Absorption Edge Of GaN Research Articles

Improved light emission through an AlGaN coalescence layer of 365-nm ultraviolet lighting-emitting diodes on patterned sapphire substrates

Ultraviolet (UV), 365-nm light-emitting diodes (LEDs) were grown on an AlGaN template on patterned sapphire substrates (PSS) and then compared with those grown on a conventional GaN template. Complete coalescence was achieved by using AlGaN up to 4.9% Al composition, although the AlGaN template displayed more dislocations than the GaN template. The 365-nm UV LEDs on the AlGaN template showed no emission absorption in the template, as seen in the GaN template, because AlGaN is sufficiently far from the absorption edge of GaN. As a result, a total emission enhancement about 4 times stronger than that obtained w hat using the GaN template was obtained at 80 mA.

Fabrication and device characteristics of Schottky-type bulk GaN-based “visible-blind” ultraviolet photodetectors

The authors present the fabrication and characterization of vertical-geometry Schottky-type ultraviolet (UV) photodetectors based on a bulk n-GaN substrate. By using low temperature rapid thermal annealing of the semitransparent Schottky contacts (nickel with 7% vanadium), they obtained an ultralow dark current of 0.56pA at −10V reverse bias. A responsivity of ∼0.09A∕W at zero bias was measured for wavelength shorter than the absorption edge of GaN, and it was found to be independent of the incident power in the range measured (50mW∕m2–2.2kW∕m2). The devices are visible blind, with an UV/visible contrast of over six orders of magnitude. An open-circuit voltage of 0.3V was also obtained under a broadband UV illumination.

The fundamental absorption edge of high crystalline quality GaN and that of amorphous GaN grown at low temperature

An amorphous and two crystalline GaN layers have been grown by metalorganic vapor phase epitaxy on sapphire substrates at the growth temperature of 500, 800, and 1075 °C, respectively. The fundamental absorption edge of amorphous GaN starts at 2.8 eV. The absorption coefficient of that layer increases approximately linearly with energy, which is qualitatively different from the sharp excitonic absorption edge starting at 3.5 eV in crystalline GaN. The fundamental absorption edge of GaN has also been investigated in a 200 nm thick GaN layer grown on a 2 µm thick Al0.08Ga0.92N layer. A broad excitonic line is observed in this GaN layer after the sample has been cooled in darkness. A metastable narrowing occurs and three excitonic absorption lines are observed after the sample has been illuminated with UV light. Measurements of this effect have been made as a function of temperature and photon energy. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Schottky barrier detectors on GaN for visible–blind ultraviolet detection

We report on the fabrication and characterization of vertical geometry transparent Schottky barrier ultraviolet detectors based on n−/n+-GaN structures grown over sapphire substrates. Spectral responsivity measurements were made using illumination through the UV transparent Schottky barrier metal. A responsitivity as high as 0.18 A/W was measured for wavelengths shorter than the absorption edge of GaN. The detector speed was RC limited and the fall time was 118 ns. The 1/f noise is identified to be the main noise contribution. At 300 Hz, we measure the noise equivalent power at less than 4×10−9 W.