3.3.1. MIM Properties
The current-voltage (I-V) curves of the MIM devices and the leakage current density @1 MV/cm of the films are shown in
Figure 9a,b. The elemental composition in the high entropy film exhibited a significant influence on the leakage current density of the film. AGHTZ and AGHTY films displayed large leakage current densities (
>1 × 10
−5 A/cm
2@1 MV/cm). These two groups of films have a rough surface as well as loose internal structure, with only approximately 20% of well-bonded oxygen content. The presence of a large number of oxygen-related defects primarily contributed to the increase in leakage current density. The AGTYZ film exhibited the lowest leakage current density (1.2 × 10
−8 A/cm
2@1 MV/cm), which is conjectured to its larger average difference in binding energy in the film and lower percentage of non-well-bound oxygen.
In factor analysis, each element’s impact was quantified by comparing the average property values between compositions with and without the specific element. The factor of each element and property was calculated as follows: To determine the impact of element X on property Y, the average value of property Y for all compositions containing X (designated as X = 1) was compared with the average value of property Y for all compositions without X (designated as X = 0). A factor value of X = 1 indicates that the element enhances the specific property under investigation, while a factor value of X = 0 suggests an inhibitory effect. This systematic analysis enables the isolation and quantification of each element’s contribution to each property of HEMO films.
Factor analysis was performed to evaluate the impact of six metal oxide (AlO
x, GaO
x, HfO
x, TiO
x, YO
x, and ZrO
x) on the leakage current density in HEMOs films, as presented in
Figure 9c. The presence of AlO
x and GaO
x was found to reduce the leakage current density. The introduction of Al helps to reduce the film roughness and the interface trap density, while the wide forbidden bandgap of Al
2O
3 suppresses the electron leap. Meanwhile, Ga
2O
3 has a large positive Gibbs free energy, which can effectively suppress the deterioration of the film dielectric properties caused by the hygroscopic reaction [
6]. Hf and Ti addition significantly increased the leakage current density of HEMOs films, which can be attributed to the residual chloride ions, nitrate ions, and organicsintroduced in the precursor solutions (HfCl
4, C
16H
36O
4Ti, EGME) require higher thermal energy for decomposition and remain partially undecomposed after annealing, creating additional electron transport pathways in the dielectric films. However, the large positive Gibbs free energy of TiO
2 may promote film deterioration through hygroscopic reaction, while the narrow forbidden bandgap of TiO
2 facilitates electron leap in the films, increasing leakage current density in HEMOs films [
45].
The capacitance-voltage (C-V) and capacitance-frequency (C-F) curves are shown in
Figure 10a,b. Under a 1000 Hz electric field, the capacitance density and dielectric constant of HEMOs films were investigated, as summarized in
Figure 10c. From the C-V curves, it can be observed that under a fixed-frequency electric field, the capacitance of the film is almostconstant with varying applied voltage, showing good voltage stability. The C-F measurements up to 1 MHz demonstrate that the capacitance decreases as frequency increases, suggesting a decrease in capacitance density and dielectric constant. This can be attributed to the polarization in the dielectric layer under an alternating electric field, where the dielectric constant is closely related to different types of polarization occurring at various frequencies within the film [
46]. At low frequencies (<10
2 Hz), almost all polarization mechanisms can respond to the change in electric field, resulting in higher polarization and consequently larger dielectric constant. As the frequency increases, some polarization processes fail to keep pace with the field variations, decreasing polarization and then the dielectric constant [
47].
The HEMOs films exhibit a highly disordered and asymmetric internal structure due to the incorporation of various smetallic elements with different atomic radii, enhancing atomic polarization under external electric field, contributing to higher dielectric constants. Among the studied films, AGHTZ film exhibits the largest capacitance density and dielectric constant at low frequencies, but undergoes rapid degradation with increasing frequency. The capacitance decreases directly from 4.89 × 10
−10 to 1.28 × 10
−12 at frequencies above 1000 Hz. As depicted in
Figure 9b, the AGHTZ film exhibits a large leakage current density, possibly due to residual nitrates, hydroxides, or organic groups, as well as potential absorption of moisture from the air, increasing mobile charges. The presence of this mobile charges increase the overall polarization in the low-frequency range, but the capacitance-frequency stability decreases in the mid to high-frequency range. To better analyze the role of different elements in the HEMOs films on the dielectric properties, the effects of six metal oxide, AlO
x, GaO
x, HfO
x, TiO
x, YO
x, and ZrO
x, on the capacitance density and dielectric constant of the films were under factor analysis, as shown in
Figure 10d,e. The result indicates that Ti significantly enhances capacitance density and dielectric constant, consistent with the high dielectric constant (60~80) in previous studies [
16].
Considering all the above analysis, it leads to a conclusion that good surface properties result in excellent dielectric properties. YOx and ZrOx show good compatibility with other metal oxides, forming HEMOs film with good surface morphology. This superior quality also facilitates the formation of a dielectric HEMO layer and metal oxide semiconductors with low interface trap density. The wide bandgap of Al2O3 suppress the electron leaps within HEMOs film, while the large positive Gibbs free energy of Ga inhibits the degradation of dielectric properties caused by hygroscopic reaction. Additionally, the high crystallization temperature of Al2O3 and Ga2O3 enables the HEMOs film to maintain an amorphous state after 400 °C annealing, thereby reduces the formation of conductive channelsand the leakage current. Among 6 components, TiO2 has the largest dielectric constant, significantly enhancing the dielectric constant of HEMOs films. The optimized films demonstrated excellent optical and electrical properties, including a high visible light transmission of 93.8%, low leakage current density of 1.2 × 10−8 A/cm2@1 MV/cm, a high dielectric constant of 29.95 at 1000 Hz, and good frequency stability.
Previous studies have shown that while single TiO
2 dielectric layers exhibit high dielectric constants (k: 60
–80), their relatively low bandgap (3.45 eV) leads to large leakage currents, limiting their application as high-performance dielectric layer. However, the AGTYZ films containing Ti maintain both a high dielectric constant and a high optical bandgap of 5.26 eV. To further investigate the role of Ti in modulating the dielectric constant and optical bandgap, the Ti concentrations in AGTYZ films were varied, as detailed in
Table 2.
The optical and capacitive properties of HEMOs films with different concentrations of Ti were measured, as shown in
Figure 11a–c. The optical bandgap of the films was fitted, the capacitance density and dielectric constant were calculated, as shown in
Figure 11d–f. Results indicate that Ti concentration significantly modulates both optical band gap and dielectric properties of the films. Specifically, Ti concentration exhibits an inverse relationship with the optical band gap and a positive correlation with capacitance density and dielectric constant, confirming our previous factor analysis results. As the concentration of Ti increases from 0 to 0.28 M, the capacitance density of the films increases from 259.72 nF/cm
2 to 1133.50 nF/cm
2, and the dielectric constant rises significantly from 10.63 to 52.74, while the optical band gap shows a modest decrease from 5.62 eV to 5.05 eV. These findings suggest that adjusting Ti content in AGTYZ film enables dielectric HEMO layers to combine high dielectric constant with wide optical bandgap.
By comparing the performance of MIM devices prepared from six metal oxides, including leakage current and dielectric constant, we found that AGTYZ has the best performance. And in the factor analysis, Ti has the effect of increasing the dielectric constant, so we investigate the effect of Ti content in AGTYZ on the film properties. The dielectric constant of the final dielectric film increased to 52.74.
