p型掺杂晶体硅的低温液相(001)外延生长研究

为寻求以低成本制备n型太阳电池的pn结,进行了Al-17.6wt;Si合金熔体中(001)n型单晶硅衬底上液相外延生长p型掺杂硅实验.所用方法为垂直浸渍法,实验了过冷恒温生长与回熔处理后连续冷却生长两种模式,过程中体系以流动高纯氩保护.对所得外延生长晶体结构、形貌及所得pn结开路电压进行了分析和测定.结果显示,合金熔体中硅晶体(001)液相外延生长能够实现,但一般呈离散分布的金字塔型岛状生长;只有衬底回熔处理后原位连续降温生长模式可获得连续外延薄膜,之后在其上出现岛状生长,呈现Stranski-Krastanov生长模式.所得连续外延薄膜形成的pn结开路电压比恒温生长所得的提升约100 mV;连续外延薄膜形成后期出现的岛状生长使开路电压明显下降;生长速度提高会使连续降温外延生长pn结开路电压略有降低.     

SiC衬底上生长的GaN外延层的高分辨X射线衍射分析

通过高分辨X射线衍射(HRXRD)技术,对金属有机化合物气相外延(MOCVD)生长的GaN外延膜及SiC衬底的相对取向,晶格常数和应力情况,位错密度等进行了分析.分析表明,GaN和SiC具有一致的a轴取向,GaN外延层弛豫度超过90;,GaN外延层的晶格常数与体块材料相近,在GaN中存在压应力,SiC衬底和GaN外延层中的位错密度分别为107和108量级.     

β-BBO薄膜的液相外延生长

采用液相外延法在掺Sr2+的α-BBO(001)衬底上进行了生长β-BBO薄膜的实验,生长出表面光滑、无色透明的薄膜.应用X射线粉末衍射(XRD)、分光光谱仪和原子力显微镜(AFM)对外延膜进行了分析测试.结果表明,所制备的β-BBO薄膜择优取向为(001)面,在1μm×1μm的面积内,外延膜的表面粗糙度为2.279nm,其紫外吸收边同β-BBO单晶一样也为190nm,但外延膜的透过率略有下降.采用调Q脉冲Nd: YAG激光器观察了β-BBO外延膜的倍频效应.     

GaSb/GaAs异质结热光伏电池材料的MBE生长

利用分子束外延方法(MBE)在GaAs (001)衬底上外延生长了GaSb薄膜,并对GaSb薄膜进行了高温退火研究,利用高分辨透射电子显微镜(HRTEM)、原子力显微镜(AFM)、Hall效应(Hall Effect)和低温光荧光谱(LTPL)等手段对薄膜的晶体质量、电学性质和光学性质进行了研究.发现直接生长的GaSb膜表面平整,空穴迁移率较高.研究发现30 s、650℃的快速热退火可消除位错等缺陷,显著提高GaSb薄膜的光学质量.     

GaSb薄膜生长的RHEED研究

采用分子束外延技术,在GaAs衬底上生长GaSb薄膜时,利用反射式高能电子衍射仪(RHEED)对衬底表面清洁状况、外延层厚度等进行在线监控.通过RHEED讨论低温缓冲层对GaSb薄膜表面结构和生长机制的作用,可以估算衬底温度,并能计算出薄膜的生长速率.实验测量GaSb的生长周期为1.96s,每秒沉积0.51单分子层.低温缓冲层提高了在GaAs衬底上外延GaSb薄膜的生长质量.     

在Si(100)上以AlN作过渡层低温外延生长ZnO薄膜

采用脉冲激光沉积(PLD)方法在Si(100)上成功生长了高度c轴取向的AlN薄膜,并以此为衬底,实现了ZnO薄膜的低温准外延生长.通过X射线衍射(XRD)、原子力显微镜(AFM)以及荧光分光光度计表征ZnO薄膜的结构、表面形貌和发光性能.结果表明,ZnO薄膜能在AlN过渡层上沿c轴准外延生长,采用AlN过渡层后,其荧光强度也有大幅提高.     

热丝CVD法在单晶硅衬底上低温外延生长Si和Ge薄膜的研究

采用热丝CVD法在单晶Si衬底上进行了Si和Ge 薄膜的低温外延生长,用XRD和Raman谱对其结构性能进行了分析.结果表明:在衬底温度200 ℃时,Si(111)单晶衬底上外延生长出了Raman峰位置为521.0 cm-1;X射线半峰宽(FWHM)为5.04 cm-1.结晶质量非常接近于体单晶的(111)取向的本征Si薄膜;在衬底温度为300 ℃时,在Si(100)单晶衬底上异质外延,得到了Raman峰位置为300.3 cm-1的Ge薄膜,Ge薄膜的晶体取向为Ge(220).研究表明热丝CVD是一种很好的低温外延薄膜的方法.     

NiFe2O4/(0.8BaTiO3-0.2Na0.5Bi0.5TiO3)异质结层状复合薄膜的磁电性能

采用脉冲激光沉积法在(001)-SrRuO3/SrTiO3(SRO/STO)衬底上生长了2-2型NiFe2O4/(0.8BaTiO3-0.2Na0.5 Bi0.5TiO3)(NFO/BT-NBT)磁电复合薄膜.X射线衍射仪(XRD)结果显示所有薄膜均为(00l)择优取向结构.物理性能测试结果表明:NFO/BT-NBT复合薄膜同时具有良好的铁磁性和铁电性,但不同的沉积顺序对复合薄膜的磁电耦合性能产生重要影响.以铁磁材料NFO为顶层的NFO/BT-NBT/SRO/STO异质结的磁电耦合系数(αE～110 mV·cm-1·Oe-1)大于以铁电材料BT-NBT为顶层的BT-NBT/NFO/SRO/STO异质结的磁电耦合系数(αE～80 mV·cm-1·Oe-1),这是NFO层受到衬底束缚作用不同的结果.     

低温超晶格YSZ/STO/YSZ电解质薄膜的制备及表征

采用脉冲激光沉积法(PLD),在Al2O3 (ALO)衬底上,将Y2O3∶ZrO2(YSZ)和SrTiO3 (STO)按照YSZ/STO/YSZ的顺序依次沉积,形成超晶格YSZ/STO/YSZ电解质薄膜,利用SEM、XRD和交流阻抗对其形貌、相结构和电学性能进行了表征.结果表明,衬底温度为700℃形成的超晶格YSZ/STO/YSZ电解质薄膜颗粒大且均匀,排列紧密且呈规律圆柱状;YSZ、STO均沿(111)方向择优生长;低温时电导率比单层YSZ电解质薄膜高出4个数量级,是较为理想的低温固体燃料电池电解质.     

异质衬底上HVPE法生长GaN厚膜的研究进展

氮化镓基(GaN)光电器件的快速发展,对GaN的质量提出更高的要求,同质外延可以避免由于失配引起的缺陷,厚膜生长是解决GaN体材料生长困难的有效手段.氢化物气相外延(HVPE)是目前最普遍的制备氮化镓厚膜的方法.衬底对于GaN厚膜的影响不可忽视,本文总结了在蓝宝石、碳化硅和铝酸锂衬底上制备GaN厚膜的研究进展,讨论了今后的研究方向.     

Epitaxial growth of the CoFe2O4 film on SrTiO3 and its characterization

Cobalt ferrite (CoFe₂O₄) thin film is epitaxially grown on (0 0 1) SrTiO₃ (STO) by laser molecular beam epitaxy (LMBE). The growth modes of CoFe₂O₄ (CFO) film are found to be sensitive to laser repetition, the transitions from layer-by-layer mode to Stranski–Krastanov (SK) mode and then to island mode occur at the laser repetition of 3 and 5 Hz at 700 °C, respectively. The X-ray diffraction (XRD) results show that the CFO film on (0 0 1) SrTiO₃ is compressively strained by the underlying substrate and exhibits high crystallinity with a full-width at half-maximum of 0.86°. Microstructural studies indicate that the as-deposited CFO film is c-oriented island structure with rough surface morphology and the magnetic measurements reveal that the compressive strained CoFe₂O₄ film exhibits an enhanced out-of-plane magnetization (190 emu/cm³) with a large coercivity (3.8 kOe).     

Growth of vertical and defect free InP nanowires on SrTiO3(001) substrate and comparison with growth on silicon

We present a study of the molecular beam epitaxy of InP nanowires (NWs) on (001) oriented SrTiO₃ (STO) substrates using vapor liquid solid mechanism and gold–indium as metal catalyst. The growth direction of InP NWs grown on STO(001) is compared with NWs grown on (001) and (111) oriented silicon substrates. Gold–indium dewetting under a flux of indium results in the majority of InP NWs growing vertically from the surface of STO(001). With the growth parameters we have used the NWs have a pure wurtzite structure and are free of stacking faults and cubic segments. The structural quality of the NWs is confirmed by micro-photoluminescence measurements showing a narrow peak linewidth of 6.5 meV.     

Study on the growth and interfacial strain of CoFe2O4/BaTiO3 bilayer films

CoFe₂O₄/BaTiO₃ bilayer films were epitaxially deposited on SrTiO₃ substrates by laser molecular beam epitaxy (LMBE). The growth process of the bilayer films was in-situ monitored by reflection high-energy electron diffraction (RHEED). Sixty nanometer thick-BTO layer was firstly fabricated in a layer-by-layer growth mode with an atomic smooth surface. CFO films with a varying thickness ranging from 5 to 60 nm were subsequently deposited on BTO-coated STO substrates. The different growth behaviors of CFO films were observed due to the lattice mismatch strain. Between two short stages of the growth mode transforming, a long duration with Stransky and Krastonov growth mode was maintained. Strainfully relaxed CFO film in the island growth mode was finally formed. High-resolution X-ray diffraction (HRXRD) was used to further analyze the strain effect. It was found that the tensile stress imposed on BTO by CFO was strengthened with increasing the thickness of CFO films, which could lessen the distortion of BTO by counteracting the compressive stress caused by STO substrates. The strengthened tensile stress weakened the ferroelectric property of BTO films by reducing structural tetragonality, which was demonstrated by polarization-electric (P-E) measurement.     

Growth and properties of Pb[(Zn1/3Nb2/3)0.91Ti0.09]O3 single crystals directly from melt

Pb[(Zn_1/3Nb_2/3)_0.91Ti_0.09]O₃ (PZNT91/9) single crystals were grown by a modified Bridgman method directly from melt using an allomeric Pb[(Mg_1/3Nb_2/3)_0.69Ti_0.31]O₃ (PMNT69/31) single crystal as a seed. X-ray diffraction (XRD) measurement confirmed that the as-grown PZNT91/9 single crystals are of pure perovskite structure. Electrical properties and thermal stabilization of PZNT91/9 crystals grown directly from melt exhibit different characters from those of PZNT91/9 crystals grown from flux, although segregation and the variation of chemical composition are not seriously confirmed by X-ray fluorescence analysis (XPS). The [0 0 1]-oriented PZNT91/9 crystals cut from the middle part of the as-grown crystal boules exhibit broad dielectric-response peaks at around 105 °C, accompanied by apparent frequency dispersion. The values of piezoelectric constant d₃₃, remnant polarization P_r, and induced strain are about 1800–2200 pC/N, 38.8 μC/cm², and 0.3%, respectively, indicating that the quality of PZNT crystals grown directly from melt can be comparable to those of PZNT91/9 single crystals grown from flux. However, further work deserves attention to improve the dielectric properties of PZNT crystals grown directly from melt. Such unusual characterizations of dielectric properties of PZNT crystals grown directly from melt are considered as correlating with defects, microinhomogeneities, and polar regions.     

Surface and interfacial structure of epitaxial SrTiO3 thin films on (0 0 1) Si grown by molecular beam epitaxy

Effects of relaxation of interfacial misfit strain and non-stoichiometry on surface morphology and surface and interfacial structures of epitaxial SrTiO₃ (STO) thin films on (0 0 1) Si during initial growth by molecular beam epitaxy (MBE) were investigated. In situ reflection high-energy electron diffraction (RHEED) in combination with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectrometry (XPS) and transmission electron microscopy (TEM) techniques were employed. Relaxation of the interfacial misfit strain between STO and Si as measured by in situ RHEED indicates initial growth is not pseudomorphic, and the interfacial misfit strain is relaxed during and immediately after the first monolayer (ML) deposition. The interfacial strain up to 15 ML results from thermal mismatch strain rather than lattice mismatch strain. Stoichiometry of STO affects not only surface morphology but interfacial structure. We have identified a nanoscale Sr₄Ti₃O₁₀ second phase at the STO/Si interface in a Sr-rich film.     

Growth and characterization of highly c‐axis textured SrTiO3 thin films directly grown on Si(001) substrates by ion beam sputter deposition

Highly c‐axis textured SrTiO₃ (STO) thin films have been directly grown on Si(001) substrates using ion beam sputter deposition technique without any buffer layer. The substrate temperature was varied, while other parameters were fixed in order to study effect of substrate temperature on morphology and texture evolution of STO films. X‐ray diffraction, pole figure analysis, atomic force microscope, and high‐resolution electron microscopy were used to characterize and confirm quality and texture of the STO films. The experimental results show that optimum substrate temperature to achieve highly c‐axis textured films is at 700 °C. The full width at half maximum (FWHM) of 002_STO was found to be 2° and fraction of (011) orientation was as low as 1%. The surface morphology was Volmer‐Weber growth mode with a small roughness ∼1 nm. The lowest leakage current density (5.8 μA/cm² at 2 V) and the highest dielectric constant (ε_STO ∼ 98) were found for highly c‐axis textured films grown at 700 °C. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth defects in a PZNT93/7 crystal prepared by directional solidification

Novel relaxor ferroelectric crystal 0.93Pb(Zn_1/3Nb_2/3)O₃‐0.07PbTiO₃ (PZNT93/7) with dimensions about Ø40× 70 mm³ was obtained by directional solidification technique. The growth defects of the crystal were investigated. Rocking curve analysis revealed the crystalline quality of PZNT93/7 crystal was not perfect and the FWHM value was measured to be about 0.7°. Some pits and oxide particles in micro‐size were formed in the crystal due to the growth conditions. A series of growth steps parallel to (001) face were observed which were attributed to the growth behavior. Moreover, it was found the average chemical composition of the crystal was deviated slightly to the stoichiometric value of PZNT93/7. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation on growth defects in Pb(Zn1/3Nb2/3)O3‐PbTiO3 crystals

Relaxor ferroelectric crystal (1‐x)Pb(Zn_1/3Nb_2/3)O₃‐xPbTiO₃ (PZNT) with x=0.07 (PZNT93/7) has been grown by the vertical Bridgman method from the high temperature solution of PZNT‐PbO system. The growth defects, such as nucleation core, inclusions, boundaries and particles, were investigated by optical microscope and scanning electron microscope. Sub‐structures were found in the flux inclusions and the lack of ZnO component in PZNT crystals was attributed to the existence of ZnO particles in the inclusions. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth anisotropy in the GaN/Al2O3 system

The crystallographic aspect of gallium nitride epitaxy on sapphire were investigated for the vapor phase epitaxy system GaCl/NH₃/HCl/N₂. For this purpose, thick layers of doped gallium nitride were deposited on hemispheres of Al₂O₃ single crystal with the three fold axis of the corundum structure perpendicular to the basal plane. Complete characterization of these layers has been carried out including Laue back diffraction, scanning electron microscopy and cathodoluminescence. After growth, sections were cut along the main crystallographic planes and the growth rate was measured as a function of the substrate orientation. The observed relationship for epitaxy has been interpreted in terms of three-dimensional matching between the gallium nitride and corundum structures.     

Effects of surface reconstruction on CdTe/GaAs(001) interface structure

CdTe/GaAs(001) heterostructures were fabricated by molecular beam epitaxy on chemically etched and thermally deoxidized GaAs(001) substrates, as well as GaAs(001) (3×1) buffer layers grown in situ by molecular beam epitaxy. Different growth protocols were also explored, leading to Te-induced (6×1) or (2×1) surface reconstructions during the early growth stage. High-resolution cross-sectional transmission electron microscopy was used to examine the final interface structure resulting from the different substrate preparations, and surface reconstructions. The (2×1) surface reconstruction led to pure (001) growth, while the (6×1) reconstruction led to an interface which included small (111)-oriented inclusions. In addition, deposition on etched and deoxidized GaAs(001) wafers led to preferential CdTe growth within etch pits and resulted in a macroscopically rough interface region.