**3.4. Electrical characterization of Ge0.83Sn0.17 p-MOSFETs**

*I DS* - *VGS* curves of Ge0.83Sn0.17 p-MOSFETs with and without sulfur passivation are shown in **Figure 11(a)**. The blue circles represent the sulfur-passivated sample and the black circles are the data points of the non-passivated one. Both devices have *LG* of 4 μm and gate width *WG* of 100 μm. The sulfurpassivated GeSn p-MOSFET exhibits *S* of 100 mV/decade. This is also the smallest reported *S* for any GeSn p-MOSFETs (non-passivated control shows *S* of 118 mV/decade). **Figure 11(b)** shows the output characteristics of the same devices in **Figure 11(a)**. 10% on-state current enhancement was demonstrated by sulfur-passivated Ge0.83Sn0.17 p-MOSFET as compared to non-passivated control. Drive current of 32 μA/μm was achieved at a gate over drive of −1.0 V and *VDS* of −1.0 V by the sulfur-passivated device. **Table 1** benchmarks *S* of the sulfur-passivated Ge0.83Sn0.17 p-MOSFETs realized in this work with other GeSn p-MOSFETs reported using various

**Figure 11.** (a) *I DS - VGS* curves of the sulfur-passivated Ge0.83Sn0.17 p-MOSFET show *S* of 100 mV/decade and *I ON/IOFF* ratio of more than 3 orders of magnitude. *S* of the sulfur-passivated sample is smaller than that of the non-passivated one. (b) *I DS - VDS* curves of the same devices in (a).

Ge0.83Sn0.17 P-Channel Metal-Oxide-Semiconductor Field-Effect Transistors: Impact of Sulfur… http://dx.doi.org/10.5772/intechopen.74532 109


**Table 1.** *S* values of GeSn p-MOSFETs with different Sn compositions and passivation techniques.

**3.4. Electrical characterization of Ge0.83Sn0.17 p-MOSFETs**

108 Design, Simulation and Construction of Field Effect Transistors

GeSn sample demonstrates reduced midgap *Dit* as compared to the non-passivated control.

*DS* - *VGS* curves of Ge0.83Sn0.17 p-MOSFETs with and without sulfur passivation are shown in **Figure 11(a)**. The blue circles represent the sulfur-passivated sample and the black circles are the data points of the non-passivated one. Both devices have *LG* of 4 μm and gate width *WG* of 100 μm. The sulfurpassivated GeSn p-MOSFET exhibits *S* of 100 mV/decade. This is also the smallest reported *S* for any GeSn p-MOSFETs (non-passivated control shows *S* of 118 mV/decade). **Figure 11(b)** shows the output characteristics of the same devices in **Figure 11(a)**. 10% on-state current enhancement was demonstrated by sulfur-passivated Ge0.83Sn0.17 p-MOSFET as compared to non-passivated control. Drive current of 32 μA/μm was achieved at a gate over drive of −1.0 V and *VDS* of −1.0 V by the sulfur-passivated device. **Table 1** benchmarks *S* of the sulfur-passivated Ge0.83Sn0.17 p-MOSFETs realized in this work with other GeSn p-MOSFETs reported using various

**Figure 10.** *Dit* distribution from valence band edge to the midgap of GeSn as a function of energy. The sulfur-passivated

*DS - VGS* curves of the sulfur-passivated Ge0.83Sn0.17 p-MOSFET show *S* of 100 mV/decade and *I*

of more than 3 orders of magnitude. *S* of the sulfur-passivated sample is smaller than that of the non-passivated one.

*ON/IOFF* ratio

*I*

**Figure 11.** (a) *I*

*DS - VDS* curves of the same devices in (a).

(b) *I*

passivation techniques [12–18, 53]. Despite the highest Sn composition, the Ge0.83Sn0.17 p-MOSFET realized in this work shows the smallest *S* as compared with the other GeSn p-MOSFETs. This could be attributed to the relative low *Dit* at the mid-gap (3.4 × 10<sup>12</sup> cm−2 eV−1) as compared with the other passivation techniques (7–9 × 10<sup>12</sup> cm−2 eV−1) [17]. This indicates the high quality of the Ge0.83Sn0.17 film grown by MBE which was maintained throughout the fabrication process using low processing temperatures, as well as the Ge and Sn oxides formation between Ge0.83Sn0.17 and high-*k* dielectric enabled by sulfur passivation. However, the *Dit* value near the valence band is still high (~1 × 10<sup>13</sup> cm−2 eV−1) as shown in the *Dit* plot in **Figure 10**. This may degrade the effective hole mobility which will be discussed in the following sections.

**Figure 12** shows the capacitance *C* as a function of gate voltage *VGS* for the sulfur-passivated Ge0.83Sn0.17 p-MOSFET (*LG* = 8 μm, *WG* = 100 μm) measured at frequency of 50 kHz, 100 kHz, and 1 MHz. The schematic in the inset illustrates the configuration for *C-V* measurement. A quantum-mechanical *C-V* simulator [54] was used to fit the measured inversion *C-V* curve at 100 kHz and the simulated data were plotted using solid line in **Figure 12**. In the *C-V* simulator, the *C*-*V* characteristics are obtained through the calculation of hole and electron distributions by solving Schrödinger's and Poisson's equations self-consistently with the Fermi-Dirac distribution. In the simulation, the heavy hole effective mass of 0.27 m<sup>0</sup> and light hole effective mass of 0.025 m<sup>0</sup> (m<sup>0</sup> is the free electron mass) were used for Ge0.83Sn0.17 channel [11]. From the simulated *C-V* curve, the equivalent oxide thickness (EOT) is extracted to be 7.5 Å. **Figure 13** shows the forward and backward inversion *C-V* sweeps of one Ge0.83Sn0.17 p-MOSFET (*LG* = 8 μm, *WG* = 100 μm) measured at a frequency of 100 kHz. The hysteresis is small, which indicates good dielectric quality with low density of oxide traps.

The *Gm,int* curves versus *VGS* at *VDS* of −0.05 V for both sulfur-passivated and non-passivated devices are shown in **Figure 14**. *LG* is 4 μm. *Gm,int* is extracted using:

$$\mathbf{G}\_{m,tot} = \frac{\mathbf{G}\_{m,out}}{1 - 0.5 \cdot \mathbf{R}\_{\text{SD}} \cdot \mathbf{G}\_{m,out}} \tag{8}$$

**Figure 12.** *C vs. VGS* plot of a sulfur-passivated Ge0.83Sn0.17 p-MOSFET measured at frequency of 50 kHz, 100 kHz, and 1 MHz. The measured data points are plotted as symbols. The solid curve is obtained using a quantum-mechanical *C-V* simulator to fit 100 kHz *C-V* curve. The inset shows the *C-V* measurement configuration.

where *Gm,ext* is the measured extrinsic transconductance and *RSD* is the source/drain resistance. Higher peak *Gm,int* was achieved in sulfur-passivated Ge0.83Sn0.17 p-MOSFET as compared with that of the non-passivated control. Improvement in *Gm,int* is attributed to better HfO<sup>2</sup> /GeSn interface achieved using sulfur passivation.

The *μeff* of Ge0.83Sn0.17 p-MOSFETs with and without sulfur passivation is extracted using the split *C-V* method:

$$
\mu\_{gf} = \frac{1}{\mathcal{W}\_{\mathcal{G}} \mathcal{Q}\_{im} \frac{\Lambda \mathcal{R}\_{rad}}{\Lambda \mathcal{L}\_{\mathcal{G}}}} \tag{9}
$$

where *Qinv* is the inversion charge density in the GeSn channel, and *ΔRTotal*/*ΔLG* is the slope of total resistance (*RTotal*) versus *LG*. *Qinv* can be obtained by integrating the measured inversion *C-V* curve as shown in **Figure 12**. Using this approach, the impact of *RSD* on extraction of hole mobility is taken out. **Figure 15** shows the extracted *μeff* versus the inversion carrier density (*Ninv*) for both the sulfur-passivated and non-passivated Ge0.83Sn0.17 p-MOSFETs. The sulfur-passivated

**Figure 14.** Intrinsic transconductance *Gm,int vs. VGS* characteristics for Ge0.83Sn0.17 p-MOSFETs with and without sulfur

Ge0.83Sn0.17 P-Channel Metal-Oxide-Semiconductor Field-Effect Transistors: Impact of Sulfur…

*Ninv* of 1 × 10<sup>13</sup> cm−2, 25% higher hole mobility is achieved by the sulfur-passivated Ge0.83Sn0.17 p-MOSFET as compared with the non-passivated one. This is consistent with the peak intrinsic

**Figure 15.** *μeff vs. Ninv* for Ge0.83Sn0.17 p-MOSFETs with and without sulfur passivation. The impact of *RSD* on μ*eff* extraction

/V·s at *Ninv* of ~2 × 10<sup>12</sup> cm−2. At

http://dx.doi.org/10.5772/intechopen.74532

111

Ge0.83Sn0.17 p-MOSFET shows a peak hole mobility of 478 cm<sup>2</sup>

was taken out using the inset equation through the total resistance slope method.

passivation at *VDS* = −0.05 V. The *LG* of the device is 4 μm and *WG* is 100 μm.

transconductance results shown in **Figure 14**.

**Figure 13.** Forward and backward inversion *C-V* sweeps at 100 kHz for one Ge0.83Sn0.17 p-MOSFET with a *LG* of 8 μm. The hysteresis is small.

Ge0.83Sn0.17 P-Channel Metal-Oxide-Semiconductor Field-Effect Transistors: Impact of Sulfur… http://dx.doi.org/10.5772/intechopen.74532 111

**Figure 14.** Intrinsic transconductance *Gm,int vs. VGS* characteristics for Ge0.83Sn0.17 p-MOSFETs with and without sulfur passivation at *VDS* = −0.05 V. The *LG* of the device is 4 μm and *WG* is 100 μm.

where *Qinv* is the inversion charge density in the GeSn channel, and *ΔRTotal*/*ΔLG* is the slope of total resistance (*RTotal*) versus *LG*. *Qinv* can be obtained by integrating the measured inversion *C-V* curve as shown in **Figure 12**. Using this approach, the impact of *RSD* on extraction of hole mobility is taken out. **Figure 15** shows the extracted *μeff* versus the inversion carrier density (*Ninv*) for both the sulfur-passivated and non-passivated Ge0.83Sn0.17 p-MOSFETs. The sulfur-passivated Ge0.83Sn0.17 p-MOSFET shows a peak hole mobility of 478 cm<sup>2</sup> /V·s at *Ninv* of ~2 × 10<sup>12</sup> cm−2. At *Ninv* of 1 × 10<sup>13</sup> cm−2, 25% higher hole mobility is achieved by the sulfur-passivated Ge0.83Sn0.17 p-MOSFET as compared with the non-passivated one. This is consistent with the peak intrinsic transconductance results shown in **Figure 14**.

**Figure 15.** *μeff vs. Ninv* for Ge0.83Sn0.17 p-MOSFETs with and without sulfur passivation. The impact of *RSD* on μ*eff* extraction was taken out using the inset equation through the total resistance slope method.

**Figure 13.** Forward and backward inversion *C-V* sweeps at 100 kHz for one Ge0.83Sn0.17 p-MOSFET with a *LG* of 8 μm. The

where *Gm,ext* is the measured extrinsic transconductance and *RSD* is the source/drain resistance. Higher peak *Gm,int* was achieved in sulfur-passivated Ge0.83Sn0.17 p-MOSFET as compared with that of the non-passivated control. Improvement in *Gm,int* is attributed to better HfO<sup>2</sup>

**Figure 12.** *C vs. VGS* plot of a sulfur-passivated Ge0.83Sn0.17 p-MOSFET measured at frequency of 50 kHz, 100 kHz, and 1 MHz. The measured data points are plotted as symbols. The solid curve is obtained using a quantum-mechanical *C-V*

The *μeff* of Ge0.83Sn0.17 p-MOSFETs with and without sulfur passivation is extracted using the

*WG Qinv*

*<sup>Δ</sup> <sup>R</sup>* \_\_\_\_\_ *Total Δ LG*

interface achieved using sulfur passivation.

110 Design, Simulation and Construction of Field Effect Transistors

*<sup>μ</sup>eff* <sup>=</sup> \_\_\_\_\_\_\_\_\_\_ <sup>1</sup>

simulator to fit 100 kHz *C-V* curve. The inset shows the *C-V* measurement configuration.

/GeSn

, (9)

hysteresis is small.

split *C-V* method:
