Piezoelectric Coefficients of Aluminum Nitride and Gallium Nitride
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as GaN. These parameters are important since both of them have potential use in microactuators, microwave acoustic and other microelectromechanical (MEM) devices [5]. EXPERIMENT AIN film grown by MBE AIN thin films were grown by using a SVTA BLT-N35 MBE system. The nitrogen molecules in a nitrogen gas stream were broken into atomic nitrogen by a SVTA RF plasma source operating at 13.56 MHz. Aluminum was evaporated by the K-cells. The atomic nitrogen and aluminum react to form AIN on a (11) silicon substrate. The silicon substrate was cleaned by a degreasing process and etched in buffered HF. After the substrate had been transferred into the deposition chamber, the substrate surface was thermally cleaned at 940 'C for 1 hour. The substrate temperature was then decreased to about 600 'C and AIN thin film started to grow. The final thickness of the AIN film was about 450nm. Sample geometry In this experiment, two samples were studied. One is a 450nm thick AIN thin film while the other is a composite film, comprising a 140nm thick GaN film grown on a 30nm thick AIN 389
Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
buffer layer. Both of the films were grown on silicon substrates with (111) orientation. Figures 1 and 2 show the X-ray diffraction (XRD) patterns of the AIN and the composite film, respectively. The peak at 35.9' corresponds to the (0002) reflection of wurtzite AIN [6]. Another peak at 34.60 corresponds to the (0002) reflection of wurtzite GaN [7]. The XRD patterns confirmed that the films have the wurtzite structure, and the c axis is oriented along the normal of the substrate.
SSi(11)
-
Si(l 11)
GaN0002)
2(0002)
_N C
C
,
1'
25
30
35
40
25
Two Theta (deg)
3'0
AN(0002)
35
40
Two Theta (deg)
Figure 2: The XRD pattern of the composite film.
Figure 1: The XRD pattern of the AIN film.
Circular dots of aluminum of diameter 1mm were thermally evaporated at different positions on the top surface of the films. These dots act as top electrodes as well as mirrors to reflect the laser beam from the interferometer. The silicon substrate was attached to an aluminum block (connected to ground) fixed on a translation stage. An a.c. electric field was applied across the top electrode and the aluminum block. The thickness of the film changes due to the converse piezoelectric effect and the small displacement was measured using a Mach-Zehnder type heterodyne interferometer. Measurement of d33 using laser interferometry Figure 3 shows the schematic diagram of the Mach-Zehnder type heterodyne interferometer (SH-120 from B. M. Industries, France) used to measure the displacement. A linearly polarized laser beam, L (frequencyfL; wave number k=2,r/A, A = 632.8 nm for a He-Ne laser) is split into a reference beam, R, and a probe beam, P, by a beam splitter (BS). R propagates through a Dove prism and a polarizing beam splitter (PBS) into a photodiode. The frequency of P is shifted by a frequency fB (70 MHz) in a Bragg cell, and then this beam (now labeled S), is phase modulated by th
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