The graded-gap Alx Ga1 − x As–GaAs heterojunction

Abstract

The electrical properties of n‐n (AlGa)As–GaAs heterojunctions and the photoresponse of n‐p and p‐n (AlGa)As–GaAs heterojunctions, all grown by liquid‐epitaxy techniques, are explained in terms of a graded‐gap model of the heterojunction interface. This model, in which the grading occurs over a few hundred angstroms now makes possible a consistent explanation of the operation of double‐heterostructure lasers in contrast to the abrupt model of the heterojunction which predicts a series resistance due to reverse biasing of the n‐nheterojunction. Most n‐nheterojunctions measured were Ohmic with the percentage of non‐Ohmic (rectification ratio ∼ 5 : 1) junctions decreasing with increasing growth temperature. The grading and its statistical nature are explained in terms of the initial nonequilibrium growth condition in liquid‐phaseepitaxy when the (AlGa)As melt is placed in contact with a GaAs solid rather than the (AlGa)As solid alloy determined from the equilibrium phase diagram. The measured photoresponse of the n‐p junction extends from the (AlGa)As band energy to the GaAs band energy as predicted by the graded‐gap model and is not consistent with previous abrupt models in which there is a spike in the conduction band.