book of abstracts - IM2NP

A B S T R A C T S MONDAY, JUNE 28 N A N O S E A 2 0 1 0
Room Calendal
17H00-17H30
Nitride nanowires: from nucleation to optoelectrical measurements
R. Calarco, Toma Stoica1, Eli Sutter2, Ralph Meijers1, Thomas Schäpers1, Thomas
Richter1, Ratan K. Debnath1, Michel Marso1, Hans Lüth1, A. Cavallini3, L.
Polenta3, M. Rossi3. (1 Institute of Bio- and Nanosystems (IBN-1), Research Centre Jülich GmbH, 52425
Jülich, Germany, and JARA-FIT Fundamentals of Future Information Technology. 2 Center for Functional
Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973. 3 Phys. Department and CNISM,
University of Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy.)
1 – Introduction
In recent years III-nitride based nanowires have attracted a lot of interest because of their potential
applications for nanoelectronic devices [1-5]. III-nitride NWs are indeed extremely interesting due to the fact
that on the one hand they possess impressive material properties and on the other hand they constitute a 1D
structure. The combination of both aspects offers the possibility of studying fundamental issues of transport
properties, often with tight feed back to the optimization of growth.
2 – Abstract
We showed that the growth of Nitride nanowires by MBE occurs on various substrates under appropriate
growth conditions without using a liquid-phase catalyst, i.e. a droplet, at the top of the growing nanowire.
We addressed the optimization [6, 7] of the crystalline and optical quality of nitride nanowires as well as the
nucleation, kinetics and growth mechanisms [8, 9]. We observed that the growth is characterized by a long
nucleation period [8], which results in a broad distribution of the nanowire sizes. A diffusion mechanism was
found to dominate the NW growth after the nucleation period and can explain the diameter dependence of
the NW length [9]. In addition, it was demonstrated that nanowire growth occurs perpendicular to the
substrate surface and along the crystallographic c-axis even on non-crystalline amourphous SiO2 substrates
[10]. This last statement has a tremendous importance and opens new possibilities for NW growth on a
variety of nonconventional substrates of interest for applications relying on low-cost substrates. Numerous
structures, including GaN and InN nanowires, GaN/AlGaN hetero- and quantum structures as well as
InN/GaN core-shell structures were realized over the past years.
The high quality of the material allowed for studying fundamental transport properties. In our work
particular emphasis was given to the investigation of effects due to surface space charge layers. Therefore,
we especially focused on both narrow gap (InN) and wide band gap (GaN) materials in which accumulation
and depletion space charge layers are present at the surface, respectively.
The investigation of the electrical properties of GaN nanowires showed that the band-to-band photoelectric
effect varies by orders of magnitude as a function of the nanowire diameter [4]. For these studies, single
nanowire devices were fabricated using e-beam lithography for the preparation of metallic contacts. The
unusual effect which we found was explained by modeling of the effect of the electron depletion region at
the nanowire surface. Photovoltage Spectroscopy and Spectral Photoconductivity (SPC) measurements have
been carried out to analyze the near band-edge absorption in GaN nanowires [11]. A strong diameter
dependence of the band absorption tail was found by SPC measurements. The band-edge tailoring and its
wire-diameter dependence can be explained by the Franz-Keldysh effect induced by the electric field at the
wire surface.
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