Ran Cheng, Zhuo Chen, Sicong Yuan, Mitsuru Takenaka, Shinichi Takagi, Genquan Han,and Rui Zhang,

1School of Micro-Nano Electronics, Zhejiang University, Hangzhou 310058, China

2State Key Discipline Laboratory of Wide Bandgap Semiconductor Technology, School of Microelectronics, Xidian University,Xi’an 710071, China

3School of Engineering, Tokyo University, Yayoi 2-11-16, Tokyo 113-8656, Japan

Abstract: The performance enhancement of conventional Si MOSFETs through device scaling is becoming increasingly difficult. The application of high mobility channel materials is one of the most promising solutions to overcome the bottleneck.The Ge and GeSn channels attract a lot of interest as the alternative channel materials, not only because of the high carrier mobility but also the superior compatibility with typical Si CMOS technology. In this paper, the recent progress of high mobility Ge and GeSn MOSFETs has been investigated, providing feasible approaches to improve the performance of Ge and GeSn devices for future CMOS technologies.

Key words: germanium; germanium-tin; MOSFET; mobility

1.Introduction

In the past few decades, the device scaling of Si metaloxide-semiconductor field-effect transistors (MOSFETs), following Moore’s Law, drives the fast development of complementary metal-oxide-semiconductor (CMOS) integrated circuits[1-3]. Recently, the performance enhancement of Si MOSFETs is increasingly difficult to achieve as the conventional device scaling is approaching its physical limit[4]. Higher channel mobility is effective to improve the MOSFETs performance, which has been well demonstrated by the application of strained-Si techniques[5,6]. However, advanced MOSFET technology is still desired to further improve the performance of CMOS' devices. Alternative channel materials with mobility higher than Si have been attracting a lot of interest for the potential to improve the MOSFET performance. Among these high mobility materials, Ge and GeSn are promising candidates owing to their high mobility as well as the superior integrability on the Si platform[7-12].

It is essential to employ the high-k dielectrics (such as Al2O3, HfO2, etc.) for thinner equivalent oxide thickness (EOT),to satisfy the requirement of device scaling. Although the thin EOT high-k/Ge and high-k/GeSn MOSFETs have been demonstrated, the relatively low inversion carrier mobility severely limits the application of Ge and GeSn channels. This phenomenon attributes to the large interface state density(Dit) at the Ge and GeSn metal-oxide-semiconductor (MOS)interfaces, especially for the ultrathin Ge and GeSn gate stacks[13-18]. Thus the suppression of Ditin ultrathin EOT highk/Ge and high-k/GeSn gate stacks would be one of the key issues to be resolved, in order to obtain high performance Ge and GeSn MOSFETs.

In this paper, the MOS interface passivation techniques from our previous work will be summarized for Ge and GeSn MOSFETs. Ge and GeSn MOSFETs with ultrathin EOT and superior channel mobility have been demonstrated, suggesting the great potential of Ge and GeSn MOSFETs as the alternative device structures in future advanced CMOS technologies.

2.Ge MOSFETs

2.1.EOT scaling and MOS interface passivation

Because of the large Ditat direct high-k/Ge interface, an interfacial layer (IL) is necessary to obtain the high mobility in Ge MOSFETs. It has been confirmed that the thermally oxidized GeO2/Ge interfaces are effective to passivate the Ge MOS interface, with a GeO2thickness of ～20 nm[19-21]. However,the relatively low permittivity of GeO2(5-6) requires the aggressive scaling down of the GeO2thickness to achieve an ultrathin EOT high-k/GeO2/Ge gate stack. Unfortunately, it is difficult to fabricate a thin GeO2/Ge MOS interface with superior interface qualities, since the Ditin the IL significantly increases as the GeO2thickness decreases[22,23]. Therefore, the Ditreduction for thin GeO2/Ge MOS interfaces is one of the technical bottlenecks for the realization of high-performance Ge pMOSFETs.

To fabricate ultrathin Ge MOS interfaces with low Dit, a plasma post oxidation (PPO) method was proposed by using oxygen plasma exposure to oxidize the high-k/Ge interface(Fig.1), yielding a high-k/GeOx/Ge gate stack[22,24-25]. Here the high-k layer serves as a protecting layer to prevent damage to GeOxIL from the subsequent device fabrication process, and also as a sufficient oxygen barrier that suppresses the growth of unnecessarily thick GeOxIL. Fig.2 shows the angel-resolved X-ray photoelectron spectroscopy (AR-XPS) spectra taken from an Al2O3(1 nm)/Ge structure after PPO. The intensity of the Ge 3d peak corresponding to GeOxdecreases more pronounced than that of the Al 2p peak corresponding to Al2O3, from which it is confirmed that the GeOxis formed underneath the Al2O3layer after PPO.

Fig.1. (Color online) The fabrication process of the high-k/GeOx/Ge gate stacks with PPO method.

Fig.2. (Color online) The AR-XPS spectra taken from an 1-nm-thick Al2O3/Ge structure with 650 W PPO for 10 s.

Fig.3. (Color online) (a) The C-V characteristics of an Au/Al2O3 (1 nm)/GeOx (1.2 nm)/Ge MOS capacitor fabricated with the PPO method.(b) The Dit at the Al2O3/Ge MOS interfaces w/ and w/o PPO treatment.

Fig.4. (Color online) The Dit at Ei - 0.2 eV taken from PPO Al2O3/GeOx/Ge MOS interfaces fabricated with different Al2O3 capping thickness,plasma power and oxidation time.

The electrical properties of the PPO GeOx/Ge MOS interfaces were investigated using the Au/Al2O3/GeOx/Ge MOS capacitors. Fig.3(a) shows the C-V curves of an Au/Al2O3(1 nm)/GeOx(1.2 nm)/Ge MOS capacitor fabricated by PPO.The superior C-V characteristics are observed with an EOT of 1.06 nm. The Ditof this MOS capacitor was evaluated by the low temperature conductance method with the correction of surface potential fluctuation[26-29]. It is found that the reduction in Ditby one order of the magnitude is obtained after the PPO, as compared with that of the direct Al2O3/Ge MOS interface (Fig.3(b)). The minimum Ditdetected in this MOS capacitor is 5 × 1010cm-2eV-1. These results show a clear correlation between the high-k/Ge MOS interface quality with the existence of GeOxIL. The relationship between the GeOx/Ge MOS interface quality and the GeOxIL thickness is further studied. Here, the Al2O3layer thickness (1 to 1.5 nm), oxygen plasma power (300 to 650 W), and plasma oxidation time (5 to 30 s) were varied to obtain different GeOxIL thicknesses. A thinner GeOxIL can be realized by lowering the plasma power, shortening the oxidation time, and increasing the thickness of the Al2O3layer, which is attributable to the reduced diffusion of oxygen species through the Al2O3layer. The GeOxILs with the thickness ranging from 1.2 to 0.23 nm can be grown by changing the PPO conditions. Fig.4 shows the Ditat the energy of Ei- 0.2 eV for the Au/Al2O3/GeOx/Ge MOS capacitors as a function of the GeOxIL thickness, fabricated with various conditions. It is found that the Ditvalues of the Au/Al2O3/GeOx/Ge MOS capacitors fabricated with different conditions generally obey the universal relationship against the GeOxthickness. These results indicate that Ditat GeOx/Ge MOS interface is determined by neither the Al2O3thickness nor the plasma conditions, but by the thickness of GeOxIL only. It is observed from this relationship that a significant degradation of Ditat the MOS interfaces starts around 0.5 nm.This phenomenon suggests that a GeOxIL with the thickness of one GeO2unit cell (0.5-0.57 nm[30]), can sufficiently passivate a Ge MOS interface. This physical thickness of 0.5 nm corresponds roughly to a EOT of 0.35 nm under an assumption that the permittivity of GeOxis 5.5. As a result, the GeOx/Ge MOS interfaces with Ditlower than half of 1011cm-2eV-1can be realized at the expense of increasing EOT by ～0.35 nm.

Fig.5. (Color online) (a) The C-V curves of the Au/HfO2/p-Ge MOS capacitors w/ and w/o PPO, compared with that of Au/Al2O3/GeOx/p-Ge MOS capacitor. (b) The C-V curves of of the Au/HfO2/Al2O3/p-Ge MOS capacitor after PPO. The inset of it shows the energy distribution of Dit of this MOS capacitor.

Fig.6. The cross section TEM image of an HfO2 (2.2 nm)/Al2O3 (0.2 nm)/Ge structure after 15 s’ PPO using 500 W plasma.

The GeOxIL shows a superior scalability to maintain the low Ditfor Ge MOS interfaces, indicating a possibility to realize high quality Ge gate stacks with ultrathin EOT. The suppression of EOT contributed by a high-k layer is also important.The formation of high quality GeOx/Ge MOS interfaces has been studied for HfO2/Ge structures. The strong inter-mixing between HfO2and Ge at high temperature induces the generation of MOS interface defects[31,32]. Thus, the HfO2/Al2O3/Ge structure was used as the starting sample for PPO instead of the direct HfO2/Ge structure. Fig.5(a) shows the C-V characteristic of an Au/Al2O3/GeOx/p-Ge MOS capacitor after PPO, as compared with that of the Au/HfO2(2.2 nm)/p-Ge MOS capacitors w/ and w/o PPO. It is found that the capacitance increases for the gate stack using the HfO2dielectrics, thanks to its large permittivity. However, the Au/HfO2(2.2 nm)/p-Ge MOS capacitor shows a large stretch-out of the C-V curve even after PPO, suggesting that the PPO method is not effective to the HfO2/Ge gate stacks. This phenomenon is attributable to the significant degradation of Ge MOS interface quality and generation of Ditdue to the HfGeOxformation induced by the HfO2-Ge inter-mixing. In contrast, the good C-V characteristics of the Au/HfO2/Al2O3/GeOx/p-Ge MOS capacitor indicate the sufficient passivation of the Ge MOS interface. The hysteresis of the C-V curves is ～150 mV for this MOS capacitor with a sweep scan from 1 to -1.5 V (data not shown), which is attributable to the slow traps in the HfO2layer. A much smaller hysteresis was confirmed for the Al2O3/GeOx/Ge gate stack even with a much thicker EOT[22]. It is evaluated from a germanium based C-V simulator that the EOT of this MOS capacitor is 0.76 nm. The evaluated Ditvalues are plotted as a function of energy in the inset of Fig.5(b). It is found that Ditof the Au/HfO2/Al2O3/GeOx/Ge MOS capacitor is suppressed by around one order of magnitude through PPO and that a minimum Ditvalue of 2 ×1011cm-2eV-1is detected in this MOS capacitor. It is confirmed from the transmission electron microscopy (TEM) image that this gate stack is composed of HfO2/Al2O3/GeOx/Ge structure (Fig.6). The ～0.55-nm-thick interfacial layer is observed between the 2.2-nm-thick HfO2layer and the Ge substrate, corresponding to the 0.2-nm-thick Al2O3diffusion control layer and the 0.35-nm-thick GeOxinterfacial layer.

Fig.7. (Color online) The Dit at the energy of Ei - 0.2 eV of the HfO2/Al2O3/GeOx/Ge and Al2O3/GeOx/Ge gate stacks, as a function of EOT.

The impact of the PPO time on Ditand EOT of the Au/HfO2(2.2 nm)/Al2O3(0.2 nm)/GeOx/p-Ge MOS capacitors were examined. The Ditvalues at Ei- 0.2 eV of these MOS capacitors are summarized in Fig.7 as a function of EOT. The Dittaken from the PPO Al2O3/GeOx/Ge gate stacks is also shown for comparison. Due to the drastic increase of the gate leakage of the Al2O3/GeOx/Ge gate stacks with decreasing Al2O3thickness, the thickness of GeOxIL has to be reduced to further scale down the EOT of Al2O3/GeOx/Ge gate stacks. As a result, the clear degradation of the MOS interface quality is observed as EOT approaches ～1 nm for Al2O3/GeOx/Ge gate stacks because of the insufficient GeOxIL thickness[24]. On the other hand, further EOT scaling is realized down to 0.7-0.8 nm for the HfO2/Al2O3/GeOx/Ge gate stacks because of the higher permittivity of HfO2. A rapid reduction of Ditat Ei- 0.2 eV is observed from 5.3 to 3.9, 3.2, until 2.8 ×1011cm-2eV-1with an increase in the PPO time from 5 to 10,15, until 25 s for the HfO2(2.2 nm)/Al2O3(0.2 nm)/Ge gate stacks which have EOT of 0.72, 0.75, 0.76 and 0.82 nm, respectively. It is also confirmed from these results that the GeOxIL is necessary to maintain low Ditat the Ge MOS interface.

It is noted that in the HfO2/Al2O3/GeOx/Ge gate stacks,the thickness of the HfO2layer was fixed at 2.2 nm to sufficiently suppress the gate leakage, while the EOT scaling capability of this gate stack structure is dominated by the GeOxIL thickness. Thus, it is still challenging to realize Ge gate stacks with thinner EOT but decently passivated MOS interface. Additionally, due to the directional nature of the oxygen plasma,it is difficult to realize an isotropic oxidation to 3D-structured Ge channels with the oxygen plasma. This phenomenon reduces the effectiveness of the PPO method to Ge FinFET or Ge gate-all-around devices, which are the most promising device structures for future Ge CMOS technology.

Fig.8. (Color online) The schematic illusion of the ozone post oxidation process.

Fig.9. Cross section TEM of an HfO2 (2 nm)/Al2O3 (0.3 nm)/Ge structure after OPO for 60 s at 300 °C.

To solve these problems, an ozone post oxidation (OPO)was employed to form ultrathin Ge gate stacks with an isotropic oxidation (Fig.8). After the ALD deposition of HfO2/Al2O3/Ge structures, the in-situ OPO was carried out at 300 °C in the 10% O3/O2ambient with a pressure of ～100 Pa.The formation of the GeOx/Ge interface by OPO was examined by the cross-sectional TEM observation. As shown in Fig.9, the gate stack with 60 s OPO shows a total physical thickness of 2.7 nm after 60 s OPO, indicating the growth of 0.4-nm-thick GeOxIL compared with the gate stack before OPO (2 nm HfO2/0.3 nm Al2O3). Here, the 0.3-nm-thick Al2O3layer serves as a blocking layer to prevent the inter-mixing between HfO2and GeOxduring OPO. The partially crystallized HfO2regions were observed in the HfO2layer, which results in the increase of k value for HfO2[33]. The crystallization of HfO2after OPO might be attributed to the slight Ge diffusion into the HfO2film. Similar phenomena have also been reported in literature that the cubic phase HfO2could be stabilized by Ge doping[34-37]. As a result, the EOT of the HfO2/Al2O3/GeOx/Ge gate stack is suppressed by the OPO treatment, and the ultrathin EOT of 0.6 nm has been obtained for the HfO2/Al2O3/GeOx/Ge gate stack with 60 s OPO. On the other hand, the EOT of the OPO HfO2/Al2O3/GeOx/Ge gate stack is thinner than the as-deposited HfO2/Al2O3/Ge gate stack(0.82 nm), although the total physical thickness of the gate stack increases after OPO due to the GeOxIL growth. This phenomenon is also observed for the HfO2/Al2O3/GeOx/Ge gate stacks with different Al2O3layer thicknesses of 0.1 and 0.2 nm(Fig.10). The relationship between Ditand EOT of the OPO HfO2/Al2O3/GeOx/Ge gate stacks is shown in Fig.11, as compared with the PPO gate stacks. It is found that the Ditin the PPO HfO2/Al2O3/GeOx/Ge and Al2O3/GeOx/Ge gate stacks increase rapidly with the scaling down of the total EOT because it is mandatory to sacrifice the GeOxIL thickness during EOT scaling. The trade-off between EOT and Ditseverely limits the scalability of these PPO gate stacks. In contrast, the lower Ditis obtained for the OPO HfO2/Al2O3/GeOx/Ge gate stacks with thinner EOT by increasing the OPO time, since the HfO2crystallization compensates the EOT increase contributed by GeOxIL. These phenomena indicate that the OPO HfO2/Al2O3/GeOx/Ge gate stack features the superiority of both Ditsuppression and EOT scalability, which is a promising gate stack technique for the future high performance Ge MOSFETs.

Fig.10. (Color online) The EOT of the OPO HfO2/Al2O3/GeOx/Ge gate stacks with different Al2O3 thicknesses and OPO times.

Fig.11. (Color online) The Dit at the energy of Ei - 0.2 eV in OPO HfO2/Al2O3/GeOx gate stacks, compared with the PPO gate stacks as a function of EOT.

The effectiveness of the OPO method to the 3D-structured Ge channels was also examined using a series of Ge fins with the same fin height and different fin widths. The EOT on the sidewall and the EOT on the fin top are extracted for the PPO Al2O3/GeOx/Ge and the OPO HfO2/Al2O3/GeOx/Ge gate stacks. As shown in Fig.12, the PPO Al2O3/GeOx/Ge gate stack has a thinner oxide thickness on the sidewall region than that at the bottom region, due to the insufficient formation of GeOxIL at the sidewall. In contrast, the OPO HfO2/Al2O3/GeOx/Ge gate stack shows almost the same oxide thickness for sidewall and bottom regions, meaning that the OPO exhibits an isotropic formation of GeOxIL. These results indicate the feasibility of OPO passivation for 3D-structured Ge channels.

2.2.Ultrathin EOT and high mobility Ge MOSFETs

Fig.12. (Color online) The oxide thickness of the PPO and OPO gate stacks at side wall and top regions of a 3D structured Ge channel.

Fig.13. (Color online) The fabrication process of the Ge MOSFETs with OPO HfO2/Al2O3/GeOx gate stacks.

Fig.14. (Color online) The Id-Vd (a) and Id-Vg (b) characteristics of an(100)/＜110＞ HfO2/Al2O3/GeOx/Ge pMOSFET fabricated by 60 s OPO.

The Ge MOSFETs have been demonstrated with the ultrathin EOT HfO2/Al2O3/GeOxgate stacks with OPO treatment.Gate first process was employed to fabricate the Ge MOSFETs with a structure shown in Fig.13. The (100) n-Ge substrates with a resistivity of 1-10 Ω·cm were used. The SiO2field oxide was deposited and the active areas were defined by etching off the SiO2. The HfO2/Al2O3/GeOxgate stacks were fabricated, and TiN gate electrodes were sputtered for the compatibility of process integration. The NiGe metal S/D structures were formed by Ni deposition and

NiGe metallization at 400 °C for 1 min. Finally, Ni contact pads were deposited on gate and S/D regions by thermal evaporation, for electrical characterization.

Fig.15. (Color online) The hole mobility in HfO2/Al2O3/GeOx/Ge pMOSFET fabricated by OPO with different oxidation times, compared with that in the HfO2/Al2O3/Ge pMOSFET.

Figs. 14(a) and 14(b) show the Id-Vdand Id-Vgcharacteristics of an HfO2/Al2O3/GeOx/Ge pMOSFETs with 60 s OPO, respectively. The ON/OFF ratios of the NiGe S/D pMOSFETs are～3.5 orders of magnitude. The S factor of 85 mV/decade could be obtained for the device, which corresponds to a Ditof 2.3 × 1012cm-2eV-1and agrees well with the Ditmeasured by the conductance method. The normal operations of the Ge pMOSFETs with different OPO times are also observed (data not shown). The effective hole mobility of the Ge pMOSFETs were evaluated by the split C-V method. Fig.15 shows the hole mobility in HfO2/Al2O3/GeOx/Ge pMOSFETs with different OPO times. The Ge pMOSFETs with OPO HfO2/Al2O3/GeOx/Ge gate stacks exhibit a much better improved hole mobility than that in HfO2/Al2O3/Ge pMOSFETs, attributing to the MOS interface passivation by OPO treatment. The peak hole mobility of 130, 332, 417 cm2/(V·s) were achieved with ultrathin EOT of 0.78, 0.72, and 0.58 nm, respectively. Conventionally, the mobility decreases with the scaling down of EOT for Ge MOSFETs, due to the degradation of MOS interfaces in the thinner gate stacks. However, it is interestingly noted that the hole mobility in the OPO HfO2/Al2O3/GeOx/Ge pMOSFETs increases with the decrease of EOT. This phenomenon is attributable to the different mechanisms of EOT scaling by OPO treatment and the conventional interface engineering techniques for Ge gate stacks. The EOT scaling was achieved by the permittivity enhancement of HfO2after OPO treatment for the HfO2/Al2O3/GeOx/Ge gate stacks, rather than the decrease of GeOxinterfacial layer thickness obtained from the conventional Ge gate stack treatments. Thus,the EOT scaling and mobility improvement are both realized for the OPO HfO2/Al2O3/GeOx/Ge MOSFETs.

3.GeSn MOSFETs

3.1.MOS interface passivation

From the viewpoint of pursuing high mobility transistors,the alternative semiconductor materials are desired to further improve the CMOS performance. Although the Ge MOSFETs show promising properties to achieve an enhanced performance than the Si MOSFETs, the alternative channel materials are still mandatory for higher mobility MOSFETs. The GeSn channel MOSFETs are investigated as one of the possible solutions for future high mobility MOSFETs, especially for the high mobility pMOSFET applications. In contrast to the superior interface qualities of oxidation SiO2/Si and GeO2/Ge MOS interfaces, the oxidation GeSnOx/GeSn interface exhibits relatively high defect density[17]. Thus the passivation of GeSn MOS interface is one of the key issues that needs to be resolved in order to realize high-performance GeSn MOSFETs.Thanks to the different electrical potentials of the valence band of Si and GeSn, the Si IL passivation indicates a promising solution for GeSn MOS devices[38-40]. Due to the carrier confinement effect, the holes are repelled from the gate dielectric interface, resulting in a GeSn quantum well layer with a decreased inversion carrier scattering with reduced Coulomb and surface roughness scattering centers (Fig.16).

Fig.16. (Color online) The mechanism of suppressed carrier scattering in the Si passivated GeSn channel, compared with the direct oxide/GeSn channel.

Fig.17. The cross section TEM image of a GeSn MOS structure having the Si passivation.

The Si passivation layers are grown on the surface of 6-nm-thick (100), (110) and (111) Ge0.92Sn0.08/Ge structures using MBE, with Si2H6as the precursor. Fig.17 shows the crosssection TEM image of a GeSn pMOSFET fabricated with the Si passivated GeSn substrate. Uniform layer thickness and sharp interface are observed for the Si/GeSn structure, indicating the successful formation of the GeSn quantum well channel.The pMOSFET devices with Si passivated Ge0.92Sn0.08quantum well channels were fabricated with a gate first process.The TaN/HfO2gate stacks were deposited on the Si/GeSn/Ge structures, followed by a post deposition annealing at 450 °C.After the gate patterning, the boron implantation and activation annealing were carried out to form the source/drain regions. The fabricated GeSn pMOSFETs exhibit normal operations as indicated by the Id-Vgand Id-Vdcharacteristics(Fig.18). The peak mobility of 685, 745, and 845 cm2/(V·s) are obtained for (100), (110), and (111) GeSn pMOSFETs, respectively (Fig.19). Additionally, the (100), (110), and (111) GeSn pMOSFETs exhibit the mobility of 445, 571, and 576 cm2/(V·s)at an Nsof 1013cm-2, which is improved by 1.5 times as compared with those in the Ge pMOSFET control sample. The much higher hole mobility in the GeSn pMOSFETs as compared with that in Ge pMOSFETs are attributable to the high bulk mobility in GeSn and the carrier confinement effect by the Si passivation layer.

Fig.18. (Color online) (a) Id-Vg and (b) Id-Vd characteristics of the (100)GeSn QW pMOSFET with Si passivation.

Fig.19. (Color online) The hole mobility in the Si passivated GeSn QW pMOSFETs with different channel orientations of (100), (110) and(111).

3.2.Channel strain engineering

Beside of the MOS interface passivation, the strain engineering is also an effective performance booster to improve the mobility for GeSn pMOSFETs. It is found that the compressive strain induces the reduction of the hole effective mass and valence band edge lifting for GeSn channels, resulting in a higher hole mobility, which is similar to those in strained SiGe and Ge channels[5,41-45]. Thus, it is essential to obtain the higher hole mobility in GeSn channels by enhancing the compressive strain. Since the GeSn channels are typically epitaxially grown on Si or Ge surfaces, the advantage of compressive strain for hole mobility improvement is expected to increase by improving the Sn composition in the GeSn channel.The 11-nm-thick GeSn channels were grown on (100) Ge surfaces with different Sn contents of 2.7%, 4.0%, and 7.5%, respectively. As a result, the biaxial compressive strain of 0.50%, 0.73%, and 1.35% were obtained in the GeSn channels. The in-situ Si2H6treatment was employed to form the SiO2/Si passivation for GeSn channels, to suppress the inversion carrier scattering by MOS interface defects. It is confirmed from the cross-sectional TEM image that the total thickness of SiO2/Si passivation layers is ～1 nm for all samples, giving the same MOS interface passivation effect for GeSn channels with different Sn contents.

Fig.20. (Color online) The comparison of (a) Id-Vd curves, (b) Gm curves and (c) hole mobility of GeSn QW pMOSFETs with different Sn contents of 2.7%, 4.0% and 7.5%.

Fig.20(a) shows the Id-Vdcharacteristics of the GeSn pMOSFETs with different Sn contents of 2.7%, 4.0% and 7.5%.The Idincrease has been obtained for the GeSn pMOSFETs with the increased Sn content, which is attributable to the mobility enhancement. A similar phenomenon is also observed in the transconductance curves of these devices (Fig.20(b)).It is also noted that the OFF-state current is larger for the GeSn pMOSFET with a higher Sn content, possibly caused by the reduced band gap due to the valent band edge lifting.The hole mobility in the GeSn pMOSFETs with different Sn contents are shown in Fig.20(c). The improved hole mobility is confirmed for the GeSn pMOSFETs with an increase of Sn content, as theoretically predicted. The peak mobility of 340, 378,and 496 cm2/(V·s) are achieved for the GeSn pMOSFETs with Sn contents of 2.7%, 4.0%, and 7.5%, respectively.

The physical origin of the enhanced hole mobility in the GeSn channel with increased Sn content is investigated by performing the subband calculation using the 8 × 8 k·p method,Schrödinger equation, and Poisson equation, with the material parameters taken from the Refs. [46, 47]. Fig.21 shows the equi-energy contour plots of heavy hole (HH) bands for GeSn with different Sn contents of 2.7%, 4.0%, and 7.5%. It is found that the effective mass of hole decreases with the increase of Sn content, which is responsible for the increase of hole mobility in GeSn pMOSFETs.

Fig.21. (Color online) The equienergy contours of heavy hole subband for GeSn pMOSFETs with different Sn contents of 2.7%, 4.0%and 7.5%, at a Ns of 5 × 1012 cm-2. The equienergy contour lines are for multiples of 20 meV.

4.Conclusion

The Ge and GeSn channel MOSFETs have been examined to further improve the performance of CMOS technology. It is found that through the interface passivation with ultrathin GeOxlayer grown by plasma post oxidation method,the mobility of Ge MOSFETs is sufficiently improved due to the Ditreduction. In addition, it is also confirmed that the Si passivation technique is an effective technique to enhance the mobility in the GeSn channel by suppressing the carrier scattering and reducing the hole effective mass especially for transistors with higher Sn content. These results indicate that the Ge and GeSn channels are one of the most promising solutions to realize future high-performance CMOS devices.

Acknowledgements

This work was supported, in part, by the Zhejiang Provincial Natural Science Foundation of China under Grant LR18F040001 and in part by the Fundamental Research Funds for the Central Universities.