Three-dimensional memory device including a composite semiconductor channel and a horizontal source contact layer and method of making the same

A three-dimensional memory device, comprising:

• an alternating stack of insulating layers and electrically conductive layers located over a substrate, wherein the electrically conductive layers comprise source-select-level electrically conductive layers and word-line-level electrically conductive layers overlying the source-select-level electrically conductive layers;
• a source contact layer located between the substrate and the alternating stack;
• a memory opening vertically extending through the alternating stack and the source contact layer with a width-modulated vertical cross-sectional profile that has a first width at and below levels of the source-select-level electrically conductive layers, and has a second width that is greater than the first width at levels of a first subset of the word-line-level electrically conductive layers;
• and
• a memory opening fill structure located within the memory opening and comprising a composite semiconductor channel and a memory film laterally surrounding the composite semiconductor channel, wherein the composite semiconductor channel comprises:
• a pedestal channel portion having a first semiconductor material composition and located within a portion of the memory opening having the first width and having a cylindrical sidewall segment that contacts the source contact layer;
• and
• a vertical semiconductor channel having a second semiconductor material composition that is different from the first semiconductor material composition, comprising a bottom surface that contains a central segment contacting an entirety of a top surface of the pedestal channel portion and contains a peripheral surface segment contacting a laterally-extending surface of a portion of the memory film having the second width, and extending through the word-line-level electrically conductive layers.