Substrate Effects on The Growth of Oriented MgO Thin Films
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Table I. XRD data for MgO deposited on (100) SrTi0 T (0 C)
MgO/SrTiO 3 (200) Intensity
MgO/SrTiO 3 (200) Rocking
2000C 300 0C 4000C 5000 C 600 0C 7000C
(cps) 85,000 149,000 164,000 155,000 128,000 95,000
3
and (100) LaAl03
MgO/LaA10 3 (200) Intensity
MgO/LaA10 3 (200) Rocking
Curve FWHM
(cps)
Curve FWHM
0.950 1.040 0.980 0.900 0.910 1.0700
12,780 45,000
2.511.410
48,000 29,000 6,500
1.450 1.600 1.690
these block temperatures which are probably higher than the actual substrate temperatures. MgO was sputtered from a 2-inch polycrystalline MgO target at a rf power of 100W which yielded a self-bias of -50V and a deposition rate of -12A/min. The sputtering gas used was an 80%Ar - 20%02 mixture (by flow) with a total spu'vtering pressure of 2.5 mTorr. 10003000A thick films were deposited at temperatures betwee.i 200*C and 7000 C. The chamber was flooded with 500 Torr of 02 upon the completion of each deposition run and the substrates held at temperature or 400°C (whichever was lower) for 30 minutes before cooling. Film orientation was studied using 0/20 and four-circle X-ray diffraction (XRD), and rocking curve measurements. The microstructure of the films was examined using scanning electron microscopy (SEM). Atomic force microscopy (AFM) was used to study the surface morphologies of the substrates.
RESULTS AND DISCUSSION MgO thin films grown on (100) SrTiO 3 at temperatures between 200*C and 700 0C all possess a (100) orientation as determined by 0/20 XRD. Full width half maximum (FWHM) values from rocking curve measurements of the MgO (200) peak range from 0.9' to 1.10 (see Table 1). Epitaxial growth of the films at all temperatures was confirmed by four-circle
35000 30000
MgO (220)
"j25000 '20000
S10000 5000 0 0
10 20 30 40 50 60 70 80 90 100
phi (deg.) Figure 1. X-ray ý-scan of the MgO (220) peak of film grown on (100) SrTiO 3 at 500*C.
456
(a)
(b)
(c)
(d)
Figure 2. SEM micrographs of MgO sputtered onto (100) SrTiO3 at (a) 7000 C, (b) 4000 C, (c)
300*C, and (d) 200MC. diffractometry 0 scans which showed MgO [100] 11SrTiO 3 [100], or "cube on cube" growth (see Figure 1). At deposition temperatures above 435°C, MgO films display a smooth and dense microstructure as observed by SEM (see Figure 2a). A deterioration in the microstructure begins to occur below 4350 C, however. Figure 2b shows the emergence of a faceted plate-like microstructure mixed with the smooth and dense material. This second microstructure increases with decreasing deposition temperature and dominates completely by 200'C (see Figures 2c and 2d). The "plates" appear to be oriented with respect to each other. The film density also decreases with decreasing temperature. Thicker films were obtained at lower temperatures even though the same amount of material was deposited at all temperatures as measured by a quartz crystal thickness monitor. MgO also grows epitaxially on (110) SrTiO 3 at 3000 C and 500'C. The films have a smooth and dense appearance at 5000 C but have a rougher surface at 300'C, similar to the (100) fil
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