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Abstract
Graphene is an ideal material for wide spectrum detector owing to its special band structure, but its low light absorption and fast composite of photogenerated carriers lead to a weak response performance. In this paper, we designed a unique photoconductive graphene-InGaAs photodetector. The built-in electric field was formed between graphene and InGaAs, which can prolong the lifetime of photogenerated carriers and improve the response of devices by confining the holes. Compared with graphene-Si structure, a higher built-in electric field and reach to 0.54 eV is formed. It enables the device to achieve a responsivity of 60 AW−1 and a photoconductive gain of 79.4 at 792 nm. In the 1550 nm communication band, the responsivity of the device is also greater than 10 AW−1 and response speed is less than 2 ms. Meanwhile, the saturation phenomenon of light response was also found in this photoconductive graphene heterojunction detector during the experiment, we have explained the phenomenon by the capacitance theory of the built-in electric field, and the maximum optical responsivity of the detector is calculated theoretically, which is in good agreement with the measurement result.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene is a hexagonal lattice material composed of monolayer carbon atoms with sp2 hybridization. Since its discovery in 2004, graphene has attracted wide attention worldwide for its unique combination of high carrier mobility, high light transmittance and high mechanical strength [1–3]. Graphene usually has a wide absorption spectrum (UV to far infrared band) and a rapid response (less than 50 fs to produce photogenerated electron-hole pairs) [4,5], a high carrier mobility at room temperature and environmentally friendly property. Therefore, graphene has important application prospect in wide spectrum, high speed and uncooled photodetectors. A large number of related works have been reported [6–11]. However, due to the low light absorption (only 2.3%) and short photo-generated carrier lifetime of graphene, the responsivity of graphene photodetectors with ordinary structure is usually only in the order of mAW−1 [12], which greatly limits their applications. In order to enhance the light absorption, micro-cavity [13], metal nanostructures [14] and silicon optical waveguide [10] are integrated on graphene, but the absorption of graphene can only be enhanced in the narrow spectral range with these structures and the process is complex, which may lead to interface contamination and affect the device performance. Prolonging the lifetime of the photo-generated carrier in graphene can effectively improve the responsivity of graphene devices. Therefore, other novel nanomaterials are introduced to construct heterojunctions with graphene such as quantum dots, two-dimensional materials, carbon nanotubes and etc [15–19]. The conventional semiconductor has the nanoscale flatness of the wafer surface because of its mature manufacturing process, which will benefit to form uniform and flat contact with graphene. At present, some heterojunctions such as graphene-silicon and graphene-GaAs structure have been reported [20–23]. The III-V group materials are very suitable for the construction of heterojunction with graphene. By regulating the elemental components, a large work function difference with graphene can be effectively formed, and the bandgap can be also designed to improve the responsivity in a certain band.
In this work, a unique near-infrared graphene-InGaAs photodetector is reported. The InGaAs thin layer was epitaxial grown on the surface of InP substrate by Metal Organic Chemical Vapor Deposition (MOCVD). The bandgap of InGaAs is adjusted to 0.75 eV by controlling the composition so that a significant light absorption is obtained in near infrared. Therefore, the InGaAs layer can contribute a part of the photocurrent in the infrared band to improve the device response. The special feature of the device structure is that graphene only contacts with InGaAs in the middle region of the channel. In this way, there will be no current in InGaAs, and the InGaAs only serves to form a built-in electric field with graphene to trap photo-generated holes. Therefore, the lifetime of photo-generated carriers is effectively prolonged, which will improve the responsivity of devices prominently. The work function difference at the photodetector junction is enlarged by replacing silicon with InGaAs so that the barrier height is enlarged from 0.41 eV to 0.54 eV, which will be benefit to separate more photogenerated carriers and obtain higher responsivity. Finally, the device achieves a responsivity of 60 AW−1 in the 792 nm laser irradiation, and a responsivity of more than 10 AW−1 and a response speed of less than 2 ms in the 1550 nm communication band. Compare with pure InGaAs photodetector, the responsivity is obviously enhanced [23]. In addition, the saturation phenomenon of photocurrent occurred during the experiment. The saturated photocurrent of the photodetector is explained and calculated theoretically base on the built-in electric field capacitance theory, and the measurement results are consistent with theoretical predictions. This work provides guidance for further optimization of graphene photodetectors.
2. Results and discussions
Figures 1(a) and 1(c) show the schematic structure and optical images of the graphene-InGaAs photodetector. The graphene-InGaAs heterojunction structure was constructed in the middle area of the graphene channel. The fluorescence spectrum of InGaAs is shown in Fig. S1 (Supporting material). we can calculate the band gap of InGaAs is about 0.75 eV. The energy band structure of the graphene-InGaAs is shown in Fig. 1(d). The contact between graphene and InGaAs can form a built-in electric field, which can effectively separate the photogenerated electron-hole pairs, slow down their recombination, and thus prolong the lifetime of photogenerated carriers. The photocurrent of the photoconductive detector can be expressed as: Ipc= Iph · G, where Iph is the initial photocurrent and G is the photocurrent gain factor (G = τn/τt, where τn is the lifetime of the photogenerated carrier and τt is the transit time of the photogenerated carrier between the source and drain electrodes) [24]. Therefore, prolonging the life of photogenerated carriers can improve the photoconductance gain and increase the responsivity of the device. Besides, InGaAs will also absorb the incident photons and generate electron and hole pairs. The conductivity of the graphene will change when the holes generated in InGaAs enter graphene under the effect of built-in electric field, and enhance the responsivity significantly.
Fig. 1. (a) Schematic sketch of the graphene-InGaAs photodetector. (b) Cross-sectional view of the device. (c) Optical image of the device surface; (d) Energy band diagram of graphene-InGaAs heterostructure. Ef(G) is the Fermi energy level of graphene. EC, EV and EF respectively represent the minimum conduction band, maximum valence band and Fermi energy level of InGaAs. The green arrow indicates the direction of the built-in electric field Ein.
In order to obtain the electrical parameters of the graphene, we prepared graphene field-effect transistors (Fig. S2, Supporting material) and measured the transfer characteristic curve of the graphene field-effect transistor, as shown in Fig. 2(a). The Dirac point of graphene is at the position of +7V. This indicates that graphene exposed to air produces p-type doping due to the adsorption of water and oxygen molecules, while when graphene is placed in a vacuum, it can be restored to its original state [25–29]. According to the expression of the field effect mobility µ = (L/WCox Vd)(ΔId/ΔVg) where Cox is the gate capacitance per unit area of the gate dielectric. The obtained carrier mobility is 103 cm2V−1s−1.
Fig. 2. (a) Transfer characteristic curve of graphene field-effect transistor. (b) Graphene Raman spectra. (The red curve is the graphene Raman spectra on the surface of SiO2, and the black curve is the graphene Raman spectra on the surface of InGaAs). (c) 2D peak displacement of graphene. (d) Displacement change and strength change of D-peak of graphene.
In Fig. 2(b), the red curve represents the graphene Raman on the surface of InGaAs and the black curve representing the graphene Raman spectrum on the surface of SiO2. The G and 2D peaks of the Raman spectra shifted to the direction of low wave number and the intensity of G peak increased in the red curve (Figs. 2(c)–2(d)). This phenomenon indicates that the electron concentration in graphene increases and significant N-type doping appears on the graphene [30–33]. The reason for this phenomenon is that the electrons in InGaAs flow into graphene, while the holes in graphene flow into InGaAs due to the difference in work function between p-type graphene and InGaAs.
The response performance of the graphene-InGaAs photodetector was measured using the laser sources with different wavelengths from light visible to near-infrared light. As shown in Fig. 3(a), under the irradiation of 792 nm laser, the device produces obvious changes in photoconductance. We measured the photocurrent dynamic response of the device (Fig. S3, Supporting material) with 405 nm, 792 nm and 1550 nm laser sources respectively, under different wavelength laser sources, the device exhibits a faster and more obvious response signal in the switching cycle than the device without graphene (Fig. S4, Supporting material). Compared the device without InGaAs (Fig. 3(b)), under the irradiation of 792 nm laser, the responsivity of the graphene-InGaAs device was improved 3 orders of magnitude. It is noted that the responsivity of the device is negative. As shown in Fig. 3(c), when light irradiate on the device surface, photogenerated electrons and holes will be generated in graphene and InGaAs. Under the action of built-in electric field, electron-hole pairs will be separated, electrons will flow to InGaAs, and holes will flow to graphene, which results in the decrease of the electrons concentration and electrical conductivity of N-typed graphene. Therefore, the response of the device is negative.
Fig. 3. (a) Output characteristics of the device illuminated by 792 nm laser with different laser power (The insert: shows the zoom-in current at 500 mV voltage). (b) Dynamic output characteristics of the conventional graphene photodetector with field effect structure (black) and the graphene-InGaAs photodetector (red) illuminated by 792 nm(140 mWcm−2) laser. (c) Energy band diagram of graphene-InGaAs heterostructures under illumination. (d) Device responsivity and photocurrent changes with incident optical power. The device power dependence of the photocurrent under the irradiation of 792 nm laser (VDS = 0.5 V). The red curve shows the relationship between photocurrent and laser power, the black curve shows the relationship between responsivity and laser power.
The measurement results show that the device exhibits a significant responsivity, the absolute value of more than 60 AW−1 at 0.5 V bias and when illuminated by 792 nm laser. Under the illumination of 405 nm and 1550 nm laser, the absolute value of responsivity is also greater than 10 AW−1. The photoconductive gain of the device is calculated by the formula G = R×(hc /λe), The photoconductive gain refers to the number of photoexcited particles generated by each absorbed photon per unit time [34–36], Where h is Planck's constant, c is the speed of light, λ is the wavelength of light, e is the electron charge, R is the responsivity. The gain is 79.4 under the above conditions. We can rewrite above function as G=τn/τt, compare with the conventional graphene field effect transistor (Fig. S2, Supporting material), the photogenerated carrier lifetime of graphene is extended by 1500 times assuming the same transit time between the source leakage electrodes in our device. Although there have been reports of graphene-InGaAs heterojunction photodetector before [23], due to the device structure, the lifetime of photo-generated carriers cannot be effectively extended in the device, so the responsivity is lower than this work. We compared the response performance of the graphene-InGaAs photodetector with other graphene photodetectors in the near infrared band (Table 1). The graphene InGaAs photodetector show a wider spectral response range and a higher responsivity at 1550 nm.
Table 1. Comparison of near infrared graphene photodetector performances
We found an interesting phenomenon of the saturation of the light response in the experiment. The increment of photocurrent gradually flattened with the increase of incident power, as shown in Fig. 3(d). The responsivity of the device also gradually decreased with the increase of incident power. We speculate that this is due to the limited number of photogenerated carriers that can be separated by the built-in electric field, the photogenerated carriers separated by the built-in electric field will accumulate on the two sides of the built-in electric field and gradually offset the built-in electric field, when the carriers accumulate to a certain extent, the built-in electric field cannot continue to separate the photogenerated carriers. Base on this mechanism, the maximum responsivity of the device depend on the intensity of the built-in electric fields. In order to verify this theory, the saturated photocurrent of the device is calculated. The built-in electric field can be thought of as a capacitor, the distance between the capacitance plates is the width of the built-in electric field. The maximum number of carriers that can be separated in the built-in electrical field is calculated by the capacitance formula and then the saturated photocurrent is achieved.
Through the measurement of the I-V curve of the graphene-InGaAs Schottky junction, the barrier height and width of the built-in electric field can be obtained. Therefore, as shown in Fig. 4(a), we connect one electrode of the device to the graphene and the other electrode to the InP substrate. According to the theory of thermal electron emission, the I–V characteristics can be described by the following functions:
where T is the Kelvin temperature, q is the elementary charge, K is the Boltzmann constant, n is the ideal coefficient, A is the effective area of the diode, ${\Phi _B}$ is the effective barrier height at zero bias, RS is the series resistance, and A** is the effective Richardson constant.Fig. 4. Graphene-InGaAs Schottky junction device interface diagram. (b) I-V curve of the device in the absence of light. (c)(d) Curves of dV/d(lnI) (c) and H(I) (d) versus current I.
When ${V_D} > 3kT/q$, function (1) can be approximated as
It can be rewritten as
Then
Define function
We can rewrite it as
According to the I-V curve drawn in Fig. 4(b), dV/d(lnI) versus I and H(I) versus I for the device can be drawn as show in Figs. 4(c) and 4(d). Through the linear fitting of the data in Figs. 4(c) and 4(d), the calculated parameters by Eqs. (6) and (8) are n=3.65, ΦB=0.54eV and Rs=6000Ω. According to the obtained height of the potential barrier, the thickness of the depletion layer of InGaAs under the dark field can be calculated by the following formula:
where ${{X}_{D}}$ is the distance between capacitor plates, C is capacitance, ${\varepsilon _r}$ is the relative dielectric constant, ${\varepsilon _o}$ is vacuum dielectric constant. Δt is charge separation time. K is electrostatic constant, ND is the doping concentration of InGaAs. From Eqs. (9–11), the XD = 0.9 µm and the saturated photocurrent are estimated to be 40 µA. This value larger than the measured result. The following two factors may be responsible for it. Firstly, the mobility of graphene was calculated on the surface of SiO2 through the FET transfer characteristic curve, while the actual mobility of the graphene on InGaAs may be lower. Another, the light response of the device is negative and the photoconductive response of graphene itself is positive. Therefore, the light response of graphene will offset part of the saturation photocurrent. Besides, some other reason such as the defects on the surface of InGaAs and the pollution during the process also effect the performance. However, it can be seen that the calculated results and the measured results are in the same order of magnitude, indicating that the ability of built-in electric field to separate carriers is one of the main factor affecting the optical response of the device. Therefore, as the incident light power gradually increases, the additional carriers will recombination partly, the rate of photocurrent change decreases. When the number of photogenerated carriers exceeds the rated value, the effect of built-in electric field reaches its limitation, and the current almost no longer changes with the light intensity.3. Conclusions
In this paper, a photoconductive graphene-InGaAs photodetector is introduced. The built-in electric field between graphene and InGaAs can realize the hole trapping, prolonging the lifetime of the photogenerated carriers and improving the responsivity of devices. The responsivity reaches 60 AW−1 and the response speed is less than 2 ms illuminated by 792 nm laser. For 405 nm and 1550 nm laser, the responsivity of the device is also greater than 10 AW−1. In the experiment, the phenomenon of light response saturation was found, and explained by the capacitance theory of built-in electric field. The saturated photocurrent of the device is calculated theoretically. Which are in good agreement with the experiment. Base on this theory, graphene doping or increasing the number of graphene layers to increase the barrier height may help to realize separate more photogenerated carriers and obtain larger responsivity.
4. Experimental methods
4.1 Materials and device fabrication
The device preparation process is as follows: First, InGaAs layer was epitaxial grown on InP substrate by MOCVD at a temperature of 650 °C. The III-V group scource is Trimethylgallium (TMGa), Trimethylindium (TMIn) and AsH3, high purity H2 is carrier gas. The ratio of In/Ga fixed to give In0.53Ga0.47As. Next, 300 nm SiO2 was deposited on the surface of InGaAs by PECVD as a dielectric layer to avoid the contact between the metal electrode and the InGaAs layer. The SiO2 surface was graphed and etched out of a window with BOE solution to expose the underlying InGaAs. Then the metal electrodes are deposited on the SiO2 surface. At last, monolayer graphene grown on Cu foil by chemical vapor deposition (CVD) is transferred onto the surface followed by lithography and oxygen plasma etching to make the graphene patterned. The graphene is transferred through a commonly used wet transfer method. First, coating polymethyl methacrylate (PMMA) on the surface of graphene. Second, the copper foil is etched with the aqueous solution of copper sulfate and hydrochloric acid (CuSO4:HCl:H2O=5 g: 20 ml:120 ml), and then the graphene is transferred to the device surface. Finally, the PMMA is removed by acetone and isopropanol. Figure 1(b) shows a schematic section of the device.
4.2 Electrical and photoelectrical characterization
Electrical and photoelectrical measurements were conducted on a probe station with an Agilent B1500A semiconductor characterization system at room temperature. The probe station is placed in a black box to avoid interference from ambient light. The intensities of the lasers were measured using a power meter (PM400, Thorlabs Company). The laser diodes (405 nm, 792 nm and 1550 nm) were used as the illumination sources. Raman and PL investigations were performed with a Horiba LabRAM HR Evolution Raman microscope equipped with a 532 nm laser.
Funding
National Key Research and Development Program of China (2018YFA0209000); National Natural Science Foundation of China (61874145, 62074011); Beijing Municipal Natural Science Foundation (4182012); Beijing Nova Program (Z201100006820096); China Postdoctoral Science Foundation (2021M692136).
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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