Ultrahigh sensitivity in the amorphous ZnSnO UV photodetector

Abstract

An ultraviolet (UV) photodetector based on amorphous ZnSnO (α-ZTO) channel thin-film transistors (TFTs) with ultrahigh sensitivity was fabricated by a simple sol–gel method. The photodetectors are investigated at various intensities of 365 nm UV illumination, which exhibit ultrahigh sensitivity and responsivity. A reasonable mechanism is proposed for the superior UV detecting performance. Moreover, our device is promising for UV detection under weak UV illumination. The fabrication of the α-ZTO UV photodetector represents a significant step toward future multifunctional optoelectronic applications.

---

Introduction

Among photodetectors, ultraviolet (UV) detectors have received tremendous attention due to their various potential applications in many fields, including environmental and biological monitoring, flame detection, space communication, missile launch detection, etc.^1–8 Generally, high UV sensitivity, high responsivity, a fast response, good stability, low cost and easy fabrication are the basic factors for practical UV photodetectors. Among various wide band gap semiconductors, ZnO, with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, has been regarded as one of the most important materials for UV photodetectors. However, pure ZnO materials show poor UV sensing performance due to the large background n-type carrier concentration. Dai et al.⁷ fabricated ZnO nanorod (NR) array film UV photodetector, which exhibits relatively low UV sensitivity and slow response.

In order to acquire high UV sensitivity and quick response photodetectors, various p–n junctions or nano-photoconductivity systems have been investigated.^7–11 However, the UV sensitivity around 10² to 10³ is still unsatisfied.^7,8,12–14 Compared with p–n junctions or nano-structure systems, field effect transistors (FETs) show unique characteristics, which own extremely low off-state current and huge on–off ratio. In particular, thin-film transistors applied in flat panel display have exhibited good UV sensing character.^15–19 Thus, thin-film transistors (TFTs) have potential for integrating a UV sensor in display applications.

Recently, amorphous zinc–tin oxide has been reported to replace the indium–gallium–zinc oxide (IGZO) as TFT channels. Due to the special electron configuration of 4d¹⁰5s⁰ of Sn⁴⁺, the In-free amorphous ZnSnO (α-ZTO) could also achieve a high field-effect mobility without the rare and expensive In element.^20–22 Considering future multifunctional optoelectronic applications, attempts to use α-ZTO TFTs as UV photodetectors are highly expected. In this work, we fabricate UV photodetectors using α-ZTO TFTs, which exhibit high sensitivity even under very weak UV illumination.

Experimental section

In this work, the ZTO precursor was synthesized via combustion processing.²³ Zn and Sn precursors were prepared, respectively, by dissolving 0.001 mol Zn(NO₃)₂·6H₂O or 0.001 mol SnCl₂·2H₂O with 0.001 mol NH₄NO₃ in 5 ml 2-methoxyethanol, then adding 0.2 ml acetylacetone as fuel. After completely dissolving the metal salt, 114 μl (for Zn) or 57 μl (for Sn) of 14.5 M NH₃ (aq.) was added as stabilizer. The mixture was stirred for 12 h at room temperature, and then filtered through 0.22 μm microfilter. The ZTO precursor was acquired by mixing the individual precursors with molar ratio of Zn : Sn = 1 : 2 and aging for at least 12 h. Then, spin-coating processes were conducted on p++ Si/300 nm SiO₂ wafers at 4000 rpm for 35 s. Subsequently, a pre-annealing process was carried out at 300 °C for 30 min under air to remove the 2-methoxyethanol solvent. The spin-coating and pre-annealing processes were repeated several times to acquire the desired film thickness. After that, annealing at 600 °C for 1 h under air was carried out. The final 100 nm thick Al electrodes as the source and drain electrodes were evaporated by electron beam evaporation technology with shadow mask. The channel width and length were 1000 and 150 μm, respectively.

Characterization

The crystallinity of the ZTO film was spin-coated on the glass substrate and analyzed by X-ray diffraction (XRD) with a Cu Kα radiation source (λ = 1.54056 Å). The surface morphology and cross-section of the devices were characterized by field-emission scanning electron microscopy (FE-SEM Hitachi S-4800). The electrical properties of the devices were studied by current–voltage (I–V) measurements, which were carried out using an Agilent E5270B parameter analyser under ambient conditions. The temporal response of the UV detector was measured by illuminating the devices with a UVA-LED (365 nm).

Results and discussion

Fig. 1a depicts the XRD pattern of ZTO film. The halo peak around 30° originates from the glass substrate. No obvious diffraction peaks of crystalline phase are observed, indicating that ZTO film is amorphous. Fig. 1b shows the cross-section SEM image of the fabricated device. The ZTO film is about 20 nm. From the cross-section image, the film is dense and compact. The higher modification focused on the ZTO layer is shown on the top left corner of Fig. 1b. The surface of the ZTO film is shown in the inset of Fig. 1b on the top right corner. There is no cluster-ordering area from the surface image, which indicates that the amorphous phase has been completely formed.

Fig. 1 (a) XRD pattern of α-ZTO films, (b) cross-section and surface SEM images of α-ZTO films.

Fig. 2a shows typical transfer characteristics of the ZTO TFTs under dark and 365 nm UV illumination with different UV intensities (P). The transfer characteristics show typical n-type behavior and good switch characteristics when the UV light is off. The depletion region, which starts from about −8 V of gate voltage (V_GS) to the more negative V_GS, is known from the dark curve. The transfer curves change differently when the device is illuminated by the ultraviolet. By the increasing of UV intensity, the curves shift up, especially in the depletion region. The mechanism would be discussed below. The devices are measured at the constant source–drain voltage (V_DS) of 10 V and the sweeping V_GS. The leakage current is at the 10⁻⁹ A order at depletion region, and the source–drain current (I_DS) varies from 10⁻⁵ to 10⁻⁴ A under 365 nm UV illumination with different UV intensity. The maximum on-to-off current ratio reaches 10⁶. The output characteristics of the device are shown in Fig. S1.†

Fig. 2 (a) Typical transfer characteristics of the α-ZTO TFTs under dark and 365 nm UV illumination with different UV intensities, (b) I–V characteristics curves of α-ZTO photodetector under dark and 365 nm UV illumination with different UV intensities.

Like IGZO TFTs, the reverse region can hardly appear in ZTO devices due to abundant defect states.^24,25 Thus, to ensure the photodetector working in the depletion region, we control V_GS at −20 V. The I–V characteristics curves of ZTO photodetector are obtained by picking out the I_DS–V_DS curves at −20 V from output characteristics, as shown in the Fig. 2b. It is found that the dark current (I_dark) has nearly no gain by the increasing of V_DS with no UV illumination. Even at the 40 V bias of V_DS, the I_dark is only 1.45 nA. Under UV irradiation, the curves of photocurrent (I_photo) and I_dark have obvious separation, which means that our device can achieve UV detection even under weak ultraviolet. The photocurrent increases by 6 orders of magnitude upon 124 μW cm⁻² UV illumination, indicating a highly UV sensitive photodetection.

The sensitivities [sensitivity = (I_photo − I_dark)/I_dark] are calculated at V_DS of 40 V, as shown in Fig. 3a. The sensitivity boosts rapidly by the increasing of UV intensity, which is attributed to the enhancement of I_photo. It is noteworthy that sensitivity can acquire 12 581 even under weak UV illumination with intensity of 19 μW cm⁻². When the intensity increases to 124 μW cm⁻², the sensitivity boosts to 103 654. This value is much larger than previous reports. The sensitivity of most UV photodetectors is in the range of 10² to 10³.^7,8,12–14

Fig. 3 (a) Sensitivity and (b) responsivity of the device at various UV intensities.

Another key parameter of photodetectors is responsivity (R). Responsivity is defined as the generated photocurrent per unit of incident UV power, R = (I_photo − I_dark)/PS, where S is the surface area of channel.^26–28 Our device exhibits a relatively high responsivity of 808 A W⁻¹ at intensity of 124 μW cm⁻². From Fig. 3b, R keeps an approximate 800 A W⁻¹ on average. Compared with the responsivity value of most commercial UV photodetectors ranging from 0.1 to 0.2 A W⁻¹, the value in our work is large enough for practical application, indicating the highly efficient use of ultraviolet light per unit.²⁹

Fig. 4 exhibits the time-resolved UV photocurrent on/off measurements of the device at V_GS of −20 V and V_DS of 40 V with different UV intensities. As seen in Fig. 4a, all curves show exponential rise and decay under illumination. When the UV light is on, the I_photo rapidly rises to saturation. Under 19 and 26 μW cm⁻² UV intensities, the photocurrent directly drops to the I_dark level after turning off the excitation. However, under higher intensities, the photocurrent decay starts with a fast decay component and then falls to bottom with about 10 s, which will be explained as follow. In the curves of intensities of 61 and 124 μW cm⁻², the rises of saturation are due to the fluctuation of UV light. The growth time constants and the decay time constants are fitted from Fig. 4a and listed in Table 1. The growth time constant ranges from 2 to 4 s, and the decay time constant is slightly longer, ranging from 2 to 6 s. Although the response time is a little longer, it has been greatly decreased compared to other ZnO-based UV photodetector, which still needs to be further improved. Five repeated cycles are displayed in Fig. 4b, in which the photocurrent is observed to be consistent and repeatable with no degenerate effect during the detection processes. It confirms that our device has good stability and reliability. The performance of the device nearly has no degradation within 2 months. After exposed in ambient environment for 8 months, the degradation of performance is puny. The detail is found in the ESI.† Here, we compare the photodetector performance with other reported devices in Table 2.^{7,8,12–14,26,27,30–32} The UV photodetector we made has the premium comprehensive performance compared with the other type detectors. It is noteworthy that the FETs type photodetectors have the relatively high responsivity. Besides, our devices show higher sensitivity, which means the obvious separation of I_photo and I_dark signals.

Fig. 4 The growth and decay of photocurrent of α-ZTO photodetector under different UV intensities: (a) single-pulse and (b) multi-pulse observation.

Table 1 The growth time constants and the decay time constants of the fabricated UV photodetector with different UV intensities

UV power (μW cm^−2)	τg (s)	τd (s)
19	2.24	2.25
26	2.44	2.62
42	3.93	5.48
61	3.23	5.31
102	3.22	4.94
124	2.18	2.72

---

Table 2 Comparison of the performance of the ZnO-based UV photodetectors

Material	Structure	Power (μW cm^−2)	Sensitivity	Responsivity (A W^−1)	τg (s)	τd (s)	Ref.
Cu:ZnO NR array film	p–n junction		2080	—	9.2	5.0	7
n-ZnO/p-NiO	p–n junction	80 000	165.7	—	6.4	12.7	8
Polycrystalline Cu:ZnO film	MSM		∼2000	0.3	7.8	5.8	12
ZnO NP	MSM	1060	∼105	61	0.1	∼1 s	13
Cu:ZnO NR	Photoconductor	—	79	—	∼300	∼600	14
Ag NP/ZnO NW	Photoelectrochemical cell	61	—	0.37	0.14	0.52	30
ZnO QDs/graphene	FETs	∼7.7	∼1000	∼2.4 × 10^7	—	—	31
ZnO NRs/graphene	FETs	6 × 10^15	—	2.5 × 10^6	—	—	27
ZnO NRs/graphene	FETs	480	—	3 × 10^5	1.2	1.9	26
ZnO/ZnMgO	FETs	16	2	—	—	—	32
ZTO	TFTs	19	12 581	640	2.24	2.25	This work
ZTO	TFTs	124	103 654	808	2.18	2.72	This work

---

A reasonable mechanism of ultrahigh sensitive UV detection is proposed. Fig. 5 depicts the schematic of the UV detection mechanism. As shown in Fig. 5, we set the V_GS as −20 V to form the depletion region in ZTO channel. At the same time, ambient oxygen chemically adsorbs on the surface of ZTO in the dark condition. These oxygen molecules capture free electrons from ZTO to become adsorbed anions [O₂ (g) + e⁻ → O₂⁻ (ad)], which make the ZTO band bend upward and cause depletion on the surface. Benefitted from the low-conductivity depletion region, carriers can hardly go through the channel and restrain the I_dark when there is no UV irradiation. Under UV illumination with photon energy higher than the ZTO band gap, electron–hole pairs are generated by UV absorption [hν → e⁻ + h⁺]. Photo-generated holes discharge the negatively charged adsorbed O₂⁻ and free the neutral oxygen molecules and enhance surface carrier concentration and conductivity [h⁺ + O₂⁻ (ad) → O₂ (g)]. Photo-generated electrons form a carrier passage like accumulation region, which is the reason of transfer curves shifting up in Fig. 2a while illuminated by UV light. With the bias of V_DS, the huge current forms, namely photocurrent. When the UV light is turned off, oxygen molecules are readsorbed on the surface of ZTO channel and the depletion regions are reformed. It needs extra time for the transition from quasi-accumulation to depletion due to excess photo-generated electrons under the high intensity of UV illumination. This is the reason why current cannot reduce to the I_dark level immediately under high UV intensity conditions in Fig. 4.

Fig. 5 (a and b) Schematic diagrams illustrating the mechanism of UV photodetector based on α-ZTO channel TFTs before and after illuminated by UV, respectively. (c and d) The energy band diagrams of α-ZTO before and after illuminated by UV, respectively.

Conclusion

In summary, α-ZTO TFT photodetectors of ultrahigh UV sensitivity have been fabricated by simple solution process. The photodetectors show low dark current of 1.45 nA at the 40 V bias of V_DS in the absence of UV illumination, which is attributed to low-conductivity depletion region. Under UV irradiation, the photocurrent increases by 6 orders of magnitude upon 124 μW cm⁻² UV illumination. The related sensitivity and responsivity are calculated to be 103 654 and 808 A W⁻¹, respectively. With the ultrahigh sensitivity and responsivity, the UV detecting performance of our photodetectors is much more superior to the previously reported UV detectors. The results demonstrate that it is possible to use α-ZTO TFTs as UV photodetectors, which is important for future multifunctional optoelectronic applications.

Acknowledgements

This work was supported by National Natural Science Foundation of China under Grant No. 51302244 and 91333203, and the Fundamental Research Funds for the Central Universities under Grant No. 2016FZA4004.

References

1. S. Park, S. An, Y. Mun and C. Lee, Appl. Phys. A: Mater. Sci. Process., 2014, 114, 903–910.
2. K. Liu, M. Sakurai, M. Liao and M. Aono, J. Mater. Chem. C, 2010, 114, 19835–19839.
3. X. Zhang, X. Han, J. Su, Q. Zhang and Y. Gao, Appl. Phys. A: Mater. Sci. Process., 2012, 107, 255–260.
4. L. Zhuang and K. H. Wong, Appl. Phys. A: Mater. Sci. Process., 2007, 87, 787–791.
5. C. Soci, A. Zhang, B. Xiang, S. A. Dayeh, D. P. R. Aplin, J. Park, X. Y. Bao, Y. H. Lo and D. Wang, Nano Lett., 2007, 7, 1003–1009.
6. D. Kim, G. Shin, J. Yoon, D. Jang, S. J. Lee, G. Zi and J. S. Ha, Nanotechnology, 2013, 24, 315502.
7. W. Dai, X. H. Pan, C. Chen, S. S. Chen, W. Chen, H. H. Zhang and Z. Z. Ye, RSC Adv., 2014, 4, 31969–31972.
8. W. Dai, X. H. Pan, S. S. Chen, C. Chen, Z. Wen, H. H. Zhang and Z. Z. Ye, J. Mater. Chem. C, 2014, 2, 4606–4614.
9. P. N. Ni, C. X. Shan, S. P. Wang, X. Y. Liu and D. Z. Shen, J. Mater. Chem. C, 2013, 1, 4445–4449.
10. G. Wang, S. Chu, N. Zhan, Y. Lin, L. Chernyak and J. Liu, Appl. Phys. Lett., 2011, 98, 041107.
11. A. Echresh, C. O. Chey, M. Z. Shoushtari, V. Khranovskyy, O. Nur and M. Willander, J. Alloys Compd., 2015, 632, 165–171.
12. L. Hu, L. Zhu, H. He, Y. Guo, G. Pan, J. Jiang, Y. Jin, L. Sun and Z. Ye, Nanoscale, 2013, 5, 9577–9581.
13. Y. Jin, J. Wang, B. Sun, J. C. Blakesley and N. C. Greenham, Nano Lett., 2008, 8, 1649–1653.
14. W. L. Ong, H. Huang, J. Xiao, K. Zeng and G. W. Ho, Nanoscale, 2014, 6, 1680–1690.
15. C. J. Chiu, W. Y. Weng, S. J. Chang, S. P. Chang and T. H. Chang, IEEE Sens. J., 2011, 11, 2902–2905.
16. Y. Wu, E. Girgis, V. Ström, W. Voit, L. Belova and K. V. Rao, Phys. Status Solidi A, 2011, 208, 206–209.
17. T. H. Chang, C. J. Chiu, S. J. Chang, T. Y. Tsai, T. H. Yang, Z. D. Huang and W. Y. Weng, Appl. Phys. Lett., 2013, 102, 221104.
18. S. J. Chang, T. H. Chang, W. Y. Weng, C. J. Chiu and S. P. Chang, IEEE J. Quantum Electron., 2014, 20, 125–129.
19. H. M. Chen, T. C. Chang, Y. H. Tai, Y. C. Chen, M. C. Yang, C. H. Chou, J. F. Chang and S. Z. Deng, IEEE Trans. Electron Devices, 2014, 61, 3186–3190.
20. Q. Jiang, J. Lu, J. Cheng, X. Li, R. Sun, L. Feng, W. Dai, W. Yan and Z. Ye, Appl. Phys. Lett., 2014, 105, 132105.
21. Q. Jiang, L. Feng, C. Wu, R. Sun, X. Li, B. Lu, Z. Ye and J. Lu, Appl. Phys. Lett., 2015, 106, 053503.
22. K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano and H. Hosono, Nature, 2004, 432, 488–492.
23. M. G. Kim, M. G. Kanatzidis, A. Facchetti and T. J. Marks, Nat. Mater., 2011, 10, 382–388.
24. T. Kamiya, K. Nomura, M. Hirano and H. Hosono, Phys. Status Solidi C, 2008, 5, 3098–3100.
25. K. Nomura, T. Kamiya, H. Yanagi, E. Ikenaga, K. Yang, K. Kobayashi, M. Hirano and H. Hosono, Appl. Phys. Lett., 2008, 92, 202117.
26. V. Q. Dang, T. Q. Trung, D. I. Kim, T. Duy le, B. U. Hwang, D. W. Lee, B. Y. Kim, D. Toan le and N. E. Lee, Small, 2015, 11, 3054–3065.
27. V. Q. Dang, T. Q. Trung, L. T. Duy, B. Y. Kim, S. Siddiqui, W. Lee and N. E. Lee, ACS Appl. Mater. Interfaces, 2015, 7, 11032–11040.
28. B. D. Boruah, A. Mukherjee, S. Sridhar and A. Misra, ACS Appl. Mater. Interfaces, 2015, 7, 10606–10611.
29. E. Monroy, F. Omnes and F. Calle, Semicond. Sci. Technol., 2003, 18, 33–51.
30. Y. Y. Zeng, X. H. Pan, W. Dai, Y. C. Chen and Z. Z. Ye, RSC Adv., 2015, 5, 66738–66741.
31. D. Shao, J. Gao, P. Chow, H. Sun, G. Xin, P. Sharma, J. Lian, N. A. Koratkar and S. Sawyer, Nano Lett., 2015, 15, 3787–3792.
32. S. Mondal, R. R. Ghimire and A. K. Raychaudhuri, Appl. Phys. Lett., 2015, 106, 041102.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02924h

---