A monolithic three-dimensional memory device, comprising: a substrate (9; 10) including a semiconductor material; a stack of alternating layers comprising insulator layers and electrically conductive layers located over the semiconductor material of the substrate (9; 10); a memory opening extending through the stack; a semiconductor channel located within the memory opening; a memory film located within the memory opening; and a metal-semiconductor alloy region which is located between and contacts the semiconductor material of the substrate (9; 10) and the semiconductor channel wherein the metal-semiconductor alloy region comprises an alloy of the semiconductor material and a plurality of metal elements, characterized in that: the metal-semiconductor alloy region comprises a variable composition of a first metal element and a second metal element that are among the plurality of metal elements; the atomic concentration of the first metal element in the metal-semiconductor alloy region decreases with the distance from an interface between the semiconductor material of the substrate (9; 10) and the metal-semiconductor alloy region; and the atomic concentration of the second metal element in the metal-semiconductor alloy region increases with the distance from the interface between the semiconductor material of the substrate (9; 10) and the metal-semiconductor alloy region. a substrate (9; 10) including a semiconductor material; a stack of alternating layers comprising insulator layers and electrically conductive layers located over the semiconductor material of the substrate (9; 10); a memory opening extending through the stack; a semiconductor channel located within the memory opening; a memory film located within the memory opening; and a metal-semiconductor alloy region which is located between and contacts the semiconductor material of the substrate (9; 10) and the semiconductor channel wherein the metal-semiconductor alloy region comprises an alloy of the semiconductor material and a plurality of metal elements, characterized in that: the metal-semiconductor alloy region comprises a variable composition of a first metal element and a second metal element that are among the plurality of metal elements; the atomic concentration of the first metal element in the metal-semiconductor alloy region decreases with the distance from an interface between the semiconductor material of the substrate (9; 10) and the metal-semiconductor alloy region; and the atomic concentration of the second metal element in the metal-semiconductor alloy region increases with the distance from the interface between the semiconductor material of the substrate (9; 10) and the metal-semiconductor alloy region. the metal-semiconductor alloy region comprises a variable composition of a first metal element and a second metal element that are among the plurality of metal elements; the atomic concentration of the first metal element in the metal-semiconductor alloy region decreases with the distance from an interface between the semiconductor material of the substrate (9; 10) and the metal-semiconductor alloy region; and the atomic concentration of the second metal element in the metal-semiconductor alloy region increases with the distance from the interface between the semiconductor material of the substrate (9; 10) and the metal-semiconductor alloy region.