Results and Discussions The dark current and photocurrent of the SB-PIN device are both smaller than that of the PIN device under a UV light (275 nm) with a power density of 100.9 μW/cm². Under a bias voltage of -10 V, the maximum responsivity is 0.12 A/W, and the external quantum efficiency is more than 50%. Different from the PIN device, in a positive bias voltage (+2.5 V-+10 V), the photocurrent of the SB-PIN device is larger than the dark current, and as the bias voltage increases, the change is more and more obvious. Under a bias voltage of +10 V, the photocurrent to dark current ratio is up to 15 times, and the maximum responsivity is 10 A/W.The external quantum efficiency is over 4500%, and the detectivity reaches up to 5×10¹⁰ cm·Hz^1/2·W^-1. Due to the existence of the Schottky barrier on the surface, the SB-PIN device also responds to a UV light of 365 nm.Under a 365 nm LED with a power density of 100.9 μW/cm² and a bias voltage of +10 V, the maximum responsivity is 14.4 A/W, and the external quantum efficiency is more than 4800%. The detectivity reaches 8×10¹⁰ cm·Hz^1/2·W^-1 (Fig. 2). By exploring the relationship between the responsivity and bias voltage and the incident optical power (Fig. 3), it is explained that the operating mechanism of the SB-PIN device is photoconductive under a positive bias voltage (≥5 V) and a UV light of 275 nm and 365 nm, respectively. The response speed τ_rise equals 2.0 ms (275 nm) and 2.3 ms (365 nm), respectively (Fig. 4). Under a UV light of 275 nm and a negative bias voltage, the operating mechanism is photovoltaic, and the response speed τ_rise equals 190 μs (Fig. 4).

Solar blind ultraviolet (UV) detectors based on AlGaN ternary compound semiconductors have attracted much attention due to their great application potential in fields such as precision guidance, missile warning, spacecraft tracking, open flame monitoring, bioimaging, and UV secure communication. In increasingly complex target environments and short-range non-line-of-sight optical communication systems, UV detectors with high sensitivity and wide working bandwidth are required. At the same time, new material structure designs and device structure research make the UV detectors have higher performance and wider application. In this work, metal Pt with a work function of 5.36 eV is deposited on the surface of a p-GaN layer with a work function of 7.5 eV on the upper surface of GaN/AlGaN material without annealing. The Schottky contact is formed to replace the Ohmic contact formed by the traditional deposition of Ni/Au, Ti/Pt/Au, and other multilayer metals in an AlGaN-based PIN device and annealed at a high temperature. The p-GaN material forms a Schottky barrier with an energy band bending downward on the side contacting with Pt and combined with the PIN structure of the AlGaN material itself. An SB-PIN heterojunction structure is formed in the device, which changes the energy band, the built-in electric field, and the carrier transport mechanism of the device compared with PIN and SBD devices and results in a new operating mechanism and photoelectric characteristics of the device. The device has a high responsivity under a positive bias voltage and realizes dual-band detection (275 nm and 365 nm).

After the wafer cleaning, a device mesa with a diameter of 700 μm is defined by reactive ion etching (RIE). Ti/Al/Ni/Au metal layers are deposited on the n+-AlGaN layer by an e-beam evaporator, and the sample is then annealed at 550 ℃ to form an ohmic contact. Then, SiO2/SiNx composite dielectric film is grown to passivate the side wall of the device and the n+-AlGaN surface of the lower mesa surface to reduce surface leakage. After the window in the upper mesa surface is opened by lithography and etching process, Pt is deposited on the surface of the p-GaN layer to form a device with an SB-PIN structure. In order to compare the differences between the device prepared in this work and the traditional PIN device, a traditional PIN device is simultaneously fabricated with the same AlGaN material. The PIN device is prepared by depositing multiple layers of Ni/Au/Ni/Au metal on the p-GaN surface of the upper mesa surface after a lower electrode is prepared, and then an Ohmic contact upper electrode is developed after rapid annealing at 850 ℃ in O2 atmosphere. Finally, the device is prepared after passivation film growth and electrode opening.

The UV photodetector based on Pt/p-GAN/AlGaN heterojunction proposed in this paper can realize dual-band (solar blind UV and visible blind UV) detection, and the device can be switched between photovoltaic and photoconductive modes by adjusting the bias voltage. In negative bias voltage, the PIN barrier becomes stronger, and the external voltage drop mainly acts on the PIN depletion region. The surface Schottky junction is smaller. As the direction of the external electric field and the Schottky junction electric field is opposite, the Schottky junction which reduces the resistance of photon-generated carriers under a light of 275 nm is weakened. The device has a responsivity and detectivity that are slightly smaller than those of the PIN structure detector, which can be used as a high-speed solar blind UV photovoltaic detector. Under a high positive bias voltage, the direction of the Schottky junction built-in electric field and the external electric field is the same, and the band bending of p-GaN contacting with Pt is stronger.At the same time, the PIN depletion region is narrowed, which makes the overall built-in electric field of the device smaller andlets transmission and collection of photon-generated carriers controlled by the external electric field. As a result, the device operating mechanism is changed to the photoconductive mode, and the detector operates as a high-sensitivity, high-gain, solar-blind, and vision-blind UV photoconductive detector, which makes the proposed UV photodetector more promising for dual-band, high-speed, and high-gain applications.