PCT TRACON Controller reported receiving an aircraft from Tower at 2000 feet; in an area with a 2100 foot Minimum Vectoring Altitude.

Narrative

Aircraft Y was departing north from DCA. My aircraft; Aircraft X; was subsequently departing 2.5 miles in trail behind Aircraft Y. I did not receive a strip that indicated Aircraft X was maintaining Visual Separation with Aircraft Y and communications transfer was delayed. I gathered Tower was trying to establish Visual Separation before frequency transfer since they coordinated with me that they had stopped Aircraft X at 2000 ft. Aircraft X was already 3 miles north of DCA in an area where the Minimum Vectoring Altitude is 2100 ft. On initial contact; I climbed Aircraft X to 4000 ft reference Aircraft Y. Aircraft X continued on his flight without further incident. Tower should better understand that 2000 ft is not an appropriate altitude to level off a departure in a north operation because of the 2100 ft Minimum Vectoring Altitude.