Leakage Current Mechanism of InN-Based Metal-Insulator-Semiconductor Structures with Al2O3 as Dielectric Layers
© Wang et al. 2016
Received: 26 September 2015
Accepted: 5 January 2016
Published: 13 January 2016
InN-based metal-insulator-semiconductor (MIS) structures were prepared with Al2O3 as the gate oxides. Surface morphologies of InN films are improved with increasing Mg doping concentrations. At high frequencies, the measured capacitance densities deviate from the real ones with turning frequencies inversely proportional to series resistances. An ultralow leakage current density of 1.35 × 10−9 A/cm2 at 1 V is obtained. Fowler-Nordheim tunneling is the main mechanism of the leakage current at high fields, while Schottky emission dominates at low fields. Capacitance densities shift with different biases, indicating that the InN-based MIS structures can serve as potential candidates for MIS field-effect transistors.
III-Nitrides, with excellent optic and electronic properties, can be widely used for solar cells, optical wave guides, high-speed electronics, and terahertz emitters . Among them, InN has the lowest effective mass of electrons and the highest mobility, and thus, it is a promising semiconductor for applications in high-speed electronics such as field-effect transistors (FETs) and high-electron-mobility transistors (HEMTs). One of the major obstacles that limit the performance and reliability of these transistors for high-power radio-frequency (rf) applications is the high gate leakage . To solve this problem, structures like metal-insulator-semiconductor (MIS) and metal-oxide-semiconductor (MOS) have been developed by using SiO2 and Al2O3 as the dielectric layers [3, 4]. However, none of MIS or MOS structures has been reported on InN electronic devices yet.
In recent years, high-quality InN films have been grown by molecular beam epitaxy (MBE) [5, 6]. Although the surface electron accumulation is not completely explained and solved, p-type carriers have been confirmed in Mg-doped InN by indirect evidences such as measurements of electrolyte-based capacitance-voltage (ECV) , temperature-dependent Hall effect , thermopower , and photoconductivity . All efforts mentioned above lay a good foundation for the fabrication of high-quality Mg-doped InN MISFETs and MIS-HEMTs.
The Al2O3 dielectric layer has been widely used in MIS and MOS structures due to its relatively larger dielectric constant compared to that of SiO2. Atom layer deposition (ALD) has many advantages in growing the Al2O3 dielectric layer such as low temperatures and pinhole-free growth. Hence p-type InN-based MOS and MIS structures with Al2O3 as the dielectric layers are promising to be applied for FETs, HEMTs, and other kinds of thin-film transistors (TFTs).
In this work, Mg-doped InN films were grown on c-plane sapphire with GaN buffer layers by MBE. Al2O3 dielectric layers were then grown by ALD. Top Cr/Au electrodes were made by thermal evaporation, while bottom electrodes were welded In dots. Surface morphology of InN films was improved with increasing Mg doping concentrations. An ultralow leakage current density of 1.35 × 10−9 A/cm2 at 1 V was obtained. The leakage mechanism, capacitance density versus frequency (C-F), and capacitance density versus voltage (C-V) of this InN-based MIS structure were also investigated.
Mg-doped InN films were grown on c-plane sapphire with GaN buffer layers by using radio-frequency plasma-assisted molecular beam epitaxy (rf-MBE, SVTA 35-V-2). Thin GaN buffer layers, with a thickness of 50 nm, were grown under the optimized conditions with the substrate temperature at 760 °C and Ga source temperature at 1020 °C on c-plane sapphire [11, 12]. InN films were then grown for 2 h under the optimized conditions reported previously, i.e., setting the In source temperature at 770 °C, substrate temperature at 450 °C, and N flow rate at 2.65 sccm [11, 12]. Mg doping in InN films was performed with Mg source temperatures at 300, 310, 320, 330, 335, and 340 °C, respectively. Al2O3 dielectric films, with a thickness of 50 nm, were prepared with a growth rate of 0.1 nm/cycle by ALD (Beneq TFS-200) by using the precursors of trimethyl aluminum (TMA) and H2O. The detailed growth conditions can be found in our previous work [13–15]. Cr/Au (15-nm Cr and 50-nm Au) were fabricated on Al2O3 layers by thermal evaporation with templates of 150 × 150 μm2 in area as the top electrodes. In dots were welded on InN layers as the bottom electrodes. InN films were examined by high-resolution x-ray diffraction (HRXRD, Bede D1) and atomic force microscopy (AFM, SPM-9500J3, Shimadzu). The C-F, C-V, and leakage current density versus voltage (I-V) characteristics were measured by using a semiconductor device analyzer (Keithley 4200, Keithley Instruments).
Results and Discussion
The leakage mechanism is investigated by using models of Schottky, Fowler-Nordheim (F-N), and Frenkel-Poole (F-P) tunneling emissions. Figure 4c shows the relationship between ln(J/E 2) and the reciprocal of electric field (E −1). When the field was above 1.3 MV/cm for Mg source temperature at 340 °C, above 1.25 MV/cm for that at 330 °C, or above 2 MV/cm for that at 310 °C, a linear relationship was observed, which meant that the conduction mechanism was governed by F-N tunneling at high fields. At low fields, ln(J) versus E 1/2 was also linear in Fig. 4d, meaning that the conduction was governed by Schottky emission. As Mg source temperature decreased, the conduction mechanism changed at fields of 1.21 MV/cm for 340 °C, 1.18 MV/cm for 330 °C, and 1.9 MV/cm for 310 °C, which also proved that the leakage mechanism of the Al2O3/InN structure followed the F-N tunneling mechanism at high fields and the Schottky emission mechanism at low fields. The fitted relative dielectric constants were 10.2, 12.3, and 21.6 for different Mg concentrations in InN films. Furthermore, the Frenkel-Poole (F-P) emission model was applied to analyze the leakage mechanism of the Al2O3/InN structure (not shown). No linear relationship between ln(J/E) versus E 1/2 was found. Hence, it can be concluded that the leakage mechanism of the Al2O3/InN structure follows the F-N tunneling when the field is above 1.2 MV/cm and the Schottky emission when the field is lower than 1.2 MV/cm.
In conclusion, InN-based MIS structures were fabricated with high-quality Al2O3 thin films as dielectrics. An ultralow leakage current density of 1.35 × 10−9 A/cm2 at 1 V was achieved. At high frequencies, the measured capacitance densities deviated from the real ones with turning frequencies inversely proportional to series resistances. It can be concluded that Fowler-Nordheim tunneling is the main mechanism of the leakage current at high fields, while Schottky emission dominates at low fields. The Mg-doped InN MIS structures can serve as potential candidates for MISFETs.
This work is supported by the NSFC under Grant Nos. 11175135 and J1210061. The authors would like to thank L. H. Bai and M. C. Wei for the technical support.
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