Zn diffusion behavior at the InGaAsP/InP heterointerface grown using MOCVD

Abstract

Zinc (Zn) diffusion through MOCVD-fabricated InP and InGaAsP layers, and the corresponding Zn doping profile at the heterojunction interface were studied as part of the doping profile control for laser diodes. It was found that the Zn doping profile has specific discontinuity at the heterojunction in InGaAsP/InP heterostructures. Using secondary ionization mass spectrometer (SIMS) and Boltzmann–Matano analysis for different composition ratios ( x) of (1− x)InP– xInGaAs epitaxial layers grown on Zn-doped InP substrates, it was found that the Zn diffusion coefficients were proportional to the square of the concentration in the InP and InGaAsP layers. The Zn diffusion coefficient strongly depends on the component ratio ( x) for the (1− x)InP– xInGaAs layer. It was concluded that this diffusion property is based on the higher stability of the substitutional Zn content in the InGaAs layer compared to that in the InP layer. The dependence of the Zn diffusion coefficient on Zn concentration in the InGaAsP/InP layers is explained based on the main diffusion source being Zn at the interstitial sites. The thermal equilibrium between Zn-interstitial and Zn-substitutional (interstitial–substitutional model) describes these Zn diffusion properties. Marked Zn concentrations at InGaAsP/InP heterojunctions are referred to as “pileups”, and are affirmed due to the difference in physical properties between InP and InGaAs. With different composition ratios ( x) of InP/InGaAsP layer growth, pre-control of the Zn concentration and strict limitations on the growth conditions before diffusion are indispensable for lasers requiring a precisely controlled doping concentration.