C-plane Al2O3 Research Articles

Epitaxial growth of high-quality Mg3Sb2 thin films on annealed c-plane Al2O3 substrates and their thermoelectric properties

High-quality epitaxial Mg3Sb2 thin films are promising thermoelectric materials to enable practical applications of compact and environmentally friendly thermoelectric conversion at RT. In this study, high-quality single-crystal Mg3Sb2 with high c-plane orientation was epitaxially grown directly on annealed c-Al2O3 substrates without passive layers. These thin films exhibited about three times higher thermoelectric power factor than any previously reported values due to high carrier mobility. The ultra-smooth surface of the annealed c-Al2O3 substrate facilitated the formation of high-quality Mg3Sb2 thin films without passive layers or polycrystalline interfaces that could be carrier scatters.

In-situ spectral reflectance investigation of hetero-epitaxially grown β-Ga2O3 thin films on c-plane Al2O3 via MOVPE process

Metalorganic vapor-phase epitaxy of β-Ga2O3/c-plane Al2O3 heterostructures was monitored in-situ by spectral reflectance in different wavelengths. The reflectance spectrum was analysed as a function of the growth time and the incident wavelength to estimate the growth rate and the refractive index at the growth temperatures. The obtained values are validated by ex-situ methods such as secondary ion mass spectrum measurement and spectroscopic ellipsometry. A theoretical simulation of the reflectance spectrum was carried out by combining a transfer matrix method with a multilayer model, and a good agreement with the experimental results is presented.

Comparative X-ray Diffractometry of the Defect Structure of ZnO Epitaxial Films Deposited by Magnetron Sputtering on C-Plane Al2O3 Substrates in Inhomogeneous Electric Field

Comparative X-ray Diffractometry of the Defect Structure of ZnO Epitaxial Films Deposited by Magnetron Sputtering on C-Plane Al2O3 Substrates in Inhomogeneous Electric Field

Epitaxial Grown VO2 with Suppressed Hysteresis and Low Room Temperature Resistivity for High-Performance Thermal Sensor Applications

The ability of VO2 to undergo semiconductor-to-metal phase transition (SMT) upon heating makes it a very attractive material for uncooled bolometers. The SMT of VO2 represents a large temperature coefficient of resistance, which is an important parameter for the development of highly responsive microbolometers. However, other characteristics of the SMT of VO2 such as its high transition temperature (341.2 K), the sharpness of the transition, its hysteresis, and the high room temperature resistivity limit the performance of this material in microbolometers. In this work, we grow a high-quality epitaxial ultrathin film VO2 on c-plane Al2O3 by pulsed laser deposition. The low deposition temperature and tuning the oxygen partial pressure during the growth process enable control over the grain size and oxygen vacancy concentration. This allowed controlling the SMT parameters of the samples. In particular, we show that the high density of grain boundaries associated with nanosized grains suppresses the thermal hysteresis of the SMT. Simultaneous control over the density of oxygen vacancies and the size of grains enables the adjustment of the temperature coefficient of resistance, room temperature resistivity, SMT temperature, sharpness, and thermal hysteresis toward suitable values for the fabrication of efficient VO2-based uncooled bolometers. Compared with other VO2 fabrication methods, this approach can be viewed as a simpler alternative for VO2 fabrication with favorable properties for practical bolometer applications.

Chemical, structural and electronic properties of ultrathin V2O3 films on Al2O3 substrate: Implications in Mott-like transitions

V2O3 presents a complex interrelationship between the metal–insulator transition and the structural rhombohedral-monoclinic one in temperature, as a function of sample thickness. Whilst in bulk V2O3 the two transitions coincide on the temperature scale, at 15 nm thickness a fully independent Mott-like transition occurs at lower temperature, with no corresponding structural changes perhaps related to epitaxial strain. It is therefore of relevance to investigate the thin and ultrathin film growth to pinpoint the chemical, electronic and structural phase phenomenology and the role of the interface with the substrate. Here we present results on the thickness dependent properties of V2O3 from 1 nm up to 40 nm thick as grown on c-plane Al2O3 substrates by exploiting variable sampling depth probes. The surface morphology of stoichiometric ultra-thin V2O3 layers evolves from islands-like to continuous flat film with thickness, with implications on the overall properties.

Growth and characterization of β-Ga2O3 thin films grown on off-angled Al2O3 substrates by metal-organic chemical vapor deposition

Beta-gallium oxide (β-Ga2O3) thin films were deposited on c-plane (0001) sapphire substrates with different mis-cut angles along <> by metal-organic chemical vapor deposition (MOCVD). The structural properties and surface morphology of as-grown β-Ga2O3 thin films were investigated in detail. It was found that by using thin buffer layer and mis-cut substrate technology, the full width at half maximum (FWHM) of the () diffraction peak of the β-Ga2O3 film is decreased from 2° on c-plane (0001) Al2O3 substrate to 0.64° on an 8° off-angled c-plane (0001) Al2O3 substrate. The surface root-mean-square (RMS) roughness can also be improved greatly and the value is 1.27 nm for 8° off-angled c-plane (0001) Al2O3 substrate. Room temperature photoluminescence (PL) was observed, which was attributed to the self-trapped excitons formed by oxygen and gallium vacancies in the film. The ultraviolet–blue PL intensity related with oxygen and gallium vacancies is decreased with the increasing mis-cut angle, which is in agreement with the improved crystal quality measured by high resolution X-ray diffraction (HR-XRD). The present results provide a route for growing high quality β-Ga2O3 film on Al2O3 substrate.

Study of SnO/ɛ-Ga2O3 p–n diodes in planar geometry

SnO/ε-Ga2O3 vertical p–n diodes with planar geometry have been fabricated on c-plane Al2O3 and investigated by current–voltage measurements. The effects of the in-plane conduction through the Si-doped ε-Ga2O3 layer on the diode performance and their relevance have been evaluated. A significant series resistance is observed, which shows typical features of the variable range hopping transport observed in Si-doped ε-Ga2O3; this in-plane transport mechanism is probably induced by the columnar domain structure of this polymorph. The dependence of the series resistance on the geometry of the diode supports the interpretation. A simple equivalent model is presented to describe the experimental behavior of the diode, supported by preliminary impedance spectroscopy investigation.

Synthesis of hexagonal boron nitride films by dual temperature zone low-pressure chemical vapor deposition

Hexagonal boron nitride (h-BN) films are synthesized by dual temperature zone low-pressure chemical vapor deposition (LPCVD) through using a single ammonia borane precursor on non-catalytic c-plane Al2O3 substrates. The grown films are confirmed to be h-BN films by various characterization methods. Meanwhile, the growth rates and crystal quality of h-BN films at different positions in the dual temperature zone are studied. It is found that the growth rates and crystal quality of the h-BN films at different positions on the substrate are significantly different. The growth rates of the h-BN thin films show their decreasing trends with the rearward position, while the crystal quality is improved. This work provides an experimental basis for the preparation of large area wafer thick h-BN films by LPCVD.

Molecular beam epitaxy of PtSe2 using a co-deposition approach

The structural properties of co-deposited ultrathin PtSe2 films grown at low temperatures by molecular beam epitaxy on c-plane Al2O3 are studied. By simultaneously supplying a Se flux from a Knudsen cell and Pt atoms from an electron-beam evaporator, crystalline (001)-oriented PtSe2 films were formed between 200 °C and 300 °C. The long separation between substrate and electron beam evaporator of about 60 cm ensured minimal thermal load. At optimum deposition temperatures, a ten times or even higher supply rate of Se compared to Pt ensured that the pronounced volatility of the Se was compensated and the PtSe2 phase was formed and stabilized at the growth front. Postgrowth anneals under a Se flux was found to dramatically improve the crystalline quality of the films. Even before the postgrowth anneal in Se, the crystallinity of PtSe2 films grown with the co-deposition method was superior to films realized by thermal assisted conversion. Postgrowth annealed films showed Raman modes with narrower peaks and more than twice the intensity. Transmission electron microscopy investigations revealed that the deposited material transitioned to a two-dimensional layered structure only after the postgrowth anneal. PtSe2 growth was found to start as single layer islands that preferentially nucleated at atomic steps of the substrate and progressed in a layer-by-layer like fashion. A close to ideal wetting behavior resulted in coalesced PtSe2 films after depositing about 1.5 PtSe2 layers. Detailed Raman investigation of the observed PtSe2 layer breathing modes of films grown under optimized co-deposition conditions revealed an interlayer coupling force constant of 5.0–5.6 × 1019 N m−3.

Investigation of Structural and Optical Properties of ZnO Thin Films Grown on Different Substrates by Mist-CVD Enhanced with Ozone Gas Produced by Corona Discharge Plasma

This study focuses on the growth and physical properties of ZnO thin films on different substrates grown by mist-CVD enhanced with ozone (O3) gas produced by corona discharge plasma using O2. Here, O3 is used to eliminate the defects related to oxygen in ZnO thin films. ZnO thin films are grown on amorphous soda-lime glass (SLG) and single crystals SiO2/Si (100) and c-plane Al2O3 substrates at 350°C of low growth temperature. All ZnO thin films show dominant (0002) diffraction peaks from X-ray diffraction (XRD). As expected, full width at half maximum (FWHM) of (0002) is decreasing in ZnO thin films on single-crystal substrates, especially c-Al2O3 due to similar crystal structure. It is found that the strain in the films is lowest in ZnO/c-Al2O3. The surface morphologies of the thin films are studied with atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements. Grown ZnO films have a hexagonal and triangular nanostructure with different nanostructure sizes depending on the used substrate types. The calculated surface roughness is dramatically decreased in ZnO/c-Al2O3 compared to the other grown structures. The confocal Raman measurements show the E2(H) peak of ZnO thin films at 437 cm−1. It is suggested that O3 gas produced by corona discharge plasma using O2 can be useful to obtain better crystal quality and physical properties in ZnO thin films.

Structural and electrical properties of V2O3 thin films on c-plane Al2O3 fabricated by reactive-HiPIMS and dcMS techniques

The study presents the synthesis of epitaxial V2O3 thin films on c-plane Al2O3 substrates by reactive high-power impulse (HiPIMS) and direct current (dcMS) magnetron sputtering. The results reveal that for fixed deposition conditions and discharge power, well defined epitaxial layers can be attained using both HiPIMS and dcMS. For dcMS we observe the formation of these epitaxial films down to flow rate values of 1.3 sccm while for HiPIMS we observe an extended operation window down to much lower oxygen flow of 0.5 sccm without sacrificing the structural quality of the films. Furthermore, the effect of varying HiPIMS discharge parameters i.e. repetition frequency and average power for fixed O2 flow setting, were explored in order to determine their effect on the structural quality and electrical characteristics of the films. The magnitude of the metal–insulator transition (MIT) and the transition temperature is correlated to the film stoichiometry which can be fine-tuned by mapping the HiPIMS discharge parameters, displaying a total change in resistance of ∼7 decades over the studied temperature range. The MIT temperature and magnitude obtained for films fabricated by HiPIMS, even for low O2 flow settings (down to 0.6–0.5 sccm), displayed superior characteristics compared to films fabricated by dcMS where a minimum O2 flow of 1.3 sccm was needed.

Epitaxial growth of β-Ga2O3 nanowires from horizontal to obliquely upward evolution

Changing the direction of horizontal nanowires to extend them in the z direction and grow obliquely has important practical significance for achieving higher density integration and making it have more free space in multi-dimension. However, how to guide the horizontal nanowires to extend in the z direction and grow obliquely is still a challenge. In this paper, we report the regular and horizontal β-Ga2O3 nanostructure grown on c-plane Al2O3 substrates, and the evolution of β-Ga2O3 nanowires from horizontal growth to obliquely upward growth through the chemical vapor deposition (CVD) method. Field-emission scanning electron microscopy, field-emission transmission electron microscopy, X-ray diffraction, Raman spectrum and photoluminescence spectroscopy were used to investigate the morphology, structure, components, and optical properties of the as-synthesized β-Ga2O3 nanowires. Due to the change of the growth interface at the Au catalysts, there are four different growth modes for the growth of β-Ga2O3 nanowires. The preparation route reported in this study is more simple, general and low-cost compared to other common growth methods for Ga2O3 nanomaterials, and these studies about the evolution of nanowires growth can provide a reference for the change of growth direction of nanowires and simplify planar device processing technology.

Substrate dependent epitaxy of superconducting niobium nitride thin films grown by pulsed laser deposition

Niobium nitride (NbN) has suitable mechanical properties for application as protective coatings in mechanical engineering, and also its superconductivity can be utilized in thin film devices for sensorics or in combination with ferromagnet in spintronics. Long-range superconducting proximity effect at the heterostructures with a weak ferromagnet can be used for generation of spin-polarized current. For any operational heterostructure application the high quality NbN thin films need to be prepared. In this work we have investigated impact of the substrates on the structure and preferential orientation of niobium nitride thin films fabricated by pulsed laser deposition at 600 °C on Si (001), MgO (001), C-plane and R-plane Al2O3 substrates. Growth parameters have been tuned in order to obtain single superconducting δ-NbN phase. Microstructure was analyzed by X-ray diffraction and transmission electron microscopy. Si substrate does not induce the film preferential orientation. Films on MgO are epitaxial (001) oriented. Films on R-Al2O3 show (135) orientation with twinned crystallites on the lower symmetry substrate surface. The (111) epitaxial growth with the largest crystallites and their smallest tilting was achieved on C-Al2O3 substrate leading to the best superconducting properties.

The Effect of Doping on the Structure of Zinc Oxide Obtained by Atomic Layer Deposition

Zinc oxide (ZnO) is an interesting inexpensive transparent conductive oxide for use as a transport layer in multilayer solar cells. We present the results of a study of the effect of aluminum doping on the structure of 50-65 nm thin films of zinc oxide obtained by atomic layer deposition (ALD) on different substrates (glass, sapphire with a-plane and c-plane orientations). Analysis of thin films by IR spectra and X-ray diffraction showed the effect of doping on the crystallographic texture of films grown by the ALD method by comparing substrates of simple glass, c-plane Al2O3 and a-plane Al2O3. We found that when doping zinc oxide, the orientation of the substrate plays a substantial role, which affects the mechanism of interaction of zinc oxide with a doping agent, leading to changes in the structure and properties of ZnO due to the interaction between ZnO molecules and water to form a Zn-O-H bond. Doping with aluminum leads to a strain stress, resulting in distortion of the structure and a decrease in the concentration of oxygen vacancies. This is turn facilitates the flow of water molecules into the interlayer space and an increase in the density of the positive charge on the metal cation, which leads to an increase in the coordination number, i.e. to an increase in the number of hydroxyl groups.

Growth of ZnO:Al by atomic layer deposition: Deconvoluting the contribution of hydrogen interstitials and crystallographic texture on the conductivity

Aluminium doped ZnO (AZO) is an interesting low cost transparent conducting oxide with further use as an inorganic transport layer in multilayer solar cells. Here we present our work on atomic layer deposited (ALD) thin films where, with optimised growth conditions, we can obtain resistivities of 1 × 10−3 Ωcm even in 50–80 nm thin films grown at low temperatures (250 °C). We discuss the influence of crystallographic texture for ALD grown films by comparing plain glass, c-plane Al2O3, and a-plane Al2O3 substrates. We show that the doping mechanism in ALD grown AZO is more complex than for e.g. sputtered material as a substantial hydrogen interstitial related background doping occurs. We compare results from as grown samples with those briefly annealed at 320 °C in nitrogen. This process leads to an increased Hall mobility due to improved grain boundary passivation, but reduced carrier concentration due to partial loss of hydrogen interstitials.

BaFe12O19 films prepared on Al2O3 (0 0 0 1) by direct current magnetron sputtering

M-type hexaferrite BaFe12O19 films that exhibit a uniaxial magnetic-crystalline anisotropy hold great application promise in the field of magnetic data-storage and microwave devices. In this letter, BaFe12O19 thin films that show an out-of-plane magnetic anisotropy were prepared on c-plane Al2O3 substrates by direct current magnetron sputtering, which is high-efficiency but seldom adopted in the fabrication of ceramic materials including most hexaferrites. In addition, the textured BaFe12O19 film with hexagonal c-axis lying in the film surface was also discussed in this work. The epitaxial-like growth of the BaFe12O19 films was characterized by the combination of x-ray diffractions and pole figures. Self-biasing properties of the films induced by the epitaxial-like structure were reflected in the magnetic measurement of hysteresis loops. The film sample deposited at high temperature of 600 °C exhibited a relatively large remanent magnetization (65% of the saturation magnetization) normal to the film surface, and a small one of <10% along in-plane directions, showing their potential for application in planar magnetic devices with light weight.

Effect of impurities on morphology, growth mode, and thermoelectric properties of (1 1 1) and (0 0 1) epitaxial-like ScN films

ScN is an emerging semiconductor with an indirect bandgap. It has attracted attention for its thermoelectric properties, use as seed layers, and for alloys for piezoelectric application. ScN and other transition metal nitride semiconductors used for their interesting electrical properties are sensitive to contaminants, such as oxygen or fluorine. In this present article, the influence of depositions conditions on the amount of oxygen contaminants incorporated in ScN films were investigated and their effects on the electrical properties (electrical resistivity and Seebeck coefficient) were studied. Epitaxial-like films of thickness 125 ± 5 nm to 155 ± 5 nm were deposited by DC-magnetron sputtering on c-plane Al2O3, MgO(1 1 1) and r-plane Al2O3 at substrate temperatures ranging from 700 °C to 950 °C. The amount of oxygen contaminants in the film, dissolved into ScN or as an oxide, was related to the adatom mobility during growth, which is affected by the deposition temperature and the presence of twin domain growth. The lowest values of electrical resistivity of 50 µΩ cm were obtained on ScN(1 1 1)/MgO(1 1 1) and on ScN(0 0 1)/r-plane Al2O3 grown at 950 °C with no twin domains and the lowest amount of oxygen contaminant. At the best, the films exhibited an electrical resistivity of 50 µΩ cm with Seebeck coefficient values maintained at −40 µV K−1, thus a power factor estimated at 3.2 × 10−3 W m−1 K−2 (at room temperature).

Effect of TMAl flow rate on AlInGaN/GaN heterostructures grown by MOCVD

In this work, lattice-matched AlInGaN/GaN epilayers are grown on c-plane Al2O3 substrate by metal organic chemical vapor deposition technique. The structural and optical properties of AlInGaN/GaN epilayers have been studied and compared. TMIn and TMAl flow is believed to have a direct impact on the quality of AlInGaN epilayer. By varying In and Al composition in AlInGaN epilayer, dislocations (V-pit density) have been found to decrease, indicating lower threading dislocation density as confirmed by HRXRD and AFM. Reciprocal space mapping analysis has confirmed that the growth of AlInGaN on GaN buffer is a fully lattice matched growth. AlInGaN exhibits two emissions originating from In(Ga)N clusters and the AlInGaN random matrix is confirmed by photoluminescence. In the Raman spectra of AlInGaN, A1 (LO) mode at 751 cm−1 and In(Ga)N clustering A1(LO) mode at 680 cm−1 is observed. These findings open the new advancement in the field of opto-electronic applications.

Comprehensive study on Sb-doped zinc oxide films deposited on c-plane Al2O3 substrates

Undoped and Sb-doped p-type ZnO films were grown on (001) sapphire using metal organic chemical vapor deposition (MOCVD) technology. The crystal, optical, and electrical properties of the undoped and Sb-doped ZnO film were investigated by X-ray diffraction (XRD), photoluminescence, optical transmittance spectroscopy and Hall measurement system. The XRD patterns indicated both films exhibited preferred orientation along c axis and had a compressive stress. Strong near-band gap emissions of both films were observed in the room temperature photoluminescence spectra. Low temperature photoluminescence spectrum of the ZnO film was dominated by a donor-bound exciton with peak at 3.365 eV. To the Sb-doped ZnO film, donor-bound exciton was buried into the free exciton with peak at 3.371 eV, and emission intensities associated acceptor enhanced. The acceptor binding energy was calculated to be 0.19 eV. The optical bandgap of the Sb-doped ZnO film was estimated to be 3.302 eV. The Sb-doped ZnO film exhibited p-type conductivity with the resistivity of 0.6729 Ω cm, mobility of 6.150 cm2 V−1 s−1 and carrier concentration of 1.508 × 1018 cm−3.

Growth and characterization of porous sp2-BN films with hollow spheres under hydrogen etching effect via borazane thermal CVD

Porous sp2-boron nitride (sp2-BN) films were directly grown on c-plane Al2O3 substrates by low pressure thermal chemical vapor deposition (CVD) using borazane (NH3-BH3) as a single precursor and without any catalyst. It was found that the BN films grown under 20 mbar are porous and consist of nano-scaled hollow BN spheres of various sizes, with outer radius (ro) ranging from 7.20 nm to 13.32 nm. The B-N bonding was confirmed to be sp2-bonded by the Raman scattering whereas, the crystal structure was determined by TEM and XRD to be turbostratic BN (t-BN) having a lattice constant ‘c’ larger than the hexagonal BN (h-BN) bulk crystal. Homogenous nucleation played a vital role in the porous sp2-BN formation that is contrary to normal CVD growth under such low pressure i.e. 2 mbar for denser BN films. Additionally, because of the H2 chemical etching effect, the H2/N2 ratio shows a strong influence on the film growth. All of the as-grown films have shown good ultraviolet (UV) absorption edge near 210 nm, hence providing a feasible route towards the synthesis of porous sp2-BN with higher surface-volume ratio for high performance photo sensors and smart energy storage devices.