Sc-doped AlN Research Articles

Piezoelectric MEMS Energy Harvester for Low-Power Applications

With the global market value of sensors on the rise, this paper focuses on the fabrication and testing of a proof-of-concept piezoelectric energy harvester which is able to harvest mechanical energy from the ambient environment and convert it into electrical energy in order to power wireless sensor networks. We focused on obtaining a new device structure based on a comb-type array of piezoelectric MEMS cantilevers (2 × 10) for a resonant frequency in the environmental application domain (a few hundred Hz) and a chip area of only 1 cm2. The configuration of the lead-free piezoelectric cantilever consists of a Si substrate, a pair of Ti-Pt electrodes and a sputtered piezoelectric layer of 12% Sc-doped AlN with a thickness of 1000 nm, a dielectric constant of ~13 and e31,f = 1.3 C/m2. At a resonant frequency of 465.2 Hz and an acceleration of 1 g, the maximum value for the collected power was 2.53 µW for an optimal load resistance of 1 MΩ resulting in a power density of 60.2 nW/mm3 for the unpacked device, without taking into account the vibration volume. By increasing the excitation acceleration to 2 g RMS and using LTC3588-1 for the power circuitry we were able to obtain a stabilized output voltage of 1.8 V.

Low temperature sputtering deposition of Al1−xScxN thin films: Physical, chemical, and piezoelectric properties evolution by tuning the nitrogen flux in (Ar + N2) reactive atmosphere

This work investigates the physical properties of Al1−xScxN thin films sputtered at low temperatures by varying the process conditions. Specifically, the films were deposited at room temperature by applying a radio frequency power equal to 150 W to an AlSc alloy (60:40) target, varying the nitrogen flux percentage in the (Ar + N2) sputtering atmosphere (30%, 40%, 50%, and 60%) and keeping constant the working pressure at 5 × 10−3 mbar. The structural and chemical properties of the Al1−xScxN films were studied by x-ray diffraction and Rutherford backscattering spectrometry techniques, respectively. The piezoelectric response was investigated by piezoresponse force microscopy. In addition, the surface potential was evaluated for the first time for Sc-doped AlN thin films by Kelvin probe force microscopy, providing piezoelectric coefficients free from the no-piezoelectric additional effect to the mechanical deformation, i.e., the electrostatic force. By alloying AlN with scandium, the piezoelectric response was strongly enhanced (up to 200% compared to undoped AlN), despite the low deposition temperature and the absence of any other additional energy source supplied to the adatoms during thin film growth, which generally promotes a better structural arrangement of polycrystalline film. This is a strategic result in the field of microelectromechanical systems completely fabricated at low temperatures.

Measured optical losses of Sc doped AlN waveguides.

Although Sc doped AlN (ScAlN) has been used extensively in micro-electro-mechanical systems (MEMS) devices and more recently in optical devices, there have not been thorough studies of its intrinsic optical losses. Here we explore the optical losses of the Sc0.30Al0.70N waveguide system by observing racetrack resonator waveguide quality factors. Using a partial physical etch, we fabricate waveguides and extract propagation losses as low as 1.6 ± 0.3 dB/cm at wavelengths around 1550 nm, mostly dominated by intrinsic material absorption from the Sc0.30Al0.70N thin film layer. The highest quality factor of the resonators was greater than 87,000. The propagation loss value is lower than any value previously published and shows that this material can be broadly used in optical modulators without significant loss.

Multiphysics Modeling and Analysis of Sc-Doped AlN Thin Film Based Piezoelectric Micromachined Ultrasonic Transducer by Finite Element Method.

This paper presents a Piezoelectric micromechanical ultrasonic transducer (PMUT) based on a Pt/ScAlN/Mo/SiO2/Si/SiO2/Si multilayer structure with a circular suspension film of scandium doped aluminum nitride (ScAlN). Multiphysics modeling using the finite element method and analysis of the effect of different Sc doping concentrations on the resonant frequency, the effective electromechanical coupling coefficient (keff2) and the station sensitivity of the PMUT cell are performed. The calculation results show that the resonant frequency of the ScAlN-based PMUT can be above 20 MHz and its keff2 monotonically rise with the increasing doping concentrations in ScAlN. In comparison to the pure AlN thin film-based PMUT, the static receiving sensitivity of the PMUT based on ScAlN thin film with 35% Sc doping concentration is up to 1.61 mV/kPa. Meanwhile, the static transmitting sensitivity of the PMUT is improved by 152.95 pm/V. Furthermore, the relative pulse-echo sensitivity level of the 2 × 2 PMUT array based on the Sc doping concentration of 35% AlN film is improved by 16 dB compared with that of the cell with the same Sc concentration. The investigation results demonstrate that the performance of PMUT on the proposed structure can be tunable and enhanced by a reasonable choice of the Sc doping concentration in ScAlN films and structure optimization, which provides important guidelines for the design of PMUT for practical applications.

Doping effects on the ferroelectric properties of wurtzite nitrides

Ferroelectric materials have been explored for a long time for easy integration with state-of-the-art semiconductor technologies. Doped wurtzite nitrides have been reported as promising candidates due to their high stability, compatibility, and scalability. We investigate doping effects on ferroelectric properties of Sc-doped AlN (AlScN) and B-doped AlN (AlBN) by first-principles methods. The energy barrier against polarization switching is observed to decrease with increasing doping concentration at low concentration ranges, which is the origin of the emerging ferroelectricity in doped AlN. Further increasing the doping concentration to a critical value, the ferroelectric wurtzite phase transforms into paraelectric phases (a rock salt phase for AlScN and a zinc blende phase for AlBN), making it invalid to decrease the coercivity by increasing the doping concentration. Furthermore, it is revealed that different nonpolar structures (a hexagonal phase for AlScN and a β-BeO phase for AlBN) appear in the ferroelectric switching pathway, generating different switching features in doped AlN. Our results give a microscopic understanding of the ferroelectricity in doped wurtzite materials and broaden the route to improve their ferroelectric properties.

Study of the Performance Enhancement of Sc-Doped AlN Super High Frequency Cross-Sectional Lamé Mode Resonators

The increasing use of mobile broadband requires new acoustic filtering technologies that can operate efficiently at frequencies above 6 GHz. Previous research has shown that AlN Super High Frequency (SHF) Cross-Sectional Lamé Mode resonators (CLMRs) can address this challenge, but their performance is limited by the piezoelectric strength of AlN. In this work, we explore the use of substitutional doping of Al in AlN with Sc to enhance the kt2 values of SHF CLMRs. Our results showed that the measured kt2·Qm product of Al72Sc28N CLMRs was four times greater than that of AlN CLMRs operating at the same frequency. Additionally, the measured fractional bandwidth (FWB) of Al72Sc28N 2nd order ladder filters was 4.13%, a fourfold improvement over AlN filters with the same design. We also discuss other aspects of the technology, such as power handling, losses, and spurious mode suppression, and identify potential areas for future research.

Influence of AlN/ScAlN piezoelectric multilayer on the electromechanical coupling of FBAR

Based on the truth, the electromechanical coupling in the top and bottom Mo electrodes caped AlN can be enhanced by replacing the Sc doped AlN (ScAlN). We tune the electromechanical coupling (kt2) of the film bulk acoustic resonator (FBAR) by varying the thickness ratio in AlN and ScAlN piezoelectric multilayer. We found that the electromechanical coupling of the piezoelectric-multilayer FBAR is area size dependent. In order to investigate the multilayer electromechanical coupling in various thickness ratios, we propose a theory to simulate both materials piezoelectric constants e varying with the area size. In various thickness ratios, the effective piezoelectric constant of the AlN layer increases with the area of FBAR, but the ScAlN layer exhibits an optimal piezoelectric constant in increasing area. Eventually, we realized that increasing the significant piezoelectric constant is inevitable for reducing the size of FBARs.

Native and radiation induced point defects in AlN and Sc-doped AlN

We have performed first-principles calculations to investigate the electronic structure, configurations, formation, and binding energies of native and radiation induced point defects in pristine and Sc-doped wurtzite AlN. For the native defects, the nitrogen vacancy has the lowest formation energy in $p$-type material while the aluminum vacancy has the lowest formation energy in $n$-type material which is consistent with the previous studies. Several interstitial defect structures were modeled for Al, N, and Sc atoms. The effects of charge state on their relative stability were investigated. The binding energy of Sc with point defects was calculated and found to be dependent strongly on the defect type and charge state. The results obtained are discussed in light of the possible Sc effects on the radiation damage evolution in AlN. Thus the attraction of Sc atom to N vacancy and both Al and N interstitials reduces their mobility and increases Frenkel pair recombination distance.

(Invited) Ferroelectric Materials Study Using Materials Informatics and First-Principles Calculation-a Computational Search for Wurtzite-Structured Ferroelectrics with Low Coercive Voltages

In this talk, I will explain our resent achievements for ferroelectric materials studies using first-principles calculations and materials informatics. Especially focusing on wurtzite-structured ferroelectrics materials. Ferroelectricity has recently been observed in wurtzite-structured Sc-doped AlN thin films, five years after our initial prediction of ferroelectricity in wurtzite compounds based on first-principles calculations. The thin films, however, exhibited a much higher coercive voltage (3 MV/cm) than that of conventional perovskite-structured ferroelectric material PbTiO3, making it difficult to switch the films’ polarity and limiting their practical application. To identify tetrahedral ferroelectric materials with low coercive voltages, we have carried out a wider exploration of candidate binary compounds, from halides to chalcogenides to pnictogenides, using first-principles methods. Our calculations results are summarized in Fig.1. The overall trend is for polarization switching barriers to decrease with decreasing anion-to-cation radius ratio, with the lowest barriers found in monovalent compounds such as the copper and silver halides; e.g., CuCl is calculated to have a switching barrier of 0.17 eV/f.u. and AgI of 0.22 eV/f.u., values similar in magnitude to that of PbTiO3 (0.20 eV/f.u.). Applying an epitaxial tensile strain in the (0001) plane is also calculated to be effective for lowering the potential barrier further, with barriers in both AgI and CuCl decreasing to 0.04 eV/f.u. when a 5% in-plane expansion is applied. The results suggest that tetrahedral ferroelectrics with moderate coercive voltages (below 100 kV/cm) should be achievable. Figure 1

Impact of Deposition Temperature on Crystal Structure and Ferroelectric Properties of (Al1−xScx)N Films Prepared by Sputtering Method

Switchable Ferroelectric (Al1−xScx)N Films In article number 2100302 by Hiroshi Funakubo and co-workers, the deposition temperature dependence of the crystal structure and ferroelectricity for Sc-doped AlN [(Al0.78Sc0.22)N] films is investigated. The background of the cover image is the XRD 2θ-Ψ mapping of AlScN films. Two crystal structures show polarization switching models of AlScN. P-E hysteresis response is ascertained for films deposited without heating substrates.

Electronic Structure of Ternary Alloys of Group III and Rare Earth Nitrides.

Electronic structures of ternary alloys of group III (Al, Ga, In) and rare earth (Sc, Y, Lu) nitrides were investigated from first principles. The general gradient approximation (GGA) was employed in predictions of structural parameters, whereas electronic properties of the alloys were studied with the modified Becke–Johnson GGA approach. The evolution of structural parameters in the materials reveals a strong tendency to flattening of the wurtzite type atomic layers. The introduction of rare earth (RE) ions into Al- and In-based nitrides leads to narrowing and widening of a band gap, respectively. Al-based materials doped with Y and Lu may also exhibit a strong band gap bowing. The increase of a band gap was obtained for GaScN alloys. Relatively small modifications of electronic structure related to a RE ion content are expected in GaYN and GaLuN systems. The findings presented in this work may encourage further experimental investigations of electronic structures of mixed group III and RE nitride materials because, except for Sc-doped GaN and AlN systems, these novel semiconductors were not obtained up to now.

NDIR CO2 gas sensing using CMOS compatible MEMS ScAlN-based pyroelectric detector

We demonstrate NDIR CO2 gas sensing using CMOS compatible MEMS ScAlN-based pyroelectric detectors. The ScAlN-based pyroelectric detectors are fabricated using 8-inch wafer level technology with 12 % Sc-doped AlN deposited at a temperature of ∼200 °C. Together with a blackbody thermal emitter, a 10 cm long enclosed gas channel with only inlet and outlet holes connected to tubings, and testing using 2 different reference gases (N2 and synthetic air), measurements show voltage signal drop due to CO2 gas absorption at the 4.26 μm wavelength at CO2 gas concentrations ranging from 5000 ppm down to 25 ppm. The signal change due to the CO2 gas response ranges from ∼2% at 100 ppm CO2 concentration to ∼40 % at 5000 ppm CO2 gas concentration for both CO2 gas measured in N2 and in synthetic air. CO2 gas response times are also measured for CO2 gas in N2 and in synthetic air at concentrations of 5000 ppm, 1000 ppm and 400 ppm. The gas response times measured around 2 s and lower. Introduction of humidity show some minor effect (<3%) to the CO2 gas response and seems most perturbed at 10 % relative humidity. To the best of our knowledge, this is the first demonstration using ScAlN-based pyroelectric detectors in NDIR CO2 gas sensing, towards practical sensor applications. The results obtained show promise in using CMOS-compatible MEMS ScAlN-based pyroelectric detectors for NDIR gas sensing, opening up possibilities for low cost, wafer-level, monolithic NDIR gas sensors with small footprint integrated with CMOS circuits.

A computational search for wurtzite-structured ferroelectrics with low coercive voltages

Ferroelectricity has recently been observed in wurtzite-structured Sc-doped AlN thin films, five years after our initial prediction of ferroelectricity in wurtzite compounds based on first-principles calculations. The thin films exhibited a much higher coercive voltage (3 MV/cm) than that of conventional perovskite-structured ferroelectric material PbTiO3, however, making it difficult to switch the films’ polarity and limiting their practical application. To identify tetrahedral ferroelectric materials with low coercive voltages, we have carried out a wider exploration of candidate binary compounds, from halides to chalcogenides to pnictogenides, using first-principles methods. The overall trend is for polarization switching barriers to decrease with decreasing anion-to-cation radius ratio, with the lowest barriers found in monovalent compounds such as the copper and silver halides; e.g., CuCl is calculated to have a switching barrier of 0.17 eV/f.u. and that of AgI is 0.22 eV/f.u., values similar in magnitude to that of PbTiO3 (0.20 eV/f.u.). Applying an epitaxial tensile strain to the basal plane is also effective for lowering the potential barrier further, with barriers in both AgI and CuCl decreasing to 0.04 eV/f.u. when a 5% in-plane expansion is applied. The results suggest that tetrahedral ferroelectrics with moderate coercive voltages (below 100 kV/cm) should be achievable.

Thermodynamic assessment of the Al–Sc–N ternary system and phase-separated region of the strained wurtzite phase

Sc-doped AlN with the wurtzite structure has excellent performance as a piezoelectric material. The phase stability of the (Al, Sc)N wurtzite phase is very important in designing highly functional piezoelectric materials. In this work, we assessed the thermodynamic parameters of the Al–Sc–N ternary system by incorporating the results of first-principles calculations into the calculation of phase diagrams (CALPHAD) method. The calculated Sc–N binary and Al–Sc–N ternary phase diagrams were consistent with the reported experimental results, where the (Al, Sc)N wurtzite phase exhibited phase separation. Because Sc-doped AlN is normally deposited on a substrate, we also evaluated the stability of the (Al, Sc)N wurtzite phase by considering the strain energy generated between the substrate and the thin film. It was found that the tendency to phase separation can be suppressed by decreasing the thickness of the thin film, which is qualitatively consistent with experimental reports.

Ferromagnetic anisotropy in scandium-doped AlN hierarchical nanostructures

We experimentally fabricated uniformly distributed single-, double- and six-sided Sc-doped AlN (AlN:Sc) nanodendrites by a plasma-assisted direct current arc discharge method. The crystal structure, composition, morphological and microstructures were characterized, and the physical mechanisms governing the formation of AlN:Sc nanodendrites were examined. It was found that the morphology and the crystal orientation of the AlN nanodendrites can be controlled by tuning the N2 pressure. The incorporation of Sc induces native point defects in AlN that contribute to the growth of hierarchical structures and affect the room-temperature photoluminescence. The AlN:Sc nanodendrites exhibit room-temperature anisotropic ferromagnetism, and this is strongly depend on the orientation of the nanodendrites. This work provides a facile strategy to fabricate complex hierarchical AlN:Sc nanostructures with manipulated magnetic properties which may find promising applications in spintronic nanodevices.

Dopant concentration dependent magnetism of Sc-doped AlN nanowires

A series of room temperature ferromagnetic (RTFM) Al1-xScxN nanowires with Sc-dopant concentration x ranging from 0 to 4.7% were prepared by the direct reaction of different weight ratios of Al:Sc alloy with N2 gas using plasma assisted direct current (DC) arc discharge method. Powder X-ray diffraction (XRD) and Raman scattering spectra revealed that Sc incorporates into the AlN crystal lattice. With the increase of the Sc content, the wurtzite-structure AlN maintained with increasing volume of unit cell even when the Sc content reached 4.7 at.%, meanwhile, the Sc dopant concentration reached saturation, indicating the successful incorporation of Sc ions into AlN lattice. Scanning and transmission electron microscopy measurements show that the nanowires have uniform diameters of about 50–100 nm and are several tens of microns in length. Magnetic measurements revealed that the room temperature ferromagnetism increased with the increase of Sc dopant concentration and the ferromagnetism remained unchanged when the Sc content reached saturation.

Quantitative thickness measurement of polarity-inverted piezoelectric thin-film layer by scanning nonlinear dielectric microscopy

A quantitative measurement method for a polarity-inverted layer in ferroelectric or piezoelectric thin film is proposed. It is performed nondestructively by scanning nonlinear dielectric microscopy (SNDM). In SNDM, linear and nonlinear dielectric constants are measured using a probe that converts the variation of capacitance related to these constants into the variation of electrical oscillation frequency. In this paper, we describe a principle for determining the layer thickness and some calculation results of the output signal, which are related to the radius of the probe tip and the thickness of the inverted layer. Moreover, we derive an equation that represents the relationship between the output signal and the oscillation frequency of the probe and explain how to determine the thickness from the measured frequency. Experimental results in Sc-doped AlN piezoelectric thin films that have a polarity-inverted layer with a thickness of 1.5 µm fabricated by radio frequency magnetron sputtering showed a fairly good value of 1.38 µm for the thickness of the polarity-inverted layer.

Parameter Optimization for Preparing c-Oriented ScAlN Thin Films

Sc doped AlN (ScAlN) films with c-axis orientation have a very high acoustic velocity and other advantages which could be used for film bulk acoustic resonator (FBAR), surface acoustic wave (SAW) and other piezoelectric devices. In this paper, ScAlN films were prepared by reactive direct-current magnetron sputtering. The degree of preferential orientation (PO), the surface topography and cross-section views of ScAlN thin films grown on Si (100) substrates at various sputtering pressure, sputtering power and substrate temperature have been investigated by X-ray diffraction, atomic force microscopy and scan electron microscope respectively. Results show that the sputtering parameters have significant effects on the PO of ScAlN thin films. The optimum properties that the full width at half maximum (FWHM) of rocking curve of 1.8° and the RMS roughness of 1.811 nm were acquired with sputtering pressure of 0.45 Pa, sputtering power of 130 W, substrate temperature of 600 °C respectively.

Preparation of (100) oriented Sc-doped AlN film on flexible substrate under different pressures using RF reactive sputtering

Abstract Sc-doped AlN films with (100) orientation have high surface acoustic wave velocity and excellent optical properties. Sc-doped AlN thin films based on flexible substrate were prepared by RF reactive magnetron sputtering at pressure levels ranging from 0.3 Pa to 1.5 Pa. The sputtering rate, crystal quality, and electric properties of the Sc-doped AlN films were investigated. The results show that pressure greatly influences the preparation of Sc-doped AlN thin films. Under low pressure, (002) oriented Sc-doped AlN film was prepared; with increasing pressure, the fraction of the (100) plane increased. Furthermore, the electrical properties were closely related to the crystal quality, achieving a maximum resistivity of 3.5 × 1012 Ω · cm, a minimum leakage current of 0.65 × 10−8 A, and a maximum piezoelectric response of 5.2 pC · N−1.

Effects of temperature on PO and resistivity of ScAlN film

Sc doped AlN (ScAlN) films were prepared by dc reactive magnetron sputtering. The crystal quality, surface topography, cross-sectional view and the resistivity of ScAlN thin films grown on Si (100) substrates at various substrate temperatures from room temperature to 650°C have been investigated. Results show that the temperature has a significant effect on the properties of ScAlN thin films. According to the research, the crystal quality of ScAlN film first increases and then decreases, reaching the best crystalline state at 600°C. The best surface morphology and cross-sectional view of ScAlN thin film was obtained at 600°C. The resistivity first increased to a maximum value of 3·36 × 1012 Ω·cm and then decreased with the temperature increasing.