of 0.16 eV has been observed in the PL spectrum, showing good size uniformity of
nanocrystals. The band gap has also been verified in the analysis of I-V characteristics of an
ITO/ZnO/SiO2/Al structure, high bias, holes or electrons may tunnel through the ZnO
nanodot layer and contribute an appreciable current that follows the Fowler-Nordheim (FN)
tunneling equation, from which the band alignment of the ZnO nanodots on ITO has been
determined. This work demonstrates an efficient synthesis of nanodots and an easy
approach to studying physical properties of nanocrystals that will help the material
optimization in device application. This chapter also outlines the application of ZnO QDs
optoelectronic property, such as EL and NST device. The results described in this chapter
are important for the future development of ZnO technology and optoelectronic
applications.
9. Acknowledgments
The authors gratefully acknowledge the financial support the Singapore National Research
Foundation under CRP Award No. NRF-G-CRP 2007-01, T. Mei acknowledges support from
Department of Education of Guangdong Province, China (Grant No. C10131). Y. Hu
acknowledges support from the Educational Commission of Zhejiang Province of China
(Grant No. Z200909406) and Zhejiang Qianjiang Talent Project (2010R10025).
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12
In-Situ Analysis of Optoelectronic Properties of
Semiconductor Nanostructures and Defects in
Transmission Electron Microscopes
Yutaka Ohno1, Ichiro Yonenega1 and Seiji Takeda2
1Institute for Materials Research, Tohoku University
2Osaka University
Japan
1. Introduction
A wide variety of optoelectronic devices such as photovoltaic (including solar cells and
photo detectors), photoemitting (including lasers) and photocatalytic devices have been
developed for more than five decades. The physical nature of such devices, especially
operated with visible and near-infrared light, is electronic transitions between pairs of
energy levels, typically in semiconductors. In addition to the conduction band and the
valence band, defect levels, i.e., localized energy levels associated with nanostructures and
lattice defects, are responsible for the electronic transitions. Various kinds of nanostructures
can be fabricated, spontaneously or artificially, inside and onto semiconductors. Also, lattice
defects, such as point defects (including vacancies, interstitials, dopant and impurity atoms)
and extended defects (including grain boundaries, stacking faults, dislocations, and point
defect clusters), can be introduced, inevitably or accidentally, during crystal growth and
device fabrication processes. Therefore, in order to fabricate optoelectronic devices with
advanced and ultimate functions, the structural properties of semiconductor nanostructures
and defects, as well as their optoelectronic properties such as the possible presence of defect
levels, should be understood with a high spatial resolution simultaneously with a high
spectral resolution.
Optical measurements such as luminescence and photoabsorption spectroscopy are
powerful techniques to determine defect levels. Since the spectral resolution is higher than
the order of 10−3 eV, this techniques are useful to study the energy levels in the low energy
range between the band edges of semiconductors (of the order of 100 eV at most), which
dominate the optoelectronic properties of the device products made of the materials.
Therefore, when the measurements are performed in a transmission electron microscope
(TEM), such as near-field optical measurements in a TEM (Ohno, 2010a), we can examine
the optoelectronic properties simultaneously with the structural properties in small regions
observed in-situ with the TEM.
With an extremely high spectral resolution in comparison with the other spectroscopic
techniques in TEM, such as energy dispersive x-ray spectroscopy (e.g., Terauchi et al, 2010)
and electron energy-loss spectroscopy (e.g., Kikkawa et al, 2007) with a resolution less than
about 10-1 eV, the optical measurements in TEM enable us to assess in detail defect levels in
242
Optoelectronic Devices and Properties
small regions. It is interesting to note that we can examine in-situ the optoelectronic and
structural properties of nanostructures and defects inside a material, which properties are
affected by the surrounding material and are not determined in-situ by the optical
measurements in scanning probe microscopes, such as scanning near-field optical
microscopy. As the continuing miniaturization and integration of optoelectronic devices, the
optical measurements in TEM have been established as an indispensable micro-
characterization technique. In this chapter, principles of the optical measurements in TEM,
i.e., cathodoluminescence (CL) spectroscopy and transmission electron microscopy under
light illumination are briefly summarized, and the resent analysis of some semiconductors
for optoelectronic devices are reviewed.
2. Principles of the optical measurements in TEM
2.1 CL spectroscopy in TEM
CL is a phenomenon of light emission induced by electron irradiation. CL light is emitted
from a region in which electrons are irradiated, and the optical parameters, such as the
photon energy, intensity and polarization, vary depending on the electronic structure in the
region. Therefore, CL spectroscopy performed in a TEM enables us to examine the electronic
structure simultaneously with the atomic structure in small regions observed in-situ with
the TEM, where electrons are irradiated. For example, the structural and compositional
variation, as well as the defect concentration and distribution, can be determined. Also, the
electronic properties such as defect levels and their carrier capture cross sections, which are
associated with the carrier lifetime and diffusion length, can be analyzed. In this subsection,
the principles underlying the generation and interpretation of CL signals are summarized.
The detailed descriptions of the principles including the pioneer work in 1978 (Petroff et al,
1978) are provided in a review (Yacobi and Holt, 1990).
2.1.1 Spatial resolution of CL measurements
Electrons irradiated into a material can undergo elastic and inelastic scattering. The
irradiated material is excited via inelastic electron scattering, and this excitation results in
the formation of x-rays, Auger electrons, secondary electrons, electron-hole pairs, and so
forth. CL lights are emitted via the recombination of electron-hole pairs. The spatial
resolution of CL measurements is, therefore, determined by the distribution of electron-hole
pairs.
When electrons are irradiated into a material, each electron changes its direction via an
elastic scattering, and it reduces its kinetic energy via an inelastic scattering. As a result of
the scattering processes, the original trajectories of the electrons are randomized. For a thin
solid material through which most incident electrons can transmit, used as a specimen in a
TEM, the shape of the electron penetration range (a so called generation volume) is conical
and the radius of a generation volume, which is the maximum at the electron exit surface, is
determined (Goldstein, 1979). Electrons and holes are generated inside a generation volume,
via some electron-electron interactions. They can diffuse in a material, and the distribution
of electron-hole pairs is dominated by the diffusion of minority carriers. The stationary
density of minority carriers at a position r, Δ n(r) obeys the differential equation of
continuity, div[grad Δ n(r)] D - Δ n(r)/τ(r) + g(r) = 0, in which D is the diffusion constant for minority carriers, τ is the mean recombination lifetime, and g is the generation rate of
electron-hole pairs. The distribution of minority carriers has been discussed theoretically
In-Situ Analysis of Optoelectronic Properties of Semiconductor Nanostructures
and Defects in Transmission Electron Microscopes
243
(e.g., Everhart & Hoff, 1971, Donolato & Venturi, 1982), and the range in which minority
carriers exist is expected to be twice as large as a generation volume at most, for thin
materials. Therefore, the spatial resolution of CL measurements can be approximated to the
maximum diameter of a generation volume. The typical resolution is the order of 102 nm.
2.1.2 CL spectroscopy and analysis
CL lights are emitted via various radiative electronic transitions. One transition is the
recombination between an electron in the conduction band and a hole in the valence band (a
band-to-band transition), which is typical in direct gap semiconductors at high
temperatures. At a temperature T at which kT ( k is the Boltzma