Structural and electronic properties of heavily phosphorus doped polycrystalline silicon thin films

In the last few years, silicon has drawn increased attention due to the possible application as thermoelectric material. In this paper we are presenting the results of the investigation on several heavily phosphorus doped polycrystalline silicon thin films. The samples were prepared in LPCVD reactor and then subjected to rapid thermal annealing in different time intervals (20, 30 and 45 seconds). They are characterized using different techniques. Four point probe resistivity measurements, scanning electron microscopy and Raman spectroscopy are used to give a valuable insight into the structural characteristics such as the grain size, degree of crystallinity and the free carrier concentration. On the basis of the room temperature resistivity, concentration of free carriers was determined to be around 1·10²⁰ cm^-3 for all the samples. A special stress was set to the low temperature measurements which include resistivity measurements down to 4.2 K together with Seebeck coefficient data down to 80 K. These measurements enabled us to reveal some essential features of the electronic structure of the investigated systems. For all the samples, electrical resistivity shows the T^3/2 dependence in the wide temperature range (for some samples even from 50 to 300 K). The resistivities of the samples show that the samples are in metallic regime from the lowest measured temperatures. The thermopower behavior suggests that we can treat the transport properties within the free electron model. A comprehensive study was made to combine suitable theories appropriate in the analysis of results.