OTFTs Analysis of Electrical Characteristics and Discussion

3.3.1 The current density-electric field (J-E) characteristics

The leakage current densities of HfO2 films after different treatments are shown as a function of applied negative gate bias voltage to positive gate bias voltage in Fig.

3-16. Among various post-treatments, the baking-treated HfO2 film exhibits the most serious leakage current, inferentially due to its poor dielectric characteristics with numerous traps inside the HfO2 film and the interface between parasitical SiOx and Si wafer. The improvement of electrical characteristics is observed by using H2O vapor process, however, a high leakage current density still appears at larger applied voltages. It could be inferred reasonably dependent on the defect passivation efficiency. The most indicating that H2O vapor can passivate the traps (or defects) and alter dielectric properties of the low-temperature-deposited HfO2 film. After H2O vapor treatment, effective improvement of electrical characteristic is obtained by the 3000 psi-SCCO2 treatment, exhibiting the lowest leakage current density among all samples. Low leakage current density (~2×10^-6 A/cm²) is kept constantly, even biased

at an electric field of 1.33 MV/cm.

3.3.2 The capacitance-voltage (C-V) characteristics

The capacitance-voltage (C-V) characteristics are also generally used to judge the quality of dielectric films. Figure 3-17 shows capacitance-voltage characteristics of HfO2 films after different treatment, measuring at 1M Hz with gate bias swing from negative voltage to positive voltage (forward) and from positive voltage to negative voltage (reverse). The slope of C-V curve in transient region, i.e. from Cmax to Cmin, is relative to the interface states, for example, the sharp slope indicates fewer defects exist in the interface between HfO2 and Si wafer. In Fig. 3-17, the baking-treated HfO2 film presents the worst C-V curve and lower capacitance. This expresses the larger number of interface states exist and lead to the smooth C-V curve. With H2O vapor treatment, the sharper C-V curve and higher capacitance are obtained, and it could be attributed to the reduction of defects in HfO2 film and the interface.

Furthermore, the best improvement is achieved by 3000 psi-SCCO2 treatment. This exhibits that the SCCO2 treatment possesses excellent ability to passivate the defects, including Hf dangling bonds and interface states.

Besides, from Fig. 3-17, the shift of C-V curve under forward and reverse swing is also appears in baking-treated and H2O vapor-treated HfO2 films. It is resulted from the trapped carrier in defects of HfO2 films, and that is not expected for gate insulator of transistors. Under negative gate bias, the electric inject from Al gate into HfO2

films and trapped by defects, leading to the larger gate bias is required for inducing electron-inversion layer. It is evidently observed that the baking-treated HfO2 film hold numerous defects because of the extensive shift of flat-band voltage, and the defects almost disappear after 3000 psi-SCCO2 treatment.

These results conform to the tendency in current-voltage characteristics and again verify that the SCCO2 technology could effectively deactivate defects in HfO2

films.

In another interesting detection, in Fig. 3-17. The baking- treated HfO2 film, the flat-band voltage (= -0.3 volt.) is away from ideal gate bias voltage (about 0~0.3 volt.), and that of 3000 psi-SCCO2 treated HfO2 is zeroed nearly. The main reason could be referred to (1) the positively charged Hf dangling bonds are passivated, (2) the fixed positive charges are removed by SCCO2 fluids. The mechanism of extracting of fixed charge is shown in Fig. 3-6, including positive and negative fixed charge [57]. The polarized-H2O molecule is taken as a dipole which would attract the fixed charge in HfO2 films. Afterward, the H2O molecule and fixed charge are carried away by SCCO2 fluids mixed with propyl alcohol. For H2O vapor-treated HfO2 film, the un-zeroed flat-band voltage could be attributed to (1) partial positively charged Hf dangling bonds remain, (2) the poorer capability for H2O vapor to remove fixed charge. Hence, it is necessary for H2O molecule to be driven into HfO2 films and carried away by SCCO2 fluids.

3.3.3 Secondary Ions Mass Spectrometer (SIMS) Analysis

The means of secondary ion mass spectroscopy (SIMS) analyzed by using various elements profiles in the dielectrics. During SIMS analysis [61], the surface of the sample is subjected to Cs+ ion bombardment with energy of 5.78 KeV and negative secondary ions. The bombarding primary ion beam (Cs+) produces monatomic and polyatomic particles of sample material and re-sputtered primary ions, along with electrons and photons. This leads to the ejection of both neutral and charged species from the surface. Monitoring the secondary ion count rate of the selected element as a function of time leads to depth profiles.

Fig. 3-18 (a), (b) and (c) show the SIMS profiles of the hafnium oxide film after various post-treatments, including Baking-only and 3000 psi-SCCO2 treatment, The first group labeled as Baking-only treatment, was designed as the control sample, and was only baked on a hot plate at 150 °C for 2 hrs. The second group labeled as 3000psi-SCCO2 treatment, was placed in the supercritical fluid system at 150°C for 2 hrs, where was injected with 3000psi of SCCO2 fluids mixed with 5 vol.% of propyl alcohol and 5 vol.% of pure H2O. In Fig. 3-17 (a) and (b), the baking-only-treated and 3000 psi-SCCO2-treated films had the nearly same quantity of Silicon and Hafnium atoms. The expression after 3000psi-SCCO2 treatment, Silicon and Hafnium atoms quantity had not evident increased. Hafnium atoms had few diffused to the Silicon substrate. In Fig. 3-17 (c), the 3000 psi-SCCO2-treated films had more quantity Oxygen atoms. The expression after 3000psi-SCCO2 treatment, Oxygen atoms quantity had evident increased and be delivered into the HfO2 film for passivating defects.

3.3.4 The current-voltage (I-V) characteristics

In this series of experiment, we compare devices of HfO2 film with different various post-treatments, including Baking-only, H2O vapor and 3000 psi-SCCO2

treatment and PECVD SiO2 film is control group. The channel width is 800μm and length is 600μm. In Fig. 3-18 (a) and (b) for ID-VGcharacteristics, Using baking only and H2O vapor treatment, the device are not accomplished. The gate current is almost the same as drain current. It indicates that the leakage current is still large after baking only and H2O vapor treatment on HfO2 gate dielectrics. In Fig. 3-19(a) and (b) for drain current-drain voltage (ID-VG ) characteristics, the gate bias ranged from 10 to -20V, and the drain voltage -10V are provided for the device. We can also compare devices with different insulator. From the ID-VG chart, the turn-on current of 3000

psi-SCCO2 treatment for HfO2 insulator of OTFTs great than PECVD SiO2 insulator for OTFTs. The turn-off current of 3000 psi-SCCO2 treatment for HfO2 insulator for OTFTs is lower than SiO2 insulator for OTFTs. From the ID-VG data, we can extract the mobility and threshold voltage from the square-root of drain current versus gate voltage curves and extract the on/off current ratio from the ratio of maximum turn-on current versus minimum turn-off current. The mobility of 0.05cm²/V．s、threshold voltage of -0.1V、 sub-threshold swing of 2.2 volt/dec and on/off current ratio of 2 x 10⁵. The PECVD SiO2 insulator of OTFTs had mobility of 0.026cm²/V．s 、threshold voltage of 2.1V、sub-threshold swing of 3.6 volt/dec and on/off current ratio of 10⁴. The 3000 psi-SCCO2 treatment for HfO2 insulator for OTFT has good performance.

In Fig. 3-20 for ID-VD characteristics, we provide the drain voltage from 0 to -20V and change the gate voltage from 0 to -30V, step by -10V. The ID-VD curves show the turn-on operation of the device. We can observe that the turn-on current is largest in the 3000 psi-SCCO2 treatment for HfO2 insulator for OTFT. In Fig. 3-21 for normalized of ID-VD characteristics, we can observe the 3000 psi-SCCO2 treatment for HfO2 insulator for OTFT had higher turn-on current and better on/off current ratio.

3.3.5 The DC bias and current stress characteristics

Under DC bias and current stress, the gate bias stress measurements were performed in atmosphere and darkness on OTFT on 3000 psi-SCCO2 treatment for HfO2 insulator and PECVD SiO2 insulator. The stress gate voltage was -20V. The stress drain voltage and current were 0V, -20V and -200nA. Fig. 3-22 show a series of transfer curves at t = 0、50、100、500 and 1000 sec on OTFT of 3000 psi-SCCO2

treatment for HfO2 insulator, between which VD = 0V, -20V, and ID= -200nA

and 1000 sec, respectively. Fig. 3.23 plots show the mobility decay with different stress times. Fig. 3.24 plots show the threshold voltage shift tended toward more negative with different stress times. Fig. 3-25 show a series of transfer curves at t = 0、50、100、500 and 1000 sec on OTFT of PECVD SiO2 insulator, between which VD

= 0 V, -20V, and ID= -200nA respectively. In Fig. 3-25 indicates the increase of stress time from 0 to 50、100、500 and 1000 sec, respectively. Fig. 3-26 plots show the mobility decay with different stress times. Fig. 3-27 plots show the threshold voltage shift tended toward more negative with different stress times. The negative threshold voltage shift due to the negative gate bias stress may be attributed to the decrease of mobile holes in the channel via the charge trapping in the insulator. The two reasons would likely be concerned about negative threshold voltage shift. First, the dangling bonds in grain boundary would be considered for the reason of the mixed phase in thin film. The accumulated carriers hopping in channel through π bond facilitate charge trapping in the dangling bonds after a long operational period. Second, due to molecular extending structure in electrical filed, π bonds happened to out of control so that the hopping behavior was temporally abnormal and the less mobile states formed.

If the stress is operated for the long time, the defect sites are more likely to form and trapping charges appear. The OTFT of PECVD SiO2 insulator has good performance on threshold voltage shift and mobility decay because the SiO2 has higher barrier than HfO2. The Stress is not serious for a long time.