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Abstract
Self-powered UV photodetectors and imaging arrays based on p-type NiO/n-type InGaZnO (IGZO) heterojunctions are fabricated at room temperature by using ratio-frequency magnetron sputtering. The p-n heterojunction exhibits typical rectifying characteristics with a rectification ratio of 7.4×104 at a ±4 V applied bias. A high photo-responsivity of 28.8 mA/W is observed under zero bias at a wavelength of 365 nm. The photodetector possesses a fast response time of 15 ms which is among the best in reported oxide-based p-n junction-based UV photodetectors. Finally, recognition of an “H” pattern is demonstrated by a 10×10 photodetector array at zero bias. The results indicate that the NiO/IGZO based photodetectors may have a great potential in constructing large-scale self-powered UV imaging systems.
Published by Optica Publishing Group under the terms of the Creative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
1. Introduction
During the past few years, ultraviolet (UV) photodetectors (PDs) have received intensive attention for applications including chemical analysis, optical communications, flame detection, transmitter calibration, astronomy and environmental monitoring [1–5]. Si-based UV PDs have been commercialized but require additional filters to filter out low-energy photons due to the small band gap of Si (1.1 eV). Wide-bandgap semiconductors, such as ZnO [6], GaN [7], Ga2O3 [8], diamond [9], SiC [10], and ZnMgO [11], have drawn an increasing interest due to their bandgaps corresponding to UV wavelengths, intrinsic visible blindness, large breakdown voltage, and high temperature stability.
Until now, many types of UV PDs have been demonstrated, including p-n junctions [12–14], Schottky junctions [15–17] and photoconductive detectors [18,19]. The photoconductive detectors have attracted much attention owing to its simple structure, low fabrication cost and high photoresponsivity. Nevertheless, a high photoconductive gain is generally accompanied by a long recovery time due to the persistent photoconductivity effect, hence unsuitable for detection when the light intensity changes at a high speed [20]. Wide bandgap semiconductor-based p-n junction UV PDs can achieve a much faster response speed and lower dark current. The built-in electric field in p-n junctions allows the separation of photo-excited electron-hole pairs effectively, generating photocurrent at zero bias and hence enabling the so-called self-powered operation without any external power supply. The p-n junction PDs also generally have a higher built-in barrier and lower saturation current than Schottky junctions [21]. To date, many p-n junction-based UV PDs require complicated manufacturing processes and high fabrication temperatures [4–6,12,22–26]. With the rapid development of wearable electronics, it would be highly desirable to fabricate UV PDs based on semiconductors that are deposited and processed at low temperatures and can hence be made on flexible plastic substrates.
Oxide semiconductors have been developed very rapidly since the invention of the first flexible, transparent thin-film transistors [27]. Due to lacking of high-performance p-type oxide semiconductors, very limited number of p-n junction-based UV PDs were reported [5,6,12,13]. Luo et al. reported a self-powered p-type NiO and n-type ZnO-nanowire-heterojunction UV PD which possess a photo-responsivity of 17 mA/W and a fall time of 2.9 s at a process temperature of 600 °C [5]. Ji et al. exploited hollow p-type CuO nanospheres and n-type ZnO nanorods to fabricate a p-n junction-based UV PD with an on/off current ratio of 4.58 and a rising time of 1.8 s at a process temperature of 500 °C [6]. Among these reported experiments, NiO is an intrinsic p-type oxide semiconductor due to the nickel vacancies with a direct bandgap of 3.7 eV. Furthermore, n-type InGaZnO (IGZO) material has already been commercially used in flat panel display back-plane drivers due to its excellent properties. IGZO has also been used for UV detection using diode (with p-silicon nanowire) and field-effect transistor structures [28,29].
In this study, self-powered UV PDs based on NiO/IGZO heterojunctions are fabricated at room temperature using radio-frequency (RF) magnetron sputtering. The photocurrent also exhibits a highly linear dependency on the light intensity at low UV powers. A high photo-responsivity of 28.8 mA/W, a low dark current of 5 pA and a response time of 15 ms are achieved, which are among the best in reported oxide-based p-n junction-based UV photodetectors. A 10×10 imaging array is also fabricated and demonstrated for imaging.
2. Experimental section
The structure diagram of the p-NiO/n-IGZO PD is shown in Fig. 1(a). All the layers were deposited by radio-frequency (RF) magnetron sputtering at room temperature. Firstly, a 50 nm-thick ITO bottom electrode was deposited with a RF power of 60 W in Ar ambient on a Si/SiO2 substrate. Then, a 100 nm-thick IGZO layer was deposited with a RF power of 70 W in Ar/O2 ambient with an O2 partial pressure of 2.5% to reduce oxygen vacancies [30]. A 150 nm-thick NiO layer was deposited with a RF power of 150 W in Ar ambient using a NiO target. Finally, a 50-nm-thick ITO layer was deposited as the top electrode. All layers were patterned by UV lithography and lift-off process. The active area of the UV PD is 300×300 µm2. The transmittance spectra were measured using an UV-Vis spectrophotometer system (Parsee-TU 1901) from 260 to 900 nm. The X-ray diffraction (XRD) spectra were measured by Smartlab 3 kW X-ray diffractometer. Atomic force microscope (AFM) images of the IGZO and NiO films were measured by Benyuan CSPM5500. The current-voltage (I-V) characteristics and UV response properties were measured using a source/measure unit (Agilent 2902A) and a 365 nm UV source at room temperature. The UV power density was measured by a laser power meter (Ophir Nova II).
Fig. 1. (a) Device structure of the fabricated NiO/IGZO p-n junction. (b) n and VTH extracted from the I-V curve measured in dark.
3. Results and discussion
Figure 1(b) shows the I-V characteristics of the fabricated p-n junction. The device exhibits rectifying characteristics with a high rectification ratio of 7.4×104 at ${\pm} $ 4 V and possesses a turn-on voltage (VTH) of 1.63 V. The ideality factor n is found to be 2.7 which is calculated using [31] :
where $q{\; }$ is the elemental charge, ${I_S}$ is a constant, $k\; $ is the Boltzmann constant, and T is the temperature.To characterize the optical performances of the fabricated NiO and IGZO films, 100 nm IGZO and 150 nm NiO films were deposited on quartz substrates. Figures 2(a) and (b) show the fitting of optical bandgap for IGZO and NiO by using Tauc plots method, and the insets are transmittance spectra of IGZO and NiO films, respectively. The optical bandgap can be calculated by Tauc relationship [12] :
where $\alpha $ is the absorption coefficient of the corresponding film, ${E_g}$ is the optical bandgap, $h\upsilon $ is the photon energy, and m = 2 for direct bandgap semiconductors. The NiO and IGZO are direct bandgap semiconductors. And the absorption coefficient $\alpha $ can be calculated by [32] : where T is the transmittance (%), d is the film thickness. The optical bandgaps of IGZO and NiO films extracted from Tauc plots are estimated to be 3.9 eV and 3.7 eV, respectively.Fig. 2. Fitting of optical bandgap of the IGZO (a) and NiO (b) films (Insets are the the transmittance spectra of the films). (c) The XRD results of the NiO and IGZO films. (d) AFM images of the NiO and IGZO films.
Figures 2(a) and (b) also show the optical transmittance spectra of IGZO and NiO films, respectively. All the films possess high visible-light transparency and clear transmission edges in the UV band. Figure 2(c) show the XRD results of fabricated IGZO and NiO films. The XRD results show that there are no diffraction peaks, indicating that the IGZO and NiO films are amorphous. The AFM results in Fig. 2(d) show that the NiO and IGZO films have rather flat surface with root mean square (RMS) roughness of 0.25 and 0.36 nm, respectively. It agrees well with the amorphous states of the XRD results of the two films, and confirms that the interface between the NiO/IGZO heterojunction is highly uniform and homogeneous.
Figure 3(a) shows the current-voltage characteristics measured under dark and 365 nm UV illumination. Both the reverse current and forward current under UV illumination are obviously higher than these in dark conditions, which proves that the fabricated p-n heterojunction can be used for UV light detection and can operate in a self-powered mode. Figure 3(b) is the dynamic current response of the fabricated PD under a range of irradiances of 0.059 to 0.190 mW/cm2 light intensities at 0 V bias. The UV PD has a low dark current of 5 pA at 0 V bias which indicates greater sensitivity. The photocurrent increases rapidly when the UV light source is turned on, and there is no obvious photocurrent degradation with repeated measurement. It can be seen that the photocurrent of the fabricated UV PD is positively associated with the UV illumination power density. The photoresponsivity ($R$) and detectivity (${D_\mathrm{\ast }}$) are calculated by [33] :
where ${I_L}$ and ${I_D}{\; }$ is the diode current under light and in dark, respectively. And P is light power density, e is the elemental charge and A is the effective area of the device. Here, at P = 0.190 mW/cm2 and applied bias = 0 V, the photoresponsivity ($R$) and detectivity (${D_\mathrm{\ast }}$) are calculated as 28.8 mA/W and 6.99×1011 Jones, respectively. To our best knowledge, these two values are among the highest for reported self-powered, oxide-based, p-n heterojunction UV PDs [5,12,24,34]. The sensitivity of the UV PD defined as $({I_L} - {I_D})/{I_D}$ is calculated about 1×103. Figure 3(c) shows the relation between the photocurrent and the UV illumination power density. The fitting result demonstrates that the photocurrent increases very linearly (with an excellent linearity R2 = 0.9989) with the UV illumination power density. The response speed of the UV PD is measured to evaluate its ability to follow a varied optical signal. As is shown in Fig. 3(d), the rising time (defined as the time for the photocurrent increase from 10%${\; }{I_L}{\; }$ to 90%${\; }{I_L}$) and the decaying time (defined as the time required for the photocurrent decrease from 90%${\; }{I_L}$ to 10% ${I_L}$) are extracted to be 15 ms and 31 ms, respectively [35]. Such a fast response speed is in a leading position among the reported oxide-based p-n heterojunction UV PDs. In Table 1, the response time parameters of the device were compared with UV PDs reported in previous works. It can be seen that the UV PD shows great performance despite having the simplest and room temperature fabrication process. Such a fast response speed under 0 V bias is attributable to the built-in voltage of the fabricated p-n heterojunction. The mechanism here will be explained in the next paragraph.Fig. 3. (a) I-V curve under dark and under 365 nm UV illumination. (b) Dynamic response of the UV PD under different UV power illumination at 0 V bias. (c) The dependences of photocurrent on UV power density. (d) One cycle of the current response, showing the corresponding rise(τr) and decay time(τd).
Table 1. Comparison of response time between this work and other reported oxide UV PDs.
In order to understand the generation, separation and recombination processes of electron-hole pairs in the heterojunction when the UV PD is stimulated by UV light, the schematic energy band diagram of the UV PD under UV illumination has been demonstrated in Fig. 4(a). The electron affinity (χ) of IGZO is 4.2 eV [36]. The electron affinity (χ) of NiO is 1.8 eV [37]. The electrons in IGZO layer diffuse into NiO layer, and the holes in NiO layer diffuse into IGZO layer until this process reaches a thermal equilibrium state. At this time, a depletion region forms at the heterojunction interface, and the depletion region generates a built-in electric field directed from IGZO to NiO. The built-in electric field will drive the separation of photogenerated electron-hole pairs. When the UV light is on, the photogenerated electrons will transfer from NiO to IGZO, whereas the photogenerated holes will transfer from IGZO to NiO due to the built-in voltage, leading to a reverse photocurrent under 0 V bias as shown as in Fig. 3(b).
Fig. 4. (a) Schematic energy band diagram of the UV PDs under UV illumination, showing the mechanism of photocurrent generation at 0 V bias; (b) C–V measurement of the p-n junction at 1 MHz.
To obtain the value of the built-in voltage, the capacitance-voltage (CV) characteristic of the fabricated p-n heterojunction is measured at 1 MHz in a dark environment, as shown in Fig. 4(b). Here, the capacitance of a p-n junction is given by [38] :
Due to the rapid response speed, high UV sensitivity, responsivity and detectivity, simple and uniform film preparation process, the NiO/IGZO p-n junction-based UV PD demonstrates significant application potential in UV sensitive imaging. This work demonstrated a 10×10 array based on the NiO/IGZO UV PDs with a device density of 100 PDs /cm2 as shown in Fig. 5(a). In order to image an “H” pattern, a black H-shaped mask is placed above the imaging array to block 365 nm UV light. And the currents between every row and column electrodes are measured at 0 V bias. In Fig. 5(b), the corresponding current distribution of the 10×10 PDs array indicates that the pattern “H” is imaged precisely. This result demonstrates that the fabricated array based on NiO/IGZO UV PDs exhibits a reliable UV imaging capability, and the fabricated UV PD can be an excellent candidate for the UV imaging sensor.
Fig. 5. (a) Digital photo and (b) the corresponding current distribution of the 10 × 10 photodiode arrays aiming to image a letter “H” pattern.
4. Conclusion
In summary, a self-powered UV PD based on the p-NiO/n-IGZO heterojunction has been fabricated at room temperature by using RF magnetron sputtering. The p-n heterojunction exhibits high rectification ratio of 7.4×104 at ${\pm} $ 4 V and a high built-in voltage of 1.61 V. Additionally the photoelectric response characteristics are studied. The UV PD based on the p-n heterojunction show a high responsivity, detectivity and sensitivity about 28.8 mA/W, 6.99×1011 Jones and 1×103 respectively, without external applied bias, under 365 nm UV light illumination. The PD also possesses an ultra-fast response time of 15 ms which is among the best the reported oxide-based p-n heterojunction UV PDs. A 10×10 array based on the UV PDs with a high uniformity is fabricated, which image a letter “H” precisely with zero power consumption. All the fabrication processes are carried out at room temperature. Thus, all the results indicate that the UV PD based on the NiO/IGZO heterojunction fabricated at room temperature exhibits great potential in constructing large-scale integrated UV imaging systems and flexible electronics.
Funding
Royal Society (IEC\R2\170155, NA170415); Engineering and Physical Sciences Research Council (EPSRC, EP/N021258/1); Natural Science Foundation of Shandong Province (ZR2018MF029, ZR2020ZD03); National Natural Science Foundation of China (No. 62074094); National Key Research and Development Program of China (No. 2016YFA0301200).
Disclosures
The authors declare no any conflicts of interest.
Data availability
Date underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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