Turgut Tut

Solar-blind AlGaN-based p-i-n photodetectors with high breakdown voltage and detectivity

We report on the high performance solar-blind AlGaN-based p-i-n photodetectors that are grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The dark current of the 200μm diameter devices was measured to be on the order of 5fA for bias voltages up to 10V. The breakdown voltages were higher than 200V. The responsivities of the photodetectors were 0.052 and 0.093A∕W at 280nm under 0 and 40V reverse biases, respectively. We achieved a detectivity of 7.5×1014cmHz1∕2∕W for 200μm diameter AlGaN p-i-n detectors.

High-performance solar-blind photodetectors based on Al/sub x/Ga/sub 1-x/N heterostructures

Design, fabrication, and characterization of high-performance Al/sub x/Ga/sub 1-x/N-based photodetectors for solar-blind applications are reported. Al/sub x/Ga/sub 1-x/N heterostructures were designed for Schottky, p-i-n, and metal-semiconductor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3fA, leakage currents at 6-V reverse bias were measured on p-i-n samples. The excellent current-voltage (I--V) characteristics led to a detectivity performance of 4.9/spl times/10/sup 14/ cmHz/sup 1/2/W/sup -1/. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.09 and 0.11 A/W peak responsivity values at 267 and 261 nm, respectively. A visible rejection of 2/spl times/10/sup 4/ was achieved with Schottky samples. High-speed measurements at 267 nm resulted in fast pulse responses with greater than gigahertz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.

Solar-blind AlGaN-based Schottky photodiodes with low noise and high detectivity

We report on the design, fabrication, and characterization of solar-blind Schottky photodiodes with low noise and high detectivity. The devices were fabricated on n−/n+ AlGaN/GaN heterostructures using a microwave compatible fabrication process. True solar-blind operation with a cutoff wavelength of ∼274 nm was achieved with AlxGa1−xN (x=0.38) absorption layer. The solar-blind detectors exhibited <1.8 nA/cm2 dark current density in the 0–25 V reverse bias regime, and a maximum quantum efficiency of 42% around 267 nm. The photovoltaic detectivity of the devices were in excess of 2.6×1012 cm Hz1/2/W, and the detector noise was 1/f limited with a noise power density less than 3×10−29 A2/Hz at 10 kHz.

AlxGa1-xN-based avalanche photodiodes with high reproducible avalanche gain

The authors report high performance solar-blind photodetectors with reproducible avalanche gain as high as 1560 under ultraviolet illumination. The solar-blind photodetectors have a sharp cutoff around 276nm. The dark currents of the 40μm diameter devices are measured to be lower than 8fA for bias voltages up to 20V. The responsivity of the photodetectors is 0.13A∕W at 272nm under 20V reverse bias. The thermally limited detectivity is calculated as D*=1.4×1014cmHz1∕2W−1 for a 40μm diameter device.

High-performance visible-blind GaN-based p-i-n photodetectors

We report high performance visible-blind GaN-based p-i-n photodetectors grown by metal-organic chemical vapor deposition on c-plane sapphire substrates. The dark current of the 200μm diameter devices was measured to be lower than 20pA for bias voltages up to 5V. The breakdown voltages were higher than 120V. The responsivity of the photodetectors was ∼0.23A∕W at 356nm under 5V bias. The ultraviolet-visible rejection ratio was 6.7×103 for wavelengths longer than 400nm.

Experimental evaluation of impact ionization coefficients in AlxGa1−xN based avalanche photodiodes

The authors report on the metal-organic chemical vapor deposition growth, fabrication, and characterization of high performance solar-blind avalanche photodetectors and the experimental evaluation of the impact ionization coefficients that are obtained from the photomultiplication data. A Schottky barrier, suitable for back and front illuminations, is used to determine the impact ionization coefficients of electrons and holes in an AlGaN based avalanche photodiode.

Solar-blind AlxGa1−xN-based avalanche photodiodes

We report the Metalorganic Chemical Vapor Deposition (MOCVD) growth, fabrication, and characterization of solar blind AlxGa1−xN∕GaN-based avalanche photodiodes. The photocurrent voltage characteristics indicate a reproducible avalanche gain higher than 25 at a 72 V applied reverse bias. Under a 25 V reverse bias voltage, the 100 μm diameter devices had a maximum quantum efficiency of 55% and a peak responsivity of 0.11A∕W at 254 nm, and a NEP of 1.89x10−16 W∕Hz1∕2.

High-speed characterization of solar-blind AlxGa1−xN p–i–n photodiodes

We report on the temporal pulse response measurements of solar-blind AlxGa1−xN-based heterojunction p–i–n photodiodes. High-speed characterization of the fabricated photodiodes was carried out at 267 nm. The bandwidth performance was enhanced by an order of magnitude with the removal of the absorbing p+ GaN cap layer. 30 µm diameter devices exhibited pulse responses with ~70 ps pulse width and a corresponding 3 dB bandwidth of 1.65 GHz.

Deep-ultraviolet Al0.75Ga0.25N photodiodes with low cutoff wavelength

Deep ultraviolet Al0.75Ga0.25N metal-semiconductor-metal photodetectors with high Al concentration have been demonstrated. A metal-organic chemical vapor deposition grown high quality Al0.75Ga0.25N layer was used as a template. Spectral responsivity, current-voltage, optical transmission, and noise measurements were carried out. The photodetectors exhibited a 229nm cutoff wavelength and a peak responsivity of 0.53A∕W at 222nm. Some 100×100μm2 devices have shown a dark current density of 5.79×10−10A∕cm2 under 50V bias. An ultraviolet-visible rejection ratio of seven orders of magnitude was obtained from the fabricated devices.

Dual-color ultraviolet metal-semiconductor-metal AlGaN photodetectors

Backilluminated ultraviolet metal-semiconductor-metal photodetectors with different spectral responsivity bands were demonstrated on a single AlxGa1−xN heterostructure. This was accomplished by the incorporation of an epitaxial filter layer and the recess etching of the surface. The 11nm full width at half maximum (FWHM) responsivity peak of the detector that was fabricated on the as-grown surface was 0.12A∕W at 310nm with 10V bias, whereas the 22nm FWHM responsivity peak of the detector fabricated on the recess-etched surface was 0.1A∕W at 254nm with 25V bias. Both detectors exhibited excellent dark current characteristics with less than 10fA leakage current.