Growth and optical characterization of
`aluminum nitride thin films deposited on
`silicon by radio-frequency sputtering
`
`Cite as: Appl. Phys. Lett. 74, 1209 (1999); https://doi.org/10.1063/1.123501
`Submitted: 24 July 1998 . Accepted: 05 January 1999 . Published Online: 23 February 1999
`
`E. Dogheche, D. Rémiens, A. Boudrioua, and J. C. Loulergue
`
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`Appl. Phys. Lett. 74, 1209 (1999); https://doi.org/10.1063/1.123501
`
`74, 1209
`
`© 1999 American Institute of Physics.
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`Page 1 of 4
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`APPLIED MATERIALS EXHIBIT 1029
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`

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`APPLIED PHYSICS LETTERS
`
`VOLUME 74, NUMBER 9
`
`1 MARCH 1999
`
`Growth and optical characterization of aluminum nitride thin films
`deposited on silicon by radio-frequency sputtering
`E. Doghechea) and D. Re´miens
`Laboratoire des Mate´riaux Avance´s Ce´ramiques, Universite´ de Valenciennes et du Hainaut,
`Cambre´sis Le Mont-Houy BP311, Valenciennes F-59304, France
`A. Boudrioua and J. C. Loulergue
`Laboratoire Mate´riaux Optiques a` Proprie´te´s Spe´cifiques (MOPS), Centre Lorrain d’Optique et
`d’Electronique du Solide (CLOES) Universite´ de Metz et Supe´lec, 2 rue E. Belin 57070 Metz, France
`共Received 24 July 1998; accepted for publication 5 January 1999兲
`Highly textured hexagonal aluminum nitride 共AlN兲 thin films were deposited on silicon substrates
`by radio-frequency magnetron sputtering at a substrate temperature below 400 °C and annealed in
`the temperature range of 400–450 °C by rapid thermal annealing. The optical and the electro-optical
`properties have been investigated using the prism-coupling technique. Both ordinary and
`extraordinary refractive indices (n 0⫽2.0058 and n e⫽2.0374 at 632.8 nm兲 were respectively
`determined from the transverse electric and the transverse magnetic mode excitations. Furthermore,
`refractive index profiles analysis by using an improved inverse Wentzel–Kramer–Brillouin method
`reveals a step-like behavior of AlN thin films. The optical losses have been evaluated to be around
`7 dB cm⫺1. The electro-optic coefficient r 13 of 0.98 pm/V has been measured from the variation of
`the shift of guided-modes spectrum as a function of the applied electric field in the experiment.
`© 1999 American Institute of Physics. 关S0003-6951共99兲03709-2兴
`
`Aluminum nitride 共AlN兲 thin films are of increasing in-
`terest for a large number of applications in microelectronic
`field. Because of its wide direct band gap, AlN is a promis-
`ing material for integrated optics in the ultraviolet 共UV兲 re-
`gion, i.e., laser diode,1 detectors,2 and fabrication of high
`frequency surface acoustic wave 共SAW兲 devices.3 Moreover,
`the high insulating and conducting properties of AlN can be
`used in the fabrication of III–V based electronic structures.
`Various techniques have been developed for growing thin
`films of AlN.4,5 Most recent works have been performed on
`epitaxial growth of aluminum nitride using sapphire as a
`substrate.6 However, silicon substrates offer more advan-
`tages for nitride devices over sapphire: it is ideal in integrat-
`ing electronic and optical devices at low costs using estab-
`lished silicon technology.
`In this letter we report
`the
`investigation of the optical and the electro-optical properties
`of AlN thin films by using the prism-coupling technique.7,8
`This work is both focused on the optimization of the growth
`process and the comprehensive study of the relationship be-
`tween the structure and the optical characteristics.
`For our study, AlN films were deposited by radio-
`frequency 共rf兲 magnetron sputtering on Si/SiO2 substrates
`from aluminum nitride targets, in a gas mixture of argon 共Ar兲
`and nitrogen (N2) with a purity of 99.999%, respectively. In
`order to improve the structural properties of nitride com-
`pounds, a two-step process was used. First, the substrate
`temperature was maintained at a low temperature of 400 °C
`in the in situ growth. Second, a rapid thermal annealing
`共RTA兲 process was achieved for enhancement of the crystal-
`line quality of the material. The optimization of the growth
`conditions leads to a single crystal AlN with a relatively
`smooth surface. We found that the growth rate was about 5
`
`a兲Electronic mail: elhadj.dogheche@univ-valenciennes.fr
`
`nm/min. The sputtering conditions are listed in Table I.
`To investigate the structural properties of our AlN thin
`films, we used the x-ray diffractions 共XRD兲 patterns in the
`␪–2␪configuration, as reported in Fig. 1. The growth direc-
`tion for AlN films is 共002兲, the best texture is obtained at a
`low temperature 共400 °C兲 with a N2 content of 30%. The
`orientation of the film is generally controlled by its interac-
`tion with the substrate and by kinetics of the growth process.
`As reported by Dovidenko et al.,5 this effect is probably
`caused by the grain-boundary separating individual grains.
`These planar defects are mainly generated in the closed-
`packed plane of AlN during the growth process.
`For the optical characterizations, we have used the
`prism-coupling technique. We report in Figs. 2 and 3, on
`transverse electric 共TE兲 and transverse magnetic 共TM兲
`guided-modes spectra, respectively. Five guided modes have
`been excited for each case. We notice the sharpness of the
`reflectivity dips indicating a good confinement of the light
`into the wave guide. Therefore, moderate optical losses in
`the AlN film can be suggested. Indeed, those later have been
`evaluated by using the charge coupled device 共CCD兲 camera
`
`TABLE I. Sputtering conditions for deposition of AlN thin films on silicon
`substrates.
`
`Parameter
`
`Condition
`
`Target
`Substrate
`Target-substrate spacing
`Substrate temperature
`rf power
`Gas pressure
`Ar/N2
`Deposition rate
`
`AlN 99.999%, 3 in. ⭋
`SiO2 /Si 共100兲
`50 mm
`400 °C
`300 W
`1.2 mbar
`70/30
`5 nm/min
`
`0003-6951/99/74(9)/1209/3/$15.00
`
`1209
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`© 1999 American Institute of Physics
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`Page 2 of 4
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`1210
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`Appl. Phys. Lett., Vol. 74, No. 9, 1 March 1999
`
`Dogheche et al.
`
`FIG. 1. X-ray diffractions patterns of aluminum nitride thin films deposited
`on silicon substrate.
`
`technique9 to be in the order of 7.3 dB cm⫺1. Higher losses
`have been obtained for higher-order modes excited in the
`film. The wave guide losses are known to be strongly depen-
`dent on the surface roughness of the film. The surface mor-
`phology of AlN samples has been examined by atomic force
`microscopies 共AFM兲 showing a good surface quality for our
`films (r ms about of 40 Å兲. In order to minimize the optical
`losses, the epitaxial growth of AlN thin films is required.
`From the angular position of the guided modes, we com-
`puted the corresponding effective indices and hence the re-
`fractive indices and the film thickness. For our samples, the
`ordinary (n 0) and the extraordinary (n e) refractive indices
`are 2.0058⫾0.0004 and 2.0374⫾0.0006 (␭⫽632.8 nm) re-
`spectively. These values are similar to those reported in the
`literature.6,10 However, slight deviations of refractive indices
`are obtained in comparison with the corresponding AlN
`single crystal ordinary refractive index n 0 which is around
`2.16.11 This is mainly attributed to the nitrogen vacancy or
`oxygen impurities. The thickness was determined to be
`1.28⫾0.04 ␮m which is in agreement with the scanning
`electron microscope 共SEM兲 investigation.
`Note that in this study, we focused our attention to the
`simple case of light propagation in anisotropic uniaxial thin
`film deposited onto an isotropic substrate. In this configura-
`tion, TE and TM modes can exist separately. Therefore, the
`problem was treated using the well-known guided-modes
`dispersion equation. However, the deposition technique may
`
`FIG. 3. TM guided mode spectra obtained by measuring the reflected inten-
`sity vs the angle of incidence (n e excitations兲.
`
`yield thin films with the optical axis tilted from normal to the
`substrate surface. In this situation, TM modes are affected by
`the tilt angle 共␸兲 of the optical axis which determines the
`refractive index seen by the optical wave propagating within
`the guiding structure. The calculation procedure12 provides a
`weak tilt angle 共␸兲 of nearly 6° with respect to the normal to
`the substrate surface, confirming the uniaxial nature of our
`AlN thin films deposited by rf sputtering, with the optical
`axis very likely oriented perpendicular to the substrate sur-
`face.
`To complete this analysis, we have reconstructed the re-
`fractive index profiles directly from the measured effective
`indices by using an improved version of
`the inverse
`Wentzel–Kramer–Brillouin (iWKB) method. This method
`only depends on the refractive index distributions within the
`guiding layer. More details of calculation are given by
`Chiang.13 Using a polynomial interpolation of the measured
`effective indices, we computed the refractive index profiles
`as a smooth function of the thickness. As shown in Fig. 4,
`the refractive index profiles indicate a step-index variation
`which is synonymous of a good optical homogeneity along
`the film thickness. Indeed, the refractive index remains con-
`stant within the guiding region and decreases rapidly near the
`film–substrate interface. Therefore, this result did not show
`any clear influence of the substrate on the growth process.
`
`FIG. 2. TE guided mode spectra obtained by measuring the reflected inten-
`sity vs the angle of incidence (n 0 excitation兲.
`
`FIG. 4. Ordinary (n 0) and extraordinary (n e) refractive index profiles ob-
`tained by an improved inverse WKB method for AlN thin films deposited on
`silicon substrates.
`
`Page 3 of 4
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`

`

`Appl. Phys. Lett., Vol. 74, No. 9, 1 March 1999
`
`Dogheche et al.
`
`1211
`
`Using TE guided modes, we have investigated the elec-
`trooptic 共EO兲 coefficient using the angular shift technique as
`described by Boudrioua et al.14 The top electrode consists of
`a semitransparent gold film with a thickness of 10 nm. By
`applying a transverse electric field through the AlN layer, a
`change of the resonant coupling angle 共⌬␪兲 in the guided-
`modes spectrum has been observed. This effect is directly
`correlated to the variation of the refractive index (⌬n) due to
`the EO effect. Finally, the linear EO coefficient r 13 obtained
`is evaluated to be 0.98 pm/V.
`In summary, AlN thin films have been grown on Si/SiO2
`substrates by radio-frequency magnetron sputtering from an
`aluminum nitride target. The deposition parameters and an-
`nealing process were optimized for the elaboration of highly
`textured AlN thin films. We have investigated the optical
`performances of the films using the prism-coupling tech-
`nique. Refractive indices were therefore determined to be
`n 0⫽2.0058 and n e⫽2.0374 at 632.8 nm. From the effective
`guided-mode indices, the analysis of the optical anisotropy
`confirmed the uniaxial nature of the AlN thin film with the
`optical axis likely oriented normal to the surface of the sub-
`strate. The optical losses were evaluated to be around 7
`dB cm⫺1. The EO measurements using the angular shift
`
`method showed a linear electro-optic coefficient r 13 of about
`0.98 pm/V. These results demonstrate the interest of AlN
`thin films to be used in integrated optics applications.
`
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