CL study of yellow emission in ZnO nanostructures annealed in Ar and O2 atmospheres

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Abstract

The yellow emission of thermally treated undoped and In doped ZnO nanostructures was studied by the cathodoluminescence (CL) technique. CL spectra acquired at room temperature of the as-grown samples revealed two emissions at about 3.2 eV and 2.13 eV, corresponding to the near band edge and defect related emissions, respectively. On annealing the samples at 600  C in Ar and O2 atmospheres, the defect emission suffers a red shift, irrespective of the annealing atmosphere. This red shift is explained in terms of variations in the relative intensities of the two component bands centered at about 2.24 eV and 1.77 eV, which were clearly resolved in the CL spectra acquired at low temperature of the annealed samples. A decrease of the relative intensity of the yellow emission (2.24 eV) was observed for all thermally annealed samples. The annealing of zinc interstitial point defects is proposed as a possible mechanism to explain this intensity decrease.

Introduction

Luminescence properties of ZnO nanostructures have been extensively studied in the last few years due to their potential applications in the fabrication of highly efficient blue lasers and light-emitting diodes [1], [2], gas sensors [3], [4] and photocatalysts [5]. While the large exciton binding energy of ZnO (60 meV) facilitates the gain mechanism in such devices, an increase in the number of shallow states by adequate doping can decrease its excitation energy with better electronic transport. Therefore, a great effort is being devoted in doping ZnO nanostructures by several impurities like In, Ga, and Sb [6], [7], [8], [9]. Incorporation of impurities in semiconductors, however, regularly produces crystalline defects that generate undesired luminescence in their radiative spectra.

Different luminescent techniques have been used to characterize the optical properties of defects in semiconductors, with enough sensitivity even for low concentrations of such defects [10], [11], [12]. In particular, cathodoluminescence (CL) in the scanning electron microscope (SEM) has been applied to study the spectral and spatial distribution of defect emissions in ZnO nanostructures [13], [14]. These reports indicate that in addition to their morphology, growth orientation and preparation methods, the defects strongly influence the luminescent behaviour of ZnO nanostructures. The most common defect related emission in ZnO is the broad green–yellow luminescence centered about of 2.2 eV, which has been widely attributed to native point defects like oxygen vacancies [15] and intrinsic or complex defects [16]. Recently we reported the presence of defect states at 2.2 eV above the valence band in ZnO nanorods grown by the hydrothermal method, and attributed them to point defects formed by a surface strain relaxation process [17].

Understanding the nature of crystal defects together with their spatial distribution and their effects on the luminescent properties of the ZnO would enable us to get insight onto the emission mechanisms in nanostructures. In order to investigate the effects of crystal defects on the luminescent properties of undoped and In doped ZnO nanostructures, CL microscopy and spectroscopy were used in this work.

Section snippets

Experimental

Undoped and In doped ZnO nanostructured samples were grown by a low-temperature hydrothermal process following the procedure reported earlier [18]. Briefly, ethylenediamine, zinc acetate, and sodium hydroxide were used as reactives in the synthesis process at 90  C. For doping, the desired amount of indium chloride (nominally 0.5, 1.0 and 2.0 mol%) was added to the reaction mixture. Under our experimental conditions, indium doping of ZnO through the hydrothermal process resulted in the

Results and discussion

Secondary electron images of doped and undoped samples show a radial growth of tapered needle-shaped ZnO structures with diameter between 240 and 580 nm, and average length of 6 μm grouped in spherical clusters [Fig. 1(a) and (b)]. Particles of about 400 nm in diameter were also present in the In-doped samples as a by-product of the synthesis process, which could be easily separated from the needle-shaped ZnO crystals after a brief sonication in water [Fig. 1(c)]. Such nanoparticles are indium

Conclusions

Doped and undoped ZnO nanostructures with nominal 0.5, 1.0, and 2.0 mol% indium doping were grown by a low temperature hydrothermal technique. Quenching of CL emission occurs in the doped samples either due to the reduction of radiative point defects or by the formation of non-radiative defects. CL spectra of the as-grown samples reveal a broad defect emission band at about 2.12 eV apart from the near band edge emission at 3.2 eV. On annealing the samples at 400  C and 600  C either in Ar and

Acknowledgements

This work was partially supported by grants from UC-MEXUS (No. CN-05-215), CONACyT (Grant # 47505 and 46269), PAPIIT-UNAM (Grant # IN107208) and VIEP-BUAP (Grant # 93/EXC/2008-1), Mexico. Technical helps of E. Flores and E. Aparicio are also acknowledged.

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