Epitaxial aluminum nitride films on sapphire formed by pulsed laser deposition

Highly oriented aluminum nitride thin films were grown on sapphire (0001)-substrate by pulsed laser deposition technique. Characterization was done by X-ray-diffraction, elastic recoil detection analysis and Rutherford backscattering/channeling measurements. The epitaxial properties were studied as function of the substrate temperature and the deposition rate. An epitaxial relation to the sapphire substrate is found to be AlN [0001] || Al₂O₃ [0001] and AlN [11 0] || Al₂O₃ [10 0]. XRD-texture-analysis on films deposited at 850°C shows a full width half maximum Δω of 0.13° (rocking curve) and Δ of 1.1° (in-plane).     

高分辨X射线衍射表征氮化镓外延层缺陷密度

利用高分辨X射线衍射方法, 分析了在4H-SiC(0001)面上采用金属有机物化学气相沉积(MOCVD)生长的GaN薄膜的位错。采用对称面衍射和斜对称面衍射等方法研究了晶面倾转角、面内扭转角、晶粒尺寸和晶面弯曲半径等参数, 通过排除仪器、晶粒尺寸及晶面弯曲对摇摆曲线半高宽的影响, 从而获得GaN薄膜的螺位错密度和刃位错密度分别为4.62×10⁷ cm^-2和5.20×10⁹ cm^-2, 总位错密度为5.25×10⁹ cm^-2。     

Growth of GaN and AlN thin films by laser induced molecular beam epitaxy

Hexagonal GaN and AlN thin films were grown by laser induced molecular beam epitaxy using Al or Ga metal as target material and N₂as nitrogen source. The films were deposited on sapphire (0001) and SiC (0001) substrates. Epitaxial growth of GaN has been achieved at 730°C and 10⁻³ mbar N₂ pressure. The AlN films were polycrystalline with predominant (0001) orientation.     

Epitaxial growth of RuO2 thin films by metal-organic chemical vapor deposition

Conductive RuO₂ thin films were epitaxially grown on LaAlO₃(100) and MgO(100) substrates by metal-organic chemical vapor deposition (MOCVD). The deposited RuO₂ films were crack-free, and well adhered to the substrates. The RuO₂ film is (200) oriented on LaAlO₃ (100) substrates at deposition temperature of 600°C and (110) oriented on MgO(100) substrates at deposition temperature of 350°C and above. The epitaxial growth of RuO₂ on MgO and LaAlO₃ is demonstrated by strong in-plane orientation of thin films with respect to the major axes of the substrates. The RuO₂ films on MgO(100) contain two variants and form an orientation relationship with MgO given by RuO₂(110)//MgO(100) and RuO₂[001]//MgO[011]. The RuO₂ films on LaAlO₃(100), on the other hand, contain four variants and form an orientation relationship with LaAlO₃ given by RuO₂(200)//LaAlO₃(100) and RuO₂[011]//LaAlO₃[011]. Electrical measurements on the RuO₂ thin films deposited at 600°C show room-temperature resistivities of 40 and 50 μΩ cm for the films deposited on the MgO and LaAlO₃ substrates, respectively.     

Deposition of CeO2 films including areas with the different orientation and sharp border between them

Epitaxial films from one material, with sharp borders between contacting regions having different film orientation are grown on one surface of the substrate for the first time. The main reason for the deposition of thin ceria layers with mixed (001) and (111) orientations on a (1 02) sapphire substrate is determined. We suggest that this is related to the availability of surface defects which, in thin near-surface layers, deviate from stoichiometric composition. This in turn is connected with the loss of oxygen.

A technique for influencing CeO₂ film orientation is demonstrated. This involves specific preliminary processing of the substrate, and the selection of oxygen partial pressure during the deposition process.

High quality thin (30–50 nm) “protective” (001) CeO₂ epitaxial layers are prepared on (1 02) Al₂O₃. Structures comprising two epitaxial protective CeO₂ layers, orientations (001) and (111), are made on the base of (0001) and (1 02) sapphire substrates. The interface between the epitaxial layers is <1 000 nm.

Preliminary results using this method are described, and the possibility of creating a “bi-epitaxial” transition in thin YBa₂Cu₃O_7−x layers is explored.     

Fabrication of GaN epitaxial thin film on InGaZnO4 single-crystalline buffer layer

Epitaxial (0001) films of GaN were grown on (111) YSZ substrates using single-crystalline InGaZnO₄ (sc-IGZO) lattice-matched buffer layers by molecular beam epitaxy with a NH₃ source. The epitaxial relationships are (0001)_GaN//(0001)_IGZO//(111)_YSZ in out-of-plane and [112¯0]_GaN//[112¯0]_IGZO//[11¯0]_YSZ in in-plane. This is different from those reported for GaN on many oxide crystals; the in-plane orientation of GaN crystal lattice is rotated by 30° with respect to those of oxide substrates except for ZnO. Although these GaN films showed relatively large tilting and twisting angles, which would be due to the reaction between GaN and IGZO, the GaN films grown on the sc-IGZO buffer layers exhibited stronger band-edge photoluminescence than GaN grown on a low-temperature GaN buffer layer.     

In incorporation efficiency and composition fluctuations in MOVPE grown GaInN/GaN hetero structures and quantum wells

GaInN layers play a key role in short wavelength optoelectronic devices for the visible spectrum. However, the epitaxial growth of In containing nitrides is more problematic than that of GaN and AlGaN. In order to increase the In incorporation efficiency, lower growth temperatures of around 700–800°C are needed. We have optimized the metalorganic vapor-phase epitaxial growth of GaInN by decreasing the H₂/N₂ ratio in the gas-phase and increasing the growth rate. However, the deposited films showed strong indications for compositional fluctuations. Besides a large miscibility gap predicted for GaInN, the mismatch induced strain for GaN may play a major role in these growth problems.     

Structure and surface termination of ZnO films grown on (0001)- and (112¯0)-oriented Al2O3

We have studied the surface termination of ZnO(0001¯) films grown on Al₂O₃ substrates with high epitaxial quality. The structural properties of the ZnO films were investigated by X-ray scattering, revealing a predominant (0001¯)ZnO out-of-plane texture with the [112¯0]_ZnO[0001]_Al2O3 and [112¯0]_ZnO[101¯0]_Al2O3 azimuthal orientations for (112¯0)Al₂O₃ and(0001)Al₂O₃ substrates, respectively. The surface termination was determined by X-ray photoemission spectroscopy (XPS) via pyridine (C₅H₅N) adsorption at the ZnO surface. XPS data recorded at different temperatures after exposure to pyridine revealed that for both orientations of the Al₂O₃ substrates the deposited ZnO films were terminated by oxygen atoms, i.e. corresponding to a ZnO (0001¯) surface.     

Low-temperature epitaxial growth of conductive LaNiO3 thin films by RF magnetron sputtering

Epitaxial growth of LaNiO₃ (LNO) thin films was successful on CeO₂/YSZ/Si(100), MgO(100) and SrTiO₃ (STO)(100) substrates by RF magnetron sputtering at 300 °C, although pulsed laser deposition requires 600 °C to prepare epitaxial LNO films according to the literature. Epitaxial LNO films deposited on CeO₂/YSZ/Si(100) and STO(100) had single orientation of LNO[100]//CeO₂[110]//YSZ[110]//Si[110]) and LNO[100]//STO[100], respectively. On the other hand, epitaxial LNO films deposited on MgO(100) had mixed orientations of LNO[100]//MgO[100] and LNO[100]//MgO[110]. The lattice parameter, composition and resistivity of the LNO thin films were strongly dependent on the substrate temperature. The minimum resistivity of LNO films was approximately 5×10⁻⁶ Ω m, which value almost agrees with the resistivity in the literature. It was found that the temperature to achieve minimum resistivity was 200 °C, irrespective of the type of substrate. The surface of the LNO films was smooth and flat.     

Molecular beam epitaxy of c-plane wurtzite GaN on nitridized a-plane β-Ga2O3

Epitaxial growth of GaN on β-Ga₂O₃ single crystal substrates by the molecular beam epitaxy technique is demonstrated for the first time. Standard and in-plane X-ray diffraction evidence the growth of c-plane wurtzite GaN on a-plane β-Ga₂O₃, the epitaxial relationship being 0 1 0_Ga2O3 1 1 2¯ 0_GaN and 0 0 1_Ga2O31¯ 1 0 0_GaN. Epitaxial growth without any buffer layer is achieved after an effective surface nitridation under NH₃ gas.     

III-Nitrides growth and AlGaN/GaN heterostructures on ferroelectric materials

The growth of III-nitrides on the ferroelectric materials lithium niobate (LN) and lithium tantalate (LT) via molecular beam epitaxy (MBE) using rf plasma source has been investigated. We have found that gallium nitride (GaN) epitaxial layers have a crystalline relationship with lithium niobate (tantalate) as follows: (0 0 0 1) GaN || (0 0 0 1) LN (LT) with [10−10] GaN || [11−20] LN (LT). The surface stability of LN and LT substrates has been monitored by in situ spectroscopic ellipsometry in the vacuum chamber. Three different temperature zones have been discerned; surface degas and loss of OH group (100–350 °C); surface segregation/accumulation of Li and O-species (400–700 °C); surface evaporation of O-species and Li desorption (over 750 °C). However, LT shows only surface degassing in the range of 100–800 °C. Therefore, congruent LN substrates were chemically unstable at the growth temperature of 550–650 °C, and therefore developed an additional phase of Li-deficient lithium niobate (LiNb₃O₈) along with lithium niobate (LiNbO₃), confirmed by X-ray diffraction. On the other hand, LT showed better chemical stability at these temperatures, with no additional phase development. The structural quality of GaN epitaxial layers has shown slight improvement on LT substrates over LN substrates, according to X-ray diffraction. Herein, we demonstrate AlGaN/GaN heterostructure devices on ferroelectric materials that will allow future development of multifunctional electrical and optical applications.     

Growth of SrTiO3 thin epitaxial films by aerosol MOCVD

SrTiO₃ thin films were prepared by aerosol metal-organic chemical vapour deposition on (001) MgO, R-plane Al₂O₃ and (001) Si single-crystal substrates. Strontium tetramethyl heptadionate and titanium n-butoxide dissolved in diethyleneglycol dimethyl ether were used as precursors. The structure of the films was investigated by X-ray diffraction and transmission electron microscopy. Epitaxial films with [001] and [111] orientation perpendicular to the substrate surface were obtained on MgO and Al₂O₃, respectively. The epitaxial films on the MgO substrate were found to be in a relaxed state with lattice parameters corresponding to the bulk values. SrTiO₃ films on the Si substrate were grown as highly textured in the [011]direction and randomly oriented in the plane parrallel to the substrate surface.     

Effects of p(O2) and p(CO2) on epitaxial growth of BaTiO3 thin films on MgO(100) substrates by using metal organic acid salts

BaTiO₃ thin films were prepared by using metal organic acid salts on MgO(100) substrates, which have large lattice-misfit with BaTiO₃. Amorphous films prefired at 470°C were crystallized to BaTiO₃ phase by heat treatment at higher temperature. Crystallinity and in-plane alignment of the prepared films were found to depend on the heat-treatment conditions. BaTiO₃ films with high crystallinity but poor (100)-orientation were obtained in air at higher than 1200°C. Whereas, (100)-oriented epitaxial BaTiO₃ film was fabricated by annealing at 900°C under low oxygen partial pressure (p(O₂)). Low carbon dioxide partial pressure (p(CO₂)) is also found to be essential for preparation of epitaxial BaTiO₃ films on MgO substrates by using metal organic acid salts.     

The structure of GaN layers grown on SiC and sapphire by molecular beam epitaxy

A comparative study of the defects at the interfaces and inside the layers was carried out in GaN/AlN epitaxial layers on SiC and sapphire. Whereas surface cleaning of the sapphire substrates is rather standardised now, the SiC substrates cleaning is still to optimise conditions, as the high densities of defects inside the epitaxial layers cannot be explained solely by the 3.54% lattice mismatch. The investigated specimens were grown by molecular beam epitaxy (MBE), either assisted by electron cyclotron resonance or an NH₃ gas source system to provide atomic nitrogen. Assuming that MBE is a growth technique more or less close to equilibrium, the observed defects are interpreted and a growth mechanism, for GaN layers on the stepped (0001) SiC and sapphire surfaces, is proposed.     

织构化ZrB2-SiC超高温陶瓷中取向关系的EBSD研究

以放电等离子体烧结的织构化ZrB₂-SiC复相陶瓷为研究对象, 利用SEM和EDS方法对相组成进行分析。研究发现, 由于初始粉体与杂质之间存在多种反应, 陶瓷中出现相当含量的ZrC新相及少量的ZrO₂、BN相。与利用TEM研究新生成相与主相间取向关系的常规方法相比, SEM中EBSD方法不但能研究该取向关系, 还可同时对大量相界进行研究以获得统计性结果, 从而避免人为选择性。利用该方法, 对ZrB₂与ZrC相间可能存在的三种取向关系(011¯0)||(111)&[21¯1¯0]||[101¯]、(112¯0)||( 2¯02)&[0001]||[111]和(1¯21¯0)||(2 2¯0)&[0001]||[110]进行验证, 确定本实验中所得复相陶瓷中两相间不存在特定取向关系, 从而推断ZrC的成相遵循均匀成核模式, 而非外延成核。     

High-resolution and analytical transmission electron microscopy of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa₂Cu₃O_7−x thin films and YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa₂Cu₃O_7−x thin films With varying oxygen stoichiometry deposited onto SrTiO₃ are discussed. Then, YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices deposited onto SrTiO₃ and MgO are investigated. Finally, an interface analysis of high-quality YBa₂Cu₃O_7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.     

Characterization of GaN epitaxial layers on SiC substrates with AlxGa1−xN buffer layers

High quality GaN epitaxial layers were obtained with Al_xGa_1−xN buffer layers on 6H–SiC substrates. The low-pressure metalorganic chemical vapor deposition (LP-MOCVD) method was used. The 500 Å thick buffer layers of Al_xGa_1−xN (0≤x≤1) were deposited on SiC substrates at 1025°C. The FWHM of GaN (0004) X-ray curves are 2–3 arcmin, which vary with the Al content in Al_xGa_1−xN buffer layers. An optimum Al content is found to be 0.18. The best GaN epitaxial film has the mobility and carrier concentration about 564 cm² V⁻¹ s⁻¹ and 1.6×10¹⁷ cm⁻³ at 300 K. The splitting diffraction angle between GaN and Al_xGa_1−xN were also analyzed from X-ray diffraction curves.     

CeO2缓冲层层数对YBa2Cu3O7-σ超导薄膜载流能力的影响

为了提高超导薄膜的载流能力,本文通过MOCVD法,在1-5层(100)取向的氧化铈缓冲层薄膜上制备YBa₂Cu₃O_7-σ薄膜。研究了缓冲层层数对YBa₂Cu₃O_7-σ薄膜物相组成、结晶度、面内取向、微观形貌和超导性能的影响。结果表明,当缓冲层数为1-2时,薄膜呈现高度(001)取向生长;当缓冲层数为4-5时,薄膜呈现高度(110)取向生长。其中氧化铈薄膜层数为1时,YBCO薄膜结晶度和面内取向最好,其ω和Ф扫描曲线的FWHM值分别为2.63°和5.21°;随着缓冲层数的增加,YBCO薄膜结晶度和面内取向均呈现变差的趋势。氧化铈缓冲层与YBCO薄膜之间的外延生长关系为YBCO[001]//CeO₂[100]和YBCO[010]//CeO₂[011]。缓冲层数为1的YBCO薄膜表面主要由大面积c轴取向的晶粒组成,其厚度为485 nm,沉积速率为4.85μm/h,其液氮温度(77 K)下的临界电流为20.8 A,相比于缓冲层数为0的超导薄膜临界电流有所提升。     

Nitride-based Schottky diodes and HFETs fabricated by photo-enhanced chemical wet etching

Photo-enhanced chemical (PEC) wet etching technology was used to etch GaN and AlGaN epitaxial layers. It was found that the maximum etch rates were 510, 1960, 300, and 0 nm/mm for GaN, Al_0.175Ga_0.825N, Al_0.23Ga_0.77N, and Al_0.4Ga_0.6N, respectively. It was also found that we could achieve a high Al_0.175Ga_0.825N to GaN etch rate ratio of 12.6. Nitride-based Schottky diodes and heterostructure field effect transistors (HFETs) were also fabricated by PEC wet etching. It was found that we could achieve a saturated I_D larger than 850 mA/mm and a maximum g_m about 163 mS/mm from PEC wet etched HFET with a 0.5 μm gate length. Compared with dry etched devices, the leakage currents observed from the PEC wet etched devices were also found to be smaller.