Anomalous Photovoltaic Response of Graphene-on-GaN Schottky

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Anomalous Photovoltaic Response of Graphene-on-GaN Schottky Photodiodes Jae Hyung Lee, Won Woo Lee, Dong Won Yang, Won Jun Chang, Sun Sang Kwon, and Won Il Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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ACS Applied Materials & Interfaces

Anomalous Photovoltaic Response of Graphene-onGaN Schottky Photodiodes Jae Hyung Lee,† Won Woo Lee,† Dong Won Yang,

†

Won Jun Chang,† Sun Sang Kwon†

and

Won Il Park*,†

†Division

of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea.

ABSTRACT

Graphene has attracted great attention as an alternative to conventional metallic or transparent conducting electrodes. Despite its similarities with conventional electrodes, recent studies have shown that a single-atom layer of graphene possesses unique characteristics, such as a tunable work function and transparencies for electric potential, reactivity, and wetting. Nevertheless, a systematic analysis of graphene and semiconductor junction characteristics has not yet been carried out. Here we report the photo-response characteristics of graphene on GaN Schottky junction photodiodes (Gr-GaN SJPDs), showing a typical rectifying behavior and distinct photovoltaic and photoelectric responses. Following the initial abrupt response to UV illumination, the Gr-GaN SJPDs exhibited a distinct difference in photo-carrier dynamics

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depending on applied bias voltage, characterized by either a negative or positive change in photocurrent with time. We propose underlying mechanisms for the anomalous photo-carrier dynamics based on the interplay between electrostatic molecular interactions over the one-atomthick graphene and GaN junction and trapped photocarriers at defect states in the GaN thin film.

Keywords: GaN-graphene Schottky junction, UV photodetector, photovoltaic/photoelectric response, gas desorption, photocurrent, internal photoemission, photo-carrier dynamic, molecular interaction

INTRODUCTION Excessive skin exposure to ultraviolet (UV) radiation is harmful to health, weakening the immune system and causing skin cancer, sunburn, aging, and cataracts, whereas moderate UV exposure can be beneficial, causing disinfection, sterilization, and the production of vitamin D.1-2 Given these positive and negative effects of UV radiation as well as increasing concerns about the destruction of the ozone layer, the demand for personalized UV monitoring has grown rapidly in recent years. UV detection also has additional applications, such as in advanced optical communications, high-density photomemory, ozone sensing and leak detection, and flame detection. Innovations in epitaxial growth and device fabrication have made GaN and its alloys with InN and AlN promising candidates for filter-free, visible-blind UV detection. Typical photodetectors use p-n junctions3 or Schottky junctions4-8; Schottky junction photodetectors (SJPDs) possess the advantages of high responsivity and fast response despite the simple device structure and processing. At present, most conventional SJPDs use vacuum-evaporated metal grids9-10;


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transparent conducting electrodes based on carbon nanotubes, metal nanowires11-12, and graphene1-2, 13-19 have also been tested as Schottky contact electrodes on semiconductors. The use of graphene electrodes has advantages over other Schottky electrodes due to the build-up of optically active built-in potential over regions just beneath the electrode, without substantial loss of light absorption by the metal grid. Specifically, the graphene produces an atomically thin, flat and uniform Schottky contact with the semiconductor, enabling a large, well-defined photoactive area. Taking advantage of these benefits, a variety of different graphene-based Schottky diodes have been explored as solar cells and photodetectors. In particular, Lin et al.2 first described the graphene-on-GaN (Gr-GaN) SJPD and its photoelectric response to UV and visible lights, suggesting variation of the Schottky barrier at the Gr-GaN interface with photo-illumination as the physical mechanism. In contrast, Xu et al.1 observed a photovoltaic response, with considerable open-circuit voltage (Voc) of 0.46 V and short circuit current (Isc) of 4.9ⅹ10-6 A, but in this case the diode exhibited the response only to UV light (wavelength less than ~365 nm). The performance of SJPDs might vary from case to case due to interface defective states, such as interface trapped charges. Nevertheless, the different results from substantially identical structures raise a number of fundamental questions. For instance, what are the similarities and differences between graphene-semiconductor contacts and conventional metal-semiconductor contacts? How do the characteristics of Gr-GaN contacts affect their photo-response properties? To address these questions, we fabricated a Gr-GaN SJPD with particular attention to the interface of the Schottky contact. This allowed us to create a high-performance SJPD showing distinct photovoltaic and photoelectric responses, with a Voc of 0.57 V and a maximum responsivity of ~110 mA/W at Va = −1 V. Time-dependent analysis of the UV response of the photocurrent (Ip) allows a direct observation of the photo-carrier dynamics under various applied



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bias (Va). Importantly, this study reveals distinctly different responses of Ip as a function of Va. The underlying mechanism of the anomalous response is elucidated by considering the interplay between UV irradiation-induced electrostatic molecular interactions over the graphene sheet and trapped charges at the defect state in the GaN thin film.

EXPERIMENTAL SECTION Graphene Synthesis and Device Fabrication. The Gr-GaN Schottky junction heterostructure was made by transferring single-layer graphene onto n-type GaN thin film. We first synthesized the graphene sheet on thin copper foil (25 µm thick, 99.999%, Alfa Aesar) by chemical vapor deposition (CVD).14, 20 The copper foil was annealed under H2 (35 sccm) for 30 min at 1000 °C, and then kept under CH4 and H2 flow (35 sccm and 15 sccm, respectively) for 10 min. After graphene growth, the tube was rapidly cooled down to room temperature. The as-grown graphene was coated with a polymethyl methacrylate (PMMA) protective layer for wet-transfer. On the other hand, a Si-doped, 3 µm-thick GaN epitaxial film grown on a sapphire (0001) substrate by metal organic chemical vapor deposition was used for the n-type GaN (supplied from Korea Advanced Nano Fab Center). The top surface of the GaN thin film was cleaned in an ultrasonic bath using acetone, iso-propyl alcohol and deionized water. Immediately after cleaning, the sample was immersed in diluted HF solution (HF: H2O = 1: 10) for 5 min to remove the native oxide layers, followed by cleaning in deionized water and drying under nitrogen gas flow. After spin-coating a 3 µm-thick SU-8 (MicroChem Corp.) polymer layer on the n-type GaN layer, a 3 mm × 3 mm open window was defined by photolithography and development processes. Then, the PMMA-coated graphene sheet was transferred over the open SU-8 window using methods described elsewhere. The PMMA layer was then removed with 4 ACS Paragon Plus Environment

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acetone solution, followed by vacuum annealing at 300 °C for 30 min. Finally, indium and silver electrodes were made on the graphene and GaN surface, respectively. Device

Characterization.

The

GaN-Gr

PD

was

characterized

electrically

and

photoelectrically using a probe station with semiconductor parameter analyzer (HP4145) and picoammeter (Keithley 6485) with solar simulator (100 mW/cm2, Newport Oriel 92250A1000w), UV Laser (λ : 325 nm, 8W/cm2, KIMMON KR1801C) and tungsten-halogen lamp (λ : 400-1100 nm, 100 W/cm2, lamp osram). Finite Element Analysis. The electrostatic potentials across the Gr-GaN Schottky junction structures were calculated using the COMSOL Multiphysics modeling software equipped with semiconductor modules. In this simulation, the size of Gr-GaN Schottky junction structures was 500 nm x 600 nm. The following parameters were used for n-type GaN and graphene: GaN (band gap energy of 3.39 eV, electron affinity of 4.1 eV, doping concentration Nd of 1x1016/cm3) and graphene (work function of 4.6 eV). The applied bias was set at -1 V for Type I modeling and 0 V for Type II modeling. In addition, it was assumed that the work function of graphene decreases by 0.15 eV upon UV illumination.

RESULTS AND DISCUSSION Figures 1a and b show a schematic diagram and photograph of the Gr-GaN SJPD, in which the active region consisting of the graphene and n-GaN junction was defined by a 3 µm-thick SU-8 polymer with a 3 mm × 3 mm open window. The current versus voltage (I-V) characteristic curve measured in darkness exhibits a typical rectifying behavior with a forward current of 5ⅹ10-6 A at 0.6 V, 103 times higher than that (> 6ⅹ10-9 A) under reverse bias of −0.6 V (Figure 1c and black curve in Figure 1d). The I-V characteristic of the Schottky junction can be



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described by the following function:1 = exp − 1, where Is the reverse bias saturation current, and e, k, and T are the elementary charge, Boltzmann constant, and absolute temperature, respectively. The ideality factor n derived through logarithmic fitting of the I-V curves was ~1.7. In addition, the Schottky barrier height ( ∅ ) can be written as ∅ =

(

∗

), where A and A* are the effective junction area and Richardson’s constant,

respectively, and Is is determined by extrapolating the logarithmic I-V curve to V = 0 V. The Schottky barrier height was extracted to be 0.49 ± 0.1 eV, which is close to the potential difference between the graphene work function (~4.6 eV) and the GaN electron affinity (~4.1 eV). These I-V analysis results indicate that the atomically sharp heterojunction of Gr-GaN exhibited a reasonable diode characteristic behavior, in accordance with classical Schottky diode theory. When exposed to UV illumination, the Gr-GaN SJPD exhibited a photovoltaic response with characteristic values of Isc, fill factor (FF) and Voc. Intriguingly, the SJPD had a Voc of 0.57 V and FF of 0.5 regardless of the UV source, whereas the Isc depended on the power and wavelength (λ) of the UV source; the Isc of 11.46 µA was achieved under AM 1.5G light (irradiance, 100 mW/cm2; λ, 200-2300 nm), a value roughly twice that of a UV laser (irradiance, 8 W/cm2; λ, 325 nm). It should be noted that the Voc of our SJPD was greater than those previously reported for similar structures (no Voc for ref. 2 and Voc of 0.46 V for ref.1). Given that Voc increases for larger built-in potential and smaller carrier recombination across the junction, this result illustrates an improvement in the junction characteristics.21 Meanwhile, when exposed to visible light (tungsten-halogen lamp), our device exhibited a photoconductive response (i.e., a Ip increases under a forward bias) while the photovoltaic response was negligible (Figure S2). Lin et al. reported the similar photoconductive response to visible light from a Gr6 ACS Paragon Plus Environment

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GaN Schottky junction device and attributed it to the hot electron excitation in graphene via internal photoemission process.2 However, different from our device, their device shows only photoconductive responses to both visible and UV lights. Given that photovoltaic characteristics are very sensitive to the junction characteristics, we believe this difference in device behavior may be associated with the interfacial junction characteristics and carrier-dynamic.21 In order to further investigate the device characteristics, we performed a time-dependent analysis of the Ip under various Va in the range of -1 V to 1 V, with repeated 2-s exposures to AM 1.5G and 4-s intervals between exposures (Figure 2). The Va-dependent Ip versus time (Ip–t) curves showed two response components: an initial abrupt and fast response (within 0.02 s), and subsequent slow response. Importantly, the first responses were always towards the negative direction so that the absolute value of Ip, , increased for Va less than Voc, whereas decreased for Va greater than Voc. In contrast, the slow responses of the Ip were sensitive to Va, as classified into the following cases: after the abrupt initial decrease in Ip, it decreased gradually with time for Va < −0.6 V (Type I), whereas it increased with time for Va > −0.2 V (Type II); in the intermittent regime (−0.6 V < Va < −0.2 V), it first decreased and then increased (Transient). Figure 3a shows logarithmic plots of versus time for Type I, Transient, and Type II responses, respectively. In these plots, the slow responses were fit to exponential decay/rise curves, for which the response time constants of the Types I and II cases were estimated to be 0.67 s and 1.23 s, respectively (discussed later). Equivalent behavior and similar response time constants can be found with UV laser illumination (Figure S3), illustrating that this usually occurs at the Gr-GaN junction only when exposed to UV light. The on/off ratio and photo-responsivity are important parameters used to measure the photoelectrical properties of the device. These parameters were calculated and plotted as a



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function of Va in Figure 3b. Despite the slight change with time, the on/off ratio increased with decreasing bias and had values larger than ~5.4ⅹ103 for small bias voltages (-0.2 V < Va < 0.2 V), enabling the device to operate in a self-powered mode. On the other hand, the responsivity generally increased with negative bias voltage, with a maximum value of ~110 mA/W at Va = −1 V. Accordingly, depending on the operation circumstances and/or performance requirements, an appropriate Va can be applied for optimal operation. Our observation of the ambitendent time-dependence of the photovoltaic response of the GrGaN SJPD indicates an interplay between two competing mechanisms. Given that these behaviors depend on the Va, which determines the depletion width of the space charge region at the junction, we conclude that each mechanism dominates at a different position. The timedependence of the Type I response is similar to those observed in conventional diodes, where electron-hole-pair generation gives rise to the behavior (discussed in the following paragraph). In contrast, the Type II response is distinct from those of conventional metal/semiconductor (including metal/GaN) diodes and becomes dominant at smaller depletion widths. On the basis of these considerations, the Type II response can be considered to be related to the Gr-GaN junction. The underlying mechanisms and related processes causing Type I and Type II behaviors are attributed to the defect-mediated electron-hole-pair generation22 in the space charge region and the gas-molecular interaction23 at the graphene surface, as schematically illustrated in Figures 4a and b, respectively. To test our hypothesis, we performed finite element analysis (FEA) modeling for Types I and II, and the resulting potential distributions are included in the schematics. Although both processes occur simultaneously, each process becomes dominant under certain bias conditions. When the junction is under a large reverse bias (Type I), UV-


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induced electron and hole carriers, generated in the region below the depletion width plus diffusion lengths, are separated by a large potential gradient developed at the depletion region. However, during this process, some of those carriers are captured by defects in the GaN, and the following electron-hole-pair generation process causes the gradual increase of with time. In Type II behavior, a small reverse bias or forward bias is applied to the junction and the depletion region becomes narrow, thereby making the process at the Gr-GaN junction or at the graphene surface dominant. In such a situation, the graphene surface tends to adsorb oxygen and water molecules in normal atmospheric conditions. These molecular adsorptions cause an increase of the hole concentration in graphene via O2 (g) + e- → O2¯and lead to p-type doping. However, when the AM 1.5G illumination is turned on, the UV light can liberate the oxygen molecule via reduction process (e.g., O2¯ → O2 (g) + e-), releasing electrons that then contribute to n-doping. On the basis of Lin et al.’s report23 on the work function change of graphene by UV illumination, we can estimate a 0.15 eV decrease of the graphene work function, giving rise to the decrease of the built-in-potential in the junction by the same magnitude, together with the change in the depletion width. Assuming the donor carrier concentration of n-GaN (ND) of 1ⅹ1016 /cm3, the depletion width (w) can be estimated by w = "

#·%& ·% ·'(

()*+ + )- ), where ε0 and

εs are the vacuum permittivity and relative permittivity of the n-GaN junction, respectively and Vbi is the built-in-potential in the Schottky junction. When the Va = 0 V, the depletion width is calculated be 88 nm. Following the gradual gas molecular desorption upon UV exposure, the depletion width was estimated to have decreased by as much as 73 nm, resulting in a gradual decrease in . It is also important to note that although gas desorption occurs in the Type I case, its contribution to the total change in is smaller for the larger reverse bias. At Va = −1 9 ACS Paragon Plus Environment


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V, the depletion width and built-in-potential by gas desorption were estimated to change by 5 % (from 150 nm to 145 nm) and by 11 % (from 1.49 eV to 1.34 eV), respectively; these relative changes are much smaller than those (20 % and 44 %, respectively) at Va = 0 V. This consideration supports the crossover of the two opposite phenomena with the Va. As mentioned earlier, the response time constant for a Type II response is twice that for a Type I response. This result also supports our hypothesis given that the gas molecular interaction process that is associated with Type II behavior is generally slower than the defect-mediated photocarrier dynamics.

Conclusion In conclusion, we developed a Gr-GaN SJPD showing a typical rectifying behavior and distinct photovoltaic and photoelectric responses, with a Voc of 0.57 V and a responsivity of ~110 mA/W at Va = −1 V. Time-dependent analysis of the photoelectric response to UV illumination revealed anomalous photo-carrier dynamics, characterized by negative or positive change of Ip with time depending on the Va. By correlating experimental results and FEA modeling, we have proposed the underlying mechanism, highlighting the interplay between electrostatic molecular interactions over the graphene sheet and trapped charges in the GaN thin film. This study provides insight into the permeation of electrostatic potential involving graphene-gas molecular interactions on the GaN surface through one-atom thick graphene.


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Figures

Figure 1. (a) Schematic illustration and (b) photograph of Gr-GaN SJPD. (c) Logarithmic I-V plot of Gr-GaN SJPD in darkness, showing Schottky junction behavior with an ideality factor of 1.7. (d) I-V characteristic curves of Gr-GaN SJPD in darkness (black), under UV laser (blue) and under AM 1.5G light (red).



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Figure 2. Ip versus time curves of Gr-GaN SJPD with Va in the range of -1 V to 1 V (0.2 V step) during repeated exposure to AM1.5G illumination (2-s exposure and 4-s intervals between exposures).


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Figure 3. (a) Logarithmic plots of the Ip–time characteristic curves in Figure 2 for Va = -1 V and -0.8 V (left), Va = -0.6 V and -0.4 V (mid) and Va = -0.2 V and 0 V (right). (b) On/off ratio (top) and photo-responsivity (bottom) as a function of Va.



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Figure 4. Schematics illustrating the proposed mechanisms for Type I and Type II behaviors, overlapped with the electric potential distributions based on FEA modeling. (a, c) Schematics illustrating the Type I response, involving the defect-mediated electron-hole pair generation in the space charge area region of the GaN thin film (a) and corresponding energy band diagram (c), before and after UV exposure. (b, d) Schematics illustrating the Type II response, involving the gas-molecular interaction at the graphene surface (b) and corresponding energy band diagram (d), before and after UV exposure.


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ASSOCIATED CONTENT

Supporting Information. The supporting information is available free of charge on the ACS Publications website. The spectra of AM 1.5G light, current versus voltage curves of Gr-GaN SJPD in darkness and under tungsten-halogen lamp, photocurrent versus time of Gr-GaN SJPD during cyclic exposure UV laser. (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Won Il Park: 0000-0001-8312-4815

Author Contributions Authors J.H.L. and W.W.L. contributed equally to this work.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) (No. 2015R1A2A2A11001426, No. 15 ACS Paragon Plus Environment


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2016K1A4A3914691, No. 2009-0082580) and the Korea Institute of Energy of Trade, Industry & Energy, Republic of Korea (No. 20174030201750). We also thank professor Hong-Gyu Park at Korea University for the assistance of FEA simulation.


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Anomalous Photovoltaic Response of Graphene-on-GaN Schottky

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