Gallium Nitride-on-Silicon High Electron Mobility Transistors (HEMT)
GaN-on-Si has emerged as an important material for the production of high electron mobility transistors. It and related binaries InN and AlN, as well as ternary compounds, are well known for their use as light emitters of UV through green wavelengths, as well as for optical detectors. The material is also of great interest for high electron mobility transistors (HEMT) for microwave communication and other high power applications. However, point and structural defects arising primarily from lattice and stacking mismatch with substrates can degrade the performance and lifetime of devices made from the material. This phenomenon is well studied [1] and continues to be of great interest to the research and development community.
Klar’s spectroscopic mapping system is ideally suited for capturing the effects of defects on the bandgap of the material and can produce a map, at each emission energy arising from the sample, that quantifies the spatial variation due to the presence of impurities.
To demonstrate the use of the Klar instrument for such studies, we acquired a commercial wafer from University Wafer and mapped a portion of the surface. A typical spectrum shows strong emission in the UV, but also a weaker peak at 2.8 eV that appears to arise from the presence of Mg in the sample. Other peaks are seen as well, including the shoulder near 3.3 eV and one in the green at 2.1 eV. Either or both of these may represent other elements, defects, or structural anomalies.
Sample of GaN-on-Si HEMT from a wafer purchased from University Wafer.
![Typical photoluminescence spectrum from a GaN-on-Si HEMT wafer. The peak at 2.8 eV is typical of an Mg defect in the GaN layer [1]](https://images.squarespace-cdn.com/content/v1/56c5ef57859fd08f9874b6cc/1621027377702-WHIMFC74RP0E7DENO3YD/Example+spectrum+and+fit+cropped.jpg)
Typical photoluminescence spectrum from a GaN-on-Si HEMT wafer. The peak at 2.8 eV is typical of an Mg defect in the GaN layer [1]
A portion of the surface of the wafer was mapped using a Klar spectroscopic mapping instrument with a 349 nm UV source. The figure below shows the near-uniform emission around 2.8 eV but with a speckling indicative of a random distribution of Mg across the sample. The energy variation is extremely small, under 0.5%, but is easily captured by the instrument.
Map of the peak energy across a section of the GaN-on-Si HEMT wafer. The fine structure shows the variation of Mg in the GaN layer across the crystal.
[1] M.A. Reshchikov and H. Morkoc, Luminescence properties of defects in GaN, J. Appl. Phys. 97, 061301 (2005).