Background

Gallium nitride (GaN)-based wide bandgap semiconductors have seen enormous success during the past few decades due to their intriguing properties such as high breakdown electric field (4.2 MV/cm), high saturation velocity (~3 × 107 cm/s) [1], excellent chemical stability, and the ability to resist radiation damage [2]. Owing to these characteristics, GaN and its alloys can be applied in high-power electronics, such as thin film transistors and high electron-mobility transistors (HEMTs) [3, 4]. The conventional HEMTs apply Schottky barrier as the control gate which usually produces a large leakage current and, thus, results in a declined breakdown voltage and an enlarged power consumption as well as an increased noise coefficient [5, 6]. To reduce the leakage current, metal-oxide-semiconductor (MOS) structures are proposed to replace the ordinary Schottky gate [7]. However, the surface passivation of GaN is usually difficult due to the existing surface defects, dangling bonds, and some impurities, which distort the interface energy band. Hence, it is crucial to analyze and optimize the MOS structures before fabricating the MOS HEMTs.

In terms of dielectric films, high-k oxides are always among the candidates, because the high dielectric constant means much smaller featured size and lower power consumption [8, 9]. Many high-k materials such as Al2O3 [10], ZrO2 [11], MgO [12], and TiO2 [13] have been investigated for GaN-based MOS capacitors. Among them, ZrO2 is attractive because it has a high permittivity (~24), large bandgap (5.8 eV), and more importantly, an appropriate value of band offset with GaN (valance band offset ~1.6 eV, conduction band offset ~1.1 eV) [14]. As for the deposition methods, atomic layer deposition (ALD) has become the mainstream technique due to its unique self-limited reactions, which demonstrates many advantages such as precise thickness control, high uniformity over a large area, and excellent conformity in many complex nanostructures. In 2013, P. von Hauff et al. reported a kind of GaN-based MOS capacitors with ZrO2 as the dielectric layer. The capacitance reached 3.8 μF/cm2 in the accumulation region. However, the leakage current reached an enormous 0.88 A/cm2 at 1 V which was too large to be applied in electronic devices [15]. In 2014, Gand Ye et al. investigated the band alignment of ZrO2 and GaN using X-ray photoelectron spectroscopy [16]. However, the related devices were not fabricated.

In this work, we systematically studied the properties of ZrO2 films grown on n-GaN substrates by ALD. H2O or O3 was used as the oxidizer to examine which precursor was more effective to grow high-quality ZrO2 films. In addition, post-annealing treatments were carried out to improve the electrical performances of the MOS capacitors. Meanwhile, the leakage mechanisms of the MOS capacitors were also discussed.

Methods

Commercially available n-type GaN substrates were purchased from Huacan Semitek. A 3-μm-thick Si-doped n-type GaN layer with a carrier concentration of 2 × 1018/cm3 was grown on c-plane sapphire substrate by metal organic chemical vapor deposition with an un-doped GaN buffer layer. Firstly, the n-GaN substrates were ultrasonically cleaned in acetone for 10 min, followed in ethanol also for 10 min. The samples were purged by high-pressure N2 gas to remove any remaining particles on the surface. Then, n-GaN substrates were transferred into ALD (Beneq TFS-200) chamber, and ZrO2 films with a thickness of 20 nm were deposited at 200 °C. Tetrakis-dimethylamino zirconium (TDMAZ) was used as the Zr precursor which was heated to 85 °C to produce enough vapor pressure. Water or ozone was used as the oxidizer. H2O was heated to 40 °C, and O3 was produced by an ozone generator. For H2O oxidant, the growth rate was about 0.885 Å/cycle, and 226 cycles were performed to grow 20 nm thick ZrO2. For O3 oxidant, the growth rate was about 0.95 Å/cycle and a total of 210 cycles was needed for the same thickness. The top isolated Cr/Au (20/50 nm) electrodes were formed on ZrO2 films by using an ordinary lift-off process. Figure 1 shows the device diagram of the GaN-based MOS capacitors. In order to improve the ZrO2 films and the interfacial qualities, a post rapid thermal annealing (RTA) process was performed in a temperature range from 300 to 600 °C in N2 atmosphere. A spectroscopic ellipsometer (J. A. Woollam alpha-SE) system was used to measure the ZrO2 film thickness. The crystallization and interface of MOS capacitors were evaluated by high-resolution transmission electron microscopy (HRTEM, JEOL JEM2010 FEF UHR). The capacitance density versus voltage (C-V) and current density versus voltage (I-V) characteristics were measured at room temperature by using a Keithley 4200 semiconductor analyzer.

Fig. 1
figure 1

The schematic diagram of Cr/Au/ZrO2/n-GaN MOS capacitors

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Results and Discussion

Figure 2 shows the cross-sectional HRTEM image of MOS capacitors with O3 as the oxidizer. As is shown, the interface is distinct and the thickness of the ZrO2 films is about 20 nm. Obviously, the ZrO2 films exhibit an amorphous phase. Besides, the growth direction of GaN is along [0001], deduced from the corresponding FFT image (shown in the inset of HRTEM image).

Fig. 2
figure 2

Cross-sectional HRTEM image of MOS capacitors with O3 as the oxidizer (the inset is the related FFT image)

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Figure 3 shows the C-V characteristics of ZrO2/GaN structure measured at 10 kHz with different annealing temperatures. A depletion region (low capacitance state) in the negative voltage range and an accumulation region (high capacitance state) in the positive voltage range were clearly observed for MOS capacitors with either H2O or O3 as oxidizer. The capacitance density went up gradually as the annealing temperature increased, which can be attributed to the improved crystalline quality of ZrO2 films [17]. However, the MOS capacitors with H2O as the oxidizer showed a higher capacitance than that with O3 oxidant. The reason was probably due to that the former have more oxygen vacancies and hydroxyl residuals [18]. These defects and impurities, on the one hand, can provide extra inherent electric dipoles which contributed to a part of capacitance. On the other hand, a larger leakage current may be produced, as seen from Fig. 4. For H2O as the oxidizer, the capacitance density was enhanced from the initial 1.24 to 1.50 μF/cm2 when the annealing temperature reached at 500 °C. This is a better inclination because the low and high capacitance states became much more distinguishable, and thus, the switching speed which can be described by the slopes of C-V curves from low to high states increased gradually, as shown in Fig. 5. For O3 as the oxidizer, the variation tendency was similar to that of H2O oxidizer. The capacitance increased from 0.95 to 1.36 μF/cm2 when the device was annealed at 500 °C. The slope also reached to the maximum at 500 °C, as seen from Fig. 5. However, the capacitance began to decline when the annealing temperature reached at 600 °C, which can be ascribed to the formation of interfacial layers such as GaO x [19].

Fig. 3
figure 3

C-V characteristics of MOS capacitors under different annealing temperatures. a H2O as the oxidizer. b O3 as the oxidizer

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Fig. 4
figure 4

I-V characteristics of MOS capacitors under different annealing temperatures. a H2O as the oxidizer. b O3 as the oxidizer

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Fig. 5
figure 5

The maximum ratio of capacitance density and voltage of the capacitors with different annealing temperatures

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Figure 6 shows the C-F features of the capacitors under a DC bias of 4 V. Both capacitors can work normally in a frequency range from 1 kHz to 1 MHz. An abrupt increase of the capacitance was observed for both capacitors with H2O and O3 as oxidizers, which can be attributed to the resonance effect at high frequencies.

Fig. 6
figure 6

C-F plot of as-deposited films with H2O or O3 as the oxidizer

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In terms of leakage properties, as shown in Fig. 4, the leakage current in the negative voltage range was remarkably lower than that in the positive voltage range for capacitors with H2O or O3 as the oxidizer. This phenomenon can be attributed to the formation of the depletion region when the negative voltage was added. The depletion region decreases the electric field intensity on the dielectric layer, which resulted in a much lower leakage current density. The leakage current density of the capacitors without annealing was improved from 1 × 10−8 to 3 × 10−9 A/cm2 at 1 V when O3 was used as the oxidizer instead of H2O. This could be explained in this way that O3 is highly volatile and has a stronger oxidizing ability than H2O, based on the fact that the prepared ZrO2 films using O3 oxidizer have less impurities [18] and more accurate stoichiometry composition. The improved ZrO2 qualities enhanced the insulating properties. As the annealing temperature gradually increased from 300 to 600 °C, the leakage current increased correspondingly because of the increased crystalline boundaries which served as the leakage channels.

It is important to understand the leakage mechanism because it helps us to find out effective ways to reduce the leakage current. Here, Schottky emission and Fowler-Nordheim (F-N) tunneling models [20] are used to analyze the I-V curves. Schottky emission is primarily applied in low or moderate electric fields. The relationship of ln(J/E) vs E1/2 should be linear if the leakage conduction follows the Schottky emission. F-N tunneling is mainly applied to describe the leakage current in high electric fields. The plot of ln(J/E2) vs E−1 should be a straight line if the F-N tunneling exists. Note the fact that the maximum capacitance density was obtained at the annealing temperature of 500 °C; the leakage current for this case was studied in detail in the following. For capacitors with H2O oxidant, as shown in Fig. 7a, the plot of ln(J/E) vs E1/2 presented a straight line when the electric field was over 0.5 MV/cm. However, the Schottky emission occurred when the electric field exceeded 0.7 MV/cm for capacitors with O3 oxidant. This indicated that ZrO2 films grown with O3 have a higher Schottky barrier than that with H2O oxidant. The increased Schottky barrier height was mostly due to less impurities and defects. Figure 7b shows the ln(J/E2) versus E−1 plots of the capacitors. It was found that F-N tunneling dominated when the electric field exceeded 0.9 MV/cm for the capacitors with H2O as the oxidant. For capacitors with O3 oxidant, however, this bar increased to 1.1 MV/cm, above which the F-N tunneling dominated. This demonstrated again that ZrO2 films with O3 as the oxidant have better performance.

Fig. 7
figure 7

The conductive mechanism of MOS capacitors annealed at 500 °C. a Schottky emission. b F-N tunneling

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Conclusions

In summary, we have fabricated GaN-based MOS capacitors with ZrO2 as dielectrics. We observed clearly distinguished accumulation and depletion regions from C-V results. The accumulated capacitance increased gradually as the annealing temperature increased. In addition, a low leakage current density of 3 × 10−9 A/cm2 at 1 V was obtained by using O3 oxidant. Based on the leakage current analyses, it can be concluded that Schottky emission dominated at low fields, while F-N tunneling governed at high fields.