Laser Threshold

A high-growth-temperature GaAs spacer layer (HGTSL) is shown to significantly improve the performance of 1.3 mm multilayer InAs/GaAs quantum-dot (QD) lasers. The HGTSL inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics and hence improved performance of QD lasers. To further reduce the threshold current density and improve the room-temperature characteristic temperature (T 0 ), the highreflection (HR) coating and p-type modulation doping have been incorporated with the HGTSL technique. A very low continuous-wave room-temperature threshold current of 1.5 mA and a threshold current density of 18.8 A cm 22 are achieved for a three-layer device with a 1 mm HR/HR cavity, while a very low threshold current density of 48 A cm 22 and a negative T 0 are achieved in the p-doped lasers.

Amodification of the thickness of the low-growth-temperature component of the GaAs spacer layers in multilayer 1.3um InAs/GaAs quantum-dot (QD) lasers has been used to significantly improve device performance. For a p-doped seven-layer device, a reduction in the thickness of this component from 15 to 2 nm results in a reduced reverse bias leakage current and an increase in the intensity of the spontaneous emission. In addition, a significant reduction of the threshold current density and an increase of the external differential efficiency at room temperature are obtained. These improvements indicate a reduced defect density, most probably a combination of the selective elimination of a very low density of dislocated dots and a smaller number of defects in the thinner low-growth-temperature component of the GaAs spacer layer.

The combination of a high-growth-temperature GaAs spacer layer and p-type modulation doping has been utilised to obtain a low-threshold- current-density, high-temperature-stability 1.3 mm InAs/GaAs quantum-dot laser. A room-temperature threshold current density of 48 A/cm2 and a negative characteristic temperature over the tempera- ture range from -50 to 40oC is achieved for a five-layer device with as-cleaved facets.

The relative contributions of the photon and thermal coupling mechanisms to the behavior of self-assembled InAs/GaAs quantum dot lasers are studied. A theoretical model, which takes into account a photon coupling process between the ground and first excited states of different sized dots, is proposed to fully explain the temperature dependence of the threshold current density (Jth) of both undoped and p-doped lasers. The simu- lation results suggest that the carrier distribution between the different energy states in a dot is modulated by the intradot thermal excitation of carriers. This process, when combined with the photon coupling mechanism, can account for the negative characteristic temperature (T0) appearing in different temperature ranges for undoped and p-doped devices. Thermal coupling, which involves thermal carrier escape and recapture among different dots, has also been studied. Below threshold, thermal coupling is found to be significant but is weakened as threshold is approached because of the decreased carrier lifetime. Near and above threshold, the photon coupling mechanism is important and can be used to model the different temperature behaviors of the lasing spectra observed experimentally for the undoped and p-doped lasers.