QDIPs technology
Removing strain promises to boost detector performance
Quantum dot infrared photodetectors suffer from strain in their nanostructures that culminates in various performance-degrading defects. However, many of these defects can be avoided by turning to a novel, strain-free growth method based on the deposition of droplets, says Jiang Wu from University of Arkansas Fayetteville.
I
nfrared photodetectors continue to attract a great deal of interest thanks to the numerous applications that they can serve. These detectors can be used for night vision, optical communication, target identification, fire fighting, medical diagnostics and surveillance.
The first infrared photodetectors that appeared on the market were fabricated from materials with a narrow bandgap, such as lead salt, InAs1–x
Sbx , and Hg1–x Cdx Te
(MCT). Detectors fabricated from these alloys have experienced a great deal of success and they are still selling today. However, they are plagued with growth-related issues, which has stimulated the development of intersubband quantum infrared photodetectors. During the last two decades, infrared photodetectors based on quantum wells and quantum dots have undergone dramatic development. Of the two types, the quantum dot infrared photodetector is the more promising due to intrinsic advantages associated with three-dimensional confinement. These include sensitivity to normal incidence radiation and high temperature operation.
One exciting aspect of the quantum dot infrared photodetector (QDIPs) is its potential to combine high resolution with multicolor detection capability. Traditionally these types of detector are fabricated from either InAs or InGaAs quantum dots. Coherent nanoscale islands are generally formed when a certain amount of In(Ga)As is deposited on the (Al)GaAs surface. However, other lattice- mismatched materials have been investigated as well.
Quantum dots are formed by a growth procedure known as Stranski–Krastanov (S-K) growth. Transformation from a two-dimensional growth mode to a three-dimensional one depends on the strain of deposited materials. The inevitable strain arising in S-K quantum dots introduces various defects, including long stacking faults, short stacking faults and dislocations. These defects impair the
optical and electronic properties of QDIPs and are one of the biggest factors behind their low quantum efficiency.
At the University of Arkansas Fayetteville we employ a novel growth process for produing strain-free dots: droplet epitaxy. This approach separately supplies source elements. Generally growth begins by forming nanosize droplets of group V materials. These structures are crystallized by group III vapor transforming droplets to yield a process that creates semiconductor nanostructures.
One of the great strengths of the droplet epitaxy approach is its versatility. It can construct quantum dot pairs, quantum molecules, quantum rings and nanoholes. In all cases, these tiny structures are free from strain, which is promising for the fabrication of high-performance devices. The QDIPs that we are developing feature strain-free
Figure 1. Atomic force microscopy image of
quantum dot pairs grown by high
temperature droplet epitaxy
November / December 2010
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