Concentration dependent Zn diffusion in InP during metalorganic vapor phase epitaxy

Abstract

Concentration dependent diffusion of Zn during metalorganic vapor phase epitaxy from a Zn-doped InP layer into the adjacent undoped InP buffer layer were studied systematically using secondary ion mass spectroscopy and carrier concentration profiling. Under the condition that the growth rate of the Zn-doped film is faster than the interdiffusion of Zn into the underlying undoped buffer layer, the diffusion problem can be treated as a one-dimensional diffusion couple between two semi-infinite media. Furthermore, Zn diffusion during the optimized growth condition for InP completely eliminates the thermal decomposition problem encountered in the sealed ampoule and open tube diffusions and also maintains all the intrinsic point defects at their thermodynamic equilibrium concentrations. With an optimal growth temperature at 625 °C and a maximum Zn flow below the incorporation limit for substitutional Zn to ensure that the dominant Zn are incorporated substitutionally, the diffusion profiles of Zn across the interface in this simple and clean system are simulated using a concentration dependent diffusivity. A third power concentration dependence of the effective diffusion coefficient has been confirmed, which applies to both Frank–Turnbull and kickout interstitial-substitutional equilibrium mechanisms using an interstitial-substitutional diffusion model. This indicates a +2 charge state of the fast diffusing Zn interstitials. The extrapolated curve into high-concentration diffusion source regime used by sealed ampoule diffusion experiments generally agrees with the published results although the dominant Zn atoms found in the high-concentration diffusion source regime form complexes with phosphorous vacancies in a neutral state. The enhanced diffusion due to excess interstitials is discussed.