Part number D962C for SATA and D981C for SAS is what you want.
I have Dell 840 harddisk cage, and want to buy some caddy tray. I'm not sure caddy tray part number N6747 can be support for Cage Rack on Power Edge 840. Thank you.
Dell Caddy Tray part number : N6747 can be use with Power Edge 840 harddisk 
cage.