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International Journal of Photoenergy
Volume 2012 (2012), Article ID 858350, 7 pages
Current Mechanism in -Gated Metal-Oxide-Semiconductor Devices
Department of Physics, Faculty of Sciences and Arts, Batman University, 72000 Batman, Turkey
Received 9 January 2012; Revised 27 March 2012; Accepted 10 April 2012
Academic Editor: Xie Quan
Copyright © 2012 Osman Pakma. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The present study aimed to examine the current density-voltage (J-V) characteristics of Al/HfO2/p-Si (MOS) structure at temperatures ranging between 100 and 320 K and to determine the structure’s current transport mechanism. The HfO2 film was coated on a single side of the p-Si (111) crystal using the spin coating method. The J-V measurements of the obtained structure at the temperatures between 100 and 320 K revealed that the current transport mechanism in the structure was compatible with the Schottky emission theory. The Schottky emission theory was also used to calculate the structure’s Schottky barrier heights (), dielectric constants () and refractive index values of the thin films at each temperature value. The dielectric constant and refractive index values were observed to decrease at decreasing temperatures. The capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of Al/HfO2/p-Si (MOS) structure was measured in the temperature range of 100–320 K. The values of measured C and G/ω decrease in accumulation and depletion regions with decreasing temperature due to localized at Si/HfO2 interface.
As an important group of thin-film electronic materials for microelectronics, dielectrics have a broad range of device applications. They are instrumental both in active devices like transistors and their electrical isolation and in passive devices like capacitors. In these days, when Si-based device technologies are widely put to use, thin film dielectric materials have properties that could be useful in many areas. They have recently been used in both high-permittivity applications such as transistor gate and capacitor dielectrics and low-permittivity materials like inter-level metal dielectrics which operate at switching frequencies in the gigahertz regime for the most demanding applications.
For over forty years, silicon dioxide (SiO2) films have been preferred for gate dielectric in metal-oxide-semiconductor (MOS) devices. SiO2 films get thinner in smaller MOS devices. Currently, the technology is aiming to obtain node technologies thinner than 100 nm , to which an SiO2 gate oxide thickness of 1.5–2 nm corresponds. Since direct tunneling has to be made through the thin oxide, high leakage current and reliability problems occur. The solution is high dielectric materials. A good example for future MOS technologies is HfO2, while a thickness less than 100 nm in silicon demands a gate dielectric other than SiO2 . With a high dielectric constant (20–25) and a high band gap (5.1–6.0 eV) [3, 4], it is compatible with polycrystalline silicon gate process and is also thermally stable with silicon . The methods used to prepare HfO2 thin films have a wide variety. To mention a few, they include Rf magnetron sputtering , chemical vapor deposition, and sol-gel process [7, 8]. Of all these methods, the sol-gel method is among the most promising ones because this method could easily control the optical and other properties of thin film when the solution composition and deposition condition are changed.
Thin Hf-layer-coated structures have been reported to include various types of current transport mechanisms, which are Fowler-Nordheim (FN) tunneling, trap-assisted tunneling (TAT), Schottky emission, Poole-Frenkel (PF), direct tunneling (DT), and tunneling-assisted PF emission (TAPF). Pan et al. performed current-voltage (I-V) measurements at high temperatures on HfO2/TaN-based metal-insulator-metal (MIM) structures whose HfO2 dielectric material was prepared using the atomic-layer-deposition (ALD) method, and on the basis of their measurements, they demonstrated that the Schottky emission current dominates in the low-field range of this structure, while the Poole-Frenkel (PF) emission current dominates in its high-field range . As a result of their I-V measurements on Al/HfO2/p-Si metal-oxide-semiconductor structures whose HfO2 dielectric material was prepared by the RF magnetron sputtering method, Chiu and colleagues reported that the Schottky emission current dominates in the structure at high temperatures and in low-field range, whereas Fowler-Nordheim (FN) tunneling dominates at low temperatures and in high-field range .
The present study examined the I-V characteristics in a temperature range of 100–320 K of Al/HfO2/p-Si (MOS) structure whose HfO2 thin film was prepared by the sol-gel immersion method.
2. Design and Fabrication of Al/HfO2/p-Si Structures
To prepare the HfO2 solution, 0.0063 mol hafnium tetrachloride was dissolved in 15 mL ethanol, to which 0.08 mol H2O and 0.013 mol HNO3 were added and the solution was kept in a magnetic stirrer for 2 hours. Finally, before coating the film, the solution was kept at 50°C for 4 hours. Several metal-oxide-semiconductor (Al/HfO2/p-Si) structures were fabricated on the 3-inch diameter float zone <111> p-type (boron-doped) single crystal silicon wafer with a thickness of 600 μm and a resistivity of 5–10 Ωcm. For the fabrication process, Si wafer was degreased through the RCA cleaning procedure (i.e., a 10-minute boiling in NH4OH + H2O2 + 6 DI (18 MΩ deionised water), which was followed by a 10-minute boiling in HCl + H2O2 + 6 DI) . Next, it was subjected to the drying process in N2 atmosphere for a prolonged time. Following the drying process, high-purity aluminum with a thickness of 1500 was thermally evaporated from the tungsten filament onto the whole back surface of the Si wafer under the pressure of 10−7 Torr. In order to obtain a low-resistivity ohmic back contact, Si wafer was sintered at 580°C for 3 minutes in N2 atmosphere. The native oxide on the front surface of the substrate was removed in HF : H2O (1 : 10) solution, and finally, the wafer was rinsed in deionised water for 30 s before forming an organic layer on the p-type Si substrate. The prepared HFO2 solution was coated on the shiny side of the cleaned p-Si surface at 2000 rpm by the sol-gel spinning technique. The HfO2/p-Si structure was annealed for five minutes at 300°C in N2 atmosphere after each coating process. The procedure was repeated in the same way until the required film thickness was obtained and the structure was finally subjected to annealing at 500°C in N2 atmosphere for 1 h. The interfacial oxide layer thickness was estimated to be about 8.9 nm by spectroscopic ellipsometry (VASE M2000). In order to obtain a rectifying contact on the front surface of p-Si coated with HfO2, a high-purity aluminum layer was coated on the surface in a high vacuum under the pressure of 10−7 Torr. The Al/HfO2/p-Si (MOS) structure is given in Figure 1. The current-voltage (I-V) characteristics of the samples were measured in the temperature range of 100–320 K using a temperature controlled Janis CCS-350S cryostat, which allowed us to perform the measurements in the temperature range of 10–325 K, and using a Keithley 2420 programmable constant current source under dark conditions. The sample temperature was continually monitored using a GaAlAs sensor and a Lakeshore 330 auto-tuning temperature controller with a sensitivity better than ±0.1 K. The forward and reverse bias capacitance-voltage (C-V) and conductance-voltage (G-V) measurements were performed in the 1 MHz by using an HP 4192 A LF impedance analyzer (5 Hz to 13 MHz) and the test signal of 50 mVrms. All measurements were carried out in the temperature range of 100–320 K and in the dark.
3. Measurement and Experimental Results
Figure 2 presents the current density-voltage (J-V) characteristics of the Al/HfO2/p-Si (MOS) structure measured in the temperature range of 100–320 K and from −3 V to +3 V. It is found that reverse leakage current density at −1 V is 5.63 × 10−4 A/cm2 for 100 K and 1.35 × 10−2 A/cm2 for 320 K, respectively. The reverse bias leakage current was observed to be 26 times higher at 320 K than that of 100 K. It is noted that the leakage current increases with increase in temperature. As can be seen in this figure, the lnJ-V plots are linear on a semilogarithmic scale at low forward bias voltages but deviate considerably from linearity at high forward bias voltages due to the effect of series resistance on the interfacial insulator layer (HfO2). The current density across an ideal Schottky barrier diode (SBD), based on the thermionic emission (TE) theory, is given by the following relation () : where is the reverse saturation current density: where , , , , , and are the zero bias barrier height, series resistance of structure, the effective Richardson constant (32 A/cm2 K2 for p-type Si ), the ideality factor, the electron charge, and the temperature in Kelvin, respectively.
As it is known, the characteristic lnJ-V graph of a diode usually has three regions, which are low-voltage region ( V), medium-voltage region (0.1 V 0.8 V), and high-voltage region ( V), respectively. The electrical parameters except for were calculated in the linear region in the medium voltage region, while was calculated in the region where the lnJ-V curve in the high-voltage region bended. As can be seen from Figure 2, each semilogarithmic J-V curve consists of a linear range with different slopes in between the intermediate-bias voltage () regions. The experimental values of and were determined from (1) and (2), respectively, and they are shown at each temperature in Table 1. As can be seen in Table 1, the values of and range from 1.24 and 0.516 eV (320 K) to 4.32 and 0.172 eV (100 K), respectively. Both parameters exhibit a strong temperature dependency as seen in Figure 3. As seen from Figure 3, while increased, decreased with decreasing temperature. Because of the temperature-activated process, the current transport will be dominated by current flowing through the lower BH and a larger ideality factor . That is, more electrons have sufficient energy to overcome the higher barrier when temperature increases, and then, BH increases with temperature and bias voltage.
Figure 4 shows the conventional Richardson (ln() versus 1/T plot. The Richardson constant and effective barrier height at 0 K are obtained from the intercept and slope of linear fit. ln() versus 1/T plot has two linear temperature ranges. In the first region (180–320 K), the values of effective barrier height obtained from the slope of this straight line come out to be 0.096 eV. Likewise, the value Richardson constant determined from the intercept at the ordinate is 2.52 × 10−7 A/cm2 K2. In the second region (100–160 K), the values of effective barrier height and Richardson constant were derived from the slope and intercept of this straight line as 0.0013 eV and 6.01 × 10−10 A/cm2 K2, respectively. The Richardson constant values are much lower than the known value of 32 A/cm2 K2 for p-type Si. As explained before, the deviation in the Richardson plots might be a result of the spatially inhomogeneous barrier heights and potential fluctuations at the interface, which consist of low and high barrier areas [14–17].
Figure 5 shows a plot of the experimental barrier height versus the ideality factor for various temperatures. As can be seen in Figure 5, there are linear regions between the experimental barrier heights and ideality factors of the Al/HfO2/p-Si (MOS) structure, which can be explained by lateral inhomogeneities of the barrier heights [14, 17]. The exploration of the experimental barrier height versus the ideality factor plot to has given a value of 0.503 eV, which is close to half of the band gap in Si. These results confirm that the predominant current transport is not the TE in our samples.
The standard Schottky emission equation is as follows : where (A/cm2K2), is the effective Richardson constant, is the electronic charge, is the electric field, is Planck’s constant, is the permittivity of free space, is the dynamic dielectric constant, is the free electron mass, and is the electron effective mass in HfO2 ( = 0.4 for Al/HfO2 and = 0.18 for HfO2/Si) [5, 18].
For the standard Schottky emission, a plot of ln() versus should be linear. As seen in Figures 6 and 7, there is a very good fit between the experimental data in the region for each measurement temperature and high electric field and the Schottky emission theory for both gate injection and substrate injection. At each temperature in Table 2, the experimental refractive index values , and were determined from (3), respectively. Table 2 reveals that experimentally determined dynamic dielectric constants at room temperature in standard Schottky plots are so close to 4, the square of the refractive index 2 [3, 19]. This points out to the consistency between the conduction mechanism under high electric fields and Schottky emission. Another observation was that the extracted refractive index value gradually increases with increased measurement temperature, which may result from the effects of thermal stress, thermal expansion, and electronic polarizability [20–22]. The barrier height at the Al/HfO2 and HfO2/p-Si interfaces was determined from 0.482 and 0.342 eV (100 K) to 0.936 and 0.778 eV (320 K), respectively.
There are a number of suggested methods which help to extract the of structure, and among them the most important one is the conductance method, developed by Nicollian and Goetzberger [23–26]. In this method, the frequency-dependent forward and reverse bias C-V and G/ω-V measurements give the detail information on the distribution profile of of MOS structure. When the MOS structure is biased into strong accumulation, the impedance is given by  where and are the measured capacitance and conductance, in strong accumulation region. Series resistance is the real part of the impedance () or The oxide layer capacitance () is obtained by substituting from (5) into the following relations: where and are the capacitance and conductance measured across the MOS structure at any bias voltage.
The capacitance-voltage (C-V) and conductance-voltage (G/ω-V) characteristics of Al/HfO2/p-Si (MOS) structure were measured in the temperature range of 100–320 K (1 MHz) and are given in Figures 8(a) and 8(b), respectively. As can be seen from Figures 8(a) and 8(b) both curves have three distinct regimes of accumulation-depletion-inversion. The values of measured and G/ω decrease in accumulation and depletion regions with decreasing temperature due to localized at Si/HfO2 interface. Such behavior of the and G/ω is attributed to particular distribution of interface states at p-Si/HfO2 interfaces and of structure .
Using (5) the values of were calculated as a function of bias in the temperature range of 100–320 K and are given in Figure 9. As can be clearly seen from Figure 9, the gives a peak. The peak position of is shifting toward inversion region with increasing temperature. Such behavior of is attributed to the particular distribution of localized at p-Si/HfO2 interface states and interfacial insulator layer at Al/p-Si interface.
The density of interface states () at p-Si/HfO2 can be derived from Hill-Coleman method . According to this method, is given by where is the area of the structure, and is the maximum measured conductance value. Figure 10 shows that the increase with increasing temperature. The high values of and at high temperatures were attributed to the excess capacitance resulting from the , which is in equilibrium with the semiconductor that follows the ac signal.
The forward and reverse J-V, C-V, and G/ω-V characteristics of Al/HfO2/p-Si (MOS) structure derived using the sol-gel method were measured at temperatures ranging between 100 and 320 K. The J-V measurements of the obtained Al/HfO2/p-Si (MOS) structure at the temperatures between 100 and 320 K revealed that the current transport mechanism in the structure was compatible with the Schottky emission theory. The Schottky emission theory was also used to calculate the structure’s Schottky barrier heights (), dielectric constants (), and refractive index values of the thin films at each temperature value. The dielectric constant and refractive index values were observed to decrease at decreasing temperatures. The values of measured and G/ω decrease in accumulation and depletion regions with decreasing temperature due to localized at Si/HfO2 interface. The increase with increasing temperature.
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