Building a Better Infrared Detector

Hank Hogan

If Robert Frost had been an optical engineer and not a poet, he might have said “good barriers make good detectors” upon seeing the work of Shimon Maimon and Gary W. Wicks of the University of Rochester in New York. The scientists have demonstrated a new type of midwave (2 to 5 μm) infrared detector with reduced dark current and higher operating temperature than PN photodiodes.

Figure 1. A band diagram depicts an infrared detector composed of an InAs-based nBn structure (biased under operating conditions), in which a barrier layer reduces dark current. The inset shows the flat-band condition in the barrier junction. Images reprinted with permission from Applied Physics Letters.

Dubbed the “nBn” detector, the device consists of two N-type semiconductors sandwiching a barrier layer. The barrier layer lessens dark current and passivates, or protects, the active layer of the device. That could benefit materials lacking a good passivation choice, such as InAs. “Elimination of the passivation step makes this device more important in the InAs material system, for which a good passivation process has not been developed,” Maimon said.

The depletion layer found at the PN junction in photodiodes blocks majority carriers, such as electrons in N-type semiconductors, while permitting minority carriers, such as holes, to flow freely. Although that characteristic helps detector performance, the depletion layer creates Shockley-Reed-Hall currents, which are a source of noise.

Figure 2. Arrhenius plot of the current of an InAs nBn exposed to room temperature background radiation via 2π steradians. The inset shows a schematic of an nBn device after processing.

The resulting dark current limits the maximum detector operating temperature of a photodiode-based focal plane array to 77 K. However, a properly designed barrier inserted between two N-type semiconductors acts as a depletion layer while not suffering from Shockley-Reed-Hall currents.

Using molecular beam epitaxy, the researchers deposited a 3-μm-thick N-doped InAs layer for the N-type semiconductor, then a 100-nm-thick AlAsSb barrier, then a final contact layer of InAs a few tens of nanometers thick.

Employing a scanning Fourier transform infrared detector from Thermo Fisher Scientific Inc. of Waltham, Mass., they measured the spectral response of the detectors, which were 100 μm square. When cooled to <230 K, the device’s dark current dropped below the photocurrent generated from the 295-K-room background. That temperature, the investigators noted, was at least 100 K above the similar point for commercial InAs photodiodes.

The detectors should be cooled to about 200 K and, for the technology to be competitive, the cooler must be inexpensive, reliable and highly efficient.

The researchers are developing devices for both shorter and longer wavelengths, along with focal plane arrays and detectors that might not be based on InAs. “Future work will involve using the nBn concept as a basis to develop devices that incorporate different material systems and structures,” Maimon said.

Applied Physics Letters, Oct. 9, 2006, 151109.