Fig. 1. ：（a） A STM image of Hg_0.72Cd_0.28Te； （b） Typical STS data by the I-V measurement.

Fig. 2. （a） and （b） are energy band diagrams for the positive and negative sample biases，respectively. E_F，S and E_F，T represent the Fermi levels of sample and tip，respectively； （c） The blue lines are the calculated variation of surface potential of HgCdTe with the sample bias by TIBB model. The dashed line designates the variation of Fermi level of tip，whose intercrosses with the blue lines define the onsets of tunneling current and their distance as designated by the horizontal arrow predicts the apparent band gap measured.

Fig. 3. ：The comparison of the calculation （red dotted line） with four experimental I-V spectra （black solid lines） measured under the imaging bias of 0.32 V.

Fig. 4. ：The comparison of the calculation （red solid line） with four experimental I-V spectra （black solid lines） measured under the imaging bias of 1.60 V.