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High Responsivity and Detectivity Graphene-Silicon Majority Carrier Tunneling Photodiodes with a Thin Native Oxide Layer Hong-Ki Park, and Jaewu Choi ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00247 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018
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High Responsivity and Detectivity Graphene-Silicon Majority Carrier Tunneling Photodiodes with a Thin Native Oxide Layer Hong-Ki Park and Jaewu Choi* Quantum Information Display Laboratory, Department of Information Display, Kyung Hee University, 26, Kyungheedae-ro, Dongdaemun-gu, Seoul, 02447, Republic of Korea. KEYWORDS: Photo-induced amplification, Schottky barrier height lowering, Tunneling, Graphene, Silicon
ABSTRACT: A photocurrent amplifier operable at low bias voltages with high responsivity and detectivity is highly demanding for various optoelectronic applications. This study shows majority carrier graphene-native oxide-silicon (GOS) photocurrent amplifiers complying with the demands. The photocurrent amplification is primarily attributed to the photo-induced Schottky barrier height (SBH) lowering for majority carriers. The unavoidably formed thin native oxide layer between graphene and silicon during the wet graphene transfer process plays significant roles in lowering of the dark leakage current as well as photo-induced SBH lowering. As a result, the photocurrent to dark current ratio is as high as ~ 12.7 at the optical power density of 1.45 mWcm-2. These GOS devices show a high responsivity of 5.5 AW-1 at an optical power (458 nm in wavelength) of 15 µWcm-2, which corresponds to ~ 1400 % quantum efficiency. Further the response speed is as fast as a few ten-microseconds. Thus these GOS majority carrier photodiodes show the highest detectivity (2.35 x 1010 cm Hz1/2 W-1) among previously reported graphene-silicon photodiodes. However, the responsivity decreases with the optical power density due to the increasing recombination rate through the interface states proportional to the optical power density.
Graphene having the unique electronic structural properties is a strong candidate for ideal transparent electrode materials in various optoelectronic applications because its uniform high optical transmittance (~ 97.7 %) over a broad spectral range, the thinnest metallic twodimensional layer with the highest carrier mobility, and the chemical inertness in the ambient atmosphere to name a few.1-4 Among graphene based optoelectronic devices harnessing the unique physical properties of graphene mentioned above, graphene based photodetectors have been heavily explored.1-4 Graphene based photoconductive mode photodetectors show high response speed due to the high carrier mobility but are suffering from relatively very low responsivity due to the poor light absorption by the graphene itself and high power consumption due to the high dark current originated from the metallicity of graphene. To improve the poor responsivity, an internal multiplication process has been considered but the amplification rate is very limited.5-14 Photoconductive modes can take the advantages of the photo-generated carrier type dependent motility difference, which can be adapted for the amplification. However, the intrinsic carrier mobility difference in graphene is almost negligible. Thus there are significant efforts to intentionally slow down or hold the one type of charge carriers by the other materials located near to the graphene by mimicking phototransistors.6-9 Among them, quantum dot-graphene heterostructure is
employed to hold the one type charge carriers in the quantum dots sitting on graphene.7 As a result, the quantum dot-graphene heterostructure shows incredibly high responsivity (~107 AW-1).7 However, the photodetectors based on this approach shows intrinsically serious weaknesses due to its high power consumption due to the unacceptably high leakage current and slow response speed (slower than seconds in timescale) due to the employed amplification mechanism holding the charge carriers in the quantum dots.7 A recent study shows a high responsivity (~103 AW-1) as well as a high response speed (~ 400 ns) at low optical power (< 1 nW) by employing a mechanically exfoliated graphene and holding the one type of photo-induced charge carriers in the silicon rather than a quantum dot.15 However, the leakage current is extremely large compared to the light induced photocurrent due to the intrinsic attribute of the employed photoconductive mode or the modified photoconductive mode. As a result, they show poor detectivity.15 Graphene based heterostructure photodiodes with semiconductors show relatively high response speed but moderated responsivity.5 In the graphene-semiconductor heterostructures, graphene is mainly employed as a transparent electrode due to its low light absorption (2.3%) and relatively high electrical conductivity.4 Thus, in this structure, light absorption and photo-induced charge carrier generation are mostly occurred in the sem-
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iconducting layer contacted with the graphene.5 One can expect almost 100% detection efficiency from the gra-
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phene-semiconductor photodiodes when the depletion width is wider than the light
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Figure 1. (a) Schematic of the employed experimental set-up for photo-response measurements of GOS diodes. The light is illuminated through the aperture only to the active area of GOS devices. (b) The experimental dark J-V curve (black open circles) of the GOS diode and the fitted curves (dotted red line) based on the modified Schottky emission theory (Equation 1). The inset shows Cheung’s plots of d(V)/d(lnJ) (Black circle) and H(J) (Red square) versus J based on the experimental J-V curves in the forward bias region. The dashed lines are fitted data. absorption depth. Further one can expect a response time faster than sub-microsecond scale because it is largely governed on the transient time of the photo-generated carrier across the depletion layer in the semiconductor.5However, unlike the expectations, the experimentally achieved highest photo-responsivity of graphenesemiconductor heterostructure photodiodes is 0.435 AW-1 even for infrared light and their response speed is limited as ~ 1.2 ms.5 Thus alternative approaches for a graphene based photodetector showing low power consumption, high responsivity, high response speed and high detectivity should be searched. This can be hinted by metal-oxide-silicon semiconductor (MOS) majority carrier photodiodes, which are showing the demanding properties as mentioned above.1618 The majority carrier devices are peculiar with the small work function difference between the metal electrode and the Si.16-18 This study adapted the scheme of majority carrier devices for the development of graphene-native oxidesilicon (GOS) photodiodes with high responsivity, high response speed, low leakage current and high detectivity. The employed GOS majority carrier devices are having a very thin native oxide layer and the small work function difference between graphene and Si. Typically, MOS photodiodes are requiring a very thin metallic layers to minimize the optical power loss due to the high light reflection and absorption by the metallic layers but the minimal thickness is limited by the required conductance of the metallic layer. Compared to the MOS majority carrier devices, GOS majority carrier devices developed in this study have strengths in the high transparency and the high conductivity. In addition to this, the controllable graphene Fermi level can be another strength over the conventional MOS majority carrier devices. In this study, the GOS majority carrier devices show that the high responsivity, ~ 14 times higher than that at 100 % quantum efficiency, the high response speed of sub milliseconds, low leakage current and high detectivity. Thus the GOS
majority carrier photodiodes can be promising for various optoelectronic applications. A graphene layer was synthesized on a polished copper (Cu) foil (~ 100 μmin thickness) using a conventional chemical vapor deposition (CVD) growth method.19-21 The Cu foil (area of 7 mm x 7 mm) was etched by Fe2Cl3 etchant solution after deposition a poly methyl methacrylate (PMMA) layer on the grown graphene/Cu layer. The residue of Fe2Cl3 etchant was rinsed by deionized (DI) water in several times.20-21 The patterned aluminum (Al) electrodes (2 mm x 5 mm) were deposited on a 300 nm thick SiO2/Si (p-type) substrate by a DC magnetron sputtering method. The Ohmic back contact to the Si substrate was achieved by deposition of gold (Au) on the back side of the etched Si. The PMMA/graphene layer (7 mm x 7 mm) was transferred on the 1.2 mm x 1.2 mm Si (resistivity of 1-10 Ωcm) window pre-defined by etching the 300 nm thick SiO2 on the Si, and on the pre-deposited Al electrodes (2 mm x 5 mm). As a result, graphene-native oxide-silicon (GOS) devices is formed as schematically shown in Figure 1a. For optical response studies, an opaque mask with an aperture of 1.2 mm x 1.2 mm was overlaid on the GOS contact area of fabricated devices to allow only the light transmission through the window mentioned above as shown in Figure 1a. The employed light source was a 458 nm wavelength light emitting diode (LED), of which the exposed light power was controlled from 14.7 µWcm-2 to 1.45 mWcm-2. The dark and photo current-voltage (I-V) characteristics were investigated using a Keithley 6430 Subfemtoamp SourceMeter as schematically shown in Figure 1a. The bias voltage (V) was applied between Au (+) and Al (-) electrodes in a voltage range of –5 V to 1 V with 0.1 V step voltage. The capacitance-voltage (C-V) characteristics were studied at the modulation frequency of 1 kHz using an HP 4192A LF impedance analyzer while the applied bias voltage range was scanned from -5 V to 3 V with 0.1 V step
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voltage. The time-dependent photo-response was measured by
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Figure 2. (a) The experimental J-V curves as a function of optical power density. The inset shows J-V curves in a low bias voltage region. (b) The ratio of photocurrent density to dark current density as a function of the reverse bias voltage (VR). (c) Effective SBH (ϕ ) as a function of illuminated optical power density based on Equation 2. (d) Capacitance-voltage (CV) characteristics as a function of optical power densities measured at the modulation frequency of 1 kHz. The voltage is varied from 3 V to -5 V. (e) The experimental photo-capacitances at the illuminated optical power density of 1.45 mW/cm2 are fitted with a distributed circuit model (See Figure S3, Supporting Information) with parallel connection of the depletion capacitance (CD), photo-induced inversion capacitance (Cinv) and interface state related capacitance (Cint). (f) and (g) The optical power dependent capacitance peak position (Black), height (Red) and width (Blue) of the photo-induced surface state related capacitance (Cint) and the photo-induced inversion capacitance (Cinv), respectively. (See Figure S3 and Table S1, Supporting Information). The power-dependent ideality factor (η) is marked by green diamonds in (f). monitoring the voltage across a resistor (1 kΩ), which is serially connected to the GOS device by using a Tektronix TDS 1012B oscilloscope while illuminating the LED light to the GOS device at -5 V reverse bias voltage.22
ation from the experimental one in the low bias region. This indicates that the effective SBH in the low bias region is higher than at the higher bias voltage. This can be attributed to the high density interface states.20
The dark current density density-voltage (J-V) curve of a GOS device exhibits a current rectification behavior acting like a Schottky diode as shown in Figure 1b. The J-V characteristics of the GOS diodes can be expressed by the modified thermionic emission theory accounting the tunneling through the native oxide layer as shown in Equation 1
Additionally, the Cheung’s method is employed (Inset of Figure 1b) when the bias voltage is limited in the forward bias voltage range (1.3 V ~3 V). The extracted physical parameters show a difference in SBH, serial resistance, and ideality factor from those extracted by the direct fitting as shown in Table 1.20,23,24 This clearly indicates that the physical parameters depend on the applied bias voltage while the native oxide thickness (d) is estimated as ~0.42 nm in both cases.
J A∗ T expξd exp
exp
!" # $
% 1'
(1)
Where A∗ is the effective Richardson constant, T is temperature in Kelvin (K), ξ is the tunneling barrier height in eV, d is the thickness of the oxide layer in Angstrom (Å), ϕ( is the SBH in eV, R ( is the series resistance in ohm (Ω), and β=q/kBT where kB is Boltzmann constant. η is the ideality factor, which depends on the applied bias voltage. The series resistance, Rs, ~ 3.9 kΩ derived from the linear slope in the forward bias region (2.5 V ~ 3 V). The J-V curve in the wide bias voltage range (-5 V ~ 3 V) is directly fitted based on Equation 1 as shown in Figure 1b and the extracted Schottky emission physical parameters are shown in Table 1. However, the fitted curve shows a devi-
Figure 2a also shows that the experimental J-V characteristic curves taken by varying the optical power densities of the illuminating light while the inset of Figure 2a shows the J-V characteristics in the low bias voltage region of -0.2 V ~ 0.2 V. The inset curves clearly show that the open-circuit voltage and the short-circuit current increases with the illuminating optical power density. However, in the 4th quadrant of the J-V curves, the poor fill factor is attributed to the high serial resistance (Rs = 3.8 kΩ~ 3.9 kΩ) and the high density interface states. The maximum possible open-circuit voltage corresponds to the work function difference of the junction structure. At illumination of the relatively high optical
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power density, the open cell voltage of the GOS structure suggests that the is ~ 0.1 V as estimated from the inset of Figure 2a.25 This Table 1. Summary of extracted physical parameters using the Schottky emission theory for the bias voltage range of -5V ~ 3V and the Cheung's method for the bias range of 1.3V ~ 3V. Method
Voltage range (V)
A (cm2)
A*(Acm-2K-2)
T (K)
ξ (eV)
d (nm)
ΦS (eV)
RS (kΩ)
η
Schottky
-5 ~ 3
1.44 x 10-2
30
288
3.3
0.42
0.38
3.9
6.88
1.3 ~ 3
-2
30
288
3.3
0.42
0.3
3.5
7.52
Cheung
1.44 x 10
work function difference between graphene and Si is as low as ~ 0.1 eV. This clearly indicates that the GOS structures correspond to majority carrier devices. This is also confirmed by the Mott-Schottky plot of the C-V curves. (See Figure S1 in Supporting Information) The leakage current density at the dark state is significantly low even though the Schottky barrier height (SBH) is very low. The low leakage current at the dark state is attributed to the native thin oxide layer. The dark leakage current increases with the reverse bias voltage in the low bias region. Above a critical voltage, the dark leakage current is almost saturated but slightly increases with the reverse bias voltage. This can be attributed to the increasing thermally generated minority carriers in the widen depletion region and the possible SBH lowering due to the increasing oxide voltage and the voltage dependent graphene Fermi level shift as the reverse bias voltage increases. However, the photo-induced leakage current shown in Figure 2a is unexpectedly higher than the expected possible maximum photo-current at 100 % quantum efficiency (See also Figure 3a, Figure 3b and Table 2). This indicates that a photocurrent amplification occurs in the employed GOS devices. The photocurrent to the dark current density ratio, Jp/JD, where Jp and JD are the photocurrent and dark current density, respectively, is as high as ~ 12.7 at the optical power density of ~ 1.45 mWcm-2 as shown in Figure 2b. Typically the photoconductivity mode graphene photodetectors show high responsivity but the average current ratio is much lower than 1 due to the intrinsic high leakage current caused by gapless graphene.6,7,26 The amplified photocurrent beyond the 100% quantum efficiency is attributed to the photo-induced SBH lowering as shown in Figure 2c, which is plotted based on the Equation 1. The SBH can be calculated by q⁄β# ln J# ϕ( 2# ∗ A T expξd At 0 V, the extracted SBH (ϕ( ) is ~ 0.51 eV at dark state. This could imply that the graphene Fermi level (EF(G)) is located at ~ 0.51 eV above from the valance band edge of Si (EV(Si)) and the corresponding graphene work function(EF(G)) is ~ 4.68 eV where the valence band edge of bulk Si (EV(Si)) is assumed to be located 5.19 eV from the vacuum level. The estimated work function of graphene is higher than the Dirac point and this indicates that the graphene is p-doped.
Based on the SBH and the work function difference between graphene and silicon, the silicon Fermi level may be at 4.69 eV, which is inconsistent with 4.86 eV estimated from the C-V study (See Figure S1, Supporting Information) and the extracted information of the silicon conductivity provided by the vendor. The inconsistency is attributed to Fermi level pinning caused by the high density interface states at the silicon surface.20 However, the SBH becomes lower due to the oxide voltage (VOX) and the voltage dependent graphene Fermi level shift (∆E34 ) with the reverse bias voltages. The oxide layer allows to de-pin the Fermi level from the interface states with the reverse bias voltage.27 However, the SBH variation with the reverse bias voltage is almost saturated at above a critical voltage. This indicates that the oxide voltage and the graphene Fermi level shift is nonlinearly varied with the reverse bias voltage due to the interplay with the nonlinearly varying voltage acting on silicon. (See Raman spectra of Figure S2 shown in Supporting Information) In addition to the reverse bias voltage-dependent SBH variation in the dark state, the photo-induced SBH lowering causes the exponentially high leakage current by the majority carriers. The SBH lowering can be triggered by the photo-induced oxide voltage enhancement as well as the graphene Fermi level shift (∆FS = q∆V67 + ∆E34 ). However, the graphene Fermi level shift due to the light illumination may not be significant compared to the oxide voltage at the employed optical power density as indicated by the optical power density dependent Raman shift and thus the photo-induced SBH lowering can be implicitly expressed by (∆FS ~ q∆V67 ) at the employed optical power density in this study. (See Figure S2 in Supporting Information) The photo-induced oxide voltage enhancement (∆V67 ) at an applied reversed bias voltage can be attributed to the conductivity enhancement of the depletion region by the photo-generated electron-hole (e-h) pairs. As a result, some portion of the applied bias voltage acting on the depletion region at the dark state is reduced and transferred to the oxide layer under light illumination. Due to the enhanced oxide voltage (∆V67 ) at a given bias voltage, the effective SBH becomes lower by q∆V67 . The optical power density dependent capacitance of a GOS is shown in Figure 2d. In the forward bias region, the capacitance decreases slowly as the forward bias voltage decreases but does not depend on the optical light illumination as expected. The plateau shown between 0 V and 1 V is related to the interface states as mentioned above,
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which are significantly large in the graphene-silicon interface as identified in the precious study.20 In the dark state, the capacitance is suddenly reduced within a few tenth volt of the applied reverse bias voltage and then slowly decreases. The sudden decrease in the low reverse bias voltage range is also attributed to the interface states while the slow decrease beyond the initial sudden change is attributed to the depletion width widening as the reverse bias voltage increases. However, the strong inversion behavior was not observed even though the measurement was done at relatively high reverse bias voltage of -5 V and the relatively low AC modulation frequency of 1 kHz. This is attributed to the higher tunneling rate of the minority carriers through the thin native oxide layer at the higher reverse bias voltages as well as the higher recombination rate by the higher density interface states as shown in the previous impedance spectroscopic study.20 The optical power density dependent C-V curves shown in Figure 2d and Figure 2e are distinct from those at the dark state and show that the depletion dominant capacitance is observed at the higher reverse bias voltage range with increasing the optical power density. At the relative low reverse bias voltage range, the capacitance is largely dominated by the interface states rather than the depletion as mentioned above. The interface states act as a charge storage as well as recombination centers at low reverse bias voltage range and its effect is appeared over the broader low bias voltage range as the optical power density increases (See Figure S3).20 As the optical power density increases, photo-induced inversion is clearly observed at relatively high reverse bias voltage. At the relatively high reverse bias voltages, the depletion width is wide enough for photo-generated minority carriers to be accumulated at the interface even though there are significant tunneling through the oxide layer and recombination through the interface states. Thus the photo-induced inversion behavior is observed at the relatively high reverse voltage when the photogenerated minority carriers in the depletion region are large enough. However, as the reverse bias voltage becomes higher than a critical voltage, the photo-induced inversion capacitance becomes weaker. This suggests that the recombination of photo-generated minority carriers becomes significant due to the increasing majority carriers tunneled from the graphene electrodes due to the lowered SBH with increasing the reverse bias voltage. Simultaneously the tunneling of minority carriers also increases with the reverse bias voltage. The excess photo-carriers in the depletion region reduce the resistance of the depletion region. As a result, at a given reverse bias voltage, the relative oxide voltage increases while the junction voltage is reduced as the optical power density increases. As a result, the depletion width at a given reverse bias voltage is reduced as the optical power density increases. This is indicated by the relatively high depletion capacitance at the high reverse
bias voltage as the optical power density increases as shown in Figure 2d. (See Figure S3, Supporting Information) To illustrate the above observations, the experimental photo-capacitances are fitted with a lumped capacitance (including oxide capacitance) serially connected with three capacitances, which are connected in parallel as shown in Figure S3e (See Supporting Information). The three capacitances depend on the reverse bias voltage and are the depletion capacitance (CD), interface capacitance (Cint) and the inversion capacitance (Cinv). The represented fitted example of the experimental photo-capacitances at illumination of the optical power density of 1.45 mW/cm-2 is shown in Figure 2e. Based on the fitting of the experimental photocapacitances shown in Figure 2d, the peak position, height and width of the photo-induced interface capacitance (Cint) and the photo-induced inversion capacitance (Cinv) are plotted as a function of the optical power density as shown in Figure 2f. As the optical power density increases, the peak position and the intensity of the interface capacitance (Cint) are not much varied but the peak width becomes wider in the low reverse voltage range as shown in Figure 2f. This suggests that the interface capacitance acts as the charge storage as long as the excess charge carriers are supplied by the photo-generated minority carriers even though the reverse bias voltage increases up to a critical voltage. However, the inversion capacitance becomes stronger at the intermediate voltage range, the peak position shifts to the higher reverse bias voltage and the peak width becomes wider as the optical power density increases as shown in Figure 2g. Once the interface states are filled with minority carriers, the photo-generated excess charges are accumulated at the interface as the optical power increases the interface states as indicated by the growing inversion capacitance with the optical power density. This indicates the minority carrier current becomes tunneling limited. The shift of the peak position to the higher reverse bias voltage with the optical power density indicates that the voltage portion acting on the semiconductor at a given bias voltage becomes lower with the optical power density. This indicates that the conductivity enhancement of the silicon region by the light illumination reduces the effective bias voltage acting on the semiconductor while the effectively acting voltage on the oxide layer increases as the optical power density does. This effect is appeared as the enhancement of the ideality factor (η) as the optical power density increases as shown in Figure 2f. The net photocurrent densities are plotted as a function of the illuminated optical power density as shown in Figure 3a. As expected, the photocurrent increases with the optical power density. Due to the very thin and highly transparent graphene (~ 97 %) for the visible light (458 nm in wavelength), in these GOS junction devices, most illuminated light is absorbed in Si.4 Accordingly, the photo-generated e-h pairs in the Si becomes larger as the optical power density increases. As a result, the conductivity
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of the silicon in the light illuminated region increases. In the GOS device configuration, graphene acts as a minority
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carrier collector as well as a tunable majority carrier supplier to the Si at a reverse bias voltage.
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Figure 3. (a) Photocurrent density as a function of optical power density in a reverse bias voltage region. Region I: the relatively low photocurrent region (light blue). Region II: the linearly increasing photocurrent region with the applied bias voltage (pale blue). Region III: the almost saturated photocurrent region (dark blue). (b) Photo-responsivity (R) as a function of optical power density and reverse bias voltage. The blue plane grid shows the photo-responsivity (R = 0.37 A/W) corresponding to 100% quantum efficiency (Q.E. = 1). (c) Time-dependent photocurrent taken by varying the illuminating optical power density at -5V. (d) Optical power density dependent decay time constants, τ9 and τ( , for fast and slow response, respectively. The reverse bias voltage-dependent photocurrent behaviors shown in Figure 3a can be classified into three regions. The Region I corresponds to the low reverse bias region (0 V ~ -0.6 V) and marked by light-blue in Figure 3a. The detailed view of the Region I is shown in the inset of Figure 3a. In general, the photocurrent is low, and less depends on the applied reverse bias voltage, and the quantum efficiency is less than 100 %. This is attributed to the relatively narrow depletion width at the low reverse bias voltage as well as the high density interface states, which are equilibrium with silicon as shown in the previous study.20 The interface states act as a charge storage as shown in Figure 2 and the recombination centers of the photo-generated carriers. As a result, the collection efficiency of the photo-generated carriers is low. The Region II (pale-blue region) of Figure 3a shows that the photocurrent linearly increases as the reverse bias voltage increases. The linearly increasing behavior indicates that the depletion width increases accordingly with the reverse bias voltage unlike the Region I. Particularly the quantum efficiency in this region is higher than the ideal limit, 100 %. This strongly suggests that there is pho-
to-induced current amplification process, which is attributed to the voltage dependent depletion width widening and the photo-induced gating for the majority carrier current by the SBH lowering, which is caused by the photo-induced oxide voltage enhancement as explained above. The Region III (dark-blue region) of Figure 3a corresponds to the region showing the almost saturated photocurrent even though the photocurrent slightly increases with the reverse bias voltage. This indicates that the depletion layer width at the end of the Region II is already wider than the penetration depth of the incident light. As a result, beyond the end of the Region II, the current slowly increases with the reverse bias voltage due to the widening of the depletion width and the residual light absorption in the widen region. In this region, the influence of the interface states acting as a charge storage and the photo-induced inversion becomes weak due to the higher recombination rate and tunneling rate of the majority and minority carriers. As a result, there is no significant enhancement of the photocurrent even though the reverse bias voltage increases.
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Photo-responsivity (R), the photocurrent density (JPh) per optical power density (OPD), can be expressed by R=JPh/OPD, respectively. Figure 3b shows the photoresponsivity, R=JPh/OPD, as a function of the reverse bias voltage and the optical power density. In the low reverse bias voltage region (Region I: -0.1 V ~ -0.6 V), the responsivity is lower than ~ 0.37 AW-1 corresponding to the 100 % quantum efficiency at 458nm in wavelength and is typically observed in the conventional MOS photodiodes.28 The detailed photocurrent of the Region I shown in the inset of Figure 3a increases with optical power density but hardly depends on the applied bias voltages. However, at the higher reverse bias voltage (-0.7 V ~ -5 V: corresponding to the Region II and III in Figure 3a), the photo-responsivity linearly increases with the bias voltage (Region II in Figure 3a) and is almost saturated above a critical voltage (Region III in Figure 3a). For the given GOS devices, the highest responsivity of ~ 5.5 AW-1 was observed at relatively low optical power density of 15 µWcm-2 at the reverse bias voltage of -5 V. The corresponding quantum efficiency of the GOS diode is ~ 1400 %. Figure 3b shows that the photo-responsivity largely decreases as the optical power increases when the optical power density is less than 0.15 mWcm-2. However, there is no significant change in the photo-responsivity at the optical power density higher than 0.15 mWcm-2. This can be considered as the interplay of the interface states equilibrium with silicon or graphene.20 At low optical power, the most photo-generated electrons are highly captured by the interface states.20 As a result, the charged interface states are maintained as long as the excess charges are provided and this induces the relatively high oxide voltage across the native oxide layer. This causes lowering of the effective SBH for the majority carriers. Even though the absolute magnitude of the SBH change is not significant, this will induce a tangible current change compared to the dark current because the leakage current varies exponentially with the SBH change (See Equation 2 and Figure 2c). Further, the low density of the photogenerated carriers causes the lower recombination rate through the interface states. Thus the responsivity at low optical power density becomes high. As the optical power density increases, the extra minority carriers are accumulated at the interface in the intermediate reverse bias voltage region, and this causes the formation of inversion layer. At high optical power illumination, many photo-generated electrons and holes are available in the conduction band and valance band, respectively. As a result, the Schottky-Read-Hall (SRH) like recombination rate through the interface states becomes higher as the photo-generated e-h pairs increase.27 As a result, the responsivity decreases with the optical power of illumination. In addition to this, the recombination rate becomes higher with the higher majority carrier tunneling as indicated by the low capacitance at the high reverse bias voltage as shown in Figure 2d.
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Figure 3c shows the time-dependent photo-response of the employed GOS devices. The time-dependent photoresponse was measured at the reverse bias voltage of -5 V with exposure of various optical power to the GOS junction as shown in Figure 3c.22 The rising time (τ: ) of the GOS device is estimated by fitting the time dependent photo-response with an exponential function of ~e ;⁄
