Transcript A Multi-Standard Mobile Digital Video Receiver in 0.18um

NATURE | VOL 408 | 23 NOVEMBER 2000 |www.nature.com
Optical gain in silicon nanocrystals
L. Pavesi*, L. Dal Negro*, C. Mazzoleni*, G. FranzoÁ² & F. Priolo²
* INFM & Dipartimento di Fisica, UniversitaÁ di Trento, Via Sommarive 14, 38050 Povo, Italy
² INFM & Dipartimento di Fisica e Astronomia, UniversitaÁ di Catania, Corso Italia 57, 95129 Catania, Italy
2011. 04. 27.
Kim Yeo-myung
RFAD LAB, YONSEI University
CONTENTS
 I. Introduction
 II. Silicon nanocrystals
 III. Light Amplification
 IV. Gain cross-section per nanocrystal
 V. Origin of gain
 VI. Conclusion
RFAD LAB, YONSEI University
Introduction
 Light emission from silicon at R.T.
– 1. Low-Dimensional
– 2. Active impurities (such as erbium)
– 3. New phases (such as iron disilicide)
 Low Dimensional Silicon system are being actively investigated
as a means of improving the light-emission properties of
silicon.(quantum confinement) → But, Silicon laser has
remained unlikely.
 To produce a silicon-based laser, we should demonstrate its
light amplification or stimulated emission
– Efficient free carrier absorption → reduces the net gain
– Auger saturation of the luminescence intensity at high power
– significant size-dependence of the radiative energies in Si
nanostructures → inhomogeneous broadening and optical losses
 Light Amplification using silicon, in the form of quantum dots
dispersed in silicon dioxide matrix!!
RFAD LAB, YONSEI University
Silicon Nanocrystals
 Low-dimensional silicon nanocrystals have been produced by
– 1. Negative ion implantation into ultra-pure quartz substrates
– 2. Into thermally grown silicon dioxide layers on Si substrates, followed
by high-temperature thermal annealing.
Silicon Nanocrystals
 Absorbance and luminescence spectra at R.T for quartz sub.
– A single wide emission band peaked at 800 nm
– characteristic of the radiative recombination of carriers in Si
nanocrystals
– The rising edge is due to absorption in the quantum confined states of
the nanocrystals
– the peculiar feature of the near-infrared absorption band is caused by a
Si.O interface state
 Interface state : interface between Si nanocrystals and SiO2
matrix
– Microscopic nature of these interface states is still under debate
– We note the spectral coincidence of the emission band and the interface
state absorption band, suggesting that radiative emission in Si
nanocrystals occurs through a radiative state associated with the
nanocrystal-oxide interface
Silicon Nanocrystals
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Figure 3(suplement): Electric field profile for the fundamental TE mode of a four layers waveguide at a wavelength
of 0.8 μm. The waveguide was formed by a 0.1 μm thick NS layer on top of a quartz substrate and capped by a
0.06 μm thick SiO2 layer. The external medium was air. The effective refractive index of the NS implanted region,
for the full line plot, was estimated by using the Maxwell Garnett approximation , which is valid for spherical
particles of dielectric constant e (here we use e =15.21 as for Si) embedded in a medium of dielectric constant e M
(here we use e M =2.102 as for SiO2) with a volumetric fraction f=0.28 (corresponding to Si nanocrystals with a
diameter of 3 nm and a density of 2x1019 cm-3) and yielded an effective dielectric constant =3.57. Then =1.89. For
the dotted line, we used an effective refractive index for the NS layer of 1.71 which was measured by ellipsometry
on PE-CVD deposited NS. The profile of the refractive indexes of the two resulting structures is also shown. The
optical filling factor of the mode is defined as G.
Light Amplification
Modal Gain: which is the material gain adjusted to
take into account the poor overlap that always exists
between the optical mode and the electron envelope
function in the quantum well
 Veriable strip length method : to measure light amplification
The sample is optically excited by a
doubled Ti:sapphire laser beam
(ʎ=390 nm, 2-ps pulse width, 82-MHz
repetition rate) in a stripe-like geometry
with variable length (l)
The amplified spontaneous emission
intensity IASE that is emitted from the
sample edge (observation angle = 0) is
measured as a function of l
By assuming a onedimensional amplifier model,
IASE can be related to g by
(Modal gain)
Light Amplification
 At L < 0.05 cm,
– Exponential increase of IASE is observed that indicates the occurrence
of amplified spontaneous emission.
 At L > 0.05 cm,
– IASE saturates as expected for any finite power supply amplification
mechanism
 At low power density, we measured absorption
– when the pump power was increased, the peak net modal gain
increased and then saturated at values of about 100 cm-1
Light Amplification
 Gain spectrum of measuring the amplified signal for various
wavelengths
spectrally overlaps the
wavelength range of the
luminescence → demonstrating
that amplification is produced by
the radiative state associated
with the nanocrystal-oxide
interface
both samples yielded similar
shapes and values for the gain
curve
Light Amplification
 Amplified spontaneous emission spectra of sample A for
different measurement conditions.
Light Amplification
 Direct evidence of light amplification from our systems was
provided by pump and probe transmission measurements
In the presence (absence) of the
pump beam the probe beam is
amplified (absorbed) (population
inversion)
This is the first evidence of light
amplification in transmission,
usually named single-pass gain, in
Si-based systems
They are high enough to compare
with those of self-assembled
quantum dots made of III-V
semiconductors
Gain cross-section per nanocrystal
 To compare gain cross-section per nanocrystal with the photon
absorption cross-section per nanocrystal.
gain cross-section per nanocrystal
= 5 X 10-16 cm2
absorption cross-section per
nanocrystal = 3 X 10-16 cm2
Another issue concerns the
comparison of the gain crosssections that are derived from the
modal and the material gain.
Modal gain = 5 X 10-16 cm2
Material gain = 3 X 10-16 cm2
Origin of gain
 Three level model to explain the observed gain
Third-level :
Due to the radiative interface
state observed in absorption
and responsible for the
luminescence emission band
at 800 nm
Electrons from the LUMO
relax very rapidly to the
interface state. Electrons in
the interface state have long
lifetimes
Conclusion
 Modal and net material optical gains have been
observed unambiguously in Si nanocrystals.
 Quantitative estimates of gain crosssection per
nanocrystal are orders of magnitude lower than
those found in III-V semiconductor quantum dots.
 However, owing to the much higher stacking density
of Si nanocrystals with respect to direct-bandgap
quantum dots, similar values for the material gain
are observed.
 These findings open a route towards the realization
of a silicon-based laser.
RFAD LAB, YONSEI University
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Quantum dot
From Wikipedia, the free encyclopedia
A quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions.
Consequently, such materials have electronic properties intermediate between those of bulk
semiconductors and those of discrete molecules.[1][2][3] They were discovered at the beginning of
the 1980s by Alexei Ekimov[4] in a glass matrix and by Louis E. Brus in colloidal solutions. The
term "quantum dot" was coined by Mark Reed.
Researchers have studied quantum dots in transistors, solar cells, LEDs, and diode lasers. They
have also investigated quantum dots as agents for medical imaging and hope to use them as
qubits.
Stated simply, quantum dots are semiconductors whose electronic characteristics are closely
related to the size and shape of the individual crystal. Generally, the smaller the size of the crystal,
the larger the band gap, the greater the difference in energy between the highest valence band
and the lowest conduction band becomes, therefore more energy is needed to excite the dot, and
concurrently, more energy is released when the crystal returns to its resting state. For example, in
fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of
the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light
emitted. In addition to such tuning, a main advantage with quantum dots is that, because of the
high level of control possible over the size of the crystals produced, it is possible to have very
precise control over the conductive properties of the material.[5] Quantum dots of different sizes
can be assembled into a gradient multi-layer nanofilm.
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Quantum confinement in semiconductors
Main article: Potential well
In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a
characteristic length, called the exciton Bohr radius. This is estimated by replacing the positively
charged atomic core with the hole in the Bohr formula. If the electron and hole are constrained
further, then properties of the semiconductor change. For example, the absorption and emission
wavelength of light shifts towards smaller wavelengths.[6] This effect is a form of quantum
confinement, and it is a key feature in many emerging electronic structures.[7][8]
Besides confinement in all three dimensions i.e. Quantum Dot - other quantum confined
semiconductors include:
quantum wires, which confine electrons or holes in two spatial dimensions and allow free
propagation in the third.
quantum wells, which confine electrons or holes in one dimension and allow free propagation in
two dimensions.
Radiative recombination
Radiative recombination is the process by which an ion in state-i binds an electron from the
electron sea to produce state-(i-1) with the subsequent radiation of photons. Both continuum and
line photons can be produced in such a recombination event as the electron passes from the
continuum (i.e. free) levels into the upper bound levels of the ion and then cascades down to form
a ground state ion.
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Nanocrystal
From Wikipedia, the free encyclopedia
Fahlman, B. D. has described a nanocrystal as any nanomaterial with at least one dimension ≤
100nm and that is singlecrystalline.[1] More properly, any material with a dimension of less than 1
micrometre, i.e., 1000 nanometers, should be referred to as a nanoparticle, not a nanocrystal. For
example, any particle which exhibits regions of crystallinity should be termed nanoparticle or
nanocluster based on dimensions. These materials are of huge technological interest since many
of their electrical and thermodynamic properties show strong size dependence and can therefore
be controlled through careful manufacturing processes.
Crystalline nanoparticles are also of interest because they often provide single-domain crystalline
systems that can be studied to provide information that can help explain the behaviour of
macroscopic samples of similar materials, without the complicating presence of grain boundaries
and other defects. Semiconductor nanocrystals in the sub-10nm size range are often referred to
as quantum dots.
Crystalline nanoparticles made with zeolite are used as a filter to turn crude oil onto diesel fuel at
an ExxonMobil oil refinery in Louisiana, a method cheaper than the conventional way.
A layer of crystalline nanoparticles is used in a new type of solar panel named SolarPly made by
Nanosolar. It is cheaper than other solar panels, more flexible, and claims 12% efficiency.
(Conventionally inexpensive organic solar panels convert 9% of the sun's energy into electricity.)
Crystal tetrapods 40 nanometers wide convert photons into electricity, but only have 3% efficiency.
(Source: National Geographic June 2006)
The term NanoCrystal is a registered trademark[2] of Elan Pharma International Limited (Ireland)
used in relation to Elan’s proprietary milling process and nanoparticulate drug formulations.
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Inhomogeneous Broadening
Inhomogeneous broadening is an increase of the Linewidth of an atomic transition caused by
effects which act differently on different radiating or absorbing atoms. This can be caused e.g. by
the different velocities of the atoms of a gas, or by different lattice locations of atoms in a solid
medium. Inhomogeneous broadening is strongly related to Inhomogeneous Saturation in Laser
Gain media.
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Tansmission electron microscopy
From Wikipedia, the free encyclopedia
Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of
electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes
through. An image is formed from the interaction of the electrons transmitted through the
specimen; the image is magnified and focused onto an imaging device, such as a fluorescent
screen, on a layer of photographic film, or to be detected by a sensor such as a CCD camera.
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Definition of Gain
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Optical Gain: In terms of the difference between the stimulated emission and absorption rates.
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Modal Gain: which is the material gain adjusted to take into account the poor overlap that always
exists between the optical mode and the electron envelope function in the quantum well. (I.e:
modal gain=material gain* confinement factor)
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Differential gain: The rate at which gain increases as we inject more carriers, dg/dN.