Hall mobility profiling in high electron mobility transistor structures

Abstract

We present results on Hall mobility and carrier density profiles as a function of etched depth in pseudomorphic high electron mobility transistor (HEMT) structures. The profiling technique involved the repetitive etching of a thin layer of semiconductor material by the process of anodic oxidation and oxide strip followed by a measurement of sheet resistivity and Hall coefficient at both room temperature and 77 K. The work was carried out on a dedicated instrument, the HL5900 + Hall profiler from Bio-Rad Microscience. The results show that anodic oxidation offers very good control on the etch step and hence good reproducibility of Hall profiles in any given HEMT structure. The structures investigated were conventional pseudomorphic HEMTs grown on GaAs substrates. As expected, the sheet Hall mobility increased as the top layer was gradually removed, reaching a well defined maximum value before the two-dimensional electron gas (2DEG) layer was fully depleted. Peak values ranged from 4500 to 38 000 cm 2 V −1 s −1 at 77 K and 2700 to 5900 cm 2 V −1 s −1 at room temperature. The 2DEG sheet carrier densities yielded equal values at room temperature and 77 K that ranged from 1.2 to 2.3 × 10 12 cm −2 . These results indicate that dual temperature Hall profiling is a very useful tool for characterizing the 2DEG layer parameters in HEMT structures and could help determine the optimal depth for the gate recess in real devices.