p-Type Doping of GaN Nanowires Characterized by Photoelectrochemical Measurements

GaN-based nanowires have emerged as a promising approach to light emission and solar energy harvesting applications. In this framework, p-type doping of GaN by Mg is one of the most important prerequisites for the fabrication of such devices. However, characterizing the electrical properties of nanowires is generally difficult for any material system, because to this end typically individual dispersed nanowires are contacted for electrical measurements, which necessitates challenging processing procedures. Furthermore, that approach is not practical for measuring large numbers of nanowires. Besides, the growth of long Mg-doped GaN nanowires, which are needed for such single nanowire measurements, is hindered by the fact that Mg induces lateral growth and coalescence in nanowires grown by molecular beam epitaxy (the synthesis technique for the majority of studies).

Figure 1: Schematic of the photoelectrochemical measurements setup (top) and obtained ΔOCP and net ionized Mg density as a function of Mg cell temperatures (Mg doping levels) (bottom).

Here, we employ photoelectrochemical measurements to systematically investigate Mg doping of GaN nanowires. In this approach, nanowires are contacted simply by immersing as-grown nanowire ensembles into a liquid electrolyte. Complicated processing steps involving expensive facilities such as electron beam lithography are not required. Furthermore, such measurements provide directly an average value for the entire nanowire ensemble. More specifically, measurements of the open-circuit potential in the dark and under illumination (ΔOCP) allow the identification of the conductivity type, and a Mott-Schottky analysis of capacitance voltage measurements provides in addition the density of ionized dopants. In fact, both methods showed the expected transition from n-type to p-type conductivity with increasing Mg doping level, and the latter characterization technique allowed us to quantify the charge carrier concentration. Beyond the quantitative information obtained for Mg doping of GaN nanowires, our systematic and comprehensive investigation demonstrates the benefit of photoelectrochemical methods for the analysis of doping in semiconductor nanowires in general.

 

This work was carried out in collaboration with the Institute for Solar Fuels at Helmholtz-Zentrum Berlin.

https://www.helmholtz-berlin.de/forschung/oe/ee/solare-brennstoffe/index_en.html

 

Jumpei Kamimura, Peter Bogdanoff, Manfred Ramsteiner, Pierre Corfdir, Felix Feix, Lutz Geelhaar, and Henning Riechert, “p-type doping of GaN nanowires characterized by photoelectrochemical measurements,” published in Nano Letters, Feb. 6, 2017. doi: 10.1021/acs.nanolett.6b04560