In this work, we present a detailed study on GaN nanowire doping, which is vital for device fabrication. The nanowires (NWs) are grown by means of molecular beam epitaxy on diamond (111) substrates. Dopant atoms are found to facilitate nucleation, thus an increasing NW density is observed for increasing dopant fluxes. While maintaining nanowire morphology, we demonstrate the incorporation of Si and Mg up to concentrations of 9× 1020cm−3 and 1 × 1020cm−3, respectively. The dopant concentration in the nanowire cores is determined by the thermodynamic solubility limit, whereas excess dopants are found to segregate to the nanowire surface. The strain state of the NWs is investigated by X-ray diffraction, which confirms a negligible strain compared to planar thin films. Doping-related emissions are identified in low-temperature photoluminescence spectroscopy and the temperature quenching yields ionization energies of Si donors and Mg acceptors of 17 meV and 167 meV, respectively. At room temperature, luminescence and absorption spectra are found to coincide and the sub-band gap absorption is suppressed in n-type NWs. The charge carrier distribution in doped GaN nanowires is simulated under consideration of surface states at the non-polar side facets. For doping concentrations below 1017cm−3, the nanowires are depleted of charge carriers, whereas they become highly conductive above 1019cm−3.
In this work, we combine conductance and contact potential difference measurements in a consistent and systematic way, in steady-state and transient modes, both in the dark and under illumination. With this we obtain valuable information about the kinetics of charges at and close to the surface of GaN. We compare the processes involved in the accumulation and the decay of charge carriers generated via excitation with above and below band-gap light with varying light intensity. In particular, we probed the role played by localized defect states in the kinetics of photogenerated charges. These states are responsible for the trapping of photogenerated electrons in the space-charge region close to the surface, which explains the slow response of the photocurrent to illumination. These states are also involved in the transfer of electrons back to the surface after illumination, which results in the slow recovery of the photocurrent and the contact potential difference in the dark.A. WINNERL, R. N. PEREIRA, AND M. STUTZMANN PHYSICAL REVIEW B 91, 075316 (2015)
In this work, we investigate the fundamental role of the substrate material, surface orientation, and termination on GaN nanowire (NW) nucleation and growth. First of all, the use of a patterned a-Si/diamond substrate confirms that NW shape and dimension are mainly determined by the applied growth conditions instead of the nature of the substrate. More important is the surface orientation as it defines growth direction and epitaxial relationship towards the GaN NWs, where both (111) and (100) surfaces yield NW growth for equivalent growth conditions. (110) substrates are found to be not suited for NW growth. Finally, the surface termination of diamond is demonstrated to survive the employed growth conditions and, therefore, to affect the nucleation of nanowires and the electronic properties of the heterointerface by its surface dipoles. This difference in nucleation is exploited as an alternative approach for selective area growth without deposition of a foreign mask material, which might also be transferable to other substrates.
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