High-Sensitivity Visible-Blind AlGaN Photodiodes and Photodiode Arrays
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p-type GaN or AlGaN Undoped GaN or AlGaN n-type AlGaN
Sapphire Substrate Polished on Both Sides
Figure 1 Schematic of p-i-n photodiode structure. polished sapphire wafer to permit illumination of the device through the substrate. The photodiode structure employed in our initial experiments responds to UV light in the wavelength band from about 320 nm to 365 nm. At wavelengths shorter than 320 nm, the incoming light is absorbed in the thick AlGaN base layer (~20% Al) and the junction is not illuminated. Likewise, the diode does not respond to wavelengths greater than 365 nm, since this corresponds to the optical absorption edge of GaN at 300K. By increasing the Al content of the base layer it is possible to increase the optical bandwidth of the diode’s UV responsivity. Likewise, by adding Al to the top layers, it is possible to change the diode UV responsivity band to other wavelength regions in the UV. Thus, UV detectors that sense different UV “colors” are possible. Diode structures of the type shown were prepared by MOVPE both at North Carolina State University (NCSU) and at the Honeywell Technology Center using low-pressure, vertical-flow MOVPE reactors that employ high speed substrate rotation during film growth. The photodiode structures were deposited onto 2 in diameter c-plane sapphire substrates. The growth was initiated by depositing a thin AlN buffer layer at 500-650 oC; all subsequent layers were grown at 1050-1080 oC. All device processing was completed at NCSU using standard semiconductor processing techniques which included photolithography using appropriately-designed masks, reactive ion etching to define mesa structures, and metallizations to provide ohmic contacts to the n-type and p-type layers of the device. Spectral responsivity measurements [8] were completed at NCSU on selected discrete photodiodes. The 300K dynamic resistance of the photodiode at zero-bias, R0, was measured for selected devices using a shielded low-noise enclosure and shielded probe tips. These measurements were combined with the device area A to obtain the R0A product which was then used to obtain an estimate of the detector detectivity D* [8]. In order to use the GaN/AlGaN photodiodes as the basis for a new visible-blind UV digital camera it is necessary to employ flip-chip bonding techniques to hybridize an appropriately sized photodiode array to a silicon readout integrated circuit (ROIC). In the present case, NCSU designed and purchased a mask set for a photodiode array that matched a 32 x 32 ROIC chip provided by the Night Vision Laboratory (NVL) at Ft. Belvoir. Prior to hybridization at NVL, In bumps were deposited onto each of the mesas and n-contact layers of the photodiode array and onto the corresponding areas of the ROIC using NCSU facilities. Each 32 x 32 GaN/AlGaN
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