AlGaN/GaN/InGaN/GaN DH-HEMTs structure with an AlN interlayer grown by MOCVD

A novel AlGaN/GaN/GaN/GaN double heterojunction high electron mobility transistors (DH-HEMTS) structure with an AlN interlayer on sapphire substrate has been grown by MOCVD. The structure featured a 6-10 nm In0.1Ga0.9N layer inserted between the GaN channel and GaN buffer. And wer also inserted one ultrathin. AlN interlayer into the Al/GaN/GaN interface, which significantly enhanced the mobility of two-dimensional electron gas (2DEG) existed in the GaN channel. AFM result of this structure shows a good surface morphology and a low dislocation density, with the root-mean-square roughness (RMS) of 0.196 nm for a scan area of 5 mu m x 5 mu m. Temperature dependent Hall measurement was performed on this sample, and a mobility as high as 1950 cm(2)/Vs at room temperature (RT) was obtained. The sheet carrier density was 9.89 x10(12) cm(2), and average sheet resistance of 327 Omega/sq was achieved. The mobility obtained in this paper is about 50% higher than other results of similar structures which have been reported. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

High epitaxial growth rate of 4H-SiC using TCS as silicon precursor

High homoepitaxial growth of 4H-SiC has been performed in a home-made horizontal hot wall CVD reactor on n-type 4H-SiC 8 degrees off-oriented substrates in the size of 10 mm x 10 mm, using trichlorosilane (TCS) as silicon precursor source together with ethylene as carbon precursor source. Cross-section Scanning Electron Microscopy (SEM), Raman scattering spectroscopy and Atomic Force Microscopy (AFM) were used to determine the growth rate, structural property and surface morphology, respectively. The growth rate reached to 23 mu m/h and the optimal epilayer was obtained at 1600 degrees C with TCS flow rate of 12 seem in C/Si of 0.42, which has a good surface morphology with a low Rms of 0.64 nm in 10 mu mx10 mu m area.

AlGaN/AlN/GaN/InGaN/GaN DH-HEMTs with improved mobility grown by MOCVD

AlGaN/AlN/GaN/InGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) structures with improved buffer isolation have been investigated. The structures were grown by MOCVD on sapphire substrate. AFM result of this structure shows a good surface morphology with the root-mean-square roughness (RMS) of 0.196 nm for a scan area of 5 mu mx5 mu m. A mobility as high as 1950 cm(2)/Vs with the sheet carrier density of 9.89x10(12) cm(-2) was obtained, which was about 50% higher than other results of similar structures which have been reported. Average sheet resistance of 327 Omega/sq was achieved. The HEMTs device using the materials was fabricated, and a maximum drain current density of 718.5 mA/mm, an extrinsic transconductance of 248 mS/mm, a current gain cutoff frequency of 16 GHz and a maximum frequency of oscillation 35 GHz were achieved.

Heteroepitaxial Growth of 3C-SiC on Si (111) Substrate using AlN as a Buffer Layer

Using AlN as a buffer layer, 3C-SiC film has been grown on Si substrate by low pressure chemical vapor deposition (LPCVD). Firstly growth of AlN thin films on Si substrates under varied V/III ratios at 1100 degrees was investigated and the (002) preferred orientational growth with good crystallinity was obtained at the V/III ratio of 10000. Annealing at 1300 degrees C indicated the surface morphology and crystallinity stability of AlN film. Secondly the 3C-SiC film was grown on Si substrate with AlN buffer layer. Compared to that without AlN buffer layer, the crystal quality of the 3C-SiC film was improved on the AlN/Si substrate, characterized by X-ray diffraction (XRD) and Raman measurements.

Influence of Al content on electrical and structural properties of Si-doped AlxGa1-xN/GaN HEMT structures

Improved electrical properties of AlxGa1-xN/GaN high electron mobility transistor (HEMT) structures grown by metalorganic chemical vapor deposition (MOCVD) were achieved through increasing the Al mole fraction in the AlGaN barrier layers. An average sheet resistance of 326.6 Omega/sq and a good resistance uniformity of 98% were obtained for a 2-inch Al0.38Ga0 62N/GaN HEMT structure. The surface morphology of AlxGa1-xN/GaN HEMT structures strongly correlates with the Al content. More defects were formed with increasing Al content due to the increase of tensile strain, which limits further reduction of the sheet resistance. (c) 2006 WILEY-VCH Verlag GmbH & Co KGaA, Weinheim.

Optical and structural properties of ZnO films grown on Si(100) substrates by MOCVD - art. no. 60290G

High quality ZnO films have been successfully grown on Si(100) substrates by Metal-organic chemical vapor deposition (MOCVD) technique. The optimization of growth conditions (II-VI ratio, growth temperature, etc) and the effects of film thickness and thermal treatment on ZnO films' crystal quality, surface morphology and optical properties were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), and photoluminescence (PL) spectrum, respectively. The XRD patterns of the films grown at the optimized temperature (300 degrees C) show only a sharp peak at about 34.4 degrees corresponding to the (0002) peak of hexagonal ZnO, and the FWHM was lower than 0.4 degrees. We find that under the optimized growth conditions, the increase of the ZnO films' thickness cannot improve their structural and optical properties. We suggest that if the film's thickness exceeds an optimum value, the crystal quality will be degraded due to the large differences of lattice constant and thermal expansion coefficient between Si and ZnO. In PL analysis, samples all displayed only ultraviolet emission peaks and no observable deep-level emission, which indicated high-quality ZnO films obtained. Thermal treatments were performed in oxygen and nitrogen atmosphere, respectively. Through the analysis of PL spectra, we found that ZnO films annealing in oxygen have the strongest intensity and the low FWHM of 10.44 nm(106 meV) which is smaller than other reported values on ZnO films grown by MOCVD.

The study of high temperature annealing of a-SiC : H films

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 A) gold film was evaporated on the half area of the aSiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100 degrees C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

Growth and characterization of semi-insulating GaN films grown by MOCVD

High resistivity unintentionally doped GaN films were grown on (0001) sapphire substrates by metalorganic chemical vapor deposition. The surface morphology of the layer was measured by both atomic force microscopy and scanning electron microscopy. The results show that the films have mirror-like surface morphology with root mean square of 0.3 nm. The full width at half maximum of double crystal X-ray diffraction rocking curve for (0002) GaN is about 5.22 arc-min, indicative of high crystal quality. The resistivity of the GaN epilayers at room temperature and at 250 degrees C was measured to be approximate 10(9) and 10(6) Omega(.)cm respectively, by variable temperature Hall measurement. Deep level traps in the GaN epilayers were investigated by thermally stimulated current and resistivity measurements.

Homoepitaxial growth and MOS structures of 4H-SiC on off oriented n-type (0001)Si-faces

Homoepitaxial growth of 4H-SiC on off-oriented n-type Si-face (0001) substrates was performed in a home-made hot-wall low pressure chemical vapor deposition (LPCVD) reactor with SiH4 and C2H4 at temperature of 1500 C and pressure of 20 Torr. The surface morphology and intentional in-situ NH3 doping in 4H-SiC epilayers were investigated by using atomic force microscopy (AFM) and secondary ion mass spectroscopy (SIMS). Thermal oxidization of 4H-SiC homoepitaxial layers was conducted in a dry O-2 and H-2 atmosphere at temperature of 1150 C. The oxide was investigated by employing x-ray photoelectron spectroscopy (XPS). 4H-SiC MOS structures were obtained and their C-V characteristics were presented.

Hetero-epitaxial growth of ZnO films on silicon by low-pressure metal organic chemical vapor deposition

Quality ZnO films were successfully grown on Si(100) substrate by low-pressure metal organic chemical vapor deposition method in temperature range of 300-500 degrees C using DEZn and N2O as precursor and oxygen source respectively. The crystal structure, optical properties and surface morphology of ZnO films were characterized by X-ray diffraction, optical refection and atomic force microscopy technologies. It was demonstrated that the crystalline structure and surface morphology of ZnO films strongly depend on the growth temperature.

Substrate temperature dependence of ZnTe epilayers grown on GaAs(001) by molecular beam epitaxy

ZnTe thin films have been grown on GaAs(0 0 1) substrates at different temperatures with constant Zn and Te beam equivalent pressures (BEPs) by molecular beam epitaxy (MBE). In situ reflection high-energy electron diffraction (RHEED) observation indicates that two-dimensional (2D) growth mode can be established after around one-minute three-dimensional (3D) nucleation by increasing the substrate temperature to 340 degrees C. We found that Zn desorption from the ZnTe surface is much greater than that of Te at higher temperatures, and estimated the Zn sticking coefficient by the evolution of growth rate. The Zn sticking coefficient decreases from 0.93 to 0.58 as the temperature is elevated from 320 to 400 degrees C. The ZnTe epilayer grown at 360 degrees C displays the narrowest full-width at half-maximum (FWHM) of 660 arcsec from (0 0 4) reflection in double-crystal X-ray rocking curve (DCXRC) measurements. The surface morphology of ZnTe epilayers is strongly dependent on the substrate temperature, and the root-mean-square (RMS) roughness diminishes drastically with the increase in temperature.

Preparation and optical property of a novel hydrophobic anti-reflection coating for KDP crystal

A novel inorganic-organic hybrid hydrophobic anti-reflection silica film used for laser crystal was obtained by sol-gel process. The film consisted of silica sols mixed with a small amount of polymethyl methacrylate (PMMA) or polystyrene (PS). The optical transparency, hydrophobic property and surface morphology of the film were characterized by UV-VIS-NIR spectrophotometer; contact angle instrument and Scanning Electron Microscopy (SEM), respectively. The results showed that the anti-reflection coating had good hydrophobility and optical transparency from 400 nm to 1200 nm. The contact angle reached to 130-140 degrees. SEM images indicated the hydrophobic films modified with PMMA or PS had compact structure compared to the pure silica sol film. (C) 2008 Elsevier B.V. All rights reserved.

聚合物超分子薄膜表面形貌及性质研究

与本体和溶液中的相行为相比，在薄膜状态下，分子链的运动是在其内在因素和外场的共同作用下进行的。微相分离的最终结构形态除了与超分子的组成，分子量，相分离强度有关外，还与薄膜厚度，表面和界面对组分的选择性亲和作用等因素有关。因此，所提出的问题就是如何利用这些影响薄膜结构的因素，构建具有新颖结构和有序结构的薄膜，以及如何实现薄膜形貌及结构尺寸的调控。另外一个重要方面就是如何利用聚合物超分子本身特殊性质，实现超分子薄膜表面结构随外界条件下的响应性变化。在薄膜的功能性方面，薄膜结构以及组成与薄膜性质有着怎样的关系，如何通过调节薄膜的结构以及组成，来优化薄膜的性质。 本论文利用高分子之间，以及高分子与无机金属离子之间的的非共价键相互作用（静电相互作用，氢键，络合作用）得到聚合物超分子体系。研究其自组装形成纳米结构的影响因素和机理，掌握调控纳米结构的形态、尺寸、以及薄膜光学性质的规律，实现通过调控薄膜微结构来优化薄膜性质的目的。 首先，本论文对聚合物超分子薄膜表面形貌的构建和调控方面进行了研究。利用两种嵌段共聚物之间的氢键作用，实现聚合物超分子络合体溶解性在共溶剂中的下降。在动力学控制的条件下，体系中未形成氢键的组分充当了“桥梁”作用，使形成的纳米球状聚集体发生在一维方向的聚集，得到柱状纳米聚集体。当两种聚合物之间全部形成氢键时，发生柱状纳米聚集体到球状胶束聚集体的转变。研究了溶剂挥发速度，聚合物溶液粘度等对形成纳米结构薄膜的影响。利用静电相互作用，得到聚合物超分子结构。实现了具有规则纳米孔洞结构的聚合物超分子薄膜的制备。研究了溶剂性质，溶剂挥发速度，聚合物溶液粘度，环境温度等对形成规则孔洞的影响，以及薄膜表面结构随外界条件的响应性变化， 并提出了孔洞形成及演变机理。 基于对薄膜表面形貌和结构的调控，我们对薄膜结构与薄膜的光学性质之间的关系进行了研究。利用两种均聚物之间的氢键相互作用，得到超分子嵌段共聚物。超分子嵌段共聚物发生微观相分离可得到自组装薄膜，利用氢键是一种弱的相互作用，使用选择性溶剂将某一组分除去，得到具有纳米孔洞的薄膜。薄膜表现出很好的抗反射性，在可见光区实现了98.00%的单波长高透过。在近红外区实现了宽波抗反射效果，透光率可达到99.00%以上，而且波段可调。深入研究了薄膜的厚度，刻蚀时间对薄膜透光率的影响。利用金属和聚合物之间的络合相互作用，得到被聚合物稳定的银纳米粒子。受聚合物与银纳米粒子之间的电子转移吸收的影响，复合纳米材料在薄膜状态下表现出很好的光致发光性质。研究了材料之间的配比，溶液的浓度，聚合物的分子量对形成的复合纳米粒子的尺寸及光致发光性能的影响。

高分子混合物薄膜表面形态的形成与稳定性

本工作主要从实验上探索了组分的分子量、薄膜厚度、混合物的组成等因素对不相容高分子混合物薄膜表面形态的形成和稳定性的影响规律。在此基础上，利用软印刷的方法对基底表面进行图案化，来控制高分子混合物的相分离行为，以制备规则排列的有序图案。用原子力显微镜（AFM）、偏光显微镜（OM）、X-射线光电子能谱（XPS）等手段，系统研究了PS护MMA混合物薄膜的初始形态及其在退火过程中的形态演变规律。（1）随着PS分子量的增加，研究发现其形态的变化存在一过渡区域并予以合理解释；即：薄膜的初始形态由纳米尺度的相分离向宏观相分离的表面形态过渡；高温下退火时，其形态的演变存在两种过程。（2）对不同厚度的PS／PMMA混合物薄膜，在退火过程中观察到两种能长时间存在的新的表面形态，并阐明其稳定存在的原因；对PS护MMA混合物超薄膜，长时间退火时，在薄膜表面观察到二次相分离。（3）首次提出了利用混合物薄膜的相分离抑制去润湿的方法，即低浓度的PMMA可以抑制PS薄膜的去润湿行为，其稳定机理归结为PMMA优先向基底的表面分离而形成的厚度为少L个纳米的PMMA富相层。（4）研究了PS／P MMA混合物薄膜在OTS图案化基底上的形态变化，结果表明，薄膜的形态演变主要由横向受限和基底的表面自由能决定。通过控制受限程度和退火时间，可得到多种形态。以PS／P 4VP夕昆合体系为例，探索了如何利用高分子混合物薄膜的相分离制备具有规则排列的有序结构。研究表明，只有当PS的分子量相对较小时，在SAM图案化的基底表面，才比较容易形成规则排列的相分离图案。该规则排列的相分离图案具有良好的热稳定性。为深刻理解在旋涂过程中高分子聚集结构的变化情况，以SESS为例，用DFM研究了旋涂高分子薄膜的形成过程。当溶液浓度较低时，首次观察到了条状取向结构并提出该结构的形成机理，说明在旋涂过程中其复杂的剪切拉伸场可以引起柔顺性高分子线团或聚集体的取向。