Gallium arsenide-based microcoolers and self-cooled laser structure
Two types of GaAs-based microcoolers have been demonstrated for the first time. One has a 2 μm thermal barrier made from a 100-period Al0.10 Ga0.90As - Al0.20Ga0.80As superlattice. Another has a thermal barrier made from a 5.2 μm Al0.10Ga 0.90As layer. Microcoolers of these two types with a size of 60 μm x 60 μm show maximum cooling temperatures around 0.8°C and 2°C at heatsink temperatures of 25°C and 100°C respectively. The microcoolers have potential to provide in situ cooling for semiconductor devices.
As an application of in situ cooling, a unipolar self-cooled laser structure is proposed and has been fabricated. The structure integrates a graded-index (GRIN) separate confining heterostructure (SCH) strained-layer InGaAs quantum well (QW) laser unit and a single-layer AlGaAs cooler using an Esaki tunnel junction as a connector. In this scheme, cooling exists on two sides of the tunnel junction. Broad-area lasers of this structure with a cavity length of 500 μm have an average threshold current density around 212 A/cm 2. A preliminary method is proposed to evaluate the cooling effect of the integrated cooler. With this method, cooling can be estimated from the movement of the spectrum of the laser, excited by pulsed current, as the pulse width varies. Using this method, potential 2∼4°C temperature reduction as a result of the integrated cooler is found in the active layer of the self-cooled laser.
Some technological platforms have been built-up to support the investigations on the microcoolers and self-cooled laser structure. Firstly, some primary MOCVD technologies such as controlling compositions, fabricating high quality interfaces, and doping have been developed for the MOCVD system itself and the compound semiconductor group. Secondly, annealing parameters—temperature and time—have been optimized for making ohmic contacts in a homemade carbon-stripe furnace. Thirdly, the SiH4 flow rate for doping the n-side of GaAs tunnel junction has been optimized to obtain a tunnel junction with low zero-bias tunnel resistance. A low zero-bias specific tunnel resistance of 9.59 × 10−5 Ω·cm2 has been achieved, which is the best reported result for the tunnel junction grown at normal growth temperatures. Theoretical evaluation and experimental results indicate that tunneling of the tunnel junction with the n-side doped with optimal SiH4 flow rate is mainly defect-assisted. Finally, a nominal-980-nm In0.2Ga0.8As GRIN-SCH strained-layer QW laser has been grown, fabricated, and characterized.
0794: Materials science