**4.1 Diodes and FETs based on single CuO nanowires**

In the following, CuO nanowires prepared by thermal oxidation in air at 500°C were used to develop diodes and FETs based on single CuO nanowires, these nanowires being chosen owed to the smaller diameter of about 60 nm and the lengths of about 30 μm for the nanowires.

Single CuO nanowire based Schottky diodes were fabricated by contacting single CuO nanowires at one end with Ti/Au by EBL and at the other end with Pt using FIBID. This outcome can be explained considering that a Pt–CuO nanowire structure exhibits an Ohmic behavior, and that a CuO nanowire–Ti/Au structure discloses a Schottky rectifying behavior [11]. Thus, **Figure 8(a)** presents a SEM image of a single CuO nanowire prepared by thermal oxidation in air and contacted at one end with Ti/Au (100 nm/300 nm) by EBL, RF magnetron sputtering and thermal vacuum evaporation and at the other end with Pt (300 nm) by FIBID, evidencing a distance between the metallic contacts of about 11 μm. **Figure 8(b)** displays the EDX elemental mapping analysis of the metallic electrodes that contact

#### **Figure 7.**

*(a) SEM image of a Si/SiO2 substrate containing Ti/Au metallic interdigitated electrodes and (b) SEM image of Ti/Au metallic interdigitated electrodes having metal oxide nanowires placed between the Ti/Au electrodes.*

**35**

**Figure 8.**

in the literature [45].

*prepared by a dry method and contacted by EBL and FIBID.*

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices*

contact formed between the CuO nanowire and Pt electrodes.

nanowire based FET (**Figure 10(d)**) disclose an ION/IOFF ratio ≈ 103

also ZnO nanowires chemically synthesized in aqueous solution.

**4.2 Diodes and FETs based on single ZnO nanowires**

with data reported in the literature for FETs based on single nanowires [11].

the nanowire to the metallic interdigitated electrodes, confirming the presence of Ti and Au elements in the case of EBL and Pt for FIBID. The current–voltage characteristics of a single CuO nanowire contacted by EBL and FIBID (**Figure 8(c)**) reveal a rectifying Schottky behavior, typical for a Schottky diode [11]. Moreover, the values of the specific parameters for diodes were estimated from the current–voltage depen-

In order to develop FETs based on single CuO nanowires, FIBID was used to contact single CuO nanowires at both ends of the nanowire with Pt (300 nm), the SEM image of a single CuO nanowire contacted by this technique being illustrated in **Figure 9(a)**. The current–voltage characteristic of a single CuO nanowire contacted by FIBID (**Figure 9(b)**) evidences a linear dependence, indicating an Ohmic

**Figure 10(a)** and **(b)** present a SEM image and the corresponding EDX mapping, proving the presence of the Pt element into the source and drain electrodes and Cu in the CuO nanowire. The length of the p-type semiconductor channel between the source and drain is about 8 μm. The output characteristics (**Figure 10(c)**) exhibit an increase in the source-drain current towards higher negative gate voltages, typical for a p-type semiconductor channel. Also, it can be noticed a change in the shape of the output characteristic at −12 V applied gate voltage, indicating the saturation region of the FET. The semilogarithmic plot of the transfer characteristic of the single CuO

In the fabrication of diodes and FETs based on single ZnO nanowires were used ZnO nanowires prepared by thermal oxidation in air at 500°C, these being chosen due to their smaller diameter of about 30 nm and the lengths of about 30 μm and

In order to develop diodes based on ZnO nanowires obtained by a dry method, single ZnO nanowire were contacted at the both ends of the nanowire Ti/Au by EBL. Hence, **Figure 11(a)** and **(b)** displays two SEM images, at different magnifications, of a single ZnO nanowire prepared by thermal oxidation in air and contacted at both ends with Ti/Pt (100 nm/200 nm) by EBL and RF magnetron sputtering, evidencing a distance between the metallic contacts of about 2 μm. The current–voltage measurement of a single ZnO nanowire contacted by EBL (**Figure 11(c)**) exhibits a non-liniar symmetrical shape indicating a structure having two back-to-back Schottky diodes, similar type of behavior being reported

*(a) SEM image, (b) EDX elemental mapping and (c) current–voltage characteristic of a single CuO nanowire* 

and the ideality factor n ≈ 1.8, being in accordance

, in agreement

*DOI: http://dx.doi.org/10.5772/intechopen.94011*

dence to be: ION/IOFF ratio ≈ 103

with data reported in the literature [11].

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94011*

*Nanowires - Recent Progress*

employed.

Ti/Au electrodes.

transistors, lithographic techniques and thin films deposition techniques were

Firstly, photolithography together with radio-frequency magnetron sputtering and thermal vacuum evaporation were used to pattern Si/SiO2 wafers with Ti/Au (10 nm/100 nm) metallic interdigitated electrodes systems. Subsequently, suspensions of CuO or ZnO nanowires in ultrapure isopropyl alcohol are prepared by ultrasonication and then drop-cast onto the Si/SiO2 substrates patterned with Ti-Au metallic interdigitated electrodes. Afterwards, single CuO or ZnO nanowires are contacted by EBL, FIBID or EBL combined with FIBID. **Figure 7(a)** illustrates a SEM image of a Si/SiO2 substrate patterned with Ti/Au metallic interdigitated electrode system, while **Figure 7(b)** presents a SEM image of Ti/Au metallic interdigitated electrodes having metal oxide nanowires, transferred by drop-casting, between the

The electrical measurements of the single CuO or ZnO nanowires contacted by

In the following, CuO nanowires prepared by thermal oxidation in air at 500°C

lithographic techniques were carried out at room temperature in a typical twopoints configuration for diodes and a three-points configuration for FETs.

were used to develop diodes and FETs based on single CuO nanowires, these nanowires being chosen owed to the smaller diameter of about 60 nm and the

Single CuO nanowire based Schottky diodes were fabricated by contacting single CuO nanowires at one end with Ti/Au by EBL and at the other end with Pt using FIBID. This outcome can be explained considering that a Pt–CuO nanowire structure exhibits an Ohmic behavior, and that a CuO nanowire–Ti/Au structure discloses a Schottky rectifying behavior [11]. Thus, **Figure 8(a)** presents a SEM image of a single CuO nanowire prepared by thermal oxidation in air and contacted at one end with Ti/Au (100 nm/300 nm) by EBL, RF magnetron sputtering and thermal vacuum evaporation and at the other end with Pt (300 nm) by FIBID, evidencing a distance between the metallic contacts of about 11 μm. **Figure 8(b)** displays the EDX elemental mapping analysis of the metallic electrodes that contact

*(a) SEM image of a Si/SiO2 substrate containing Ti/Au metallic interdigitated electrodes and (b) SEM image of Ti/Au metallic interdigitated electrodes having metal oxide nanowires placed between the Ti/Au electrodes.*

**4.1 Diodes and FETs based on single CuO nanowires**

lengths of about 30 μm for the nanowires.

**34**

**Figure 7.**

the nanowire to the metallic interdigitated electrodes, confirming the presence of Ti and Au elements in the case of EBL and Pt for FIBID. The current–voltage characteristics of a single CuO nanowire contacted by EBL and FIBID (**Figure 8(c)**) reveal a rectifying Schottky behavior, typical for a Schottky diode [11]. Moreover, the values of the specific parameters for diodes were estimated from the current–voltage dependence to be: ION/IOFF ratio ≈ 103 and the ideality factor n ≈ 1.8, being in accordance with data reported in the literature [11].

In order to develop FETs based on single CuO nanowires, FIBID was used to contact single CuO nanowires at both ends of the nanowire with Pt (300 nm), the SEM image of a single CuO nanowire contacted by this technique being illustrated in **Figure 9(a)**. The current–voltage characteristic of a single CuO nanowire contacted by FIBID (**Figure 9(b)**) evidences a linear dependence, indicating an Ohmic contact formed between the CuO nanowire and Pt electrodes.

**Figure 10(a)** and **(b)** present a SEM image and the corresponding EDX mapping, proving the presence of the Pt element into the source and drain electrodes and Cu in the CuO nanowire. The length of the p-type semiconductor channel between the source and drain is about 8 μm. The output characteristics (**Figure 10(c)**) exhibit an increase in the source-drain current towards higher negative gate voltages, typical for a p-type semiconductor channel. Also, it can be noticed a change in the shape of the output characteristic at −12 V applied gate voltage, indicating the saturation region of the FET. The semilogarithmic plot of the transfer characteristic of the single CuO nanowire based FET (**Figure 10(d)**) disclose an ION/IOFF ratio ≈ 103 , in agreement with data reported in the literature for FETs based on single nanowires [11].

#### **4.2 Diodes and FETs based on single ZnO nanowires**

In the fabrication of diodes and FETs based on single ZnO nanowires were used ZnO nanowires prepared by thermal oxidation in air at 500°C, these being chosen due to their smaller diameter of about 30 nm and the lengths of about 30 μm and also ZnO nanowires chemically synthesized in aqueous solution.

In order to develop diodes based on ZnO nanowires obtained by a dry method, single ZnO nanowire were contacted at the both ends of the nanowire Ti/Au by EBL. Hence, **Figure 11(a)** and **(b)** displays two SEM images, at different magnifications, of a single ZnO nanowire prepared by thermal oxidation in air and contacted at both ends with Ti/Pt (100 nm/200 nm) by EBL and RF magnetron sputtering, evidencing a distance between the metallic contacts of about 2 μm. The current–voltage measurement of a single ZnO nanowire contacted by EBL (**Figure 11(c)**) exhibits a non-liniar symmetrical shape indicating a structure having two back-to-back Schottky diodes, similar type of behavior being reported in the literature [45].

#### **Figure 8.**

*(a) SEM image, (b) EDX elemental mapping and (c) current–voltage characteristic of a single CuO nanowire prepared by a dry method and contacted by EBL and FIBID.*

#### **Figure 9.**

*(a) SEM image and (b) current–voltage of a single CuO nanowire prepared by a dry method and contacted by FIBID.*

**Figure 10.**

*(a) SEM image, (b) EDX elemental mapping, (c) output and (d) transfer characteristics of a FET based on a single CuO nanowire prepared by a dry method and contacted by FIBID.*

FETs based on single ZnO nanowires prepared by a dry method were fabricated by contacting single ZnO nanowires at both ends with Ti/Au (100 nm/300 nm) by EBL, RF magnetron sputtering and thermal vacuum evaporation, the SEM image of a single ZnO nanowire contacted by EBL being illustrated in **Figure 12(a)**. The current–voltage dependence of a single ZnO nanowire contacted by EBL (**Figure 12(b)**) put in evidence a linear shape, typical for an Ohmic contact formed between the ZnO nanowire and the two Ti/Au contacts.

**37**

**Figure 13.**

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices*

*(a), (b) SEM images and (c) current–voltage characteristic of a single ZnO nanowire prepared by a dry* 

*(a) SEM image and (b) current–voltage characteristic of a single ZnO nanowire prepared by a dry method* 

**Figure 13(a)** displays a SEM image of a single ZnO nanowire contacted by EBL, indicating that the length of the n-type semiconductor channel between the source and drain is about 1 μm. The equivalent EDX mapping image (**Figure 13(b)**) proves

*(a) SEM image, (b) EDX elemental mapping, (c) output and (d) transfer characteristics of a FET based on a* 

*single ZnO nanowire prepared by a dry method and contacted by EBL.*

*DOI: http://dx.doi.org/10.5772/intechopen.94011*

**Figure 11.**

**Figure 12.**

*and contacted by EBL.*

*method and contacted by EBL.*

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94011*

#### **Figure 11.**

*Nanowires - Recent Progress*

**Figure 9.**

*FIBID.*

**36**

**Figure 10.**

and the two Ti/Au contacts.

FETs based on single ZnO nanowires prepared by a dry method were fabricated by contacting single ZnO nanowires at both ends with Ti/Au (100 nm/300 nm) by EBL, RF magnetron sputtering and thermal vacuum evaporation, the SEM image of a single ZnO nanowire contacted by EBL being illustrated in **Figure 12(a)**. The current–voltage dependence of a single ZnO nanowire contacted by EBL (**Figure 12(b)**) put in evidence a linear shape, typical for an Ohmic contact formed between the ZnO nanowire

*(a) SEM image, (b) EDX elemental mapping, (c) output and (d) transfer characteristics of a FET based on a* 

*single CuO nanowire prepared by a dry method and contacted by FIBID.*

*(a) SEM image and (b) current–voltage of a single CuO nanowire prepared by a dry method and contacted by* 

*(a), (b) SEM images and (c) current–voltage characteristic of a single ZnO nanowire prepared by a dry method and contacted by EBL.*

#### **Figure 12.**

*(a) SEM image and (b) current–voltage characteristic of a single ZnO nanowire prepared by a dry method and contacted by EBL.*

#### **Figure 13.**

*(a) SEM image, (b) EDX elemental mapping, (c) output and (d) transfer characteristics of a FET based on a single ZnO nanowire prepared by a dry method and contacted by EBL.*

**Figure 13(a)** displays a SEM image of a single ZnO nanowire contacted by EBL, indicating that the length of the n-type semiconductor channel between the source and drain is about 1 μm. The equivalent EDX mapping image (**Figure 13(b)**) proves

#### **Figure 14.**

*(a) SEM image and (b) current–voltage characteristic of a single ZnO nanowire obtained by a wet method and contacted by EBL.*

#### **Figure 15.**

*(a) SEM image, (b) EDX elemental mapping, (c) output and (d) transfer characteristics of a FET based on a single ZnO nanowire obtained by a wet method and contacted by EBL.*

the presence of the Ti and Au elements into the source and drain electrodes provided by EBL.

The output characteristics of a FET based on a single ZnO nanowire (**Figure 13(c)**) evidence an increase in the source-drain current towards higher positive gate voltages, typical for an n-type semiconductor channel. It can be observed that at 1.5 V applied drain-source voltage, the FET reaches the saturation region. The semilogarithmic plot of the transfer characteristic of a FET having a single ZnO nanowire as a channel (**Figure 13(d)**) exhibits an ION/IOFF ratio ≈ 105 , in accordance with data reported in the literature for FETs based on single nanowires [13].

In the following, single ZnO nanowires synthesized by a wet technique were used as n-type channels into FET devices. Accordingly, FETs based on single ZnO nanowires were developed by contacting single ZnO nanowires at both ends with Ti/Au (100 nm/200 nm) by EBL, RF magnetron sputtering and thermal vacuum evaporation. **Figure 14(a)** reveals the SEM image of a single ZnO nanowire

**39**

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices*

synthesized by a wet method contacted by EBL. Similar to the ZnO nanowires prepared by a dry path, the current–voltage characteristic of a single ZnO nanowire obtained by a wet approach contacted by EBL (**Figure 14(b)**) exhibits a linear shape, with an Ohmic contact between the ZnO nanowire and the two Ti/Au

The SEM image at a higher magnification of a single ZnO nanowire contacted by EBL (**Figure 15(a)**) evidences that the length of the nanowire channel between the source and drain is about 2 μm. The corresponding EDX mapping image (**Figure 15(b)**) confirms the presence of the Ti and Au elements into the source

The output characteristics of a FET based on a single ZnO nanowire prepared by a wet method (**Figure 15(c)**) disclose an increase in the source-drain current towards higher positive gate voltages, demonstrating that the channel is an n-type semiconductor. In addition, it can be noticed that at 2 V applied drain-source voltage, the FET reaches the saturation region. The semilogarithmic plot of the transfer characteristic of a FET having a single ZnO nanowire as an n-type channel

data reported in the literature for ZnO single nanowires based FETs [13].

Metal oxide, ZnO and CuO, nanowire arrays were obtained by using two straightforward and cost-effective preparation methods: chemical synthesis in aqueous solution (wet) and thermal oxidation in air (dry). The influence of the preparation technique on the morphological, structural and optical properties of the metal oxide nanowire arrays were investigated. Further, ZnO and CuO nanowires prepared by these wet and dry approaches were successfully integrated as active elements into electronic devices, such as Schottky diodes and FETs by using lithographic techniques (photolithography, EBL and FIBID) and thin film deposition techniques (RF magnetron sputtering and thermal vacuum evaporation). Additionally, the characteristic parameters for the diodes and FETs were estimated

The main advantage of fabricating electronic devices based on single metal oxide nanowires is represented by the possibility to integrate them in optoelectronic

This work has been funded by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI), Romania, Project code: Project code: PN-III-P1-1.1-PD-2019-1102 and by the Core Program, contract no. PN19-03 (contract no. 21 N/08.02.2019) supported from the Romanian Ministry

from the electrical measurements: n ≈ 1.8 and ION/IOFF ratio ≈ 103

for the FETs.

, the value being in accordance with

for diodes and

*DOI: http://dx.doi.org/10.5772/intechopen.94011*

and drain metallic electrodes fabricated by EBL.

(**Figure 15(d)**) reveals an ION/IOFF ratio ≈ 104

–105

The authors declare no conflict of interest.

contacts.

**5. Conclusions**

ION/IOFF ratio ≈ 104

**Acknowledgements**

of Research and Innovation.

**Conflict of interest**

applications.

*Metal Oxide Nanowires as Building Blocks for Optoelectronic Devices DOI: http://dx.doi.org/10.5772/intechopen.94011*

synthesized by a wet method contacted by EBL. Similar to the ZnO nanowires prepared by a dry path, the current–voltage characteristic of a single ZnO nanowire obtained by a wet approach contacted by EBL (**Figure 14(b)**) exhibits a linear shape, with an Ohmic contact between the ZnO nanowire and the two Ti/Au contacts.

The SEM image at a higher magnification of a single ZnO nanowire contacted by EBL (**Figure 15(a)**) evidences that the length of the nanowire channel between the source and drain is about 2 μm. The corresponding EDX mapping image (**Figure 15(b)**) confirms the presence of the Ti and Au elements into the source and drain metallic electrodes fabricated by EBL.

The output characteristics of a FET based on a single ZnO nanowire prepared by a wet method (**Figure 15(c)**) disclose an increase in the source-drain current towards higher positive gate voltages, demonstrating that the channel is an n-type semiconductor. In addition, it can be noticed that at 2 V applied drain-source voltage, the FET reaches the saturation region. The semilogarithmic plot of the transfer characteristic of a FET having a single ZnO nanowire as an n-type channel (**Figure 15(d)**) reveals an ION/IOFF ratio ≈ 104 , the value being in accordance with data reported in the literature for ZnO single nanowires based FETs [13].

### **5. Conclusions**

*Nanowires - Recent Progress*

**Figure 14.**

*and contacted by EBL.*

**38**

vided by EBL.

**Figure 15.**

(**Figure 13(d)**) exhibits an ION/IOFF ratio ≈ 105

literature for FETs based on single nanowires [13].

*single ZnO nanowire obtained by a wet method and contacted by EBL.*

the presence of the Ti and Au elements into the source and drain electrodes pro-

*(a) SEM image, (b) EDX elemental mapping, (c) output and (d) transfer characteristics of a FET based on a* 

*(a) SEM image and (b) current–voltage characteristic of a single ZnO nanowire obtained by a wet method* 

The output characteristics of a FET based on a single ZnO nanowire (**Figure 13(c)**) evidence an increase in the source-drain current towards higher positive gate voltages, typical for an n-type semiconductor channel. It can be observed that at 1.5 V applied drain-source voltage, the FET reaches the saturation region. The semilogarithmic plot of the transfer characteristic of a FET having a single ZnO nanowire as a channel

In the following, single ZnO nanowires synthesized by a wet technique were used as n-type channels into FET devices. Accordingly, FETs based on single ZnO nanowires were developed by contacting single ZnO nanowires at both ends with Ti/Au (100 nm/200 nm) by EBL, RF magnetron sputtering and thermal vacuum evaporation. **Figure 14(a)** reveals the SEM image of a single ZnO nanowire

, in accordance with data reported in the

Metal oxide, ZnO and CuO, nanowire arrays were obtained by using two straightforward and cost-effective preparation methods: chemical synthesis in aqueous solution (wet) and thermal oxidation in air (dry). The influence of the preparation technique on the morphological, structural and optical properties of the metal oxide nanowire arrays were investigated. Further, ZnO and CuO nanowires prepared by these wet and dry approaches were successfully integrated as active elements into electronic devices, such as Schottky diodes and FETs by using lithographic techniques (photolithography, EBL and FIBID) and thin film deposition techniques (RF magnetron sputtering and thermal vacuum evaporation). Additionally, the characteristic parameters for the diodes and FETs were estimated from the electrical measurements: n ≈ 1.8 and ION/IOFF ratio ≈ 103 for diodes and ION/IOFF ratio ≈ 104 –105 for the FETs.

The main advantage of fabricating electronic devices based on single metal oxide nanowires is represented by the possibility to integrate them in optoelectronic applications.

#### **Acknowledgements**

This work has been funded by the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI), Romania, Project code: Project code: PN-III-P1-1.1-PD-2019-1102 and by the Core Program, contract no. PN19-03 (contract no. 21 N/08.02.2019) supported from the Romanian Ministry of Research and Innovation.

#### **Conflict of interest**

The authors declare no conflict of interest.

*Nanowires - Recent Progress*
