Silicon-based InAs quantum dot lasers operate at temperatures higher than 150°C

The China Institute of Semiconductors and the University of Chinese Academy of Sciences claim that the continuous wave (CW) operating temperature of a 1.3μm wavelength Indium Arsenide (InAs) quantum dot (QD) laser grown directly on an on-axis silicon (001) substrate reaches a record high of 150°C [Zunren Lv et al., Optics Express, v31, p24173, 2023]. Silicon (Si) devices also exhibit ultra-high thermal stability, with effectively constant threshold currents and slope efficiencies over a wide temperature range.

The team attributed their results to a combination of a low penetration dislocation density (TDD) gallium arsenide (GaAs) buffer layer, a high-gain QD active region, and P-type modulation doping.

"We believe this work demonstrates the great promise of silicon-based direct epitaxy QD lasers in enabling low-power, miniaturized and low-cost silicon photonic chips, providing a strong driving force for the development of low-cost, high-performance silicon photonic integrated circuits (PIC)," the researchers comment.

The team anticipates that low-cost, low-power, and highly integrated QD laser diodes (LD), combined with complementary metal-oxide semiconductor (CMOS) electronic processing units, could be used for high-capacity data transmission, high-performance optical computing, and high-precision light detection and ranging (LiDAR). The stability of the laser diode at high temperatures is particularly important when it is placed next to a hot high-speed CMOS computer processing unit (CPU).

The researchers used molecular beam epitaxy (MBE) on a silicon-based gallium phosphide (GaP/Si) template to produce QD laser materials (Figure 1). Templates do not contain inverting domains.

The team was particularly careful when growing the buffer layer to reduce the defect density of the upper device layer. The first three layers grow at low temperature (LT, 30nm), medium temperature (IT, 70nm) and high temperature (HT, 1500nm) of 400-500 °C, 500-600 °C and 600-700 °C, respectively. Injection of the IT step reduced the half-peak full width (FWHM) of one X-ray swing curve to 173.3 arcseconds, while the two-step 100 nm LT + 1500 nm HT buffer had a half-peak full width (FWHM) of 182.3 arcseconds. Using the three-step buffering process, the penetration dislocation density (TDD) was also reduced by 10.9% to 1.01x10 8 /cm 2.

Further measures to promote dislocation annihilation include the growth of the InGaAs/GaAs strained-layer superlattice site miss filter and cyclic thermal annealing. ECCI analysis showed that the TDD of complete 3 μm GaAs buffer was 4.3x106 / cm-2. According to AFM, the surface roughness root mean square is 2.46nm.

The active region in the device layer consists of eight layers of self-assembled InAs/GaAs QD. The InAs points in the first layer are covered by a 4nm InGaAs strain reduction layer and 45nm GaAs. These layers are separated by a 6nm P-type modulated doping barrier. The hole concentration is of the order of 1x10 18 /cm, or about 14 holes per QD.

The coating layer is suitably doped 1.4μm aluminum gallium arsenide (Al 0.4 Ga 0.6 As). The top contact layer consists of 300nm p-GaAs.

The material is fabricated into a ridged waveguide laser using titanium/platinum/gold (Ti/Pt/Au) and gold-germanium/nickel/gold (AuGe/Ni/Au) as P-type and N-type electrodes, respectively. Electrical isolation is provided by 350nm silica. The material was thinned to 100 nanometers thick and split to produce different cavity lengths. One side is coated with 95% reflective material.

The room temperature CW threshold current density for the 6 μmx1000 μm laser is 933.3A/cm 2, compared to 654.9A/cm 2 for the wider 30 μmx1500 μm device. The emission wavelength is about 1316nm. The wider laser diode achieved an optical output power saturation of 91.6mW and A slope efficiency of 0.2W/A. The narrower device has A slightly higher slope efficiency of 0.22W/A and an output power of almost 50mW.

The 6 μmx2000 μm device is capable of sustaining continuous laser emission at temperatures up to 150°C with a maximum output of 0.13mW (Figure 2). "To our knowledge, these results represent the highest O-band CW operating temperature of any laser grown directly on a silicon substrate, with 119°C for edge cut silicon and 108°C for on-axis silicon," the researchers comment.

The device's emission wavelength redshifts to longer wavelengths at higher temperatures, from 1313.1nm at 15°C to 1345nm at 85°C. At 125℃ and 145℃, the wavelength reaches 1375nm and 1377.5nm, respectively.

To avoid self-heating effects, the team also studied lasers under pulsed current injection at different temperatures (Figure 3). For example, at 150°C, a 6 μmx2000 μm QD laser has a saturated light output power of 12.3mW, instead of 0.13mW for CW operation.

The threshold current for pulsed operation is essentially constant in the range of 10-75 °C, corresponding to an infinite characteristic temperature (T 0 =∞).

"This is consistent with the highest temperature stability results reported for GAAS-based QD lasers and has important value for silicon PIC," the researchers comment.

The characteristic temperature (T1) at which the slope efficiency decreases with temperature is infinite, i.e. there is no decline over a wider range of 10-140 °C.

At pulse injection, the wider 10 μmx2000 μm QD laser has an output power of more than 25 MW at 150°C and more than 2mW at 160°C.

The final change reported by the team is to reduce the number of quantum dot layers to five. Comparing the performance of 10 μmx2000 μm devices, the pulse threshold current density of 5 layers at 25℃ is 90A/cm 2, which is slightly lower than the threshold of 8 layers 99A/cm 2. However, the 5-layer laser diode only reached 120°C and failed to emit the laser. The T 0 characteristic is also reduced from infinity for 8-layer devices in the 15-75°C range to 142K for 5-layer devices.

"These results suggest that high-quality multilayer quantum dots can significantly improve the high-temperature performance of silicon-based devices," the team comments.

Source: Laser Network