*1.5.2 Efficiency of phosphorus doping from a GaP source*

**Figure 12** displays the evolution of the room-temperature photoluminescence spectrum versus the temperature of the GaP cell. For all samples, the substrate temperature is chosen to be 300°C and the film thickness is 100 nm. The temperature of the GaP source increases from 600 to 750°C. Note that based on the ref. [51], we avoid doping Ge films at 800°C. After growth, all samples were annealed in the growth chamber at 750°C during 1 min to activate dopants. As can be seen from the figure, the photoluminescence intensity increases with increasing the temperature of the GaP source temperature from 600 to 725°C and the highest PL intensity is obtained at 725°C. For the GaP source temperature at 750°C, the PL is found to decrease. Thus, the PL result, indicating that above 725°C the Ga trap from the GaP cell becomes less efficient.

### *1.5.3 Dependence of the substrate temperature on the doping level*

To investigate the effect of the doping level versus the substrate temperature, we have therefore kept the GaP source at a constant temperature of 725°C. We display in **Figure 13** the evolution of the photoluminescence spectrum with the

#### **Figure 12.**

*Evolution of photoluminescence spectrum versus the GaP source temperature. All PL measurements were carried out at room temperature.*

**83**

**Figure 13.**

*New Material for Si-Based Light Source Application for CMOS Technology*

substrate temperature. The reference sample is a 600 nm thick undoped Ge layer deposited at a substrate temperature of 170°C. The temperature of the GaP source is 725°C. The spectrum of the reference sample exhibits a very weak intensity and the emission from direct band gap is not clearly observed, as expected in an indirect band gap semiconductor. The PL intensity, which is very weak for the sample grown at 300°C, is found to increase with decreasing the substrate temperature and the highest PL intensity is obtained for a substrate temperature of 170°C. The evolution of the PL signal follows almost the same trend already observed for the electrical measurements not show here and thus confirms a high efficiency of P doping at a low substrate temperature of 170°C. The 170°C PL spectrum peaks at around 1624 nm (the corresponding energy is 0.765 eV). This transition can be attributed to arise from the direct band gap radiative recombination in the n-doped Ge layer. It is worth noting that if we compare the photoluminescence intensity of the sample doped at 170°C (blue curve) with that of the undoped sample (black curve), an intensity enhancement of about 50 times is obtained. Another noteworthy point is that when comparing the energy maximum at around 0.810 eV, arising from the direct band gap emission of unstrained and undoped Ge, we observe here a redshift of 45 meV, which can be attributed to band gap narrowing at high n-doping levels [52]. Indeed, a low temperature growth at 170°C does not induce tensile strain and after annealing at 750°C for 1 min, the corresponding Ge films only exhibit a tensile strain as low as 0.13%. Thus, the above optical redshift can be directly correlated to the effect of n-doping. According to ref. [52], a redshift of 45 meV corresponds to a doping

. The separate investigation of the effect of the substrate

can be obtained at a

temperature on the tensile strain and n-doping of the Ge layer brings fruitful information. The substrate temperature is shown to produce an opposite effect on these two properties of the Ge film. Higher growth temperatures (up to 770°C) induce larger tensile strain while low temperatures favor n doping. It is notewor-

growth temperature of 170°C. In addition, it should be pointed out that if Ge is directly deposited on Si substrates at a substrate temperature of 170°C, the Ge/Si growth does proceed via the Stranski-Krastanov mode. However, once a smooth Ge buffer has been formed on Si at 300°C, further Ge deposition on this Ge buffer layer will behave similarly as a homoepitaxial Ge growth and the growth is

thy that an activated n-doping level higher than 1019 cm−<sup>3</sup>

*Evolution of the photoluminescence spectrum versus the growth temperature.*

two-dimensional in a very large range of substrate temperatures.

*DOI: http://dx.doi.org/10.5772/intechopen.84994*

level of ~2 × 1019 cm−<sup>3</sup>

### *New Material for Si-Based Light Source Application for CMOS Technology DOI: http://dx.doi.org/10.5772/intechopen.84994*

*Silicon Materials*

**Figure 11.**

**82**

**Figure 12.**

*carried out at room temperature.*

*Evolution of photoluminescence spectrum versus the GaP source temperature. All PL measurements were* 

Ga atoms are trapped by the cap and only the P2 beam can escape. According to the

**Figure 12** displays the evolution of the room-temperature photoluminescence spectrum versus the temperature of the GaP cell. For all samples, the substrate temperature is chosen to be 300°C and the film thickness is 100 nm. The temperature of the GaP source increases from 600 to 750°C. Note that based on the ref. [51], we avoid doping Ge films at 800°C. After growth, all samples were annealed in the growth chamber at 750°C during 1 min to activate dopants. As can be seen from the figure, the photoluminescence intensity increases with increasing the temperature of the GaP source temperature from 600 to 725°C and the highest PL intensity is obtained at 725°C. For the GaP source temperature at 750°C, the PL is found to decrease. Thus, the PL result, indicating that above 725°C the Ga trap

To investigate the effect of the doping level versus the substrate temperature, we have therefore kept the GaP source at a constant temperature of 725°C. We display in **Figure 13** the evolution of the photoluminescence spectrum with the

supplier [49], a P2/P4 ratio of about 150:1 can be achieved.

*(a) Photograph of a GaP decomposition cell and (b) schema of the Ga trapping cap [50].*

*1.5.3 Dependence of the substrate temperature on the doping level*

*1.5.2 Efficiency of phosphorus doping from a GaP source*

from the GaP cell becomes less efficient.

substrate temperature. The reference sample is a 600 nm thick undoped Ge layer deposited at a substrate temperature of 170°C. The temperature of the GaP source is 725°C. The spectrum of the reference sample exhibits a very weak intensity and the emission from direct band gap is not clearly observed, as expected in an indirect band gap semiconductor. The PL intensity, which is very weak for the sample grown at 300°C, is found to increase with decreasing the substrate temperature and the highest PL intensity is obtained for a substrate temperature of 170°C. The evolution of the PL signal follows almost the same trend already observed for the electrical measurements not show here and thus confirms a high efficiency of P doping at a low substrate temperature of 170°C. The 170°C PL spectrum peaks at around 1624 nm (the corresponding energy is 0.765 eV). This transition can be attributed to arise from the direct band gap radiative recombination in the n-doped Ge layer. It is worth noting that if we compare the photoluminescence intensity of the sample doped at 170°C (blue curve) with that of the undoped sample (black curve), an intensity enhancement of about 50 times is obtained. Another noteworthy point is that when comparing the energy maximum at around 0.810 eV, arising from the direct band gap emission of unstrained and undoped Ge, we observe here a redshift of 45 meV, which can be attributed to band gap narrowing at high n-doping levels [52]. Indeed, a low temperature growth at 170°C does not induce tensile strain and after annealing at 750°C for 1 min, the corresponding Ge films only exhibit a tensile strain as low as 0.13%. Thus, the above optical redshift can be directly correlated to the effect of n-doping. According to ref. [52], a redshift of 45 meV corresponds to a doping level of ~2 × 1019 cm−<sup>3</sup> . The separate investigation of the effect of the substrate temperature on the tensile strain and n-doping of the Ge layer brings fruitful information. The substrate temperature is shown to produce an opposite effect on these two properties of the Ge film. Higher growth temperatures (up to 770°C) induce larger tensile strain while low temperatures favor n doping. It is noteworthy that an activated n-doping level higher than 1019 cm−<sup>3</sup> can be obtained at a growth temperature of 170°C. In addition, it should be pointed out that if Ge is directly deposited on Si substrates at a substrate temperature of 170°C, the Ge/Si growth does proceed via the Stranski-Krastanov mode. However, once a smooth Ge buffer has been formed on Si at 300°C, further Ge deposition on this Ge buffer layer will behave similarly as a homoepitaxial Ge growth and the growth is two-dimensional in a very large range of substrate temperatures.

**Figure 13.** *Evolution of the photoluminescence spectrum versus the growth temperature.*
