This reveals that the thickness of the ZnO
sublayer in the ZnO/Al2O3 composite films is a crucial parameter for the control of the formation of ZnO and ZnAl2O4 phases during the thermal annealing process. Taking into account of the etching during the Al2O3 cycle, the measured ZnO sublayer thickness is 0.91 and 2.01 Å in the samples with the ZnO/Al2O3 cycle ratios of 2:1 and 1:1. Comparing to the reported length of the Zn-O bond (1.98 Å) , the critical thickness of the ZnO sublayer is limited within one atomic layer for the formation of the ZnAl2O4 phase. This can be interpreted by the chemical reaction for synthesis of the ZnAl2O4, ZnO + Al2O3 = ZnAl2O4, where one monolayer of selleck chemicals Al2O3 Selleckchem PND-1186 consumes one atomic layer of ZnO. Thicker ZnO sublayer containing excess atomic layers has a priority forming in the ZnO crystal phase of the annealed ZnO/Al2O3 multilayers,
because the crystallization of ZnO need much lower energy than that for the ZnAl2O4 crystallization. Figure 9 XRD patterns of the compound films at different ZnO/Al 2 O 3 cycle ratios. Room temperature PL spectroscopy was used to analyze and control traceable amount of the crystalline ZnO phase in the annealed samples. Figure 10 shows the MK-8931 PL spectra from the ZnO/Al2O3 mutilayers annealed at 1,000°C with different cycle ratios of ZnO/Al2O3 from 1:2 to 5:1. No PL signal from the crystalline ZnO is observed for the annealed samples with the ZnO/Al2O3 cycle ratios at 2:1, 1:1, and 1:2, respectively; this is supported by the XRD results in Figure 9, which showed only diffraction peaks of spinel ZnAl2O4 without ZnO impurity phase in these samples. The PL intensity from ZnO near-band-edge emission increases strongly as the CYTH4 ZnO sublayer thickness increases above three ALD cycles; this is also in good agreement with the formation of ZnO phase in the samples with ZnO/Al2O3 ratios of 3:1 to 5:1. These results reveal that the presence of excess ZnO bonds leads to the formation
of the ZnO crystal phase due to the easy crystallization of ZnO. The specific multilayers containing alternative monatomic layers of ZnO and Al2O3 are crucial as the starting composite for synthesis of pure ZnAl2O4 films. The composite can only be deposited precisely through layer by layer ALD technology. Preformation of Zn-O-Al-O bonds at the interface of two ZnO/Al2O3 multilayers during the ALD process may play an important role for the crystallization of pure ZnAl2O4 films in the subsequent high-temperature annealing. Figure 10 Room temperature PL spectra of the ZnO/Al 2 O 3 composite films with different ZnO/Al 2 O 3 cycle ratios. Figure 11 shows the XRD patterns of the composite films after annealed at different temperatures ranging from 400 to 1,100°C, in which the ZnO/Al2O3 cycle ratio of the composite film was set to 1:1.