The doping of bulk semiconductors has enabled a wealth of microelectronic and optoelectronic devices. But while semiconductor nanocrystals could herald a new generation of devices produced through cheap and easy wet-chemical approaches, doping has remained elusive.
Now, however, researchers from Hebrew and Tel Aviv Universities in Israel claim to have developed a simple, room-temperature chemical method for doping semiconductor nanocrystal quantum dots [D. Mocatta et al., Science 332 (2011) 77]. The new method simply relies on mixing a solution of the dopant metal salt dissolved in toluene and a surfactant with preformed quantum dots. Using this solid state diffusion method, the researchers believe that Cu and Ag metal impurities are introduced into InAs quantum dots, enabling control of the band gap and the Fermi energy to yield both n- and p-doped nanocrystals.
The doping of quantum dots presents a unique scenario—–introducing even a single impurity atom into a 4nm nanocrystal of around 1000 atoms results in a doping level of 7×1019 cm−3, well into the heavily doped regime of bulk semiconductors. But the results of such heavy doping in nanocrystals are very different to the effects in bulk semiconductors. Instead of producing metallic (or ‘degenerate’) behavior, the doping of nanocrystals appears to introduce impurity states that significantly change the density of states (DOS) because of quantum confinement effects. Multiple impurities in the confined nanocrystals also appear to enhance disorder effects, altering the electronic structure via a process known as the Urbach tail mechanism.
The researchers report that introducing Ag atoms produces a red shift in the absorption and emission spectra of the InAs nanocrystals, while Cu results in a blue shift. The addition of Ag, which is a substitutional impurity replacing the In ion in the lattice, shifts the Fermi energy down towards the valence band, characteristic of p-doping. Cu impurities have the opposite effect, squeezing into the spaces between atoms in the crystal lattice as an interstitial impurity, shifting the Fermi energy up towards the conduction band, typical of n-doping.
‘‘This is the first time that colloidal quantum dots have been doped with heterovalent impurities leading to heavy doping effects corresponding to phenomena seen in bulk semiconductors,’’ says Uri Banin of Hebrew University. ‘‘It is also the first time that impurities have been seen to shift the Fermi level in single quantum dots. The successful doping of colloidal nanocrystals will enable their electronic properties to be altered allowing for more efficient quantum dot devices such as p—n junctions, transistors and solar cells.’’ Todd D. Krauss of the University of Rochester believes that the work could prove significant in the progress towards devices based on doped semiconductor nanocrystals. ‘‘It is very interesting that you can dope semiconductor nanocrystals with metal atoms that are not isovalent with the cations,’’ he told Nano Today, adding that the simplicity of the approach is remarkable.
The researchers hypothesize that the approach could be applied to all dopant—quantum dot systems that undergo diffusion and are now working on expanding the technique to other semiconductor types and different impurities. But Krauss cautions that it remains to be seen whether the approach will be as easy with other materials systems.
Figure. A Cu impurity in an interstitial site in the InAs lattice. Credit: P. Huey/Science ©2011 AAAS.