- Email: [email protected]
Si-quantum-dot photodetectors with TiOx back-surface passivation layer
Dyes and Pigments 170 (2019) 107587
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High-detectivity and -stability multilayer-graphene/Si-quantum-dot photodetectors with TiOx back-surface passivation layer
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Dong Hee Shin1, Dong Hwan Jung1, Suk-Ho Choi∗ Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin, 17104, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Multilayer Si quantum dot Photodetector TiOx Detectivity Stability
We first report Si quantum dots (SQDs)-embedded SiO2 (SQDs:SiO2) multilayers (MLs)/Si photodetectors (PDs) using multilayer graphene (MLG) transparent conductive electrode (TCE) and TiOx back-surface passivation layer. With increasing number of graphene layer (Ln) to 5, the sheet resistance of the MLG TCE sharply decreases to ∼273 Ω/sq whilst the work function gradually increases to ∼4.71 eV, indicating p-type doping, useful for collection of photo-excited carriers at the MLG/SQDs:SiO2 MLs interface. At Ln = 4, the PD without TiOx exhibits best performance of 0.340 AW-1 responsivity (R) and 3.21 × 109 cm Hz1/2 W−1 specific detectivity (D*) at the peak wavelength. With TiOx on the back side of Si wafer (Ln = 4), the R slightly increases to 0.351 AW-1, but the D* is greatly enhanced to 9.42 × 1011 cm Hz1/2 W−1, resulting from the sharp reduction of the dark current by the suppression of the carrier recombination at the Si/TiOx interface. The PD performance is almost consistent for 1000 h in air, irrespective of the use of the TiOx layer. These behaviors are much better than ever achieved in graphene/Si wafer PDs.
1. Introduction Optoelectronic devices including photodetectors (PDs) are highly attractive because they provide platforms for studying fundamental optical/electrical mechanisms and can be applied in versatile practical areas [1–3]. Among various functional transparent conductive electrodes (TCEs) for optoelectronic devices, graphene has received much attention due to its unique two-dimensional structure and excellent electronic/optical properties [4–6]. Since graphene/Si heterojunction solar cells showed interesting rectification and photovoltaic effects in 2011 [7], subsequent studies on PDs have been actively done based on the graphene/Si heterojunctions [8–10] with some of them exhibiting high performances [11–13]. However, relatively-high density of interface states possibly pinning the Fermi level exists between graphene and Si, resulting in large leakage-current noise, which limits further improvement of the overall performance [13]. To solve this problem, several alterations of the graphene/Si heterojunctions were done, leading to significant enhancements in the performances [14–18]. As one of them, the interface of the graphene/Si junction was greatly improved by employing Si quantum dots (SQDs)-embedded SiO2 (SQDs:SiO2) multilayers (MLs) on Si, thereby enhancing the device performances [17,18]. As shown in the previous studies [17,18], pristine graphene is not
effective as a TCE for enhancing the photoresponse due to the low conductivity and work function despite the excellent transmittance. Thus, the doping of impurities in graphene was employed to enhance its work function and conductivity, thereby improving the performance of optoelectronic devices. However, the doping effect on graphene becomes less effective over time, resulting in the deterioration of the device performance [19,20]. It is well known that the use of multilayer graphene (MLG) can induce similar effect with the real doping, resulting in the enhancement of conductivity and work function [19,21]. On the other hand, the leakage current can be reduced by inserting a layer such as TiOx [22–24] and Cs2CO3 [25] between the Si and the cathode in the PD, resulting in the decrease of the rear recombination because the interlayer plays as a role of effective electron-selective contact or hole blocking. In this work, we employ MLG as a TCE for SQDs:SiO2 MLs/Si PDs by varying the layer number (Ln) of MLG from 1 to 5. The work function and sheet resistance of MLG are changed to ∼4.71 eV and 273 Ω/sq, respectively by increasing Ln to 5. The MLG/ SQDs:SiO2 MLs/Si PDs show maximum responsivity (R)/noise equivalent power (NEP)/specific detectivity (D*) of 0.340 AW-1/ 156.88 pW Hz−1/2/3.21 × 109 cm Hz1/2 W−1 for Ln = 4 at the peak wavelength. By employing TiOx layer on the back of Si wafer, the R/ NEP/D*/linear dynamic range (LDR) of the PDs reach 0.351 AW-1/ 0.53 pW Hz−1/2/9.42 × 1011 cm Hz1/2 W−1/78 dB, respectively. The R
∗
Corresponding author. E-mail address: [email protected] (S.-H. Choi). 1 Two authors have contributed equally to this study. https://doi.org/10.1016/j.dyepig.2019.107587 Received 15 November 2018; Received in revised form 22 May 2019; Accepted 25 May 2019 Available online 31 May 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.
Dyes and Pigments 170 (2019) 107587
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Fig. 1. (a) A schematic of a typical MLG/SQDs:SiO2 MLs/Si/TiOx PD with Au and InGa as top and bottom electrodes, respectively. (b) Dark/photo J-V curves for various λ from 400 to 1000 nm. (c) PC/DC ratio and (d) responsivity at λ = 600 nm as functions of bias for various Ln. (e) Spectral responsivity and (f) quantum efficiency at a bias of −3 V for various Ln.
bottom electrodes of the PDs were made of Au and InGa films, respectively. The front electrodes were fabricated through photolithography and standard lift-off processes to obtain a 5 × 5 mm2 illumination window.
is consistent under 1000 h operations in air, irrespective of the use of TiOx, indicating extremely excellent long-term stabilities of the PDs. 2. Experimental section 2.1. Preparation of SQDs
2.4. Characterizations Reactive ion beam sputtering system was used to grow SiO1·0/SiO2 MLs with 50 periods of 2 nm thin layers on Si wafers, as described in our previous work [17,18]. Subsequently, the samples were annealed at 1100 °C in an ultra-high purity nitrogen atmosphere using a horizontal furnace to form SQDs:SiO2 MLs embedded in SiO2.
The existence of the SQDs within SiO2 were confirmed by high resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) was measured using a 325 nm HeCd laser line as the excitation source. The optical characteristics of the graphene film were analyzed by Raman spectroscopy using a 532 nm laser line, and the Raman peaks were corrected by the reference Si peak. The transmittance, sheet resistance, carrier mobility, and work function of the MLG were measured by UV–visible–near-infrared optical spectrometer (Agilent Varian, model cary 5000), 4 probe van der Pauw method (Dasol eng, model FPP-HS8-40K), Hall-effect technique (Ecopia model HEM-2000), and Kelvin probe force microscopy (Park systems, model XE 100), respectively. Here, the calibration of the work function was done by using gold as a reference. Current density-voltage (J-V) measurements for the PDs were performed using a Keithley 2400 source meter controlled by the LabView program. The dark current (DC) noise of the PDs was measured by a dynamic signal analyzer (Agilent 35670A) connected to a low-noise current preamplifier (Stanford Research SR570) in the frequency range of 1 Hz–10 kHz. The spectral response of the PDs was measured from 300 to 1100 nm using a 450 W xenon light source and monochromator (Newport Cornerstone, medel 260 1/4 m). The intensity of the light incident on the PD was monitored by a UV-enhanced Si photodiode. The photosensitivity linearity of the PD was measured using a 532 nm monochromatic diode laser, and the intensity of the light was controlled by neutral density filters. Time decay of a PD was analyzed using a 532-nm pulse laser of 10 μWcm−2 light intensity at a pulse frequency of 50 kHz.
2.2. Preparation of MLG Single-layer graphene was synthesized on Cu foil under mixed gas flow of CH4 and H2 by chemical vapor deposition. Subsequently, graphene sheet was transferred onto a 100 nm SQD: SiO2 MLs/Si substrate using a generally-known poly(methyl methacrylate) (PMMA) supporting film [26]. The PMMA layer was then removed in an acetone bath for 1 h to obtain graphene/SQD:SiO2 MLs/Si wafer. This step was repeated up to 5 times to produce single-to quintuple-layer graphene on the SQD:SiO2 MLs/Si substrates. 2.3. Fabrication of MLG/SQD:SiO2 MLs/Si/TiOx PDs Titanium isopropoxide in isopropyl alcohol (IPA) and HCl in IPA were prepared to produce the TiOx precursor. The HCl/IPA solution was added dropwise to the titanium isopropoxide/IPA solution while stirred, and the resulting mixed solution was further stirred for 2 h. Subsequently, the precursor solution was diluted by different amount of IPA, and filtered through a polytetrafluoroethylene filter. Finally, the TiOx precursor solution was set at a concentration of 1.0 mg/mL, and was spin-coated on the backside of Si at 3000 rpm for 40 s. The top and 2
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Fig. 2. (a) Energy band diagram of a typical MLG/SQDs:SiO2 MLs/Si/TiOx PD. (b) Dark/photo J-V curves at λ = 600 nm for the PD with/without TiOx. (c) Spectral responsivity/quantum efficiency and (d) detectivity at bias of −3 V for the PD with/without TiOx.
few carriers flow from the MLG to n-Si due to the large barrier from the Fermi level of the MLG. Incident photons create electron-hole pairs in the SQDs, thereby generating photocurrent (PC) under forward bias as well as under reverse bias, as can be easily understood based on the energy-band diagram, as previously reported by us [17,18]. The PC is expected to increase with increasing Ln because the available density of graphene at the Fermi level (EF) increases by the downward movement of EF caused by the increased work function at larger Ln. However, the PC under forward bias is negligible because the DC is too high. These results suggest that the photoresponse of the PD is much more sensitive under reverse bias. On the other hand, the photoresponse of the PD at Ln = 1 is nearly negligible, as shown in Fig. 1c–f, due to the high sheet resistance and low work function of graphene despite the excellent transmittance. Fig. 1c shows Ln–dependent PC/DC (on/off) ratio. The ratios reach maxima at a bias voltage of about −0.4 V, irrespective of Ln, and show largest value of 173 at Ln = 4, possibly resulting from Ln–dependent trade–off relation between the sheet resistance, work function, mobility, and transmittance, as explained above. Fig. 1d shows R as functions of bias voltage at λ = 600 nm for various Ln. The R was calculated by the following equation: R = Iph/Llight, where Iph is the PC and Llight is the incident light power. The power density was about 720 μW-cm−2 at λ = 600 nm. The R is maximized at Va = ∼ −3.2 V for all Ln. Especially, the maximum R value is about 0.340 A/W at Ln = 4 and λ = 630 nm, as shown in Fig. 1e (for other biases at Ln = 4, Fig. S5a). Ln–dependence of the spectral external quantum efficiency (EQE) is similar to that of the spectral R, except small differences between their peak wavelengths, as shown in Fig. 1f (for other biases at Ln = 4, Fig. S5b). Based on these results, Ln was fixed at 4 for further studying the effect of the back surface passivation on the performance of the PD, as shown in Fig. 1a. For enhancing D* of a PD, lowering DC is crucial. Fig. 2a shows the energy band diagram of the PD with TiOx layer, useful as a backside passivation layer of Si for preventing the holes from being collected to the cathode (InGa), resulting in the reduction of DC. The conduction band and valence band edges of TiOx are located at −4.0 and −7.0 eV, respectively from the vacuum level [22–24]. This makes negligible band offset for electrons moving from the Si to the TiOx layer
3. Results and discussion Fig. 1a shows a schematic diagram of a typical Au/MLG/SQDs:SiO2 MLs/n-Si/TiOx/InGa PD. The active area of a PD irradiated with light was defined as 25 mm2 by photolithography process. The average size of the SQDs used as the active layer was estimated to be ∼3.5 nm, as analyzed by HRTEM (Fig. S1). The PL spectrum peaked at ∼730 nm also provided another evidence for the formation of the SQDs (Fig. S1) [17,18]. Structural, optical, and electrical properties of MLG were analyzed to check their dependence on Ln (Fig. S2). Three intense D, G, and 2D peaks at ∼1350, ∼1580, and ∼2700 cm−1, respectively were observed in the Raman spectrum at Ln = 1, indicating unique characteristics of graphene [27]. As Ln increased, both the G and 2D bands were red-shifted due to the change of the electronic band towards graphitic structure [28,29]. The Raman intensity ratio of the G to 2D peaks, I(G/2D) is related to the number of stacked graphene layers. As Ln varied from 1 to 5, I(G/2D) increased from 0.47 to 1.21, consistent with the increase of Ln. The transmittance spectra of the five MLG films were measured from 300 to 1000 nm (Fig. S2). By fitting the data based on Beer's law, the attenuation coefficient per layer was calculated to be 2.5%, very close to the theoretical attenuation coefficient (2.3%) of graphene per layer [30,31]. As Ln increased from 1 to 5, the sheet resistance of graphene was reduced from 705 ± 33 to 273 ± 12 Ω/sq (Fig. S2), consistent with the previous reports [32]. To study the change of the electric properties as functions of Ln, mobility and work function were measured (Fig. S2). The mobility/work function at Ln = 1 were 2150 cm2-V−1s−1/4.48 eV, respectively, and monotonically increased/ decreased to 215 cm2-V−1s−1/4.71 eV with increasing Ln to 5, respectively. These behaviors resulted from the changes of carrier density and the band structure depending on Ln, as previously reported [21,33]. As a first step, we characterized the PDs without TiOx. Fig. 1b shows typical J-V curves under dark and illumination for Ln = 4 (for other Ln at λ = 600 nm and power density = 720 μW-cm−2, Fig. S3). Based on the energy band diagram of the MLG/SQDs:SiO2 MLs/n-Si PD under dark (Fig. S4), majority carriers (electrons) flow from n-Si through the SQDs MLs towards the p-type MLG by direct tunneling, resulting in high DC. Under reverse bias (V < 0), the PD exhibits low DC because only a 3
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under reverse bias, and inhibits carrier recombination at the interface by blocking holes from the Si to the cathode, resulting in reduction of DC. The use of a titanium isopropoxide precursor can partially terminate the pendant bonding of the Si surface [34] by combination of Si and TiOx (for the schematic diagram, Fig. S6a). The chemical bond of Si–O–Ti formed by the precursor can significantly reduce the pendant bonding of the Si surface and suppress surface charge recombination. We measured the Si 2p XPS spectrum to check the bonding of Si to the precursor (Fig. S6b). By linearly shaping the XPS spectrum based on the appropriate combination of Gaussian and Lorentz functions, the bond energies of the Si 2p core levels were shown to be 101.7 and 102.7 eV, corresponding to Si–O–Ti and Si–O–Si bonds, respectively [34]. Fig. 2b shows typical dark and photo J–V curves for the PDs with/ without TiOx on the back side of the Si (photo J-V curves for other λ, Fig. S7a), indicating a big suppression of DC under reverse bias by the use of TiOx film without almost any change in total current. As a result, the PC/DC reaches more than 103. (Fig. S7b). The dark J-V curve of a nonideal diode is determined by the following equation [35]: J = Js [exp(eV/nkT) - 1], expressed based on the thermal ion emission model, where Js is the ideal reverse saturation current, e is electron charge, T is temperature, k is the Boltzmann constant, and n is diode ideality factor. The n is estimated to be 2.39 and 3.65 for the PDs with/without TiOx, respectively, meaning the PD with TiOx is better-quality diode. Fig. 2c shows spectral R and EQE of the PDs with/without TiOx (R and EQE for other Va, Figs. S7c–d). The R and EQE slightly increase from 0.340 AW−1 and 70% to 0.351 A-W−1 and 72%, respectively by the use of TiOx. Another figure-of-merit of PD is NEP, indicating minimum detectable power. The NEP is expressed by the following equation: NEP = In/R [36], where In is noise current. The NEP can be calculated by measuring In (Figs. S8a and d) of PDs with/without TiOx as functions of Va (Figs. S8b and e). The NEP is related to the specific D* representing the detection limit of a PD by the following equation: D* = Af /NEP [36], where A is the absorbing area (0.25 cm2) and f is the electrical bandwidth in Hz. Fig. 2d shows spectral D* of the PDs with/without TiOx at Va = - 3V (for other biases, Figs. S8c and f). The minimum NEP is 0.53/156.88 pW Hz−1/2 at Va = - 3V and λ = 630 nm for the PDs with/without TiOx, resulting in D* of 9.42 × 1011/3.21 × 109 cm Hz1/ 2 W−1, respectively. These results suggest that the D* is greatly enhanced by the use of TiOx layer in the MLG/SQDs PDs, and this maximum D* value is much larger than ever achieved in graphene/Si wafer PDs in the visible region [8,14,16–18]. We measured additional figure-of-merits to further characterize the PDs with TiOx. Fig. 3a shows normalized responses as functions of pulse frequency under various reverse biases by chopping a continuous-wave 532-nm laser line with a spot size of 25 mm2. The 3 dB bandwidth ranges from 0.12 to 0.15 MHz for various Va. Fig. 3b shows bias-dependent fast decay curves of normalized PC, from which the rise time (tr) and decay time (td) for various Va are extracted to be 0.65–0.71 μs and 6.8–7.3 μs, respectively, consistent with the results of 3 dB bandwidth. These behaviors are much faster than or comparable to those of graphene/Si wafer PDs [8,9,13,14,16–18]. Fig. 3c shows J-V curves under illumination at 532 nm, measured by varying the light intensity from 2.1 × 10−4 to 4.5 mW/cm2. Here, the light intensity was controlled using a neutral density filters. The PC is almost linearly proportional to light power under all biases. The LDR is expressed as LDR = 20 log (J*PC/Jd), where J*PC is obtained at a power density of 1 mW cm−2 and Jd is the dark current density, resulting in the LDR over 78 dB, comparable to those obtained from graphene/Si PDs [13,17,18]. We also studied the long-term stability of the PD with TiOx in the atmosphere for about 1000 h. Fig. 4a shows time-dependent dark and photo J-V curves at λ = 400, 600, and 800 (for the PD without TiOx, Fig. S9a). As shown in Fig. 4b, the R is almost constant for 1000 h regardless of λ and Va (for the PD without TiOx, Fig. S9b). These results suggest that the MLG/SQDs:SiO2 MLs/n-Si structures are very promising as next-generation PDs with high detectivity and long-term stability in the visible region.
Fig. 3. (a) Normalized response of the PD with TiOx as a function of pulse frequency for various Va. (b) Normalized transient PC of the PD with TiOx for various Va. (c) Photo-response linearity of the PD with TiOx as functions of irradiance for various Va.
4. Conclusion MLG/SQDs:SiO2/Si/TiOx PDs were successfully fabricated to show highest performance of 0.351 A-W−1 R, 9.42 × 1011 cm Hz1/2-W−1 D*, ∼0.7/∼7.0 μs rise/decay times, and 78 dB LDR at Ln = 4, much better than ever achieved in graphene/Si wafer PDs. Especially, the D* was enhanced almost 300 times by the use of TiOx as a back-surface passivation layer of the PDs, resulting from the sharp reduction of DC caused by the suppressed carrier recombination at the interface of Si with the TiOx layer. In addition, the R was almost consistent for 1000 h in air, regardless of the bias voltage and λ. These results are very promising for potential applications of the MLG/SQDs:SiO2/Si/TiOx PDs in next-generation optoelectronic areas.
Acknowledgments This work was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science & ICT (NRF-2017R1A2B3006054).
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Fig. 4. Long-term stabilities of the PD with TiOx during 1000 h under ambient air. (a) Time-dependent dark and photo J-V curves at λ = 400, 600, and 800 nm. (b) Temporal behaviors of responsivities at λ = 400, 600, and 800 nm under Va = −3 V for the PD with TiOx.
Appendix A. Supplementary data [18]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.107587.
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High-detectivity and -stability multilayer-graphene/Si-quantum-dot photodetectors with TiOx back-surface passivation layer
T
Dong Hee Shin1, Dong Hwan Jung1, Suk-Ho Choi∗ Department of Applied Physics and Institute of Natural Sciences, Kyung Hee University, Yongin, 17104, South Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Graphene Multilayer Si quantum dot Photodetector TiOx Detectivity Stability
We first report Si quantum dots (SQDs)-embedded SiO2 (SQDs:SiO2) multilayers (MLs)/Si photodetectors (PDs) using multilayer graphene (MLG) transparent conductive electrode (TCE) and TiOx back-surface passivation layer. With increasing number of graphene layer (Ln) to 5, the sheet resistance of the MLG TCE sharply decreases to ∼273 Ω/sq whilst the work function gradually increases to ∼4.71 eV, indicating p-type doping, useful for collection of photo-excited carriers at the MLG/SQDs:SiO2 MLs interface. At Ln = 4, the PD without TiOx exhibits best performance of 0.340 AW-1 responsivity (R) and 3.21 × 109 cm Hz1/2 W−1 specific detectivity (D*) at the peak wavelength. With TiOx on the back side of Si wafer (Ln = 4), the R slightly increases to 0.351 AW-1, but the D* is greatly enhanced to 9.42 × 1011 cm Hz1/2 W−1, resulting from the sharp reduction of the dark current by the suppression of the carrier recombination at the Si/TiOx interface. The PD performance is almost consistent for 1000 h in air, irrespective of the use of the TiOx layer. These behaviors are much better than ever achieved in graphene/Si wafer PDs.
1. Introduction Optoelectronic devices including photodetectors (PDs) are highly attractive because they provide platforms for studying fundamental optical/electrical mechanisms and can be applied in versatile practical areas [1–3]. Among various functional transparent conductive electrodes (TCEs) for optoelectronic devices, graphene has received much attention due to its unique two-dimensional structure and excellent electronic/optical properties [4–6]. Since graphene/Si heterojunction solar cells showed interesting rectification and photovoltaic effects in 2011 [7], subsequent studies on PDs have been actively done based on the graphene/Si heterojunctions [8–10] with some of them exhibiting high performances [11–13]. However, relatively-high density of interface states possibly pinning the Fermi level exists between graphene and Si, resulting in large leakage-current noise, which limits further improvement of the overall performance [13]. To solve this problem, several alterations of the graphene/Si heterojunctions were done, leading to significant enhancements in the performances [14–18]. As one of them, the interface of the graphene/Si junction was greatly improved by employing Si quantum dots (SQDs)-embedded SiO2 (SQDs:SiO2) multilayers (MLs) on Si, thereby enhancing the device performances [17,18]. As shown in the previous studies [17,18], pristine graphene is not
effective as a TCE for enhancing the photoresponse due to the low conductivity and work function despite the excellent transmittance. Thus, the doping of impurities in graphene was employed to enhance its work function and conductivity, thereby improving the performance of optoelectronic devices. However, the doping effect on graphene becomes less effective over time, resulting in the deterioration of the device performance [19,20]. It is well known that the use of multilayer graphene (MLG) can induce similar effect with the real doping, resulting in the enhancement of conductivity and work function [19,21]. On the other hand, the leakage current can be reduced by inserting a layer such as TiOx [22–24] and Cs2CO3 [25] between the Si and the cathode in the PD, resulting in the decrease of the rear recombination because the interlayer plays as a role of effective electron-selective contact or hole blocking. In this work, we employ MLG as a TCE for SQDs:SiO2 MLs/Si PDs by varying the layer number (Ln) of MLG from 1 to 5. The work function and sheet resistance of MLG are changed to ∼4.71 eV and 273 Ω/sq, respectively by increasing Ln to 5. The MLG/ SQDs:SiO2 MLs/Si PDs show maximum responsivity (R)/noise equivalent power (NEP)/specific detectivity (D*) of 0.340 AW-1/ 156.88 pW Hz−1/2/3.21 × 109 cm Hz1/2 W−1 for Ln = 4 at the peak wavelength. By employing TiOx layer on the back of Si wafer, the R/ NEP/D*/linear dynamic range (LDR) of the PDs reach 0.351 AW-1/ 0.53 pW Hz−1/2/9.42 × 1011 cm Hz1/2 W−1/78 dB, respectively. The R
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Corresponding author. E-mail address: [email protected] (S.-H. Choi). 1 Two authors have contributed equally to this study. https://doi.org/10.1016/j.dyepig.2019.107587 Received 15 November 2018; Received in revised form 22 May 2019; Accepted 25 May 2019 Available online 31 May 2019 0143-7208/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. (a) A schematic of a typical MLG/SQDs:SiO2 MLs/Si/TiOx PD with Au and InGa as top and bottom electrodes, respectively. (b) Dark/photo J-V curves for various λ from 400 to 1000 nm. (c) PC/DC ratio and (d) responsivity at λ = 600 nm as functions of bias for various Ln. (e) Spectral responsivity and (f) quantum efficiency at a bias of −3 V for various Ln.
bottom electrodes of the PDs were made of Au and InGa films, respectively. The front electrodes were fabricated through photolithography and standard lift-off processes to obtain a 5 × 5 mm2 illumination window.
is consistent under 1000 h operations in air, irrespective of the use of TiOx, indicating extremely excellent long-term stabilities of the PDs. 2. Experimental section 2.1. Preparation of SQDs
2.4. Characterizations Reactive ion beam sputtering system was used to grow SiO1·0/SiO2 MLs with 50 periods of 2 nm thin layers on Si wafers, as described in our previous work [17,18]. Subsequently, the samples were annealed at 1100 °C in an ultra-high purity nitrogen atmosphere using a horizontal furnace to form SQDs:SiO2 MLs embedded in SiO2.
The existence of the SQDs within SiO2 were confirmed by high resolution transmission electron microscopy (HRTEM). Photoluminescence (PL) was measured using a 325 nm HeCd laser line as the excitation source. The optical characteristics of the graphene film were analyzed by Raman spectroscopy using a 532 nm laser line, and the Raman peaks were corrected by the reference Si peak. The transmittance, sheet resistance, carrier mobility, and work function of the MLG were measured by UV–visible–near-infrared optical spectrometer (Agilent Varian, model cary 5000), 4 probe van der Pauw method (Dasol eng, model FPP-HS8-40K), Hall-effect technique (Ecopia model HEM-2000), and Kelvin probe force microscopy (Park systems, model XE 100), respectively. Here, the calibration of the work function was done by using gold as a reference. Current density-voltage (J-V) measurements for the PDs were performed using a Keithley 2400 source meter controlled by the LabView program. The dark current (DC) noise of the PDs was measured by a dynamic signal analyzer (Agilent 35670A) connected to a low-noise current preamplifier (Stanford Research SR570) in the frequency range of 1 Hz–10 kHz. The spectral response of the PDs was measured from 300 to 1100 nm using a 450 W xenon light source and monochromator (Newport Cornerstone, medel 260 1/4 m). The intensity of the light incident on the PD was monitored by a UV-enhanced Si photodiode. The photosensitivity linearity of the PD was measured using a 532 nm monochromatic diode laser, and the intensity of the light was controlled by neutral density filters. Time decay of a PD was analyzed using a 532-nm pulse laser of 10 μWcm−2 light intensity at a pulse frequency of 50 kHz.
2.2. Preparation of MLG Single-layer graphene was synthesized on Cu foil under mixed gas flow of CH4 and H2 by chemical vapor deposition. Subsequently, graphene sheet was transferred onto a 100 nm SQD: SiO2 MLs/Si substrate using a generally-known poly(methyl methacrylate) (PMMA) supporting film [26]. The PMMA layer was then removed in an acetone bath for 1 h to obtain graphene/SQD:SiO2 MLs/Si wafer. This step was repeated up to 5 times to produce single-to quintuple-layer graphene on the SQD:SiO2 MLs/Si substrates. 2.3. Fabrication of MLG/SQD:SiO2 MLs/Si/TiOx PDs Titanium isopropoxide in isopropyl alcohol (IPA) and HCl in IPA were prepared to produce the TiOx precursor. The HCl/IPA solution was added dropwise to the titanium isopropoxide/IPA solution while stirred, and the resulting mixed solution was further stirred for 2 h. Subsequently, the precursor solution was diluted by different amount of IPA, and filtered through a polytetrafluoroethylene filter. Finally, the TiOx precursor solution was set at a concentration of 1.0 mg/mL, and was spin-coated on the backside of Si at 3000 rpm for 40 s. The top and 2
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Fig. 2. (a) Energy band diagram of a typical MLG/SQDs:SiO2 MLs/Si/TiOx PD. (b) Dark/photo J-V curves at λ = 600 nm for the PD with/without TiOx. (c) Spectral responsivity/quantum efficiency and (d) detectivity at bias of −3 V for the PD with/without TiOx.
few carriers flow from the MLG to n-Si due to the large barrier from the Fermi level of the MLG. Incident photons create electron-hole pairs in the SQDs, thereby generating photocurrent (PC) under forward bias as well as under reverse bias, as can be easily understood based on the energy-band diagram, as previously reported by us [17,18]. The PC is expected to increase with increasing Ln because the available density of graphene at the Fermi level (EF) increases by the downward movement of EF caused by the increased work function at larger Ln. However, the PC under forward bias is negligible because the DC is too high. These results suggest that the photoresponse of the PD is much more sensitive under reverse bias. On the other hand, the photoresponse of the PD at Ln = 1 is nearly negligible, as shown in Fig. 1c–f, due to the high sheet resistance and low work function of graphene despite the excellent transmittance. Fig. 1c shows Ln–dependent PC/DC (on/off) ratio. The ratios reach maxima at a bias voltage of about −0.4 V, irrespective of Ln, and show largest value of 173 at Ln = 4, possibly resulting from Ln–dependent trade–off relation between the sheet resistance, work function, mobility, and transmittance, as explained above. Fig. 1d shows R as functions of bias voltage at λ = 600 nm for various Ln. The R was calculated by the following equation: R = Iph/Llight, where Iph is the PC and Llight is the incident light power. The power density was about 720 μW-cm−2 at λ = 600 nm. The R is maximized at Va = ∼ −3.2 V for all Ln. Especially, the maximum R value is about 0.340 A/W at Ln = 4 and λ = 630 nm, as shown in Fig. 1e (for other biases at Ln = 4, Fig. S5a). Ln–dependence of the spectral external quantum efficiency (EQE) is similar to that of the spectral R, except small differences between their peak wavelengths, as shown in Fig. 1f (for other biases at Ln = 4, Fig. S5b). Based on these results, Ln was fixed at 4 for further studying the effect of the back surface passivation on the performance of the PD, as shown in Fig. 1a. For enhancing D* of a PD, lowering DC is crucial. Fig. 2a shows the energy band diagram of the PD with TiOx layer, useful as a backside passivation layer of Si for preventing the holes from being collected to the cathode (InGa), resulting in the reduction of DC. The conduction band and valence band edges of TiOx are located at −4.0 and −7.0 eV, respectively from the vacuum level [22–24]. This makes negligible band offset for electrons moving from the Si to the TiOx layer
3. Results and discussion Fig. 1a shows a schematic diagram of a typical Au/MLG/SQDs:SiO2 MLs/n-Si/TiOx/InGa PD. The active area of a PD irradiated with light was defined as 25 mm2 by photolithography process. The average size of the SQDs used as the active layer was estimated to be ∼3.5 nm, as analyzed by HRTEM (Fig. S1). The PL spectrum peaked at ∼730 nm also provided another evidence for the formation of the SQDs (Fig. S1) [17,18]. Structural, optical, and electrical properties of MLG were analyzed to check their dependence on Ln (Fig. S2). Three intense D, G, and 2D peaks at ∼1350, ∼1580, and ∼2700 cm−1, respectively were observed in the Raman spectrum at Ln = 1, indicating unique characteristics of graphene [27]. As Ln increased, both the G and 2D bands were red-shifted due to the change of the electronic band towards graphitic structure [28,29]. The Raman intensity ratio of the G to 2D peaks, I(G/2D) is related to the number of stacked graphene layers. As Ln varied from 1 to 5, I(G/2D) increased from 0.47 to 1.21, consistent with the increase of Ln. The transmittance spectra of the five MLG films were measured from 300 to 1000 nm (Fig. S2). By fitting the data based on Beer's law, the attenuation coefficient per layer was calculated to be 2.5%, very close to the theoretical attenuation coefficient (2.3%) of graphene per layer [30,31]. As Ln increased from 1 to 5, the sheet resistance of graphene was reduced from 705 ± 33 to 273 ± 12 Ω/sq (Fig. S2), consistent with the previous reports [32]. To study the change of the electric properties as functions of Ln, mobility and work function were measured (Fig. S2). The mobility/work function at Ln = 1 were 2150 cm2-V−1s−1/4.48 eV, respectively, and monotonically increased/ decreased to 215 cm2-V−1s−1/4.71 eV with increasing Ln to 5, respectively. These behaviors resulted from the changes of carrier density and the band structure depending on Ln, as previously reported [21,33]. As a first step, we characterized the PDs without TiOx. Fig. 1b shows typical J-V curves under dark and illumination for Ln = 4 (for other Ln at λ = 600 nm and power density = 720 μW-cm−2, Fig. S3). Based on the energy band diagram of the MLG/SQDs:SiO2 MLs/n-Si PD under dark (Fig. S4), majority carriers (electrons) flow from n-Si through the SQDs MLs towards the p-type MLG by direct tunneling, resulting in high DC. Under reverse bias (V < 0), the PD exhibits low DC because only a 3
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under reverse bias, and inhibits carrier recombination at the interface by blocking holes from the Si to the cathode, resulting in reduction of DC. The use of a titanium isopropoxide precursor can partially terminate the pendant bonding of the Si surface [34] by combination of Si and TiOx (for the schematic diagram, Fig. S6a). The chemical bond of Si–O–Ti formed by the precursor can significantly reduce the pendant bonding of the Si surface and suppress surface charge recombination. We measured the Si 2p XPS spectrum to check the bonding of Si to the precursor (Fig. S6b). By linearly shaping the XPS spectrum based on the appropriate combination of Gaussian and Lorentz functions, the bond energies of the Si 2p core levels were shown to be 101.7 and 102.7 eV, corresponding to Si–O–Ti and Si–O–Si bonds, respectively [34]. Fig. 2b shows typical dark and photo J–V curves for the PDs with/ without TiOx on the back side of the Si (photo J-V curves for other λ, Fig. S7a), indicating a big suppression of DC under reverse bias by the use of TiOx film without almost any change in total current. As a result, the PC/DC reaches more than 103. (Fig. S7b). The dark J-V curve of a nonideal diode is determined by the following equation [35]: J = Js [exp(eV/nkT) - 1], expressed based on the thermal ion emission model, where Js is the ideal reverse saturation current, e is electron charge, T is temperature, k is the Boltzmann constant, and n is diode ideality factor. The n is estimated to be 2.39 and 3.65 for the PDs with/without TiOx, respectively, meaning the PD with TiOx is better-quality diode. Fig. 2c shows spectral R and EQE of the PDs with/without TiOx (R and EQE for other Va, Figs. S7c–d). The R and EQE slightly increase from 0.340 AW−1 and 70% to 0.351 A-W−1 and 72%, respectively by the use of TiOx. Another figure-of-merit of PD is NEP, indicating minimum detectable power. The NEP is expressed by the following equation: NEP = In/R [36], where In is noise current. The NEP can be calculated by measuring In (Figs. S8a and d) of PDs with/without TiOx as functions of Va (Figs. S8b and e). The NEP is related to the specific D* representing the detection limit of a PD by the following equation: D* = Af /NEP [36], where A is the absorbing area (0.25 cm2) and f is the electrical bandwidth in Hz. Fig. 2d shows spectral D* of the PDs with/without TiOx at Va = - 3V (for other biases, Figs. S8c and f). The minimum NEP is 0.53/156.88 pW Hz−1/2 at Va = - 3V and λ = 630 nm for the PDs with/without TiOx, resulting in D* of 9.42 × 1011/3.21 × 109 cm Hz1/ 2 W−1, respectively. These results suggest that the D* is greatly enhanced by the use of TiOx layer in the MLG/SQDs PDs, and this maximum D* value is much larger than ever achieved in graphene/Si wafer PDs in the visible region [8,14,16–18]. We measured additional figure-of-merits to further characterize the PDs with TiOx. Fig. 3a shows normalized responses as functions of pulse frequency under various reverse biases by chopping a continuous-wave 532-nm laser line with a spot size of 25 mm2. The 3 dB bandwidth ranges from 0.12 to 0.15 MHz for various Va. Fig. 3b shows bias-dependent fast decay curves of normalized PC, from which the rise time (tr) and decay time (td) for various Va are extracted to be 0.65–0.71 μs and 6.8–7.3 μs, respectively, consistent with the results of 3 dB bandwidth. These behaviors are much faster than or comparable to those of graphene/Si wafer PDs [8,9,13,14,16–18]. Fig. 3c shows J-V curves under illumination at 532 nm, measured by varying the light intensity from 2.1 × 10−4 to 4.5 mW/cm2. Here, the light intensity was controlled using a neutral density filters. The PC is almost linearly proportional to light power under all biases. The LDR is expressed as LDR = 20 log (J*PC/Jd), where J*PC is obtained at a power density of 1 mW cm−2 and Jd is the dark current density, resulting in the LDR over 78 dB, comparable to those obtained from graphene/Si PDs [13,17,18]. We also studied the long-term stability of the PD with TiOx in the atmosphere for about 1000 h. Fig. 4a shows time-dependent dark and photo J-V curves at λ = 400, 600, and 800 (for the PD without TiOx, Fig. S9a). As shown in Fig. 4b, the R is almost constant for 1000 h regardless of λ and Va (for the PD without TiOx, Fig. S9b). These results suggest that the MLG/SQDs:SiO2 MLs/n-Si structures are very promising as next-generation PDs with high detectivity and long-term stability in the visible region.
Fig. 3. (a) Normalized response of the PD with TiOx as a function of pulse frequency for various Va. (b) Normalized transient PC of the PD with TiOx for various Va. (c) Photo-response linearity of the PD with TiOx as functions of irradiance for various Va.
4. Conclusion MLG/SQDs:SiO2/Si/TiOx PDs were successfully fabricated to show highest performance of 0.351 A-W−1 R, 9.42 × 1011 cm Hz1/2-W−1 D*, ∼0.7/∼7.0 μs rise/decay times, and 78 dB LDR at Ln = 4, much better than ever achieved in graphene/Si wafer PDs. Especially, the D* was enhanced almost 300 times by the use of TiOx as a back-surface passivation layer of the PDs, resulting from the sharp reduction of DC caused by the suppressed carrier recombination at the interface of Si with the TiOx layer. In addition, the R was almost consistent for 1000 h in air, regardless of the bias voltage and λ. These results are very promising for potential applications of the MLG/SQDs:SiO2/Si/TiOx PDs in next-generation optoelectronic areas.
Acknowledgments This work was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Science & ICT (NRF-2017R1A2B3006054).
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Fig. 4. Long-term stabilities of the PD with TiOx during 1000 h under ambient air. (a) Time-dependent dark and photo J-V curves at λ = 400, 600, and 800 nm. (b) Temporal behaviors of responsivities at λ = 400, 600, and 800 nm under Va = −3 V for the PD with TiOx.
Appendix A. Supplementary data [18]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.dyepig.2019.107587.
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