Tsuyoshi Yokoyama

Effect of Mg and Zr co-doping on piezoelectric AlN thin films for bulk acoustic wave resonators

This paper reports the crystal structures and piezoelectric properties of Mg and Zr co-doped AlN (MgZr-doped AlN) thin films. MgZr-doped AlN thin films on Si (100) substrates were fabricated by using a radio-frequency magnetron reactive cosputtering system. The relations between dopant concentration and the crystal structures measured by X-ray diffraction, transmission electron microscopy, and the piezoelectric constant d33 measured with a piezometer were investigated. Up to total Mg and Zr concentrations of 34.8 at%, the MgZr-doped AlN films maintained the c-axis-oriented wurtzite structure and were likely formed by substituting Mg and Zr atoms into Al atom sites. The d33 of the MgZr-doped AlN with a total Mg and Zr concentration (Mg + Zr) of 34.8 at% was about three times that of pure AlN. The experimental results on the relation between total Mg and Zr concentration and the crystal structure and the d33 were in close agreement with the results of first-principles calculations. Finally, thin film bulk acoustic wave resonators (FBARs) that used MgZr-doped AlN as a piezoelectric thin film were fabricated and compared with the AlN-based FBAR. A total Mg and Zr concentration of 13 at% was found to improve the electromechanical coupling coefficient of AlN from 7.1% (for pure AlN) to 8.5%. The results from this study suggest that the MgZr-doped AlN films have potential as piezoelectric thin films for wideband and high-frequency RF applications.

Highly piezoelectric co-doped AlN thin films for bulk acoustic wave resonators

In this paper, the piezoelectric properties of Mg and Zr co-doped AlN (MgZr doped AlN) thin films are reported. MgZr doped AlN thin films were prepared on Si (100) substrates with a radio frequency magnetron reactive cosputtering system. The crystal structures and piezoelectric constants d33 of the films were investigated as a function of their concentrations, which was measured by X-ray diffraction and with a piezometer. The d33 of the MgZr doped AlN at total Mg and Zr concentrations of 35 atomic % was about three times larger than that of pure AlN. The experimental results of the crystal structure and d33 as a function of total Mg and Zr concentrations were in very close agreement with first-principle calculations. Finally, thin film bulk acoustic wave resonators (FBARs) that used MgZr doped AlN as a piezoelectric thin film were fabricated and compared with the AlN based FBAR. As a result, the electromechanical coupling coefficient improved from 7.1 to 8.5% with the Mg and Zr concentration at 13atomic % doped into AlN. The results from this study suggest that the MgZr doped AlN films have potential as a piezoelectric thin film for wide band and high frequency RF applications.

Analysis on electromechanical coupling coefficients in AlN-based bulk acoustic wave resonators based on first-principle calculations

We discuss the effect of aluminum nitride (AlN) film stress on the electromechanical coupling coefficient (k2) of film bulk acoustic resonators (FBAR) based on first-principle calculations and experiments. Our calculations indicate that the compressive stress of AlN film induces the expansion of the c-axis lattice constant and affected the piezoelectric constant of AlN. We found that the piezoelectric constant of AlN and k2 of FBAR estimated using first-principle calculation increase with decreasing compressive stress of AlN films. We also confirmed from experiments that the piezoelectric constant increased from 6.5 to 7.1pC/N due to the decrease in compressive stress. Finally, we fabricated air-gap type FBAR that have different AlN film stresses. As a result, k2 improved from 6.0 to 6.5% by controlling the film stress from -1200 to 0MPa.

Highly piezoelectric co-doped AlN thin films for wideband FBAR applications

In this paper, we report new piezoelectric materials composed of charge-compensated co-doped (Mg, β) x Al 1−x N (β = Zr or Hf). The effect of the dopant element into AlN on chemical stability, crystal structure, and piezoelectric property of co-doped AlN was determined on the basis of first-principles calculation, and the theoretical piezoelectric property was confirmed by experimentally depositing thin films of magnesium (Mg) and zirconium (Zr) co-doped AlN (Mg-Zr-doped AlN). The Mg-Zr-doped AlN thin films were prepared on Si (100) substrates by a triple-radio-frequency magnetron reactive co-sputtering system. The crystal structures and piezoelectric coefficients (d 33 ) of the films were investigated as a function of their concentrations, which were measured by X-ray diffraction and a piezometer. The investigation results show that d 33 of Mg-Zr-doped AlN at total Mg and Zr concentrations (both expressed as x) of 0.35 is 280% larger than that of pure AlN. The experimentally measured parameter of the crystal structure and d 33 of Mg-Zr-doped AlN (plotted as functions of total Mg and Zr concentrations) are in very close agreement with the corresponding values obtained by first-principle calculations. Thin film bulk acoustic wave resonators (FBARs) employing (Mg,Zr) 0.13 Al 0.87 N and (Mg,Hf) 0.13 Al 0.87 N as a piezoelectric thin film were fabricated, and their resonant characteristics were evaluated. As a result, the measured electromechanical coupling coefficient was found to increase from 7.1% (for pure AlN) to 8.5% for Mg-Zr-doped AlN and 10.0% for Mg-Hf-doped AlN. These results indicate that co-doped (Mg, β) x Al 1−x N (β = Zr or Hf) films have potential as piezoelectric thin films for wideband RF applications.

Temperature behavior of sound velocity of fluorine-doped vitreous silica thin films studied by picosecond ultrasonics

Vitreous silica (v-SiO2) shows anomalous temperature dependence of velocity, including positive temperature coefficient of velocity (TCV) and velocity minimum around 70u2009K. The former characteristic allows its application in acoustic-resonator devices as a temperature compensating material. In this paper, we study the temperature dependence of velocity of fluorine-doped v-SiO2 (v-SiO2–xFx) thin films using picosecond ultrasonic spectroscopy. To correct the temperature increase caused by irradiation with light pulses, we calculated the steady temperature increase in the measuring volume with a finite volume method, considering the temperature dependence of thermal conductivity, and find that temperature in the measurement region remains high even when the back surface is cryogenically cooled. Using the corrected temperature, we determine TCV of v-SiO2–xFx thin films for 0<x<0.264, which increases as x increases and is smaller than reported bulk values by a factor of 0.5−0.7. The velocity minimum is absent f...

Highly piezoelectric MgZr co-doped aluminum nitride-based vibrational energy harvesters [Correspondence]

The first MgZr co-doped AlN-based vibrational energy harvester (VEH) is presented. (MgZr)AlN, which is a new class of doped AlN, provides high piezoelectricity and cost advantage. Using 13%-(MgZr)-doped AlN for micromachined VEHs, maximum output power of 1.3 μW was achieved with a Q-factor of 400 when resonant frequency, vibration acceleration, load resistance were 792 Hz, 8 m/s2, and 1.1 MΩ, respectively. Normalized power density was 8.1 kW.g-2.m-3. This was one of the highest values among the currently available piezoelectric VEHs.

Acoustic properties of co-doped AlN thin films at low temperatures studied by picosecond ultrasonics

(Mg0.5Zr0.5)xAl1?xN and (Mg0.5Hf0.5)xAl1?xN thin films are AlN-base piezoelectric materials, and their piezoelectric coefficients are higher than those of pure AlN, being promising materials for acoustic devices. However, their acoustic properties remain unknown because of measurement difficulty for deposited thin films. In this study, we measure their longitudinal-wave elastic constants C33 and their temperature coefficients using picosecond ultrasound spectroscopy for 0 < x < 0.13; we obtain C33 = 398.2 ? 0.7 GPa for pure AlN, and it largely decreases by doping Mg, Zr, and Hf, leading to a minimum values of 316.8 ? 1.6 GPa for (Mg0.5Zr0.5)0.126Al0.874N.

Dopant concentration dependence of electromechanical coupling coefficients of co-doped AlN thin films for BAW devices

In this paper, we present piezoelectric co-doped (Mg,Hf)_xAl_1-xN thin films, where the dopant concentration x is in the range of 0-0.13. The (Mg,Hf)_xAl_1-xN thin films were prepared on Si (100) substrates with a dual magnetron with AC power applied between two ring targets. The compositions of the targets were pure Al and Al-Mg-Hf metals. The composition of the films was controlled by adjusting a DC bias power between the two ring targets. The crystal structures and piezoelectric coefficients d₃₃ of the films were investigated by X-ray diffraction (XRD) and with a piezometer. XRD measurement showed that the increases of concentration x did not affect the c-axis orientation of AlN and induced a decrease of lattice constant ratio c/a. The d₃₃ of the (Mg,Hf)_xAl_1-xN thin films increases as the concentration x increases. The d₃₃ of the (Mg,Hf)_xAl_1-xN and our previously reported (Mg,Zr)_xAl_1-xN have the same linear relationship in respect to the change of the c/a. Furthermore, the film bulk acoustic resonators (FBARs) using these films were fabricated and characterized as a function of the concentration x. The electromechanical coupling coefficients k² of fabricated FBARs increased in proportion to the concentration x. These improved k² are the direct consequence of the increased piezoelectric constant e₃₃ in conjunction with the decreased elastic constant C₃₃. These behaviors of the (Mg,Hf)_xAl_1-xN are consistent with those of Sc_xAl_1-xN, indicating that the Mg-Hf co-doping and Sc doping have the same effect as improving of the piezoelectric properties and k² of AlN.

Piezoelectric micro energy harvesters employing advanced (Mg,Zr)-codoped AlN thin film

We report the new doped-AlN thin film, (Mg,Zr)AlN, based micro energy harvester. By co-doping Mg and Zr into AlN crystal, (Mg,Zr)AlN shows giant piezoelectricity and preserves low permittivity. (Mg,Zr)AlN has higher figure of merit (FOM = e₃₁²/(ε₀ε)) than conventional PZT. The 13 at.%-(Mg,Zr)AlN had the experimental FOM of up to 16.7 GPa. The micromachining harvester provided the high normalized power density of 3.72 mW.g^-2.cm^-3. This achievement was 1.5-fold increase compared to state of the art.