High-quality Aluminum Nitride Research Articles

Effect of stress control by growth adjustment on the edge thread dislocation density of AlN grown on the sapphire

The relationship between stress and dislocation density in MOCVD epitaxial AlN was studied. It has been found that the aluminum nitride (AlN) epitaxial layer generates tensile stress when the crystal islands are merged. By controlling the size and density of crystal islands at the end of 3D growth, the tensile stress generated during epitaxy can be effectively reduced. Mechanical calculations show that there is a linear relationship between the edge thread dislocations density of AlN and the tensile stress during growth. By controlling the stress during AlN growth below 0.1 Gpa, a high-quality AlN sample with an edge thread dislocation density of 6.31 × 107 cm−2 was obtained.

Synchrotron radiation x-ray topography applied to nitride semiconductor crystals

Gallium nitride (GaN) and aluminum nitride (AlN), as examples of third-generation semiconductors, have attracted significant interest due to their remarkable physical attributes, including a wide bandgap, high breakdown voltage, exceptional chemical stability, and high thermal conductivity. These characteristics render GaN and AlN highly promising for use in power and (opto)-electronic devices. Consequently, there is a growing demand for high-quality GaN and AlN crystals on the centimeter scale. As the dislocation density in these materials decreases, the need for a reliable method of dislocation characterization becomes more pressing. Synchrotron radiation x-ray topography (SR-XRT) has emerged as a superior, nondestructive technique for the precise characterization of crystal defects. This review briefly introduced the principle of XRT, and its application in the analysis of dislocations in GaN and AlN crystals is summarized. By examining the relationship between the SR-XRT image contrast and the Burgers vectors of dislocations, it is possible to categorize wafer dislocations and determine the magnitude and direction of Burgers vectors. Additionally, SR-XRT facilitates the analysis of interactions between dislocations in GaN and AlN crystals. These analyses are instrumental in advancing the development of superior crystals. This review concludes with a discussion of the current challenges faced by SR-XRT and a projection of its future applications in characterizing third-generation semiconductor crystal materials. This review offers significant guidance for the characterization of nitride crystal defects using SR-XRT.

A solar-blind vacuum-ultraviolet photodetector based on free-standing lamellar aluminum nitride single crystal

High-quality aluminum nitride (AlN) crystals are the key material for the development of high-performance solid-state solar-blind vacuum-ultraviolet (VUV) photodetectors. However, the commonly used epitaxial method to grow AlN crystals would limit this development due to the existence of indispensable substrates. This study addresses this issue using free-standing lamellar AlN single crystals that are grown using the physical vapor transport method. The large lateral dimension of the crystal enables the construction of an Au-AlN-graphene van der Waals heterojunction, which can function as a vertical VUV photodetector with the graphene serving as the light window. The asymmetric junctions formed on the two sides of the crystal and the limited penetration of the VUV endow the device with a bias polarity-dependent photoresponse feature arising from different photoelectric processes. Furthermore, the device demonstrates a high responsivity of 5.77 A W−1 and a high specific detectivity of 1.71 × 1013 cm Hz1/2 W−1 under the illumination of a 193 nm laser. The high crystallinity of the AlN guarantees a high spectral selectivity of responsivity with a 193 nm/280 nm rejection ratio of 3 × 102. This work would inspire the development of wide-bandgap-semiconductor-based VUV photodetectors in terms of methodology and mechanism.

Optical and Structural Properties of Aluminum Nitride Epi-Films at Room and High Temperature

The high-quality aluminum nitride (AlN) epilayer is the key factor that directly affects the performance of semiconductor deep-ultraviolet (DUV) photoelectronic devices. In this work, to investigate the influence of thickness on the quality of the AlN epilayer, two AlN-thick epi-film samples were grown on c-plane sapphire substrates. The optical and structural characteristics of AlN films are meticulously examined by using high-resolution X-ray diffraction (HR-XRD), scanning electron microscopy (SEM), a dual-beam ultraviolet-visible spectrophotometer, and spectroscopic ellipsometry (SE). It has been found that the quality of AlN can be controlled by adjusting the AlN film thickness. The phenomenon, in which the thicker AlNn film exhibits lower dislocations than the thinner one, demonstrates that thick AlN epitaxial samples can work as a strain relief layer and, in the meantime, help significantly bend the dislocations and decrease total dislocation density with the thicker epi-film. The Urbach’s binding energy and optical bandgap (Eg) derived by optical transmission (OT) and SE depend on crystallite size, crystalline alignment, and film thickness, which are in good agreement with XRD and SEM results. It is concluded that under the treatment of thickening film, the essence of crystal quality is improved. The bandgap energies of AlN samples obtained from SE possess larger values and higher accuracy than those extracted from OT. The Bose–Einstein relation is used to demonstrate the bandgap variation with temperature, and it is indicated that the thermal stability of bandgap energy can be improved with an increase in film thickness. It is revealed that when the thickness increases to micrometer order, the thickness has little effect on the change of Eg with temperature.

Effects of growth temperature and reactor pressure on AlN thin film grown by metal-organic chemical vapor deposition

Following the development of the semiconductor industry, advanced and multifunctional material aluminum nitride (AlN) is widely used. However, AlN thin film faces the dilemma of low crystal quality or cracking in practical applications. This paper presents a high crystal quality and crack-free AlN thin film with a thickness of over 510 nm grown on a sapphire substrate by metal-organic chemical vapor deposition (MOCVD). We have analyzed the thin film deposition mechanism of the MOCVD reaction system using different growth temperatures and reactor pressures and optimized the growth mode of AlN thin film. Wherein the growth temperature refers to the substrate temperature while the film is deposited. It was shown that the deposition rate of the AlN thin film is significantly affected by the reactor pressure. As shown by the x-ray diffraction patterns, the AlN thin films exhibit a (002) preferred orientation while non-(002) reflections are extremely weak. At constant pressure, the intensity of the diffraction peak of (002) orientation increased with the growth temperature. Furthermore, the surface morphology observed by atomic force microscopy revealed a decrease in the surface roughness with increasing growth temperature. The AlN thin film grown at a growth temperature of 1300 ℃ under 10,000 Pa exhibited better (002) orientation in this work. The full width at half maximum of its rocking curve was 0.033° for (002) planes. Beside, the sample showed a flat and dense surface with a root-mean-square surface roughness of 8.4 nm. This study offers an improvement in the deposition of AlN thin films, promising for AlN-based devices operated in various conditions.

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).

Structural, Surface, and Optical Properties of AlN Thin Films Grown on Different Substrates by PEALD

Plasma-enhanced atomic layer deposition was employed to grow aluminum nitride (AlN) thin films on Si (100), Si (111), and c-plane sapphire substrates at 250 °C. Trimethylaluminum and Ar/N2/H2 plasma were utilized as Al and N precursors, respectively. The properties of AlN thin films grown on various substrates were comparatively analyzed. The investigation revealed that the as-grown AlN thin films exhibit a hexagonal wurtzite structure with preferred c-axis orientation and were polycrystalline, regardless of the substrates. The sharp AlN/substrate interfaces of the as-grown AlN are indicated by the clearly resolved Kiessig fringes measured through X-ray reflectivity. The surface morphology analysis indicated that the AlN grown on sapphire displays the largest crystal grain size and surface roughness value. Additionally, AlN/Si (100) shows the highest refractive index at a wavelength of 532 nm. Compared to AlN/sapphire, AlN/Si has a lower wavelength with an extinction coefficient of zero, indicating that AlN/Si has higher transmittance in the visible range. Overall, the study offers valuable insights into the properties of AlN thin films and their potential applications in optoelectronic devices, and provides a new technical idea for realizing high-quality AlN thin films with sharp AlN/substrate interfaces and smooth surfaces.

Improved crystallinity and surface morphology of a-plane AlN grown on high temperature annealed AlN/sapphire template by pulsed-flow mode metal-organic vapor phase epitaxy

Preparing high quality non-polar aluminum nitride (AlN) templates is the key to improving the performance of non-polar deep-ultraviolet light-emitting diodes. In this study, we investigated the effect of buffer layer on the crystallinity and surface morphology of a-plane AlN films regrown by pulsed-flow mode (PM) metal-organic vapor phase epitaxy (MOVPE). Three buffer layers were compared including low-temperature AlN buffer layer grown by MOVPE (MO-buffer), sputtered AlN buffer layer (SP-buffer), and high-temperature annealed sputtered AlN buffer layer (HTA-buffer). It is found that the (11-20) plane x-ray rocking curve-full width at half maximum (XRC-FWHM) values of a-plane AlN films are significantly reduced after the regrowth process. Thanks to the high crystalline quality of HTA-buffer, AlN regrown on HTA-buffer exhibits the smallest (11-20) plane XRC-FWHM values of 1260/1440 arcsec along the [0001]/[1-100] direction. The mosaic tilt and basal plane stacking fault density are estimated to be 0.41° and 1.76 106 cm−1, respectively. The surface shows a uniform stripe-like pattern with the lowest root mean square value of 0.82 nm. Furthermore, the a-plane AlN epilayer regrown on HTA-buffer displays a weak in-plane stress anisotropy and high optical transmittance in the ultraviolet-visible region. Our work suggests that combining the PM regrowth with HTA-AlN buffer layer is a promising route to prepare high-quality a-plane AlN templates for efficient non-polar deep-ultraviolet light emitters.

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.

Investigation of V/III ratio dependencies for optimizing AlN growth during reduced parasitic reaction in metalorganic vapor phase epitaxy

AlGaN-based ultraviolet (UV) light-emitting diodes (LEDs) are expected to have various applications, including sensing and printing, and light with ultraviolet-C (UVC) wavelengths has a virus inactivation effect. The metalorganic vapor phase epitaxy (MOVPE) method has been used to fabricate LED devices with film control and impurity doping. However, to achieve high luminous efficiency, highly crystalline aluminum nitride (AlN) must be grown in the underlying layer. Although high temperatures are required to grow high-quality AlN for strong migration at the surface, there is a trade-off in the high temperature promoting parasitic reactions. These parasitic reactions are more dominant at a high V/III ratio with more raw material in the case of using the conventional MOVPE. Here, we used jet stream gas flow MOVPE to investigate the effect of V/III ratio dependencies in optimizing AlN growth and without affecting parasitic reaction conditions. As a result, trends of typical AlN crystal growth at V/III-ratio dependencies were obtained. AlN is more stable at a higher V/III ratio of 1000, exhibiting a double atomic step surface, and the crystal orientation is further improved at 1700 °C compared to that at a lower V/III ratio.

The interplay of process parameters and influence on the AlN films on sapphire fabricated by DC magnetron sputtering and annealing

This paper reports a high-quality aluminium nitride (AlN) film deposited on c-plane sapphire using a direct current reactive sputtering system. A comprehensive investigation on the effects of substrate temperature, sputtering power, Ar/N2 flow ratio, and process pressure on the morphology and crystalline quality of the AlN thin films was performed. Optimal process parameters including substrate temperature of 500 °C, process pressure of 0.5 Pa, gas flow ratio of Ar/N2 = 1/6, and sputtering power of 400 W was demonstrated. In addition, the crystallization quality was further improved after face-to-face annealing and homoepitaxial regrowth by metal–organic chemical vapor deposition. The full widths at half maximum values of the x-ray rocking curves of the (0002) and () diffraction of AlN after the re-growth process reach as low as 49 and 360 arcsec, respectively, and the corresponding surface morphologies of the films were covered with atomic bi-layer steps. Finally, a ultraviolet-C light emitting diode with decent performance was demonstrated on top of the sputtered AlN thin film.

Direct Determination of Band Gap of Defects in a Wide Band Gap Semiconductor.

Crystal defects play an important role in the degradation and failure of semiconductor materials and devices. Direct determination of band gap of defects is a critical step for clarifying how the defects affect the physical properties of semiconductors. Here, high-quality aluminum nitride (AlN) thin films were grown epitaxially on single-crystal Al2O3 substrates via pulsed laser deposition. The atomic structure and band gap of three types of inversion domain boundaries (IDBs) in AlN were determined using aberration-corrected transmission electron microscopy and atomic-resolution valence electron energy-loss spectroscopy. It was found that the band gap of all of the IDBs reduces evidently compared to that of the bulk AlN. The maximum band gap reduction of the IDBs is 1.0 eV. First-principles calculations revealed that the band gap reduction of the IDBs is mainly due to the rise of pz orbital at the valence band maximum, which originates from the elongated Al-N bonds along the [0001] direction at the IDBs. The successful band gap determination of defects paves an avenue for quantitatively evaluating the effect of defects on the performance of semiconductor materials and devices.

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.

Influence of sublimation surface on mass transport in AlN crystal growth by physical vapor transport process

A series of numerical experiments were performed to investigate the influence of the sublimation surface on mass transport during the aluminum nitride (AlN) growth process. The distribution of Al partial pressure is strongly affected by the cover of the sublimation interface. The morphology of the growth interface can be controlled by the cover of the sublimation interface to achieve the growth of a specific crystal shape. Based on the same temperature field, the influence of sublimation interface cover on the growth rate indicates that temperature and temperature gradient are not the main limiting factors of the growth rate and further verifies that Al partial pressure gradient is the rate-limiting step. Under the growth system and specific growth conditions, a smooth growth interface can be obtained by using [0, 2/6] sublimation interface cover, so as to realize the rapid growth of high quality AlN crystals.

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.

An insight into growth transition in AlN epitaxial films produced by metal-organic chemical vapour deposition at different growth temperatures

This work demonstrates a clear picture growth transition of aluminium nitride (AlN) films from the three-dimensional (3D) to the two-dimensional (2D) regime on the sapphire substrate at various temperatures using metal-organic chemical vapour deposition (MOCVD) under low reactor pressure. The high deposition rate of large 3D AlN islands that isolated each other change to 2D growth mode with a smoother surface as temperature increases from 800 °C to 1340 °C. From x-ray diffraction measurement , the AlN (100), AlN (002), and AlN (101) planes exhibit strong peak monocrystalline AlN (002) films as the temperature increase. It found that the AlN film grew at 1100 °C in the Frank–van der Merwe or 2D growth mode exhibits the highest crystalline quality with the threading dislocation density around 2.21 × 10 9 cm −2 . In addition, the lattice vibrational parameters of the AlN films at 1100 °C shows the lowest phonon damping from IR spectra results. Thus, this study details the AlN epitaxial films growth transition, which is crucial for growing high crystalline quality AlN layer using the MOCVD technique. • AlN thin films transition from 3D to 2D as the growth temperatures increase at lower reactor pressure. • The smoothest surface was observed at a higher growth rate but not at a higher growth temperature. • The intermediate growth temperature produced the high-quality AlN thin films under Frank–van der Merwe or 2D growth mode. • Theoretical modelling of the infrared reflection spectrum reveals a stressed layer between AlN film and sapphire substrate.

The Effect of the Crucible on the Temperature Distribution for the Growth of a Large Size AlN Single Crystal.

The appropriate distribution of temperature in the growth system is critical for obtaining a large size high quality aluminum nitride (AlN) single crystal by the physical vapor transport (PVT) method. As the crystal size increases, the influence of the crucible on the temperature distribution inside the growth chamber becomes greater. In order to optimize the field of temperature and study the specific effects of various parts of the crucible on the large size AlN single crystal growth system, this study carried out a series of numerical simulations of the temperature field of two crucibles of different materials and put forward the concept of a composite crucible, which combines different materials in the crucible parts. Four composite crucible models were established with different proportions and positions of tantalum carbide (TaC) parts and graphite parts in the crucible. Calculations reveal that different parts of the crucible have different effects on the internal temperature distribution. The axial temperature gradient at the crystal was mainly governed by the crucible wall, whereas the temperature gradient was determined by the integrated effect of the crucible lid and the crucible wall in the radial direction. One type of composite crucible was chosen to minimize the thermal stress in grown AlN crystal, which is applicable to the growth of large sized AlN crystals in the future; it can also be used to grow AlN single crystals at present as well.

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.

High quality aluminum nitride layer grown with a combined step of nitridation and trimethylaluminum preflow

In this work, aluminum nitride (AlN) layers were grown on sapphire substrate with different times and temperatures of nitridation through metal organics chemical vapor deposition growth technique. Besides, during the growth process, trimethylaluminum (TMAl) preflow was introduced, in particular, after the nucleation. From x-ray diffraction measurement in (002) and (102) reflections, it was found that an average threading dislocation density (TDD) for the AlN layer without nitridation was around 1.07 × 109 cm−2. By applying the nitridation for 20 minutes at 850 ºC, the average TDD in the corresponding AlN layer reduced down to ∼9.33 × 108 cm−2. Moreover, the layer exhibited a smoother surface, as observed from atomic force microscopy measurement. Overall, the results from this work suggested that a proper time and temperature of nitridation with the introduction of TMAl preflow could be a promising alternative for obtaining a high quality AlN layer.