High-Performance Solar-Blind AlGaN Schottky Photodiodes
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Internet Journal Nitride Semiconductor Research
High-Performance Solar-Blind AlGaN Schottky Photodiodes Necmi Biyikli1, Tolga Kartaloglu1, Orhan Aytur1, Ibrahim Kimukin2 and Ekmel Ozbay2 1Bilkent 2Bilkent
University Dept. of Electrical and Electronics Engineering, University Dept. of Physics,
(Received Friday, January 10, 2003; accepted Tuesday, February 25, 2003)
High-performance solar-blind AlGaN-based Schottky photodiodes have been demonstrated. The detectors were fabricated on MOCVD-grown AlGaN/GaN hetero-structures using a microwavecompatible fabrication process. Current-voltage, spectral responsivity, noise, and high-speed characteristics of the detectors were measured and analyzed. Dark currents lower than 1 pA at bias voltages as high as 30 V were obtained. True solar-blind detection was achieved with a cut-off wavelength lower than 266 nm. A peak device responsivity of 78 mA/W at 250 nm was measured under 15 V reverse bias. A visible rejection of more than 4 orders of magnitude was observed. The solar-blind photodiodes exhibited noise densities below the measurement setup noise floor of 3x1029 A2/Hz around 10 KHz. High-speed measurements at the solar-blind wavelength of 267 nm resulted in 3-dB bandwidths as high as 870 MHz.
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Introduction
With its wide bandgap and intrinsic visible-blind absorption spectrum, AlxGa1-xN ternary alloy is the most promising material system for high-performance photodetectors operating in the ultraviolet (UV) spectrum [1]. Their cut-off wavelength (λc) can be tuned from 365 nm to 200 nm, which covers the solar-blind spectrum (λ50 V breakdown voltage. Excellent low-leakage performance was achieved with a dark current below 400 fA under reverse bias voltages as high as 25 V. The dark current fluctuated between 150-400 fA in the 0-25 V reverse bias range (see inset figure). These leakage values correspond to dark current density values of 0.7-1.8 nA/cm2. The differential resistance (R = dV/dI) of these devices was calculated and their dark impedance was in excess of 1013 Ω in the 0-25 V range. These measurement results correspond to the best I-V performance reported for solar-blind AlGaN Schottky photodiodes. To indicate the performance variation, Figure 4 shows the I-V characteristics of a 200 µm diameter photodiode
MRS Internet J. Nitride Semicond. Res. 8, 2 (2003). © 2003 The Materials Research Society
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fabricated on a different wafer piece. Few pA dark current at 5 V reverse bias and a breakdown around 20 V was observed. I-V measurement under UV illumination at 256 nm showed a clear UV photoresponse when compared with the detector dark current (see inset figure). Spectral photoresponse measurements of the AlGaN Schottky photodiodes were carried out in the 250-350 nm spectral range. A bias-dependent spectral responsivity with true sol
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