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Scientific Report 2007-2009<br />
Condensed matter physics and biophysics<br />
C32. Hydrogen-mediated nanostructuring of the electronic and<br />
structural properties of nitrogen-containing III-V semiconductors<br />
The synthesis of nanostructured semiconductors is incessantly<br />
boosting the number of opportunities in the<br />
field of electronics and photonics, as well as in the investigation<br />
of fundamental quantum phenomena in topbench<br />
experiments. The control and modification of the<br />
physical properties of semiconductor heterostructures at<br />
nanometre scale lengths can be obtained by several approaches.<br />
Layer-by-layer deposition of materials with<br />
different chemical composition and thickness, which is<br />
typical of modern epitaxial growth techniques, allows<br />
achieving carrier confinement in a two-dimensional potential<br />
(or quantum well). The attainment of nanostructures<br />
with lower dimensionality, such as quantum<br />
wires (QWRs) and quantum dots (QDs), is not as easy<br />
and mainly two approaches have been attempted, so<br />
far. Top-down methods achieve carrier lateral confinement<br />
by chemically removing small portions of quantum<br />
well heterostructures previously processed by lithography.<br />
This leads to QDs and QWRs characterized by a<br />
large degree of uniformity, flexibility, and reproducibility<br />
but at the expense of very lengthy and costly processes.<br />
Alternatively, bottom-up methods exploit the<br />
self-aggregation of QDs in highly-strained heterostructures<br />
or the spontaneous formation of colloidal nanocrystals.<br />
The resulting nanostructures have very high optical<br />
efficiency and thus are very attractive for the fabrication<br />
of optoelectronic devices, optical imaging in biological<br />
systems, or for the generation of single photon sources<br />
for quantum computation. However, the lack of a control<br />
in the spatial arrangement of the single nanostructure<br />
and the large dispersion in QD size have so far hindered<br />
a full exploitation of self-formed QDs.<br />
Figure 1: a. Schematic representation of the method leading<br />
to the formation of Ga(As,N) QD. b. Distribution of<br />
the N concentration (red: maximum; blue: minimum) in a<br />
Ga(As,N) QD. The vertical axis is 5 times exaggerated.<br />
Dilute nitrides, such as Ga(As,N), are a new class<br />
of semiconductors with surprising physical properties<br />
and qualitatively new alloy phenomena, e.g., a giant<br />
negative bowing of the band gap energy ( 200 meV<br />
upon incorporation of 1% of N atoms in GaAs) and a<br />
large deformation of the conduction band structure [1].<br />
This renders this alloy of high potential in several fields,<br />
such as optical fiber telecommunications, multi-junction<br />
solar cells, and Terahertz applications. Within this<br />
framework, we have developed a new method for achieving<br />
a band gap modulation in the sample growth plane<br />
without incurring in the main drawbacks of previous<br />
methods [2]. We showed that the incorporation of a<br />
suitable amount of hydrogen in Ga(As,N) modifies in<br />
a fully controllable and reversible way the band gap<br />
energy as well as the transport, spin and structural<br />
properties, which can be tuned on demand at any value<br />
intermediate between that of the as-grown material and<br />
that of the N-free lattice (GaAs) [3].<br />
Figure 2: Comparison of the photoluminescence spectra of a<br />
bulk (black line) and a single Ga(As,N) QD (red line). Inset:<br />
Light emission from an ordered arrays of QDs. The red circle<br />
highlights the dot, whose emission spectrum is shown in the<br />
main part of the figure.<br />
Hydrogen irradiation of these alloys performed<br />
through H stopping masks made of Ti and deposited by<br />
electron-beam lithography allows us to tailor the band<br />
gap in selected parts of the sample growth plane (see<br />
Fig. 1a). The size of the Ga(As,N)/GaAs heterostructures<br />
so achieved is limited only by H diffusion, whose<br />
front edge can be sharper than 5 nm thanks to the<br />
peculiar kinetics of H in these materials (see Fig. 1b)<br />
[4]. Finally, micro-photoluminescence shows that a true<br />
zero-dimensional confinement and an elevated degree of<br />
spatial ordering can be obtained by this approach (Fig.<br />
2).<br />
References<br />
1. G. Pettinari et al., Phys. Rev. Lett. 98, 146402 (2007).<br />
2. L. Felisari et al., Appl. Phys. Lett. 93, 102116 (2008).<br />
3. R. Trotta et al., Appl. Phys. Lett. 94, 261905 (2009).<br />
4. R. Trotta et al., Phys. Rev. B 80, 195206 (2009).<br />
Authors<br />
A. Polimeni, R. Trotta, M. Capizzi<br />
http://chimera.roma1.infn.it/G29<br />
<strong>Sapienza</strong> Università di Roma 85 Dipartimento di Fisica