Optical Characterization of Doped Zinc Oxide Nanowires

Synopsis

ZnO is a promising semiconductor material with a direct band gap energy of 3.3 eV . Such a large band gap energy makes it a good candidate for UV and visible range light emitting devices. UV and visible light emitting devices are now primarily made with GaN, however the higher earth abundance of ZnO coupled with the use of lower toxicity precursors such as oxygen make it highly desirable from an industrial point of view. Nevertheless, it is difficult to make stable and reproducible p-type ZnO, and this has greatly hampered the commercial development of this material. The details of this are still not fully understood. Metalorganic chemical vapour epitaxy (MOVPE) provides the possibility of industrial scale growth of ZnO, and has the ability to achieve very fine control of impurity dopants. Despite the vast recent literature on ZnO, there are very few studies of systematic intentional doping, and even fewer by MOVPE. ZnO nanowires (NWs) can be grown easily on various substrates with high crystalline quality and low defect densities and tend to exhibit reduced substrate induced strain. Because of these factors, it is possible to perform careful spectroscopic analysis of impurity related optical transitions without significant inhomogeneous broadening. This enables us to identify the physical nature of various dopant species.

This work presents a detailed study of low temperature photoluminescence (PL) transitions in doped ZnO NWs, thin films, and bulk crystals grown by MOVPE and chemical vapor transport (CVT) methods. The standard group III donors were first investigated over a wide range of doping levels. Donor bound exciton (D0X) transitions previously assigned to Ga, Al, and In were confirmed in intentionally doped samples grown by MOVPE.

Lower energy transitions related to these donors were identified and confirmed to be due to phonon replicas of the exciton transitions and not donor-acceptor pair transitions as commonly stated. D0X linewidths for lightly doped or undoped samples were as low as 0.17 meV, which is close to the best values so far reported in bulk single crystals. At higher group III dopant concentrations, the D0X linewidth increases and an asymmetric broadening is observed with a tail at lower energy. For Ga doped ZnO NWs this was explained by the transfer of excitons between donor impurities in the shell region of the intentionally doped NWs. The model was used to estimate the donor concentrations in doped material. Electrical transport measurements confirmed the estimated donor concentration trends in these samples.

Group IV dopants such as carbon, and tin are interesting since they can act in principle as double donors or double acceptors. We report four new shallow D0X transitions (Z-lines), at 3360.8 (Z1), 3361.2 (Z2), 3361.7 (Z3) and 3361.9 (Z4) meV, which can be greatly enhanced by co-doping with carbon tetrachloride and hydrogen.The D0X nature was confirmed by temperature dependent PL and the observation of so-called two electron satellites due to excited states of the final state donor. These shallow donors appear to be due to carbon impurities complexed with other unknown defects in four distinct configurations. Carbon doped samples also exhibit two distinct acceptors with binding energies of 133 ± 5 and 181 ± 5 meV. These transitions were identified as free-to-bound and donor-acceptor pair transitions and appear to have a different origin from the Z-lines.

PL studies of unintentionally doped and Sn-doped ZnO single crystals grown by CVT showed increased emission from the I10 bound exciton transition which was recently proven to contain Sn on a Zn site. Temperature dependent PL measurements of the I10 reveal a behavior that is similar to other shallow donors in ZnO. D+X and two electron satellite (TES) transitions of the Sn-related transition were unambiguously identified and yield a donor binding energy of 71 meV.

Sb doped ZnO was grown in an attempt to produce p-type material as reported by some groups. Rather than p-doped material, the addition of small amounts of Sb-dopant resulted in a new PL transition at 3364.3 meV, which turns out to be the shallowest D0X transition so far observed in ZnO with a significant negative central cell correction. This transition thermalizes in a way similar to group III donors. Magneto PL measurements and temperature dependent measurements confirm the D0X nature of this transition and rule out the possibility that ISb is related to acceptor bound exciton (A0X) transitions. A model consisting of a triple Sb donor paired with a single Zn vacancy is presented.

The estimated binding energies of C, Sb and Sn impurities show a dependence of exciton binding energy on donor energy (Haynes rule) that is similar to the group III dopants. This is surprising since all three defects are clearly complexes. The fact that Sb and Sn have such widely different central cell corrections (negative vs strongly positive) argues to a fundamental difference in the nature of the compensating defects.