A Sensitive AC System for Measuring Changes in Small Hall Voltages at High Temperatures

A simple ac sysyem which incorprates a phase-sensitive detector for measuring changes in small Hall voltages at high temperatures is described. Under optimum operating conditions, the system has a sensitivity of better than 10-8 V. The apparatus has been used to directly investigate the change of charge carrier concentration with time in very heavily doped semiconductor material while undergoing high-temperature isothermal heat treatment.1 A chart recording of a < 10-7-V change in Hall voltage at a temperature of 1040 K is given as an indication of the system's performance. The results of Hall-voltage measurements on zone levelled n-type Si63.5 Ge36.5 alloy doped with phosphorus to ~1.2 X 1026 m-3 over a range of heat-treatment temperatures and as a function of time are reported. The results indicate that over a certain temperature range, precipitation of phosphorus takes place very rapidly and the rate of change of carrier concentration exhibits a maximum in the temperature range around 750 K.