Sapphire evolution of the vicinal (0001) sapphire surface upon annealing in air

The evolution of a terrace-step nanostructure (TSN) on the sapphire (0001) surface misoriented by an angle of 0.1° with respect to the (\(10\bar 10\)) plane was observed by atomic force microscopy (AFM) at temperatures from 1273 to 1673 K. It was established that, with an increase in the annealing temperature to 1373 K, the step height attains 0.44 nm at a distance of 220 nm between steps; i.e., heating by 100 K doubles these parameters. In this case, the relief periodicity is retained. Rapid cooling of the substrate to 973 K leads to partial freezing of the surface structure, which makes it possible to observe the transition from one TSN to another. It was established that two steps coalescence upon annealing to 1373 K toward the (\(10\bar 10\)) plane, which has the lowest rigidity and, consequently, the lowest atomic density. The coalescence of two steps at a specified temperature is completed at a sufficiently large distance between the steps, at which their interaction energy is negligible. Upon further annealing of the samples above 1373 K, the steps overgrow to 1 nm; however, their periodicity is broken in this case.     

Manifestation of the Sapphire Crystal Structure in the Surface Nanopattern and Its Application in the Nitride Film Growth

The “quenching” technique was used to investigate the initial stage of the formation of a terracestep nanostructure on the R-cut surface of sapphire crystal upon high-temperature annealing in air. The morphological features of the formation of A- and R-sapphire planes are experimentally shown in dependence on the misorientation direction and qualitatively interpreted with allowance for the surface energy density for the main sapphire faces. The possibilities of forming AlN layers on the R-sapphire surface with a terrace-step nanostructure under thermochemical effect and high-temperature substrate annealing in a mixture of nitrogen and reducing gases are considered.     

Characterizations of GaN films and AlGaN/GaN heterostructures on vicinal sapphire (0 0 0 1) substrates grown by MOCVD

GaN films and AlGaN/GaN heterostructures grown on vicinal sapphire (0 0 0 1) substrates by metalorganic chemical vapor deposition (MOCVD) are investigated. It is found that surface morphologies of GaN films depend on the vicinal angle, however, they are not sensitive to the inclination directions of the substrate. The optimized vicinal angle for obtaining excellent surface morphology is around 0.5°. This conclusion is also confirmed by characterizing the electrical property of two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure.     

Structural contribution to the roughness of supersmooth crystal surface

Technological advances in processing crystals (Si, sapphire α-Al₂O₃, SiC, GaN, LiNbO₃, SrTiO₃, etc.) of substrate materials and X-ray optics elements make it possible to obtain supersmooth surfaces with a periodicity characteristic of the crystal structure. These periodic structures are formed by atomically smooth terraces and steps of nano- and subnanometer sizes, respectively. A model surface with such nanostructures is proposed, and the relations between its roughness parameters and the height of atomic steps are determined. The roughness parameters calculated from these relations almost coincide with the experimental atomic force microscopy (AFM) data obtained from 1 × 1 and 10 × 10 μm areas on the surface of sapphire plates with steps. The minimum roughness parameters for vicinal crystal surfaces, which are due to the structural contribution, are calculated based on the approach proposed. A comparative analysis of the relief and roughness parameters of sapphire plate surfaces with different degrees of polishing is performed. A size effect is established: the relief height distribution changes from stochastic to regular with a decrease in the surface roughness.     

Suppression of growth domains in epitaxial ZnO films on structured (0001) sapphire surface

Zinc oxide layers have been grown by magnetron sputtering in an oxygen atmosphere on structured sapphire surfaces. The formation of ZnO islands oriented in two directions (the so-called domains) was observed on (0001) Al₂O₃ surfaces with steps spaced by a distance from several thousands to several tens of thousands of nanometers. The islands formed along steps on (0001) Al₂O₃ surfaces with an ordered terrace-step structure (and, subsequently, ZnO films) have only one orientation. Another method is proposed for suppressing domains during ZnO growth on (0001) Al₂O₃ d.     

Sapphire shaped crystals for waveguiding,sensing and exposure applications

Second half of the XX century was marked by a rapid development of sapphire shaped crystal growth technologies, driven by the demands for fast, low-cost, and technologically reliable methods of producing sapphire crystals of complex shape. Numerous techniques of shaped crystal growth from a melt have been proposed relying on the Stepanov concept of crystal shaping. In this review, we briefly describe the development of growth techniques, with a strong emphasize on those that yield sapphire crystals featuring high volumetric and surface quality. A favorable combination of physical properties of sapphire (superior hardness and tensile strength, impressive thermal conductivity and chemical inertness, high melting point and thermal shock resistance, transparency to electromagnetic waves in a wide spectral range) with advantages of shaped crystal growth techniques (primarily, an ability to produce sapphire crystals with a complex geometry of cross-section, along with high volumetric and surface quality) allows fabricating various instruments for waveguiding, sensing, and exposure technologies. We discuss recent developments of high-tech instruments, which are based on sapphire shaped crystals and vigorously employed in biomedical and material sciences, optics and photonics, nuclear physics and plasma sciences.     

Epitaxy of CdTe on sapphire substrates with titanium buffer layers

The formation of a developed electrical relief on the sapphire substrate surface is investigated. A technique is proposed for introducing Ti⁴⁺ impurity atoms into the sapphire crystal lattice by depositing titanium layers with a thickness of about 5 nm and their annealing in air (oxidizing atmosphere) to a temperature of 1400°C. It is shown that this preliminary treatment of the sapphire substrate surface results in epitaxial growth of (111) СdTe films parallel to the sapphire (0001) plane at a temperature of 350°C.     

Structure‐controlled growth of ZnO nanonails by thermal evaporation technique

Wurtzite ZnO nanonail structures have been grown on sapphire substrate by simple thermal evaporation of Zn powder in oxygen ambient. Growth parameters such as growth temperature and oxygen gas flow have been examined for the growth of nanonail structure. It is found that the nanonail structures repeatedly grow under a certain relation between the growth temperature and the oxygen flow. Also, at higher growth temperature, the nanonails grow in the form of branched‐structures. The grown ZnO nanonails have hexagonally well‐faceted cap and grow mostly perpendicular to the sapphire substrate. Excellent luminescence properties of a strong UV emission peak with negligible green band have been obtained at room temperature. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Sapphire substrates with a regular surface relief

The lithography-free technique is proposed to obtain a regular relief in the form of a regular 2D system of 25-nm protrusions on a sapphire plate surface. The use of grid masks allows one to form a regular relief on the sapphire substrates of arbitrary area. The structure of crystal films formed by the sputtering of metal aluminum onto sapphire substrates with subsequent oxidation and annealing is thoroughly investigated and compared with the nanostructured (0001) sapphire wafer surface in the form of regular steps up to 5 nm in height with atomically smooth terraces.     

Optical and structural characterisation of zinc oxide thin films prepared by sol‐gel process

Zinc oxide thin films have been prepared on different substrates by the sol‐gel method using 2‐methoxyethanol solution of zinc acetate dihydrate stabilized by monoethanolamine. The photoluminescence spectra of the films show the band‐edge and sub‐band transitions. The intensity of the band edge emission peak increases, while the intensity of the deep level emission peak decreases in the films coated on sapphire substrate. Transmittance spectra show that the films are transparent beyond 400 nm. The structural property of the films has been evaluated using X‐ray diffraction. The X‐ray peak intensity of the film (002) grown on sapphire substrate is higher than the films grown on glass and quartz substrates. The AFM images show improvement in the surface of the annealed films as compared to the as‐grown ZnO films coated on sapphire substrates. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Specific features of the formation of submicron inclusion in Ti:sapphire

Specific features of the formation of submicron (70–300 nm) inclusions in Ti:sapphire (Al₂O₃:Ti) grown in a carbon-containing medium have been investigated. These inclusions are caused by deviation from the melt stoichiometry and are formed during the melt-crystal phase transition. These defects are submicropores containing excess aluminum and its suboxides; they can be destroyed by thermal loading of a crystal.     

单晶硅外延生长晶化硅薄膜的研究

采用P型单晶硅片为衬底,并经混合酸溶液腐蚀抛光、清洗后,利用射频磁控溅射镀膜系统在其表面制备非晶硅薄膜;再结合快速光热退火工艺,于N2气氛下480℃退火30 min,得到晶化硅薄膜;利用光学金相显微镜、XRD衍射仪和拉曼散射光谱(Raman)仪对单晶硅衬底和晶化硅薄膜进行结构和性能表征.研究了混合酸溶液对单晶硅表面腐蚀效果、籽晶诱导外延生长晶化硅薄膜的物相结构和薄膜带隙.结果表明:采用混合酸溶液腐蚀后得到表面平整、光滑的单晶硅衬底;非晶硅薄膜经过快速退火后受籽晶诱导生成晶化硅薄膜,其晶相沿单晶硅衬底取向择优生长;随着非晶硅薄膜厚度从80 nm增加到280 nm,晶化后硅薄膜的表面粗糙度逐渐减小,晶化率从90.0;逐渐降低到37.0;;晶粒尺寸从6.65 nm逐渐减小到1.71 nm;带隙从1.18 eV逐渐升高到1.52 eV.     

Specific features of the growth of A II B VI films on (0001)Al2O3 substrates

The structure and orientation of CdTe and ZnO films on sapphire have been investigated for different techniques of pregrowth substrate treatment. Polycrystalline CdTe films are found to grow of substrates unannealed or annealed in vacuum at a residual pressure P < 0.13 Pa. Epitaxial CdTe films with the sphalerite cubic structure, oriented parallel to the substrate by the (111) plane, grow on substrates annealed in air at a temperature of 1000°C or more and having a system of smooth terraces and steps on the surface. For ZnO films with a wurtzite hexagonal structure obtained by magnetron sputtering, a similar correlation between the structural quality and the regime of treatment of sapphire substrates is observed. It is shown that thermal annealing of (0001) sapphire plates in air is the optimal way of substrate preparation for growing epitaxial ZnO films with the base orientation. The obtained epitaxial CdTe films contain a certain amount of structural defects (mosaicity and twins), while the epitaixal ZnO films treated in the same way are close to perfect.     

Multiatomic step formation on GaAs(001) vicinal surfaces during thermal treatment

We observed the surface morphology of vicinal GaAs(001) after thermal treatment in AsH₃/H₂ atmosphere by atomic force microscopy (AFM). Clear multiatomic steps were formed under the high temperature thermal treatment. Next, we investigated the mechanism of step bunching during thermal treatment by two experiments from the view point of Ga atom evaporation. One is the selective thermal treatment using a partially masked GaAs wafer, and the evaporation amount of Ga atoms was estimated by AFM. The other is the investigation of photoluminescence (PL) peak energy shifts for AlGaAs/GaAs single quantum wells with a thermal treatment process at the top of the GaAs quantum well layer, compared to those without thermal treatment. These results indicate that the evaporation hardly occurs during the thermal treatment process. Therefore, step bunching phenomena on GaAs(001) vicinal surfaces during thermal treatment are probably caused by migration of the atoms detached from upside steps and their re-incorporation to downside steps.     

Correlation between the luminescence properties and the surface structures of submicron silica particles

We investigated the correlation between the luminescence properties and the surface structures of submicron silica particles prepared by the Stöber method. After annealing in a non-oxidizing atmosphere, the submicron-sized silica particles show a broad photoluminescence (PL) band at 500–540 nm by excitation at an ultraviolet wavelengths (254 and 365 nm), and the one at the 600 nm by excitation an Ar⁺ laser (488 nm). The PL appeared to result from the removal of impurities and subsequent formation of several luminescent structures on the internal surface of the primary particles by thermal annealing.     

Supersmooth and modified surface of sapphire crystals: Formation,characterization, and applications in nanotechnologies

The results of studying the state of the surface of sapphire crystals by a complex of methods in different stages of crystal treatment are considered by an example of preparing sapphire substrates with a supersmooth surface. The possibility of purposefully forming regular micro- and nanoreliefs and thin transition layers using thermal and thermochemical impacts are considered. The advantages of sapphire substrates with a modified surface for forming heteroepitaxial CdTe and ZnO semiconductor films and ordered ensembles of gold nanoparticles are described. The results of the experiments on the application of crystalline sapphire as a material for X-ray optical elements are reported. These elements include total external reflection mirrors and substrates for multilayer mirrors, output windows for synchrotron radiation, and monochromators working in the reflection geometry in X-ray spectrometers. In the latter case, the problems of the defect structure of bulk crystals sapphire and the choice of a method for growing sapphire crystals of the highest structural quality are considered.     

Low-pressure HVPE growth of crack-free thick AlN on a trench-patterned AlN template

A thick AlN layer was grown on a trench-patterned AlN/sapphire template by low-presssure hydride vapor phase epitaxy (LP-HVPE). Compared with the AlN layer grown on a flat AlN/sapphire template, the AlN layer grown on the trench-patterned AlN/sapphire template had a crack-free and smooth surface. The typical full-widths at half-maximum (FWHMs) of X-ray rocking curves (XRC) for the (0 0 0 2), (1 0 1¯ 2), and (1 0 1¯ 0) diffractions of the AlN layer on the trench-patterned AlN/sapphire template were 132, 489, and 594 arcsec, respectively. In addition, atomic steps were observed on the AlN layer on the trench-patterned AlN/sapphire template, and the root-mean-square (RMS) roughness of the AlN layer was determined to be 0.602 nm by atomic force microscopy (AFM).     

热屏间距对泡生法蓝宝石单晶生长影响研究

太空、军事和科研等高科技领域的持续发展极大促进了对蓝宝石晶体的需求,泡生法是蓝宝石晶体的主要制造方法之一;热场结构对所得蓝宝石晶体的质量具有重要影响.本文对采用泡生法工艺制造蓝宝石单晶过程中,具有内置7层氧化锆外置8层钼金属的新型热屏结构间距进行研究.通过数值模拟考察热屏间距对单晶炉功率、固-液界面形状和晶体热应力的影响确定了合理的热场结构;并与试验生产结果进行对比验证.结果表明:热屏间距增大使得单晶炉功率明显提升,并引起固-液界面凸度增大;而蓝宝石晶体热应力出现减小.综合考察三个影响因素的影响,最后确定热屏间距为5 mm时单晶炉能耗较低,可用于制造高质量的蓝宝石晶体.     

蓝宝石衬底上磁控溅射法室温制备外延ZnO薄膜

在室温条件下,采用磁控溅射方法在蓝宝石(0001)衬底上制备了外延的ZnO薄膜.采用原子力显微镜(AFM)、X射线衍射仪(XRD)、可见-紫外分光光度计系统研究了ZnO薄膜微观结构和光学特性.AFM测量结果表明ZnO薄膜具有较为均匀的ZnO晶粒,表面平整,具有较小的均方根粗糙度(0.9 nm);X射线衍射结果表明制备的ZnO薄膜为具有六角纤锌矿结构的外延薄膜;光学透射谱显示样品在可见光范围内具有较高的透过性,并在370 nm附近出现一个较陡的吸收边,表明在室温下制备出了具有较高质量的ZnO薄膜.     

Growth of oriented GaN nanowires by controlling nucleation conditions

GaN nanowires (NWs) were grown on GaN(0001) coated sapphire substrate with Ni/Au catalyst by using metalorganic chemical vapor deposition. Nucleation conditions were investigated for improving the growth orientation of NWs. With decreasing catalyst thickness from 5nm/5nm to 2nm/2nm, the NW orientation was improved and the NW morphology was changed from taper to cylindrical, due to the varying of growth mode. Vertical alignment of NWs can be further improved by inserting an additional high‐temperature (850 ℃) nucleation step with an optimum V/III flow ratio of 20.