Acta Materialia 100 (2015) 81–89
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Acta Materialia
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Microstructure and piezoelectric response of YxAl1 xN thin films ⇑ P.M. Mayrhofer a, , H. Riedl b, H. Euchner b, M. Stöger-Pollach c, P.H. Mayrhofer b, A. Bittner a, U. Schmid a a Institute of Sensor and Actuator Systems, TU Wien, Floragasse 7, 1040 Vienna, Austria b Institute of Materials Science and Technology, TU Wien, Getreidemarkt 9, 1060 Vienna, Austria c University Service Center for Transmission Electron Microscopy (USTEM), TU Wien, Wiedner Hauptstrasse 8-10/052, 1040 Vienna, Austria article info abstract
Article history: Transition metal doping of aluminium nitride (AlN) type thin films was recently employed to increase the Received 6 July 2015 piezoelectric constants for application in micro electromechanical systems. YxAl1 xN thin films were Revised 7 August 2015 synthesized with varying x up to 11.6% by reactive co-sputtering from elemental Al and Y targets. Ab initio Accepted 11 August 2015 density functional theory studies up to x = 50% yttrium in YxAl1 xN suggest increasing piezoelectric constants d33 and d31 with increasing yttrium content. Experimental measurements of d33 yield increasing values for increasing yttrium content though, remaining below the prediction from ab initio Keywords: calculations. Detailed X-ray diffraction and transmission electron microscopy studies prove that, Yttrium aluminum nitride although the thin films are highly c-axis oriented, their nucleation at the Si (100) substrates leads to Piezoelectric the formation of an amorphous interface region with increasing Y. d33 PFM Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND Reactive sputtering license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction properties. The highest increase was achieved by incorporation of
scandium (Sc) into the AlN lattice with reported values for d33 of Piezoelectric actuation in micro electromechanical systems 27.6 pm/V for ScxAl1 xN thin films with x = 42.5% [18,19]. So far, (MEMS) based on thin films is a lively research area with high mar- doping of AlN with other elements did not yield a significant ket potential [1]. Group III nitride type thin films, foremost alu- increase of the piezoelectric constants. Yttrium (Y) may provide minium nitride (AlN), are increasingly applied as piezoelectric an alternative to Sc as dopant, with substantially lower material materials. Piezoelectric cantilevers with high Q-factors were costs. Ab initio calculations on the mixing enthalpy of YxAl1 xN recently used to measure viscosity and density of liquids [2,3]. showed higher phase stability of the wurtzite type crystal phase Other applications include accelerometers [4], surface and bulk as compared to cubic and hexagonal layered phases up to approx- acoustic resonators [5], atomic force microscopy (AFM) cantilevers imately x = 75% [20]. The same work reported on the fabrication of
[6], RF oscillators [7], contour-mode resonators for band pass filters YxAl1 xN thin films up to x = 22% via reactive co-sputter deposition. [8–10], or energy harvesting systems [11]. Reasons for applications They found strongly decreasing crystalline quality with yttrium of AlN in MEMS devices are its beneficial properties, such as high incorporation into AlN [20]. However, studies on the influence of temperature stability and high Curie temperature (Tc = 1150 °C) the sputter parameter on the crystal quality and measurements [12], low dielectric permittivity (er 10 [13]), or high sound veloc- of piezoelectric constants are yet to be reported. Furthermore, ity (v 6000 m/s) [14], as well as complementary metal–oxide–s the optical bandgap decreases from Eg = 6.2 eV down to Eg = 4.5 eV emiconductor (CMOS) compatibility. However, the piezoelectric for YxAl1 xN with x = 22% [21]. For protective layer applications, constants of AlN thin films are relatively low compared to other introduction of yttrium into the cubic TixAl1 xN system was piezoelectric materials. For example, values for the out-of-plane realized to enhance the oxidation resistance, finding an optimum piezoelectric strain constant d33 of reactively sputtered AlN films at low concentrations of about x = 2%. Higher yttrium concentra- of up to 6.5 pm/V are reported [15–17]. In that context transition tions tend to form wurtzite TixAl1 x yYyN, which exhibits deterio- metal doping of AlN proved to enhance the piezoelectric actuation rated oxidation resistance as well as mechanical properties [22]. potential of AlN, while retaining most other beneficial material Crucial requirement for an enhanced piezoelectric response in AlN type thin films is a highly columnar microstructure with a strong degree of c-axis orientation of the wurtzite type structure. X-ray diffraction (XRD) analysis or transmission electron micro- scopy (TEM) can be employed to determine the degree of ⇑ Corresponding author. c-axis orientation [22,23]. Hence, it is important to optimize the E-mail address: [email protected] (P.M. Mayrhofer). http://dx.doi.org/10.1016/j.actamat.2015.08.019 1359-6454/Ó 2015 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 82 P.M. Mayrhofer et al. / Acta Materialia 100 (2015) 81–89 deposition conditions to reach theoretical predicted values for the increase of the piezoelectric constants in transition metal alloyed AlN thin films. This works presents the impact of various deposition conditions on the formation of c-axis oriented wurtzite YxAl1 xN on Si (100), being a standard substrate in MEMS. Reactive DC magnetron co-sputtering from yttrium and aluminium targets enabled for a wide variety in yttrium concentration x. Furthermore, this work employs density functional theory based ab initio results to predict piezoelectric and elastic properties of wurtzite YxAl1 xN.
2. Theoretical results
The piezoelectric and elastic properties of YxAl1 xN are calcu- lated employing ab initio density functional theory (DFT). The latter Fig. 1. Elastic moduli along the a-axis: E1, and along the c-axis: E3 from DFT with cell-sizes of 32 atoms and 128 atoms. Fitted line (32 atoms) to guide the eye. approach was applied by Zukauskaite et al. who reported on the phase stability of wurtzite YxAl1 xN compared to cubic and layered for w-ScxAl1 xN thin films, where the Young’s modulus is still hexagonal crystal structures [20]. The wurtzite phase showed above 220 GPa at a scandium concentration of 40% [29]. The piezo- favourable mixing enthalpy up to an yttrium concentration of electric stress constant e31 increases only slightly while e33 75% of the metal sublattice, hence, the subsequent discussion in increases more pronounced with incorporation of Y, as shown in this work only considers the wurtzite phase. The simulations were Fig. 2(a). The piezoelectric strain tensor elements dij are calculated performed with the Vienna ab initio Simulation Package (VASP) from the results of the piezoelectric stress tensor eij and the elastic- [24], applying the projector augmented wave method and the ity matrix C , employing d ¼ e C 1 [30]. Consequently, the strong generalized gradient approximation (PAW-GGA) [25]. The special ij decrease of the elastic constants C together with the moderate quasi-random structure (SQS) approach was used to allow for a ij increase of e lead to a pronounced increase of the piezoelectric random distribution of the Y and Al atoms within the metal sublat- ij strain constants d31 and d33, as depicted in Fig. 2(b). Thus, ab initio tice of the solid solution w-YxAl1 xN [26]. For the determination of calculations predict an increase of d33 up to 700% and of d31 up to elastic and piezoelectric properties of w-Y Al N with lower Y x 1 x 600% when Y substitutes for Al within w-AlN. These values exceed concentration (6.25% and 12.5% of the metal sublattice) 4 4 2 the previously reported experimental and ab initio obtained data supercells with altogether 128 atoms and thus 64 atoms on the for d in Sc Al N of 500%. metal sublattice were investigated. For higher Y contents 33 x 1 x 2 2 2 (25%, 37.5% and 50%) supercells, containing 32 atoms were considered. This was necessary since replacing aluminium 3. Experimental details by yttrium results in a drastic increase of computational effort, making piezoelectric calculations extremely time consuming and Yttrium doped AlN films were synthesized via DC reactive hence, computationally expensive. Before accessing elastic and co-sputtering, applying an AJA Orion 5 laboratory-scaled piezo-electric properties, the respective supercells were optimized magnetron sputtering system equipped with a 3-inch Al and a by relaxing both lattice and atomic positions, using a 2 2 3 2-inch Y target (Plansee Composite Materials GmbH). Prior to the (4 4 3) C-centred k-point mesh and an energy cut-off of depositions, the targets as well as the substrates were ion etched 600 eV for the 128 (32) atom supercell. To obtain the elastic in a pure Ar atmosphere at a total pressure of 800 mPa for 5 and response of the respective supercells, the ground state structures 10 min, respectively. The depositions were performed in pure were strained, using the universal independent coupling strain nitrogen or mixed N2 to Ar atmosphere (ratio of 75/25 sccm/sccm approach [27]. Then, the strained configurations were again and 55/45 sccm/sccm) at a total pressure of 40 mPa and with relaxed, however, with fixed lattice constants, such that only an deposition temperatures of 250 °C and 800 °C. The aluminium internal relaxation of the atomic positions was conducted. From target was supplied with a power density of 11 W/cm2, whereas the corresponding stress–strain relationship the elastic tensor the yttrium target power density was varied between 5, 10, and was determined by a linear least square fit procedure using single 15 W/cm2 to obtain different chemical compositions. In addition, value decomposition [27]. Finally, the hexagonal projection of the the influence of different substrate potentials (0 V, 85 V, and elastic tensor was determined to restore symmetry [28]. This is floating potential) was investigated. For all depositions, a base necessary since the introduction of Y atoms distorts the crystal pressure lower than 10 4 Pa was guaranteed, to reduce the matrix and therefore the supercells slightly deviate from the exact influence of residual gases, such as oxygen, within the deposition hexagonal symmetry. In a further step the piezoelectric tensor was process. All coatings were deposited on (100) oriented Si sub- calculated for the different relaxed supercells, applying density strates (< 50 X cm). YN thin films crystallize, similarly to ScN, in functional perturbation theory as implemented in VASP. To ensure face centered cubic structure (NaCl type, B1) unit cell – having a the reliability of the results for different supercell sizes, piezoelec- lower energy of formation than the competing hexagonal crystal tric and elastic properties for an yttrium concentration of 6.25%, structure (wurtzite ZnS type, B4) [31]. Upon exposure to humid were also investigated for the small 32 atom supercell. The slight air YN forms YOOH groups [32]. Therefore, a sputter deposited deviations can clearly be attributed to the different SQS structures, 250 nm thin TiN capping layer was employed as a capping layer thus suggesting reliable results for our calculations for higher to prevent oxidation and formation of hydroxide groups. For the yttrium concentrations. deposition of the TiN layer a 2-inch Ti target was used in mixed
The ab initio obtained elastic constants of w-YxAl1 xN, elastic N2/Ar atmosphere at 0.4 Pa, applying deposition temperatures of moduli in-plane (E1) and out of plane (E3), strongly decrease with 250 °C or 500 °C, respectively. increasing yttrium concentration, as illustrated in Fig. 1. The value In order to determine electrical and piezoelectrical material of both moduli is nearly identical, reaching 100 GPa at a concentra- properties disc shaped platinum electrodes with 500 lm diameter tion of 37% yttrium, which is a third of their original value of were sputter-deposited directly on the YxAl1 xN films and 300 GPa for w-AlN. Hence, the decrease is more pronounced than patterned via a lift-off process. P.M. Mayrhofer et al. / Acta Materialia 100 (2015) 81–89 83
Fig. 2. Piezoelectric constants d33 and d31 obtained from ab initio calculations as well as e33 and e31 for YxAl1 xN as a function of their Y content, x. Fitted parabolic lines serve as guide to the eye.
The yttrium concentration x was determined employing both a scanning electron microscope (SEM) based energy dispersive X-ray Table 1 Deposition results and deposition conditions for co-sputtered Y Al N thin films: (EDX) detector and transmission electron microscopy (TEM) based x 1 x Power on yttrium target: PY, argon concentration in the sputtering gas Ar, substrate electron energy loss spectrometry (EELS). In SEM (Hitachi SU8030) temperature Ts, S substrate bias condition (F...Floating), film thickness d, sputter rate based EDX, concentration determination without employing a YAl r, YN concentration x, piezoelectric coefficient d33. standard is difficult because of the low X-ray yield at 20 kV accel- eration voltage for the yttrium peak corresponding to the K-edge at approximately 14 keV. Therefore, first TEM based EELS measure- ments were conducted and further employed as a standard for EDX measurements. TEM analyses were conducted using a FEI Tec- nai G20 employing a selected area aperture with a diameter of 200 nm for electron diffraction images. Structural properties were investigated by means of X-ray diffraction (XRD) powder spectra in Bragg–Brentano configuration using a PANalytical X’Pert PRO with a copper tube operated at 40 kV and 40 mA. Pole figures were measured with a Panalytical Empyrian equipped with an Eulerian cradle and a parallel beam monochromator set to CuKa. The piezo- electric constant d33 and the test structure related capacitance were determined with a piezometer (Piezotest PM 300) that employs the Berlincourt method [33]. For comparison, piezoelec- tric constants were also determined via a Laser Doppler Vibrome- ter (MSA 400), as described in a previous work employing Bull’s Eye type test structures [34]. An atomic force microscope (AFM, Bruker Dimension Edge) was used in contact mode to determine topological properties. Furthermore, the piezo force mode (PFM) was employed for investigations of piezoelectric properties on the nanoscale by measuring the deflection amplitude upon simul- taneous application of an AC voltage in the kHz range between cantilever tip and silicon substrate at constant force (cantilever deflection). 9). Thereby, the sputter rate only slightly increases, but the yttrium 4. Results and discussion content stays nearly unchanged within the measurement accuracy, when comparing samples prepared with otherwise identical depo- The deposition parameters of the sputter process have a high sition conditions. Grounding the substrate potential from initially impact on the deposition rate and yttrium concentration in the slightly negative floating potential causes a slight decrease of the sputter rate accompanied by a further decrease of the yttrium con- prepared YxAl1 xN thin films. In order to achieve a highly crys- talline and c-axis oriented microstructure, a wide parameter study centration, as can be seen in Table 1 when comparing samples pre- ° was conducted. For that purpose, films were deposited at three dif- pared at Ts = 800 C (No. 10–12: 0 V with Nr. 4–6: floating) or at 250 °C (No. 13–15: 0 V with Nr. 7–9: floating). ferent power levels applied on the yttrium target: PY = 100 W, Further variations of the deposition conditions are conducted PY = 200 W, PY = 300 W. Moreover, different substrate bias condi- tions, two temperature levels (250 °C and 800 °C) as well as vary- by increasing the Ar flow rate (N2/Ar = 11 sccm/9 sccm, 45% Ar) and increasing the negative substrate bias to 85 V. Thereby, the ing argon fractions in the reactive gas (in addition to N2) were investigated. Table 1 gives an overview of the applied sputter con- deposition rate decreases significantly. While the films prepared ditions and summarizes measured thickness t, sputter rate r, with 25% Ar and floating bias (samples 4–9) as well as 0 V bias yttrium concentration x and piezoelectric constants. Initial deposi- (samples 10–15) potentials exhibit a crystalline and columnar microstructure, the films deposited with 85 V (samples 19–21) tions in pure N2 (0% Ar) atmosphere yield an yttrium concentration of up to x = 11.6% and low sputter rates (samples 1–3). Adding 25% and the samples prepared with 45% Ar and 0 V bias (samples 16– argon to the process gas at constant overall gas volume flow 18) showed an amorphous structure from XRD patterns. (20 sccm) and constant pressure (40 mPa), leads to a significant increase of the sputter rate, being in good agreement with results 4.1. X-ray diffraction analysis previously reported for pure AlN [35], combined with a decreasing yttrium concentration (samples 4–6). Furthermore, the substrate To maximize the piezoelectric response of w-AlN type thin temperature (Ts) was reduced from 800 °C to 250 °C (samples 7– films, crystalline films are required with pronounced columnar 84 P.M. Mayrhofer et al. / Acta Materialia 100 (2015) 81–89 grains and a high degree of c-axis orientation. Consequently, their X-ray diffraction patterns should exhibit only orders of (002) reflections. Analyses of the XRD pattern reveal a hexagonal wurt- zite structure of all coatings investigated in this work. Fig. 3 depicts XRD patterns of the YAlN (002) peak for all crystalline films obtained at varying sputter deposition conditions and different values for the yttrium concentration. With increasing yttrium con- tent of YxAl1 xN, the diffraction angle of the major (002) reflex decreases from 36° to 35.4°. Also, the (100) peak shifts to lower values of 2h from 33° to 31.6° for x = 11%. The contribution at 36.7° is from the TiN capping layer and the sharper peaks at 33° originates from the Si substrate.
The XRD patterns of the YxAl1 xN films with low yttrium con- ° centration (x < 8%) deposited at 250 C and 25% Ar (see Fig. 3(a)) Fig. 4. FWHM values of YAlN (002) XRD peaks of YxAl1 xN films deposited at exhibit a similar high degree of c-axis orientation as the films varying deposition conditions. Corresponding XRD patterns are shown in Fig. 3. deposited at 800 °C and 25% Ar, Fig. 3(b). The full width at half maximum (FWHM) of the (002) peak is a key parameter to quan- tify the degree of crystallinity and the quality of the c-axis orienta- tion. The FWHM values, shown in Fig. 4 for the films at low yttrium concentration x deposited at 800 °C, are at a similarly low level compared to the FWHM of those deposited at 250 °C. For the sam- ples deposited at 800 °C the FWHM is initially lower, for lower yttrium contents, and increases up to 0.25° for the samples with highest x = 6%. Consequently, the thin films deposited at 800 °C show a slightly higher degree of c-axis orientation, as in both cases exclusively the YAlN (002) reflex is present. The XRD patterns of films prepared at 800 °C with x between 6.2% and 11.6% in Fig. 3 (c), also exhibit increasing intensity of the YAlN (100) reflex with increasing Y content. Also their FWHM of the YAlN (002) reflex increases, as shown in Fig. 4, suggesting decreasing c-axis orienta- tion, reaching 0.7° at x = 11.6%. Fig. 5 gives an overview of the XRD patterns for w-YxAl1 xN films with highest c-axis orientation at each yttrium concentration. The vertical dashed lines give the ab initio obtained peak positions for the (100) and (002) reflexes for w-YxAl1 xN from x =0%tox = 11.6%. Furthermore, the patterns show that a high degree of c-axis orientation could be obtained up to an yttrium concentration of x = 6%, while the film with 11.6% shows traces of other orientations, for instance the (100) reflex. Fig. 5. XRD patterns of w-YxAl1 xN films up to x = 0.116 and with best c-axis orientation at given yttrium concentration x. The vertical dashed lines indicate the In addition, the larger FWHM of the YAlN (002) peak indicates peak positions from ab intio calculations corresponding to YxAl1 xN (100) and poor crystalline quality. The experimentally obtained lattice (002) respectively, for compareable chemical compositions. parameters, along the c-axis, agree very well with results from ab initio calculations for large (128 atoms) and small (32 atoms) supercells, see Fig. 6. Consequently the agreement suggests, analysis requires crystallographic information of tilted planes as yttrium is substituting for aluminium in the wurtzite type crystal well. For that reason, pole figures of the diffraction planes corre- H structure, as indicated by the nomenclature YxAl1 xN. Our results sponding to the 2 angles of YxAl1 xN (100), (101) and (002) are also in good agreement with previously reported results of are measured. Fig. 7 shows a measured pole figure of an YxAl1 xN ab initio calculations [20]. sample as a contour plot and an inset with important crystallo- While XRD patterns provide a tool for fast evaluation of the graphic directions on the lower right. The thin film with x =6% crystal planes nearly parallel to the substrate, complete texture was prepared at the optimal process conditions for high c-axis
Fig. 3. XRD patterns of YxAl1 xN films prepared at varying sputter conditions: (a) is for low yttrium concentration x prepared at 250 °C and 25% Ar, (b) also low x prepared at 800 °C and 25% Ar, (c) are higher x prepared at 800 °C and 25% Ar (samples No. 5 and 3) and 800 °C and 0% Ar (samples No. 2 and 6).The yttrium target power is indicated by
PY. All patterns are normed to the YAlN (002) peak and scaled. P.M. Mayrhofer et al. / Acta Materialia 100 (2015) 81–89 85
orientation, at 25% Ar, PY = 300 W, TS = 800 °C and showed a strong (002) peak and low FWHM in the XRD pattern (see Fig. 7). Evalu- ation and graphic illustration are performed employing MTEX [36]. The (002) contribution has the highest intensity located exclu- sively at the center of the pole figure. Furthermore, the (101) planes are observed at an angle of 60° corresponding to the angle between the (001) and (101) planes. The lower right graphic indi- cates the expected peak positions in a c-axis oriented wurtzite Yx- Al1 xN crystal. A weaker contribution of the (100) orientation is also present and may be due to the initial growth phase.
4.2. Piezoelectric properties
The key property of this work, the out-of-plane piezoelectric Fig. 6. Lattice constant along the c-axis from XRD analyses compared to results of ab initio calculations. The inserted lines serve as guide to the eye. strain constant d33 was determined for all thin films, as described in Section 2. The results are presented in Table 1 for each YxAl1 xN film together with the corresponding co-sputter parameters. Results of both evaluation techniques based on the direct and indi- manner as the XRD analysis in Fig. 3(a–c). The experimentally rect piezoelectric effect agree well. The expectation based on ab ini- obtained d33 values are lower than the ab intio predicted values. tio calculations (see Section 2) is a constant increase for YxAl1 xN Nevertheless, highly c-axis oriented films exhibit an increase in with x, starting at d33 = 5.0 pm/V for x = 0%, d33 = 6.5 pm/V for d33 with increasing Y-content, as predicted by ab initio theory. x = 6% and d33 = 7.8 pm/V for x = 12%. Films deposited at a substrate temperature of 800 °C, as illustrated The piezoelectric constants d33 for the YxAl1 xN thin films with in Fig. 8(b), exhibit the highest d33, which slightly increases with varying yttrium content are given in Fig. 8(a–c) in the same increasing Y-content from 1% to 5.9%. However, as the films with
— — Fig. 7. YAlN (0002), YAlN (1010) and YAlN (1011) pole figure XRD measurements of YxAl1 xN prepared at PY = 300 W, 25% Ar, 800 °C and grounded substrate condition. 86 P.M. Mayrhofer et al. / Acta Materialia 100 (2015) 81–89
Fig. 8. Piezoelectric constant d33 and sputter rate r shown as a function of yttrium concentration x in YxAl1 xN. (a) Films with low yttrium concentration x prepared at 250 °C and 25% Ar, (b) low yttrium content x prepared at 800 °C and 25% Ar, (c) films with higher yttrium content x prepared at 800 °C and 25% Ar (samples with x 6% and 11%) and 0% Ar (samples with x 8% and 9%). even higher yttrium contents exhibit a mixed crystalline orienta- tion, their piezoelectric constants are close to zero, as shown in
Fig. 8(c). All films with significant values of d33 also exhibit a high deposition rate (see Fig. 8), which also increases slightly with yttrium concentration due to the higher power levels applied on the yttrium target. Unfavourable deposition conditions that lead to mixed crystallite orientations suppress the increase of d33 with the incorporation of yttrium in AlN.
YxAl1 xN thin films with highest c-axis orientation clearly exhi- bit increasing d33 values from d33 = 3.2 pm/V at x =1% to d33 = 3.7 pm/V at x = 5.9%, with increasing yttrium content, as e given in Fig. 9. Fig. 10. Dielectric constant r of YxAl1 xN from electrical capacitance measurements as a function of yttrium concentration x. Furthermore, the relative dielectric permittivities er of the YxAl1 xN thin films were determined from capacitance measure- ments employing a piezometer. Fig. 10 shows the measured values in dependence of the yttrium concentration for the films with amorphous-like regions located close to the silicon substrate inter- highest c-axis orientation at varying concentration. A weak face, the films exhibit a pronounced columnar structure with e e e increase of r is observed with x from r =10at x =0%to r =13 increasing column diameter for increasing coating thickness, corre- at x = 11.6%. This increase is in particular similar to the results sponding to competitive growth morphologies. On top of the presented for scandium doped AlN thin films [29]. YxAl1 xN thin films the TiN capping layer can easily be identified by the darker contrast and the distinct interface. Moreover, the crystallographic phase and orientation was stud- 4.3. Transmission electron microscopy ied by selected area electron diffraction (SAED) of the YxAl1 xN thin films. In Fig. 12(a), the SAED image taken in the centre of the film More detailed analysis of the microstructure of Y Al N thin x 1 x illustrates a pronounced (002) growth orientation. Peak indices in films are performed by cross-sectional transmission electron Fig. 12(a and b) are allocated along the YxAl1 xN (110) zone axis microscopy. As the measured piezoelectric constant d33 always thus, confirming the wurtzite type lattice, as shown on the left side was below the ab initio predicted values, we selected the three of Fig. 12. The SAED patterns of each film suggest for lattice films with the highest d33 values. Hence, also with the highest constants of c = 4.99 Å, a = 3.09 Å at x = 1%, c = 5.03 Å, a = 3.15 Å degree of c-axis orientation of each deposition series, prepared at at x = 3.5% and c = 5.06 Å, a = 3.17 at x = 5.9% of our w-YxAl1 xN thin PY = 100 W, 200 W, 300 W, at 800 °C, 25% Ar and grounded sub- films. These results are in good agreement with the c-lattice strate potential (0 V). The corresponding TEM cross sections in parameters obtained from XRD investigations, as shown in Fig. 6. Fig. 11(a –c) show an increasing thickness of the amorphous inter- Furthermore, the cross-sectional TEM studies also allow for the face region close to the Si substrate with increasing yttrium con- determination of the a-lattice parameters, which are also in excel- tent ranging from 20 nm for x 1% to 50 nm for x 3.5% and lent agreement with ab initio predicted values of a = 3.13 Å for up to 100 nm for x 6%. These amorphous layers influence the x = 0% and a = 3.17 Å for x = 5.9%. The SAED image in Fig. 12(b) of measured piezoelectric constants significantly and can explain the substrate near region, of the Y Al N thin film with x 5.9% the slightly lower increase in d with increasing yttrium content x 1 x 33 and hence with the thickest amorphous-like interface region of compared to theoretical predictions. However, above these 100 nm, clearly exhibits pronounced diffraction spots from the Si substrate and weak diffraction contributions from the film. As the aperture size is 100 nm in diameter, these weak contributions also originate from the crystalline thin film regions and proof the amorphous-like nature of the first 100 nm. In Fig. 13(a), EDX linescans across the coating thickness of the sample with x = 5.9% during cross sectional TEM studies demon- strate, that the amorphous near interface region is characterized by a higher oxygen content. This can especially be seen by the sig- nificant decrease in the Al signal EDX elemental peak intensity within this region close to the Si substrate, when comparing with the EDX line scan across the overall film thickness in Fig. 13(b). The aluminum and yttrium intensities decrease from the crys- talline area by roughly one half, while the nitrogen intensity Fig. 9. Piezoelectric constant d33 of YxAl1 xN films with highest degree of c-axis orientation versus the yttrium concentration x. decreases only slightly, accompanied by a sharp increase of the P.M. Mayrhofer et al. / Acta Materialia 100 (2015) 81–89 87
Fig. 11. TEM cross sections for YxAl1 xN thin films with TiN capping layer. (a) x = 1%, (b) x = 3.5%, (c) x = 5.9% deposited at 800 °C and 25% Ar and grounded substrate bias conditions.
Fig. 12. SAED pattern for YxAl1 xN with x = 5.9% deposited at 25% Ar, 800 °C and grounded substrate conditions. Observed regions: central on YxAl1 xN film and (b) Si substrate near region including of YxAl1 xN. The inset on the left is an illustration of the wurtzite type YxAl1 xN crystal structure with green Y atoms, red Al and blue N atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) oxygen EDX peak intensity. Based on our studies we envision, that also exhibits pronounced (100) XRD reflection. The grain size as the higher oxygen content near the interface to the Si substrate well as the roughness values increase with increasing yttrium stems from the deposition conditions and leads to the formation incorporation. The mean roughness values rq for the shown images of an amorphous like region, which, in turn, leads to reduced effec- are: (a) rq = 1 nm, (c) rq = 15 nm, (e) rq = 13 nm. For mapping of the tive values for the piezoelectric constants. piezoresponse amplitude a voltage of 6 V at 150 kHz was applied across the film and the induced deflection is recorded at this given 4.4. AFM analysis frequency. The piezo response images are shown next to the corre- sponding topology images in Fig. 14(b, d and f), levelled to show Surface topology and piezoelectric response on the nanoscale the amplitude contrast. The mean amplitude level in all three films were studied applying atomic force microscopy in contact PFM remains nearly constant. Variations from this amplitude level on mode. Fig. 14(a, c and e) show 5 5 lm2 surface topology images the lateral scale are generally similarly sized as grains in the corre- of YxAl1 xN samples with x = 3.5%, 6.5% and 9% prepared at 800 °C, sponding topographic image. The piezoelectric amplitude variation 25% and floating substrate potential. The YxAl1 xN thin films with between individual grains increases with increasing incorporation x 3.5% and 6.5% are c-axis oriented, while the films with x 9% of yttrium. This indicates misorientation between individual grains
Fig. 13. Elemental peak intensity along EDX linescans of (a) Si-substrate near region and (b) overall cross section of the YxAl1 xN thin film with x = 5.9%, deposited at 25% Ar, 800 °C and grounded substrate conditions. 88 P.M. Mayrhofer et al. / Acta Materialia 100 (2015) 81–89
Fig. 14. AFM topography (a, c, e) and PFM amplitude (b, d, f) of YxAl1 xN from (a and b) x = 3.5%, (c and d) x = 6.5%, (e and f) x = 9% prepared at floating substrate conditions, 25% Ar, 800 °C, d 1300 nm. The PRM amplitudes are levelled from: (b) 301 pm, (d) 297 pm, (c) 295 pm, measured at 6 V AC amplitude across the films.
on the surface, which besides the amorphous interface layer to the intio predictions the d33 value increases with increasing Y-content, silicon substrate may in addition, reduce the effective piezoelectric however, only for films with highest degree of c-axis orientation response measured with electrodes. and from d33 = 3.2 pm/V with x 1% to d33 = 3.7 pm/V with x = 5.9%. Cross-sectional TEM studies highlight, that the deviation 5. Conclusions to ab initio predictions is mainly based on the formation of an amor- phous region near the interface to the Si substrate. The thickness of
YxAl1 xN thin films are promising candidates to achieve an this amorphous region increases with increasing Y content, up to increase of the electromechanical coupling in AlN. Ab intio calcula- 100 nm for x = 5.9% and is characterized by an higher oxygen content tions suggest an increase of the piezoelectric strain constants d33 being present during the initial stage of film growth. AFM studies of and d31 due to a significant decrease of the elastic constants and a the w-YxAl1 xN thin films showed an increased roughness with an slight increase of the piezoelectric stress constants e33 and e31. The increasing contrast in the piezoelectric amplitude between individ- theoretical results suggest an increase of d33 by 700% for wurtzite ual grains with increasing yttrium content. In summary, yttrium type YxAl1 xN with x = 50% which is higher than the reported 500% doped AlN thin films offer a high potential for a significant increase increase of d33 reported for ScxAl1 xN with x = 42.5% [18]. The high- of the piezoelectric constants. However, high quality wurtzite thin est c-axis orientation of w- YxAl1 xN thin films, prepared (with up to films with a high degree of c-axis orientation and pronounced x = 11.6%) via co-sputter deposition from an Al and Y target, was columnar growth are needed. Especially challenging is the mini- obtained when using a mixed Ar/N2 atmosphere with 25% Ar and, mization of the amorphous interface near region, easily formed with 40 mPa with grounded substrate potential. In agreement with ab increasing Y content due to the high affinity to oxygen. P.M. Mayrhofer et al. / Acta Materialia 100 (2015) 81–89 89
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