**2.3. PdGe contacts**

**Figure 15.** Schematic energy band diagrams of: (i) an NiGe/*n*-Ge interface; (ii) an NiGe/*n*-Ge interface with the incorporation of P atoms; (iii) an NiGe/*n*-Ge interface with the incorporation of P atoms and the insertion of a silicon layer.

**Figure 15(i)** shows this energy band bending at the NiGe/*n*-Ge interface. **Figure 15(ii)** and **(iii)** show the energy band bending at the NiGe:P/*n*-Ge and NiGe:P/Si/*n*-Ge interfaces respectively. We see the successive reduction in the Schottky potential barrier heights from 0.54 to 0.51 eV in **Figure 15(i)** and **(ii)** respectively and then 0.42 eV in the ohmic contact of **Figure 15(iii)**. The energy band bending due to the doping by diffused P atoms is shown in **Figure 15(ii)** and **(iii)**. **Figure 15(i)** shows that the Fermi level is pinned. As explained in Section 1.1.5 of the previous chapter, this Fermi-level pinning is caused by dipole formation due to the large amount of interface states. The diameter of a silicon atom is approximately 0.1 nm and the thickness of the silicon layer in **Figure 15(iii)** is also 0.1 nm, meaning that it was essentially a monolayer deposition of silicon which released the Fermi-level pinning and achieved an ohmic contact. The large modulation of the Schottky potential barrier height due to the insertion of a 0.1-nmthick Si monolayer is explainable by considering that the Si atoms caused the formation of chemical bonds between NiGe, Si, and *n*-Ge, thereby reducing the number of dangling bonds (valence mending) that are responsible for the dipole formation which is explained in Section

1.1.5 of the previous chapter.

80 Advanced Material and Device Applications with Germanium

Chawanda et al. [13] used *n*-type Ge(111) doped with antimony (Sb) at a density of 2.5 × 1015 cm−3. Pd was deposited onto the substrates by vacuum resistive evaporation as explained in Section 2.1.1 of the previous chapter. This was done through a mechanical mask which had circular windows of diameter, 0.6 ± 0.05 mm. In this way, 24 circular contacts were prepared in a single evaporation. The thickness of each deposited layer of Pd was 30 nm. Room temperature forward and reverse bias current-voltage characteristics were obtained for five of the Pd/*n*-Ge contacts, which acted as Schottky barrier diodes. The results are shown in **Figure 16**.

Rectifying characteristics are seen for all samples in **Figure 16**. The height of the Schottky potential barrier, Φ*Bn* and the ideality *n*-factor were extracted from the forward bias I-V characteristics of the Schottky diodes at room temperature using the thermionic emission model, as explained in Section 1.1.4 of the previous chapter. This was done for several samples and a histogram was produced to show the statistical distribution of the effective potential barrier heights from the forward bias I-V characteristics, and this is presented in **Figure 17**.

The effective potential barrier heights obtained from the I-V characteristics varied from 0.492 to 0.550 eV. A Gaussian distribution function was used to obtain fits to the histogram. The statistical analysis yielded a mean Schottky potential barrier height value of 0.529 eV with a standard deviation of 0.019 eV.

A histogram was also produced for the values of the ideality factors determined from the I-V characteristics. **Figure 18** shows the statistical distribution of ideality factors from the forward bias I-V characteristics.

**Figure 16.** Room temperature forward and reverse bias current-voltage characteristics obtained for five of the Pd/*n*-Ge contacts [13].

**Figure 17.** Room temperature Schottky potential barrier height distribution derived from forward bias I-V characteristics [13].

The ideality factor ranged from 1.140 to 1.950. A Gaussian distribution function was used to obtain a fit to the histogram. The statistical analysis of the ideality factor yielded an average value of 1.414 with a standard deviation of 0.270.

It is seen that the experimental effective potential barrier heights and ideality factors differ from contact to contact even though they were identically prepared in a single evaporation and on the same substrate. A plot of the effective potential barrier heights as a function of the respective ideality factors is shown in **Figure 19**.

The experimental effective potential barrier height decreases as the ideality factor increases. We see a linear relationship and the straight line drawn in the figure is the least-squares fit to the experimental data.

Five Pd/*n*-Ge contacts were experimentally examined at room temperature to obtain the reverse bias C−2-V characteristics. The results are shown in **Figure 20**.

We see in **Figure 20** that each contact gives a straight line in the C−2-V graphs. The value of the capacitance-voltage derived potential barrier height, Φ*B (C - V)* can be obtained from **Figure 20** using,

$$
\boldsymbol{\Phi}\_{\text{B(C-V)}} = \boldsymbol{V}\_{\text{D}} + \boldsymbol{E}\_{\text{f}} - \boldsymbol{\Delta} \, \boldsymbol{\Phi}\_{\text{g}} \tag{1}
$$

The capacitance-voltage potential barrier heights for the Pd/*n*-Ge (111) Schottky structures varied from 0.427 to 0.509 eV. The statistical analysis yields a mean barrier height of 0.463 eV with a standard deviation of 0.023 eV. The difference between the mean Schottky potential barrier height obtained using the C−2–V characteristics and that from the I-V characteristics

**Figure 19.** A linear plot of the Schottky potential barrier heights against the ideality factors [13].

**Figure 18.** Statistical distribution of ideality factors from the forward bias I-V characteristics [13].

Interface Control Processes for Ni/Ge and Pd/Ge Schottky and Ohmic Contact Fabrication: Part Two

http://dx.doi.org/10.5772/intechopen.79318

83

where *EF* is the energy difference between the bulk Fermi level of Ge and the conduction band edge, *VD* is the diffusion potential, and *Δ*Φ*B* is the image-force barrier lowering given by Eq. (16) in Section 1.1.4 of the previous chapter, where the maximum electric field *Em* is calculated using Eq. (18) of the previous chapter.

The reverse bias C−2-V characteristics were obtained for several samples and a histogram was produced to show the statistical distribution of the capacitance-voltage-derived potential barrier heights, and this is presented in **Figure 21**.

Interface Control Processes for Ni/Ge and Pd/Ge Schottky and Ohmic Contact Fabrication: Part Two http://dx.doi.org/10.5772/intechopen.79318 83

**Figure 18.** Statistical distribution of ideality factors from the forward bias I-V characteristics [13].

**Figure 17.** Room temperature Schottky potential barrier height distribution derived from forward bias I-V characteristics

The ideality factor ranged from 1.140 to 1.950. A Gaussian distribution function was used to obtain a fit to the histogram. The statistical analysis of the ideality factor yielded an average

It is seen that the experimental effective potential barrier heights and ideality factors differ from contact to contact even though they were identically prepared in a single evaporation and on the same substrate. A plot of the effective potential barrier heights as a function of the

The experimental effective potential barrier height decreases as the ideality factor increases. We see a linear relationship and the straight line drawn in the figure is the least-squares fit to

Five Pd/*n*-Ge contacts were experimentally examined at room temperature to obtain the

We see in **Figure 20** that each contact gives a straight line in the C−2-V graphs. The value of the capacitance-voltage derived potential barrier height, Φ*B (C - V)* can be obtained from **Figure 20** using, Φ*B*(*C*−*V*) = *VD* + *EF* − Δ Φ*<sup>B</sup>* (1)

edge, *VD* is the diffusion potential, and *Δ*Φ*B* is the image-force barrier lowering given by Eq. (16) in Section 1.1.4 of the previous chapter, where the maximum electric field *Em* is calculated using

The reverse bias C−2-V characteristics were obtained for several samples and a histogram was produced to show the statistical distribution of the capacitance-voltage-derived potential bar-

is the energy difference between the bulk Fermi level of Ge and the conduction band

reverse bias C−2-V characteristics. The results are shown in **Figure 20**.

value of 1.414 with a standard deviation of 0.270.

82 Advanced Material and Device Applications with Germanium

respective ideality factors is shown in **Figure 19**.

the experimental data.

Eq. (18) of the previous chapter.

rier heights, and this is presented in **Figure 21**.

where *EF*

[13].

**Figure 19.** A linear plot of the Schottky potential barrier heights against the ideality factors [13].

The capacitance-voltage potential barrier heights for the Pd/*n*-Ge (111) Schottky structures varied from 0.427 to 0.509 eV. The statistical analysis yields a mean barrier height of 0.463 eV with a standard deviation of 0.023 eV. The difference between the mean Schottky potential barrier height obtained using the C−2–V characteristics and that from the I-V characteristics

as explained in section 2.2 of the previous chapter. The samples were then vacuum annealed at a pressure of 4 × 10−5 Torr using a rapid thermal annealing (RTA) setup at 700°C for 30 min. This annealing was done to activate some diffusion of the Se dopant atoms further into the semiconductor surface, before metallization with Pd. A Pd film with a thickness of 20 nm was then deposited at room temperature onto the surface of the sample using magnetron sputtering at a base pressure of 10−8 Torr. The second type of samples was prepared in exactly the same way as the first type but with no Se implantation and activation. All the samples were then vacuum annealed at a pressure of 10−6 Torr to induce solid state reactions, resulting in the formation of the PdGe phase. X-ray diffraction (XRD) measurements were made in-situ. The heating ramp rate was 5°C per min steps and these steps were separated by 5 min-long XRD measurements at a constant temperature.

Interface Control Processes for Ni/Ge and Pd/Ge Schottky and Ohmic Contact Fabrication: Part Two

http://dx.doi.org/10.5772/intechopen.79318

85

For the samples which were implanted with Se, the distribution of Se atoms in the surface region was determined at three stages of the sample preparation using secondary ion mass spectrometry (SIMS). The distribution was first obtained immediately after the Se implantation (as implanted). It was also obtained after the rapid thermal annealing at 700°C for 30 min, which was done to activate some diffusion of the Se dopant atoms. The third SIMS determination of the Se distribution was carried out after the annealing ramp which resulted in the formation and growth of the PdGe phase. All secondary ion mass spectrometry results are presented in **Figure 22**. The Se SIMS profile measured immediately after the Se implantation is represented by open triangles. The profile after the activation annealing was performed at

The Se SIMS profile measured after the annealing ramp to form PdGe is represented by open circles in **Figure 22**. If we compare this profile to the one obtained after the activation annealing was performed at 700°C for 30 min (open squares), we see that Se atoms did not diffuse any further into the depth of the substrate during the annealing ramp. The Se profile immediately after the implantation corresponds to a Gaussian distribution with a maximum concentration

700°C for 30 min and is represented by the open squares.

**Figure 22.** Secondary ion mass spectrometry results [15].

**Figure 20.** Schottky reverse bias C−2-V characteristics for five Pd/*n*-Ge samples [13].

**Figure 21.** Schottky potential barrier height distribution derived from reverse bias C−2-V characteristics [13].

is 0.066 eV. Potential barrier heights obtained from the I-V and C−2-V characteristics are not always the same [14] because of the different nature of the two measurement techniques.

#### *2.3.1. Interface dopant implantation*

Descoins et al. [15] used Ge (001) substrates to form two types of Pd/Ge contacts. In the first type of samples the surface of the substrates were implanted with Se atoms at an energy of 130 keV as explained in section 2.2 of the previous chapter. The samples were then vacuum annealed at a pressure of 4 × 10−5 Torr using a rapid thermal annealing (RTA) setup at 700°C for 30 min. This annealing was done to activate some diffusion of the Se dopant atoms further into the semiconductor surface, before metallization with Pd. A Pd film with a thickness of 20 nm was then deposited at room temperature onto the surface of the sample using magnetron sputtering at a base pressure of 10−8 Torr. The second type of samples was prepared in exactly the same way as the first type but with no Se implantation and activation. All the samples were then vacuum annealed at a pressure of 10−6 Torr to induce solid state reactions, resulting in the formation of the PdGe phase. X-ray diffraction (XRD) measurements were made in-situ. The heating ramp rate was 5°C per min steps and these steps were separated by 5 min-long XRD measurements at a constant temperature.

For the samples which were implanted with Se, the distribution of Se atoms in the surface region was determined at three stages of the sample preparation using secondary ion mass spectrometry (SIMS). The distribution was first obtained immediately after the Se implantation (as implanted). It was also obtained after the rapid thermal annealing at 700°C for 30 min, which was done to activate some diffusion of the Se dopant atoms. The third SIMS determination of the Se distribution was carried out after the annealing ramp which resulted in the formation and growth of the PdGe phase. All secondary ion mass spectrometry results are presented in **Figure 22**. The Se SIMS profile measured immediately after the Se implantation is represented by open triangles. The profile after the activation annealing was performed at 700°C for 30 min and is represented by the open squares.

The Se SIMS profile measured after the annealing ramp to form PdGe is represented by open circles in **Figure 22**. If we compare this profile to the one obtained after the activation annealing was performed at 700°C for 30 min (open squares), we see that Se atoms did not diffuse any further into the depth of the substrate during the annealing ramp. The Se profile immediately after the implantation corresponds to a Gaussian distribution with a maximum concentration

**Figure 22.** Secondary ion mass spectrometry results [15].

**Figure 21.** Schottky potential barrier height distribution derived from reverse bias C−2-V characteristics [13].

**Figure 20.** Schottky reverse bias C−2-V characteristics for five Pd/*n*-Ge samples [13].

84 Advanced Material and Device Applications with Germanium

*2.3.1. Interface dopant implantation*

is 0.066 eV. Potential barrier heights obtained from the I-V and C−2-V characteristics are not always the same [14] because of the different nature of the two measurement techniques.

Descoins et al. [15] used Ge (001) substrates to form two types of Pd/Ge contacts. In the first type of samples the surface of the substrates were implanted with Se atoms at an energy of 130 keV

**Figure 23(a)** shows the in-situ X-ray diffractogram obtained from an Se-doped sample. This diffractogram evolved during the in-situ XRD annealing process. Initially only a single Pd(111)

peaks increased until the Pd(111) peak disappeared after which five new peaks correspond-

get consumed, giving way to PdGe growth. This evolution is displayed in **Figure 23(b)** for the sample with Se doping and in **Figure 23(c)** for the sample without Se doping. To get the results in **Figure 23(b)** and **(c)**, the XRD peak intensities corresponding to various phases were integrated and normalized. The normalized integrated intensities were then plotted against the temperatures of the ramp annealing. We see from **Figure 23(b)** and **(c)** that at the end of the experiment we have a layer of PdGe in contact with an Se-doped Ge substrate and another in contact with an Se-free Ge substrate. Sheet resistivity measurements were carried out on both, the samples with Se interface doping and those without Se doping. The resistivity of the PdGe film grown on the Se-free Ge substrate was found to be, *ρ*sh = 13 ± 1 μΩ cm and that on the Ge substrate with interface Se doping was, *ρ*sh = 6 ± 0.8 μΩ cm. This means that interface Se doping reduces the sheet resistivity by half which should result in a nearly ohmic contact

Some of the novel interface control processes developed for the fabrication of NiGe and PdGe

NiGe grown using the cyclic stacking of Ni/Ge films on an *n*-Ge substrate showed a stable sheet resistivity in the annealing temperature range from 275 to around 500°C. This temperature range was much wider than the corresponding stable-sheet-resistance annealing temperature range obtained from NiGe grown under similar conditions but without the cyclic stacking. The Schottky potential barrier heights for the contacts with cyclically stacked NiGe exhibited stable values which were less than 0.54 eV, even after annealing at temperatures of up to 600°C. The ideality factors of these contacts were less than 1.2, even after annealing at temperatures of up to 500°C. NiGe contacts with the interface incorporation of phosphorus atoms and the insertion of a silicon film at the interface were explained. Ohmic characteristics have been observed for contacts with substantial P interface incorporation and those with minimal P interface incorporation coupled with the insertion of a 0.1 nm-thick Si film, which

A linear relationship was observed between the potential barrier heights and corresponding ideality factors for Schottky contacts of Pd grown on Sb-doped Ge(111) with a doping density of about 2.5 × 1015 cm−3. Current-voltage and capacitance-voltage characteristics were obtained at room temperature. The effective potential barrier heights obtained from these I-V characteristics varied from 0.492 to 0.550 eV, while the ideality factor varied from 1.140 to 1.950. The

Ge(002) peaks appeared at 2θ ≈ 37.5 and 53.7°, respectively. The intensity of the

Interface Control Processes for Ni/Ge and Pd/Ge Schottky and Ohmic Contact Fabrication: Part Two

Ge(111)

87

Ge (002)

Ge then starts to

Ge(111) and Pd<sup>2</sup>

http://dx.doi.org/10.5772/intechopen.79318

diffraction peak is detected at a diffraction angle of 2θ ≈ 40°. Upon annealing, the Pd<sup>2</sup>

Pd(111) peak decreased during further annealing and that of the Pd<sup>2</sup>

because the sheet resistivity is closely related to the contact resistivity.

Schottky and ohmic contacts on *n*-type germanium have been reviewed.

**3. Summary and conclusion**

is essentially a monoatomic Si layer.

ing to the PdGe(101), (111), (211), (121), and (002) planes appeared. The Pd<sup>2</sup>

and Pd<sup>2</sup>

**Figure 23.** (a) In-situ X-ray diffractogram obtained from an Se-doped sample. (b) Pd(111), Pd<sup>2</sup> Ge(002) and PdGe(101) integrated and normalized XRD peak data extracted in-situ during the annealing of a sample with Se doping. (c) Integrated and normalized XRD peak data for a sample without Se doping [15].

of about 5 × 1020 atoms cm−3, which is located at around 60 nm below the surface of the sample. As a result of the activation annealing, the Se atoms diffused further into the substrate decreasing the maximum Se concentration at a depth of 60 nm from 5 × 10<sup>20</sup> to about 1 × 1020 at cm−3.

**Figure 23(a)** shows the in-situ X-ray diffractogram obtained from an Se-doped sample. This diffractogram evolved during the in-situ XRD annealing process. Initially only a single Pd(111) diffraction peak is detected at a diffraction angle of 2θ ≈ 40°. Upon annealing, the Pd<sup>2</sup> Ge(111) and Pd<sup>2</sup> Ge(002) peaks appeared at 2θ ≈ 37.5 and 53.7°, respectively. The intensity of the Pd(111) peak decreased during further annealing and that of the Pd<sup>2</sup> Ge(111) and Pd<sup>2</sup> Ge (002) peaks increased until the Pd(111) peak disappeared after which five new peaks corresponding to the PdGe(101), (111), (211), (121), and (002) planes appeared. The Pd<sup>2</sup> Ge then starts to get consumed, giving way to PdGe growth. This evolution is displayed in **Figure 23(b)** for the sample with Se doping and in **Figure 23(c)** for the sample without Se doping. To get the results in **Figure 23(b)** and **(c)**, the XRD peak intensities corresponding to various phases were integrated and normalized. The normalized integrated intensities were then plotted against the temperatures of the ramp annealing. We see from **Figure 23(b)** and **(c)** that at the end of the experiment we have a layer of PdGe in contact with an Se-doped Ge substrate and another in contact with an Se-free Ge substrate. Sheet resistivity measurements were carried out on both, the samples with Se interface doping and those without Se doping. The resistivity of the PdGe film grown on the Se-free Ge substrate was found to be, *ρ*sh = 13 ± 1 μΩ cm and that on the Ge substrate with interface Se doping was, *ρ*sh = 6 ± 0.8 μΩ cm. This means that interface Se doping reduces the sheet resistivity by half which should result in a nearly ohmic contact because the sheet resistivity is closely related to the contact resistivity.
