Results And Discussions

Figure 3.1 shows typical I-V curves of the fabricated n-type Si/Alq₃/Al structure. As can be seen, this device exhibits two different conductance states at an identical applied voltage. The silicon electrode is kept at 0 V, and all bias conditions are applied on the aluminum electrode. At the first bias (the black square curve in Fig. 3.1), the voltage sweeps from 0 V to 10 V. Initially, the device exhibits a low conductance state (OFF state). However,

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with an increased bias, a transition from the low conductance state to a high conductance state (ON state) occurs at a threshold voltage of about 5 V, and then the device is maintained at the high conductance state. At the next bias (the red circle curve in Fig. 3.1), the device still holds at the high conductance state. Therefore, this device possesses the nature of bistablity.

Furthermore, by applying a negative bias form 0 V to -10 V, the device can be switched from the high conductance state back to the low conductance state. The plot of ON/OFF current ratios as a function of reading bias is shown in the inset of Fig. 3.1. It is obvious that the device has a very wide reading voltage range with large ON/OFF current ratios which may reduce reading errors and increase the reliability of the device. For this reason, the tolerance of this device is large enough for external surrounding circuitry to adopt. The corresponding reading currents after „„writing” and „„erasing” for the first four cycles are shown in Fig. 3.2.

At the low voltage of the first bias, the current is very small because electrons are obstructed by a barrier formed between the Si substrate and Alq3. Thus, only a few electrons are injected into the Alq₃ thin film. Then most of them are further trapped by the defects in the Fig. 3.1 The I-V curves of the OBD with the n-type Si/Alq3/Al structure. The black square and red circle curves represent writing and reading biases, respectively. The inset shows the voltage-dependent ON/OFF current ratio curve.

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Alq3 thin film and at the Alq3/Al interface. As a result, the device stays at the low conductance state. By applying a bias above the threshold, the barrier can be overcame, and this enables numerous electrons to be injected into the Alq3 layer and the defects can be filled.

Accordingly, electrons are transported easily into the Alq₃ thin film and drift unobstructedly towards the other end of the device. At the next bias, the device exhibits a resistance-like characteristic when the reading voltage is larger than the energy barrier between Si and Alq₃. That is, the ON state can be obtained for any reading voltage larger than the iso-type hetero-junction barrier between n-type silicon and Alq₃, which is about 0.65 eV estimated from Fig. 3.3.

Fig. 3.4 shows the C-V curve of the device. It can be seen from the curve that the device keeps at some capacitance value while the applied voltage is below the threshold. Then, the value changes into another lower capacitance value when the voltage exceeds its threshold.

The variation of capacitance could be ascribed to the defects in the Alq3 thin film and at the Alq₃/Al interface. At the initial stage of the applied voltage, few electrons are trapped by the defects in the low electrical field. Then, more and more electrons are trapped by the defects as Fig. 3.2 The reading currents after “writing” and “erasing” of the OBD with the n-type Si/Alq3/Al structure for the first four cycles.

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the voltage increases. While the applied voltage is near the threshold, the defects are filled sufficiently to make the device possess a metallike property; consequently, the capacitance is converted into a lower value. In addition, the I-V curve of the n-type Si/Alq3 structure with one small Al drop as the top electrode does not exhibit bistability but rather diode behavior.

This indicates that the interface property between the Al electrode and the Alq3 thin film plays an important role in bistability.

Mason et. al. report that a significant chemical reaction occurs at the Alq3/Al interface when Al is thermally deposited on the Alq₃ thin film [167]. The resulting product, supportively consisting of Al-O interactions, serves as interface traps and makes carriers be poorly injected through the Alq₃/Al interface. Fig. 3.5 shows the XPS curves of the Al electrode and the Alq3/Al interface of the reported device, which clearly confirm the existence of an Al-O compound at the Alq₃/Al interface of the device. Therefore, we can conclude that trapping charges at the interface between Alq3 and Al primarily controls the switching mechanism.

The electrical behavior of the device can be modified by varying the deposition rate of Fig. 3.3 The linear I-V curve of the OBD with the n-type Si/Alq3/Al structure at the high conductance.

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the Alq3 thin film. Fig. 3.6(a) show the threshold voltages (the black square curve) and ON/OFF current ratios of OBDs (the red circle curve) using the same structure whose Alq₃ thin films were deposited at different deposition rates: both parameters decrease with an Fig. 3.5 The XPS curves of the Al electrode and Alq₃/Al interface of the OBD with the n-type Si/Alq₃/Al structure.

Fig. 3.4 The C-V curve of the OBD with the n-type Si/Alq₃/Al structure at a frequency of 1 MHz. The Si electrode is kept at 0 V, and the bias on the Al electrode is swept form -5 V to 7 V.

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increase in the deposition rate. In addition, as can be seen in Fig. 3.6(b), the retention time is dependent on the threshold voltages of the OBDs. Since the threshold voltages can be tuned by adjusting the deposition rates of the Alq3 thin films, the retention time can be extended by Fig. 3.6 The electrical properties of OBDs with the n-type Si/Alq3/Al structure whose Alq3

thin films are deposited at different deposition rates: (a) the black square and red circle curves represent deposition-rate dependent threshold voltage, and deposition-rate dependent ON/OFF current ratios, respectively, and (b) threshold-voltage dependent retention time. Data points shown in (a) are average values measured from the OBDs.

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reducing the deposition rates of the Alq3 thin films.

Previous reports on the morphology of the Alq₃ thin films indicate that roughness decreases with the deposition rate [168], [169]. That is to say, the effective contact surface area between Alq₃ and Al can be adjusted by regulating the deposition rate of Alq₃. For that reason, a higher deposition rate introduces a relatively small effective contact area at the Alq₃/Al interface. Fig. 3.7 shows the AFM images of the Alq₃ thin films deposited at 0.05 nm/s, 0.15 nm/s, 0.2 nm/s, and 0.3 nm/s. The corresponding surface roughness means are 0.38 nm, 0.35 nm, 0.31 nm, and 0.17 nm. These confirm that the deposition rates of the Alq₃ thin films are a major factor in the adjustment of the effective contact surface areas between Alq3

and Al, which may affect the amount of the interface defects of the corresponding OBDs. It suggests that the relative amount of the defects at the Alq3/Al interfaces can be modified by controlling the deposition rates of the Alq₃ thin films. Furthermore, measuring the

Fig. 3.7 The AFM images of the Alq3 thin films deposited on n-type Si substrates at the four different deposition rates. Surface roughness means are 0.38 nm, 0.35 nm, 0.31 nm, and 0.17 nm for deposition rates at 0.05 nm/s, 0.15 nm/s, 0.2 nm/s, and 0.3 nm/s, respectively.

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GIXRD curves of the Alq3 thin films deposited at the four deposition rates, as shown in Fig.

3.8, we find that crystallization does not occur in all Alq₃ thin films. Consequently, the threshold voltages, ON/OFF current ratios, and retention time are not related to the crystallization quality of the Alq₃ thin films. They are closely related to the film roughness, as shown by the AFM images in Fig. 3.7.

It has been demonstrated that the atomic N/C ratios of the Alq₃ thin films changes with the deposition rates of the Alq3 thin films. [168], [169] At a higher deposition rate (a higher temperature condition), the Alq₃ molecule structure disintegrates to release N-containing species due to the decomposition energy of Alq3 being smaller than its sublimation energy. It has been also shown that the Alq₃ thin film deposited at a lower deposition rate contains a greater atomic concentration of nitrogen and a higher atomic N/C ratio. From XPS measurements, the corresponding concentrations of N and the atomic N/C ratios of the four Alq3 thin films in this study are given in Table 2.1, which clearly indicates the same trend discussed above.

When electrons are injected into an Alq3 thin film, they undergo a repulsive force generated by the negatively charged nitrogen atoms which is the result of the

Fig. 3.8 The GIXRD curves of the Alq3 thin films deposited at the four deposition rates.

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electronegativity of a nitrogen atom being larger than that of a carbon or oxygen atom for a neutral Alq₃ molecule [170]. Hence, electrons in an Alq₃ thin film with a smaller N/C ratio experience less repulsive force [169]. In other words, an increase in the deposition rate of the Alq₃ thin film can extend the hopping distance of the electrons in the Alq₃ thin film and can raise the hopping frequency.

From above discussions, two findings can be made to explain the results obtained in Fig.

3.6(a). First, it is obvious that the threshold voltages decrease with the increasing deposition rates because of a smaller amount of defects at the Alq₃/Al interface at a higher deposition rate. Second, the same relationship for the ON/OFF current ratios is because a smaller amount of nitrogen atoms are available to prevent the electrons from hopping in the Alq₃ thin film, hence increase the low conductance state current and decrease the ON/OFF current ratio at a higher deposition rate.

Because the distribution of the defects at the interface corresponds to the trapping energy [171], we may simply classify the defects at the Alq₃/Al interface into two groups: low trapping energy defects (Elow) and high trapping energy defects (Ehigh), as shown in Fig. 3.9.

Fig. 3.9 The illustration of the defect states in the Al-O compound layer: E_low and E_high.

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In order to address those two groups, the I-V curves of the OBD, whose Alq3 thin film deposited at 0.05nm/s, are measured at three different temperatures (room temperature, 50°C, and 90°C), as shown in Fig. 3.10. At room temperature and 50°C, the device is operated normally, but its electrical characteristic changes, e.g. the smaller threshold voltage, at 50°C.

This indicates that Elow are compensated effectively at 50°C. As a result, lower threshold is observed. As the temperature increases to 90°C, the device reveals a diode property instead of bistable behavior. This means that Elow and Ehigh are both compensated. The measured temperature-dependent results can be evidence that there are two types of defects and that the bistability of the reported device arises from these defect states.

A higher deposition rate introduces a smoother Alq₃ surface, and may produce less E_high at the Alq3/Al interface. Also, the corresponding OBD, which has low threshold voltage and the smoother Alq₃ surface, exhibits shorter retention time probably because of less E_high. In other words, after the corresponding OBD being switched into the high conductance state, most trapped charges are in the E_low, and they are easily released at the Alq₃/Al interface due to the lower confinement barrier. As a result, the trapped charges cannot be kept longer in

Fig. 3.10 The temperature-dependent I-V curves of the OBD whose Alq₃ thin film deposited at 0.05nm/s.

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the corresponding OBD so it has shorter retention time, as shown in Fig. 3.6(b).