Room-temperature self-organized in0.5Ga0.5As quantum dot lasers on silicon

Abstract

Future high-speed systems require the monolithic integration of electronic circuits and optoelectronic components on silicon substrates. An urgent need for such technology is the development of high performance and reliable electrically-injected light sources that can be integrated on silicon in a CMOS-compatible process. Recently, self-organized In(Ga)As/GaAs quantum dot (QD) lasers on GaAs and InP substrates have become appealing due to their ultra-low threshold current (Jth ≤ 100 A/cm2) and extremely high temperature stability (T0 ≈ ∞) [1, 2]. It is recognized that the strong strain field surrounding self-organized QDs will inhibit the propagation of dislocations through the dots [3]. Therefore, it is most likely that such quantum dots buried in highly mismatched layers will have a smaller defect density than quantum well (QW) active regions. In this context, we have investigated the growth and properties of self-organized InGaAs QD lasers grown on Si substrates by molecular beam epitaxy (MBE). We have achieved the first room temperature operation of an InGaAs/GaAs QD laser grown directly on Si substrates with a thin (≤ 2 μm) GaAs buffer layer, which exhibits relatively low threshold current (Jth ∼ 1500 A/cm2), high output power (> 50 mW) and high temperature operation (>100°C). A thin (≤ 2 μm) GaAs buffer layer was first grown by metal organic vapor phase epitaxy on (001)-oriented Si substrates, misoriented 4° towards [111] to eliminate the formation of antiphase domains and stacking faults at the GaAs/Si interface. This buffer layer has surface dislocation densities of 2 ∼ 5×107 cm-2. GaAs/GaAlAs separate confinement heterostructure QD lasers, as shown in Fig. 1(a), were then grown by MBE. The active region consists of three coupled In0.5Ga0.5As QD layers, separated by 350 Å GaAs barriers. The photoluminescence (PL) spectra measured at room temperature is shown in Fig. 1(b). It has a full width half maximum of 60 meV, comparable to those grown on GaAs substrates. From cross-sectional transmission electron microscopy (XTEM) of the laser structure, as shown in the inset of Fig. 1(a), it appears that the device active region may be almost defect free due to the strain field surrounding the QDs [3]. This is consistent with the strong and narrow PL spectra measured at room temperature. Single mode ridge waveguide lasers with ridge widths varying from 3 to 8 μm were fabricated by standard photolithography, wet and dry etching and contact metallization techniques. Light-current (L-I) measurements were performed under pulsed bias conditions (1% duty cycle). These lasers demonstrate threshold current ∼ 1500 A/cm2 and high output power (>50 mW), as shown in Fig. 2(a), for a 400×8 μm2 device. Temperature-dependent L-I measurements were performed on these devices, and we measured a T 0 of 103 K in the temperature range of 5 to 95°C and reasonably high output slope efficiency of 0.37 W/A, as shown in Fig. 2(b). These results are comparable to typical InGaAs QD lasers grown on GaAs substrates. We performed lifetime measurements on the lasers at 20°C under pulsed bias (1% duty cycle). Devices were operated under constant output power with the change in injection current automatically recorded. The plots of the injection current versus time for output powers of 1 mW and 10 m W are shown in Fig. 3(a) and 3(b), respectively. The injection current doubles in about 20 hours for 1 mW output and 15 hours for 10 mW output. This gradual failure is most likely due to the propagation and formation of dark-line defects in the laser active region during operation. Similar failure behavior has been reported by Groenert et. al. [4], who measured a lifetime of 4 hours for GaAs/AlGaAs QW lasers on Si substrates under continuous wave operation. It is clear that the lifetime needs to be further improved. In order to do that, we will investigate QD lasers grown directly on relaxed graded GexSi1-x buffer layers, which have lower dislocation densities (≤ 106 cm2) [4]. In summary, we have demonstrated the first room temperature operational InGaAs/GaAs QD lasers grown directly on Si substrates utilizing a thin GaAs buffer layer. These devices have displayed relatively low threshold current, high output power, and high temperature operation. The thin GaAs buffer also allows easy integration with other electronic components on Si substrates. These preliminary results show that QD lasers offer a promising route for monolithic integration on Si substrates. Improved device performance and reliability is also expected with the optimization of the GaAs buffer layer and utilization of special techniques such as tunnel injection and p-doping. Results on these devices will also be presented. © 2005 IEEE.