Abstract
Some approaches have been reported to improve operation characteristics of flash devices such as high-k materials, nanowire channel, junctionless channel, poly-Ge and SiGe buried channel. The carrier mobility of Ge is higher than that of Si. Poly-Ge devices can be fabricated by low temperature process (<600。C), which can reduce process thermal budget. The electric field in a low dielectric constant material such as SiO2 is higher than that in a higher dielectric constant material like Al2O3 under the same operating voltage. Device with SiO2 tunneling layer has higher program/erase speed but poor retention characteristics as compared to Al2O3 tunneling layer. In this thesis, characteristics of poly-Ge flash memory device with SiO2 and Al2O3 tunneling layer are investigated and compared. With the advantages of SiO2 and Al2O3 tunneling layer, operation speed and retention characteristics can be both improved. In the first part of this thesis, inductively coupled plasma chemical vapor deposition (ICPCVD) and atomic layer deposition (ALD) are applied on N-type polycrystalline germanium junctionless flash memory devices. Different materials are deposited as tunneling oxide on polycrystalline germanium flash memory devices. It is found that N-channel polycrystalline germanium devices with SiO2 tunneling layer have no significant difference in program characteristics as compared to those with Al2O3 tunneling layer. Since the electric field in SiO2 tunneling layer is larger and the carrier injection current is enhanced, devices with SiO2 tunneling layer have better erase characteristics at the same operating voltage. In the second part, SiO2 tunneling layer and SiO2/Al2O3 stacked one on characteristics of flash devices with P-type polycrystalline germanium channel are compared. Program and erase speeds of devices with stacked tunneling layer are slower than those with single-layer silicon dioxide one. However, devices with SiO2/Al2O3 have better reliability performance. Devices with stacked tunneling layer can suppress leakage current effectively. Therefore, the retention characteristics are also improved. In the third part, we can find the similar results as found before, namely, devices with single-layer of SiO2 as a tunneling layer have higher program and erase speeds. It can be seen that the leakage current can be suppressed very effectively in the stack-type tunneling layer, resulting in very good retention. Between polysilicon and poly germanium devices using the same stacked tunneling layer, program speed of poly germanium devices with SiO2/Al2O3 stacked tunneling layer is only 2% degradation at 10 μs and erase speed has 0.027 s improvement at same window measurement which keep in 2 V. It can also be seen from this experiment that poly-Ge junctionless flash memory devices already have quite good operating characteristics, not inferior to the poly-Si ones. Therefore, poly-Ge flash devices are promising for applications in 3D high-density memory.