Aluminum Nitride Films Research Articles

Toward crack-free AlN growth on silicon (111) by introducing boron incorporated buffer layer via MOCVD

High-quality aluminum nitride (AlN) films on silicon substrates are crucial for various applications due to their inherent properties as wide-bandgap semiconductors, cost-effectiveness, and compatibility with silicon-based circuits. Nonetheless, producing high-quality and crack-free AlN on silicon presents significant challenges due to the stress caused by lattice and thermal expansion mismatches. This study introduces a method to mitigate these challenges by incorporating a boron precursor during the metalorganic chemical vapor deposition process to form a BAlN buffer layer. Analytical techniques, such as secondary ion mass spectrometry, atomic force microscopy imaging, XRD rocking curves, reciprocal space map, and Raman spectroscopy, indicate that the BAlN buffer layer promotes the enlargement of seed crystal size, which effectively delays AlN coalescence, mitigates accumulated tensile stress, and enhances the overall crystal quality. Employing this technique has produced a 520 nm thick, crack-free AlN film on silicon (111) with high crystal quality, achieving full width at half maximum values of only 0.2° and 0.3° for XRC (002) and (102), respectively.

Recent Advances in Aluminum Nitride (AlN) Growth by Magnetron Sputtering Techniques and Its Applications

This review explores the processes involved in enhancing AlN film quality through various magnetron sputtering techniques, crucial for optimizing performance and expanding their application scope. It presents recent advancements in growing AlN thin films via magnetron sputtering, elucidating the mechanisms of AlN growth and navigating the complexities of thin-film fabrication. Emphasis is placed on different sputtering methods such as DC, RF, pulsed DC, and high-power impulse DC, highlighting how tailored sputtering conditions enhance film characteristics in each method. Additionally, the review discusses recent research findings showcasing the dynamic potential of these techniques. The practical applications of AlN thin films, including wave resonators, energy harvesting devices, and thermal management solutions, are outlined, demonstrating their relevance in addressing real-world engineering challenges.

Исследование пьезоэлектрических свойств пленок нитрида алюминия, сформированных на слоях алюминия и молибдена для создания микроэлектронных ОАВ-резонаторов

The paper presents the results of AFM measurements of piezomodulus d33 in aluminum nitride films obtained by vacuum magnetron sputtering in a reactive gas atmosphere of argon and nitrogen. Dependences of piezomodule d33 on technological parameters are presented of sputtering AlN films, such as substrate temperature, magnetron discharge power, and the Ar/N2 ratio. Based on the analysis of the obtained data, the optimal technological modes of formation of aluminum nitride films have been determined aluminum nitride films for the formation of the piezoelectric layer of the FBAR.

Systematic approach for high piezoelectric AlN deposition

Aluminum nitride (AlN) is a wide band gap semiconductor with interesting piezoelectric properties for many applications, including micromechanical systems (MEMS), acoustic wave sensors or energy harvesting devices. The influence of DC reactive magnetron sputtering (DCRMS) deposition parameters on the structural, crystalline, and piezoelectric properties of AlN thin films are presented. The systematic approach of Design of Experiments (DoE) has been used to evaluate the role of magnetron power, nitrogen and argon flows on the deposition process and the film properties. Magnetron power and argon flow have resulted to be the parameters causing the most significant effects on the deposition rate. AlN films have been deposited with a high crystal quality, showing low values of FWHM (0.28) and c-axis orientation parallel oriented respect to the growth direction, as evidenced by X-ray diffraction (XRD) analysis and high-resolution transmission electron microscopy (HRTEM). These samples also showed a high piezoelectric coefficient (d33) of 5 pC/N for AlN thin films. This work highlights the importance of deposition parameters on the properties of the film and the important role that DoE play for its optimization with a minimum number of depositions.

Enhancing cardiovascular health monitoring: Simultaneous multi-artery cardiac markers recording with flexible and bio-compatible AlN piezoelectric sensors

Continuous monitoring of cardiovascular parameters like pulse wave velocity (PWV), blood pressure wave (BPW), stiffness index (SI), reflection index (RI), mean arterial pressure (MAP), and cardio-ankle vascular index (CAVI) has significant clinical importance for the early diagnosis of cardiovascular diseases (CVDs). Standard approaches, including echocardiography, impedance cardiography, or hemodynamic monitoring, are hindered by expensive and bulky apparatus and accessibility only in specialized facilities. Moreover, noninvasive techniques like sphygmomanometry, electrocardiography, and arterial tonometry often lack accuracy due to external electrical interferences, artifacts produced by unreliable electrode contacts, misreading from placement errors, or failure in detecting transient issues and trends. Here, we report a bio-compatible, flexible, noninvasive, low-cost piezoelectric sensor for continuous and real-time cardiovascular monitoring. The sensor, utilizing a thin aluminum nitride film on a flexible Kapton substrate, is used to extract heart rate, blood pressure waves, pulse wave velocities, and cardio-ankle vascular index from four arterial pulse sites: carotid, brachial, radial, and posterior tibial arteries. This simultaneous recording, for the first time in the same experiment, allows to provide a comprehensive cardiovascular patient's health profile. In a test with a 28-year-old male subject, the sensor yielded the SI = 7.1 ± 0.2 m/s, RI = 54.4 ± 0.5 %, MAP = 86.2 ± 1.5 mmHg, CAVI = 7.8 ± 0.2, and seven PWVs from the combination of the four different arterial positions, in good agreement with the typical values reported in the literature. These findings make the proposed technology a powerful tool to facilitate personalized medical diagnosis in preventing CVDs.

A High Q Lamb Wave Resonator with Dummy Toes Based on 20% ScAlN Film

Abstract The lamb wave resonator (LWR) based on aluminum nitride (AlN) films are preferred strategy for high-frequency filters in 5G communication due to the tunable electromechanical coupling and high Q value. However, since the low electromechanical coupling and piezoelectric coefficient of AlN films, conventional lamb filters are difficult to meet the requirements of high bandwidth and low loss. This work proposes a lamb wave resonator with dummy toes (DT-LWR) based on scandium-doped aluminum nitride (ScAlN) film to improve the quality factor(Q) meanwhile suppress the spurious modes. The processed DT-LWR was tested using a vector network analyser (VNA, Keysight N5222B), and the MBVD model was used to fit the Z parameters. The Q value of DT-LWR is 1437.23, while that of LWR is 1250.09. It is increased by 14.97%. The enhancements make the DT-IWR as an ideal for filters with minimal insertion loss.

Effects of sputtering pressure on microstructure and secondary electron emission properties of AlN film prepared by magnetron sputtering

In this paper, aluminum nitride (AlN) thin films were prepared by reactive radio-frequency magnetron sputtering at different sputtering pressures and the crystalline structure, surface morphology and secondary electron emission (SEE) properties of the AlN films were studied in detail. The experimental results reveal that AlN films present the preferred orientation of a-axis (100), and the grain size and surface roughness of the film decrease with the increase of sputtering pressure. Additionally, the SEE coefficient of the film reaches a maximum of 4.3 at 200 eV when the sputtering pressure is 0.55 Pa and is relatively stable under the continuous bombardment of an incident electron with the energy of 200 eV. Thus, the AlN thin film prepared under the optimized sputtering pressure has a relatively high and stable coefficient at a low incident electron energy, which promotes its potential application in the high-gain and long-life electron multipliers.

High-k2eff AlSc0.095N-based two-dimensional coupled mode resonators with transducer design toward 5G application

This paper presents the two-dimensional coupled mode resonators (TCMRs) based on 9.5 % scandium-doped aluminum nitride (AlScN) films. We report the design, fabrication, and characterization of 770 nm-thick AlSc0.095N TCMRs, which utilize the e31 and e33 piezoelectric coefficients to jointly excite the coupled mode with high electromechanical coupling coefficient (k2eff). The TCMRs with different electrical boundary conditions (TCMR-I and TCMR-II) are proposed, and the dependence of the resonant mode on the electrode period was studied. The effect of electrode duty factor (DF) on the resonator performance was explored, and both TCMR-Ⅰ and TCMR-II with DF = 0.5 exhibit the maximum k2eff value. Through measured characterization and simulation analysis, the effect of interdigitated transducer (IDT) thickness on resonator performance was studied. In addition, the equivalent electrical parameters of the prepared resonator were extracted using the MBVD equivalent circuit model. The Qs values of TCMR-Ⅰ and TCMR-II fabricated in this paper can reach 1134 and 165, and the k2eff values can reach 3.5 % and 7.86 %, respectively.

Investigation of Piezoelectric Properties of Wurtzite AlN Films under In-Plane Strain: A First-Principles Study

This research article presents a comprehensive first-principles study on the piezoelectric properties of Wurtzite Aluminum Nitride (AlN) films under in-plane strain conditions. By calculating the piezoelectric tensor coefficients (e33, e31, and e15), we investigate the variation patterns of these constants with respect to in-plane strain. Our results indicate significant changes in the piezoelectric constants within the range of in-plane strain considered, exhibiting a linear trend despite opposite trends for e33 compared to e31 and e15. This study highlights the extreme sensitivity of AlN films’ piezoelectric performance to in-plane strain, suggesting its potential as an effective means for tuning and optimizing the piezoelectric properties of AlN-based devices.

Design and Fabrication of High-Performance Piezoelectric Micromachined Ultrasonic Transducers Based on Aluminum Nitride Thin Films.

Ultrasound is widely applied in diverse domains, such as medical imaging, non-destructive evaluation, and acoustic communication. Piezoelectric micromachined ultrasonic transducers (PMUTs) capable of generating and receiving ultrasonic signals at the micrometer level have become a prominent technology in the field of ultrasound. It is important to enrich the models of the PMUTs to meet the varied applications. In this study, a series of PMUT devices featured with various top electrode configurations, square, circular, and doughnut, were designed to assess the influence of shape on the emission efficacy. It was demonstrated that the PMUTs with a circular top electrode were outperformed, which was calculated from the external acoustic pressure produced by the PMUTs operating in the fundamental resonant mode at a specified distance. Furthermore, the superior performance of PMUT arrays were exhibited through computational simulations for the circular top electrode geometries. Conventional microelectromechanical systems (MEMS) techniques were used to fabricate an array of PMUTs based on aluminum nitride (AlN) films. These findings make great contributions for enhancing the signal transmission sensitivity and bandwidth of PMUTs, which have significant potential in non-destructive testing and medical imaging applications.

Quantum features of low-energy photoluminescence of aluminum nitride films

Photoluminescence of aluminum nitride films at the below bandgap excitation has been studied. It has been found that low-energy (up to 2.02 eV) photoluminescence spectra of the AlN films contain a series of equidistant maxima, the intensities of which decrease with energy. Theoretical analysis has shown that the observed photoluminescence features may be caused by strong electron-phonon interaction (long-range interaction of electrons in the band gap with Al3+ ions in the lattice sites). This interaction presumably leads to appearance of quasi-particles in the band gap of AlN, which are a bound state of an electron with an ion in a crystal lattice site. Such quasi-particles have been called “elions”. The energy of an elion is quantized. An elion quantum is equal to the longitudinal optical phonon energy. The low-energy photoluminescence is based on the elion generation and subsequent annihilation mechanism.

Silver Nanowire Networks Coated with a Few Nanometer Thick Aluminum Nitride Films for Ultra-Transparent and Robust Heating Applications

Silver Nanowire Networks Coated with a Few Nanometer Thick Aluminum Nitride Films for Ultra-Transparent and Robust Heating Applications

Improving quality of AlN films on GaAs substrate via in-situ plasma pre-treatment in plasma enhanced atomic layer deposition

Using plasma-enhanced atomic layer deposition (PEALD), we achieved the low-temperature growth of aluminum nitride (AlN) films on GaAs substrate. The effect of in-situ plasma pre-treatment on AlN films growth has been thoroughly investigated in detail. After in-situ plasma pre-treatment, the surface roughness closely matched that of the substrate, the interfaces between AlN films and GaAs substrate were extremely sharp, and the film density increased from 3.05 g/cm3 to 3.15 g/cm3. Additionally, the oxygen content in the AlN films decreased from 35.5 % to 18.8 %. This indicates that in-situ plasma pre-treatment is an effective method for reducing impurity content and improving film quality.

Design and Optimization of Piezoelectric Pressure Sensor with AlN as piezo electric material for High-Temperature Application using COMSOL 5.3

Dynamic pressure sensors in contrast to static pressure sensors measure pressure changes in liquids or gases generated due to a blast, a propulsion or an explosion, where the temperature is normally high which is above 700°C[1]. Piezoelectric pressure sensors with their inherent advantage of direct transduction capability are drawing attention for high-temperature applications. Lead Zirconate Titanate (PZT) and Zinc Oxide (ZnO) are popular ferroelectric materials for Piezoelectric sensor applications. Aluminum Nitride (AlN) is a suitable candidate for high-temperature applications with its high melting point of 2673 K, the piezoelectric property remaining stable even up to 1423K, the energy band gap of 6.2eV, piezoelectric coefficient d33 of 7pC/N and pressure handling capacity up to 10 MPa. In this study, COMSOL Multiphysics 5.3 was used to analyse the pressure sensing capability of AlN film by optimizing the crystal orientation and the dimension of AlN in addition to studying suitability of using at high temperature. Also a comparison is done on the high temperature performance of pressure sensor using Silicon and Silicon Carbide as diaphragm.

Design and Fabrication of a Film Bulk Acoustic Wave Filter for 3.0 GHz-3.2 GHz S-Band.

Film bulk acoustic-wave resonators (FBARs) are widely utilized in the field of radio frequency (RF) filters due to their excellent performance, such as high operation frequency and high quality. In this paper, we present the design, fabrication, and characterization of an FBAR filter for the 3.0 GHz-3.2 GHz S-band. Using a scandium-doped aluminum nitride (Sc0.2Al0.8N) film, the filter is designed through a combined acoustic-electromagnetic simulation method, and the FBAR and filter are fabricated using an eight-step lithographic process. The measured FBAR presents an effective electromechanical coupling coefficient (keff2) value up to 13.3%, and the measured filter demonstrates a -3 dB bandwidth of 115 MHz (from 3.013 GHz to 3.128 GHz), a low insertion loss of -2.4 dB, and good out-of-band rejection of -30 dB. The measured 1 dB compression point of the fabricated filter is 30.5 dBm, and the first series resonator burns out first as the input power increases. This work paves the way for research on high-power RF filters in mobile communication.

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.

Simultaneously realizing reversal of piezoelectric coefficient and enhancement of piezoresponse by chromium-doping in aluminum nitride films

Polarity inversion is an interesting phenomenon in non-centrosymmetric wurtzite-structured aluminum nitride (AlN), which offers an important platform to establish acoustic devices with heteropolar junctions. However, previous studies showed that switching polarity generally resulted in only reversing the piezoelectric coefficient (d33) in AlN films. Here, we discovered that appropriate Cr-doping would not only allow to reverse d33 but also improve piezoelectric response in the c-axis oriented AlN films by co-sputtering dual targets of Cr and Al. Specifically, the d33 was reversed from +2.6 pC/N for the undoped AlN to −2.6 pC/N for the 6.2 at. % Cr-doped CrxAl(1−x)N films. As the Cr-doping ratio increased to 9.3 at. %, d33 was −7.0 pC/N, which was 1.7 times higher than that of the undoped AlN films. Independent PFM phase image measurement offered further evidence of the polarity inversion by comparing the undoped and 9.3 at. % Cr-doped CrxAl(1−x)N films. This work offers a simple doping strategy that allows for simultaneous reversal of piezoelectric coefficient and enhancement of piezoresponse. As a result, it establishes a promising foundation for the design and development of acoustic wave devices featuring heteropolar junctions.

Deep‐Ultraviolet Luminescence Properties of AlN

High‐resolution, low‐excitation photoluminescence (PL) spectroscopy is performed for unintentionally doped, silicon‐doped, and magnesium‐doped homoepitaxial aluminum nitride (AlN) films, using a wavelength‐tunable high‐repetition‐rate laser. The wavelength‐tunable laser is used to distinguish between the luminescence and scattering signals from AlN. Providing the high‐resolution, low‐excitation PL spectra, the current understanding of the deep‐ultraviolet luminescence properties of AlN is reviewed and potential assignments for the unknown luminescence lines and bands are discussed. Although previous studies have led to a consensus on the origins of some emission peaks and bands such as the neutral silicon donor‐bound exciton transition and free exciton transitions involving longitudinal optical phonons, it is shown that many of the emission peaks are still unidentified. The origins of all the emission peaks should be elucidated to enable control of the electronic and optoelectronic properties of AlN.

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.

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.