Improvement of Fermi-Level Pinning and Contact Resistivity in Ti/Ge Contact Using Carbon Implantation

Effects of carbon implantation (C-imp) on the contact characteristics of Ti/Ge contact were investigated. The C-imp into Ti/Ge system was developed to reduce severe Fermi-level pinning (FLP) and to improve the thermal stability of Ti/Ge contact. The current density (J)-voltage (V) characteristics showed that the rectifying behavior of Ti/Ge contact into an Ohmic-like behavior with C-imp. The lowering of Schottky barrier height (SBH) indicated that the C-imp could mitigate FLP. In addition, it allows a lower specific contact resistivity (ρc) at the rapid thermal annealing (RTA) temperatures in a range of 450–600 °C. A secondary ion mass spectrometry (SIMS) showed that C-imp facilitates the dopant segregation at the interface. In addition, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping showed that after RTA at 600 °C, C-imp enhances the diffusion of Ge atoms into Ti layer at the interface of Ti/Ge. Thus, carbon implantation into Ge substrate can effectively reduce FLP and improve contact characteristics.


Introduction
As a channel material for the next-generation field-effect transistors (FETs), Germanium (Ge) is considered a promising alternative to silicon (Si) owing to its higher carrier mobility and the process compatibility with the advanced Si microfabrication. However, the low-solid solubility and the high-diffusion coefficient of n-type dopants in Ge hinder the realization of low specific contact resistivity (ρ c ) [1]. Moreover, Fermi-level pinning (FLP) caused by the metal-induced gap states (MIGS) at the metal/Ge interface is another problem to be solved [2][3][4][5]. FLP strongly occurs near the Ge valence band (E v ) and forces the electron Schottky barrier height (e-SBH) above 0.5 eV irrespective of the metal workfunction [6]. Several approaches, including dopant segregation [7], dipole formation [8], and surface treatment [9] were proposed to mitigate FLP phenomena. Recently, the use of an ultra-thin insulator between the metal and Ge showed an effective reduction of FLP but the degradation of ρ c due to a high tunneling resistance [10][11][12][13]. The formation of metal germanide can be another approach because the MIGS from metal dangling bond states in germanide can lead to an FLP reduction [14,15].
Ion implantation is another approach to achieving low ρ c and suppressing dopantdiffusion behaviors. For example, Germanium implantation before silicidation induces surface amorphization to aid an epitaxial regrowth on the semiconductor surface [16]. Carbon implantation (C-imp) has been introduced in Ni-silicide and Ni-germinide contacts to reduce contact resistivity [17,18]. However, Ti/Ge contact with carbon implantation has been rarely reported.
Here, we investigated the effects of C-imp on the FLP reduction of a Ti/Ge contact and the related contact characteristics. Electrical characteristics were measured using the Figure 1 shows the J-V characteristics of the Ti/Ge contacts with and without C-imp at RTA temperatures in a range of 450-600 • C for 60 s in N 2 ambient. The Ti/Ge contact without C-imp shows a typical rectifying behavior attributed to a strong FLP near the E v , which leads to a significantly high e-SBH and reduces the reverse current density. On the other hand, the Ti/Ge contact with C-imp shows an Ohmic-like behavior with relatively high current density under the reverse regime, indicating the alleviation of FLP.
Carbon implantation (C-imp) has been introduced in Ni-silicide and Ni-germinide contacts to reduce contact resistivity [17,18]. However, Ti/Ge contact with carbon implantation has been rarely reported.
Here, we investigated the effects of C-imp on the FLP reduction of a Ti/Ge contact and the related contact characteristics. Electrical characteristics were measured using the multiring-circular transmission line model (MR-CTLM) structure and Schottky barrier diode (SBD). Physical and structural properties of Ti/Ge contact with C-imp were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and secondary ion mass spectrometry (SIMS).

Materials and Methods
N-type Ge wafers moderately doped with phosphorus (~10 18 cm −3 ) were cleaned in a 1:100 diluted HF (dHF) solution and deionized (DI) water to remove native oxide. Subsequently, C + ions were implanted into the Ge substrate at a dose of 1 × 10 15 cm −2 and an implantation energy of 10 keV. A reference sample without C-imp was also prepared. A SBD of Ti/Ge structure and a MR-CTLM structure were fabricated on the Ge substrate to characterize electrical properties. First, a 100 nm thick SiO2 was deposited to isolate the contact holes using a plasma-enhanced chemical vapor deposition (PECVD). Then, the metal contact was formed using the conventional photolithography process. Sequentially, the oxide was etched using a dry etcher, and a Ti (5 nm)/TiN (5 nm) was deposited using a DC sputtering system. After a metal lift-off process, rapid thermal annealing (RTA) was performed in N2 ambient for 60 s at 450-600 °C . Finally, a 100 nm thick Al was deposited as contact pad metal. The electrical measurements of current (I)-bias voltage (V) were performed using Keithley 4200-SCS. TEM images of the Ti/Ge structure without and with Cimp were obtained using a JEOL JEM 2200FS with an image Cs-corrector. Figure 1 shows the J-V characteristics of the Ti/Ge contacts with and without C-imp at RTA temperatures in a range of 450-600 °C for 60 s in N2 ambient. The Ti/Ge contact without C-imp shows a typical rectifying behavior attributed to a strong FLP near the Ev, which leads to a significantly high e-SBH and reduces the reverse current density. On the other hand, the Ti/Ge contact with C-imp shows an Ohmic-like behavior with relatively high current density under the reverse regime, indicating the alleviation of FLP.    Figure 2a shows the extracted e-SBHs of the Ti/Ge contacts without (blue box) and with (red box) C-imp after RTA at 550 • C and 600 • C, respectively, for 60 s in N 2 ambient. The e-SBHs were extracted from the current-temperature (I-T) curves in a range of 300-378 K. The I-V relationship of a Schottky barrier diode is represented by [19] I = AA * T 2 e −q∅ B /kT e qV/nkT − 1 = I S1 e −q∅ B /kT e qV/nkT − 1 = I S e qV/nkT − 1

Results
where I s is the saturation current, A is the diode area, A* = 4πqk 2 m*/h 3 = 120 (m*/m) A/cm 2 ·K 2 Richardson's constant, Φ B is the barrier height, and n is the ideality factor. For V kT/q Equation (1) can be written as follows: Richardson's constant, ΦB is the barrier height, and n is the ideality factor. For ≫ / Equation (1) can be written as follows: Therefore, the barrier height is calculated from the slope (=d[ln(I/T 2 )]/d(1/T)). The bandgap and electron affinity in eV of Ge at 300 K are 0.66 and 4.0 eV, respectively. The workfunction of Ti metals is about 4.3 eV. When Fermi level is pinned near Ev of Ge, ΦB of ~0.6 eV is calculated. If there is negligible FLP, ΦB of ~0.3 eV is obtained.
Without C-imp, the SBH of ~0.48 eV was obtained for both 550 °C and 600 °C RTA, indicating the occurrence of FLP. In contrast, the SBH with C-imp was significantly reduced from 0.31 eV at 550 °C to 0.27 eV at 600 °C . Figure 2b,c show schematics of the energy band diagrams for Ti/Ge contacts. Without C-imp, Fermi-level on the Ti side is pinned with the charge neutrality level (ECNL) due to FLP [6].  Without C-imp, the SBH of~0.48 eV was obtained for both 550 • C and 600 • C RTA, indicating the occurrence of FLP. In contrast, the SBH with C-imp was significantly reduced from 0.31 eV at 550 • C to 0.27 eV at 600 • C. Figure 2b,c show schematics of the energy band diagrams for Ti/Ge contacts. Without C-imp, Fermi-level on the Ti side is pinned with the charge neutrality level (E CNL ) due to FLP [6]. Figure 3 shows a top-view SEM image of the fabricated MR-CTLM structure to extract ρ c and the sheet resistance beneath the metal (R S ). The current flows through multiple metal-semiconductor structures from the center region to the outer-circle region. From the I-V curve of MR-CTLM, the total resistance (R tot ) is expressed as the sum of the effective resistance (R eff ) and the parasitic resistance (R pr ) as follows [20]: where r 0~r9 are the inner radius of the serial CTLM. S m and S s are the spacing among metal rings and dielectric rings, respectively. L t is the transfer length. S s = 10 µm, r 0 = 50 µm, and S m , from 0.5 to 10 µm were defined using an i-line stepper. ρ c was calculated from the L t (= ρ c /R s ) which was extracted by fitting a set of R t -S m data using Equations (4)- (6). where r0~r9 are the inner radius of the serial CTLM. Sm and Ss are the spacing among metal rings and dielectric rings, respectively. Lt is the transfer length. Ss = 10 μm, r0 = 50 μm, and Sm, from 0.5 to 10 μm were defined using an i-line stepper. ρc was calculated from the Lt (= √ / ) which was extracted by fitting a set of Rt-Sm data using equation (4)-(6).  Figure 4 shows the extracted ρc values versus RTA temperature. ρc was obtained using a MR-CTLM test structure [20]. A relatively high ρc value seems mainly because of the low activation of a substrate doping concentration of ~1 × 10 18 cm −3 [21,22]. After RTA annealing at 600 °C , the ρc values of the Ti/Ge with and without C imp were 1.3 × 10 −5 and 8.4 × 10 −4 Ω•cm 2 , respectively. Owing to the FLP effect, the Ti/Ge contact without C-imp shows ρc values higher than 1.0 × 10 −4 Ω•cm 2 .   Figure 4 shows the extracted ρ c values versus RTA temperature. ρ c was obtained using a MR-CTLM test structure [20]. A relatively high ρ c value seems mainly because of the low activation of a substrate doping concentration of~1 × 10 18 cm −3 [21,22]. After RTA annealing at 600 • C, the ρ c values of the Ti/Ge with and without C imp were 1.3 × 10 −5 and 8.4 × 10 −4 Ω·cm 2 , respectively. Owing to the FLP effect, the Ti/Ge contact without C-imp shows ρ c values higher than 1.0 × 10 −4 Ω·cm 2 .
To further analyze the effect of C-imp on the Ti/Ge composition, TEM and SIMS were conducted. The decrease of ρ c is mainly attributed to the dopant segregation in the Ti/Ge interface [23]. In particular, for the Ti/Ge contact with C-imp after RTA at 600 • C, a further reduction of ρ c is observed. These results can be expected by TiGe x formation. The low resistive C54-TiGe x is formed at a temperature above 600 • C [24], which mitigates FLP and improves the contact resistivity [14,15]. Figure 5a,b show SIMS profiles for Ti/Ge contacts without and with C-imp, respectively. At the Ti/Ge interface with C-imp, the peak P concentration increases from 1.6 × 10 18 cm −3 to 3.6 × 10 18 cm −3 , attributed to the dopant segregation facilitated by carbon [18]. This dopant segregation can increase the tunneling current by reducing the depletion thickness at the interface and lowering the contact resistivity.
To directly observe the microstructure of Ti/Ge contact, the cross-sectional TEM images and the corresponding EELS were analyzed. The samples were prepared after RTA at 600 • C for 60 s in N 2 ambient, as shown in Figure 6. In EELS maps, a bright region represents the area that the element of interest is abundant. With C-imp, Ge element is considerably observed in the Ti layer (red box in Figure 6b). The diffused Ge reacts with Ti and forms the Ti-germanide during the RTA process, which is beneficial to reduce the contact resistivity [14,15]. These results show that the C-imp is a promising approach to lower the contact resistivity in Ti/Ge contact by inducing the dopant segregation and Ge diffusion into the Ti layer. Figure 4 shows the extracted ρc values versus RTA temperature. ρc was obtained using a MR-CTLM test structure [20]. A relatively high ρc value seems mainly because of the low activation of a substrate doping concentration of ~1 × 10 18 cm −3 [21,22]. After RTA annealing at 600 °C , the ρc values of the Ti/Ge with and without C imp were 1.3 × 10 −5 and 8.4 × 10 −4 Ω•cm 2 , respectively. Owing to the FLP effect, the Ti/Ge contact without C-imp shows ρc values higher than 1.0 × 10 −4 Ω•cm 2 .  To further analyze the effect of C-imp on the Ti/Ge composition, TEM and SIMS were conducted. The decrease of ρc is mainly attributed to the dopant segregation in the Ti/Ge interface [23]. In particular, for the Ti/Ge contact with C-imp after RTA at 600 °C, a further reduction of ρc is observed. These results can be expected by TiGex formation. The low resistive C54-TiGex is formed at a temperature above 600 °C [24], which mitigates FLP and improves the contact resistivity [14,15]. Figure 5a,b show SIMS profiles for Ti/Ge contacts without and with C-imp, respectively. At the Ti/Ge interface with C-imp, the peak P concentration increases from 1.6 × 10 18 cm −3 to 3.6 × 10 18 cm −3 , attributed to the dopant segregation facilitated by carbon [18]. This dopant segregation can increase the tunneling current by reducing the depletion thickness at the interface and lowering the contact resistivity. To directly observe the microstructure of Ti/Ge contact, the cross-sectional TEM images and the corresponding EELS were analyzed. The samples were prepared after RTA at 600 °C for 60 s in N2 ambient, as shown in Figure 6. In EELS maps, a bright region represents the area that the element of interest is abundant. With C-imp, Ge element is considerably observed in the Ti layer (red box in Figure 6b). The diffused Ge reacts with Ti and forms the Ti-germanide during the RTA process, which is beneficial to reduce the contact resistivity [14,15]. These results show that the C-imp is a promising approach to lower the contact resistivity in Ti/Ge contact by inducing the dopant segregation and Ge diffusion into the Ti layer. To further analyze the effect of C-imp on the Ti/Ge composition, TEM and SIMS were conducted. The decrease of ρc is mainly attributed to the dopant segregation in the Ti/Ge interface [23]. In particular, for the Ti/Ge contact with C-imp after RTA at 600 °C, a further reduction of ρc is observed. These results can be expected by TiGex formation. The low resistive C54-TiGex is formed at a temperature above 600 °C [24], which mitigates FLP and improves the contact resistivity [14,15]. Figure 5a,b show SIMS profiles for Ti/Ge contacts without and with C-imp, respectively. At the Ti/Ge interface with C-imp, the peak P concentration increases from 1.6 × 10 18 cm −3 to 3.6 × 10 18 cm −3 , attributed to the dopant segregation facilitated by carbon [18]. This dopant segregation can increase the tunneling current by reducing the depletion thickness at the interface and lowering the contact resistivity. To directly observe the microstructure of Ti/Ge contact, the cross-sectional TEM images and the corresponding EELS were analyzed. The samples were prepared after RTA at 600 °C for 60 s in N2 ambient, as shown in Figure 6. In EELS maps, a bright region represents the area that the element of interest is abundant. With C-imp, Ge element is considerably observed in the Ti layer (red box in Figure 6b). The diffused Ge reacts with Ti and forms the Ti-germanide during the RTA process, which is beneficial to reduce the contact resistivity [14,15]. These results show that the C-imp is a promising approach to lower the contact resistivity in Ti/Ge contact by inducing the dopant segregation and Ge diffusion into the Ti layer.

Conclusions
We investigated the electrical and material characteristics of a Ti/Ge contact with C-imp. The current-voltage behavior shows that the carbon implantation changes the Ti/Ge rectifying behavior into an Ohmic-like behavior above RTA at 450 • C. The extracted Schottky barrier height was also decreased due to the mitigation of Fermi-level pinning. The specific contact resistivity of the Ti/Ge contact with C-imp was significantly reduced by approximately two orders of magnitude. Transmission electron microscopy and secondary ion mass spectrometry showed that carbon element at the Ti/Ge interface facilitates the dopant segregation and induces the diffusion of Ge into Ti layer. Therefore, the carbon implantation is promising to improve the Ti/Ge contact properties for high-performance Ge-FET applications.