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The effect of electric field on near-band-edge (NBE) photoluminescence (PL) of a sol-gel derived ZnO film has been investigated via a SiO2/ZnO/SiOx(x<2) double-barrier structure on Si under different forward biases. A forward current-voltage curve is characterized by a negative-differential-resistance (NDR) region, which follows a normal region. With an increase of forward bias the NBE PL of the ZnO film is enhanced in the normal region, but it is attenuated in the NDR region. The increase of forward bias also causes the NBE PL of the ZnO film to blueshift from ~377.6 to ~374.9 nm no matter how current changes. The mechanism for the effect of bias on the intensity and position of NBE PL of the ZnO film is discussed.
©2009 Optical Society of America
1. Introduction
In recent years research enthusiasm on ZnO has been greatly spurred due to the fact that ZnO is a wide-band-gap semiconductor with a direct band gap of ~3.37 eV at room temperature (RT) and a considerably large exciton binding energy of ~60 meV [1,2]. It has been demonstrated that the photoluminescence (PL) of ZnO usually consists a narrow near-band-edge (NBE) emission peak around ~380 nm and a broad defect-related emission band in the visible spectrum region [1]. The PL position and intensity of ZnO can be significantly influenced by extrinsic factors [3–14]. We have recently studied the electrical-field-controlled PL, i.e., electrophotoluminescence (EPL) of ZnO film [15]. It was found that the electric field with appropriate direction and amplitude would influence the distribution of photo-excited carriers in the ZnO film, leading to the enhancement of NBE emission and the attenuation of visible emission. The ZnO film deposited by sputtering was highly c-axis oriented in Ref. 15, and it could be treated as ZnO nanorod array. The longitude of ZnO nanorods was parallel with the direction of electric field. The electric field would drive the photo-excited carriers to transport inside a single ZnO nanorod.
Sol-gel process, with the virtues of simplicity and low cost, has been widely used for the growth of ZnO polycrystalline films. The sol-gel derived ZnO films feature unoriented growth. Distinguished from the carrier transport in the sputtered ZnO film, the carriers in the sol-gel derived ZnO film under electric field will transport through several grains. Therefore, how the sol-gel derived ZnO film exhibits its EPL is indeed a concern of fundamental research interest. In this work we have investigated the EPL of sol-gel derived ZnO film in a wide range of forward bias voltages. The results show that above a critical bias voltage the device exhibits a negative differential resistance (NDR) following a normal current-voltage (I–V) characteristic. In the normal region, the EPL of sol-gel derived ZnO film is similar to that of sputtered one. In addition, with the increase of forward bias the NBE PL of sol-gel derived ZnO film is attenuated in the NDR region, while it is blue-shifted in the whole measured voltage range. The physical origin of the effect of electric field on the intensity and position of NBE PL of sol-gel derived ZnO film has been tentatively discussed.
2. Sample fabrication
To induce the electric field on the sol-gel derived ZnO film, a SiO2/ZnO/SiOx(x<2) double barrier (DB) structure on Si was prepared according to the procedures as follows. (1) A polished n-type silicon wafer was cleaned by the standard RCA process. (2) ~5 nm thick SiOx film was deposited onto the silicon substrate by electron-beam evaporation. (3) ~300 nm thick ZnO film was deposited onto the SiOx film by a sol-gel process as follows: (i) zinc acetate [Zn(CH3COO)2·2H2O] was dissolved completely into 2-methoxyethanol solution with addition of certain amount of ethanolamine as stabilizer; (ii) the mixed solution was stirred for 12 h at RT to form transparent precursor sol; (iii) the precursor film was spin-coated onto the SiOx film for 3 cycles, each spun at 3000 rpm for 30 s (after the spin-coating, each layer was immediately baked at 80 °C for 10 min and then annealed in air at 300 °C for 20 min before spinning the next layer). (4) ~100 nm thick SiO2 film was deposited onto the ZnO film by a sol-gel process and then annealed at 800 °C for 2 h under O2 ambient. (5) ~20 nm thick semitransparent Au film on the SiO2 film and ~100 nm thick Au film on the backside of the silicon substrate were respectively sputtered as electrodes. It is defined as forward bias when the n-type silicon substrate is connected to negative voltage.
3. Results and discussion
Figure 1 shows the x-ray diffraction (XRD) pattern of the sol-gel derived ZnO film on a silicon substrate annealed at 800 °C. Besides the Si (200) diffraction peak, all the others are associated with ZnO, indicating that the ZnO film is polycrystalline and unoriented. The inset of Fig. 1 shows the field emission scanning electron microscope (FESEM) image of the surface morphology of the sol-gel derived ZnO film. The grains, with an average size of ~80 nm, distribute randomly in the plane of the ZnO film.
Fig. 1. XRD pattern of the sol-gel derived ZnO film on Si annealed at 800 °C. The inset shows the FESEM image of the surface morphology of the sol-gel derived ZnO film.
Figure 2 shows the forward I–V characteristic of the device based on SiO2/ZnO/SiOx DB structure. The I–V curve can be divided into two regions in terms of the dependence of current on the bias voltage. In the range of 0–12.5 V, denoted as a normal region, the current increases rapidly with the voltage. In the range of 12.5–25.0 V the current decreases rapidly, exhibiting an obvious NDR effect with a peak-to-valley current ratio of ~3.2. Owing to the highly insulative SiO2 film the forward current density is quite small (≤0.01 A/cm2). In this case the electrically injected carriers are considerably few.
The inset of Fig. 3 shows the schematic diagram of the device under test. As can be seen, to study the EPL behavior of the sol-gel derived ZnO film, the device, under optical pump of a 325 nm He-Cd laser, was simultaneously applied with different forward biases. The front-Au and SiO2 films were nearly transparent for UV and visible lights. Thus the ZnO film could be excited by the laser, and PL of that was recorded by a detector. Figure 3(a) shows the PL spectra of the sol-gel derived ZnO film as the device is applied with different forward biases of 0 and 12.0 V. In the case without bias (0 V) on the device, the PL spectrum (red dash line) reveals a UV NBE emission of ZnO film at ~377.6 nm accompanied with a defect-related visible emission band centering at ~535.0 nm. The intensity of UV emission is similar to that of visible emission. As the device is applied with a forward bias of 12.0 V, shown in Fig. 3(a) as blue solid line, the UV emission is remarkably enhanced while the visible emission is attenuated. Figure 3(b) shows the PL spectra of the sol-gel derived ZnO film as the device applied with a forward bias of 12.0 V was illuminated by the laser or not. As can be seen, once the laser is turned off, shown in Fig. 3(b) as green dash line, nearly no light is detected even though the forward bias is still applied on the device. It is reasonably believed that almost all the carriers contributing to the light emissions of ZnO film are generated by laser excitation rather than electrically injection. The significant forward-bias-dependent PL of ZnO is originated from the electric-field effect on the distribution of photo-excited carriers in the sol-gel derived ZnO film.
Fig. 3. (a) PL spectra of the sol-gel derived ZnO film as the device is applied with different forward biases of 0 and 12.0 V. The inset shows the schematic diagram of the device under test. (b) PL spectra of the sol-gel derived ZnO film as the device applied with a forward bias of 12.0 V is illuminated by a He-Cd laser or not.
Fig. 4. PL spectra in 360–400 nm of the sol-gel derived ZnO film in the (a) normal and (b) NDR regions.
To demonstrate the EPL of the sol-gel derived ZnO film in more details, Figs. 4(a) and 4(b) show the PL spectra in 360–400 nm of the ZnO film in the normal and NDR regions, respectively. In the normal region, as shown in Fig. 4(a), with the increase of forward bias from 0 to 13.0 V the NBE PL of ZnO film is significantly enhanced and meanwhile slightly blue-shifted from ~377.6 to ~376.0 nm. In the NDR region, as shown in Fig. 4(b), the increase of forward bias from 13.0 to 25.0 V causes the NBE PL of ZnO film to progressively attenuate and still slightly blue-shift from ~376.0 to ~374.9 nm. Figure 5 shows the dependences of spectrally integrated intensity and peak position of the NBE PL of ZnO film on the forward bias voltage. The PL intensity-voltage curve (red solid squares) largely conforms to the I–V characteristic; in contrast, the peak position of the NBE PL (blue hollow circles) exhibits almost linear reduction with the forward bias voltage.
Fig. 5. Dependences of spectrally integrated intensity (red solid squares) and peak position (blue hollow circles) of the NBE PL of ZnO flim on the forward bias voltage.
Regarding the mechanisms for the NDR effect and the enhancement of NBE PL by forward bias in the normal region, they have been discussed in our previous reports [15,16]. In the following, the emphasis will be placed on the physical origin of the attenuation of NBE PL by forward bias in the NDR region and the blue-shift of that in the whole measured voltage range.
As the device is applied with bias voltage (V all), most of V all is assumed by the SiO2 film (V SiO2) and only a small proportion by the ZnO film (V ZnO). It is V ZnO that controls the carrier distribution and PL of ZnO film. The higher the V ZnO, the stronger the NBE PL of ZnO. In the normal region, V ZnO is increased with V all, leading to the enhancement of NBE PL of ZnO film. However, in the NDR region, as can be seen in Fig. 2, along with the bias voltage increasing twice from 12.5 to 25.0 V the corresponding current decreases from 8.3 to 2.6 mA. The resistance of the device can be roughly estimated to rise about 6 times. The resistance increases much more quickly than the bias voltage. It is known that the increase of resistance results from the shut-off of electron-tunneling channels by trapped electrons in SiO2 film. Consequently, much larger proportion of V all will be applied on the SiO2 film; while, V ZnO is actually decreased although V all is increased in the NDR region. In this case, fewer electrons accumulate near the SiO2/ZnO interface, leading to the weaker NBE PL of ZnO film.
The blue-shift of NBE PL of ZnO film will be discussed in terms of carrier distribution in ZnO film and energy band diagrams of the device, which are schematically shown in Fig. 6. As mentioned in Ref. 15, the mobility of hole is much smaller than that of electron, and the carrier recombination lifetime is extremely short for ZnO. Therefore, on the scale of carrier recombination lifetime, the concentration of photo-excited holes in ZnO film is not influenced by electrical field. The position and intensity of NBE PL of ZnO are determined by the distribution of photo-excited electrons in ZnO film. In the normal region, forward bias drives the photo-excited electrons to accumulate near the SiO2/ZnO interface. Some of them should be filled into the higher energy states in conduction band of ZnO, as shown in Fig. 6(a). The recombination of these higher energy electrons with holes in the valence band of ZnO generates higher energy photons. The NBE PL of ZnO exhibits slight blue-shift. As the forward bias is increased, the electrons accumulation becomes stronger and more electrons occupy much higher energy states, leading to further blue-shift of NBE PL of ZnO. In the NDR region, the NBE PL of ZnO film in Fig. 4(b) features intensity attenuation and position blue-shift, indicating that the electron concentration in accumulation layer is decreased, whereas the proportion of higher energy electrons in total electrons is increased. The lower inset of Fig. 6(b) shows the carrier distribution in ZnO film in this case. As described above, the weaker accumulation results from the smaller V ZnO in the NDR region. However, to essentially understand the increase of the proportion of higher energy electrons is still a challenge. A tentative explanation is proposed concerning the capacitance of SiO2 film. As forward bias is applied on the device, SiO2 film is charged with metal cations in the Au/SiO2 interface and electrons accumulation in the SiO2/ZnO interface, respectively. In the NDR region V SiO2 is greatly increased with V all, therefore SiO2 film is further charged. Abundant electrons accumulate in an extremely thin layer close to the SiO2/ZnO interface, where some electrons are filled into much higher energy states in the conduction band of ZnO, leading to the blue-shifted NBE PL.
Fig. 6. Energy band diagrams of the device and carrier distribution in ZnO film in the (a) normal and (b) NDR regions.
4. Conclusion
In conclusion, we have systematically investigated the EPL behavior of sol-gel derived ZnO film under forward bias taking advantage of the SiO2/ZnO/SiOx DB structure on Si. It is found that the device exhibits a NDR effect under sufficiently high forward bias following the normal I–V characteristic. With the increase of forward bias, the NBE PL of ZnO in the normal region is enhanced and blue-shifted; while, that in the NDR region is attenuated and further blue-shifted. Based on the energy band diagrams of the device and the carrier distribution in ZnO film in the normal and NDR regions, the mechanism for the effect of electric field on the intensity and position of NBE PL of ZnO has been explained. It is believed that this work would contribute to the comprehensive understanding of optoelectronic properties of ZnO.
Acknowledgements
The authors would like to thank Dr. Xiaodong Pi for helpful discussion. This work was supported by “973 Program” (No. 2007CB613403), China Postdoctoral Science Foundation funded project (No. 20080441223), Natural Science Foundation of China (No. 60776045) and Changjiang Scholars and Innovation Teams in Universities.
References and links
1. Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, and H. Morkoç, “A comprehensive review of ZnO materials and devices,” J. Appl. Phys. 98(4), 041301 (2005). [CrossRef]
2. S. J. Pearton, D. P. Norton, K. Ip, Y. W. Heo, and T. Steiner, “Recent progress in processing and properties of ZnO,” Prog. Mater. Sci. 50(3), 293–340 (2005). [CrossRef]
3. Z. Fu, B. Yang, L. Li, W. Dong, C. Jia, and W. Wu, “An intense ultraviolet photoluminescence in sol-gel ZnO-SiO2 nanocomposites,” J. Phys. Condens. Matter 15(17), 2867–2873 (2003). [CrossRef]
4. K. C. Hui, J. An, X. Y. Zhang, J. B. Xu, J. Y. Dai, and H. C. Ong, “Electron beam induced light emission and charge conduction patterning in ZnO by using an AlOx Layer,” Adv. Mater. 17(16), 1960–1964 (2005). [CrossRef]
5. K. Kim, H. Tampo, J. Song, T. Seong, S. Park, J. Lee, S. Kim, S. Fujita, and S. Niki, “Effect of rapid thermal annealing on Al doped n-ZnO films grown by RF-magnetron sputtering,” Jpn. J. Appl. Phys. 44(No. 7A), 4776–4779 (2005). [CrossRef]
6. H. Y. Lin, C. L. Cheng, Y. Y. Chou, L. L. Huang, Y. F. Chen, and K. T. Tsen, “Enhancement of band gap emission stimulated by defect loss,” Opt. Express 14(6), 2372–2379 (2006). [CrossRef]
7. V. A. Coleman, J. E. Bradby, C. Jagadish, and M. R. Phillips, “Observation of enhanced defect emission and excitonic quenching from spherically indented ZnO,” Appl. Phys. Lett. 89(8), 082102 (2006). [CrossRef]
8. J. V. Foreman, J. Li, H. Peng, S. Choi, H. O. Everitt, and J. Liu, “Time-resolved investigation of bright visible wavelength luminescence from sulfur-doped ZnO nanowires and micropowders,” Nano Lett. 6(6), 1126–1130 (2006). [CrossRef]
9. S. Gao, H. Zhang, R. Deng, X. Wang, D. Sun, and G. Zheng, “Engineering white light-emitting Eu-doped ZnO urchins by biopolymer-assisted hydrothermal method,” Appl. Phys. Lett. 89(12), 123125 (2006). [CrossRef]
10. J. W. P. Hsu, D. R. Tallant, R. L. Simpson, N. A. Missert, and R. G. Copeland, “Luminescent properties of solution-grown ZnO nanorods,” Appl. Phys. Lett. 88(25), 252103 (2006). [CrossRef]
11. P. Cheng, D. Li, Z. Yuan, P. Chen, and D. Yang, “Enhancement of ZnO light emission via coupling with localized surface plasmon of Ag island film,” Appl. Phys. Lett. 92(4), 041119 (2008). [CrossRef]
12. P. Cheng, D. Li, and D. Yang, “Influence of substrates in ZnO devices on the surface plasmon enhanced light emission,” Opt. Express 16(12), 8896–8901 (2008). [CrossRef]
13. S. Kim, C. Oh Kim, H. Taek Oh, and S.-H. Choi, “Strong enhancement of near-band-edge photoluminescence from ZnO by assembling ZnO/SiOx heterostructures,” J. Phys. D Appl. Phys. 41(23), 235403 (2008). [CrossRef]
14. B. Chen, H. Zhang, N. Du, D. Li, X. Ma, and D. Yang, “Hybrid nanostructures of Au nanocrystals and ZnO nanorods: Layer-by-layer assembly and tunable blue-shift band gap emission,” Mater. Res. Bull. 44(4), 889–892 (2009). [CrossRef]
15. X. Ma, P. Chen, D. Li, Y. Zhang, and D. Yang, “Electrophotoluminescence of ZnO film,” Appl. Phys. Lett. 91(2), 021105 (2007). [CrossRef]
16. P. Chen, X. Ma, D. Li, Y. Zhang, and D. Yang, “Electrically pumped ultraviolet random lasing from ZnO-based metal-insulator-semiconductor devices: dependence on carrier transport,” Opt. Express 17(6), 4712–4717 (2009). [CrossRef]
