Mingshan Liu

Design of GeSn-Based Heterojunction-Enhanced N-Channel Tunneling FET With Improved Subthreshold Swing and ON-State Current

We design a heterojunction-enhanced n-channel tunneling FET (HE-NTFET) employing a Ge_1-xSn_x/Ge_1-ySn_y (x > y) heterojunction located in the channel region with a distance of L_T-H from the source-channel tunneling junction. We investigate the impact of L_T-H on the performance of HE-NTFETs by simulation. HE-NTFETs achieve a positive shift of VONSET, a steeper subthreshold swing (SS), and an enhanced ION compared with homo-NTFETs, which is attributed to the modulating effect of heterojunction on band-to-band tunneling (BTBT). At a supply voltage of 0.3 V, 304% ION enhancement is demonstrated in the Ge_0.92Sn_0.08/Ge_0.94Sn_0.06 HE-NTFET with a 4 nm L_T-H over Ge0.92Sn0.08 homo-NTFET due to the steeper average SS. The impact of Sn composition on the performance of HE-NTFETs is also studied. As we increase the difference in Sn composition x - y across the heterojunction, ION and SS of HE-NTFETs are improved due to the increase in band offsets at the Ge_1-xSn_x/Ge_1-ySn_y interface, which leads to the enhanced modulating effect of heterojunction on BTBT.

GeSn Quantum Well P-Channel Tunneling FETs Fabricated on Si(001) and (111) With Improved Subthreshold Swing

Ultrathin GeSn channels were epitaxially grown on Si(111) and (001) substrates using solid source molecular beam epitaxy. Well-behaved GeSn quantum well (QW) pTFETs and pMOSFETs were fabricated on Si. GeSn QW pMOSFETs on Si(111) demonstrate a high effective hole mobility of 505 cm2/Vs, indicating the high crystallinity of the GeSn material. GeSn QW pTFETs on Si(111) outperform the devices on Si(001) on subthreshold swing (SS) and ON-state current ION. (111)-oriented GeSn pTFET with a 4-nm-thick channel achieves a steep SS of ~60 mV/decade and a high ON-state/OFF-state current ratio of 107, which are superior to those of the other reported non-Si pTFETs with a small bandgap.

Strained GeSn p-Channel Metal–Oxide–Semiconductor Field-Effect Transistors With In Situ Si 2 H 6 Surface Passivation: Impact of Sn Composition

We report a study about the impact of Sn composition on the performance of strained germanium-tin (GeSn) pMOSFETs. GeSn pMOSFETs with Sn compositions of 0.027, 0.040, and 0.075 were fabricated on Ge(001) with an in situ Si₂H₆ passivation. Enhancement in drive current and transconductance is obtained for GeSn pMOSFETs with higher Sn composition due to the smaller capacitance equivalent thickness, the reduced source/drain resistance, and the improved effective hole mobility μeff. Right shift of threshold voltage with Sn composition is observed for the devices. Ge_0.973Sn_0.027, Ge_0.960Sn_0.040, and Ge_0.925Sn_0.075 pMOSFETs demonstrate the peak μ_eff of 340, 378, and 496 cm²/Vs, respectively. At an inversion charge density of 5 × 10¹² cm^-2, Ge_0.925Sn_0.075 pMOSFETs demonstrate 36% and 24% enhancement in μ_eff compared with Ge_0.973Sn_0.027 and Ge_0.960Sn_0.040 devices, respectively. Simulation shows that the enhancement in μeff with Sn composition is resulted from the reduction of hole effective mass and intervalley scattering between heavy and light holes caused by the increased compressive strain.

Germanium-Tin P-channel tunneling field-effect transistors: Impacts of biaxial tensile strain and surface orientation

This work investigates the impacts of biaxial tensile strain and surface orientation on performance of GeSn pTFET. Multi-bands k·p method is used to calculate the band structure of biaxially tensile strained GeSn on various orientations. The electrical characteristics of tensile strained GeSn line- and point-pTFETs are computed implementing the dynamic nonlocal BTBT algorithm. Our simulation demonstrates that 1) tensile strained GeSn pTFETs achieve significantly improved |ION| over relaxed devices; 2) With the same tensile strain, GeSn pTFETs on (011) and (111) orientations demonstrate higher |ION| compared to (001)-oriented device.

Relaxed germanium-tin P-channel tunneling field-effect transistors fabricated on Si: impacts of Sn composition and uniaxial tensile strain

In this work, relaxed GeSn p-channel tunneling field-effect transistors (pTFETs) with various Sn compositions are fabricated on Si. Enhancement of on-state current ION with the increase of Sn composition is observed in transistors, due to the reduction of direct bandgap EG. Ge0.93Sn0.07 and Ge0.95Sn0.05 pTFETs achieve 110% and 75% enhancement in ION, respectively, compared to Ge0.97Sn0.03 devices, at VGS - VTH = VDS = - 1.0 V. For the first time, ION enhancement in GeSn pTFET utilizing uniaxial tensile strain is reported. By applying 0.14% uniaxial tensile strain along [110] channel direction, Ge0.95Sn0.05 pTFETs achieve 12% ION improvement, over unstrained control devices at VGS - VTH = VDS = - 1.0 V. Theoretical study demonstrates that uniaxial tensile strain leads to the reduction of direct EG and affects the reduced tunneling mass, which bring the GBTBT rising, benefiting the tunneling current enhancement in GeSn TFETs.

Mobility enhancement in undoped Ge0.92Sn0.08 quantum well p-channel metal-oxide-semiconductor field-effect transistor fabricated on (111)-oriented substrate

We report the dependence of the electrical performance on surface orientations of undoped Ge0.92Sn0.08 quantum well (QW) pMOSFETs on Ge(111) and (001) substrates. (111)-oriented Ge0.92Sn0.08 QW pMOSFETs show a peak μeff of 845 cm2V−1 s−1 and demonstrate a μeff improvement of 25% over (001)-oriented control at an inversion charge density of 5 × 1012 cm−2. We also report that undoped Ge0.92Sn0.08 QW pMOSFETs show a higher μeff than the doped GeSn devices reported in the literature. The high μeff achieved in undoped QW devices is enabled by incorporating high biaxial compressive strain (1.43%) and eliminating dopant impurity scattering in the defect-free channel.

Relaxed Ge 0.97 Sn 0.03 P-channel tunneling FETs with high drive current fabricated on Si and further improvement enabled by uniaxial tensile strain

We fabricated relaxed Ge_0.97Sn_0.03 pTFETs on Si(001). The devices show much higher I_ON than SiGe, Ge, and compressively strained GeSn planer pTFETs in literatures. For the first time, I_ON enhancement in GeSn pTFET utilizing uniaxial strain is reported. By applying 0.14% uniaxial tensile strain along channel direction, Ge_0.97Sn_0.03 [110] pTFETs achieve ~ 10% I_ON improvement, over relaxed devices at |V_GS - V_TH| = |V_DS| = 1.0 V. Calculation demonstrates that the reduction of direct E_G by tensile strain results in an enhanced G_BTBT in GeSn, leading to improvement of I_ON in uniaxially tensile strained pTFET.

Strained Ge 0.96 Sn 0.04 P-channel MOSFETs with in situ low temperature Si 2 H 6 surface passivation

We developed process flow for GeSn pMOSFET fabrication with in situ low temperature Si₂H₆ passivation module. High performance Ge_0.96Sn_0.04 pMOSFETs were fabricated. At a Q_mv of 6×10¹² cm^-2, a 24% enhancement in μ_eff is demonstrated in Ge_0.96Sn_0.04 pMOSFETs compared to Ge control.