A polysilane-polysilazane copolymer contains a polysilane unit of formula (I), and a polysilazane unit of formula (II), where each R1 and each R2 are each independently selected from H, Si, and N atoms, R3 is selected from H, Si, or C atoms, a≧1, b≧1, and a quantity (a+b)≧2. The polysilane-polysilazane copolymer may be formulated in a composition with a solvent. The polysilane-polysilazane copolymer may be used in PMD and STI applications for trench filling, where the trenches have widths of 100 nm or less and aspect ratios of at least (6). The polysilane-polysilazane copolymer can be prepared by amination of a perchloro polysilane having (2) or more silicon atoms per molecule with a primary amine.