AlN Epitaxial Layers Research Articles

High-quality temperature-complementary bulk acoustic wave resonators fabricated with strippable single-crystalline AlN films grown on sapphire

To satisfy the strict demands of 5G radio frequency communication, we propose high-quality, flexible temperature-compensated single-crystalline AlN film bulk acoustic wave resonators (TC-SABARs) based on a 6-inch sapphire substrate. An AlGaN sacrificial layer and a 600-nm-thick single-crystalline AlN epitaxial layer are deposited on a sapphire substrate by metal organic chemical vapor deposition (MOCVD). Two types of TC-SABARs are fabricated and their performances are compared with published results. The results indicate that one of the TC-SABARs has a maximum Bode Q of 3406, an effective coefficient (Keff2) of 6.21%, and a temperature coefficient of frequency (TCF) of −9.5 ppm/°C. The other TC-SABAR exhibits a maximum Bode Q of 3022, a Keff2 of 5.99%, and a TCF of +0.7 ppm/°C. This performance can be attributed to the high-quality single-crystalline AlN film and the temperature-compensation structure with nonmetallic flip-chip bonding film transfer process and a thick SiO2 layer.

High quality AlN film assisted by graphene/sputtered AlN buffer layer for deep-ultraviolet-LED

The advantages of van der Waals epitaxial nitrides have become a research hot topic. It is worth noting that graphene plays an important role in the research of epitaxial AlN epitaxial layer. In this work, we demonstrate a method to obtain high-quality and low-dislocation AlN epitaxial layer by combining graphene and sputtered AlN as the nucleation layer on the C-sapphire substrate via metal organic chemical vapor deposition, and successfully fabricated a 277 nm AlGaN-based deep ultraviolet light emitting diode (DUV-LED) based on the obtained AlN epitaxial layer. The presence of graphene promotes the stress release of AlN. Compared with the AlN epitaxial layer directly grown on graphene/sapphire substrate, the exist of sputtered AlN/graphene nucleation layer facilitates most of the threading dislocations in AlN can annihilate each other in the range of about 100 nm. Thus, as grown AlN epitaxial layer shows the decreasing of the screw dislocation from 2.31 × 109 to 2.08 × 108 cm−2 significantly. We manufacture an DUV-LED with 277 nm emission wavelength by using high-quality AlN films, which shows that magnitude of the leakage current is only on the order of nanoamperes and the forward turn on voltage is 3.5 V at room temperature. This study provides a meaningful strategy to achieve high-quality AlN film and high-performance DUV-LED.

High-quality AlN epilayers prepared by atomic layer deposition and large-area rapid electron beam annealing

Low-temperature atomic layer deposition and high-temperature electron beam annealing (EBA) are used to achieve high-quality epitaxial AlN layers. Efficient energy transfer from the single exposure for only 1 min of electron irradiation with a large area contributes to substantial improvement in the film density and crystal quality of AlN thin films, as demonstrated by the X-ray reflectivity and the θ-2θ/ω-scan X-ray diffraction. High-resolution transmission electron microscopy indicates well-arranged lattice fringes in the epitaxial AlN layer on a sapphire substrate. A low surface roughness of the AlN epilayer, as revealed by the atomic force microscopy, suggests that the damage induced by the EBA treatment is insignificant. The outcomes indicate that the large-area rapid EBA is a low-damage and efficient technique for the recrystallization of thin films prepared at low temperatures.

Cathodoluminescence studies of point defects in aluminum nitride

There is near-ultraviolet and yellow light absorption in AlN crystal. The defect of this absorption hampers the transmittance of ultraviolet light, which, in turn, limits the application of aluminum nitride substrate in deep ultraviolet electronic devices. In this paper, the sources of absorption defects are determined through the first principle simulation method and the cathodoluminescence test of four samples with different growth modes on two substrates. The peak positions of the two AlN grown by metal–organic chemical vapor deposition were observed at 370 and 480 nm; two AlN samples grown by hydride vapor phase epitaxy only observed luminescence near 370 nm. Through the characterization of sample components by x-ray photoelectron spectroscopy, it is speculated that O impurity component and Al vacancy in AlN crystal form donor–acceptor pairs, which are the source of 370 nm luminescence; with the increase in O concentration, a composite defect with Al vacancy is formed, which is the source of 480 nm luminescence. This is very important for further research on eliminating impurity absorption in an AlN epitaxial layer.

Kinetic Monte Carlo simulations for AlN and AlGaN epitaxial growth on AlN

With increasing availability of AlN substrates, growth of AlN and AlGaN epitaxial layers on AlN(0001) has become a subject of extended experimental investigations. For a deeper understanding of the growth process, we developed a kinetic Monte Carlo (KMC) model for the adsorption, desorption, and diffusion on a stepped Al(001) surface under MOVPE conditions. Nitrogen is provided via ammonia and we assumed that the surface is covered by N in a 2 × 2 N reconstruction. This reconstruction could be obtained in our calculations for a flat surface. We performed several runs for a vicinal AlN(0001) surface with a miscut of 3° in m-direction [11¯00] direction. The results are analysed in detail both for AlN and AlGaN epitaxial layers. In addition, we investigate the influence of dislocations by a simple approach for bond and diffusion energies.

High-quality AlN growth on flat sapphire at relatively low temperature by crystal island shape control method

In this study, we found that the two-step method for heteroepitaxial AlN on sapphire has some limitations. Growing the buffer layer for a too long time will make it challenging to obtain a flat film surface. So, three-step-growth was introduced and an intermediate layer was inserted. In this step, the surface morphology of samples is controlled by NH3 flow rate and the dislocation density is further reduced. Finally, we are able to obtain high-quality AlN samples grown by metalorganic chemical vapor deposition at 1120 ℃, which is a relatively low temperature for the AlN epitaxial layers. The X-ray diffraction ω-scan full width at half maximum values for (002) and (102) reflections are 172 and 145 arcsec, respectively.

The Effect of Si (111) Substrate Surface Cleaning on Growth Rate and Crystal Quality of MOVPE Grown AlN

In this work, the effect of Si (111) substrate surface cleaning by RCA (Radio Corporation of America) method on growth rate and crystalline quality of epitaxially grown AlN thin films by MOVPE (Metal Organic Vapor Phase Epitaxy) technique is investigated. In situ reflectance system and high resolution X-ray diffraction (HRXRD) technique are used for the analysis of growth rate and crystal quality of epitaxial AlN layers, respectively. Also, The Raman measurement is done to show the effect of the RCA cleaning procedure on the position of the peaks that occurred in the Raman spectra. The results have shown that the surface cleaning of Si (111) substrate by the RCA method removes the oxide layer formed on the surface, also helps to decrease the parasitic reactions and increases the adatom efficiency, results in an increased growth rate of the AlN layer. Besides, surface cleaning of Si (111) substrate by the RCA method has reduced the FWHM value ~5% for ω-2θ scan and ~60% for ω scan of AlN epilayer, indicating an improvement in crystal quality.

Influence of pulsed Al deposition on quality of Al-rich Al(Ga)N structures grown by molecular beam epitaxy

Surfaces of AlN epitaxial layers grown by molecular beam epitaxy (MBE) in continuous mode were analyzed by means of X-ray photoelectron spectroscopy, atomic force microscopy and reflection high-energy electron diffraction without breaking the ultra-high vacuum. Our studies show the presence of the Al-Al and N-N bonds on the AlN samples, as well as a rather rough 3D-like structure. Significant improvement of the surface quality was achieved by employing a low flux Al deposition in pulsed mode at the end of the MBE growth (still under a nitrogen background pressure). Subsequently, on the obtained AlN buffer layers, Al0.85Ga0.15N/GaN multiple quantum wells (MQWs) were grown to show the influence of the buffer layers quality on the entire structure properties. The photoluminescence studies indicate that the introduction of pulsed low flux Al deposition improves optical properties of the grown MQWs layers, i.e., the relative intensity between the QW emission and the low energy emission is clearly in favor of pulsed Al deposition.

Defect-related photoluminescence and photoluminescence excitation as a method to study the excitonic bandgap of AlN epitaxial layers: Experimental and ab initio analysis

We report defect-related photoluminescence (PL) and its vacuum ultraviolet photoluminescence excitation (PLE) spectra of aluminum nitride layers with various layer thicknesses and dislocation densities grown on two different substrates: sapphire and silicon. The defect-related transitions have been distinguished and examined in the emission and excitation spectra investigated under synchrotron radiation. The broad PL bands of two defect levels in the AlN were detected at around 3 eV and 4 eV. In the PLE spectra of these bands, a sharp excitonic peak originating most probably from the A-exciton of AlN was clearly visible. Taking into account the exciton binding energy, the measurements allow determination of the bandgaps of the investigated AlN samples and their temperature dependencies. Next, they are compared with the literature data obtained by other experimental techniques for bulk AlN crystals and layers grown on different substrates. The obtained results revealed that the AlN bandgap depends on the substrate. The theoretical analysis using density functional theory calculations showed that the effect is induced by the tetragonal strain related to the lattice mismatch between the substrate and the AlN layer, which has a strong influence on the spectral positions of the intrinsic excitons, and consequently on the bandgap of AlN layers.

Ultraviolet Stimulated Emission in AlGaN Layers Grown on Sapphire Substrates Using Ammonia and Plasma‐Assisted Molecular Beam Epitaxy

Ammonia and plasma‐assisted (PA) molecular beam epitaxy modes are used to grow AlN and AlGaN epitaxial layers on sapphire substrates. It is determined that the increase in the thickness of the AlN buffer layer grown by ammonia molecular beam epitaxy (MBE) from 0.32 to 1.25 μm results in the narrowing of 101 X‐Ray rocking curves (XRCs), whereas no clear effect on the 002 XRC width is observed. It is shown that strong GaN decomposition during growth by ammonia MBE causes AlGaN surface roughening and compositional inhomogeneity, which leads to deterioration of its lasing properties. AlGaN layers grown by ammonia MBE at an optimized temperature demonstrate stimulated emission (SE) which peaks at λ = 330, 323, 303, and 297 nm, with SE threshold values of 0.7, 1.1, 1.4, and 1.4 MW cm−2, respectively. In comparison with these, AlGaN layers grown using PA MBE pulsed modes (migration‐enhanced epitaxy, metal‐modulated epitaxy, and droplet elimination by thermal annealing) show SE with a relatively low threshold (0.8 MW cm−2) at a considerably shorter wavelength of λ = 267 nm.

GaN-on-diamond technology platform: Bonding-free membrane manufacturing process

GaN-on-diamond samples were demonstrated using a membrane-based technology. This was achieved by selective area Si substrate removal of areas of up to 1 cm × 1 cm from a GaN-on-Si wafer, followed by direct growth of a polycrystalline diamond using microwave plasma chemical vapor deposition on etch exposed N-polar AlN epitaxial nucleation layers. Atomic force microscopy and transmission electron microscopy were used to confirm the formation of high quality, void-free AlN/diamond interfaces. The bond between the III-nitride layers and the diamond was validated by strain measurements of the GaN buffer layer. Demonstration of this technology platform is an important step forward for the creation of next generation high power electronic devices.

Singularities of the Thin AlN Layers Formation by Molecular Beam Epitaxy On 3C-SiC/Si(111) Templates with On-Axis and 4° Off-Axis Disorientation

Abstract—In this article, we studied the growth characteristics of AlN epitaxial layers on 3С-SiC/Si(111) templates at various values of atomic flux of aluminum at a constant growth temperature and a constant flow of atomic nitrogen. The AFM method was used to study the morphology of the resulting structures. The minimum roughness was achieved at a growth rate of 150 nm/h on on-axis templates, and at 90 nm/h on off-axis templates. Epitaxial layers of hexagonal AlN with root mean square roughness of less than 3 nm were obtained on 3С-SiC/Si(111) templates with a diameter of 100 mm, in which there was no grain structure. Single-crystal AlN (0002) layers with FWHM (ω‑geometry) values of about 1.4° were obtained.

Formation of conductive AlN buffer layer using spontaneous via-holes and realization of vertical AlGaN Schottky diode on a Si substrate

A conductive AlN epitaxial layer is successfully realized by spontaneously forming via-holes filled with n-AlGaN inside an AlN buffer layer on a Si substrate. The via-holes are found to originate from the formation of an Al–Si alloy, produced from a small amount of Al supplied to the Si substrate at the initial stage of the crystal growth of the n-AlN buffer layer using metal organic chemical vapor deposition and successive selective growth of n-AlN on the Si surface. The via-holes are filled with conductive n-AlGaN by successive epitaxial growth of n-AlGaN, making the insulating n-AlN buffer layer conductive. The vertical conductivity through this n-AlN buffer layer is enhanced more than 540 times compared with an n-AlN buffer layer without via-holes. Using this conductive n-AlN buffer layer on the Si substrate, we successfully fabricated a vertical n-AlGaN Schottky diode on the Si substrate for the first time.

Enhanced lateral growth of AlN epitaxial layer on sapphire by introducing periodically pulsed-TMGa flows

Crystal quality improvements of AlN epilayers grown on sapphire substrates has been achieved by using periodically pulsed-trimethylgallium (TMGa) flows (PTFs) in the initial growth stage of high-temperature AlN. The 400-nm-thick AlN layer deposited with this method demonstrated atomically flat surface, and the line widths of x-ray rocking curves were 40 and 245 arcsec for (002) and (105) reflections, respectively. GaN mole fraction was measured to be less than 0.6% in the AlN layer, indicating a surfactant effect of the PTFs. A combination of experimental characterizations and density functional theory calculations unveils that the improvements are ascribed to the supplement of Ga atoms and the enhanced surface migration of Al adatoms by using the PTFs which promotes the transition of growth mode from three-dimension to two-dimension one.

AlGaN/GaN high electron mobility transistor heterostructures grown by ammonia and combined plasma-assisted ammonia molecular beam epitaxy

The structural properties and surface morphology of AlN epitaxial layers grown by ammonia (NH3) and plasma-assisted (PA) molecular beam epitaxy (MBE) at different growth conditions on (0001) sapphire were investigated. The lowest RMS roughness of ∼0.7 nm was achieved for the sample grown by NH3 MBE at a substrate temperature of 1085 °C and NH3 flow of 100 standard cm3 min−1. Atomic force microscopy measurements demonstrated a terrace-monolayer step-like surface morphology. Furthermore, the optimal substrate temperature for growth of GaN and AlGaN layers was determined from analysis of the GaN thermal decomposition rate. Using the optimized growth conditions, high electron mobility transistor heterostructures were grown by NH3 MBE on different types of AlN nucleation layer deposited by NH3 MBE or PA MBE. The grown heterostructures demonstrated comparable two-dimensional electron gas (2DEG) properties. The maximum 2DEG mobility of ∼2000 cm2 V–1 s–1) at a 2DEG density of ∼1.17 × 1013 cm−2 was achieved for the heterostructure with a PA MBE-grown AlN nucleation layer. The obtained results demonstrate the possibility of successful combination of different epitaxial approaches within a single growth process, which will contribute to the development of a new type of hybrid epitaxy that exploits the advantages of several technologies.

Morphology and element composition profiles of alumina films obtained by mixed thermal oxidation of AlN epitaxial layers

The mixed thermal oxidation of the high temperature aluminium nitride (HT-AlN) epitaxial layer leads to the formation of the stable aluminium oxide having a developed surface area, potentially suitable for sensor applications. In this work, we studied the process of oxide formation on HT-AlN layers versus oxidation time. We applied scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to perform analysis of the morphology and chemical depth profiles of the oxide/AlN structures obtained at gradually increased time of oxidation. The SEM images revealed porous, semi-crystalline columnar structure of HT-AlN layers and largely developed surface of aluminium oxide films. We recognized two oxidized regions with different features: stoichiometric Al2O3 layer near the top structure surface and partially oxidized, gradually changing Al-oxynitride region between Al2O3 and AlN, without clearly determined boundaries. We estimated the thicknesses of these regions and compared the results with the ellipsometric and energy-dispersive X-ray spectroscopy measurements. We proposed a morphological and chemical model of the oxidized structures, including both the fully oxidized alumina film and oxynitride region adjacent to the AlN region. We found two competitive components of the oxidation process, namely an almost linear Al2O3 growth and gradual decay of the Al-oxynitride layer with oxidation prolongation.

Graphene-assisted quasi-van der Waals epitaxy of AlN film for ultraviolet light emitting diodes on nano-patterned sapphire substrate

We report the growth of high-quality AlN films on nano-patterned sapphire substrates (NPSSs) by graphene-assisted quasi-van der Waals epitaxy, which enables rapid coalescence to shorten the growth time. Due to the presence of graphene (Gr), AlN tends to be two-dimensional laterally expanded on the NPSS, leading to the reduction of dislocation density and strain release in the AlN epitaxial layer. Using first-principles calculations, we confirm that Gr can reduce the surface migration barrier and promote the lateral migration of metal Al atoms. Furthermore, the electroluminescence results of deep ultraviolet light emitting diodes (DUV-LEDs) have exhibited greatly enhanced emission located at 280 nm by inserting the Gr interlayer. The present work may provide the potential to solve the bottleneck of high efficiency DUV-LED.

AlN epitaxy on SiC by low-temperature atomic layer deposition via layer-by-layer, in situ atomic layer annealing.

AlN thin films were epitaxially grown on a 4H-SiC substrate via atomic layer deposition (ALD) along with atomic layer annealing (ALA). By applying the layer-by-layer, in situ ALA treatment using helium/argon plasma in each ALD cycle, the as-deposited film gets crystallization energy from the plasma, which results in significant enhancement of the crystal quality to achieve a highly crystalline AlN epitaxial layer at a deposition temperature as low as 300 °C. In a nanoscale AlN epitaxial layer with a thickness of ∼30 nm, X-ray diffraction reveals a low full-width-at-half-maximum of the AlN (0002) peak of only 176.4 arcsec. Atomic force microscopy, high-resolution transmission electron microscopy, and Fourier diffractograms indicate a smooth surface and high-quality hetero-epitaxial growth of a nanoscale AlN layer on 4H-SiC. This research demonstrates the impact of the ALA treatment on the evolution of ALD techniques from conventional thin film deposition to low-temperature atomic layer epitaxy.

MBE AlGaN/GaN HEMT Heterostructures with Optimized AlN Buffer on Al2O3

The effect of molecular-beam epitaxy (MBE) growth conditions on properties of AlN epitaxial layers was investigated resulting in determination of optimal substrate temperature and ammonia flow. Optimal substrate temperature for growth of GaN and AlGaN layers was determined analyzing thermal decomposition rate of GaN. Based on the information, high electron mobility transistor heterostructures were grown on sapphire substrates using both ammonia and combined plasma-assisted/ammonia MBE modes. The highest achieved 2DEG mobility was 1992 cm2/(V s) (at 2DEG density of 1.17 × 1013 cm–2) which is the current state-of-the-art level.

Intentional polarity conversion of AlN epitaxial layers by oxygen

Nitride materials (AlN, GaN, InN and their alloys) are commonly used in optoelectronics, high-power and high-frequency electronics. Polarity is the essential characteristic of these materials: when grown along c-direction, the films may exhibit either N- or metal-polar surface, which strongly influences their physical properties. The possibility to manipulate the polarity during growth allows to establish unique polarity in nitride thin films and nanowires for existing applications but also opens up new opportunities for device applications, e.g., in non-linear optics. In this work, we show that the polarity of an AlN film can intentionally be inverted by applying an oxygen plasma. We anneal an initially mixed-polar AlN film, grown on sapphire substrate by metal-organic vapor phase epitaxy (MOVPE), with an oxygen plasma in a molecular beam epitaxy (MBE) chamber; then, back in MOVPE, we deposit a 200 nm thick AlN film on top of the oxygen-treated surface. Analysis by high-resolution probe-corrected scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS) evidences a switch of the N-polar domains to metal polarity. The polarity inversion is mediated through the formation of a thin AlxOyNz layer on the surface of the initial mixed polar film, induced by the oxygen annealing.