Coulomb gap shrinkage in compensated Si:(P, B) in high magnetic fields

Abstract

Magnetoresistance measurements on heavily doped but still insulating n-type Si : (P, B) are reported for the temperature range 1.6 to 300 K and in magnetic fields up to 35 T. In zero magnetic field, the samples investigated (carrier concentration 1.5, 2.5 and 2.95×10 18 cm −3, compensation ratio ∼0.65) show Efros–Shklovskii (ES) variable range hopping (VRH), ρ= ρ 0 exp[( T 0/ T) p ] with p= 1 2 . For the sample with 2.95×10 18 cm −3, the ES hopping conduction remains unchanged in the low-field regime (up to 8 T). In addition, the observed B 2 dependence of ln[ ρ(B)] in this field range is in good agreement with the expected behavior, with the proportionality constant depending on temperature as K s ∝T −3/2. However, in high fields (>25 T) p changes to 1 3 as expected for Mott VRH with a finite density of states at the Fermi level, indicating that the Coulomb gap becomes ineffective in large fields. This surprising result was previously found for Ge : As.