Disclosed are methods of depositing a transition metal such as tungsten on a semiconductor substrate. The method includes providing a gas mixture of diborane with a balance of hydrogen, where the hydrogen serves to stabilize the diborane in the gas mixture. The method further includes delivering the gas mixture to the semiconductor substrate to form a boron layer, where the boron layer serves as a reducing agent layer to convert a metal-containing precursor to metal, such as a tungsten-containing precursor to tungsten. In some implementations, the semiconductor substrate includes a vertical structure, such as a three-dimensional vertical NAND structure, with horizontal features or wordlines having openings in sidewalls of the vertical structure, where the boron layer may be conformally deposited in the horizontal features of the vertical structure.