Structural and optical properties of sputtered AlN thin films Padmalochan Panda, R. Ramaseshan, S. Tripura Sundari, R. Rajaraman, H. Suematsu, and S. Dash
Citation: AIP Conference Proceedings 1832, 080061 (2017); doi: 10.1063/1.4980521 View online: http://dx.doi.org/10.1063/1.4980521 View Table of Contents: http://aip.scitation.org/toc/apc/1832/1 Published by the American Institute of Physics
Structural and Optical properties of Sputtered AlN Thin Films Padmalochan Panda1, R. Ramaseshan1*, S. Tripura Sundari1, R. Rajaraman1, H. Suematsu2 and S. Dash1 1
Materials Science Group, Indira Gandhi Center for Atomic Research, HBNI, Kalpakkam-603102, India. 2Extreme Energy Density Research Institute, Nagaoka University of Technology, Nagaoka, Japan. *
Email: [email protected]
Abstract. Wurtzite type AlN thin films were grown on Si (100) substrate using DC reactive sputtering with varying substrate temperatures (35-600 °C). The phonon vibrational study of these films was performed by Raman Spectroscopy, where E2 (High) mode was observed with a movement of peak position from red shift to blue shift due to change in residual stress. Cross sectional TEM micrograph of these AlN films showed a columnar structure. Using phase modulated ellipsometry, refractive index and extinction co-efficient of the film were extracted based on new amorphous model and found to be 2.2 and 0.03 at a wavelength of 250 nm, respectively. Keywords: Thin films, Sputtering, Raman, Cross-sectional TEM, Ellipsometry. PACS: 81.15.-z; 81.15.Cd; 78.30.Ly; 68.37.Lp; 07.60.Fs.
INTRODUCTION Aluminum nitride (AlN) is a promising material for optoelectronic devices and semiconductor industries due to its wide band gap, high thermal conductivity and small thermal coefficient of expansion. Oriented AlN films are expected to play an important role in deep-ultraviolet light-emitting diode in the range of 200 to 300 nm wavelengths as well as electro-acoustic devices due to high refractive index, low-absorption coefficient and excellent piezoelectric response [1-3]. According to the application point of view, it is important to study the optical properties of AlN film in wide range of wavelength (NIR – UV). So, in this report, we are discussing the vibrational properties, microstructure and optical properties of AlN thin films deposited on Si (100) substrate by reactive sputtering technique.
EXPERIMENTAL DETAILS AlN thin films were deposited on Si (100) substrates by DC reactive magnetron sputtering (M/s. MECA 2000, France) by using a high pure Al targets with constant Ar to N2 ratio (16:4 SCCM) by varying substrate temperatures (Ts, 35 - 600 °C). The deposition parameter like target to substrate distance, deposition pressure and time as 14 cm, 5 x 10-3 mbar,
and 30 min, respectively were kept constant. These films thickness was measured by a surface profiler (M/s. Dektak 6M, Veeco, USA) and it was around 800 nm. Raman Spectroscopy (M/s. InVia, Renishaw, UK) was performed by using an excitation of 514.5 nm Ar+ laser source in the back scattering geometry. The analyzer was kept to analyze the perpendicular polarization scattered light through a polarizer to the incident light at Z ( XY ) Z configuration to reduce the substrate information. A transmission electron microscopy (M/s JEM-2100F, JEOL, Japan) was used for cross sectional micro-structural studies of these films. The optical properties were studied by a phase modulated spectroscopic ellipsometer (M/s. Horiba, Uvisel, France) at 70q angle of photon incidence with energy range 0.6 to 6.5 eV under ambient conditions.
RESULTS AND DISCUSSIONS Polarized Raman spectra of AlN films with different substrate temperatures varying from 35 to 600 °C are shown in Fig.1. According to the group theory prediction, AlN structure shows A1, E1 and two E2 modes of phonon vibration in Raman scattering . However, we have observed E2 (High) vibrational mode of AlN films with two more peaks around 620 cm-1 and 810 cm-1, which corresponds to Si substrate bands. An increase in substrate temperature to 600 °C
DAE Solid State Physics Symposium 2016 AIP Conf. Proc. 1832, 080061-1–080061-3; doi: 10.1063/1.4980521 Published by AIP Publishing. 978-0-7354-1500-3/$30.00
results in a significant increase in the intensity of the E2 Raman peak, where the peak shift is ranged from 638 to 658 cm-1. Prokofyeva et al reported that Raman shift of a stress free AlN film is around 653.6 cm-1 . A red shift in E2 (High) mode peak at lower temperature (35 to 300 °C) implies that these films have tensile residual stress. However at 400 °C onwards a blue shift was observed, corresponding to a compressive residual stress in these films. This clearly shows that the Raman spectra of AlN films are strongly dependent on the residual stress of these films and indirectly on the substrate temperature.
phonon scattering effect at grain boundaries due to smaller crystallite size and defects in these films. In this case, the peak broadening is also enhanced by the Si substrate peak at 620 cm-1 . The dark-field TEM cross sectional image of AlN film deposited at a ST of 100 °C is shown in Fig.3. This confirms a columnar AlN structure with a thin interface between the coating and the Si substrate. This is a thin layer of Al deposited on the substrate to improve the bonding between AlN and Si substrate. The thickness of AlN is around 720 nm with a smooth surface topography.
FIGURE.3. Dark-field cross-sectional TEM micrograph of AlN film on Si (100) substrate deposited at 100 °C substrate temperature.
FIGURE.1. Raman spectra of Si (100) substrate and AlN films at substrate temperatures (35 to 300 °C).
Spectroscopic ellipsometry is a non-contact and sensitive characterization technique that has been used to determine the optical properties of thin films. Based on the input from cross sectional TEM, a five layer model (air / roughness / AlN / interface (Al+AlN) / cSi) has been employed to fit the ellipsometry data using Bruggeman effective medium approximation. Using phase modulated ellipsometry, a measurement of ellipsometric parameters defined by Is and Ic Eq. 1 were obtained. (1) Is = Sin (2