Epitaxial AlN Films Research Articles

Epitaxial AlN film with improved quality on Si (111) substrates realized by boron pretreatment via MOCVD

This study investigated the growth of AlN epitaxial films on 2-in. Si(111) via metal-organic chemical vapor deposition. By introducing triethylboron (TEB) during trimethylaluminum pretreatment, a nearly crack free AlN epilayer with a thickness of 500 nm was acquired. The x-ray diffraction rocking curves of AlN (002) and (102) exhibited full width at half maximum values of 0.22° and 0.36°, respectively. Atomic force microscopy image analysis showed that after the introduction of TEB, larger grains appeared on the surface of Si(111) substrate, promoting the 3D growth pattern of the subsequent AlN buffer layer. Laytec reflection curves depicted the morphological transition from 3D to 2D growth mode during AlN deposition. At the same time, the curvature value was significantly reduced by 20 km−1, and the Raman spectrum peak of E2(high) shifted from 648.7 to 652.5 cm−1, indicating that the surface tensile stress was greatly reduced, effectively suppressing the crack problem of AlN on Si(111).

Influence of Si(111) substrate off-cut on AlN film crystallinity grown by magnetron sputter epitaxy

The increasing demand for More than Moore devices requires epitaxy technology to keep up with the discovery and deployment of new semiconductors. An emerging technology for cost-effective, device-quality growth is magnetron sputter epitaxy, though detailed studies on the process itself remain scarce. Here, we report an extensive study on the correlation between the substrate off-cut and film quality in AlN-on-Si heteroepitaxy. Controlled reactive pulsed magnetron sputtering is used to grow epitaxial AlN(0001) films on in situ Ar plasma etched off-cut Si(111) substrates with growth rates above 1.5 nm/s. Substrate off-cut angles in the range of 0.02°–0.30° are investigated and precisely determined by high-resolution x-ray diffraction. Structural examination of the AlN films is carried out by transmission electron microscopy and high-resolution x-ray diffraction. The AlN/Si interface is well-defined and two types of AlN domains with epitaxial relationships are observed. The formation of secondary rotation domains deteriorates the crystal quality substantially. Substrates with small off-cuts, ideally no off-cut substrates, appear to be crucial for suppressing the formation of secondary domains and further result in a better overall crystal quality of AlN films. We discuss this effect in relation to the AlN/Si interface, the substrate pre-treatment, and nucleation.

The 3.4 GHz BAW RF Filter Based on Single Crystal AlN Resonator for 5G Application.

To meet the stringent requirements of 5G communication, we proposed a high-performance bulk acoustic wave (BAW) filter based on single crystal AlN piezoelectric films on a SiC substrate. The fabrication of the BAW filter is compatible with the GaN high electron mobility transistor (HEMT) process, enabling the implementation of the integration of the BAW device and high-performance monolithic microwave integrated circuit (MMIC). The single crystal AlN piezoelectric film with 650-nm thickness was epitaxially grown on the SiC substrate by Metal Organic Chemical Vapor Deposition (MOCVD). After wafer bonding and substrate removal, the single crystal AlN film with electrode layers was transferred to another SiC wafer to form an air gap type BAW. Testing results showed that the fabricated resonators have a maximum Q-factor up to 837 at 3.3 GHz resonant frequency and electromechanical coupling coefficient up to 7.2%. Ladder-type filters were developed to verify the capabilities of the BAW and process, which has a center frequency of 3.38 GHz with 160 MHz 3 dB bandwidth. The filter achieved a minimum 1.5 dB insertion loss and more than 31 dB out-of-band rejection. The high performance of the filters is attributed to the high crystallinity and low defects of epitaxial single crystal AlN films.

Non-polar a-plane AlN epitaxial films on r-plane sapphire with greatly reduced defect densities obtained by high-temperature annealing

This study addresses the difficulty of obtaining high crystalline quality non-polar a-plane AlN films grown epitaxially on r-plane sapphire substrates by applying a high-temperature annealing process to films grown via standard-temperature metal organic chemical vapor deposition. Atomic force microscopy results demonstrate the occurrence of a significant recrystallization process in the AlN films during annealing. Raman spectroscopy measurements reveal that the AlN films are subject to a transition from tensile strain to compressive strain after the annealing process. Transmission electron microscopy analyses demonstrate that the non-polar a-plane AlN films provide an appropriately low dislocation density of 3.7 × 109 cm−2.

The In-Plane-Two-Folders Symmetric a-Plane AlN Epitaxy on r-Plane Sapphire Substrate

In the present work, a single-crystalline epitaxial nonpolar a-plane AlN film with in-plane two-folder symmetries was successfully achieved on an r-plane sapphire substrate, by combining physical vapor deposition and a high-temperature annealing technique. Moreover, by varying the AlN thickness, the evolution of crystalline quality and structure were systematically investigated using X-ray diffraction, Raman spectroscopy, and atomic force microscopy. The crystalline quality was much improved by the annealing treatment. Most importantly, when the thickness of AlN was increased up to 1000 nm, the AlN lattice was found to endure strong distortion along the out-of-plane direction, and the lattice showed an obvious expansion. The change of the surface morphology induced by high-temperature annealing was also tracked, and the morphology displayed structural anisotropy along the [11¯00] direction. Our results act as a crucial platform to better understand and employ the nonpolar AlN template; in particular, it is of importance for subsequent device fabrication.

Growth and characterization of PALE Si-doped AlN on sapphire substrate by MOVPE

In this study, we report different SiH4 flow condition effects on crystal, surface, optical, and electrical characteristics of heteroepitaxial Metal-Organic Vapor Phase Epitaxy (MOVPE) grown AlN layers on sapphire substrates. Adjustment of growth kinetics is very important to control the doping. Therefore, pulsed atomic layer epitaxy (PALE) was used to control the growth kinetics and reduce parasitic reactions that inevitably caused adverse impact on the properties of the epitaxial AlN films. As a result of HRXRD (high resolution x-ray diffraction) analysis, the (002) ω FWHM decreased significantly with the PALE method, while the increase occurred due to the development of V defects for the (102) ω scan. Atomic force microscopy (AFM) analyzes showed that SiH4 led to a 3D-like growth mode. It was demonstrated that the increased SiH4 flow increased Si incorporation into the Si-doped AlN layer while increased the sheet resistance due to the self-compensating effect obtained from secondary ion mass spectroscopy (SIMS) and I–V measurement results.

Non-Polar A-Plane Aln Epitaxial Films on R-Plane Sapphire with Greatly Reduced Defect Densities Obtained by High-Temperature Annealing

Non-Polar A-Plane Aln Epitaxial Films on R-Plane Sapphire with Greatly Reduced Defect Densities Obtained by High-Temperature Annealing

Role of oxygen diffusion in the dislocation reduction of epitaxial AlN on sapphire during high-temperature annealing

Recovery of epitaxial AlN films on sapphire at high temperatures is now an established process to produce pseudo-substrates with high crystalline perfection, which can be used to grow epitaxial structures for UV-light-emitting devices. To elucidate the elementary mechanisms taking place during the thermal treatment of MOVPE-grown films, we studied as-grown and annealed samples combining transmission electron microscopy techniques and secondary ion mass spectrometry (SIMS). By using SIMS, we find a temperature-dependent increase in the overall oxygen content of the films, which cannot be explained quantitatively with either simple bulk or pure pipe-diffusion from the sapphire substrate. Instead, we propose a lateral outdiffusion from the dislocation cores to explain qualitatively and quantitatively the presence of observed oxygen concentration plateaus. Based on the formation enthalpy of various atomic defects and complexes found in literature, we conclude that the di-oxygen/aluminum vacancy complex (VAl–2ON) is the dominant point defect controlling the annealing process. The formation of this defect at high temperatures promotes a dislocation core climb process, which causes the annihilation/fusion of the threading dislocation segments.

Semi-polar (11–22) AlN epitaxial films on m-plane sapphire substrates with greatly improved crystalline quality obtained by high-temperature annealing

This study addresses the difficulty of obtaining relatively low-cost semi-polar (11–22) AlN films epitaxially grown on m-plane sapphire substrates with high crystalline quality by applying a cost-effective high-temperature annealing process conducted at 1700 °C to films grown via standard-temperature metal organic chemical vapor deposition. The X-ray diffraction rocking curves of the annealed films along the [11–23] and [10–10] directions obtain full width at half maximum values of 0.152° and 0.193°, respectively. Atomic force microscopy results demonstrate the occurrence of a significant recrystallization process in the columnar formations of AlN films during annealing, resulting in increased AlN crystal grain size. Raman spectroscopy measurements reveal that the semi-polar (11–22) AlN films are subject to increased compressive stress after the annealing process. Transmission electron microscopy analyses confirm that our semi-polar AlN films provide appropriately low dislocation and basal stacking fault densities of 5.6 × 109 cm−2 and 1.05 × 105 cm−1, respectively. The proposed approach therefore obtains low-cost AlN films that are suitable for the mass manufacture of efficient optoelectronic devices.

Microstructure and Interfaces of Ultra-Thin Epitaxial AlN Films Grown by Plasma-Enhanced Atomic Layer Deposition at Relatively Low Temperatures

We demonstrate the growth of ultra-thin AlN films on Si (111) and on a GaN/sapphire (0001) substrate using atomic layer epitaxy in the temperature range of 360 to 420 °C. Transmission electron microscopy and X-ray diffraction were used to characterize the interfaces, fine scale microstructure, and the crystalline quality of thin films. Films were deposited epitaxily on Si (111) with a hexagonal structure, while on the GaN/sapphire (0001) substrate, the AlN film is epitaxial and has been deposited in a metastable zinc-blende cubic phase. Transmission electron microscopy reveals that the interface is not sharp, containing an intermixing layer with cubic AlN. We show that the substrate, particularly the strain, plays a major role in dictating the crystal structure of AlN. The strain, estimated in the observed orientation relation, is significantly lower for cubic AlN on hexagonal GaN as compared to the hexagonal AlN on hexagonal GaN. On the Si (111) substrate, on the other hand, the strain in the observed orientation relation is 0.8% for hexagonal AlN, which is substantially lower than the strain estimated for the cubic AlN on Si(111).

Tuning the electromechanical properties and polarization of Aluminium Nitride by ion beam-induced point defects

We report the evolution of uniaxial strain, resulting in an expansion of the c-axis in the wurtzite structure by up to 1 %, without significant degradation of the crystal structure of 30 keV Zr+ implanted epitaxial AlN films, grown on Si substrates. Raman and X-ray absorption spectroscopies demonstrated that the dominant defects are ZrAl, VAl and VN. The uniaxial strain can be attributed to a weakening of the Metal-N π bond along the c-axis. Monte Carlo simulations further predict the formation of a cation-rich region within the Zr implantation range, along with a buried anion-rich layer for all investigated fluences. The anion-rich layer undergoes a polarity inversion, which was experimentally confirmed by high-resolution high-angle annular dark field scanning transmission electron microscopy. Those microstructural changes influence the macroscopic electromechanical properties of AlN. The effective piezoelectric coefficient, d33, reduces from (7.0 ± 0.5) pm/V to (5.2 ± 0.5) pm/V at a fluence of 1015 at./cm2 Zr+. At higher fluences AlN undergoes a strain- and compositionally-induced polarity change, and the piezoelectric coefficient decreases due to crystalline damage. These results provide a pathway to optimise the performance of AlN by ion implantation for applications such as energy harvesting, light-emitting diodes and acoustic wave devices. In addition, the capability to engineer a buried polarisation inversion after AlN growth may enable novel device design.

Annealing behaviors of vacancy-type defects in AlN deposited by radio-frequency sputtering and metalorganic vapor phase epitaxy studied using monoenergetic positron beams

Vacancy-type defects in AlN films were probed by using monoenergetic positron beams. The AlN films were deposited on sapphire substrates by using a radio-frequency sputtering technique. Epitaxial films grown by metalorganic vapor phase epitaxy on the sputtered AlN films were also characterized. For the sputtered AlN, the major defect species was identified to be complexes between Al-vacancy and oxygen atoms located at nitrogen sites. Vacancy clusters were introduced after annealing at 1300 °C in the N2 atmosphere but their concentration decreased with a higher annealing temperature. The vacancy–oxygen complexes, however, still existed in the AlN film after annealing at 1700 °C. For the AlN epitaxial films, the concentration of vacancy clusters increased as the growth temperature increased up to 1300 °C but it decreased with the post-growth annealing at 1700 °C.

Рост объемных эпитаксиальных пленок AlN полуполярной ориентации на подложках Si (001) и гибридных подложках SiC/Si (001)

Abstract The possibility of growing bulk (more than 7 μm thick) epitaxial semipolar AlN films on Si(001) and hybrid SiC/Si(001) substrates without cracks has been investigated. It is found that an AlN layer grown on the Si substrate is extended, whereas an AlN layer grown on the hybrid SiC/Si substrate is compressed. The limiting (critical) thickness of the semipolar AlN layer on the Si(001) substrate is determined to be ~7.5 μm. When the film thickness exceeds this value, an ensemble of cracks is formed in the film, leading to its total cracking and exfoliation from the substrate. The semipolar epitaxial AlN films with a thickness of more than 40 μm are grown on hybrid SiC/Si substrates without cracking and exfoliation from the substrate.

Temperature dependent He-enhanced damage and strain in He-implanted AlN

He implantation was performed at RT and 550 °C into AlN epitaxial films. The resulting microstructural damage and elastic strain were investigated using TEM and XRD. Basal stacking faults, clusters of point defects and bubbles aligned along (0001) planes are observed at both temperatures. Apart from slight variations of size and densities of defects, no significant difference is observed on the microstructure after implantation at RT and 550 °C. In particular, He bubbles are observed with similar size, showing that the long-range migration of vacancies is not triggered at 550 °C. An important effect of He on the enhancement of damage and strain build-up during implantation is evidenced. He bubbles are observed to strongly favor the nucleation of extended defects, both basal stacking faults and point defect clusters, which results from the increase of free interstitial concentration due to stabilization of vacancies by He that could be enhanced by the bubble growth by loop punching or trap mutation to relieve the gas pressure. At an earlier stage of implantation-induced damage, the presence of He is also seen to induce a strong increase of strain with increasing implantation temperature, whilst efficient point defect recombination takes place in the regions free of He, or with low concentration of He, resulting in a decrease of strain. The strong increase of strain with increasing temperature in the deep implanted region, concomitant with a strong reduction of scattered intensity, is ascribed to the growth of He–V complexes, probably induced by a thermally enhanced mobility of He, and only occurs for sufficient He concentration.

Plasma assisted-MBE of GaN and AlN on graphene buffer layers

The possibility of using chemical vapor deposition (CVD) graphene as a 2D buffer layer for epitaxial growth of III-nitrides by plasma assisted-MBE on amorphous substrates (SiO2 prepared by thermal oxidation of Si wafer) was investigated. The comparative study of graphene-coated parts of the wafers and the parts without graphene was carried out by scanning electron microscopy and X-ray diffractometry. It was shown that epitaxial GaN and AlN films with close to 2D surface morphology can be obtained by plasma assisted-MBE on amorphous SiO2 substrates with a multilayer graphene buffer using the HT AlN nucleation layer.

Formation of m-plane AlN on plasma-nitrided m-plane sapphire

Microwave plasma using a gas mixture of N2 and H2 has been applied for the nitridation of m-plane sapphire substrate to form a thick epitaxial AlN film. The X-ray diffraction results show that the AlN films formed on the sapphire surface by nitridation for a period from 10–60 min are in (100) orientation and have an epitaxial relationship with the substrate. The thickness of the nitride film increases with nitridation time and approaches about 0.5 μm after nitridation for 1 h, while the film surface becomes rough. The film quality is reasonably good, as evaluated with the X-ray rocking curve of (100) AlN. Faceted voids in the sapphire substrate underneath the AlN are also observed with inclined a-plane facets after nitridation.

Epitaxial AlN on c-plane sapphire by plasma nitriding

Nitridation of sapphire to form epitaxial AlN by microwave plasma containing nitrogen gas has been investigated to be an efficient process. Structural characterization with chemical analysis by X-ray diffraction and X-ray photoelectron spectroscopy shows that an epitaxial AlN film containing oxygen can form on sapphire by using pure nitrogen plasma. Furthermore, nitridation with N2/H2 plasma not only improves the crystallinity of the epitaxial AlN film without oxygen, but also significantly increases the nitridation rate to 9 nm min−1, resulting in 270 nm thick AlN for 30 min plasma nitriding.

High-temperature annealing induced evolution of strain in AlN epitaxial films grown on sapphire substrates

High-temperature (HT) annealing effects on the evolution of strain in AlN films grown on sapphire have been investigated. It is found that there is a significant transition behavior from tensile to compressive strain in AlN before and after HT annealing at an optimal temperature of 1700 °C. Based on a microstructural analysis, it is clarified that the HT annealing will result in the (1) disappearance of grains that account for the tensile stress before HT annealing, (2) generation of a new interface that has little influence on the lattice constant upper/below this interface, and (3) regular 8/9 arrangement of misfit dislocation at the AlN/sapphire interface that relieves almost all stress associated with lattice mismatch. It is thus deduced that the remnant compressive strain in AlN after HT annealing mainly comes from the cooling down process due to thermal mismatch between sapphire and AlN. This understanding of the annealing effect is certainly of great significance in AlN materials science and technology.

Growth of Si-doped AlN on sapphire (0001) via pulsed sputtering

The Si-doped AlN films were grown on sapphire (0001) by pulsed sputtering deposition (PSD), and their structural and electrical properties were investigated. A combination of PSD and high-temperature annealing process enabled the growth of high-quality AlN (0001) epitaxial films on sapphire (0001) substrates with atomically flat stepped and terraced surfaces. The transmission electron microscopy observations revealed that the majority of the threading dislocations in AlN belonged to the mixed- or edge-type, with densities of 2.8 × 108 and 4.4 × 109 cm−2, respectively. The Si-doping of AlN by PSD yielded a clear n-type conductivity with a maximum electron mobility of 44 cm2 V−1 s−1, which was the highest value reported for AlN that was grown on sapphire. These results clearly demonstrated the strong potential of the PSD technique for growing high-quality conductive n-type AlN on sapphire.

Influence of temperature and Al/N ratio on structural, chemical & electronic properties of epitaxial AlN films grown via PAMBE

The present article investigates structural, chemical and electronic properties of epitaxial AlN films grown via plasma assisted molecular beam epitaxy on atomically clean Si (1 1 1) substrates. An inclusive optimization process of growth parameters by varying the substrate temperature (790–825 °C) and Al/N (III/V) ratio is demonstrated. The AlN film grown with optimized parameters yielded an FWHM of 24.6 arcmin, crystallite size of 11.6 nm, screw dislocation density of 4.43 × 109/cm2 and a surface roughness of 3.11 nm. Besides, the chemical states and electronic structure analysis displayed absence of remnant metallic aluminium and native surface oxide (∼2%) with Fermi level (3.0 eV) pinned near its intrinsic value. A growth mechanism has been proposed for the optimized growth of AlN. Further, the high quality AlN film can potentially be used for the fabrication of smart optoelectronics for deep UV application and field emission devices.