Sub-microsecond response time deep-ultraviolet photodetectors using α-Ga2O3 thin films grown via low-temperature atomic layer deposition

A photodetector responding to deep-ultraviolet (DUV) wavelengths (lambda = 200-300 nm) is developed via the growth of crystalline alpha-Ga2O3 thin films with a bandgap of 5.2 eV on sapphire (0006) substrates using atomic layer deposition (ALD), at a low temperature of approximately 250 degrees C. The achievement of crystalline alpha-Ga2O3 films at such a low temperature is viable owing to the lattice match between alpha-Ga2O3 and sapphire crystals, which is confirmed by X-ray diffraction measurements and high-resolution transmission electron microscopy analysis. Metal-semiconductor-metal photodetectors (active area of 30 x 30 mu m(2)) using 10-nm-thick alpha-Ga2O3 films exhibit a rise time (time required for the photocurrent to increase from 10% to 90% of its final value under illumination) of 539 ns at lambda = 266 nm. Such an ultrafast response to DUV with lambda = 266 nm is maintained for 3-nm-thick alpha-Ga2O3 photodetectors, suggesting that our ALD process is adequate for obtaining high-quality ultrathin alpha-Ga2O3 films. Measurements of the wavelength-resolved photocurrents reveal that the alpha-Ga2O3 photodetectors respond selectively to DUV wavelengths (lambda = 200-300 nm), without responding to other longer wavelengths (lambda > 300 nm). The responsivity is maximized to 0.76 A/W at lambda = 253 nm, and drops off at lambda approximate to 300 nm (i.e., a cutoff wavelength). The dark current measured at 10 V is as low as 0.5 pA, and the signal-to-noise ratio reaches 10(4), both of which underpin the pristine material quality of the ALD-grown alpha-Ga2O3 films. We believe that the fabrication of photodetectors using alpha-Ga2O3 thin films at such a low temperature will provide an economically feasible solution for high-performance DUV detection and sensing applications. (C) 2018 Elsevier B.V. All rights reserved.