Growth Of Heteroepitaxial Films Research Articles

Atomic structure of self-buffered BaZr(S, Se)3 epitaxial thin film interfaces

Understanding and controlling the growth of chalcogenide perovskite thin films through interface design is important for tailoring film properties. Here, the film and interface structure of BaZr(S,Se)3 thin films grown on LaAlO3 by molecular beam epitaxy and postgrowth anion exchange is resolved using aberration-corrected scanning transmission electron microscopy. Epitaxial films are achieved from self-assembly of an interface “buffer” layer, which accommodates the large film/substrate lattice mismatch of nearly 40% for the alloy film studied here. The self-assembled buffer layer, occurring for both the as-grown sulfide and post-selenization alloy films, is shown to have rock-salt-like atomic stacking akin to a Ruddlesden–Popper phase. These results provide insights into oxide-chalcogenide heteroepitaxial film growth, illustrating a process that yields relaxed, crystalline, epitaxial chalcogenide perovskite films that support ongoing studies of optoelectronic and device properties.

MgO (1 0 0) as an affordable support for heteroepitaxial growth of high-quality β-Ga2O3 thin films and related highly-sensitive solar-blind UV photodetectors

Heteroepitaxial growth of high-quality β-Ga2O3 thin films on foreign substrates is of crucial importance for achieving high-performance power-electronic and optoelectronic devices with affordable prices. Herein, we demonstrate realization of heteroepitaxial growth of high-quality β-Ga2O3 films on MgO(100) via pulsed laser deposition, and achievement of related highly-sensitive solar-blind ultraviolet photodetectors (SBUVPDs). Under optimized oxygen pressure (7 Pa), single-phase, (100)-oriented, and atomically-smooth β-Ga2O3 films were grown epitaxially on MgO(100), consisting of unique 8–12 nm-wide columnar nanodomains. Atomically-resolvable aberration-corrected transmission electron microscopy unveiled initial growth of a cubic γ-phase transition layer of ∼ 8 nm thickness, and directly visualized occurrence and preferable location of oxygen vacancies at domain walls in the further grown β-Ga2O3 film. Density-functional-theory simulations rationalized the formation of columnar nanodomains in terms of energetics, and presumably identified the nanodomains interface orientations as (112¯)/(11¯2¯). The heteroepitaxial β-Ga2O3(100)/MgO(100) film-based photodetector demonstrates excellent performance, featured with exceedingly-low dark current, remarkably-high light-to-dark current ratio, and ultrahigh responsivity and detectivity (Idark = 0.6 pA, Ilight/Idark = 1.47 × 107, R = 191 A/W, D*=3.08 × 1015 Jones) as well as fast response speed towards 255 nm-UV detection. This work highlights MgO(100) as a suited yet affordable foreign substrate for heteroepitaxial growth of high-quality β-Ga2O3 films towards further development of high-performance devices such as SBUVPDs.

Exploring heteroepitaxial growth and electrical properties of α-Ga2O3 films on differently oriented sapphire substrates

This study explores the epitaxial relationship and electrical properties of α-Ga2O3 thin films deposited on a-plane, m-plane, and r-plane sapphire substrates. We characterize the thin films by X-ray diffraction and Raman spectroscopy, and elucidate thin film epitaxial relationships with the underlying sapphire substrates. The oxygen vacancy concentration of α-Ga2O3 thin films on m-plane and r-plane sapphire substrates are higher than α-Ga2O3 thin film on a-plane sapphire substrates. All three thin films have a high transmission of over 80% in the visible and near-ultraviolet regions, and their optical bandgaps stay around 5.02–5.16 eV. Hall measurements show that the α-Ga2O3 thin film grown on r-plane sapphire has the highest conductivity of 2.71 S/cm, which is at least 90 times higher than the film on a-plane sapphire. A similar orientation-dependence is seen in their activation energy as revealed by temperature-dependent conductivity measurements, with 0.266, 0.079, and 0.075 eV for the film on a-, m-, r-plane, respectively. The origin of the distinct transport behavior of films on differently oriented substrates is suggested to relate with the distinct evolution of oxygen vacancies at differently oriented substrates. This study provides insights for the substrate selection when growing α-Ga2O3 films with tunable transport properties.

A New Method for Relaxation of Elastic Stresses During the Growth of Heteroepitaxial Films

A New Method for Relaxation of Elastic Stresses During the Growth of Heteroepitaxial Films

A New Method for Relaxation of Elastic Stresses During the Growth of Heteroepitaxial Films

In the article, using the example of growing aluminum nitride (AlN) on (110) orientation silicon (Si) with a silicon carbide (SiC) buffer layer, a method for growing a new type of substrates is developed which makes it possible to obtain mechanically unstressed semiconductor heterostructures. A specific feature of the synthesis of this kind of substrates is that the SiC layers used for the growth of AlN films were synthesized by the coordinated atoms substitution method. When this method of growth is used in the Si substrate, some of the Si atoms are replaced by carbon atoms. As a result of the substitution of atoms, the initially smooth Si(110) surface transforms into a SiC surface covered with prism-like growth figures, one side of which is the (111) face and the other is the face. These faces are "substrates" for the further growth of semipolar AlN. The structure and morphology of AlN films have been studied by X-ray diffraction, electron microscopy, and Raman spectroscopy. It was found that the AlN layer is formed by intergrown hexagonal microcrystals, which grow in two directions, and for both orientations of the crystals the following relation is approximately satisfied: AlN || Si(110). It is shown that the half-width of the X-ray rocking curve (FWHM) for the diffraction peak from AlN microcrystals, averaged over the area of the sample, is 20 arc minutes. Raman spectroscopy and X-ray diffraction studies have shown the almost complete absence of mechanical stresses in the AlN layer. A theoretical model is constructed to explain the presence of two orientations of the AlN film on SiC/Si(110) found in the experiment, and a method for controlling their orientation is proposed. It is shown that this morphology of the AlN film makes it possible to use it as a buffer layer for the growth of heterostructures based on gallium nitride and aluminum nitride.

Heteroepitaxial growth of CaGe2 films on high-resistivity Si(111) substrates and its application for germanane synthesizing

Realizing a germanane (GeH) film on insulating substrates is strongly required to understand its intrinsic electric properties in discussing the potential applications for the next-generation field-effect transistor channels. This study investigates the heteroepitaxial growth of CaGe2—the precursor of the GeH—films on a high resistive substrate of FZ-Si(111) using a molecular beam epitaxy with the deposition temperature (Tdepo) as a variable. X-ray diffraction and Raman spectroscopy analyses indicated that a metastable polytype of 2H was dominated in the epitaxial CaGe2 films; the Tdepo must be less than 700 °C to prevent Si diffusion from the substrates. We also synthesize GeH films by immersing some CaGe2 films in HCl acid solutions at a low temperature of −40 °C. Although the obtained grain size of GeH was still as small as several hundred nanometers, we believe that the present study is the first step in obtaining the GeH continuous films directly on insulators necessary for electrical characterizations.

Step flow growth of β-Ga2O3 films on off-axis 4H-SiC substrates by LPCVD

In this work, β-Ga2O3 films were prepared on 4H-SiC (0001) substrates with 4° off-axis by low-pressure chemical vapor deposition (LPCVD). The crystalline structures, chemical compositions, optical properties, and surface/interface of the as-prepared films and heterostructures were all comprehensively investigated. X-ray diffraction and room temperature Raman spectrometry demonstrated β-Ga2O3 films are pure phase with highly preferential growth orientation along the (-201) plane. The Ga/O ratio estimated by X-ray photoelectron spectroscopy was 0.7, indicating the existence of Ga-O bonding. In addition, β-Ga2O3 films showed high transmittance at wavelengths over 300 nm, corresponding to an optical bandgap of about 4.88 eV. Remarkably, the growth of β-Ga2O3 films on 4H-SiC substrates with 4° off-axis followed the step-flow growth mode, reported for the first time. The relevant growth mechanism was discussed. These results demonstrated the feasible formation of heteroepitaxial growth of β-Ga2O3 films with high crystal quality and superior electrical properties on 4H-SiC substrates with off-axis angle by LPCVD, promising for applications in Ga2O3 based high power and optoelectronic devices.

Phase-Pure Epitaxial b-Axis-Oriented Bronze TiO2 Films

We demonstrate the heteroepitaxial growth of phase-pure bronze-phase TiO2 films using pulsed laser deposition on MgAl2O4 single-crystal substrates. While the growth on cubic substrates with smaller lattice parameters favors the stabilization of an out-of-plane-oriented anatase phase, and the use of substrates with larger lattice parameters leads to formation of the rutile phase, MgAl2O4 lies in a narrow intermediate range where the bronze phase is stabilized. X-ray diffraction shows that the b-axis is oriented out-of-plane, while the a–c lattice plane lies within the film plane. The bronze films show twinned domains due to their monoclinic structure that are aligned along all four in-plane directions of the MgAl2O4 lattice. In a subsequent step, TiO2 films are grown on top of MgAl2O4-buffered MgO single crystals in order to demonstrate a route to stabilize the bronze phase on a larger variety of substrates. The growth of bronze-type TiO2 films with the unique, open, one-dimensional framework aligned along the film normally may allow for the investigation of its basic functional properties related to ion diffusion that cannot otherwise be studied easily in other crystal forms.

2D-Antimonene-assisted hetero-epitaxial growth of perovskite films for efficient solar cells

There are many grain boundaries and defects in polycrystalline perovskite films, resulting in sacrificed efficiency and instability for perovskite solar cells (PSCs). By regulating the growth of perovskite grains along the vertical direction through epitaxial growth, one may expect fewer grain-boundaries, effective charge transport, improved crystalline quality, and reduced defect density. However, there is still no suitable epitaxial growth substrate for perovskite. Here, we developed an electrochemical lithiation intercalation and ultrasonication method to prepare high-quality antimonene nanosheets (ANs). It is found that the perovskite film grows preferentially along the (012) planes of the ANs that have perfect lattice match with the (001) planes of the perovskite, leading to a high-quality perovskite film with a preferential orientation along the [001] direction and greatly enlarged grain size. Consequently, the oriented perovskite-based PSC achieves a remarkable PCE of 24.54% and shows an enhanced stability under ambient conditions, thermal annealing or light illumination. This work opens an effective avenue to effectively control the oriented growth of perovskite film for high-performance perovskite optoelectrical devices.

Role of surface chemistry in determining the heteroepitaxial growth of Ir films on a-plane α-Al2O3 single crystals

Heteroepitaxy has increased in importance as the development of the semiconductor industry has advanced and a number of heteroepitaxial devices have been developed. In this study, Ir films of (001) or (111) preferred orientation were deposited onto a-plane α-Al2O3 surfaces pretreated via annealing in air and/or Ar+ ion etching. Ir (001) films were epitaxially grown on α-Al2O3 substrates with an in-plane orientation relationship of Ir (001)[110]//α-Al2O3 (112¯0)[0001]. In contrast, (111) films were non-epitaxial with the out-of-plane orientation relationship of Ir (111)// α-Al2O3 (112¯0). The effects of annealing in air and Ar+ ion etching on Ir film growth were then analyzed. Annealing in air can lead to a terrace morphology and a lower Al/O atomic ratio of 1/2 than 2/3 for α-Al2O3 surfaces, and a better Ir film crystallinity due to the step-flow mechanism; however, this was not the key factor for Ir heteroepitaxy. Ar+ ion etching was regarded as an irreplaceable pretreatment procedure. The surface chemistry of α-Al2O3 substrates was characterized via X-ray photoelectron spectroscopy. Carbon adsorbate (C = O, CC, CO, etc.) on substrate surfaces was drastically reduced after etching. Interfacial structures were observed by transmission electron spectroscopy. Two different stacking sequences were observed along the out-of-plane direction of α-Al2O3 (112¯0) surface, representing the 〈111〉 and 〈001〉 crystal directions, respectively; C element enrichment was observed at the interface without etching. Finally, the interaction between Ir films and α-Al2O3 substrates, which is probably influenced by the surface chemistry, is considered to be the growth mechanism of Ir films. This study provides significant insights into the role of the surface chemistry in the epitaxial growth of films.

Enhancement of optical property and crystal structure for GaN films on 2D MoS2 buffer layer by nitridation treatment

The growth of GaN films on layered 2D-MoS2/c-sapphire substrate by plasma-assisted molecular beam epitaxy system was investigated. Pre-nitridation process and deposition time as growth parameters were employed to exploit the heteroepitaxial growth of GaN films at the temperature of 700 °C. The characterizations of surface condition, surface morphology, chemical composition, optical properties, crystal orientation and quality of GaN epitaxial layers were conducted by reflection high energy electron diffraction (RHEED), scanning electron microscope (SEM), atomic force microscope (AFM), X-ray photoelectron Spectroscopy (XPS), Raman spectroscopy, photoluminescence spectroscopy (PL) and high-resolution X-ray diffraction (HR-XRD), respectively. The pre-nitridation process and growth duration can enhance the optical properties and crystal quality of GaN films on 2D-MoS2/c-plane sapphire. The pre-treatment of MoS2 layer by nitrogen plasma before the growth, providing a wetting layer of nitrogen for GaN growth, can eliminate the oxygen contamination and reduce the defects in the films. Band-to-band emission of Wurtzite GaN with the bandgap of 3.5 eV can be improved by pre-nitridation process, and yellow band emission can be suppressed due to the elimination of defect states in the films. Meanwhile, the crystal orientation and quality can be improved by the pre-nitridation process and longer duration of growth time. This study demonstrates a van der Waals epitaxial GaN films on 2D-MoS2 layer with higher crystal quality by pre-nitridation process, and it provides a potential solution for the future applications of GaN-based devices.

Hybrid molecular beam epitaxy growth of BaTiO3 films

The ability to reproducibly synthesize thin films with precise composition and controlled structure is essential for fundamental study and mass production. Here, we demonstrate the hybrid molecular beam epitaxy (MBE) growth of epitaxial, single crystalline BaTiO3 films with different thicknesses on Nb-doped SrTiO3 substrates with atomically smooth surfaces. By combining scanning transmission electron microscopy, temperature-dependent high-resolution x-ray diffraction, reflection high-energy electron diffraction, and atomic force microscopy, we study the effect of growth conditions and the interplay between stoichiometry and epitaxial strain on the resulting structure. Furthermore, we demonstrate a close to bulk-like ferroelectric phase transition in thicker films and highlight the effect of strain on the phase transition temperature. This work establishes the hybrid MBE approach for the growth of heteroepitaxial BaTiO3 films on conducting substrates with scalable thickness and controlled stoichiometry.

Roughness and correlations in the transition from island to film growth: Simulations and application to CdTe deposition

Using kinetic Monte Carlo simulations, we relate morphological properties and microscopic dynamics during island growth, coalescence, and initial formation of continuous heteroepitaxial films. The average island width is controlled by adatom mobility on substrate. Subsequent evolution strongly depends on the Ehrlich-Schwöebel energy barrier EES of the deposited material. As EES decreases, islands become taller and their coalescence is delayed. For small islands and large EES, the global roughness increases as W~thickness1/2 and the local roughness increases at short scales (apparent anomalous scaling) before and after island coalescence. If the islands are wide, W may have a plateau for large EES and has a maximum for small EES when the islands coalesce. This framework is applied to atomic-force microscopy data of the initial stages of CdTe deposition on Kapton: a maximum of W during island coalescence indicates negligible ES barrier, consistently with scaling properties of much thicker films, and the diffusion coefficient 10-7—10-5cm2/s on the Kapton surface at 150°C is estimated. Applications of the framework to other materials are suggested, in which the expected roles of ES barriers are highlighted.

Oxygen vacancy induced phase and conductivity transition of epitaxial BaTiO3−δ films directly grown on Ge (001) without surface passivation

The heterogeneous epitaxial system of BaTiO3/Ge (BTO/Ge) is of great interest for both fundamental research and device applications, thanks to its quasi-lattice-matching feature and the integration of functional oxides on semiconductors. Currently, the heteroepitaxial growth of crystalline BTO films on Ge includes the utilization of ultrahigh vacuum tools and complex surface passivation pre-treatment as well as careful control of oxygen partial pressure during the growth. Meanwhile, oxygen vacancies in oxides strongly impact their structural and electrical properties. Here, we report a facile method to directly grow single crystalline BTO films on Ge using pulsed laser deposition. The strict control of oxygen partial pressure ensures a sharp interface with an atom-to-atom registry and also leads to the oxygen-deficient characteristics of BTO. The epitaxial relationship of BTO and Ge is [110] BTO (001)//[100] Ge (001). Detailed crystallographic studies on BTO films with different thicknesses show that, for the films with a thickness less than 20 nm, BTO shows a mixture of tetragonal and cubic phases due to the oxygen vacancies and the strain from the Ge substrate and the cubic phase eventually dominates as the film thickness increases. Such oxygen-deficient BTO films reveal conducting characteristics rather than dielectric properties. The oxygen vacancies can be partly “cured” after a low temperature annealing process. These results not only demonstrate the possibility to directly grow single crystalline oxides on semiconductors without surface passivation but also highlight the importance of oxygen vacancies and lattice strain on the crystallographic and electrical properties of BTO films.

Heteroepitaxial growth of wide bandgap cuprous iodide films exhibiting clear free-exciton emission

Cuprous iodide (CuI) is an emerging wide-bandgap semiconductor of superior optical and transport properties. In particular, CuI shows high stability and large oscillator strength of free excitons that are of great advantage for optoelectronic applications. However, thin films of CuI reported so far have not been genuine single crystals, containing a sizable density of impurity and defect. Here, we demonstrate a dramatic improvement in the quality of CuI films grown by molecular beam epitaxy on a lattice-matched InAs substrate. The film is revealed to be in a single-crystal structure with high lattice coherence and an atomically flat surface. The low-temperature photoluminescence spectra exhibit extremely sharp emission from free excitons and much-suppressed emission from trapped states. The high-quality CuI films realized in the present study will not only facilitate the device application of CuI films but also provide unprecedented functionalities in halide semiconductors at the atomically sharp heterointerfaces.

Mechanisms of Stress Generation in Thin Films and Coatings

Analysis of the literature data on the reasons for the development of mechanical stresses in epitaxial, polycrystalline, and amorphous films during their formation and under various external influences is carried out. The mechanism of the appearance of internal stresses during heteroepitaxial growth of films caused by the mismatch of the crystal lattice constants of the film and substrate is described. The relationship between the development of mismatch stresses in heteroepitaxial films and changes in the nature of their growth is shown. Models of the occurrence of compressive and tensile stresses in polycrystalline films due to the formation and coalescence of islands at the initial stage of their growth are considered. The regularities of the evolution of internal stresses in continuous films are discussed depending on the conditions of their deposition, as well as their chemical composition, structure, and mechanical properties. The mechanisms of development of internal stresses in thin films associated with the formation of point defects in them, the incorporation of impurities, and phase transformations occurring in the deposition process are reviewed. External factors that lead to the appearance of stresses in thin films during their storage and operation are considered in detail.

Heteroepitaxial growth of β-Ga2O3 films on SiC via molecular beam epitaxy

β-Ga2O3 is a promising ultrawide bandgap semiconductor for next generation radio frequency electronics. However, its low thermal conductivity and inherent thermal resistance provide additional challenges in managing the thermal response of β-Ga2O3 electronics, limiting its power performance. In this paper, we report the heteroepitaxial growth of β-Ga2O3 films on high thermal conductivity 4H-SiC substrates by molecular beam epitaxy (MBE) at 650 °C. Optimized MBE growth conditions were first determined on sapphire substrates and then used to grow β-Ga2O3 on 4H-SiC. X-ray diffraction measurements showed single phase (2¯01) β-Ga2O3 on (0001) SiC substrates, which was also confirmed by TEM measurements. These thin films are electrically insulating with a (4¯02) peak rocking curve full-width-at-half-maximum of 694 arc sec and root mean square surface roughness of ∼2.5 nm. Broad emission bands observed in the luminescence spectra, acquired in the spectral region between near infrared and deep ultraviolet, have been attributed to donor-acceptor pair transitions possibly related to Ga vacancies and its complex with O vacancies. The thermal conductivity of an 81 nm thick Ga2O3 layer on 4H-SiC was determined to be 3.1 ± 0.5 W/m K, while the measured thermal boundary conductance (TBC) of the Ga2O3/SiC interface is 140 ± 60 MW/m2 K. This high TBC value enables the integration of thin β-Ga2O3 layers with high thermal conductivity substrates to meliorate thermal dissipation and improve device thermal management.

First formed dislocations in microcompressed c-oriented GaN micropillars and their subsequent interactions

c-oriented GaN micropillars created from single crystals containing ∼103 or ∼106 dislocations/cm2 and a thick heteroepitaxially grown film containing ∼109 were compressed to study methods to accommodate strain during heteroepitaxial growth. The yield stress in the 103 samples was found to be the highest, and it was the lowest in the 109 samples. The 103 and 106 pillars often failed catastrophically but the 109 pillars almost never did. This was linked to the high stresses required to generate sufficient pyramidal dislocations to accommodate plastic strain and dislocation interactions, which precipitated axial fracture. Transmission electron microscopy analysis shows categorically that the first formed dislocations are ⅓⟨1123_⟩{1122} dislocations, and that a few ⅓⟨1123_⟩{0111} dislocations found were formed by a cross slip in the vicinity of where the former dislocations interacted. When compared with the similar stress patterns created in the heteroepitaxial growth of AlGaN films on GaN substrates, the analysis suggests that there is no pathway for creating basal plane dislocations during growth from the pyramidal dislocations, which require high applied stresses; the basal plane dislocations would provide relief for the mismatch strain while not penetrating the region where active devices are fabricated in the film. Rather, it will be necessary to find a method for creating shear stress in the basal plane during growth to form them directly.

Parameter boundaries for the heteroepitaxial growth of REBCO films by e-beam quantitative evaporation on inclined substrate deposited MgO buffered Hastelloy tapes

The technology of electron beam (e-beam) quantitative evaporation of oxide powders at THEVA is quite unique among the many standard in-situ growth methods of high-temperature superconducting (HTS) films. As well known for YBa2Cu3O7-x (YBCO) film growth the temperature and oxygen pressure should be in proximity of the YBCO stability line in the Bormann diagram. In case of e-beam quantitative evaporation, we observe a shift in the DyBa2Cu3O7-x (DyBCO) stability line towards lower pressures and higher temperatures compared to YBCO. We investigated the temperature and oxygen partial pressure boundaries of epitaxial DyBCO growth on inclined substrate deposited MgO (ISD-MgO) buffered Hastelloy tapes. For completeness, the influence of powder stoichiometry and evaporation rate on HTS epitaxial growth is shown. For the first time high-quality HTS films have been grown by e-beam quantitative evaporation without employing an oxygen shuttle and with oxygen in the chamber background instead.

Epitaxial growth and characterization of high quality Bi2O2Se thin films on SrTiO3 substrates by pulsed laser deposition

Recently, Bi2O2Se was revealed as a promising two-dimensional (2D) semiconductor for next generation electronics, due to its moderate bandgap size, high electron mobility and pronounced ambient stability. Meanwhile, it has been predicted that high-quality Bi2O2Se-related heterostructures may possess exotic physical phenomena, such as piezoelectricity and topological superconductivity. Herein, we report the first successful heteroepitaxial growth of Bi2O2Se films on SrTiO3 substrates via pulsed laser deposition (PLD) method. Films obtained under optimal conditions show an epitaxial growth with the c axis perpendicular to the film surface and the a and b axes parallel to the substrate. The growth mode transition to three-dimensional (3D) island from quasi-2D layer of the heteroepitaxial Bi2O2Se films on SrTiO3 (001) substrates is observed as prolonging deposition time of films. The maximum value of electron mobility reaches 160 cm2 V−1 s−1 at room temperature in a 70 nm thick film. The thickness dependent mobility provides evidence that interface-scattering is likely to be the limiting factor for the relatively low electron mobility at low temperature, implying that the interface engineering as an effective method to tune the low temperature electron mobility. Our work suggests the epitaxial Bi2O2Se films grown by PLD are promising for both fundamental study and practical applications.