In-plane Relationship Research Articles

Epitaxial properties of ZnO thin films on LaAlO3 substrates by pulsed laser deposition

Wurtzite ZnO thin films with different epitaxial relationships have been grown on (001)-, (011)-, and (111)LaAlO3 (LAO) single-crystal substrates by pulsed laser deposition. Nonpolar (112¯0)ZnO films with two orthogonal domains were obtained on (001) LAO, in which the in-plane orientation relationship is demonstrated to be 〈0001〉ZnO//〈110〉LAO. For ZnO on (011)- and (111)LAO substrates, a single-domain epitaxy with c axial orientation is observed, in which the in-plane relationships were 〈11¯00〉ZnO//〈011¯〉LAO irrespective of the substrate orientations. Based on the in-plane orientation relationship, the lattice mismatch has been obtained for these three oriented ZnO/LAO heterointerfaces.

Low temperature epitaxy of Ge-Sb-Te films on BaF2 (111) by pulsed laser deposition

Pulsed laser deposition was employed to deposit epitaxial Ge2Sb2Te5-layers on the (111) plane of BaF2 single crystal substrates. X-ray diffraction measurements show a process temperature window for epitaxial growth between 85 °C and 295 °C. No crystalline growth is observed for lower temperatures, whereas higher temperatures lead to strong desorption of the film constituents. The films are of hexagonal structure with lattice parameters consistent with existing models. X-ray pole figure measurements reveal that the films grow with one single out-of-plane crystal orientation, but rotational twin domains are present. The out-of-plane epitaxial relationship is determined to be Ge2Sb2Te5(0001) || BaF2(111), whereas the in-plane relationship is characterized by two directions, i.e., Ge2Sb2Te5 [-12-10] || BaF2[1-10] and Ge2Sb2Te5[1-210] || BaF2[1-10]. Aberration-corrected high-resolution scanning transmission electron microscopy was used to resolve the local atomic structure and confirm the hexagonal structure of the films.

Electrical and optical characterizations of β-Ga2O3:Sn films deposited on MgO (110) substrate by MOCVD

Tin-doped β-Ga2O3 (β-Ga2O3:Sn) films doped with different tin concentrations were deposited on MgO (110) substrates by metal organic chemical vapor deposition (MOCVD) at 700 °C. The effect of doping on the structural, electrical and optical properties of the films was investigated. The 10% Sn-doped film exhibited the best electrical conductivity properties with the lowest resistivity about 5.21 × 10−2 Ω cm, which is over ten orders of magnitude lower than the un-doped film. Micro-structural analysis revealed that the film with 10% Sn content had a clear in-plane relationship of β-Ga2O3 (100) ‖ MgO (110) with β-Ga2O3 (01) ‖ MgO (111). The average transmittance of the samples in the visible range exceeded 87% and the optical band gap of the films varied from 4.12 to 4.80 eV.

Epitaxy of triangular GaN nanoislands on (010) β-LiGaO2 substrate

Aligned triangular GaN nanoislands were successfully grown on closely lattice-matched Au coated (010) β-LiGaO2 substrates by chemical vapor deposition method. The morphologies of GaN nanostructures were characterized by scanning electron microscopy. Structural properties of the GaN nanostructures were investigated by X-ray diffraction and transmission electron microscopy. Cross-section transmission electron microscopy results shown direct evidences of the in-plane structural relationship between GaN nanostructure and (010) β-LiGaO2 substrate. The optical properties of GaN nanostructures were studied by room temperature photoluminescence.

Ferroelectric BaTiO3/SrTiO3 multilayered thin films for room-temperature tunable microwave elements.

Ferroelectric BaTiO3/SrTiO3 with optimized c-axis-oriented multilayered thin films were epitaxially fabricated on (001) MgO substrates. The microstructural studies indicate that the in-plane interface relationships between the films as well as the substrate are determined to be (001)SrTiO3//(001)BaTiO3//(001)MgO and [100]SrTiO3//[100]BaTiO3//[100]MgO. The microwave (5 to 18 GHz) dielectric measurements reveal that the multilayered thin films have excellent dielectric properties with large dielectric constant, low dielectric loss, and high dielectric tunability, which suggests that the as-grown ferroelectric multilayered thin films can be developed for room-temperature tunable microwave elements and related device applications.

Structural and optical properties of semipolar [formula omitted] AlGaN grown on [formula omitted] sapphire by metal–organic vapor phase epitaxy

We report on the growth of semipolar (112¯2) AlGaN in the entire composition range on (101¯0) sapphire by metal–organic vapor phase epitaxy. Growth rates increase linearly with the metal–organic supply and have been realized up to 2.5μm/h. The Al content determined by photoluminescence and transmission measurements depends linearly on the Al/Al+Ga ratio in the gas phase. The growth rate and the Al incorporation are higher for (112¯2) AlGaN than for (0001) AlGaN. This is attributed to different adatom mobility and desorption. The in-plane relationship of (112¯2) AlGaN is [1¯100]AlGaN∥[12¯10]Sap. and [1¯1¯23]AlGaN∥ [0001]Sap., as typical for semipolar nitrides on m-plane sapphire. For Al contents up to 60% triangle-like structures dominate the surface. For Al contents above 70% it transforms into an undulation along [1¯100] and additional dot-like structures. The smoothest surface morphologies with a rms roughnesses of 2nm were achieved for Al mole fractions of ∼50%.

Enhanced dielectric properties of (Ba,Sr)TiO3//Ba(Zr,Ti)O3 heterostructures with optimized structure design

Environment-friendly ferroelectric Mn:BST//Mn:BZT heterostructures were epitaxially deposited on (001) MgO substrates by pulsed laser deposition with different combination ratios of Mn:BST over Mn:BZT, 2.5:7.5; 1:1; and 7.5:2.5. The microstructural studies indicate that the in-plane interface relationships between the heterostructures and the substrate are determined to be (001)Mn:BST//(001)Mn:BZT//(001)MgO and [100]Mn:BST//[100]Mn:BZT//[100]MgO. The Ku band microwave (15.5–18 GHz) dielectric property characterizations show that the dielectric constant, dielectric loss, and tunabilities of the heterostructures are highly dependent upon the sequences of Mn:BST to Mn:BZT. The optimized dielectric tunability with a low loss tangent was found to be 58.1%–74.4% under the combination ratio of 1:1 for Mn:BST to Mn:BZT, which suggests that the as-grown ferroelectric heterostructures can be developed for room temperature tunable microwave elements and related device applications.

ZnO Heteroepitaxy on Sapphire Using a Novel Buffer Layer of Titanium Oxide: Crystallographic Behavior

A novel buffer layer consists of titanium oxide grown on a-sapphire by low-pressure chemical vapor deposition using titanum-tetra-iso-propoxide and oxygen gas was used for ZnO epitaxial growth at temperature as low as 340℃ by plasma-assisted epitaxy using radio-frequency oxygen-gas plasma. XRD and RHEED indicated (0001)Ti2O3 layer in corundum crystal system was epitaxially grown on the substrate in an in-plane relationship of [1-100]Ti2O3// [0001]Al2O3 by uniaxial phase-lock system. Growth behavior of ZnO layer was significantly dependent on the Ti2O3 buffer-layer thickness, for example, dense columnar ZnO-grains were grown on the buffer layer thinner than 10 nm but the hexagonal pyramid-like grains were formed on the thin buffer layers below 2 nm. RHEED observations showed ZnO layer including the pyramid-like grains was epitaxially grown with single-domain on the thin buffer layer of 0.8 nm in the in-plane relationship of [1-100]ZnO//[1-100]Ti2O3//[0001]Al2O3, whereas the multi-domain was included in ZnO layer on the buffer layer above 10 nm.

Epitaxial growth of (104)- and (018)-oriented LiCoO2 films on MgO single crystals prepared by chemical vapor deposition

The lithium cobaltate (LiCoO2) films were deposited onto single-crystal magnesium oxide (MgO) substrates by metal organic chemical vapor deposition, and the in-plane orientation relationship, morphology and deposition rates were investigated. Hetero-epitaxial LiCoO2 films oriented to (104) and (018) were obtained on (100) and (110) MgO substrates. The surface morphology of the (104) LiCoO2 film showed diagonally arrayed rectangular facets, whereas the (018) LiCoO2 film exhibited a roof-like structure extended unidirectionally to [1¯10] MgO. Twin structures formed with in-plane relationships of [010] LiCoO2 // [01¯0] LiCoO2 and [421¯] LiCoO2 // [010] LiCoO2 in the (104) LiCoO2 film and with in-plane relationships of [100] LiCoO2 // [1¯00] LiCoO2 in the (018) LiCoO2 film. The deposition rates of the (104)- and (018)-oriented LiCoO2 films were 10.8μmh−1 and 8.8μmh−1, respectively, which were almost 100 times greater than those reported for LiCoO2 epitaxial films fabricated by pulsed laser deposition.

Growth and characterization of a-plane (11[formula omitted]0) GaN films on (010) LiGaO2 substrate by chemical vapor deposition

Nonpolar (112¯0) GaN films were successfully grown on closely lattice-matched (010) LiGaO2 (LGO) substrates by chemical vapor deposition (CVD) method. The dependence of growth characteristics on the growth temperatures was investigated. The surface morphologies of GaN films were characterized by scanning electron microscopy. Structural properties of the GaN epilayers were investigated by X-ray diffraction and transmission electron microscopy. Cross-section transmission electron microscopy results showed direct evidence of the in-plane structural relationship between the GaN epilayer and (010) LGO substrate. Low temperature photoluminescence was dominated by neutral donor bound excitons emission at 3.472eV and the defect-related yellow emission was negligible.

MOVPE growth of semipolar [formula omitted] AlN on m-plane [formula omitted] sapphire

We report on the growth of semipolar (112¯2) AlN by metal-organic vapor phase epitaxy on (101¯0) sapphire. High temperature nitridation and nucleation at a V/III ratio of 1050 provide single phase (112¯2) AlN with mirror like crack free surfaces, even for layer thicknesses larger than 1μm. Increasing reactor pressure results in lower growth rates because of gas phase particle formation. The in-plane relationship is [0001]Sap.∥[1¯1¯23]AlN and [12¯10]Sap.∥[11¯00]AlN and therewith similar to MBE grown AlN and MOVPE and HVPE grown GaN on m-plane sapphire. The (112¯2) AlN reflex is anisotropically broadened in XRD. The surface shows an undulation along [11¯00] as typical for (112¯2) oriented nitride epilayers. Transmission spectroscopy measurements exhibit a sharp absorption edge at 5.95eV, which is 130meV shifted towards lower energies compared to c-plane AlN.

Growth and characterizations of semipolar (112¯2) InN

We report on metal-organic vapor phase epitaxial growth of (112¯2) InN on (112¯2) GaN templates on m-plane (101¯0) sapphire substrates. The in-plane relationship of the (112¯2) InN samples is [1¯1¯23]InN||[0001]sapphire and [11¯00]InN||[12¯10]sapphire, replicating the in-plane relationship of the (112¯2) GaN templates. The surface of the (112¯2) InN samples and the (112¯2) GaN templates shows an undulation along [11¯00]InN,GaN, which is attributed to anisotropic diffusion of indium/gallium atoms on the (112¯2) surfaces. The growth rate of the (112¯2) InN layers was 3-4 times lower compared to c-plane (0001) InN. High resolution transmission electron microscopy showed a relaxed interface between the (112¯2) InN layers and the (112¯2) GaN templates, consistent with x-ray diffraction results. Basal plane stacking faults were found in the (112¯2) GaN templates but they were terminated at the InN/(112¯2) GaN interface due to the presence of misfit dislocations along the entire InN/GaN interface. The misfit dislocations were contributed to the fully relaxation and the tilts of the (112¯2) InN layers. X-ray photoelectron spectroscopy was used to determine the polarity of the grown (112¯2) InN sample, indicating an In-polar (112¯2) InN. The valence band maximum was determined to be at (1.7 ± 0.1) eV for the (112¯2) InN sample, comparable to In-polar c-plane InN.

Epitaxial recrystallization and luminescence of CaAl2O4:Eu2+ thin films prepared on sapphire substrates

Eu2+ doped CaAl2O4 thin films were prepared on c-, a-, and r-plane sapphire substrates by a sputtering method, and then post-annealed for the recrystallization. Post-annealed films have out-of-plane epitaxial orientations of (010) and (001) CaAl2O4 on c- and a-plane sapphire substrates, respectively, whereas the films on r-plane were composed of two epitaxial phases of (210) CaAl2O4 and (210) CaAl4O7. In-plane relationships between films and substrates were also determined by an X-ray pole figure method, demonstrating that the films were epitaxially recrystallized, but not single crystals. The emission wavelengths were independent of the orientations of epitaxial films, showing a blue emission at 448 nm under 325 nm excitation, but the emission intensity depended on the film structure.

Twin structures of epitaxial SnO2 films grown on a-cut sapphire by metalorganic chemical vapor deposition

SnO2 films have been grown on a-cut (112¯0) sapphire substrates by metalorganic chemical vapor deposition. X-ray diffraction and transmission electron microscopy were employed to characterize the epitaxial relationship and film structure. The films were (101) oriented with pure rutile structure. The in-plane relationship was determined to be SnO2 [010]//Al2O3 [0001] and SnO2 [101¯]//Al2O3 [11¯00]. There are three kinds of {101} twins in the SnO2 film. These twins caused high density of planar defects in the film and slight misorientation of the growth plane. The film/substrate interface was flat, while the film surface was rough with steps and inclinations.

Strong biaxial texture and polymorph nature in TiO 2 thin film formed by ex-situ annealing on c-plane Al 2O 3 surface

Many applications based on TiO 2 thin films are strongly influenced by the film crystalline quality, microstructure and surface morphology. We have utilized x-ray scattering methods for crystal structure study of TiO 2 films, grown by DC reactive magnetron sputtering without heating. We have observed the formation of ordered phases after post-deposition ex-situ annealing in air at 600 °C. The distinct biaxial texture with respect to the c-plane (0001) Al 2O 3 substrate surface, which developed during heating, shows remarkable patterns in pole figures. We have identified epitaxial relationships of anatase A(001)[110]‖sub(001)[11̄00] and rutile R(100)[001]‖sub(0001)[100] within single TiO 2 layer. In addition, within the same sample, we have observed the anatase in (112) orientation with two in-plane relationship options A(112)[11̄0]‖sub(0001)[21̄1̄0] and A(112)[11̄0]‖sub(0001)[11̄00].

Heteroepitaxy of SnO 2 thin films on m-plane sapphire by MOCVD

SnO 2 thin films have been deposited on (1 0 1̄ 0) m-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD) in the temperature range 500–750 °C. X-ray diffraction showed that the films were pure SnO 2 with rutile structure and the film orientation was dependent on the growth temperature. Single crystalline SnO 2 (0 0 1) film was fabricated at 700 °C, which was confirmed by cross-sectional transmission electron microscopy and electron diffraction. A clear in-plane relationship of [0 1 0]SnO 2‖[0 0 0 1] Al 2O 3 and [1 0 0]SnO 2‖[1 2̄ 1 0] Al 2O 3 was determined between the film and the substrate. The samples showed high transparency of ∼80% in the visible range. Photoluminescence (PL) spectra measured at room temperature revealed that the film grown at 700 °C showed a weak PL band near 600 nm, while a strong PL band centered at about 480 nm was detected at 13 K. The PL mechanism is discussed in detail.

Microwave properties of epitaxial (111)-oriented Ba0.6Sr0.4TiO3 thin films on Al2O3(0001) up to 40 GHz

Perovskite Ba0.6Sr0.4TiO3 (BST) thin films have been grown on Al2O3(0001) substrates without/with inserting an ultrathin TiOx seeding layer by rf magnetron sputtering. X-ray diffraction and pole figure studies reveal that the film with the TiOx layer (12-Å-thick) is highly oriented along the (111) direction and exhibits a good in-plane relationship of BST(111)∥Al2O3(0001). The high frequency dielectric measurements demonstrate that the complex permittivity (ε=ε′−jε″) is well described by a Curie–von Scheidler dispersion with an exponent of 0.40. The resulting epitaxial BST films show high permittivity (∼428) and tunability (∼41%, at 300 kV/cm and 40 GHz) and their microwave properties (1–40 GHz) potentially could be made suitable for tunable devices.

Epitaxial Growth of MgO and CoFe/MgO on Ge(001) Substrates by Molecular Beam Epitaxy

We report the epitaxial growth of MgO and CoFe/MgO on Ge (001) substrates using molecular beam epitaxy. It was found that the epitaxial growth of a MgO film on Ge could be realized at a low growth temperature of 125 ± 5 °C and the MgO matches the Ge with a cell ratio of 21/2:1, which renders MgO rotated by 45° relative to Ge. In-situ and ex-situ structural characterizations reveal the epitaxial crystal growth of bcc CoFe/MgO on Ge with the in-plane crystallographic relationship of CoFe(001)[100]∥MgO(001)[110]∥Ge(001)[100], exhibiting sharp interfaces in the (001) matching planes. The saturation magnetization of the sample is 1430 ± 20 emu/cc, which is comparable to the value of bulk CoFe.

Structural and optical properties of epitaxial ZnO thin films on 4H–SiC (0001) substrates prepared by pulsed laser deposition

Abstract Epitaxially grown ZnO thin films on 4H–SiC (0 0 0 1) substrates were prepared by using a pulsed laser deposition (PLD) technique at various substrate temperatures from room temperature to 600 °C. The crystallinity, in-plane relationship, surface morphology and optical properties of the ZnO films were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements, respectively. XRD analysis showed that highly c -axis oriented ZnO films were grown epitaxially on 4H–SiC (0 0 0 1) with no lattice rotation at all substrate temperatures, unlike on other hexagonal-structured substrates, due to the very small lattice mismatch between ZnO and 4H–SiC of ∼5.49%. Further characterization showed that the substrate temperature has a great influence on the properties of the ZnO films on 4H–SiC substrates. The crystalline quality of the films was improved, and surfaces became denser and smoother as the substrate temperature increased. The temperature-dependent PL measurements revealed the strong near-band-edge (NBE) ultraviolet (UV) emission and the weak deep-level (DL) blue-green band emission at a substrate temperature of 400 °C.

Electrical properties of ferroelectric BaTiO3 thin film on SrTiO3 buffered GaAs by laser molecular beam epitaxy

Ferroelectric BaTiO3 thin films were epitaxially grown on (001) GaAs substrate using SrTiO3 as a buffer layer by laser molecular beam epitaxy. The perovskite SrTiO3 buffer layer present a body centered cubic structure by formation of an interfacial layer with the [100]SrTiO3∥[110]GaAs in-plane relationship. Thereupon, a highly c-oriented BaTiO3 thin film was grown on SrTiO3/GaAs in a layer by layer mode. Enhanced electrical properties of the heteroepitaxial structure were demonstrated. The BaTiO3(150 nm)/SrTiO3/GaAs system demonstrates hysteresis loops with a remnant polarization of 2.5 μC/cm2 at 600 kV/cm and a small leakage current density of 2.9×10−7 A/cm2 at 200 kV/cm.