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Accepted: Jul. 31, 2019

Posted: Sep. 9, 2019

Published Online: Sep. 9, 2019

The Author Email: Xinke Liu (liuxinke@ciomp.ac.cn), Dabing Li (lidb@ciomp.ac.cn)

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You Wu, Zhiwen Li, Kah-Wee Ang, Yuping Jia, Zhiming Shi, Zhi Huang, Wenjie Yu, Xiaojuan Sun, Xinke Liu, Dabing Li. Monolithic integration of MoS2-based visible detectors and GaN-based UV detectors[J]. Photonics Research, 2019, 7(10): 10001127

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## Abstract

With the increasing demand for high integration and multi-color photodetection for both military and civilian applications, the research of multi-wavelength detectors has become a new research hotspot. However, current research has been mainly in visible dual- or multi-wavelength detectors, while integration of both visible light and ultraviolet (UV) dual-wavelength detectors has rarely been studied. In this work, large-scale and high-quality monolayer $MoS2$ was grown by the chemical vapor deposition method on transparent free-standing GaN substrate. Monolithic integration of $MoS2$-based visible detectors and GaN-based UV detectors was demonstrated using common semiconductor fabrication technologies such as photolithography, argon plasma etching, and metal deposition. High performance of a 280 nm and 405 nm dual-wavelength photodetector was realized. The responsivity of the UV detector reached 172.12 A/W, while that of the visible detector reached 17.5 A/W. Meanwhile, both photodetectors achieved high photocurrent gain, high external quantum efficiency, high normalized detection rate, and low noise equivalent power. Our study extends the future application of dual-wavelength detectors for image sensing and optical communication.

## 1 引言

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## 2 单应性矩阵估计

In this work, large-scale SL $MoS2$ was grown on a lattice-matched and transparent free-standing (FS) GaN wafer by the CVD method. Common but efficient semiconductor fabrication processes were utilized to obtain monolithic integration of $MoS2$-based visible and GaN-based UV detectors. Both the materials and devices were fully characterized. Raman spectroscopy, transmission electron microscopy (TEM), and absorption spectroscopy were used to characterize the as-grown $MoS2$ material, proving that a high-quality SL $MoS2$ material was obtained. The photoresponsive properties of $MoS2$-based visible and GaN-based UV detectors under different incident light power conditions have been investigated. The photoresponsivity $R$, photoconductive gain $G$, external quantum efficiency (EQE), noise equivalent power (NEP), normalized detectivity $D*$, and the switching characteristics of the monolithic integration dual-wavelength detector were systematically studied in this work.

## A. Sample Preparation and Device Fabrication

Figure 1. Stereo vision measurement principle diagram of zoom lens

$H*=argmin∑k=1nm2k-Hm1k2。(1)$

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Figure 2. Diagram of homography matrix in zoom process

$HLR2=HL12HLR1(HR12)-1。(2)$

## 3.1 内参线性估计

A (0001)-oriented Ga face FS-GaN substrate with thickness of $∼350 μm$ was grown by the hydride vapor phase epitaxy (HVPE) method. Ammonia ($NH3$) and metal Ga/HCl were used as the nitrogen source and gallium source, respectively, and $N2/H2$ mixed gas was used as the carrier gas. After HVPE growth, it was further polished by chemical mechanical polishing (CMP) to obtain a surface with a surface roughness of 0.2 nm. A two-inch FS-GaN substrate was cut into pieces (1 cm by 1 cm) for the growth process, which is limited by the CVD chamber. The GaN substrate was ultrasonically cleaned with acetone, isopropanol, and deionized water for 10 min each time before being placed in the CVD chamber. 20 mg of molybdenum oxide ($MoO3$) was located in an alumina crucible with an FS-GaN substrate upside down on the center of the crucible. 8 g of sulfur (S) powder was laid upstream, and the alumina crucible was located near the center in 100 sccm of argon (Ar) gas flow. The tube was thoroughly cleaned by Ar gas flow to remove air before the growth process. The SL $MoS2$ was grown at 750°C for 10 min. The growth temperature has a significant effect on the thickness and shape of the $MoS2$ layer. The optimized growth conditions were selected in this study. After the SL $MoS2$ was grown on the GaN sample, a photolithography process was performed, and then part of the $MoS2$ was etched by the Ar plasma without the protection of the photoresist. Standard photolithography and electron beam evaporation (EBE) for Au electrode (50 nm) preparation were used to define the electrode.

## B. Materials and Device Characterizations

The incident laser wavelength of the Raman spectra was 514 nm from an Ar ion laser with laser power of 0.025 mW to avoid laser-induced heat. The absorbance spectra were tested by a UV-visible spectrometer (Shimadzu-2450) from 350 to 800 nm. Current-voltage (I-V) characteristics of the devices were measured by a Keithley 4200-SCS semiconductor analyzer. All measurements of materials and devices were carried out at room temperature.

## 3. RESULTS AND DISCUSSION

Figure 2(a) schematically illustrates the $MoS2$ growth process by CVD. $S$ powder and $MoO3$ were used as precursors, while a Ga-face FS-GaN substrate ($1 cm×1 cm$) was placed upside down on the center of the alumina crucible. The $MoS2$ growth was completed in an Ar atmosphere with a growth temperature of 750°C for 10 min. The detailed growth process was described in the previous work [35]. The $MoS2$ grown on GaN was characterized, and the Raman spectrum was shown in Fig. 2(b). Raman spectrum measurement is based on the Raman scattering effect of the laser passing through the sample, reflecting the interlaminar vibration and intralayer vibration of the material, which is an important means for structural characterization of 2D layered materials [36]. For the Raman spectrum of 2D $MoS2$, only $E2g1$ and $A1g$ optical modes can be observed, which represent the in-plane vibration modes of $S$ and Mo atoms and the out-of-plane vibration modes of $S$ atoms. In addition, the full width at half-maximum (FWHM) of the $E2g1$ peak characterizes the crystalline quality of 2D $MoS2$. The value of FWHM for peak $E2g1$ here is about $4.6 cm−1$, which indicates the high quality of $MoS2$. Although the FWHM here is slightly larger than that of mechanical exfoliated $MoS2$, large-scale continuous $MoS2$ on GaN was obtained by the CVD method in this work. The frequency difference ($Δk$) between the $E2g1$ and $A1g$ Raman peaks is also important information for analyzing the thickness of $MoS2$. The positions of the $E2g1$ and $A1g$ peaks are at 384.5 and $403.5 cm−1$, respectively, and the frequency difference $Δk$ is about $19 cm−1$, corresponding to monolayer $MoS2$ which is consistent with other literatures [29,37,38]. An additional strong peak centered at around $418 cm−1$ was observed, which is in accord with the high-order harmonic frequency of an acoustic phonon of GaN, and is not a Raman active mode [39,40]. Figure 2(c) is the cross-sectional transmission electron microscopy (TEM) image of the $MoS2$ grown on GaN substrate. The measured $MoS2$ film thickness is $∼0.7 nm$, indicating a single layer of $MoS2$, which is consistent with the Raman measurement result. Figure 2(d) shows the absorbance of the SL-$MoS2$ grown on GaN as a function of incident wavelength. There are several absorption peaks in the visible region for SL $MoS2$: 430.9, 611.8, and 658.12 nm, respectively, and the absorption at 430.9 nm is the strongest, which is the exciton peak associated with the band edge. The excitonic absorption peaks between 600 and 700 nm observed for $MoS2$ arise from the $K$ point of the Brillouin zone [41,42]. The spin-orbit splitting of the valence band of $MoS2$ results in an energy difference between the two exciton peaks. Absorbance is the quantification of the absorption intensity, which is defined as $A=log10(I0/It)$, where $I0$ is the intensity of original light and $It$ is the intensity of transmitted light [43]. According to the Beer–Lambert law, the relationship between $It$ and $I0$ is described as $It=I0e−αt$, where $t$ is the thickness of material and $α$ is the absorption coefficient. The absorption coefficient $α$ is calculated using the formula: $α=ln(10A)/t$. $A$ is 0.15 at 405 nm, indicating 29.2% of incident light intensity is absorbed. The thickness of SL $MoS2$ is about 0.617 nm. The value of absorption coefficient $α$ calculated in this work is $5.6×106 cm−1$, which is similar to the absorption coefficient of the multilayer $MoS2$ we studied before [28]. The obtained absorption coefficient $α$ of the $MoS2$ grown on FS-GaN is about one order of magnitude higher than that of the 2D $MoS2$ grown on $SiO2/Si$ substrate, which may result in a highly responsive $MoS2$ photodetector. The high absorption coefficient $α$ in this work could be due to growth on a nearly lattice-matched and a small thermal expansion mismatch FS-GaN substrate, resulting in a high quality of $MoS2$.

Figure 2. (a) Illustration of SL $MoS2$ growth process by CVD under Ar atmosphere. The FS-GaN substrate was placed upside down on the center of the crucible, and the SL-$MoS2$ was grown on the Ga-face. The growth condition was 750°C lasting for 10 min. (b) Raman spectrum of SL-$MoS2$ on FS-GaN. (c) Cross-sectional transmission electron microscopy (TEM) image of the $MoS2$ grown on the GaN substrate. The measured $MoS2$ film thickness is $∼0.7 nm$, indicating a single layer of $MoS2$. (d) Absorbance of SL $MoS2$ on the FS-GaN substrate as a function of incident wavelength. The strongest absorption is at around 430 nm.

The schematic diagram of the experimental process for monolithic integration of GaN-based UV detectors and $MoS2$-based visible detectors is shown in Fig. 3(a). A large-scale SL $MoS2$ was grown on the pre-cleaned FS-GaN substrate by CVD. Then the SL $MoS2$ on the FS-GaN sample was etched by Ar plasma to remove part of the $MoS2$ that was not protected by photoresist. After the photoresist was removed, an interdigitated Au electrode (50 nm) was prepared on the entire sample to obtain the monolithic integrated GaN PDs and $MoS2$ PDs by standard photolithography and metal evaporation. Figure 3(b) is the optical microscope image of monolithic integration of $MoS2$-based and GaN-based detectors, and the reference scale in the image is 100 μm. Each photodetector owns an active area of 47,376$μm2$. Figure 3(c) is the 3D schematic view of $MoS2/GaN$ monolithic integration device. The top view photo image of the fabricated detector is shown in the inset. In this work, monolithic integration of GaN-based detectors and $MoS2$-based detectors was achieved in a feasible and low-cost way.

Figure 3. (a) Schematic diagram of the preparation steps of monolithic integration of GaN-based UV detectors and $MoS2$-based visible detectors. Photoresist was used for mask, and part of the SL $MoS2$ was etched by Ar plasma. Standard photolithography was applied, and a 50 nm Au electrode was fabricated by electron beam evaporation for the whole wafer. (b) Optical microscope image of monolithic integration of GaN-based and MoS2-based detectors. The reference scale in the image is 100 μm. (c) 3D schematic view of the $MoS2/GaN$ monolithic integration device. Top view photo image of $1 cm×1 cm$ size sample fabricated is shown in the inset.

Figure 4(a) shows dark current and light current under 280 nm incident light with different incident power of GaN PDs as a function of voltage, while Fig. 4(b) shows that of $MoS2$ PDs under 405 nm laser illumination. Our study before has compared the three different laser wavelengths, 405, 532, and 638 nm. It was found that the $MoS2$ PD under the incident light of 405 nm has the best photodetector performance, which is because of a high-photocurrent gain mechanism and high absorption ratio [28]. For 280 nm UV light, it can be absorbed by GaN and then generate photo-generated carriers to achieve UV detection. Thereby, dual-color detection was achieved in this study. The ratio of $Ilight/Idark$ for GaN PDs is about $101$$102$ at a bias voltage of 20 V. It shows as good response to UV light, which is comparable to the results of previous studies [44,45]. As for $MoS2$ PDs, the ratio of $Ilight/Idark$ here is $∼100$$101$ at a bias voltage of 3 V.

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Figure 4. (a) Dark current and light current for 280 nm incident light under different powers of the GaN PD. (b) Dark current and light current for a 405 nm laser under different incident powers of the $MoS2$ PD.

The photoelectric characteristics of both the GaN-based UV detector and $MoS2$ visible detector are calculated as shown in Fig. 5. The responsivity ($R$), the quantification of the photosensitivity of PDs under certain wavelength light, is described as $R=IphPA$, where photocurrent $Iph$ is calculated by $Iph=Ilight−Idark$, $A$ is the active area of PD, and $P$ is the power density of incident light. Figures 5(a) and 5(d) are photocurrent and responsivity of the GaN PD and $MoS2$ PD at 20 V and 3 V, respectively. The peak photoresponsivity of the GaN PD is 172.12 A/W under incident light power of 15 nW, while that of $MoS2$ PD is 17.5 A/W under 2 mW laser illumination. This rather high photoresponsivity may be associated with longer carrier lifetime and less carrier recombination in the 2D material under weaker illumination. As shown in Fig. 5(d), as the incident laser power decreases, the $R$ of $MoS2$ PD increases slightly, which is in accord with previous reports in $MoS2$ and other 2D-based devices [46,47]. When the light power density increased enough, the trap states would be saturated, resulting in the photoresponsivity decrease [48].

Figure 5. (a) Responsivity $R$ (A/W) and photocurrent (μA), (b) photoconductive gain $G$ and external quantum efficiency (EQE), and (c) noise equivalent power (NEP) and normalized detectivity $D*$ of the GaN PD as functions of incident power under a fixed voltage of 20 V. (d) Responsivity $R$ (A/W) and photocurrent (mA), (e) photoconductive gain $G$ and external quantum efficiency (EQE), and (f) noise equivalent power (NEP) and normalized detectivity $D*$ of the $MoS2$ PD as functions of incident power under a fixed voltage of 3 V.

Photoconductive gain ($G$) and EQE of the GaN-based PDs and $MoS2$-based PDs were also calculated as shown in Fig. 5(b) and Fig. 5(e), respectively. Photoconductive gain indicates the number of photocurrent carriers generated by absorbing each photon, which can be calculated by the formula $G=Iph/eμP/hν$, where $e$ is the elementary charge, $ν$ is the frequency of the incident light, $h$ is the Planck constant, and $μ$ is the absorption percentage. $Iph$ has been calculated in Figs. 5(a) and 5(d). In theory, GaN can absorb light with energy higher than its band gap, so for 280 nm light, $μ=1$. The absorption percentage $μ$ of SL $MoS2$ at 405 nm is 29.2%, which has been discussed before. The trend of photoconductive gain under different incident power is consistent with that of responsivity. For a GaN UV detector, the maximum photoconductive gain $Gmax$ of 763.71 was obtained at a voltage of 20 V and an incident light power of 15 nW. The $MoS2$ visible detector achieved a $Gmax$ of 183.88 at 3 V and an incident light power of 2 mW. EQE is the percentage of collected electrons to the total number of incident photons, which can be described as $EQE=Iph/eP/hν=Rhνq$. The maximum EQE of GaN UV detectors reached 76,370.1%, and for $MoS2$ visible detectors the maximum EQE is 5369%. Both the GaN-based UV detector and $MoS2$-based visible detector exhibit high photoelectric conversion efficiency.

To further evaluate the detectivity and detection limit of our devices, normalized detectivity ($D*$) and noise equivalent power (NEP) were calculated as shown in Figs. 5(c) and 5(f) for the GaN UV PD and the $MoS2$ visible PD. NEP can evaluate the signal-to-noise ratio more quantitatively, which is defined as the incident optical power at unit signal-to-noise ratio, described as $NEP=PAS/N$, where $S$ is signal power and $N$ is noise power [49]. In this work, we can assume that the signal current is the illumination current and the noise current is the dark current ($Id$). Therefore, NEP can be calculated by the formula $NEP=(2eIdΔf)12/R$ [49], in which $e$ is the elementary charge amount and $Δf$ is the amplifier bandwidth. A small NEP means that the noise is small, indicating the better performance of the device. The normalized detectivity ($D*$), indicating the detection limit to detect the minimum optical signal, is calculated by the equation $D*=A1/2/NEP$. The larger $D*$ means the stronger detection capability of the detector. For a GaN UV detector under 280 nm illumination, NEP is $8.09×10−15−4.25×10−14 W/Hz1/2$, and $D*$ is $5.12×109−2.69×1010$ Jones. For a $MoS2$ visible detector, NEP is $3.10×10−13−7.14×10−13 W/Hz1/2$, and $D*$ is $7.02×1011−3.05×1011$ Jones. The performance of the $MoS2$-based PD in this work is much lower than that of Si-based PDs ($R$ of $∼300 A/W$ and $D*$ of $∼1013$ Jones) [50,51], but it is better than the SL $MoS2$-based PD ($R$ of 7.5 mA/W) [16]. This work achieved monolithic integration of GaN-based UV PD and $MoS2$-based visible PD for the first time to our knowledge in a simple process; however, the performance of the detectors is still not ideal, and further optimization of CVD growth details and device fabrication optimization are necessary.

Photo-switching characteristics have also been investigated for both GaN-based and $MoS2$-based PDs at a fixed voltage under 280 nm and 405 nm wavelength of incident light illumination, respectively. Figures 6(a) and 6(c) show the photocurrent of the GaN PD and $MoS2$ PD under periodic changes in dark and illumination conditions. When under-illuminated, the current rises to the on state and then falls to the off state under the dark condition. The switch ratio of devices is described as $ONOFF=IlightIdark$. For GaN-based UV PD, the ratio of photocurrent versus dark is about 27.4, while that of $MoS2$-based visible PD is about 13.5. The switching behavior is quite stable through multiple times of changing the illumination conditions. The rise time ($trise$) of the photodetectors is usually defined as the time it takes for the current to increase from 10% to 90% of the maximum, and the fall time ($tfall$) is the time for current decreasing from 90% to 10%. For a GaN UV detector, $trise$ is 0.21 s and $tfall$ is 0.22 s as shown in Fig. 6(b); for $MoS2$ visible detector, $trise$ is 1.36 s and $tfall$ is 8.31 s as shown in Fig. 6(d). $trise$ and $tfall$ obtained in this work are shorter than that of graphene-based devices [52], but are comparable to the reported SL $MoS2$-based PDs [24]. For $MoS2$-based PD, when the laser turns off, the photocurrent decays to the dark level exponentially. It is observed that the sustained photocurrent is because of the long-range Coulomb potential captured, which may be related to the charged impurities in the interface of $MoS2/GaN$ or inside $MoS2$, therefore greatly affecting the recombination of electron-hole pairs. The proper device passivation or packaging technology can further improve the photocurrent switching performance of the device, which is also the next step for us to improve the performance.

Figure 6. Photocurrent as a function of time under alternative dark and illumination. (a) Photocurrent-time curve of GaN illuminated by a 280 nm light source with the incident power of 15.01 nW at 20 V. (b) The rise time (from 10% to 90% of maximum photocurrent) and the fall time (from 90% to 10% of maximum photocurrent) of the GaN PD. (c) Photocurrent-time curve of $MoS2/GaN$ illuminated by a 405 nm laser with the incident power of 10 mW at 3 V. (d) The rise and fall time of the $MoS2/GaN$ PD.

In this work, the CVD growth method was used to obtain a high-quality and large-scale monolayer $MoS2$ on a nearly lattice matched free-standing GaN substrate. And by a low-cost and feasible process, monolithic integration of $MoS2$-based visible detectors and GaN-based UV detectors was realized. The responsivity of the UV detector reached 172.12 A/W under 280 nm illumination, while that of the visible detector reached 17.5 A/W under 405 nm laser. Meanwhile, high-photocurrent gain $G$, high external quantum efficiency, high normalized detectivity, and low noise equivalent power have been achieved for both two kinds of photodetectors. This study provides a low-cost, easy-to-process method for the preparation of dual-wavelength detectors with monolithic integrated UV and visible light, which has promising applications in image sensing and optical communication devices.