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