High Mobility Ge pMOSFETs with ZrO2 Dielectric: Impacts of Post Annealing
This paper investigates the impacts of post metal annealing (PMA) and post deposition annealing (PDA) on the electrical performance of Ge p-type metal-oxide-semiconductor field-effect transistors (pMOSFETs) with ZrO2 dielectric. For the transistors without PDA, on-state current (ION), subthreshold swing (SS), and capacitance equivalent thickness (CET) characteristics are improved with PMA temperature increasing from 350 to 500 °C. Crystallization of ZrO2 dielectric at the higher PMA temperature contributes to the increase of the permittivity of ZrO2 and the decrease of the density of interface states (Dit), resulting in a reduced CET and high effective hole mobility (μeff). It is demonstrated that Ge pMOSFETs with a PDA treatment at 400 °C have a lower CET and a steeper SS but a lower μeff compared to devices without PDA.
KeywordsGermanium MOSFET ZrO2 PMA PDA Mobility
Atomic layer deposition
Boron fluoride ion
Capacitive effective thickness
High-resolution transmission electron microscope
Metal-oxide-semiconductor field-effect transistors
Post deposition annealing
Post metal annealing
Tetrakis (dimethylamido) hafnium
Effective carrier mobility
Germanium (Ge) has been regarded as one of the attractive p-channel materials for advanced CMOS because it offers much higher hole mobility than does Si [1, 2, 3]. A high-quality gate dielectric and effective passivation of Ge surface are the keys to realizing the superior effective carrier mobility (μeff) and high drive current in Ge transistor [4, 5, 6, 7]. Several high-κ materials such as HfO2 , ZrO2 [7, 9], La2O3 , and Y2O3  have been studied as the alternative gate dielectrics for Ge p-type metal-oxide-semiconductor field-effect transistors (pMOSFETs) to achieve capacitance equivalent thickness (CET) scalability toward sub-1 nm. Among these, ZrO2 dielectric has attracted most attention due to the much higher κ value [12, 13] and the better interfacial quality  compared to the Hf-based ones. It has widely been reported that crystallization of ZrO2 can further improve the electrical performance of Ge pMOSFET, e.g., reducing CET and boosting μeff [15, 16]. However, there is a lack of study on the impacts of process steps for ZrO2 crystallization on device performance of Ge transistors.
In this paper, we investigate the impacts of the post metal annealing (PMA) and the post deposition annealing (PDA) on the electrical performance of Ge pMOSFETs with ZrO2 dielectric. Significantly improved μeff and reduced CET can be achieved in devices at higher PMA temperature.
Figure 1b shows the scanning electron microscope (SEM) image of a fabricated Ge pMOSFET. Figure 1c shows the cross-sectional transmission electron microscope (XTEM) image of Ge pMOSFET, showing the source/drain region, metal gate, and ZrO2 dielectric. Figure 1d and e show the high-resolution TEM (HRTEM) images of the gate stacks of Ge pMOSFETs with a PMA at 400 and 500 °C, respectively, on wafer I. It is observed that the ZrO2 dielectric was fully crystallized and underwent a PMA at 500 °C. The thickness of Al2O3 interfacial layer is about 0.7 nm.
Results and Discussion
In summary, the impacts of PMA and PDA on Ge pMOSFET with ZrO2 dielectric were extensively investigated. Crystallization of ZrO2 gate dielectric provides for significantly enhanced hole mobility and reduced CET compared to devices at the lower PMA temperature. A peak hole mobility of 384 cm2/V·s and enhanced drive current have been achieved in devices with PMA at 500 °C. Devices with PDA at 400 °C exhibited the lower CET and the smaller Dit but the poor hole mobility and the larger leakage current compared with transistors without PDA.
HL carried out the experiments and drafted the manuscript. GQH and YL supported the study and helped to revise the manuscript. YH provided constructive advice in the drafting. All the authors read and approved the final manuscript.
The authors acknowledge the support from the National Natural Science Foundation of China under Grant No. 61534004, 61604112, 61622405, 61874081, and 61851406
The authors declare that they have no competing interests.
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