Shallow donor and DX state in Si doped AlN nanowires grown by molecular beam epitaxy

Abstract

Si doping of AlN nanowires grown by plasma assisted molecular beam epitaxy was investigated with the objective of fabricating efficient AlN based deep ultra-violet light-emitting-diodes. The Si concentration ranged from 10 16 to 1.8x10 21 cm -3 . Current-voltage measurements performed on nanowire ensembles revealed an ohmic regime at low bias (below 0.1 V) and a space charge limited regime for higher bias. From temperature dependent current-voltage measurements, the presence of Si donors is evidenced in both shallow and deep DX states with ionization energy of 75 meV and 270 meV respectively. The role of Fermi level pinning on NWs sidewalls is discussed in terms of near surface depletion inducing a favorable formation of shallow Si donors.

DX center formation in highly Si doped AlN nanowires revealed by trap assisted space-charge limited current

“…On the other side, the ohmic regime observed for those samples points towards a conduction exclusively related to the DX state. 20 Thus, it highlights a correlation between the increase of Si doping and DX concentration, and the apparition of a regime of trap filling, in the line of other studies 7,10 .…”

“…Breckenridge and coworkers reported an ionization energy of 70 meV after annealing at 1200°C while a 270 meV ionization energy was found for samples annealed at higher temperature. In a previous article, we reported the presence of shallow Si states in AlN nanowires (NWs) with an ionization energy of 75 meV 20 . Such states are dominant at room temperature for NWs in the low Si flux range whereas at higher Si flux, it is no longer the case.…”

“…After the growth of a 350 nm Si-doped GaN stem, a Si-doped AlN section of around 450 nm was grown and capped by a 20 nm Si-doped GaN section. In all samples, GaN sections are doped with the same Si flux corresponding to ~2x10 20 Si/cm 3 measured by energy dispersive X-ray spectroscopy (EDX). The silicon cell temperature (TSi)…”

“…was varied from sample to sample during the AlN section growth, ranging from 750°C up to 1300°C. Silicon concentration in AlN was also measured by EDX and was ranging from a few 10 15 Si/cm 3 to 6x10 20 Si/cm 3 . Widening and shortening of the NWs were observed at TSi > 1100°C assigned to an enhancement of adatom diffusion on lateral m-planes of the wires, consistent with previous observations 25,26 .…”

“…It is worth noting that these dimensions are large enough to discard the effect of free carrier quantum confinement in NWs, supporting the assumption that the band structure is the same as in bulk. More details on NW growth and morphology can be found in ref 20. The description of the investigated samples is given in Table 1.…”