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.

Optimization of VI/II pressure ratio in ZnTe growth on GaAs(001) by molecular beam epitaxy

ZnTe epilayers were grown on GaAs(0 0 1) substrates by molecular beam epitaxy (MBE) at different VI/II beam equivalent pressure (BEP) ratios (R-VI/II) in a wide range of 0.96-11 with constant Zn flux. Based on in situ reflection high-energy electron diffraction (RHEED) observation, two-dimensional (2D) growth mode can be formed by increasing the R-VI/II to 2.8. The Te/Zn pressure ratios lower than 4.0 correspond to Zn-rich growth state, while the ratios over 6.4 correspond to Te-rich one. The Zn sticking coefficient at various VI/II ratios are derived by the growth rate measurement. The ZnTe epilayer grown at a R-VI/II of 6.4 displays the narrowest full-width at half-maximum (FWHM) of double-crystal X-ray rocking curve (DCXRC) for (0 0 4) reflection. Atomic force microscopy (AFM) characterization shows that the grain size enlarges drastically with the R-VI/II. The surface root-mean-square (RMS) roughness decreases firstly, attains a minimum of 1.14 nm at a R-VI/II of 4.0 and then increases at higher ratios. It is suggested that the most suitable R-VI/II be controlled between 4.0 and 6.4 in order to grow high-quality ZnTe epitaxial thin films.

Multistage etching process for microscopically smooth tellurite glass surfaces in optical fibers

Bulk samples of tellurite glass with composition 75TeO(2)-20ZnO-5Na(2)O (TZN) were fabricated by melting and quenching techniques. In order to improve the surface quality of optical fiber preform made with this tellurite glass, the authors developed a multistage etching process. The relationship between successive etching treatments and roughness of the TZN glass surface was probed by using an atomic force microscope. The results demonstrate that this multistage etching method effectively improves this tellurite glass surface smoothness to a level comparable with that of a reference silica glass slide, and the corresponding chemical micromechanisms and fundamentals are discussed and confirmed by atomic force microscopy, potentially contributing to the development of multicomponent soft glass fibers and devices. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3437017]

Au(111)自组装膜表面原子力显微镜纳米刻蚀

在Au(111)硫醇自组装膜上通过原子力显微镜（AFM）纳米刻蚀加工了各种功能化纳米结构。本论文取得的主要成果如下： 1. “蘸笔”技术移除自组装膜及金基底 以烯丙基溴为墨水，用蘸笔技术在16－巯基十六羧酸（MHA）修饰的Au(111)基底上可以实现对金基底的直接加工，形成阴刻图案；并提出了详细的加工机理。 2. Au(111)自组膜表面的可逆加工 （1）利用乙醇修饰的导电AFM针尖通过电致刻蚀在Au(111)基底的烷基硫醇自组装膜上实现了阴阳刻图案的可逆加工。X射线光电子能谱分析表明阳刻图案的化学组成为氧化金，且此氧化金可被乙醇还原生成金得到阴刻图案。此阴刻图案可用作纳米模板，我们利用此模板加工了氧化铁磁性纳米结构和溶菌酶蛋白质纳米结构。 （2）用修饰硫醇的导电AFM针尖在Au(111)基底上实现了硫醇的可逆书写。 3. Au(111)自组膜表面官能团转化 用硫醇自组装膜修饰的Pt针在Au(111)基底的烷基硫醇自组装膜上实现了表面端基官能团转化。所得亲水性纳米图案可用作物理（如直接吸附纳米粒子）和化学纳米模板（如诱导硅烷自组装）。加工过程中基底自组装膜保持完整，未破坏Au-S键。 4. AFM研究适配子与血小板衍生生长因子－BB的相互作用 利用AFM电致刻蚀在HS-(CH2)12-(OCH2CH2)4-OH（EG4）/Au(111)上选择性移除EG4，在新暴露的金基底上生长血小板衍生生长因子－BB（PDGF-BB）适配子与EG4的混合自组装膜，通过PDGF-BB适配子与 PDGF-BB的特异性键合固定PDGF-BB，得到PDGF-BB的纳米图案。

平面酞菁弱取向外延薄膜的KPFM研究

载流子传输一直是有机半导体研究中最核心的问题，通过弱外延生长方法可制备高取向、大面积连续的平面酞菁化合物薄膜，其迁移率可达到单晶水平。本论文中应用建立起来的有机半导体开尔文探针力显微镜(KPFM)表征方法，从微观水平研究了这种弱取向外延薄膜和常规的多晶有机半导体薄膜中结构和电特性的关系。 1. 建立了有机半导体的KPFM表征方法。KPFM用微探针方法同时获得样品的表面形貌和电势，本文第二章中论述了KPFM的工作原理、操作方法和成像机制，解释了相关的有机半导体表面态的问题。并且定量验证我们所建立的这种方法的可靠性。 2. 运用KPFM研究了多晶态的有机半导体CuPc和F16CuPc，直接观察到晶界势垒的存在，说明CuPc晶界处存在类施主的缺陷能级，F16CuPc晶界处存在类受主的缺陷能级，这两两种缺陷态分别俘获空穴和电子，使晶界周围的载流子耗尽而形成空间电荷区，限制了载流子的传输，我们从实验的角度证明了有机半导体中晶界限制载流子传输的理论。另外得到多晶CuPc高能量分辨的局域功函图像，越是π电子暴露的表面其功函越高。 3. 在导电的H-Si(111)衬底上制备出6P诱导弱取向生长的CuPc和F16CuPc薄膜，比较了和SiO2衬底上的弱外延生长薄膜的特征。KPFM观察到在CuPc界面处的能带弯曲和空穴累积，这大大降低了载流子传输沟道内CuPc晶界的势垒，从而其迁移率得到很大提高，这说明6P除了具有在结构上诱导使酞菁取向的作用外，还具有电子结构上的效应。另外研究了6P弱外延生长的亚单层F16CuPc薄膜，直接观察到在6P-F16CuPc界面存在强烈的电荷累积，这从微观上说明了有机异质结的电特性。

PMMA/SAN共混物薄膜表面相分离动力学的原位AFM研究

本论文主要借助原子力显微镜（AFM）、X-射线光电子能谱（XPS）等实验手段，纳米尺度上在线原位地研究了高温临界组成PMMMA/SAN（50／50，w／w）共混薄膜体系，分析并探讨了此共混薄膜表面相分离及其超薄膜体系的润湿／去润湿和相分离行为。首先，从敲击式AFM中相位图的成像原理出发，建立了一个高温下用原位AFM定性鉴别不同聚合物的纳米尺寸微区的方法，即：先在基底（硅）上铺展一层非常平坦的单一组分（SAN）薄膜，再在此千膜上，通过旋涂由选择性溶剂（冰醋酸）配制的另一组分（PMMA）的极稀溶液，使之不能形成一层连续的完整膜，最后在高温（175℃）下用敲击模式的原子力显微镜（TM-AFM）检测相位图随退火时间的变化。得到在175℃的相位图中，PMMA富相比SAN的富相显得更暗，这为后续工作提供了定性鉴别相区的方法。其次，高温下用原位AFM研究了PMMA/SAN薄膜表面相分离过程，在线观察了相分离的归并过程，定量地得到了临界相分离温度，并给出了特征波矢对时间依赖关系的标度指数，划分了相分离动力学演变的不同阶段。对于膜厚约为130nm的体系，表面相分离的临界温度大约为165℃，其表面相分离的特征波矢与时间的标度关系q+（t）-t-n，在整个实验时间内，随时间的演变过程中，显示了两个不同的指数变化区，即前期很慢的n=0.13和后期的n＝1／3。0.13的指数关系可能是由于表面聚合物链的几何受限及表面富集相把部分新生成的另一相覆盖所致，1／3的指数关系可以认为是由普遍的Brownian扩散所致。而对于膜厚为50曲的此组成共混薄膜，得到的标度关系与C汕n线性理论吻合得非常好的Spinodal Deposition（SD）表面相分离的初期，即n＝0，这是由于降低膜厚 增加了共混物的相容性，提高了临界温度，从而减缓了相分离过程，使得在我 们观测的时间范围内更易观察到表面相分离的初期。再次，用阶梯式降温的好M和XPS，发现了PMMA/SAN体系原位和离位实验结果存在巨大差别的主要原因之一—润湿温度的存在。对PMMA/SAN（50／50，w／w，-130nm）共混薄膜体系，原位AFM的退火和准淬火实验表明， 虽然在高温和室温、真空和常态、原位和离位，相分离的聚合物共混薄膜表面 形貌变化不大，但是表面物理性质却有很大的区别。原位XPS实验表明，未经 任何处理的样品在185℃退火时，很快在离表面很薄的下面形成一层PMMA含 量远低于本体值的薄层，之后最表面的PMMA也往本体迁移，直至表面SAN 的含量远高于它的本体值。对已在175℃退火20hr的此样品，逐渐降温退火过 程的原位XPS表明，当退火温度降至145oC时，样品表面几乎完全被PMMA覆盖，很好地验证了Conlposto等提出的润湿温度的假设，即对此体系，润湿温 度是原位和离位存在巨大的差异的主要因素之一，这与原位AFM的实验结果也，存在很好的一致性。最后，高温下原位观察了硅基底上PMMA/SAN超薄膜（膜厚-Rg）的去润湿、相分离过程和PMMA在基底上的润湿过程，以及云母基底上的该体系高温下的稳定润湿行为。以硅为基底的PMM刀sAN共混超薄膜在155”c下发生了类 似sPinodal dewetting的去润湿，其原因可能是在垂直于基底的浓度梯度引起的表面组成的涨落，对这种去润湿的动力学还进行了探讨。以硅为基底的PMMA/SAN共混超薄膜在175℃下不仅发生了类似spinodal dewetting的去润 湿，而且还发生了去润湿液滴内部的相分离以及相分离析出的PMMA润湿硅基底的过程，提出了一个简单的模型描述了这种复杂的过程。此外，还研究了不同基底对此共混超薄膜体系的润湿行为进行了研究，以云母为基底的此共混体系在175℃下长时间内是稳定的，其原因可能是云母与PMMA和SAN均有很强的作用力，足于补偿聚合物链在构象嫡上的损失。

Structural ordering and interface morphology in symmetrically strained (GaIn)As/Ga(PAs) superlattices grown on off-oriented GaAs(100)

In this work we investigate the structural properties of symmetrically strained (GaIn)As/GaAs/Ga(PAs)/GaAs superlattices by means of x-ray diffraction, reciprocal-space mapping, and x-ray reflectivity. The multilayers were grown by metalorganic vapor-phase epitaxy on (001) GaAs substrates intentionally off-oriented towards one of the nearest [110] directions. High-resolution triple-crystal reciprocal-space maps recorded for different azimuth angles in the vicinity of the (004) Bragg diffraction clearly show a double periodicity of the x-ray peak intensity that can be ascribed to a lateral and a vertical periodicity occurring parallel and perpendicular to the growth surface. Moreover, from the intensity modulation of the satellite peaks, a lateral-strain gradient within the epilayer unit cell is found, varying from a tensile to a compressive strain. Thus, the substrate off-orientation promotes a lateral modulation of the layer thickness (ordered interface roughness) and of the lattice strain, giving rise to laterally ordered macrosteps. In this respect, contour maps of the specular reflected beam in the vicinity of the (000) reciprocal lattice point were recorded in order to inspect the vertical and lateral interface roughness correlation, A semiquantitative analysis of our results shows that the interface morphology and roughness is greatly influenced by the off-orientation angle and the lateral strain distribution. Two mean spatial wavelengths can be determined, one corresponding exactly to the macrostep periodicity and the other indicating a further interface waviness along the macrosteps. The same spatial periodicities were found on the surface by atomic-force-microscopy images confirming the x-ray results and revealing a strong vertical correlation of the interfaces up to the outer surface.

Boron-doped silicon film as a recombination layer in the tunnel junction of a tandem solar cell

Boron-doped hydrogenated silicon films with different gaseous doping ratios (B_2H_6/SiH_4) were deposited in a plasma-enhanced chemical vapor deposition (PECVD) system. The microstructure of the films was investigated by atomic force microscopy (AFM) and Raman scattering spectroscopy. The electrical properties of the films were characterized by their room temperature electrical conductivity (σ) and the activation energy (E_a). The results show that with an increasing gaseous doping ratio, the silicon films transfer from a microcrystalline to an amorphous phase, and corresponding changes in the electrical properties were observed. The thin boron-doped silicon layers were fabricated as recombination layers in tunnel junctions. The measurements of the Ⅰ-Ⅴ characteristics and the transparency spectra of the junctions indicate that the best gaseous doping ratio of the recombination layer is 0.04, and the film deposited under that condition is amorphous silicon with a small amount of crystallites embedded in it. The junction with such a recombination layer has a small resistance, a nearly ohmic contact, and a negligible optical absorption.

Growth of 0.55eV-GaInAsSb Quaternary Alloy Films for a Thermophotovoltaic Device by Liquid Phase Epitaxy

Lattice matched Ga_(1-x)In_xAs_ySb_(1-y) quaternary alloy films for thermophotovoltaic cells were successfully grown on n-type GaSb substrates by liquid phase epitaxy. Mirror-like surfaces for the epitaxial layers were achieved and evaluated by atomic force microscopy. The composition of the Ga_(1-x)In_xAs_ySb_(1-y) layer was characterized by energy dispersive X-ray analysis with the result that x = 0.2, y = 0.17. The absorption edges of the Ga_(1-x)In_xAs_ySb_(1-y) films were determined to be 2. 256μm at room temperature by Fourier transform infrared transmission spectrum analysis, corresponding to an energy gap of 0.55eV. Hall measurements show that the highest obtained electron mobility in the undoped p-type samples is 512cm2~/(V·s) and the carrier density is 6. 1×10~(16)cm~(-3) at room temperature. Finally, GaInAsSb based thermophotovoltaic cells in different structures with quantum efficiency values of around 60% were fabricated and the spectrum response characteristics of the cells are discussed.

Fabrication of Silicon Crystal-Facet-Dependent Nanostructures by Electron-Beam Lithography

Silicon crystal-facet-dependent nanostructures have been successfully fabricated on a (100)-oriented silicon-on-insulator wafer using electron-beam lithography and the silicon anisotropic wet etching technique. This technique takes ad-vantage of the large difference in etching properties for different crystallographic planes in alkaline solution. The mini-mum size of the trapezoidal top for those Si nanostructures can be reduced to less than 10nm. Scanning electron microscopy(SEM) and atomic force microscopy (AFM) observations indicate that the etched nanostructures have controllable shapes and smooth surfaces.

DCXRD Investigation of a Ge/Si（001） Island Multilayer Structure

A Ge/Si（001） island multilayer structure is investigated by double crystal X-ray diffraction, transmission electron microscopy,and atomic force microscopy. We fit the satellite peaks in the rocking curve by two Lorentz lineshapes, which originate from the wetting layer region and the island region. Then from the ratio of the thick- nesses of the Si and Ge （GeSi） layers as determined by TEM,tbe Ge compositions of the wetting layer and islands are estimated to be about 0. 51 and 0. 67, respectively,according to the positions of the fitted peaks. This proves to be a relatively simple way to investigate the Ge/Si （001） island multilayer structure.

MOCVD-Grown AlGaN/AlN/GaN HEMT Structure with High Mobility GaN Thin Layer as Channel on SiC

AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with a high-mobility GaN thin layer as a channel are grown on high resistive 6H-SiC substrates by metalorganic chemical vapor deposition. The HEMT structure exhibits a typical two-dimensional electron gas (2DEG) mobility of 1944cm2/(V · s) at room temperature and 11588cm2/(V· s) at 80K with almost equal 2DEG concentrations of about 1.03 × 1013 cm-2 High crystal quality of the HEMT structures is confirmed by triple-crystal X-ray diffraction analysis. Atomic force microscopy measurements reveal a smooth AlGaN surface with a root-mean-square roughness of 0. 27nm for a scan area of 10μm × 10μm. HEMT devices with 0.8μm gate length and 1.2mm gate width are fabricated using the structures. A maximum drain current density of 957mA/mm and an extrinsic transconductance of 267mS/mm are obtained.

1.3μm InGaAs/InAs/GaAs Self-Assembled Quantum Dot Laser Diode Grown by Molecular Beam Epitaxy

The growth of multi-layer InGaAs/InAs/GaAs self-assembled quantum dots （QDs） by molecular beam epitaxy （MBE） is investigated,and a QD laser diode lasing at 1.33μm in continuous operation mode at room temperature is reported. The full width at half maximum of the band edge emitting peaks of the photoluminescence （PL） spectra at room temperature is less than 35meV for most of the multi-layer QD samples,revealing good,reproducible MBE growth conditions. Moreover,atomic force microscopy images show that the QD surface density can be controlled in the range from 1×10^10 to 7 ×10^10 cm^-2 . The best PL properties are obtained at a QD surface density of about 4×10^10cm^-2. Edge emitting lasers containing 3 and 5 stacked QD layers as the active layer lasing at room temperature in continuous wave operation mode are reported.

Homoepitaxial Growth and Characterization of 4H-SiC Epilayers by Low-Pressure Hot-Wall Chemical Vapor Deposition

Horizontal air-cooled low-pressure hot-wall CVD (LP-HWCVD) system is developed to get highly qualitical 4H-SiC epilayers.Homoepitaxial growth of 4H-SiC on off-oriented Si-face (0001) 4H-SiC substrates is performed at 1500℃ with a pressure of 1.3×103Pa by using the step-controlled epitaxy.The growth rate is controlled to be about 1.0μm/h.The surface morphologies and structural and optical properties of 4H-SiC epilayers are characterized with Nomarski optical microscope,atomic force microscopy (AFM),X-ray diffraction,Raman scattering,and low temperature photoluminescence (LTPL).N-type 4H-SiC epilayers are obtained by in-situ doping of NH3 with the flow rate ranging from 0.1 to 3sccm.SiC p-n junctions are obtained on these epitaxial layers and their electrical and optical characteristics are presented.The obtained p-n junction diodes can be operated at the temperature up to 400℃,which provides a potential for high-temperature applications.

Single Layer Growth of Strained Epitaxy at Low Temperature

Contacting mode atomic force microscopy (AFM) is used to measure the In0.asGao.65As/GaAs epilayer grown at low temperature (460°C). Unlike the normal layer-by-layer growth (FvdM mode) or self-organized islands growth (SK mode) ,samples grown under 460 C are found to be large islands with atomic thick terraces. AFM measurements reveale near one monolayer high steps. This kind of growth is good between FvdM and SK growth modes and can be used to understand the evolution of strained epitaxy from FvdM to SK mode.