Enhancement of intrinsic emission from ultrathin ZnO films using Si nanopillar template
© Chiang and Dai; licensee Springer. 2012
Received: 28 March 2012
Accepted: 23 April 2012
Published: 22 May 2012
Highly efficient room-temperature ultraviolet (UV) luminescence is obtained in heterostructures consisting of 10-nm-thick ultrathin ZnO films grown on Si nanopillars fabricated using self-assembled silver nanoislands as a natural metal nanomask during a subsequent dry etching process. Atomic layer deposition was applied for depositing the ZnO films on the Si nanopillars under an ambient temperature of 200°C. Based on measurements of photoluminescence (PL), an intensive UV emission corresponding to free-exciton recombination (approximately 3.31 eV) was observed with a nearly complete suppression of the defect-associated, broad-range visible emission peak. As compared to the ZnO/Si substrate, the almost five-times-of-magnitude enhancement in the intensity of PL, which peaked around 3.31 eV in the present ultrathin ZnO/Si nanopillars, is presumably attributed to the high surface/volume ratio inherent to the Si nanopillars. This allowed considerably more amount of ZnO material to be grown on the template and led to markedly more efficient intrinsic emission.
Due to the wide application of, for instance, transistors [1, 2], ultraviolet photodetectors , and piezoelectric transducers [4, 5] in the industry, all of which need to use ZnO thin film, which is regarded as a promising material for optoelectronic applications. In particular, the wide bandgap of 3.37 eV and large exciton binding energy of 60 meV (much higher than that of ZnSe (40 meV) and GaN (25 meV)) inherent to ZnO have been demonstrated to exhibit efficient excitonic emission at room temperature. Consequently, considerable efforts have been devoted to this particular material in order to harvest efficient ultraviolet (UV) emission from ZnO [6–8]. These approaches, however, required not only the thickness of 100 nm or more for ZnO thin film, but also much more time in fabricating processes. Thus, ultrathin ZnO film (<30 nm), with its low fabrication cost and time consumption, has a great potential for manufacturing optoelectronic devices. Unfortunately, owing to the relatively small amount of emissive materials involved, light emission from the planar thin films is often very inefficient with weak intensity, and therefore, it may seriously hinder the possibility of any practical applications. For that matter, it is desirable to develop a viable approach that can harvest UV emission from ultrathin ZnO films in a more efficient fashion.
In this report, a time-saving and cost-effective technique is introduced to fabricate Si nanopillars by etching a Si substrate. An alternative method of using the self-assembled silver (Ag) nanoislands as the metal nanomask to manufacture the Si nanopillars was employed. Through the Volmer-Weber growth mode during sputtering , Ag nanoislands were formed. With these Ag nanoislands, Si nanopillars were subsequently produced by dry etching. In general, the Si nanopillars can be obtained by the duration of about 10 s of sputter time and 5 min of dry etching. This kind of fabrication method is thus advantageous because not only the manufacturing process was very effective - only 10 s of duration for obtaining the self-assembled Ag nanomask by sputtering, but also the fabrication cost was reduced through the lithography-free, anisotropic dry etching. In addition, the ZnO/Si nanopillar heterostructures have a high surface/volume ratio, which greatly contributes to the increase of the area of UV emission from ultrathin ZnO films.
A field-emission scanning electron microscope (FESEM, JEOL JSM-6700 F, Tokyo, Japan) was used to examine the morphology of the Ag nanoislands and Si nanopillars, and energy-dispersive X-ray spectroscopy (EDS) was performed to examine the composition of the obtained samples. The structural characteristics of the ultrathin ZnO films were determined by X-ray diffraction (XRD) technique using Cu Kα radiation (λ = 1.54 Å) (PANalytical X'Pert Pro, Philips Inc., Almelo, The Netherlands), which was operated at 45 kV and 40 mA for grazing angle scan. Moreover, the photoluminescence (PL) was measured at room temperature using a He-Cd laser (325 nm) for excitation and a CCD with a monochromator for detection.
Results and discussion
In summary, we have demonstrated that by the combination of self-assembled Ag nanoislands and the subsequent dry etching process to form a Si nanopillar array, ZnO films were deposited on Si nanopillars by ALD, forming ZnO/Si nanopillar heterostructures. According to such results, it was found that compared to the ZnO/Si substrate, the UV emission intensity was strongly enhanced in the ZnO/Si nanopillar heterostructures, which is mainly ascribed to the large surface-to-volume ratio. It is obvious that compared to most of the techniques employed in obtaining similar structures, the current process of using these self-assembled Ag islands as metal nanomasks with subsequent dry etching can produce Si nanopillars, which also has the advantages of both simplicity and effectiveness.
Mr. TYC received his MS degree in mechanical engineering from Feng Chia University, Taiwan, in 1999. He is currently a candidate for a doctor's degree at the Department of Mechanical Engineering, National Chung Hsing University and an instructor at the Department of Mechanical Engineering, National Chin-Yi University of Technology, Taiwan. His research interests are nanotechnology and thin film materials.
Dr. CLD received his MS degree in applied mechanics from National Taiwan University, Taiwan, in 1993 and his Ph.D. degree in mechanical engineering from Nation Taiwan University, in 1997. He is currently a professor at the Department of Mechanical Engineering, National Chung Hsing University, Taiwan. His research interests are microsystem technology, nanotechnology, microsensors, and microactuators.
This work was partially supported by the National Science Council of Taiwan under grant no. NSC 100-2221-E-005-0122. The authors would like to thank Prof. Hsi-Fu Shih and Jyun-Hao Wu for the sputtering (NCHU), Prof. Hsin-Yi Lee (NSRRC and NCTU) for the PL measurements, and Prof. Chih-Ming Lin (NHUE) for the ALD system.
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