Abstract
Direct plasma enhanced-atomic layer deposition (PE-ALD) is adopted for the growth of ZnO on c-Si with native oxide at room temperature. The initial stages of growth both in terms of thickness evolution and crystallization onset are followed ex-situ by a combination of spectroscopic ellipsometry and X-ray based techniques (diffraction, reflectivity, and fluorescence). Differently from the growth mode usually reported for thermal ALD ZnO (i.e., substrate-inhibited island growth), the effect of plasma surface activation resulted in a substrate-enhanced island growth. A transient region of accelerated island formation was found within the first 2 nm of deposition, resulting in the growth of amorphous ZnO as witnessed with grazing incidence X-ray diffraction. After the islands coalesced and a continuous layer formed, the first crystallites were found to grow, starting the layer-by-layer growth mode. High-temperature ALD ZnO layers were also investigated in terms of crystallization onset, showing that layers are amorphous up to a thickness of 3 nm, irrespective of the deposition temperature and growth orientation.
Highlights
Zinc oxide (ZnO) is a II-VI semiconductor, well-studied because of its unique qualities
ZnO thin films were prepared at room temperature by plasma enhanced-atomic layer deposition (PE-atomic layer deposition (ALD)), as previously reported [18], and the thickness was varied by varying the number of cycles from 1 to 50 resulting in a maximum thickness of about 8 nm
In a recent in-situ study on the effect of annealing on PE-ALD ZnO, we demonstrated the presence of an amorphous fraction of material in the room-temperature-deposited
Summary
Zinc oxide (ZnO) is a II-VI semiconductor, well-studied because of its unique qualities. X-ray fluorescence and X-ray absorption near-edge structure spectroscopy) identified the onset of crystallization to start from the very first cycle when deposited on c-Al2 O3 , adopting a 2D-like growth and developing in-plane crystallinity; a different growth behavior was instead observed when deposited on a-SiO2 and In0.53 Ga0.47 As, on which the crystallinity starts after the development of an initial amorphous layer. In this contribution, the initial growth and onset of crystallization of ZnO is investigated by adopting a direct plasma-enhanced ALD (PE-ALD) approach, highlighting the role of plasma surface activation. Following the studies on room [18] and high temperature deposition of ZnO by direct PE-ALD, the evolution of the material properties is investigated with a combination of several ex-situ analysis techniques
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