COULOMB GAP SHRINKAGE IN COMPENSATED SI(P,B) IN HIGH MAGNETIC-FIELDS

Magnetoresistance measurements on heavily doped but still insulating n -type Si:(P, B) are reported for the temperature range 1.6 to 300 K an d in magnetic fields up to 35 T. In zero magnetic held, the samples in vestigated (carrier concentration 1.5, 2.5 and 2.95 x 10(18) cm(-3), c ompensation ratio similar to 0.65) show Efros-Shklovskii (ES) variable range hopping (VRH), rho = rho(0) exp[(T-0/T)(p)] with p = 1/2. For t he sample with 2.95 x 10(18) cm(-3), the ES hopping conduction remains unchanged in the low-held regime (up to 8 T). In addition, the observ ed B-2 dependence of In[rho(B)] in this field range is in good agreeme nt with the expected behavior, with the proportionality constant depen ding on temperature as K-s proportional to T-3/2. However, in high fie lds (> 25 T) p changes to 1/3 as expected for Mott VRH with a finite d ensity of states at the Fermi level, indicating that the Coulomb gap b ecomes ineffective in large fields. This surprising result was previou sly found for Ge:As. (C) 1998 Elsevier Science B.V. All rights reserve d.