1 Introduction

ZnO is one of the promising materials for an UV light emitter because of its merits as a wide band gap of 3.37 eV and a large exciton energy of 60 meV at room temperature. However, it is very difficult to fabricate p-type ZnO because of the self-compensating effect from native defects [1–3]. In recent studies, doping of the group V elements N [4, 5], P [6], As [7, 8] and Sb [9, 10] have been performed to fabricate the p-type ZnO. In the group V elements, N is seemed to be suitable for synthesis of p-type ZnO because its atomic radius is similar to that of O. However, it was reported that fabrication of p-type ZnO by N-doping is difficult as N is a deep acceptor in ZnO crystals [11]. On the other hand, reports on Sb doping are increasing recently. It has been predicted that Sb would occupy the Zn site and simultaneously induce two Zn vacancies to form a complex (SbZn-2VZn), which is serving as an acceptor with both low formation energy and low ionization energy [12]. Sb-doped ZnO is fabricated by using chemical vapor deposition [13, 14], pulsed laser deposition [15], molecular beam epitaxy [16], and hydrothermal method [17]. However, there were few reports about the doping in ZnO using a laser doping technique [18].)

In this study, we performed laser doping of Sb into ZnO nanowires for realization of Sb-doped p-type ZnO, where laser doping was performed in Sb nanoparticles-dispersed ethanol solution. ZnO nanowires were grown on an a-cut sapphire substrate by nanoparticle-assisted pulsed laser deposition (NAPLD), and Sb nanoparticles (NPs) were fabricated by laser irradiation in ethanol. Here, we report the morphological, optical, and electronic properties of Sb-doped ZnO nanowires after laser doping.

2 Experimental procedure

2.1 Preparation of ZnO nanowires

ZnO nanowires for laser doping were fabricated by NAPLD, which had been reported [19]. A sintered ZnO target with 99.99 % purity was used as a source material in synthesizing ZnO nanowires. This material was ablated with the third harmonic of a Nd:YAG laser with a fluence of 1.5 J/cm2 and a repetition rate of 10 Hz. The substrate-target distance was fixed at 40 mm. Before synthesis of ZnO nanowires, a ZnO buffer layer, which works for aligned growth of the nanowires with low density, was prepared by pulsed laser deposition (PLD) [19]. The buffer layer was deposited on the sapphire substrate with an oxygen gas pressure of 3.0 Pa and a substrate temperature of 500 °C for 10 min. After deposition of the buffer layer, ZnO nanowires were synthesized with an argon gas pressure of 27 kPa and a substrate temperature of 750 °C for 10 min, where nanoparticles generated in the laser ablation in a high pressure play an important role in the growth of ZnO nanowires.

Figure 1 shows the scanning electron microscope (SEM) image of the ZnO nanowires synthesized by NAPLD. Vertically aligned ZnO nanowires were grown on the buffer layer. The hexagonal pyramid structure formed at the bottom of each nanowire acts as a nucleus for the growth of the nanowire. The nanowires have an average length of 2 μm and an average diameter of 100 nm. We have confirmed that each ZnO nanowire consists of a single crystal [20].

Fig. 1
figure 1

SEM image of the ZnO nanowires

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2.2 Fabrication and characterization of Sb NPs

Sb NPs were synthesized by laser ablation in ethanol. The third harmonic of a Nd:YAG laser (355 nm, 250 mJ/cm2, 10 Hz) was used as the ablation source. Sb powders (10 mg, average particle size of 30 µm) and ethanol (3 ml) were contained in a quartz cell (1 cm × 1 cm × 5 cm) and were laser-irradiated for 15 or 30 min under continuous stirring with a magnetic stirrer. The color of the Sb-dispersed ethanol changed from silver gray to brown after laser ablation. Large-sized Sb powders were precipitated to the bottom after stopping the stirring for 60 min, and then, supernatant liquid which contains the Sb NPs was decanted off.

2.3 Laser doping into ZnO nanowires using Sb NPs

Using the Sb NPs dispersed in ethanol, laser doping with a pulsed laser of the third harmonic of a Nd:YAG laser (355 nm, 5 ns) was performed in half of the area. As the absorption coefficient of ZnO at 355 nm is around 2 × 105 cm−1 [21],) the absorption length of 50 nm is estimated with the doping laser beam. In the previous report, it is demonstrated that tips of the ZnO nanowires are melted and changed into a spherical shape by laser irradiation [22]. Therefore, the rapid heating and cooling process by the laser irradiation can be expected to apply to the Sb doping into ZnO. Before laser doping, a spin-on-glass (SOG) layer was prepared to expose only tips of the ZnO nanowires to the Sb-dispersed ethanol. Figure 2 shows the schematic of laser doping. ZnO nanowires were immersed in the Sb-dispersed ethanol with a depth of 1.5 mm, and the laser beam at a fluence of 100 mJ/cm2 was irradiated with ten shots at intervals of 10 s. The doping fluence was fixed in considering of the light extinction by the Sb-dispersed ethanol with a thickness of 1.5 mm, which was approximately 9.1 % at 355 nm. By this process, p-n homojunction can be expected to form in each single-crystalline ZnO nanowire.

Fig. 2
figure 2

Schematic of laser doping

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3 Results and discussion

Figure 3 shows the particle size distribution of the Sb powders (before laser irradiation) and the Sb NPs (after laser irradiation for 15 min) measured by a particle size analyzer. The particle size distribution of the Sb NPs with laser irradiation times of 30 min was too small to measure by the analyzer. Figure 4 shows the transmission electron microscope (TEM) images of Sb NPs with an irradiation times of 15 and 30 min. It is found that the size of Sb NPs was decreased with increase in the irradiation time, which suggests that the size distribution of Sb NPs can be controlled by the laser irradiation time. Typical sizes of Sb NPs were 50 and 30 nm for the ablation time of 15 and 30 min, respectively.

Fig. 3
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Particle size distribution of the Sb powder (before laser irradiation) and NPs (after laser irradiation for 15 min)

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Fig. 4
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TEM images of the Sb NPs with irradiation times of 15 min and 30 min

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Figure 5 shows the SEM images of the as-grown, SOG-coated, and laser-doped ZnO nanowires. The tips of the laser-doped ZnO nanowires were changed into spherical shapes due to melting and resolidification. Figure 6 shows the photoluminescence (PL) spectra of the ZnO nanowires at each process. After SOG coating, PL emission in UV region was enhanced which was attributed to the surface passivation by the SOG (SiO2) layer. On the other hand, PL emission of the laser-doped ZnO nanowires in UV region was significantly enhanced and that in visible region was decreased which were attributed to laser annealing effect. The shoulder peak around 386 nm is provably P-band emission, which is exciton–exciton scattering [23]. The UV emission peak of 372 nm was slightly redshifted to 376 nm after laser doping. The redshift of the UV peak after Sb doping was reported by several researchers [24, 25]. The redshift to lower energy was the narrowing in the band gap with Sb doping, which may be due to the strong sp-d exchange interactions between the band electrons of ZnO and the localized ‘‘d’’ electrons of Sb3+ ions [25]. In addition, green emission of 530 nm was also redshifted to 560 nm after laser doping. It originates from the recombination of holes with the electrons occupying the singly ionized O vacancies [24, 25].

Fig. 5
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SEM images of a as-grown, b SOG-coated and c laser-doped ZnO nanowires

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Fig. 6
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PL spectra of as-grown ZnO nanowires, after SOG coating, and after laser doping

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An I–V characteristic of the Sb-doped ZnO nanowires was investigated using needle electrodes, which are a tungsten needle on the n-side and a gold-coated tungsten one on the p-side for Ohmic contact. Each needle electrode was attached to tip of the non-doped ZnO nanowires and of Sb-doped nanowires. Figure 7 shows I–V characteristic of the non-doped ZnO nanowires/Sb-doped ZnO nanowires. Rectifying property was observed and a threshold voltage was 4.5 V, suggesting that Sb-doped p-type ZnO nanowires were fabricated by laser doping. However, it is not enough to verify fabrication of p-type. Some researchers have proved the Sb-doped p-type ZnO by Raman spectroscopy [26] and low-temperature PL [27]. Thus, these measurements will be taken and provided in the future research. Electroluminescence (EL) was not observed in the experimental configuration using needle electrodes. EL property of the p-n homojunction ZnO will be also investigated in the future work.

Fig. 7
figure 7

I–V characteristic of non-doped ZnO nanowires/Sb-doped ZnO nanowires

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4 Conclusions

To summarize, we have succeed to fabricate the Sb-doped ZnO nanowires by laser doping using Sb NPs. ZnO nanowires synthesized by NAPLD were a single crystal and high optical quality. Sb NPs were fabricated by laser irradiation in ethanol. The Sb NPs have an average size of 30 nm for an irradiation time of 30 min. PL emissions of the Sb-doped ZnO nanowires in UV region were strongly enhanced after laser doping which was attributed to the rapid laser annealing effect. Furthermore, a rectifying property with a threshold voltage of 4.5 V was observed between non-doped ZnO nanowires and Sb-doped ZnO nanowires. Thus, Sb NPs and laser doping technique show potential for realization of p-type ZnO nanowires.