Nonvolatile semiconductor storage device
According to one embodiment, a control gate is formed on the semiconductor substrate and includes a cylindrical through hole. A block insulating film, a charge storage film, a tunnel insulating film, and a semiconductor layer are formed on a side surface of the control gate inside the through hole. The tunnel insulating film comprises a first insulating film having SiO 2 as a base material and containing an element that lowers a band gap of the base material by being added. A density and a density gradient of the element monotonously increase from the semiconductor layer toward the charge storage film.
1. A nonvolatile semiconductor storage device, comprising:
a semiconductor substrate;
a control gate formed on the semiconductor substrate and having a cylindrical through hole extending from a top surface to a bottom surface thereof;
a block insulating film formed on a side surface of the control gate inside the through hole;
a charge storage film formed on the side surface of the block insulating film inside the through hole;
a tunnel insulating film formed on the side surface of the charge storage film inside the through hole; and
a semiconductor layer formed on the side surface of the tunnel insulating film inside the through hole,
wherein the tunnel insulating film includes a first insulating film having SiO 2 as a base material and containing an element that lowers a band gap of the base material by being added and
a density and a density gradient of the element monotonously increase from the semiconductor layer toward the charge storage film.
2. The device of claim 1 , wherein the element of the first insulating film is N.
3. The device of claim 1 , wherein the element of the first insulating film is one of Al and Hf.
4. The device of claim 1 , wherein the tunnel insulating film further includes a second insulating film formed between the first insulating film and the semiconductor layer and containing SiO 2 .
5. The device of claim 4 , wherein the tunnel insulating film further includes an Si microcrystal formed in an interface between the first insulating film and the second insulating film.
6. The device of claim 1 , wherein the charge storage film comprises a material having a band gap smaller than the band gap of the tunnel insulating film.
7. The device of claim 2 , wherein the first insulating film has a composition ratio of (SiO 2 ) x (Si 3 N 4 ) 1-x (0.75≦x≦1).
8. The device of claim 3 , wherein the first insulating film has a composition ratio of (SiO 2 ) x (Al 2 O 3 ) 1-x (0≦x≦1) or (SiO 2 ) x (HfO 2 ) 1-x (0≦x≦1).
9. The device of claim 4 , wherein the second insulating film has a constant composition ratio.
10. The device of claim 5 , wherein the Si microcrystal has a constant existing density.
11. The device of claim 5 , wherein the Si microcrystal is formed also inside the second insulating film.