Raman Studies on Oxygen Doped GaN Grown by Molecular Beam Epitaxy

Abstract

High quality Ga-polarity GaN films were grown by plasma-assisted molecular beam epitaxy to study strain effects due to oxygen incorporation. Oxygen concentrations up to 2 × 10²² cm⁻³ were studied. Layers containing oxygen at levels above 10²² cm⁻³ exhibit severe cracking while oxygen concentrations less than 10²¹ cm⁻³ apparently does not introduce significant strain. Raman spectra of O₂-doped films were evaluated with respect to spectra of unintentionally doped GaN films (n=4×10¹⁴ cm⁻³) grown under the same conditions except for the O₂-flux. Analysis of the E₂ (high frequency phonon mode near 570 cm⁻¹) Raman band indicated the heaviest doped samples exhibit compressive strain. Frequency-shifts between 0.7 and 0.9 cm⁻¹ were observed as a function of the distance from the crack edges in the heavily doped samples, implying a strain (δα/α) of the order of 5×10⁻⁴.