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.

Electrical and optical properties of iron in GaN, AlN, and InN

“…Contamination may arise from the presence of silica or alumina components in the system or from the reaction of amides and halides formed from gas sources with stainless steel components [8]. Memory effects following the intentional incorporation of iron can also lead to unintentional incorporation of iron in subsequent growth processes [9]. Secondary ion mass spectroscopy (SIMS) on bulk GaN samples grown using hydride vapor phase epitaxy (HVPE) [10,11] revealed the presence of iron incorporated unintentionally at concentrations of 10 15 cm −3 [10].…”

Optimization of Metal‐Organic Chemical Vapor Deposition Regrown n‐GaN

“…As shown in Figure , the morphology of these defects was dependent on the dopant atoms. The benefit of low growth temperatures to reduce atom diffusion in GaN layers, which is valid not only for Mg but also for Fe, is well known. Based on these reports and on the experience gained with the optimization of GaN layers for ohmic contacts, we applied similar growth conditions (LT‐LC process) as those optimized for the ohmic contact layer also for the deposition of n‐doped GaN on pin structures.…”

The Effect of Heavy Fe-Doping on 3D Growth Mode and Fe Diffusion in GaN for High Power HEMT Application

“…Moreover, these Fe atoms tend to diffuse into the GaN channel and cause the capture of conducting electrons during the on/off device switching operation, further resulting in current collapse and high R ON [ 23 , 25 ]. Several groups proposed different approaches for solving this Fe diffusion issue [ 10 , 25 , 26 , 27 ]. A u-GaN buffer layer was inserted between the GaN channel and the Fe-doped buffer to reduce the effect of Fe diffusion and avoid impact on device characteristics [ 10 ].…”