Transport Behaviors in Semiconductor Nanonanowires

Nanonanowires

It is already learned in Section 3.3.1 that with the shrinkage of nanocontact area on nanodevices, the contact resistance will contribute extremely to the total resistance of the two-probe NW devices. In the below discussion, we have made the best efforts in facilitating the Ohmic contact so that the contact resistance can be neglected. Here, ZnO, InP, and GaP NW devices were investigated in determination of the intrinsic temperature dependent resistance.

Fig. 3.7 (a) displays a schematic diagram of a two-probe ZnO NW measurement system and the inset is a typical FE-SEM image of a ZnO NW device.

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The scheme demonstrates that the NW was buried under the leads (electrodes) and contacted with a Ti metal, having a contact area of ~1×0.04 μm². The separation distance between two probes is a constant of ~1 μm. The targeted NWs were selected from the same source sample. Since these NWs have the same diameter and length, they should exhibit the same resistance. Fig. 3.7 (b) presents data collected from three two-probe devices in which the NWs were detached from the same source sample. It was observed that not only the room-temperature resistances on the three devices, ZnO-1, ZnO-2, and ZnO-3, are largely different (up to four orders of magnitudes), but their temperature behaviors are also disparate.

According to above-mentioned assumption, that is, NWs of the same sample source should have defect concentration and resistances in the same order of magnitude, we propose that the higher resistance from ZnO-1 and ZnO-2 devices could be due to the nanocontact. This conjecture has been confirmed by current-voltage measurements which exhibit a nonlinear, non-Ohmic behavior [24]. As reported in previous section, the nanocontact can be treated as a disordered system and the electron transport in nanocontact follows a theory of Mott-VRH (More theoretical and experimental detail could be referred to Section 2.2 and Section 3.3.1, respectively) temperature-dependent constant, and p is the exponent parameter. The dashed lines give the best fits to data in Fig. 3.7 (b) to derive the exponent p of 2 and 4 for ZnO-2 and ZnO-1 devices, respectively. The increase of room-temperature resistance as well as disorder in the nanocontact can raise the exponent parameter p from 2 to 4 and the nanocontact-constrained electron system from one- to

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three-dimensional Mott-VRH.

On the other hand, if a device, such as ZnO-3, holds the lowest room-temperature resistance, it implies that the intrinsically electrical property of the NW and the temperature-dependent resistivity may follow a thermally activated transport equation

The solid line in Fig. 3.7 (b) presents the best fit to the data of ZnO-3 device according to the thermally activated transport equation. The room-temperature resistance of ZnO-3 device is 16.8 kΩ and the length and diameter of the NW are 1 μm and 40 nm, respectively, resulting in a resistivity of ~0.002 Ω cm. Assuming that the electron mobility is 50 cm² V^-1 s^-1 [25], we can estimate the carrier concentration to be 10¹⁹ cm^-3 at room temperature. At temperatures lower than 140 K, it is amazing to see that the temperature-dependent resistance deviates from the theoretically predicted values. Since a random distribution of native defects and a disorder are introduced, the electron transport in ZnO NWs, having a low carrier concentration, should follow Mott-VRH at very low temperatures. The resistance of ZnO-3 device agrees well with the three-dimensional Mott-VRH theory (the dashed line, p = 4) at temperatures lower than 140 K. To verify this conjecture, we adopted a four-probe measurement method (see Fig. 3.7 (c)) and confirmed that the intrinsic NW transport follows the three-dimensional Mott-VRH theory described in Eq. (3.1) with the exponent parameter p = 4. This result is in consistent with a determination reported recently [20]. As a consequence, the two-probe measurement with two Ohmic contacts and low resistance can be employed in acquiring carrier concentration and electron transport properties in ZnO NWs.

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Furthermore, the source sample of ZnO-4 is different from that of ZnO-3 and the room-temperature resistivity and carrier concentration of ZnO-4 were evaluated to be 17 Ω cm and 10¹⁶ cm^-3, respectively. The carrier concentration of ZnO-4 NW is about three orders of magnitude lower than that of ZnO-3 NW. It should be pointed out that the ZnO NWs picked up from different source samples can exhibit extraordinarily discrepant resistivities and carrier concentrations, even though they are synthesized by the same growth method. The change in carrier concentrations among NWs picked up from different source samples is expected to come from varied concentrations of structure defects such as oxygen vacancies and Zn interstitials.

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Figure 3.7: (a) Schematic diagram of a two-probe ZnO NW device with a typical SEM image shown in the inset. Temperature-dependent resistance of (b) two- and (c) four-probe ZnO NW devices. A schematic diagram of a four-probe device is drawn in the inset of Fig. (c). Solid and dashed lines delineate the best fits to the mathematical forms of thermally activated transport and Mott-VRH, respectively.

After fitting to Mott-VRH (dashed lines), the exponents, p's, of ZnO-1, ZnO-2, ZnO-3, and ZnO-4 devices are estimated to be 4, 2, 4, and 4, respectively.

To extend the application of this two-probe measurement method to other semiconductor NWs, InP NW devices were fabricated through a solution-based growth¹⁵ and a typical SEM image is presented in the inset of Fig. 3.8. InP NWs, like ZnO NWs, are natively n-type doped with a relatively low carrier concentration, which have been proved by the back gate effect (not shown in this

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report). Fig. 3.8 illustrates temperature-dependent resistances of InP-1, InP-2, and InP-3 devices. The InP NWs were picked up from the same source but their room-temperature resistances, 1100, 420, and 30 MΩ for InP-1, InP-2, and InP-3 devices, varied considerably. Like ZnO-1 and ZnO-2 devices, the high resistance of InP-1 and InP-2 might be resulted from the nanocontact. The temperature behaviors at higher temperatures can be fitted by the thermally activated transport theory (Eq.

(3.2), solid lines in Fig. 3.8), whereas those at temperatures lower than 150 K follow the Mott-VRH theory (Eq. (3.1), dashed lines). The exponent parameter p's are 2.4 and 4 for InP-2 and InP-1 devices, respectively, after fitting to Mott-VRH in a low temperature range. It is noted that the Mott-VRH theory can be solely used in fitting the data of ZnO-1 device in the whole range of temperatures, whereas both the Mott-VRH and thermally activated transport theories must be employed in fitting the data of InP-1 device. The difference comes from the NW resistance (resistivity). If the resistance (resistivity) of a NW is much lower, such as that of ZnO-3, the nanocontact resistance should dominate. Else, if they are comparable, the device characteristics will depend on contributions from both the NW and the nanocontact. Moreover, when thermally activated transport was used in fitting the data of InP NWs (InP-1, InP-2, and InP-3 devices) at high temperatures, the same slope of temperature-dependent resistance indicates the same activation energy (EA) for carriers in InP NWs. On the other hand, since InP-3 device holds the lowest resistance, it implies an intrinsic electron transport in the InP NW. In the case of InP-3, the NW was ~1 μm in length and ~20 nm in diameter and the resistivity was determined as ~0.94 Ω cm. Assuming that the electron mobility is 1000 cm² V^-1 s^-1 [26], the carrier concentration at room temperature can be estimated as ~6.6×10¹⁵ cm^-3. The temperature-dependent resistance of InP-3 device indicates that electron transport at high temperatures can be well described by the thermally activated

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transport theory, and at low temperatures it follows the three-dimensional Mott-VRH theory (p = 4). It should be emphasized that, in nanocontact-dominated devices such as InP-1 and InP-2, the exponent parameter p of Mott-VRH rises from 2.4 to 4 with an increasing room-temperature resistances of these InP NW devices.

In contrast, in NW-dominated devices such as InP-3, the exponent parameter p of Mott-VRH remains as a constant of ~4, implying three-dimensional Mott-VRH in InP NWs.

Figure 3.8: (a) Resistance as a function of inverse temperature for two-probe InP NW devices with a typical SEM image shown in the inset. The solid and dashed lines delineate the best fits to mathematical equations of thermally activated transport and Mott-VRH, respectively. After fitting to Mott-VRH (dashed lines), the exponents, p's, for InP-1, InP-2, and InP-3 are estimated to be 4, 2.4, and 4, respectively.

As mentioned above, it was observed that, as the NW resistance (resistivity) increases, the resistance of a two-probe NW device will be dominated by the

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intrinsic NW resistance even though the contact is non-Ohmic and poor. We conclude, therefore, that the higher the NW resistance is, the easier the intrinsic electrical property of the NW can be drawn out. As for a contact area of ~1×0.04 μm², the contact resistance is typically no bigger than ~1 GΩ. Thus, NWs of room-temperature resistance no less than 1 GΩ could easily be determined by using the two-probe technique. This idea has been materialized in GaP NWs. I-V curves of all GaP devices are displayed in Fig. 3.9. The inset is a temperature-dependent resistance of GaP-5. It is worth noting that the I-V curves of all devices are linear in a wide range of voltage (from -3 to 3 V). This behavior can be observed in devices of high-resistance nanostructures such as NWs and nanocrystals. If electron transport in GaP NW follows the thermally activated transport theory, the I-V should reveal an Ohmic characteristic of this NW rather than of hopping conduction in a disordered system of the nanocontacts. As expected, the temperature-dependent resistance, shown in the inset of Fig. 3.9, implies thermally activated transport. For example, GaP-5 which are 1 μm and 20 nm in length and diameter, respectively, exhibits a room-temperature resistance of 8.1 GΩ and a resistivity of ~254 Ω cm. Assuming that the electron mobility is 160 cm² V^-1 s^-1 [27], we can therefore estimate the carrier concentration as 1.53×10¹⁴ cm^-3 at room temperature.

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Figure 3.9: I-V curves, taken at room temperature, from six different GaP NW devices. The inset shows temperature-dependent resistance of GaP-5 device. The solid line in the inset demonstrates the best fit to the thermally activated transport equation.

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Chapter 4

Enhanced Photoresponse and Gas Sensing of InP Nanowire Device

4.1 Introduction

Indium phosphide (InP) is a direct and narrow gap III-V group semiconductor with a room-temperature (RT) bandgap energy of ~ 1.34 eV [1]. InP possessing a relatively high electron mobility ~ 1000 cm² V^-1 s^-1 at room temperature [2], with respect to the more common semiconductor such as silicon and gallium arsenide, is a suitable candidate to make modern communications and high-frequency electronic devices. Moreover, due to a lattice match, InP used as a substrate for the development of long-wavelength optoelectronic devices has been manifested.

Latterly, by using laser-assisted catalytic growth [3], n- and p-type doped InP materials have been transferred to form quasi-one-dimensional nanowires (NWs).

It was then pointed out that InP NWs as building blocks could be applied to realize a sensitively polarized photodetection [4] and a light-emitting diode [5]. InP NWs were also exercised as a spacer and barrier in InAs NWs for observation of a single-electron tunneling effect [6]. Schottky diode based on a single InP

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nanoneedle has also been demonstrated [7]. Most recently, it was presented in a theoretical prediction that, using an appropriate surface passivation, InP NWs might be applied as a gas sensor [8]. Until now electrical properties of InP NWs have not been broadly studied, in particular the understanding of temperature dependent resistance and current-voltage (I-V) behaviors. Most of nano electronic devices possess two-probe configuration as the source and drain electrodes so that intrinsic behaviors in NWs and contact resistance always cannot be distinguished explicitly [9]. In this work, we report a series of systematical experiments to explore the electrical properties in more than thirty InP NW devices with two-probe electrical configurations. The NWs for device fabrication are picked up from the same source sample. According to temperature behaviors of resistances in InP NW devices, contact- and NW-dominated devices could be separated each other. The electron transport and carrier concentration of native doping in InP nanowires can be determined. Furthermore, contact- and NW-dominated two-probe nanodevices are exposed to light (green laser) and gas (oxygen) to identify the influence of contact effect.