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.
We report solar-blind AlxGa1−xN photovoltaic detectors with cutoff wavelengths as short as 290 nm. Mesa geometry devices of different active areas are fabricated and characterized for spectral responsitivity, speed, and noise performance. The responsivity of the devices near the cutoff wavelength is 0.07 A/W. The detector noise is found to be 1/f limited, with a noise equivalent power of 6.6×10−9 W over the total response bandwidth of 100 kHz.
We report novel GaN detectors grown by molecular beam epitaxy on Si(111) substrates. Wurtzite structure epitaxial GaN exhibits room-temperature photoluminescence with a band-edge-related emission width as narrow as 7 nm and intensities comparable to high quality layers grown on sapphire by metalorganic chemical vapor deposition. Spectral response of lateral geometry Schottky detectors shows a sharp cutoff at 365 nm with peak responsivities of ∼0.05 A/W at 0 V, and ∼0.1 A/W with a −4 V bias. The dark current is ∼60 nA at −2 V bias. The noise equivalent power is estimated to be 3.7×10−9 W over the response bandwidth of 2.2 MHz.
Simultaneous low-frequency noise characterization of gate and drain currents in AlGaN/GaN high electron mobility transistorsLow-frequency noise in AlGaN/GaN heterojunction field effect transistors on SiC and sapphire substrates
We report on low-frequency noise characteristics of visible-blind GaN p-n junction photodetectors. Carrier hopping through defect states in the space charge region, believed to be associated with dislocations, is identified as the main mechanism responsible for the dark conductivity of the photodiodes. Under reverse bias, the dark current noise has the 1/f character and obeys the Hooge relation with α≈3. Under forward bias, we observe generation-recombination noise related to a trap level with the activation energy of 0.49 eV. Under illumination, detectivity is found to be shot noise limited. The noise equivalent power of a 200×200 μm2 photodetector is estimated at 6.6×10−15 W/Hz1/2 at a bias of −3 V.
We observe a polarization instability in circularly symmetric vertical-cavity surface-emitting lasers. A relatively long time, 3–5 ns, is required to establish a dominant polarization state. Under high-speed digital modulation this leads to strong enhancement, 20–30 dB, in polarization resolved low-frequency relative intensity noise. This polarization instability is accurately described by a simple rate-equation model. A similar increase in relative intensity noise, under dc bias, is caused by energy partition between orthogonally polarized modes.
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