Determination of the thermal band gap from the change of the Seebeck-coefficient at the pn-transition in (Bi0.5Sb0.5)2)Te3

The range of existence of the homogeneous /spl delta/-phase of (Bi/sub 0.5/Sb/sub 0.5/)/sub 2/Te/sub 3/ in the phase diagram (Bi,Sb)-Te intersects the stoichiometry line. n-type samples can be obtained by Bridgman growth with excess of tellurium. Due to the strong tellurium segregation the transition from the p-extrinsic region through intrinsic material to n-extrinsic behavior proceeds within a sample section of several millimeters along the axial direction. The positive and negative maximum values S/sub max.p/ and S/sub max.n/ can be read out from a scan of the Seebeck coefficient by the micro-thermoprobe technique over the transition area. The p-type maximum scan value is in good agreement with the value deduced from the integral measurement of homogeneous bulk samples. The width of the thermal band gap and the ratio of the electronic parameters /spl mu//sub B/(m/sub d//m/sub o/)/sup 3/2/ (/spl mu//sub B/-non-degeneracy value of the carrier mobility (reduced to Boltzmann statistics); m/sub d//m/sub o/-reduced d.o.s. mass) of holes and electrons are obtained from S/sub max.p/ and S/sub max.n/. The mobility of holes as minority carriers exhibits a strong sensitivity on doping impurities.