*1.4.1 Ge band gap engineering induced by tensile strain*

Semiconductor diode lasers are conventionally based on direct band gap materials due to the efficient radiative recombination of direct gap transitions. As discrete devices, direct band gap III-V semiconductors are the main materials used to fabricate semiconductor lasers in telecommunications. On the other hand, indirect semiconductors, such as Si, Ge and their SiGe alloys are traditionally considered unsuitable for laser diodes due to their indirect band structure. The radiative recombination through the indirect band to-band optical transition is inefficient as a result of a phonon-assisted process. However, the direct band-to-band optical transition in Ge was shown to be very fast, exhibiting radiative recombination rate of 4–5 orders of magnitude higher than that of the indirect transition [13, 15]. Thus, if we can highly increase the number of electrons in the direct band gap, Ge could be used to fabricate diode lasers. Among group-IV indirect semiconductors (Si, and SiC), Ge exhibits interesting properties both from the transport and optical points of view. Regarding the transport properties, as can be seen from the **Table 1**, the hole mobility of Ge is highest in all semiconductors and its electron mobility is about 2.7 times higher than that of Si. Thus, Ge is of great interest for high mobility CMOS transistors, in particular p-CMOS transistors. Concerning the optical properties, Ge can be considered as a pseudo direct band gap material because of a small difference in energy between its direct gap and indirect gap, which is 0.140 eV or ∼ 5 kBT at room temperature (kB is the Boltzmann constant). Theoretically, it has been shown that Ge can be transformed into a direct gap material with ∼1.8–1.9% tensile strain

**77**

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

[14, 16–18]. Indeed, upon application of biaxial tensile strain, both direct and indirect gaps reduce but the direct gap reduces faster. Thus, Ge can progressively transform from an indirect gap semiconductor toward a direct gap semiconductor with the increase of tensile strain. Furthermore, the tensile strain also lifts the degeneracy between heavy hole and light hole valence bands. The small effective mass of the light hole band reduces the density of states in the valence band, which,

*Values of the forbidden band gap and mobilities of holes and electrons in group-IV and III-V semiconductors.*

in turn, decreases the threshold for optical transparency and lasing [14, 18].

dependence of the strain level on the substrate temperature.

sample grown at 300°C of the same thickness.

*1.4.2 Effect of the growth temperature on the value of tensile strain*

Tensile strain can be induced in Ge via several approaches: applying external mechanical stress [19], growing Ge on a larger lattice parameter substrate, such as InGaAs/GaAs [20, 21] or on relaxed GeSn buffer layers on Si [22, 23], or by taking benefit of the thermal mismatch between Ge and Si [18, 24–26]. An interesting result recently demonstrated by Jain et al. [27], who realized a Ge-MEMS device with 1% tensile strain introduced by a Si3N4 stressor. Compared to bulk Ge, the authors observed an enhancement of the photoluminescence intensity up to 260 times. In the frame of this work, we focus on the effect of band gap engineering of Ge films directly grown on Si substrates. The main advantage of the Ge/Si system is that it allows direct integration of Ge-based optoelectronic devices into the mainstream Si technology. In view of device applications, it is vital to obtain high-quality epitaxial material growth, i.e., to get Ge epilayers, which have a smooth surface and a low density of threading dislocations. To reach such an objective, the challenge is to control the Ge/Si growth process to overcome the limitation imposed by 4.2% lattice mismatch between these two materials. Thus, in the following part, we present results on the study of Ge growth on Si using a two-step method and on the

After the low-temperature growth step, we have deposited another Ge film at higher substrate temperatures. In the following, we will consider the effect of the two-step growth process on the tensile strain in the Ge films. All samples have a total thickness of ~ 300 nm, which consists of a ~50 nm thick Ge film deposited at 300°C followed by a 250 nm thick Ge layer grown at various temperatures: 400, 500, 600, 650, 700, 750 and 770°C. As the tensile strain in Ge layers is induced by the difference of the thermal expansion coefficients between Ge and Si, it is natural to expect that the level of tensile strain should increase with increasing the growth temperature. **Figure 7** displays some representative θ–2θ XRD scans taken around the Ge(004) reflection. For comparison, we report in dotted lines a XRD scan of a

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

**Table 1.**

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


**Table 1.**

*Silicon Materials*

**Figure 6.**

solved by n-type doping in Ge.

requires different thermal expansion coefficient between Ge and the substrate material, which is adopted in this research. Up to 0.25–0.30% tensile strain has been achieved in Ge epitaxially grown on Si substrate, which will be discussed later. The second issue is the excessive change of the band gap in highly tensile-strained in Ge. Both the direct band gap and the indirect band gap become 0.53 eV at 1.8% tensile strain, as shown in **Figure 6**. This optical band gap is corresponding to an emission wavelength of about 2300 nm, which is far away from the 1550 nm telecommunication wavelength band, which is also the primary choice for Si photonics. These two issues suggest that very high tensile strains are not favorable in both material growth and photonics applications. Thus, the increase of the number of the injected excess electrons in the Γ valley owing to strain effect is limited. This problem can be

*Electron and hole distributions and light emission at 1.8% tensile-strained Ge.*

**1.4 Growth of tensile-strained Germanium on silicon substrates**

Semiconductor diode lasers are conventionally based on direct band gap materials due to the efficient radiative recombination of direct gap transitions. As discrete devices, direct band gap III-V semiconductors are the main materials used to fabricate semiconductor lasers in telecommunications. On the other hand, indirect semiconductors, such as Si, Ge and their SiGe alloys are traditionally considered unsuitable for laser diodes due to their indirect band structure. The radiative recombination through the indirect band to-band optical transition is inefficient as a result of a phonon-assisted process. However, the direct band-to-band optical transition in Ge was shown to be very fast, exhibiting radiative recombination rate of 4–5 orders of magnitude higher than that of the indirect transition [13, 15]. Thus, if we can highly increase the number of electrons in the direct band gap, Ge could be used to fabricate diode lasers. Among group-IV indirect semiconductors (Si, and SiC), Ge exhibits interesting properties both from the transport and optical points of view. Regarding the transport properties, as can be seen from the **Table 1**, the hole mobility of Ge is highest in all semiconductors and its electron mobility is about 2.7 times higher than that of Si. Thus, Ge is of great interest for high mobility CMOS transistors, in particular p-CMOS transistors. Concerning the optical properties, Ge can be considered as a pseudo direct band gap material because of a small difference in energy between its direct gap and indirect gap, which is 0.140 eV or ∼ 5 kBT at room temperature (kB is the Boltzmann constant). Theoretically, it has been shown that Ge can be transformed into a direct gap material with ∼1.8–1.9% tensile strain

*1.4.1 Ge band gap engineering induced by tensile strain*

**76**

*Values of the forbidden band gap and mobilities of holes and electrons in group-IV and III-V semiconductors.*

[14, 16–18]. Indeed, upon application of biaxial tensile strain, both direct and indirect gaps reduce but the direct gap reduces faster. Thus, Ge can progressively transform from an indirect gap semiconductor toward a direct gap semiconductor with the increase of tensile strain. Furthermore, the tensile strain also lifts the degeneracy between heavy hole and light hole valence bands. The small effective mass of the light hole band reduces the density of states in the valence band, which, in turn, decreases the threshold for optical transparency and lasing [14, 18].

Tensile strain can be induced in Ge via several approaches: applying external mechanical stress [19], growing Ge on a larger lattice parameter substrate, such as InGaAs/GaAs [20, 21] or on relaxed GeSn buffer layers on Si [22, 23], or by taking benefit of the thermal mismatch between Ge and Si [18, 24–26]. An interesting result recently demonstrated by Jain et al. [27], who realized a Ge-MEMS device with 1% tensile strain introduced by a Si3N4 stressor. Compared to bulk Ge, the authors observed an enhancement of the photoluminescence intensity up to 260 times. In the frame of this work, we focus on the effect of band gap engineering of Ge films directly grown on Si substrates. The main advantage of the Ge/Si system is that it allows direct integration of Ge-based optoelectronic devices into the mainstream Si technology. In view of device applications, it is vital to obtain high-quality epitaxial material growth, i.e., to get Ge epilayers, which have a smooth surface and a low density of threading dislocations. To reach such an objective, the challenge is to control the Ge/Si growth process to overcome the limitation imposed by 4.2% lattice mismatch between these two materials. Thus, in the following part, we present results on the study of Ge growth on Si using a two-step method and on the dependence of the strain level on the substrate temperature.
