Breakdown of the quantized Hall effect in the vicinity of current contacts

Spatial distribution of the cyclotron radiation emitted from nonequilibrium electrons is studied on 1.5 mm-wide Hall bars in the quantized Hall effect (QHE) regime. At low-current levels where the two terminal resistance is quantized, the cyclotron emission (CE) is observed at the electron entry and exit corners formed between the metallic current contacts and the two-dimensional electron gas (2DEG) layer. As the current increases, additional CE occurs in the vicinity of the corner of the source contact that is opposite to the electron entry corner. The additional CE is accompanied by a finite voltage drop. The region over which the finite voltage drop takes place is localized to the limited region adjacent to the source contact where the CE is observed, and shows that the QHE breaks down locally in the vicinity of the source contact before the breakdown develops in the entire 2DEG channel. Studies of device-width dependence reveal that this local breakdown of the QHE in the vicinity of the source contact takes place only in sufficiently wide Hall bars (with a width well larger than 200 $\mu \mathrm{m}).\mathrm{}$ All the experimentally observed features are reasonably interpreted by assuming that electron-hole pairs are generated in cascading process due to strong polarization fields formed along the interface between the metallic source contact and the 2DEG.