Semiconductor devices containing a CVD BPSG layer and an undoped CVD oxide cap layer are described. The cap layer can be any silicon oxide material with a thickness between about 50 Å and about 350 Å. The cap layer may be formed using a low temperature CVD process that is controlled for density by adjusting the amount of silicon precursor in the gas-phase. In some embodiments, the cap layer is deposited on the BPSG layer followed immediately by the BPSG film deposition prior to any annealing of the BPSG layer. The cap layer may prevent dopant out-diffusion and/or out-gassing during storage and high-temperature annealing, and moisture penetration into the BPSG layer, as well as suppress defect nucleation on the as-deposited BPSG surface and defect formation during high temperature annealing, while still allowing flow ability of the BPSG layer. Other embodiments are also described.