Thermal stability of ultra-wide-bandgap MgZnO alloys with wurtzite structure

In this study, Mg_xZn_1−xO with composition x = 0, 0.17, 0.34, 0.52, 0.72 and 0.80 were grown utilizing a DC/RF reactive magnetron sputtering. The chamber was evacuated to a base pressure of 10⁻⁶ Torr before deposition of the films. Sputtering was performed at a total working pressure of 11 mTorr, achieved by passing high purity Ar and O₂ inside the chamber as working and reactive gases respectively. Flow of the gas inside the chamber was regulated using MKS p4b digital mass flow controllers. During sputtering, custom Mg-Zn metallic targets were used, and the delivered power was 30 W. Films were grown for ~ 2 h on (0001) sapphire and (111) CaF₂ substrates. The growth temperatures were 250 °C. The thickness of the films were estimated to be ~ 400 nm as determined via thin film interference [18].

The as-grown films were subjected to post annealing treatments at 750 and 900 °C for 1 h under controlled Ar environment in a Lindberg furnace. Structural phase of the alloy films was investigated by XRD using a Siemens Diffractometer D5000 with the CuKα1 line in 2θ mode with scan step size of 0.02°. At all steps of the annealing experiments, the alloy compositions of the films were determined via energy dispersive X-ray spectroscopy (EDS). Only minor changes were observed relative to the as-grown films. For the transmission experiments, an Agilent 300 Cary UV–vis transmission system was used in double beam mode.

We first address the study of alloys with relatively low-Mg compositions. Figures 1 and 2a, b present the XRD and the transmission spectra, respectively, of the Mg_0.17Zn_0.83O and Mg_0.34Zn_0.66O samples. The bandgaps were estimated via the transmission derivative method, dT/dE, that was shown to be a useful approach for ascertaining bandgaps of alloys [19], and in particular provides a suitable definition of bandgap for the alloys studied in this paper. Usually, the transmission spectrum of the alloy is not decreasing in a sharp manner at the spectral-range of the bandgap, but rather exhibits an Urbach-type band-edge tailing. This band-edge for the alloys can be explained in terms of in-gap density of states due to structural and morphology defects as well as alloy inhomogeneities [20, 21]. We previously found that the derivative method yields a good measure of the alloy’s bandgap; sometime referred to as the optical gap rather than the fundamental gap [19]. The energies of the optical gaps are usually somewhat lower than those of the fundamental gaps. The XRD spectra indicate that both as-grown samples have a single-phase wurtzite structure; no cubic diffraction is detectible. Moreover, upon annealing at up to 900 °C both samples retain their original single-phase wurtzite structure. The transmission spectra in Fig. 2 concur with the XRD analysis. Specifically, no significant shift in the bandgap energy is evident as a result of the annealing process. If the alloy were phase-separated, the bandgap would have a noticeable shift to lower energy relative to that of the as-grown, corresponding to a bandgap of precipitates with a Zn-rich component. Moreover, the transmission spectra in principle should also have an absorption signature at the high-energy regime, corresponding to the bandgap of a Mg-rich phase precipitates. Examination of the transmission spectrum and its derivative at high energies did not reveal any such features. In light of these findings it can be concluded that for Mg composition up to ~ 34%, and for a temperature of 900 °C, these alloys are thermally stable.

In the following section results regarding alloys at the high Mg-composition regime are discussed. Figure 3a presents the XRD of Mg_0.52Zn_0.48O, Mg_0.72Zn_0.28O, and Mg_0.80Zn_0.20O, and Fig. 3b is the scanning electron microscopy (SEM) image showing the characteristic granular morphology of our samples. As can be seen in Fig. 3a, the as-grown Mg_0.52Zn_0.48O, and Mg_0.72Zn_0.28O, both have the wurtzite structure. Upon annealing at 900 °C the Mg_0.52Zn_0.48O alloy is found to exhibit an additional small diffraction peak corresponding to the cubic structure, while the Mg_0.72Zn_0.28O is found to be phase separated already at 750 °C. In contrast, the Mg_0.80Zn_0.20O has sufficient Mg-concentration that enabled it to be crystalized with the cubic structure; however, 750 °C annealing caused it to be phase separated into the cubic and wurtzite structures. In a similar manner to the low Mg-composition samples, transmission studies were conducted. Figure 4 shows the transmission spectra characteristics of one of the high Mg composition samples, as an example where features due to phase separation after annealing are present. Figure 5 shows the transmission analysis from which the bandgaps of the high Mg-composition alloys were determined. As can be seen in Fig. 5, after annealing the bandgaps of Mg_0.52Zn_0.48O and Mg_0.72Zn_0.28O shifted to lower energies corresponding to Zn-rich phases, and moreover bandgaps in the deep-UV ~ 6 eV due to Mg-rich phase are detectable. The transmission results correlate well with those of the XRD, and as such the deep-UV bandgaps can be assigned to originate from the cubic structures.

In order to further discuss the thermal stability of the Mg_xZn_1−xO, the properties of the alloys are summarized in Figs. 6 and 7. Figure 6 shows the compositional dependence of the optical gap for the as-grown alloys as well as the optical gap structural transition of the annealed alloys, and in Fig. 7 the temperature behavior as a function of composition is depicted. Specifically, the open data points in Fig. 7, starting at 1100 °C and above, are adapted from phase-diagram studies of Mg_xZn_1−xO ceramics that were grown under thermodynamics equilibrium conditions [10, 11, 22‐24], while the data points below that temperature are from the present study. As can be seen in Fig. 7, the as-grown films (temperature 250 °C) of composition x = 0.17 to 0.72 were determined to be a single-phase wurtzite structure, while the x = 0.8 alloy was found to crystalize with the cubic structure. Thus the growth method used for this research resulted in a single-phase wurtzite alloys for Mg compositions up to ~ 72%. This composition is significantly larger than those predicted for Mg_xZn_1−xO ceramics that were grown under thermodynamics equilibrium conditions, to be discussed later.

In the following discussion we focus on the alloys with the wurtzite structure. As was mentioned above, after annealing at 750 and 900 °C, the alloys with composition up to x = 0.34 retain their original wurtzite structure, while the alloys with the high Mg content, x = 0.52 and 0.72, undergo structural changes in which wurtzite as well as cubic phases co-exist. Notably, the x = 0.34 alloy does not have a detectible XRD after annealing, corresponding to the cubic phase; however, due to the small bandgap shift (see Fig. 6) we surmise that this composition is the onset for a phase separation. With regard to the high Mg composition, the alloy with x = 0.52 shows structural stability at 750 °C with a single phase wurtzite, and is phase separated at 900 °C, while the alloy with x = 0.72 is phase separated already at 750 °C. Ab-initio density functional theory (DFT) studies regarding the structural properties of Mg_xZn_1−xO have determined that a phase transition from the wurtzite to the cubic phase should occur for a critical Mg concentration ~ 50% [25]. This result explains the occurrence of the observed phase separation for the alloy with x = 0.52 (and that with x = 0.72); upon providing the thermodynamics condition, the alloy precipitates into its more preferable stable equilibrium state.

An interesting issue that is evident from Fig. 7 is that the experimental phase diagram for the ceramics has somewhat conflicting results pertaining to the solubility limit of MgO in the ZnO wurtzite matrix. Some experimental data show a very limited solubility x ~ 0.05 [10, 23, 24], while others found it to be much a more substantial x ~ 0.15 (at 1100 °C) [11, 22]. In order to gain insight into this topic we chose the Mg_0.17Zn_0.83O that was found to exhibit no evidence for phase separation, and drove it to a further equilibrium state that approached that of the ceramics via high-temperature annealing at 1000 °C. As can be seen in Fig. 7, this temperature is in close proximity to the growth temperature ~ 1100 °C of the ceramics at the low Mg-composition regime. Figures 8 and 9 present the XRD and the bandgap characteristics, respectively, which indicate that no structural changes took place after the high-temperature annealing process, implying the stability of this sample. The data point for this alloy is included in the diagram of Fig. 7 (orange diamond) and is in the range of the solubility limit for the ~ 15% ceramics, which is of similar composition to our alloy. Our results agree with the prediction of the higher limit for Mg incorporation given by the groups in references [11, 22], suggesting that wurtzite Mg_xZn_1−xO with relatively high Mg-composition are achievable, not only as metastable alloys but even as thermodynamics equilibrium alloys.

Lastly the stability limit of our alloys is discussed, and supporting evidence from Raman scattering analysis and a model calculation will be presented. Annealing studies have been previously reported with a variety of research focuses [6‐8, 26‐30]; in the following we will summarize some of those which are closely related to our work. As is depicted in Fig. 6, the value of the bandgaps of the high-Mg composition alloys (x = 0.52, 0.72, and 0.8) after annealing all converge to the value of ~ 3.5 eV (see Fig. 6). This bandgap corresponds approximately to an alloy with Mg composition in the range of x ~ 0.25 ± 0.05, implying that the solubility limit at 900 °C of our samples is for Mg composition in that range. Studies on the phase stability of cubic Mg_0.55Zn_0.45O grown by metal–organic chemical vapor deposition found that the phase separation occurred after annealing at 850 °C [6]. The stability limit of sol–gel Mg_xZn_1−xO was determined to be ~ x = 0.23 [7]. Additionally, a phase stability study of wurtzite Mg_0.22Zn_0.78O film grown via PLD found that with post-growth annealing at 1000 °C, the bandgap of this alloy reduced to 3.56 eV, corresponding to an alloy with x = 0.15 [8]. This study thus concluded that the thermodynamic stability limit of MgO in Mg_xZn_1−xO is about x = 0.15 [8]. Our predicted value for the stability limit that was inferred from several samples is at a higher range of x ~ 0.25 ± 0.05, as is indicated in Fig. 6. As can be seen in Fig. 6, the bandgap of the lower Mg alloys is not a strong function of the composition, which imposes some uncertainty in the experimental approach for pinpointing a single value for the solubility limit.

Additionally, previous work on the thermal stability of RF sputtered Mg_0.4Zn_0.6O thin film and superlattices has been conducted at annealing temperatures of 600 and 800 °C as a function of time [27]. Their annealing studies of Mg_0.4Zn_0.6O film have shown that the hexagonal crystallinity of the film was improved. Moreover, upon annealing at 800 °C for 5 min, MgZnO cubic precipitation was observed, while annealing at 600 °C has required a longer time of at least 3 h to initiate the cubic precipitation [27]. In our case, the Mg_0.52Zn_0.48O that was annealed at 900 °C was found to be phase separated, a result which agrees with their study. A different study of rapid and high-efficiency laser alloying of MgZnO nanocrystals examined the relationship between annealing temperature and the limit value of homogeneous Mg in the nanocrystals [28]. Their study concluded that annealing at temperatures of 400 °C, 600 °C, and 800 °C, a MgO cubic phase appeared for nanocrystals with Mg composition of 62 at.%, 53 at.%, and 39 at.%, respectively. Specifically, for annealing at 800 °C the limit value of Mg in their nanoalloys was indicated to be ~ 22 at.% [28]. Our result of the solubility limit in the sputtered thin films, which is 25% ± 5%, is similar to theirs.

To further investigate the solubility limit of the alloys, Raman spectroscopy was employed. Our UV-lasers, at 325 nm (3.8 eV) and 244 nm (5.1 eV), due to their resonance with the bandgaps of the alloys, enables strong Raman scattering from the LO mode of the wurtzite structure [31]. Figure 10 shows the Raman spectra of the as-grown and annealed Mg_xZn_1−xO alloys. In Fig. 11 the LO Raman frequency as a function of composition is plotted for the as-grown alloys and for the alloys after annealing at 900 °C. As can be seen in Fig. 11, the LO mode frequency of the as-grown samples exhibits a linear increase with Mg composition, while that of the annealed alloys was found to exhibit a saturation-type behavior. This mode behavior can be modeled with a saturation function of the form [32, 33]:

where \({\omega }_{0}\) and b are constants, and \({x}_{0}\) is the Mg composition for which the LO mode becomes saturated. A fit to the data resulted in \({\omega }_{0}\) = 575 cm⁻¹, b = 28.47, and \({x}_{0} =\)0.23 (23%). The Raman analysis thus indicates that the solubility limit is for Mg composition x ~ 0.23. This result, up to the experimental error, concurs with the solubility limit of x ~ 0.25 ± 0.05 obtained via the optical gap analysis, shown in Fig. 6.

A key point of the above studies is that Mg has considerable solubility in the wurtzite matrix, which makes Mg_xZn_1−xO a viable alloy system for UV-applications.