Near atomic scale quantification of a diffusive phase transformation in (Zn,Mg)O/Al 2 O 3 using dynamic atom probe tomog
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The onset of a diffusive phase transformation in thin film Zn0.70Mg0.29Ga0.01O deposited on c-oriented sapphire (a-Al2O3) was explored using dynamic heating experiments in a laser pulsed atom probe tomography (APT) instrument and correlated with transmission electron microscopy (TEM). Specimens were laser irradiated using 100–1000 pJ pulse energies with initial temperatures between 50 and 300 K for up to 8.64 1010 pulses. Using a finite element model, it was possible to estimate the temperatures reached by the specimen during laser pulsing, which were calculated to be 300 K to above 1000 K. Due to the small sample volume, quench rates were estimated to be 1013 K/s, allowing for nanosecond temporal resolution during the in situ heating experiments. The formation of Mg-spinel (MgAl2O4) at the transparent conductive oxide/a-Al2O3 substrate interface was observed using electron diffraction and confirmed by atom probe analysis. Subnanometer spatial resolution in the atom probe data reconstructions allowed for near atomic level diffusion to be observed. This work demonstrates the feasibility of conducting these experiments in situ using a combined TEM and APT instrument.
I. INTRODUCTION
Nanoscale characterization of materials is required to understand the onset of physical phenomena such as diffusion across a boundary. Atom probe tomography (APT) has advantages to transmission electron microscopy (TEM) in combined chemical and spatial resolution. However, for ceramic materials, unlike some metals,1 information about the specimen crystallography cannot be obtained easily in the atom probe. Furthermore, analysis of ceramic materials in the atom probe requires the use of laser pulsing to induce field emission of ions. It is generally accepted that the mechanism of field emission for oxides is due to thermal pulsing2,3; hence, temperature increases are induced in the specimen for a short period of time. Due to the small thermal mass of atom probe specimens, temperature changes from ;20 to .300 K in nominally 10–200 ps are possible.3–5 The thermal cycling depends on laser fluence, laser absorption, and material properties such as thermal conductivity, specimen geometry, and thermal mass.6–8 In this paper, the performance of a dynamic experiment, a time-dependent phenomenon that can be observed in the atom probe, is explored. A system consisting of a transparent conductive oxide thin film with nominal composition Zn0.70Mg0.29Ga0.01O Contributing Editor: Scott T. Misture a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.86 J. Mater. Res., Vol. 30, No. 8, Apr 28, 2015
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deposited on c-oriented sapphire (a-Al2O3) was chosen. Ga is added to the ZnO thin film to improve carrier concentration while Mg additions are used to tune the band gap energy (Eg).9,10 The presence of Mg changes the lattice spacing of ZnO by ,1%,11,12 therefore, the film can still grow epitaxially on the substrate. It is known that the maximum solubility of Mg in ZnO is
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