PROBING MOBILITY GAPS AT RESISTIVITY MINIMA IN THE INTEGER QUANTUM HALL EFFECT C.-T. Liang 1 , K. Y. Chen 1 , Jau-Yang Wu 2 , S. D. Lin 2 , Li-Hung Lin 3 , Yu-Ru Li 1 , Yen Shung Tseng 1 , Chun-Kai Yang 1 , Po-Tsun Lin 3 , K. A. Cheng 4 , and C. F. Huang 5* 1 Department of Physics, National Taiwan University, Taipei, Taiwan, R. O. C. 2 Department of Electronics Engineering, National Chiao Tung University, Hsin Chu, Taiwan, R.O.C 3 Department of Applied Physics, National Chiayi University, Chiayi, Taiwan, R.O.C. 4 Department of Electronic Engineering, Lung Hwa University of Science and Technology, Taoyuan County, Taiwan, R.O.C. 5 National Measurement Laboratory, Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu 300, Taiwan, R. O. C. Abstract Magneto-transport measurements are performed on the AlGaAs/GaAs quantum Hall (QH) devices fabricated recently by our group. A series of Hall plateaus are observed with increasing the perpendicular magnetic field, and the mobility gaps resulting from localization effects are investigated at the minima in the longitudinal resistivity. Only the gap corresponding to the filling factor i=2 is close to the expected cyclotron energy, and our study supports that the low-field QH conductors may suffer problems due to insufficient localization. The anomalous change on the Hall slope is observed when the i=3 plateau is destroyed by the large current. Introduction The integer quantum Hall effect (IQHE) has been used to maintain the resistance standard. [1,2] Such an effect can be observed as we apply a magnetic field B perpendicular to the two- dimensional electron system (2DES) in a semiconductor heterostructure. The i=2 and 4 plateaus in ρ xy are accurate enough for the resistance metrology if the QH devices are well-fabricated. Here i represents the Landau level filling factor, and ρ xy denotes the Hall resistivity. In addition, the minima of the longitudinal resistivity ρ xx can reveal whether the temperature is low enough or not. However, more studies are necessary to improve our understanding on QH devices, especially on the optimal condition for the precision measurements. Experimental details The inset to Fig. 1 shows the structures of the samples used for this study, and we denote them as LM4656 and LM4640. The samples are grown by molecular beam epitaxy. To probe the IQHE, they are made into the Hall pattern with the channel width 80 μm, and Ohmic contacts are fabricated by alloying AuGeNi in forming gas. We deposit Au/Ti on the Hall bar of sample LM4656 to form Schottky gate. The transport measurements are performed by using the top-loading He 3 system with the superconducting magnet, and the AC excited current I with the frequency 87 Hz is applied to the devices. Figure 1 shows ρ xx and ρ xy at the gate voltage V g =0 for sample LM4656. Fig. 1 . The curves of ρ xx and ρ xy in sample LM4656. The inset shows the sample structures, where the doping concentration n=3.66×10 18 cm -3 and 2.65×10 18 cm -3 in the Si-doped layers for samples LM4656 and LM4640, respectively. Results In Fig. 1, there are well-developed QH states characterized by the Hall plateaus. It is expected in the intermediate temperature range that [2] ρ xx (T) ∝ exp(–ΔE/2k B T) (1) at the minimum of ρ xx in each QH state. Here ΔE represents the mobility gap, k B is the Boltzmann 1-4244-2399-6/08/$20.00 ©2008 IEEE