•N-polar face-to-face annealed sputtered AlN was fabricated with Al sputtering target.•The controllability of the polarity by changing the sputtering target was demonstrated.•With increasing AlN film ...thickness, a reduction in the TDDs of N-polar FFA Sp-AlN was observed.
N-polar face-to-face annealed sputtered AlN (FFA Sp-AlN) was fabricated by sputtering with an Al metal target and high-temperature annealing in a face-to-face configuration. The polarities of the FFA Sp-AlN samples fabricated with different sputtering targets (i.e., Al metal or sintered AlN targets) were checked by KOH etching and cross-sectional scanning transmission electron microscope images. As a result, samples sputtered with a sintered AlN target and an Al metal target resulted in Al-polar and N-polar FFA Sp-AlN, respectively. Then, we fabricated FFA Sp-AlN with N-polar AlN on the top-most layers (N-polar FFA Sp-AlN) with different total film thicknesses. The threading dislocation densities (TDDs) of N-polar FFA Sp-AlN were estimated from the full width at half maximum values of the X-ray rocking curves. Consequently, the TDD of N-polar FFA Sp-AlN decreased with increasing AlN film thickness, which was the same trend as that of Al-polar FFA Sp-AlN. The minimum TDD of 1.7 × 108 cm−2 was obtained from N-polar FFA Sp-AlN with a total thickness of 730 nm.
Combination of sputter deposition and high-temperature annealing is a promising technique for preparing AlN templates with a low threading dislocation density (TDD) at a lower film thickness compared ...to those prepared by the conventional metalorganic vapor phase epitaxy. However, cracking of AlN films during annealing is a critical issue. In this study, we controlled the residual stress of the sputter-deposited AlN films by modifying the sputtering conditions. Consequently, the occurrence of cracking was effectively suppressed. By optimizing the fabricating conditions, a TDD of 2.07 × 108 cm−2 was achieved for the AlN template with a thickness of 480 nm.
•Strain-relaxation of annealed sputtered AlN film on c-plane sapphire was revealed.•Sputtered AlN films before annealing have different strains and tilt mosaics.•The AlN film after annealing consists ...of a layer that have a compressive strain.•After MOVPE growth, the compressive strain was inherited.
The strain relaxation of sputter-deposited AlN (sp-AlN) films on c-plane sapphire substrates before and after face-to-face annealing (FFA) was evaluated. After FFA, the AlN films consisted of a layer with compressive strain and an extremely low dislocation density, which was ascribed to solid-state growth during FFA. In addition, the crystallinity was further improved after the homoepitaxial growth of AlN by metal-organic vapor phase expitaxy (MOVPE). The full widths at half maximum of the X-ray rocking curves of AlN (0 0 0 2) and (1 0 −1 2) for the MOVPE-grown AlN layer on the FFA-sp-AlN film were 15 and 240 arcsec, respectively. The surface morphology of the MOVPE-grown AlN layer on the FFA-sp-AlN film was covered with an atomically flat step-and-terrace structure. The compressive strain of the MOVPE-grown AlN layer was inherited from the underlying FFA-sp-AlN film.
This study comprehensively investigates the properties of metalorganic vapor phase epitaxy (MOVPE)‐grown AlN films on high‐quality face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) templates on ...sapphire substrates, which are highly important to control the surface morphology for various applications, such as UV light‐emitting diodes and laser diodes. The conditions of thermal cleaning and AlN growth by MOVPE are investigated to remove numerous small islands on as‐annealed FFA Sp‐AlN. Subsequent to thermal cleaning in H2 + NH3 at 1300 °C, MOVPE growth is performed with varying NH3 flow rate and growth temperature (Tg) under a constant pressure and group‐III flow rate. An atomically flat surface with an atomic step‐and‐terrace structure is obtained at a growth rate of ≈1.0 μm h−1 and a Tg of 1300 °C. Transmission electron microscopy images and secondary‐ion mass spectrometry reveal low dislocation densities and impurity concentrations. Finally, the effects of compressive strain in FFA Sp‐AlN on the lattice constant and curvature of the MOVPE‐grown AlN film on FFA Sp‐AlN are investigated. The compressive strain of the AlN film, which is carried over from FFA Sp‐AlN, can prevent crack formation but leads to a large wafer curvature after cooling down from the Tg.
Properties of a face‐to‐face annealed sputtered AlN (FFA Sp‐AlN) template on a sapphire substrate and metalorganic vapor phase epitaxy (MOVPE)‐grown AlN films on FFA Sp‐AlN templates are comprehensively investigated. As a result, the AlN films on FFA Sp‐AlN templates exhibit superior properties than those on the conventional AlN templates directly MOVPE‐grown on sapphire substrates.
•Curvature-controllable and crack-free AlN template realized.•Double-sided sputtering and high-temperature annealing employed to achieve this.•Twist component of X-ray rocking curve markedly improved ...after annealing.•Experimental and calculated curvature and in-plane stress agreed well.
Post-growth high-temperature annealing of sputtered AlN films is a promising approach to realize high-quality AlN templates for deep-ultraviolet light-emitting diode applications. This paper proposes the double-sided sputtering of AlN on double-sided polished sapphire to suppress the wafer bowing caused by high-temperature annealing. Similar to the case of single-sided sputtering, the twist component of the X-ray rocking curve was markedly improved after annealing, yielding rocking curve widths of 256–321 arcsec in (10–12). By varying the backside thickness, the curvature was controlled from −27 km−1 to 29 km−1, while a crack-free surface was maintained on the front side of AlN. The experimentally obtained curvature and in-plane stress agreed well with the calculation based on thermal stress analysis in the multilayer system.
The impact of a face-to-face annealed sputtered AlN/sapphire (FFA Sp-AlN) template with threading-dislocation densities (TDDs) of 2 × 108 cm−2 and an n-type AlGaN (n-AlGaN) underlayer on optical ...properties of AlGaN multiple quantum wells (MQWs) with an ultraviolet C (UVC) emission is investigated comprehensively. For comparison of the FFA Sp-AlN template with low TDDs, a conventional MOVPE (metalorganic vapor phase epitaxially)-grown AlN/sapphire (MOVPE-AlN) template with TDDs of 1 × 109 cm−2 was prepared. Consequently, cathodoluminescence (CL), temperature-dependent photoluminescence (PL), and time-resolved PL (TR-PL) measurements verified that both the FFA Sp-AlN template and n-AlGaN underlayer are indispensable for obtaining MQWs with high internal quantum efficiencies, which decrease the TDDs and point defect (PD) densities. Our results revealed that 10-period quantum wells (10QWs)/n-AlGaN/AlN grown on the FFA Sp-AlN template exhibit a lower dark spot density in CL panchromatic intensity maps, a higher integrated emission intensity ratio from the temperature-dependent PL (from 15 to 300 K), and a longer nonradiative lifetime from the TR-PL measurements at 300 K compared with those grown on the MOVPE-AlN template. Moreover, we found that the optical properties of 10QWs/AlN in FFA Sp-AlN and MOVPE-AlN templates do not exhibit a significant difference because of the existence of numerous PDs. Our experimental results demonstrate the favorable impact of the FFA Sp-AlN template for low-TDDs and the n-AlGaN underlayer for low-PDs, which holds promise for highly efficient AlGaN deep-ultraviolet light-emitting devices.
•AlN films were fabricated on SiC substrates by sputtering deposition.•High temperature annealing improved crystalline quality of the AlN films.•Strain relaxation in AlN films occurred during high ...temperature annealing.•Homoepitaxially grown AlN films on the template exhibited an atomically flat surface.
High-quality AlN templates fabricated by sputtering-deposition and post-deposition high-temperature annealing have great potential for deep ultraviolet light-emitting device applications. In this work, we fabricated AlN films on 6H-SiC substrates by sputtering and face-to-face annealing and characterized the structural quality of the AlN films before and after annealing. As reported in previous studies, to accomplish high-quality AlN films on SiC substrates using conventional methods, such as molecular beam epitaxy or metalorganic vapor phase epitaxy (MOVPE), it is important to grow the AlN on the SiC coherently. However, in this work, although the annealed AlN films were fully relaxed from the SiC substrates, or even had tensile strain, the AlN films indicated high crystallinity. The X-ray rocking curve full width at half maximum (XRC-FWHM) values of the 200-nm-thick annealed AlN film were 17 and 246 arcsec for the AlN (0002) and (10–12) diffraction, respectively. Though the annealed AlN film indicated rough surfaces with bunched step structures, the surface morphology was remarkably improved by MOVPE growth and clear atomic step-and-terrace structures were formed. The XRC-FWHM values of the MOVPE-grown AlN were 90 and 239 arcsec for the AlN (0002) and (10–12) diffraction, respectively.
A novel fabrication process of hexagonal aluminum nitride (AlN)/(001) diamond by means of sputtering and high‐temperature annealing is proposed. Crystallinity of the AlN is significantly improved by ...the annealing, leading to full width at half maximum (FWHM) of the AlN(0002) diffraction X‐ray rocking curve (XRC) of 1490 arcsec for the AlN thickness of 400 nm. The in‐plane epitaxial relationship in AlN/diamond structure is stabilized in the single domain of (0001) 11–20 AlN || (001) 110 diamond on single‐crystal diamond substrates with off‐cut angle along 110. Transmission electron microscopy and nano‐beam electron diffraction indicate the existence of an epitaxial graphite layer exhibiting an epitaxial relationship of (0001) 11–20 AlN || (1–100) 0001 graphite || (001) 110 diamond. The demonstration of single‐domain hexagonal AlN/(001) diamond structure provides a cost‐effective, mass production approach for realizing AlxGa1−xN/diamond devices for electronic and optoelectronic applications.
A novel fabrication process of hexagonal aluminum nitride (AlN)/(001) diamond by means of sputtering and high‐temperature annealing is proposed. The in‐plane epitaxial relationship in AlN/diamond structure is stabilized in the single domain of (0001) 11–20 AlN || (001) 110 diamond on single‐crystal diamond substrates with off‐cut angle along 110.
High-temperature annealing of sputtered AlN (Sp-AlN) using a face-to-face configuration is a novel technique that has attracted considerable attention because it can reduce the threading dislocation ...density of Sp-AlN to 107 cm−2. However, drawbacks such as cracking, residual stress, and wafer curvature remain because of a high annealing temperature of 1700 °C. We previously developed a thermal strain analysis model that uses an elastic multilayer system to describe the elastic behavior of Sp-AlN on sapphire under high-temperature annealing. In this study, we expand this model to consider in-plane anisotropy. By performing thermal strain analysis of the curvature, strain, stress, and strain energy of c-plane AlN grown on c- and a-plane sapphire, our calculation successfully approximates the experimental results, even for an in-plane anisotropic structure. The proposed model is, therefore, useful for quantitative evaluation of the residual strain and can contribute to strain engineering of AlGaN-based deep-ultraviolet light-emitting diodes.
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