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First observation of the trion in a nitride-based quantum dot.

In short: Nitride-based quantum dots populated with two or four charge carriers have previously been studied. Rather surprisingly, no such quantum dots populated with odd number of charge carriers have been evidenced earlier. In our work, we investigate an indium gallium nitride quantum dot populated with three charge carriers, two electrons and one hole, forming a so-called negative trion. Our research improves the fundamental understanding of these quantum dots, which are highly potential for future applications, such as quantum cryptography.

Knowledge of the behavior of charge carriers in semiconductor based quantum dots is important for optoelectronic applications, both in the classical case and in the quantum mechanical regime. A semiconductor quantum dot is a nanometer-sized semiconductor crystal embedded in another semiconductor crystal, small enough to inhibit quantum mechanical properties. Two types of charge carriers, negative electrons and positive holes, may populate a quantum dot. When an electron and a hole in the quantum dot share the same point in space and recombine, a photon (single quantum of light) is emitted. This process is utilized in light emitting diodes. Indium gallium nitride is a particularly important semiconductor material for light emitting diodes, as it can emit light spanning from the ultraviolet to the infrared region of the electromagnetic spectrum, depending on the composition and the size of the quantum dots. Furthermore, individual quantum dots are promising sources of single photons for quantum cryptography applications used for secure communication. In order to realize devices for such applications, a comprehensive understanding of the optical properties of the quantum dots is crucial.

Nitride-based quantum dots populated by one or two electron-hole pairs, forming so-called excitons and biexcitons, have previously been studied. Rather surprisingly, no convincing evidence of such quantum dots populated with unequal number of electrons and holes have been reported. In our work, we investigate an indium gallium nitride quantum dot populated with three charge carriers, two electrons and one hole, forming a so-called negative trion.

Our quantum dot sample is fabricated by a chemical growth technique, at approximately 1000 degrees Celsius. Initially, micrometer-sized gallium nitride pyramids were grown. On the pyramids, a nanometer thin layer of indium gallium nitride (a quantum well) was deposited, which subsequently was coated by a layer of gallium nitride. Quantum dots can form spontaneously at the top of the pyramids due to three-dimensional encapsulations of nanometer-sized indium gallium nitride crystals.

The properties of the quantum dots can then be probed by optical spectroscopy. We have illuminated a pyramid containing a quantum dot with a laser beam to populate the quantum dot with electrons and holes. When these electrons and holes recombine, light known as photoluminescence is emitted. The photoluminescence spectrum of a single quantum dot contains one or more characteristic sharp peaks, depending on its exact population. We have analyzed this spectrum for sample temperatures from 4 Kelvin up to room temperature.

 
Figure 1: Photoluminescence spectrum of one pyramid containing quantum wells and a quantum dot. The part of the spectrum containing the photoluminescence from the quantum wells and the quantum dot is magnified ten times for clarity. The insert shows a schematic drawing of the crossection of a pyramid.

The photoluminescence spectrum of this quantum dot, as well as its neighboring quantum wells and bulk gallium nitride pyramid are shown in figure 1. The details of this spectrum and its temperature evolution provide convincing evidences that this pyramid hosts a quantum dot containing a trion, in addition to the exciton, as shown in Figure 2(a). The time dependence of the photoluminescence obtained by pulsed laser illumination is illustrated by the transients in Figure 2(b). From the decay slopes in these curves, the lifetimes of the exciton and the trion have been determined to be 310 picoseconds and 120 picoseconds, respectively. Thus, we report on the unique observation of the trion in an indium gallium nitride quantum dot, with spectral features distinguished from those of the exciton and those expected for a biexciton. Our result carries important knowledge about the optical properties of the quantum dots needed for future applications.

(a) Photoluminescence spectrum of the quantum dot with the emission from the exciton and the trion indicated. (b) Corresponding time dependent photoluminescence spectra of the two peaks in (a), with the lifetimes of the two states indicated.

Details of the research are described in Applied Physics Letters 103 013109 (2013)
doi:10.1063/1.4812984

Authors

Chih-Wei Hsu, Evgenii S. Moskalenko, Martin O. Eriksson, Anders Lundskog, K. Fredrik Karlsson, Urban Forsberg, Erik Janzén and Per Olof Holtz

Contact

Martin Eriksson, PhD student
Phone: +46 (0)13 28 56 72
E-mail: marer@ifm.liu.se

Funding

  • Nano-N consortium funded by the Swedish Foundation for Strategic Research (SSF).

 


Responsible for this page: Fredrik Karlsson
Last updated: 02/11/14