This essay has been submitted by a student. This is not an example of the work written by our professional essay writers.
In this paper, we present the dimensional effects observed for TiO2 films deposited by DC circular magnetron sputtering. TiO2 thin films are deposited in an Al-TiO2-Al structure to investigate their non-linear characteristics, from which the carrier effective mass and the barrier potential at the Al-TiO2 interface is calculated, and an important relationship between the effective carrier mass and film thickness is observed. The dielectric constant of TiO2 thin films is also investigated, and is observed to vary with TiO2 film thickness. Further, TiO2 bandgap is observed to vary with film thickness.
Titanium dioxide is one of the most extensively studied oxides because of its remarkable optical and electronic properties [1-3]. TiO2 films have attracted attention for use in fabricating capacitors for microelectronics devices due to their unusually high dielectric constant [4, 5]. Many deposition methods are used to prepare titanium oxides film: thermal  or anodic  oxidation of titanium, electron beam evaporation , chemical vapour deposition , plasma-enhanced chemical vapour deposition , sol-gel method [11, 12] and reactive sputtering methods [13-16]. Among these methods, D.C. reactive magnetron sputtering has permitted the fabrication of insulator films with desirable and reproducible properties. A comprehensive review of DC reactive magnetron sputtering has been conducted by Safi . In this paper, the dielectric properties of DC magnetron sputtered TiO2 films for metal - insulator -metal (MIM) and metal-insulator-semiconductor (MIS) structures are investigated. We demonstrate that the dielectric properties of polycrystalline TiO2 thin films were deposited in an Al-TiO2-Al structure to investigate their non-linear characteristics, from which the carrier effective mass and the barrier potential at the Al-TiO2 interface is calculated. The dielectric constant and the bandgap of TiO2 thin films is also investigated.
TiO2 thin films were prepared by DC magnetron sputtering using a custom built magnetron installation . The deposition chamber consists of an 80 litre stainless steel chamber and a circular magnetron with a 60 mm diameter erosion zone used as the cathode. The discharge characteristics are controlled using a variable DC power supply at 3kV and 500 mA. A 99.95% pure Ti target, 130 mm in diameter and 3 mm thickness was used as the sputtering target. 99.99% pure Ar and O2 were used as the sputtering and reactive gases respectively. The gases were mixed prior to injection into the sputtering chamber at a proportion of 75% Ar and 25% O2. The target to substrate distance was 35 mm. The sputtering pressure was maintained at 2.10-3 torr. Prior to the deposition the target was well cleaned in order to remove the surface oxide layers into an ultrasonic vessel. The substrate temperature was held at 300oC using a quartz halogen lamp, whose power was controlled by varying the lamp input voltage. TiO2 films were deposited on a vacuum evaporated aluminium bottom electrode made on a well-cleaned microscope glass slide with dimensions (75x25x1) mm. The deposition time was selected to obtain films of several different thicknesses. The sputtering power was 110 W (200 mA x 550 V), corresponding to a power density of 1.25 W/cm2. The top electrode was vacuum evaporated aluminium. Film thicknesses were calculated to an accuracy of ± 10nm by using multiple beam interferometry. The Al thickness was 2mm. The area of the metal-TiO2 -metal surface was 10 mm2. The structure of the films was examined by using X-ray diffraction with a Cu Ka radiation source in a standard X-ray diffractometer. I-V characteristics of the produced films were recorded using an X-Y recorder with the voltage and current as inputs. Dielectric properties were recorded using an RLC bridge Tesla BM 439, with a 100 pF to 100 mF scanning range and the loss tangent were recorded at different applied voltages and signal frequencies.
Experimental results and discussion
The TiO2 thin films deposited by the DC magnetron sputtering system were initially amorphous. X-ray diffraction analysis reveals that crystalline diffraction peaks corresponding to the polycrystalline films were observed only after annealing of the film. Figure 1 depicts the X-ray diffraction spectra for TiO2 films as-deposited and post-annealed. The XRD diffraction pattern reveals that the post-annealed films contain primarily anatase TiO2 with some small amounts of TiO1+x (0,95≤x≥1) . Measurements performed on films after several anneals in air reveal only anatase peaks.
3.1. Dimensional effects observed for electrical properties
The I-V characteristics investigated for different TiO2 film thicknesses in Al-TiO2-Al structures are presented in Fig. 2. The experimental results demonstrate symmetrical I-V characteristics. Figure 2 only represents the positive polarity component. It can be clearly seen that the I-V characteristics are highly nonlinear and thickness dependent. The I-V characteristic analysis was conducted by decomposing the characteristic into three regions. Each of these regions has been found to correspond to a specific carrier conduction mechanism.
Figure 3 depicts the linear region of the current-voltage dependencies. The linear domain limits present a slight thicknesses dependency. Table 1 presents the electrical parameters of the TiO2 films in the linear region of the I-V characteristic. Nonlinear effects occur for thinner films at lower values of applied voltage. The applied voltage corresponding to where non-linear effects begin occur, is 0.65 V for 0.3 μm TiO2 films, 2.66 V for 0.9 μm TiO2 films and is 3.71 V for 1.44 μm films. Table 1 revels that the slope of I-V linear region, is larger for thinner films and decreases by almost one order of magnitude for the 1.66 μm thick films. This behaviour can not be explained exclusively by a film thickness difference since there is also a difference in electrical carrier properties. The electric field is observed to behave where the non-linear effects occur. 0.9 μm thick films present a larger applied electrical field. This behaviour can be related to a saturation process that may be responsible for the larger value where non-linear effects occur.
Figure 4 depicts the features of the second domain for current-voltage characteristics. This nonlinear behaviour is well-fitted by a squared voltage dependence for the electrical current (I=f (U2)). This dependence corresponds to a space charge limited current conduction mechanism :
where d is the distance between electrodes, mis the carrier mobility, er is the relative dielectric constant, e0 is the vacuum permittivity, Nc is the effective state density in the conduction band, Nt is the trap density located in the forbidden band at level Et, and k is Boltzmann's constant.
From Fig. 4 we notice that films with smaller thicknesses have a larger rate of variation for I-U2 characteristics, suggesting that film trap density varies with film thickness. Further it is observed that the trap density is lower for films with larger thicknesses. This behaviour can be explained by a better cristallinity of thicker films. Table 2 presents the TiO2 electrical parameters in the space-charge limited current conduction domain. A large slope is observed for films with small thicknesses. This difference in slopes value cannot be explained only by a difference in thickness. Table 1 depicts film thickness dependencies of the maximum applied electrical fields corresponding to the limits of the current carrier mechanism. An unusual behaviour is observed for films with thicknesses less than 1 mm. These facts have to be related also to the changes that occur in carriers properties, especially for changes that occur in the carrier effective mass.
Figure 5 depicts the third domain of the current-voltage characteristics for the three thickness different TiO2 thin films. Figure 5 reveals that, for some values of electric fields, the conduction mechanism changes from one that corresponds to a space-charge-limited current conduction mechanism to a trapping conduction mechanism. This type of carrier conduction mechanism is well described by the Schottky theory , and is characterised by the Richardson-Schottky law:
Where F0 is the extraction work function at the metal-semiconductor interface and A is the Richardson-Dushmann expression :
Using the I-V characteristic we have evaluated the carrier effective mass using a method proposed by Vodenicharov . According to Vodenicharov the carrier effective mass may be calculated from the current-voltage characteristic domain that corresponds to a transition in the carrier conduction mechanism, as per:
where h is Planck's constant, e is the elementary charge, er is the dielectric constant at low frequency calculated elsewhere , k is Boltzmann's constant, T is the absolute temperature, and Ek is the electric field intensity that is corresponding to the transition observed in conduction mechanism. By extrapolating of the data in Fig. 5, (ln I=f (U1/2))) for U=0 V, we obtain the value of the potential barrier. Table 3 depicts the electric field intensity corresponding to the transition in conduction mechanism, the calculated values for the carrier effective mass, and the barrier potential for different TiO2 film thickness. From this data, that the carrier effective mass and potential barrier is observed to be film thickness dependent. For thicker films a slight decrease in the carrier effective mass is observed. The larger effective mass observed for thinner films may be related to poor film crystallisation. The lower barrier potential for thicker films can be correlated to a decrease in surface charge states due to a better cristallinity of the thicker films. The values calculated for the carrier effective mass and the barrier potential are consistent with those reported in literature [21,22,23], and are correspond to the anatase polycrystalline structure of TiO2 films.
3.2. Dimensional effects observed for dielectric properties
Figure 6 depicts the capacitance variations observed for two different thicknesses of metal-TiO2- metal structures. We have varied the frequency of an external signal applied to the measurement bridge, and have observed a completely different behaviour of the two films. Thinner films are observed to have an order of magnitude decrease in electrical capacitance as a function of frequency, while thicker films are observed to also have a decrease in electric capacitance, but with only half the magnitude at low frequencies. At high frequencies, a saturation effect is observed and the electric capacitance and dielectric constant are no longer frequency dependent for thicker films. Figure 7 depicts the frequency variation for the M-I-M TiO2 dielectric loss or loss tangent. The loss tangent provides a measure for charge leakage in for TiO2 films. At the low frequencies the dielectric loss is greater for thinner films. This loss behaviour could be related to insufficient crystallisation or grain size effects [25, 26].
We have calculated the TiO2 dielectric constant from the accumulation regime of the capacitance-voltage characteristics for metal-oxide-semiconductors (MOS) structures. A thickness-dependant dielectric constant has been observed for TiO2 thin films. 300 nm thick TiO2 films have a dielectric constant of 70, while 600 nm thick films have a dielectric constant of 50. We have also observed that the dielectric loss is thickness dependent for TiO2 thin films. Films 600 nm in thickness were observed to have a dielectric constant of 50, which was similarly obtained for films up to 900 nm in thickness. Other observed dielectric constants of amorphous TiO2 films are 73 for a 200 nm thick film , 13.7 for a 94 nm thick film and 18.4 for a 105 nm thick film . Figure 8 summarises the thickness variation for the TiO2 dielectric constant based on our values and the existing data. Figure 8 reveals a strong variation in dielectric constant with polycrystalline TiO2 thin films thickness.
For films 80 to 100 nm thick, small dielectric constant were observed. These small dielectric constants are most likely a function of the growth process and are due to an incomplete packing of the grains (average dimensions for anatase grain are about 40 nm ). 200 to 300 nm thick films are observed to have a larger dielectric constant. This value is most likely attributed to enhanced grain packing corresponding to a higher degree of polycrystallinity for these films. For thicker TiO2 thin films, we notice a small decrease in dielectric constant. After a certain critical thickness, the dielectric constant becomes independent of film thickness. Variance in dielectric constant can most likely be explained structural modifications in film, i.e. films with lower dielectric constants are amorphous and those with higher dielectric constants are partially crystallised . XRD data revealed polycrystalline anatase peaks and some TiOx peaks. Measurements performed for films after several anneals in air revealed only anatase peaks. The TiO2 dielectric constant presents a saturation effect at films with thickness around 200 to 250 nm, which is most likely correlated to insufficient crystallisation and the small degree grain packing.
3.2. Dimensional effects observed for optical properties
We have also investigated the optical property thickness dependencies of TiO2 thin films. Figure 9 depicts the thickness variation of the TiO2 band gap. We have calculated the optical band gap from the fundamental absorption region of the visible transmission spectra, and have found that the optical gap for TiO2 thin films increase with decreasing film thickness. The variation in the optical band gap is larger than 10% and can not be attributed to the recrystallisation process. This behaviour is most likely related to the nanostructure characteristics. TiO2 thin films thermally annealed in air for one hour, reveal a 2% increase in optical band gap.
We have analysed the nonlinear I-V characteristics of DC magnetron deposited TiO2 thin films. We have determined that transitions occur in the carrier conduction mechanism from a linear or ohmic regime to a trapping and Shottky conduction mechanism. We have calculated the carrier effective mass and the barrier potential for an Al-TiO2 interface, and we found that the TiO2 films are composed of anatase polycrystalline TiO2. We have also found a thickness dependency for the carrier effective mass. We have studied the dielectric properties of DC magnetron TiO2 thin films in MOM and MOS structures. We have studied the variation of the TiO2 dielectric constant and loss tangent with the signal frequency. We have observed a thickness dependency of the dielectric constant and loss tangent. The largest value for the dielectric constant of TiO2 films is observed for films 200 to 300 nm thick. If we assume that the dependency of dielectric constant and dielectric loss on film thickness is due to the structural modifications in film, we can conclude that films structure is also thickness dependent. We have found that there is a thickness dependency for the bandgap of TiO2 thin films. We have found that the bandgap for TiO2 thin films decreases for the thicker films. This behaviour may be related to the nanostructure characteristics and to the smallest degree of crystallisation.