Wing Tilt Research Articles

Influences of mask width, fill factor, HCl addition and C doping on wing tilts in the epitaxial laterally overgrown GaN films by hydride vapor phase epitaxy

Crystal tilts in the epitaxial lateral overgrown (ELO) GaN region over SiO2 mask by hydride vapor phase epitaxial away from the opening region were investigated by scanning electron microscope and the four-circle x-ray diffraction method. The increased mask width and ratio of stripe opening width to stripe period (fill factor) lead to increased wing tilts. The introduction of additional HCl and C3H8/H2 can decrease wing tilts and improve the ELO GaN surface morphologies significantly. Moreover, it has been observed that a high growth rate will bring more defects and large wing tilts into the GaN materials.

Determination of crystal misorientation in epitaxial lateral overgrowth of GaN

Epitaxial lateral overgrowth of GaN on Al 2O 3 using a SiO 2 mask with different fill factors (ratio of stripe opening width to stripe period) is examined with synchrotron X-ray topography (SXRT) and X-ray diffraction (XRD) techniques. The crystal misorientation in the lateral overgrown region (wing) and the normal region (window region and beneath the seed layer) is determined with SXRT. The wings tilt asymmetrically around the window and the tilts increase as the fill factor increases. XRD measurements confirm the same wing tilt tendency as the fill factor changes. The average wing tilt reaches approximately 1600 arcsec measured using the X-ray rocking curve method at a fill factor of 0.625, but the maximum wing tilts can reach values as large as 2400 arcsec measured by SXRT when the fill factor is only 0.571. The significance of this is explained. The crystal misorientation in the normal region is approximately an order of magnitude less than the wing tilt.

Maskless pendeo-epitaxial growth of GaN films

High-resolution x-ray diffraction (XRD) and atomic force microscopy (AFM) of pendeo-epitaxial (PE) GaN films confirmed transmission electron microscopy (TEM) results regarding the reduction in dislocations in the wings. Wing tilt ≤0.15° was due to tensile stresses in the stripes induced by thermal expansion mismatch between the GaN and the SiC substrate. A strong D°X peak at ≈3.466 eV (full-width half-maximum (FWHM) ≤ 300 µeV) was measured in the wing material. Films grown at 1020°C exhibited similar vertical [0001] and lateral [1120] growth rates. Increasing the temperature increased the latter due to the higher thermal stability of the GaN(1120). The (1130) surface was atomically smooth under all growth conditions with a root mean square (RMS) = 0.17 nm.

Strain and crystallographic tilt in uncoalesced GaN layers grown by maskless pendeoepitaxy

The strain in thin GaN layers grown by maskless pendeoepitaxy has been investigated using high-resolution x-ray diffraction and finite-element simulations. The crystallographic tilt of the free-hanging wings was determined to result from the strain relaxation of the seed stripes along [0001]. The impact of the dimensions of the pendeostructure and of the formation of crystal defects on the expected wing tilt is discussed.

Observation of crystallographic wing tilt in cantilever epitaxy of GaN on silicon carbide and silicon (111) substrates

We have characterized the growth of GaN on both SiC and Si (111) using the cantilever epitaxy technique. Cantilever epitaxy employs growth from periodic, parallel mesas which are formed by etching the substrate [C.I.H. Ashby et al. Appl. Phys. Lett. 77, 3233 (2000)]. GaN/AlN/Si and GaN/AlN/SiC layers were grown via metalorganic chemical vapor deposition on substrates with reactive ion etched trenches. This single step approach allows GaN to laterally overgrow the substrate trenches yielding low defect density material over the trench, while avoiding use of a mask which is commonly employed in lateral epitaxial overgrowth. Scanning electron microscopy, atomic force microscopy, and x-ray diffraction were used to characterize the resulting material. Significant threading dislocation reduction was observed in the “wing” regions (the material directly over the trenches). Contrary to previous reports, crystallographic wing tilt ranging from 0.22° to 0.77° was also observed, despite the absence of a wing/mask interface using this technique.

Channeling contrast microscopy on lateral epitaxial overgrown GaN

High resolution (<1 μm) channeling contrast microscopy (CCM) is employed to map variations of crystallographic orientation across micron-sized regions of lateral epitaxial overgrown (LEO) GaN thin film structures. The sample consists of 6 μm thick GaN stripes, aligned along the [1 1 ̄ 0 0] GaN direction, grown by LEO from 3 μm wide Si 3N 4 windows spaced 13 μm apart. Axial CCM using 2 MeV He + is employed to investigate the crystalline structure of the LEO GaN layer. A low χ min of ∼2.8% and a critical angle ψ 1/2 of ∼0.85° is found, comparable with data obtained from broad beam channeling [J. Portmann, C. Huag, R. Benn, T. Frey, B. Schottker, D.J. As, Nucl. Instr. and Meth. B 155 (1999) 489]. 5 μm wide bands of high and low yield at a periodicity of 13 μm are observed in the CCM maps. This contrast is due to the opposing tilts of the [0 0 0 1] axis in the overgrown regions (the so-called wings) on either side of the window regions. From angular scan curves this tilt is found to be ±0.45° in the direction perpendicular to the GaN stripes and no measurable wing tilt is found along the stripe direction.

White Beam Synchrotron X-ray Topography and X-ray Diffraction Measurements of Epitaxial Lateral Overgrowth of GaN

AbstractThe Epitaxial Lateral Overgrowth (ELO) of GaN on Al2O3 using a SiO2 mask with different fill factors (ratio of stripe opening width to stripe period) is examined with White Beam Synchrotron X-ray Topography (WBSXT) and X-ray rocking curve analysis. The sapphire substrate was identified with a dislocation density of the order of ~106cm-2. WBSXT in both transmission and back reflection mode is used to image the ELO GaN and confirms that crystal planes in the lateral overgrown part (wing) are tilted, and that the wing tilt increases as the fill factor increases. X-ray rocking curve and WBSXT measurements confirm the same wing tilt tendency as the fill factor changes. The WBSXT method provides a measure of the maximum wing tilt, while the X-ray rocking curve method gives the average wing tilt. The average wing tilt reaches about 1602 arcsec at a fill factor of 0.625, but the maximum wing tilts can reach values as large as 2372 arcsec when the fill factor is only 0.571. This study shows that WBSXT is an effective method in dislocation and wing tilt determination for the GaN on Al2O3 ELO epilayer system or indeed for similar systems. The tilted wings induce a slightly lower compressive stress in the coalesced region of the GaN epilayer.

Distortions on Length Perception in a Combination of Illusionary Figures

In psychophysiological experiments, subjects were asked to compare the lengths of the test and control parts of stimuli constructed of two variants of superimposed illusionary figures: (1) two Muller–Lyer figures of the same body length but different in contrast, wing lengths, or tilt angles or (2) Muller–Lyer and Oppel–Kundt figures. Experiments with superimposed Muller–Lyer figures showed that the strength of illusion is determined by differences in the wing lengths and tilts, as well as the difference in the absolute contrast levels of the figures. Similar dependences were also found in experiments with stimuli formed by superimposing the Muller–Lyer and Oppel–Kundt figures. The data showed that simultaneous presentation of superimposed figures causes an illusion, which cannot be decomposed into parts, and the results of these experiments cannot be explained by summation of the effects of the figures. It is suggested that illusions caused by figures with different spatial organizations, such as Muller–Lyer and Oppel–Kundt figures, have common mechanisms. Calculations based on our model are fully consistent with experimental data and support the hypothesis based on the principles of spatial–frequency filtration of the image at the level of the primary visual cortex.

High-quality GaN films obtained by air-bridged lateral epitaxial growth

High-quality GaN films with low dislocation density and low wing tilt of c-axis orientation have been successfully obtained by a promising technique of selected area growth, namely air-bridged lateral epitaxial growth (ABLEG). A GaN film was grown from the exposed (0 0 0 1) top facet of the ridged GaN seed structures, whose side walls and etched bottoms were covered with silicon nitride mask, using low-pressure metalorganic vapor-phase epitaxy. The ridge-stripe structures of the GaN seed were constructed in the 1 1 ̄ 00 GaN direction. At the optimum growth temperature of 950°C, only the 1 1 2 ̄ 0 and {0 0 0 1} facets were obtained. Continuing the growth led to fabricating the air-bridged structure, where the coalescence of the wing region occurred. From the transmission electron microscopy study, it was found that most of the vertical dislocations along the c-axis were confined to the seed region, while the horizontal dislocations were newly generated in the vicinity of coalescence boundary. The densities of the vertical dislocations were about 9×10 8 cm −2 in the seed region, while below 1×10 6 cm −2 in other regions. The densities of the horizontal dislocations were about 1×10 6 cm −2 in the wing region and 4×10 7 cm −2 in the vicinity of the coalescence boundary, respectively. The X-ray diffraction (XRD) measurements revealed that the tilt angle of c-axis relative to underlying seed GaN was about 297 arcsec (0.083°), and the full-width at half-maximum of the XRD curve for the wing region was 138 arcsec, indicating that the wing region has high uniformity of c-axis orientation. Both of the wing and the coalescence boundary region exhibited atomically smooth surfaces with stepped terraces, whose root mean square roughness was found to be 0.089 nm by atomic force microscopy measurements.

In situ, real-time measurement of wing tilt during lateral epitaxial overgrowth of GaN

By performing in situ, real-time x-ray diffraction measurements in the metalorganic chemical-vapor deposition environment, we have directly observed the emergence and evolution of wing tilt that occurs during the lateral overgrowth of GaN from stripes patterned in a SiO2 mask. This was done by repeatedly performing line scans through the 101̄3 peak in the direction perpendicular to the [101̄0]GaN stripe direction. The wing tilt developed as soon as the wings started forming, and increased slightly thereafter to reach a value of ∼1.19° after 3600 s of growth. Upon cooldown to room temperature, the tilt increased to ∼1.36°, indicating that thermally induced stresses during cooldown have only a small effect on wing tilt. However, changes in mask density, composition, and stress state during early lateral overgrowth must be considered as possible origins of wing tilt.

Air-bridged lateral epitaxial overgrowth of GaN thin films

A promising technique of selective lateral epitaxy, namely air-bridged lateral epitaxial overgrowth, is demonstrated in order to reduce the wing tilt as well as the threading dislocation density in GaN thin films. A seed GaN layer was etched to make ridge-stripe along 〈11̄00〉GaN direction and a GaN material was regrown from the exposed (0001) top facet of the ridged GaN seed structures, whose sidewalls and etched bottoms were covered with silicon nitride mask, using low-pressure metalorganic vapor phase epitaxy. The density of dislocations in the wing region was reduced to be &amp;lt;107 cm−2, which was at least two orders of magnitude lower than that of underlying GaN. The magnitude of the wing tilt was determined to be 0.08° by x-ray diffraction (XRD) measurements, which was smaller than other lateral epitaxial overgrown GaN thin films. The full width at half maximum of XRD for the wing region was 138 arc sec, indicating high uniformity of c-axis orientation.

Determination of tilt in the lateral epitaxial overgrowth of GaN using X-ray diffraction

Tilt that occurs during the lateral overgrowth of GaN over an SiO 2 mask from 〈1 1̄ 0 0〉 GaN-oriented stripe openings is systematically measured using X-ray diffraction. This tilt is measured for a range of `fill factors’ (ratio of stripe opening width to stripe period) and for changes in growth temperature and input V/III ratio. It is found that changes in these parameters have a significant effect on the extent and distribution of tilt in the overgrown regions relative to GaN directly above the underlying `seed’ layer. Correlations between tilts and stripe morphologies as observed in scanning electron microscopy are made, and an explanation based on the `local’ V/III ratio around the growing stripes is proposed to account for these results. The aspect ratio of the stripes in cross section (which is affected by fill factor and growth conditions) is found to be directly related to the degree of wing tilt, suggesting that tilt may be affected by GaN wing/SiO 2 interfacial forces.

Dislocation Arrangement in a Thick LEO GaN Film on Sapphire

Diffraction-contrast TEM, focused probe electron diffraction, and high-resolution X-ray diffraction were used to characterize the dislocation arrangements in a 16µm thick coalesced GaN film grown by MOVPE LEO. As is commonly observed, the threading dislocations that are duplicated from the template above the window bend toward (0001). At the coalescence plane they bend back to lie along [0001] and thread to the surface. In addition, three other sets of dislocations were observed. The first set consists of a wall of parallel dislocations lying in the coalescence plane and nearly parallel to the substrate, with Burgers vector (b) in the (0001) plane. The second set is comprised of rectangular loops with b = 1/3 [110] (perpendicular to the coalescence boundary) which originate in the coalescence boundary and extend laterally into the film on the (100). The third set of dislocations threads laterally through the film along the [100] bar axis with 1/3&lt;110&gt;-type Burgers vectors These sets result in a dislocation density of ∼109 cm−2. High resolution X-ray reciprocal space maps indicate wing tilt of ∼0.5º.

Dislocation Arrangement in a Thick LEO GaN Film on Sapphire

AbstractDiffraction-contrast TEM, focused probe electron diffraction, and high-resolution X-ray diffraction were used to characterize the dislocation arrangements in a 16[.proportional]m thick coalesced GaN film grown by MOVPE LEO. As is commonly observed, the threading dislocations that are duplicated from the template above the window bend toward (0001). At the coalescence plane they bend back to lie along [0001] and thread to the surface. In addition, three other sets of dislocations were observed. The first set consists of a wall of parallel dislocations lying in the coalescence plane and nearly parallel to the substrate, with Burgers vector (b) in the (0001) plane. The second set is comprised of rectangular loops with b = 1/3 [11 20] (perpendicular to the coalescence boundary) which originate in the coalescence boundary and extend laterally into the film on the (1 100). The third set of dislocations threads laterally through the film along the [1 100] bar axis with 1/3&lt;11 20&gt;-type Burgers vectors These sets result in a dislocation density of ∼109 cm−2. High resolution X-ray reciprocal space maps indicate wing tilt of ∼0.5°.

Aerodynamic Interactions Between a Rotor and Wing in Hover

An experimental investigation of rotor/wing aerodynamic interactions in hover is described. The investigation consisted of both a large-scale and a small-scale test. A 0.658-scale V-22 rotor and wing was used in the large-scale test. Wing download, wing surface pressure, rotor performance, and rotor downwash data from the large-scale test are presented. A small-scale experiment was conducted to determine how changes in the rotor/wing geometry affected the aerodynamic interactions. These geometry variations included the distance between the rotor and wing, wing incidence angle, wing flap angle, rotor rotation direction, and configurations both with the rotor axis at the tip of the wing (tilt rotor configuration) and with the rotor axis at the center of the wing (compound helicopter configuration).

Review of V/STOL Aircraft with Tilt-Propellers and Tilt-Rotors

SummaryTests of the first experimental VTOL aircraft using tiltable wings and propellers indicated that practical operational aircraft are possible and can operate with a high degree of effectiveness.The flight tests accomplished up to the present time with the second generation tilt-propeller and tilt-rotor aircraft are discussed and analysed. The XC-142 and the CL-84 tilt-wing aircraft have been performing well throughout their projected flight envelope and good pilot ratings have been obtained with each in hover, transition, and the normal cruise mode. The X-19 tandem wing tilt propeller was flown only up to 100 knots and was lost due to a structural failure in one nacelle. Although the operation of the aircraft had been satisfactory up to this point, this programme was terminated due to shortage of funds.A study of the various VTOL aircraft configurations operating at low radius of action indicates that the tilt wing/ tilt propeller type are the most cost-effective as long as speed is an important parameter in the operating mission. There are missions where speed is of secondary importance, and therefore, the helicopter will continue to be one of the most effective VTOL for these missions.Since the tilt-wing /tilt-rotor aircraft tend to be only slightly higher in cost than the helicopter and with development the complexity and maintainability will be reduced to acceptable levels, it is anticipated that further work will be done in the development of these vehicles, and successful and useful machines will be built in the future.