(Invited) Electron Channeling Contrast Imaging for Rapid Characterization of Compound Semiconductors
Authors: Deitz Julia , Ruggles Timothy , Lee Stephen , Allerman Andrew A. , Carter C Barry , Michael Joseph 
Published: November 2022

As compound semiconductor devices become more advanced, their ultimate performance becomes increasingly dependent on nano to micro scale structural defects in their constituent materials. These defects, such as dislocations, stacking faults, and anti-phase boundaries often act as recombination centers in devices, limiting efficiency. Characterization of these defects – their type (such as edge vs. screw vs. mixed dislocations), their relative densities, how they interact with other defects, what growth conditions are more or less likely to propagate them, how they relate to device efficiencies and performance, etc. – is vital to mitigate their negative effects and even sometimes exploit their potential benefits depending on the desired application. In this contribution, we will discuss the use of electron channeling contrast imaging (ECCI) as an ideal characterization method for compound semiconductors and devices as well as some recent applications which include ECCI characterization in tandem with other characterization methods for a broader material understanding.

Transmission electron microscopy (TEM) is a widely used technique for characterizing extended defects and local crystal orientation. However, TEM analysis usually entails thinning a sample via focused ion beam milling or chemical/mechanical polishing to around 50-100 nm, which is time consuming and only allows a relatively small area of the sample to be characterized. These drawbacks can cause bottlenecks in research progress and even render TEM impractical for certain applications. ECCI performed in a scanning electron microscope (SEM) can bypass these issues as it requires little to no sample preparation. This means ECCI can easily characterize over large areas in a fraction of the time as TEM analysis. [1-3] These benefits mean that ECCI gives more accurate analysis by not missing larger trends that can be missed with the small samples in TEM as well as by not introducing artifacts from the sample preparation.

Here we highlight applications of ECCI for characterization of two very different compound semiconductors of growing importance -- Cd3As2 and GaN. Firstly, we consider Cd3As2 thin films of use as topological quantum materials, where the Cd3As2 can be either a compound semiconductor or topological Dirac semimetal, depending on the structural phase or mode of use. The ECCI of MOCVD-grown Cd3As2 shown in this work finds that the developmental thin films are not completely single crystalline. [4] Moreover, two different spatial scales of domain formation are seen. At lower magnification, we see roughly circular-shaped larger domains that are ~ 2-5 mm in diameter. Some of these domains are more prominently defined and feature single dark spots at their center, which at first suggests threading dislocations, but turns out to be occasional defect-pits formed in concert with numerous, indistinct threads. At higher magnification, we see much smaller dot-like or speck-like domains that are ~ 100 nm or less in diameter. The wide-area electron channeling patterns used to orient the sample for the ECCI imaging were visible, but rather indistinctly resolved, which is consistent with small crystallographic misorientations of the ensemble of domains composing the film. Secondly, we consider bulk GaN wafers entering use in a wide variety of functional applications, including GaN high-voltage diodes for power electronics. We will show novel “star” defects appearing in the GaN substrates when characterized by both ECCI and high-resolution electron backscattered diffraction (EBSD). [5] Through a more complete structural understanding of these star defects, we seek to improve understanding of the impact of these defects on device performance, which may in turn facilitate development of strategies for their mitigation or removal.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.