Shallow Donor and DX State in Si Doped AlN Nanowires

Thanks to its ultra-wide band gap, AlGaN and AlN nanowires (NWs) are attractive candidates for deep UV light emitting diodes (LED). The absence of extended defects, such as dislocations, as well as the morphology of nitride NWs, are favorable to improve the dopants incorporation, the internal quantum efficiency and light extraction. In this context, some of us recently reported axial pn junction realization with Mg acceptors in the p-region and Si donors in the n-region [1]. Thus, it becomes crucial to understand and describe in detail the transport mechanisms at the origin of the current flow inside the individual nanodevices. The n-type doping of AlN nanowires remains a blind spot in recent research and is as important as its p-type counterpart in order to achieve the best performances for nanowire-based DUV LEDs.<br/>In this work, we investigate the electrical properties of n-type AlN NWs grown by molecular beam epitaxy with a Si concentration ranging from 10¹⁶ cm^-3 up to 10²¹ cm^-3. The current voltage measurements show that all the samples exhibit an Ohmic regime at low bias (&lt; 0.2 V) and a space charge limited current (SCLC) regime assisted by traps for higher voltage (&gt; 0.2 V). Conductivity values versus silicon doping extracted from Ohmic regime describe a bell-like curve peaking at a maximum of 6.10^-6 S/cm for [Si] of 10¹⁷ cm^-3 before dropping down to 4.10^-9 S/cm at larger flux, in good agreement with high compensation effects observed in bulk Si:AlN [2]. Temperature dependent measurements from 180 K to 600 K revealed that NW conductivity is a combination of the two identified silicon levels at 75 meV and 270 meV [3]. Simplified theory of SCLC was used to extract trap properties and showed the presence of traps located at 160 meV below CB and with a density increasing with Si flux. This later is attributed to another DX state involving Si atom in a different configuration [4]. The full analysis and the model used will be presented and the results discussed. Fermi level pinning and strain relaxation at the NW sidewalls will be pointed as candidates to explain the presence of the three levels in the same set of samples.<br/>[1] A.-M. Siladie et al, <i>Nano Lett.</i> 19, 8357-8364 (2019)<br/>[2] J. S. Harris et al, <i>Appl. Phys. Lett.</i> 112, 152101 (2018)<br/>[3] M.H. Brenckenridge et al, <i>Appl. Phys. Lett.</i> 118, 112104 (2021)<br/>[4] I. Aleksandrov et al, <i>Journal of Physics Condensed Matter</i> 32(43) (2020)