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High Mobility Ge pMOSFETs with ZrO2 Dielectric: Impacts of Post Annealing
Nanoscale Research Letters volume 14, Article number: 202 (2019)
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.
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.
Key process steps for fabricating Ge pMOSFETs with ZrO2 dielectric are shown in Fig. 1a. The Ge pMOSFETs were fabricated on n-type Ge(001) wafer with a resistivity of 0.088–0.14 Ω∙cm. After the several cycles of chemical cleaning in the diluted HF (1:50) solution and rinsing in DI water. Ge wafer was loaded into an atomic layer deposition (ALD) chamber. The Ge surface was passivated by an ozone post oxidation (OPO), i.e., an ultrathin Al2O3 layer was deposited at 300 °C, and then, the in situ OPO was carried out at 300 °C for 15 min. After that, a 5-nm-thick ZrO2 was deposited at 250 °C in the same ALD chamber using TDMAZr and H2O as precursors of Zr and O, respectively. During the deposition, Zr[N(CH3)2]4 source was heated to 85 °C. PDA process was carried out on some sample at 400 °C for 60 s using the rapid thermal annealing. Samples with and without PDA were denoted wafer II and I, respectively. Then, a 100-nm-thick TaN gate electrode was deposited by reactive sputtering. After the gate patterning and etching, the source/drain (S/D) regions were formed by BF2+ implantation at an energy of 30 keV and a dose of 1 × 1015 cm−2. Fifteen-nanometer nickel S/D contacts were formed by a lift-off process. Finally, the PMA at 350, 400, 450, and 500 °C for 30 s was carried out for dopant activation and S/D metallization.
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
Inversion capacitance Cinv vs. VGS curves measured at a frequency of 300 kHz for the devices on wafer I are shown in Fig. 2. The CET values are extracted to be ∼ 1.95, 1.80, 1.67, and 1.52 nm for the devices with PMA at 350, 400, 450, and 500 °C, respectively. The smaller CET is achieved at a higher PMA temperature due to the crystallization of ZrO2. In general, the κ values for amorphous and crystalline ZrO2 are about 20–23 and 28–30, respectively. A 5-nm-thick crystalline ZrO2 contributes an EOT of ~ 0.7 nm. The shift of C-V curves with various PMA temperature is due to the fact that crystallization reduces the density of bulk traps in ZrO2 dielectric.
Figure 3a shows the measured transfer characteristics and gate leakage currents IG of Ge pMOSFETs on wafer I with the different PMA temperatures. All the devices have a gate length LG of 4 μm and a gate width W of 100 μm. Ge pMOSFETs exhibit the much lower IG compared to IDS for all the PMA temperatures. An ION/IOFF ratio above 104 is achieved for the device with a PMA at 500 °C. The IDS-VDS curves of the devices measured at the different gate overdrive |VGS-VTH| are shown in Fig. 3b. It is noted that the threshold voltage VTH is defined as the VGS at IDS of 10−7 A/μm. The Ge transistor with a PMA at 500 °C obtains the ~ 47% and 118% drive current improvement compared to the devices annealed at 450 °C and 350 °C, respectively, at a VDS of − 1.0 V and a |VGS-VTH| of 0.8 V. Figure 3c shows the statistical plot of the ION at a VDS of − 0.5 V and a VGS-VTH of − 1 V for Ge pMOSFETs with the various PMA temperatures. All the transistors in this plot have an LG of 4 μm and a W of 100 μm. Devices with a PMA at 500 °C exhibit an improved ION as compared to those with the lower PMA temperatures, which is attributed to the decreased S/D resistance, the reduced CET, and the higher μeff, which will be discussed later.
Figure 4 shows the statistical plots of midgap Dit, SS, and VTH characteristics for the devices with the different PMA temperatures. As shown in Fig. 4a, based on the maximum conductance method , the midgap Dit values are extracted to be 1.3 × 1013, 9.5 × 1012, 9.2 × 1012, and 6.3 × 1012 cm−2 eV−1 for the devices with the PMA at 350, 400, 450, and 500 °C, respectively. Figure 4b presents that Ge pMOSFETs annealed at 500 °C have the improved SS characteristics than the transistors annealed at the lower temperatures, due to the smaller midgap Dit and CET. The values of Dit and SS of Ge pMOSFETs with PMA are still higher than those of the best reported Ge transistors. It could possibly be reduced by optimizing the OPO passivation module, e.g., Al2O3 thickness and ozone oxidation temperature and duration. VTH shifts to the positive VGS with the increasing of PMA temperature, which is originated from the reduced CET and Dit. It is concluded that the best electrical performance is achieved for Ge pMOSFETs with a PMA at 500 °C.
μeff, as a crucial factor affecting device drive current and transconductance in Ge pMOSFETs, was measured using the ΔRtot/ΔLG method . A large number of devices were measured with LG ranging from 1.5 to 9 μm. Figure 5a illustrates the total resistance Rtot extracted at a |VGS-VTH| of − 1 V and a VDS of − 0.05 V as a function of LG. The RSD is the value at which the fitted line intersects at the y-axis. The RSD values were estimated about to be 7.85, 7.15, 6.10, and 4.35 kΩ ·μm for devices with PMA at 350, 400, 450, and 500 °C, respectively. This is indicative of the better dopant activation of S/D at higher PMA temperature. μeff can be extracted by μeff = 1/[WQinv(ΔRtot/ΔLG)], where Qinv is the inversion charge density in Ge channel and ΔRtot/ΔLG is the slope of the Rtot vs. LG as shown in Fig. 5a. The smaller ΔRtot/ΔLG for devices with PMA at 500 °C indicates an enhancement in μeff as compared with transistors with PMA at 450 °C. Figure 5b shows μeff as a function of Qinv curves, extracted using the split C-V method. The peak hole mobility is 384 cm2/V ·s for devices with a PMA at 500 °C, which is 31% higher than that of the devices with a PMA at 400 °C. At a high Qinv of 1 × 1013 cm−2, Ge pMOSFETs which underwent a PMA at 500 °C achieve a mobility enhancement in comparison with the devices annealed at 400 °C. Ge transistors with crystalline ZrO2 have the lower density of bulk trap charge resulting in the lower remote Coulomb scattering of holes, compared to the devices with amorphous ZrO2. Owing to the smooth interface between crystalline ZrO2 and Ge, Ge devices annealed at 500 °C have a lower surface roughness scattering and show a shift of peak mobility to the higher Qinv.
Next, we discuss the impacts of PDA on the electrical characteristics of Ge pMOSFETs. Figure 6 shows the measured Cinv vs. VGS of the Ge pMOSFETs on wafer I and wafer II with a PMA at 400 °C. The device which underwent a PDA at 400 °C has a much lower CET value of 1.29 nm compared to the device without PDA, 1.80 nm. Figure 7a shows the ID, IS, and IG-VGS characteristic curves of Ge pMOSFETs on wafer I and wafer II, and the devices which underwent a PMA at 400 °C. A larger gate leakage current is obtained for the device with PDA compared to the transistor without PDA, which is due to the lower CET. The corresponding IDS-VDS curves of the devices measured at different gate overdrive VGS-VTH are shown in Fig. 7b. The Ge transistor without PDA shows a ~ 24% improvement in drive current over the one with PDA at 400 °C at the same overdrive of − 0.8 V in the saturation region.
Figure 8 plots the statistical results of midgap Dit, SS, and VTH of the Ge pMOSFETs with and without PDA. Figure 8a shows that the smaller Dit is achieved in Ge pMOSFETs with PDA at 400 °C compared to devices without PDA. In Fig. 8b, the lower value of mean subthreshold swing of 142 mV/decade is achieved for devices with PDA at 400 °C, corresponding to the lower CET and the lower Dit. It indicates that devices with PDA at 400 °C have a superior ZrO2/Ge interface. Figure 8c shows that devices with and without PDA have a different VTH; it may be attributed to the density of traps in the lower bandgap half dominant in the VTH.
Figure 9a shows the Rtot vs. LG curves at a gate overdrive of − 1 V and VDS of − 0.05 V for devices with a PMA at 400 °C. The RSD values are estimated about to be 7.15 and 7.30 kΩ·μm for devices without and with PDA at 400 °C, respectively. As shown in Fig. 9b, a remarkable higher peak μeff is achieved for Ge pMOSFETs without PDA, corresponding the smaller ΔRtot/ΔLG in Fig. 9a, compared to devices with PDA. The devices with a PDA at 400 °C exhibit a peak μeff of 211 cm2/V·s; the lower hole mobility might be mainly attributed to the strong remote Coulomb scattering contributed by the fixed charge in ZrO2 dielectric.
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.
Availability of Data and Materials
The datasets supporting the conclusions of this article are included in the article.
Atomic layer deposition
- BF2 + :
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
- ZrO2 :
- μ eff :
Effective carrier mobility
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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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