**3.1 The Development of QD laser on Si**

The advantages of high data rate, broad bandwidth, mature fabrication processes and low power consumption make Si photonics become a desirable approach, meeting the future demands of optical interconnections. To date, significant achievements have been made in Si photonics and most of key components have been well demonstrated, including low-loss waveguides, high-speed modulators and high-performance photodetectors [70–74]. However, the realization of highperformance Si-based on-chip light sources still remains challenging for the full integration of optoelectronics integrated circuits [75]. Among various of approaches, monolithically integrating high-performance III-V QD lasers on Si substrate has been considered as a promising method to develop an on-chip optical source for Si photonics [76–78]. The advanced properties of low threshold, high defects tolerance and high temperature stability contribute largely to the development of QD lasers [27, 79–81].

Before illustrating the recent progress of QD lasers grown on Si (001) substrates, it is worth to discuss briefly about some key milestones in the development of monolithic integration of III-V lasers on Si. Although some optimized heteroepitaxy techniques have reduced the TDD of III-V on Si from originally <sup>10</sup><sup>9</sup> cm<sup>2</sup> to <sup>10</sup><sup>6</sup> cm<sup>2</sup> , QW lasers directly grown on Si still suffered on their high threshold and limited lifetime due to the enhanced TD generation [82–85]. An early result presented a QW laser with InP buffer as thick as 15 μm with decent performance and lifetime [86]. However, due to the difference of thermal expansion coefficient between III-V epi-layer and Si substrate, the thick buffer is also vulnerable to the formation of micro-cracks, which destroys the yield of Si-based devices [87]. The research of III-V QD lasers on Si (001) comes out since early 2000s. After the early attempt by using droplet epitaxy to grow QD lasers, the successful address of Stranski-Krastanov growth mode on the growth of QDs presents significant advantages on emitting light with the presence of high TDD caused by mismatch in lattice constants and thermal expansion coefficients [88, 89]. By taking the benefits of ultra-high vacuum and precise control, MBE system has been widely considered as a suitable technique for the growth of high-performance QDs.

Recently, numerous achievements that pursuing high performance III-V QD lasers on Si have been demonstrated. The offcut Si substrate was addressed initially to prevent the formation of APB. In 2001, the first QD laser on Si emitting at 855 nm at room temperature under continuous-wave operation was presented by growing InGaAs QDs on Si substrate with MOCVD [90]. More importantly, the aging test illustrated the advantage of reliability for QD lasers on Si compared with QW counterparts. By further optimizing the active region and III-V buffer, such as utilizing DFLs and P-type modulation doped QD region, the performance of QD lasers on Si was highly improved, realizing a characteristic temperature (T0) of 244 K between operation temperature of 25–95°C and a reduced threshold current density of 900A/cm<sup>2</sup> at that time [48, 91]. These results suggest the possibility of QD lasers directly grown on Si substrate as an efficient and reliable light source for Si photonics.

The aforementioned works of QD lasers were all operated under emission of 1.1 μm. However, the recent ever-growing demands on telecommunication and data-communication system, led to significant achievements on 1.3 μm InAs/GaAs QD lasers on Si substrate. The first room temperature 1.3 μm emission of QDs on Si grown by MOCVD was achieved by Li et al. at 2008, with the help of Sb [92].

The SAE approach entails a large number of variants, including epitaxial lateral overgrowth ELO [67] and confined epitaxial lateral overgrowth (CELO) [68]. These alternatives often use a "3D" confinement of defects. However, if they are in certain cases, very efficient, they generally require a complex and cost consuming patterning of the substrates. For an overview of the latter methods one can refer to the

*Spatial correlation between mappings: (a) cross section STEM, (b) top-view CL intensity, (c) CL peak positions, and (d) cross section ε [110], ε[001], and ε [110, 001] strain distortions realized on a single III-V QWF. The high luminescent area is bounded by dislocations d3 and d4 and associated with a peak position shift toward higher wavelength. ε [110] shows a 0.5% distortion along this III-V QWF, and no significant distortion*

*(a) low magnification cross-sectional STEM image of a GaAs layer grown in 140 nm wide SiO2 trenches on (001)-oriented Si substrate showing a good uniformity of the selective growth. The trenches are oriented along the [1–10] direction and are 180 nm deep. (b) Normalized room temperature lPL spectra of different InGaAs. QWs having different composition of Indium of (#1) 10%, (#2) 20%, (#3) 30%, and (#4) 40%. (c) Crosssectional TEM image of the top layers showing the stack of GaAs/AlAs/InGaAs/AlAs/GaAs layers with no*

*crystalline defects. (d) 5 K panchromatic CL mapping of the nanoribbons array. From [64].*

**In the past 30 years, efforts have been made to decrease the TDs induced by the lattice mismatch between GaAs and Si. Introduction of DFL method as well as the use of Aspect Ratio trapping method allow to decrease the threading disloca-**

**–106 cm**�**<sup>2</sup> range, value required to obtain efficient devices.**

references [59, 69].

*for ε[001] and ε [110, 001]. From [64].*

**tion density in the 105**

**2.4 Summary**

**148**

**Figure 23.**

**Figure 22.**

*Post-Transition Metals*

However, due to the high TDD in the GaAs buffer, its PL intensity was eight times weaker than QDs grown on the native GaAs substrate, even with a high QD density obtained of 7 <sup>10</sup><sup>10</sup> cm<sup>2</sup> . This also suggested the importance of developing improved GaAs buffer on Si substrate associated with well-performed DFLs. The first electrically pumped 1.3 μm InAs/GaAs QD laser directly grown on Si substrate by MBE was successfully demonstrated by Wang et al. in 2011 [93]. The laser structure was grown on an offcut Si substrate with 4° miscut angle to [110] orientation. An improvement initialized from the growth of AlAs nucleation layer instead of GaAs nucleation layer, realizing a reduction of defects observed at the interface of AlAs/Si [94]. The threshold current density was reduced to 725A/cm<sup>2</sup> at room temperature under pulsed operation, with a single facet output power of 26 mW achieved at room temperature. The highest operation temperature was 42°C with a T0 of 44 K.

300 nm GaAs spacer layer. The TDD after DFLs was successfully reduced to the

*GaAs Compounds Heteroepitaxy on Silicon for Opto and Nano Electronic Applications*

DWELL active region was developed upon this platform. A TEM image of active region was shown in **Figure 24b** where QDs were coherently grown, without any visible defects. The two inset images presented a 1 � <sup>1</sup> <sup>μ</sup>m<sup>2</sup> AFM image which show a good uniformity with 3 � <sup>10</sup><sup>10</sup> cm�<sup>2</sup> dot density and the typical shape of a single QD. Broad-area lasers were fabricated as shown schematically by a scanning electron microscope (SEM) image in **Figure 24c**. The light-current–voltage curve of the device was shown in **Figure 24d**. An ultra-low threshold current density of 62.5 A/ cm2 under continuous wave at room temperature was obtained, which was the lowest threshold current density value achieved for any kind of lasers on Si substrate at that time. The single facet output power measured under injection current density of 650 A/cm<sup>2</sup> was exceeded 105 mW. The highest operation can be achieved up to 75°C under continuous-wave mode and 120°C under pulsed mode. Moreover, negligible degradation was observed after 3100 h aging test, realizing an extraordinaire mean time to failure lifetime more than 100,158 h. After that, Si-based monolithically integrated narrow-ridge Fabry-Perot and distributed feedback QDs laser

These previous discussions on QD lasers were all fabricated on offcut Si substrate, which are not fully compatible to the CMOS technique. The commercialized on-axis Si (001) platform demands an miscut angle less than 0.5°. As discussed in the first section of this chapter, the heteroepitaxy technique on on-axis Si (001) was satisfied by forming APB-free GaAs buffer. Beyond the successful demonstration of QD lasers on offcut Si platform, QD lasers grown on CMOS-compatible Si (001) substrate were successfully developed in recent years [98–105], owing to the dem-

The first electrically pumped continuous-wave InAs/GaAs QD laser monolithically grown on-axis GaAs/Si (001) substrate was demonstrated in 2017 [98]. Following by a 400 nm APB-free on-axis GaAs/Si (001) platform grown by MOCVD, MBE system was employed to grow QD laser structure with four repeats of DFLs, which consist of five sets of InGaAs/GaAs SLSs. The five stacks of InAs/GaAs QD layers sandwiched by AlGaAs cladding layers were grown subsequently. A <sup>1</sup> � <sup>1</sup> <sup>μ</sup>m<sup>2</sup> AFM image of uncapped InAs QD on Si (001) substrate was shown in **Figure 25a**, realizing a good uniformity and a dot density of �3.5 � 1010 cm�<sup>2</sup>

sample was fabricated to broad-area laser devices with as-cleaved facets for laser characteristic measurements. A comparison of room-temperature continuous-wave light-current–voltage characteristics between QD laser on on-axis GaAs/Si (001) platform and native GaAs substrate was shown in **Figure 25b**. The GaAs-based QD

quantum efficiency of GaAs-based QD laser were � 0.12 W/A and 12.7%, respectively. The QD laser on on-axis Si (001) also show decent results on corresponding characteristics, which the calculated slope-efficiency was 0.068 W/A and differential quantum efficiency was 7.2%. **Figure 25c** presents a temperature dependent light-current curve of Si-based QD laser operated under continuous-wave mode. The maximum operating temperature achieved was 36°C. As shown in **Figure 25d**, the pulsed results of light-current characteristic at various heatsink temperature presented a highest operation temperature of 102°C, which was the first demonstration of QD laser directly grown on on-axis Si (001) substrate that observed lasing over 100°C. The inset image of **Figure 25d** shows the characteristic

. The

, while that of on-axis Si-

. The calculated slope-efficiency and differential

are fabricated based on these outstanding outcome [96, 97].

**3.2 InAs/GaAs QD laser on on-axis Si (001) substrate**

onstration of the APB-free GaAs and GaP templates on Si.

laser presented a threshold current density of 210 A/cm2

based QD laser was 425 A/cm<sup>2</sup>

**151**

. High-performance laser structure with five stacks of InAs/GaAs

level of 10<sup>5</sup> cm�<sup>2</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.94609*

Extensive studies were devoted following the first demonstration of electrically pumped 1.3 μm QD laser on Si. In 2012, by utilizing Ge-on-Si virtual substrate, the first room-temperature continuous-wave electrically pumped InAs/GaAs QD laser monolithically grown on Si substrate with a Ge buffer layer was demonstrated by MBE [95]. A low threshold current density of 162 A/cm<sup>2</sup> was achieved at continuous-wave mode with a room temperature lasing emission of 1.28 μm. The operation temperature was as high as 84°C under pulsed mode. Although these results were outstanding, a 1.3 μm InAs/GaAs QD laser directly grown on Si substrate was still far from practice, until the successful demonstration by us in 2016. By applying unique epitaxial method and improved fabrication process, the first high-performance and long-lifetime 1.3 μm QD laser directly grown on Si was achieved [29]. As shown in **Figure 24a, 1** μm GaAs buffer was grown by three steps on a deoxidized Si substrate to improve the material quality, followed by four sets of DFLs consisted of five sets of InGaAs/GaAs SLSs and high temperature annealed

#### **Figure 24.**

*(a) TEM image of GaAs buffer on Si including dislocation filter layers. (b) TEM image of active region, upper inset: 1 <sup>1</sup> <sup>μ</sup>m<sup>2</sup> AFM image of uncapped QDs, and bottom inset: TEM image of a single QD. (c) SEM image of fabricated broad-area laser. (d) Light-current–voltage curve of lasing characteristics under continuous-wave condition at room temperature. Reproduce from [29].*

*GaAs Compounds Heteroepitaxy on Silicon for Opto and Nano Electronic Applications DOI: http://dx.doi.org/10.5772/intechopen.94609*

300 nm GaAs spacer layer. The TDD after DFLs was successfully reduced to the level of 10<sup>5</sup> cm�<sup>2</sup> . High-performance laser structure with five stacks of InAs/GaAs DWELL active region was developed upon this platform. A TEM image of active region was shown in **Figure 24b** where QDs were coherently grown, without any visible defects. The two inset images presented a 1 � <sup>1</sup> <sup>μ</sup>m<sup>2</sup> AFM image which show a good uniformity with 3 � <sup>10</sup><sup>10</sup> cm�<sup>2</sup> dot density and the typical shape of a single QD. Broad-area lasers were fabricated as shown schematically by a scanning electron microscope (SEM) image in **Figure 24c**. The light-current–voltage curve of the device was shown in **Figure 24d**. An ultra-low threshold current density of 62.5 A/ cm2 under continuous wave at room temperature was obtained, which was the lowest threshold current density value achieved for any kind of lasers on Si substrate at that time. The single facet output power measured under injection current density of 650 A/cm<sup>2</sup> was exceeded 105 mW. The highest operation can be achieved up to 75°C under continuous-wave mode and 120°C under pulsed mode. Moreover, negligible degradation was observed after 3100 h aging test, realizing an extraordinaire mean time to failure lifetime more than 100,158 h. After that, Si-based monolithically integrated narrow-ridge Fabry-Perot and distributed feedback QDs laser are fabricated based on these outstanding outcome [96, 97].

#### **3.2 InAs/GaAs QD laser on on-axis Si (001) substrate**

These previous discussions on QD lasers were all fabricated on offcut Si substrate, which are not fully compatible to the CMOS technique. The commercialized on-axis Si (001) platform demands an miscut angle less than 0.5°. As discussed in the first section of this chapter, the heteroepitaxy technique on on-axis Si (001) was satisfied by forming APB-free GaAs buffer. Beyond the successful demonstration of QD lasers on offcut Si platform, QD lasers grown on CMOS-compatible Si (001) substrate were successfully developed in recent years [98–105], owing to the demonstration of the APB-free GaAs and GaP templates on Si.

The first electrically pumped continuous-wave InAs/GaAs QD laser monolithically grown on-axis GaAs/Si (001) substrate was demonstrated in 2017 [98]. Following by a 400 nm APB-free on-axis GaAs/Si (001) platform grown by MOCVD, MBE system was employed to grow QD laser structure with four repeats of DFLs, which consist of five sets of InGaAs/GaAs SLSs. The five stacks of InAs/GaAs QD layers sandwiched by AlGaAs cladding layers were grown subsequently. A <sup>1</sup> � <sup>1</sup> <sup>μ</sup>m<sup>2</sup> AFM image of uncapped InAs QD on Si (001) substrate was shown in **Figure 25a**, realizing a good uniformity and a dot density of �3.5 � 1010 cm�<sup>2</sup> . The sample was fabricated to broad-area laser devices with as-cleaved facets for laser characteristic measurements. A comparison of room-temperature continuous-wave light-current–voltage characteristics between QD laser on on-axis GaAs/Si (001) platform and native GaAs substrate was shown in **Figure 25b**. The GaAs-based QD laser presented a threshold current density of 210 A/cm2 , while that of on-axis Sibased QD laser was 425 A/cm<sup>2</sup> . The calculated slope-efficiency and differential quantum efficiency of GaAs-based QD laser were � 0.12 W/A and 12.7%, respectively. The QD laser on on-axis Si (001) also show decent results on corresponding characteristics, which the calculated slope-efficiency was 0.068 W/A and differential quantum efficiency was 7.2%. **Figure 25c** presents a temperature dependent light-current curve of Si-based QD laser operated under continuous-wave mode. The maximum operating temperature achieved was 36°C. As shown in **Figure 25d**, the pulsed results of light-current characteristic at various heatsink temperature presented a highest operation temperature of 102°C, which was the first demonstration of QD laser directly grown on on-axis Si (001) substrate that observed lasing over 100°C. The inset image of **Figure 25d** shows the characteristic

However, due to the high TDD in the GaAs buffer, its PL intensity was eight times weaker than QDs grown on the native GaAs substrate, even with a high QD density

improved GaAs buffer on Si substrate associated with well-performed DFLs. The first electrically pumped 1.3 μm InAs/GaAs QD laser directly grown on Si substrate by MBE was successfully demonstrated by Wang et al. in 2011 [93]. The laser structure was grown on an offcut Si substrate with 4° miscut angle to [110] orientation. An improvement initialized from the growth of AlAs nucleation layer instead of GaAs nucleation layer, realizing a reduction of defects observed at the interface of AlAs/Si [94]. The threshold current density was reduced to 725A/cm<sup>2</sup> at room temperature under pulsed operation, with a single facet output power of 26 mW achieved at room temperature. The highest operation temperature was

Extensive studies were devoted following the first demonstration of electrically pumped 1.3 μm QD laser on Si. In 2012, by utilizing Ge-on-Si virtual substrate, the first room-temperature continuous-wave electrically pumped InAs/GaAs QD laser monolithically grown on Si substrate with a Ge buffer layer was demonstrated by

MBE [95]. A low threshold current density of 162 A/cm<sup>2</sup> was achieved at

continuous-wave mode with a room temperature lasing emission of 1.28 μm. The operation temperature was as high as 84°C under pulsed mode. Although these results were outstanding, a 1.3 μm InAs/GaAs QD laser directly grown on Si substrate was still far from practice, until the successful demonstration by us in 2016. By applying unique epitaxial method and improved fabrication process, the first high-performance and long-lifetime 1.3 μm QD laser directly grown on Si was achieved [29]. As shown in **Figure 24a, 1** μm GaAs buffer was grown by three steps on a deoxidized Si substrate to improve the material quality, followed by four sets of DFLs consisted of five sets of InGaAs/GaAs SLSs and high temperature annealed

*(a) TEM image of GaAs buffer on Si including dislocation filter layers. (b) TEM image of active region, upper inset: 1 <sup>1</sup> <sup>μ</sup>m<sup>2</sup> AFM image of uncapped QDs, and bottom inset: TEM image of a single QD. (c) SEM image of fabricated broad-area laser. (d) Light-current–voltage curve of lasing characteristics under continuous-wave*

. This also suggested the importance of developing

obtained of 7 <sup>10</sup><sup>10</sup> cm<sup>2</sup>

*Post-Transition Metals*

42°C with a T0 of 44 K.

**Figure 24.**

**150**

*condition at room temperature. Reproduce from [29].*

on-axis Si (001) and native GaAs substrates was shown in **Figure 26b**, the inset image shows an AFM image of uncapped InAs/GaAs QD layer with about

*GaAs Compounds Heteroepitaxy on Silicon for Opto and Nano Electronic Applications*

60.8 K between 16–36°C.

670 A/cm<sup>2</sup>

**Figure 27.**

**153**

and thermal reliability have been proved.

*DOI: http://dx.doi.org/10.5772/intechopen.94609*

on-chip optical source for Si photonics.

<sup>4</sup> <sup>10</sup><sup>10</sup> cm<sup>2</sup> dot density. The laser samples were fabricated into 50 <sup>μ</sup><sup>m</sup> 3 mm broad-area laser devices. The characterization of laser devices was all measured under continuous wave. As shown in the inset image of **Figure 26c**, the threshold current density as low as 160 A/cm<sup>2</sup> has been achieved at room temperature, which was improved compare to previous result. A single facet output power of 48 mW was obtained at an injection current density of 500 A/cm<sup>2</sup> without any thermal rollover. The threshold current density increased with the rising of operation temperature and laser operation was observed up to 52°C. The T0 obtained was

In order to investigate the defect tolerance of QD and QW structure, an InAs/ GaAs QD laser directly grown on on-axis GaAs/Si (001) platform and an InGaAs QW laser in the same structure except active region were grown for comparison [100]. By further analyzing the performance of QD and QW laser and their thermal activation energy (Ea), the great characteristics of QD laser on dislocation tolerance

Temperature dependent PL measurements were performed for both QD and QW samples. As shown in **Figure 27a**, PL intensity of the QD sample at roomtemperature was about six times lower than the PL intensity at 20 K. In contrast, the difference for the QW sample shown in **Figure 27b** was 1000 times between 20 K and room temperature. Moreover, the integrated PL intensity for both samples was measured in order to estimate their Ea. The results were shown in **Figure 27c**, which were 240 meV and 35 meV for QD and QW lasers, respectively. The significantly higher Ea observed for the QD could contribute to its higher optical intensity at high temperatures. As shown in **Figure 28a, 25** μm 3 mm broad-area lasers were fabricated for both QD and QW samples. The room-temperature characteristics of them under continuous-wave mode were illustrated in **Figure 28b**. The threshold current density of 173 A/cm<sup>2</sup> for QD laser was achieved. In addition, over

100 mW single-facet output power was obtained under injection current density of

temperature even at higher injection levels. After comparing with modelling results, this study indicated that QW laser cannot work properly above 10<sup>7</sup> cm<sup>2</sup> of TDD [100, 101]. **Figure 28c** presented a temperature dependent light-current curve of QD laser on Si (001). The highest continuous-wave operation was observed over 65°C. These results quantitively suggested that QD laser had its natural advantages on defect tolerance and temperature insensitivity. It also demonstrated that QD laser monolithically integrated on on-axis Si (001) substrate can be a promising

*Comparison of PL spectra at room-temperature (300 K) and 20 K for (a) the QD laser, and (b) the QW laser. (c) Temperature-dependent integrated PL intensities of the InAs QD and InGaAs QW lasers from the*

*temperature region of 20 K to 300 K, showing Ea of both samples. Reproduce from [100].*

. In contrast, there was no lasing observed for the QW device at room-

#### **Figure 25.**

*(a) 1 <sup>1</sup> <sup>μ</sup>m<sup>2</sup> AFM image of uncapped InAs QDs grown on on-axis Si (001) substrate. (b) Light-current– voltage characteristic comparison of an InAs/GaAs QD laser grown on on-axis Si (001) and native GaAs substrate at room temperature under continuous-wave operation. (c) Single facet light-current curve for InAs/ GaAs QD laser on on-axis Si (001) as a function of temperature under continuous-wave operation, inset: lightcurrent curve at a heat sink temperature of 36°C. (d) Single facet light-current curve for InAs/GaAs QD laser grown on on-axis Si (001) substrate at different heat sink temperatures under pulsed condition, inset: natural logarithm of threshold current density against temperature in the ranges of 16–102°C. Reproduce from [98].*

temperature T0 of 32 K between 16–102°C. This result is further improved by K. Li et al. with an optimized DFLs and QDs [99].

As shown in **Figure 26a**, four repeats of In0.18Ga0.82As/GaAs SLSs DFLs were well performed to annihilate TDs with total buffer thickness of 2 μm. The active region of laser was consisted of five repeats of InAs/GaAs DWELL structure, realizing a room temperature peak PL emission of 1308 nm with a linewidth of 32 meV. A comparison of room temperature PL results of InAs/GaAs QD on

#### **Figure 26.**

*(a) Cross-sectional TEM image for whole buffer; (b) A comparison of room temperature PL results, inset: an AFM image of uncapped InAs/GaAs QD layer; (c) Light-current characteristics of InAs/GaAs QD laser grown on Si exact (001) at various operation temperature, inset: light-current–voltage characteristic at room temperature. Reproduce from [99].*

on-axis Si (001) and native GaAs substrates was shown in **Figure 26b**, the inset image shows an AFM image of uncapped InAs/GaAs QD layer with about <sup>4</sup> <sup>10</sup><sup>10</sup> cm<sup>2</sup> dot density. The laser samples were fabricated into 50 <sup>μ</sup><sup>m</sup> 3 mm broad-area laser devices. The characterization of laser devices was all measured under continuous wave. As shown in the inset image of **Figure 26c**, the threshold current density as low as 160 A/cm<sup>2</sup> has been achieved at room temperature, which was improved compare to previous result. A single facet output power of 48 mW was obtained at an injection current density of 500 A/cm<sup>2</sup> without any thermal rollover. The threshold current density increased with the rising of operation temperature and laser operation was observed up to 52°C. The T0 obtained was 60.8 K between 16–36°C.

In order to investigate the defect tolerance of QD and QW structure, an InAs/ GaAs QD laser directly grown on on-axis GaAs/Si (001) platform and an InGaAs QW laser in the same structure except active region were grown for comparison [100]. By further analyzing the performance of QD and QW laser and their thermal activation energy (Ea), the great characteristics of QD laser on dislocation tolerance and thermal reliability have been proved.

Temperature dependent PL measurements were performed for both QD and QW samples. As shown in **Figure 27a**, PL intensity of the QD sample at roomtemperature was about six times lower than the PL intensity at 20 K. In contrast, the difference for the QW sample shown in **Figure 27b** was 1000 times between 20 K and room temperature. Moreover, the integrated PL intensity for both samples was measured in order to estimate their Ea. The results were shown in **Figure 27c**, which were 240 meV and 35 meV for QD and QW lasers, respectively. The significantly higher Ea observed for the QD could contribute to its higher optical intensity at high temperatures. As shown in **Figure 28a, 25** μm 3 mm broad-area lasers were fabricated for both QD and QW samples. The room-temperature characteristics of them under continuous-wave mode were illustrated in **Figure 28b**. The threshold current density of 173 A/cm<sup>2</sup> for QD laser was achieved. In addition, over 100 mW single-facet output power was obtained under injection current density of 670 A/cm<sup>2</sup> . In contrast, there was no lasing observed for the QW device at roomtemperature even at higher injection levels. After comparing with modelling results, this study indicated that QW laser cannot work properly above 10<sup>7</sup> cm<sup>2</sup> of TDD [100, 101]. **Figure 28c** presented a temperature dependent light-current curve of QD laser on Si (001). The highest continuous-wave operation was observed over 65°C. These results quantitively suggested that QD laser had its natural advantages on defect tolerance and temperature insensitivity. It also demonstrated that QD laser monolithically integrated on on-axis Si (001) substrate can be a promising on-chip optical source for Si photonics.

#### **Figure 27.**

*Comparison of PL spectra at room-temperature (300 K) and 20 K for (a) the QD laser, and (b) the QW laser. (c) Temperature-dependent integrated PL intensities of the InAs QD and InGaAs QW lasers from the temperature region of 20 K to 300 K, showing Ea of both samples. Reproduce from [100].*

temperature T0 of 32 K between 16–102°C. This result is further improved by K. Li

*(a) 1 <sup>1</sup> <sup>μ</sup>m<sup>2</sup> AFM image of uncapped InAs QDs grown on on-axis Si (001) substrate. (b) Light-current– voltage characteristic comparison of an InAs/GaAs QD laser grown on on-axis Si (001) and native GaAs substrate at room temperature under continuous-wave operation. (c) Single facet light-current curve for InAs/ GaAs QD laser on on-axis Si (001) as a function of temperature under continuous-wave operation, inset: lightcurrent curve at a heat sink temperature of 36°C. (d) Single facet light-current curve for InAs/GaAs QD laser grown on on-axis Si (001) substrate at different heat sink temperatures under pulsed condition, inset: natural logarithm of threshold current density against temperature in the ranges of 16–102°C. Reproduce from [98].*

As shown in **Figure 26a**, four repeats of In0.18Ga0.82As/GaAs SLSs DFLs were well performed to annihilate TDs with total buffer thickness of 2 μm. The active region of laser was consisted of five repeats of InAs/GaAs DWELL structure, realizing a room temperature peak PL emission of 1308 nm with a linewidth of 32 meV. A comparison of room temperature PL results of InAs/GaAs QD on

*(a) Cross-sectional TEM image for whole buffer; (b) A comparison of room temperature PL results, inset: an AFM image of uncapped InAs/GaAs QD layer; (c) Light-current characteristics of InAs/GaAs QD laser grown on Si exact (001) at various operation temperature, inset: light-current–voltage characteristic at room*

et al. with an optimized DFLs and QDs [99].

**Figure 25.**

*Post-Transition Metals*

**Figure 26.**

**152**

*temperature. Reproduce from [99].*

**Figure 28.**

*(a) SEM image of an example of broad area laser fabricated by QD and QW samples with 25 μm ridge width and 3 mm cavity length. (b) Comparison of room-temperature light-current–voltage characteristics for QD and QW lasers directly grown on on-axis Si (001) substrate. (c) Temperature-dependent light-current measurement of the QD laser under continuous-wave mode. Reproduce from [100].*

89 nm between adjacent whispering gallery modes. Both ground state and excited state emission were observed. A main peak wavelength of 1263 nm was located at the first excited state. **Figure 30b** shows the collected intensity and linewidth as a function of input optical power for the corresponding peak emission at 1263 nm. An ultra-low threshold of 2.6 0.4 μW and a clear narrowing trend of FWHM was obtained. The threshold of this result was even lower than the InAs QD microdisk lasers on native GaAs and InP substrates [109–111]. Additionally, the sample was fabricated into microdisk lasers with variable diameter from 1 μm to 2 μm. The corresponding threshold of main peak of microdisk lasers were presented as a function of diameter in **Figure 30c**. All the results of threshold were below 3.5 μW. The fluctuation of threshold versus the diameter of microdisk may result from the

*(a) Schematic diagram of a QD microdisk laser fabricated on on-axis Si (001) substrate. (b) SEM image of a QD microdisk laser with 1.9 μm diameter. (c) Cross section TEM image of the epitaxial structure of QD*

*GaAs Compounds Heteroepitaxy on Silicon for Opto and Nano Electronic Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.94609*

**3.4 Continuous-wave QD photonic crystal lasers on on-axis Si (001)**

crystal laser emitting at 1.3 μm on CMOS-compatible Si (001) substrate [108]. A single mode operation with ultra-low threshold down to 0.6 μm and a large

As a promising ultra-compact on-chip light source, III-V photonic crystal lasers on Si benefits on their ultralow power consumption and small footprint. Most recently, Zhou et al. demonstrated an optically pumped InAs QD photonic crystal laser on on-axis GaAs/Si (001) substrate, which was the first monolithic integration of photonic

slight factor difference in fabrication process.

*microdisk laser on on-axis Si (001) substrate. Reproduce from [107].*

**Figure 29.**

**155**

#### **3.3 Microdisk QD laser grown on on-axis Si (001) substrate**

Despite the outstanding progress has been made on edge-emitting QD lasers on on-axis GaAs/Si (001) substrate. For the dense integration with light source on Si that compatible to CMOS technique, microdisk lasers with small footprint has been considered as a promising approach for realizing nanophotonic integrated circuits. Additionally, compared with Fabry-Perot laser cavity, microdisk lasers also benefit from their advantages on low threshold and high quality factor which could bring less optical loss [106]. Recently, by applying the well-performed on-axis GaAs/Si (001) platform and optimized DFLs, a monolithically grown InAs/GaAs QD microdisk laser on on-axis Si (001) substrate with ultra-low threshold at room temperature was successfully demonstrated [107]. The device was optically pumped under continuous-wave mode. **Figure 29a** presented a schematic structure of this fabricated microdisk laser where the top of disk was the active region that consisted of three stacks of InAs/GaAs DWELL layers separated by 50 nm of GaAs space layer and 69 nm of AlGaAs cladding layer. A typical fabricated microdisk laser with disk diameter of 1.9 μm was shown in the SEM image of **Figure 29b**, which indicated a smooth etched surface with 73.5° sidewall tilt. The cross-sectional TEM image in **Figure 29c** shows the whole epilayer structure on on-axis GaAs/Si (001) substrate.

The collected lasing spectra for the microdisk laser with 1.9 μm diameter was illustrated in **Figure 30a**. The results presented a free spectral range of 76 nm –

*GaAs Compounds Heteroepitaxy on Silicon for Opto and Nano Electronic Applications DOI: http://dx.doi.org/10.5772/intechopen.94609*

#### **Figure 29.**

**3.3 Microdisk QD laser grown on on-axis Si (001) substrate**

*measurement of the QD laser under continuous-wave mode. Reproduce from [100].*

substrate.

**154**

**Figure 28.**

*Post-Transition Metals*

Despite the outstanding progress has been made on edge-emitting QD lasers on on-axis GaAs/Si (001) substrate. For the dense integration with light source on Si that compatible to CMOS technique, microdisk lasers with small footprint has been considered as a promising approach for realizing nanophotonic integrated circuits. Additionally, compared with Fabry-Perot laser cavity, microdisk lasers also benefit from their advantages on low threshold and high quality factor which could bring less optical loss [106]. Recently, by applying the well-performed on-axis GaAs/Si (001) platform and optimized DFLs, a monolithically grown InAs/GaAs QD microdisk laser on on-axis Si (001) substrate with ultra-low threshold at room temperature was successfully demonstrated [107]. The device was optically

*(a) SEM image of an example of broad area laser fabricated by QD and QW samples with 25 μm ridge width and 3 mm cavity length. (b) Comparison of room-temperature light-current–voltage characteristics for QD and*

*QW lasers directly grown on on-axis Si (001) substrate. (c) Temperature-dependent light-current*

pumped under continuous-wave mode. **Figure 29a** presented a schematic structure of this fabricated microdisk laser where the top of disk was the active region that consisted of three stacks of InAs/GaAs DWELL layers separated by 50 nm of GaAs space layer and 69 nm of AlGaAs cladding layer. A typical fabricated microdisk laser with disk diameter of 1.9 μm was shown in the SEM image of **Figure 29b**, which indicated a smooth etched surface with 73.5° sidewall tilt. The cross-sectional TEM image in **Figure 29c** shows the whole epilayer structure on on-axis GaAs/Si (001)

The collected lasing spectra for the microdisk laser with 1.9 μm diameter was illustrated in **Figure 30a**. The results presented a free spectral range of 76 nm –

*(a) Schematic diagram of a QD microdisk laser fabricated on on-axis Si (001) substrate. (b) SEM image of a QD microdisk laser with 1.9 μm diameter. (c) Cross section TEM image of the epitaxial structure of QD microdisk laser on on-axis Si (001) substrate. Reproduce from [107].*

89 nm between adjacent whispering gallery modes. Both ground state and excited state emission were observed. A main peak wavelength of 1263 nm was located at the first excited state. **Figure 30b** shows the collected intensity and linewidth as a function of input optical power for the corresponding peak emission at 1263 nm. An ultra-low threshold of 2.6 0.4 μW and a clear narrowing trend of FWHM was obtained. The threshold of this result was even lower than the InAs QD microdisk lasers on native GaAs and InP substrates [109–111]. Additionally, the sample was fabricated into microdisk lasers with variable diameter from 1 μm to 2 μm. The corresponding threshold of main peak of microdisk lasers were presented as a function of diameter in **Figure 30c**. All the results of threshold were below 3.5 μW. The fluctuation of threshold versus the diameter of microdisk may result from the slight factor difference in fabrication process.

## **3.4 Continuous-wave QD photonic crystal lasers on on-axis Si (001)**

As a promising ultra-compact on-chip light source, III-V photonic crystal lasers on Si benefits on their ultralow power consumption and small footprint. Most recently, Zhou et al. demonstrated an optically pumped InAs QD photonic crystal laser on on-axis GaAs/Si (001) substrate, which was the first monolithic integration of photonic crystal laser emitting at 1.3 μm on CMOS-compatible Si (001) substrate [108]. A single mode operation with ultra-low threshold down to 0.6 μm and a large

#### **Figure 30.**

*(a) Collected intensity as a function of wavelength with different pumped power below and above the threshold of QD microdisk laser on on-axis Si (001). (b) The corresponding collected intensity and linewidth versus pumped power for the first excited state emission at 1263 nm. (c) Threshold of main lasing peak of QD microdisk laser as a function of various diameter. Reproduce from [108].*

Si (001) substrate can be a promising on-chip optical source for Si photonics. Different approaches also provide new routes to form the basis of future monolithic light sources for the application of optical interconnects in large-scale silicon

*GaAs Compounds Heteroepitaxy on Silicon for Opto and Nano Electronic Applications*

*DOI: http://dx.doi.org/10.5772/intechopen.94609*

*(a) Schematic structure of QD photonic crystal laser on on-axis Si (001). (b) A diagram of active region in our photonic crystal laser. (c) Collected light–light curve and linewidth of the lasing peak at 1306 nm, inset: Lorentzian fitting of data below the threshold. (d) Logarithmic light–light plot of fitted and collected data.*

Heterogeneous integration of III–V compound semiconductors is promising to realize functionalities such as laser sources and photodetectors, and silicon based waveguides on Si platform. The direct heteroepitaxy of GaAs on nominal Si(100) wafers used by the microelectronics industry faces several issues to produce high quality material. In this chapter, we discussed the recent advances to tackle the formation of antiphase domains and to reduce the threading dislocation density. Currently, APB is no more an issue, as solutions have been proposed to obtain thin (<400 nm) GaAs film without APB, solutions based on dedicated cleaning and annealing processes of Si substrate before the GaAs epitaxy. The threading dislocations have hindered the development of GaAs devices on a Si CMOS platform and many solutions have been studied in th epast. We have reviewed the most efficient methods that used interchangeably the insertion of a Ge buffer between silicon and GaAs, the insertion of dislocation filter layers in the GaAs, or selective epitaxy in a cavity with a proper aspect ratio. All these progresses allowed reaching the range of

–10<sup>5</sup> cm�<sup>2</sup> TDD required to elaborate performant optoelectronics devices. Next we developed the fabrication of InAs QDs/GaAs laser emitters in the infrared region integrating GaAs buffer without APB grown on nominal Si(100) wafers and DFL to reduce the TDD. Different type of devices were fabricated such as broad area laser

optoelectronics integrated circuits.

**4. Conclusions**

*Reproduce from Ref. [108].*

**Figure 31.**

10<sup>6</sup>

**157**

coupling efficiency for room temperature spontaneous emission under continuouswave condition were achieved. 3D finite-difference time- domain (FDTD) simulation method was applied in order to obtain a high-quality factor for the resonance among QDs emission spectrum. **Figure 31a** shows a schematic structure of fabricated photonic crystal laser with 1 μm thickness of air slab underneath the cavity to enhance the vertical light confinement. The structure of active region that consists of four repeats of InAs/InGaAs/GaAs DWELL layers sandwiched by 50 nm GaAs space layers and 40 nm AlGaAs cladding layers was shown in **Figure 31b**. The collected intensity and linewidth of photonic crystal laser as a function of input power were shown in **Figure 31c**. The optically pumped QD photonic crystal lasers exhibited single-mode operation with an ultra-low threshold of �0.6 μW. The inset image shows a peak wavelength at �1306 nm with different pumped power. The Lorentzian fitting curve indicated a linewidth of �0.68 nm and a calculated cavity quality factor of 2177. The soft turn on process shown in **Figure 31c** also presented a typical behavior of laser with high spontaneous emission coupling efficiency (β). The logarithmic plot of light–light curve with fitting results of this QD photonic crystal laser were shown in **Figure 31d**. It indicated the best fitting data obtained at β = 0.18, realizing a large spontaneous emission coupling efficiency under continuous-wave condition at room temperature.

#### **3.5 Summary**

QD laser on Si has attracted great research interests in recent years, which brings new approach for achieving efficient light source of Si-based photonics integration. These works discussed in this section with established epitaxy technique of APBfree on-axis GaAs/Si (001) platform, effective DFLs and optimized QD layers demonstrate that high-performance QD laser monolithically integrated on on-axis

*GaAs Compounds Heteroepitaxy on Silicon for Opto and Nano Electronic Applications DOI: http://dx.doi.org/10.5772/intechopen.94609*

**Figure 31.**

coupling efficiency for room temperature spontaneous emission under continuouswave condition were achieved. 3D finite-difference time- domain (FDTD) simulation method was applied in order to obtain a high-quality factor for the resonance among QDs emission spectrum. **Figure 31a** shows a schematic structure of fabricated photonic crystal laser with 1 μm thickness of air slab underneath the cavity to enhance the vertical light confinement. The structure of active region that consists of four repeats of InAs/InGaAs/GaAs DWELL layers sandwiched by 50 nm GaAs space layers and 40 nm AlGaAs cladding layers was shown in **Figure 31b**. The collected intensity and linewidth of photonic crystal laser as a function of input power were shown in **Figure 31c**. The optically pumped QD photonic crystal lasers exhibited single-mode operation with an ultra-low threshold of �0.6 μW. The inset image shows a peak wavelength at �1306 nm with different pumped power. The Lorentzian fitting curve indicated a linewidth of �0.68 nm and a calculated cavity quality factor of 2177. The soft turn on process shown in **Figure 31c** also presented a typical behavior of laser with high spontaneous emission coupling efficiency (β). The logarithmic plot of light–light curve with fitting results of this QD photonic crystal laser were shown in **Figure 31d**. It indicated the best fitting data obtained at

*(a) Collected intensity as a function of wavelength with different pumped power below and above the threshold of QD microdisk laser on on-axis Si (001). (b) The corresponding collected intensity and linewidth versus pumped power for the first excited state emission at 1263 nm. (c) Threshold of main lasing peak of QD*

β = 0.18, realizing a large spontaneous emission coupling efficiency under

QD laser on Si has attracted great research interests in recent years, which brings new approach for achieving efficient light source of Si-based photonics integration. These works discussed in this section with established epitaxy technique of APBfree on-axis GaAs/Si (001) platform, effective DFLs and optimized QD layers demonstrate that high-performance QD laser monolithically integrated on on-axis

continuous-wave condition at room temperature.

*microdisk laser as a function of various diameter. Reproduce from [108].*

**3.5 Summary**

**156**

**Figure 30.**

*Post-Transition Metals*

*(a) Schematic structure of QD photonic crystal laser on on-axis Si (001). (b) A diagram of active region in our photonic crystal laser. (c) Collected light–light curve and linewidth of the lasing peak at 1306 nm, inset: Lorentzian fitting of data below the threshold. (d) Logarithmic light–light plot of fitted and collected data. Reproduce from Ref. [108].*

Si (001) substrate can be a promising on-chip optical source for Si photonics. Different approaches also provide new routes to form the basis of future monolithic light sources for the application of optical interconnects in large-scale silicon optoelectronics integrated circuits.

## **4. Conclusions**

Heterogeneous integration of III–V compound semiconductors is promising to realize functionalities such as laser sources and photodetectors, and silicon based waveguides on Si platform. The direct heteroepitaxy of GaAs on nominal Si(100) wafers used by the microelectronics industry faces several issues to produce high quality material. In this chapter, we discussed the recent advances to tackle the formation of antiphase domains and to reduce the threading dislocation density. Currently, APB is no more an issue, as solutions have been proposed to obtain thin (<400 nm) GaAs film without APB, solutions based on dedicated cleaning and annealing processes of Si substrate before the GaAs epitaxy. The threading dislocations have hindered the development of GaAs devices on a Si CMOS platform and many solutions have been studied in th epast. We have reviewed the most efficient methods that used interchangeably the insertion of a Ge buffer between silicon and GaAs, the insertion of dislocation filter layers in the GaAs, or selective epitaxy in a cavity with a proper aspect ratio. All these progresses allowed reaching the range of 10<sup>6</sup> –10<sup>5</sup> cm�<sup>2</sup> TDD required to elaborate performant optoelectronics devices. Next we developed the fabrication of InAs QDs/GaAs laser emitters in the infrared region integrating GaAs buffer without APB grown on nominal Si(100) wafers and DFL to reduce the TDD. Different type of devices were fabricated such as broad area laser

electrically pumped and operating at room temperature and up to 65°C, microdisk QDs lasers and continuous-wave QD photonic crystal lasers.

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This paves the way towards the monolithic integration of optoelectronics and microelectronics functionalities on the same silicon CMOS platform, promising tremendous evolution in the data treatment and computing fields.
