The current transport mechanism of graphene formed on AlxGa1−xN/GaN heterostructures with various Al mole fractions (x = 0.15, 0.20, 0.30, and 0.40) is investigated. The current–voltage measurement from graphene to AlGaN/GaN shows an excellent rectifying property. The extracted Schottky barrier height of the graphene/AlGaN/GaN contacts increases with the Al mole fraction in AlGaN. However, the current transport mechanism deviates from the Schottky-Mott theory owing to the deterioration of AlGaN crystal quality at high Al mole fractions confirmed by reverse leakage current measurement.

AlGaN semiconductors are promising materials in the field of ultraviolet (UV) detection. We fabricated AlGaN/GaN UV metal–semiconductor–metal (MSM) photodiodes with two back-to-back interdigitated finger electrodes comprising reduced graphene oxide (rGO). The rGO showed high transparency below the wavelength of 380 nm, which is necessary for a visible-blind photodetector, and showed outstanding Schottky behavior on AlGaN. As the photocurrent, dark current, photoresponsivity, detectivity, and cut-off wavelength were investigated with the rGO/AlGaN MSM photodiodes with various Al mole fractions, systematic variations in the device characteristics with the Al mole fraction were confirmed, proving the potential utility of the device architecture combining two-dimensional materials, rGO, and nitride semiconductors.

A dual-functional ultraviolet (UV) photodetector with a large UV-to-visible rejection ratio is presented, in which interdigitated finger-type two-dimensional graphene electrodes are introduced to an AlGaN/GaN heterostructure. Two photocurrent generation mechanisms of photovoltaic and photoconductive dominances coexist in the device. The dominance of the mechanisms changes with the induced bias voltage. Below a threshold voltage, the device showed fairly low responsivities but fast response times, as well as a constant photocurrent against the induced bias. However, the opposite characteristics appeared with high bias voltage. Specifically, above the threshold voltage, the device showed high responsivities with additional gain, but slow rise and recovery times. For instance, the responsivity of 10.9 A/W was observed with the gain of 760 at the induced bias voltage of 5 V. This unique multifunctionality enabled by the combination of an AlGaN/GaN heterostructure with graphene electrodes facilitates the development of a single device that can achieve multiple purposes of photodetection.

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PbS, HgCdTe, etc. [4] These materials are currently dominating the industry due to their mature technology readiness level. The high stiffness of these materials in bulk crystal or epitaxial layer form has prompted interest on low-dimensional materials for flexible photodetectors, an emerging prospective toward wearable electronics. [5,6] In order to meet with the speediness at which the present electronics industry is evolving, photodetectors featuring diverse functionalities with excellent figures of merits (high photo gain, ultrafast photoresponse, broad spectral selectivity) and facile integration with existing universal platforms like complementary metal-oxide semiconductor (CMOS) and silicon photonics, are highly desired. [7][8][9] Given the atomically thin nature, strong light-matter coupling, ultrafast carrier dynamics, flexibility, dangling bond free surface, and effective property manipulation by artificial stacking of different layered materials one over the other (called vdW stacking) without the restraints of lattice matching, 2D materials stand out as promising candidates for high performance photodetectors. During the past one decade, several 2D materials have witnessed a remarkable journey in this arena and have been the topic of several review articles. [10][11][12][13][14][15][16][17][18][19] The nearly constant light absorption in the entire electromagnetic spectrum witnessed in graphene due to gapless energy-momentum dispersion has permitted realization of first 2D material based photodetectors operating over a broad spectral range from UV to terahertz limit. [20][21][22][23] Furthermore, the high carrier mobility and associated ultrafast carrier dynamics in graphene enabled extremely fast photodetection, [24] which has been found beneficial to process images faster than existing photodetectors. [25,26] Meanwhile, layered transition metal dichalcogenides (TMDs), represented generically as MX 2 , where M is a transition metal element of groups 4-10 and X is a chalcogen atom (S, Se, Te), have become more popular due to their exotic electronic and optical properties. [27][28][29][30] In contrast to graphene, most sulfides-and selenides-based Group VI and Group VII TMDs (MoS 2 , WS 2 , ReS 2 , MoSe 2 , WSe 2 , ReSe 2 ) have a band gap covering the visible-near infrared spectral range. [31,32] For instance, the most extensively studied Group VI TMDs such as MoS 2 and WS 2 at monolayer thickness limit are direct bandgap