Three-dimensional photoluminescence imaging of threading dislocations in GaN by sub-band optical excitation

Abstract

GaN is rapidly gaining attention for implementation in power electronics but is still impacted by its high density of threading dislocations (TDs), which have been shown to facilitate current leakage through devices limiting their performance and reliability. Here, we discuss a novel implementation of photoluminescence (PL) imaging to study TDs in regions within vertically structured p-i-n GaN (PIN) diodes consisting of metalorganic chemical vapor deposition (MOCVD) epitaxial layers grown on ammonothermal GaN (am-GaN) substrates. PL imaging with a sub-bandgap excitation energy (3.1 eV) reveals TDs with excellent clarity in three dimensions within the am-GaN substrate. Galvanometric-driven PL imaging allows the microstructure of hundreds of devices to be characterized in a single session, enhancing the screening process through the addition of device specific TD location tracking and density mapping. The visibility, structural characteristics, luminescent nature and evolution of TDs through the GaN growth process are described, potentially providing the ability to define TD structures associated with leakage current.

Fig. 1

(a) Photoluminescence imaging experimental setup. (b) A section of the device array layout showing the designed fiducial grid (red crosses). Each array is identified by a letter-number combination. In this image, array C3 is shown. Fiducial rows and columns are numbered from 0 to 20. Devices are centered between each set of fiducials. (c) A mosaic PL intensity map of a section of device array C3 on an imaged sample.

Fig. 2

A high-resolution z-stack imaging sequence through MOCVD epitaxial layers grown on an am-GaN substrate. Images (a–e) are taken with focal depth increments of 2.5 μm through the epitaxial layers to the top of the bulk am-GaN substrate while (f–j) are taken every 25 μm within the bulk am-GaN substrate. Approximate scan locations are shown in k).

Fig. 3

Cross-sectional images of (a) bare am-GaN, (b) annealed am-GaN and (c) fully processed am-GaN with MOCVD epitaxial growth, along with their (d–f) plan view counterparts and (g) a plot of the average PL intensity of each sample within its first 80 μm. The red line in (c) indicates the boundary between am-GaN and the epitaxial layer.

Fig. 4

High-resolution images of GaN’s microstructure (a) 25 and (b) 50 μm beneath the sample surface as well as (c) a comparison of selected TDs at both depths. Distribution of TD (d) eccentricity and (e) dislocation densities across 200 imaged sites. (f) Summary of TD inclination direction with each point color-coded by their degree of eccentricity.