Solar-blind photodetectors respond only to ultraviolet light at wavelengths shorter than those of the solar radiation that can penetrate atmosphere of the Earth. This wavelength range, traditionally defined as wavelengths

Figure 2. The Photovoltaic Spectral Responsivity of an AlGaN UV-C Schottky photodiode is overlaid on solar irradiance spectra from above the atmosphere and from sea level to show that the AlGaN UV-C photodiode is blind to solar light that penetrates the atmosphere.

The material system that consists of nitrides of elements of column III of the periodic table offers the potential for solid-state photonic devices that are inherently solar-blind. These materials are a subset of the semiconducting compounds that comprise elements from columns III and V of the periodic table. More familiar III-V semiconductors include GaAs, GaP, and such ternary variations as AlGaAs. The distinctive aspect of the III-nitrides is that their bandgaps fall in the energy range that corresponds to blue and ultraviolet wavelengths. The bandgap of a ternary III-nitride material, (e.g., InGaN or AlGaN) can be tailored by varying the proportions of the column-III elements. For example, the bandgap of Al0.37Ga0.63N is 4.4 eV, which, as a photon energy, corresponds to a wavelength of 280 nm — the long wavelength edge of the UV-C band. In comparison, silicon, the most common semiconductor material for photodetectors, has a bandgap of 1.1 eV, corresponding to a wavelength of 1.1 µm, which is in the infrared region.

AlGaN Schottky photodiodes that respond primarily to radiation in the UV-C spectral band have been developed. Single-element Schottky photodiodes, each having an active area of 0.5 mm2, have been fabricated. The AlGaN layers of such a device are deposited by molecular beam epitaxy (MBE) on a sapphire substrate. The device consists of an underlying 0.5-µm-thick n+ layer of AlGaN doped with silicon and an overlying 1.0-µm-thick layer of undoped AlGaN. Standard photolithographic techniques are used for patterning the device. Inductively coupled plasma (ICP) etching is used to form a mesa of undoped AlGaN layer and expose the underlying n+ layer for ohmic-contact metallization. A semitransparent nickel layer is used for the Schottky contact on the undoped AlGaN layer.

Figure 1 depicts the photovoltaic spectral responsivity of a UV-C-responsive photodiode of this type. Also shown for comparison is the responsivity of an Al0.17Ga0.83N Schottky photodiode that exhibits a peak response at 320 nm (the long-wavelength cutoff of UV-B) and the responsivity of a GaN p-type/intrinsic/n-type (PIN) photodiode that exhibits a peak response at 365 nm.

The logarithmic responsivity scale of Figure 1 helps to emphasize the long-wavelength rejection of the UV-C-responsive photodiode. The response of this photodiode at a wavelength of 370 nm is only 10–3 times its peak response. As the wavelength increases to 500 nm, the response falls another order of magnitude, and as the wavelength increases further to 650 nm, the response decreases to 10–5 times the peak response. With its peak response at 280 nm, the UV-C AlGaN Schottky photodiode is the shortest-wavelength group-III nitride photovoltaic device demonstrated to date. The absolute response at 280 nm is considered to be fair. At 0.022 A/W it exhibits an external quantum efficiency of about 10 percent; this level of quantum efficiency is comparable to the quantum efficiencies of photocathodes in this spectral range.

Figure 1. The Photovoltaic Spectral Responsivities of three column-IIInitride photodiodes are plotted together for comparison.

Figure 2 shows an overlay of the responsivity of the UV-C sensor to the solar irradiance spectra at sea level and at high altitude. The AlGaN UV-C photodiode is blind to solar light at sea level because the portion of the sunlight in its spectral-response range is absorbed in the atmosphere.

Photodetectors made from column-III nitride compounds are uniquely well suited for numerous ultraviolet-detection applications, including sensing of flames in industrial settings, monitoring UV curing and drying, detection of arcs, controlling UV sterilization, monitoring UV in phototherapy, and solar-blind detection of fires. Because the column-III nitride material system is stable at high temperatures and during exposure to intense radiation, it is a good choice for photodetectors that must operate in harsh environments.

This work was done under a under 1998 NASA Phase I Small Business Innovation Research (SBIR) Contract by Jody J. Klaassen, James VanHove, Robert Hickman II, Andrew Wowchak, Christina Polley, David King, Matt Rosamond, and Peter P. Chow of SVT Associates/Blue Lotus Micro Devices, Inc., for Dryden Flight Research Center.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Jody Klaassen
SVT Associates, Inc.
7620 Executive Drive
Eden Prairie, MN 55344-3677
Tel. No. (612) 934-2100 Ext. 247

Refer to DRC-99-25, volume and number of this NASA Tech Briefs issue, and the page number.