Growth of ZnO/Al2O3 alloy films using atomic layer deposition techniques

Atomic layer deposition (ALD) is an ideal technique for fabricating composite thin films. The thickness and stoichiometry of composite thin films prepared using ALD is dependent on the underlying surface chemistry during ALD film growth. A set of ZnO/Al2O3 alloy films was prepared by ALD in a viscous flow reactor using alternating Zn(CH2CH3)(2)/H2O exposures for ZnO ALD and alternating Al(CH3)(3)/H2O exposures for Al2O3 ALD. The ZnO reaction cycle percentage was varied from 0 to 100%. The composite film thicknesses were measured using ex situ stylus profilometry and ellipsometry. The atomic composition of the composite films was established by atomic emission spectroscopy. Large deviations were found when the measured thicknesses and compositions were compared with rule of mixtures predictions. Many of the ZnO/Al2O3 alloy films showed lower than expected Zn film content and were thinner than predicted by the ALD growth rates of the pure ZnO and Al2O3 films. To understand these discrepancies, in situ quartz crystal microbalance measurements were performed during the ZnO/Al2O3 alloy film growth. The QCM measurements revealed that the Zn deficiency may result from the etching of Zn by the Al(CH3)(3) precursor during the Al2O3 ALD cycles. In addition, the lower than expected film thicknesses are caused by a reduced initial growth rate for ZnO ALD on Al2O3 or Al-doped ZnO surfaces and Al2O3 ALD on ZnO or Zn-doped Al2O3 surfaces. A nucleation period of 4-6 cycles for ZnO ALD was observed following one or more Al2O3 reaction cycles. Similarly, a 2-3 cycle nucleation period was monitored for Al2O3 ALD following the ZnO reaction cycles. Understanding the underlying surface chemistry during ALD helps predict the thickness and composition of composite thin films grown using ALD techniques.