**3.1. Sulfur-passivated gate stack study**

The (001) surface of a diamond-structure semiconductor has two dangling bonds per surface atom. GeSn grown on Ge (001) surface has a (001) surface as shown in the atomic structure in **Figure 5(a)** viewed into the [110] direction. One monolayer (ML) of a Group VI element can passivate all the dangling bonds by occupying the bridge site in a (1 × 1) geometry [34, 35]. Sulfur atoms could obtain an ideal (1 × 1) termination of the bivalent (001) surfaces of silicon and Ge. Although sulfur could desorb from the Si surface at room temperature or diffuse into the Si bulk during heating [36], Weser et al. found that an ordered (1 × 1) structure with one sulfur atom bonded on a bulk-like bridge site could be achieved by introducing elemental sulfur atoms on the Ge (001) surface under ultrahigh vacuum (UHV) condition [34, 35]. The formation of Ge-S-Ge bridge bonds after a treatment in (NH<sup>4</sup> ) 2 S solution has also been reported based on various characterization techniques, such as photoelectron spectroscopy [37], ion scattering spectroscopy [38], as well as X-ray standing wave measurements [39]. Similarly, sulfur passivation should also be able to passivate the GeSn (001) surface through the formation of S-Ge and S-Sn covalent bonds which suppress the formation of Ge and Sn oxides at the surface, as illustrated in the atomic schematic shown in **Figure 5(b)**. In this Section, the

**Figure 5.** Side view into the [110] direction of (a) non-passivated (1 × 1) and (b) the sulfur-passivated GeSn (001) surfaces.

peak (399.0 ± 0.02 eV) [41] on the sulfur-passivated sample is not observed, indicating that nitrogen is not incorporated. The sulfur passivation layer thickness is calculated using two

The circles, blue lines and gray lines are the raw data points, the overall fitting curves, and the S 2*p*1/2 or S 2*p*3/2 peak components, respectively. The black curve represents the S 2*p* signal obtained from the non-passivated sample. (b) N 1*s*

> *<sup>λ</sup>A*(*E*1) <sup>⋅</sup> *<sup>λ</sup>A*(*E*2) \_\_\_\_\_\_\_\_\_\_\_\_ *<sup>λ</sup>A*(*E*1) <sup>−</sup> *<sup>λ</sup>A*(*E*2) <sup>⋅</sup> ln

) are the attenuation length of Ge 3*d* and Ge 2*p3*

corrected photoelectron intensities, and *θ* is take-off angle. The sulfur passivation layer thick-

ples with and without sulfur passivation, respectively. The circles, blue lines, and gray lines shown in **Figure 7** are the raw data points, the overall fitting curves, and fitted peak components, respectively. Due to spin orbit splitting, two separated Sn *3d* peaks (Sn *3d*3/2 and Sn *3d*5/2) can be observed on both samples. The left shoulders, binding energy at 486.7 ± 0.2 eV and

both samples. The Ge and Sn oxide signals can be detected on both samples and could come

with a temperature of 250°C, (2) sample transfer before loading into the XPS chamber as the samples were exposed to the air ambient. However, the intensities of both Ge oxides and SnO*<sup>x</sup>* are reduced significantly after the sulfur passivation, indicating the effectiveness of sulfur

The native Ge and Sn oxides formation at the high-*k*/GeSn interface could result in high *Dit* value and gate leakage current. Lee et al. reported that the native Ge oxide could react with Ge at the interface and form GeO which is easily desorbed during thermal processing [44].

deposition as H<sup>2</sup>

*I B* '

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

(*E*1) \_\_\_\_\_ *I B* '

(*E*2)

XPS signals.

and GeO*<sup>x</sup>*

formation.

and *E2*

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


data (not shown) are used and the

, (1)


) [41]. Similarly, **Figure 7(c)**

) can also be observed in


O pulses were introduced in the chamber

, *I'B*(*E*<sup>1</sup>

(*E1*

and *E2*

) and *I'B*(*E*<sup>2</sup>

have

103

) are

different photoelectron peaks of the same element at kinetic energies *E1*

sufficiently large differences in λ) [42, 43]. Ge *3d* and Ge *2p3*

ness is calculated to be 0.44 nm using the Ge 3*d* and Ge 2*p3*

**Figure 7(a)** and **(b)** show the Sn *3d* core level spectra of HfO<sup>2</sup>

494.9 ± 0.2 eV, can be attributed to the formation of Sn oxides (SnO*<sup>x</sup>*

and **(d)** show the Ge *3d* core level spectra of HfO<sup>2</sup>

passivation in suppressing both Ge oxides and SnO*<sup>x</sup>*

without sulfur passivation, respectively. Ge oxides (GeO<sup>2</sup>

equation is shown below:

core level spectra for both samples.

where *λA*(*E*<sup>1</sup>

*d* = cos*θ* ⋅

**Figure 6.** (a) S 2*p* core level XPS spectra for ~1 nm HfO<sup>2</sup>

) and *λA*(*E*<sup>2</sup>

from two sources: (1) ALD HfO<sup>2</sup>

effectiveness of sulfur passivation on the gate stack of GeSn p-MOSFETs is investigated using XPS. The interface trap density value is also extracted using conductance method and compared with the non-passivated control.

#### **3.2. XPS study on the effect of sulfur passivation**

To investigate the interfacial property between the high-*k* dielectric and Ge0.83Sn0.17 after sulfur passivation, XPS measurement was carried out to study the change of the interfacial chemical bonds. Two blanket Ge0.83Sn0.17 samples were prepared for the measurement. After the cyclic DHF (1:50) cleaning, one of the sample went through 10 minutes aqueous (NH<sup>4</sup> )2 S solution (24% by weight) and the other one did not. After that, an ultra-thin (~1 nm) HfO<sup>2</sup> layer was deposited by ALD on these two samples. The HfO<sup>2</sup> layer thickness should be smaller than the XPS information depth [40]. XPS characterization was then performed using a VG ESCALAB 220i–XL imaging XPS system. Monochromatic aluminum (Al) K*α* X-ray (1486.7 eV) was used to obtain the core level spectra of these samples. Binding energy was calibrated with standard samples for some pure metals. The binding energy of Carbon (C) *1 s* from adventitious hydrocarbon surface contamination was set at 285.0 eV for further charge correction.

In order to confirm the incorporation of S after the (NH<sup>4</sup> )2 S passivation, core level XPS spectra of the S *2p* peak were captured for HfO<sup>2</sup> -capped Ge0.83Sn0.17 blanket samples with and without sulfur passivation, as shown in **Figure 6(a)**. The black curve represents the S *2p* signal obtained from the non-passivated Ge0.83Sn0.17 sample. The circles are the raw data points obtained from the sulfur-passivated sample. Gaussian and Lorentzian line shapes with a Shirley background subtraction were used to fit the raw data. The blue line is the overall fitting of the core level spectra and the gray lines are the fitted peak components. For the S *2p* core level spectra, the well-resolved two peaks correspond to S *2p*1/2 (163.4 ± 0.02 eV) and S *2p*3/2 (162.0 ± 0.02 eV) [41]. The S *2p* signal obtained from the sulfur-passivated Ge0.83Sn0.17 sample indicates that S is introduced onto the GeSn surface by the (NH<sup>4</sup> ) 2 S treatment and is still present after the deposition of HfO<sup>2</sup> . **Figure 6(b)** shows N *1 s* core level spectra for both samples. The N *1 s*

**Figure 6.** (a) S 2*p* core level XPS spectra for ~1 nm HfO<sup>2</sup> -capped Ge0.83Sn0.17 samples with and without sulfur passivation. The circles, blue lines and gray lines are the raw data points, the overall fitting curves, and the S 2*p*1/2 or S 2*p*3/2 peak components, respectively. The black curve represents the S 2*p* signal obtained from the non-passivated sample. (b) N 1*s* core level spectra for both samples.

peak (399.0 ± 0.02 eV) [41] on the sulfur-passivated sample is not observed, indicating that nitrogen is not incorporated. The sulfur passivation layer thickness is calculated using two different photoelectron peaks of the same element at kinetic energies *E1* and *E2* (*E1* and *E2* have sufficiently large differences in λ) [42, 43]. Ge *3d* and Ge *2p3* data (not shown) are used and the equation is shown below:

effectiveness of sulfur passivation on the gate stack of GeSn p-MOSFETs is investigated using XPS. The interface trap density value is also extracted using conductance method and com-

**Figure 5.** Side view into the [110] direction of (a) non-passivated (1 × 1) and (b) the sulfur-passivated GeSn (001) surfaces.

To investigate the interfacial property between the high-*k* dielectric and Ge0.83Sn0.17 after sulfur passivation, XPS measurement was carried out to study the change of the interfacial chemical bonds. Two blanket Ge0.83Sn0.17 samples were prepared for the measurement. After the cyclic

XPS information depth [40]. XPS characterization was then performed using a VG ESCALAB 220i–XL imaging XPS system. Monochromatic aluminum (Al) K*α* X-ray (1486.7 eV) was used to obtain the core level spectra of these samples. Binding energy was calibrated with standard samples for some pure metals. The binding energy of Carbon (C) *1 s* from adventitious hydro-

sulfur passivation, as shown in **Figure 6(a)**. The black curve represents the S *2p* signal obtained from the non-passivated Ge0.83Sn0.17 sample. The circles are the raw data points obtained from the sulfur-passivated sample. Gaussian and Lorentzian line shapes with a Shirley background subtraction were used to fit the raw data. The blue line is the overall fitting of the core level spectra and the gray lines are the fitted peak components. For the S *2p* core level spectra, the well-resolved two peaks correspond to S *2p*1/2 (163.4 ± 0.02 eV) and S *2p*3/2 (162.0 ± 0.02 eV) [41]. The S *2p* signal obtained from the sulfur-passivated Ge0.83Sn0.17 sample indicates that S

> ) 2

. **Figure 6(b)** shows N *1 s* core level spectra for both samples. The N *1 s*

)2

)2

layer thickness should be smaller than the

S passivation, core level XPS spectra

S treatment and is still present after the


S solution

layer was

DHF (1:50) cleaning, one of the sample went through 10 minutes aqueous (NH<sup>4</sup>

(24% by weight) and the other one did not. After that, an ultra-thin (~1 nm) HfO<sup>2</sup>

carbon surface contamination was set at 285.0 eV for further charge correction.

pared with the non-passivated control.

102 Design, Simulation and Construction of Field Effect Transistors

**3.2. XPS study on the effect of sulfur passivation**

deposited by ALD on these two samples. The HfO<sup>2</sup>

In order to confirm the incorporation of S after the (NH<sup>4</sup>

is introduced onto the GeSn surface by the (NH<sup>4</sup>

of the S *2p* peak were captured for HfO<sup>2</sup>

deposition of HfO<sup>2</sup>

$$d = \cos\Theta \cdot \frac{\lambda\_{\lambda}(\mathbf{E\_1}) \cdot \lambda\_{\lambda}(\mathbf{E\_2})}{\lambda\_{\lambda}(\mathbf{E\_1}) - \lambda\_{\lambda}(\mathbf{E\_2})} \cdot \ln \frac{I\_{\mathbf{s}}(\mathbf{E\_1})}{I\_{\mathbf{s}}(\mathbf{E\_2})} \tag{1}$$

where *λA*(*E*<sup>1</sup> ) and *λA*(*E*<sup>2</sup> ) are the attenuation length of Ge 3*d* and Ge 2*p3* , *I'B*(*E*<sup>1</sup> ) and *I'B*(*E*<sup>2</sup> ) are corrected photoelectron intensities, and *θ* is take-off angle. The sulfur passivation layer thickness is calculated to be 0.44 nm using the Ge 3*d* and Ge 2*p3* XPS signals.

**Figure 7(a)** and **(b)** show the Sn *3d* core level spectra of HfO<sup>2</sup> -capped Ge0.83Sn0.17 blanket samples with and without sulfur passivation, respectively. The circles, blue lines, and gray lines shown in **Figure 7** are the raw data points, the overall fitting curves, and fitted peak components, respectively. Due to spin orbit splitting, two separated Sn *3d* peaks (Sn *3d*3/2 and Sn *3d*5/2) can be observed on both samples. The left shoulders, binding energy at 486.7 ± 0.2 eV and 494.9 ± 0.2 eV, can be attributed to the formation of Sn oxides (SnO*<sup>x</sup>* ) [41]. Similarly, **Figure 7(c)** and **(d)** show the Ge *3d* core level spectra of HfO<sup>2</sup> -capped Ge0.83Sn0.17 blanket samples with and without sulfur passivation, respectively. Ge oxides (GeO<sup>2</sup> and GeO*<sup>x</sup>* ) can also be observed in both samples. The Ge and Sn oxide signals can be detected on both samples and could come from two sources: (1) ALD HfO<sup>2</sup> deposition as H<sup>2</sup> O pulses were introduced in the chamber with a temperature of 250°C, (2) sample transfer before loading into the XPS chamber as the samples were exposed to the air ambient. However, the intensities of both Ge oxides and SnO*<sup>x</sup>* are reduced significantly after the sulfur passivation, indicating the effectiveness of sulfur passivation in suppressing both Ge oxides and SnO*<sup>x</sup>* formation.

The native Ge and Sn oxides formation at the high-*k*/GeSn interface could result in high *Dit* value and gate leakage current. Lee et al. reported that the native Ge oxide could react with Ge at the interface and form GeO which is easily desorbed during thermal processing [44].

$$\text{GeO}\_z \text{+Ge} \to 2\text{GeO}(\uparrow). \tag{2}$$

This could generate a huge amount of interface states which degrade the gate stack quality [45]. The Sn oxide could also be detrimental to the GeSn gate stack as SnO<sup>2</sup> is known to exhibit metallic behaviour, which leads to high gate leakage current [46, 47]. Therefore, suppressing the Ge and Sn oxides formation is important for achieving good gate quality for Ge0.83Sn0.17 p-MOSFETs.

To quantify the impact of sulfur passivation on Ge oxides and SnO*<sup>x</sup>* formation at the high-*k*/ GeSn interface, angle-resolved XPS (ARXPS) was performed. Both SnO*<sup>x</sup>* and Ge oxide signals can be detected. The ratio of oxidized Sn (or Ge) atoms to the total Sn (or Ge) atoms can be calculated using

$$\gamma\_{s\text{-}o\text{-}O} = \frac{A\_{s\text{-}O}}{A\_{s\text{-}6\text{-}} + A\_{s\text{-}O}} \tag{3}$$

$$\gamma\_{\text{Ge-O}} = \frac{A\_{\text{Ge-O}}}{A\_{\text{Ge-O}} + A\_{\text{Ge-O}}} \tag{4}$$

using the obtained ARXPS data. The Ge and Sn atomic concentrations can be calculated using the stabilized Ge *3d* and Sn *3d* spectra. The atomic concentrations of Sn (*γSn/(Ge <sup>+</sup> Sn)*) and Ge

**Figure 7.** Sn 3*d* and Ge 3*d* core-level spectra of (a) and (c) sulfur passivated and (b) and (d) non-passivated Ge0.83Sn0.17

was deposited on both samples.

and plotted as a function of θ. **Figure 9** shows *γGe/(Ge <sup>+</sup> Sn)* and *γSn/(Ge <sup>+</sup> Sn)* near the surface of Ge0.83Sn0.17 as a function of θ for both the sulfur-passivated and non-passivated samples. The calculated *γSn/(Ge <sup>+</sup> Sn)* increases with the increase of θ, indicating that surface segregation of Sn occurred in both samples. The Sn composition of the non-passivated Ge0.83Sn0.17 sample even exceeds 20% at θ of 60°. This is because Sn tends to segregate toward the surface, with the severity increasing at higher Sn compositions. Wang et al. reported that Sn segregation can occur at a temperature as low as 200°C for strained Ge0.915Sn0.085 grown on Ge [24]. Since our

 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>A</sup> Sn*−*Sn* <sup>+</sup> *ASn*−*<sup>O</sup>* <sup>+</sup> *AGe*−*Ge* <sup>+</sup> *AGe*−*<sup>O</sup>*

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

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

105

 \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>A</sup> Sn*−*Sn* <sup>+</sup> *ASn*−*<sup>O</sup>* <sup>+</sup> *AGe*−*Ge* <sup>+</sup> *AGe*−*<sup>O</sup>*

, (5)

, (6)

(*γGe/(Ge <sup>+</sup> Sn)*) can be expressed as

samples obtained by XPS. A 1 nm-thick HfO<sup>2</sup>

*<sup>γ</sup>Sn*/(*Ge*+*Sn*) <sup>=</sup> *ASn*−*Sn* <sup>+</sup> *ASn*−*<sup>O</sup>*

*<sup>γ</sup>Ge*/(*Ge*+*Sn*) <sup>=</sup> *AGe*−*Ge* <sup>+</sup> *AGe*−*<sup>O</sup>*

where *ASn-O*, *ASn-Sn*, *AGe-O*, and *AGe-Ge* are the normalized Sn-O peak area, normalized Sn-Sn peak area (also including Sn-Ge bonding), normalized Ge-O peak area, and normalized Ge-Ge peak area (also including Ge-Sn bonding), respectively [48]. With consideration of Scofield photoionization cross-sections and the transmission function of the spectrometer, *γSn-O* and *γGe-O* can be plotted as a function of the photoelectron take-off angle θ and are shown in **Figure 8**. The inset in **Figure 8** illustrates the definition of θ. θ of 0°, 30°, 45°, and 60° were used. It is observed that *γSn-O* and *γGe-O* increase with increasing θ. This is due to the fact that more Ge and Sn atoms at the surface get oxidized than those at the sub-surface. For larger θ, the information depth is smaller and ARXPS becomes more surface sensitive. For Ge oxides, the oxide percentages of the sulfur-passivated Ge0.83Sn0.17 sample increase from 20 to 42% when θ increases from 0 to 60°. However, all the values are 10–20% smaller than those of the non-passivated one. A similar trend is also observed for SnO*<sup>x</sup>* , and the sulfurpassivated Ge0.83Sn0.17 sample shows more than 50% reduction in SnO*<sup>x</sup>* percentage than the non-passivated one at all take-off angles. The reduction of oxide formation is more obvious in Sn atoms than Ge atoms. This reveals that sulfur passivation is more effective in suppressing Sn oxide formation than Ge oxide formation. The reduction of both Ge and Sn oxides can be ascribed to the formation of S-Ge and S-Sn bonds on the sample surface. Since both samples went through the DHF treatment, most native oxides were removed and the sample surface becomes H-terminated. As a result, the Ge0.83Sn0.17 sample surface has Ge-H, Sn-H bonds, and possibly Ge-O and Sn-O bonds due to the incomplete surface oxide removal in DHF [49]. After sulfur passivation, Ge-H bond (bond energy: 263 kJ/mol [50]) and Sn-H bond (bond energy: 264 kJ/mol) are replaced by more stable Ge-S bond (bond energy: 534 kJ/ mol) and Sn-S bond (bond energy: 467 kJ/mol), respectively. The S passivation layer formed at the GeSn surface can suppress the further oxidation of sub-surface Ge and Sn atoms.

To further investigate the effect of sulfur passivation on the interface quality between the high-*k* dielectric and Ge0.83Sn0.17, the extent of surface segregation of Sn atom was analyzed Ge0.83Sn0.17 P-Channel Metal-Oxide-Semiconductor Field-Effect Transistors: Impact of Sulfur… http://dx.doi.org/10.5772/intechopen.74532 105

GeO<sup>2</sup> + Ge → 2GeO(↑). (2)

This could generate a huge amount of interface states which degrade the gate stack quality [45].

lic behaviour, which leads to high gate leakage current [46, 47]. Therefore, suppressing the Ge and Sn oxides formation is important for achieving good gate quality for Ge0.83Sn0.17 p-MOSFETs.

can be detected. The ratio of oxidized Sn (or Ge) atoms to the total Sn (or Ge) atoms can be

*ASn*−*Sn* + *ASn*−*<sup>O</sup>*

*AGe*−*Ge* + *AGe*−*<sup>O</sup>*

where *ASn-O*, *ASn-Sn*, *AGe-O*, and *AGe-Ge* are the normalized Sn-O peak area, normalized Sn-Sn peak area (also including Sn-Ge bonding), normalized Ge-O peak area, and normalized Ge-Ge peak area (also including Ge-Sn bonding), respectively [48]. With consideration of Scofield photoionization cross-sections and the transmission function of the spectrometer, *γSn-O* and *γGe-O* can be plotted as a function of the photoelectron take-off angle θ and are shown in **Figure 8**. The inset in **Figure 8** illustrates the definition of θ. θ of 0°, 30°, 45°, and 60° were used. It is observed that *γSn-O* and *γGe-O* increase with increasing θ. This is due to the fact that more Ge and Sn atoms at the surface get oxidized than those at the sub-surface. For larger θ, the information depth is smaller and ARXPS becomes more surface sensitive. For Ge oxides, the oxide percentages of the sulfur-passivated Ge0.83Sn0.17 sample increase from 20 to 42% when θ increases from 0 to 60°. However, all the values are 10–20% smaller than

non-passivated one at all take-off angles. The reduction of oxide formation is more obvious in Sn atoms than Ge atoms. This reveals that sulfur passivation is more effective in suppressing Sn oxide formation than Ge oxide formation. The reduction of both Ge and Sn oxides can be ascribed to the formation of S-Ge and S-Sn bonds on the sample surface. Since both samples went through the DHF treatment, most native oxides were removed and the sample surface becomes H-terminated. As a result, the Ge0.83Sn0.17 sample surface has Ge-H, Sn-H bonds, and possibly Ge-O and Sn-O bonds due to the incomplete surface oxide removal in DHF [49]. After sulfur passivation, Ge-H bond (bond energy: 263 kJ/mol [50]) and Sn-H bond (bond energy: 264 kJ/mol) are replaced by more stable Ge-S bond (bond energy: 534 kJ/ mol) and Sn-S bond (bond energy: 467 kJ/mol), respectively. The S passivation layer formed at the GeSn surface can suppress the further oxidation of sub-surface Ge and Sn atoms.

To further investigate the effect of sulfur passivation on the interface quality between the high-*k* dielectric and Ge0.83Sn0.17, the extent of surface segregation of Sn atom was analyzed

those of the non-passivated one. A similar trend is also observed for SnO*<sup>x</sup>*

passivated Ge0.83Sn0.17 sample shows more than 50% reduction in SnO*<sup>x</sup>*

is known to exhibit metal-

formation at the high-*k*/

, (3)

, (4)

and Ge oxide signals

, and the sulfur-

percentage than the

The Sn oxide could also be detrimental to the GeSn gate stack as SnO<sup>2</sup>

To quantify the impact of sulfur passivation on Ge oxides and SnO*<sup>x</sup>*

*<sup>γ</sup>Sn*−*<sup>O</sup>* <sup>=</sup> \_\_\_\_\_\_\_\_\_\_ *ASn*−*<sup>O</sup>*

104 Design, Simulation and Construction of Field Effect Transistors

*<sup>γ</sup>Ge*−*<sup>O</sup>* <sup>=</sup> \_\_\_\_\_\_\_\_\_\_ *AGe*−*<sup>O</sup>*

calculated using

GeSn interface, angle-resolved XPS (ARXPS) was performed. Both SnO*<sup>x</sup>*

**Figure 7.** Sn 3*d* and Ge 3*d* core-level spectra of (a) and (c) sulfur passivated and (b) and (d) non-passivated Ge0.83Sn0.17 samples obtained by XPS. A 1 nm-thick HfO<sup>2</sup> was deposited on both samples.

using the obtained ARXPS data. The Ge and Sn atomic concentrations can be calculated using the stabilized Ge *3d* and Sn *3d* spectra. The atomic concentrations of Sn (*γSn/(Ge <sup>+</sup> Sn)*) and Ge (*γGe/(Ge <sup>+</sup> Sn)*) can be expressed as

Ine sauonizeu Ge  $\gg u$  and  $\gg u$  specura. The aomic coucenirallons ou  $\mathrm{SI}$  ( $\mathcal{V}\_{\mathrm{Su@G} + \mathrm{Su@}}$ ) and  $\mathrm{Ge}$  ( $\mathcal{V}\_{\mathrm{Su@G} + \mathrm{Su@}}$ ) can be expressed as

$$\mathcal{V}\_{\mathrm{Su@G} + \mathrm{Su@}} = \frac{A\_{\mathrm{Su@}} + A\_{\mathrm{Su@}}}{A\_{\mathrm{Su@}} + A\_{\mathrm{Su@}} + A\_{\mathrm{Su@}} + A\_{\mathrm{Su@}}}\tag{5}$$

 *<sup>γ</sup>Ge*/(*Ge*+*Sn*) <sup>=</sup> *AGe*−*Ge* <sup>+</sup> *AGe*−*<sup>O</sup>* \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>A</sup> Sn*−*Sn* <sup>+</sup> *ASn*−*<sup>O</sup>* <sup>+</sup> *AGe*−*Ge* <sup>+</sup> *AGe*−*<sup>O</sup>* , (6)

and plotted as a function of θ. **Figure 9** shows *γGe/(Ge <sup>+</sup> Sn)* and *γSn/(Ge <sup>+</sup> Sn)* near the surface of Ge0.83Sn0.17 as a function of θ for both the sulfur-passivated and non-passivated samples. The calculated *γSn/(Ge <sup>+</sup> Sn)* increases with the increase of θ, indicating that surface segregation of Sn occurred in both samples. The Sn composition of the non-passivated Ge0.83Sn0.17 sample even exceeds 20% at θ of 60°. This is because Sn tends to segregate toward the surface, with the severity increasing at higher Sn compositions. Wang et al. reported that Sn segregation can occur at a temperature as low as 200°C for strained Ge0.915Sn0.085 grown on Ge [24]. Since our

**Figure 8.** *γ*Sn-O and *γ*Ge-O calculated from angle-resolved XPS measurement for both the sulfur-passivated and nonpassivated GeSn samples as a function of photoelectron take-off angle *θ*. The inset shows the definition of *θ*, which is set to be 0°, 30°, 45°, or 60° in the measurements.

interface and its maximum occurs when the energy level of the trap states is aligned with the semiconductor surface Fermi-level. The value of *Dit* can be extracted using the following

**Figure 9.** The Ge and Sn atomic concentrations at surface region of Ge0.83Sn0.17 as a function of photoelectron take-off

capacitor. The band-gap of fully compressively strained Ge0.83Sn0.17 on Ge (100) substrate is ~0.45 eV [52]. *Dit* values from the valence band edge to the midgap of GeSn for both sulfurpassivated and non-passivated GeSn capacitors are extracted and plotted as a function of energy in the GeSn band-gap as shown in **Figure 10**. For the sample with sulfur passivation,

non-passivated sample which has *Dit* of 6 × 10<sup>13</sup> cm−2·eV−1. In addition, sulfur passivation also leads to significant reduction in *Dit* near the valence band edge. Sulfur passivation suppresses Ge and Sn oxide formation and Sn out-diffusion, leading to the reduction of *Dit*. As a result, *S* of the sulfur-passivated GeSn p-MOSFETs is improved as compared with the non-passivated sample. In terms of the Ge0.83Sn0.17 p-MOSFETs, high density interface traps near the valence band edge can be charged when the device is biased to strong inversion, and degrade the effective hole mobility. In order to further improve the effective hole mobility of the Ge0.83Sn0.17 p-MOSFETs, further optimization and significant improvement are needed to reduce the *Dit*

(*Gp* /*ω*) \_\_\_\_\_\_\_\_*max*

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

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

107

*/ω*)*max* is the peak energy loss value, *q* is the electronic charge, and *A* is the area of

*qA* , (7)

of 0.13 eV. This is much smaller as compared with the

equation:

where (*Gp*

*Dit* = 2.5 ⋅

angle *θ*, as determined by angle-resolved XPS.

*Dit* of 10<sup>13</sup> cm−2·eV−1 was obtained at *E-Ev*

near the valence band for the high-*k*/GeSn gate stack.

Ge0.83Sn0.17 sample went through the ALD deposition process with a temperature of 250°C, Sn segregation could also occur. Although the segregation of Sn occurs on both GeSn samples, the calculated *γSn/(Ge <sup>+</sup> Sn)* of the sulfur-passivated GeSn sample is smaller than that of the nonpassivated one at all take-off angles. The S passivation layer appears to suppress the underlying Sn atoms from segregating to the surface and from further oxidation during ALD. The good integrity of high-*k* dielectric/GeSn interface maintained by sulfur passivation through the prevention of Sn out-diffusion and interfacial oxidation may help to improve the carrier transport characteristics in transistors.

#### **3.3. Extraction of interface trap density**

In order to extract the *Dit* of HfO<sup>2</sup> /Ge0.83Sn0.17 interfaces with and without sulfur passivation, Ge0.83Sn0.17 MOS capacitors (MOSCAPs) with 4 nm-thick HfO<sup>2</sup> were fabricated. TaN and Al were deposited as the front gate and backside metals by reactive sputtering, respectively. Low temperature *C-V* measurement with frequencies ranging from 10 kHz to 1 MHz was performed on the Ge0.83Sn0.17 capacitors. *Dit* was extracted using the conductance method [51]. At a particular gate bias, the peak of the *Gp /ω* versus frequency curve can be obtained at one sweeping frequency and is referring to the maximum of per-cycle energy loss. The percycle energy loss is due to charge trapping and detrapping at certain oxide-semiconductor

**Figure 9.** The Ge and Sn atomic concentrations at surface region of Ge0.83Sn0.17 as a function of photoelectron take-off angle *θ*, as determined by angle-resolved XPS.

interface and its maximum occurs when the energy level of the trap states is aligned with the semiconductor surface Fermi-level. The value of *Dit* can be extracted using the following equation:

Ge0.83Sn0.17 sample went through the ALD deposition process with a temperature of 250°C, Sn segregation could also occur. Although the segregation of Sn occurs on both GeSn samples, the calculated *γSn/(Ge <sup>+</sup> Sn)* of the sulfur-passivated GeSn sample is smaller than that of the nonpassivated one at all take-off angles. The S passivation layer appears to suppress the underlying Sn atoms from segregating to the surface and from further oxidation during ALD. The good integrity of high-*k* dielectric/GeSn interface maintained by sulfur passivation through the prevention of Sn out-diffusion and interfacial oxidation may help to improve the carrier

**Figure 8.** *γ*Sn-O and *γ*Ge-O calculated from angle-resolved XPS measurement for both the sulfur-passivated and nonpassivated GeSn samples as a function of photoelectron take-off angle *θ*. The inset shows the definition of *θ*, which is set

were deposited as the front gate and backside metals by reactive sputtering, respectively. Low temperature *C-V* measurement with frequencies ranging from 10 kHz to 1 MHz was performed on the Ge0.83Sn0.17 capacitors. *Dit* was extracted using the conductance method

one sweeping frequency and is referring to the maximum of per-cycle energy loss. The percycle energy loss is due to charge trapping and detrapping at certain oxide-semiconductor

/Ge0.83Sn0.17 interfaces with and without sulfur passivation,

*/ω* versus frequency curve can be obtained at

were fabricated. TaN and Al

transport characteristics in transistors.

to be 0°, 30°, 45°, or 60° in the measurements.

106 Design, Simulation and Construction of Field Effect Transistors

**3.3. Extraction of interface trap density**

[51]. At a particular gate bias, the peak of the *Gp*

Ge0.83Sn0.17 MOS capacitors (MOSCAPs) with 4 nm-thick HfO<sup>2</sup>

In order to extract the *Dit* of HfO<sup>2</sup>

$$D\_u = 2.5 \cdot \frac{\left(\mathcal{G}\_r/\omega\right)\_{mu}}{qA} \tag{7}$$

where (*Gp /ω*)*max* is the peak energy loss value, *q* is the electronic charge, and *A* is the area of capacitor. The band-gap of fully compressively strained Ge0.83Sn0.17 on Ge (100) substrate is ~0.45 eV [52]. *Dit* values from the valence band edge to the midgap of GeSn for both sulfurpassivated and non-passivated GeSn capacitors are extracted and plotted as a function of energy in the GeSn band-gap as shown in **Figure 10**. For the sample with sulfur passivation, *Dit* of 10<sup>13</sup> cm−2·eV−1 was obtained at *E-Ev* of 0.13 eV. This is much smaller as compared with the non-passivated sample which has *Dit* of 6 × 10<sup>13</sup> cm−2·eV−1. In addition, sulfur passivation also leads to significant reduction in *Dit* near the valence band edge. Sulfur passivation suppresses Ge and Sn oxide formation and Sn out-diffusion, leading to the reduction of *Dit*. As a result, *S* of the sulfur-passivated GeSn p-MOSFETs is improved as compared with the non-passivated sample. In terms of the Ge0.83Sn0.17 p-MOSFETs, high density interface traps near the valence band edge can be charged when the device is biased to strong inversion, and degrade the effective hole mobility. In order to further improve the effective hole mobility of the Ge0.83Sn0.17 p-MOSFETs, further optimization and significant improvement are needed to reduce the *Dit* near the valence band for the high-*k*/GeSn gate stack.

**Figure 10.** *Dit* distribution from valence band edge to the midgap of GeSn as a function of energy. The sulfur-passivated GeSn sample demonstrates reduced midgap *Dit* as compared to the non-passivated control.

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

**Work Passivation Technique Sn composition (%)** *S* **value** [12] Si passivation 5.3 250 [15] Sulfur passivation 4.2 220 [53] Si passivation 3 113 [13] No passivation 3 250 [16] Si passivation 4.2 135 [17] Si passivation 3 158 [14] Si passivation 8 198 [18] Ge capping 9 160 This work Sulfur passivation 17 100

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.

**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

[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

The *Gm,int* curves versus *VGS* at *VDS* of −0.05 V for both sulfur-passivated and non-passivated

1 <sup>−</sup> 0.5 <sup>⋅</sup> *RSD* <sup>⋅</sup> *Gm*,*ext*

is the free electron mass) were used for Ge0.83Sn0.17 channel

, (8)

and light

Fermi-Dirac distribution. In the simulation, the heavy hole effective mass of 0.27 m<sup>0</sup>

small, which indicates good dielectric quality with low density of oxide traps.

hole mobility which will be discussed in the following sections.

(m<sup>0</sup>

devices are shown in **Figure 14**. *LG* is 4 μm. *Gm,int* is extracted using:

*Gm*,*int* <sup>=</sup> \_\_\_\_\_\_\_\_\_\_\_\_\_\_ *Gm*,*ext*

hole effective mass of 0.025 m<sup>0</sup>
