Non-polar a-plane oriented ZnO:Al thin films for optoelectronic applications
Introduction
Over two decades, enormous attention has been laid on the transparent conducting thin films (TCFs) due to their effective applications in gas sensors, flat panel displays and optoelectronic devices such as light-emitting diodes, photodiodes and solar cells [[1], [2], [3], [4], [5]]. The fluorinated tin oxide and indium tin oxide TCFs have been extensively used in such applications over years. However, group II-VI semiconducting zinc oxide (ZnO) has proven to be best alternate and potential material due to its suitable material properties such as wide band gap (3.37 eV), large excitation binding energy (60 meV), and low processing cost in the fabrication of TCF based optoelectronic devices.
Several researchers have put efforts to understand the effect of doping with different elements such as Al, Ga, In, Co, and Fe in tailoring the physical properties of ZnO thin films. Amongst different dopants, Al-doped ZnO (AZO) has played an important role in enhancing the optoelectronic properties of ZnO based TCFs' [1,2]. AZO (wurtzite structure) generally grows along polar (c-axis) and non-polar (m-plane (101) and a-plane (110)) orientations [1,3,4]. The crystallite orientation in the films plays a decisive role in modifying the physical properties of TCFs and optoelectronic devices. However, the induced electric field in ZnO would negatively impact on the absorption and emission spectrum (quantum confined stark effect (QCSE)). This deteriorates the quantum efficiency of ZnO based LEDs. The QCSE results in the shifting of electrons and holes energy states towards lower and higher energy states respectively. Further, such a shift in energy values of electrons and holes leads to the reduction in the permitted light absorption or emission wavelengths [[4], [5], [6]]. The exhibition of spontaneous polarization of charges (built-in electric field) along the growth direction in polar ZnO, affects the optoelectronic properties of LEDs [7,8]. Non-polar a-plane and m-plane oriented ZnO nanostructures have no spontaneous polarization along the growth direction. Therefore, these nanostructures play a significant role to improve the devices’ properties by reducing the QCSE.
The method of fabrication and film growth conditions such as type of substrates and substrate's temperature play a considerable role in the fabrication of thin films. So far, the growth of a-plane orientated ZnO and AZO thin film has been achieved by Molecular Beam Epitaxy, Radio Frequency Magnetron Sputtering, Chemical spray pyrolysis and, Pulsed laser deposition techniques [[9], [10], [11], [12], [13], [14], [15]]. L Long et al. reviewed the difficulties faced in the fabrication of high-quality non-polar ZnO and AZO films for LED device applications [4]. X Chen et al. and Dong et al. have fabricated c-axis oriented AZO thin films on glass substrates by PLD but they failed to attain a-plane oriented AZO films [16,17]. To the best of our knowledge, no investigation has been reported on the a-plane oriented AZO grown on a glass substrate by employing PLD technique till date. The pulsed laser deposition (PLD) technique enables to achieve the desired orientation of metal oxide and semiconducting thin films by tuning the growth conditions. It is important to grow a non-polar a plane oriented AZO film on glass substrates to use them as TCFs for LED applications. Further, the low cost and ease of the process of glass substrates offer advantages over other substrates such as r-sapphire, Si, and LiAlO2.
In the present work, a non-polar, a-plane oriented AZO thin film has been successfully grown on corning glass substrate by PLD technique. The modifications in optoelectronic properties of AZO films due to variation in laser fluence are analyzed and discussed. To the best of our knowledge, for the first time we are reporting the fabrication of a-plane oriented AZO film using pulsed laser deposition technique. The deposited non-polar, a-plane oriented AZO thin films possess no electrostatic field along the surface and have several advantages over polar ZnO thin films. Also, AZO thin film is grown on glass substrates, reduces the cost of fabrication of LED devices.
Section snippets
Material and methods
The ZnO (99.999% purity) and Al2O3 (99.997% purity) powders procured from Sigma Aldrich were used in the preparation of the target. 5 wt% of Al doped ZnO target (diameter 2.5 cm) was prepared by applying 1000 kg/cm2 pressure by employing a hydraulic pressing technique. The target was sintered at 1000 °C for 10 h in a muffle furnace, and a rigorous polishing routine was followed to smoothen the surface to avoid the problem of particulates. AZO pellet was ablated by second harmonic Q-switched
Structural studies
The crystal structure of deposited AZO thin films were investigated by X-ray diffraction. Fig. 1 shows the XRD diffractograms of AZO thin films deposited on glass substrate at different fluences. The XRD diffractogram of AZO1 shows no peaks under the detection limit of X-ray diffractometer employed in this study. The absence of AZO related peaks in AZO1 is ascribed to its lower thickness value (80 nm). However, with increasing thickness, AZO2 and AZO3 show some peaks confirming the crystalline
Conclusion
A non-polar a-plane oriented AZO thin film has been grown on amorphous glass substrates for the first time, using the pulsed laser deposition technique. The linear optical properties showed optimized transmittance and optical energy gap values in the films. The lower resistivity and higher figure of merit values suggest the films well suited for electronic applications. An n-type semi conductivity with increased mobility is observed in the AZO3 film from the Hall measurements. These
Credit author statement
Prasad Kumar: Conceptualization, Methodology, Data curation, Writing – original draft preparation, Formal analysis, Parvathy Venu M: Conceptualization, Visualization, K. M. Sandeep: Investigation, Writing – review & editing, Veena Shivadas Kindalkar: Data curation, Ananya Kote M: Methodology, Writing – review & editing. S. M. Dharmaprakash: Supervision, Visualization, Investigation, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge the Coordinator, DST FIST, UGC CPEPA, CeNSE–IISc, Department of Physics Karnatak University Dharwad and Department of Physics Mangalore University for providing the experimental and characterizations facility.
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