Estimation of carrier concentration in In and Ga doped zinc oxide nanowires: a comprehensive analysis of optical spectra

Zinc oxide is a wide direct bandgap (3.34 eV) compound semiconductor with a large exciton binding energy of 60 meV. It has potential applications in optoelectronic devices and is a promising candidate for quantum information applications. Metalorganic chemical vapor deposition provides an inexpensive and comparatively environmentally friendly method for growing ZnO on various substrates. ZnO is naturally an n-type semiconductor due to its native defects, and it appears that stable p-type ZnO cannot be manufactured with current methods. Shallow donors in ZnO can be detected from optical emission of electron-hole pairs bound to specific donor impurities. Doping ZnO NWs results in an asymmetric broadening at the lower energy side of the donor-bound exciton peak in photoluminescence spectroscopy. In this study, we investigated the optical and morphological properties of In and Ga doping on ZnO nanowires. These elements substitute the Zn site and form shallow hydrogenic donors. Characterizing donor concentration in NWs is challenging with current methods, such as Hall effect and nanoprobe measurements due to their geometry. Based on the correlation between the dopant flow rate and the change in the tail of the donor-bound exciton peak, we studied the optical linewidth of doped NWs to characterize donor concentration. As the effect of substrate-induced strain on the linewidth of donor-bound excitons is negligible in NWs it is possible to spectroscopically measure the donor concentration. In this work, we explored two mechanisms for the inhomogeneous broadening effect observed in our samples: (1) the Stark effect due to random fields from charged impurities, and (2) a pair model where the energies of neutral donor bound excitons are modified by wavefunction overlap with their closest neighbours. Specifically, we modeled this for the Ga D0X transition at various doping concentrations, comparing the results to nanoprobe measurements. The results suggest that the method is promising, however, further verification is required. Finally, we investigated methods to control donor dopant concentration in ZnO NWs while simultaneously controlling their morphology. We employed a direct growth from the vapor phase, as well as a core-shell method involving the growth of an undoped core followed by a lateral shell. We obtained more uniform NWs with the core-shell method.