Scanning Probe Techniques Identify GaN HEMT Reliability Issues
Researchers led by Leonard Brillson at The Ohio State University in Columbus, can monitor the evolution of HEMT device failure with nanoscale scanning measurements inside the gallium nitride (GaN) operating device.
Results extracted from cathodoluminescence spectroscopy using a scanning electron microscope (SEM) and Kelvin probe force microscopy (KPFM) not only describe a major degradation mechanism of III-based HEMTs, but also predict device failure without time-consuming lifetime tests. These tools now provide clues to improve III-nitride HEMT device reliability.
These scanning probe techniques gave the Ohio State group cross-sectional temperature and field-induced-stress maps near the GaN HEMT’s 2-dimensional electron gas (2DEG) within the extrinsic drain during operation. Simultaneous atomic force microscopy (AFM) and KPFM monitor the surface potential evolution in devices operated to failure, while depth-resolved cathodoluminescence spectroscopy (DRCLS) tracks the stress and formation of electrically active defects responsible for the potential changes. Above a critical stress, surface potential changes accelerate with failure occurring at patches that change the fastest.
An SEM coupled with optical lens system, monochromator, and photo multiplier achieves cathodoluminescence localized to sub-50 nm lateral resolution, with the semiconductor band gap emission depending on temperature and stress. Tuning the electron beam energy from 0.1 – 25 keV enables DRCLS to probe from a few atomic surface layers to microns with depth resolution below 100 nm.
A non-contact AFM with conducting tip supplies maps of surface topography and, operated as a Kelvin probe, electric potential that change with on-state (high current, low voltage bias) and off-state (low current, high voltage bias) stress conditions. The surface potential changes in localized patches (fig. 1) correlate directly with optical emissions from deep level defects in the GaN band gap. The stress-induced shifts in band gap emission with external stress support an inverse piezoelectric stress-induced defect model proposed by MIT researchers. From a practical standpoint, nanoscale scanning probe techniques can now be used to predict operating conditions as well as the location within the HEMT where catastrophic failure will occur.
Next steps for the researchers include charting cross-sectional temperatures and field-induced stress with a range of operating parameters. They also plan to use Auger electron spectroscopy to detect chemical reactions microscopically at the point of failure.
Further details of this work can be obtained in the paper “Field-induced strain degradation of AlGaN/GaN high electron mobility transistors on a nanometer scale,” by Chung-Han Lin, D. R. Doutt, U. K. Mishra, T. A. Merz, and L. J. Brillson, Appl. Phys. Lett. 97, 223502 (2010).
Figure 1. KPFM maps showing how surface potential within the extrinsic drain and source regions of an AlGaN/GaN HEMT changes with increasing OFF-state stress. AFM image shows surface topography at VDS = 30 V, VGS = - 6 V. The red dashed circles show regions where potentials change fastest. SEM image shows the corresponding AFM/KPFM scanning area and indicates where failure occurs with increasing OFF-state stress.
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Figure 2 (a) DRCLS cross-sectional temperature distribution of a GaN HEMT during operation (VDS = 6 V, VGS = -1 V, IDS = 1 A/mm) showing hot spot localized at drain-side gate foot and variations below. Inset: Field-induced-stress distribution from source to drain before and during off-state stress, maximizing near drain-side gate. (b) Plume generated by 5 keV, 10 nm electron beam which shows it is primarily within 50 nm.
