M-plane Sapphire Research Articles

Effect of ultrasonic elliptical vibration assistance on the surface layer defect of M-plane sapphire in microcutting

Surface-layer defects of M-plane sapphire caused by ultrasonic vibration-assisted cutting were investigated in this paper through comparison cutting experiments. Topography analysis results indicate that when the cutting direction is not along a-axis or c-axis, the spalling defects of conventional cutting groove are asymmetric because the probabilities of activated cleavage/twinning systems vary with machining direction; however, the vibrational cutting groove presents better symmetry and less spalling owning to the propagation of stress wave. The cross-section of cutting groove was obtained by focus ion beam cutting. The cross-section analysis shows that the subsurface defects of conventional cutting groove involve median cracks and lateral cracks, and the main propagating path of cracks is towards downward, which is the cause of the generation of serious spalling; however, lateral cracks are the main defect in the subsurface of the vibrational cutting groove, and they mainly propagate towards the free surface and give rise to material removal. The subsurface quality of cutting groove with ultrasonic assistance is better than that of the conventional cutting groove. It is evident that ultrasonic vibration can prevent the median crack from extending towards bulk material and contribute to improve the uniformity of groove quality.

Structural characterization of AlN (11-22) films prepared by sputtering and thermal annealing on m-plane sapphire substrates

Abstract (11-22)-oriented AlN films are sputtered on m-plane sapphire substrates at different temperatures. With an optimized sputtering temperature of 800 °C and high-temperature annealing at 1600 °C, the full widths at half maximum (FWHMs) of X-ray diffraction of AlN are 0.186° and 0.243° along [11-23] AlN and [1-100]AlN. The annealed AlN shows smooth surface morphology with a roughness of 0.425 nm. Convergent beam electron diffraction patterns display that the annealed AlN (11-22) film is grown with Al-face sense polarity rather than mixed polarity in c-plane sputtered AlN, which may result from less oxygen in the AlN/sapphire interface. Threading dislocations with a density of 7.5 × 109 cm−2 and stacking faults with a density of 1.1 × 105 cm−1 are identified in the transmission electron microscopy cross-sectional images. The crystal quality of AlN in this work is attractive for growth and fabrication of AlN or AlGaN-based semipolar devices.

Single-phase high-quality semipolar (10–13) AlN epilayers on m-plane (10–10) sapphire substrates

Single-phase flat semipolar (10–13) AlN epilayers on m-plane (10–10) sapphire substrates grown by ammonia-free high-temperature metalorganic vapor phase epitaxy are firstly demonstrated. It is found that twins and basal-plane stacking faults are undetectable by X-ray diffraction and cross-sectional characterization. An X-ray rocking curve (XRC) shows the full widths at half maximum (FWHM) of the (10–13) and the (0002) diffraction peaks from a ∼2.3 μm thick AlN film as narrow as 322 and 373 arcsec, respectively, indicating a high structural quality. Semipolar AlN epilayers hold great promise for high performance deep-ultraviolet (DUV) optoelectronic device applications.

Effect of Temperature on the Structural and Optical Properties of Ga2O3 Thin Films Grown on m-plane Sapphire Substrates by Low-Pressure MOCVD

In this paper, Ga2O3 films were grown on m-sapphire substances at different temperatures of 600 °C, 700 °C and 800 °C by low pressure MOCVD. By changing the temperature, the crystal phase change of the Ga2O3 thin films grown on the m-sapphire substrates were studied. The effects of temperature on structural characteristics, surface morphology and optical properties of the films were studied systematically. XRD results indicated that the mixture of α-phase and β-phase Ga2O3 films were obtained on the m-sapphire substrates. As the temperature increased to 800 °C, the growth of α-Ga2O3 was weakened. The degree of polycrystallization of β-Ga2O3 decreased, and it gradually changed to preferential growth along the (−402) direction. With increasing of the temperature, the surface morphology of films was obviously changed. AFM measurement showed that the growth temperature has an important effect on the formation of Ga2O3 grains. As the temperature increased, the grain size of the film gradually increased. The analysis results of Raman spectroscopy detected the Raman vibration modes of α-Ga2O3 and β-Ga2O3. In addition, the photoluminescence properties of Ga2O3 thin films have also been systematically studied. Compared with c-sapphire, Ga2O3 thin films grown on m-sapphire substrates have more photoluminescence peaks, and the photoluminescence intensity increased with increasing temperature.

Breaking Lattice Symmetry in Highly Strained Epitaxial VO2 Films on Faceted Nanosurface.

The lattice symmetry of strongly correlated oxide heterostructures determines their exotic physical properties by coupling the degrees of freedom between lattices and electrons, orbitals, and spin states. Systematic studies on VO2, a Mott insulator, have previously revealed that lattice distortion can be manipulated by the interfacial strain and electronic phase separation can emerge. However, typical epitaxial film-substrate interface strain provides a very limited range for exploring such interface-engineered phenomena. Herein, epitaxially grown VO2 thin films on asymmetrically faceted m-plane sapphire substrates with the hill-and-valley type surfaces have been demonstrated. Interestingly, lattice symmetry breaking has been proven based on the large residual strain from the different faceted planes. By this lattice symmetry breaking, electronic phase separation and metal-insulator transition in the VO2 films are modulated, and anisotropy in optical responses is exhibited. These results on asymmetrical interfacial engineering in oxide heterostructures open up new routes for novel functional materials design and functional electro/optic device nanofabrication.

Untwinned semipolar (101̅3) AlxGa1-xN layers grown on m-plane sapphire

Heteroepitaxial growth of untwinned mono-crystalline semipolar (101̅3) Alx layers on (101̅3) AlN template was investigated by metalorganic vapour phase epitaxy. The templates were initially deposited on (101̅0) m-plane sapphire substrates by directional sputtering. Different Al/Ga ratios in gas phase were used to adjust the AlN mole fraction over the entire range of composition. All the layers show a triclinic distortion in the wurtzite unit cell due to anisotropic in-plane strain. The AlN mole fraction of the (101̅3) layers and c-plane co-loaded layers estimated by x-ray diffraction is comparable. This is consistent both with their comparable effective bandgap energy estimated from optical transmission measurements and their near band-edge emission energy obtained from room-temperature photoluminescence. The dependence of the bandgap and near band-edge emission energies on the AlN mole fraction indicates a bowing parameter of 0.9 eV.

Electric polarizations in PVDF-TrFE nanorods under lateral nanoshaping

Horizontal, single-dimensional, trianglelike poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) nanorods are prepared under lateral nanoconfinement induced by faceted m-plane sapphire surfaces. X-ray diffraction reciprocal space mappings reveal that the polymer contains polycrystalline ferroelectric β-phase PVDF. Depolarizing fields induce different polarizations in polymer nanorods along the out-of-plane and in-plane directions. The differences are characterized using vector piezoresponse force microscopy. The depolarizing fields in the lateral PVDF-TrFE nanorods are further discussed in theory. Our results suggest that a nanofaceted m-plane sapphire can be an alternative template to induce the depolarizing fields for future nanosacle devices.

Impact of 3D growth and SiNx interlayer on the quality of (11–22) semi-polar GaN grown on m-plane sapphire

We report the growth of semi-polar (11−22) GaN films with low defect densities on m-plane sapphire substrates by incorporating a porous SiNx interlayer in a three-stage growth process. The linewidth of X-ray diffraction rocking curves for our semi-polar GaN films decrease with increasing SiNx deposition time. Cross-sectional transmission electron microscopy analyzes confirm that the proposed three-stage growth process greatly reduces the semi-polar GaN threading dislocation densities down to 7 × 108 cm−2. Photoluminescence measurements demonstrate that InGaN/GaN quantum wells grown on our semi-polar GaN films provide peak intensities that are 26-fold greater than those grown on semi-polar GaN by a standard method.

Crystalline orientation effects on material removal of sapphire by femtosecond laser irradiation

C-, M − , A- and R-plane sapphire plates have been irradiated by a femtosecond laser with different laser parameters. The influences of crystal orientations on irradiation damage of single- and multiple-pulse have been investigated. The experimental results illustrate that the damage morphology, damage threshold, material removal rate and accumulation of laser damage depend on the crystal orientation. This dependence is related to the different bonding energies between adjacent atomic layers for different crystal orientations. The theoretical analysis indicates that bonding energy between adjacent atomic layers of C-, M − , A- and R-plane sapphire plates increases in the order of C-plane < M-plane < A-plane < R-plane. The experimental damage threshold of different crystal planes increases in the order of C-plane < M-plane < A-plane < R-plane and material removal rate decreases in the order of C-plane > M-plane > A-plane > R-plane. The analytical result is in agreement with the experimental result. The experiment results also present that the damage accumulation of M-plane sapphire plates is the largest. Therefore, M-plane is more easily to form cracks under multiple pulse irradiation.

Controlling the orientations of directional sputtered non- and semi-polar GaN/AlN layers

We have studied the impact on the surface orientation of different sputtered Al layers followed by AlN layers sputtered on m-plane sapphire. These initial layers were then overgrown by AlN and followed by GaN using metal-organic vapor phase epitaxy (MOVPE). By increasing sputtering time of the initial Al layer from 0 s to 15 s, we obtained single phase (10-10), (10-13) and (10-14) GaN/AlN layers. The thickness of the initial Al layer was estimated by optical transmission measurements to be about 0.5–1 nm for the (10-13) orientation, and >1 nm for the (10-14) orientation. After MOVPE growth, no trace of metallic Al was found by transmission electron microscopy, indicating that this layer was fully converted to AlN.

Controlled crystal orientations of semipolar AlN grown on an m-plane sapphire by MOCVD

The way to control AlN orientations on an m-plane sapphire in metal-organic chemical vapor deposition has not been clarified. We investigated the effect of the nitridation process on the crystal orientation of semipolar AlN on an m-plane sapphire. In terms of substrate heating, (10-1-3) AlN was obtained under H2 without NH3, whereas (11–22) AlN was obtained by heating with H2 and NH3. Temperature-dependent nitridation demonstrated that the AlN orientation changed from (10-1-3) to (11–22) at the nitridation temperature of 500 °C, which can be explained by the formation of r-plane nanofacets on m-plane sapphire under significant nitridation at a high temperature. At a nitridation temperature of 1100 °C, a small amount of NH3 led to the formation of (11–22) AlN, indicating that care must be taken to keep the reactor conditions for producing desired AlN orientations with reproducibility.

Reducing the grain density in semipolar (11-22) AlGaN surfaces on m-plane sapphire substrates

The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented AlxGa1-xN with x ∼ 0.80 on m-plane sapphire are studied with the intention of mitigating the expansion of misoriented grains, composed of the (1-10-3) crystal orientation and achieving a flat surface with only the (11-22) orientation. An increase in reactor pressure, metalorganic supply, and V/III ratio led to a decrease in the grain density from 1.0 × 109 cm−2 to 1.5 × 107 cm−2. By comparing different growth regimes, we found that the main factor suppressing the growth of the (1-10-3) orientation and decreasing the grain density in the AlGaN layers is the growth rate, which decreased with increasing reactor pressure, MO supply, and V/III ratio due to increasing pre-reactions in the gas phase. To mitigate pre-reactions even with lower growth rates, growth conditions with higher total flow and lower TMAl flow were chosen, yielding low growth rates of 0.13 μm h−1 and a grain density of 3.0 × 107 cm−2 at an aluminum mole fraction of 84%. To allow the growth of thick LED heterostructures we demonstrated that such a buffer can be overgrown with higher growth rate AlGaN, yielding a low grain density of 1.0 × 106 cm−2 and a smooth morphology with a rms roughness of 2.5 nm by avoiding misoriented crystal propagation during nucleation.

Formation of m-plane AlN on plasma-nitrided m-plane sapphire

Microwave plasma using a gas mixture of N2 and H2 has been applied for the nitridation of m-plane sapphire substrate to form a thick epitaxial AlN film. The X-ray diffraction results show that the AlN films formed on the sapphire surface by nitridation for a period from 10–60 min are in (100) orientation and have an epitaxial relationship with the substrate. The thickness of the nitride film increases with nitridation time and approaches about 0.5 μm after nitridation for 1 h, while the film surface becomes rough. The film quality is reasonably good, as evaluated with the X-ray rocking curve of (100) AlN. Faceted voids in the sapphire substrate underneath the AlN are also observed with inclined a-plane facets after nitridation.

Semi-polar (11–22) GaN epitaxial films with significantly reduced defect densities grown on m-plane sapphire using a sequence of two in situ SiNx interlayers

This letter reports an approach for growing semipolar (11–22) GaN films with significantly reduced defect densities on m-plane sapphire substrates by incorporating a sequence of two in situ SiNx layers. The first SiNx layer is deposited on an initial epitaxial GaN layer and acts as a nanomask for preventing the propagation of extended defects. The second SiNx layer is deposited just after subsequent epitaxial GaN overgrowth begins to form self-organized GaN islands, which encourages further GaN growth to initiate from the exposed island sidewalls while blocking the penetration of the remaining defects. X-ray rocking curve measurements show that our semipolar GaN films provide relatively low full width at half maximum values at 0.119° along both the [11–23] and [10–10] directions. Additionally, transmission electron microscopy analyses confirm that our semipolar GaN films provide a significantly reduced density of threading dislocations down to ∼6 × 108 cm−2, which is two orders of magnitude less than those of conventionally deposited films.

Formation mechanism and thermoelectric properties of CaMnO3 thin films synthesized by annealing of Ca0.5Mn0.5O films

A two-step synthesis approach was utilized to grow CaMnO3 on M-, R- and C-plane sapphire substrates. Radio-frequency reactive magnetron sputtering was used to grow rock-salt-structured (Ca, Mn)O followed by a 3-h annealing step at 800 °C in oxygen flow to form the distorted perovskite phase CaMnO3. The effect of temperature in the post-annealing step was investigated using x-ray diffraction. The phase transformation to CaMnO3 started at 450 °C and was completed at 550 °C. Films grown on R- and C-plane sapphire showed similar structure with a mixed orientation, whereas the film grown on M-plane sapphire was epitaxially grown with an out-of-plane orientation in the [202] direction. The thermoelectric characterization showed that the film grown on M-plane sapphire has about 3.5 times lower resistivity compared to the other films with a resistivity of 0.077 Ωcm at 500 °C. The difference in resistivity is a result from difference in crystal structure, single orientation for M-plane sapphire compared to mixed for R- and C-plane sapphire. The highest absolute Seebeck coefficient value is − 350 µV K−1 for all films and is decreasing with temperature.

Controlled growth and magnetic property of a-plane-oriented Mn3Sn thin films

Highly a-plane-oriented Mn3Sn thin films were grown on m-plane sapphire substrates with low-temperature grown Mn3+xSn buffer layers by sputtering deposition technique, and their crystallinity and magnetic properties were investigated by X-ray diffraction and SQUID magnetometer, respectively. The crystallographic orientations of Mn3Sn domains are found to be sensitively influenced by substrate temperature, thickness and composition ratio of Mn3+αSn buffer layer. The highly a-plane-oriented Mn3Sn film shows slightly different magnetization behavior from randomly oriented Mn3Sn film, while the saturation magnetization Ms and coercivity of the highly a-plane-oriented Mn3Sn film are corresponding to that of single crystal bulk.

Nonpolar m-plane [formula omitted]N layers grown on m-plane sapphire by MOVPE

Heteroepitaxial growth of single-phase nonpolar (101¯0) m-plane AlxGa1-xN layers on m-plane sapphire substrates was investigated by metalorganic vapour phase epitaxy. Different Al/Ga gas phase ratios were used to adjust the AlN mole fraction over the entire range of composition. All m-plane AlGaN layers show an orthorhombic distortion in the wurtzite unit cell due to anisotropic in-plane strain. This distortion decreases with increasing AlN mole fraction due to a decreased anisotropic biaxial strain. The AlN mole fraction of m-plane layers and c-plane co-loaded layers estimated by X-ray diffraction is comparable. This is consistent with their comparable energy bandgaps, which were estimated from room-temperature dielectric functions. The dependence of the energy bandgap on composition indicates a bowing parameter of about 0.9 eV. The crystal-field splitting energy is found for the m-plane layers with a bowing parameter bcf of about −0.15 eV.

Mn valence state mediated room temperature ferromagnetism in nonpolar Mn doped GaN

In this work, we demonstrate room temperature ferromagnetism in nonpolar, m-plane manganese doped gallium nitride (m-GaN:Mn) epitaxial thin films synthesized using plasma-assisted molecular beam epitaxy on nonpolar, m-plane sapphire (Al2O3) substrates. We observed that Mn doping concentration plays a key role in stabilizing the valence state of Mn ions thereby influencing the magnetic and transport properties. Chemical characterization by X-ray photoelectron spectroscopy, X-ray absorption near edge spectra, and energy dispersive X-ray spectroscopy affirm homogeneous valence state Mn+3 in lightly doped samples, while higher doping levels cause a heterogeneous Mn+0 and Mn+2 mixed valence states. Magnetic measurements indicate that films with low doping levels show the signature of room temperature ferromagnetism, while films with higher doping show antiferromagnetic/spin glass behavior besides the indication of MnGa alloys. Temperature dependent resistivity measurements show that both the samples are semiconducting with deep level defect states. Our findings suggest the possibility of incorporating spin functionality in non-polar optoelectronic devices.

Crucial influential factor on background electron concentration in semi-polar (11[formula omitted]2) plane AlGaN epi-layers

The semi-polar (112¯2) plane Al0.3Ga0.7N epi-layers with different background electron concentrations were deposited on (101¯0) m-plane sapphire substrates with metal-organic chemical vapor deposition technology, and characterized with Hall effect measurement, photoluminescence spectroscopy, high-resolution X-ray diffraction, and atomic force microscopy. It was found that there is a strong positive correlation between the background electron concentrations and the anisotropy in two-dimensional (2D)-growth rate of the triangular structures on the surface of semi-polar Al0.3Ga0.7N epi-layers. That is to say, the anisotropy in the 2D-growth rate is indeed the crucial influential factor on the background electron concentration. In fact, by carefully suppressing the anisotropy in 2D-growth rate, the background electron concentration in semi-polar Al0.3Ga0.7N epi-layers was reduced successfully from 1.38×1019 cm−3 to 4.20×1015 cm−3. This work should open a new way to realize the p-type semi-polar AlGaN epi-layers, which is of great significance for the manufacturing of semi-polar AlGaN-based DUV-LEDs.

Improved crystal quality of semipolar AlN by employing a thermal annealing technique with MOVPE

AlGaN light-emitting diodes (LEDs) have been studied for their use as highly efficient light sources in the deep ultraviolet (DUV) region. The spontaneous and piezoelectric polarization in c-plane AlGaN, produces a strong electric field, which degrades the performance of LEDs. To overcome this polarization problem, growth on less widely used substrates, such as those with semipolar/nonpolar orientations has been carried out. However, the expected improvements have not been forthcoming for AlGaN alloys in the DUV region because of the poor crystallinity obtained. Here, we report on the improved crystallinity of semipolar AlN grown on m-plane sapphire using a high-temperature annealing process. For (10-1-3) and (11-22) AlN layers grown on m-plane sapphire, the crystal quality improved with increasing annealing temperature up to 1700 °C. The crystallinity of (11-22) AlN layer was further improved by further growth of AlN. A best FWHM of 280arcsec was obtained for a 3-μm thick (11-22) AlN layer grown after high-temperature annealing.