Micro-Raman characterization of Germanium thin films evaporated on various substrates

We perform an extensive micro-Raman analysis of Germanium thin films physically evaporated on several substrates including silicon, silicon oxide and glass. We investigate the dependence of crystal quality on thin film deposition parameters such as substrate temperature and growth rate. We also study the continuous transitional change of the material structure from amorphous to crystalline phases. Ge films obtained by this simple and low cost technique are a viable solution towards the realization of virtual substrates and devices.     

Germanium islands embedded in strained silicon quantum wells grown on patterned substrates

Germanium islands were embedded in strained silicon quantum wells in order to provide an improved electron confinement in vicinity of the islands. Growth was performed on relaxed SiGe layers. Patterned substrates were used, favouring lattice relaxation as well permitting the fabrication of small Ge islands at deposition temperatures above 500 °C. Photoluminescence analysis reveals a strongly reduced dislocation related signal. The low temperature spectra are dominated by intense signals from the germanium islands. The origin of these signals were investigated by removing the islands by etching, analysing reference samples without a silicon quantum well, varying the germanium deposition and the growth temperature.     

Silicon-germanium alloy growth control and characterization

Silicon-germanium alloy has been grown on silicon substrates by the hydrogen reduction of silicon tetrachloride and germanium tetrachloride as well as the pyrolysis of silane and germane. The film growth characteristics in the first few minutes have been studied using an SEM. Techniques have been developed for growing high germanium content films which are single crystal and have surfaces suitable for MOS device fabrication.     

薄层SOS薄膜材料外延生长及其器件应用

亚微米 CMOS/ SOS器件发展对高质量的 1 0 0 - 2 0 0纳米厚度的薄层 SOS薄膜提出了更高的要求 .实验证实 :采用 CVD方法生长的原生 SOS薄膜的晶体质量可以通过固相外延工艺得到明显改进 .该工艺包括 :硅离子自注入和热退火 .X射线双晶衍射和器件电学测量表明 :多晶化的 SOS薄膜固相外延生长导致硅外延层晶体质量改进和载流子迁移率提高 .固相外延改进的薄层 SOS薄膜材料能够应用于先进的 CMOS电路 .     

TIPS-pentacene crystalline thin film growth

The crystalline film growth of TIPS-pentacene thin films by confined solution deposition is investigated. The crystalline thin films grow dendritic in the initial stage and continue to grow to elongated plate-like crystals when the solution is deposited in a confined space in-plane. The majority of the thin film, containing smaller thin crystals, is formed within the first 10 s after depositing the solution and continues to grow in minutes to millimeter sized single crystals. By atomic force microscopy we show that impurities are expelled by the growing crystals and clusters accumulate at step edges on the surface of the larger crystals. The influence of crystal thickness and orientation on the electronic transport in field-effect transistors is studied, and shows an optimum performance for devices with thin elongated crystals that are aligned parallel to the electric field between the source–drain electrodes.     

薄层SOS薄膜材料外延生长及其器件应用

亚微米CMOS/SOS器件发展对高质量的100-200纳米厚度的薄层SOS薄膜提出了更高的要求.实验证实;采用CVD方法生长的原生SOS薄膜的晶体质量可以通过固相外延工艺得到明显改进.该工艺包括:硅离子自注入和热退火.X射线双晶衍射和器件电学测量表明:多晶化的SOS薄膜固相外延生长导致硅外延层晶体质量改进和载流子迁移率提高.固相外延改进的薄层SOS薄膜材料能够应用于先进的CMOS电路.     

SiC/Si(111) film quality as a function of GeH4 flow in an MOCVD reactor

Due to large lattice and thermal expansion coefficient mismatches, SiC films grown on Si are usually low quality. To provide a more stable growth front we added Ge in the form of GeH₄ to the reactant gases in a MOCVD reactor. Several SiC films with Ge flow rates ranging from 0–50 sccm were grown on (111) Si substrates at 1000°C. TEM results show that the crystalline quality is amorphous or polycrystalline for Ge flow rates at or below 15 sccm. Samples grown at Ge flow rates at or exceeding 20 sccm have an initial layer of single crystalline 3C SiC followed by heavily twinned crystalline 3C SiC. In particular, the samples grown with 20–30 sccm Ge contain an 80 nm initial layer of reasonably high quality single crystal 3C SiC.     

A Facile Method for the Growth of Organic Semiconductor Single Crystal Arrays on Polymer Dielectric toward Flexible Field‐Effect Transistors

Polymer dielectrics with intrinsic mechanical flexibility are considered as a key component for flexible organic field‐effect transistors (OFETs). However, it remains a challenge to fabricate highly aligned organic semiconductor single crystal (OSSC) arrays on the polymer dielectrics. Herein, for the first time, a facile and universal strategy, polar surface‐confined crystallization (PSCC), is proposed to grow highly aligned OSSC arrays on poly(4‐vinylphenol) (PVP) dielectric layer. The surface polarity of PVP is altered periodically with oxygen‐plasma treatment, enabling the preferential nucleation of organic crystals on the strong‐polarity regions. Moreover, a geometrical confinement effect of the patterned regions can also prevent multiple nucleation and misaligned molecular packing, enabling the highly aligned growth of OSSC arrays with uniform morphology and unitary crystallographic orientation. Using 2,7‐dioctyl[1]benzothieno[3,2‐b]benzothiophene (C8‐BTBT) as an example, highly aligned C8‐BTBT single crystal arrays with uniform molecular packing and crystal orientation are successfully fabricated on the PVP layer, which can guarantee their uniform electrical properties. OFETs made from the C8‐BTBT single crystal arrays on flexible substrates exhibit a mobility as high as 2.25 cm² V^?1 s^?1, which has surpassed the C8‐BTBT polycrystalline film‐based flexible devices. This work paves the way toward the fabrication of highly aligned OSSCs on polymer dielectrics for high‐performance, flexible organic devices.     

Influence of substrate on the growth of microcrystalline silicon thin films deposited by plasma enhanced chemical vapor deposition

Microcrystalline silicon (μc-Si) thin films are widely used for silicon thin film solar cells, especially in the high performance tandem solar cells which comprise an amorphous silicon junction at the top and a μc-Si junction at the bottom. One of the major factors affecting the photovoltaic properties of μc-Si thin film solar cells of thin films is the quality of the μc-Si thin films. In this work, we investigated the effect of substrates on the crystallization characteristics and growth behaviors of μc-Si thin films grown by the plasma enhanced chemical vapor deposition method (PECVD), and found that substrates have a strong effect on the crystallization characteristics of μc-Si thin films. In addition, the growth rate of μc-Si thin films was also highly influenced by the substrates. Three types of substrates, quartz glass, single crystalline silicon and thermally oxidized single crystalline silicon, were used for growing μc-Si thin films from SiH₄/H₂ with a flow rate ratio 2:98 at different temperatures. Crystallization characteristics of these μc-Si thin films were studied by Raman scattering and X-ray diffraction techniques.     

Effect of As Passivation on Vapor-Phase Epitaxial Growth of Ge on (211)Si as a Buffer Layer for CdTe Epitaxy

We report an investigation of epitaxial germanium grown by chemical vapor deposition (CVD) on arsenic-terminated (211)Si, which is the preferred substrate in the USA for fabrication of night-vision devices based on mercury cadmium telluride (MCT) grown by molecular-beam epitaxy (MBE). The films were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), cross-sectional transmission electron microscopy (XTEM), and x-ray diffraction (XRD). Arsenic passivation was found to be effective in preventing cross-contamination of unwanted residual species present inside the reactor chamber and also in prolonging the evolution of layer-by-layer growth of Ge for significantly more monolayers than on nonpassivated Si. The two-dimensional (2D) to three-dimensional (3D) transition resulted in Ge islands, the density and morphology of which showed a clear distinction between passivated and nonpassivated (211)Si. Finally, thick Ge layers (∼250 nm) were grown at 525°C and 675°C with and without As passivation, where the layers grown with As passivation resulted in higher crystal quality and smooth surface morphology.     

Confocal Raman Spectroscopy of α‐Sexithiophene: From Bulk Crystals to Field‐Effect Transistors

We report confocal micro‐Raman spectra of the organic semiconductor α‐sexithiophene (T6) on bulk crystals and on thin films grown on technologically relevant substrates and devices. We show that the two polymorphs, which are clearly identified by their lattice phonon spectra, may coexist as physical impurities of one inside the other in the same crystallite. Spatial distribution of the two phases is monitored by Raman phonon mapping of crystals grown upon different conditions. Raman microscopy has then been extended to T6 thin films grown on silicon oxide wafers. We identify the crystal phase in thin films whose thickness is just 18 nm. The most intense total‐symmetric Raman vibration is still detectable for a two‐monolayer thick film. Comparative analysis between micro‐Raman and AFM of T6 thin films grown on field effect transistors shows that electrode‐channel steps favour the nucleation and growth of T6 molecules on the substrate, at least below 50 nm.     

Ultra-short pulsed laser deposition and patterning of SiC thin films for MEMS fabrication

A Ti:Sapphire (IR 800-nm) femtosecond (fs) pulsed laser was used to ablate a sputtering grade of silicon carbide (SiC) in an ultra-high vacuum chamber. The laser-induced plasma species were then driven and grown to form 3C-SiC films of about 1 μm thick on single crystal silicon wafers at 20 °C (room temperature) and 500 °C. Scanning electron microscopy, atomic force microscopy, X-ray photoelectron microscopy, X-ray diffraction and nanoindentation were used to characterize the structure, composition, thickness and properties of the SiC films. Results of the femtosecond-pulse laser deposited (fs-PLD) films were compared with those obtained by atmospheric pressure chemical vapor deposition (APCVD) and nanosecond-pulse laser (excimer laser at 248-nm) deposition (ns-PLD). The distinctive features of fs-PLD films are their extremely smooth surfaces, stoichiometry, amorphous structure and low defect density compared to APCVD films, along with better film quality and higher growth rates than ns-PLD films. In addition to film growth studies, a SiC microgripper (to grab 20-μm-sized objects) was micromachined by use of the fs-pulsed laser to demonstrate the utility of ultra-short PLD in SiC-device fabrication.     

Fluorinated Graphene in Interface Engineering of Ge‐Based Nanoelectronics

Germanium is a promising candidate to replace silicon in nanoelectronics due to its significantly higher electron and hole mobilities. However, the unstable germanium oxide formed at the interface between the channel and dielectric layer has impeded the progress of Ge‐based nanoelectronics. By taking advantage of the impermeability of graphene, it is discovered that the insulating fluorinated graphene is able to act as an efficient diffusion barrier layer to suppress the formation of the unstable interfacial oxide in Ge‐based devices. The Ge‐based device with the fluorinated graphene exhibits negligible capacitance versus voltage hysteresis, extremely low leakage, and superior equivalent oxide thickness. First‐principles calculations reveal that interfacial diffusion, which would otherwise be unmanageable, is sufficiently obstructed by the fluorinated graphene. This new structure is expected to expedite the implementation of germanium as a channel material in next‐generation nanoelectronic devices.     

Evaluation of 1/f Noise Characteristics for Si‐Based Infrared Detection Materials

Silicon antimony films are studied as resistors for uncooled microbolometers. We present the fabrication of silicon films and their alloy films using sputtering and plasma‐enhanced chemical vapor deposition. The sputtered silicon antimony films show a low 1/f noise level compared to plasma‐enhanced chemical vapor deposition (PECVD)‐deposited amorphous silicon due to their very fine nanostructure. Material parameter K is controlled using the sputtering conditions to obtain a low 1/f noise. The calculation for specific detectivity assuming similar properties of silicon antimony and PECVD amorphous silicon shows that silicon antimony film demonstrates an outstanding value compared with PECVD Si film.     

中红外完全光子带隙Ge反蛋白石三维光子晶体的制备

采用溶剂蒸发对流自组装法将单分散二氧化硅（SiO2）微球组装形成三维有序胶体晶体模板。以锗烷（GeH4）为先驱体气,用等离子增强化学气相沉积法向胶体晶体的空隙中填充高折射率材料Ge。酸洗去除二氧化硅微球,得到Ge反蛋白石三维光子晶体。通过扫描电镜、X射线衍射仪和傅立叶变换显微红外光谱仪对锗反蛋白石的形貌、成分和光学性能进行了表征。结果表明：Ge在SiO2微球空隙内填充致密均匀,得到的锗为多晶态,锗反蛋白石为三维有序多孔结构。锗反蛋白石的测试光谱图有明显的光学反射峰,表现出光子带隙效应。测试的完全光子带隙位于中红外3.4µm处,测试的光学性能与理论计算基本吻合。     

Comparative Studies of the Growth and Characterization of Germanium Epitaxial Film on Silicon (001) with 0° and 6° Offcut

The quality of germanium (Ge) epitaxial films grown directly on silicon (Si) (001) with 0° and 6° offcut orientation using a reduced-pressure chemical vapor deposition system is studied and compared. Ge film grown on Si (001) with 6° offcut presents ～65% higher threading dislocation density and higher root-mean-square (RMS) surface roughness (1.92 nm versus 0.98 nm) than Ge film grown on Si (001) with 0° offcut. Plan-view transmission electron microscopy also reveals that threading dislocations are more severe (in terms of contrast and density) for the 6° offcut. In addition, both high-resolution x-ray diffraction and Raman spectroscopy analyses show that the Ge epilayer on 6° offcut wafer presents higher tensile strain. The poorer quality of the Ge film on Si (001) with 6° offcut is a result of an imbalance in Burgers vectors that favors dislocation nucleation over annihilation.     

Self-organization of germanium highly ordered nanoclusters by the deposition of polycrystalline silicon films doped with germanium

Aspects of the formation of self-organized clusters of germanium (Ge) and solid solution of SiGe, first formed in the mode of deposition of subthin polycrystalline silicon films doped with Ge on the nanoscale film of dielectrics were studied by techniques of atomic force microscopy and Raman scattering by optical phonons in germanium clusters. We found that in subthin polycrystalline silicon films (PSF) doped with Ge on the nanoscale films of dielectrics in conditions of PSF film deposition, there appeared correlated spatial distribution of germanium clusters, as well as silicon rich clusters in certain conditions, i.e., clusters of SiGe solid solution, with the modes of their formation. The relationship of the form, size, and density of Ge nanoclusters (NC), with the conditions of their self-organization is considered. The influence of interdiffusion processes on self-organizing of clusters is established, which is significant at high temperatures of the deposition and doping of PSF. It was found that clusters (islands) could occur on the cleavage surface in the form of four types of topographic features in a classical pyramid form, flat-topped pyramids, and domes and sharp spines, depending on the conditions of the deposition of PSF doped with Ge. The systems of highly organized Ge NC, measuring 3.5–40 nm and with a density of 2.7 × 10⁷–3.5 × 10 cm⁻² were obtained. The possibility, in principle, of managing the geometric parameters of self-organizing NC (nanoislands) by selecting the conditions of their self-organization in the mode of deposition of PSF doped with Ge, was shown.     

Structural and electrical analysis of poly-Ge films fabricated by e-beam evaporation for optoelectronic applications

We have investigated the relationship between structural and electrical properties of Ge thin films deposited on single crystal silicon (100) substrates by electron beam evaporation at room temperature. Post-thermal annealing was applied to obtain poly-crystalline Ge thin films. The structural effects of the annealing temperature and annealing time on the crystallization of Ge films were analyzed using Raman and X-ray diffraction measurements. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increasing annealing temperature and annealing time. It was found that high quality poly-crystalline Ge films were obtained with crystallization ratio of 90% at an annealing temperarure of 500 °C following the crystallization threshold of 450 °C. Effects of structural ordering on the electrical properties were investigated through current-voltage characteristics of fabricated heterostructure devices (Ge/p-Si). Smooth cathode-anode interchange in the diode behavior has been clearly observed following the structural ordering as a function of annealing temperature in a systematic way. These outcomes could be exploited for engineering of low-cost Ge based novel electronic and opto-electronic devices.     

Pure germanium epitaxial growth on thin strained silicon-germanium graded layers on bulk silicon substrate for high-mobility channel metal-oxide-semiconductor field-effect transistors

We demonstrate epitaxially grown high-quality pure germanium (Ge) on bulk silicon (Si) substrates by ultra-high-vacuum chemical vapor deposition (UHVCVD) without involving growth of thick relaxed SiGe buffer layers. The Ge layer is grown on thin compressively strained SiGe layers with rapidly varying Ge mole fraction on Si substrates resulting in several SiGe interfaces between the Si substrate and the pure Ge layer at the surface. The presence of such interfaces between the Si substrate and the Ge layer results in blocking threading dislocation defects, leading to a defect-free pure Ge epitaxial layer on the top. Results from various material characterization techniques on these grown films are shown. In addition, capacitance-voltage (CV) measurements of metal-oxide-semiconductor (MOS) capacitors fabricated on this structure are also presented, showing that the grown structure is ideal for high-mobility metal-oxide-semiconductor field-effect transistor applications.     

基于不同单光子能量拉曼谱的氢化硅薄膜微观特性研究

用等离子体增强化学汽相沉积法（PECVD）在玻璃和单晶硅（c-Si）衬底上分别制备了氢化纳米硅（nc-Si：H）和非晶硅（a-Si：H）薄膜,用紫外、可见和近红外3种不同波长的激光线对不同形态的Si薄膜进行拉曼散射实验研究。研究发现,这些Si薄膜在不同的单光子能量的激光线激发下的拉曼谱线形也不同。进而通过对Si薄膜材料...