ZnO nanowires and related materials

Semiconductor nanowires (NWs) have been proposed as basic “building blocks” in a variety of devices in many applications, such as photonics, electronics and chemical sensing . For instance, the large surface area offered by NWs is very attractive for solar cells where high surface chemical sensitivity and efficient light trapping and absorption are needed.

ZnO offers Interesting optical, electronic and
electro-mechanical properties, such as

1. Large bandgap 3.3 eV (UV)
2. Large excitonic binding energy (60meV, which means stable excitonic emission)
3. Large piezoelectricity (opportunity for MEMS devices
4. High surface reactivity (catalysis, photocatalysis, electrolytic solar cells…)

ZnO Nanowire growth

Most studies report on efforts to grow vertically aligned ZnO NW arrays on various substrates (notably on Si and saphire). An important requirement for cost-effectiveness is the possibility of growing NWs on inexpensive and transparent substrates, such as glass. In such an amorphous substrate, growth of vertically aligned ZnO NWs on a vacuum deposited SiO₂ film on a Si substrate can be achieved by fabricating a thin seed ZnO layer on the SiO₂ film prior to NW growth.

ZnO nanowires grown by the NANO project team on SiO₂

The image above shows partially aligned, high crystallinity ZnO nanowires that grew on an amorphous SiO₂ substrate. The partial alignment is believed to be due to a very thin ZnO layer that grew at the SiO₂ and helped seed ordered nanowire growths.

The growth system (LAFISO)

NW growths were performed at the LAFISO vapor-transport system, which was designed and set up using an existing furnace especially for this project.
It consists of a 1 3/8 " diameter, 1 m long quartz tube placed within a 0.6 m length tubular furnace. One side of the quartz tube is seal-connected through a vacuum valve to a rotary pump while its other side to a high purity (99.999%) Ar line made of a 1/4 " diameter stainless steel tube. An alumina crucible containing ZnO and graphite powders (2:1 volume ratio) was placed inside the quartz tube at a position corresponding to the furnace center. Pairs of Si (100) and SiO₂/Si substrate pieces were placed parallel to the tube axis facing upwards at two different distances downstream the tube (16 and 19 cm from the furnace center), which resulted in two different substrate temperatures due to the natural temperature gradient profile along the tube.
The image above shows typical process variables used in the fabrications.

(From D. Comedi et al., J. Alloys and Compounds (2009) in Press)

ZnO nanostructured 2D layers

The film shown in the image above is a result of coalescence of individual nanopillars after each one of them grew radially during the growth process. Such a film was obtained by the NANO project team and has interesting applications for photocatalysis, solar cells, and sensors.

ZnO NW networks

An interesting and potentially useful assembly is in the form of ramdom networks of interconnected NWs on insulating substrates. Direct growth of randomly-oriented ZnO NW networks on thermal SiO₂ layers on Si using the metal catalysis method. Has been achieved. The electrical properties of  the NW networks can be easily tested right on the insulating SiO₂/Si growth substrates and exhibit macroscopic conduction and photoconduction with potential sensor, photovoltaic and other optoelectronic applications

A ZnO nanowire network grown by the NANO project team on SiO₂

(From D. Comedi et al., J. Alloys and Compounds 495, 439-442 (2010))

ZnO nanoflowers

Marvelous ZnO structures grow as a result of combined 3D, 2D and 1D growth. This structure was obtained accidentally in our system in Tucuman on top of a Si substrate containing Si nanowires (which can be seen as tiny fingers at the back) grown by our Collaborators at the Polytechnique University of Madrid.

Impedance spectroscopy of nanowires

Typically, PV device performance is assessed through DC current-voltage characteristic measurements. However, a much more complete device characterization can be achieved by studying the frequency dispersion of electrical properties. In this work, we present an impedance spectroscopy study of core-shell GaAs NW p-n junction structures fabricated by gas source molecular beam epitaxy (GS-MBE) for PV applications where Te has been used as the n-type dopant. The use of Te instead of the more common Si as the n-dopant derives from the amphoteric behavior that Si may exhibit in GaAs NWs.

Impedance spectroscopy studies in the 10-10⁷ Hz and -40, +40 V frequency and bias ranges can be performed at the Dielectric Properties of Matter Laboratory (LPDM), at the Physics Department, FACET-UNT.

Impedance Data

The impedance data in the complex plane show strong differences which are related to the different n-p spatial distributions of samples A and D above.

The results below show that, under proper fabrication conditions, the experimental impedance follows the behaviour expected for a p-n junction but, nevertheless, they emphasize the role of surface traps in the transport and recombination mechanisms and interface space-charge regions in the studied devices.

Conductance and Capacitance as extracted from the impedance data of Figs. 5 and 6, for samples A and D. The core-shell capacitance for sample D in Fig. 8(b) is modelled by a coaxial cylindrical charge sheet capacitor for (1) Vbi=0.65 V and (2) Vbi= 1.2 V, where Vbi is the built=in potential at the p-n junction.
From J. Caram et al. in Nanotechnology (http://iopscience.iop.org/0957-4484/21/13/134007/?rss=2.0 )

Luminescent Si nanostructures

Bulk silicon (Si) is the basis of our telecommunications
and computational technology. The highly developed
Si technology has enabled the fabrication of excellent
transistors and photodetectors and, by combining Si with
its natural oxide—silicon dioxide (SiO₂) —outperforming
waveguides have been designed and produced as well.
However, Si possesses an indirect band gap and therefore it
does not exhibit room-temperature luminescence. This has
been the major obstacle to completion of an all-Si photonic
circuit. Although most of the Si-based computing and
communication functions have already been developed,
a practical Si light source is still lacking. That is why huge
efforts have been directed in recent years towards making Si
an efficient, reliable luminescent material.

Fortunately, when silicon is prepared in special nanoscopic
forms, such as Si nanoparticles or nanocrystals (Si-nc),
bright luminescence at room-temperature eventually
occurs. The “particle-in-a-box” or “quantum confi nement”
concept has been used to rationalize the effect. According
to Heisenberg’s uncertainty principle, the confi nement
of an electron to a nanometer space region blurs out its
momentum, thus enabling “quasi-direct” luminescent
transitions to occur even across an indirect bandgap.
Furthermore, the energy released in these transitions, which
determines the wavelength of the emitted light, should scale
with the “box” size, namely, with the Si-nc characteristic size.

Si nanoparticles embedded in a SiO₂ matrix

An interesting approach to prepare Si-ncs, which is fully
compatible with standard Si technology, involves thermal
promotion of Si-nc formation within a SiO₂ layer. This
can be accomplished by (a) fabricating a Si “suboxide” thin
fi lm, i.e., a Si oxide where the Si concentration is tailored to
be slightly larger than that of SiO₂ and (2) promoting the
formation of the stable SiO₂ phase by a high-temperature

heat treatment of the suboxide fi lm and the precipitation of Si nanoclusters withn the SiO₂ matrix.

From D. Comedi et al.

Synchrotron radiation excited luminescence and total electron yield from Si nanoparticles embedded in SiO₂ as a function of the excitation energy. (in collaboration with Prof. P. Mascher and coworkers from CEDT, McMaster University).

Porous Si

Si can be nanostructured by electrochemical etching of Si wafers, producing porous Si (P-Si). We have a collaboration with Dr. Koropecki, Dr. Arce and co-workers from INTEC, and Prof. Yuri Pusep from Sao Carlos Physics Institute, USP Brazil to study the physical properties of this material. The porous structure in P-Si forms a random system where the degree of disorder can be controlled by porosity. Since the Si nanostructures in P-Si are naturally oxidized in ambient air, an important well-known problem in this material is the pinning of the PL by Si-SiO2 interface states, seriously limiting the tuning of PL over a wide spectral range, just as in Si-nc embedded in SiO2 systems described above. In oxide-free porous Si, a considerable change of PL over wide spectral range caused by quantum confinement is observed, demonstrating the fundamental significance of the Si-SiO2 interface electronic states. In one of our papers (Y. Pusep et al., J. Raman Spectrosc. 42, 1405-1407 (2011)), we have observed the Fano-type typical asymmetry on the Si longitudinal optical(LO) phonons peak by Raman spectroscopy in heavily doped P-Si. Such an asymmetry is usually assigned to the interference of continuum electrons with the LO phonons; a quantum phenomenon. Our analysis showed an unexpected very sharp Fano resonance that could only be explained by the quantum interference of conduction electrons trapped at the Si-SiO2 interfaces with the LO phonons. From this analysis and the experimental Raman data, the gap energy associated with these electronic interface states could be determined.

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