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Abstract
With vacuum thermal evaporation, the CuI film was deposited on quartz and n-GaN substrates, and the morphology, crystalline structure and optical properties of the CuI films were investigated. According to the XRD results, the CuI film preferentially grew along [111] crystal orientation on the GaN epilayer. With Au and Ni/Au ohmic contact electrodes fabricated on CuI and n-GaN, a prototype p-CuI/n-GaN heterojunction UV photodetector strong UV spectral selectivity was created. At 0 V and 360 nm front illumination (0.32 mW/cm2), the heterojunction photodetector displayed outstanding self-powered detection performance with the responsivity (R), specific detectivity (D*), and on/off ratio up to 75.5 mA/W, 1.27×1012 Jones, and ∼2320, respectively. Meanwhile, the p-CuI/n-GaN heterojunction photodetector had excellent atmosphere stability.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultraviolet (UV) detection technology, which was created after infrared detection technology, is utilized in fire warning, environmental monitoring, biochemical analysis, security communications, intercontinental missile, and stealth vehicle detection, space exploration, and other fields because of its low false alarm rate, good concealment, simple structure, small size, easy integration, and other incomparable technical advantages of photoelectric detection methods. UV photodetectors (PDs) based on broadband semiconductors (general bandwidth Eg > 3.0 eV) feature a wide spectral response range, high quantum efficiency, outstanding linearity, and high-frequency operation, especially the self-powered heterojunction PDs. Self-powered UV PDs have received a lot of attention in recent years due to their ability to operate at zero bias without requiring external power and their wide range of applications [1–10]. In recent decades, AlN (6.2 eV) [11], β-Ga2O3 (4.9 eV) [12], ZnMgO (3.9 eV) [13], ZnO (3.37 eV) [14], and GaN (3.4 eV) [15] have all been explored for UV photodetection. Among them, GaN has been used to manufacture UV PDs because of its high breakdown electric field strength [16], high operating temperature [17], low device on-resistance [18], and high electron density [19]. However, the relatively low hole mobility limited the photoresponse performance.
In the last few years, g-CuI, a direct bandgap wide band semiconductor material with a wide band gap of 3.1 eV, big exciton binding energy (62 meV), and large hole mobility [14,20–22], has gotten a lot of interest. The hole mobility of CuI is up to 43.9 cm2V-1s-1 in single crystals, and even in polycrystalline CuI films, the hole mobility can reach 25 cm2V-1s-1 [23], which is significantly greater than that of other broadband semiconductors. Moreover, thanks to interband absorption and exciton absorption, CuI can achieve very strong UV absorption at room temperature with absorption coefficients up to 105 cm-1, which is very advantageous for boosting the quantum efficiency of solar cells and UV detectors [24]. In addition, CuI is non-toxic, abundant, and low-cost, making it a new choice for the fabrication of high-performance, low-cost optoelectronic devices. The high hole mobility and wide band gap of p-CuI can be combined with an n-GaN epitaxial layer with a matching energy band structure to construct a heterojunction PD that should be able to achieve good photoresponse and self-powered features.
Herein, the high-quality CuI thin films, which were close to single crystal, were deposited on GaN epilayers by vacuum thermal evaporation approach, and p-CuI/n-GaN heterojunction PDs were constructed. The crystalline structure, morphology and photoelectric properties of the PD have been investigated. The self-powered p-CuI/n-GaN heterojunction PD features a high on/off ratio, high responsivity, and self-powered performance. And the heterojunction PD still worked well after being exposed to the atmosphere for 100 days.
2. Experiments
2.1 Materials
CuI powder (Alfa Aesar, 99.998%) was used as source material. The double-sided polished quartz wafers (15mm×15 mm, Advanced Election Technology Co., Ltd) and n-GaN epitaxial films on Al2O3 substrate (10mm×5 mm, Suzhou Nanowin Science and Technology Co., Ltd) were used as substrates. All of the substrates were cleaned for 15 minutes in acetone with an ultrasonic cleaner before being dried with high purity nitrogen flow
2.2 Preparation of the CuI films
During CuI thermal evaporation growth, the vacuum chamber pressure was 1.0×10−4 Pa, and the substrate temperature was 100°C. A rate/thickness monitor with a quartz crystal sensor (INFICON STM-2XM) was used to measure the growth rate and film thickness while the films were being deposited. The CuI film grew at a rate of 0.1 Å/s. CuI thin films with thickness of 200 nm were deposited on the quartz wafer and the n-GaN epilayer.
2.3 Fabrication of heterojunction photodetector
About 30 nm ohmic contact Au and Ni/Au electrodes were prepared on p-CuI and n-GaN substrate by thermal evaporation, respectively, and the p-CuI/n-GaN heterojunction PD was fabricated, as shown in Fig. 1(a). The linear I-V curves of Au/Ni-GaN and Au-CuI, as shown in Fig. 1(b), indicated that ohmic contacts were formed between the electrodes and the device.
Fig. 1. (a) Schematic diagram of the p-CuI/n-GaN photodetector. (b) I-V curves of Au electrode-CuI and Au/Ni electrode -GaN.
3. Results and discussion
Using vacuum thermal evaporation, 200 nm thick CuI thin films were prepared on quartz wafer and n-GaN epilayer. An X-ray diffractometer (Bruker AXS D8 ADVANCE) with Cu Ka radiation (l=0.15406 nm), was used to analyze the crystalline structure of the CuI thin film and the p-CuI/n-GaN heterojunction. Figure 2 showed the XRD results of CuI thin films on different substrates. For the CuI on quartz wafer, the XRD patterns in Fig. 2(a) included a strong diffraction peak at 25.67° and a much weaker diffraction peak at 52.48°, corresponding to (111) and (222) lattice planes of the face-centered cubic (fcc) structure g-CuI (PDF#75-0831), indicating that the CuI film grew along the [111] orientation preferentially. Meanwhile, the tiny FWHM (full width at half maximum) of 0.165° of the (111) peak indicated that the CuI thin film had good crystallization quality.
The XRD patterns of the CuI/GaN heterojunction as shown in Fig. 2(b), included two diffraction peaks at 25.67° and 34.6° corresponded to the (111) plane of fcc g-CuI (PDF#75-0831) and (002) plane of GaN (PDF#74-0243), respectively. The positions of the diffraction peaks of CuI were the same as those of CuI on quartz, indicating that CuI films grown on these two substrates had nearly identical crystal structure, and the CuI also grew preferentially along the [111] crystal direction on the GaN surface. On the other hand, they should also have almost the same photoelectric properties. Therefore, we can use the CuI thin film on quartz wafer to study the optical properties of CuI, such as absorption and photoluminescence (PL) properties.
The XRD patterns of GaN substrate was shown in Supplement 1, Fig. S1.
The optical properties of CuI films were explored using 200 nm thick CuI films deposited on quartz substrates. The absorbance spectra of the CuI films were obtained using a UV-visible spectrophotometer (UV-2600i, SHIMADZU). Figure 3(a) depicts the absorption spectrum of CuI film. Tauc plot [25,26] can be used to compute the direct optical band gap of CuI,
where α is the absorption coefficient of the material, hν is the photon energy, Eg is the energy gap of CuI, and A is a constant. As illustrated in the inset of Fig. 3(a), the optical bandgap of CuI thin film is 3.01 eV, which is consistent with earlier findings [27].Fig. 3. (a) Absorption spectrum of CuI film, the inset is optical band gap of the CuI thin film calculated by Tauc Plot. (b) PL spectrum of CuI thin film, the inset is the schematic diagram of the PL mechanism of CuI thin film.
The PL spectra were captured using a homemade setup consisting of a He-Cd laser (operating at 325 nm, Kimmon Koha Co., Ltd) and an imaging spectrometer (SR-500i, ANDOR). Figure 3(b) presented the PL spectrum of a CuI film stimulated with a 325 nm He-Cd laser. As shown in the figure, there were three emission bands in the PL spectrum in the range of 350nm∼850 nm. The blue emission band can be divided into two peaks, centered at 410 nm (Peak1) and 420 nm (Peak2), respectively, as shown in the insert of Fig. 3(b). The emission at 410 nm was linked with the transition of interband free excitons [28,29], while the emission at 420 nm was associated with the trap level resulting from Cu vacancies [28,30]. The Tauc plot result matched the PL result. Peak2 was identical in intensity to peak1 according to the fitting curves of the PL spectra in Fig. 3(b), showing that the CuI film had many Cu vacancies, due to the vacuum thermal evaporation technology's inherent properties. As for the broadened red emission centered at 700 nm, the majority believe that it is attributable to the iodine vacancy [28,31]. The infrared band was the second-order diffraction of the blue band caused by the working principle of the grating monochromator.
The PL spectra of n-GaN epilayer and CuI/GaN heterostructure stimulated with 325 nm He-Cd laser were shown in Fig. 4. The PL spectrum of GaN (Fig. 4(a)) contained a narrow UV emission peak and a broadened yellow-green emission band. The former was derived from interband exciton recombination, while the latter corresponded to the n-type impurity levels. However, in the PL spectrum of CuI/GaN heterostructure, as shown in Fig. 4(b), there was no UV emission peak corresponding to the interband exciton recombination of GaN, and just one broadened yellow-green emission band and the PL peak of CuI. We believe that the UV emission of GaN was absorbed by the upper CuI layer and produced more PL emission of CuI. On the other hand, it should be noted that due to the small thickness of CuI thin film, the absorption of excitation light by CuI was low, a considerable amount of excitation light can be introduced into the GaN epilayer through the CuI layer to activate PL of GaN.
A field-emission scanning electron microscope (FE-SEM, FEI Nano Nova 450) was employed to examine the morphology of the CuI thin film and the heterojunction device. The surface morphology and cross-sections of the GaN epilayer and p-CuI/n-GaN heterojunction PD were depicted in Fig. 5(a) and 5(b). As can be seen from Fig. 5(a), the GaN epilayer was compact with a smooth surface, without obvious structural defects. The thickness of GaN epilayer was ∼5mm, which was much bigger than the thickness of the CuI film, as shown in the inset of Fig. 5(b). From Fig. 5(b), it can be observed that the CuI film comprised of crystal grains of various sizes with a thickness of ∼200 nm, and the surface of the CuI layer was uniform. In addition, as indicated in the inset of Fig. 5(b), the interface was clear and there were no broken tiny grains, which indicated that the density of interface defects was low. And more, the CuI crystal grains grew perpendicular to the GaN surface, which corroborated the XRD results. The cross-sectional distribution of Cu, I, Ga, and N elements in the heterojunction device was illustrated in Fig. 5(c).
Fig. 5. (a) The surface and cross-section of the n-GaN epilayer. (b) The surface and cross-section of the p-CuI/n-GaN heterojunction photodetector. (c) EDS mapping of p-CuI/n-GaN heterojunction photodetector.
The self-powered photoresponse characteristics were investigated after the p-CuI/n-GaN PD was fabricated. The current-voltage (I-V) data was collected with a Keithley 2612B digital sourcemeter. A 150W Xe lamp with a grating monochromator (RF5301PC, SHIMADZU Corporation, 150W) was used for photoresponse measurements of the p-CuI/n-GaN heterojunction PD. The spectrum of the Xe lamp was shown in Supplement 1, Fig. S2.
The responsivity (R in A/W) and specific detectivity (D* in Jones) of a heterojunction PD operated at 0 V can be calculated using the following equations [32,33],
where Iph and Idark are the photocurrent and dark current generated by the PD, Plight is the power of the incident illumination, q is the electron charge (1.602×10−19C), and S is the effective area in cm2.According to the above two equations, the spectral responsivity and specific detectivity curves of the p-CuI/n-GaN detector can be obtained at 0 V with weaker incident light, as shown in Fig. 6(a). The PD displayed a robust response and specific detectivity in the wavelength range of 275-370 nm, indicating that the p-CuI/n-GaN detector was a UV PD, according to the figure. At 0 V, the maximum responsivity, measured at 360 nm with a light power density of 0.32 mW/cm2, was 75.5 mA/W, with a peak specific detectivity of 1.27×1012 Jones. And for the reduced peaks in the 550-740 nm region, we believe that they are due to second-order diffraction of 275-370 nm light from the grating monochromator. Furthermore, R and D* exhibited a steep decline between 360 nm (R360 = 75.5 mA/W) and 400 nm(R400 = 0.20 mA/W), indicating strong UV spectral selectivity with a UV/visible rejection ratio (R360/ R400) of ∼380.
Fig. 6. (a) Responsivity (red curve) and specific detectivity rate curve (blue curve in the inset) of p-CuI/n-GaN heterojunction at 0 V. (b) I-V curves of p-CuI/n-GaN heterojunction at 0 V with 360 nm illumination(0.32 mW/cm2). The inset is on/off ratio curve. (c) Time-dependent photoresponse curve of the photodetector at 0 V with 360 nm illumination(0.32 mW/cm2). (d) The response and recovery times of the photodetector at 0 V with 360 nm illumination(0.32 mW/cm2).
The I-V curve under dark and 360 nm front illumination (0 V, 0.32 mW/cm2) was depicted in Fig. 6(b), illustrating the evident rectification features of the p-CuI/n-GaN heterojunction device. At 0 V, the photocurrent and dark current were ∼3.24 × 10−10A and ∼7.51 × 10−7A, respectively, resulting in an on/off ratio of ∼2320, as illustrated in the inset of Fig. 6(b), demonstrating a high level of self-powered photo response characteristic.
For comparison, the performances of CuI-related photodetectors were listed in Table 1.
Table 1. Comparison of the key parameters for CuI-based photodetectors
By repeatedly turning the light source (360 nm, 0.32 mW/cm2) on and off, a time-dependent photoresponse test at 0 V was performed to further explore the photodetector's stability and reproducibility. As demonstrated in Fig. 6(c), the p-CuI/n-GaN detector had excellent stability and repeatability at 0 V, and the photodetector's time-dependent photoresponse curve at various biases was provided in Supplement 1, Fig. S3. In addition, the response and recovery times of the heterostructure device were 160 ms and 158 ms, respectively, as shown in Fig. 6(d).
The photoresponse characteristics of various light power densities were also examined. The I-V curves of the PD under 360 nm UV irradiation with varying light power densities were shown in Fig. 7(a). At 0 V, the photocurrent(Iph) increased rapidly as the light power density increased. At the same light power density, the Iph increased with increasing negative bias.
Fig. 7. (a) The photocurrent under 360 nm ultraviolet light illumination with different light power density. (b) The fitting curve of the photocurrent and light power density of the photodetector at 0 V under 360 nm illumination. (c) The on/off ratio at different light power density under 0 V and 360 nm illumination. (d) The responsivity (R) and specific detectivity (D*) at different light power density under 0 V and 360 nm illumination.
To better understand the electrical reaction of the device features to light, Fig. 7(b) depicted the dependency of the photocurrent on the light power density, which can be characterized by a power law [37,38],
where C is a constant for a certain wavelength, and q is the exponent that determines the response of photocurrent to light power density.The photoconducting mechanism type for photodiodes is determined by the value of <ιτ>q</it>. If <ιτ>q </it>is between 0.5 and 1, there is a continuous distribution of trap levels, and these continuous localized states regulate the photoconduction mechanism. In this work, the value of <ιτ>q</it> was 0.83, which was in the range of 0.5 to 1, implying that localized states in the mobility-gap of structures were present [39–41], and that more photogenerated electron-hole pairs would result in a higher photocurrent if the light intensity was increased [42,43]. As a result, the on/off ratio of the device increased as well, with a linear relationship to the light power density, and the on/off ratio can reach 5.9 × 104 under 18.38 mW/cm2 illumination, as shown in Fig. 7(c).
However, the responsivity and specific detectivity dropped when the light power density rose, as illustrated in Fig. 7(d).
The band diagram based on the Anderson model will help to understand the mechanisms for the photoresponse and carrier transport of the p-CuI/n-GaN PD. The band gaps (Eg) of g-CuI and GaN are 3.1 eV and 3.4 eV, whereas their electron affinities are 2.1 eV and 4.2 eV [15], respectively. The carrier concentrations of the materials were measured by the Hall effect test (Lake Shore Model 8404 Hall Effect Measurement System). According to Hall measurement, g-CuI deposited by vacuum thermal evaporation technique is p-type with a hole concentration of pCuI = 1.2×1018/cm3, and the electron concentration of n-GaN epilayer is nGaN = 1.0×1018/cm3. The effective masses of g-CuI and GaN are taken as 0.3m0 [44] and 0.37m0 [45]. So the effective density of states in the valence band (NV) and the effective density of states in the conduction band (NC) can be calculated to be 4.1×1018/cm3 and 5.6×1018/cm3 for g-CuI and n-GaN. For the nondegenerate semiconductors (pCuI< NV, nGaN< NC), the Fermi levels of -5.17 eV and -4.22 eV for p-CuI and n-GaN can be obtained [46], yielding a built-in voltage of 0.95 eV. Thus we can get the schematic band diagram for the p-CuI/n-GaN heterojunction, as shown in Fig. 8.
When the surface of the PD was illuminated, the incident light passed through the CuI layer with minimal absorption, and light absorption largely occurred in the GaN epilayer, generating electrons and holes. The photogenerated carriers were swiftly separated by the large built-in electric field, which drove the photogenerated holes through the CuI layer and onto the Au electrode, while the electrons were driven to the Ni/Au electrode. Meanwhile, the photogenerated electrons were prevented from entering the CuI layer to form self-recombination due to the significant conduction band offsets (ΔEC = 2.1 eV). Without any bias applied, the photocurrent was generated.
Furthermore, the long-term stability of the film and PD was also examined after 100 days of storage in the atmosphere. The crystalline structure and PL of the CuI thin film and PD were almost unchanged, as shown in Supplement 1, Fig. S4. After 100 days, the photocurrent of the PD remained quite steady at 360 nm (0 V, 0.32 mW/cm2), as shown in Supplement 1, Fig. S5. As indicated by the aforementioned results, the CuI thin film and CuI/GaN heterojunction PD prepared by vacuum thermal evaporation demonstrated excellent atmosphere stability.
4. Conclusion
In summary, high-quality CuI films were produced on quartz and GaN substrates utilizing a vacuum thermal evaporation approach, and a p-CuI/n-GaN heterojunction PD was fabricated. On quartz and GaN substrates, the CuI film was a polycrystalline g-phase and grew along the [111] orientation preferentially. The p-CuI/n-GaN heterojunction PD demonstrated remarkable self-powered photoresponse characteristics and spectrum selectivity. At 0 V and 360 nm UV illumination (0.32 mW/cm2), the peak responsivity and corresponding specific detectivity were 75.5 mA/W and 1.27×1012 Jones, respectively, with an on/off ratio of ∼2320. The p-CuI/n-GaN heterojunction PD also demonstrated outstanding atmospheric stability, with the crystalline structure and photoresponse performance remaining nearly intact after 100 days in the atmosphere.
Funding
Natural Science Foundation of Shandong Province (ZR2021MF121); National Natural Science Foundation of China (62075092); Yantai City-University Integration Development Project (2020XDRHXMP11, 2021XDRHXMXK26).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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