The purpose of using rare earth dopants in semiconductors is to improve their electrical properties.

From Wikipedia:

"SnO2 crystallises with the rutile structure, wherein the tin atoms are six coordinate and the oxygen atoms three coordinate. SnO2 is usually regarded as an oxygen-deficient n-type semiconductor."

Doping with cobalt and manganese, leads to the formation of oxygen vacancies. This behavior increases oxygen diffusion at grain boundary, promoting SnO2 densification. These dopants added to SnO2 are very active to the promotion of SnO2 densification. However, the compound obtained is very electrically resistive. In order to improve the electrical conductivity,  other rare earth additives such as Nb2O5 and Ta2O5 are used in doping SnO2.. 

Thank you sir. Is there any possibility to enhance the optical properties by adding rare earth dopants?

Sorry for the delay. Of course, for example rare-earth ions are incorporated into semiconductors to improve their luminescence efficiency. 

You may take a look to the following links below.

https://books.google.fr/books?id=WcLBAAAAQBAJ&pg=PA526&lpg=PA526&dq=enhance+the+optical+properties+of+semiconductors+by+adding+rare+earth+dopants?&source=bl&ots=i8yAdIIgkX&sig=vilw9L5mCTKq1r-zMIsQu0r7MAA&hl=fr&sa=X&ved=0ahUKEwiGlt32jsvMAhUBXRoKHcVODHIQ6AEIWjAG#v=onepage&q=enhance%20the%20optical%20properties%20of%20semiconductors%20by%20adding%20rare%20earth%20dopants%3F&f=false

http://link.springer.com/article/10.1007/s40097-015-0151-4

thank  you sir.

What is the significance of doping rare earth oxides in amorphous system?

sir i read your ideas really useful to me and one more question belongs to my area of research .

what are the rare element material has to be use for solid oxide fuel cell.?

the incorporation of metal atoms introduces more sublevels into energy hierarchy which permits more allowed electronic transition to take place and hence a enhancement in the properties