LOW TEMPERATURE GROWN GAAS AND ITS APPLICATION ON LASER DIODES

Abstract

The properties of low temperature (LT) grown GaAs and Al0.3Ga0.7As epilayers by molecular beam epitaxy (MBE) had been investigated. The lattice constant of the as-grown LT GaAs epilayer is estimated to be about 0.14% larger than that of the bulk GaAs. After ex situ rapid thermal annealing (RTA) at 600°C for 60 seconds, the lattice constant became the same as the bulk GaAs. Arsenic precipitates were observed in both the LT GaAs and the LT Al0.3Ga0.7As epilayers grown on GaAs substrates. However, no precipitate was observed in LT GaAs grown on Si substrates. The resistivity of LT Al0.3Ga0.7As is of the order of 109 Q-cm, which is two-order higher than the resistivity of the LT GaAs grown under the similar conditions. The luminescence efficiency of both the LT GaAs and the LT Al0.3Ga0.7As were at least 200 times weaker than that of the GaAs grown at 600°C, It was found that low temperature grown epilayers can reduce the threading dislocation density in GaAs on Si material system. Among the samples studied, the best results obtained were from the sample with two 10-pair stacks of LT GaAs/L T InxGa1-xAs (100 Å/100 Å) strained layer superlattice buffer layer. In wet selective etchant studies, succinic acid/H20 2 solution with pH value of 4.2 was found to be the most selective etchant for both the LT GaAs and the GaAs grown at 600°C over AlAs, with selectivities of greater than 1100. As for the Al0.3Ga0.7As over GaAs, the best slow etchant, KI/I2/H2O/H2SO4 solution at 0.3°C, was found to have higher than 20 selectivities for both the LT Al0.3Ga0.7As and the Al0.3Ga0.7As grown at 600°C. The effectiveness of LT GaAs and LT Al0.3Ga0.7As as current confinement layers for laser diode structures were simulated by a commercial software, MEDICI. The simulation results showed that both the LT GaAs and the LT Al0.3Ga0.7As epilayers were effective in confining the current spreading. In comparison, LT Al0.3Ga0.7As is more effective in current confinement as compared to the LT GaAs. The minimum thicknesses for LT GaAs and LT Al0.3Ga0. 7As epilayers to effectively confine the current spreading are 0.4 µm and 0.1 µm, respectively. Graded-index (GRIN) separate confinement (SCH) heterostructure single quantum well (SQW) AlxGa1-xAs/GaAs laser diodes grown on both the n+-GaAs and the n+-Si substrates were investigated. Among the laser diode samples on n+ -GaAs substrate, it was found that the samples with low temperature grown current confinement layers showed significant improvements in threshold current performance over the sample without any current confinement layer. The laser diode samples with the LT GaAs and the LT Al0.3Ga0.7As current confinement layers gave threshold current reductions of 37% and 60% over the laser diode sample without any current confinement layer. These results are attributed to the leakage currents reduced by current confinement layers. As for the laser diode samples on n+-Si substrate with different buffer layer structures, the sample with LT InxGa1-xAs/LT GaAs SLS buffer layer showed 10% lower threshold current as compared to the one with only LT GaAs buffer layer. The better result is attributed to the lower threading dislocation density. For the samples with the same buffer layer structures, the one with the LT GaAs current confinement layer showed 30% lower threshold current than the one without. In comparison, the threshold current reduction through the current confinement structure is more effective than that through the LT InxGa1-xAs /LT GaAs SLS buffer layer.