Effects of Post-Deposition Annealing on ZrO2/n-GaN MOS Capacitors with H2O and O3 as the Oxidizers
© The Author(s). 2017
Received: 1 January 2017
Accepted: 29 March 2017
Published: 11 April 2017
GaN-based metal-oxide-semiconductor capacitors with ZrO2 as the dielectric layer have been prepared by atomic layer deposition. The accumulation and depletion regions can be clearly distinguished when the voltage was swept from −4 to 4 V. Post-annealing results suggested that the capacitance in accumulation region went up gradually as the annealing temperature increased from 300 to 500 °C. A minimum leakage current density of 3 × 10−9 A/cm2 at 1 V was obtained when O3 was used for the growth of ZrO2. Leakage analysis revealed that Schottky emission and Fowler-Nordheim tunneling were the main leakage mechanisms.
KeywordsZrO2 GaN MOS Atomic layer deposition Post-annealing
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) , excellent chemical stability, and the ability to resist radiation damage . 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 . 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 , ZrO2 , MgO , and TiO2  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) . 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 . In 2014, Gand Ye et al. investigated the band alignment of ZrO2 and GaN using X-ray photoelectron spectroscopy . 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.
Results and Discussion
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  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.
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.
This work is supported by the NSFC under Grant No. 11574235. The authors would like to thank D. X. Zhang and F. P. Zhang for the technical support.
MJZ carried out the experiments and drafted the manuscript. GZZ participated in the design of the study and performed the analysis. XW and JXW participated in the measurements. HW conceived the study and participated in its design. CL supervised the overall study and polished the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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