A method for forming a three-dimensional NAND memory device, comprising: forming an alternating layer stack on a substrate; forming a plurality of channel holes in the alternating layer stack, each penetrating vertically through the alternating layer stack; forming a functional layer (310, 400) including a storage layer on a sidewall of each channel hole, wherein the storage layer has an uneven surface, the method further comprising dividing the storage layer into a plurality of segments; forming a channel layer to cover the functional layer (310, 400) in each channel hole; and forming a filling structure (500, 600) to cover the channel layer and fill each channel hole, characterized in that forming the functional layer (310, 400) comprises: forming a barrier layer on the sidewall of each channel hole for blocking an outflow of the electronic charges during operation, wherein the barrier layer includes an Al 2 O 3 sublayer and a SiO 2 sublayer; forming the storage layer on a surface of the barrier layer for storing electronic charges during operation, wherein the storage layer includes a first SiN sublayer, a first SiON sublayer, a second SiN sublayer, a second SiON sublayer, and a third SiN sublayer; wherein the first SiON sublayer is arranged between the first SiN sublayer and the second SiN sublayer and wherein the second SiON sublayer is arranged between the second SiN sublayer and the third SiN sublayer; wherein the SiO 2 sublayer of the barrier layer is between the Al 2 O 3 sublayer and the first SiN sublayer of the storage layer; forming a tunneling layer on a surface of the storage layer for tunneling electronic charges during operation, wherein the tunneling layer includes a first SiO sublayer, a first SiON sublayer, a second SiON sublayer, a third SiON sublayer, and a second SiO sublayer, wherein the first SiON sublayer, the second SiON sublayer and the third SiON sublayer are between the first SiO sublayer and the second SiO sublayer, wherein the nitrogen concentration is increased following a certain gradient from the first SiON sublayer to the second SiON sublayer and to the third SiON sublayer. forming an alternating layer stack on a substrate; forming a plurality of channel holes in the alternating layer stack, each penetrating vertically through the alternating layer stack; forming a functional layer (310, 400) including a storage layer on a sidewall of each channel hole, wherein the storage layer has an uneven surface, the method further comprising dividing the storage layer into a plurality of segments; forming a channel layer to cover the functional layer (310, 400) in each channel hole; and forming a filling structure (500, 600) to cover the channel layer and fill each channel hole, characterized in that forming the functional layer (310, 400) comprises: forming a barrier layer on the sidewall of each channel hole for blocking an outflow of the electronic charges during operation, wherein the barrier layer includes an Al 2 O 3 sublayer and a SiO 2 sublayer; forming the storage layer on a surface of the barrier layer for storing electronic charges during operation, wherein the storage layer includes a first SiN sublayer, a first SiON sublayer, a second SiN sublayer, a second SiON sublayer, and a third SiN sublayer; wherein the first SiON sublayer is arranged between the first SiN sublayer and the second SiN sublayer and wherein the second SiON sublayer is arranged between the second SiN sublayer and the third SiN sublayer; wherein the SiO 2 sublayer of the barrier layer is between the Al 2 O 3 sublayer and the first SiN sublayer of the storage layer; forming a tunneling layer on a surface of the storage layer for tunneling electronic charges during operation, wherein the tunneling layer includes a first SiO sublayer, a first SiON sublayer, a second SiON sublayer, a third SiON sublayer, and a second SiO sublayer, wherein the first SiON sublayer, the second SiON sublayer and the third SiON sublayer are between the first SiO sublayer and the second SiO sublayer, wherein the nitrogen concentration is increased following a certain gradient from the first SiON sublayer to the second SiON sublayer and to the third SiON sublayer. forming a barrier layer on the sidewall of each channel hole for blocking an outflow of the electronic charges during operation, wherein the barrier layer includes an Al 2 O 3 sublayer and a SiO 2 sublayer; forming the storage layer on a surface of the barrier layer for storing electronic charges during operation, wherein the storage layer includes a first SiN sublayer, a first SiON sublayer, a second SiN sublayer, a second SiON sublayer, and a third SiN sublayer; wherein the first SiON sublayer is arranged between the first SiN sublayer and the second SiN sublayer and wherein the second SiON sublayer is arranged between the second SiN sublayer and the third SiN sublayer; wherein the SiO 2 sublayer of the barrier layer is between the Al 2 O 3 sublayer and the first SiN sublayer of the storage layer; forming a tunneling layer on a surface of the storage layer for tunneling electronic charges during operation, wherein the tunneling layer includes a first SiO sublayer, a first SiON sublayer, a second SiON sublayer, a third SiON sublayer, and a second SiO sublayer, wherein the first SiON sublayer, the second SiON sublayer and the third SiON sublayer are between the first SiO sublayer and the second SiO sublayer, wherein the nitrogen concentration is increased following a certain gradient from the first SiON sublayer to the second SiON sublayer and to the third SiON sublayer.