Nonvolatile semiconductor memory device having floating gate that includes two layers

A nonvolatile semiconductor memory device comprising:

• a substrate;
• a base insulating layer over the substrate;
• a semiconductor layer over the base insulating layer, the semiconductor layer including a channel forming region between a first impurity region and a second impurity region which are formed apart from each other;
• a first insulating layer over the channel forming region;
• a floating gate over the channel forming region with the first insulating layer interposed therebetween;
• a second insulating layer over the floating gate;
• and
• a control gate over the floating gate with the second insulating layer interposed therebetween,
• wherein the floating gate includes at least a first layer which is in direct contact with the first insulating layer and a second layer formed over the first layer,
• wherein the first layer comprises germanium or a germanium compound,
• wherein the second layer is formed of a material selected from the group consisting of a metal, an alloy, and a metal compound which is different from the germanium and the germanium compound, and
• wherein the germanium compound is germanium oxide or germanium nitride.

A nonvolatile semiconductor memory device comprising:

• a substrate;
• a base insulating layer over the substrate;
• an island-like semiconductor layer over the base insulating layer, the island-like semiconductor layer including a channel forming region between a first impurity region and a second impurity region which are formed apart from each other;
• a first insulating layer over the channel forming region;
• a floating gate over the channel forming region with the first insulating layer interposed therebetween;
• a second insulating layer over the floating gate;
• and
• a control gate over the floating gate with the second insulating layer interposed therebetween,
• wherein the floating gate includes at least a first layer which is in direct contact with the first insulating layer and a second layer formed over the first layer,
• wherein the first layer comprises germanium or a germanium compound,
• wherein the second layer is formed of a material selected from the group consisting of a metal, an alloy, and a metal compound which is different from the germanium and the germanium compound, and
• wherein a plurality of the floating gates and the control gates are arranged over the island-like semiconductor layer, each control gate being stacked over the floating gate.