Influence of the Growth Atmosphere on the Properties of Ain Grown by Plasma - Assisted Pulsed Laser Deposition
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substrate temperature. The deposition of AIN films was carried out in three types of environments: (a) in vacuum (10-v Torr), (b) in nitrogen gas ambient, and, (c) in nitrogen plasma ambient for 10 hours. The AIN deposition in nitrogen gas was performed at 40 mTorr. The dc glow discharge nitrogen plasma was generated by using a ring-shaped electrode biased negatively to about 600 V placed between the target and the substrate as shown in Fig. 1. In this case, the substrate and the target were grounded. The discharge current was adjusted to be less than 1 mA. A faint violet glow could be observed near the ring. LASER TARGEr
-450 V
N,
Fig. 1 Schematic diagram of the PLD apparatus. The AIN thin films were characterized by the X-ray diffraction (XRD) technique, scanning electron microscopy (SEM), Rutherford backscattering spectrometry (RBS), micro-Raman scattering spectroscopy, and cathodoluminescence (CL). The x-ray source is Cu Ka. RBS measurements were performed using 2.1 MeV He2" ions. Raman spectra were excited by an argon-ion laser operated at 514.5 nm with 200 mW. CL spectra were measured at room temperature using a 20 keV electron beam with a current of the order of 10 nA as the excitation source. RESULTS AND DISCUSSIONS The XRD measurements were carried out to confirm the growth of AIN thin films. For all types of AIN films, dominant (0002) diffraction peaks were observed, indicating the oriented growth of AIN films on the Si (100) substrate. The XRD pattern of the AIN film grown in high vacuum is shown in Fig. 2.
z-C
r 20
30
40
50
60
20 (deg.)
70
80
Fig. 2 0-20 x-ray diffraction pattern of the AIN film grown in high vacuum (10 7 Torr) on Si (100) of 800 'C.
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Figures 3 (a), (b), and (c) show the SEM micrographs of the AIN films grown in the vacuum, nitrogen gas ambient and nitrogen plasma ambient, respectively. Comparison of these figures indicates that growth of AIN film in vacuum resulted in a rough surface because large particles could reach the substrate during the deposition. However, the presence of nitrogen gas during deposition seems to prevent large particles from depositing on the substrate as shown in Figs. 3 (b) and (c). Moreover, the generation of nitrogen plasma may enhance the chemical reactivity o" nitrogen on the surface. This effect seems to reduce grain size as shown in Fig. 3 (c). This smaller grain may lead to a smoother surface.
(a)
(b)
(c) Fig. 3 SEM micrographs of AIN films deposited (a) in high vacuum, (b) in nitrogen gas, and (c) in nitrogen plasma.
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In general, the film fabricated using the PLD technique most nearly maintains the target composition ratio. The target used in this work is deficient in nitrogen and contains yttrium as an impurity (Al : 62.8%, N : 32.1%, Y : 3.4 %, 0: 1.7%). Nitrogen gas and nitrogen plasma ambient were used in order to compensate for the shortage of nitrogen. The effects of these ambient were characterized by a RBS study which determined the relative N/Al ratio in the grown films. The results are shown in Fig. 4. Figures 4 (a
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