Epitaxial Aluminum-Doped Zinc Oxide Thin Films on Sapphire: II, Defect Equilibria and Electrical Properties

The electrical transport properties of epitaxial ZnO films grown on different orientations of sapphire substrates have been measured as a function of partial pressure of oxygen. After equilibration, the carrier concentration is found to change from a p ^-1/4_O2 to a p ^-3/8_O2 dependence with increasing oxygen partial pressure. The partial pressure dependence is shown to be consistent with zinc vacancies being the rate-controlling diffusive species. In addition, the carrier concentration in ZnO films grown on A-, C-, and M-plane sapphire are the same but that of R-plane sapphire is systematically lower. Electron Hall mobility measurements as a function of carrier concentration for all the substrate orientations exhibit a transition from "single-crystal" behavior at high carrier concentrations to "polycrystalline" behavior at low carrier concentrations. This behavior is attributed to the effective height of potential barriers formed at the low-angle grain boundaries in the epitaxial ZnO films. The trap density at the grain boundaries is deduced to be 7 × 10¹²/cm². The electron mobility, at constant carrier concentration, varies with the substrate orientation on which the ZnO films were grown. The difference is attributed to the difference in dislocation density in the films produced as a result of lattice mismatch with the different sapphire orientations.     

Crystallographic Orientation in Pure and Aluminum-Doped Zinc Oxide Thin Films Prepared by Sol–Gel Technique

Novel sol–gel processes for the preparation of grain-oriented thin films of pure and aluminum-doped zinc oxide (AZO) are reported. Using various stabilizing ligands, oxide films with different degree of c -axis orientation could be prepared on amorphous substrates. The origin of orientation in sol–gel-derived ZnO and AZO films is investigated. The texture selection of ZnO films is independent of the heat-treatment schedule but sensitive to the precursor system. The electrical properties of the films do not depend significantly on the crystallographic orientation. The film visible transmittance remained higher than 90% even after vacuum annealing.     

Sol-Gel Preparation of Transparent and Conductive Aluminum-Doped Zinc Oxide Films with Highly Preferential Crystal Orientation

Transparent aluminum-doped zinc oxide (ZnO) films were prepared via the sol-gel method on silica-glass substrates from 2-methoxyethanol solutions of zinc acetate and aluminum chloride that contained monoethanolamine. Dip coating was conducted at room temperature, with substrate withdrawal rates of 1.2-7.0 cm/min. After each deposition, the films were heat-treated in air at 200°-450°C for 10 min (pre-heat-treatment). After six to fourteen layers had been deposited, the films were then subjected to annealing in air at 500°-800°C for 1 h (the first post-heat-treatment), followed by annealing in nitrogen at 500°-700°C for 15 min to 4 h (the second post-heat-treatment). All the films obtained were transparent and showed only an extremely sharp ZnO (002) peak in the X-ray diffractometry (XRD) patterns. The effects of the aluminum content, the substrate withdrawal speed, and the heat-treatment conditions on the electrical resistivity of the films were studied. All these factors strongly affected the resistivity. The lowest resistivity value (6.5 10^-3 Omegacm) was achieved in a film that contained 0.5 at.% aluminum, prepared with a low substrate withdrawal speed (1.2 cm/min), and a pre-heat-treatment of individual layer at 400°C in air and a post-heat-treatment of the entire film at 600°C in air, followed by a post-heat-treatment at 600°C in nitrogen. These preparation parameters also affected the degree of crystal orientation, which was revealed by the intensity of the ZnO (002) XRD peak. Higher crystal orientation was effective in reducing the film resistivity, whereas the higher grain-packing density and possible aluminum segregation were thought to have positive and negative effects, respectively, in reducing the resistivity.     

Preparation of Aluminum-Doped Zinc Oxide Films by a Normal-Pressure CVD Method

Aluminum-doped zinc oxide films prepared using a normal-pressure CVD (NP-CVD) method were studied as a transparent electrode material. The films were prepared on fused quartz substrates at 500°C using bis(2,4-pentanedionato)zinc and tris(2,4-pentanedionato)aluminum as source material. The transparent films, ∼0.3 μm in thickness, with a transmittance above 80% at a wavelength between 400 and 820 nm, were easily obtained. The optical band gap of the films increased from 3.3 to 3.6 eV with increasing amounts of aluminum dopant. The minimum resistivity of the film was about 4.9 × 10⁻⁵Ω·m.     

制备过程对掺铝氧化锌薄膜性质的影响

文章采用溶胶-凝胶法在玻璃基底上制备得到了高透光率,高导电性的掺铝氧化锌薄膜（AZO）,利用原子力显微镜、扫描电镜、X射多次退火的制备过程不仅可以得到可见光区透光率大于85%,电阻率为7.2×10-5Ω·m的透明导电薄膜,而且因省略了热处理过程而简化了制备工艺,缩短了制膜时间。线衍射仪和紫外可见分光光度计等研究了三种不同的制备过程对AZO性质的影响。结果表明,制备过程的不同导致了薄膜的表面形貌、结构和导电性质方面的差别,其中分层多次退火过程有利于晶粒长大,形成结晶度高且具有单一（002）取向的AZO薄膜,同时改善薄膜的导电性能。分层     

Effect of Microstructural Control on Thermoelectric Properties of Hot-Pressed Aluminum-Doped Zinc Oxide

The thermal conductivity of polycrystalline Al-doped ZnO was controlled through the fabrication of nanostructured polycrystalline materials, by hot-pressing nanosized Zn_{1– x} Al _x O ( x = 0.01, 0.02) particles, which were synthesized by a coprecipitation and spray-drying method. This process resulted in an improved thermoelectric power factor because of the nanosized Zn_0.99Al_0.01O particles of the polycrystalline products. The thermal conductivity also was decreased as a result of the formation of nanocrystalline Zn_0.99Al_0.01O.     

Effect of Substrate Orientation on Interfacial and Bulk Character of Chemically Vapor Deposited Monocrystalline Silicon Carbide Thin Films

The high breakdown electric field, saturated electron drift velocity, and melting (decomposition) point of SiC have given continual impetus to research concerned with the development of thin films having minimum concentrations of line and planar defects and electronic devices for severe environments. To this end, epitaxial growth via chemical vapor deposition of monocrystalline films of β-SiC on Si (100) and 6 H -SiC {0001} substrates and 6 H -SiC on vicinal 6 H -SiC {0001} substrates have been conducted. High concentrations of stacking faults, microtwins, and inversion domain boundaries were produced in films grown directly on Si (100) as a result of a lattice parameter difference of ∼ 20% and the presence of single (or odd number) atomic steps on the substrate surface. Growth on Si (100) oriented 3° to 4° toward [011] completely eliminated the IDBs (but not the other defects) due to the preferential formation of double steps with dimerization axes on the upper terraces parallel to the step edges. Growth of β-SiC films on 6 H {0001} lowered the density of all defects but resulted in the formation of a new defect, namely, double positioning boundaries. The latter were eliminated by using 6 H {0001} oriented 3° toward [1120]. The defect density in these last films, relative to those grown on on-axis Si (100), was reduced substantially (to ∼10⁵ cm/cm³). However, the resulting film was 6 H -SiC. Significant improvements in electrical properties of simple devices were obtained as the defect density was progressively decreased.     

衬底取向对 Al 掺杂3C-SiC 薄膜微结构的影响

采用低压化学气相沉积(LPCVD)法分别在 Si(100)和 Si(111)衬底上制备了 Al 掺杂的3C-SiC 薄膜。采用 X 射线衍射、扫描电子显微镜、Raman 光谱对所制备薄膜的微结构、形貌以及内部应力的演变进行分析。结果表明：在 Si(100)衬底上制备的 Al 掺杂 SiC 薄膜具有较好的结晶质量,而且结晶质量受 Al 掺杂浓度的影响比较大。Al 掺杂 SiC 薄膜的生长模式为二维层状生长模式。Si(100)衬底上所制备的 Al 掺杂 SiC 薄膜表面为层状的四边形结构,而 Si(111)衬底上的 Al 掺杂 SiC 薄膜表面为层状的截角三角形结构。Si(100)衬底上的薄膜厚度略大于 Si(111)衬底上的。由于 Al 离子的掺入和薄膜厚度的增加,Si(100)衬底上所制备的 Al 掺杂 SiC 薄膜内部的应力得到很好的释放。Si(111)衬底上的 Al 掺杂 SiC 薄膜内部的应力则由张应力模式转为压应力模式,而且纵光学声子(LO)、横光学声子(TO)特征峰分离变大,出现这种现象的原因可能与 Al3+替代 Si4+使 SiC离子性增强和生长模式的转变有关。     

Morphological Control of Centimeter Long Aluminum-Doped Zinc Oxide Nanofibers Prepared by Electrospinning

Centimeter long aluminum-doped zinc oxide (AZO) nanofibers were prepared by electrospinning from polyvinyl alcohol (PVA) aqueous solutions, and the fibers with controlled morphology were expected to have potential applications in gas-sensing and optoelectronic devices. In our research, the diameter and morphology of the nanofibers can be controlled by the applied voltage, the concentration of PVA, and the aluminum (Al) dopant. The introduction of Al resulted in a decrease of the zinc oxide lattice constant by 0.41% and uniform distribution of the nanofiber diameters. Possible mechanisms were proposed to discuss the variations of diameter and morphology.     

聚合物在取向聚合物薄膜上的附生结晶研究进展

总结了近年来以取向聚合物为基底的聚合物附生结晶的部分研究成果,重点综述了聚合物附生结晶的机理及晶格匹配、链段规整度、结晶动力学、二次成核等因素对聚合物附生结晶的影响,并对其今后的发展进行了展望。     

Aluminum Nitride Thin Films on an LTCC Substrate

Aluminum nitride thin films deposited on a low-temperature co-fired ceramics substrate by reactive magnetron sputtering were investigated with regard to their crystal orientation and microstructural characteristics. Strong c -axis orientations of AlN thin films were observed when either a higher deposition temperature or an RF bias was adopted. This orientation was believed to be responsible for the high thermal conductivity of 26 W/mK for the AlN films deposited at 700°C under 25-W bias. Photoluminescence spectrum in the wavelength range of 350–650 nm was analyzed to prove the involvement of potential oxygen-related defects in the thin films.     

Formation of Aluminum-Doped Zinc Oxide Nanocrystals via the Benzylamine Route at Low Reaction Kinetics

The influence of essential process parameters on the adjustability of specific process and particulate properties of aluminum-doped zinc oxide (AZO) nanocrystals during synthesis via the benzylamine route at low reaction kinetics is demonstrated by enabling time-resolved access of the selected measurement technique. It is shown that the validity of the pseudo-first-order process kinetics could be extended to the minimum operable reaction kinetics. On the other hand, the impacts of the process temperature and the initial precursor concentration on both the process kinetics and the particle morphology are discussed. The obtained data provide a versatile tool for precise process control by adjusting defined application-specific particle properties of AZO during synthesis.     

Effect of Substrate Orientation, Roughness, and Film Deposition Mode on the Tensile Strength and Toughness of Niobium–Sapphire Interfaces

The strength and toughness of interfaces between sputter-deposited polycrystalline niobium films and sapphire substrates with basal and prismatic orientations were measured. The effect of deposition parameters and substrate roughness on these interface properties also was investigated. Substrates of polycrystalline alumina with two different surface morphologies were chosen for studying the effect of interface roughness. The interface strength was measured using a previously developed laser spallation experiment in which a laser-generated compressive stress pulse in the substrate, upon reflection into a tensile stress pulse from the coating's free surface, pulls the interface apart. The interface toughness was obtained using a controlled delamination technique, in which a residually stressed loading layer was used to buckle the underlying test layer from its substrate. The energy balance in the prebuckeled and postbuckled states provided a direct measure of the interface toughness. These values were independently obtained by another experiment in which well-characterized, artificially generated, interfacial flaws were loaded using a stress pulse in the laser spallation assembly. The coating's free surface velocity upon crack initiation was related to the critical energy release rate via a numerical simulation. The results of the two toughness experiments conformed to each other and related fairly well to the independently obtained strength measurements.     

Effect of Substrate on Structure and Multiferrocity of (La,Mn) CoSubstituted BiFeO3 Thin Films

In this article, we report the substrate effect on ferroelectric and magnetic properties of epitaxial BiFeO₃‐based thin films at room temperature. (La, Mn) cosubstituted BiFeO₃ (BFOLM) thin films were deposited on differently lattice mismatched single‐crystal substrates to manipulate the strain states in the as‐deposited films. All the films with 30‐nm thick CaRuO₃ bottom electrodes exhibited highly epitaxial growth behavior with a slightly monoclinic distorted lattice structure while their strain states are drastically different as confirmed by X‐ray reciprocal space mapping. These films possessed significantly different macroscopic ferroelectric properties with giant remanent polarization of 101 ± 2, 65 ± 2, and 48 ± 2 μC/cm² for the films grown on SrTiO₃, (La, Sr)(Al, Ta)O₃, and LaAlO₃, respectively. It is found that the room‐temperature magnetic properties are also in accordance with their strain state, having a reciprocal relationship with polarization. For example, the enhanced magnetization is associated with the suppressed polarization and vice versa. The stain tunability of multiferroic properties in BFOLM thin films are presumably ascribed to the polarization rotation and oxygen octahedral tilts.     

氧化铋薄膜的制备及其性能表征

利用化学水浴沉积法在室温下制备了多晶氧化铋薄膜,并在350℃的空气气氛条件下进行退火处理,得到纯的α-Bi2O3,薄膜.对它们的晶相结构、光电性能进行测试.从X衍射分析结果来看,在退火过程中,薄膜结构由四方相和非化学计量相向单斜相转变.光学性质显示退火处理的薄膜吸收边缘明显的向长波的方向移动,发生红移现象,而且禁带宽度减少了0.4eV.退火后的薄膜在室温下暗电导率下降2个数量级.电导率的变化显示具有半导体行为的性质.     

Novel Spray-Pyrolysis Deposition of Cuprous Oxide Thin Films

Spray pyrolysis of aqueous solutions that include copper(II) acetate, glucose, and 2-propanol was studied for the formation of cuprous oxide (Cu₂O) thin films on glass substrates. The deposition conditions, based on the phase relations of the films, were investigated in terms of solution concentration and substrate temperature. Also, the formation process was kinetically discussed. The Cu₂O thin film obtained here was composed of rounded grains ∼50 nm in size with a surface roughness of ∼30 nm. This film was reddish yellow and showed indirect and direct bandgap energies of 1.95 and 2.60 eV, respectively. Furthermore, the film exhibited p -type conduction, with a resistivity of ∼100 Omegacm.     

Humidity-Sensing Characteristics of Divalent-Metal-Doped Indium Oxide Thin Films

Metal-doped indium oxide thin films that have been prepared on glass substrates via a dip-coating method from an aqueous sol show a rapid decrease in electrical conductivity at room temperature (by approximately three orders of magnitude) when brought into contact with moist air. This observation is in contrast to the conductivities of nondoped indium oxide films that are independent of moisture. Thus, the doped films can be applied as a humidity sensor. The film thickness has no substantial effect on the sensing properties, probably because of the porous nature of the films that are prepared by a sol-gel process. To examine the effects of film morphology, a comparison between the humidity-sensing properties of the films that have been prepared from organic and aqueous sols also has been conducted. The roles of the doped metal ions are also discussed.     

SEM Analysis of Oxide Thin Films and Reactions

The determination of orientation relationships between thin oxide films and the underlying single-crystal oxide substrates using electron backscattered diffraction (EBSD) in scanning electron microscopy (SEM) is discussed. In these well-defined systems, complementary information concerning the local chemistry can be obtained using high-resolution backscattered electron (BSE) imaging. Recent studies have shown the value of using EBSD patterns (EBSPs) in SEM.     

Effects of Individual Layer Thickness on the Microstructure and Optoelectronic Properties of Sol–Gel-Derived Zinc Oxide Thin Films

Zinc oxide (ZnO) thin films were prepared under different conditions on glass substrates using a sol–gel process. The microstructure of ZnO films was investigated by means of diffraction analysis, and plan-view and cross-sectional scanning electron microscopy. It was found that the preparation conditions strongly affected the structure and the optoelectronic properties of the films. A structural evolution in morphology from spherical to columnar growth was observed. The crystallinity of the films was improved and columnar film growth became more dominant as the zinc concentration and the substrate withdrawal speed decreased. The individual layer thickness for layer-by-layer homoepitaxy growth that resulted in columnar grains was <20 nm. The grain columns are grown through the entire film with a nearly unchanged lateral dimension through the full film thickness. The columnar ZnO grains are c -axis oriented perpendicular to the interface and possess a polycrystalline structure. Optical transmittance up to 90% in the visible range and electrical resistivity as low as 6.8 × 10⁻³·Ω·cm were obtained under optimal deposition conditions.