Structure Of Epitaxial Films Research Articles

Morphology and spatial distribution of ordered domains in GaInP/GaAs(001) according to transmission electron microscopy

The structure of epitaxial films of the GaInP solid solution, in which ordering occurs, was studied using transmission electron microscopy. The films were grown by metalorganic vapor phase epitaxy on GaAs (001) substrates near the half-composition point. During the study, dark-field images obtained using superstructure reflections for cross-sectional and plan-view specimens of films were analyzed. The morphology and relative spatial arrangement of ordered domains have been determined. The phenomenon of spontaneous self-organization of regions with CuPt–B+ and CuPt–B– ordering near the surface was discovered, while in the bulk of the film the domains are uniformly located and mutually overlap each other. The effect of spatial separation of domains is associated with the lattice relaxation, leading to a change in the surface topology.

Enhanced Broadband Light Harvesting in Ultrathin Absorbers Enabled by Epitaxial Stabilization of Silver Thin Film Mirrors.

Silver thin film mirrors are attractive candidates for use as specular back reflectors to enhance broadband light absorption via strong optical interference in ultrathin film semiconductor photoabsorbers. However, deposition of metal-oxide absorbers often requires exposure to high temperature in an oxygen atmosphere, conditions that cause thermal etching and degrade the specular reflectance of silver films. Here, we overcome this challenge and demonstrate that epitaxial growth of silver mitigates thermal etching under the high-temperature oxygen-containing environments that cause polycrystalline films to degrade. The degree of thermal etching resistance is related to the epitaxial film structure, where high-quality films completely prevent thermal etching, allowing for direct deposition of metal-oxide thin film photoabsorbers at elevated temperatures without any degradation of the optical properties of the silver layer. As a proof of concept for device applications, a metal-oxide photoanode for photoelectrochemical water splitting is fabricated by directly growing epitaxial SnO2 and Ti-doped α-Fe2O3 (hematite) thin films onto stabilized silver reflectors by pulsed laser deposition. The photoanode displays enhanced broadband light absorption due to strong interference effects enabled by the highly reflective silver film and demonstrates stable operation in a photoelectrochemical cell under conditions of water photo-oxidation in alkaline electrolyte.

Strong strain gradients and phase coexistence at the metal-insulator transition in VO2 epitaxial films

The proximity of a thermodynamic triple point and the formation of transient metastable phases may result in complex phase and microstructural trajectories across the metal-insulator transition in strained VO2 films. A detailed analysis using in-situ synchrotron X-ray diffraction unveils subtle fingerprints of this complexity in the structure of epitaxial films. During phase transition the low-temperature monoclinic M1 phase is constrained along the {111}R planes by the coexisting high-temperature R phase domains, which remain epitaxially clamped to the substrate. This geometrical constraint induces counteracting local stresses that result in a combined tilt and uniaxial in-plane compression of M1 domains, and a concomitant anomalous cR-axis elongation. This mechanism progressively transforms the M1 phase into the transitional triclinic phase (T), and ultimately into the monoclinic M2 phase, generating strong strain and tilt gradients that remain frozen after the complete transformation of the R phase upon cooling to RT. The transformation path of VO2 films, the complex competition between stable and metastable VO2 polymorphs and its impact on the structure of the low temperature monoclinic state, provide essential insights for understanding the electronic and mechanical properties of the films at the nanoscale, as well as to control their use in functional devices.

Strain effect on orbital and magnetic structures of Mn ions in epitaxial Nd0.35Sr0.65MnO3/SrTiO3 films using X-ray diffraction and absorption

This study probes the temperature-dependent strain that is strongly correlated with the orbital and magnetic structures of epitaxial films of Nd0.35Sr0.65MnO3 (NSMO) that are fabricated by pulsed laser deposition with two thicknesses, 17 (NS17) and 103 nm (NS103) on SrTiO3 (STO) substrate. This investigation is probed using X-ray diffraction (XRD) and absorption-based techniques, X-ray linear dichroism (XLD) and the X-ray magnetic circular dichroism (XMCD). XRD indicates a significant shift in the (004) peak position that is associated with larger strain in NS17 relative to that of NS103 at both 30 and 300 K. Experimental and atomic multiplet simulated temperature-dependent Mn L3,2-edge XLD results reveal that the stronger strain in a thinner NS17 film causes less splitting of Mn 3d eg state at low temperature, indicating an enhancement of orbital fluctuations in the band above the Fermi level. This greater Mn 3d orbital fluctuation can be the cause of both the enhanced ferromagnetism (FM) as a result of spin moments and the reduced Néel temperature of C-type antiferromagnetism (AFM) in NS17, leading to the FM coupling of the canted-antiferromagnetism (FM-cAFM) state in NSMO/STO epitaxial films at low temperature (T = 30 K). These findings are also confirmed by Mn L3,2-edge XMCD measurements.

Atomic visualization of copper oxide structure in the infinite-layer cuprate SrCuO2

We report the atomic-scale structure of epitaxial films of the parent infinite-layer compound $\mathrm{SrCu}{\mathrm{O}}_{2}$ prepared on ${\mathrm{SrTiO}}_{3}$ by molecular beam epitaxy. In situ scanning tunneling microscopy study reveals a stoichiometric copper oxide (${\mathrm{CuO}}_{2}$) -terminated surface featured by $2\ifmmode\times\else\texttimes\fi{}2$ reconstruction, caused primarily by structural distortions of four adjacent ${\mathrm{CuO}}_{2}$ plaquettes. Furthermore, the subsurface Sr atoms have been rarely discernible, showing intra-unit-cell rotational symmetry breaking. These observations can be reasonably modeled by a periodic up-down buckling of oxygen ions on the ${\mathrm{CuO}}_{2}$ plane. Further postannealing leads to the removal of surface oxygens and an incommensurate stripe phase. Our study provides indispensable structural information to help understand the exotic properties of cuprate superconductors.

Magnetic Hyperfine Structure of Epitaxial Films of Nickel Ferrite

Comparative NGR study of the parameters of the magnetic hyperfine structure of epitaxial films grown on single-crystal substrates of orientations of (100) and (111) by liquid phase epitaxy (LPE) and chemical transport reactions (CTR) is conducted. It is established that in the films obtained by the method of CTR, there is a significant number of ions of bivalent iron, which is in phase magnetite . After heat treatment of these films in air at T = 1273 K, the magnetite becomes the oxide. In the films of , grown by the LPE method the divalent iron was not detected.
 As shown by calculations, the vector direction of the magnetization of films obtained by the method of LPE, lies in the film plane regardless of the orientation (100) or (111) substrates. In films obtained by the CTR method, it forms with the substrate 41 - 450.

Growth of epitaxial Cd1–x Mn x Te films

We have studied the growth and structure of epitaxial films of Cd1–x Mn x (x = 0.03) diluted magnetic solid solutions grown on mica substrates by molecular beam epitaxy and identified conditions for producing n- and p-type epitaxial films. Using an additional Te vapor source and optimizing the substrate temperature in the growth process, we were able to obtain structurally perfect p-type Cd1–x Mn x Te (x = 0.03) films with clean, smooth surfaces. The growth plane of the films on the mica substrates is (111) of a face-centered cubic lattice and their unit-cell parameter is а = 6.477 A.

β-Ga2O3 solar-blind deep-ultraviolet photodetector based on annealed sapphire substrate

A vacuum annealing was conducted on c-plane sapphire substrate in a plasma-assisted MBE system prior to the epitaxial growth of β-Ga2O3 film. Accordingly, both the crystal structures of epitaxial films and the device performance of fabricated metal-semiconductor-metal photodetectors were comparatively investigated based on the as-supplied and annealed sapphire substrates. The AFM result showed a large number of protrusions distributed uniformly on the surface of annealed sapphire substrate, proving an effective high-temperature reconstruction by such a pretreatment. Moreover, the β-Ga2O3 epitaxial film exhibited a significantly improved crystalline quality by adopting the annealed sapphire substrate as revealed by the XRD measurement. Accordingly, the carrier mobility could be improved due to the suppression of impurity scattering related to structural disorders, resulting in the observed significant increase of both dark and photo currents in photodetector. As a result, its DUV responsibility was improved from 44 A/W to 153 A/W. However, some degradations in photo-to-dark current ratio and response time appeared, which can be attributed to the reduction in the crystallographic defects impacting internal gain and carrier recombination, respectively. This work may pave an alternative way for the fabrication of high-performance β-Ga2O3 solar-blind DUV photodetector, especially for high responsibility.

Special features of reconstruction of defect structure of epitaxial films of CeO2 and La2Zr2O7 in alternating magnetic field

Results obtained by methods of X-ray spectroscopy and atomic force microscopy (AFM) and special features of reconstruction of the defect structure of epitaxial films of cerium oxide CeO2 and lanthanum zirconate La2Zr2O7 during their processing in an alternating magnetic field or during their magnetic structural processing (MSP) are reported. The epitaxial films of CeO2 and La2Zr2O7 are seed and barrier layers of a buffer current-carrying element of a high-temperature superconducting second-generation wire (HTSC-2 wires). Changes in the crystalline structure of epitaxial films of CeO2 and La2Zr2O7 are reflected in changes in their X-ray diffraction patterns and AFM data. The X-ray spectroscopy indicated a change in the crystal structure of a biaxially textured substrate of Ni–5% W alloy as a result of MSP of sample buffer CeO2/La2Zr2O7. The obtained data indicate the existence of complex and fast processes in the crystal structure of epitaxial films of CeO2 and La2Zr2O7 and of Ni–5% W alloy at MSP, which result in irreversible changes in the crystal structure of these materials; in other words, it is found that, in the epitaxial films of CeO2 and La2Zr2O7 and in the Ni–5% W alloy, a magnetic structural effect is observed in an alternating magnetic field [1–3].

High-rate and wide-area deposition of epitaxial Si films by mesoplasma chemical vapor deposition

Homoepitaxial Si films have been deposited at a high rate of 200 nm s−1 over a wide area of 20 mm × 80 mm by cluster-assisted mesoplasma chemical vapor deposition (MPCVD) on a moving substrate. The obtained epitaxial Si films exhibited a uniform roughness of 0.1–0.3 nm (1 × 1 μm2) and a Hall mobility of ∼240 cm2 V−1 s−1. The results suggested that under the MPCVD the deposition precursors formed at the plasma edge could be small enough not to influence either epitaxial film structure or the film quality provided the substrate temperature is maintained above 500 °C.

Impact of lattice mismatch and stoichiometry on the structure and bandgap of (Fe,Cr)2O3 epitaxial thin films

The structural properties of phase-pure epitaxial (Fe1–xCrx)2O3 thin films deposited on α-Al2O3(0 0 0 1) substrates by oxygen-plasma-assisted molecular beam epitaxy are investigated across the composition range using x-ray photoelectron spectroscopy, high-resolution x-ray diffraction, scanning transmission electron microscopy and electron energy loss spectroscopy, and non-Rutherford resonant elastic scattering measurements. The films possess a columnar grain structure with uniform mixing of cations on the nanometer scale. Fe-rich films are relaxed and appear to be slightly oxygen-rich, while Cr-rich films remain partially strained to the Al2O3 substrate and are found to be oxygen deficient. A model is proposed to explain the oxygen stoichiometry results based on the energetics of oxygen defect formation and rate of oxygen diffusion in the corundum lattice, and the dependence on the cation composition. Deliberately introducing residual compressive biaxial strain into (Fe1–xCrx)2O3 thin films (x = 0, 0.41, 0.52) by employing a Cr2O3 buffer layer is shown to narrow the optical bandgap, from 1.80(1) eV for relaxed (Fe0.47Cr0.53)2O3 to 1.77(1) eV for partially strained (Fe0.48Cr0.52)2O3. The relationships which are elucidated between epitaxial film structure and optical properties can be applied to bandgap optimization in the (Fe,Cr)2O3 system.

In-line Si1-xGex epitaxial process monitoring and diagnostics using multiwavelength high resolution micro-Raman spectroscopy

Multiwavelength, high resolution micro-Raman spectroscopy was applied to in-line process monitoring and diagnostics of undoped and B-doped Si1-xGex epitaxy on Si(100) device wafers. This noncontact technique was used to monitor the Ge content, B concentration and thickness of single and double Si1-xGex epitaxial layers. Epitaxial process problems were diagnosed nondestructively. Raman peak positions and full-width-at-half-maximum of the Si-Si peak(s) from the Si1-xGex epitaxial layer(s) and Si substrates, in the wavenumber range of 475 ∼ 535 cm-1, were monitored under ultraviolet and visible excitation wavelengths. The Ge content, B concentration and Si1-xGex epitaxial film structures were verified by secondary ion mass spectroscopy (SIMS) depth profiling results. In-line monitoring of Si-Si and Si Raman peaks is very effective in noncontact material property characterization, epitaxial process optimization, and quality control applications.

Long-range chemical interaction in solid-state synthesis: The formation of a CuAu alloy in Au/β-Co(001)/Cu(001) epitaxial film structures

The effect of an inert Co layer (0, 210, 480 nm) on the chemical interaction between Cu and Au in Au/β-Co(001)/Cu(001) epitaxial films has been investigated by X-ray diffraction, nuclear magnetic resonance, photoelectron spectroscopy, and magnetic structure measurements. Mixing at interfaces in Cu/β-Co(001) and Au/β-Co(001) bilayer films has not been revealed up to a temperature of 600°C. The solid-state synthesis of ordered CuAu| and CuAu∥ phases occurs through the Co inert buffer layer in Au/β-Co(001)/Cu(001) trilayer film systems with an increase in the annealing temperature. The initiation temperatures of the CuAu| and CuAu∥ phases increase only slightly with the thickness of the Co buffer layer. The assumption of the long range of the chemical interaction between Cu and Au through the chemically inert Co layer is justified using the performed investigations.

Modification and Conversion of E-Beam Co-Evaporated Precursors for Fabricating High Critical Current ${\rm YBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$ Films

Ex situ conversion of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</i> -beam co-evaporated precursors was studied in an effort to fabricate high critical current YBCO films using the BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> process. The precursors were deposited on CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> buffered single crystal YSZ substrates. It was shown that an intermediate oxygenation annealing prior to the conversion modifies the precursor crystallinity and promotes c-axis epitaxial growth while randomly-oriented film formation is suppressed. With the modified precursors, a critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , 77 K & 0 T) of 2.1 MA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> measured by SQUID magnetometry was obtained in 1.8 mum thick YBCO films. The mechanism of the pre-annealing effect was investigated by characterization of the precursors, quenched films, and fully converted films using XRD, SEM, and TEM. Cross-sectional TEM was used to study the early nucleation of YBCO film at the precursor/substrate interface. The significant effect of the precursor modification indicated that, in addition to optimizing conversion processing parameters, modifying the precursor is an effective way to achieve the desired epitaxial film structure and to obtain higher critical currents, I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> .

Strain-driven magnetism inLaCoO3thin films

The electronic structure of epitaxial films on ${\text{LaCoO}}_{3}$ (LCO) has been studied within first-principles electronic structure calculations. A spin state transition is found to take place as a function of lattice strain which freezes in the intermediate spin state for nonzero strain. In contrast to earlier speculations this is found to arise from substrate strain alone and angle variations are small. The intermediate spin state also stabilizes ferromagnetism in the ground state. The anomalous temperature dependence of the x-ray absorption spectra for films of LCO on LCO and its absence in films of LCO on ${({\text{LaAlO}}_{3})}_{0.3}{({\text{SrAlTaO}}_{6})}_{0.7}$ is also explained.

Soft x-ray emission spectra and ferromagnetism in wide-gap doped semiconductors

A study is made of the resonant and nonresonant L x-ray emission spectra of impurities in the semiconducting compounds ZnS:Mn, ZnO:Mn, ZnO:Co, and Co2O:Mn. An analysis of the Mn L2,3 x-ray emission spectra of Zn1−xMnxS (x=0.1–0.3) reveals that the Mn impurities do not form clusters in the ZnS lattice. Studies of the Mn L2,3 spectra and electronic structure of epitaxial films of Zn0.8Mn0.2O annealed at different temperatures show that the cause of the observed suppression of ferromagnetism at T&amp;gt;600°C is segregation of Mn atoms. In this case the Mn atoms occupy both Zn sites and interstitial positions. For Zn1−xCoxO (x=0.02, 0.06, and 0.10) the absence of free carriers that could mediate an exchange interaction between Co ions is established. Mn L2,3 x-ray emission measurements show that in Mn-doped oxides Cu2O synthesized at 650 and 800°C the Mn atoms are found both in interstitial positions and occupy Cu sites, but the configurations of these defects depend on the synthesis temperature. A decrease of the Curie temperature with increasing synthesis temperature may be explained as a manifestation of antiferromagnetic superexchange between substituent Mn atoms via oxygen.

Direct determination of epitaxial film and interface structure: Gd 2O 3 on GaAs (1 0 0)

We present a new method of sub-Angstrom resolution imaging of the 3D structure of epitaxial films and their interface with the substrate. The method utilizes the diffraction intensities along the substrate-defined Bragg rods and some crude knowledge of the system structure to determine the complex scattering factors (CSFs) along the Bragg rods. The system electron density and the structure is obtained by Fourier transforming the CSFs into real space. We have applied this method to study the structure of a Gd 2O 3 film and its interface with the GaAs substrate. The results show that the Gd 2O 3 abandons the bulk stacking order and adopts that of GaAs. Moreover, the atoms in the first few layers move to in-plane positions that overlap those of the underlying Ga and As. This behavior may be at the heart of its ability to passivate GaAs.

In situ observation of low temperature growth of crystalline silicon using reflection high-energy electron diffraction

In situ observations of reflection high-energy electron diffraction (RHEED) have been performed to investigate the microcrystalline silicon formation mechanism in plasma-enhanced-chemical-vapor-deposition (PECVD) using a mixture of SiH 4 and H 2 at a temperature T s between 27°C and 560°C. In low temperature epitaxy (LTE) on Si(0 0 1), we found that the epitaxial structure of films as a function of temperature had, at 120°C, a surface structure Si1 × 1 and at 430°C Si2 × 1, depending on hydrogen dilution. This bistable epitaxy is mediated by the surface hydrogen mode. In polycrystalline silicon growth on a ZnO coated glass substrate, we observed in situ the change in the order and disorder of the structure and the change in the orientation during the growth. These results can be utilized to control crystal growth by a real-time feed-back to deposition parameters. Thus, we have demonstrated that in situ reflection high-energy electron diffraction observation is useful for the study of microcrystalline silicon growth.

Surface-Enhanced Raman Scattering of Ultrathin Cadmium Chalcogenide Films on Gold Formed by Electrochemical Atomic-Layer Epitaxy: Thickness-Dependent Phonon Characteristics

The utility of surface-enhanced Raman scattering (SERS) for characterizing phonon and related interfacial vibrations within ultrathin semiconductor layers on gold substrates is demonstrated, specifically for cadmium chalcogenide (CdSe, CdS) films formed by electrochemical atomic layer epitaxy. The occurrence of quantum-confinement effects for CdSe films less than ca. 10 monolayers thick is clearly evident from the observed ca. 12 cm-1 phonon redshifts and bandshape changes. Milder phonon redshifts are seen for the ultrathin CdS films, suggesting the presence of offsetting blueshifts from lattice compression. The observed sensitivity of the phonon spectra to the order of initial Cd/Se or Cd/S monolayer electrodeposition, along with the retention of Au−Se and Au−S vibrational bands upon layer growth, signal the importance of the initial chemisorbate to the epitaxial film structure and vibronic properties.

Nonmetallic conductivity of epitaxial monolayers of Ag at low temperatures

Epitaxial metallic monolayers are models for two-dimensional conduction. They provide well-ordered monatomic films with an atom distance as in the bulk. Measurements of the dc conductivity of epitaxial Ag films on a clean Si(111) $7\ifmmode\times\else\texttimes\fi{}7$ substrate at about 100 K reveal that the conductance for a thickness down to nearly a monolayer is well described by a simple Drude model with a mean free path given by the film thickness. For lower temperatures (down to nearly 4 K) the conductance of very thin films is reversibly decreased by orders of magnitude. Whereas annealing increases the mean free path for thick films up to twice the thickness, the films with a thickness of less than 2 monolayers show no annealing effect. The results may be discussed with models of amorphous or granular films. The lack of agreement with theoretically predicted temperature dependences may be due to the special structure of epitaxial films.