Electron leakage and the excess voltage at p-P heterojunctions in InGaAsP/InP lasers

The observation of electron leakage from the active layer to the p-binary layer in 1.3 /spl mu/m InGaAsP/InP Covered Mesa Buried Heterostructure lasers is linked to the problem of acceptor deficiency at the p-P junction via measurements of the "excess voltage", V/sub x/. The experimental method entails transparency point measurements which accurately track the active layer voltage V/sub /spl lambda// along with the terminal voltage V/sub t/ over a substantial subthreshold bias range. A plot of V/sub t/-V/sub /spl lambda// versus bias current reveals strong rectification at the heterobarrier in devices with a strong component of electron leakage. This result is consistent with the theory of hole transport across the space charge region at the p-P heterointerface. In leakage-free devices the total absence of rectification is evidenced by the subthreshold V/sub x/ measurements and also by the signature "flat" dV/dI curves exhibited by such devices above laser threshold. Extension of the transport theory to higher acceptor levels however does not satisfactorily predict the linear character of the p-P interface observed by experiment.