Superlattices and Microstructures 112 (2017) 210e217
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Band alignment and optical response of facile grown NiO/ZnO nano-heterojunctions Muhammad Sultan a, *, Sundas Mumtaz a, b, Asad Ali a, Muhammad Yaqoob Khan c, Tahir Iqbal b, ** a
Nanoscience and Technology Department, National Centre for Physics, Quaid-I-Azam University Campus, 44000, Islamabad, Pakistan Department of Physics, Faculty of Science, University of Gujrat, Gujrat, 50700, Punjab, Pakistan c Department of Physics, Kohat University of Science and Technology (KUST), Kohat, 26000, Khyber Pakhtoon Khwa, Pakistan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 June 2017 Received in revised form 9 September 2017 Accepted 10 September 2017 Available online 12 September 2017
ZnO nanorods decorated by NiO nanostructures were fabricated using facile chemical route. The nanorods of ZnO were prepared by using chemical bath deposition technique and subsequently decorated by NiO using sol-gel spin coating. The density and orientation of the ZnO nanorods was controlled through the seed layer with preferential growth along c-axis and hexagonal face. X-Ray Photoelectron Spectroscopy (XPS) analysis was used to confirm stoichiometry of the materials and band alignment study of the heterostructures. Type-II band alignment was observed from the experimental results. The IV characteristics of the device depicting rectifying behavior at different temperatures were observed with photocurrent generation in response to light excitation. The electrical properties reported in this study are in line with earlier work where heterojunctions were fabricated by physical deposition techniques. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Metal oxide semiconductor Heterojunction Band alignment Nanostructured materials Sol-gel processes Electronic properties
1. Introduction Metal oxide semiconductors have the potential to be used for next-generation electronic, optoelectronic and photovoltaic devices [1]. Zinc oxide (ZnO) has got significant attention due to large exciton binding energy, intrinsic n-type conductivity, good carrier confinement and wide direct band gap (3.37 eV) with high transmittance in visible region [2e4]. These novel properties makes it a potential candidate for device applications such as sensors [5,6], electronics [7], UV detectors [8], solar cells [9,10] and solar water splitting [11]. Furthermore, low-temperature-processed high crystalline nature, stability and ease of fabrication makes ZnO important candidate for economical device fabrication. A versatile range of ZnO nanostructures has been reported such as nanorods [12], nanowires [13], and nanotubes [14] in different device architectures. A p-n junction is basic building block for many device applications. Use of metal oxide as electrode in p-n junction devices is under investigation. However development of oxide based homojunction is still challenging because of low solubility of dopants and self-compensation process in the development of p-type oxides. One way to overcome this limitation is the growth of heterojunction. A heterojunction utilizes two distinct materials for the formation of p-n junction utilizing best properties of each constituent. For fabrication of p-n heterojunction with ZnO, various materials such as GaN [15], Si [16], CuO
* Corresponding author. ** Corresponding author. E-mail addresses:
[email protected] (M. Sultan),
[email protected] (T. Iqbal). http://dx.doi.org/10.1016/j.spmi.2017.09.019 0749-6036/© 2017 Elsevier Ltd. All rights reserved.
M. Sultan et al. / Superlattices and Microstructures 112 (2017) 210e217
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[17] and NiO [18] are used in different geometries as p-type counterpart. Amongst these combinations, NiO with direct and wide band gap of 3.6 eV, electrochemical stability, high durability and low cost makes it feasible to be used in various applications [19]. Particularly cathode and anode developed from NiO and ZnO has attracted the interest of researchers in recent years for their possible application in the next generation solar cells and electronic applications [9,20,21]. NiO is intrinsically p-type semiconductor while ZnO is intrinsically n-type semiconductor and their band alignments make them a promising candidate for heterojunction based thin film devices for optoelectronics applications such as LEDs and photodectors [22] [23], [24]. For the fabrication of NiO/ZnO heterostructure devices reported so far, mostly high-temperature and expensive techniques like e-beam evaporation [25], thermal evaporation, sputtering [26], molecular beam epitaxy (MBE) or pulse laser deposition have been utilized [18,25,27,28]. Band alignment studies were also reported on thin films of NiO and ZnO which were prepared by e-beam evaporation and plasma assisted MBE, respectively [29]. Here, we report solution based growth of n-ZnO nanorods decorated with p-NiO nanostructures. Their optical, electrical and spectroscopic studies were performed. The properties of chemically grown heterojunctions were found similar to the recently reported devices grown by physical deposition methods. 2. Experimental 2.1. Synthesis of ZnO nanorods For the growth of semiconductor hybrid nanostructures we used commercially coated FTO glass substrates. First the FTO substrates were cleaned by soaking in soapy water for couple of minutes and then sonicated for half an hour in acetone, ethanol, methanol and in distilled water. Prior to the growth of ZnO nanorods a seed layer of ZnO was deposited on FTO substrates by spin coating method using sol gel of zinc acetate. To prepare the sol gel solution, 0.4 M solution of zinc acetate dehydrate was prepared in ethylene glycol and the solution was stirred for 45 min. After few minutes stirring the solution turned milky and few drops of di-ethanolamine were added as a stabilizer until solution became transparent and subsequently spin coated it at the rate of 3000 rpm for 30 s. After each spin coat the substrates were dried at 300 C for 10 min to remove residual organics from the surface of the coated seed layer. This process was repeated several times to get uniform coverage of the substrate with ZnO seed layer and finally the samples were annealed at 500 C for 1 h [30]. For the growth of ZnO nanorods equimolar (0.1 M) aqueous solution of hexamethyltetramine (HMTA) and zinc nitrate hexahydrate were prepared separately. After stirring each solution separately for few minutes HMTA solution was added to zinc nitrate hexahydrate solution drop wise and the obtained solution was used for the growth of ZnO nanorods. The ZnO seed layer coated substrates were immersed in the growth solution such that the seed layer face of the substrates was in downward direction. We carried out the reaction at 90 C for 3 h. After the growth of the nanorods the substrates were removed from the oven and washed well with distilled water. Finally we annealed our samples at 400 C for 1 h [31e33]. The effect of seed layer and molarity of solution was optimized with different concentrations. For simplicity, only results of above mentioned parameters are presented here. 2.2. Synthesis of NiO thin film For the growth of NiO film, 0.1 M solution of mono-ethanolamine and nickel acetate tetrahydrate in methanol was prepared with 1:1 M ratio. The solution was stirred for 5 h at 90 C. This solution was spin coated on substrate at the rate of 3000 rpm for 30 s. Then NiO substrates were dried for 10 min at 300 C on hot plate. Finally samples were annealed at 550 C for 2 h [34]. 2.3. Characterization methods Morphology of the prepared NiO/ZnO structures was studied by Scanning Electron Microscope (SEM). Crystal structure of ZnO nanorods and NiO thin film were confirmed by X-ray diffraction (Model D8 Advance Bruker Company, wavelength of Cu Ka is 0.1541 nm). Optical transmittance of NiO thin film deposited on glass and ZnO nanorods deposited on FTO were investigated by ultraviolet-near infrared spectrometer UVeVis (Lambda 950 spectrometer). Current-voltage (IV) characteristics were analyzed by Keithley Picoammeter 2440 with standard AM 1.5 solar simulator at different excitation powers. Moreover, individual layers of ZnO and NiO were separately analyzed to make sure that the reported IV characteristics are from NiO/ZnO heterojunction only. Tungsten tips attached with micropositioner were used for direct contact on the NiO side unless otherwise mentioned. For second contact steel clips were directly attached on the bare FTO side. For IV characteristics of ZnO nanorods additional Al foil was introduced to make sure the Ohmic contact under the clip on top of the ZnO nanorods. To avoid the contact errors IV measurements were repeated at different positions of the samples and only reproducible results are reported in this article. The XPS spectroscopy was performed in ultra high vacuum conditions using standard omicron system equipped with monochromatic Al Ka 1486.7 eV X-ray source and Argus hemispherical electron spectrometer with 128 channels MCP detector. CASA XPS software was used for data analysis and curve fitting. C1s was used for the calibration of binding energy. Measurements were repeated three times and uncertainty in the measurements was
