phys. stat. sol. (c) 3, No. 6, 1671–1674 (2006) / DOI 10.1002/pssc.200565183

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Evaluation of strain in AlN thin films grown on sapphire and6H-SiC by metalorganic chemical vapor deposition

Naoto Kato and Takashi Inushima∗

Department of Electronics, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292, Japan

Received 13 August 2005, accepted 10 May 2006

Published online 31 May 2006

PACS 61.10.Nz, 78.30.Fs, 79.30.Fs, 81.15.Gh, 81.70.Fy, 83.85.St

The effect of growth rate on the strain in AlN films grown on sapphire and 6H-SiC c-planes at 1150 ◦Cby metalorganic chemical vapor deposition was investigated. The strain was measured by X-ray diffractionand Raman spectroscopy. It was found that the strain in the films was expressed by the biaxial stress andthat it depended on the growth rate of the films and on the surface pre-treatments. When the growth ratewas lower than 2.4 nm/min, the strain was constant, and it was compressive for the films grown on sapphire,and tensile for those on 6H-SiC. These stresses were produced by the differences of the thermal expansioncoefficients between the films and substrates. The low temperature AlN-buffer layer, which was useful forthe high temperature growth of AlN on sapphire, was revealed not to be applicable for 6H-SiC.

1 Introduction III-nitride AlN for device applications has been primarily grown on sapphire and SiC

substrates by metalorganic chemical vapor deposition (MOCVD). In this case, as the deposition tempera-

ture is higher than 1000 ◦C, compressive or tensile stress greater than 1 GPa is expected to develop in the

AlN films, causing serious problems for device applications [1]. To avoid these problems, substrate-surface

treatment was proposed in order to control the initial crystal growth of AlN. The result was, however, not

successful for the low temperature buffer layer on 6H-SiC. It is considered that because of little lattice mis-

match (<1%) between AlN and 6H-SiC, the initial nucleation of AlN depended on the surface morphology

of 6H-SiC [2].

Recently we reported on the validity of in situ monitoring for controlling the crystal growth of AlN,

where pyrometer signals from the substrate were used not only for detecting the substrate temperature, but

also for monitoring the film growth condition, and showed that the crystal quality, especially its breakdown

voltage, depended strongly on the growth rate and growth mode of the film [3].

At present it is not known if a buffer layer is useful for the crystal growth of AlN on 6H-SiC. In this

report we investigate the growth rate dependence of the residual stress in AlN films, grown on sapphire

and 6H-SiC, controlling the growth condition by the in situ monitoring method. Ex situ Raman and X-ray

diffraction measurements of AlN on sapphire and 6H-SiC films are employed for the evaluation of the

residual stress in AlN films. Finally we consider the validity of the buffer layer and pre-surface treatments

for the reduction of residual stress in the films.

2 Experimental details The samples used in the experiments were grown by a horizontal two-flow-

zone MOCVD system. The substrates were on-axis Si-terminated and CMP polished 6H-SiC(0001) sup-

plied by Sixon Co. and c-plane sapphire. The source gases were trimethylaluminium (TMA) and ammonia

(NH3), and the carrier gas was hydrogen (H2). The AlN growth temperature was kept at 1150 ◦C and the

reaction pressure at 20 Torr during the experiments. In order to control the growth condition, a pyrometer

∗ Corresponding author: e-mail: inushima@keyaki.cc.u-tokai.ac.jp, Phone: +81 463 1211, Fax: +81 463 58 8320

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1672 Naoto Kato and Takashi Inushima: Evaluation of strain in AlN thin films grown on sapphire

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

(a combination of an InGaAs detector and a narrow band-pass filter with center= 1.33 µm and FWHM=

0.045 µm) was set 20 cm above the Mo susceptor, and infrared radiation from the AlN films was detected

through a hole (2 mm in diameter) made in a quartz inner-tube. The growth mode and the film thickness

were monitored and controlled by the pyrometer signals as was reported before [3]. The growth rate of

AlN was controlled by changing the flow rate of TMA from 0.32 to 13 µmol/min, where NH3 flow rate

was kept constant at 1 SLM. Accordingly, the growth rate was from 1.2 nm to 45 nm/min. The growth

temperature was measured and kept constant by a thermocouple located underneath the susceptor. The film

thickness was fixed at 150 nm throughout the measurements. For the evaluation of the low temperature

buffer layer, a 10 nm-thick AlN layer was deposited prior to the main growth on the substrate at 1000 ◦C

at the growth rate of 3 nm/min.

3 Results and discussion The strain in the film was evaluated by the lattice constants of the film. The

c-axis lattice constants were measured by (0002) reflection using a high resolution four-Ge(022)-crystal

XRD. Figure 1 shows the XRD charts of AlN grown on sapphire (0001) and 6H-SiC. The vertical dashed

lines in the figures indicate the peak position of the unstrained powder AlN(0002), which corresponds

to c0 = 4.979 A [4]. According to the decrease of the growth rate, the c-axis lattice constant increases

and approaches the ideal value for the 6H-SiC and exceeds that for the sapphire. When the substrates are

coated with low temperature AlN, the lattice constant becomes shorter for both cases. When the growth

rate was 45 nm/min, the AlN film became polycrystalline and the X-ray intensity decreased. We hence

increased the film thickness to 2400 nm in order to increase the scattering intensity. The film grown under

this condition contained reflections of AlN, which were not expected from 2θ − ω scan, suggesting that

the film was poly-crystalline. Although for the evaluation of the strain in the a–b plane, the a-axis lattice

a) b)

Fig. 1 XRD charts of 150 nm-thick (2400 nm-thick for AlN-BL) AlN grown on the c-plane of sapphire (a) and on

6H-SiC (b). The (0002) reflections of AlN grown on SiC are asymmetric due to the strong (0006) reflection from the

substrate. In both measurements, the direction of the X-ray beam is taken along [1010]. At the vertical dashed lines

c0 = 4.979 A.

constants should be determined from asymmetric Bragg spots, the film is not thick enough to determine

them accurately. Hence we determine them using the E2 (high) Raman active mode, because it is sensitive

to the strain in the a–b plane, and Saura et al. [5] have determined the in-plane strain of AlN layers grown

on (111)-oriented silicon substrates from the shift, and obtained good agreements with the deformation

potentials by ab initio calculation [6]. Figure 2 shows the Raman spectra of the samples given in Fig.

1. In the back scattering configuration, only A1(LO) and E2 modes are observed, and the intensity of

the A1(LO) is weak. In Fig. 2, non-strained E2(high) frequency (ω0) of 655 cm−1, which is obtained

using non-strained AlN grown by sublimation method [7], is indicated as a vertical dotted line. As for the

AlN on 6H-SiC, when the growth rate decreases, the Raman shift increases and approaches ω0. When the

phys. stat. sol. (c) 3, No. 6 (2006) 1673

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low temperature buffer layer is applied, the Raman intensity degrades and Raman shift becomes smaller

than 10 cm−1 from ω0 though the film is 20 times thicker than those of other samples. As for the Raman

shift of the films grown on sapphire, Raman spectra are observed around 655 cm−1 and their FWHM is

similar to each other including those of the films grown on the low temperature buffer layer. The line shape

representing the films grown at 45 nm/min are asymmetric for both substrates, which suggests that there

are distributions of Raman shifts, and especially on the sapphire, the Raman shift exceeds ω0, for several

cm−1. The strain in the a-b plane is derived from E2(high) phonon frequency shifts (∆ωλ), where the film

a) b)

Fig. 2 Micro-Raman scattering spectra of AlN on sapphire (a) and on 6H-SiC (b). The polarized incident and reflected

beams (λ = 0.633 nm) are parallel to the crystal c-axis. Resolution of the experiment is 1 cm−1. At the vertical dashed

lines ω0 = 655 cm−1.

is assumed to be under biaxial stress. The ∆ωλ can be written as [5]

∆ωλ = ω − ω0 = 2aλεxx + bλεzz, (1)

where aλ and bλ are the phonon deformation potential components of the E2(high) phonon mode of AlN,

and εxx is the in-plane (εxx=εyy) component of the strain tensor determined by εxx = (a − a0)/a0, where

a0 = 3.111 A, cited from the powder data [4]. εzz is the normal component of the strain tensor and is

expressed by εzz = (c − c0)/c0. The values of aλ = −877 cm−1 and bλ = −911 cm−1 are reported by

ab initio calculation [6]. Using these parameters, εxx of each sample is obtained from Eq. (1). Finally, the

growth rate dependence of the lattice constants (a, c), which are derived from εxx and εzz , is obtained and

is shown in Fig. 3.

When we plot the lattice constants a as x-axis and c as y-axis as shown in Fig. 3, (a, c) of the AlN films

grown on 6H-SiC and sapphire under the growth rate lower than 3 nm/min fall on a straight line, which is

drawn as a dashed line. This line is the prediction by the theory of elasticity, for bi-axially strained films

with the lattice constants a0 = 3.111 A and c0 = 4.979 A. A different choice of bulk lattice constants

would produce a line parallel to the one shown in the figure. When the growth rate is 45 nm/min, the

AlN films either on sapphire or on SiC are poly-crystalline and their (a, c) is located in the compressed

conditions as is shown in Fig. 3.

As Fig. 3 shows, the AlN film on sapphire is in a tensile stress condition when the growth rate is 3

nm/min, but when the growth rate becomes lower than 2.4 nm/min, it goes into a compressed condition

and then is stabilized. The most stress-free condition is obtained when the low temperature buffer layer is

applied prior to the growth at high temperatures. As for the AlN film grown on 6H-SiC, it is in a tensile

stress condition when the growth rate is 3nm/min, and when the growth rate is lowered, the film goes

into a less stressed condition and is stabilized. When the low temperature buffer layer is introduced on

6H-SiC, the film becomes the most tensile stressed condition, which suggests that the crystal nucleation

1674 Naoto Kato and Takashi Inushima: Evaluation of strain in AlN thin films grown on sapphire

© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-c.com

Fig. 3 Lattice constants a and c in A for

AlN grown on sapphire (open symbols)

and on 6H-SiC (closed symbols). The

underlined figures are the growth rate in

nm/min. The dashed line is the predic-

tion from elasticity theory for strained films

with lattice constants a0 = 3.111 A and

c0 = 4.979 A. A different choice of bulk

lattice constants would produce a line par-

allel to the one shown in the figure.

mechanism of low temperature buffer on 6H-SiC is different from that on sapphire. When the film becomes

poly-crystalline, it does not follow the theory of elasticity.

At present it is reported that the thermal expansion coefficients of AlN, 6H-SiC and sapphire in high

temperature regions are 2×10−6/◦C [8], 4×10−6/◦C [8] and 8×10−6/◦C [9], respectively. If the value

of AlN is correct, both of the films on sapphire and on 6H-SiC should be in a compressed condition at

room temperature. The experimental data shown in Fig. 3, however, suggest that the thermal expansion

coefficient of AlN should be somewhere between those of sapphire and 6H-SiC, and should be ∼ 6×10−6

◦C.

4 Conclusion The effect of growth rate on the strain in AlN films was investigated. When the growth

rate was lower than 2.4 nm/min, the strain in the film was constant and was compressive for the film grown

on sapphire and it was tensile for that on 6H-SiC. These stresses were produced by the difference of the

thermal expansion coefficients between the films and substrates. The thermal expansion coefficient of AlN

should be between that of sapphire and 6H-SiC and should be ∼ 6× 10−6/◦C. The low temperature AlN-

buffer layer, which was useful for the high temperature growth of AlN on sapphire, was revealed not to be

applicable for 6H-SiC.

Acknowledgements The authors are greateful to the Ministry of Education, Science, Sports, and Culture of Japan

for the Grant-in-Aid for Scientific Research No. 17560293, and to the Nippon Sheet Glass Foundation for the financial

supports.

References[1] S. Nakamura and S. F. Chichibu (eds.), Introduction to Nitride Semiconductor Blue Lasers and Light Emitting

Diodes (Taylor and Francis, London, 2000).[2] J. Suda, K. Miura, M. Honaga, Y. Nichi, N. Onojima, and H. Matsunami, Appl. Phys. Lett. 81, 5141 (2002).[3] T. Suzuki and T. Inushima, Mater. Sci. Forum 457-460, 1565 (2004)[4] JCPDS X-ray database, No. 25-1133.[5] A. Sarua, M. Kuball, and J. E. Van Nostrand, Appl. Phys. Lett. 81, 1426 (2002).[6] J.-M. Wagner and F. Bechstedt, Appl. Phys. Lett. 77, 346 (2000).[7] M. Strassburgh, J. Senawiratne, N. Dietz, U. Haboeck, A. Hoffmann, V. Noveski, R. Dalmau, R. Schlesser, and

Z. Sitar, J. Appl. Phys. 96, 5870 (2004).[8] Landolt-Bornstein, New Series, III/17a, edited by O. Madelung (Springer, New York, 1982), pp. 136–156.[9] Saint-Gobain Crystals Corporation (EFGTM sapphire) data sheet.