Density-functional investigation of the electronic structure at surfaces and interfaces of gallium phosphide

Gallium phosphide is a compound semiconductor with an indirect band gap of 2.26 eV. It is commonly employed for manufacturing low-cost red, orange, and green light-emitting diodes (LEDs) with low to medium brightness. As pure material GaP is transparent for yellow and red light, thus GaP is a better suited as substrate for GaAsP LEDs than GaAs. GaP itself may also be employed as active LED material, which emits green light in the pure phase and shifts to yellow-green upon nitrogen-doping and further to the red by doping with zinc-oxide. Thus, GaP thin films on cheaper silicon substrates promise a high application potential for LED engineering.
The present study addresses both material discontinuities which occur at the boundaries of such a GaP thin functional layer, the surface and the interface with the silicon substrate. Density-functional calculations have been performed with the pseudopotential plane-wave code ABINIT [1]. The most stable (001) surface is P-terminated and exhibits the formation of P dimers in analogy with the common (1x1)-, (2x1)-, and c(4x2)-symmetric reconstructions known for the Si(001) surface. Those reconstructions give rise to STM patterns with differing symmetries, especially when terminated by additional hydrogen atoms. At the interface with the silicon substrate both the Ga-rich and the P-rich terminations of the GaP layer are studied along with partially Si-occupied boundary layers. Substantially negative values of the work of separation are obtained for all investigated interfaces, which indicate the high stability and the low remanent stresses at the GaP-Si interface.

[1] www.abinit.org.