Field and Hot Carrier Enhanced Leakage in InGaAsPAnP Heterojunctions

A model calculation for the field and hot carrier enhanced electron leakage in InGaAsP/InP LED’s and lasers is presented. The significant influence of the doping level in the P-InP confining layer on leakage current is confirmed. T HE temperature sensitivity and power saturation of InGaAsP lasers and light emitting diodes (LEDs) have triggered an extensive search for the responsible mechanisms. Prime candidates are nonradiative Auger recombination and carrier leakage over the heterobarrier. These are believed to be important factors affecting the performance of the family of quaternary optoelectronic devices emitting in the spectral range of 1.1-1.6 pm. A number of new structures have been designed to isolate and thus directly demonstrate the role played by carrier leakage in LEDs [1] , [2] and lasers [3] . An interesting experimental observation of Chen et al. [3] is the continued increase of leakage current with total current above lasing threshold. Leakage models based on diffusion of electrons [4] [6] alone cannot explain the observed phenomenon, as clamping of carrier density leads to a constant leakage current above lasing threshold. However, the observed behavior can be explained by invoking the electric field dominated drift component of electron current in the cladding layer. Lee and Dentai had included the field term in their calculation of the carrier confinement factor in GaAs/GaAIAs LED’s [7], but Anthony and Schumaker [8], [9] were the first to recognize the significance of this field. However, their theory predicted a linear dependence of leakage current on inManuscript received December 16, 1982; revised April 5, 1983. This work was supported by the U.S. Office of Naval Research and the National Science Foundation. The authors are with the California Institute of Technology, Pasadena, CA 91125. jection current above threshold, contrary to the experimental result of [3] . The observed result can be explained by a theory analogous to the minority carrier injection in a Schottky diode [IO] , when the band discontinuity at the heterojunction is taken into account. We will show that in the high injection regime where lasers normally operate, the electron leakage current J , will be proportional to J p a , the injected hole current where 01 is a positive number between 2 and 3 . This would result in an “unclamped” leakage current which increases superlinearly with total injection current beyond lasing threshold. In this work, we present an electron leakage over the heterobarrier which incorporates the influence of both the electric field and carrier heating effect [ 1 11 ~ [ 123 . Under high injection conditions in double heterostructure InGaAsP LED’s and lasers, if the doping level in the P-InP cladding layer is low, it can easily be shown that there exists a substantial electric field across the InP layer. In such a case, the hole current can be written as where e is the electric charge, pp the hole mobility, NA the doping level in the P-InP layer, and 3 the electric field strength. For simplicity, only the component transverse to the junction will be considered. The ratio of the drift and diffusion components of the electron leakage current across the p-InGaAsPP-InP heterojunction is then given approximately by where L is the thickness of the cladding layer, and is assumed to be small compared to the diffusion length of electrons in the P cladding layer, p N the electron mobility, and DN the 0018-9197/83/0900-1335$01 .OO