A Higher-Temperature Quantum Dot Infrared Photodetector

Hank Hogan

Researchers at the University of Massachusetts Lowell and at Raytheon Missile Systems in Tucson, Ariz., have produced a long-wave infrared quantum dot photodetector with a responsivity of 2.5 A/W at operating temperatures as high as 190 K (–83 °C). Although cold, that is actually pretty hot for a quantum dot infrared photodetector. The advance holds promise for tracking, sensing and imaging applications.

The investigators constructed the quantum dot device using molecular beam epitaxy to grow layers on a GaAs wafer, then creating a heterostructure of a 1-nm-thick layer of indium, gallium and arsenic followed by 2.4 molecular layers of InAs capped with 30 molecular layers of InGaAs. They repeated this heterostructuring 10 times between current-blocking layers of AlGaAs. After growing the structures, they processed them into 100-μm-diameter circular mesas, with top and bottom electrodes.

The devices were wire-bonded so that they could be driven by outside electrical sources; the researchers mounted them in a temperature-controllable Dewar with a ZnSe window that allowed infrared measurements to be taken of the devices inside.

Using a spectrometer from Bruker Optics of Billerica, Mass., the group measured the photocurrent spectrum of the quantum dot infrared detectors. At 78 K, which is just above the temperature of liquid nitrogen, the devices had a peak at 9.9 μm with a response width of about 1.6 μm for a bias of –1.0 V. These values were the same for various bias conditions and at temperatures up to 190 K.

The researchers found that the dark current density of the devices changed with the temperature, which was expected because of the increase in thermal electrons with increasing temperature. The dark current density at 190 K, for example, was several orders of magnitude higher than at 78 K.

However, this did not prevent the devices from working because there also was a strong temperature-dependent device photoresponsivity. Using a blackbody source at 1000 K, the researchers measured the photoresponsivity of the devices and normalized the values for temperature by dividing it by photoconductive gain. The results were nearly a constant.

Because they operate in the long-wave infrared range, such devices possibly could be used in a variety of surveillance-related and other applications.

Applied Physics Letters, July 30, 2007, Vol. 91, 051115.