Impact of near-surface native point defects, chemical reactions, and surface morphology on ZnO interfaces

Abstract

The authors used a complement of depth-resolved cathodoluminescence spectroscopy (DRCLS), atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM) to correlate the formation of native point defects with interface chemical reactions as well as surface morphology. A wide array of ZnO crystals grown by both melt and hydrothermal growth methods display orders-of-magnitude variation in 2.1, 2.5, and 3.0eV native point defect optical transitions at their free surface and as a function of depth on a nanometer scale. AFM surface morphology scans taken simultaneously with KPFM potential maps reveal large variations in surface morphology related to the growth method and subsequent processing. Notably, when DRCLS defect emissions are low, the surface roughness is low and the morphology matches its respective KPFM potential map. When DRCLS emissions vary with depth, the morphology and potential maps do not correlate. Indeed, the latter can vary by hundreds of meV across micron square areas. These subsurface electrical changes are consistent with DRCLS features and emphasize the contribution of surface morphology to electrically active interface defects. The relative strength of near band edge to deep level defect emissions exhibit a threshold dependence on surface roughness.

Intrinsic photoluminescence from low temperature deposited zinc oxide thin films as a function of laser and thermal annealing

“…Poly crystalline thin films of ZnO are deposited by a variety of physical and chemical vapour methods, with sputtering [1] being a preferred choice for low cost and scalability. However, to achieve the desired thin film properties, particularly for low temperature deposited films, it is critical to control grain microstructure, surface morphology, and internal defects [3]. Techniques that have been previously reported to improve these properties of ZnO thin films include post deposition thermal annealing [4,5], rapid thermal annealing [6,7], and laser annealing [8,9,10].…”

ZnO Surface Properties and Schottky Contacts

“…In Figure 4.10(a), DRCLS spectra show relatively low defect emissions within 40 nm of the free surface but orders-of-magnitude higher defects deeper into the crystal. [47] Similar potential variations across hundreds of nanometers in other crystals can easily account for the distribution of Schottky barrier heights discussed in Notwithstanding this apparently smooth surface, a KFPM map obtained simultaneously with the AFM map shows surface potentials that vary by well over 100 meV on the same surface on a scale of hundreds of nanometers.…”

“…Besides the defect variations with depth into the surface space charge regions shown in Figure 4.7, sizeable variations in ZnO surface potential are evident that depend sensitively on surface cleaning and polishing. [47] This reduction is due to a chemomechanical polishing that achieves near-monolayer surface roughness as pictured in the AFM map in Figure 4.10(b). [47] This reduction is due to a chemomechanical polishing that achieves near-monolayer surface roughness as pictured in the AFM map in Figure 4.10(b).…”

Surface chemistry and surface electronic properties of ZnO single crystals and nanorods

“…47 Low surface densities of defects, such as zinc and oxygen vacancies, have been shown to drastically change the electronic properties of the ZnO surface. 48 Ultraviolet light illumination has previously been shown to cause persistent photoconductivity as well as to desorb oxygen and create defects on the surface of single crystal ZnO. [49][50][51] Recently, the effect of persistent photoconductivity has also been seen on single ZnO nanorods.…”