1 Introduction

Random laser, a unique light source, can be realized through the multiple light scattering in disordered optical gain materials, which makes it have advantages over the conventional laser such as simple processing technology and low cost [1,2,3]. Due to its broad angular distribution, random laser has the potential application in display and lighting. Besides, it is also suitable for data storage, communication, medical diagnostics, etc [4, 5]. Zinc oxide (ZnO) has been proved to be the excellent material for random laser by many researches in recent years. It possesses the characteristics of direct bandgap, large exciton binding energy, abundant nanostructures, and high refractive index, which meet the fundamental requirement for generating random laser [6,7,8,9,10,11,12,13,14,15,16,17]. At present, the ZnO-based device structures for electrically pumped random laser are mainly classified into two types, i.e., heterojunction [6,7,8,9,10], and Schottky junction [11,12,13,14,15,16,17]. Especially, the Schottky junction owns the outstanding advantage of simple fabrication procedure, which makes it become a better alternative device structure for ZnO random laser. Till now, there are lots of reports on ZnO Schottky random laser devices. These devices are based on vavious ZnO nanostructures, such as nanoparticle [12], nanowire [13, 14], nanorod [16], and nanocolumn [17]. Zhu et al. have demonstrated that the electrically pumped random laser can be realized in the ZnO nanoparticle with the Au/i–ZnO/MgO/ZnO Schottky structure. The threshold current of the laser is about 6.5 mA, and the FWHM of the laser peaks can reach less than 0.8 nm [12]. Liu et al. have constructed the Au/MgO/ZnO-nanowire Schottky structure, and the electrically pumped random lasing has been observed from the structure. The threshold current of the laser is around 7.6 mA, and the FWHM of the laser peaks can reach 1.0 nm [13]. Liu et al. have reported that ectrically pumped near-ultraviolet lasing can be achieved in the ZnO/MgO core/shell nanowire with the Au/MgO/ZnO Schottky structure. The lasing peaks are observed in a spectral range from 390 to 415 nm, and their linewidths are less than 0.6 nm. The lasing threshold current density is about 2.3 A/cm2 [14]. Ma et al. have realized an electrically pumped random laser from the Au/SiO2/ZnO-nanorod-array Schottky structure fabricated on Si, whose threshold current is around 88 mA [16]. Liu et al. have fabricated the Au/MgO/ZnO-nanocolumn Schottky structured random laser device on copper substrate, whose lasing threshold is determined as 29.3 A/cm2 [17].

From the above-mentioned cases, it can be noted that most of reported ZnO Schottky random laser devices employ the insulating layer (e.g., MgO or SiO2), which increases the difficulty and complexity in device fabrication. In this paper, electrically pumped random laser has been demonstrated in the Au/ZnO nanocolumn/Au back-to-back Schottky structure. The threshold of the lasing is about 2 A/cm2, which is among the lowest threshold current of ZnO random lasing. The lasing peaks are randomly distributed between 375 and 420 nm, whose full width at half maximum can reach as narrow as 2 nm. Compared with the ZnO Schottky random laser devices with the insulating layer, the device in our case has a simple preparation process. Besides, due to its symmetric back-to-back Schottky structure, it can be operated without considering its electrode polarity. Hence, the proposed device provides a potentially valuable way to realize electrically pumped random laser.

2 Experimental

A magnetron sputtering system was used to deposit the ZnO nanocolumn on top of c-sapphire substrate. The sputtering parameters were listed as follows: metal zinc target (99.999%), the sputtering gas chosen as oxygen and argon, the sputtering power set as 100 W, the growth temperature set as 300 °C, and the chamber pressure fixed at 1 Pa. Then the Au film was coated on the top of ZnO nanocolumn in a vacuum coating equipment. After that, the interdigital Au electrodes were constructed using the photolithography and wet etching method. The surface and cross-sectional morphology of the ZnO nanocolumn were characterized by scanning electron microscope (SEM) (Hitachi S4800). The X-ray diffraction (XRD) was performed to characterize the structure of the ZnO nanocolumn using a X-ray diffractometer (Bruker D8). The optical absorption of the ZnO nanocolumn was measured in an UV–Vis spectrophotometer (Shimadzu UV-3101). The photoluminescence (PL) spectrum of the ZnO nanocolumn was investigated using He–Cd laser (325 nm). The electrical characteristic of the device was studied using a Hall test equipment (Lakeshore 7707). The electroluminescence (EL) measurement of the device was performed under the drive of a continuous current source using a spectrometer (Hitachi F4500). All the above measurements were carried out under the condition of room temperature.

3 Results and discussion

The sectional morphology of the ZnO nanocolumn is shown in the inset of Fig. 1a. One can see that closely packed ZnO nanocolumn is grown vertically on the substrate, and the mean length of the ZnO nanocolumn can reach 200 nm. The surface morphology of the ZnO nanocolumn is shown in Fig. 1a. By analyzing 106 nanorods from the surface SEM image of the ZnO nanocolumn, the mean diameter of ZnO nanocolumn can be determined to be 50.0 ± 6.5 nm (σ = 13%), which is shown in the Supplementary Fig. 1. It has been reported that the nanocolumn is beneficial for generating random lasing since the nanostructure can act as a random medium for light scattering [18]. The XRD pattern of the ZnO nanocolumn is illustrated in Fig. 1b. There are two diffraction peaks located at 34.36° and 41.66° in the pattern, which correspond to the (002) facet of wurtzite ZnO and the sapphire substrate, respectively. The XRD data indicates that the preferential growth of the ZnO nanocolumn is along c-axis. The full width at a half maximum of the (002) peak is around 0.34°, which indicates that the ZnO nanocolumn has a good crystalline quality. The inset of Fig. 1c presents the PL spectrum of the ZnO nanocolumn. A strong emission appers at around 375 nm, which is related to the near band edge emission of ZnO material, and a weak emssion centered at about 524 nm can be observed, which corresponds to the defect-related emission of ZnO material. The absorption spectrum of the ZnO nanocolumn is presented in the Fig. 1c. The excitonic absorption peak of ZnO material located at about 364 nm can be observed, which is attributed to the good crystalline quality of ZnO nanocolumn.

Fig. 1
figure 1

a The surface SEM image of the ZnO nanocolumn, and the inset shows the cross-sectional SEM image of the ZnO nanocolumn. b The XRD pattern of the ZnO nanocolumn. c The absorption spectrum of ZnO nanocolumn, and the inset shows the PL spectrum of the ZnO nanocolumn

Full size image

The schematic illustration of the device is presented in the inset of Fig. 2. The electrode length is 500 µm, and the electrode width and gap between electrodes are both 5 µm, and the active area of the device is about 1.2 × 10− 3 cm2. Since the electron affinity potential of ZnO (~ 4.35 eV) has the discrepancy with the Au work function (~ 5.1 eV), a Schottky barrier will be generated at the interface of Au/ZnO junction [19]. Therefore, the Au/ZnO nanocolumn/Au structure is regarded as two Schottky barriers connected back to back [20]. As a bias is applied to the structure, one Schottky barrier is reverse-biased (at the cathode) and the other one is forward-biased (at the anode). The total current flowing through the device is composed of the electron current generated by the injected electron from the reverse biased Schottky contact and the hole current formed by the injected hole from the forward biased Schottky contact. The I–V characteristic of the structure is shown in Fig. 2. It can be noted from the I–V curve that the structure shows the obvious rectifying performance. Also, the structure exhibits almost symmetric I–V curve in forward and reverse bias operation, which indicates that Schottky barrier height at the two Au/ZnO contacts is almost identical in our case.

Fig. 2
figure 2

The I–V characteristic of the device, with the inset showing a schematic diagram

Full size image

Figure 3a shows the EL spectra in the wavelength range of 330–650 nm for the device under the drive of a continuous current. It can be observed that the weak spontaneous emission appears in the spectrum under a current density of 1.2 A/cm2. As the injected current is increased to 2 A/cm2, the spontaneous emission evolves into a few discrete sharp peaks. With the increase of the injection current, the intensity of the sharp peaks becomes stonger. Further increase of the injected current density to 6.7 A/cm2, the intensity of the sharp peak located at 386 nm increases remarkably, whose full width at half maximum can reach as narrow as 2 nm. From the above phenomenon, we can conclude that random lasing has been obtained in the device [6,7,8,9,10,11,12,13,14,15,16,17]. The dependence of the integrated emission of the device on the injection current is shown in Fig. 3b. The threshold of the lasing in our case can be determined to be about 2 A/cm2, which is among the lowest threshold current of ZnO random lasing [6,7,8,9,10,11,12,13,14,15,16,17]. To verify the random lasing action obtained from the structure, the schematic diagram of the angle-dependent EL measurement configuration is shown in Fig. 4a, and the EL spectra of the device from different detection angles have been investigated under a current density of 6.7 A/cm2, which is presented in Fig. 4b. It can be noted that the spectra obtained from different detection angles exhibit different lasing peaks, which accords with the typical feature of a random laser. For the study on time stability of laser radiation, the EL spectra of the device driven at a current density of 6.5 A/cm2 are taken at three successive measurements, as shown in the Fig. 5. In the three spectra, it can be observed that the number, the position, and the intensity of the sharp peaks change randomly. This temporal chaotic behavior is the typical feature of a random laser.

Fig. 3
figure 3

a The EL spectra of the device under the drive of a continuous current. b The dependence of the integrated emission of the device on the injection current

Full size image
Fig. 4
figure 4

a The schematic diagram of the angle-dependent EL measurement configuration. b The EL spectra of the device from different detection angles under the injection current density of 6.7 A/cm2

Full size image
Fig. 5
figure 5

EL spectra of the Au/ZnO nanocolumn/Au structure under a current density of 6.5 A/cm2 obtained in three successive measurements

Full size image

To explain the lasing mechanism, the band diagram of the Au/ZnO nanocolumn/Au structure under a bias voltage is illustrated in Fig. 6. The electron injected into ZnO nanocolumn is mainly from the reverse-biased Au/ZnO Schottky contact, and the hole injected into ZnO nanocolumn is mainly from the forward-biased Au/ZnO Schottky contact [20]. As a result, the injected electron and hole will recombine radiatively within the ZnO nanocolumn. Subsequently, the light generated in the ZnO nanocolumn may experience a strong scattering process among the nanocolumn boundaries with air gaps since the refraction index of ZnO (n = 2.45) is much higher than that of air (n = 1.0) [12]. A closed-loop resonant cavity will be formed on condition that the scattered light returns to its initial position. In this resonant cavity, the light can be amplified. As the injection current density is increased to 2 A/cm2, the optical gain exceeds the optical loss in the closed-loops, and eventually random lasing is achieved.

Fig. 6
figure 6

The band diagram of the Au/ZnO nanocolumn/Au structure under a bias voltage

Full size image

4 Conclusions

In summary, the electrically pumped random laser has been demonstrated in the Au/ZnO nanocolumn/Au back–to-back Schottky structure. The threshold of the lasing is about 2 A/cm2, which is among the lowest threshold current of ZnO random lasing. The lasing peaks are randomly distributed between 375 and 420 nm, whose full width at half maximum can reach as narrow as 2 nm. The results reported in this paper indicate that ZnO nanocolumn based on back–to-back Schottky structure is feasible to realize random laser.