# AlGaN Ultraviolet Metal-Semiconductor-Metal Photodetectors with Reduced Graphene Oxide Contacts
## 1 Introduction  
The III-nitride semiconductors and alloys thereof have been studied extensively because of their wide and direct bandgaps, high breakdown voltages, high mechanical and chemical stabilities, and low reverse leakage currents. These promising properties have inspired the use of III-nitride semiconductors for many applications such as light-emitting diodes, laser diodes, photodetectors, high electron mobility transistors, field-effect transistors, Schottky diodes, and solar cells [1; 2; 3; 4; 5]. Among these applications, ultraviolet (UV) photodetectors that are usable in visible- to solar-blind conditions have drawn significant attention for use in missile plume detection, flame engine sensors, ozone layer monitoring, and secure intersatellite communication systems [6; 7]. With a bandgap between those of GaN (3.4 eV) and AlN (6.2 eV) [8], AlGaN can be used in both visible- and solar-blind UV photodetectors without bulky filter systems or heavy cooling systems such as those required for Si-based photodetectors, thus reducing the footprint and the production cost of the devices [7]. Regarding the photodetector device architecture, the easy and cost-effective fabrication of metal-semiconductor-metal (MSM) photodiodes with two interdigitated Schottky electrode contacts on their planar semiconductor surfaces has attracted significant attention. Such devices present low dark currents, low noise, high response speeds, and no doped layers [9; 10].  
Another important factor of planar UV MSM photodiodes is the transparent conducting electrode [11]. The transparency of existing metals and metal oxides declines sharply with decreasing illumination wavelength [12]. Therefore, high transparency in the UV spectral region, high material conductivity, and linear Ohmic contact properties are essential for electrodes in UV photodetectors [7]. Graphene oxide (GO), a derivative of graphene, possesses significant potential as a conductive electrode material with UV transparency [13]. GO can be fabricated via inexpensive and facileprocesses while retaining graphene's properties. Furthermore, by adding thermal or chemical reduction processing [14,15], so-called reduced GO (rGO) can be obtained, which presents variable optical and electrical properties depending on the degree of the oxygen functional groups contained on the basal plane of the rGO. Recently, [PERSON] et al. and [PERSON] et al. demonstrated the potential of rGO electrodes in the Si photodiodes [16,17]. [PERSON] et al. studied rGO Schottky contacts on a GaN layer, presenting the dependency of the Schottky barrier height on the GO reduction temperature [18]. Hence, the combination of AlGaN and GO-based materials for UV-enhanced optical sensors seems to be an interesting and attractive topic for exploration. An rGO/AlGaN-based photodiode may provide an enhanced photo-to-dark current ratio and tunable cut-off wavelength depending on the Al mole fraction. Here, we investigated rGO/Al\({}_{\chi}\)Ga\({}_{1-\chi}\)N MSM photodiodes with various Al mole fractions, \(x\). Measurements of the dark current, photocurrent, and photoresponsivity of the fabricated devices showed systematic correlations of these properties with the Al mole fraction in the AlGaN layer.