Device Characterization - Fabrication And Characterization Methods

Chapter 2 Fabrication And Characterization Methods

2.6 Device Characterization

2.6.1 Current-Voltage Measurement

The I–V curves of OBDs (the writing process, the erasing process, the reading process, the retention test, and the write-read-erase-read cycle) were measured in an ambient environment using a measurement system which is composed of a personal computer, a Hewlett Packard 4156A semiconductor parameter analyzer, a switching box, and a probe station. The writing bias, the erasing bias, and the reading bias are sweeping voltages and/or (quesi-)pulse voltages.

2.6.2 Capacitance-Voltage Measurement

A measurement system which includes a personal computer, an Agilent 4284A Precision LCR meter, a switching box, and a probe station was adopted to record the high frequency capacitance-voltage (C-V) curves of OBDs and MIS solar cells.

Fig. 2.15 The setup of the transmitted IR imaging system.

2.6.3 Photocurrent Measurement

The J-V curves of MIS solar cells were obtained using a solar simulator system which consists of a laptop, a Keithley 2400 source-measure unit, a probe station, and a solar simulator with an Air Mass 1.5 source with a power of 100 mW/cm²(Oriel Class A). The illuminated light intensity was calibrated by a polycrystalline silicon solar cell certified by the National Renewable Energy Laboratory (NREL).

Chapter 3 Organic Bistable Devices With Interfacial Oxide Layers

3.1 Organic Bistable Devices With Interfacial Layers

Interface layers between the (metal) electrodes and organic layers of OBDs play an important role in the characteristics of organic electronics. [161] For understanding the interface effects on OBDs, some have paid attention on the interface engineering of OBDs.

[35], [161]-[166] Kondo et al. demonstrate that the ON/OFF ratio of OBDs is significantly enhanced by modifying the ITO electrodes (introducing Ag nanodots between the organic layer and the indium tin oxide surface). The Ag nanodots act as trapping sites, reducing the current of the OFF state. [162] Cho et al. form a self-assembled monolayer (4-nitrophenyl dichloride phosphate) on the hydroxyl-terminated Al electrode surface with native oxide to enhance switching reproducibility with an improved current level distribution. [163]

Moreover, when an aluminum electrode is deliberately oxidized or unintentionally is passivated with native oxide, some organic devices display written, reproducible resistance switching. [35], [164]-[166] Cölle et al. show that the switching mechanism of OBDs with a (metal) electrode/organic/(metal) electrode structure (at least one electrode using Al) is due to the oxide layer on the Al electrode, and carriers transport through filaments, whereas the organic materials have only minor influences on it. [35] Verbakel et al. realize reproducible OBDs by deliberately adding thin sputtering Al2O3 layers to the surfaces of the bottom electrodes of nominal electron-only, hole-only, and bipolar organic diodes. They attribute the operation of the OBDs to the soft breakdown of the sputtering Al2O3 layers. In addition, the

OBDs exhibit negative differential resistance (NDR) and local maximum currents in the I-V curves, which are dependent on the thickness of the sputtering Al₂O₃ layers. In their case, the polymers play a role as a current limiting series resistance. [165] Cho et al. study OBDs with a interfacial AlO_x layer, using an O₂ plasma treatment method, formed on the surface of the bottom Al electrode. The OBDs give higher ON/OFF ratios than those without the interfacial AlO_x layer because of the relatively small resistance at the OFF states. Nevertheless, the duration of the O2 plasma treatment makes a great influence on the threshold voltage distribution and the switching reproducibility. [166]

In this chapter, an OBD with Alq3/Al deposited on an n-type Si substrate, as shown in Fig. 2.1, is fabricated and investigated. The OBD shows distinct bistability with an ON/OFF current ratio over 10⁶ and a wide reading voltage range for differentiating between „„ON” and

„„OFF” states. The formation of electrically bistable states is the result of electrons being trapped in the defects at the Alq3/Al interface during electrical field stressing. This study also provides a simple approach, varying the deposition rate of the Alq₃ thin film, using which the electrical characteristics of the OBD, e.g., threshold voltage, can be tuned or controlled.

HRXPS, AFM, and GIXRD measurements are performed to help us understand the properties of Alq3 thin films and the Alq3/Al interface and explain the experimental results obtained.

Besides, the simple structure of the OBD indicates that the OBD can be easily embedded into the well-developed semiconductor fabrication processes.

3.2 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,

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

3.3 Summary

The I-V characteristics of the OBD, n-type Si/Alq₃/Al, are investigated. The bistability results from the defects of the Al-O compound layer at the Alq3/Al interface. The electrical characteristics of the OBD can be optimized and tuned according to our needs for different situations based on the trends obtained in these experiments. Of course some tradeoffs must be made. Owing to its simple structure, the OBD can be embedded into the conventional silicon-based fabrication processes. Furthermore, the OBD has great potential for high-density data storage, low-cost memory applications in future nanoelectronics.

Chapter 4 Organic Bistable Devices Using Metal Oxide Nanocluster Layers

4.1 Organic Bistable Devices Using Nanostructured Materials

In organic memory, many published results have focused on ORM with nanostructured materials. [7], [26], [62]-[64], [67], [74], [172]-[178] Because of the distinct properties of the nanostructured materials, ORM with high density, large ON/OFF ratios, and other superior performances will be obtained in the near future. The nanostructured materials can consist of a nanocluster/organic-metal composite layer fabricated by a thermal evaporation method [26], [62], [63], [70] (e.g., Al cores covered with Al oxide shells, [26] and Ag islands in TPD, [70]).

The nanostructured materials can also be composed of a nanocomposite of polymer-Au NPs fabricated by chemical synthesis. [7], [64]-[67] (e.g., Au NPs capped with 2-naphthalenthiol, [67] Au NPs capped with dodecanethiol [64]-[67]). In addition, metal NPs, [62], [175] oxide NPs, [74], [172], [117], [178] C60 NPs, [173], [174] and core/shell type CdSe/ZnS NPs [176]

have been introduced to be the nanostructured materials.

MoOx has been extensively applied in organic electronics (e.g., as a doping layer to raise conductivity, [179] as a buffer layer to increase carrier injection [180] or carrier collection, [181] as an intermediate layer to protect underlayers, [182] and as a capping layer to enhance light coupling [181]). Such wide applications attribute to the characteristics of the MoOx thin films: high work function, high conductivity for holes, and high transparency.

In this chapter, we use a nanostructured MoOx layer as the charge storage layer of an OBD.

The OBD consists of a p⁺-Si/Alq3/nanostructured MoOx/Alq3/Al structure, as shown in Fig.

2.4. The OBD exhibits a high ON/OFF ratio up to 10⁵, long retention time over 4000 s, and a rewritable/reerasable feature. The formation of the bistable switching of the OBD is ascribed to the charge trapping effect of the nanostructured MoOx layer. Moreover, the I-V characteristics of the OBD are quite different from those of an OBD using a MoO_x NPs layer.

[74] No NDR is observed in the I-V curves of our OBD. This phenomenon probably results from the dissimilar surface morphology of the MoO_x layer deposited on the Alq₃ thin film.

Because of both the simple structure of our OBD and the merits of Alq3 (e.g., low cost, easiness of preparation, and high stability), the complexity of fabrication and the production cost have potential to greatly reduce. Furthermore, our OBD can be easily embedded into the well developed semiconductor fabrication processes.

4.2 Results And Discussions

The I-V curves of the OBD with the p⁺-SiAlq3/nanostructured MoO_xAlq3Al structure are shown in Fig. 4.1(a). At first sweeping bias (the olive open square curve), a sweeping bias from 0 to 10 V is applied to the OBD with 100 mA current compliance. Initially, the OBD is in a low conductance state which has a current level of 10⁻¹⁰-10⁻⁵ A. An abrupt increase in the current level is observed when the applied voltage is about 3.5 V. Then, the OBD holds at a high conductance state when the applied bias sweeps to higher voltage. It is clear that the OBD undergoes an electrical transition from an OFF state to an ON state. At next sweeping bias, the OBD still maintains at the high conductance state (the red open circle curve).

Obviously, the OBD clearly exhibits two different conductance states with an ON/OFF current ratio up to 10⁵. That is to say, the OBD possesses the nature of bistability. In addition,

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