Hydrogen-dependent lattice dilation in GaN

Abstract

Using Raman spectroscopy we have analysed the strain status ofGaN films grown on sapphire substrates by NH3 source molecularbeam epitaxy (MBE). In addition to the expected compressive biaxial strain,in some cases GaN films grown on c-face sapphire substrates suffer fromserious tensile biaxial strain. This anomalous behaviour has been wellinterpreted in terms of interstitial hydrogen-dependent lattice dilation.The hydrogen concentration in the films is measured by nuclear reactionanalysis (NRA). With increasing hydrogen incorporation, the residualcompressive biaxial strain is first further relaxed, and then turns intotensile strain when the hydrogen contaminant exceeds a criticalconcentration. The hydrogen incorporation during the growth process isfound to be growth-rate dependent, and is supposed to be strain driven. Webelieve that the strain-induced interstitial incorporation is another wayfor strain relaxation during heteroepitaxy, besides the two currently wellknown mechanisms: formation of dislocations and growth front roughening.