*1.2.1 Photonics in optical communications*

Photonics is about the science and the technology of generating, manipulating, and detecting photons. Silicon, while dominating the semiconductor electronics for decades, is naturally a challenge for developing photonics in many communication applications [7]. Silicon photonics offer a promising platform for the monolithic integration of optics and microelectronics, aiming for many applications including the optical interconnects solution to the microelectronics bottleneck [8]. Because of its importance and rapid growth, a roadmap of silicon photonics, assessed by academic and industrial experts, has been proposed [9]. Light or photon has been used to transmit information for over three decades, mainly by using optical fibers to form optical interconnection between places in distance. In the past three decades, photons have been widely used in optical fiber communication systems, especially in long-distance communications. The fundamental reason for the optical interconnect advantage is the zero rest mass of a photon, which can greatly reduce the required energy. In addition, compared to electrical interconnections, the most important advantage using optical fiber communications is the high speed. However, many challenges need to be solved to achieve on-chip optical interconnects. The first issue is to integrate all the optical devices with the silicon microelectronic devices on silicon chips. Another challenge is materials used in each device. For traditional optical components, III-V materials, such as GaAs and InP, are exclusively used due to their excellent optical properties. However, all the materials and their fabrication are not compatible with the Si exciting Si-CMOS technology.

## *1.2.2 Key devices for Si photonics*

The on-chip interconnect system contains an integrated light source, a modulator to transfer the electrical signal to an optical signal, a waveguide or waveguide device to direct the optical signal to the destination, and finally a photodetector to convert the optical signal back to an electrical signal. Most of these devices are already developed on a Si platform with high bandwidth capability.

**Figure 2** Si photonics key devices are needed to realize on-chip optical interconnects: Integrated light source, modulator, waveguide and photo-detector [10]. The only missing key device is an electrically pumped laser source, which is the compatible with the integrated CMOS technology. As we have mentioned above, numerous approaches have been investigated, including porous Si, semiconducting silicides, Er-doped Si, Si nanocrystals or Ge/Si self-assembled quantum dots but none of them could produce a high emission efficiency at room temperature. From the material point of view, in order to achieve a CMOS compatible light source, the

**73**

**Figure 3.**

*Ge band structure at 300 K.*

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

use of group IV materials (such as Si, Ge, or Sn) is highly expected. In order to have an efficient light source, a direct band gap semiconductor is preferred. At least, a local minimum at the Γ point of the conduction band is required to accumulate electrons and achieve efficient radiative recombination. This requirement makes Si impossible to be an efficient light source because the difference between the direct and indirect valleys in Si is larger than 2 eV. Fortunately, germanium, which is a group IV material, has a local minimum at the Γ point of the conduction band. More attractively is that the lowest energy point in the Ge conduction band is at the L point, which is only 0.140 eV lower than the lowest energy at the Γ point at room temperature. Therefore, Ge has the potential to be engineered to become a direct

band gap material and used as an on-chip integrated light source [11, 12].

As previously discussed, Ge is the most interesting group-IV material for the light emitting process. However, achieving direct band gap in Ge and improving the Ge light emitting efficiency are still huge challenges. Indeed, Ge is normally recognized as a poor light emitting material due to its indirect band structure. The radiative recombination through the indirect band-to-band optical transition is inefficient as a result of a phonon-assisted process. The direct band-to-band optical transition in Ge is however a very fast process with a radiative recombination rate of 4–5 orders of magnitude higher than that of the indirect transition [13]. Thus, the

Of Ge can be, in principle, as efficient as that from direct gap III-V materials. The challenge is that the number of the electrons for the direct optical transition is deficient due to an indirect band structure. Fortunately, Ge is a pseudo direct band gap material because of a small energy difference (140 meV) between its direct gap and indirect gap. It has been shown that with a combination of tensile strain and

The band structure of bulk Ge is shown in **Figure 3**. The valence band is composed of a light-hole band, a heavy-hole band, and a split-off band from spin-orbit interaction. The light-hole band and the heavy-hole band are degenerate at wave

k = <111> or L point. The energy difference between the conduction band at L point and the valence band at Γ point determines the narrowest band gap in Ge:

n-type doping Ge can be engineered to be a direct band gap material.

vector k = 0 or Γ point, which is the maximum of valence band. The lowest energy point of the conduction band is located at

*1.3.1 Ge band structure at equilibrium and under injection*

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

**1.3 Ge band structure engineering**

direct gap emission

**Figure 2.** *Illustrates several key photonics devices needed to realize Si on-chip interconnections [10].*

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

use of group IV materials (such as Si, Ge, or Sn) is highly expected. In order to have an efficient light source, a direct band gap semiconductor is preferred. At least, a local minimum at the Γ point of the conduction band is required to accumulate electrons and achieve efficient radiative recombination. This requirement makes Si impossible to be an efficient light source because the difference between the direct and indirect valleys in Si is larger than 2 eV. Fortunately, germanium, which is a group IV material, has a local minimum at the Γ point of the conduction band. More attractively is that the lowest energy point in the Ge conduction band is at the L point, which is only 0.140 eV lower than the lowest energy at the Γ point at room temperature. Therefore, Ge has the potential to be engineered to become a direct band gap material and used as an on-chip integrated light source [11, 12].

#### **1.3 Ge band structure engineering**

*Silicon Materials*

**1.2 Silicon photonics**

*1.2.1 Photonics in optical communications*

*1.2.2 Key devices for Si photonics*

Photonics is about the science and the technology of generating, manipulating, and detecting photons. Silicon, while dominating the semiconductor electronics for decades, is naturally a challenge for developing photonics in many communication applications [7]. Silicon photonics offer a promising platform for the monolithic integration of optics and microelectronics, aiming for many applications including the optical interconnects solution to the microelectronics bottleneck [8]. Because of its importance and rapid growth, a roadmap of silicon photonics, assessed by academic and industrial experts, has been proposed [9]. Light or photon has been used to transmit information for over three decades, mainly by using optical fibers to form optical interconnection between places in distance. In the past three decades, photons have been widely used in optical fiber communication systems, especially in long-distance communications. The fundamental reason for the optical interconnect advantage is the zero rest mass of a photon, which can greatly reduce the required energy. In addition, compared to electrical interconnections, the most important advantage using optical fiber communications is the high speed. However, many challenges need to be solved to achieve on-chip optical interconnects. The first issue is to integrate all the optical devices with the silicon microelectronic devices on silicon chips. Another challenge is materials used in each device. For traditional optical components, III-V materials, such as GaAs and InP, are exclusively used due to their excellent optical properties. However, all the materials and their fabrication are not compatible with the Si exciting Si-CMOS technology.

The on-chip interconnect system contains an integrated light source, a modulator to transfer the electrical signal to an optical signal, a waveguide or waveguide device to direct the optical signal to the destination, and finally a photodetector to convert the optical signal back to an electrical signal. Most of these devices are

**Figure 2** Si photonics key devices are needed to realize on-chip optical interconnects: Integrated light source, modulator, waveguide and photo-detector [10]. The only missing key device is an electrically pumped laser source, which is the compatible with the integrated CMOS technology. As we have mentioned above, numerous approaches have been investigated, including porous Si, semiconducting silicides, Er-doped Si, Si nanocrystals or Ge/Si self-assembled quantum dots but none of them could produce a high emission efficiency at room temperature. From the material point of view, in order to achieve a CMOS compatible light source, the

already developed on a Si platform with high bandwidth capability.

*Illustrates several key photonics devices needed to realize Si on-chip interconnections [10].*

**72**

**Figure 2.**

As previously discussed, Ge is the most interesting group-IV material for the light emitting process. However, achieving direct band gap in Ge and improving the Ge light emitting efficiency are still huge challenges. Indeed, Ge is normally recognized as a poor light emitting material due to its indirect band structure. The radiative recombination through the indirect band-to-band optical transition is inefficient as a result of a phonon-assisted process. The direct band-to-band optical transition in Ge is however a very fast process with a radiative recombination rate of 4–5 orders of magnitude higher than that of the indirect transition [13]. Thus, the direct gap emission

Of Ge can be, in principle, as efficient as that from direct gap III-V materials. The challenge is that the number of the electrons for the direct optical transition is deficient due to an indirect band structure. Fortunately, Ge is a pseudo direct band gap material because of a small energy difference (140 meV) between its direct gap and indirect gap. It has been shown that with a combination of tensile strain and n-type doping Ge can be engineered to be a direct band gap material.

#### *1.3.1 Ge band structure at equilibrium and under injection*

The band structure of bulk Ge is shown in **Figure 3**. The valence band is composed of a light-hole band, a heavy-hole band, and a split-off band from spin-orbit interaction. The light-hole band and the heavy-hole band are degenerate at wave vector k = 0 or Γ point, which is the maximum of valence band.

The lowest energy point of the conduction band is located at k = <111> or L point. The energy difference between the conduction band at L point and the valence band at Γ point determines the narrowest band gap in Ge:

**Figure 3.** *Ge band structure at 300 K.*

Eg = 0.664 eV. Because the direct energy gap EΓ2 is much larger than EΓ1 and Eg, almost no electrons can occupy such high energy levels. Therefore, we refer direct band gap only to EΓ1 throughout this thesis. The part of the conduction band near Γ point is called direct valley and the part near L point is called indirect valley. Since the energy is 4-fold degenerate with regard to the changes of the secondary total angular-momentum quantum number, four L valleys are considered.

**Figure 4** shows the electron and the hole distributions of Ge at equilibrium at equilibrium and under injection conditions. At equilibrium, most of the thermally activated electrons occupy the lowest energy states in the indirect L valleys while it is worth noting that in a direct band gap material such as GaAs or InGaAs most of the electrons stay in the direct Γ valley.

Under injection conditions, there are a non-negligible amount of electrons in the Γ valley owing to the small energy difference (140 meV) between the direct band gap and the indirect band gap of Ge, as shown in **Figure 4(b)**. The excess electrons in the Γ valley lead to recombination with the holes in the valence band, which is a highly efficient light emission process because that the direct band-to-band radiative recombination is generally faster than the nonradiative recombinations, such as Auger and defect-assisted processes. But the overall light emission efficiency is very low because most of the injected electrons, staying in the L valleys, recombine nonradiatively due to a slower indirect phonon-assisted radiative recombination than the non-radiative recombinations. On the contrary, the light emission in a direct band gap material such as InGaAs is very efficient because almost all injected electrons are in the Γ valley thus recombine radiatively. To improve the light emission efficiency in Ge, more injected electrons are required to be pumped into Γ valley at the same carrier injection level. Thus, the band structure of Ge can be engineered to accomplish this goal.
