Abstract
In this work, undoped, aluminum-, and gallium-doped ZnO thin films (ZnO-H, AZO-H, and GZO-H, respectively) deposited on soda-lime glass substrates by magnetron sputtering method in a gas mixture of hydrogen and argon are annealed at various temperatures in the range of 200–500°C in air to evaluate the durability of those films under annealing temperature. From photoluminescence spectra, formation of point defects, especially oxygen vacancies, when hydrogen diffuses out of the films at high annealing temperature is exhibited via a significant increase of visible emissions. We find out that carrier concentration and Hall mobility of AZO-H and ZnO-H films dramatically decrease, while those of GZO-H film are still stable as the annealing temperature increased from 200°C to 300°C. We proposed a model for interpreting the thermal durability of GZO-H film that, at an annealing temperature of 300°C, Ga3+ ions located at adjacent Zn sites can push hydrogen atoms, which are broken out of the antibonding sites which are perpendicular to the c -axis (AB┴), into bond center sites paralleled to the c -axis (BC//). The movement of hydrogen from AB┴ to BC// site also gives rise to the durability of electrical properties of GZO-H films at the high annealing temperature.
Highlights
ZnO, a native n-type oxide semiconductor, owning a wide bandgap (3.37 eV) along with a high exciton energy of 60 meV, has been extensively investigated in recent years due to its ability for UV detectors, photoelectric electronic devices, or thin film solar cells [1, 2]
There are numerous experimental as well as theoretical publications reported that hydrogen behaves as a shallow donor, which gives rise to the increase of carrier concentration [8, 14, 23,24,25,26,27,28], and passivates defects [29,30,31] such as VO, zinc interstitials (Zni), or VZn in ZnO thin films
If the annealing temperature is higher than 400°C, n as well as μ has a dramatic decrease for all films, which indicates that hydrogen completely diffuses out of the host lattice, even HO defects, and results in a large amount of defect
Summary
ZnO, a native n-type oxide semiconductor, owning a wide bandgap (3.37 eV) along with a high exciton energy of 60 meV, has been extensively investigated in recent years due to its ability for UV detectors, photoelectric electronic devices, or thin film solar cells [1, 2] Efficiency of those devices is strongly affected by intrinsic or extrinsic defects, which are generated during depositing, such as zinc interstitials (Zni), oxygen vacancies (VO), or zinc vacancies (VZn) [1,2,3]. Hydrogen can diffuse out of ZnO thin films at low temperature, which leads to decreased electrical properties of ZnO thin films Among those hydrogenrelated defects, to 475°C before. This study investigates the effects of annealing temperature on durability of un-, aluminum-, or gallium-doped ZnO thin films and compares the thermal durability between them
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