High-quality Aluminum Nitride Films Research Articles

HiPIMS-grown AlN buffer for threading dislocation reduction in DC-magnetron sputtered GaN epifilm on sapphire substrate

Gallium nitride (GaN) epitaxial films on sapphire (Al2O3) substrates have been grown using reactive magnetron sputter epitaxy with a liquid Ga target. Threading dislocations density (TDD) of sputtered GaN films was reduced by using an inserted high-quality aluminum nitride (AlN) buffer layer grown by reactive high power impulse magnetron sputtering (R-HiPIMS) in a gas mixture of Ar and N2. After optimizing the Ar/N2 pressure ratio and deposition power, a high-quality AlN film exhibiting a narrow full-width at half-maximum (FWHM) value of the double-crystal x-ray rocking curve (DCXRC) of the AlN(0002) peak of 0.086° was obtained by R-HiPIMS. The mechanism giving rise the observed quality improvement is attributed to the enhancement of kinetic energy of the adatoms in the deposition process when operated in a transition mode. With the inserted HiPIMS-AlN as a buffer layer for direct current magnetron sputtering (DCMS) GaN growth, the FWHM values of GaN(0002) and (10 1‾ 1) XRC decrease from 0.321° to 0.087° and from 0.596° to 0.562°, compared to the direct growth of GaN on sapphire, respectively. An order of magnitude reduction from 2.7 × 109 cm−2 to 2.0 × 108 cm−2 of screw-type TDD calculated from the FWHM of the XRC data using the inserted HiPIMS-AlN buffer layer demonstrates the improvement of crystal quality of GaN. The result of TDD reduction using the HiPIMS-AlN buffer was also verified by weak beam dark-field (WBDF) cross-sectional transmission electron microscopy (TEM).

Quasi-van der Waals Epitaxy of a Stress-Released AlN Film on Thermally Annealed Hexagonal BN for Deep Ultraviolet Light-Emitting Diodes.

The heteroepitaxy of high-quality aluminum nitride (AlN) with low stress is essential for the development of energy-efficient deep ultraviolet light-emitting diodes (DUV-LEDs). In this work, we realize that quasi-van der Waals epitaxy growth of a stress-released AlN film with low dislocation density on hexagonal boron nitride (h-BN)/sapphire suffered from high-temperature annealing (HTA) treatment and demonstrate its application in a DUV-LED. It is revealed that HTA effectively improves the crystalline quality and surface morphology of monolayer h-BN. Guided by first-principles calculations, we demonstrate that h-BN can enhance lateral migration of Al atoms due to the ability to lower the surface migration barrier (less than 0.14 eV), resulting in the rapid coalescence of the AlN film. The HTA h-BN is also proved to be efficient in reducing the dislocation density and releasing the large strain in the AlN epilayer. Based on the low-stress and high-quality AlN film on HTA h-BN, the as-fabricated 290 nm DUV-LED exhibits 80% luminescence enhancement compared to that without h-BN, as well as good reliability with a negligible wavelength shift under high current. These findings broaden the applications of h-BN in favor of III-nitride and provide an opportunity for further developing DUV optoelectronic devices on large mismatched heterogeneous substrates.

Impact of Pulse Parameters of a DC Power Generator on the Microstructural and Mechanical Properties of Sputtered AlN Film with In-Situ OES Data Analysis.

In the present study, the sputtered aluminum nitride (AlN) films were processed in a reactive pulsed DC magnetron system. We applied a total of 15 different design of experiments (DOEs) on DC pulsed parameters (reverse voltage, pulse frequency, and duty cycle) with Box-Behnken experimental method and response surface method (RSM) to establish a mathematical model by experimental data for interpreting the relationship between independent and response variables. For the characterization of AlN films on the crystal quality, microstructure, thickness, and surface roughness, X-ray diffraction (XRD), atomic force microscopy (AFM), and field emission-scanning electron microscopy (FE-SEM) were utilized. AlN films have different microstructures and surface roughness under different pulse parameters. In addition, in-situ optical emission spectroscopy (OES) was employed to monitor the plasma in real-time, and its data were analyzed by principal component analysis (PCA) for dimensionality reduction and data preprocessing. Through the CatBoost modeling and analysis, we predicted results from XRD in full width at half maximum (FWHM) and SEM in grain size. This investigation identified the optimal pulse parameters for producing high-quality AlN films as a reverse voltage of 50 V, a pulse frequency of 250 kHz, and a duty cycle of 80.6061%. Additionally, a predictive CatBoost model for obtaining film FWHM and grain size was successfully trained.

Optimization of growth parameters of AlN thin films and investigation of electrical and electroluminescence characteristics from Au/i-AlN/n-GaN UV light-emitting diode

Aluminum nitride (AlN) film has a wide range of applications optoelectronic devices. In this paper, AlN thin films were prepared by magnetron sputtering method, the influence of substrate temperature and nitrogen-argon ratio on the structure, morphology and optical properties of AlN films were evaluated by X-ray diffractometer, scanning electron microscope and ultraviolet–visible spectrophotometer. The analysis showed that the AlN film exhibited better crystallization quality under the substrate temperature of 350 °C with a nitrogen-argon ratio of 5:40. In addition, an Au/i-AlN/n-GaN metal–insulator-semiconductor (MIS) structure device was fabricated based on high-quality AlN films. The current–voltage and electroluminescence characteristics of the device under different temperatures were studied in detail. The results showed that the device exhibited excellent rectification behavior at all operating temperatures, producing an intense UV emission (∼372 nm). This paper proposes a hole generation model and a carrier transport and recombination mechanism. The research results provide a new reference for the practical application of AlN films.

Growth of self-assembled nanovoids embedded AlN layer on a low- temperature buffer by metal organic chemical vapor deposition

In this study, we demonstrate a high-quality aluminum nitride (AlN) film with a thickness of 2.5 μm grown on 4H-SiC substrate using a three-dimensional (3D) buffer via high-temperature metal-organic chemical vapor deposition. For the buffer layer, 3D structure morphology without polycrystals was selected by the temperature change from 850 to 1350°C. The main layer was grown on the 3D buffer, and self-assembled nanovoids were formed, enabling the growth of a high-quality AlN film without cracks. However, many cracks occurred in the sample without a buffer. The full width at half maximum of the X-ray rocking curve was 144/368 arcsec for (002)/(102) planes, respectively. Cross-sectional transmission electron microscopy showed nanovoids and a rapid defect reduction effect at the interface of the buffer layer.

Flexible thin-film acoustic wave devices with off-axis bending characteristics for multisensing applications

Flexible surface acoustic wave (SAW) devices have recently attracted tremendous attention for their widespread application in sensing and microfluidics. However, for these applications, SAW devices often need to be bent into off-axis deformations between the acoustic wave propagation direction and bending direction. Currently, there are few studies on this topic, and the bending mechanisms during off-axis bending deformations have remained unexplored for multisensing applications. Herein, we fabricated aluminum nitride (AlN) flexible SAW devices by using high-quality AlN films deposited on flexible glass substrates and systematically investigated their complex deformation behaviors. A theoretical model was first developed using coupling wave equations and the boundary condition method to analyze the characteristics of the device with bending and off-axis deformation under elastic strains. The relationships between the frequency shifts of the SAW device and the bending strain and off-axis angle were obtained, and the results were identical to those from the theoretical calculations. Finally, we performed proof-of-concept demonstrations of its multisensing potential by monitoring human wrist movements at various off-axis angles and detecting UV light intensities on a curved surface, thus paving the way for the application of versatile flexible electronics.

Experimental Optical Properties of Single-Photon Emitters in Aluminum Nitride Films

Single-photon emitters (SPEs) are one of the building blocks in quantum information processing. Here, we report detailed experimental optical properties of the SPEs in aluminum nitride (AlN) films at 10 K. The high-quality AlN films are grown by metal–organic chemical vapor deposition on graphene/sapphire substrates, which can conquer the large lattice and thermal mismatches between the sapphire and AlN. We report the defects in AlN with a relatively high Debye–Waller factor up to ∼29% and near-perfect linear polarization SPEs with a saturation count rate of 1.43 × 10⁶ counts/s. The power-dependent second-order autocorrelation measurements are used to study the transition kinetics, which can be described using a three-level model. The polarization measurements of absorption and emission reveal the optical cycle mechanism, where a particular zero-phonon line may be excited via multiple mechanisms. This work provides some insight into the nature of the optical properties and energy-level structures of AlN defects, which pave the way to integrated on-chip quantum photonics.

Low-Defect-Density Aluminum Nitride (AlN) Thin Films Realized by Zigzag Macrostep-Induced Dislocation Redirection

Owing to the lack of large-diameter and affordable AlN bulk crystals, high-quality AlN films grown on sapphire substrates have been intensively analyzed for the potential applications in optoelectr...

Quasi-2D Growth of Aluminum Nitride Film on Graphene for Boosting Deep Ultraviolet Light-Emitting Diodes.

Efficient and low‐cost production of high‐quality aluminum nitride (AlN) films during heteroepitaxy is the key for the development of deep ultraviolet light‐emitting diodes (DUV‐LEDs). Here, the quasi‐2D growth of high‐quality AlN film with low strain and low dislocation density on graphene (Gr) is presented and a high‐performance 272 nm DUV‐LED is demonstrated. Guided by first‐principles calculations, it is found that AlN grown on Gr prefers lateral growth both energetically and kinetically, thereby resulting in a Gr‐driven quasi‐2D growth mode. The strong lateral growth mode enables most of dislocations to annihilate each other at the AlN/Gr interface, and therefore the AlN epilayer can quickly coalesce and flatten the nanopatterned sapphire substrate. Based on the high quality and low strain of AlN film grown on Gr, the as‐fabricated 272 nm DUV‐LED shows a 22% enhancement of output power than that with low‐temperature AlN buffer, following a negligible wavelength shift under high current. This facile strategy opens a pathway to drastically improve the performance of DUV‐LEDs.

Bulk-like Intrinsic Phonon Thermal Conductivity of Micrometer-Thick AlN Films.

Aluminum nitride (AlN) has garnered much attention due to its intrinsically high thermal conductivity. However, engineering thin films of AlN with these high thermal conductivities can be challenging due to vacancies and defects that can form during the synthesis. In this work, we report on the cross-plane thermal conductivity of ultra-high-purity single-crystal AlN films with different thicknesses (∼3-22 μm) via time-domain thermoreflectance (TDTR) and steady-state thermoreflectance (SSTR) from 80 to 500 K. At room temperature, we report a thermal conductivity of ∼320 ± 42 W m-1 K-1, surpassing the values of prior measurements on AlN thin films and one of the highest cross-plane thermal conductivities of any material for films with equivalent thicknesses, surpassed only by diamond. By conducting first-principles calculations, we show that the thermal conductivity measurements on our thin films in the 250-500 K temperature range agree well with the predicted values for the bulk thermal conductivity of pure single-crystal AlN. Thus, our results demonstrate the viability of high-quality AlN films as promising candidates for the high-thermal-conductivity layers in high-power microelectronic devices. Our results also provide insight into the intrinsic thermal conductivity of thin films and the nature of phonon-boundary scattering in single-crystal epitaxially grown AlN thin films. The measured thermal conductivities in high-quality AlN thin films are found to be constant and similar to bulk AlN, regardless of the thermal penetration depth, film thickness, or laser spot size, even when these characteristic length scales are less than the mean free paths of a considerable portion of thermal phonons. Collectively, our data suggest that the intrinsic thermal conductivity of thin films with thicknesses less than the thermal phonon mean free paths is the same as bulk so long as the thermal conductivity of the film is sampled independent of the film/substrate interface.

Heteroepitaxial Growth of High-Quality and Crack-Free AlN Film on Sapphire Substrate with Nanometer-Scale-Thick AlN Nucleation Layer for AlGaN-Based Deep Ultraviolet Light-Emitting Diodes.

High-quality and crack-free aluminum nitride (AlN) film on sapphire substrate is the foundation for high-efficiency aluminum gallium nitride (AlGaN)-based deep ultraviolet light-emitting diodes (DUV LEDs). We reported the growth of high-quality and crack-free AlN film on sapphire substrate with a nanometer-scale-thick AlN nucleation layer (NL). Three kinds of nanometer-scale-thick AlN NLs, including in situ low-temperature AlN (LT-AlN) NL, oxygen-undoped ex situ sputtered AlN NL, and oxygen-doped ex situ sputtered AlN NL, were prepared for epitaxial growth of AlN films on sapphire substrates. The influence of nanoscale AlN NL thickness on the optical transmittance, strain state, surface morphology, and threading dislocation (TD) density of the grown AlN film on sapphire substrate were carefully investigated. The average optical transmittance of AlN film on sapphire substrate with oxygen-doped sputtered AlN NL was higher than that of AlN films on sapphire substrates with LT-AlN NL and oxygen-undoped sputtered AlN NL in the 200–270 nm wavelength region. However, the AlN film on sapphire substrate with oxygen-undoped sputtered AlN NL had the lowest TD density among AlN films on sapphire substrates. The AlN film on sapphire substrate with the optimum thickness of sputtered AlN NL showed weak tensile stress, a crack-free surface, and low TD density. Furthermore, a 270-nm AlGaN-based DUV LED was grown on the high-quality and crack-free AlN film. We believe that our results offer a promising and practical route for obtaining high-quality and crack-free AlN film for DUV LED.

Effect of working pressure and annealing temperature on single-phase AlN films

Single-phase aluminum nitride (AlN) films were synthesized on Si substrates by RF sputtering at different working pressures and different annealing temperatures. Effect of two factors on crystal quality, surface roughness and molecular structure was investigated. As the working pressure rose, the crystal quality of AlN films first increased and then decreased, reaching the best quality at the pressure of 0.7 Pa in the range of 0.3–2.5 Pa. When the annealing temperature was 100 °C, root-mean-square surface roughness was greatly reduced from 1.510 nm to 0.554 nm. In addition annealing technology could evidently enhance Al–N covalent bond density. In short, this work provides an effective approach to fabricate high-quality AlN (1 0 0) films for future applications of AlN-based devices.

Formation of graphene/SiC/AlN multilayers synthesized by pulsed laser deposition on Si(110) substrates

We have grown aluminum nitride (AlN) films on Si(110) substrates by pulsed laser deposition (PLD), and investigated the effects of laser power on the crystallinity and surface morphology of the AlN films. First, we epitaxially grew a fairly flat, high-quality AlN film, which contained no rotation domains, onto the Si(110) substrate in a well-lattice-matched relationship. Secondly, we formed a SiC interfacial buffer layer on the AlN film to grow a high-quality 3C-SiC film on the SiC buffer layer by PLD, which gave rise to a 3C-SiC(111)3×3 surface. The root-mean-square-roughness value of the SiC film was smaller than the previously reported values of SiC/AlN multilayers on Si(100) and Si(111) substrates. Thirdly, we grew graphene by annealing the SiC film at a high temperature in an ultra-high vacuum. It was demonstrated that the qualified graphene layer without rotation domains was grown on the SiC film. The formation of voids and the outdiffusion of Al and N atoms from the AlN film were successfully suppressed during the high-temperature annealing.

Effect of nitrogen flow ratio on the structural and optical properties of aluminum nitride thin films

Aluminum nitride (AlN) films have been deposited on glass substrates at various nitrogen flow ratios by rf reactive magnetron sputtering. The AlN film deposited at 10% of nitrogen flow ratio shows a strongly c-axis preferred orientation with a crystalline size of 100 nm, thickness of 1100 nm, and band gap energy of 4.38 eV. The optimum crystallographic structure occurs at a nitrogen flow ratio of 100%, where a considerable crystallinity enhancement of the AlN film is observed. The band gap energies E g calculated by the Tauc model and parabolic bands are well described by a relationship, E g =10 −3 X+4.39, where X is the nitrogen flow ratio. The results suggest that the nitrogen flow ratio plays a key role in growing the high-quality AlN films.

Single crystalline aluminum nitride films fabricated by nitriding α-Al2O3

Single crystalline aluminum nitride (AlN) is a key material for deep-ultraviolet light emitting devices. In the present study, high-quality single crystalline AlN films have been fabricated by nitriding α-Al2O3 with a precise control of driving force of nitridation reaction. This process provides the ⟨0001⟩-axis oriented AlN film as large as 50.8mm in diameter formed on (0001) and (112¯0) planes of α-Al2O3 substrates. The crystalline qualities of the films have the values of full width at half maximum of rocking curves using (0002) and (101¯0) planes of AlN for tilt and twist components as follows; Tilt=165arcsec and twist=540arcsec in the case of nitriding (0001) plane of α-Al2O3 and tilt=127arcsec and twist=237arcsec in the case of nitriding (112¯0) plane of α-Al2O3.

Structural characterization of pulsed laser-deposited AlN thin films on semiconductor substrates

Thin films of aluminum nitride (AlN) have been grown on semiconductor Si (100) substrates using the pulsed laser deposition method. The laser-deposition parameters and substrate temperature play an important role in fabricating high-quality AlN films. The films deposited at higher temperatures have shown better crystalline properties and also are highly oriented perpendicular to the substrate. The films have been characterized using X-ray diffraction, scanning electron microscope, and Fourier transform infrared spectroscopic techniques. An atomic force microscope was used to obtain three-dimensional topographs of the surfaces of these films. These maps are useful both for quantitatively characterizing the roughness of the surfaces and for gaining information about the basic mechanisms involved in the film growth process. The hardness as a function of the penetration depth has been measured using the nano-indentation method.