The role of surface states in a-axis GaN nanowires
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GaN nanowires have been the subject of intense research lately, due to the many potential ultraviolet applications and interesting properties that they possess. Because GaN has an anisotropic wurtzite crystal structure, many of its properties are dependent upon crystal orientation. For example, the photoluminescence (PL) of GaN nanowires with growth direction along the a-axis is blue-shifted relative to the PL of wires with growth direction along the c-axis. However, the origin of the difference in PL between nanowire samples of different growth directions remains unclear. To determine if surface states play a role in the dependence of GaN nanowire photoluminescence on crystal orientation, we use time-integrated photoluminescence (TIPL) and time-resolved photoluminescence (TRPL) to study the PL from GaN nanowire samples of different crystallographic orientations. We observe temporal dynamics of the blue-shifted PL feature in the a-axis GaN nanowires that is suggestive of a surface trapping process occurring, where some fraction of electron-hole pairs are prevented from recombining via the band edge emission process because carriers diffuse to the surface where they are trapped before carrier relaxation to the band edge is complete. Once a carrier is trapped and localized at a surface trap state, light emission primarily occurs only when the complementary carrier diffuses to the same surface trap. We envision that a thin oxide layer forming at the surface introduces surface traps that cause the blue emission, and that the surfaces of the a-axis GaN nanowires are more susceptible to this oxidation than the c-axis GaN nanowire surfaces.
