Ankur Saxena

Defect studies of ZnSe nanowires

During the synthesis of ZnSe nanowires various point and extended defects can form, leading to observed stacking faults and twinning defects, and strong defect related emission in photoluminescence spectra. In this paper, we report on the development of a simple thermodynamic model for estimating the defect concentration in ZnSe nanowires grown under varying Se vapour pressure and for explaining the results of our experimental findings. Positron annihilation spectroscopy was used successfully for the first time for nanowires and the results support predictions from the defect model as well as agreeing well with our structural and optical characterization results. Under very high Se vapour pressure, Se nodules were observed to form on the sidewalls of the nanowire, indicating that beyond a limit, excess Se will begin to precipitate out of the liquid alloy droplet in the vapour-liquid-solid growth of nanowires.

Excitonic and pair-related photoluminescence in ZnSe nanowires

It has been established that deviations from stoichiometry during the growth of ZnSe crystals result in point defects, which influence its electronic properties. We report on detailed photoluminescence results and their systematic analysis for ZnSe nanowires. We studied photoluminescence from vapor-phase grown undoped ZnSe nanowires grown under excess Zn conditions, and in particular the dependence on excitation intensity. Luminescence spectra were characterized by strong near-band-edge luminescence with negligible deep-level emission. We observed excitonic emission at 2.794 eV related to the neutral donor at VSe. The binding energy of the exciton was found to be 7 meV, and that of the donor was 35 meV. Two donor-acceptor pair transitions at 2.714 and 2.686 eV were also observed, which can be related to the defect complexes of native defects with other native defects or with common unintentional shallow donors and acceptors. The ionization energies of both donors were 27 meV, whereas those of the acceptor...

Crystal lattice determination of ZnSe nanowires with polarization-dependent second harmonic generation microscopy

We demonstrate a noninvasive optical microscopy technique based on polarization-dependent second harmonic generation for determining the crystal lattice structure and microscopic heterogeneities within individual nanostructures. Differentiation between periodically twinned and wurtzite ZnSe nanowires (NWs) was demonstrated, and measurement of the cubic lattice rotation orientation around the NW axis was determined within 1° accuracy. Zinc blende NWs were differentiated from wurtzite. The technique can be used for quality inspection and optimization of growth conditions for nanostructures.

Unambiguous identification of recombination lines in single zinc-blende ZnSe nanowires in direct relation to their microstructure

We present results on the low-temperature photoluminescence characterization of individual ZnSe nanowires, whose crystal structure was determined to be zinc-blende by transmission electron microscopy on the same individual nanowires as were studied optically. The photoluminescence response from single ZnSe nanowires was found to be dominated by excitonic emission due to native point defects, while no emission peaks related to the unintentional impurities were detected. Two strong photoluminescence lines were observed at 2.785 eV and 2.780 eV, assigned to the excitons bound to deep neutral acceptors related to the vacancies of Zn (V(Zn)) and complexes of V(Zn), respectively. Another recombination peak at 2.800 eV related to the free exciton emission in ZnSe was also observed. Longitudinal optical-phonon replicas of up to three orders were seen for both lines, and the average number of emitted phonons was also determined. The excitonic emission linewidths of 1.5 meV were observed from individual nanowires, which are the narrowest excitonic linewidths reported so far for ZnSe nanowires. The optical response from a single nanowire was also compared to that from a bundle of nanowires, and it was found that the linewidths of excitonic emission from the bundle of nanowires were slightly larger than those from single nanowires, due to the effects of ensemble broadening. It is also suggested that in the case of a bundle of nanowires, the broadening is limited by the nanowire which exhibits the largest excitonic linewidth.