Defect states of chemical vapor deposition grown GaN nanowires: Effects and mechanisms in the relaxation of carriers

Abstract

Carrier relaxation in GaN nanowires, grown by atmospheric pressure chemical vapor deposition, via direct nitridation of Ga with NH3 at 950 °C has been investigated in detail. Differential absorption measurements reveal a large number of defect states located within the band gap. The relaxation dynamics of the photogenerated carriers suggest three distinct regions of energy states below the band edge identified as shallow donor states, midgap states, and deep acceptor states. Measurements suggest that Auger recombination is not a contributing factor in carrier relaxation even at the highest fluence (∼1 mJ/ cm2) used in this work for carriers located within the conduction band. On the contrary, Auger recombination has been observed when probing the shallow donor states for fluences above 40μJ/cm2. Measurements at the lowest fluence reveal a biexponential relaxation for the donor states with the fast component (∼50 ps) corresponding to the relaxation of carriers into the midgap states and the slow component of 0.65 ns associated with the relaxation into the deep acceptor states. Measurements reveal free-carrier absorption contribution from the deep acceptor states to the U -valley with an observed threshold limit of 3.5 eV suggesting the U -valley is located approximately 4.7 eV from the valence band. © 2009 American Institute of Physics.