Methods of forming silicon nitride. Silicon nitride is formed on a substrate by atomic layer deposition at a temperature of less than or equal to about 275° C. The as-formed silicon nitride is exposed to a plasma. The silicon nitride may be formed as a portion of silicon nitride and at least one other portion of silicon nitride. The portion of silicon nitride and the at least one other portion of silicon nitride may be exposed to a plasma treatment. Methods of forming a semiconductor structure are also disclosed, as are semiconductor structures and silicon precursors.