Aluminum Nitride Thin Films Research Articles

Study of AlN Epitaxial Growth on Si (111) Substrate Using Pulsed Metal–Organic Chemical Vapour Deposition

A dense and smooth aluminium nitride thin film grown on a silicon (111) substrates using pulsed metal–organic chemical vapor deposition is presented. The influence of the pulsed cycle numbers on the surface morphology and crystalline quality of the aluminium nitride films are discussed in detail. It was found that 70 cycle numbers produced the most optimized aluminium nitride films. Field emission scanning electron microscopy and atomic force microscopy images show a dense and smooth morphology with a root-mean-square-roughness of 2.13 nm. The narrowest FWHM of the X-ray rocking curve for the AlN 0002 and 10–12 reflections are 2756 arcsec and 3450 arcsec, respectively. Furthermore, reciprocal space mapping reveals an in-plane tensile strain of 0.28%, which was induced by the heteroepitaxial growth on the silicon (111) substrate. This work provides an alternative approach to grow aluminium nitride for possible application in optoelectronic and power devices.

Aluminum Nitride Thin Film Pyroelectric Detector Based on Metamaterial Absorber

Highly integrated pyroelectric detectors have been widely used in infrared spectrometers and gas detection. Aluminum nitride(AlN) has excellent compatibility with CMOS processes and is often used to fabricate piezoelectric and optoelectronic devices with superior performance. This study proposes a pyroelectric detector with AlN thin film as the sensitive element and integrated metamaterial absorber. Magnetron sputtering technology was used to prepare the AlN thin film as the sensitive element. To achieve narrowband specific absorption subsequently, a three-layer metamaterial absorber structure was designed and simulated. Preliminary processing of AlN pyroelectric detectors was performed based on MEMS technology. The designed narrowband absorbing structure exhibits near unity specific absorption, with a quality factor of 17.75. This study preliminarily verifies the application potential of AlN thin films in mid-infrared pyroelectric sensors, and realizes narrowband absorption through metamaterial structures, laying the foundation for the development of mid-infrared gas detection sensors.

Aluminum Nitride Thin Film Piezoelectric Pressure Sensor for Respiratory Rate Detection.

In this study, we propose a low-cost piezoelectric flexible pressure sensor fabricated on Kapton® (Kapton™ Dupont) substrate by using aluminum nitride (AlN) thin film, designed for the monitoring of the respiration rate for a fast detection of respiratory anomalies. The device was characterized in the range of 15-30 breaths per minute (bpm), to simulate moderate difficult breathing, borderline normal breathing, and normal spontaneous breathing. These three breathing typologies were artificially reproduced by setting the expiratory to inspiratory ratios (E:I) at 1:1, 2:1, 3:1. The prototype was able to accurately recognize the breath states with a low response time (~35 ms), excellent linearity (R2 = 0.997) and low hysteresis. The piezoelectric device was also characterized by placing it in an activated carbon filter mask to evaluate the pressure generated by exhaled air through breathing acts. The results indicate suitability also for the monitoring of very weak breath, exhibiting good linearity, accuracy, and reproducibility, in very low breath pressures, ranging from 0.09 to 0.16 kPa. These preliminary results are very promising for the future development of smart wearable devices able to monitor different patients breathing patterns, also related to breathing diseases, providing a suitable real-time diagnosis in a non-invasive and fast way.

Room-temperature, RF-activated, quantum effects via engineered defect sites in thin-film AlN, Al2O3, and SnOx

The behavior of a sine wave propagated through thin films of aluminum nitride (AlN), aluminum oxide (Al2O3), and tin oxide (SnOx) with engineered buried defect sites may suggest quantum excitation and defect-mediated waveform modulations. Two distinct methods to induce these buried defects, etch pattern defects (EPD) and indentation pattern defects (IPD), were employed to detect these interactions. All the experiments were conducted at room temperature (21 °C) over a frequency range between 5 and 1000 kHz. In addition, EPD and IPD devices were composed of AlN, Al2O3, and SnOx. An inverse relationship between excitation frequency and voltage is observed for all devices. All these devices exhibited a relaxation time ranging between 0.2 and 0.75 µs. Devices without these engineered defect sites preserve the waveform integrity, emphasizing the impact of the buried defect sites. This research focuses on the relationship between defect type, excitation frequency, and voltage to understand the deeper mechanisms at play in these quantum defect-driven wave alterations in AlN, Al2O3, and SnOx thin films.

Structural and Optical Properties of Aluminium Nitride Thin Films Fabricated Using Pulsed Laser Deposition and DC Magnetron Sputtering on Various Substrates

Abstract Aluminium nitride thin films were fabricated using pulsed laser deposition and DC magnetron sputtering. Different technological parameters and the effects of different substrates on the optical and structural parameters of AlN samples were studied. An X-ray diffraction study was performed for the layer deposited on the Si3N4 substrate. A high-energy electron diffraction study was also carried out for the layer deposited on a KCl substrate. Transmission spectra of layers on quartz, sapphire, and glass substrates were obtained. An evaluation of the optical band gap of the obtained layers was carried out (Eg form 3.81 to 5.81 eV) and the refractive index was calculated (2.58). The relative density of the film (N1TN-AlN sample) is 1.26 and was calculated using the Lorentz-Lorentz relationship. Layers of aluminium nitride show an amorphous character with a polycrystalline region. It was shown that the properties of AlN films strongly depend on the method, growth conditions, and substrate used.

Effect of annealing temperature on Eu2+ and Eu3+ ratios in AlN:Eu thin films

Eu-doped aluminum nitride (AlN:Eu) thin films were fabricated and characterized in a way of white light generation. AlN exhibits blue luminescence originating from defect levels. Furthermore, Eu2+ demonstrates green luminescence, while Eu3+ emits red light. Consequently, white luminescence is anticipated in Eu-doped AIN thin films. The concept is to control the ratio of Eu2+/Eu3+ in the thin film to vary the emission colors. We aimed to manipulate the Eu valence ratio in AIN:Eu thin films by changing the annealing temperature and examining the resultant luminescence characteristics. Our findings revealed an increase in defect-related luminescence in AIN samples annealed at higher temperatures, along with a rise in the Eu2+ fraction. We explore these outcomes in relation to the structural models surrounding Al and Eu ions, as elaborated in the main text.

Fabrication of a wider bandgap θ-Al2O3 by oxidation of ultrathin AlN films for leakage current reduction

Aluminum oxide (Al2O3) films were fabricated through the oxidation of ultrathin aluminum nitride (AlN) films. The fabricated films exhibited a leakage current reduction compared to that of conventional Al2O3 films fabricated using atomic layer deposition. This reduction in the leakage current can be attributed to the formation of θ-Al2O3, which has a wider-bandgap than γ-Al2O3. The formation of θ-Al2O3 was attributed to the residual stress caused by the oxidation of the AlN thin films.

Demonstration of MHz piezoelectric micromachined ultrasonic transducers array and its potential application for biomedical treatment

In this paper, we successfully developed the 24 × 31 piezoelectric micromachined ultrasound transducers (PMUTs) array with 8 inch complementary metal oxide semiconductor (CMOS) compatible processes. Scandium aluminum nitride (ScxAl1−xN) thin film (x = 20%) was used as the piezoelectric material. The PMUTs array was fabricated on 8 inch cavity silicon-on-insulator wafers with the device silicon layer thickness of only 3 μm and a cavity pitch of 59 μm. The wafer-level electrical test was conducted using inline auto-prober and the yield rate >90% was achieved across the 8 inch wafer. The following acoustic test on our PMUTs array (1.5 mm × 2.0 mm) indicated the transmit sensitivity of 18.4 kPa V−1 @ 10 mm @ 11.6 MHz in oil with −3 dB bandwidth of 2.3 MHz. In order to validate the potential of using PMUTs array for biomedical treatments, subsequent ultrasonic heating experiments were carried out on a silicone block. A temperature rise of 4 °C was achievable in about 50 s. These results demonstrate that the potential of using PMUTs array as a disposable biomedical device in many therapeutic applications, for example, a treatment for dry eye disease.

Characterization of aluminum nitride thin films by ion beam analysis techniques

AlN thin films have been deposited on silicon substrate by vacuum arc discharge technique at different substrate temperatures. The information regarding depth profiling of AlN thin films has been determined by applying both elastic backscattering (EBS) and nuclear reaction analysis (NRA) techniques simultaneously at optimized experimental conditions. Additionally, combined SEM/EDX techniques have been employed to gain further information regarding thickness and composition of the AlN thin films. The Al/N ratio has been determined, while the oxygen content was found to be negligible. The substrate temperature influence on depth profile of AlN thin films as well as densities has been discussed. The advantages of using ion beam analysis techniques have been reviewed.

Local Environment of Sc and Y Dopant Ions in Aluminum Nitride Thin Films.

The local environments of Sc and Y in predominantly ⟨002⟩ textured, Al1-xDoxN (Do = Sc, x = 0.25, 0.30 or Y, x = 0.25) sputtered thin films with wurtzite symmetry were investigated using X-ray absorption (XAS) and photoelectron (XPS) spectroscopies. We present evidence from the X-ray absorption fine structure (XAFS) spectra that, when x = 0.25, both Sc3+ and Y3+ ions are able to substitute for Al3+, thereby acquiring four tetrahedrally coordinated nitrogen ligands, i.e., coordination number (CN) of 4. On this basis, the crystal radius of the dopant species in the wurtzite lattice, not available heretofore, could be calculated. By modeling the scandium local environment, extended XAFS (EXAFS) analysis suggests that when x increases from 0.25 to 0.30, CN for a fraction of the Sc ions increases from 4 to 6, signaling octahedral coordination. This change occurs at a dopant concentration significantly lower than the reported maximum concentration of Sc (42 mol % Sc) in wurtzite (Al, Sc)N. XPS spectra provide support for our observation that the local environment of Sc in (Al, Sc)N may include more than one type of coordination.

A strong dependence of sputtering power on c‐axis oriented aluminium nitride on Si (111): A structural and electrical study

AbstractGrowing and controlling the c‐axis orientation of the aluminium nitride (AlN) thin film on unheated Si (111) substrates using reactive magnetron sputtering are challenging. Sputtering parameters such as nitrogen concentration and sputtering power were effectively tuned to grow the c‐axis oriented AlN thin film. The results show that a low concentration of (25%) N2 is enough for forming AlN at a reduced flow rate, whereas a higher flow rate requires a higher concentration of N2. Low concentration with a low flow rate is preferred to grow AlN at low temperature and power. The poor crystallinity of AlN with (100) orientation was improved by varying the power from 75 to 175 W. The X‐ray diffraction results confirmed the improvisation of crystallinity of grown AlN films and indicated the strong dependence of preferred c‐axis orientation on sputtering power. Also, the dependence of sputtering power on microstrain and stress was analysed. The surface morphology study by field emission scanning electron microscopy and topography measured by an atomic force microscope shows a dependence on sputtering power. The high c‐axis orientation was observed at 175 W with low surface roughness, low leakage current density (2 × 10−9 A/cm2) and low dielectric constant (6.8).

Surface chemistry in atomic layer deposition of AlN thin films from Al(CH3)3 and NH3 studied by mass spectrometry

Mass spectrometry shows that the surface chemistry in ALD of AlN is dominated by reductive elimination and ligand exchange.

Influence of growth temperature on the properties of aluminum nitride thin films prepared by magnetron sputter epitaxy

High quality, uni-polar, epitaxial AlN with minimum oxygen content promises excellent surface acoustic wave and bulk acoustic wave resonator characteristics such as high electromechanical coupling coefficient and power handling capabilities, which is particularly useful for RF filter applications. By systematically varying the growth temperature, the study investigates its impact on the oxygen levels, defect states, and crystallographic texture of the AlN thin films using a combination of atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, spectroscopic ellipsometry, scanning transmission electron microscopy, as well as room temperature and temperature dependent I–V measurements. The research demonstrates that the films grown at a temperature of 700°C exhibit the most favorable results. These films exhibit the lowest oxygen levels, possess epitaxial growth, and display the highest crystalline quality (XRD AlN 0002 ω−FWHM=1.3°). Additionally, these films demonstrate a significant reduction in sub-bandgap absorption. By comparing the cathode current measured during deposition, we suggest that the presence of an impurity layer formed during idle time between depositions as a possible source of oxygen in the sputter chamber. In addition, the study presents a possible model to explain the mixed polarity observed in AlN and proposes various ways to achieve uni-polar AlN on silicon substrates.

Enhancing piezoelectric response in (002)-Oriented TaxAl(1−x)N films by magnetron-sputtering composition-tunable AlTa alloys

Aluminum nitride (AlN) films suffer from poor piezoelectric response due to the local hexagonal symmetry of wurtzite crystal structure. The fundamental mechanism of improving piezoelectric response in Ta-doped AlN thin films has not been fully understood. Here, we report that composition-tunable AlTa alloys, instead of Al and Ta individual sputtering targets, can be employed to deposit (002)-oriented TaxAl(1-x)N films by magnetron-sputtering, where Ta-doping ratio is tunable by changing Al:Ta composition ratio of the AlTa alloys. Importantly, Ta-doping enhances spontaneous polarization to improve piezoelectricity significantly, as suggested by a threefold increase of the effective piezoelectric coefficients (d33,eff) upon doping 7 at.% Ta. A better alignment of spontaneous polarization for stronger piezoelectric response originates from the reduction in point defects and crystalline disorder in the reduced-symmetrical wurtzite structure by doping Ta in the TaxAl(1−x)N. This study provides significant insights in improving piezoelectric response in AlN for the development of high-performance film-bulk-acoustic-resonators.

Structural and electrical correlation in aluminum nitride thin films grown by plasma enhanced atomic layer deposition as interface insulating layers on silicon carbide (4H-SiC)

(0001) oriented aluminum nitride (AlN) thin films have been grown by plasma enhanced atomic layer deposition (PE-ALD) on silicon carbide (4H-SiC) substrates. During different PE-ALD processes, the ammonia (NH3) plasma pulsing time has been varied and its effect on the microstructure and on the orientation degree of the AlN layers has been monitored. Structural characterization by Transmission Electron Microscopy (TEM) showed that the crystalline structure of the deposited films was strongly dependent on the NH3-plasma pulsing, so that different polymorphic structures were observed. In particular, both processes resulted in wurtzite AlN structure for few nanometers at the interface with the 4H-SiC substrate, while upon increasing thickness a poly-crystalline wurtzite phase was obtained by short-pulse NH3-plasma, whereas longer plasma exposure resulted in a mixture of wurtzite and zincblende defective phases. Phase formation mechanism were discussed and electrical nanoscopic characterization by conductive atomic force microscopy showed a clear correlation between the different AlN crystalline phases and the insulating properties.

High Thermal Conductivity of Submicrometer Aluminum Nitride Thin Films Sputter-Deposited at Low Temperature.

Aluminum nitride (AlN) is one of the few electrically insulating materials with excellent thermal conductivity, but high-quality films typically require exceedingly hot deposition temperatures (>1000 °C). For thermal management applications in dense or high-power integrated circuits, it is important to deposit heat spreaders at low temperatures (<500 °C), without affecting the underlying electronics. Here, we demonstrate 100 nm to 1.7 μm thick AlN films achieved by low-temperature (<100 °C) sputtering, correlating their thermal properties with their grain size and interfacial quality, which we analyze by X-ray diffraction, transmission X-ray microscopy, as well as Raman and Auger spectroscopy. Controlling the deposition conditions through the partial pressure of reactive N2, we achieve an ∼3× variation in thermal conductivity (∼36-104 W m-1 K-1) of ∼600 nm films, with the upper range representing one of the highest values for such film thicknesses at room temperature, especially at deposition temperatures below 100 °C. Defect densities are also estimated from the thermal conductivity measurements, providing insight into the thermal engineering of AlN that can be optimized for application-specific heat spreading or thermal confinement.

Enhancement of Piezoelectric Response in Yttrium Aluminum Nitride (YxAl1‐xN) Thin Films

Alloying rare earth elements into aluminum nitride (AlN) thin films to increase the piezoelectric response has gained a lot of attention in the past few years. Many rare earth elements are investigated in which scandium alloying resulted in the highest piezoelectric response for AlN. At the same time, researchers have also theoretically explored yttrium alloying as a feasible and economical alternative to scandium. Herein, for the first time, experimentally, the increase of the piezoelectric response of sputter‐deposited YxAl1–xN thin films as a function of increasing yttrium concentration as predicted by density functional theory calculations is demonstrated. Using differently manufactured targets, YxAl1–xN thin films with four different yttrium alloying concentrations (9, 12, 15, and 20 at%). are synthesized. Detailed thin‐film analysis is carried out and the highest value of d33 measured is 12 pC N−1 for Y0.2Al0.8N, which is a 250% increase compared to pure AlN. Even more, the Young's modulus decreases with increasing yttrium concentration in excellent agreement with theoretical predictions. Finally, Y0.15Al0.85N and Y0.2Al0.8N layers show high crystalline stability in pure oxygen environment up to 800 °C, demonstrating high oxidation resistance even under harsh environmental conditions.

Improvement of the piezoelectric response of AlN thin films through the evaluation of the contact surface potential by piezoresponse force microscopy

Sputtered aluminum nitride (AlN) thin films were characterized by Piezoresponse Force Microscopy (PFM) technique using a methodology to decrease the contribution of the electrostatic forces to obtain a pure piezoelectric response. Our method is based on the sweeping of the DC voltage applied to the Atomic Force Microscope (AFM) tip under a fixed AC field to evaluate the contact surface potential difference (VCPD) between the tip and the sample used to measure the proper AlN piezoelectric coefficient (d33,eff), minimizing the electrostatic contribution. Kelvin probe Force Microscopy (KPFM) was employed as reference standard technique to measure the surface potential, confirming the reliability of the proposed experimental procedure on ceramic piezoelectric films, and simultaneously overcoming the disadvantages of the KPFM technique. The capability to tune surface potential of materials over a wide range of values opens new perspectives for the design of devices with changeable surface potential.

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.

Recent Progress in Polymeric Flexible Surface Acoustic Wave Devices: Materials, Processing, and Applications

Recently, there has been a remarkable increase in interest in piezoelectric thin films, particularly zinc oxide (ZnO) and aluminum nitride (AlN), deposited on flexible polymeric substrates. This is due to the rapid expansion of fields such as robotics, wearable devices, and the Internet of Things (IoT). These thin‐film layered structures have a wide range of applications, such as energy harvesting, sensing and biosensing, microfluidic sorting, pumping and mixing, and telecommunications. One of the most promising platforms is piezoelectric acoustic‐based thin‐film devices on inexpensive, recyclable, or disposable polymeric substrates. Their reliability, reproducibility, conformability, small size, and wireless control capabilities make them a leading candidate for the development of new wireless, fully automated, and digitized microsystems for IoT and wearable devices. This review examines recent advancements in the engineering of ZnO and AlN thin films on polymeric, flexible, and conformable surface acoustic wave (SAW) devices. Topics covered include the materials used, piezoelectric film deposition, device fabrication, and the latest applications of this technology as sensors and actuators.