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European Microscopy Congress 2020 Abstract Database

δ-doping of ZnO through the incorporation of 2D sheets of Fe

δ-doping of ZnO through the incorporation of 2D sheets of Fe

Session

PSA.3 - Semiconductors & Devices

Authors

Cana Elgvin (1), Kevin G. Both (1), Prof. Lasse Vines (1), Prof. Øystein Prytz (1)

Affiliations

1. University of Oslo

Keywords

2D materials, electron microscopy, semiconductors

Abstract text

There is a great interest in the semiconductor ZnO as it exhibits intriguing properties for optoelectronic applications. It is a transparent conductive oxide (TCO) with its direct wide bandgap of ∼3.3 eV [1], absorbing UV-radiation while being transparent to visible light. As it also is conductive, it may replace contacts on photovoltaics, acting as a transparent electrode, increasing the conversion efficiency. 

 

It has been reported that when ZnO is doped with trivalent oxides, such as Fe₂O₃ or In₂O₃, inversion domain boundaries (IDBs) decorated with the trivalent metal are formed [2]. The IDBs invert the polar axis of ZnO, making inversion domains of ZnO with different polarities between the boundaries. The IDBs consists of only a monolayer of the oxide [2], which may allow quantum mechanical effects to arise. This is due to bandgap alignment – when semiconductors with different bandgaps are put together, the energy levels of the semiconductor will have to align. Depending on the alignment, electronic properties, like charge trapping, could emerge. The charge trapping could lead to optoelectronic properties confined in “2D” – meaning the properties will be confined essentially to a two-dimensional sheet. 

 

The samples are manufactured using pulsed laser deposition (PLD) and characterized using a JEOL 2100F transmission electron microscope (TEM) operated at 200 kV and equipped with an Oxford X-Max 80 EDS detector. Optical properties including the size of the band gap are analysed using optical transmission measurements utilizing a photospectrometer. The method requires thin samples in order for the light to be transmitted, i.e. the technique is suitable for samples grown on transparent substrates using PLD. Hall measurements will be conducted to identify charge carrier and to extract information like carrier concentration and mobility.

 

Figure 1: Schematic drawing of sample manufactured by PLD.

 

The PLD technique involves a pulsed laser beam irradiating a target [3], causing rapid evaporation and plasma formation of the target crystal. The evaporated species expands in the vacuum chamber as a plasma plume and are then deposited onto the substrate. The technique is chosen for control of the film thickness, and because it allows epitaxial growth, as one single orientation of the crystal is needed for electrical and optical characterization. Ideally, one monolayer of Fe₂O₃ between thicker layers of ZnO is deposited, as this would allow optical and electrical measurements on a single IDB.

 

Two targets of ZnO and Fe₂O₃ are used, and a substrate of Si. There will be further exploration of appropriate substrates, where a promising candidate is Al₂O₃. The deposition has been conducted in an Ar atmosphere. The distance between the substrate and targets is between 7 cm and 9 cm. A larger distance yields a more even distribution of the species from the plasma plume onto the substrate. During depostition the substrate was heated to 650 , which promotes epitaxial growth of the thin film. The result is a film of a thinner layer of Fe₂O₃ between two thicker layers of ZnO. In figure 2, one can see thickness variations on top film. Thus, an adjustment for a larger distance between target and substrate will be done for the preparation of the next sample.

 

Figure 2: Thin film sample of one layer of Fe₂O₃ between two layers of ZnO on a Si substrate. Color variations indicate thickness variations of top film. 

 

TEM can provide structural and chemical information on atomic scale. The TEM imaging techniques used for characterization are diffraction pattern imaging and dark field- and bright field imaging, to determine crystal structures and phases present, and analyse crystal orientation. Further, high resolution STEM (scanning-TEM) imaging is chosen for imaging atomic columns in the sample. With HAADF-images (high-angle annular dark-field) in STEM-mode one can identify and distinguish between Zn and Fe, as the imaging mode is sensitive to the atomic number of scattering atoms. EDS (Energy-dispersive X-ray Spectroscopy) will be used to obtain chemical information, which can be obtained spatially in STEM-mode. ABF (annular bright-field) and ADF (annular dark-field) images will be used to identify strained areas in STEM-mode, as these images shows diffraction contrast.

 

The key points of the study are to identify the desired phase of ZnO with IDBs of Fe₂O₃, with an orientation such that the c-axis of ZnO is perpendicular to the surface of the thin film. This is to have control of the orientation of the crystal when electrically and optically measured.Subsequently, we want to conduct successful optical and electrical measurements that identify charge carrier, carrier concentration, mobility and bandgap variations, and lastly, if any optoelectronic properties of the material system occur, confined in 2D.

References

[1] Ü. Ozgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doan, V. Avrutin, S. J. Cho, and 

H. Morko. A comprehensive review of ZnO materials and devices. Journal of Applied Physics,

98(4):1-103, 2005.

 

[2] H. Schmid, E. Okunishi, and W. Mader. Defect structures in ZnO studied by high-resolution

structural and spectroscopic imaging. Ultramicroscopy, 127:76-84, 2013.

 

[3] R. J. Martín-Palma and A. Lakhtakia. Vapor-Deposition Techniques. Engineered Biomimicry, 2013.