制备出多种形貌的光纤探针。与传统的静态化学腐蚀法相比 ,该法具有重复性高、探针形貌可控、操作方便、实验费用低廉、制备的探针表面光滑等优点。利用该装置 ,成功地制备出针尖尺寸 5 0～ 30 0nm ,针尖锥角在 16°～ 6 5°之间可调的光纤探针。同时 ,选择适当的液位差 ,通过一步腐蚀法制备出针尖尺寸小于 5 0nm ,针尖锥角 12 5°的双锥角光纤探针。并对可能的腐蚀机理进行了探讨。