Solar-blind Operation Research Articles

Solar blind Schottky photodiode based on an MOCVD-grown homoepitaxial β-Ga2O3 thin film

We report on a high performance Pt/n−Ga2O3/n+Ga2O3 solar blind Schottky photodiode that has been grown by metalorganic chemical vapor deposition. The active area of the photodiode was fabricated using ∼30 Å thick semi-transparent Pt that has up to 90% transparency to UV radiation with wavelengths < 260 nm. The fabricated photodiode exhibited Schottky characteristics with a turn-on voltage of ∼1 V and a rectification ratio of ∼108 at ±2 V and showed deep UV solar blind detection at 0 V. The Schottky photodiode exhibited good device characteristics such as an ideality factor of 1.23 and a breakdown voltage of ∼110 V. The spectral response showed a maximum absolute responsivity of 0.16 A/W at 222 nm at zero bias corresponding to an external quantum efficiency of ∼87.5%. The cutoff wavelength and the out of band rejection ratio of the devices were ∼260 nm and ∼104, respectively, showing a true solar blind operation with an excellent selectivity. The time response is in the millisecond range and has no long-time decay component which is common in photoconductive wide bandgap devices.

MBE-Grown <inline-formula> <tex-math notation="LaTeX">$\beta$ </tex-math> </inline-formula>-Ga2O3-Based Schottky UV-C Photodetectors With Rectification Ratio ~107

In this letter, we demonstrate high-performance vertical solar-blind Schottky photodetectors on MBE-grown homoepitaxial (010)-oriented β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> films. The structure, consisting of (100 nm) β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /(60 nm) n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">++</sup> β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , was grown on a Fe-doped insulating (010) β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> substrate. Ni/Au and indium were used as the Schottky and Ohmic contacts, respectively. The devices exhibited a rectification ratio of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sup> with turn-on voltage ~1 V and an ideality factor of 1.31. The extracted Schottky barrier height was 1.4 eV. The photodetectors showed low dark current of 0.3 nA at 5 V with a photo-to-dark current ratio of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 0 V. The devices exhibited a zero-bias responsivity of 4 mA/W at 254 nm corresponding to an external quantum efficiency ~3 %, with a UV-to-visible rejection ratio >10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , showing true solar-blind operation. The transient response of the devices indicated rise/fall times of ~100 ms. Temperature-dependent current-voltage characteristics agree with the thermionic emission model.

Experimental Evaluation of Modulation Formats’ Performance in Diffuse UV Channels

The fundamental properties of free space optical channels in the ultraviolet (UV) spectral range between 200 and 280 nm are the enhanced scattering and the solar blind operation. Exploiting these properties, point-to-point links in a non-line-of-sight (NLOS) regime were experimentally tested and the performance of pulse position modulation of fourth order (4-PPM) and flip–orthogonal frequency division multiplexing (Flip-OFDM) modulation formats was evaluated. At the transmitting side, light-emitting diodes (LEDs) emitting at 265 nm were used. At the receiving side, a photo-multiplier tube was combined with an optical filter. Distances up to 20 m were covered and kilobits/s rates were applied. The performance was also evaluated after modifying artificially the atmospheric conditions by the operation of a fog machine. In short, 4-PPM outperformed Flip-OFDM, whereas a medium that offers enriched scattering due to fog may boost the performance of short-range NLOS links in the UV spectral range.

AlxGa1-xN-based back-illuminated solar-blind photodetectors with external quantum efficiency of 89%

We report on high performance AlxGa1−xN-based solar-blind ultraviolet photodetector (PD) array grown on sapphire substrate. First, high quality, crack-free AlN template layer is grown via metalorganic chemical vapor deposition. Then, we systematically optimized the device design and material doping through the growth and processing of multiple devices. After optimization, uniform and solar-blind operation is observed throughout the array; at the peak detection wavelength of 275 nm, 729 μm2 area PD showed unbiased peak external quantum efficiency and responsivity of ∼80% and ∼176 mA/W, respectively, increasing to 89% under 5 V of reverse bias. Taking the reflection loses into consideration, the internal quantum efficiency of these optimized PD can be estimated to be as high as ∼98%. The visible rejection ratio measured to be more than six orders of magnitude. Electrical measurements yielded a low-dark current density: <2 × 10−9 A/cm2, at 10 V of reverse bias.

AlxGa1−xN-based solar-blind ultraviolet photodetector based on lateral epitaxial overgrowth of AlN on Si substrate

We report on AlxGa1−xN-based solar-blind ultraviolet (UV) photodetector (PD) grown on Si(111) substrate. First, Si(111) substrate is patterned, and then metalorganic chemical vapor deposition is implemented for a fully-coalesced ∼8.5 μm AlN template layer via a pulsed atomic layer epitaxial growth technique. A back-illuminated p-i-n PD structure is subsequently grown on the high quality AlN template layer. After processing and implementation of Si(111) substrate removal, the optical and electrical characteristic of PDs are studied. Solar-blind operation is observed throughout the array; at the peak detection wavelength of 290 nm, 625 μm2 area PD showed unbiased peak external quantum efficiency and responsivity of ∼7% and 18.3 mA/W, respectively, with a UV and visible rejection ratio of more than three orders of magnitude. Electrical measurements yielded a low-dark current density below 1.6 × 10−8 A/cm2 at 10 V reverse bias.

Al_xGa_1−xN–based deep-ultraviolet 320×256 focal plane array

We report the synthesis, fabrication, and testing of a 320×256 focal plane array (FPA) of back-illuminated, solar-blind, p-i-n, Al(x)Ga(1-x)N-based detectors, fully realized within our research laboratory. We implemented a pulse atomic layer deposition technique for the metalorganic chemical vapor deposition growth of thick, high-quality, crack-free, high Al composition Al(x)Ga(1-x)N layers. The FPA is hybridized to a matching ISC 9809 readout integrated circuit and operated in a SE-IR camera system. Solar-blind operation is observed throughout the array with peak detection occurring at wavelengths of 256 nm and lower, and falling off three orders of magnitude by ~285 nm. By developing an opaque masking technology, the visible response of the ROIC is significantly reduced; thus the need for external filtering to achieve solar- and visible-blind operation is eliminated. This allows the FPA to achieve high external quantum efficiency (EQE); at 254 nm, average pixels showed unbiased peak responsivity of 75 mA/W, which corresponds to an EQE of ~37%. Finally, the uniformity of the FPA and imaging properties are investigated.

AlGaN-Based Linear Array for UV Solar-Blind Imaging From 240 to 280 nm

The realization of a linear array of 300 pixels, with a 26-mum pitch, based on Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> N metal-semiconductor-metal photodetectors, is described. The composition of the active layer is chosen in order to optimize the solar-blind operation: A sharp cutoff at 280 nm is observed, with discrimination between far and medium ultraviolet (UV) of three orders of magnitude. The detector shows a peak responsivity of 12 mA/W and a dark current smaller than 1 fA at the typical polarization of 4 V. The maximum resolution is analyzed in terms of modulation transfer function (MTF): The best result is obtained for a front-side illumination, i.e., when an MTF of 0.45 is measured at the half Nyquist frequency (19.2 lp/mm). Some UV images, which are obtained in a pushbroom model, are reported. The visible rejection is proven by directly imaging the arc of a Xenon lamp: It is shown that only the relatively weak far-UV component contributes to the signal

High bandwidth-efficiency solar-blind AlGaN Schottky photodiodes with low dark current

Al 0.38Ga 0.62N/GaN heterojunction solar-blind Schottky photodetectors with low dark current, high responsivity, and fast pulse response were demonstrated. A five-step microwave compatible fabrication process was utilized to fabricate the devices. The solar-blind detectors displayed extremely low dark current values: 30 μm diameter devices exhibited leakage current below 3 fA under reverse bias up to 12 V. True solar-blind operation was ensured with a sharp cut-off around 266 nm. Peak responsivity of 147 mA/W was measured at 256 nm under 20 V reverse bias. A visible rejection more than 4 orders of magnitude was achieved. The thermally-limited detectivity of the devices was calculated as 1.8 × 10 13 cm Hz 1/2 W −1. Temporal pulse response measurements of the solar-blind detectors resulted in fast pulses with high 3-dB bandwidths. The best devices had 53 ps pulse-width and 4.1 GHz bandwidth. A bandwidth-efficiency product of 2.9 GHz was achieved with the AlGaN Schottky photodiodes.

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 &amp;lt;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.

Solar-Blind AlGaN-based Schottky Photodiodes With High Detectivity and Low Noise

AbstractWe report on the design, fabrication and characterization of solar-blind Schottky photodiodes with high detectivity and low noise. The devices were fabricated on n-/n+ AlGaN/GaN hetero-structures using a microwave compatible fabrication process. Using Al0.38Ga0.62N absorption layer, true solar-blind operation with a cutoff wavelength of ∼274 nm was achieved. The solar-blind detectors exhibited &lt; 400 fA dark current in the 0–25 V reverse bias regime, and a maximum responsivity of 89 mA/W around 267 nm. The photovoltaic detectivity of the devices were in excess of 2.6×1012 cmHz1/2/W, and the detector noise was 1/f limited with a noise power density less than 3×10−29 A2/Hz at 10 KHz.

Solar-blind AlGaN-based inverted heterostructure photodiodes

True solar-blind operation with a sharp responsivity cutoff at ∼300 nm has been demonstrated in AlGaN-based photodiodes using an “inverted heterostructure photodiode” design. This structure utilizes an AlxGa1−xN(x&amp;gt;0.3) intrinsic or lightly doped active layer surrounded by p- and/or n-type contact layers having a narrower band gap than the active layer. By utilizing narrow band gap (e.g., GaN) contact layers, the difficulties associated with achieving high doping efficiencies in wide band gap contact layers are circumvented. This basic structure is applicable to both front- and back-side illuminated detector geometries. Front-side illuminated solar-blind photodiodes were demonstrated with a peak responsivity of 0.08 A/W at 285 nm, while back-side illuminated detectors yielded a peak responsivity of 0.033 A/W at 275 nm (both are measured without antireflection coating). Both types of detectors offered sharp spectral responsivity cutoff of at least three orders of magnitude by 325 nm.