Oriented Sapphire Substrates Research Articles

Growth and impact of intrinsic interlayers in high temperature vapor phase epitaxy of GaN

Structural quality of GaN layers grown on foreign substrates is usually improved by applying masks or interlayers, which interact with the crystal structure defects, mainly with threading dislocations, and consequently reduce their density. In this work, GaN interlayers with intrinsic defect structures were employed as barriers impeding the propagation of threading dislocations. The GaN samples were produced in a multistage deposition process by high-temperature vapor phase epitaxy (HTVPE). They consisted of GaN seed layers deposited on 15 × 15 mm2 (001)-oriented sapphire substrates, structured GaN interlayers and GaN top layers. The structuring of the interlayers was facilitated by changing the substrate temperature and the V/III ratio during the growth. The optical and scanning electron microscopies revealed that the structured interlayers contain pyramidal facets, vertical trenches and elongated voids. The results of X-ray diffraction and Raman spectroscopy have shown that these microstructure features are able to reduce the density of threading dislocations in the top layer and the tensile residual stress that develops in the top layer during the deposition process. These results demonstrate the potential of the intrinsic interlayer concept for the in-line defect and stress engineering in HTVPE GaN layer stacks.

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.

On the anion and cation positions within and nearby R(101¯2)$R(10\overline{1}2)$, Σ(202¯4)$\mathrm{\Sigma}(20\overline{2}4)$, and Π(101¯4)$\mathrm{\Pi}(10\overline{1}4)$ planes of α‐Al2O3 corundum structure

AbstractThe positions of aluminum cations and oxygen anions within and nearby the , , , and planes of α‐Al2O3 corundum structure are determined and compared with the corresponding ion positions within and nearby their conjugate‐adjacent planes , , , and planes. The plane contains neither aluminum nor oxygen ions, while the planes , , and contain only oxygen anions, whose planar densities are and . The planes , , and contain only aluminum cations with planar densities and . The plane contains both aluminum and oxygen ions, whose planar densities are and . The nearest ions to the , , , and planes are at a normal distance of 0.58 Å from these planes, which is one‐half of the normal distance between the adjacent R and , or r and planes. The ions nearest to the and planes are also at a normal distance of 0.58 Å from these planes. The nearest ions to the and planes are 0.85 Å from these planes, which is one‐half the interplanar spacing between the adjacent Π or π planes. The obtained results are of interest for the evaluation of ionic bonding and surface energies, epitaxial film growth over differently oriented sapphire substrates, and for the analysis of plastic deformation and cleavage fracture of sapphire crystals under different loading and temperature conditions.

Stimulated emission in heavily doped Al0.68Ga0.32N:Si structures with external cavity

Room temperature broad-band stimulated emission with transverse optically pumping was demonstrated from heavily doped Al0.68Ga0.32N:Si structures with external selective and non-selective cavities. The Al0.68Ga0.3N:Si films with thickness 1.2 μm were grown by molecular beam epitaxy on (0001) oriented sapphire substrates. Stimulated emission was observed in the continuous wavelength range from 400 to 650 nm under pulse optical pumping of the YAG:Nd3+ laser with a wavelength 266 nm, 10 Hz repetition rate and 8 ns pulse width. Low optical pumping threshold power of 5 kW/cm2 at 490 nm in the external non–selective cavity has been observed. The nature of stimulated emission has been studied by means time–resolved spectroscopy. This broad band stimulated emission in the visible region is attributed with two main recombination mechanisms: slow donor–acceptor pair and fast free electron-acceptor transitions. The relative contributions of these processes are experimentally measured at all wavelength range of stimulated emission. The optical gain coefficients of (0.1–17.3) × 103 cm−1 have been determined at pump power density from 12 to 1040 kW/cm2 by direct measurement the gain of a probe radiation in the Al0.68Ga0.32N:Si structures.

Structural and electrical properties of high-performance vanadium dioxide thin layers obtained by reactive magnetron sputtering

Epitaxial vanadium dioxide (VO2) films exhibiting an abrupt metal-insulator transition with resistivity ratios of five orders of magnitude have been grown on (001)-oriented sapphire substrates by reactive magnetron sputtering. The influence of deposition and annealing temperature on the structure and morphology of the layers are discussed as well as their impact on the films’ electrical properties. Thus, the VO2 layers obtained using the optimal experimental conditions have electrical resistivity ratios over 105 and can be obtained on substrates as large as three-inch in diameter. The combination of the scalability of magnetron sputtering and the high quality of the films enables the large-scale industrial applications of high quality VO2 layers.

Heteroepitaxial β-Ga2O3 thick films on sapphire substrate by carbothermal reduction rapid growth method

The heteroepitaxial β-Ga2O3 thick films were rapidly grown on various oriented sapphire substrates using carbothermal reduction method. The β-Ga2O3 films were prepared in our home-made vertical dual temperature zone furnace. The growth direction as well as surface morphology showed the strong dependence on the orientation of the sapphire substrate. The fastest growth rate was obtained reaching approximate 15 μm h−1 on c-plane sapphire substrate according to the average 30 μm thickness of β-Ga2O3 films grown for 2 h measured by cross-section scanning electron microscope. The Raman spectra indicated the pure-phase β-Ga2O3 films without obvious strain. The bandgap for grown films were in range of 4.6–4.7 eV confirmed by x-ray photoelectron spectra and Tauc plot from absorption spectra. Secondary ion mass spectrometry was used to check the impurities indicating a limited amount of residual carbon inside the films even though graphite as the reducing agent. The results in this work give promising alternative method of rapid epitaxial β-Ga2O3 thick films for the application on high-power electronic devices.

Hexagonal and rhombohedral polytypes in indium selenide films grown on c-plane sapphire

We report on the growth of 2D layered indium selenide films on (0001)-oriented sapphire substrates by coevaporation. The θ − 2θ x-ray diffractograms reveal that the (00l) planes are preferentially oriented parallel to the substrate with a tendency to deviate from the 2D stacking as a function of the growth time. The ϕ-scans performed for the (107) and (10 10) orientations of the hexagonal (h) and rhombohedral (r) phases, respectively, reveal that both polytypes coexist in the epitaxial films. We show that the merging of the h-(100), r-(101), h-(101), and r-(102) lines results in different spectral shapes in the θ − 2θ scans according to samples, which gives qualitative information about the contribution of each polytype.

Cathodoluminescent Analysis of Sapphire Surface Etching Processes in a Medium-Energy Electron Beam.

Sapphire crystals are widely used in optics and optoelectronics. In this regard, it is important to study the stability of crystals under external influence and the possibility of modifying their surfaces by external influence. This work presents the results of studying the processes of the action of an electron beam with an average energy of 70 keV or less under vacuum conditions on the surfaces of sapphire substrates of various orientations. The effect of etching a sapphire surface by an electron beam in vacuum at room temperature was discovered. The highest etching rate was observed for A-plane sapphire (the average pit etching rate was 10−6 µm3/s). It was shown that the rate of etching of a sapphire surface increased many times over when gold is deposited. An in situ method for studying the process of etching a sapphire surface using cathodoluminescence analysis was considered. Possible mechanisms of sapphire etching by a beam of bombarding electrons were considered. The results obtained could be important in solving the problem of the stability of sapphire windows used in various conditions, including outer space. In addition, the proposed method of metal-stimulated etching of a sapphire surface can be widely used in patterned sapphire substrate (PSS) technology and further forming low-dislocation light-emitting structures on them.

Ultrafast 2 × 2 green micro-LED array for optical wireless communication beyond 5 Gbit/s

A green 2 × 2 micro light-emitting diode ( μ - LED ) array with nanostructured grating patterns grown on a semipolar (20-21)-oriented gallium nitride (GaN) buffered layer on (22-43)-oriented sapphire substrate is specially transistor-outline can (TO-can) packaged with a sub-miniature-A (SMA) connector for high-speed data communication beyond 5 Gbit/s. Through a specific design for suppressing the quantum-confined Stark effect (QCSE) in the green 2 × 2 μ - LED array with a low polarization-related electric field and flat quantum well band diagram, the green 2 × 2 μ - LED array exhibits a turn-on voltage of 2.5 V and output power of 0.3 mW at 1 A / cm 2 . The green 2 × 2 μ - LED array also reveals a wavelength shift from 543 nm to 537 nm smaller than that of conventional devices grown on c -plane buffered GaN substrate due to the inhibited QCSE. The 50 µm emission aperture of the green 2 × 2 μ - LED array ensures a lower capacitance for a larger − 3 dB modulation bandwidth, which exhibits − 1 dB power compression at a larger bias under high-speed operation, as it is less affected by the high resistance of the single μ - LED element. With a specific TO-can+SMA package, the green 2 × 2 μ - LED array exhibits maximal data rates exceeding 1.5 Gbit/s for the non-return-to-zero on–off keying format and beyond 5.02 Gbit/s for the bit-loaded discrete multitone (BL-DMT) format, which is very promising for optical wireless communication. As the sampling rate increases from 4 GSa/s to 16 GSa/s, the μ - LED array’s received signal-to-noise ratio (SNR) improves dramatically from 15.4 dB to 12.2 dB. The SNR remains about 15.4 dB, with a matching bit-error ratio (BER) of 2.7 × 10 − 3 , whereas the 10-fold oversampling of the eight-ray quadrature amplitude modulation orthogonal frequency-division multiplexing (8-QAM OFDM) data stream with 16 GSa/s appears to reduce the SNR by − 3 dB , resulting in a decoded BER of 3.3 × 10 − 3 . The green 2 × 2 μ - LED array has demonstrated greater potential in data transmission beyond 5 Gbit/s using the BL-DMT algorithm for future applications in domains of visible light communication or optical wireless communication when packaged with handed mobile devices.

Epitaxial Growth and Solar‐Blind Photoelectric Characteristic of Ga2O3 Film on Various Oriented Sapphire Substrates by Plasma‐Enhanced Chemical Vapor Deposition

The epitaxial growth of Ga2O3 thin films is important for their applications in electronic and optoelectronic devices. Herein, the growth of Ga2O3 thin films on various oriented (c‐, a‐, m‐, r‐plane) sapphire substrates by plasma‐enhanced chemical vapor deposition (PECVD) is investigated using high purity metallic Ga and oxygen (O2) as precursor materials and argon (Ar) as carrier gas under a relatively lower growth temperature compared with other CVD methods. The effects of the substrates orientation to the surface morphology, crystal orientation, growth rate, optical properties, and solar‐blind photoelectric properties of Ga2O3 thin films are studied. The epitaxial film grown on the c‐plane sapphire substrate exhibits the best crystallinity and smooth surface, while that grown on the r‐plane shows the fastest growth rate of 1.97 μm h−1. The photodetector based on the Ga2O3 film grown on the c‐plane exhibits the lowest dark current of 0.17 nA, the highest Ilight/Idark ratio of 242.47, and the fastest response time of 0.31 s, while that of grown on the m‐plane shows the highest responsivity (Rλ) of 27.71 mA W−1.

Influence of Electrodes on the Parameters of Solar-Blind Detectors of UV Radiation

The influence of the topology of electrodes on the electrical and photoelectric characteristics of metal–semiconductor–metal structures is studied. Gallium-oxide films are produced by the radio-frequency magnetron-assisted sputtering of a Ga2O3 target onto (0001)-oriented sapphire substrates. Two types of electrodes are formed on the surface of the oxide films. The first type corresponded to two parallel electrodes spaced by an interelectrode distance of 250 μm, and the second type to interdigitated electrodes. In the case of the second type of electrodes, the distance between the “fingers” is 50, 30, 10, and 5 μm. The structures possess sensitivity to ultraviolet radiation at a wavelength of λ = 254 nm, irrespective of the type of contact. The second type of detectors with an interelectrode distance of 5 μm show the largest photocurrents, Iph = 3.8 mA, and a specific detectivity of D* = 5.54 × 1015 cm Hz0.5 W–1.

Investigation of Structural, Optical and Electrical Properties of Al0.3Ga0.7N/GaN HEMT Grown by MOCVD

In this study, Al0.3Ga0.7N/GaN high electron mobility transistor (HEMT) structure is investigated grown over c- oriented sapphire substrate by using Metal Organic Chemical Vapor Deposition (MOCVD) method. Optical, morphological and electrical characteristics of this structure are determined by X-ray diffraction (XRD), Photoluminecanse (PL), Ultraviolet (UV-Vis.), Atomic Force Microscopy (AFM) and Hall- Resistivity measurements. By using XRD method, 2θ, Full Width at Half Maximun (FWHM), lattice parameters, crystallite size, strain, stress and dislocation values are calculated on symmetric and asymmetric planes. Direct band gap of GaN is determined by PL measurements as 3.24 eV. It is seen that conduction of AlGaN layer starts at 360 nm in UV-Vis. In Hall-Resistivity measurements, it is noticed that carrier density of HEMT structure is not effected by temperature and mobility value is high. Carrier density and mobility values are determined as 5.82x1015 1/cm3 and 1198 cm2/Vs at room temperature respectively. At the lowest temperature point (25 K) they are calculated as 5.19x1015 1/cm3 and 6579 cm2/Vs, respectively.

Designing sapphire surface patterns to promote AlGaN overgrowth in hydride vapor phase epitaxy

Lateral overgrowth of patterned c-plane oriented sapphire substrates (PSS) with AlGaN using hydride vapor phase epitaxy was investigated with focus on how to suppress parasitic non c-planar crystallite nucleation and propagation. To this end, trigonal PSS was fabricated with either sidewalls parallel to sapphire -facets or parallel to sapphire -facets, which are crystallographically different due to the three-fold sapphire symmetry. X-ray diffraction-based texture analysis and SEM were applied to find two types of -domains solely nucleating on PSS sidewalls close to -facets. Their occurrence effectively blocks other orientations of non c-plane AlGaN crystallites allowing for quicker coalescence of c-plane AlGaN and improving overall AlGaN material quality.

Defect-rich GaN interlayer facilitating the annihilation of threading dislocations in polar GaN crystals grown on (0001)-oriented sapphire substrates

The interaction of microstructure defects is regarded as a possible tool for the reduction of the defect density and improvement of the crystal quality. In this study, this general approach is applied to reduce the density of threading dislocations in GaN crystals grown using high-temperature vapor phase epitaxy directly on (0001)-oriented sapphire substrates. The GaN crystals under study were deposited in three steps with different process temperatures, growth rates, and ammonia flows. The first GaN layer accommodates the lattice misfit between sapphire and gallium nitride. Thus, it contains a high number of randomly distributed threading dislocations. The next GaN layer, which is internally structured and defect-rich, bends and bunches these dislocations and facilitates their annihilation. The uppermost GaN layer mainly contains bunched threading dislocations terminating large areas of almost defect-free GaN. In order to be able to visualize and to quantify the microstructure changes in individual parts of the sandwich-like structure, the samples were investigated using nanofocused synchrotron diffraction, confocal micro-Raman spectroscopy, and transmission electron microscopy. The transmission electron microscopy provided information about the kind of microstructure defects and their mutual interaction. The synchrotron diffraction and the micro-Raman spectroscopy revealed the depth profiles of dislocation density and lattice parameters.

Interface-induced d0 ferromagnetism in undoped ZnO thin films grown on different oriented sapphire substrates

The origin of d0 ferromagnetism in ZnO material still remains an open question. Here, we report a systematic study of the structural, optical, Raman and magnetic properties of undoped ZnO films grown on a-, c-, m- and r-plane sapphire substrates by radio frequency magnetron sputtering at room temperature. It is found that the polarity of the substrate does not affect the preferential growth of undoped ZnO film along the c-axis, but it will obviously affect the species and concentration of intrinsic defects in films, thereby regulating the optical and Raman properties of undoped ZnO films. Magnetic measurement reveals that all undoped ZnO films exhibit clear hysteresis loops at room temperature, confirming the presence of room temperature ferromagnetism. Furthermore, the relationship between intrinsic defects and magnetic properties was discussed, which suggests that the observed ferromagnetic order has nothing to do with the internal intrinsic defects in undoped ZnO films, but is derived from the interface effects between the undoped ZnO films and the substrates.

Grain growth and solid-state dewetting of Bi-Crystal Ni-Fe thin films on sapphire

We studied the solid-state dewetting behavior of thin Ni80Fe20 films deposited on basal plane oriented sapphire substrate and annealed in the range of temperatures of 1023–1323 K. All studied films exhibited strong <111> texture and maze bicrystal microstructure, with only two grains misoriented by 60° around the common <111> axis present in the film. The morphology of partially dewetted films changed from the one typical for polycrystalline thin films to the one typical for single crystalline heteroepitaxial films with increasing temperatures and annealing times. This change of dewetting behavior was associated with the fast grain growth in the films. The films of pure Ni of identical thickness, annealed under identical conditions exhibited significantly slower grain growth and lower thermal stability. Both the high-resolution X-ray diffraction and the cross-sectional high-resolution transmission electron microscopy observations revealed the phase separation of the Ni80Fe20 films into two parallel layers of the face-centered cubic (adjacent to the substrate) and hexagonal close-packed (on the top of the film) phases of similar compositions. Our density functional theory (DFT) calculations indicated that this phase separation is driven by the decrease of the film surface and interface energy, leading to the thermodynamically equilibrium thickness of the metastable hexagonal close-packed phase. This phase exhibits higher surface anisotropy than its stable face-centered cubic counterpart and is instrumental in accelerating the grain growth in the film via suppression of grain boundary grooving.

Ammonia flux tailoring on the quality of AlN epilayers grown by pulsed atomic-layer epitaxy techniques on (0 0 0 1)-oriented sapphire substrates via MOCVD

A smooth and dense single-crystalline AlN was successfully grown by tailoring the flux density of ammonia.

Grain Growth and Solid-State Dewetting of Bi-Crystal Ni-Fe Thin Films on Sapphire

We studied the solid-state dewetting behavior of thin Ni80Fe20 films deposited on basal plane oriented sapphire substrate and annealed in the range of temperatures of 1023-1323 K. All studied films exhibited strong texture and maze bicrystal microstructure, with only two grains misoriented by 60° around the common axis present in the film. The morphology of partially dewetted films changed from the one typical for polycrystalline thin films to the one typical for single crystalline heteroepitaxial films with increasing temperatures and annealing times. This change of dewetting behavior was associated with the fast grain growth in the films. The films of pure Ni of identical thickness, annealed under identical conditions exhibited significantly slower grain growth and lower thermal stability. Both the high-resolution X-ray diffraction and the cross-sectional high-resolution transmission electron microscopy observations revealed the phase separation of the Ni80Fe20 films into two parallel layers of the face-centered cubic (adjacent to the substrate) and hexagonal close-packed (on the top of the film) phases of similar compositions. Our density functional theory (DFT) calculations indicated that this phase separation is driven by the decrease of the film surface and interface energy, leading to the thermodynamically equilibrium thickness of the metastable hexagonal close-packed phase. This phase exhibits higher surface anisotropy than its stable face-centered cubic counterpart and is instrumental in accelerating the grain growth in the film via suppression of grain boundary grooving.

Phase stability and strain accumulation in CdO as a function of Cd/O supply during MOVPE synthesis

A prominent trend for the stabilization of zincblende or wurtzite phases in CdO is observed. Typically, CdO occurs in its “equilibrium” rocksalt phase when grown on r-plane oriented sapphire substrates employing Cd-rich growth conditions. Gradual decrease of Cd supply, i.e. moving towards O-rich condition, results in additional phases, firstly zincblende and secondly wurtzite. Importantly, the variation of Cd/O ratios at the reaction zone was tuned by changing both the precursor flow rates and the growth temperature affecting the precursor decomposition. Moreover, an interesting strain accumulation trend, correlating with the band gap variations was observed as a function of Cd/O supply ratios too. In particular, a gradual change towards O-rich conditions leads to a build-up of strain in the films and red-shifts the optical absorption edge.

UVA and UVB light emitting diodes with AlyGa1−yN quantum dot active regions covering the 305–335 nm range

Ultra-violet (UV) light emitting diodes (LEDs) using III-N quantum dot (QD) active regions have been fabricated by molecular beam epitaxy on (0001)-oriented sapphire substrates. By using the epitaxial compressive stress between the QD material and the template/barrier layers, leading to a 2D–3D growth mode transition, self-assembled AlyGa1−yN QDs with a nominal Al composition of 10% and 20% have been fabricated on Al0.6Ga0.4N. Atomic force microscopy and transmission electron microscopy measurements show high QD densities, ranging between 2 × 1011–5 × 1011 cm−2, and height and diameter distributions between 1.5–3 nm and 5–20 nm. LED structures including two different AlyGa1−yN/Al0.6Ga0.4N (0001) QD active regions have then been fabricated and processed using a standard planar geometry and squared mesa structures. Current–voltage characteristics and electroluminescence (EL) measurements have been performed at room temperature. In particular, the EL properties are investigated in terms of spectral range and wavelength shift as a function of the injection current density. Typically, an emission between 325 and 335 nm is obtained for Al0.1Ga0.9N QDs and between 305 and 320 nm for Al0.2Ga0.8N QDs. The LED characteristics (EL wavelength and broadening) are then correlated to the QD structural properties and the results, supported by calculations, show the main influence of the QD height dispersion and composition fluctuations. Finally, the light output intensity variation as a function of the injection current density has also been investigated and is discussed in terms of injection and recombination mechanisms in the devices.