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Comparing Thermal Durability and Effects of Annealing Temperature on Characteristics of Hydrogen-Doped ZnO, AZO, and GZO Thin Films
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 -axis (AB┴), into bond center sites paralleled to the -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.
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]. However, 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) [1–3]. Therefore, the intrinsic or extrinsic defects are popularly investigated by researchers via simulations or experiments to depress and enhance the useless and useful defects, respectively.
Zni and VO play role as donors in ZnO material [3–5]. Ghose et al.  proved that Zni and VO can exist in different charged states. For example, the yellow emission from photoluminescence spectra is attributed to doubly charged oxygen vacancies () (● is the Kröger–Vink notation) or the blue luminescence is attributed to Zni. Lin et al.  also proved that Zni and VO still exist in ZnO films regardless of growing in high oxygen partial pressure. Huang et al.  theoretically proved the migration barrier of Zni that has a very small value, about 0.3–0.5 eV, which could mobile down to very low temperature. Besides, those authors also showed that VZn is stable at room temperature. Apart from mentioned intrinsic defects, the un- or intentional extrinsic defects, especially defects related to hydrogen, also play a crucial role on characteristics of ZnO thin films. Van de Walle exploded the researches related to hydrogen in ZnO  after the conclusion that hydrogen behaves as a shallow donor in ZnO thin films. After that, there have been numerous experimental as well as theoretical reports which proved and expanded the role of hydrogen in ZnO [8–16]. Hydrogen could be un- or intentionally incorporated into ZnO films during depositing and it could locate at HBC or HAB as the configuration proposed by Van de Walle . Besides, hydrogen can also passivate defects like oxygen vacancy (VO) or zinc vacancy (VZn) to create VO-H (HO) or VZn-H complex, respectively [9, 17–19], which improve electron mobility in ZnO thin films. However, hydrogen can easily diffuse out of ZnO thin films at low temperature, which leads to decreased electrical properties of ZnO thin films. Among those hydrogen-related defects, HO may be the best thermal stable defect up to 475°C before diffusing out, while Hi (HBC, HAB) would diffuse out if the temperature is higher than 225°C . Therefore, finding the method to improve or maintain the electrical properties of hydrogen and/or dopants codoped ZnO thin films under high temperature conditions is very important to widen applications of ZnO materials. Thus, 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.
2. Experimental Details
2.1. Specimen Preparation
ZnO, AZO, and GZO thin films were deposited on soda-lime glass substrates by a dc magnetron sputtering on a Univex-450 system. All films were deposited in Ar-H gas from 3-inch ceramic un-, aluminum- (0.25 wt.% Al2O3), or gallium- (0.25 wt.% Ga2O3) doped ZnO targets. Prior to the deposition, the glass substrates were cleaned in dilute sodium hydroxide 1%, acetone, and distilled water by ultrasonic wave for 15 minutes at each step before plasma cleaning for 15 minutes. The vacuum chamber was evacuated to the base pressure of . The Ar-H gas was introduced into the sputtering chamber to reach the working pressure of . The hydrogen partial pressure ratio was calculated as the following expression: , where pH2 and pAr are hydrogen and argon gas partial pressure, respectively. In this work, we prepared three series of ZnO-H, AZO-H, and GZO-H thin films at H2 ratio of 1.7% and the substrate temperature of 200°C. The distance between the substrate and the target and sputtering power was fixed at 50 mm and 60 W, respectively. All films were annealed in air with temperature range of 200–500°C.
Hall effect measurement using the Van der Pauw method at room temperature on Ecopia HMS-3000 system was employed to investigate the electrical properties of the films. The transmission spectra of the films in the wavelength range of 300–1100 nm were obtained from a Jasco-730 UV-visible spectrophotometer. Film thickness was determined by using Dektak 6M surface profilometer. The film thickness of all films was maintained at 800 nm. The crystalline structure of the films was identified by X-ray diffraction (XRD) using CuKα radiation of Bruker D8 Advance system. The photoluminescence spectra obtained from NanoLog™ spectrofluorometer (Horiba) system by using the excitation wavelengths of 325 nm (He-Cd laser) were used to determinate the effects of annealing temperature on defects in ZnO thin films.
3. Results and Discussions
The dependence of carrier concentration () and electron mobility () as a function of annealing temperature is illustrated in Figure 1. In Figure 1(a), for as-deposited films, values of AZO-H and GZO-H films are higher than those of ZnO-H film, which indicates that group III elements effectively substituted for Zn atoms in ZnO lattice. The values for all films without undergoing annealing process are quite high in comparison with published reports for the similar materials and technique [21, 22]. Besides, in Figure 1(b), hydrogen shows the ability to improve the electron mobility for unannealed AZO-H and ZnO-H films better than unannealed GZO-H films. There are numerous experimental as well as theoretical publications reported that hydrogen not only behaves as a shallow donor, which gives rise to the increase of carrier concentration [8, 14, 23–28], but also passivates defects [29–31] such as VO, Zni, or VZn in ZnO thin films.
The carrier concentration and electron mobility just insignificantly decrease for all films annealed at 200°C, which means that hydrogen-related defect complexes are not influenced at anneal . With further increase of annealing temperature up to 300°C, there is a difference between ZnO-H and AZO-H films and GZO-H film in as seen in Figure 1(b). The values of ZnO-H and AZO-H films sharply drop from ca. 55 cm2/Vs at 200°C to 2 cm2/Vs and 12 cm2/Vs at 300°C, respectively. The significant decrease of in ZnO-H and AZO-H films originates from the diffusion of hydrogen out of defect complexes, which gives rise to the formation of “dangling bond” type defects, as in silicon materials, inside the crystal grains or at the grain boundaries [8, 32, 33]. As mentioned above, Hi plays role as a shallow donor, therefore the diffusion of Hi out of crystal grains triggers reduction of carrier concentration in ZnO-H and AZO-H films. In addition, the curves of ZnO-H and AZO-H films being similar to each other with the increase of annealing temperature (≤300°C) indicate that aluminum dopant has no roles in thermal resistance like gallium in ZnO materials as discussed below. Interestingly, and of GZO-H film are not only still stable at annealing temperature of 300°C but also have a slight increasing trend. This implies that hydrogen and gallium interacted with each other in a certain way to keep stable the electrical properties of GZO-H film at 300°C. However, if the annealing temperature is higher than 400°C, as well as 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.
XRD patterns, crystallite size, and full width at half maximum (FWHM) of ZnO-H, AZO-H, and GZO-H films as a function of annealing temperature are shown in Figure 2. At the first glance, all films reveal a strong preferred orientation of (002) peak (JCPDS #36-1451) and the intensity of ZnO-H and GZO-H has a decrease trend, while the intensity of AZO-H films is almost unchanged as rising annealing temperature up to 300°C, which implies that there has a variation in crystal structure of these films. The recrystallisation phenomenon is observed for all films annealed at 500°C, which interprets the increase of electron mobility for all films after being annealed at 500°C except AZO-H film which cannot be measured by Hall measurement at this temperature.
To shed light on effects of annealing temperature on the crystal structure of films, the (002) peak centers obtained from fitting these peaks based on Pearson VII function are plotted in the insets of Figures 2(a)–2(c). For ZnO-H, AZO-H, and GZO-H films without annealing treatment, the (002) peak center of AZO-H film is nearest the 2θ value of 34.42° in comparison with ZnO-H and GZO-H films, which indicates that hydrogen effectively bonds with host lattice in AZO-H film rather than in ZnO-H and GZO-H films (especially located at BC// site ) which gives rise to the significant restoration of crystal lattice and also the highest carrier concentration of AZO-H film as seen in Figure 1. However, at annealing temperature of 300°C, there has a significant decrease in the (002) peak center of ZnO-H and GZO-H films, while AZO-H film has an increase trend. Based on Bragg’s equation
where is a positive integer, is the wavelength of X-ray source (), and is the interplaner spacing of the (002) plane of ZnO crystal, we reveal that the (002) peak center of ZnO-H and GZO-H films annealed at 300°C shifts towards lower 2θ angle, which means that the lattice parameter ( for hexagonal wurtzite structure of ZnO) would increase. To interpret the increase of lattice parameter of ZnO-H and GZO-H films, we proposed some causes for this: (1)Hydrogen atoms originated from the broken bonds of HAB defects due to high temperature may move into the position between Zn-O bonds parallel -axis. Wardle et al.  proved by first-principle study that the movement of hydrogen between HAB┴ and HBC// sites has relatively low barrier and they found that HBC// was the lowest energy structure. In addition, HBC// does gives rise to the increase of lattice parameter (2)The zinc or gallium interstitials (Zni or Gai) move into the zinc vacancies’ sites (VZn—a type of electron trapping defect) which gives rise to crystal lattice restoration(3)Oxygen interstitial (Oi) atoms move into the oxygen vacancies’ sites (the existence of Oi atoms in films because of all films annealed in air ambiance). However, there is no trace of this defect in photoluminescence spectrum (discussion below) of films with annealing temperature under 300°C. Therefore, this cause may be ruled out
At annealing temperature of 300°C, the HAB bonds are easy to be broken and hydrogen diffuses out of the host lattice in case of ZnO-H and AZO-H films; however, in case of GZO-H film, hydrogen is seemingly pushed by adjacent Ga3+ ions at Zn2+ site and move into the bond-center position (HAB//). Therefore, the cause (1) interprets for the increase of the lattice parameter , while and values of GZO-H film are still stable, because HBC is also considered as a shallow donor  and GaZn-O-Zn-HBC//-O complex results in the increase of lattice parameter . The cause (2) accounts for the increase of lattice parameter of ZnO-H film annealed at 300°C. Figure 3 is outlined to illustrate causes increasing lattice parameter of ZnO-H and GZO-H films as elevating annealing temperature up to 300°C. However, in case of AZO-H film, the lattice parameter monotonically decreases with further increase of annealing temperature up to 500°C, because of the hydrogen diffusion out of bond-center positions along with the small ionic radii of Al3+ ion () in comparison with ionic radii of Ga3+ ion () and Zn2+ ions () .
At annealing temperature of 500°C, all films have a trend of increasing lattice parameter , because most of hydrogen in films, especially hydrogen at bond-center positions paralleled to the -axis (HBC//), completely diffuse out of the host lattice. Besides, Figure 2(d) shows that the crystallite size of all samples fluctuates around the value of 31–37 nm and has a slight increasing trend at high annealing temperature due to the recrystallisation phenomenon. In addition, with this large crystallite size of all films, the contribution of the boundary scattering on electrical resistivity could be omitted . In addition, Figure 2(e) shows FWHM values of ZnO-H, AZO-H, and GZO-H films at various annealing temperatures. The Scherrer equation (2) is commonly used to calculate nanoscale crystallite size, and FWHM value is one of the two parameters determining the crystallite size. Bragg angle and FWHM are inversely proportional to crystallite size, while the variation of Bragg angle is small; thus, the crystallite size as seen in Figure 2(e) has the opposite trend with FWHM values.
where is the crystallite size (nm), is a dimensionless shape factor, and is FWHM.
Figure 4 shows the surface morphology of ZnO-H, AZO-H, and GZO-H films as-deposited and annealed at various temperatures. Generally, there is no significant difference in surface morphology between as-deposited films regardless of the appearance of aluminum or gallium and that annealed at 300°C or 500°C. Besides, based on SEM micrographs, it is clearly seen that the distribution of grain size for all films is homogenous on the surface and all films have the similar morphology, which indicates that films were well prepared and homogeneous.
The transmittance spectra of ZnO-H, AZO-H, and GZO-H thin films measured in wavelength range of 300–1100 nm in Figure 5 show clear and homogenous interference fringes of all films indicating the high crystalline quality and the similar film thickness, respectively. The inset in Figure 5(a) shows a red shift of all films with further increasing annealing temperature, because the decrease of as seen in Figure 1(a) gives rise to the Burstein-Moss effect. The transmittance is larger than 82% (including film and glass substrate) when averaged in the visible and near-infrared range from 400 to 1100 nm for all films as seen in Figure 5(b). The average transmittance of ZnO-H and AZO-H films has a decreasing trend when annealing temperature rises to 300°C. This is due to the formation of defects in host lattice generated from the broken hydrogen-related bonds as mentioned above, which gives rise to the appearance of scattering centers. However, the average transmittance of GZO-H films is almost unchanged at annealing temperature of 300°C, which is consistent with electrical properties as discussed above. With further increasing annealing temperature, the average transmittance of all films increases because of the aforementioned recrystallisation phenomenon.
Figure 6 shows the variation of the photoluminescence spectra of ZnO-H, AZO-H, and GZO-H thin films at different annealing temperatures. All films have a strong emission band in wavelength range of 375–450 nm, which is known as a featured emission band of ZnO material including near-band edge emission (NBE) [37, 38] and the transition between donor levels (such as Zni or Hi) and acceptor levels (VZn). The broaden emission band in wavelength range of 450–600 nm is attributed to the transition from oxygen vacancy levels to acceptor levels or valence band (VB) [37, 38]. From Figure 6, it can be clearly seen that there is a significant variation in shape of PL spectra of all films at different annealing temperatures, which indicates that annealing temperature strongly affects defect levels. In case of AZO-H films (Figure 6(b)), the PL intensity of AZO-H films significantly increases at annealing temperature of 300°C, which indicates that hydrogen is easy to cooperate with defects in AZO-H films such as Hi or VZn to effectively improve electron mobility and carrier concentration as seen in Figure 1, but these hydrogen-related bonds are easy to be broken, which results in defects. However, in the case of ZnO-H and GZO-H films, the defect-related emission has a slight variation. The intensity of all samples in wavelength range of around 600 nm strongly increases at annealing temperature of 500°C, which indicates that hydrogen diffused out of oxygen vacancy sites.
To get more details on the influence of annealing temperature on defects in ZnO-H, AZO-H, and GZO-H films, all PL spectra were deconvoluted based on Gaussian function distribution to determinate the overlapped emissions as seen in Figure 7. There are four deconvoluted emission peaks for all films. However, there is an absence or appearance of several PL peaks because of the annealing treatment. For example, for as-deposited and 300°C-annealed films (ZnO-H, AZO-H, and GZO-H films), the emission peaks in the UV region (under 383 nm in this work) which is known as the transition of electrons between NBE states and VB were detected, while the yellow emission peak at around 600 nm which is attributed to the emission from conduction band (CB) to doubly charged oxygen vacancies () appears in the films annealed at temperature of 500°C .
The violet region (391–403 nm) for all films (as-deposited and annealed films) is attributed to the transition from the Zni levels (located at 0.2 eV below the CB for a typical bandgap of 3.37 eV of ZnO) to VB or from CB to the VZn levels (located at 0.3 eV above VB) [39, 40]. For ZnO-H films, the violet emission peaks located in wavelength range of 397 nm (3.12 eV) to 403 nm (3.07 eV) are caused by the transitions from CB and NBE to VZn levels. However, for AZO-H and GZO-H films, the violet emissions fluctuate around the wavelength range of 391 nm (3.17 eV) to 398 nm (3.11 eV) which means that these emissions are well fitting with the transition of electrons from Zni levels to VB. The transition between Zni levels and VB prevails in AZO-H and GZO-H films in comparison with ZnO-H film because of the existence of Gai or Ali which cannot completely substitute for Zn and behave as a shallow donor like Zni . The near-blue emission peaks (414–433 nm and 2.99–2.86 eV, respectively) in PL spectra of ZnO-H, AZO-H, and GZO-H films (both as-deposited and annealed films) are the result of transition of electrons from Zni to VZn levels .
From the observation of the emissions in violet and near-blue regions in Figures 6 and 7, ZnO-H film has the larger red shift in comparison with GZO-H film because the broadness of VZn level relates to the broken VZn-H complexes at annealing temperature of 300°C. However, interestingly, AZO-H film has the blue shift in the violet and near-blue regions because of the narrowness of VZn levels which is caused by the formation of AlZn-VZn complex as proved by Johansen et al. . The annealing temperature of 300°C is enough to break the VZn-H complex as well as gives rise to the combination of AlZn defect to the nearest VZn defect to form the AlZn-VZn complex. This complex limits the role of shallow AlZn donor because it acts as a compensating acceptor with the charge state of -1, which is consistent with the decrease of carrier concentration of AZO-H film annealed at temperature of 300°C as mentioned above.
The blue-green emission peaks (479–498 nm; 2.59–2.49 eV) are attributed to the transition of electrons from defect (neutral oxygen vacancies formed by the state captures an electron from CB)  or complex to VB. Besides, the blue-green emission peaks of ZnO-H films have a red-shifted tendency, whereas those of AZO-H and GZO-H films have a blue-shifted tendency at annealing temperature of 300°C, which implies that there is an influence of Al or Ga on VO-related defect level. From those PL results, we reveal that there is a small change in emission peaks of GZO-H films in comparison with ZnO-H and AZO-H films at annealing temperature of 300°C, which indicates that GZO-H film is still stable at this annealing temperature and these PL results are consistent with electrical properties.
The influence of annealing temperature on electrical, optical, structural, and morphological properties of ZnO-H, AZO-H, and GZO-H films was systematically investigated to find out the thermal durability of those films. We reveal that GZO-H films are not only durable with respect to electrical properties at annealing temperature up to 300°C but also have slightly increased carrier concentration and electron mobility. Besides, we propose that Ga3+ ions located at adjacent Zn sites along with annealing temperature of 300°C can push hydrogen atoms into bond center sites forming HBC//, which gives rise to the stability of electrical properties of GZO-H films. ZnO-H and AZO-H films have only thermal durability at annealing temperature up to 200°C.
The data that support the findings of this study are within the article and the raw data are available from the corresponding author, Vinh Cao Tran, upon reasonable request.
Conflicts of Interest
There are no conflicts of interest to declare.
This research is funded by Vietnam National University HoChiMinh City (VNU-HCM) under the grant number B2017-18-09. The Hall effect measurement was supported by the Faculty of Materials Science and Technology, University of Science, VNU-HCM.
Graphical abstract depicting a model for briefly interpreting the thermal durability of GZO-H film can be found at the system on Journal of Nanomaterials. (Supplementary Materials)
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