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  1. (School of Electronic and Electrical Engineering, Hongik University, Seoul, Korea)
  2. (Department of Electronic and Electrical Engineering, Hongik University, Sejong, Korea)

AlGaN, filter-free, photodiode, ultraviolet detection


Solid-state ultraviolet (UV) detectors have received much attention recently due to their miniature size, light weight, and low production cost. They have been developed to replace bulky, fragile photomultiplier tubes. AlxGa1-xN semiconductor alloys would be suitable materials for fabricating solid-state UV detectors for two reasons: (1) the energy bandgap can be tuned between 3.3 and 6.2 eV by adjusting the Al mole fraction and (2) this large energy bandgap allows low dark current. The energy bandgap and its cut-off wavelength as a function of the Al mole fraction are shown in Fig. 1, whose relationship is given in ref. (1). Additional advantages include direct bandgap, high quantum efficiency, physical and chemical stability, a high breakdown field, and the ability to operate at high temperature (2). AlGaN photodiodes are used in solar-blind UV detection, including flame detection, chemical and biological agent detection, partial discharge detection, ozone-hole detection, secure short-range communication, and missile plume detection (3-6). Several research groups have demonstrated solar-blind photodetectors based on AlGaN material systems (7-12).

Fig. 1. Energy bandgap and cut-off wavelength of AlxGa1-xN alloys as functions of Al mole fraction.


In this study, we designed and fabricated AlGaN PIN photodiodes that can detect UV emissions ranging from 230 nm to 270 nm. The photodiode was designed to eliminate the need for an external optical filter.


The epitaxial structure of the AlGaN photodiode consisted of an 8 nm highly-doped p-GaN layer (NA = 1 × 1020 cm-3), a 20 nm p-AlGaN grading layer (NA = 1 × 1018 cm-3), a 200 nm p-Al0.6Ga0.4N layer (NA = 8 × 1016 cm-3), a 100 nm i-Al0.46Ga0.54N layer (ND = 1 × 1016 cm-3), a 1000 nm n-Al0.6Ga0.4N layer (ND = 8 × 1018 cm-3), a 1000 nm interlayer, and a 3000 nm AlN layer on top of a 1 mm sapphire substrate. The epitaxial structure was designed for selective detection of the wavelength range of interest. The light absorption layer was i-Al0.46Ga0.54N layer, whereas the upper and lower parts had wider energy bandgaps to be transparent enough for the wavelength of interest.

The photodiode fabrication was carried out as follows. A mesa isolation was defined by reactive ion etching (RIE) with a BCl3Cl2 gas mixture. The n-type AlGaN ohmic contact was formed by Ti/Al/Ni/Au metallization followed by rapid thermal annealing at 820 °C for 30 sec in a nitrogen ambient, whereas the top p-type GaN ohmic contact was formed by Ni/Au metallization that was annealed at 500 °C for 1 min in a nitrogen ambient. A 270 nm plasma-enhanced chemical vapor deposition SiO2 film was deposited at 350 °C for surface passivation. Finally, a Ti/Au metal stack was evaporated for pad electrodes. The cross-sectional schematic of the AlGaN PIN photodiode is shown in Fig. 2(a), and the corresponding energy band diagram is shown in Fig. 2(b).

Fig. 2. (a) Cross-sectional schematic, (b) the corresponding energy band diagram.


The spectral photoresponsivity characteristics of a fabricated AlGaN photodiode with different bias conditions are shown in Fig. 3. The incident light was illuminated from the top front side. A peak photoresponsivity of 125 mA/W and external quantum efficiency of 60% were achieved at 245 nm under zero bias condition, which is the highest quantum efficiency achieved from AlGaN photodiodes to the best of our knowledge (13-16). The spectral characteristics exhibited a selective photoresponsivity band between 230 nm and 270 nm without an external optical filter, which is suitable for partial discharge detection in power plants or power transmission lines (17). When a reverse-bias voltage was applied to the photodiode, the photoresponsivity was enhanced noticeably by the drift component of photogenerated carriers. The maximum photoresponsivity was 170 mA/mm with an external quantum efficiency of 80%.

Fig. 3. (a) Photoresponsivity, (b) external quantum efficiency characteristics of a fabricated AlGaN photodiode as a function of reverse bias voltage. The inset is the microscopic image of a fabricated photodiode with a diameter of 250 μm.


The dark current and photocurrent characteristics of a fabricated photodiode as a function of the reverse bias voltage, where the photocurrent was measured at a wavelength of 245 nm, are shown in Fig. 4. Under zero bias condition, the dark current was 1.3 × 10-8 A/cm2, allowing very weak UV emission detection with zero standby power. We suggest that the gradual increase in the dark current as increasing the reverse bias voltage were attributed to the dislocation-induced leakage component. Further optimization of the epitaxial growth technique is required to suppress the leakage current.

Fig. 4. Dark current and photocurrent at 245 nm as a function of reverse bias voltage.



We developed a filter-free AlGaN photodiode for partial discharge detection. A wavelength between 230 nm and 270 nm was selectively detected by a proposed AlGaN photodiode. A peak responsivity of approximately 125 mA/W was achieved at 245 nm with a low dark current density of 1.3 × 10-8 A/cm2 at zero bias condition; these are state-of-the-art characteristics. The proposed photodiode does not need an external filter and can offer zero standby power.


This work was supported by Korea Electric Power Corporation (Grant: R18XA02) and Basic Science Research Programs (2015R1A6A1A03031833) through the National Research Foundation of Korea (NRF).


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Thu Thi Thuy Pham

Thu Thi Thuy Pham received a B.S. in Physical Engineering from Hanoi University of Science and Technology in Hanoi, Vietnam, in 2017.

She is currently pursuing an M.S. at Hongik University. Her research interests include wide-bandgap semiconductor devices.

June-Heang Choi

June-Heang Choi received his B.S. in Materials Science & Engineering from Hongik University in Seoul, South Korea.

He received his M.S. in 2018. He is pursuing a Ph.D. in the department of Electronic and Electrical Engineering at Hongik University.

His research interest is wide-bandgap semiconductor devices. Chun-

Chun-Hyung Cho

Chun-Hyung Cho received a B.S. in Electrical Engineering from the Seoul National University in Seoul, South Korea, in 1997, and an M.S. and a Ph.D. in Electrical and Computer Engineering from Auburn University in Auburn, AL, in 2001 and 2007.

In 2009, he joined Hongik University, Sejong where he is currently an assistant professor in the Department of Electronic & Electrical engineering.

His research interests include the application of analytical and experimental methods of piezoresistive sensors to problems in electronic packaging.

Ho-Young Cha

Ho-Young Cha received a B.S. and an M.S. in Electrical Engineering from the Seoul National University in Seoul, Korea, in 1996 and 1999, and a Ph.D. in Electrical and Computer Engineering from Cornell University in Ithaca, NY, in 2004.

He was a Postdoctoral Research Associate with Cornell University until 2005, where he focused on the design and fabrication of wide-bandgap semiconductor devices.

He was with the General Electric Global Research Center in Niskayuna, NY, from 2005 to 2007, developing widebandgap semiconductor sensors and high-power devices.

Since 2007, he has been a professor in the School of Electronic and Electrical Engineering.

His research interests include wide-bandgap semiconductor devices.

He has authored over 110 publications in his research area.