02 Jul 2007
High speed, high operating temperature applications could benefit from research into quantum-dot infrared photodetectors.
Quantum dots (QDs) are helping researchers in the US to reduce the dark current and increase the operating temperature of infrared photodetectors. Without the need for cooling, a detector based on QDs will become a cheaper and portable option for many medical, military and scientific applications. (Applied Physics Letters 90 201109)
"The most important factor was the incorporation of QDs into a quantum well detector. This decreased the dark current of the device by about an order of magnitude," Manijeh Razeghi, a professor at Northwestern University, told optics.org. "This is important for high temperature imaging because with these devices the operating temperature is limited partly due to high dark current saturating the readout circuitry."
Current IR photon detectors require some type of cryogenic cooling in order to operate. These cooling systems add extra size, power consumption and cost to imaging systems.
"Increasing the operating temperature will reduce or eliminate the design constraints on IR imaging systems," explained Razeghi. "These systems are useful in high speed, high operating temperature applications. One sector that needs this is the military – for example in targeting systems."
Reducing the dark current has so far been difficult to achieve because it can also lead to the reduction of the photocurrent signal. "We were able to reduce the dark current significantly enough for high temperature imaging but without decreasing the photocurrent," commented Razeghi. "This level of performance has not yet been achieved in QD-based imaging arrays."
The end result is a 320 x 256 mid-IR focal plane array that has a peak detection wavelength at 4 µm.
The array is an InP-based, InAs quantum dot/InGaAs quantum well/InAlAs barrier detector. It is grown on an InP substrate by low pressure metal organic chemical vapor deposition. The group quotes a responsivity of 34 mA/W at an operating temperature of 200 K.
This material system is less commonly investigated because it is harder to grow and control. "The primary challenge was maintaining the highest quality material," commented Razeghi. "It is necessary to optimize the growth conditions of all of the different layers in the detector especially the QDs and work on this is continuing now."
The team is now optimizing the device structure and growth conditions and is hopeful that the performance can be improved.