Effect of the chemical composition at the memory behavior of Al/BST/SiO2/Si-gate-FET structure

The effect of the chemical composition of the ferroelectric barium strontium titanate (BST) on the memory window behavior of Al/BST/SiO2/Si-gate-field effect transistor structure has been investigated. Nanocrystalline BaxSr1−xTiO3 thin films with different x values have been fabricated as metal-ferroelectric-insulator-semiconductor (MFIS) and metal-ferroelectric-metal (MFM) configurations using a sol–gel technique. The variation of the dielectric constant (ε) and tan δ with frequency for MFM samples have been studied to ensure the dielectric quality of the material. At low frequencies, ε increases as the strontium content decreases, whereas at high frequencies, it shows the opposite variation, which is attributed to the dipole dynamics. The ferroelectricity of the BST within MFM structure has been investigated using C–V characteristics, which show that the ferroelectric hysteresis strength increases as the strontium content decreases. The ferroelectric memory behavior of the MFIS samples has been investigated using C–V characteristics. The results show that the memory window width increases as the strontium content decreases; this is attributed to the grain size and dipole dynamics effect.


Introduction
In the past few years, ferroelectric random access memories (FRAMs) have been studied extensively due to their potential advantages, such as non-volatility, unlimited write cycles, and low power consumption. In particular, nondestructive read out (NDRO) FRAM, which has a transistor as a memory cell, has high attention, since the ferroelectric gate offers simpler circuits and excellent performance (Roy et al. 2008;Wang et al. 2003). Among ferroelectric materials, barium strontium titanate (BST) in thin-film form is considered to be one of the most promising candidates for FRAM applications due to its desirable properties such as high permittivity, and relatively high remnant polarization (Ru-Bing et al. 2005).
The barium-to-strontium (Ba:Sr) ratio in BST thin films plays a significant role at the nanostructure and electrical properties. The Curie temperature of BST thin films varies through a long range of values depending on the Ba:Sr ratio (Ru-Bing et al. 2005), which in turn controls the phase of the film, i.e., to be in ferroelectric (with a tetragonal lattice) or paraelectric (with a cubic lattice) phase. Furthermore, the size of BST grains is directly related to the Ba:Sr ratio, as Sr ions increases in the lattice the grain size decreases (Saif and Poopalan 2010); however, the grain size is directly related to the domain wall (Arlt et al. 1985); as a result, the ferroelectric properties of the material change.
Relatively few studies reported the electrical properties for BST as a metal-ferroelectric-semiconductor (MFS) junction (Panda et al. 2002;Jha et al. 2008;Agarwal et al. 2001). This is attributed to the difficulty in the deposition process directly onto silicon, high trap densities, and the diffusion of elements into silicon (Lee et al. 2005;Tang et al. 2007). Hence, to overcome these difficulties, an insulating buffer layer between the ferroelectric layer and the silicon layer has been suggested. In the current work, BST thin films with different Ba:Sr ratios have been fabricated in a Al/BST/ SiO 2 /Si configuration to study the effect of the chemical composition at the memory window behavior.

Experiment
Three solutions with different proportions of Ba:Sr (50:50,70:30 and 80:20) were prepared using barium acetate, strontium acetate, and titanium (IV) isopropoxide as the starting materials; the preparation details for the solutions can be found in Saif and Poopalan (2011). Two sets of samples were prepared: (1) a metal-ferroelectric-metal (MFM) configuration where a Pt/SiO 2 /Si structure was used as the substrate and (2) a metal-ferroelectric-insulatorsemiconductor (MFIS) configuration where a SiO 2 /Si structure was used as the substrate. The film preparation procedure is reported by Saif and Poopalan (2011). The film thickness has been measured using the same procedure mentioned by Saif and Poopalan (2011); the average thickness of both sample sets is 400 nm. For the electrical measurement, in both sets, dots of Al with an area of 7.85 9 10 -3 cm 2 were deposited on top of the films as the top electrode using a shadow mask via physical vapor deposition (PVD). For the MFIS samples, the backside of the silicon substrates was etched in hydrofluoric acid and was metallized by depositing a 140-nm-thick Al layer to represent the backside electrode. The crystallization of the material was determined using an X-Ray diffractometer (XRD) with a CuKa radiation source (k = 1.54 Å ), operated at a voltage 40 kV with a current of 40 mA. The dielectric characteristics were performed by an impedance/ gain-phase analyzer (Solartron 1260) in the frequency range of 10 Hz-1 MHz at room temperature. C-V and I-V measurements were performed using a Keithley 4200 semiconductor parameter analyzer.
Results and discussion Figure 1 shows XRD patterns of Ba 0.5 Sr 0.5 TiO 3 , Ba 0.7 Sr 0.3 TiO 3 , and Ba 0.8 Sr 0.2 TiO 3 films. From Fig. 1 it can be seen that the diffraction peaks are (1 0 0), (1 1 0), (1 1 1), (2 0 0), (2 1 0), and (2 1 1) within the 2h range from 20°t o 60°, which confirms that the films are crystallized with a perovskite structure. The measured lattice parameters of the samples are listed in Table 1. It is observed that the lattice parameters decrease with an increasing strontium content; this is attributed to the fact that the ionic radius of Sr is smaller than that of Ba. Table 1 shows that the lattice constants for Ba 0.5 Sr 0.5 TiO 3 are equal, which reveals that it has simple cubic structure, whereas, for Ba 0.7 Sr 0.3 TiO 3 and Ba 0.8 Sr 0.2 TiO 3 films, c-axis lattice constant is larger than the a-axis lattice constant. This suggests that the crystal structure for these films is tetragonal at room temperature.
In order to ensure the dielectric quality of the material, the dielectric permittivity (e) and loss tangent (tan d = e 00 /e 0 ) for MFM samples were studied as a function of frequency. Figure 2 shows the variation of e with frequency plots for the films used in this work at room temperature. It is observed that the value of e for all Ba:Sr ratios decreases as the frequency increases and attains a constant limiting value e ? (high-frequency value of e). This can be explained according to the behavior of the dipole movement, the dielectric permittivity related to free dipoles oscillating in the presence of an alternating electric field. At very low frequencies (f \ 1/s, s is the relaxation time), dipoles follow the electric field. As the frequency increases, dipoles begin to lag behind the field and e slightly decreases. When the frequency reaches the characteristic frequency (f = 1/s), the dielectric constant drops (relaxation process). At very high frequencies (f [ 1/s), dipoles can no longer follow the field and e & e ? (Tripathi et al. 2010).
It can be seen in Fig. 2 that at low frequencies the value of e increases as the strontium content decreases. This can be explained according to the lattice shape and the presence of the dipoles in the BST lattice. As discussed earlier in this article, the Ba 0.5 Sr 0.5 TiO 3 film is crystallized in a simple cubic structure, while Ba 0.7 Sr 0.3 TiO 3 and Ba 0.8 Sr 0.2 TiO 3 crystallized with a tetragonal perovskite structure. That explains the low dielectric constant value for Ba 0.5 Sr 0.5 TiO 3 at low frequencies compared with the other ratios, whereas the tetragonal phase for Ba 0.7 Sr 0.3 TiO 3 and Ba 0.8 Sr 0.2 TiO 3 films leads to the presence of a valuable number of permanent dipoles within their lattice, which explains the high value of their dielectric constant. Furthermore, the high value of e for Ba 0.8 Sr 0.2 TiO 3 compared with Ba 0.7 Sr 0.3 TiO 3 can be attributed to the longer permanent dipoles, since the c/a ratio for Ba 0.8 Sr 0.2 TiO 3 is larger than that for Ba 0.7 Sr 0.3 TiO 3 . On the other hand, at high frequencies, the dielectric constant variation with strontium content becomes the opposite of that observed at low frequencies, i.e. e decreases as the strontium content decreases. This may be explained considering the dipole elongation responding to the applied electric field. As an AC electric field is applied at BST lattice, it creates a new dipoles, reorients the permanent dipoles to the direction of the applied filed, and causes an induced shift to the Ti ions for the dipoles that already have the same orientation of the applied field in case of Ba 0.7 Sr 0.3 TiO 3 and Ba 0.8 Sr 0.2 TiO 3 films, which in turn increases their length. However, as the frequency increases, the longer dipoles find it harder to follow the applied field; as a result, a low dielectric constant is obtained. Furthermore, the trend of e at high frequencies agrees very well with the published results in the literature (Ru-Bing et al. 2005). The value of e in the whole frequency range is relatively high.
The variation of tan d as a function of frequency is given in Fig. 3. It can be observed from the figure that at low frequencies tan d decreases with increasing frequency and reaches a value close to zero at high frequencies. At a frequency range between 250 and 10 5 Hz, a broad peak in Ba 0.7 Sr 0.3 TiO 3 and Ba 0.8 Sr 0.2 TiO 3 is observed. This kind of peak occurs when the hopping frequency of electric charge carriers approximately equal that of the external applied AC electric field (Elkestawy et al. 2010); however, this peak becomes more noticeable and shifts toward lower frequencies as the strontium content decreases. This could be attributed to the increase of the grain sizes and dipoles present. From the above results for the MFM structure, it is shown that the dielectric properties of the films used in this work are relatively good.
In order to confirm the ferroelectric behavior of BST within the MFM samples, the capacitance-voltage (C-V) characteristics have been investigated. Figure 4 shows the C-V characteristics for Ba 0.5 Sr 0.5 TiO 3 , Ba 0.7 Sr 0.3 TiO 3 , and Ba 0.8 Sr 0.2 TiO 3 at 500 kHz and at room temperature. The capacitance was measured while a DC field was swept from -7.5 to ?7.5 V and then reversed, with a sweeping rate of 0.01 V/s. For all the tested samples, it is observed that the capacitance varies non-linearly with the applied voltage. However, a well-defined butterfly shape with two peaks of the capacitance is observed; these peaks are formed as a result of a spontaneous polarization switching (Lahiry et al. 2000). This kind of hysteresis indicates that these films have a ferroelectric nature. The strength of the hysteresis increases as strontium content decreases, which could be attributed to the grain size and dipole dynamics. Furthermore, an observed asymmetry in the C-V curves suggests that the films contain mobile ions or charges accumulated at the interface between the film and the electrode. In addition, there is a difference between the capacitance values of the two peaks, which may be due to some defect in energy levels in the film (Kumari et al. 2007).  The memory properties of the Al/BST/SiO 2 /Si-gate (MFIS) structure were characterized by C-V measurement using a Keithley 4200 semiconductor parameter analyzer. Figure 5 shows typical C-V characteristic curves for BST films within MFIS configuration, at a frequency of 1 MHz, and at room temperature. The applied DC bias swept from -20 to ?20 V and then reversed with a sweeping rate of 0.01 V/s. The C-V plots show clockwise hysteresis loops as indicated by the arrows, corresponding to the ferroelectric polarization switching. This hysteresis is known as the memory window, and it occurs due to the flat-band voltage shift (VD FB ) of the C-V curves when the bias voltage is swept from accumulation to inversion and back (Roy et al. 2008).
It is observed that the C-V curves shift toward the negative voltage axis, which indicates that a fixed positive charge is present at the interfaces, originating from oxygen vacancies that are formed during the heating and annealing treatment in ambient O 2 . This kind of shift is widely reported for different kind of ferroelectric materials (Bozgeyik et al. 2010;Juan et al. 2007). Furthermore, the sharp change in the capacitance at the accumulation and inversion region indicates that the interfaces of the junction are good.
The memory window for the Ba 0.5 Sr 0.5 TiO 3 , Ba 0.7 Sr 0.3 TiO 3 , and Ba 0.8 Sr 0.2 TiO 3 capacitors are 1.4, 3, and 3.3 V, respectively. These values reveal that the memory window value increases as the strontium content decreases, in agreement with the trend of the ferroelectric hysteresis strength obtained for MFM samples. This increment is attributed to the dipoles present and grain size effects. XRD analysis reveals that Ba 0.7 Sr 0.3 TiO 3 and Ba 0.8 Sr 0.2-TiO 3 films were crystallized with a tetragonal structure while Ba 0.5 Sr 0.5 TiO 3 crystallized with a cubic structure, which leads to the valuable number of permanent dipoles that exist within the perovskite lattice of Ba 0.7 Sr 0.3 TiO 3 and Ba 0.8 Sr 0.2 TiO 3 ; these dipoles contribute to the ferroelectric behavior subsequently at the memory window width. On the other hand, as mentioned earlier in a previous work, the grain size of BST increases with the decreasing in the strontium content (Saif and Poopalan 2010). However, it has been reported that the ferroelectric properties, such as remnant polarization and coercive field, strongly depend on the grain size (Hongwei et al. 2006). Furthermore, Arlt et al. (1985) presented theoretical calculations showing that the density of the domain walls is inversely proportional to the square root of the grain, i.e., the density of the domain walls increases as the grain size decreases. This in turn strengthens the repulsive force between neighboring domain walls. As a result, the mobility for the domain wall reduces, which in turn makes the domain reorientation more difficult (Bozgeyik et al. 2010). Leading up to higher activation energy is required for the reorientation of the domains; as a result, the remnant polarization decreases, which reflects as a narrower memory window in C-V curves.  Figure 6 shows a typical variation of leakage current density as a function of applied voltage (J-V) for Ba 0.5 Sr 0.5 TiO 3 , Ba 0.7 Sr 0.3 TiO 3 , and Ba 0.8 Sr 0.2 TiO 3 within MFIS structure at room temperature and for gate voltage swept from zero up to 10 V. It is observed that the leakage current density increases as the applied voltage increases and as the strontium content decreases. It is found that for all the tested samples the leakage current density is of the order of 10 -8 A/cm 2 , at an applied voltage of 10 V (0.23 MV/cm). These values of the current density are relatively low, indicating that the films have good insulating characteristics. The increase of the leakage current density with the decrease of strontium content is attributed to the increase in the grain size. It is well known that the grain boundaries act as trappers for crystal defects (vacancies and dislocations) that interact with free carriers. As the grain size decreases the density of the grain boundaries increase, which leads to a larger amount of vacancies and dislocations, giving rise to high density of local charge accumulations. Those charge centers near the grain boundaries act to block the current flow, leading to low leakage current (Hu et al. 2004).

Conclusion
Nanocrystalline-ferroelectric Ba x Sr 1-x TiO 3 thin films with different x values have been fabricated as MFIS and MFM configurations using a sol-gel technique. The perovskite structure of the material has been confirmed via XRD. The e and tan d have been studied for MFM samples to insure the dielectric quality of the material. At low frequencies, e increases as the strontium content decreases, whereas at high frequencies, it shows the opposite variation, which is attributed to the dipole dynamics. tan d shows low values with a peak at the mid-frequency range. The ferroelectric memory window behavior of the MFIS samples has been  Fig. 6 Leakage current density for Ba 0.5 Sr 0.5 TiO 3 , Ba 0.7 Sr 0.3 TiO 3 , and Ba 0.8 Sr 0.2 TiO 3 thin films within MFIS structure investigated using C-V characteristics. The results show that the memory window width increases as the strontium content decreases; this is attributed to the grain size and dipole dynamics effect. In addition, the leakage current density for the films was measured and was found to be of the order of 10 -8 A/cm 2 for all tested samples, indicating that the films have insulating characteristics.
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