(Invited) Atom Probe Tomography of 1D, 2D and 3D Semiconductors
Authors: Kung Patrick , Waters Joseph L , Garg Sourav , Kim Seongsin M 
Published: February 2020

Atom probe tomography (APT) is a nanoscale materials characterization technique that combines field ion microscopy with time-of-flight mass spectrometry in order to yield a three-dimension imaging of sub-micrometer structure with atomic scale spatial resolution and isotope level sensitivity. Although it was originally developed for metals, significant instrumentation advances in the past decade, including the implementation of ultrafast lasers, have made this technique also attractive for characterization of semiconductors. Today, atom probe tomography of semiconductors is increasingly being used in correlation with other techniques (e.g. electron microscopy) to investigate doping distribution and segregation, phase separation, and diffusion across heterointerfaces. Experimentally, using a local electrode atom probe (LEAP), either a precisely prepared or an already cone tip-shaped specimen is cryogenically cooled in vacuum (25~150 K), electrically biased (2~10 kV) and excited by ultrafast laser pulses (0.02~2000 pJ pulse energies) to induce field evaporation of ions from its surface. In this talk, we review and present our recent results on the atom probe tomography of 1D, 3D and 2D compound semiconductors and heterostructures. We will also address specimen preparation challenges as well as challenges in properly interpreting collected data.

We investigated the laser-assisted APT of 1D semiconductors, including ZnO nanowires and GaAs/AlGaAs core-shell nanowires. Columnar in shape, averaging ~100 nm in diameter, nanowires did not require extensive specimen preparation. Our early APT study of ZnO nanowire addressed the challenge of properly differentiate Zn and O in the mass spectrum, as the main isotopes of Zn and O (64Zn and 16O) exhibited the same mass-to-charge ratio of 32 under ionic species 64Zn++ and 16O2
+. A comparison of ZnO nanowires grown by chemical vapor deposition under Ar and N2 ambients revealed the presence of nitrogen dopant incorporated in the ZnO, which was confirmed by a lower electrical resistivity. Our APT work on GaAs/Al0.33Ga0.67As core-shell nanowires yielded a quantitative measure of the radial composition transition change from the GaAs core to AlGaAs shell, revealing a nearly linear-graded heterojunction over a distance of 6.5 nm.

A comparative study of the voltage-mode and the laser-assisted APT of GaN (3D semiconductors) showed that the latter yielded sharper mass spectra. The APT of III-V semiconductors such as GaN often suffers from the loss of nitrogen as neutral N2 species that are not detected, which we showed can be alleviated by using suitable laser pulse energies. Doing so enabled us to obtain overall near-stoichiometric field evaporation of Ga and N elements from the material. Undoped GaN, Mg-doped GaN ([Mg]~1×1019 cm-3) and Si-doped GaN ([Si]~5×1018 cm-3) were investigated and compared. Mg dopants were successfully detected by APT in GaN:Mg as evidenced by the detection of the three natural isotopes of Mg at masses 12, 12.5 and 13, corresponding to 24Mg++, 25Mg++, and 26Mg++. The detection of Si was slightly more challenging because the main isotope 28Si led to a mass spectrum peak of 28 that can be either 28Si+ or 14N2
+. However, a direct comparison between undoped and Si-doped GaN allowed us to confidently identify the Si detected and perform 3D image reconstruction of the dopant distribution. Other 3D semiconductors investigated included AlxGa1-xN and Al1-yInyN ternary compounds.

In terms of device heterostructures, we employed APT to probe the buried heterointerface in strained AlGaN/GaN and lattice-matched AlInN/GaN high electron mobility transistor structures. We compared the measured composition interface roughness and interface diffuseness with the measured electrical transport characteristics of the two-dimensional electron gas, as well as before and after gamma ray and proton irradiation under various doses.

Finally, we will also present our recent success in using APT to characterize a single monolayer transition metal dichalcogenide materials (2D semiconductors) grown by chemical vapor deposition.