Influence of c-axis orientation and scandium concentration on infraredactive modes of magnetron sputtered ScxAl1−xN thin filmsP. M. Mayrhofer, C. Eisenmenger-Sittner, H. Euchner, A. Bittner, and U. Schmid Citation: Appl. Phys. Lett. 103, 251903 (2013); doi: 10.1063/1.4850735 View online: http://dx.doi.org/10.1063/1.4850735 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v103/i25 Published by the AIP Publishing LLC. Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

Influence of c-axis orientation and scandium concentration on infrared activemodes of magnetron sputtered ScxAl12xN thin films

P. M. Mayrhofer,1 C. Eisenmenger-Sittner,2 H. Euchner,3 A. Bittner,1 and U. Schmid1

1Institute of Sensor and Actuator Systems, Vienna University of Technology, Floragasse 7, 1040 Vienna,Austria2Institute of Solid State Physics, Vienna University of Technology, Wiedner Hauptstrasse 8, 1040 Vienna,Austria3Institute 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. ScxAl1�xN thin films with different Sc concentrations (x¼ 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

orientations. VC 2013 Author(s). All article content, except where otherwise noted, is licensed undera Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4850735]

Wurtzite-type aluminium nitride (w-AlN) is a piezoelec-

tric 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

(Eg¼ 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

(Tc¼ 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 piezoelec-

tric constants, whereas the 500% increase of d33 to 27.6 pC/N

for ScxAl1�xN thin films with x¼ 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 sublat-

tice. This results in an elongation of the in-plane lattice con-

stant a while the out-of plane constant c stays roughly

constant with increasing Sc content up to x¼ 0.5.11

The influence of seed layers and deposition temperatures

on c-axis orientation and crystal quality during film prepara-

tion by reactive magnetron sputtering of ScxAl1�xN have

been discussed using XRD and transmission electron micros-

copy 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

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=? þe1k=?ðx2

LOk=? � x2LOk=?Þ

x2LOk=? � x2 � ixCk=?

: (1)

The phonons with polarization parallel to the c-axis

(xLOk,xTOk) are of A symmetry, those with polarization per-

pendicular 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 concen-

trations up to x¼ 0.15 on the IR absorption bands in

ScxAl1�xN is discussed. For each concentration x, a set of

thin films was prepared under optimized conditions to facili-

tate the growth of a columnar-type, c-axis oriented micro-

structure 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 orien-

tation 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.

ScxAl1�xN thin films were deposited by reactive magne-

tron 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

0003-6951/2013/103(25)/251903/5 VC Author(s) 2013103, 251903-1

APPLIED PHYSICS LETTERS 103, 251903 (2013)

atmosphere (ISE), resulting in a thin amorphous Si layer at the

interface between Si and AlN.18 For the deposition of

ScxAl1�xN thin films, a 2 in., 99.99% Al target with 10 circu-

larly arranged, cylindrical holes was used. These holes were

filled with either scandium or aluminium inlets in order to de-

posit thin films with different Sc concentrations, namely,

x¼ 0, 0.03, 0.06, 0.13, and 0.15. All ScxAl1�xN films were

prepared from a base pressure of less than 2 � 10�6 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 concen-

tration was verified with an energy dispersive X-ray spectros-

copy (EDX) system (Oxford Instruments X-Max 50) at a

beam energy of 10 keV and a beam current of 5 lA, respec-

tively. IR spectroscopy was conducted in a Bruker Tensor 27

absorption spectrometer. Structural properties were also inves-

tigated by XRD measurements in Bragg Brentano configura-

tion 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 ScxAl1�xN thin films after subtraction of the

Si-substrate background contribution. The E1(LO) and

A1(LO) phonon modes of AlN located around 900 cm�1 are

only pronounced in reflectance IR spectra hence, we found

nearly no evidence of them in absorbance IR spectra, as pre-

viously reported.15 Instead, our analysis focused on the

stronger modes E1(TO): �¼ 669 cm�1 and A1(TO):

�¼ 608 cm�1, 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 fre-

quencies of both phonon modes E1(TO) and A1 (TO) were found

by non-linear least square fitting the sum of two Lorentzian func-

tions to the experimental spectra. A linear least square fit of the

obtained E1(TO) peak-frequencies for ScxAl1�xN thin films up

to x¼ 0.15 and with a variety of orientations yielded the follow-

ing concentration dependence of the peak-frequencies:

d�E=dx ¼ �2 cm�1 %�1. Figure 2(a) shows the peak position

�E as a function of Sc concentration x, with the observed varia-

tion 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

� ¼ 1

2pc

k

l

� �: (2)

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

l ¼ AN ½xASc þ ð1� xÞAAl�AN þ ½xASc þ ð1� xÞAAl�

: (3)

The relation of force constant k (in N/m) and average bond

length l (in A) can be estimated by the following empirical

relationship, which was originally published for TeO glasses

and is also used for wurtzite ZnO20,21

k ¼ 17

l3: (4)

Using Eqs. (2)–(4) and the measured frequency of the pho-

non mode E1(TO), the average bond length for Al(Sc)-N was

FIG. 1. FTIR absorbance spectra of

ScxAl1�xN films for (a) films with a

high degree of c-axis orientation, (b)

randomly oriented films.

FIG. 2. (a) Frequency shift of the

E1(TO) mode as function of the Sc

concentration in ScxAl1�xN thin films.

(b) Bond length l with varying orienta-

tions calculated from Eqs. (2)–(4).

Linear fits to the data are depicted as

red lines.

251903-2 Mayrhofer et al. Appl. Phys. Lett. 103, 251903 (2013)

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 ex-

perimental 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

AlxSc1�xN, pure w-AlN as well as a high symmetry configu-

ration of w-Al0.5Sc0.5N were investigated. The high symme-

try 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 k-

point 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 split-

ting), it is necessary to take long range Coulomb interactions

into account. This was done by modifying the dynamical ma-

trix, introducing Born effective charges and dielectric con-

stants, as determined from DFT. Finally, the PHONON

code20 was used to determine phonon frequencies at q¼ 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

(FWHM¼ 1 meV). It is clearly visible that the whole spec-

trum 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 agree-

ment with the shift obtained from the calculations. A shift

from �E ¼ 644 cm�1 at x¼ 0 to �E ¼ 617 cm�1 at x¼ 0.5 is

observed, corresponding to: d�E=dx ¼ �0:5 cm�1 %�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

to extend the procedure to ScxAl1�xN 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 inte-

grated peak areas A¼AA/(AEþAA) is used as a measure

for the thin film orientation, similar to the FWHM in XRD

scans.

In addition, all ScxAl1�xN 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 x¼ 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 ScxAl1�xN 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-

lengths kCuKa1¼ 1.5405 A and kCuKa2¼ 1.5444 A. Hereafter,

FWHM denotes the FWHM of the AlN (002) peak as calcu-

lated 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 ran-

domly 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 dif-

fraction peak, corresponding to the (002), orientation is visi-

ble. 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, tak-

ing both the A1(TO) and E1(TO) modes into account, is nec-

essary 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

FIG. 3. Calculated phonon density of states, convoluted with a Gaussian re-

solution function (FWHM of 1 meV) for AlN and Sc0.5Al0.5N as a function

of energy.

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.

251903-3 Mayrhofer et al. Appl. Phys. Lett. 103, 251903 (2013)

(100) amplitude fraction as gained from XRD analyses. A

clear trend of increasing IR-A1(TO) area ratio with increas-

ing AlN (100) fraction is observed. The amplitude of the

AlN (002) XRD peak scales with 60% compared to the am-

plitude of AlN (001) as found in ICDD database for

wurtzite-type AlN,24 thus an amplitude ratio of 50% corre-

sponds 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 combina-

tion 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 con-

text of AlN orientation–is less meaningful because the resid-

ual 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 ori-

ented thin films from those which do not exhibit pronounced

texturing.

Within this work, a study on ScxAl1�xN thin films with

varying Sc concentration from x¼ 0 up to x¼ 0.15 was

conducted. The thin films were investigated by FTIR absorp-

tion spectroscopy, evidencing a spectral redshift of the

E1(TO) and A1(TO) modes with increasing Sc concentration

(d�E=dx ¼ �2cm�1 %�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 �0:5cm�1%�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 spectros-

copy is applied to measure the directional anisotropy of

E1(TO) and A1(TO) phonon modes. In addition, XRD analy-

ses were performed at several ScxAl1�xN 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 ScxAl1�xN 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-ScxAl1�xN 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).

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