🔬 materials science

Three-dimensional visualization of threading dislocation in GaN by polarized-light microscopy

This paper demonstrates a simple, nondestructive, high-throughput method using polarized-light microscopy to visualize the three-dimensional inclination and large-area distribution of threading dislocations in ammonothermal GaN wafers, revealing their climb-mediated motion associated with strain relaxation during crystal growth.

Original authors: Yukari Ishikawa, Kisara Matsumoto, Kazuki Ohnishi, Yongzhao Yao

Published 2026-06-29

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CC BY 4.0

Original authors: Yukari Ishikawa, Kisara Matsumoto, Kazuki Ohnishi, Yongzhao Yao

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a giant, perfect crystal of Gallium Nitride (GaN) as a towering skyscraper made of tiny, stacked bricks. Ideally, every brick should be perfectly aligned. However, in the real world, some bricks get pushed out of place, creating a "threading dislocation." Think of these dislocations as invisible, twisted wires running through the building, connecting the foundation to the roof. If too many of these wires exist or if they are twisted the wrong way, the building (or in this case, the electronic device) becomes weak and unreliable.

This paper is about a new, clever way to take a 3D "X-ray" of these invisible wires without breaking the crystal apart.

The Problem: Seeing the Invisible in 3D

Usually, looking at these defects is like trying to see a single thread inside a thick, opaque block of glass. You can see the thread if you look at the surface, but as soon as you try to see how it twists deep inside, it disappears. Previous methods to see the whole 3D path were slow, like scanning a document one tiny line at a time. The researchers wanted a faster way to see the whole picture at once.

The Solution: A "Flashlight" and a Polarized Lens

The team used a special microscope setup that acts like a high-speed, 3D camera.

The Light: Instead of a laser that scans slowly, they used a bright, focused LED light (like a flashlight) that shines straight down.
The Filter: They placed special sunglasses (polarizers) on the light source and the camera. This makes the light behave in a specific way that highlights the "twisted wires" (dislocations) against the clear background of the crystal.
The Trick: They took a stack of photos, moving the camera up and down in tiny steps (like focusing a camera on a flower, then the stem, then the roots). By stacking these 2D photos, they could reconstruct the 3D path of the wires.

The Discovery: The "Leaning Tower" Effect

When they looked at the 3D paths, they found something surprising. The wires weren't running straight up and down like elevator shafts. They were leaning.

Imagine a stack of books where every book is slightly shifted to the left compared to the one below it. That's what these wires were doing.

The Angle: The wires were tilting about 3.3 degrees away from the straight-up direction.
The Direction: They didn't lean randomly. They leaned in a specific direction that was perpendicular (at a right angle) to the direction of the "twist" in the wire itself.

Why Does This Happen? (The "Stress Relief" Analogy)

The researchers explain this leaning using the concept of strain relaxation.
Imagine you are stretching a rubber band. It wants to snap back. If the crystal is growing and there is too much internal stress (like a rubber band being stretched too tight), the crystal needs a way to let go of that tension.

The "wires" (dislocations) move sideways as they grow upward. This sideways movement is called climb.
By leaning over, the wires act like a safety valve, releasing the built-up stress inside the crystal, much like a person taking a deep breath to relax their shoulders.

The "Honeycomb" Clue

The researchers also studied a specific pattern called a "honeycomb defect," which looks like a hexagon made of six bundles of these wires.

They used a powerful tool called Synchrotron X-ray Topography (think of it as a super-powerful, high-energy flashlight that can see the atomic structure) to confirm the direction of the twist in each bundle.
They found that the direction the wires leaned matched perfectly with the direction of their atomic twist. This confirmed that the wires were indeed "climbing" to relieve stress, rather than just sliding sideways.

The Big Picture

The paper concludes that this new method is a fast, non-destructive way to see the 3D shape of defects in large crystal wafers. It proves that these defects aren't just straight lines; they are tilted, leaning structures that form naturally to help the crystal handle the stress of growing. By understanding this "leaning" behavior, scientists can better understand how to grow stronger, more perfect crystals for future electronics.

In short: The researchers built a fast 3D camera to see invisible wires inside a crystal. They discovered the wires lean over to "relax" the crystal's internal stress, and they proved this by matching the lean direction to the wire's atomic twist.

Technical Summary: Three-dimensional visualization of threading dislocation in GaN by polarized-light microscopy

Problem Statement
Gallium nitride (GaN) is a critical wide-bandgap semiconductor for light-emitting and power devices, yet its performance and reliability are compromised by crystalline defects, particularly threading dislocations (TDs), remaining in wafers. While nondestructive characterization of defect distributions over wafer-scale areas is essential for optimizing crystal growth and manufacturing, existing three-dimensional observation methods rely on focused-light scanning. These methods are unsuitable for rapid, large-area inspection. Optical microscopy offers high-throughput potential, but previous applications for 3D imaging of TDs have been limited in scope or speed.

Methodology
The authors demonstrate a high-throughput, nondestructive approach for 3D imaging of TDs in ammonothermal GaN wafers using polarized-light microscopy with collimated LED illumination.

Sample: A commercially available 2-inch ammonothermal GaN (0001) wafer with a chemically mechanically polished (CMP) Ga-polar surface and an additionally polished N-polar surface (thickness ~340 μm).
Optical System: The setup utilized a 405 nm collimated LED, a polarizer aligned parallel to [11 $\bar{2}$ 0], a crossed-Nicol analyzer aligned parallel to [1 $\bar{1}$ 20], and a 20× objective lens (NA 0.45).
Acquisition Strategy: To achieve 3D visualization, images were acquired by shifting the objective lens in 2.5 μm steps from the surface focus position. Accounting for the refractive index of GaN (n = 2.4), this resulted in an effective focal shift of approximately 6 μm within the crystal. A total of 42 images were captured over a depth range of 48 to 294 μm.
Data Processing: Images were processed using FIJI software with fast Fourier transform (FFT) bandpass filtering and contrast adjustment. A projected image was generated by calculating the standard deviation of image intensity along the z-direction, revealing the traces of dislocations caused by in-plane positional shifts during focal scanning.
Validation: Synchrotron radiation X-ray topography (SR-XRT) was performed at KEK Photon Factory using six equivalent g-vectors of the 11 $\bar{2}$ 4 reflection family to identify in-plane Burgers vector components and validate the optical observations.

Key Results

3D Visualization of Dislocation Inclination: The polarized-light microscopy revealed that TDs exhibit focal-depth-dependent in-plane shifts. This allowed for the visualization of 3D inclination behavior over large areas (stitched area ~1.6 × 1.4 mm²). A total of 768 dislocation traces were detected, corresponding to a TD density of approximately 3 × 10⁴ cm⁻².
Trace Geometry and Burgers Vector Correlation: SR-XRT analysis confirmed that dislocation bundles (DBs) forming "honeycomb defects" are threading edge dislocations (TEDs). The study found that dislocations with identical in-plane Burgers vector components exhibited similar trace geometries and inclination directions.
Quantitative Inclination Measurement: The traces were concentrated in six angular ranges. By comparing experimental trace angles and lengths against a geometrical model, the authors determined that the TDs were tilted by approximately 3.3° from the c-axis in directions perpendicular to their Burgers vectors. This tilt was attributed to a slight misalignment of the c-axis relative to the microscope's optical axis (tilted ~1.2° toward [1 $\bar{1}$ 00]).
Mechanism of Motion: The observed inclination directions (perpendicular to Burgers vectors) indicate climb-mediated motion rather than glide motion. As the focal position moved from the Ga-polar to the N-polar surface, dislocations shifted outward, corresponding to positive climb motion that reduces the excess half-plane area.
Strain Relaxation Estimation: Based on the assumption that TEDs constitute ~81% of the TDs in the crystal, the effective strain relaxation associated with this dislocation inclination was estimated to be approximately 5.1 × 10⁻⁶ per millimeter along the growth direction.

Significance and Claims
The paper claims to demonstrate a simple, nondestructive approach for the large-area three-dimensional characterization of dislocations in GaN wafers. The primary significance lies in the ability to visualize the 3D inclination behavior of TDs without the time-consuming nature of focused-light scanning.

The authors conclude that the observed ~3.3° tilt of TDs from the c-axis is indicative of climb-mediated motion associated with strain relaxation during crystal growth, likely driven by thermal stress, temperature inhomogeneity, or impurity concentration gradients. The study suggests that the in-plane strain distribution is approximately isotropic, while a strain gradient likely exists along the c-axis. This method provides a pathway for rapid inspection of defect distributions to aid in crystal growth optimization and device manufacturing.