Al Grain Size

XRD was employed to characterize the crystallite sizes of aluminum films in the SiOxNy/Al/glass stack post SiOxNy deposition using the full width at half maximum of the dominant Al (111) peak and Scherrer’s equation (3.1). Figs. 4.9 and 4.10 showed Al (111) peaks in SiOxNy/Al/glass stack for SiOxNy films prepared at high flow rate and low flow rate conditions, respectively. Moreover, the crystallite sizes of the Al films at various SiOxNy deposition conditions were summarized in Table 4.7. It showed that films prepared under high substrate temperature possessed

larger crystallite sizes (~60 nm) than the ones under low temperature conditions (~40 nm). It implied that the energy derived from SiOxNy deposition induced grain growth in the underlying Al film.

In this study, Al film was first deposited onto glass plate at 60 ^oC using the same modified ion beam deposition system. The microstructure of Al film was expected to be about the same as that in SiOxNy/Al deposited also at 60 ^oC as illustrated in Fig.

4.12.

In the modified ion beam deposition system, Al/glass substrate was kept at room temperature (~20 ^oC) prior to the deposition of SiOxNy layer. The deposition temperature calibrated at the top of glass plate was controlled by the lateral moving speed of glass plate. In order to avoid any damage onto the OLED devices, whose Alq3 layer cannot survive temperature > 100 ^oC, the deposition temperatures in this study were controlled at 60, 80 and 90 ^oC. However, the surface temperature of Al film during SiOxNy deposition was expected to be higher due to energy transfer by the evaporated atoms and molecules. The surface temperature could be estimated by the energy conservation described by Eq. 4.1.

ΔHAl+ ΔHbarrier= 0 (4.1) ΔH=mSΔT (4.2) where m is the mass, S is the heat of capacity, while ΔT is the difference between the final and initial temperatures for a process. In addition, SAl=0.217 cal/g℃ and Sbarrier=0.2 cal/g℃.

Hence, the Al surface temperature was estimated to be 247 ℃ and 361 ℃ for glass substrate temperature controlled at 60 ℃ and 80 ℃, respectively. Assuming the particle energy arriving to the surface of OLED structure was 25 eV as illustrated by Fig. 3.2 in the modified ion beam evaporation system, the temperature at Al surface was 387℃, which was in line with the estimation by Eq. 4.1. It was perceived that

these surface temperatures (T/Tm for Al ~0.32-0.55) were relative high to be able to drive additional Al grain growth during SiOxNy deposition.

This trend of grain size measured by XRD was closely related with the domains observed by top-view SEM as shown in Figs.4.11 (a) through (f). The cross-sectional microstructure of SiOxNy/Al/glass stack was further examined by a TEM as illustrated in Fig. 4.12. Al films exhibited columnar structure with width between 50 and 100 nm, while SiOxNy passivation showed amorphous phase and conformal coverage onto the underlying Al film. It demonstrated the morphology of SiOxNy passivation observed by top-view SEM was in response to the planar grain size of aluminum and, in some cases, hillocks. Then, the distribution of Al grain size in SEM image was analyzed by using IMAGE J software. The results of grain size distributions were listed in Figs. 4.13 (a) through (d). Fig. 4.13 (a) showed films deposited under 60 ℃ exhibited the smallest grain size and concentrated in 40-60 nm region. There was not much difference in different gas flow rates under 60℃. Fig.

4.13 (b) showed that the films deposited under 80℃ and high flow rate condition possessed grain size concentrated in 40-60 and 60-80 (nm) regions. However, the films deposited under 80℃ and low flow rate condition exhibited wide grain distribution in the range of 40-180 nm. Fig. 4.13 (c) showed that the films deposited under 90℃ and two different flow rate conditions exhibited the grain sizes concentrated in 80-120 nm. Fig. 4.13 (d) summarized the grain size distributions of Al films under the deposition of SiOxNy layer at different substrate temperatures but the ssme high gas flow rate. It showed there was not much difference in grain size distribution under 80 and 90℃, but it was apparently different at 60℃ compared to those at 80 and 90℃. Since the crystallite sizes of aluminum which derived from XRD is the average value, the distribution of Al grain size in SEM image, which analyzed by using IMAGE J software, can help us to analyze the Al grain boundary

size more quantitatively.

30 35 40 45

0 200 400 600 800 1000 1200

60oC 80oC 90oC

Intensity(cps)

2 Theta,degree

Figure 4.9 X-ray diffraction pattern of SiOxNy/Al/Alq3/glass sample, where SiOxNy

was deposited under high flow rate at various substrate temperatures.

30 35 40 45 0

200 400 600 800 1000

Intensity( cps)

2 Theta,degree

60oC 80oC 90oC

Figure 4.10 X-ray diffraction pattern of SiOxNy/Al/Alq3/glass, where SiOxNy was deposited under low flow rate at various substrate temperatures.

Table 4.7 The crystallite sizes of Al in SiOxNy/Al/Alq3/glass sample stack, where SiOxNy was deposited under different flow rate condition at various substrate temperatures.

λ=1.54Å θ FWHM

(B)

Grain size (nm)

Low flow rate 19.33 0.203π/180 41

60 ℃

High flow rate 19.27 0.228 π/180 37

Low flow rate 19.28 0.146π/180 58

80 ℃

High flow rate 19.25 0.148π/180 57

Low flow rate 19.23 0.146π/180 58

90 ℃

High flow rate 19.29 0.148π/180 57

Figure 4.11 SEM top-view graphs of the SiOxNy films deposited under high flow rate at various substrate temperatures: (a) 90 ℃, (b) 80℃, and (c) 60℃ and under low flow rate at various substrate temperatures: (d) 90 ℃, (e) 80℃, and (f) 60℃.

Figure 4.12 Cross-sectional TEM photograph of SiOxNy/Al/Alq3/glass stack for SiOxNy films prepared at high flow rate and substrate temperature 60℃ condition.

0 10 20 30 40 50 60 70

0-20 20-40 40-60 60-80 80-100 100-120

range of grain diameter(nm)

fra ctio n o f to ta l g ra in num ber (% )

Low flow rate 60℃

High flow rate 60℃

(a)

0

0

0-20 20-40 40-60 60-80 80-100 100-120 120-140 140-160 160-180 180-200 200-220

range of grain diameter(nm)

Figure 4.13 Al grain size distributions obtained fromSEM image: (a) SiOxNy films deposited under 60℃ and two different gas flow rate conditions. (b) films deposited under 80℃ and two gas flow rate conditions. (c) films deposited under 90℃ and two gas flow rate conditions. (d) films deposited under different substrate temperature and high flow rate conditions.