Study of Ag-dopants on the structural and optical properties of ZnO nanorods
In short: Zinc oxide (ZnO) is a semiconductor material promising for optoelectronics due to its efficient light emitting properties. However, the key challenge that must be solved before ZnO can be used as a LED material is to make it so-called p-type. Substitution of some Zn atoms with silver (Ag) was recently proposed as an approach for obtaining p-type ZnO. Here we investigate the structural and optical properties of ZnO nanorods doped with Ag atoms. We demonstrate that Ag promotes the generation crystal defects and significantly modifies the optical spectrum of ZnO. These results can be of high importance for further progress on p-type ZnO.
Zinc oxide (ZnO) is a semiconductor material promising for optoelectronics and photonics due to its potential for highly efficient light emitting diodes (LEDs) and laser diodes (LDs). However, fabrication of these devices requires material of both electron (n-type) and hole (p-type) conductivity. A key challenge is to obtain stable p-type ZnO, which has been unsuccessful up to now. This is because even undoped high quality ZnO material still is of n-type, with an excess of electrons, which hardly can be compensated by p-doping.
For obtaining the p-type ZnO, it has to be doped by impurity atoms, so-called acceptor dopants. Many different elements has been suggested as possible candidates – As, P, N etc. Recently, silver (Ag) was proposed to be an efficient acceptor dopant in ZnO. Ag creates a very shallow acceptor level in ZnO, which is necessary for efficient doping. Also, the difference between the atom size of Zn and Ag is very small (0.089 and 0.074 nm radius), therefore Ag tends to substitute Zn atom sites. In order to “turn” ZnO material into a p-type semiconductor, one needs to incorporate large amount of Ag. It is unknown, however, how high concentrations of Ag atoms behave in the ZnO crystal, especially if the concentration exceeds the solubility limit (0.76 mol. %).
In this work, we have grown the ZnO nanorods with a high content of Ag (from 0.4 to almost 4 mol. %) by MOCVD and investigated in details their structural and light emitting properties. We have noticed that the ZnO nanorods with high Ag content have their side facets highly corrugated in an unusual quasi-regular pattern (Fig. 1). Previously, we observed such a phenomenon for undoped ZnO nanostructures with high density of crystal defects such as basal plane stacking faults (BSF). Obviously, Ag atoms incorporated into the ZnO crystal lattice create local stress and thereby promote the generation of the BSF defects. This is confirmed by investigations of the crystal structure of individual nanorods by high-resolution transmission electron microscopy (HR TEM). A number of basal plane  stacking faults were observed, penetrating the nanorods perpendicular to the axis (Fig. 2). BSFs were found to be quasi-periodically inserted every 5 - 10 nm along the nanorods.
The optical properties of the nanorods were investigated by luminescence spectroscopy on individual nanorods and mapping of their light emitting properties (Fig. 3). These experiments reveal that the nanorods with a high Ag content (and high concentration of BSFs) possess photoluminescence spectra with two emission bands. One band is commonly observed near the band gap energy of ZnO at ~375 nm (~3.31 eV), with highest intensity in areas free from BSFs (Fig. 3). However, an additional peak appears in the spectrum at ~386 nm (~3.21 eV) in areas with BSFs.
In order to prove that the light of ~386 nm wavelength is originating from the BSFs, we have performed the cathodoluminescence mapping measurements (inset of Fig. 3). The measurements show that the conventional near band edge emission of ZnO (red color) is from the BSFs free areas, while the second emission band (green color) is related to BSFs rich areas.
To conclude, our study shows that acceptor doping of ZnO by Ag at high concentrations results in creating defects, which significantly modify the light emission spectrum of ZnO. These results can be of high importance for further progress on p-type ZnO materials and well as for the development of future LEDs in long-term perspective.
Details of the research are described Physica Status Solidi A 211 2109 (2014) and Journal of Luminescence (in press 2015).
- Linköping Linnaeus Initiative for Novel Functional Materials (LiLi-NFM)
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Last updated: 01/27/15