A Review on Microstructural, Optical, Electrical and Gas Sensing Properties of Aluminum-doped Zinc Oxide Thin Films for Gas Sensing and Optoelectronic Applications

Santus C. Onuegbu; S. S. Oluyamo; O.  I. Olusola

doi:10.20221/jnmsr.v1i.4

Authors

Santus C. Onuegbu Department of Physics, Faculty of Science, Alex Ekwueme Federal University, Ndufu-Alike, P.M.B 1010, Ebonyi State, Nigeria
S. S. Oluyamo Department of Physics, Faculty of Science, Alex Ekwueme Federal University, Ndufu-Alike, P.M.B 1010, Ebonyi State, Nigeria
O. I. Olusola Department of Physics, School of Science, The Federal University of Technology, Akure (FUTA), P.M.B. 704, Nigeria

DOI:

https://doi.org/10.20221/jnmsr.v1i.4

Keywords:

Al-doped zinc oxide, Thin films, Electrical properties, Gas sensing properties, Porous morphology

Abstract

In recent years, Al-doped ZnO nanoparticles have been proven to be a promising transparent material applicable in the fabrication of varistors, gas sensors, and thermoelectric, piezoelectric, and electro-optical devices due to its high electrical conductivity and porous morphological nature. Doping ZnO with Al-ions has demonstrated to have strong impact on the overall properties of ZnO nanostructures and significantly improve the electronic performance of ZnO nanomaterial. However, this review summarizes the effect of Aluminium ions as a dopant on the structural, optical, and electrical and gas sensing properties of zinc oxide thin films. The study observed that the presence of Aluminium ions in the ZnO film significantly improved the electronic and gas sensing properties of the films; yet it was remarkably noted from the assembled data that the structural quality of the AZO deteriorated heavily upon increasing the Al-dopants. The morphology of the Al-ZnO thin films was severely influenced, as the study showed that increasing the Al³⁺/Zn²⁺ ratio transited the AZO films from hexagonal nanocrystalline into a porous mixture of nanosheets, nanorods, nanowires, narcissus, wrinkled and cluster structures. However, considering the desired morphology and phase purity, as well as enhanced electrical properties, gas sensing properties, and optical transmittance, the optimal window of Al³⁺/Zn²⁺ ratio in the fabrication of Aluminium doped zinc oxide thin film was observed between 1at% and 6at%, beyond which the properties of the AZO as observed from the experimental data will no longer be suitable for optoelectronic and gas sensing applications. Further, the authors concluded the review with advancement, challenges, and recommendation for future improvement of Al-doped ZnO as a transparent material for optoelectronic and gas sensing applications.

References

Y. F. Hu, Y. L. Chang, P. Fei, R. L. Snyder, and Z. L. Wang, “Designing the electric transport characteristics of ZnO micro/- nanowire devices by coupling piezoelectric and photoexcitation Effects,” ACS Nano, vol. 4, no. 2, pp. 1234–1240, 2010

Z. R. Tian, J. A. Voigt, J. Liu et al., “Complex and oriented ZnO nanostructures,” Nature Materials, vol. 2, no. 12, pp. 821–826, 2003

L. Gong, J. G. Lu, and Z. Z. Ye, “Transparent conductive Ga- doped ZnO/Cu multilayers prepared on polymer substrates at room temperature,” Solar Energy Materials & Solar Cells, vol. 95, no. 7, pp. 1826–1830, 2011

Z. L. Wang and J. H. Song, “Piezoelectric nanogenerators based on zinc oxide nanowire arrays,” Science, vol. 312, no. 5771, pp. 242–246, 2006

D. Tamvakos, S. Lepadatu, V. A. Antohe et al., “Piezoelectric properties of template-free electrochemically grown ZnO nanorod arrays,” Applied Surface Science, vol. 356, pp. 1214– 1220, 2015.

P. S. Shewale and Y. S. Yu, “Structural, surface morphological and UV photodetection properties of pulsed laser deposited Mg-doped ZnO nanorods: effect of growth time,” Journal of Alloys and Compounds, vol. 654, no. 5, pp. 79–86, 2016

J. J. Li, L. T. Zhang, J. B. Zhu, Y. Liu, and W. C. Hao, “Aligned ZnO:Co nanorod arrays: Electrophoretic deposition fabrica- tion and magnetic manipulation,” Ceramics International, vol. 41, no. 3, pp. 3456–3460, 2015.

S.K. Mishra, D. Kumari, B.D. Gupta, Surface plasmon resonance based fiber optic ammonia gas sensor using ITO and polyaniline, Sens. Actuators B 171–172 (2012) 976–983

M. Sinha, R. Mahapatra, B. Mondal, T. Maruyama, R. Ghosh, Ultrafast and reversible gas-sensing properties of ZnO nanowire arrays grown by hydrothermal technique, J. Phys. Chem. C. 120 (2016) 3019–3025

M. Sundararajan, P. Sakthivel, A.C. Fernandez, Structural, optical and electrical properties of ZnO-ZnS nanocomposites prepared by simple hydrothermal method, J. Alloys Compd. 768 (2018) 553–562

A. Franco Jr., H.V.S. Pessoni, Enhanced dielectric constant of Co-doped ZnO nano particulate powders, Physica B 476 (2015) 12–18.

S. Singh, P. Dey, J.N. Roy, S.K. Mandal, Tunable dielectric constant with transition metal doping in Zn1-x(MnTM)xO (TM ¼ Co, Fe) nanocrystals, J. Alloys Compd. 642 (2015) 15–21.

Bizarro, M. High photocatalytic activity of ZnO and ZnO: Al nanostructured films deposited by spray pyrolysis. Appl. Catal., B 2010, 97 (1), 198−203.

Saravanakumar, K.; Sakthivel, B.; Ravichandran, K. Simulta- neous doping of aluminum and fluorine on zinc oxide nanopowder using a low-cost soft chemical route. Mater. Lett. 2011, 65 (14), 2278− 2280

M. Jiang, X. Liu, H. Wang, Conductive and transparent Bi-doped ZnO thin films prepared by rf magnetron sputtering, Surface & Coatings Technology, 203 (2009) 3750- 53

Y. Ammaih, A. Lfakir, B. Hartiti, A. Ridah, P. Thevenin, and M. Siadat, ‘Structural , optical and electrical properties of ZnO : Al thin films for optoelectronic applications’, pp. 229–234, 2014, doi: 10.1007/s11082-013-9757-2.

N. Srinatha, P. Raghu, H. M. Mahesh, and B. Angadi, ‘Spin-coated Al-doped ZnO thin films for optical applications : Structural, micro-structural, optical and luminescence studies’, J. Alloys Compd., vol. 722, pp. 888–895, 2017, doi: 10.1016/j.jallcom.2017.06.182.

Berginski, M.; Hupkes, J.; Schulte, M.; Schope, G.; Stiebig, H.; Rech, B.; Wuttig, M. The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells. J. Appl. Phys.2007, 101, 074903.

Minami, T.; Sato, H.; Nanto, H.; Takata, S. Heat treatment in hydrogen gas and plasma for transparent conducting oxide films such as ZnO, SnO2 and indium tin oxide. Thin Solid Films 1989, 176, 277–282

Dhere, R.G.; Bonnet-Eymard, M.; Charlet, E.; Peter, E.; Duenow, J.N.; Li, J.V.; Kuciauskas, D.; Gessert, T.A. CdTe solar cell with industrial Al:ZnO on soda-lime glass. Thin Solid Films 2011, 519, 7142–7145

Vasekar, P.S.; Dhere, N.G.; Moutinho, H. Development of CIGS2 solar cells with lower absorber thickness. Sol. Energy 2009, 83, 1566–1570

Park, S.; Ikegami, T.; Ebihara, K. Growth of transparent conductive Al-doped ZnO thin films and device applications. Jpn. J. Appl. Phys. 2006, 45, 8453–8456

Saravanakumar, B.; Mohan, R.; Thiyagarajan, K.; Kim, S. J. Investigation of UV photoresponse property of Al, N co-doped ZnO film. J. Alloys Compd. 2013, 580 (15), 538−543

Verma, A.; Khan, F.; Kumar, D.; Kar, M.; Chakravarty, B.C.; Singh, S.N.; Husain, M. Sol–gel derived aluminum doped zinc oxide for application as anti-reflection coating in terrestrial silicon solar cells. Thin Solid Films 2010, 518, 2649–2653.

Mondragon-Suarez, H.; Maldonado, A.; de la L Olvera, M.; Reyes, A.; Castanedo-Perez, R.; Torres-Delgado, G.; Asomoza, R. ZnO:Al thin films obtained by chemical spray: Effect of the Al concentration. Appl. Surf. Sci. 2002, 193, 52–59.

Ma, J.; Ji, F.; Ma, H.; Li, S. Preparation and properties of transparent conducting zinc oxide and aluminium- doped zinc oxide films prepared by evaporating method. Sol. Energy Mater. Sol. Cells 2000, 60, 341–348

Liu, Y.; Lian, J. Optical and electrical properties of aluminum doped ZnO thin films grown by pulsed laser deposition. Appl. Surf. Sci. 2007, 253, 3727–3730

Yang, W.; Wu, Z.; Liu, Z.; Pang, A.; Tu, Y.; Feng, Z.C. Room temperature deposition of Al-doped ZnO films on quartz substrates by radio-frequency magnetron sputtering and effects of thermal annealing. Thin Solid Films 2010, 519, 31–36

Tynell, T.; Yamauchi, H.; Karppinen, M.; Okazaki, R.; Terasaki, I. Atomic layer deposition of Al-doped ZnO thin films. J. Vac. Sci. Technol. A 2013, 31, A109-A1–01A109-4

C. H. Lee and D. W. Kim, ‘Preparation of Al doped ZnO thin films by MOCVD using ultrasonic atomization’, pp. 12–16, 2014, doi: 10.1007/s10832-014-9895-3.

D. T. Speaks, ‘Effect of concentration , aging , and annealing on sol gel ZnO and Al-doped ZnO thin films’, 2020.

Y. Hou, A.M. Soleimanpour, A.H. Jayatissa, Low resistive aluminum doped nanocrystalline zinc oxide for reducing gas sensor application via sol–gel process, Sens. Actuators B 177 (2013) 761–769

L. Cao, L.P. Zhu, J. Jiang, R. Zhao, Z.Z. Ye, B.H. Zhao, Highly transparent and conducting fluorine-doped ZnO thin films prepared by pulsed laser deposition, Sol. Energy Mater. Sol. Cells 95 (2011) 894–898

Dimitrov, Dimitre, Che-liang Tsai, Stefan Petrov, Vera Marinova, Dimitrina Petrova, Blagovest Napoleonov, and others, ‘Atomic Layer-Deposited Al-Doped ZnO Thin Films for Display Applications’, 2020, 1–12, doi: 10.3390/coatings10060539

E. Manikandan, V. Murugan, G. Kavitha, P. Babu, and M. Maaza, ‘Nano flower rod wire-like structures of dual metal ( Al and Cr ) doped ZnO thin fi lms : Structural , optical and electronic properties’, Mater. Lett., vol. 131, pp. 225–228, 2014, doi: 10.1016/j.matlet.2014.06.008.

A. A. Al-Ghamdi, O. A. Al-Hartony, M. El Okr, A. M. Nawar, S. El-Gazzar, Farid El-Tantawy, F. Yakuphanoglu, Spectrochemic Acta Part A; Molecular Biomolecular Spectroscopy, 131, 15 October 2014, 512-517

D. T. Speaks, ‘Effect of concentration , aging , and annealing on sol gel ZnO and Al-doped ZnO thin films’, 2020

Kathwate, L. H., Umadevi, G., Kulal, P. M., Nagaraju, P., Dubal, D. P., Nanjundan, A. K., & Mote, V. D. (2020). Ammonia gas sensing properties of Al doped ZnO thin films. Sensors and Actuators, A: Physical, 313, 112193. https://doi.org/10.1016/j.sna.2020.112193

P. Banerjee, W.-J. Lee, K.-R. Bae, S.B. Lee, G.W. Rubloff, J. Appl. Phys. 108, 043504 (2010)

H. Ahmoum, M. Boughrara, M.S. Su’ait, M. Kerouad, Chem. Phys. Lett. 719, 45 (2019)

S. P. B. K. V Bangera, ‘Fast detection and discriminative analysis of volatile organic compounds using Al doped ZnO thin films’, Appl. Phys. A, pp. 1–10, 2021, doi: 10.1007/s00339-021-04771-8.

X. Zhang, Y. Chen, S. Zhang, and C. Qiu, ‘High photocatalytic performance of high concentration Al-doped ZnO nanoparticles’, Sep. Purif. Technol., vol. 172, pp. 236–241, 2017, doi: 10.1016/j.seppur.2016.08.016

B. Sarma, D. Barman, and B. K. Sarma, ‘Applied Surface Science AZO ( Al : ZnO ) thin films with high figure of merit as stable indium free transparent conducting oxide’, vol. 479, no. January, pp. 786–795, 2019, doi: 10.1016/j.apsusc.2019.02.146.

M. Hjiri, L. El Mir, S. G. Leonardi, A. Pistone, L. Mavilia, and G. Neri, ‘Sensors and Actuators B : Chemical Al-doped ZnO for highly sensitive CO gas sensors’, Sensors Actuators B. Chem., vol. 196, pp. 413–420, 2014, doi: 10.1016/j.snb.2014.01.068

M. A. Kaid and A. Ashour, Applied Surface Science. Volume 253, Issue 6, 15 January 2007, Pages 3029-3033

E. Manikandan, V. Murugan, G. Kavitha, P. Babu, and M. Maaza, „Nano flower rod wire-like structures of dual metal ( Al and Cr ) doped ZnO thin films : Structural , optical and electronic properties‟, Mater. Lett., vol. 131, pp. 225–228, 2014, doi: 10.1016/j.matlet.2014.06.008

A. A. Al-Ghamdi, O. A. Al-Hartony, M. El Okr, A. M. Nawar, S. El-Gazzar, Farid El-Tantawy, F. Yakuphanoglu, Spectrochemic Acta Part A; Molecular Biomolecular Spectroscopy, 131, 15 October 2014, 512-517

Dominic B. Potter, Davinder S. Bhachu, Michael J. Powell, Jawwad A. Darr, Ivan P. Parkin,and Claire J. Carmalt, Phys. Status Solidi A 213,No. 5, 1346–1352 (2016)

K. Cheng et al., Applied Surface Science Surface microstructure evolution of highly transparent and conductive Al-doped ZnO thin films and its application in CIGS solar cells‟, Appl. Surf. Sci., vol. 409, pp. 124–131, 2017, doi: 10.1016/j.apsusc.2017.03.044

Y. Khaaissa et al., (2020) Ultrasonic Spray-Assisted CVD Growth of Highly Transparent and Conductive Aluminum-doped ZnO, Surface Review and Letters, doi: 10.1142/S0218625X20500249

Ji, X., Song, J., Wu, T., Tian, Y., Han, B., Liu, X., Wang, H., Gui, Y., Ding, Y., & Wang, Y. (2019). Solar Energy Materials and Solar Cells Fabrication of high-performance F and Al co-doped ZnO transparent conductive films for use in perovskite solar cells. Solar Energy Materials and Solar Cells, 190 (June 2018), 6–11. https://doi.org/10.1016/j.solmat.2018.10.009

M. Islam, M. Rahman, J. Podder, and S. Farhad, ‘Structural, optical and photocatalysis properties of sol–gel deposited Al-doped ZnO thin films’, Surfaces and Interfaces, vol. 16, 2019, doi: 10.1016/j.surfin.2019.05.007

G. Petrucci, D. Caputo, A. Nascetti, N. Lovecchio, E. Parisi, S. Alameddine, G. de Cesare, A. Zahra, Thermal characterization of thin film heater for lab-on-chip application, in: 2015 XVIII AISEM Annu. Conf., IEEE, 2015, pp. 1–4, https://doi. org/10.1109/AISEM.2015.7066835

B.W. An, E.-J. Gwak, K. Kim, Y.-C. Kim, J. Jang, J.-Y. Kim, J.-U. Park, Stretchable, transparent electrodes as wearable heaters using nanotrough networks of metallic glasses with superior mechanical properties and thermal stability, Nano Lett. 16 (2016) 471–478 https://doi.org/10.1021/acs.nanolett.5b04134

Jayathilake, D. S. Y., Sagu, J. S., & Wijayantha, K. G. U. (2019). Transparent heater based on Al , Ga co-doped ZnO thin films. Materials Letters, 237, 249–252. https://doi.org/10.1016/j.matlet.2018.11.092

S. Kurtaran, ‘Al doped ZnO thin films obtained by spray pyrolysis technique : Influence of different annealing time’, vol. 114, no. December 2020, pp. 20–25, 2021.

S. Kurtaran, S. Alda˘g, G. ¨Of¨ofo˘glu, I. Akyüz, F. Atay, On the role of Al in ultrasonically sprayed ZnO films, Mater. Chem. Phys. 185 (2017) 137–142.

Y. Zhang, Z. Fei, H. Huang, T. Lü, and R. Mu, ‘In fl uence of ZnO Cap Layer Morphology on the Electrical Properties and Thermal Stability of Al-Doped ZnO Films’, vol. 2000025, pp. 1–7, 2020, doi: 10.1002/pssa.202000025.

N. Kumar, A. Haider, B. Bahrami, and M. Reza, ‘Origin of enhanced carrier mobility and electrical conductivity in seed-layer assisted sputtered grown Al doped ZnO thin fi lms’, Thin Solid Films, vol. 700, no. May 2019, p. 137916, 2020, doi: 10.1016/j.tsf.2020.137916.

K.N. Tonny, R. Rafique, A. Sharmin, M.S. Bashar, Z.H. Mahmood, Electrical, optical and structural properties of transparent conducting Al doped ZnO (AZO) deposited by sol-gel spin coating, AIP Adv. 8 (2018) 065307

J.B. Kim, S.B. Jin, L. Wen, P. Premphet, K. Leksakul, J.G. Han, Low temperature, high conductivity Al-doped ZnO film fabrication using modified facing target sputtering, Thin Solid Films 587 (2015) 88–93.

D.K. Kim, H.B. Kim, Room temperature deposition of Al-doped ZnO thin films on glass by RF magnetron sputtering under different Ar gas pressure, J. Alloy. Compd. 509 (2011) 421–425

T. Yang, S. Song, Y. Li, Y. Xin, G. Du, M. Lv, S. Han, The enhanced conductivity and stability of AzO thin films with a TiO2 buffer layer, Physica B 407 (2012) 4518–4522

P. Sagar, M. Kumar, R.M. Mehra, Influence of hydrogen incorporation in sol-gel derived aluminum doped ZnO thin films, Thin Solid Films 489 (2005) 94–98

P.-.C. Yao, S.-.T. Hang, Y.-.S. Lin, W.-.T. Yen, Y.-.C. Lin, Optical and electrical characteristics of Al-doped ZnO thin films prepared by aqueous phase deposition, Appl. Surf. Sci. 257 (2010) 1441–1448

Y.-.S. Liu, C.-.Y. Hsieh, Y.-.J. Wu, Y.-.S. Wei, P.-.M. Lee, H.-.M. Hsieh, C.-.Y. Liu, Mechanism of conductivity degradation of AzO thin film in high humidity ambient, Appl. Surf. Sci. 282 (2013) 32–37

Z.P. Kan, Z.W. Wang, Y. Firdaus, M. Babics, H.N. Alshareef, P.M. Beaujuge, Atomic- layer-deposited AzO outperforms ITO in high-efficiency polymer solar cells, J. Mater. Chem. A 6 (2018) 10176–10183.

R.N. Chauhan, N. Tiwari, R.S. Anand, J. Kumar, Development of Al-doped ZnO thin film as a transparent cathode and anode for application in transparent organic light- emitting diodes, Rsc. Adv. 6 (2016) 86770–86781

R.C. Raju Nagiri, S.D. Yambem, Q. Lin, P.L. Burn, P. Meredith, Room-temperature tilted-target sputtering deposition of highly transparent and low sheet resistance Al doped ZnO electrodes, J. Mater. Chem. C 3 (2015) 5322–5331

A. Kumar and I. Ahmad, ‘Role of defects and microstructure on the electrical properties of solution processed Al doped ZnO transparent conducting films’, Appl. Phys. A, vol. 126, no. 8, pp. 1–11, 2020, doi: 10.1007/s00339-020-03767-0.

K. Ellmer, J. Phys. D. Appl. Phys. 34, 3097 (2001)

V. Devi, H. Pandey, D. Tripathi, M. Kumar, and B. C. Joshi, ‘Optical and electrical properties of pristine and Al doped ZnO thin films Optical and Electrical Properties of Pristine and Al Doped ZnO Thin Films’, vol. 040010, no. August, 2019.

Dimitrov, Dimitre, Che-liang Tsai, Stefan Petrov, Vera Marinova, Dimitrina Petrova, Blagovest Napoleonov, and others, „Atomic Layer-Deposited Al-Doped ZnO Thin Films for Display Applications‟, 2020, 1–12 ,doi:10.3390/coatings10060539

M.H. Hong, C.S. Park, W.S. Seo, Y.S. Lim, J.K. Lee, H.H. Park, < 2013 (2013) 131537

H.F. Hussein, G.M. Shabeeb, S.S. Hashim, J. Mater. Environ. Sci. 2 (2011) 423

Y. H. Kim, S. J. Kim, Y.-J. Kim, Y.-S. Shim, S. Y. Kim, B. H. Hong, H. W. Jang, ACS Nano 2015, 9, 10453

C. Gong, Y. Zhang, W. Chen, J. Chu, T. Lei, J. Pu, L. Dai, C. Wu, Y. Cheng, T. Zhai, L. Li, J. Xiong, Adv. Sci. 2017, 4, 1700231

B. Cho, M. G. Hahm, M. Choi, J. Yoon, A. R. Kim, Y.-J. Lee, S.-G. Park, J.-D. Kwon, C. S. Kim, M. Song, Y. Jeong, K.-S. Nam, S. Lee, T. J. Yoo, C. G. Kang, B. H. Lee, H. C. Ko, P. M. Ajayan, D.-H. Kim, Sci. Rep. 2015, 5, 8052

H. Y. Jeong, D.-S. Lee, H. K. Choi, D. H. Lee, J.-E. Kim, J. Y. Lee, W. J. Lee, S. O. Kim, S.-Y. Choi, Appl. Phys. Lett. 2010, 96, 213105

W. Yuan, A. Liu, L. Huang, C. Li, G. Shi, Adv. Mater. 2013, 25, 766.

T. Kim, Y. Kim, S. Park, S. Kim, H. Jang, Chemosensors 2017, 5, 15.

I. V. Zaporotskova, N. P. Boroznina, Y. N. Parkhomenko, L. V. Kozhitov, Mod. Electron. Mater. 2016, 2, 95

S. S. Varghese, S. Lonkar, K. K. Singh, S. Swaminathan, A. Abdala, Sens. Actuators, B 2015, 218, 160

C. Wang, L. Yin, L. Zhang, D. Xiang, R. Gao, Sensors 2010, 10, 2088 6

P. Shankar, J. B. B. Rayappan, J. Mater. Chem. C 2017, 5, 10869

Zhang, X. Liu, G. Neri, N. Pinna, Adv. Sci. 2016, 28, 795

A. Sanger et al., “Morphology-Controlled Aluminum-Doped Zinc Oxide Nanofibers for Highly Sensitive NO2 Sensors with Full Recovery at Room Temperature”, vol. 1800816, no. 2, 2018, doi: 10.1002/advs.201800816.

P. S. Kolhe, A. B. Shinde, S. G. Kulkarni, N. Maiti, P. M. Koinkar, and K. M. Sonawane, „Gas sensing performance of Al doped ZnO thin fi lm for H 2 S detection‟, J. Alloys Compd., vol. 748, pp. 6–11, 2018, doi: 10.1016/j.jallcom.2018.03.123.

Kathwate, L. H., Umadevi, G., Kulal, P. M., Nagaraju, P., Dubal, D. P., Nanjundan, A. K., & Mote, V. D. (2020). Ammonia gas sensing properties of Al doped ZnO thin films. Sensors and Actuators, A: https://doi.org/10.1016/j.sna.2020.112193 Physical, 313, 112193.

S. Bai, T. Guo, Y. Zhao, R. Luo, D. Li, A. Chen, C.C. Liu, J. Mater. Chem. A 1 (2013) 11335–11342

U. Chaitra, A. V. M. Ali, A. E. Viegas, D. Kekuda, and K. M. Rao, „Growth and characterization of undoped and aluminium doped zinc oxide thin films for SO2 gas sensing below threshold value limit‟, Appl. Surf. Sci., vol. 496, no. August, p. 143724, 2019, doi: 10.1016/j.apsusc.2019.143724.

S. Kundu, R. Majumder, R. Ghosh, and M. Pal Chowdhury, „Superior positive relative humidity sensing properties of porous nanostructured Al:ZnO thin films deposited by jet- atomizer spray pyrolysis technique‟, J. Mater. Sci. Mater. Electron., vol. 30, no. 5, pp. 4618–4625, 2019, doi: 10.1007/s10854-019-00754-x

H. Aydın, F. Yakuphanoglu, and C. Aydın, „Al-doped ZnO as a multifunctional nanomaterial: Structural, morphological, optical and low-temperature gas sensing properties‟, J. Alloys Compd., vol. 773, pp. 802–811, 2019, doi: 10.1016/j.jallcom.2018.09.327

Namgung, Q. T. H. Ta, W. Yang, and J. S. Noh, „Diffusion-Driven Al-Doping of ZnO Nanorods and Stretchable Gas Sensors Made of Doped ZnO Nanorods/Ag Nanowires Bilayers‟, ACS Appl. Mater. Interfaces, vol. 11, no. 1, pp. 1411–1419, 2019, doi: 10.1021/acsami.8b17336

T. Krishnakumar et al., „CO gas sensing of ZnO nanostructures synthesized by an assisted microwave wet chemical route‟, Sensors Actuators, B Chem., vol. 143, no. 1, pp. 198–204, 2009, doi: 10.1016/j.snb.2009.09.039.

A. Sharmin, S. Tabassum, M. S. Bashar, and Z. H. Mahmood, „Depositions and characterization of sol–gel processed Al-doped ZnO (AZO) as transparent conducting oxide (TCO) for solar cell application‟, J. Theor. Appl. Phys., vol. 13, no. 2, pp. 123–132, 2019, doi: 10.1007/s40094-019-0329-0

S. Ghosh, A. Mallick, B. Dou, M. F. A. M. van Hest, S. M. Garner, and D. Basak, „A novel blanket annealing process to achieve highly transparent and conducting Al doped ZnO thin films: Its mechanism and application in perovskite solar cells‟, Sol. Energy, vol. 174, no. April, pp. 815–825, 2018, doi: 10.1016/j.solener.2018.09.017

G. Namgung, Q. T. H. Ta, W. Yang, and J. S. Noh, „Diffusion-Driven Al-Doping of ZnO Nanorods and Stretchable Gas Sensors Made of Doped ZnO Nanorods/Ag Nanowires Bilayers‟, ACS Appl. Mater. Interfaces, vol. 11, no. 1, pp. 1411–1419, 2019, doi: 10.1021/acsami.8b17336

R. Nisha, K. N. Madhusoodanan, T. V. Vimalkumar, and K. P. Vijayakumar, „Gas sensing application of nanocrystalline zinc oxide thin films prepared by spray pyrolysis‟, Bull. Mater. Sci., vol. 38, no. 3, pp. 583–591, 2015, doi: 10.1007/s12034-015-0911-2.

S. O. El Hamali, W. M. Cranton, N. Kalfagiannis, X. Hou, R. Ranson, and D. C. Koutsogeorgis, „Enhanced electrical and optical properties of room temperature deposited Aluminium doped Zinc Oxide (AZO) thin films by excimer laser annealing‟, Opt. Lasers Eng., vol. 80, pp. 45–51, 2016, doi: 10.1016/j.optlaseng.2015.12.010.