N. DEEPAK KUMAR. Laser Photonics Technology Inc.,1576 Sweet Home Road, Amherst, NY-14 228, USA. Received January 27, 1999; Accepted May 24, 1999.
Journal of Sol-Gel Science and Technology 16, 101–107 (1999) c 1999 Kluwer Academic Publishers. Manufactured in The Netherlands. °
Electrical Properties of Sol-Gel Processed Amorphous BaTiO3 Thin Films REJI THOMAS AND D.C. DUBE∗ Department of Physics, Indian Institute of Technology, New Delhi 110 016, India M.N. KAMALASANAN National Physical Laboratory, New Delhi 110 012, India N. DEEPAK KUMAR Laser Photonics Technology Inc.,1576 Sweet Home Road, Amherst, NY-14 228, USA Received January 27, 1999; Accepted May 24, 1999
Abstract. BaTiO3 thin films were prepared on single crystal silicon (1 0 0) and platinum substrates by sol-gel technique. Amorphous films with thickness uniformity were obtained by spinning the solution at 3000 rpm for 30 s and by post-deposition annealing at 400◦ C. The films exhibited good dielectric and insulating properties. The ˚ dielectric constant and dissipation factor at a frequency of 100 kHz were 17 and 0.20, respectively, for 1400 A thick film on platinum substrate (MIM). The corresponding values were 16 and 0.015 for films on Si (MIS). Dielectric properties were also studied as functions of frequency and voltage. The C-V curve for MIS structure exhibited a hysteresis. The density of interface states recharged during the bias cycle in hysteresis measurement was estimated to be of the order of 2.10 × 1011 cm−2 and total oxide charge density was about 4.28 × 1011 cm−2 . I -V measurements were performed on films of different thicknesses. The leakage current densities at 5 V for the ˚ were 0.86 and 0.11 µA/cm2 respectively. The conduction mechanism is films having thicknesses 1400 and 2800 A found to be Poole-Frenkel and Schottky mechanisms at low and high fields, respectively. Keywords: 1.
sol-gel, electrical properties, barium titanate, conduction mechanism
Introduction
The insulating properties of dielectric materials have attracted much attention due mainly to their possibility for applications to various electronic devices; especially in memory cell capacitors [1]. Recently, the silicon-on-insulator structure is gaining importance due to its low parasitic capacitance, high voltage isolation and high packing density compared with bulk silicon [2]. Dielectric thin films also play an important role in integrated circuit technology at microwave frequency [3]. Thin film deposited multichip module packaging (MCM-D) uses low dielectric constant thin ∗ To
whom all correspondence should be addressed.
films for signal line isolation. For this kind of application, dispersion of dielectric constant in operating frequency region is undesirable and amorphous BaTiO3 films meet this requirement up to 50 GHz [4]. High and adjustable capacitance (per unit area) is realizable by using BaTiO3 as an insulating material in integrated circuit applications. However polycrystalline BaTiO3 films have several problems; 1) the tendency of high ² 0 films to have high electrical conductivity (σ ), 2) the decrease in the break down voltage (VBD ) of the capacitor with increasing ² 0 , 3) requirement for a high temperature treatment, after synthesis, is not easy in the existing device technology, and 4) poor aging and fatigue properties [5]. As an alternative, thin films having an amorphous structure seem
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to be attractive. These amorphous films can easily be prepared at relatively lower temperature (∼400◦ C) to be compatible with the existing device technology and also do not exhibit fatigue or aging effects. Further, the amorphous thin films exhibit better electrical properties as compared to thin crystalline dielectric films, since the later often possess columnar structure resulting in degradation [5–7]. As a result, interest in thin films of ferroelectric materials deposited in amorphous form has increased remarkably in recent years. Amorphous BaTiO3 thin films generally have a dielectric constant in the range 15 to 20 and this value meets the requirements for the fabrication of some memory devices [5]. In the present paper, we report the preparation of amorphous BaTiO3 thin films on p-silicon and platinum substrates by the solution-gelation (sol-gel) process. Their electrical properties were studied in terms of C-V and I-V characteristics and their suitability for memory device applications has been discussed. 2.
Experimental Details
For making BaTiO3 thin films, the sol was prepared using titanium isopropoxide and barium 2-ethyl hexanoate as precursors [8, 9]. The procedure is as follows. 4.23 g of barium 2-ethyl hexanoate was dissolved in 24 ml of methanol and 1 ml of acetyl acetone. 0.02 mol of water was added for hydrolysis and the mixture was refluxed for 1 h at 80◦ C. 2.95 ml of titanium isopropoxide was added to this mixture under constant stirring. Addition of acetyl acetone helps in getting a clear solution. The solution is then refluxed at 80◦ C for 2 h, filtered and stored in sealed bottles. Silicon wafers were etched in 10% hydrofluoric acid prior to the film deposition for the removal of SiO2 from the surface. The etched silicon wafers were held above isopropanol vapour and then dried in a nitrogen gas jet. Thoroughly cleaned and polished platinum disks (0.25 mm thick) were also used as substrate. The sol was spin-coated onto the silicon and platinum substrates at 3000 rpm for 30 s. After deposition, the samples were held in a humid and dust free atmosphere for 10 min. for gelation. The films were then fired at 350◦ C to remove organics. Multiple coatings were done to get the desired thickness of the film. After obtaining the desired thickness, the films were annealed at higher temperatures to get desired phase. In the present study, aluminium was used as the top electrode material
in fabricating the MIS and MIM structures. Circular Al gates of diameter 1 mm and thickness 1 µm were deposited on the dielectric film by vacuum evaporation using proper masks. Electrical contact to the p-type silicon was formed by depositing aluminium film on the back of the wafer by thermal evaporation, followed by low temperature annealing (150◦ C) for a short duration (5 min). Aluminum was used to make the back contact due to the fact that, Al can make p-type silicon even stronger p-type and thus reduce the barrier height for holes to surmount. This reduces the resistance for current flow in both (forward and reversed) directions. Again, to make proper ohmic contact, silicon wafers with rough back side were used. The rough region, which is electrically equivalent to a diffused generated layer, helps in achieving a good ohmic contact [10]. The film thickness was first determined with help of Surfometer SF200 and the results were then compared with the thickness calculated from the optical transmission spectra. The measured thicknesses from these two methods were in good proximity with each other. I -V measurements were done using a Keithley 228 voltage/current source and a Keithly picoammeter C-V measurements were done using HP 4192A impedance analyzer. 3. 3.1.
Results and Discussion Structure and Morphology
Figure 1 shows the XRD patterns of the BaTiO3 film on silicon annealed at different temperatures (peaks corresponding to Si are not shown). It is clear from the figure that, in the films annealed at 400◦ C (or below) the XRD pattern did not show any peaks corresponding to the crystalline BaTiO3 , which is suggestive of amorphous nature of the film. Figure 2 shows the surface morphology (using SEM ) of the film on silicon annealed at 400◦ C. The SEM pictures revealed smooth featureless and crack free surface. Further, these films were found to have refractive index of about 1.8. This value is consistent with the reported value of 1.85 for similar films by Li et al. [11]. The structure begins to transform into crystalline at a temperature of about 600◦ C. This is evident from the peaks in the XRD pattern at 600 and 650◦ C (Fig. 1). Thus we were able to get smooth, transparent and crack-free amorphous BaTiO3 films at an annealing temperature as low as 400◦ C.
Electrical Properties of Barium Titanate
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Figure 3. Frequency dependence of dielectric constant and loss tangent of a-BaTiO3 in MIM configuration. Figure 1. X-ray diffraction patterns of BaTiO3 on p-silicon annealed at various temperatures.
the resonance induced by the lead inductance at high frequencies [12]. The loss tangent also shows very small variation up to 1 MHz. But a sudden rise in its value at higher frequencies, where the ε0 shows a minimum, is presumably due to lead inductance effect as already mentioned. Variation of dielectric constant and tan δ of amorphous BaTiO3 in the MIS configuration is shown Fig. 4. The dielectric constant for the MIS structure has been calculated from the capacitance value in
Figure 2. Scanning electron micrograph of amorphous BaTiO3 thin films on silicon annealed at 400◦ C.
3.2.
Capacitance-Voltage Characteristics
3.2.1. Dielectric Response and Integrability with Silicon. The frequency dependence of dielectric constant and loss tangent (tan δ) of amorphous BaTiO3 film in MIM configuration is shown in Fig. 3. The dielectric constant was found to decrease gradually up to 106 Hz and after that ² 0 decreased to a low value due to
Figure 4. Frequency dependence of dielectric constant and loss tangent of a-BaTiO3 in MIS configuration at accumulation.
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accumulation region where the total capacitance is only due to the insulating film and the configuration resembles to an MIM structure. The dielectric constant of ˚ calamorphous BaTiO3 thin films of thickness 1400 A culated at 100 kHz was found to be 16. Almost the same value has been obtained for the amorphous films on platinum substrate (² 0 ∼ 17) at the same frequency. The dielectric behaviour was almost same as that of the MIM structure except the degree of dispersion (Figs. 3 and 4). More dispersion in the ² 0 in the MIS structure is presumably due to the existence of some interface states as will be discussed in the C-V characteristics. The response of interface traps is known to vary with frequency and is responsible for dispersion in capacitance. Therefore, the observed ² 0 dispersion seems to indicate the existence of interface states in the MIS structure, which are generally observed in MOS structures [13]. Figure 5 shows the small signal (10 mV) capacitance as a function of bias voltage in the range −10 to +10 volts at 1 MHz. The C-V characteristic (Fig. 5) clearly show the indications of the regions of accumulation, depletion and inversion. The minimum capacitance, Cmin , observed was about 107 pF. The value of Cmin may also be calculated theoretically by the relation [14], Cmin =
ε0 ε 0 A 0 d + εεs Wm
(1)
where ²0 is the permittivity of free space, ² 0 and ²s are the dielectric constants of the insulating film and silicon respectively, d is the film thickness, A is the area of the top electrode and Wm is the maximum depletion width. From the semiconductor data, the value of Wm calculated was about 650 nm [15]. Using this value of Wm , the theoretical value of Cmin calculated for film of ˚ was about 111 pF. Thus the theoretthickness 1400 A ical value of Cmin matched well with the experimental value, indicating a good interface. During strong inversion, the minimum depletion capacitance Cdmin acting in series with insulator capacitor, causes a maximum reduction in the measured capacitance and hence the following relation holds true, 1 Cmin
=
1 Cmax
+
1 Cdmin
(2)
Using the measured values of Cmin and Cmax from the C-V curve in the above relation, the value of minimum depletion capacitance is obtained as 127 pF. The value calculated theoretically from the relation [14], Cdmin =
ε0 εs A Wm
(3)
comes out to be 123. The proximity in calculated and measured values of these quantities indicate that the film capacitance was not masked much by interface capacitance [16]. These results show the effectiveness of amorphous BaTiO3 integration on bare silicon and its utilization for the fabrication of charge storage devices. 3.2.2. Oxide Properties. One of the useful parameters which can be deduced from the C-V characteristics is the flat band voltage, VFB , which in turn gives the information about the oxide charges in the insulator. The flat band capacitance, CFB , of the film was calculated from the relation, CFB =
Figure 5. C-V curve for the amorphous BaTiO3 film in the MIS configuration 1 MHz.
ε0 ε 0 A 0 d + εεs L d
(4)
where L d is the extrinsic Debye length and is calculated for P-type silicon of resistivity 10 Ä-cm, was about 70 nm [17]. Using this value, the flat band ˚ amorphous BaTiO3 thin film capacitance for 1400 A was about 475 pF at a signal frequency of 1 MHz and the flat band voltage was found about −1.56 V (from the C-V curve). This shift in the negative direction is an indication of positive charges in the insulator and interface.
Electrical Properties of Barium Titanate
The total oxide charge density in an insulating film is related to the flat band voltage VFB and the work function difference between the gate electrode material and the semiconductor (8ms ) by the following relation [14], Q i = (8ms − VFB )Ci
(5)
where Ci is the insulator capacitance per unit area. The value of 8ms for p-silicon with resistivity 10 Ä-cm and aluminium gate metal is ≈−0.8 eV. The oxide charge density was calculated as 4.28 × 1011 cm−2 which is indicating less concentration of oxide charges in the sol-gel derived amorphous BaTiO3 thin films as compared with the reported data [18]. In addition, existence of hysteresis behaviour has been observed in the C-V curve (Fig. 5) when the dc bias was swept from accumulation to inversion and back to accumulation. This kind of hysteresis window is usually observed in ferrolectric thin films in the MIS configuration and is attributed to the change in the polarization state. Since the film used in the present study is in the amorphous state, hysteresis behaviour may be due to the mobile ions present in the system or due to the charge carrier injection. The hysteresis width (1V ) was about 0.40 V and did not show any variation upon varying the sweep rate, which suggests that the flat band shift during the C-V measurement is due to charging/discharging of interfacial (near-interfacial) traps and not due to the mobile ion drift. So the charges injected into the film during the bias scan period and their subsequent trapping and re-ejection by the interfacial states seems to be the cause for the hysteresis behaviour [17]. The density of interface states recharged during the bias cycle, Nit , was calculated using the relation Nit =
Ci 1V q
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MIM structure reflect the better insulating behaviour of amorphous BaTiO3 films at room temperature. 3.3.
Leakage Current and Conduction Mechanism
Leakage current and the knowledge about the conduction mechanism in the thin films are issues of critical concern when using the films for various electronic applications. In this section, the leakage current and the charge transport mechanisms in amorphous BaTiO3 thin films will be discussed. The voltage is applied in steps and the current is measured after a delay time of 10 s. I -V characteristics of the forward biased (in the same sense as for C-V plot, where forward bias referred to accumulation) MIS structure employing amorphous BaTiO3 are shown in Fig. 6 for two different thicknesses. The leakage current density at 5 V decreased from 0.86 µA/cm2 to 0.11 µA/cm2 as the films thick˚ These values are ness increased from 1400 to 2800 A. small enough for DRAM applications [5].The leakage current, however, increases sharply for higher applied voltages (Fig. 6). The leakage current in a dielectric film may arise due one or more of the several conduction mechanisms, like Schottky emission, Poole-Frenkel emission, Fowler-Nordhein tunneling and space charge current [19]. Space charge limited current (SCLC) is well known in crystalline semi-insulators. A necessary condition of its predominance over the ohmic current is the availability of excess free carriers from the injecting contact [20]. In the case of SCLC, the plot between
(6)
where q is the electronic charge. The calculated interface state density is about 2.10 × 1011 cm−2 . Similar value of interface states in MIS devices with SrTiO3 films were reported, which indicate a good interface between Si and amorphous BaTiO3 [17]. The capacitance-voltage characteristic of the MIM structure (Fig. 5) did not show any appreciable variation as a function of applied voltage. The hysteresis is also not observed during voltage sweep from negative to positive and back to negative. The constant value of the capacitance and the absence of any hysteresis in the
Figure 6. I -V characteristic of amorphous BaTiO3 (MIS) having ˚ a thickness of (a) 1400 and (b) 2800 A.
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log I vs. log V should clearly have two regions, one in the low field region where I ∝ V and the other at the high field region where I ∝ V 2 . But in the present case, this behaviour could not be observed and so SCLC may be excluded. Direct tunneling or Fowler-Nordhein tunneling of electrons from the Si substrate through the BaTiO3 film is possible when the film is very thin ˚ As the thickness of the film in the present (
