Effect of 120 MeV Au⁹⁺ ion irradiation on the structure and surface morphology of ZnO/NiO heterojunction

Abstract

ZnO/NiO thin films, each of thickness 100 nm, were deposited on Si(100) substrate by pulsed laser deposition method. The resulting heterojunction, ZnO/NiO/Si, was irradiated by 120 MeV Au 9+ ions and characterized by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and atomic force microscopy (AFM). The GIXRD confirmed the presence of both NiO and ZnO in the samples. Ion irradiation induced suppression of crystalline nature, and the recrystallization of the same occurred at the fluence of 1 × 10 13 ions cm −2 . The occurrence of most intense band at 302 cm −1 in Raman spectra corresponds to the symmetric stretching vibration of ZnO. The linear shift of stretching mode of ZnO with ion fluence could be associated with the effect of compressive stress in the material. AFM analysis of the films indicated that the rms roughness increased when the film is irradiated at a fluence of 1 × 10 12 ions cm −2 .Beyond this fluence, the value of roughness decreased up to fluence of 1 × 10 13 ions cm −2 and increased thereafter. To see the effect of the stress of buffer layer on the surface layer, we calculated the stress for NiO layer with ion fluence form the lattice parameter. Comparing the stress of buffer layer with roughness of surface layer at the given fluence, we can say that the compressive stress in the buffer layer could possibly control the roughness of the surface layer. KEYWORDS atomic force microscope, heterostructure, ion irradiation, stress 1 | INTRODUCTION Heterojunction devices, formed by combining semiconductors with different band gap and closely matching lattice parameters, have attracted much attention in optoelectronics. 1 ZnO is a wide-band gap (3.37 eV) semiconductor of the II-VI group. 2 It exhibits intrinsic n-type semiconducting property due to oxygen vacancies and zinc interstitials. It exhibits several properties like large exciton binding energy, 3 good transparency, 4 high electron mobility, 5 high thermal conductivity, 6 and strong room-temperature luminescence 7 for various device applications like liquid crystal displays, transparent conducting electrodes in solar cells, light emitting diodes, thin film transistors, gas sensors, and spintronic applications. 8 NiO is another wide band gap semiconductor that shows p-type behaviour due to the vacancy at Ni 2+ sites. NiO is widely used for applications such as transparent conductive films, 9 anode material, 10 and electrochromic devices. 11 Heterojunctions formed out of two layers (n-type ZnO and p-type NiO) can exhibit the potential of both the materials, in addition to the junction characteristics. In 2003, Ohta et al., 12 first reported the preparation of transparent NiO/ZnO heterojunction by pulsed laser deposition (PLD) combined with a solid-phase-epitaxy technique. Since then, many developments have been reported on the preparation and application of this heterojunction. These include methods like sol-gel spin coating, 13 chemical bath deposition, 4 hydrothermal method, 14and PLD. 15 On the application front, use of the hete...

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“…The peaks centered at 235 cm −1 may correspond to undesired adulteration [54]. The peaks at 302 cm −1 , 617 cm −1 , and 670 cm −1 originate from symmetric stretching vibration, E1 (LO) modes, and the TA + LO intrinsic mode of ZnO [55,56]. The peak centered at 435 cm −1 corresponds to the E2 (high) of ZnO, corresponding to the energy band characteristic peak of wurtzite ZnO and confirming the formation of wurtzite ZnO (Figure 5i) [57,58].…”