c-Al2O3 Substrates Research Articles

Structural and optical properties of PA MBE AlGaN quantum well heterostructures grown on c-Al2O3 by using flux- and temperature-modulated techniques

Abstract

Structural and mechanical properties of YBaCo4O7+δ thin films deposited on c-Al2O3 substrates by dc magnetron sputtering

High-quality YBaCo4O7+δ powders were obtained by standard solid-state reaction. The powders were pelletized and sintered at 1300°C. The targets were subsequently coupled to a dc magnetron sputtering cannon using commercial silver epoxy. YBaCo4O7+δ thin films (~100nm) were then grown on c-Al2O3 single-crystal substrates heated at 750, 800 and 850°C. Microstructural analysis was carried out by means the X-ray diffraction measurements. A strong dependence of the crystalline quality of the thin films on the substrate temperature was evidenced. In turn, Young׳s modulus and nanohardness values of YBaCo4O7+δ, both in bulk and thin film form, were determined by means of nanoindentation measurements. The values of Young׳s modulus and hardness obtained for YBaCo4O7+δ in bulk form were in good agreement with those encountered for thin films. Before the deposition of the films, the thermal expansion coefficients of the YBaCo4O7+δ target and the sapphire substrates were determined using a dilatometry technique. The thermal expansion coefficients of the target and the sapphire substrates ended up being very close each to other, which favors the growing of the thin films. Finally, the lattice strain, the lattice deformation stress and the deformation density energy of the thin films were also estimated from the X-ray diffraction measurements using the Williamson–Hall model. The achieved results show that the Williamson–Hall methodology is a good alternative method to estimate stress and mechanical properties of materials in thin film form.

E‐beam pumped mid‐UV sources based on MBE‐grown AlGaN MQW

A 60 mW output power has been achieved in mid‐UV (λ = 270 nm) spontaneous sources with electron‐beam pulse‐scanning pumping, fabricated from AlGaN MQW heterostructures grown by PA MBE on c‐Al2O3 substrates. Under the CW pumping at much lower excitation power density the mid‐UV sources demonstrate a 4.7 mW output power. In that regime the power efficiency of the structures is about 0.24%, while their internal quantum efficiency is estimated to be as high as 50%.

Epitaxial growth of vertically aligned ZnO nanowires for bidirectional direct-current driven light-emitting diodes applications

We presented a comparative investigation on the morphological and structural properties of the produced ZnO nanowires (NWs) on c-Al2O3 substrates under different preparation conditions. The effects of a low-temperature nucleation layer, reaction pressure and light irradiation on the alignment, diameter and growth rate of ZnO NWs were evaluated in detail. It was found that the low-temperature nucleation layer acting as a supporting layer favored the formation of a transition layer featuring initially a rough surface and a high density of grain boundaries, guiding and facilitating the subsequent homoepitaxy of ZnO NWs grown on the top of developed (0001) facets of the new activation sites. The crystallinity and vertical alignment of epitaxial ZnO NWs were further optimized to achieve the best value, as the second growth stage was conducted with a higher reaction pressure and light irradiation, such that a relatively low full width at half-maximum (701 arcsec) of the rocking curve could be obtained. The thermodynamical growth kinetics of the produced ZnO NWs via the two-step growth process was also investigated by monitoring the morphology evolution at different growth stages. Coaxial n-ZnO/MgO/p-NiO-core/shell NWs heterostructured light-emitting diodes (LEDs) were then fabricated and characterized, and a unique tunability of the electroluminescence (EL) spectra depending on the bias polarities was observed. We studied in detail the bicolor EL characteristic of the bidirectional direct-current driven LEDs and tentatively proposed carrier tunneling and hole generation models to explain such interesting features.

Defect engineering in AlGaN-based UV optoelectronic heterostructures grown on c-Al2O3 by plasma-assisted molecular beam epitaxy

ABSTRACTAlGaN-based SQW heterostructures grown by plasma-assisted molecular beam epitaxy on c-Al2O3 substrates have been studied with high resolution transmission electron microscopy (HR TEM), photoluminescence spectroscopy and x-ray diffraction. The high-temperature (780°C) synthesis of the AlN buffer layer nucleated on c-Al2O3 by a migration enhanced epitaxy and including several ultra-thin GaN interlayers grown under moderate N-rich conditions was shown to be the optimum approach for lowering the threading dislocations density down to 108-109 cm-2. HR TEM study has confirmed the fine structure of single quantum wells (SQW) formed by a sub-monolayer digital alloying technique and revealed different kinds of compositional inhomogeneities in the AlxGa1-xN barrier layers of the heterostructures, including the formation of Al-rich barriers induced by the temperature-modulated epitaxy and the spontaneous compositional disordering along the growth axis for x=0.6-0.7. The influence of these phenomena on the parameters of the mid-UV stimulated emission observed in the SQW structures has been studied as well.

Investigation of the growth and properties of single-crystalline aluminum nitride nanowires

The growth of high-quality, large-scale wurtzite-structured aluminum nitride (AlN) nanowires on c-Al2O3 substrates using a catalyst-assisted chemical vapor deposition method was investigated. The morphology and structure of the nanowires were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and Raman spectroscopy. These uniform nanowires exhibit a smooth surface and are 50nm in diameter and 10–30μm long; they also exhibit a hexagonal single-crystal lattice with parameters of c=0.497nm and a=0.272nm and grow along the [100] direction. A room-temperature near-band edge emission at 6.12eV was observed in their electron excitation spectra. Our approach provides a simple and efficient method to grow high-quality AlN nanowires, and our results indicate that they are promising materials for use in various applications.

Optical and photoluminescence properties of ZnO(1-x)Se(x) thin films.

ZnO(1-x)Se(x) (0≤x≤0.11) thin films have been grown on sapphire (c-Al₂O₃) substrates at the temperature of 350°C by means of the radio-frequency magnetron sputtering technique. The optical transmission, reflectance, and luminescence spectra at room temperature were analyzed. Using the λ-modulation method gives the possibility to reveal the main features of the energy band structure and the nature of the radiative transitions that cause ultraviolet luminescence.

Single-crystalline GaN nanotube arrays grown on c-Al2O3 substrates using InN nanorods as templates

Single-crystalline GaN nanotube arrays were grown on c-Al2O3 substrates by metal-organic chemical vapor deposition. InN nanorods with In droplets at the top-ends were used as the templates. After growing GaN over the templates, InN nanorods were thermally removed and GaN nanotubes were obtained. In droplets were considered to be the pathways for the decomposed InN species to escape away and played an important role in the formation of GaN nanotubes. The nanotubes are well-aligned along the c-axis of the wurtzite GaN structure with good crystal quality and morphologies. X-ray diffraction and energy-dispersive X-ray spectrometer studies indicate that the products are absent of template related residues. Our method for fabricating GaN nanotubes can be applied to the formation of nanotube arrays of other materials (such as AlGaN and AlN).

The effect of O2 partial pressure on the photoluminescence of ZnO thin films grown by pulsed laser deposition

In this paper, photoluminescence (PL) of ZnO thin films prepared on c-Al2O3 substrates by the pulsed laser deposition (PLD) method at different O2 partial pressures is investigated. For all samples, a narrow ultraviolet (UV) emission and a broad visible emission can be observed at room-temperature (RT). With increasing O2 partial pressures from 0.2 to 5Pa, the intensity ratio of the UV to visible emissions increases, and the energy positions of the UV emission band shift to the high energy side. It is noted that the visible part includes two emission bands of green luminescence (GL) and yellow luminescence (YL), in which the GL emission is strong at low oxygen pressure and the YL emission becomes dominant at high O2 partial pressures. The temperature-dependent PL spectra show that the UV emission is composed of two bands labeled FX and FA. The dependences and possible assignments of these PL bands are briefly discussed.

Structure, band gap, and Mn-related mid-gap states in epitaxial single crystal (Zn1−xMgx)1−yMnyO thin films

Epitaxial (Zn1−xMgx)1−yMnyO thin films were grown on c-Al2O3 substrates by radio frequency oxygen plasma assisted molecular beam epitaxy. Single crystal structure of the (Zn1−xMgx)1−yMnyO films was revealed by reflection high energy electron diffraction and X-ray diffraction. The band gap of the films can be tuned dramatically with increasing the Mg concentration, while the onset energy of Mn-related mid-gap absorption band only shows a small blue shift. Photoconductivity measurements indicate the Mn-related mid-gap states in (Zn1−xMgx)1−yMnyO films can create free carriers and contribute to charge transfer transitions. The conduction band offset ΔEC = 0.13 eV and valence band offset ΔEV = 0.1 eV were obtained for ZnO/Zn0.8Mg0.2O heterostructures, which increase to ΔEC = 0.21 eV and ΔEV = 0.14 eV for ZnO/Zn0.7Mg0.3O heterostructures.

Control of threading dislocation density at the initial growth stage of AlN on c-sapphire in plasma-assisted MBE

Crystalline properties of (1–2)-μm-thick AlN buffer layers grown by plasma-assisted molecular-beam epitaxy (PA MBE) on c-Al2O3 substrates with different AlN nucleation layers have been studied. The best quality layers are obtained on 50-nm-thick nucleation AlN layers grown by a migration enhanced epitaxy (MEE) at substrate temperature of 780°C. In this case the buffer layers possess the lowest FWHM values of the symmetric AlN(0002) and skew symmetric AlN(10–15) x-ray rocking curve peaks of 469 and 1025arcsec, respectively, which correspond to the screw and edge threading dislocation densities of 4.7×108cm−2 and 5.9×109cm−2. This improvement seems to be related with the larger diameter of the flat-top grains in the AlN nucleation layers grown in the MEE mode at high substrate temperatures.

XRD Characterization of AlN Thin Films Prepared by Reactive RF-Sputter Deposition

AlN thin films have been grown on R((1-12) surface-cut)-Al2O3, SiO2-glass and C((001) surface-cut)-Al2O3 substrates, by using a reactive-RF-sputter-deposition method. X-ray diffraction (XRD) shows that AlN film has (110) orientation of wurtzite crystal structure for R-Al2O3 and (001) orientation for SiO2-glass and C-Al2O3 substrates. The film thickness was analyzed by Rutherford backscattering spectroscopy (RBS) and it appears that XRD intensity does not show a linear increase with the film thickness but a correlation with the stress, i.e., deviation of the lattice parameter of the film from that of bulk. The film composition and impurities have been analyzed by ion beam techniques. Effects of high-energy ion beams are briefly presented on atomic structure (whether stress relaxation occurs or not), surface morphology and optical properties.

Selective growth of pure magnetite thin films and/or nanowires grown in situ at a low temperature by pulsed laser deposition

Pure Fe3O4 thin films and/or nanowires were selectively grown onto c-Al2O3 (0001) substrates in situ at different substrate temperatures using pulsed laser deposition (PLD). The epitaxial Fe3O4 thin films were grown at deposition temperatures that ranged from 300 to 600 °C, while nanowires with a single-crystalline nature were grown at temperatures of more than 600 °C without catalysts. The in-plane compressive stress of the films increased as deposition temperature increased. Protrusions to relieve the compressive stress typically formed at the low-angle grain boundaries and were then developed to make nanowires. Both the films and the nanowires showed a ferromagnetic nature and a magnetic anisotropy at room temperature.

P-type conductivity control of heteroepitaxially grown ZnO films by N and Te codoping and thermal annealing

The authors report on the p-type conductivity in the ZnO films, grown on c-Al2O3 substrates by molecular-beam epitaxy, using N and Te codoping and thermal annealing. In the electrical properties, the N and Te codoping effectively suppresses the background electron concentration of ZnO films, and the thermal annealing causes the conductivity conversion from n-type to p-type. In the structural properties, the N and Te codoping deteriorates the crystalline quality of ZnO films, even if Te doping contributes to improve the crystallinity, and the thermal annealing recovers the degraded crystalline quality again. In the optical properties, the N and Te codoping simultaneously increases the donor-related emission, the acceptor-related emission, and the nonradiative recombination in ZnO films, while the thermal annealing relatively enhances the acceptor-related emission. It is proposed that (i) the N and Te codoping simultaneously induces N-related defects and donor-type defects along with free acceptors in the ZnO films, but the amount the N-related defects is larger than the donor-type defects and the free acceptors; and (ii) the thermal annealing relatively activates the N-related defects and suppresses the donor-type defects.

Structural and magnetic properties of Zn1 − xMnxO nanocrystalline powders and thin films

The structural and magnetic properties of Zn1−xMnxO (x=0, 0.02, 0.05, 0.1) nanocrystalline powders and thin epitaxial films were studied in detail. Epitaxial films were fabricated by pulsed laser deposition on c-Al2O3 substrates. To this end, ablation targets were prepared by pressing nano-sized powders of Zn1−xMnxO (x=0, 0.02, 0.1) synthesized by the wet polymeric precursor method. The method was chosen in order to improve purity and homogeneity of the grain-size distribution. The epitaxial growth was optimized with respect to crystalline quality by varying deposition parameters such as substrate temperature, laser fluence and background gas conditions. Films prepared under optimized conditions show a mosaic spread<0.3° and a bulk-like c-axis lattice parameter of 5.198Å. The structural data indicate the substitution of Zn2+ by the Mn2+ cations in the wurtzite structure of pristine ZnO. The influence of Mn on the lattice dynamics of ZnO is confirmed by Raman scattering. It is found that Mn doping increases the lattice defects and induces two Raman vibration modes of 275 and 526cm−1. Magnetic measurements, in turn, show while thin films, grown in vacuum, are ferromagnetic at RT, the growth in an oxygen atmosphere results in a dramatic decrease of the magnetic properties, demonstrating that oxygen vacancies are most likely responsible for the magnetic exchange. The co-doping with nitrogen has no major influence on the structural and magnetic properties of the ZnO films. A correct Mn doping concentration seems to be crucial for making a viable diluted magnetic semiconductor.

Effect of oxygen pressure on electrical properties of Ge-doped ZnO thin films grown by using pulsed laser deposition

ZnO and Ge-doped ZnO (GZO) thin films were fabricated on c-Al2O3 substrates at various oxygen pressures (10, 50, 100, 200, and 400 mTorr) and at 300 °C by using a pulsed laser deposition method. The transmittances of the films were measured by using ultraviolet-visible spectroscopy. The absorption of the band gap edge was calculated by using Tauc’s equation. The films showed n-type conduction characteristics. Compared to the other films, the film deposited at 10 mTorr compared showed improved electrical properties. The carrier concentration and the resistivity of the GZO film were 1021 cm−3 and 6.5 × 10−4 Ωcm, respectively.

Comparative Study of the Electronic Transport in YBaCo4O7+δ Thin Films Grown on Different Substrates

YBaCo4O7+δ thin films with thicknesses of 180 nm were grown by means of magnetron sputtering technique onto (100)-SrTiO3,(100)-LaAlO3,(100)-MgO, and (0001)-Al2O3 substrates. The structural characterization performed by X-ray diffraction shows that films deposited on MgO, SrTiO3, and c-Al2O3 substrates grow with preferential orientation in the (203) direction. For LaAlO3 substrates, the preferred growth direction is the (212). The coupling between the lattice parameter of the compound and those of the substrates seems to determine the preferred orientation for the growth onto the different substrates. We carry out measurements of the resistivity as a function of the temperature in the standard four-terminal configuration, and analyze the results in the framework of some transport models such as simple thermal activation, small polaron hopping, and Mott’s variable range hopping (VRH). A better agreement with the experimental data is observed by applying the latter model. The fitting of this model to the experimental data allows for determining important physical parameters related to the electrical conduction in these materials such as the density of states at the Fermi level.

Combination of Chemical and Physical Methods to Produce High-Quality Epitaxial Zn1−x Mn x O Thin Films

We have carefully analyzed the epitaxial growth and magnetic properties of Zn1−xMnxO (x=0–0.1) thin films fabricated by pulsed laser deposition on c-Al2O3(0001) substrates. The ablation targets are prepared by pressing nano-sized Zn1−xMnxO (x=0–0.1) powders which were previously synthesized by the polymeric precursor method. The films show excellent crystalline quality with full width at half-maximums varying from ∼0.07–1∘. Some films show well-defined magnetic hysteresis loops both at 300 and 5 K, which suggests the presence of ferromagnetic order. Nevertheless, we find that the ferromagnetism in the samples depends strongly on the oxygen partial pressure in the deposition chamber. This finding suggests that the observed ferromagnetism in Mn-doped ZnO is not an extrinsic property of the system. It is probable that unpaired electron spins, which have their origin in the oxygen vacancies, are responsible for the observed ferromagnetic behavior.

Enhancement of the Ultraviolet Luminescence Intensity from Cd-Doped ZnO Films Caused by Exciton Binding

ZnO films doped with the cadmium (0.4–0.6%) were grown on crystalline sapphire c-Al2O3 substrates applying radiofrequency magnetron sputtering at the temperature of 400 ◦C in Ar–O2 atmosphere. The as-grown films were investigated in detail using X-ray diffraction, X-ray photoelectron spectroscopy, and cathodoluminescence spectra. The X-ray diffraction analysis revealed that the films possess a hexagonal wurtzite-type structure with the dominant crystallite orientation along the c axis. It was found that the small concentration of the cadmium significantly enhances the ultraviolet emission associated with excitonic transitions. We suggest that this enhancement effect mainly results from appearance of the cadmium isoelectronic traps, which may bind an exciton, thereby increasing the probability of radiation recombination. The effect of Cd isoelectronic impurity on structural and luminescent properties of ZnO films is discussed.

Epitaxial Growth of V2O3 Thin Films on c-Plane Al2O3 in Reactive Sputtering and Its Transformation to VO2 Films by Post Annealing

Epitaxial growth of thin vanadium sesquioxide (V2O3) films on c-plane sapphire (c-Al2O3) substrates was achieved with reactive magnetron sputtering under restricted oxygen flow. Even with a film thickness of approximately 12 nm, highly c-axis textured growth of corundum V2O3 was realized because of the smaller mismatch of V2O3 against corundum Al2O3. Post annealing in O2 atmosphere for as-grown V2O3 films caused phase transformation to oxidized crystalline phases. At a moderate annealing temperature of 450 °C, the V2O3 thin films transformed to VO2 films, which show a resistivity change of over three orders of magnitude. The X-ray photoelectron spectroscopy spectra for the annealed VO2 film showed a single charge state of V4+, indicating a homogeneous crystalline structure, in contrast to the inhomogeneous feature with mixed charge states of V in addition to V3+ for as-grown V2O3 film. This method is promising to prepare thin VO2 films with metal–insulator transition in productive reactive sputtering and to examine crystalline phase transformation mechanisms, including phase coexistence.