p-n型氧化亞銅/氧化鋅異質接面奈米線陣列的合成與性質分析

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Research Express@NCKU - Articles Digest

Research Express@NCKU Volume 14 Issue 6 - June 25, 2010

[ http://research.ncku.edu.tw/re/articles/e/20100625/4.html ]

Synthesis and Characterization of Cu

₂

O/Al:ZnO radial

p-n junction nanowire arrays

Jow-Lay Huang

1,4,*

_{, Chien-Lin KuO}

1

_{, Ruey-Chi Wang}

2

_{, Chuan-Pu Liu}

1,3

1_{Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan}

70101

2_{Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung,}

Taiwan 81148

3_{Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, Taiwan} 4_{Department of Polymer Materials and Graduate School of Green Materials, Kun Shan University,}

Tainan Hsien 710, Taiwan [email protected] Nanotechnology 2009, 20, 365603

Z

inc oxide (ZnO) is a promising natural n-type semiconductor with a direct wide energy gap of 3.37 eV and a large exciton binding energy of 60 meV [1] at room temperature. While ZnO p-n homojunction devices can be used for light emitting diodes [2] and gas sensors [3], no such devices have been realized in the form of nanowires yet, due to the challenges of p-type doping and nanofabrication.

Alternatively, p-n heterojunctions are another approach with which more degree of freedom can be designed into the devices. Cuprous oxide (Cu₂O) is a natural p-type semiconductor with a direct energy gap of 2.1 eV [4], and is also a technologically important material because the theoretical energy conversion efficiency of a Cu₂O

solar cell has been predicted to be around 20% [5]. Consequently, there is interest in fabricating p–n Cu₂O–ZnO 1D nanomaterials for optoelectronic and photovoltaic applications. Although p-Cu₂O/n-ZnO multi-layers and planar solar cells have been realized [6], it remains a challenge to fabricate 1D nano-sized radial p-Cu₂O/n-ZnO heterojunctions.

In this work, we demonstrate the fabrication of vertically-aligned large-area Cu₂O/Al:ZnO radial p-n junction nanowire arrays for the first time. The well-defined current rectifying behaviors of the nanodiodes are

demonstrated. In addition, the optical properties of the ZnO nanowires can be modulated by capping Cu₂O nano-shells on the surfaces of ZnO nanowires. The radial Cu₂O/ZnO p-n heterojunction nanowire arrays thus produced pave the way for developing ZnO-based nano-pixel optoelectronic devices and solar cells.

The Al doped ZnO nanowires arrays were then synthesized on the pre-grown ZnO seed layer via thermal chemical vapor deposition (CVD). Zn (purity: 99.8%, 100 mesh) and Al (purity: 99.9%, 100 mesh) mixed powders (weight ratio=93:7) were placed in an aluminum boat located inside a 1 in. diameter horizontal quartz tube reactor. Argon was introduced as the carrier gas at the beginning with a flow rate of 8 sccm, and the working pressure was kept at 50 Torr. The sources were heated at a rate of 20℃/min from room temperature to 500℃ and held for 30 minutes to promote the alloying of Al and Zn, which will enhance the Al doping of ZnO nanowires. After the alloying

treatment, the pressure was decreased to 1 Torr and the system was heated again at a rate of 20℃/min to 650℃. Once the temperature was raised to 650℃, oxygen was introduced into the chamber with a flow rate of 1 sccm. 1 of 3

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After heating at 650℃ for one hour, the substrate was slowly cooled down to room temperature in the furnace. Cu₂O nano-shells were deposited on the periphery of pre-grown Al:ZnO nanowires in an e-beam evaporation system with electron energy of 8 keV in vacuum using a Cu target. Oxygen was introduced as the reactive gas with a flow rate of 50 sccm, and the working pressure was kept at 6.4 × 10-5_{torr. The film growth rate was around 0.8Å} s-1_.

Figure. 1. (a) A low-magnification TEM image of an Al: ZnO nanowire. (b) A corresponding diffraction pattern of an Al:ZnO nanowire. (c) A HRTEM image of an AZO nanowire. (d) A low-magnification TEM image of

the p-Cu₂O/n-AZO radial heterostructure. (e) A

corresponding diffraction pattern of the Cu₂O thin shell with the inset showing the HRTEM image of the shell layer.

Figure 1(a) shows a low-magnification TEM image of an Al:ZnO nanowire synthesized by thermal-CVD. The uniform contrast indicates uniform thickness over individual nanorods. A corresponding diffraction pattern in Figure 1(b) reveals that the AZO nanowires are single-crystalline wurtzite structures growing along the c-axis. A high-resolution TEM image of a

nanowire, as shown in Figure 1(c), confirms that the d-spacing of ZnO(0002) nanowire is 2.6 Å. The atomic ratio of Al to (Al+Zn) in the nanowire calculated from TEM-electron energy loss spectroscopy (TEM-EELS) is around 1.19 at.%. Figure 1(d) shows a

low-magnification TEM image of a coated nanowire exhibiting a radial core-shell heterostructure with thinner shells and thicker ends. The image also reveals that the interface between the core and shell is abrupt, indicating no obvious elemental intermixing. The thickness of the shell layers is around 20 nm, consistent with the SEM results. A corresponding diffraction pattern in Figure 1(e) confirms that the nano-shells are poly-crystalline cubic structures of Cu₂O and the core-shell nanowires are p-Cu₂O/n-AZO radial heterostructures. The inset in Figure 1(e) is a HRTEM image of the shell layer, where the fringe

spacing corresponding to the d-spacing of Cu₂O(111) is 2.47 Å.

Figure. 2. (a) Schematic illustration of p-n junction

Figure 2(a) shows a schematic diagram of the p-n junction device, while Figure 2(b) shows the SEM cross-sectional image of the real p-Cu₂O/n-AZO radial nanowire heterostructure devices with two electrodes on top and bottom. The bottom electrode is a highly-doped Si (111) substrate with a ZnO buffer layer connected to the n-AZO cores. The top electrode connected to the p-Cu₂O shells is a Pt layer containing slanted columnar structures, due to deposition by an oblique angle sputter deposition technique. Figure 2(c) is the I–V characteristics of the p-Cu₂O/n-AZO radial nanowire arrays. It is found that a obvious turn on

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device. (b) The SEM cross-sectional image of the

p-Cu₂O/n-AZO radial nanowires heterostructures devices.

(c) The I-V characteristics of the p-Cu₂O/n-AZO radial nanowires heterostructures. (d) The C–V characteristic

of the p-Cu₂O/n-AZO junction diode

occurs at around 2 V. The forward current of the device is around 0.2 mA with +10 V applied, and the reverse leakage current is only 0.24 uA when -10 V is applied. The rectifying behavior confirms that the p-Cu₂O/n-AZO radial nanowires behave as well-defined p-n diodes. The C–V characteristic of the p-Cu₂ O/n-AZO junction diode was measured at the frequency of 1 MHz and is plotted in the form of 1/C2_{–V, as shown in} Figure 2(d). The plot shows linear behavior, indicating that the diode is an abrupt junction. Therefore, this work not only provides a simple method to adjust the optical properties of ZnO nanowires, but also demonstrates an effective fabrication of p-n radial nanowire arrays, which are promising for use in nano-pixel electronic devices and solar cells.

In summary, p-Cu₂O/n-AZO radial nanowire arrays were fabricated on silicon without using catalysts via a combined method of PVD and thermal CVD. This work provides a simple method to fabricate superior p-Cu₂ O/n-AZO radial nanowire arrays, which have good potential for developing nano-pixel optoelectronic devices and solar cells.

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