Memory device including composite metal oxide semiconductor channels and methods for forming the same
A three-dimensional memory device includes an alternating stack of insulating layers and electrically conductive layers, a memory opening vertically extending through the alternating stack, and a memory opening fill structure located in the memory opening and including a memory film and a vertical composite metal oxide semiconductor channel that contains an outer semiconducting metal oxide channel layer having a first band gap and an inner semiconducting metal oxide channel layer having a second band gap that is different from the first band gap.
1 . A three-dimensional memory device, comprising:
an alternating stack of insulating layers and electrically conductive layers;
a memory opening vertically extending through the alternating stack; and
a memory opening fill structure located in the memory opening and comprising a memory film and a vertical composite metal oxide semiconductor channel comprising an outer semiconducting metal oxide channel layer having a first band gap and an inner semiconducting metal oxide channel layer having a second band gap that is different from the first band gap,
wherein:
a conduction band offset of a conduction band energy level of the outer semiconducting metal oxide channel layer relative to a conduction band energy level of the inner semiconducting metal oxide channel layer is positive, and a valence band offset of a valence band energy level of the outer semiconducting metal oxide layer relative to a valence band energy level of the inner semiconducting metal oxide layer is negative;
the vertical composite metal oxide semiconductor channel further comprises an intermediate semiconducting channel layer having a graded band gap that is interposed between the outer semiconducting metal oxide channel layer and the inner semiconducting metal oxide channel layer; and
the intermediate semiconducting channel layer comprises indium gallium zinc oxide or indium gallium zinc oxynitride in which an atomic percentage of gallium and indium increases with the distance from the outer semiconducting metal oxide channel layer.
2 . The three-dimensional memory device of claim 1 , wherein the first band gap is smaller than the second band gap, and the valence band offset has a greater absolute value than the conduction band offset.
3 . The three-dimensional memory device of claim 1 , wherein:
the second band gap is greater than the first band gap; and
an entire range of the graded band gap is greater than the first band gap, and is less than the second band gap.
4 . The three-dimensional memory device of claim 1 , wherein a value of the graded band gap increases with a distance from an interface between the intermediate semiconducting channel layer and the outer semiconducting metal oxide channel layer toward an interface between the intermediate semiconducting metal oxide channel layer and the inner semiconducting metal oxide channel layer.
5 . The three-dimensional memory device of claim 1 , wherein:
the outer semiconducting metal oxide channel layer comprises zinc oxynitride; and
the inner semiconducting metal oxide channel layer comprises indium gallium zinc oxide.
6 . The three-dimensional memory device of claim 1 , wherein the memory film has a cylindrical inner sidewall that vertically extends through each of the electrically conductive layers within the alternating stack and is in contact with the outer semiconducting metal oxide channel layer.
7 . The three-dimensional memory device of claim 1 , wherein an atomic concentration of free charge carriers within the outer semiconducting metal oxide channel layer is greater than an atomic concentration of free charge carriers within the inner semiconducting metal oxide channel layer.
8 . The three-dimensional memory device of claim 1 , further comprising a drain region contacting an upper end of the vertical composite metal oxide semiconductor channel.
9 . The three-dimensional memory device of claim 8 , further comprising a source region having a doping of a same conductivity type as the drain region and contacting a lower end of the vertical composite metal oxide semiconductor channel.