Effect of Ga/Si interdiffusion on optical and transport properties of GaN layers grown on Si(111) by molecular-beam epitaxy

Optical and transport properties of wurtzite GaN layers, grown by plasma-assisted molecular-beam epitaxy on Si(111) substrates, have been investigated. An emission at 3.455 eV, analyzed by continuous-wave and time-resolved luminescence in undoped and Si-doped GaN layers, is assigned to excitons bound to Si donors with an optical binding energy of 50 meV. A common origin of this peak, for undoped and Si-doped GaN, is backed by secondary-ion-mass spectroscopy that evidences a Si diffusion from the substrate into the GaN layer for growth temperatures above 660 °C. Simultaneously, Ga diffusion into the Si substrate generates a highly $p$-type conductive layer at the GaN/Si interface, leading to unreliable Hall data in undoped and lightly doped layers. Positron annihilation reveals a concomitant vacancy cluster generation at the GaN/Si interface in samples grown above 660 °C. No traces of the “yellow band” are detected either in undoped or in Si-doped samples.