Effect of Ag/Al co-doping method on optically p-type ZnO nanowires synthesized by hot-walled pulsed laser deposition
© Kim et al.; licensee Springer. 2012
Received: 4 March 2012
Accepted: 30 May 2012
Published: 30 May 2012
Silver and aluminum-co-doped zinc oxide (SAZO) nanowires (NWs) of 1, 3, and 5 at.% were grown on sapphire substrates. Low-temperature photoluminescence (PL) was studied experimentally to investigate the p-type behavior observed by the exciton bound to a neutral acceptor (A0X). The A0X was not observed in the 1 at.% SAZO NWs by low-temperature PL because 1 at.% SAZO NWs do not have a Ag-O chemical bonding as confirmed by XPS measurement. The activation energies (Ea) of the A0X were calculated to be about 18.14 and 19.77 meV for 3 and 5 at.% SAZO NWs, respectively, which are lower than the activation energy of single Ag-doped NW which is about 25 meV. These results indicate that Ag/Al co-doping method is a good candidate to make optically p-type ZnO NWs.
Zinc oxide (ZnO) has emerged as a superior n-type II-VI compound semiconductor material with high chemical and physical stability as well as a direct, wide bandgap (3.37 eV) and high exciton binding energy (60 meV) at room temperature . ZnO has potential applications due to its various attractive properties such as transparency, ferromagnetic, optical, photoelectric, gas sensing, and piezoelectric properties and a wide range of electrical properties, from dielectric to conductive materials [2–5]. Wide-bandgap ZnO optoelectronics have been stimulated by the investigation of materials for use in the next generation of short-wavelength optoelectronic devices and have revealed good results, including high transmittance and mobility [6, 7]. As-grown ZnO nanostructures typically show an n-type semiconductor due to their intrinsic defects such as oxygen vacancies and zinc interstitials . The strong n-type conductivity of ZnO restricts its application, which makes it difficult to fabricate p-type ZnO materials , and the realization of p-type ZnO is rather difficult due to its asymmetric doping limitations . Recently, research on ZnO has been focused on the synthesis of p-type ZnO using various dopants including N, P, As, Sb, and Ag [5, 10–12]. Among possible acceptor dopants, Ag (a group Ib element) is a good candidate for producing a shallow acceptor level in ZnO, if incorporated on substituted Zn sites . The Ag-doped ZnO for the various applications has been reported by Kang et al. . They demonstrated that the Ag ion can be substituted into the site of the Zn ion and a narrow window region exists to fabricate the p-type ZnO using Ag as a p-type dopant source. However, there have been no reports on the perfect fabrication of p-type ZnO nanowires (NWs) using Ag dopant.
Several research groups proposed a co-doping method where the acceptor dopants and group III elements are used such as co-doping of nitrogen and gallium by pulsed laser deposition (PLD), nitrogen and indium by ultrasonic spray pyrolysis, and nitrogen and aluminum by reactive magnetron sputtering [15–17]. Lu et al. reported to fabricate p-type ZnO structures by co-doping with nitrogen and aluminum . They demonstrated that simultaneous co-doping using acceptors and reactive donors could be expected to enhance the solubility of acceptors in ZnO and to raise shallow acceptor levels in the bandgap [17, 18]. Therefore, the incorporation of acceptors in ZnO was remarkably enhanced due to the presence of aluminum. However, controlling nitrogen at room temperature is very difficult because nitrogen in air exists in a gas state. So, we used Ag dopant for making a p-type semiconductor in a ZnO matrix instead of nitrogen.
In this work, we have investigated the growth behavior of various silver and aluminum-co-doped zinc oxide (SAZO) NWs on an Al2O3 (0001) substrate by hot-walled pulsed laser deposition (HW-PLD) which is one of the special methods for nanostructure synthesis. After optimizing the process condition for NW formation in the HW-PLD, we verified the Ag-doping status, confirmed the exciton bound to a neutral acceptor (A0X) by low-temperature (13 K) photoluminescence (PL), and compared thermal activation energies (Ea) of single-doped and co-doped ZnO-based NWs.
The fabrication methods of NWs have been developed to be realized at nanoscale such as thermal evaporation, PLD, and wet chemical processing [19, 20]. Among various growth methods, the PLD has several notable advantages which are very effective in obtaining the stoichiometry of synthesized materials on the substrate as a target than many other gas surface-based growth techniques . SAZO NWs of 1, 3, and 5 at.% have been synthesized on sapphire (0001) substrates in HW-PLD with a 20-Å Au film as a catalyst. SAZO NWs of 1, 3, and 5 at.% are grown in a furnace temperature of 800 °C with Ar gas of 90 sccm and a working pressure of 1.2 Torr. The HW-PLD has a target rotating system ensuring homogeneous target ablation. A KrF excimer laser (248 nm) operating at a pulse repetition rate of 10 Hz is focused onto pure ZnO and 1, 3, and 5 at.% SAZO targets for the deposition. The energy density of the laser is set to 1.2 J/cm2, and the shot area on the target surface is 0.042 cm2. Before synthesis, there should be a pre-deposition process with a laser shot time of 5 min on the surface of the target. The deposition process is continued for 30 min.
The target in this process is synthesized using high-purity ZnO (99.999%, Kojundo Chemical Laboratory, Sakado-shi, Japan), Ag2O (99.99%, Kojundo Chemical Laboratory), and Al2O3 (99.99%, Kojundo Chemical Laboratory) powders. The ethanol-based solutions of Ag and Al powders are ground and mixed with the ZnO powder by planetary milling for 48 h. The target is finalized by sintering at 950 °C for 3 h.
The low-temperature PL spectroscopy is a very sensitive tool for characterizing acceptor/donor impurities and is helpful to understand the optical and electrical performances of the materials. We focus on the temperature dependence of PL measurements of various SAZO NWs to reveal the role of the Ag acceptor in the optical properties of the ZnO NWs. Temperature-dependent PL (Dongwoo Fine-Chem Co., Ltd., Seoul, South Korea) spectra analyses are performed from 13K to room temperature using a 325-nm He-Cd laser, and a cycle refrigeration system is used to lower the temperature of the sample to 13 K during the low-temperature PL measurement. The crystal morphology is characterized using a field-emission scanning electron microscope (FE-SEM) and a transmission electron microscope (TEM), and the Ag element is observed in ZnO NWs by X-ray photoelectron spectroscopy (XPS) which is carried out to investigate the elemental composition of ZnO-based NWs.
Results and discussions
EDX spectra reveal the weight/atomic percent of Zn, Al, and Ag elements in SAZO NWs
In order to understand the origins of the chemical bonding, the binding energies of both related Ag and Al elements in 1, 3, and 5 at.% SAZO NWs are investigated by XPS measurement as shown in Figure 4. The binding energy of Ag2-Al metallic bond in 1 at.% SAZO NWs is observed at 368.3 eV. In other words, the chemical bonding of Ag-O (with the role of an acceptor) is not observed in 1 at.% SAZO NWs as shown in Figure 4a. The silver ions are substituted in the Zn site when the Ag/Al co-dopants are successfully doped in ZnO NWs. However, the XPS data of 1 at.% SAZO NWs show only a Ag2-Al metallic bond which acts as an interstitial defect in ZnO NWs. It means that the doping condition of 1 at.% Ag/Al co-dopants could not act as a desirable acceptor in ZnO-based NWs. However, the XPS data of 3 and 5 at.% SAZO NWs show Zn, O, and Ag orbital as shown in Figure 4b,c, respectively. A sharp, strong peak which originated from the Ag chemical bonding peak (Ag 3d5/2) of the SAZO NWs is observed at 369.2 and 369.1 eV for 3 and 5 at.% SAZO NWs, respectively, as shown in the inset of Figure 4b,c. Also, the binding energies of both 369.2 and 369.1 eV are close to Ag 3d5/2 of the Ag-O bond. The Ag 3d5/2 peaks of 3 and 5 at.% SAZO NWs show a very sharp and high intensity which means that Ag dopants are successfully doped into the ZnO structure. From these results, the doping concentrations of 3 and 5 at.% Ag/Al co-dopants are best conditions to make p-type ZnO NWs.
In summary, we have synthesized various silver/aluminum-co-doped ZnO NWs on sapphire substrates by hot-walled pulsed laser deposition with Au catalysts. The XPS data of 1 at.% SAZO NWs show Ag2-Al metallic bond. This means that this condition generates Ag-Al metal compound which acts as an interstitial defect in ZnO NWs. Therefore, 1 at.% SAZO NWs have only one peak at 3.359 eV which originated from the D0X. However, new peaks of 3 and 5 at.% SAZO NWs located at 3.355 and 3.356 eV originated from the exciton bound to neutral acceptors. Also, the activation energies of 3 and 5 at.% SAZO NWs show lower values compared with the Ea of single Ag-doped ZnO NWs. We can conclude that 3 and 5 at.% Ag/Al co-doping methods facilitate to make optically p-type ZnO NWs.
This work has been supported by a grant from the Korea Institute of Science and Technology. Also, this research was supported by a grant (code number 2011 K000208) from the Center for Nanostructured Materials Technology under 21st Century Frontier R&D Programs of the Ministry of Education, Science and Technology, Korea.
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