I did request to increase salary after 3 months and wrote in offer letter. Actually, the salary increased by 4k more than verbal when I got offer letter, and also put in writting in the offer letter for raising salary after 3 months.
The salary is not far below market, but not yet reach the highest level, and also I did good job in the past a month, it's hard to say contribuite to the company a lot. It's hard for anybody replaced my position except they hire back the person who quitted.