Low-Power Graphene/ZnO Schottky UV Photodiodes with Enhanced Lateral Schottky Barrier Homogeneity

The low-power, high-performance graphene/ZnO Schottky photodiodes were demonstrated through the direct sputter-growth of ZnO onto the thermally-cleaned graphene/SiO2/Si substrate at room temperature. Prior to the growth of ZnO, a thermal treatment of the graphene surface was performed at 280 °C for 10 min in a vacuum to desorb chemical residues that may serve as trap sites at the interface between graphene and ZnO. The device clearly showed a rectifying behavior with the Schottky barrier of ≈0.61 eV and an ideality factor of 1.16. Under UV illumination, the device exhibited the excellent photoresponse characteristics in both forward and reverse bias regions. When illuminating UV light with the optical power density of 0.62 mW/cm2, the device revealed a high on/off current ratio of >103 even at a low bias voltage of 0.1 V. For the transient characteristics upon switching of UV light pulses, the device represented a fast and stable photoresponse (i.e., rise time: 0.16 s, decay time: 0.19 s). From the temperature-dependent current–voltage characteristics, such an outstanding photoresponse characteristic was found to arise from the enhanced Schottky barrier homogeneity via the thermal treatment of the graphene surface. The results suggest that the ZnO/graphene Schottky diode holds promise for the application in high-performance low-power UV photodetectors.


Introduction
Graphene-based hybrid and heterostructures with other inorganic and/or organic materials render more fascinating functionalities compared to conventional electronic and optoelectronic devices [1][2][3]. Among various graphene-based hetero-architectures, the graphene/inorganic semiconductor contacts have attracted much attention because of their ample potential for high-performance Schottky photodiodes (PDs) [4]. For example, the enhanced photoresponse characteristics with high sensitivity and fast response were demonstrated on various graphene-based Schottky PDs that were composed of typical inorganic semiconductor materials (e.g., Si [5,6], Ge [7], GaAs [8,9], CdSe [10], ZnO [11][12][13][14][15][16][17][18][19][20][21], etc.). Among them, ZnO is one of the most prospective materials for high-performance ultraviolet (UV) PDs because of its wide band gap and excitonic properties [22,23]. Due to the c-axis preference of wurtzite ZnO, moreover, ZnO thin films [18,19] or ZnO nanorods [20] can be easily grown on the defective graphene sheet that had been grown using chemical vapor deposition (CVD). Namely, the presence of C-O nucleation sites on CVD graphene allows us to fabricate a simple device scheme of the graphene/ZnO Schottky PD through the direct growth of ZnO on graphene. Furthermore, since the work function of CVD graphene could be controlled by thermal [24,25] and chemical [26] treatments, one can easily manipulate the Schottky barrier height at the graphene/ZnO interface. Owing to such advantages, very recently, high-performance graphene/ZnO Schottky UV PDs have been realized through various techniques; for example, direct growth of ZnO onto graphene by using CVD [16], chemical bath deposition [11], radio frequency (r.f.) magnetron sputtering [18,19], hydrothermal methods [20], and dispersion of ZnO nanorods onto graphene [12][13][14]. When using graphene as an active layer of the electronic and the optoelectronic devices, there are critical issues from chemical adsorbates and residues that might remain on the graphene surface during the graphene transfer step [24]. For instance, during the growth of ZnO on graphene, the chemical adsorbates and residues will degrade the lateral homogeneity of the Schottky barrier because they act as unnecessary dopants and/or contaminants at the graphene/ZnO interface. In Schottky PDs, the inhomogeneous barrier underneath the photon collection area may degrade the ideality factor (i.e., transport characteristics) [4], and will eventually restrict the photoresponse characteristics of the Schottky PDs. To realize the high-performance graphene/ZnO Schottky UV PDs, therefore, enhancing the lateral Schottky barrier homogeneity is essential. In other words, the chemical adsorbates and residues should be effectively removed prior to the growth of ZnO onto the graphene surface.
In light of all the above, we have investigated the enhancement of the lateral Schottky barrier homogeneity in the graphene/ZnO Schottky PDs. The devices were fabricated by direct sputtering of ZnO onto thermally-cleaned single-layer graphene (SLG). Namely, to improve the ZnO/SLG interface properties, in situ thermal cleaning of CVD SLG was performed just prior to the sputtering of ZnO in a single chamber. This simple method uses neither chemical nor physical treatments that may provide additional adsorption of gas molecules during subsequent handling of the sample in air ambience. Despite such advantages, according to our best survey, no previous works have reported on the in situ thermal treatment for fabricating a high-performance SLG/ZnO Schottky PD. Through the temperature-dependent electrical characterization, we analyzed the Schottky barrier homogeneity of the fabricated ZnO/SLG Schottky PDs. In addition, we thoroughly assessed the photoresponse characteristics of the devices due to varying UV powers. Figure 1 schematically illustrates the device fabrication procedures for the SLG/ZnO Schottky PD. First, the SLG sheet was grown on Cu foil using CVD and transferred onto the SiO 2 /Si substrate by using a poly(methyl methacrylate) (PMMA) transfer method ( Figure 1a). Next, the sample was mounted in the sputtering chamber and was thermally cleaned at 280 • C for 10 min in a high vacuum (≈10 −6 Torr) to eliminate chemical residues and/or molecular oxygens that might be adsorbed onto the SLG surface during the transfer process ( Figure 1b). Nanomaterials 2019, 9, x FOR PEER REVIEW 2 of 13 example, direct growth of ZnO onto graphene by using CVD [16], chemical bath deposition [11], radio frequency (r.f.) magnetron sputtering [18,19], hydrothermal methods [20], and dispersion of ZnO nanorods onto graphene [12][13][14]. When using graphene as an active layer of the electronic and the optoelectronic devices, there are critical issues from chemical adsorbates and residues that might remain on the graphene surface during the graphene transfer step [24]. For instance, during the growth of ZnO on graphene, the chemical adsorbates and residues will degrade the lateral homogeneity of the Schottky barrier because they act as unnecessary dopants and/or contaminants at the graphene/ZnO interface. In Schottky PDs, the inhomogeneous barrier underneath the photon collection area may degrade the ideality factor (i.e., transport characteristics) [4], and will eventually restrict the photoresponse characteristics of the Schottky PDs. To realize the high-performance graphene/ZnO Schottky UV PDs, therefore, enhancing the lateral Schottky barrier homogeneity is essential. In other words, the chemical adsorbates and residues should be effectively removed prior to the growth of ZnO onto the graphene surface.

Preparation of SLG/SiO 2 /Si Substrate
In light of all the above, we have investigated the enhancement of the lateral Schottky barrier homogeneity in the graphene/ZnO Schottky PDs. The devices were fabricated by direct sputtering of ZnO onto thermally-cleaned single-layer graphene (SLG). Namely, to improve the ZnO/SLG interface properties, in situ thermal cleaning of CVD SLG was performed just prior to the sputtering of ZnO in a single chamber. This simple method uses neither chemical nor physical treatments that may provide additional adsorption of gas molecules during subsequent handling of the sample in air ambience. Despite such advantages, according to our best survey, no previous works have reported on the in situ thermal treatment for fabricating a high-performance SLG/ZnO Schottky PD. Through the temperature-dependent electrical characterization, we analyzed the Schottky barrier homogeneity of the fabricated ZnO/SLG Schottky PDs. In addition, we thoroughly assessed the photoresponse characteristics of the devices due to varying UV powers. Figure 1 schematically illustrates the device fabrication procedures for the SLG/ZnO Schottky PD. First, the SLG sheet was grown on Cu foil using CVD and transferred onto the SiO2/Si substrate by using a poly(methyl methacrylate) (PMMA) transfer method ( Figure 1a). Next, the sample was mounted in the sputtering chamber and was thermally cleaned at 280 °C for 10 min in a high vacuum (≈10 −6 Torr) to eliminate chemical residues and/or molecular oxygens that might be adsorbed onto the SLG surface during the transfer process ( Figure 1b).

Fabrication of ZnO/SLG Schottky PDs
The ZnO/SLG Schottky contacts were formed via the direct growth of the 200-nm-thick ZnO layer onto the SLG/SiO 2 /Si substrate through r.f. magnetron sputtering (Figure 1c). The sputtering process Nanomaterials 2019, 9, 799 3 of 13 was performed at room temperature in Ar plasma ambient under the following conditions: Ar gas flow rate = 30 sccm, working pressure = 30 mTorr, and r.f. power = 150 W. After the growth of the ZnO layer onto SLG, the active area (w ≈ 10 µm, l ≈ 30 µm) was defined by conventional photolithography techniques. Finally, the Al (t Al ≈ 100 nm) and Ti/Al (t Ti ≈ 10, t Al ≈ 100 nm) ohmic electrodes for ZnO and SLG layer were formed using electron-beam evaporation and standard lift-off processes (Figure 1d). We here note that two different types of ZnO/SLG Schottky PDs were prepared by using as-transferred SLG and thermally-cleaned SLG so as to examine the effects of thermal cleaning on the device characteristics. For convenience, we refer the former and the later as a ZnO/AT-SLG Schottky PD and a ZnO/TC-SLG Schottky PD, respectively.

Measurements of Material and Device Characteristics
The Raman scattering characteristics of the as-transferred and thermally-cleaned SLG samples were measured by using a Renishaw Micro Raman spectrometer (Renishaw, Wotton-under-Edge, UK) under green laser excitation (λ = 514 nm). The topographic cross-section and the crystal structure of the ZnO layer were monitored through scanning electron microscopy (SEM) using an FE SEM XL-30 system (Phillips, Eindhoven, The Netherlands) and X-ray diffractometry (XRD) using a Bede D3 system (Bede Scientific Instruments Ltd., Durham, UK), respectively. The temperature-dependent electrical characteristics of the ZnO/SLG Schottky PDs were assessed at 300-400 K by using a Keysight B1500A semiconductor device parameter analyzer (Keysight Technologies, Santa Rosa, CA, USA). The photoresponse characteristics of the PDs were examined under light illumination using a 365-nm UV light emitting diode, wherein the UV power density (P UV ) was varied from 0-0.77 mW/cm 2 during photoresponse measurements.  Figure 2b shows the Raman spectra of the SLG sheets used for the device fabrication. Both the as-transferred and the thermally-cleaned SLG sheets revealed two predominant Raman features of G and 2D bands from high-quality graphene. For as-transferred SLG, the G and 2D peaks appeared at ≈1588 and 2684 cm −1 , respectively, and the observed positions were blue-shifted from pristine graphene (i.e., G pristine ≈ 1580-1585 cm −1 , 2D pristine ≈ 2635-2645 cm −1 ) [25]. This implies that our as-transferred SLG was doped by acceptor impurities from oxygen molecules [27] and/or chemical residues [28,29] adsorbed during the graphene transfer step. After thermal cleaning at 280 • C, both G and 2D peaks were shifted by 9 and 13 cm −1 toward their pristine graphene positions, respectively. Such a red-shift of both the G and 2D peaks depicts the decrease in charge trapping effects because the unintentional acceptors were effectively removed via thermal annealing at 280 • C [24,25]. Furthermore, no appearance of the defect-mediated D band at ≈1350 cm −1 indicates that vacuum-annealing at 280 • C caused no damage to the sp 2 carbon bonds in SLG.

Results and Discussion
Onto the surface of high quality SLG, we deposited a 200-nm-thick ZnO layer at room temperature by using an r.f. magnetron sputtering technique. As shown in Figure 2c, the cross-sectional SEM image shows that the ZnO layer was effectively grown on SLG with a well-merged c-axis preferential columnar structure. The ZnO layers grown on the as-transferred and the thermally-cleaned SLG sheets exhibited a typical XRD pattern with the (000l) lattice phase (Figure 2d), which is indicative of the wurtzite structure of typical ZnO. In such a ZnO/SLG structure, the Schottky barrier (φ B = Φ SLG − χ ZnO ) would be formed at the interface between SLG and ZnO ( Figure 2e) because the work function of CVD SLG (Φ SLG > ≈4.5 + α eV) [30] is greater than electron affinity of ZnO (χ ZnO ≈ 4.1 eV) [31].
Here, the magnitude of α depends on the difference between the Dirac point and the Fermi level in CVD SLG, and it could be caused by the p-type doping effect from oxygen molecules and/or chemical residues. Thanks to the formation of the Schottky barrier, the SLG/ZnO Schottky PDs show a good rectifying behavior (Figure 2f).  To characterize the Schottky barrier homogeneity of the ZnO/AT-SLG and ZnO/TC-SLG Schottky PDs, as a primary task, we measured the current-voltage (I-V) characteristics for both samples at temperatures ranging from 300 to 400 K in a dark chamber. For both samples, the diode current increased with increasing temperature (Figure 3a,b). Particularly, the increase in the reverse saturation current became significant because of thermally-activated carrier conduction (TACC) at elevated temperatures (see also the inset of Figure 3a). According to the thermionic emission theory [32], the I-V relationship of the Schottky diode at V > 3 kT/q can be expressed as: where J0 is the reverse saturation current, q is the electron charge, η is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, A is the contact area, and A* is the Richardson constant. Based upon Equations (1) and (2), we represented the Richardson plots (i.e., ln(J0/T 2 ) vs. 1000/T) to determine the effective value of A* for our devices ( Figure 4). From linear best fitting, the values of A* were calculated to be 0.495 and 0.628 A·cm −2 K −2 for the ZnO/AT-SLG and the ZnO/TC- To characterize the Schottky barrier homogeneity of the ZnO/AT-SLG and ZnO/TC-SLG Schottky PDs, as a primary task, we measured the current-voltage (I-V) characteristics for both samples at temperatures ranging from 300 to 400 K in a dark chamber. For both samples, the diode current increased with increasing temperature (Figure 3a,b). Particularly, the increase in the reverse saturation current became significant because of thermally-activated carrier conduction (TACC) at elevated temperatures (see also the inset of Figure 3a). According to the thermionic emission theory [32], the I-V relationship of the Schottky diode at V > 3 kT/q can be expressed as: where J 0 is the reverse saturation current, q is the electron charge, η is the ideality factor, k is the Boltzmann constant, T is the absolute temperature, A is the contact area, and A* is the Richardson constant. Based upon Equations (1) and (2), we represented the Richardson plots (i.e., ln(J 0 /T 2 ) vs. 1000/T) to determine the effective value of A* for our devices (Figure 4). From linear best fitting, the values of A* were calculated to be 0.495 and 0.628 A·cm −2 K −2 for the ZnO/AT-SLG and the ZnO/TC-SLG Schottky PDs, respectively. The obtained A* values are much smaller than the theoretical value of the ZnO Schottky diode (≈32 A·cm −2 K −2 ) because of the Schottky barrier inhomogeneity [33][34][35]. In addition, the ultrathin insulating layer at the Schottky interface could be a possible scenario that may degrade the A* value because the chemical adsorbates and oxygen molecules on SLG could locally form inadvertent insulating potential barriers [36][37][38]. Although both samples showed a smaller value of A* than the theoretical calculation, the effective value of A* was higher for the ZnO/TC-SLG Schottky PD (i.e., A* = 0.628 A·cm −2 K −2 ) than the ZnO/AT-SLG Schottky PD (i.e., A* = 0.495 A·cm −2 K −2 ). From this result, one can conjecture that the Schottky barrier homogeneity was enhanced in the ZnO/TC-SLG Schottky PD because the thermally-cleaned SLG exhibited a red-shift of G and 2D peaks, which is attributed to the effective elimination of oxygen molecules and/or chemical residues via thermal annealing at 280 • C (see also Figure 2b).
Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 13 SLG Schottky PDs, respectively. The obtained A* values are much smaller than the theoretical value of the ZnO Schottky diode (≈32 A·cm −2 K −2 ) because of the Schottky barrier inhomogeneity [33][34][35]. In addition, the ultrathin insulating layer at the Schottky interface could be a possible scenario that may degrade the A* value because the chemical adsorbates and oxygen molecules on SLG could locally form inadvertent insulating potential barriers [36][37][38]. Although both samples showed a smaller value of A* than the theoretical calculation, the effective value of A* was higher for the ZnO/TC-SLG Schottky PD (i.e., A* = 0.628 A·cm −2 K −2 ) than the ZnO/AT-SLG Schottky PD (i.e., A* = 0.495 A·cm −2 K −2 ). From this result, one can conjecture that the Schottky barrier homogeneity was enhanced in the ZnO/TC-SLG Schottky PD because the thermally-cleaned SLG exhibited a red-shift of G and 2D peaks, which is attributed to the effective elimination of oxygen molecules and/or chemical residues via thermal annealing at 280 °C (see also Figure 2b).   To examine further insight into the Schottky barrier homogeneity, we investigated the temperature dependencies of ϕB and η for both samples. At 300 K, the ZnO/AT-SLG Schottky PD revealed a ϕB of 0.58 eV and η of 1.91 ( Figure 5a). As we explained earlier, the Schottky barrier with ϕB = 0.58 eV was effectively formed due to the difference between ΦSLG and χZnO. The high magnitude of η (= 1.91) is thought to be responsible for multiple recombination channels [39], which might arise from chemical residues residing at the ZnO/SLG interface. As the temperature increased, ϕB slightly increased while η suddenly decreased. These behaviors (i.e., ϕB ↗ and η ↓ with T ↗) are commonly observed in typical metal/semiconductor Schottky diodes, and are known to be attributed to the lateral Schottky barrier inhomogeneity [40,41]. We therefore expect our ZnO/AT-SLG Schottky PD to have inhomogeneous Schottky barriers with a significant ϕB fluctuation along the surface direction, particularly at impurity and/or defect sites attributing to chemical residues (see also Figure 6a). In such a device, the effective values of ϕB and η would increase and decrease with increasing temperature, respectively, because TACC through higher Schottky barriers became serious at elevated temperatures (see also Figure 6b,c). To examine further insight into the Schottky barrier homogeneity, we investigated the temperature dependencies of φ B and η for both samples. At 300 K, the ZnO/AT-SLG Schottky PD revealed a φ B of 0.58 eV and η of 1.91 ( Figure 5a). As we explained earlier, the Schottky barrier with φ B = 0.58 eV was effectively formed due to the difference between Φ SLG and χ ZnO . The high magnitude of η (= 1.91) is thought to be responsible for multiple recombination channels [39], which might arise from chemical residues residing at the ZnO/SLG interface. As the temperature increased, φ B slightly increased while η suddenly decreased. These behaviors (i.e., φ B and η ↓ with T ) are commonly observed in typical metal/semiconductor Schottky diodes, and are known to be attributed to the lateral Schottky barrier inhomogeneity [40,41]. We therefore expect our ZnO/AT-SLG Schottky PD to have inhomogeneous Schottky barriers with a significant φ B fluctuation along the surface direction, particularly at impurity and/or defect sites attributing to chemical residues (see also Figure 6a). In such a device, the effective values of φ B and η would increase and decrease with increasing temperature, respectively, because TACC through higher Schottky barriers became serious at elevated temperatures (see also Figure 6b,c). To examine further insight into the Schottky barrier homogeneity, we investigated the temperature dependencies of ϕB and η for both samples. At 300 K, the ZnO/AT-SLG Schottky PD revealed a ϕB of 0.58 eV and η of 1.91 ( Figure 5a). As we explained earlier, the Schottky barrier with ϕB = 0.58 eV was effectively formed due to the difference between ΦSLG and χZnO. The high magnitude of η (= 1.91) is thought to be responsible for multiple recombination channels [39], which might arise from chemical residues residing at the ZnO/SLG interface. As the temperature increased, ϕB slightly increased while η suddenly decreased. These behaviors (i.e., ϕB ↗ and η ↓ with T ↗) are commonly observed in typical metal/semiconductor Schottky diodes, and are known to be attributed to the lateral Schottky barrier inhomogeneity [40,41]. We therefore expect our ZnO/AT-SLG Schottky PD to have inhomogeneous Schottky barriers with a significant ϕB fluctuation along the surface direction, particularly at impurity and/or defect sites attributing to chemical residues (see also Figure 6a). In such a device, the effective values of ϕB and η would increase and decrease with increasing temperature, respectively, because TACC through higher Schottky barriers became serious at elevated temperatures (see also Figure 6b,c).  For the ZnO/TC-SLG Schottky PD, ϕB and η were determined to be 0.61 eV and 1.16, respectively, at 300 K. In Schottky diodes, the low η manifested a clean interface between metal and semiconductor for the device, resulting in a dominance of thermionic emission rather than recombination [42]. One possible reason could be an effective removal of chemical residues from the SLG surface through the thermal cleaning process. Furthermore, the magnitudes of both ϕB and η were almost independent of   For the ZnO/TC-SLG Schottky PD, ϕB and η were determined to be 0.61 eV and 1.16, respectively, at 300 K. In Schottky diodes, the low η manifested a clean interface between metal and semiconductor for the device, resulting in a dominance of thermionic emission rather than recombination [42]. One possible reason could be an effective removal of chemical residues from the SLG surface through the thermal cleaning process. Furthermore, the magnitudes of both ϕB and η were almost independent of Figure 6. Expected Schottky barrier homogeneity along the in-plane direction normal to the surface and its impact on the carrier transport characteristics at different temperatures: (a) three-dimensional illustration of the energy-band diagram for the ZnO/SLG Schottky PD consisting of inhomogeneous Schottky barriers (e.g., for the case of the ZnO/AT-SLG Schottky PD), (b) carrier transport across the inhomogeneous Schottky barriers at T = T 1 , (c) carrier transport with TACC at T = T 2 >> T 1 for the diode with inhomogeneous Schottky barriers, (d) three-dimensional illustration of the energy-band diagram for the ZnO/SLG Schottky PD with homogeneous Schottky barriers (e.g., for the case of the ZnO/TC-SLG Schottky PD), (e) carrier transport across the homogeneous Schottky barriers at T = T 1 , and (f) carrier transport with TACC at T = T 2 >> T 1 for the case of the diode with homogeneous Schottky barriers.
For the ZnO/TC-SLG Schottky PD, φ B and η were determined to be 0.61 eV and 1.16, respectively, at 300 K. In Schottky diodes, the low η manifested a clean interface between metal and semiconductor for the device, resulting in a dominance of thermionic emission rather than recombination [42].
One possible reason could be an effective removal of chemical residues from the SLG surface through the thermal cleaning process. Furthermore, the magnitudes of both φ B and η were almost independent of temperature (Figure 5b), which is totally different from the thermodynamic behavior of the ZnO/AT-SLG Schottky PD. In a homogeneous Schottky barrier system, due to the weak fluctuation of φ B along the surface direction, thermionic emission dominates its carrier transport whereas TACC becomes insignificant at elevated temperatures (see also Figure 6d,f). As a consequence, an enhanced Schottky barrier homogeneity will result in the almost invariance of both φ B and η upon varying the environmental temperature. Therefore, we expected that the Schottky barrier homogeneity was enhanced in our ZnO/TC-SLG Schottky PD via thermal cleaning (i.e., formation of uniform φ B along the surface direction by the effective removal of the chemical residues from the SLG surface).
In Schottky PDs, the Schottky barrier homogeneity is one of the most crucial factors that strongly affects the photoresponse characteristics. To confirm the effect of the lateral homogeneity of the Schottky barrier, we assessed and compared the photoresponse properties of the ZnO/AT-SLG and ZnO/TC-SLG Schottky PDs under UV light illumination with a P UV of 0-0.77 mW/cm 2 . For the ZnO/AT-SLG Schottky PD, as shown in Figure 7a, the current exponentially increased with increasing P UV even in both the positive and the negative voltage regions. Figure 7b,c shows the P UV dependence of the steady state photocurrent (i.e., I Ph = I Light − I Dark ) at various bias voltages and its corresponding I Ph /I Dark ratio, respectively. The device exhibited a large difference between I Light and I Dark , resulting in a high I Ph /I Dark ratio. Since the current level of I Dark was sufficiently low at V = 0.1 V, the high I Ph /I Dark ratio of ≈1100 was achievable at 0.1 V. SLG Schottky PD. In a homogeneous Schottky barrier system, due to the weak fluctuation of ϕB along the surface direction, thermionic emission dominates its carrier transport whereas TACC becomes insignificant at elevated temperatures (see also Figure 6d,f). As a consequence, an enhanced Schottky barrier homogeneity will result in the almost invariance of both ϕB and η upon varying the environmental temperature. Therefore, we expected that the Schottky barrier homogeneity was enhanced in our ZnO/TC-SLG Schottky PD via thermal cleaning (i.e., formation of uniform ϕB along the surface direction by the effective removal of the chemical residues from the SLG surface).
In Schottky PDs, the Schottky barrier homogeneity is one of the most crucial factors that strongly affects the photoresponse characteristics. To confirm the effect of the lateral homogeneity of the Schottky barrier, we assessed and compared the photoresponse properties of the ZnO/AT-SLG and ZnO/TC-SLG Schottky PDs under UV light illumination with a PUV of 0-0.77 mW/cm 2 . For the ZnO/AT-SLG Schottky PD, as shown in Figure 7a, the current exponentially increased with increasing PUV even in both the positive and the negative voltage regions. Figure 7b,c shows the PUV dependence of the steady state photocurrent (i.e., IPh = ILight -IDark) at various bias voltages and its corresponding IPh/IDark ratio, respectively. The device exhibited a large difference between ILight and IDark, resulting in a high IPh/IDark ratio. Since the current level of IDark was sufficiently low at V = 0.1 V, the high IPh/IDark ratio of ≈1100 was achievable at 0.1 V.
When illuminating the UV light onto the ZnO/TC-SLG Schottky PD, a similar behavior to the above was observed (Figure 7d). As can be seen from Figure 7e, however, the device exhibited a slightly lower current level of IDark (= 0.12 nA) at 0.1 V, compared to the ZnO/AT-SLG Schottky PD (IDark = 0.35 nA). We ascribe this feature to the lower contribution of carrier recombination at the small bias voltage in our ZnO/TC-SLG Schottky PD because the device revealed a quite low magnitude of η (= 1.16). Accordingly, the IPh/IDark ratio was increased by a factor of ≈4 (i.e., IPh/IDark ≈ 4200) at 0.1 V (Figure 7f), compared to the ZnO/AT-SLG Schottky PD. The low operating voltage was advantageous in both demonstrating a high on/off ratio and reducing the power consumption [43]; hence, we believe the ZnO/SLG Schottky structure is preferable for the application of low power UV photodetectors.  When illuminating the UV light onto the ZnO/TC-SLG Schottky PD, a similar behavior to the above was observed (Figure 7d). As can be seen from Figure 7e, however, the device exhibited a slightly lower current level of I Dark (= 0.12 nA) at 0.1 V, compared to the ZnO/AT-SLG Schottky PD (I Dark = 0.35 nA). We ascribe this feature to the lower contribution of carrier recombination at the small bias voltage in our ZnO/TC-SLG Schottky PD because the device revealed a quite low magnitude of η (= 1.16). Accordingly, the I Ph /I Dark ratio was increased by a factor of ≈4 (i.e., I Ph /I Dark ≈ 4200) at 0.1 V (Figure 7f), compared to the ZnO/AT-SLG Schottky PD. The low operating voltage was advantageous Nanomaterials 2019, 9, 799 9 of 13 in both demonstrating a high on/off ratio and reducing the power consumption [43]; hence, we believe the ZnO/SLG Schottky structure is preferable for the application of low power UV photodetectors. Figure 8 shows the transient waveforms of I Ph for the ZnO/AT-SLG and ZnO/TC-SLG Schottky PDs under UV light illumination (P UV = 0.77 mW/cm 2 ) at an optical switching frequency of 0.5 Hz. Both the ZnO/AT-SLG and the ZnO/TC-SLG Schottky PDs exhibited a distinct, stable, and repeatable switching characteristic of I Ph upon turning on and off the UV signal. However, the on-state photocurrent was unstable in the ZnO/AT-SLG Schottky PD (Figure 8a), whereas the ZnO/TC-SLG Schottky PD revealed a stable photoresponse of I Ph at the UV on state (Figure 8b). The rising time (τ r ) and the decay time (τ d ) of the ZnO/TC-SLG Schottky PD were τ r = 0.16 s and τ d = 0.19 s, respectively; and those of the ZnO/AT-SLG Schottky PD were τ r = 0.41 s and τ d = 0.31 s, respectively. When considering the on-and off-state resistance (R PD = 0.2-500 MΩ) and the electrostatic capacitance (C PD ≈ 320 pF) of our devices, the time constant (i.e., τ RC = R PD C PD ) was determined to be 64 µs-160 ms. Therefore, the observed photoresponse time was close to the maximum τ RC and is comparable to other graphene/ZnO Schottky PDs [11][12][13][14][15][16]44] (see also Table 1). For further improvement of the photoresponse time, a study on defect natures in ZnO can be the next step, for example, engineering of point defects and their lifetimes. Both the ZnO/AT-SLG and the ZnO/TC-SLG Schottky PDs exhibited a distinct, stable, and repeatable switching characteristic of IPh upon turning on and off the UV signal. However, the on-state photocurrent was unstable in the ZnO/AT-SLG Schottky PD (Figure 8a), whereas the ZnO/TC-SLG Schottky PD revealed a stable photoresponse of IPh at the UV on state (Figure 8b). The rising time (τr) and the decay time (τd) of the ZnO/TC-SLG Schottky PD were τr = 0.16 s and τd = 0.19 s, respectively; and those of the ZnO/AT-SLG Schottky PD were τr = 0.41 s and τd = 0.31 s, respectively. When considering the on-and off-state resistance (RPD = 0.2-500 MΩ) and the electrostatic capacitance (CPD ≈ 320 pF) of our devices, the time constant (i.e., τRC = RPDCPD) was determined to be 64 μs-160 ms. Therefore, the observed photoresponse time was close to the maximum τRC and is comparable to other graphene/ZnO Schottky PDs [11][12][13][14][15][16]44] (see also Table 1). For further improvement of the photoresponse time, a study on defect natures in ZnO can be the next step, for example, engineering of point defects and their lifetimes.    Here, one should focus on the fact that the photoresponse time of the ZnO/TC-SLG Schottky PD was outstandingly shorter than those of the ZnO/AT-SLG Schottky PD. We attribute such a discrepancy to the difference in Schottky barrier homogeneity between two devices. In the case of the inhomogeneous Schottky-barrier system (e.g., ZnO/AT-SLG Schottky PD), as illustrated in the right-hand-side inset of Figure 8a, the photocarriers would irregularly jump over the Schottky barrier due to the φ B fluctuation along the surface direction. In addition, some of photocarriers might suffer from charge-trapping at the ZnO/SLG interface due to the presence of residual chemical constituents and/or oxygen molecules. These cause the unstable on-state photocurrent and retard the photoresponse time. Contrariwise, such behaviors could be effectively diminished in the homogeneous Schottky barrier system (e.g., ZnO/TC-SLG Schottky PD) because the Schottky barrier undulation was suppressed via elimination of trap sites through thermal cleaning of the SLG surface (see the right-hand-side inset of Figure 8b). Owing to the enhanced Schottky barrier homogeneity, more stable and prompter photoresponse characteristics could be achievable from the ZnO/TC-SLG Schottky PD.
Finally, we discuss the responsivity (R) and the gain (G) of the prepared ZnO/SLG Schottky PDs. In PDs, R is defined by the ratio of the photocurrent to the optical power of incident light (P opt ), and G is given by the ratio of the photogenerated carriers (N e ) to the incident photons (N Ph ). These can be described using following equations [45]: where hν is the photon energy of the incident light. Using Equations (3) and (4), we calculated R and G for both the ZnO/AT-SLG and the ZnO/TC-SLG Schottky PDs. We here note that the average I Ph value during a single pulse duration was used for calculations of R and G. In the case of the ZnO/AT-SLG Schottky PD, R and G were calculated to be ≈101 A/W and ≈347, respectively, and these are comparable to those of the state-of-the-art graphene/ZnO nanorod Schottky PDs [12,15] (see also Table 1). Compared to the ZnO/AT-SLG Schottky PD, both R and G were increased by ≈10% for the ZnO/TC-SLG Schottky PD (i.e., R ≈ 111 A/W and G ≈ 381). We attribute the increased R and G values to the enhanced Schottky barrier homogeneity in the ZnO/TC-SLG Schottky PD. In photocarrier conduction, G can also be defined as the ratio of the photocarrier lifetime (τ PC ) in the photocarrier generation region to the carrier transit time (τ TR ) in the photocarrier collection region [3]: From this relation, we can expect τ PC at the SLG/ZnO interface to be increased in the ZnO/TC-SLG Schottky PD, when assuming that τ TR is identical in SLG. This specifies that the photocarrier diffusion length (L PC ) is increased in the ZnO/TC-SLG Schottky PD because of the following relationship [32]: where D PC is the diffusion coefficient of the photocarrier. In other words, L PC is increased due to the decreased interface trap density resulting from the effective removal of the chemical residues and/or oxygen molecules at the SLG/ZnO interface via thermal cleaning of the SLG surface prior to the deposition of ZnO onto SLG.

Summary and Conclusions
We fabricated high-performance, low-power ZnO/SLG Schottky UV PDs by sputtering ZnO onto thermally-cleaned SLG sheets. The device clearly showed a rectifying behavior and exhibited an excellent photoresponse under UV illumination. When the UV light (P UV = 0.62 mW/cm 2 ) was irradiated onto the device, the I Ph /I Dark ratio was recorded to be more than 4 × 10 3 . For the transient characteristics, the device represented a fast and stable photoresponse (i.e., τ r = 0.16 s and τ d = 0.19 s) upon switching of the UV light pulse. From analyses of the temperature-dependent I-V characteristics, we found such an outstanding photoresponse performance to arise from the enhanced Schottky barrier homogeneity due to the reduced interfacial trap density (i.e., effective removal of chemical residues and/or oxygen molecules at the ZnO/SLG interface via thermal cleaning).

Supplementary Materials:
The following are available online at http://www.mdpi.com/2079-4991/9/5/799/s1, Figure S1: I-V characteristic curves of various ZnO/AT-SLG Schottky PDs fabricated through an identical device fabrication process using as-transferred SLG, Figure S2: I-V characteristic curves of various ZnO/TC-SLG Schottky PDs fabricated through an identical device fabrication process using thermally-cleaned SLG.

Conflicts of Interest:
The authors declare no conflict of interest.