Multipeak Emissions and Electrical Properties of ZnO/Si Heterojunctions Based on ZnO Nanoflakes by Spin Coating Technique

ZnO/Si heterojunctions have been fabricated by spinning ZnO nanoﬂakes on the p-type single crystal silicon by using the spin coating technique. Photoluminescence spectra of as-grown and annealed ZnO/Si heterojunctions have been recorded under the excitation of 336nm. Multipeaks between ∼ 360nm and ∼ 430nm from annealed ZnO/Si heterojunctions have been analyzed, the origins of which have been ascribed to the eﬀects of one or multiple LO phonons. The rectifying eﬀects can be observed from the prototypical devices based on ZnO/Si heterojunctions. Although the parameters obtained by analyzing the current density-voltage characteristics are away from those from the ideal device, it is believed that ZnO/Si heterojunctions in the present work will be a potential candidate in the optoelectronic ﬁeld through modulating and optimizing the fabrication conditions.


Introduction
As an important functional oxide semiconductor, nanostructured ZnO has offered an interesting prospect in the fields of optoelectronic devices by manipulating the morphology, doping, and structural composition [1][2][3][4][5][6]. In the perovskite solar cells, ZnO has been used as an alternative to TiO 2 for its high efficiency [7]. Based on the p-type silicon, ZnO has been used as an n-type material to fabricate the ZnO/Si heterojunctions. Solar cells based on ZnO/Si heterojunctions have been researched extensively by many groups, and some exciting results have been obtained [8][9][10]. But the efficiency of photovoltaic devices based on ZnO/Si heterojunctions is still unsatisfactory. In order to improve the performance, for instance, Pietruszka and Knutsen have incorporated a Zn 1-x Mg x O layer into the ZnO-Si interface to modulate the conduction band offset, which can lead to a diminishing impact of recombination centers at the interface [9,10]. Hussain has deposited amorphous ZnO at the interface of ZnO-Si to improve the interface passivation and decrease the oxide formation, and the open circuit voltage shows more than 20% improvement [8]. Zhong's group has doped Al in ZnO to construct an efficient electron-selective contact to enhance the performance [11]. In the study of light emitting diodes, Fang's research group has incorporated HfO 2 into the heterojunction interface to improve the light emission of ZnO/Si heterojunctions [12][13][14]. Another way to improve device performance is to fabricate highquality ZnO to construct the high efficiency optoelectronic devices. e pulsed laser deposition and metal organic chemical vapor deposition have been applied, which are expensive and carried out in the high vacuum atmosphere [15]. e spin coating technique is a simple method, which is characterized with many benefits, for example, cost effectiveness, absence of vacuum, and ease of fabrication of large area film on simple or complex substrates [16,17].
In the present work, ZnO/Si heterojunctions have been fabricated through the spin coating technique. Under the excitation of 336 nm, photoluminescence (PL) spectra of asgrown and annealed ZnO/Si heterojunctions have been recorded, and the origins of multipeaks have been investigated. And, the electronic parameters of prototypical device based on annealed ZnO/Si heterojunctions have been analyzed.

Experiments
e preparation of ZnO nanoflakes is as follows. First, the analytical reagents, zinc nitrate (0.015 mol) and sodium hydroxide (0.06 mol), were mixed into 70 ml deionized water. e mixture was shifted into an autoclave which was filled with ∼70% solution and fixed in the oven. e temperature in the oven rose from room temperature to 160°C for 30 min. After lasting for 2 h at 160°C, the autoclave was taken out from the oven. After natural cooling for 1.5 h, the white precipitant in the autoclave was washed three times with the deionized water, and then it was filtered and dried. And then, the white precipitant was added into the ethanol and stirred for 20 min at room temperature. e p-type single crystal silicon was placed on the spin coating platform, and then the solution was dropped on the silicon substrate. e spin coater was set at 2000 rpm for 5 min. e abovementioned process was repeated until the required thickness was obtained. e coated silicon substrate was dried in air atmosphere for 1 h and then split into two parts. For comparison, one of them was annealed at 300°C in argon atmosphere for 2 h. In order to investigate the electrical properties of ZnO/Si heterojunctions, the Ag electrodes were fabricated on the ZnO thin films and silicon, respectively, by using the DC magnetron sputtering method, and sandwich structure of Ag/ZnO/Si/Ag was constructed. e morphological and structural properties of the ZnO nanoflakes, as-grown and annealed ZnO/Si heterojunctions, were characterized by field emission scanning electron microscopy (FE-SEM, JSM 6700F) at an acceleration voltage of 15 kV; high-resolution transmission electron microscopy (HR-TEM, JEM-2100F) at an acceleration voltage of 200 kV; and X-ray diffraction (XRD, Panalytical X'Pert Pro) with Cu-Kα as the X-ray source (λ �1.5046Å). e absorption spectrum of the annealed ZnO/Si heterojunctions was obtained using a UV-vis-IR spectrophotometer (Shimadzu, UV-3150) with an integrating sphere detector. PL spectra were measured using a double grating spectrofluorometer (HORIBA, FL3-22), with a Xe lamp as the excitation source. e current density-voltage (I-V) properties were carried out at a RST 5200 electrochemical workstation (SRS Instrument Inc., China).

Results and Discussion
XRD patterns of ZnO nanoflakes, as-grown and annealed ZnO/Si heterojunctions, are shown in Figure 1(a). e peaks located at ∼31.7°, ∼34.4°, ∼36.2°, ∼47.5°, ∼56.6°, ∼62.9°, and ∼67.9°correspond to the diffractions of (100), (002), (101), (102), (110), (103), and (112) planes from ZnO (JCPDS: 01-089-7102), respectively. rough contrasting XRD patterns of as-grown and annealed ZnO/Si heterojunctions, the full width at half maximum (FWHM) of diffraction peaks from annealed ZnO/Si is much less than that of as-grown ZnO/Si heterojunctions. It is indicated that the size of nanostructural ZnO from ZnO/Si heterojunctions is increased after annealing treatment. e inset of Figure 1(b) shows the morphology of annealed ZnO/Si heterojunctions. In order to study the fine structure, ZnO has been peeled off from the annealed ZnO/ Si heterojunctions and been shifted to a copper net to measure. Figure 1(b) gives the TEM image of ZnO nanoflakes. e HR-TEM images of locations marked by asterisk and pentagram in Figure 1(b) are shown in Figures 1(c) and  1(d), respectively. From Figures 1(c) and 1(d), we can observe many zones with the obvious lattice fringes of which distances are measured and shown. e lattice fringes with ∼0.278 nm and ∼0.259 nm can be obtained, corresponding to the (100) and (002) planes, respectively. From SEM, TEM, and HR-TEM images, it is concluded that the size contribution of ZnO nanoflakes is large by using the provided prepared method in the present work. How to obtain the uniform size of ZnO nanostructure will be done in our future work.
Under the excitation of 336 nm, two emission bands from PL spectra of as-grown and annealed ZnO/Si heterojunctions can be obtained and illustrated in Figure 2(a). e UV emission band and blue-green emission band are located between ∼360 nm and ∼430 nm and between ∼430 nm and ∼600 nm, respectively. e inset of Figure 2(a) gives the absorptance spectrum and the plot of (F(R)hv) 2 vs. hv from annealed ZnO/Si heterojunctions [18]. rough analyzing the plot of (F(R)hv) 2 vs. hv, the band gap of annealed ZnO/ Si heterojunctions can be calculated to be ∼3.316 eV (∼374 nm) [19,20]. So, it is believed that the UV emission is ascribed to the emission of band gap from ZnO nanoflakes. However, a significant red-shift compared to the calculated band gap can be observed, which is attributed to more surface defects [21]. Oxygen vacancies are located at ∼0.9 eV above the valence band of ZnO [22][23][24][25]. Combined with the obtained band gap of ZnO nanoflakes, we think that the blue-green emission is originated from the transition from the conduction band to oxygen vacancies. For PL spectrum of as-grown ZnO/Si heterojunctions, the intensity of UV emission is much less than that of blue-green emission. After the annealing treatment has been carried out, however, the intensity of UV emission has greatly been improved. e peak positions of UV and blue-green emissions are independent of the annealed treatment. While for PL of annealed ZnO/Si heterojunctions, multiple peaks can be observed between ∼360 nm and ∼430 nm, which are located at ∼375 nm (∼3.307 eV), ∼385 nm (∼3.221 eV), ∼395 nm (∼3.139 eV), ∼406 nm (∼3.054 eV), and ∼418 nm (∼2.967 eV) from short wavelength to long wavelength, respectively. e energy differences between adjacent peaks can be estimated to be ∼86 meV, ∼82 meV, ∼85 meV, and ∼87 meV, respectively. In the works of Dingle and Reynolds [26,27], LO phonon of ∼72 meV had been discussed in the emission bands of ZnO. In the present work, the multiple LO phonons have been ascribed to the multipeaks between ∼360 nm and ∼430 nm [14,28,29].
In order to investigate the emission dynamics, [30] the decay curves of the PL spectrum from annealed ZnO/Si heterojunctions at ∼393 nm and ∼500 nm under the 450 nm 2 Journal of Nanotechnology excitation are analyzed and shown in Figure 2(b). e data are fitted by the biexponential decay function as follows [29,31]:  where A(t) represents the emission band intensity and A 1 and A 1 are the relative weights of two exponential decays with the time constants τ 1 and τ 2 , respectively. rough analyzing equation (1), the estimated decay time constants τ 1 and τ 2 are ∼273.7 ps and ∼240.7 ps for the UV emission band and ∼61.8 ns and ∼8.05 ns for the blue-green emission band, respectively. For the UV emission band, the lifetime of ∼273.7 ps is attributed to bound exciton recombination, and the lifetime of ∼240.7 ps originates from free exciton recombination [29]. For the blue-green emission band, the lifetimes of ∼61.8 ns and ∼8.1 ns originate from the defects near the ZnO nanoflake surface [32].
I-V characteristics of as-grown and annealed Ag/ZnO/ Si/Ag heterojunctions have been measured at room temperature and shown in Figure 3(a). From Figure 3(a), the obvious rectification effects are observed. For the as-grown ZnO/Si heterojunctions, the forward current density of ∼0.05 mA/cm 2 has been obtained at the forward applied voltage of ∼6.36 V. For the annealed ZnO/Si heterojunctions, however, at the forward applied voltage of ∼6.36 V, the forward current density is ∼0.42 mA/cm 2 . And, it can be also observed that the onset voltage at the current density of ∼1.0 mA/cm 2 is decreased from ∼1.46 V to ∼0.81 V after the annealing treatment.
In order to study the parameters of prototypical devices based on ZnO/Si in the present work, the plots of dV/d(lnI)-I by analyzing the I-V characteristics are shown in Figure 3(b), which can be expressed as follows [33]: where I is the current density, R S is the series resistance, n is the ideality factor, k is the Boltzmann constant, q is the electron charge, and T is the temperature which is set to 300 K in the present work. According to equation (2), the slopes and intercepts can be obtained by using the linear fit and are shown in Figure 3(b). e series resistances can be estimated from the slops of fitted lines and be calculated to be ∼15.34 Ohm/cm 2 and ∼14.5 Ohm/cm 2 for as-grown and annealed ZnO/Si heterojunctions, respectively. And, the ideality factors, which can be estimated from the intercept of fitted line [34], are ∼53.7 and ∼37.2, respectively. It is indicated that the annealed treatment can enhance the performance of ZnO/Si heterojunctions. Although the obtained parameters, such as ideality factor, are away from the those of ideal devices [35], it is believed that the performance based on ZnO/Si heterojunctions by the spin coating technique at the present work can be improved through modulating and optimizing the preparation conditions.

Conclusion
ZnO/Si heterojunctions have been fabricated through the spin coating technique. In the UV emission band of annealed ZnO/Si heterojunctions, multipeaks have been observed. e origins of emission bands have been discussed in detail. After annealing treatment, the performance of ZnO/Si heterojunctions device has been improved. However, the obtained electrical parameters are away from those of the ideal device. It is believed that ZnO/Si heterojunctions in the present work will be a promising candidate in the optoelectronic device fields through optimizing the preparation conditions.

Data Availability
e data used to support the finding of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.