Optimization of the preparation of GaN-based specimens with low-energy ion milling for (S)TEM

We report on optimization of electron transparent GaN based specimens for transmission electron microscopy (TEM) and scanning TEM (STEM) studies by combining focused ion beam thinning and low-energy (≤500 eV) Ar-ion milling. Energy dependent ion milling effects on GaN based structures are investigated and the quality of ion milled samples is compared with that of specimens prepared by wet chemical etching. Defects formed during ion milling lead to amorphization of the specimen. The experimental results are compared with Monte-Carlo simulations using the SRIM (stopping and range of ions in matter) software. Specimen thickness was deduced from high-angle annular dark field STEM images by normalization of measured intensities with respect to the intensity of the scanning electron probe and comparison with multislice simulations in the frozen lattice approach. The results show that the thickness of the amorphous surface layer can be successfully reduced below 1 nm by low energy ion milling, leading to a homogeneous image contrast in TEM and STEM, so that good conditions for quantitative analysis can be achieved. For an ion energy of 400 eV the thickness measurements resulted in an etching rate of about 6-8 nm/min.     

石英衬底上GaN薄膜沉积及其光学性能的研究

GaN薄膜材料广泛应用于发光二极管(LED),激光二极管(LD)等光电器件。但是GaN基器件的制备与应用以及器件推广很大一部分取决于其器件的价格,常用的方式是在单晶蓝宝石衬底上沉积制备GaN薄膜样品,单晶蓝宝石衬底晶向择优,可以制备出高质量的GaN薄膜样品,但是单晶蓝宝石衬底价格昂贵,一定程度上限制了其GaN基器件推广使用。如何在廉价衬底上直接沉积高质量的GaN薄膜,满足器件的要求成为研究热点。石英玻璃价格廉价,但是属于非晶体,没有择优晶向取向,很难制备出高质量薄膜样品。本研究采用等离子体增强金属有机物化学气相沉积系统在非晶普通石英衬底上改变氮气反应源流量低温制备GaN薄膜材料。制备之后采用反射高能电子衍射谱、X射线衍射光谱、室温透射光谱和光致光谱对制备的薄膜进行系统的测试分析。其结果表明：在氮气流量适当的沉积参数条件下,所制备的薄膜具有高C轴的择优取向,良好的结晶质量以及优异的光学性能。     

A simple, quick and reliable method for TEM cross-section preparation of ceramic oxide films on thin metal substrates

We present a preparation method of cross-sectional thin foils for transmission electron microscopy (TEM). Samples are 0.1-1 m thick ceramic oxide films (CeO2, CeO2-YBa2Cu3O7 and CeO2-ZrO2/YO2-YBa2Cu3O7) epitaxially grown on 30-100 m highly textured nickel substrates. This method includes gluing the sample between a copper oxide dummy and a silicon dummy, followed by mechanical polishing and conventional ion milling. TEM cross-sectional samples obtained with this selection of dummies are electron-transparent up to a few tens of m parallel to the film surface. Several layer structures were analyzed by TEM and the results are shown. The preparation technique described here can be applied to any type of oxide film deposited on thin metal substrates.     

A new method for preparing powders for transmission electron microscopy examination

In this paper we describe a novel method to prepare powder specimens for transmission electron microscopy examination. The powder samples are embedded in a metallic matrix by a route based on the plastic flow of a soft metal, using a small laboratory type hand driven hydraulic press. The resulting composites are processed with the conventional procedure based on grinding polishing and ion beam milling. The resulting TEM specimens have a self-supporting structure, good thermal and electrical conductivity while showing a well-polished surface resulting from the ion milling process. The method can be applied to a large variety of samples with sufficiently strong mechanical properties; a few examples are reported. The limits, mainly due to the mechanical toughness of the powder, are discussed.     

斜切蓝宝石衬底MOCVD生长GaN薄膜的透射电镜研究

利用MOCVD技术在斜切角度为0.3°的c面蓝宝石衬底上生长了非故意掺杂 GaN 薄膜, 并采用透射电子显微镜对材料的质量和材料内部缺陷进行了分析. 研究发现斜切蓝宝石衬底上外延的GaN材料中,位错在距离衬底0.8 μm附近大量湮灭, 同时位错扎堆出现.基于上述现象, 提出了斜切衬底上GaN材料中位错的湮灭机制, 解释了斜切衬底能够提高GaN晶体质量的原因.     

High contrast hollow-cone dark field transmission electron microscopy for nanocrystalline grain size quantification

In this paper, we describe hollow-cone dark field (HCDF) transmission electron microscopy (TEM) imaging, with a slightly convergent beam, as an improved technique that is suitable to form high contrast micrographs for nanocrystalline grain size quantification. We also examine the various factors that influence the HCDF TEM image quality, including the conditions of microscopy (alignment, focus and objective aperture size), the properties of the materials imaged (e.g., atomic number, strain, defects), and the characteristics of the TEM sample itself (e.g., thickness, ion milling artifacts). Sample preparation was found to be critical and an initial thinning by wet etching of the substrate (for thin film samples) or tripod polishing (for bulk samples), followed by low-angle ion milling was found to be the preferred approach for preparing high-quality electron transparent samples for HCDF imaging.     

The application of FIB milling for specimen preparation from crystalline germanium

The effectiveness of focused ion beam (FIB) for preparation of crystalline germanium specimens has been studied. FIB milling results in strong cellular relief of the germanium surfaces on bulk specimens. This cellular relief, associated with the generation of high densities of point defects during interaction of the specimen with the high-energy gallium beam, can be reduced by using either a lower ion beam currents or a lower beam energy. Even under these milling conditions the cellular relief is, however, still evident on the surface of the TEM specimens as evidenced by so-called 'curtaining' relief. Nevertheless good quality specimens for both conventional and high-resolution imaging may be prepared using FIB milling if low currents are employed for final milling.     

Die Herstellung dünner Silizium-Einkristallschichten durch Kathodenzerstäubung

Thin silicon single-crystal films some 100 Å thick have been prepared from silicon single-crystal disks by mechanical polishing and subsequent bombardment with argon-ions of 1500 eV energy in a controlled Penning-discharge (at 1 micron argon pressure). Mechanical polishing prevented the formation of etch pits during ion bombardment, so that specimens with large thin areas of some 100 microns diameter can be obtained. The silicon disks remain single-crystal during the thinning process, the films were tested by means of electron-diffraction, the transmission patterns show sharp Kikuchi-lines with an angular width of 1 or 2 minutes.     

高阻GaN的MOCVD外延生长

利用金属有机化合物气相沉积(MOCVD)在蓝宝石衬底上生长了高阻GaN薄膜。对GaN成核层生长的反应室压力、生长时间和载气类型对GaN缓冲层电学特性的影响进行了分析。实验结果表明,延长GaN成核层的生长时间,降低成核层生长时的反应室压力,载气由H₂换为N₂都会得到高阻的GaN缓冲层。样品的方块电阻R_s最高为2.49×10¹¹ Ω/□。以高阻GaN样品为衬底制备了AlGaN/AlN/GaN结构HEMT器件,迁移率最高达1 230 cm²/(V·s)。     

Ga2O3/GaN/蓝宝石模板上β-Ga2O3薄膜的生长

为获得高质量的β-Ga2O3薄膜,将c面蓝宝石上生长的GaN薄膜进行高温氧化制成了Ga2O3/GaN/蓝宝石模板,进而在模板上利用金属有机化学气相沉积(MOCVD)工艺进行了β-Ga2O3薄膜的同质外延。通过X射线衍射仪、原子力显微镜、场发射扫描电子显微镜等方法对样品的晶体结构、表面形貌等性质进行测试与分析。结果表明,该方法获得的β-Ga2O3薄膜晶体质量受GaN薄膜氧化效果与MOCVD工艺条件等因素影响较大。通过优化实验条件,得到了质量较高的β-Ga2O3薄膜。与蓝宝石上或GaN薄膜上异质外延得到的β-Ga2O3薄膜相比,薄膜的晶体质量明显提高。通过对比不同样品的晶体质量、表面形貌和制备过程,发现该方法成功地将β-Ga2O3薄膜在蓝宝石衬底或GaN/蓝宝石模板上异质外延转化为了Ga2O3/GaN/蓝宝石模板上的同质外延,有效地减小了β-Ga2O3薄膜和蓝宝石、GaN之间较大的晶格失配和热失配,有利于提高β-Ga2O3薄膜的晶体质量。     

半球蓝宝石整流罩制造技术研究

整流罩在高速飞行中既要对空气进行整流,同时又起光学窗口的作用。蓝宝石材料硬度高,加工非常困难。分别从精磨模、精磨磨料、抛光膜层、抛光辅料、机床速度和压力等方面介绍了一种加工蓝宝石整流罩的工艺方法。     

Eu掺杂GaN薄膜的阴极荧光特性

通过MOCVD方法在蓝宝石衬底上生长GaN薄膜,利用离子注入方法将Eu³⁺离子注入到GaN基质中。X射线衍射结果表明：经过退火处理后,修复了部分离子注入所导致的晶格损伤。利用阴极荧光光谱可得到GaN∶Eu³⁺材料在623 nm处有很强的红光发射,该发射峰来源于Eu³⁺离子的内部4f能级跃迁。另外,Eu³⁺离子注入会在样品中引入电荷转移态,产生408 nm附近的发光。退火处理有助于获得更强的电荷转移态发光和Eu离子特征发光。GaN基质的黄光峰与Eu离子之间存在能量交换,将能量传递给Eu离子,促进Eu离子发光。     

Design and fabrication of single-crystal GaN nano-bridge on homogeneous substrate for nanoindentation

This study reports a simple method to design and fabricate a freestanding GaN nano-bridge over a homogeneous short column as supporting leg. Test samples were fabricated from MOCVD-grown single-crystal GaN films over sapphire substrate using a FIB milling to leave freestanding short spans. We also investigated the nanoindentation characteristics and the corresponding nanoscopic mechanism of the GaN nano-bridge and its short column with a conical indenter inside transmission electron microscopy. The stress–strain mechanical properties and Young’s modulus have also been examined and calculated as 108 GPa ± 4.8 % by the strain energy method. The significant slope switch of the L–D curve corresponds to the transition from the single-point bending indentation to the surface stretching indentation and has been interpreted with the evolution of TEM images. This freestanding fabrication and test have key advantages to characterize nanoscale behavior of one-dimensional bridge structure and greater ease of sample preparation over other micro-fabrication techniques.     

Preparation of GaN-based cross-sectional TEM specimens by laser lift-off

Laser lift-off (LLO) technology is successfully used to prepare GaN-based TEM cross-sectional specimens. Detailed procedures of the method to prepare the specimens are demonstrated. Large thin areas suitable for TEM analysis were obtained. TEM images of the resulting GaN interface are studied, and the changes in structural quality are confined to approximately the first 250 nm of the epilayer. Clear TEM images of the whole epilayer and the InGaN quantum wells and the HRTEM images of the superlattice layer are demonstrated, showing that LLO is a quick and ideal method to study the crystal structure of the epilayer, especially if only the upper layers are of interest.     

Chemical polishing method of GaAs specimens for transmission electron microscopy

A practical method for transmission electron microscopy specimen preparation of GaAs-based materials with quantum dot structures is presented to show that high-quality image observations in high-resolution transmission electron microscopy (HRTEM) can be effectively obtained. Specimens were prepared in plan-view and cross-section using ion milling, followed by two-steps chemical fine polishing with an ammonia solution (NH₄OH) and a dilute H₂SO₄ solution. Measurements of electron energy loss spectroscopy (EELS) and atomic force microscopy (AFM) proved that clean and flat specimens can be obtained without chemical residues. HRTEM images show that the amorphous regions of carbon and GaAs can be significantly reduced to enhance the contrast of lattice images of GaAs-based quantum structure.     

Elimination of Crystallographic Wing Tilt of Canti-Bridged Epitaxial Laterally Overgrown GaN Films by Optimizing Growth Procedure

Canti-bridged epitaxial lateral overgrowth （CBELO） of GaN is performed by metalorganic chemical vapour deposition （MOCVD） on maskless V-grooved sapphire substrates prepared by wet chemical etching with different mesa widths. The wing tilt usually observed in ELO is not found in the CBELO GaN with wide mesa widths, while it can be detected obviously in the GaN with narrow mesa widths. The wing tilt of CBELO GaN grown on a grooved sapphire substrate with narrow mesa can be controlled by adjusting the thickness of the nucleation layer. The dependence of the wing tilt on the nucleation layer thickness is studied. Cross-sectional scanning electron microscopy is used to characterize the geometry of the wing regions, and double crystal x-ray diffraction is used to analyse the structural characteristics and to measure the magnitude of the crystalline wing tilt. It is found that the crystalline wing tilt can be eliminated completely by first growth of a thin nucleation GaN layer then the CBELO GaN. Possible reason and the origin of the wing tilt in CBELO GaN films are also discussed.     

Synthesis of single crystalline GaN nanoribbons on sapphire (0001) substrates

Single crystalline GaN nanoribbons were synthesized through nitriding Ga₂O₃ thin films deposited on sapphire (0001) substrates by radio frequency magnetron sputtering. The component and structure of nanoribbons were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The flat and smooth ribbon-like nanostructures are high quality single crystalline hexagonal wurtzite GaN. The thickness and width-to-thickness ratio of the grown GaN nanoribbons are in the range of 8-15 nm and ∼5-10, respectively.     

硅衬底氮化镓基LED薄膜转移至柔性黏结层基板后其应力及发光性能变化的研究

本文将硅(Si)衬底上外延生长的氮化镓(GaN)基发光二极管(LED)薄膜转移至含有柔性黏结层的基板上, 获得了不受衬底和支撑基板束缚的LED薄膜. 利用高分辨率X射线衍射仪(HRXRD)研究了薄膜转移前后的应力变化, 同时对其光致发光(PL)光谱的特性进行了研究. 结果表明: 硅衬底GaN基LED薄膜转移至柔性基板后, GaN受到的应力会由转移前巨大的张应力变为转移后微小的压应力, InGaN/GaN量子阱受到的压应力则增大; 尽管LED薄膜室温无损转移至柔性基板其InGaN阱层的In组分不会改变, 然而按照HRXRD倒易空间图谱通用计算方法会得出平均铟组发生了变化; GaN基LED薄膜从外延片转移至柔性基板时其PL谱会发生明显红移.     

Focused ion beam milling and ultramicrotomy of mineralised ivory dentine for analytical transmission electron microscopy

The use of focused ion beam (FIB) milling for preparation of sections of mineralised ivory dentine for transmission electron microscopy (TEM) is investigated. Ivory dentine is essentially composed of fibrillar type-I collagen and apatite crystals. The aim of this project is to gain a clearer understanding of the relationship between the organic and inorganic components of ivory dentine using analytical TEM, in order to utilise these analytical techniques in the context of common skeletal diseases such as osteoporosis and arthritis. TEM sections were prepared in both single and dual beam FIB instruments, using two standard lift-out techniques, in situ and ex situ. The FIB sections were systematically compared with sections prepared by ultramicrotomy, the traditional preparation route in biological systems, in terms of structural and chemical differences. A clear advantage of FIB milling over ultramicrotomy is that dehydration, embedding and section flotation can be eliminated, so that partial mineral loss due to dissolution is avoided. The characteristic banding of collagen fibrils was clearly seen in FIB milled sections without the need for any chemical staining, as is commonly employed in ultramicrotomy. The FIB milling technique was able to produce high-quality TEM sections of ivory dentine, which are suitable for further investigation using electron energy-loss spectroscopy (EELS) and energy-filtering TEM (EFTEM) to probe the collagen/apatite interface.     

Sample preparation by focused ion beam micromachining for transmission electron microscopy imaging in front-view

This article deals with the development of an original sample preparation method for transmission electron microscopy (TEM) using focused ion beam (FIB) micromachining. The described method rests on the use of a removable protective shield to prevent the damaging of the sample surface during the FIB lamellae micromachining. It enables the production of thin TEM specimens that are suitable for plan view TEM imaging and analysis of the sample surface, without the deposition of a capping layer. This method is applied to an indented silicon carbide sample for which TEM analyses are presented to illustrate the potentiality of this sample preparation method.