Chapter 2 Experiment Procedures and Principle
2.3 Properties of Organic Thin Film Transistors
3.1.2 Conduction Mechanism
There may be different conduction mechanisms in the insulator thin film, including Schottky-Richardson emission [47], Frenkel-Poole emission [47,48], Fowler-Nordheim tunneling [47,48], and trap assisted tunneling [49,50] illustrated in
Fig 3-2. The Schottky-Richardson emission generated by the thermionic effect is caused by the electron transport across the potential energy barrier via field-assisted lowering at a metal-insulator interface. The leakage current governed by the Schottky-Richardson emission is as following:
⎟⎠
φSR is the contact potential barrier,E is the applied electric field, ε0 is the permittivity in vacuum, ε is the high frequency relative dielectric constant, Tis the absolute temperature, and is the Boltzmann constant. We can find the slope of the leakage current equation.
kB The Frenkel-Poole emission is due to field-enhanced thermal excitation of trapped electrons in the insulator into the conduction band. The leakage current equation is: potential barrier,E is the applied electric field, ε0 is the permittivity in vacuum, ε is the high frequency relative dielectric constant, Tis the absolute temperature, and
is the Boltzmann constant. We can find the slope of the leakage current equation.
k
[ln( )J k T The Fowler-Nordheim tunneling is the flow of electrons through a triangular potential barrier. Tunneling is a quantum mechanical process similar to throwing a ball against a wall often results that the ball goes through the wall without damaging the wall or the ball. It also loses no energy during the tunnel event. The probability of this event happening, however, is extremely low, but an electron incident on a barrier typically several nm thick has a high probability of transmission. The Fowler-Nordheim tunneling current IFN is given by the expression [51]:
(
FN ox)
are usually considered to be constant. and are given as the following:AFN BFN
where is the effective electron mass in the oxide, m is the free electron mass, is the electronic charge, and is the barrier height at the silicon-oxide interface given in units of eV in the expression for . is actually an effective barrier height that take into account barrier height lowering and quantization of electrons at the semiconductor surface. Rearranging formula gives by:
mox q
A plot of ln
(
JFN ε versus ox2) (
1 ε should be a straight line if the conduction ox)
through the oxide is pure Fowler-Nordheim conduction [51].
In the trap assisted tunneling model, it is assumed that electrons first tunnel through the SiOX interfacial layer (direct-tunneling). Then, electrons tunnel through traps located below the conduction band of the high-k thin film and leak to substrate finally [49]. The equation of leakage current density is [50]:
(
ox)
ox exp E
E
J =α −β (3.11) From the equations as shown above, leakage current behaviors of insulate films can be investigated further on the leakage current density electric field J E characteristics such as J vs. E12 plots.
The plot of the nature log of leakage current density versus the square root of the applied electric field was observed. It is found that the leakage current density is linearly related to square root of the applied electric field. The linear variations of the current correspond either to Schottky-Richardson emission or to Frenkel-Poole conduction mechanism. For trap states with coulomb potentials, the expression is virtually identical to that of the Schottky-Richardson emission. The barrier height, however, is the depth of the trap potential well, and the quantity βFP is larger than in the case of Schottky-Richardson emission by a factor of 2.
Leakage conduction mechanism is also investigated to support the comments on the electrical improvement of HfO2 film. Fig. 3-3(a) plots ln (J/E) versus reciprocal of electric field variation for the baking-only treated HfO2 film, and a schematic energy band diagram accounting for leakage transport mechanism shown in the inset. A good linear fitting explains Fowler-Nordheim (F-N) tunneling [52] occurs in the electric fields higher than 0.7 MV/cm. Also, it is consistent with the electrical behavior of
baking-only treated HfO2 film in Fig. 3-1 that leakage current density sharply increases, while gate bias voltage larger than 0.7 MV/cm. This could be attributed to the trap-assisted tunneling due to numerous traps inside the 150°C- baking treated HfO2 film [53]. For the 3000 psi-SCCO2 treated HfO2 film, a plot of leakage current density versus the square root of the applied field (E1/2) gives a good representation of the leakage behavior at high electric fields, as shown in Fig. 3-3(b). The leakage current density of the 3000 psi-SCCO2 treated HfO2 is linearly related to the square root of the applied electric field, demonstrating Schottky-Richardson emission transport mechanism [54]. The Schottky-type conduction can be verified by comparing the theoretical value of
(
3 0)
12SR q 4πε ε
β = with the calculated one obtained from the slope of the experimental curve ln J versus E1/2 [55], where q is the electronic charge, ε0 the dielectric constant of free space, ε is the high frequency relative dielectric constant. The Schottky emission generated by the thermionic effect is caused by electron transport across the potential energy barrier via field-assisted lowering at a metal-insulator interface, shown in the insert of Fig. 3-3(b), and independent of traps. From the slope of ln J versus E1/2, the calculated value of relative dielectric constant (ε) is 26.4, and which is close to the determined value of 29.4 in capacitance-voltage (C-V) measurement (referring to table 3-1). This also proves, for 3000spi-SCCO2 treated HfO2 film, the conduction mechanism is really Schottky emission, but not trap-dependent Poole-Frenkel emission [55]. Additionally, the evolution of conduction mechanisms from trap-assisted tunneling to Schottky emission can confirm these defects inside low-temperature-deposited HfO2 film is minimized effectively by implementing the proposed SCCO2 technology. The leakage current densities of HfO2 films after different treatments are shown as a function of
be acquired after 3000 psi-SCCO2 and H2O vapor treatment, especially treated with SCCO2 fluids. This could be attributed to the influence of traps in the interface between parasitical SiOx and Si wafer. Generally, in positive gate bias, the sources of electron are (1) the interface states, (2) defects in depletion region, (3) back electrode of substrate, [56] and the later two source are negligible due to the p-type signal-crystal Si wafer is used in this work. For baking-only treated HfO2 film, the great quantity of interface states still exist which generate electron-hole pair and lead to higher leakage current, as described in the inset of Fig. 3-4. After 3000 psi-SCCO2
treatment, the interface states were deactivated, hence the leakage current is reduced.
The reduction of interface states would be proved in capacitance-voltage measurement.