Ta is good for annealing at these temperatures as S.N. has suggested. We use Ta substrate to anneal nanoparticles in our lab. I assume you will do the annealing in vacuum? Ta will begin to oxidize at temperatures greater than about 200-300 degC if heated in air.

You can use Tungsten (W) or Graphite plates as substrate,

I am sure it will fulfill your requirements. Its (W) melting point around 3422* C. You can use SiC also as substrate, Beside some oxide such as Al2O3 can resist at high temperatures.

i think you and Mr. Kunal Mondal, IIT Kanpurare doing same research work, . you can contact him. https://www.researchgate.net/profile/Kunal_Mondal_IITKanpur/

best wishes

Dear Pratik

Hi, i think you can use Al2O3 substrate,because this ceramic has higher thermal resistance than other materials.

I working with this material.

Gadolinium Gallium Garnet, or GGG (Gd3Ga5O12) substrates from integratedphotonics.com will be good up to at least 1000C. Also, sapphire, possibly.