In this work, we demonstrate a high-performance ultraviolet phototransistor (UVPT) based on the AlGaN/GaN high-electron mobility transistor (HEMT) configuration. When the device is biased at off state, the peak photoresponsivity (R) of 3.6 × 107 A/W under 265 nm illumination and 1.0 × 106 A/W under 365 nm illumination can be obtained. Those two R values are one of the highest among the reported UVPTs at the same detection wavelength under off-state conditions. In addition, we investigate the gate-bias (VGS) dependent photoresponse of the fabricated device with the assistance of band structure analysis. It was found that a more negative VGS can significantly reduce the rise/decay time for 265 nm detection, especially under weak illumination. This can be attributed to a largely enhanced electric field in the absorptive AlGaN barrier that pushes the photo-generated carriers rapidly into the GaN channel. In contrast, the VGS has little impact on the switching time for 365 nm photodetection, since the GaN channel has a larger absorption depth and the entire UVPT simply acts as a photoconductive-type device. In short, the proposed AlGaN/GaN HEMT structure with the superior photodetection performance paves the way for the development of next generation UVPTs.

Demonstration of AlGaN/GaN-based ultraviolet phototransistor with a record high responsivity over 3.6 × 107 A/W

A high responsivity and controllable recovery ultraviolet (UV) photodetector based on a tungsten oxide (WO3) gate AlGaN/GaN heterostructure with an integrated micro-heater is reported for the first time. The WO3 nanolayer was deposited by physical vapor deposition (PVD) for deep UV absorption and the micro-heater was integrated for chip level heating and cooling. Our device when exposed to UV wavelength exhibits a high responsivity of 1.67 × 104 A W−1 at 240 nm and a sharp cut-off wavelength of 275 nm. More importantly, the persistent photoconductivity (PPC) effect can be eliminated by a novel method, mono-pulse heating reset (MHR), which consists in applying an appropriate pulse voltage to the micro-heater right after the removal of the UV illumination. The recovery time was reduced from hours to just seconds without reducing the high responsivity and stability of the photodetector. The UV detection, high responsivity, high stability, controllable recovery process and low production cost of GaN-based photodetectors make these devices extremely attractive for several applications, such as fire detection and missile and rocket warning.

/pdf/a-high-responsivity-and-controllable-recovery-ultraviolet-ktnnyory22.pdf

A high responsivity and controllable recovery ultraviolet detector based on a WO 3 gate AlGaN/GaN heterostructure with an integrated micro-heater

The collective oscillation of electrons located in the conduction band of metal nanostructures being still energized, with the energy up to the bulk plasmon frequency, are called nonequilibrium hot electrons. It can lead to the state-filling effect in the energy band of the neighboring semiconductor. Here, we report on the incandescent-type light source composed of Au nanorods decorated with single Ga-doped ZnO microwire (AuNRs@ZnO:Ga MW). Benefiting from Au nanorods with controlled aspect ratio, wavelength-tunable incandescent-type lighting was achieved, with the dominating emission peaks tuning from visible to near-infrared spectral regions. The intrinsic mechanism was found that tunable nonequilibrium distribution of hot electrons in ZnO:Ga MW, injected from Au nanorods, can be responsible for the tuning emission features. Apart from the modification over the composition, bandgap engineering, doping level, etc., the realization of electrically driving the generation and injection of nonequilibrium hot electrons from single ZnO:Ga MW with Au nanostructure coating may provide a promising platform to construct electronics and optoelectronics devices, such as electric spasers and hot-carrier-induced tunneling diodes.

/pdf/nonequilibrium-hot-electron-induced-wavelength-tunable-1t7fwxk42i.pdf

Nonequilibrium hot-electron-induced wavelength-tunable incandescent-type light sources

In this letter, we report the first demonstration of monolithically integrated ultraviolet (UV) light emitting diodes (LEDs) and visible-blind UV photodetectors (PDs) employing the same p-GaN/AlGaN/GaN epi-structures grown on Si. Due to the radiative recombination of holes from the p-GaN layer with electrons from the 2-D electron gas (2DEG) accumulating at the AlGaN/GaN heterointerface, the forward biased LED with p-GaN/AlGaN/GaN junction exhibits uniform light emission at 360 nm. Facilitated by the high-mobility 2DEG channel governed by a p-GaN optical gate, the visible-blind phototransistor-type PDs show a low dark current of ∼10-7 mA/mm and a high responsivity of 3.5×105 A/W. Consequently, high-sensitivity photo response with a large photo-to-dark current ratio of over 106 and a response time less than 0.5 s is achieved in the PD under the UV illumination from the on-chip adjacent LED. The demonstrated simple integration scheme of high-performance UV PDs and LEDs shows great potential for various applications such as compact opto-isolators.

Monolithic integration of ultraviolet light emitting diodes and photodetectors on a p-GaN/AlGaN/GaN/Si platform.

An AlGaN/GaN metal-heterostructure-metal (MHM) ultraviolet (UV) photodetector employing lateral Schottky contacts was fabricated and characterized at different temperatures. As the temperature increased from 25 to 250 °C, the photoresponsivity of the MHM photodetector increased 3.5 times. This was attributed to the spontaneous-polarization-induced spatial separation of the photogenerated electrons and holes and the increased optical absorption at higher temperatures. Meanwhile, the decay time constant of the photocurrent became approximately three orders of magnitude smaller. With the enhanced photoresponsivity and the decreased response time constant, kilohertz optical switching of the MHM photodetector was recorded at 250 °C. The AlGaN/GaN MHM photodetector, sharing the same GaN-on-Si electronics platform, provides an applicable candidate for an all-GaN integrated UV sensing and amplifying system for high-temperature applications.

Temperature enhanced responsivity and speed in an AlGaN/GaN metal-heterostructure-metal photodetector

In this work, a GaN p-i-n diode based on Mg ion implantation for visible-blind UV detection is demonstrated With an optimized implantation and annealing process, a p-GaN layer and corresponding GaN p-i-n photodiode are achieved via Mg implantation As revealed in the UV detection characterizations, these diodes exhibit a sharp wavelength cutoff at 365 nm, high UV/visible rejection ratio of 12×104, and high photoresponsivity of 035 A/W, and are proved to be comparable with commercially available GaN p-n photodiodes Additionally, a localized states-related gain mechanism is systematically investigated, and a relevant physics model of electric-field-assisted photocarrier hopping is proposed The demonstrated Mg ion-implantation-based approach is believed to be an applicable and CMOS-process-compatible technology for GaN-based p-i-n photodiodes

Magnesium ion-implantation-based gallium nitride p-i-n photodiode for visible-blind ultraviolet detection

In this work, the effect of very high-fluence 100 MeV proton radiation on the performance of 6H-SiC photoconductive proton detectors is studied. The irradiation fluence is up to ${1.6}\times {10}^{{{17}}}{\mathrm {cm}}^{{\text {-2}}}$ and the degradation process of the SiC detector is continuously monitored for the first time. Before proton irradiation, the detector shows a low dark current of ~0.8nA/cm2 at 1000 V bias. As the irradiation fluence increases, the dark current continuously drops, which should be caused by irradiation damage induced carrier compensation defects. Meanwhile, the output current of the SiC detector shows an exponential decay behavior and tends to saturate at irradiation fluence up to ${5}\times {10}^{{{16}}}{\mathrm {cm}}^{{\text {-2}}}$ . At the end of the very high fluence irradiation test, the detector still exhibits ~20% of its original output current value, suggesting the excellent radiation hardness of SiC for proton detection.

Effect of Very High-Fluence Proton Radiation on 6H-SiC Photoconductive Proton Detectors

In this work, a large size x-ray detector with a 25 mm2 active area is demonstrated based on a thick 4H-SiC p-i-n structure. The detector exhibits obvious merits of high photon sensitivity over 4 × 104 μC Gy−1 cm−2, good photon-response linearity, and high-temperature operation compatibility. Meanwhile, due to the ultralow leakage current level achieved, single photon detection performance for x-ray photons is further realized with energy resolutions of 1.1 and 4.9 keV at 5.9 and 59.5 keV, respectively. This work thus suggests the significant potentials of wide-bandgap SiC semiconductor for photon-resolved x-ray detection in a harsh environment.

High sensitivity x-ray detectors based on 4H-SiC p-i-n structure with 80 μm thick intrinsic layer

https://iopscience.iop.org/article/10.1088/0256-307X/37/6/068502/pdf

Effect of a Single Threading Dislocation on Electrical and Single Photon Detection Characteristics of 4H-SiC Ultraviolet Avalanche Photodiodes

To explore the potential of semi-insulating (SI) SiC substrate for radiation detector applications, p-i-n diodes with thick depletion layers are fabricated by using a dual-face ion implantation technique on SI 4H-SiC substrate. The Poole-Frenkel (P-F) emission is found responsible for the reverse leakage current transport of the p-i-n diode at elevated temperatures. The derived emission barrier height is likely induced by the residual boron-related shallow-level states, which are located ∼ 0.3 eV above the top of the valence band. Detection tests of α-particles are performed on each side of the p-i-n detector to determine the location of the depleted region, which reveals weak p-type conduction of the SI 4H-SiC substrate after high temperature post-implantation annealing. The energy resolution of the detector operating in photovoltaic mode is ∼ 4.3%, which would degrade at higher biases. The saturated charge collection efficiency (CCE) of ∼ 99% for detection of 5.48 MeV α-particles is obtained as reverse bias exceeds 80 V.

Qing Liu

Papers

Magnesium ion-implantation-based gallium nitride p-i-n photodiode for visible-blind ultraviolet detection

Effect of Very High-Fluence Proton Radiation on 6H-SiC Photoconductive Proton Detectors

High sensitivity x-ray detectors based on 4H-SiC p-i-n structure with 80 μm thick intrinsic layer

Effect of a Single Threading Dislocation on Electrical and Single Photon Detection Characteristics of 4H-SiC Ultraviolet Avalanche Photodiodes

Development of p-i-n radiation detectors based on semi-insulating 4H-SiC substrate via dual-face ion implantation