The Impact of Nanocontact on Nanodevices

In this work, more than thirty two-probe ZnO NW devices were fabricated to determine the intrinsic resistance of ZnO NWs and to explore the influence of nanocontact on nanodevices by analyzing their corresponding I-V characteristics and electron transport. It is emphasized that all of ZnO NWs used to fabricate nanodevices is detached from the same source sample. Even though this reason, we still observed a distinct RT (room-temperature) resistance of our ZnO NW devices.

Hence, according to their RT (room-temperature) resistances and I-V curves, the devices were grouped into three different types. Fig. 3.2 (a) presents the first type, Type I, ZnO NW device with characteristics of a lower RT resistance, a linear I-V

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demeanor in low voltage range, and a symmetrically downward bending feature in I-V curves. The low RT resistance and linear I-V dependence imply that both of the two nanocontacts are Ohmic type. Moreover, the temperature behavior of current at various bias voltage is analyzed with regard to the thermionic-emission theory (see Section 2.1) and shown in Fig. 3.2 (b) but we did not find any concordance. The result of inconsistence with Schottky contact supports the Ohmic type contacts for Type I devices as well. The circuit diagram of Type I devices with two Ohmic nanocontacts (indicated as circles) and one ZnO NWs (a resistance symbol) is given in the inset of Fig. 3.2 (a). Since both the two nanocontacts are Ohmic type, the device resistance could mainly come from the intrinsic ZnO NW resistance. On the other hand, the zero-voltage resistances as a function of temperature are displayed in Fig. 3.2 (c). The electron transport behavior can be well apprehended in agreement with the thermally activated transport (see Section 2.2) of electrons in ZnO NWs. The intrinsic ZnO NW resistance unveils thermally activated transport of conduction through the NW which conforms to our four-terminal measurements [18] and other groups’ results [19, 20]. Since the conduction through the nanowire has been unambiguously investigated in Type I devices, the other devices having higher RT resistances could give electrical properties at nanocontact.

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Figure 3.2: (a) I-V curves of a Type I ZnO NW device with a RT resistance of ~15 kΩ. The inset introduces a model of circuit diagram for Type I devices. (b) I/T² as a function of inverse temperature at various bias voltages. (c) Resistance as a function of temperature revealing electron transport in the Type I device.

A rectifying behavior on I-V curves, especially at low temperatures, has been unambiguously obtained and presented in Fig. 3.3 (a). We notice that the non-symmetrical I-V curves of the Type II NW device could only originate from one and only one Schottky nanocontact with the other one of Ohmic type. A circuit diagram model is offered in the inset of Fig. 3.3 (a) to depict the Schottky nanocontact, the ZnO NW, and the Ohmic nanocontact from left to right. The analyses in accordance with thermionic-emission theory (see Section 2.1) are given in Fig. 3.3 (b) and (c). For Type II devices, we observed linear dependence between ln(I/T²) and 1/T with a slope indicating the effective Schottky barrier in the Schottky nanocontact. At a high bias voltage, the slope tends to zero which denotes

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the vanishing of effective Schottky barrier φ_B when the bias approaches the break down voltage. Moreover, the I-V curve in a wide voltage range introducing in Fig 3.3(c) fits precisely with thermionic-emission theory both in forward and reverse bias voltages. Intriguingly and surprisingly, the ideality factor estimated from our fitting is very close to the ideal value of 1. Alternatively, the RT resistance of Type II devices ranges from near the upper bound RT resistance of Type I devices to that of Type III devices as will be acquainted in the next paragraphs. The temperature dependence of resistance R(T) in Fig 3.3 (d) indicates that the electron transport behavior cannot be solely interpreted by the thermally activated transport in ZnO NW. In addition, We found that electron transport in these Type II devices departs from thermal activated transport and starts to reconcile with thermionic-emission and VRH conduction. We noticed that the fittings with either Schottky contact or VRH conduction do not make a significant difference at temperatures above 100 K.

These results suggest that electrical properties, including I-V curves and zero-voltage resistances, mostly arise from the single Schottky nanocontact in the Type II NW devices.

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Figure 3.3: (a) I-V curves of a Type II ZnO NW device with a RT resistance of ~50 kΩ. The inset introduces a model of circuit diagram for Type II devices. (b) I/T² as a function of inverse temperature at various bias voltages. (c) ln(I/(1-exp(-qV/kT))) as a function of voltage. (d) Resistance as a function of temperature revealing electron transport in this Type II device. The dashed and solid lines are best fittings (see text) of Equations 4 and 5, respectively.

Typical characteristics of the last type, the Type III ZnO NW devices with analyses through thermionic-emission theory are given in Fig. 3.4. First of all, the I-V curves in Fig. 3.4 (a) reveal symmetrical and an upward bending feature, and the data can be fitted well with thermionic-emission theory in reverse bias voltage.

The linear dependence of ln(I/T²) and 1/T at various bias voltage appears

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prominently in Fig 3.4 (b) that supports our idea of Schottky contacts for Type III devices. Since no forward bias I-V reliance of a Schottky diode is spotted and the currents in both positive and negative voltages agree well with the reverse-bias thermionic-emission theory (see Fig. 3.4 (a) and (b)), we propose a model of two back-to-back Schottky contacts, which is commonly addressed to explain nonlinear I-V curves of semiconductor NW devices [21, 22], in the inset of Fig. 3.4 (a) to demonstrate the Type III devices. As for zero-voltage resistance, we obtained several different temperature dependent behaviors. Two typical sets of resistance data are presented in Fig. 3.4 (c) with the best fittings of dashed and solid lines following Schottky contact and VRH conduction, respectively. In this case, we found that the data points vary from the ideal Schottky contact form at temperatures below 200 K. Including the information of high RT resistances for Type III devices as well as the best fitting in accordance with VRH conduction form, we propose that a deteriorated contact between the Ti electrode and the ZnO NW forms due to a non-crystalline interface layer or titanium oxides. Owing to a diminished nanocontact area, a poor specific contact resistivity results in a high contact resistance and dominates electrical properties of the Type III NW devices.

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Figure 3.4: (a) I-V curves of a Type III ZnO NW device with a RT resistance of

~1.5 MΩ. The inset introduces a model of circuit diagram for Type III devices. (b) I/T² as a function of inverse temperature at various bias voltages. (c) Resistance as a function of temperature revealing electron transport in this Type III device (red circles) and in another Type III device having a RT resistance of ~128 MΩ (black squares). The dashed and solid lines are best fittings (see text) of Equations 4 and 5, respectively.

In order to discuss electron transport in ZnO NW devices and to fit their temperature dependent resistance, we adopted VRH conduction and included the exponent parameter p of 1 for thermally activated transport. The estimated exponent parameters as a function of device RT resistance are given in Fig. 3.5 (a).

An unambiguous trend of a rising exponent parameter with an increase of RT resistance has been detected that signals worsened electrical properties in the nanocontacts. Moreover, the graph can be approximately separated into Regions A,

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B, and C with an exponent parameter of about 1, 2, and bigger than 3, respectively.

We discerned that all Type I ZnO NW devices are gathering in Region A indicating two Ohmic contacts on the individual ZnO NW. It is noted that the Type I devices have RT resistances lower than 100 kΩ. In addition, Type II devices distribute in Region B and unveil one-dimensional VRH conduction in one and only one nanocontact on the ZnO NW. Unlike Type I and II devices, Type III devices, having RT resistances higher than 100 kΩ, spread over Region B and C that connotes two- and three-dimensional VRH conduction in both nanocontacts of a NW device. In Fig. 3.5 (b) we provide three nanocontact models to elaborate our thoughts. Since a direct Ti metal contacting on ZnO should be attributed to Ohmic contacts as inferred from experiments of bulk systems [23], we propose a direct contacting model for nanocontacts of ZnO NW devices in Region A. Otherwise, the RT resistance is greater for devices in Regions B and C, we argue that titanium oxide form in the nanocontacts due to poor vacuum conditions during thermal evaporation. For NW devices in Region B, we believe that the oxide layer is thin enough for electrons to transport as one-dimensional VRH conduction. When the oxide layer is thick, the conduction channels mix to form two- and three-dimensional VRH conduction such as the proposed nanocontact model for devices in Region C.

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Figure 3.5: (a) The fitting exponent parameters p as a function of RT resistance for our as-fabricated ZnO NW devices of Type I, II, and III, marking as blue circles, black triangles, and red squares, respectively. The figure is approximately separated into Regions A, B, and C according to the exponent parameters of our devices. (b) Three different nanocontact models corresponding to the ZnO NW devices belonging to Regions A, B, and C of Figure (a).

In the above discussion, we have learned that the interface problems in nanowire-based electronics play important roles due to the reason that the reduced contact area in nanoelectronics multiplies enormously the contribution of electrical contact properties. In the interfacial nanocontact system, electron transport follows Mott variable range hopping theory of the form R∝exp(( / ) )T T₀ ^1/p . Here, the

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specific contact resistivity of an interfacial nanocontact system is further considered according to the form ρ_C = ×R A , where R and A_C C represent RT resistance and contact area of nanodevices, respectively. The fitting exponent parameters, p, as a function of nanodevices contact resistivity at room temperature are given in Fig. 3.6. It should be emphasized that many two-probe nanodevices of different systems of semiconductor nanowires (The intrinsic electrical properties of these nanodevices will be illustrated in Sec. 3.3.2.) are used to extract the exponent parameters. A straightforward trend of a rising exponent parameter with an increase of specific contact resistance can be seen. The exponential parameter, p, rises from 2 to 4 with an increase of specific contact resistivity, as shown in Fig. 3.6, indicating a change from one- to three-dimensional hopping condition. Besides, a universal trend (i.e. the dashed line) is marked in Fig. 3.6, implying the nanocontact system seems to exhibit a general behavior of electron transport, even in these nanodevices fabricated by disparate nanowires. Moreover, the fitting exponent parameters as a function of T0 also depict in the inset of Fig. 3.6. Since the fitting parameter, T0, is proportional inversely to the localization length in disorder system (see Eq. (2.20)), the increasing T0 suggests strongly a reduction of localization length as well as increasing disorder in the interfacial electron system.

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Figure 3.6: The fitting exponent parameters p as a function of RT contact resistivity for NW devices. The fitting exponent parameter as a function of T0 shows in the inset.