Influence of c-axis orientation and scandium

Influence of c-axis orientation and scandium concentration on infrared active modes of magnetron sputtered ScxAl1−xN thin films P. M. Mayrhofer, C. Eisenmenger-Sittner, H. Euchner, A. Bittner, and U. Schmid Citation: Applied Physics Letters 103, 251903 (2013); doi: 10.1063/1.4850735 View online: http://dx.doi.org/10.1063/1.4850735 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/25?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Stress controlled pulsed direct current co-sputtered Al1−xScxN as piezoelectric phase for micromechanical sensor applications APL Mater. 3, 116102 (2015); 10.1063/1.4934756 Synthesis and characterization of 10 nm thick piezoelectric AlN films with high c-axis orientation for miniaturized nanoelectromechanical devices Appl. Phys. Lett. 104, 253101 (2014); 10.1063/1.4882240 The impact of argon admixture on the c-axis oriented growth of direct current magnetron sputtered ScxAl1−xN thin films J. Appl. Phys. 115, 193505 (2014); 10.1063/1.4876260 Impact of the surface-near silicon substrate properties on the microstructure of sputter-deposited AlN thin films Appl. Phys. Lett. 101, 221602 (2012); 10.1063/1.4768951 Structural, optical, and acoustic characterization of high-quality AlN thick films sputtered on Al 2 O 3 ( 0001 ) at low temperature for GHz -band electroacoustic devices applications J. Appl. Phys. 96, 2610 (2004); 10.1063/1.1777809

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APPLIED PHYSICS LETTERS 103, 251903 (2013)

Influence of c-axis orientation and scandium concentration on infrared active modes of magnetron sputtered ScxAl12xN thin films P. M. Mayrhofer,1 C. Eisenmenger-Sittner,2 H. Euchner,3 A. Bittner,1 and U. Schmid1 1

Institute of Sensor and Actuator Systems, Vienna University of Technology, Floragasse 7, 1040 Vienna, Austria 2 Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstrasse 8, 1040 Vienna, Austria 3 Institute of Materials Science and Technology, Vienna University of Technology, Karlsplatz 13, 1040 Vienna, Austria

(Received 8 October 2013; accepted 2 December 2013; published online 16 December 2013) Doping of wurtzite aluminium nitride (AlN) with scandium (Sc) significantly enhances the piezoelectric properties of AlN. ScxAl1xN thin films with different Sc concentrations (x ¼ 0 to 0.15) were deposited by DC reactive magnetron sputtering. Infrared (IR) absorbance spectroscopy was applied to investigate the Sc concentration dependent shift of the IR active modes E1(TO) and A1(TO). These results are compared to ab initio simulations, being in excellent agreement with the experimental findings. In addition, IR spectroscopy is established as an economical and fast method to distinguish between thin films with a high degree of c-axis orientation and those exhibiting mixed C 2013 Author(s). All article content, except where otherwise noted, is licensed under orientations. V a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4850735] Wurtzite-type aluminium nitride (w-AlN) is a piezoelectric group III-nitride with many applications in the field of MEMS (micro electro-mechanical systems) devices.1 Monocrystalline w-AlN is a wide bandgap dielectric (Eg ¼ 6.2 eV) that remains stable up to high temperatures (over 800  C).2 Moreover, this active material possesses one of the highest Curie temperatures reported for thin film piezoelectrics (Tc ¼ 1150  C).3 In addition, the high sound velocity (v  6000 m/s)4 makes it an ideal choice for surface and bulk acoustic resonators. The piezoelectric stress constants dij of AlN are directly dependent on the degree of c-axis orientation. For d33 and d31, values up to 6.5 pC/N and 2.9 pC/N are reported for AlN thin films prepared by reactive magnetron sputtering.5–7 The enhancement of the piezoelectric constants via transition metal doping provides an excellent opportunity to improve the efficiency of AlN based devices. To this day, several groups have achieved an enhancement of the piezoelectric constants, whereas the 500% increase of d33 to 27.6 pC/N for ScxAl1xN thin films with x ¼ 0.425 is still unsurpassed.8,9 From ab initio calculations, the change in piezoelectric response was attributed to be an intrinsic effect resulting from a phase competition between the hexagonal phase of wurtzite type AlN and cubic ScN.10 A comparison of these simulations with X-ray diffraction (XRD) measurements strongly suggests the incorporation of Sc on the Al sublattice. This results in an elongation of the in-plane lattice constant a while the out-of plane constant c stays roughly constant with increasing Sc content up to x ¼ 0.5.11 The influence of seed layers and deposition temperatures on c-axis orientation and crystal quality during film preparation by reactive magnetron sputtering of ScxAl1xN have been discussed using XRD and transmission electron microscopy techniques.12 In addition, Fourier transform infrared (FTIR) absorption spectroscopy proved to be a viable tool for determining the degree of c-axis orientation and phase purity of aluminium nitride thin films.13–15 0003-6951/2013/103(25)/251903/5

FTIR spectra of aluminium nitride can be modelled by damped harmonic oscillators taking into account the uniaxial anisotropy of AlN.16 In the latter study, the dielectric functions parallel (k) and normal (?) to the c-axis of AlN are described by ek=? ¼ e1k=? þ

e1k=? ðx2LOk=?  x2LOk=? Þ x2LOk=?  x2  ixCk=?

:

(1)

The phonons with polarization parallel to the c-axis (xLOk ,xTOk ) are of A symmetry, those with polarization perpendicular to the c-axis (xLO? ,xTO? ) are of E symmetry. The high frequency limit of the dielectric function is e1k=?  4:84 and C represents a damping constant.16,17 Both polarization directions (LO, TO) of the phonons labelled with E and A can be excited with IR-radiation and owing to the directional anisotropy of the phonon modes E(LO,TO) and A(LO,TO), the integrated area ratio of the corresponding IR absorption peaks are used as measure for the degree of c-axis orientation. In this work, the influence of scandium doping concentrations up to x ¼ 0.15 on the IR absorption bands in ScxAl1xN is discussed. For each concentration x, a set of thin films was prepared under optimized conditions to facilitate the growth of a columnar-type, c-axis oriented microstructure with random in plane orientation of grains, as previously observed for pure AlN thin films.18 In addition, other sets of samples were prepared with partial c-axis orientation up to random orientation. Finally, based on the peak positions of IR absorption bands, the Al(Sc)-N bond length variation with scandium concentration x is determined. ScxAl1xN thin films were deposited by reactive magnetron sputtering on nominally unheated silicon (100) substrates. Prior to the deposition, the Si (100) substrates were cleaned by rinsing in Aceton and Isopropanol. The native oxide on the surface was removed by ion sputter etching in pure Argon

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FIG. 1. FTIR absorbance spectra of ScxAl1xN films for (a) films with a high degree of c-axis orientation, (b) randomly oriented films.

atmosphere (ISE), resulting in a thin amorphous Si layer at the interface between Si and AlN.18 For the deposition of ScxAl1xN thin films, a 2 in., 99.99% Al target with 10 circularly arranged, cylindrical holes was used. These holes were filled with either scandium or aluminium inlets in order to deposit thin films with different Sc concentrations, namely, x ¼ 0, 0.03, 0.06, 0.13, and 0.15. All ScxAl1xN films were prepared from a base pressure of less than 2  106 Pa at a power density of 5.1 W/cm2 and pressure of 3 lbar at varying Ar/N2 ratios, ranging from 0% up to 45% Ar. The Sc concentration was verified with an energy dispersive X-ray spectroscopy (EDX) system (Oxford Instruments X-Max 50) at a beam energy of 10 keV and a beam current of 5 lA, respectively. IR spectroscopy was conducted in a Bruker Tensor 27 absorption spectrometer. Structural properties were also investigated by XRD measurements in Bragg Brentano configuration using a PANalytical X’Pert PRO with a Copper tube operated at 40 kV and 40 mA (CuKa1, CuKa2). In a next step, all samples were analyzed using FTIR absorption spectrometry. Figure 1 shows the IR spectra for a series of ScxAl1xN thin films after subtraction of the Si-substrate background contribution. The E1(LO) and A1(LO) phonon modes of AlN located around 900 cm1 are only pronounced in reflectance IR spectra hence, we found nearly no evidence of them in absorbance IR spectra, as previously reported.15 Instead, our analysis focused on the stronger modes E1(TO):  ¼ 669 cm1 and A1(TO):  ¼ 608 cm1, respectively. The exact frequencies of these modes are not easily accessible because they exhibit a shift, dependent on residual film stress.16,19 Figure 1(a) shows absorption spectra of aluminium nitride thin films—with a high degree of c-axis orientation— for Sc concentrations x up to 0.15. In contrast, Figure 1(b) shows thin films that do not exhibit pronounced growth in a preferential direction. Considering the highly c-axis oriented films in (a) only one peak A1(TO) is observed, while the other spectra show two shoulders: A1(TO) þ E1(TO). From Figure 1, it is obvious that upon doping with Sc the phonons in AlN films are shifted to lower frequencies and

simultaneously the peak widths increase drastically. The frequencies of both phonon modes E1(TO) and A1 (TO) were found by non-linear least square fitting the sum of two Lorentzian functions to the experimental spectra. A linear least square fit of the obtained E1(TO) peak-frequencies for ScxAl1xN thin films up to x ¼ 0.15 and with a variety of orientations yielded the following concentration dependence of the peak-frequencies: d E =dx ¼ 2 cm1 %1 . Figure 2(a) shows the peak position  E as a function of Sc concentration x, with the observed variation in peak position being due to the residual film stresses. The frequency change is directly related to an elongation of the Al(Sc)-N bond length, as shown in Figure 2(b). Furthermore, the frequency change of the investigated IR active modes can be attributed to the ionic sizes of Sc and Al and to structural changes. Based on the measured frequencies for the E1(TO) phonon, the bond length l can be determined, as shown in Figure 2(b). The frequency change of E1(TO) is related to the bond length and can be estimated by20   1 k : (2) ¼ 2pc l Equation (2) attributes the frequency change to the change in effective mass l and force constant k. The effective mass of the bond is defined by the concentration x and atomic masses Ai of the constituting elements i l¼

AN ½xASc þ ð1  xÞAAl  : AN þ ½xASc þ ð1  xÞAAl 

(3)

The relation of force constant k (in N/m) and average bond ˚ ) can be estimated by the following empirical length l (in A relationship, which was originally published for TeO glasses and is also used for wurtzite ZnO20,21 k¼

17 : l3

(4)

Using Eqs. (2)–(4) and the measured frequency of the phonon mode E1(TO), the average bond length for Al(Sc)-N was

FIG. 2. (a) Frequency shift of the E1(TO) mode as function of the Sc concentration in ScxAl1xN thin films. (b) Bond length l with varying orientations calculated from Eqs. (2)–(4). Linear fits to the data are depicted as red lines.

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calculated, as depicted in Figure 2(b). A linear fit to the data is shown as guide to the eye. Ab initio density functional theory (DFT) was applied to compute the frequencies of E1(TO) and A1(TO) as well as the phonon density of states of w-ScxAl1-xN with two selected Sc concentrations in order to compare with the experimental data. The simulations were conducted using the Vienna Ab initio Simulation Package (VASP),22 applying the projector augmented wave method and the generalized gradient approximation (PAW-GGA).23 To determine the influence of the Sc concentration on the lattice dynamics of AlxSc1xN, pure w-AlN as well as a high symmetry configuration of w-Al0.5Sc0.5N were investigated. The high symmetry supercell of Al0.5Sc0.5N was chosen to facilitate the calculation by reducing the number of displacements to be calculated to determine the dynamical matrix. To minimize finite size effects, 4  4  2 supercells with altogether 128 atoms were optimized by relaxing both, lattice and atomic positions, using a 3  3  3 Gamma-centred kpoint mesh and an energy cutoff at 500 eV. Next, symmetry non-equivalent displacements were introduced into the relaxed structures to determine the force constant matrix. To correctly reproduce phonons at low q values (LO/TO splitting), it is necessary to take long range Coulomb interactions into account. This was done by modifying the dynamical matrix, introducing Born effective charges and dielectric constants, as determined from DFT. Finally, the PHONON code20 was used to determine phonon frequencies at q ¼ 0 as well as the total phonon density of states. Figure 3 shows the calculated phonon density of states convoluted with a Gaussian resolution function (FWHM ¼ 1 meV). It is clearly visible that the whole spectrum is shifted to lower frequencies when Sc is introduced into the Al sublattice. Moreover, the experimentally observed frequency change of the E1(TO) phonon is in qualitatively good agreement with the shift obtained from the calculations. A shift from  E ¼ 644 cm1 at x ¼ 0 to  E ¼ 617 cm1 at x ¼ 0.5 is observed, corresponding to: d E =dx ¼ 0:5 cm1 %1 . Since the phonon modes E1(TO) and A1(TO) exhibit directional anisotropy, their respective intensities offer the possibility to analyse the growth direction of w-AlN with respect to the direction of the incident IR radiation. In order

FIG. 3. Calculated phonon density of states, convoluted with a Gaussian resolution function (FWHM of 1 meV) for AlN and Sc0.5Al0.5N as a function of energy.

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FIG. 4. XRD pattern of a Sc0.03Al0.97N thin film with highest degree of c-axis orientation. Peaks correspond to Si (400) k/2 at 2H  33 , AlN(002) at 2H  36 , Pt(111) at 2H  40 and higher order contributions of Si.

to extend the procedure to ScxAl1xN thin films a combined XRD and FTIR study was conducted. From fitting the FTIR absorption spectra to both shoulders corresponding to A1(TO) and E1(TO) transitions the resulting fraction of integrated peak areas A ¼ AA/(AE þ AA) is used as a measure for the thin film orientation, similar to the FWHM in XRD scans. In addition, all ScxAl1xN thin films were investigated by XRD in order to obtain reference values for the degree of c-axis orientation. Figure 4 gives an overview of an XRD pattern for x ¼ 0.03 and measured up to a 2H angle of 70 . The highly pronounced c-axis texturing is clearly visible as only one peak from the ScxAl1xN layer, corresponding to the (002) plane, is detected. The additional contribution in the XRD spectrum (at, e.g., 2H  40 ), not originating from the Si substrate, is due to platinum electrodes located at the edge of our sample. To obtain the location, FWHM and amplitude of the XRD peaks, two pseudo-Voigt functions with a fixed ratio of peak intensities (2:1) were used to account for the two wave˚ and kCuKa2 ¼ 1.5444 A ˚ . Hereafter, lengths kCuKa1 ¼ 1.5405 A FWHM denotes the FWHM of the AlN (002) peak as calculated for the a1 contribution only. The FTIR data were fitted with two Lorentzian functions, corresponding to the E1(TO) and A1(TO) phonon frequencies. To evaluate the degree of c-axis orientation the peak area ratio of these two peaks was taken into account. Figure 5 depicts XRD patterns and FTIR absorption spectra of two Sc0.03Al0.97N thin films, one with high degree of c-axis orientation, while the other one is randomly oriented. Figure 5(a) shows the IR spectra of a highly (002) oriented thin film together with a single Lorentzian peak, corresponding to the E1(TO) mode, while Figure 5(c) depicts the corresponding XRD pattern where only one diffraction peak, corresponding to the (002), orientation is visible. On the other hand, Figures 5(b) and 5(d) compare the IR spectrum and XRD pattern of a Sc0.03Al0.097N thin film with high amount of (100) orientation. For the latter thin film with random orientation, a fit consisting of two Lorentzians, taking both the A1(TO) and E1(TO) modes into account, is necessary to model the IR absorption. Figure 6 shows a comparison of the orientation analysis gained from the XRD measurements and the FTIR peak area ratio analysis. The fraction of the IR-peak area ratio A1(TO), regardless of Sc concentration, is plotted versus the AlN

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FIG. 5. (a) and (b) FTIR absorption spectra of Sc0.03Al0.097N thin films. (c) and (d) XRD pattern of Sc0.03Al0.097N thin films. For clarity, XRD measurements are scaled to the AlN (002) peak and the Si (400) k/2 contribution at 2H  33 is removed.

(100) amplitude fraction as gained from XRD analyses. A clear trend of increasing IR-A1(TO) area ratio with increasing AlN (100) fraction is observed. The amplitude of the AlN (002) XRD peak scales with 60% compared to the amplitude of AlN (001) as found in ICDD database for wurtzite-type AlN,24 thus an amplitude ratio of 50% corresponds to 83% c-axis orientation—rather than 50%. However, a low fraction of IR-absorption peak A1(TO) is always present for highly doped AlN, which in combination with the increased peak width of A1(TO) and E1(TO) upon Sc doping complicates the fit procedure. Nevertheless, the IR-peak ratio as obtained from FTIR spectrometry offers a fast and reliable means to determine the degree of c-axis orientation. Finally, we want to point out that a comparison with the FWHM of AlN (002)–as is common to show in context of AlN orientation–is less meaningful because the residual film stress and a finer graining result in enhanced peak broadening. A simple comparison of the FTIR absorbance spectra is shown in Figure 1, also for roughly distinguishing c-axis oriented thin films from those which do not exhibit pronounced texturing. Within this work, a study on ScxAl1xN thin films with varying Sc concentration from x ¼ 0 up to x ¼ 0.15 was

conducted. The thin films were investigated by FTIR absorption spectroscopy, evidencing a spectral redshift of the E1(TO) and A1(TO) modes with increasing Sc concentration (d E =dx ¼ 2cm1 %1 ). This shift can unambiguously be attributed to an elongation of Al(Sc)-N bonds as a result of Sc doping. The ab initio calculations are in good qualitative agreement, however, yield a lower redshift (d E =dx ¼ 0:5cm1 %1 ). This slight underestimation of phonon frequencies is yet a well-known issue and is amongst others related to the overestimation of the lattice constants in the GGA approximation. Moreover, IR absorption spectroscopy is applied to measure the directional anisotropy of E1(TO) and A1(TO) phonon modes. In addition, XRD analyses were performed at several ScxAl1xN thin films and the peak amplitude ratio of AlN (100)/AlN (002) was compared to the peak area ratio of E1(TO)/A1(TO), proving clearly that IR absorption spectroscopy is a fast and reliable technique to determine the degree of c-axis orientation in ScxAl1xN thin films. Although this method is not sensible to the basal plane orientation, it provides a strong and reliable tool to determine the degree of c-axis orientation in piezoelectric w-ScxAl1xN thin films, thus being of utmost importance for the implementation into micro-and nanomachined devices. We gratefully acknowledge the financial support from the Austrian Science Fund (FWF), No. P 25212-N30. In addition, we thank Dr. Erich Halwax from the X-ray Center (XRC) of Vienna University of Technology for help with measurements and interpretation. The computational results presented have been achieved using the Vienna Scientific Cluster (VSC). 1

FIG. 6. Comparison of FTIR peak area ratio of peak A1(TO) with amplitude fraction of AlN (100) from XRD. The inserted straight line serves as guide to the eye.

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