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Abstract
A high performance AlGaN-based back-illuminated solar-blind ultraviolet (UV) p-i-n photodetectors (PDs) are fabricated on sapphire substrates. The fabricated PD exhibits ultra-low dark current of less than 0.15 pA under -5 V bias, which corresponds to a dark current density of <1.5×10−11 A/cm2. In particular, the PD shows broad spectral response from 240 nm to 285 nm with an excellent solar-blind/UV rejection ratio of more than 103. The peak responsivity at the wavelength of 275 nm reaches 0.19 A/W at -5 V, corresponding to a maximum quantum efficiency of approximately 88%. Based on the absence of any anti-reflection coating, this corresponds to nearly 100% internal quantum efficiency. In addition, the PD shows a quite fast response of 0.62 ms. To the best of our knowledge, this is the record low dark current density and broadest response band reported for the back-illuminated AlGaN-based solar-blind UV detectors.
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1. Introduction
Recently, solar blind ultraviolet (UV) photodetectors (PDs) based on AlGaN-based wide-bandgap semiconductors have been extensively studied due to wide range of potential applications in military and civilian fields [1–5], such as missile warning systems, UV safety communication, environmental monitoring, and biomedical agent detection. A variety of AlGaN-based solar-blind PDs with different device structures have already been demonstrated, including photoconductors, photoelectrochemical (PEC), metal-semiconductor-metal (MSM) PDs, Schottky barrier PDs, and p-i-n PDs [6–17]. Among these structures, p-i-n PDs draw particular interest due to their potentially high quantum efficiency, low dark current, and capability for photovoltaic mode operation. Benefited from its unique structural features allow the PD to be hybridized to a silicon read-out integrated circuit and still collect light through the back of the transparent sapphire substrate, back-illuminated p-i-n UV PDs have become a very attractive candidate for the preparation of large-scale focal plane arrays (FPAs) which are important for imaging applications [2,13–15]. Moreover, the PDs with low dark current and spectral response band covering the whole solar blind region (240-285 nm) are demanded for low-level UV signal detection applications. However, limited by large dark current and premature edge breakdown, the previously reported PDs and FPAs typically have large dark current and narrow band spectral response. In the past decades, driven by the development of deep-UV light emitting diodes, the material quality of high Al-content AlGaN has been greatly improved, which makes it possible to achieve PDs with low dark current. In our previous work, we have successfully fabricated the solar-blind AlGaN MSM PDs with ultra-low dark current [18]. But so far, the reports on the AlGaN solar-blind PDs with broad spectral response are very limited.
In this letter, we report fabrication and characterization of back-illuminated Al0.42Ga0.58N-based p-i-n UV PDs based on metal organic chemical vapor deposition (MOCVD). The PDs exhibit ultra-low dark current, broadest response band, high solar-blind/UV rejection ratio, high quantum efficiency, and high speed.
2. Experimental details
The epitaxial layers for our device fabrication were grown on a double-side-polished c-plane sapphire substrate by MOCVD. Epitaxial growth started with the deposition of a low-temperature AlN buffer layer followed by a 1.2-µm-thick optimized high-temperature AlN layer. Subsequently, a ten-period Al0.71Ga0.29N/AlN (5 nm/5 nm) superlattice structure (SLs) was grown on top of the AlN layer to accommodate the strain and reduce the dislocation density in the epitaxial layers. The SLs layers were followed by a 100-nm-thick unintentionally (uid) doped Al0.71Ga0.29N layers. After this, a 600-nm-thick Si doped n+-Al0.71Ga0.29N layer is grown, which simultaneously serves as the window layer for short wavelength cutoff at 240 nm in back-illumination operation mode. Before the growth of a 200 nm undoped Al0.42Ga0.58N active layer, to reduce the lattice mismatch and avoid the accumulation of the photo-generated carriers due to the band offset between the ohmic contact and active layer, a 20-nm-thick composition-graded uid-AlxGa1-xN (x = 0.71-0.5) followed by a 20-nm-thick n--Al0.5Ga0.5N layer was deposited on the ohmic-contact layer. Following the i-Al0.42Ga0.58N active layer, a 40-nm-thick p-Al0.42Ga0.58N layer was grown. Finally, to reduce the contact resistance of p-type ohmic electrodes, a 200-nm-thick p-type GaN layer and a 2-nm-thick p++-GaN layer were grown on top of the p-i-n structure. The illustrations of complete device structure as well as the doping profile for each layer are shown in Fig. 1(a).
Fig. 1. (a) Schematic cross section shows the AlGaN-based back-illuminated p-i-n UV PD structure. (b) The top-view optical micrograph of the 1 mm×1 mm device.
In the device fabrication process, the mesa was first formed by inductively coupled plasma dry etching. The etch depth is ∼500 nm and down to the n+-Al0.71Ga0.29N contact layer. Then the N-electrode of Ti/Al/Ni/Au (25/200/50/100 nm) stacks are deposited on the surface of n-type Al0.71Ga0.29N by the electron beam evaporation, which is then annealed by rapid thermal annealing to form ohmic contact. The P-electrode comprises Ni/Au (10/10 nm) metal stack. After the metal contact formation, the whole device was covered by a 200 nm SiO2 passivation layer grown by plasma-enhanced chemical vapor deposition. Finally, after selectively etching the SiO2 layer to expose contact regions, a 1.2-µm-thick Ti/Au bi-layer was evaporated to form the contact pads. Figure 1(b) shows a top view image of one finished PD with an effective device area of 1×1 mm2.
3. Result and discussion
The crystalline quality of the active layer is measured by high-resolution X-ray diffraction (XRD) and atomic-force microscopy (AFM). On the one hand, XRD characterization methods were performed to determine the overall structural quality and the alloy compositions of the designed UV PD. Figure 2(a) exhibits XRD pattern with ω/2θ scan around the (002) reflection of the epitaxial structure. It can be observed that there are several main diffraction peaks corresponding to the GaN, Al0.42Ga0.58N, Al0.71Ga0.29N, and AlN buffer layers. The root mean square (RMS) roughness of the AlN buffer layer is 0.195 nm, while full-widths at half-maximum (FWHM) of (002) and (102) planes are 49.6 and 651.1 arcsec (shown in Fig. 2(b)), respectively. The screw threading dislocation (TD) density deduce from the XRD data is ∼5.36×106 cm-2 [18,19], which indicates high crystalline quality of the AlN epilayer. Besides, the FWHM values are obtained as 406.6 and 757.8 arcsec, respectively, from the ω scans for (002) and (102) planes of i-Al0.42Ga0.58N. The screw dislocation densities can be evaluated as 3.42×108 cm-2, which indicates reasonable crystalline quality of the Al0.42Ga0.58N active layer. Furthermore, the reciprocal space mapping of the p-i-n UV PD epi-structure (105) plane has been shown in Fig. 2(c). The ellipse in the lower left corresponds to the reciprocal space pattern of the AlN buffer layer. The pattern consisting of a series of ellipses above is caused by p-Al0.42Ga0.58N/i-Al0.42Ga0.58N/n-Al0.71Ga0.29N superlattice diffraction. As a result, it can be revealed that i-Al0.42Ga0.58N layer is fully strained with respect to the layer beneath and has the same in-plane lattice constant as GaN. On the other hand, Fig. 2(d) exhibits a step-flow morphology determined from a typical 5×5 µm2 scan area for the epitaxial layer surface. The RMS roughness is evaluated as 1.042 nm, which reveals a reasonable morphological quality.
Fig. 2. (a) HR-XRD (002) Omega/2theta scan of the PD epi-structure. (b) The rocking curves of 102 and 002 planes for the AlN template. (c) Asymmetric plane (105) reciprocal space pattern of epitaxial structure. (d) Typical 5×5 µm2 AFM image of the epitaxial layer surface.
Figure 3 shows the dark-current and photocurrent characteristics of the fabricated PD as a function of external bias at room temperature. The PD exhibits an ultra-low dark current of less than 0.15 pA at 5 V reverse bias, which corresponds to a dark current density of <1.5×10−11 A/cm2. This is the lowest dark current density ever reported for back-illuminated AlGaN-based solar-blind UV PDs [2,13,20–25]. The low dark current of the PDs is due mainly to the low TD density in the Al0.42Ga0.58N epilayers and the high-quality p-i-n junction. Simultaneously, the forward dark current is 3.9×10−6 A under 5 V. It can be seen that the dark current-voltage characteristics show good rectifying behavior, with a sharp turn-on at 3.4 V and a rectification exceeding 106. Furthermore, as shown in the insert of Fig. 3, the photocurrent of ∼0.6 µA can be obtained at 5 V reverse bias for an incident optical power of 3.1 µW/mm2, indicating that a high photo-to-dark current ratio of exceed 106 is observed in reverse bias range.
Fig. 3. The dark current-voltage cures of the device at RT. The inset shows the dark and 250-nm light illuminated current-voltage cures under reverse bias.
Optical transmission spectroscopy was used to confirm the Al content of the i-AlGaN absorption layer. As shown in the insert of Fig. 4, the optical transmission spectrum exhibits a cutoff wavelength of 285 nm, corresponding to an Al-content of ∼0.42 for i-AlGaN. As shown in Fig. 4, voltage-dependent response spectra calibrated by standard Si-PD exhibit reasonably high photo-response performance in the solar-blind region. The peak responsivity and corresponding quantum efficiency are 0.10 A/W and 45% at 0 V, while 0.19 A/W and 88% at -5 V, respectively. The responsivity of the PD gradually increases as a function of bias and tends to saturate at ∼1 V, possibly due to the full depletion of the i-Al0.42Ga0.58N active region. The PD shows an excellent solar-blind/UV rejection ratio of more than 103, we ascribed the high rejection ratio to the high photoresponse and the ultra-low dark current. In addition, the photoresponse curve of the PD shows a sharp cutoff at 285 nm, which is in good agreement with the band gap of Al0.42Ga0.58N. The cut-on for short wavelength of the PD is 240 nm because most UV light would be absorbed by the beneath active layer and connot form photocurrent under back-illumination conditions. It is worth noting that the response spectra show broad peaks located at region ranging from 240 nm to 285 nm, which covers the whole solar blind region. Broad band spectral response is more conducive to the probability of detecting the target UV signal.
Fig. 4. Spectral responsivities of the p-i-n PD at bias voltages of 0V to -5V at RT. The inset shows the transmission spectrum of the i-AlGaN active layer.
Finally, the temporal response of the PD at zero bias is measured under the irradiation of 275 nm UV light-emitting diode driven by a pulse generator. As shown in Fig. 5(a), the on/off switching behavior can be well maintained after several measurement cycles, which indicates the device has excellent stability and reproducibility. To estimate the response time of the PD, the normalized time-dependent photocurrent curve is show in Fig. 5(b), the decay time of the PD (defined as the falling time interval from 90% to 10% of the maximum value under illumination) is 0.62 ms, which indicates a high response speed for the AlGaN-based back-illuminated p-i-n PDs.
Fig. 5. Optical switch measurements were carried out by manually switching on/off 275 nm UV illumination with a period of 40 s at applied voltage of 0 V. (b) Response time of the PD measured by using a 265-nm LED with a 2 Hz signal.
For a better comparison, Table 1 summarizes the critical parameters of recently reported AlGaN solar-blind PDs. Our PD exhibits the lowest dark current and broadest spectral response. Furthermore, a reasonable response speed of 0.62 ms is also achieved in our PD. The overall improvement of the device performance is due to the optimized structural design to avoid short wavelength absorption in the beneath active layer as well as the use of a low TD density AlN buffer layer which allows for the growth of a high crystalline quality high-Al-content AlGaN epi-layer.
Table 1. Detector Performance Comparison for AlGaN-based p-i-n UV PDs
4. Conclusion
In summary, AlGaN-based back-illuminated solar blind p-i-n PDs have been grown by MOCVD. The PD exhibits an ultra-low reverse dark current of < 0.15 pA under -5 V and high peak responsivity of 0.19 A/W at -5 V with a maximum quantum efficiency of approximately 88%. In particular, the response curve of the PD shows a broad range from 240 to 285 nm with a quite fast response of 0.62 ms. The excellent performance can be attributed to the optimized PD design as well as the high crystalline quality of the AlGaN epi-structure.
Funding
National Natural Science Foundation of China (11874012, 12174002, 61974056); Jiangsu Provincial Key Research and Development Program (BE2020756); Key Technologies Research and Development Program of Anhui Province (202104a05020059); the University Synergy Innovation Program of Anhui Province (GXXT-2020-052); the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCY20_1769).
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.
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