Suppression of Iron Memory Effect in GaN Epitaxial Layers

Abstract

AlGaN/GaN High Electron Mobility Transistors (HEMTs) require a semi‐insulating buffer to compensate a high background donor concentration and to prevent parasitic effects, such as parallel conduction. Iron and carbon are typical impurities used for such purpose, since they can behave as deep acceptors in GaN layers. The former (Fe) brings as drawback a well‐known memory effect which consists in the segregation of Fe atoms through the GaN layers, requiring thick undoped layers to keep the two‐dimensional electron gas (2DEG) away from such electron traps. The latter (C), although easier to incorporate in the GaN layers and free of any memory effect, could cause current collapse. In this study we investigate the effect of differently strained epitaxial layers on Fe‐segregation. HEMT structures were grown on sapphire substrates by MOCVD. By growing the Fe‐doped layers under unusual growth conditions (low temperature, or as AlGaN), or by adding an interlayer (AlN, 5× AlN/GaN, or C‐doped GaN) between the doped and undoped epitaxial layers, we have succeeded in limiting the Fe segregation within 200 nm of undoped GaN layer instead of the typically required 800 nm, as proven by SIMS. While the morphology and the crystal quality of the HEMT structures have been affected to a very low extent, the electrical characteristics have benefitted from such interlayers. Higher carrier mobility and lower sheet resistance distinguish such epitaxial structures. Further benefits are expected by the device performance such as reduced soft‐subthreshold behavior (soft‐breakdown) and dispersion effects.