Modification of optical and electrical properties of zinc oxide-coated porous silicon nanostructures induced by swift heavy ion
© Kumar et al., licensee Springer. 2012
Received: 30 April 2012
Accepted: 2 July 2012
Published: 2 July 2012
Morphological and optical characteristics of radio frequency-sputtered zinc aluminum oxide over porous silicon (PS) substrates were studied before and after irradiating composite films with 130 MeV of nickel ions at different fluences varying from 1 × 1012 to 3 × 1013 ions/cm2. The effect of irradiation on the composite structure was investigated by scanning electron microscopy, X-ray diffraction (XRD), photoluminescence (PL), and cathodoluminescence spectroscopy. Current–voltage characteristics of ZnO-PS heterojunctions were also measured. As compared to the granular crystallites of zinc oxide layer, Al-doped zinc oxide (ZnO) layer showed a flaky structure. The PL spectrum of the pristine composite structure consists of the emission from the ZnO layer as well as the near-infrared emission from the PS substrate. Due to an increase in the number of deep-level defects, possibly oxygen vacancies after swift ion irradiation, PS-Al-doped ZnO nanocomposites formed with high-porosity PS are shown to demonstrate a broadening in the PL emission band, leading to the white light emission. The broadening effect is found to increase with an increase in the ion fluence and porosity. XRD study revealed the relative resistance of the film against the irradiation, i.e., the irradiation of the structure failed to completely amorphize the structure, suggesting its possible application in optoelectronics and sensing applications under harsh radiation conditions.
Nowadays, efforts are being made to look for suitable types of nanocomposites for optoelectronic applications. Semiconductor nanocrystallites have been considered as the emission source for the next-generation light-emitting diodes due to their electro-optical properties and tunable size [1, 2]. Zinc oxide (ZnO) with trivalent elements such as aluminum (Al) is a unique n-type semiconductor and transparent material with a direct bandgap of 3.37 eV, along with a large exciton binding energy of 60 meV . Al-doped ZnO (AZO) is considered as an important material for its application as a transparent electrode in flat panel displays  due to its high conductivity and good transparency. Till now, AZO films with resistivity lower than 1 to 5 × 10−4 Ω cm [5–7] and transmittance more than 85 % have been attained. On the other hand, porous silicon (PS) has been investigated due to its room-temperature luminescence, and efforts have been focused to obtain an efficient electroluminescent (EL) device  based on PS for its possible integration with the present microelectronic industry. Along with various semiconducting and piezoelectric properties suitable for various applications in the optoelectronic industry, the EL efficiency could be increased strongly by filling the pores of PS with AZO. Due to the open structure and large surface area, together with the unique optical properties, PS is a good candidate for a template. It is known that the emission energy of PS increases with a decrease in silicon nanocrystallite size covering the entire visible spectrum from red to blue . It has been reported that the blue luminescence band with a relatively fast decay time is observed in the oxidized PS samples. On the other hand, it is easy to get the red emission from the PS, and if it could be added with any other semiconductor with emission in the blue green region, it could be useful to obtain the white light through a simple route for possible applications like the display technology [4, 10]. Apart from that, our group recently demonstrated the importance of PS-ZnO composites for sensing application through a control over the spacial distribution of zinc oxide and its transport properties on the porosity of the PS substrate .
In the last few years, a considerable amount of progress has been done to enhance the optical properties and other physical characteristics of the ZnO film with different techniques. Among them, energetic ion beams have been employed to modify the electrical, optical, and structural properties of different materials [12–15]. Matsunami et al.  studied the effect of irradiation on the electrical, structural, and optical properties of indium-doped ZnO films and found an enhancement in the electrical conductivity. Sugai et al.  reported a two-order increase in the conductivity of AZO films after irradiation with Ni and Xe ions. Recently, Singh et al.  have reported an increase in the ethanol sensing response of irradiated SnO2 films with ZnO demonstrating a strong resistance to damage caused by ion irradiation. Another work on ZnO-PS nanocomposites , where ZnO deposited onto the microporous silicon with sol–gel technique, showed the suppression of X-ray diffraction (XRD) peaks after irradiating it with heavy ions (Au). Hence, the effect of high-energy light ions (such as nickel) could be interesting in studying the stability of the ZnO structure along with its optical properties. In this work, we have investigated the ion irradiation effects on AZO films deposited onto the mesoporous silicon substrate and shown a white light emission from the resulting composites after irradiating with high-energy nickel ions. Swift heavy ion (SHI)-induced morphological and structural changes, in terms of XRD and scanning electron microscopy, have also been studied. The emission after 325-nm excitation from a xenon arc lamp has been compared with the cathodoluminescence (CL) in studying the modifications in the deep-level defects induced by the high-energy radiations. Comparison between the low- and high-porosity mesoporous substrates is also presented. The nanocomposites are found to retain the crystalline structure after irradiation.
PS samples were fabricated by wet electrochemical etching of p ++ -type Si(100) wafers with a resistivity of 0.01 to 0.05 Ω cm and at different current densities of 10 (LP) and 70 mA/cm2 (HP) using a 3:7:1 solution of HF/ethanol/glycerol. The porosity of samples LP (low porosity) and HP (high porosity) was 50 % and 70 %, respectively. The thickness of both samples was kept to 7 μm. After the fabrication, the samples were rinsed with ethanol and dried in pentane.
In order to study the effect of PS on PL and other structural and transport properties, AZO films were deposited by radio frequency magnetron sputtering. A sputtered target with a mixture (2 wt.% Al2O3-ZnO) was used, and the PS substrate temperature was kept at 300 °C during the deposition of the AZO film with a thickness of 150 nm. After deposition, the low- and high-porosity PS-AZO composites were named as ZLP and ZHP, respectively. As-deposited films were later annealed at 700 °C for 1 h in the tubular furnace in argon atmosphere. The annealed films were irradiated with 130-MeV nickel ions using the 15UD Pelletron Accelerator at the Inter University Accelerator Centre, New Delhi. The samples were mounted on a rectangular-shaped ladder and were irradiated in high vacuum chamber. The focused ion beam was scanned over an area of 1 × 1 cm2. The films of low porosity (ZLP) were irradiated with fluences of 1 × 1012 and 3 × 1013 ions/cm2, and the films of high porosity (ZHP) were irradiated with fluences of 3 × 1012 and 1 × 1013 ions/cm2. The beam current was kept constant at approximately 1.5 pnA. The electronic stopping power (energy dissipated in electronic excitations) and nuclear stopping power (energy dissipated in atomic collisions) by such ions in ZnO are around 24.63 and 0.44 keV/nm, respectively (calculated using SRIM2003 simulation code). The modifications in the properties of ZnO films are expected to be mainly due to the electronic excitations, though the contributions of small nuclear stopping power could not be ignored.
The structural properties and the thickness of the PS and nanocomposites were analyzed using a high-resolution field emission scanning electron microscope (SEM; JSM-7401 F, JEOL Ltd., Akishima-shi, Japan). The orientation and crystallinity of the ZnO crystallites were analyzed using an X-ray diffractometer (X'Pert PRO, PANalytical B.V., Almelo, The Netherlands) using CuKα radiation having a wavelength of 1.54 Å. PL properties were studied using a Varian Fluorescence spectrophotometer (Cary Eclipse, Agilent Technologies, Inc., Santa Clara, CA, USA) under the excitation by 325-nm photons using a 500-W xenon lamp. CL spectroscopy was done using JEOL JSM 5300 SEM with an electron beam energy of 15 keV. CL measurements were performed at 100 K in the UV-visible spectral range using a Hamamatsu R928P photomultiplier (Hamamatsu, Japan). A SPEX 340-E computer-controlled monochromator (Metuchen, NJ, USA) was used for the spectral analysis.
Results and discussion
Measurement of bandgap by optical method
For determining the bandgap of the ZnO film, the absorption coefficient (α) is obtained from transmittance data using the following equation:
Scanning electron microscopy
Although a similar behavior was observed for the ZHP sample as well (see Figure 4e,f), an increase of fluence is shown to enhance significantly the defect-related emission centered at 2.5 eV. The peaks corresponding to 2.5 to 3.25 eV have been observed to merge, resulting in the formation of broad white emission band from 3.25 to 1.50 eV. In comparing Figure 4e,f, a further increase in the level of fluence is found to increase the signal corresponding to the defects (2.5 eV) with a simultaneous decrease in the band-edge emission (3.25 eV). The intensity of the emission band corresponding to PS (around 1.7 eV) is found to decrease slightly with an increase in the level of fluence which is in accordance to the already reported work of Singh et al. . The resulting PL spectra are observed to have an almost white emission from the composite structure which can be very useful for the display devices.
We report the substrate porosity and fluence-dependent white light emission from RF-sputtered zinc aluminum oxide deposited on PS after SHI irradiation. The structures are shown to partially retain their crystallinity, when irradiated with light Ni ions. Composites are found to have a rectifying behavior which reduces by a factor of 0.78 after irradiation. Its stability under harsh irradiation conditions makes it useful for space applications. Apart from that, the tunability in the optical properties is important for optoelectronic applications.
YK is a Ph.D. student registered at CIMAV, Chihuahua, and is doing his research work at CIICAp, UAEM, Mexico. MHZ is an associate professor at CNyN, UNAM and is working on the characterization of semiconductor nanostructures. SFOM is working as a researcher at CIMAV, Chihuahua, in the field of nanostructured materials. FS is a scientist at IUAC, New Delhi. XM is a senior scientist at CIE, UNAM working on thin film semiconductors for photovoltaic applications.VA is working as a professor investigator at CIICAp, UAEM in the field of nanostructured silicon.
The authors are thankful to CONACyT, Mexico, and DST, Government of India, for providing the bilateral exchange project (Mexico-India J000.0374 and DST/INT/MEX/RPO-09/2008). VA also acknowledges the partial support given by CONACyT project number 128593. MHZ acknowledges the support of the project CONACyT 102519. YK acknowledges the CONACyT support (179496) for doctoral scholarship and the fruitful discussion with Dr. Karl-Heinz Heinig. We acknowledge the technical support provided by MC Enrique Torres for doing the XRD and Wilber Antunez for SEM at CIMAV-Chihuahua.
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