Prepared By Dr . Ahmed Shaaban Dessouki Faculty of Engineering ...

ECE 101
Introduction to Photonic Devices
Prepared By
Dr . Ahmed Shaaban Dessouki
Faculty of Engineering – Port Said University
Dept. of Electrical Engineering
Electronics and Communications Section
Page 1

ECE 101
Introduction to Photonic Devices
Chapter (4): Introduction ِ
to Optoelectronic Devices
4.1 Introduction
Optoelectronics is the term for combined technologies of optics and
electronics.
Optoelectronic devices are devices that emit or detect optical radiation. They
considered electrical-to-optical or optical-to-electrical transducers, or instruments that
use such devices in their operation.
Optoelectronics has become an important part of our lives. Wherever light is
used to transmit information, tiny semiconductor devices are needed to transfer
electrical current into optical signals and vice versa. Examples include light-emitting
diodes in radios and other appliances, photodetectors in elevator doors and digital
cameras, and laser diodes that transmit phone calls through glass fibers. Such
optoelectronic devices take advantage of sophisticated interactions between electrons
and light.
4.1.1 The Visible Light Spectrum:
The visible light spectrum is the section of the electromagnetic radiation
spectrum that is visible to the human eye. It ranges in wavelength from
approximately 400 nm (4 x 10 -7 m) to 700 nm (7 x 10 -7 m). It is also known as the
optical spectrum of light.
The wavelength (which is related to frequency and energy) of the light
determines the perceived color. The ranges of these different colors are shown below.
The edges of the visible light spectrum blend into the ultraviolet and infrared levels
of radiation.
Page 2

ECE 101
Introduction to Photonic Devices
4.1.2 Characteristics of Light Wave:
Page 3

ECE 101
Introduction to Photonic Devices
41.3 Photon Nature of Light:
Under the photon theory of light, a photon is a discrete bundle (or quantum) of
electromagnetic (or light) energy. Photons are always in motion and, in a vacuum,
have a constant speed of light to all observers, at the vacuum speed of light (more
commonly just called the speed of light) of c = 2.998 x 108 m/s. However, in the
presence of matter, a photon can be absorbed, transferring energy and momentum
proportional to its frequency. Like all quanta, the photon has both wave and
particle properties.
According to the photon theory of light, the basic properties of photons are:
Move at a constant velocity, c = 2.9979 x 10 8 m/s (i.e. "the speed of light"), in
free space
Have zero mass and rest energy.
Carry energy and momentum, which are also related to the frequency.
Can be destroyed / created when radiation is absorbed / emitted.
The photon is massless, has no electric charge and does not decay
spontaneously in empty space.
The characteristics of a photon in free space are summarized below:
Page 4

ECE 101
Introduction to Photonic Devices
The energy of a photon that has a wavelength λ in free space can be calculated
using the following formula:
For example, at an optical wavelength of 1 μm, the photon energy is 1.2398 eV. The
energy of a photon is determined only by the frequency, or wavelength, of light, but
not by its intensity. The intensity of light is related to the flux density, or number per
unit time per unit area, of photons by:
EXAMPLE: It is found that a piece of crystal transmits light at λ = 500 nm but
absorbs light at λ = 400 nm. Make an intelligent guess of its bandgap from this
limited information.
Solution: Because a crystal transmits photons with energies below its bandgap but
absorbs those with energies above its bandgap, we can reasonably guess that the
bandgap of this crystal falls between the photon energies corresponding to 500 and
400 nm wavelengths. Using:
for the photon energy, we find that:
Assignment:
1. GaAs has an energy bandgap of 1.424 eV at room temperature and absorbs any
photon that has energy higher than this value. For what optical wavelengths is
GaAs transparent?.
2. A photon of 10.6 μm wavelength is combined with a photon of 1.06 μm
wavelength to create a photon that combines the energies of both. What is the
wavelength of the resultant photon?.
Page 5

ECE 101
Introduction to Photonic Devices
4.2 Optical (Radiative) Transitions:
Absorption: exciting an electron to a higher energy level by absorbing a photon.
Emission: electron relaxing to a lower energy state by emitting a photon.
Optical absorption and emission occur through the interaction of optical
radiation with electrons in a material system that defines the energy levels of the
electrons. In any event, the absorption or emission of a photon by an electron is
associated with a resonant transition of the electron between a lower energy level >1<
of energy E 1 and an upper energy level >2< of energy E 2 , as illustrated in the
following Figure. The resonance frequency, ν 21 , of the transition is determined by the
separation between the energy levels:
There are basically three types of processes associated with resonant optical
transitions between two energy levels in a system: absorption, stimulated emission,
and spontaneous emission, which are illustrated in the above Figure (a), (b), and (c),
respectively.
In other words, there are basically three processes for interaction between a
photon and an electron in a solid: absorption, spontaneous emission, and
stimulated emission.
We shall use a simple system to demonstrate these processes. Consider two
energy levels E 1 and E 2 of an atom; where E 1 corresponds to the ground state and E 2
corresponds to an excited state.
In quantum mechanics an excited state of a system (such as an atom, molecule or nucleus) is
any quantum state of the system that has a higher energy than the ground state (that is, more energy
than the absolute minimum).
Page 6

ECE 101
Introduction to Photonic Devices
An atom will go into an excited state when the electrons are given extra energy. Then after
the electrons have been excited it will eventually go back to ground state producing a light as it
returns to its normal state.
Any transition between these states (E 1 & E 2 ) involves the emission or
absorption of a photon with frequency ν given by hν 12 = E 2 - E 1 .
At room temperature, most of the atoms in a solid are at the ground state. This
situation is disturbed when a photon of energy exactly equal to hν 12 impinges on the
system. An atom in state E 1 absorbs the photon and thereby goes to the excited state
E 2 . The change in the energy state is the absorption process, shown in the above Fig.
(a).
The excited state of the atom is unstable; and after a short time, without any
external stimulus, it makes a transition to the ground state, giving off a photon of
energy hν 12 . This process is called spontaneous emission (c).
When a photon of energy hν 12 impinges on an atom while it is in the excited
state (b), the atom can be stimulated to make a transition to the ground state and gives
off a photon of energy hν 12 , which is in phase with the incident radiation. This process
is called stimulated emission. The radiation from stimulated emission is
monochromatic because each photon has energy of precisely hν 12 and is coherent
because all photons emitted are in phase.
Page 7

ECE 101
Introduction to Photonic Devices
The dominant operating process for the light-emitting diode (LED) is spontaneous
emission; for the laser, it is stimulated emission; and for the photo-detector
and the solar cell, it is absorption.
4.2.1 Optical Absorption:
In physics, absorption of electromagnetic radiation is the way by which the
energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the
electromagnetic energy is transformed to other forms of energy for example, to heat.
Therefore, Absorption is associated with induced transitions between the
energy levels caused by interaction of an electron with the existing optical radiation.
The follwing Figures show the basic transitions in a semiconductor. When the
semiconductor is illuminated, photons are absorbed to create electron-hole pairs as
shown at (a) if the photon energy is equal to the bandgap energy, that is, hv equals
E g . If hv is greater than E g , an electron-hole pair is generated and, in addition, the
excess energy (h v - E g ) is dissipated as heat as shown at (b). Both processes, (a) and
(b), are called intrinsic transitions (or band-to-band transitions). On the other hand,
for hv less than E g , a photon will be absorbed only if there are available energy states
in the forbidden bandgap due to chemical impurities or physical defects as shown at
(c), (d), and (e). Process (c) (as (d) and (e)) is called extrinsic transition. This
discussion also is generally true for the reverse situation. For example, an electron at
the conduction band edge combining with a hole at the valence band edge will result
in the emission of a photon with energy equal to that of the bandgap.
Page 8

ECE 101
Introduction to Photonic Devices
• Absorption simply leads to the attenuation of an optical signal.
Assume that a semiconductor is illuminated from a light source with hv greater
than E g and a photon flux of Ф o (in units of photons per square centimeter per
second). As the photon flux travels through the semiconductor, the fraction of the
photons absorbed is proportional to the intensity of the flux. Therefore, the number of
photons absorbed within an incremental distance Δx, as shown in the ollowing
Figure, is given by αФ(x) Δx, where α proportionality constant is defined as the
absorption coefficient. From the continuity of photon flux as shown in the figure, we
obtain:
Page 9

ECE 101
Introduction to Photonic Devices
The absorption coefficient α is a function of hv. The followin Figure shows the
measured absorption coefficient for some important semiconductors that are used for
photonic devices. Also shown is the absorption coefficient for amorphous silicon
(dashed curve), which is an important material for solar cells. The absorption
coefficient decreases rapidly at the cutoff wavelength Ac; that is,
Page 10

ECE 101
Introduction to Photonic Devices
Solution: From the previous chart the absorption coefficient is 4x10 4 cm -1 . The
energy absorbed per second is:
Assignment:
A gallium arsenide sample is illuminated with a light having a wavelength of
0.6 µm. The incident power is 15mW. If one thrid of the incident power is reflected
and another thrid exit from the other end of the sampl, what is the thickness of the
sample? Find the thermal energy dissipated to the lattice per second.
Page 11

ECE 101
Introduction to Photonic Devices
Applications:
The solar cell is an application example of light absorption, since it works in
three steps:
1. Photons in sunlight hit the solar panel and are
absorbed by semiconducting materials, such as
silicon.
2. Electrons (negatively charged) are knocked loose
from their atoms, allowing them to flow through
the material to produce electricity. Due to the
special composition of solar cells, the electrons are
only allowed to move in a single direction.
3. An array of solar cells converts solar energy into a
usable amount of direct current (DC) electricity.
5.2.2 Stimulated Emission of Photons:
Stimulated emission of photons is associated with induced transitions between
the energy levels caused by interaction of an electron with the existing optical
radiation. If an electron is initially in the lower level >1221

ECE 101
Introduction to Photonic Devices
and phase as the photon triggering this process. The stimulated emission therefore
amplifies the density of photons in the lasing mode, where, emitted photons will be
confined to the ridge waveguide. The stimulated emission process yields an increase
in photons as they travel along the waveguide.
When a sizable population of electrons resides in upper levels, this condition is
called a "population inversion", and it sets the stage for stimulated emission of
multiple photons. This is the precondition for the light amplification which occurs in
a laser, and since the emitted photons have a definite time and phase relation to each
other, the light has a high degree of coherence.
Optical absorption results in attenuation of an optical field, while stimulated
emission leads to amplification of an optical field.
This applet (shown on the right) illustrates a schematic operation of a laser.
The yellow photons represent the pumping radiation.
The group of red photons is the coherent laser beam.
The balls mark the atoms making transitions between
three energy levels.
The pumping radiation causes the transition of
atoms from the ground state to the high energy
excited state. From this short-living state the atoms
go by non-radiative transition to the long-living metastable state. Once in the
metastable state many atoms can be accumulated. The laser beam, stimulated
emission, arises when all atoms simultaneously make a transition to the ground state.
Page 13

ECE 101
Introduction to Photonic Devices
5.2.3 Spontaneously Emission of Photons:
Spontaneous emission is the process by which an electron initially in the upper
level >2< can spontaneously relax to the lower level >1< by emitting a spontaneous
photon, irrespective of the presence, or absence, of any existing optical radiation.
Spontaneously emitted photons are random in phase and polarization and are
emitted in all directions, though their frequencies are still dictated by the separation
between the two energy levels.
Spontaneous emission of light or luminescence is a fundamental process that
plays an essential role in many phenomena in nature and forms the basis of many
applications, such as fluorescent tubes, and light emitting diodes.
This applet (on the right) illustrates the absorption and emission of photons by
an atom. An electron revolving around the
nucleus may capture an incident photon.
Once that occurs, the electron is raised to a
higher energy level and thus changes its
orbit to a one with a larger radius. While
being in the excited state, the electron
reemits the photon after some time and
returns to the ground state. Note that one
can observe the phenomenon involving the
emission of the photon and its immediate recapture upon change of orbit (virtual
photon emission).
Page 14

ECE 101
Introduction to Photonic Devices
4.3 Semiconductors for photonic devices:
4.3.1 Introduction:
A semiconductor can be an elemental material or a compound material. The
group IV elements Si and Ge are elemental semiconductors. Crystalline C can take
the form either of diamond, which is more an insulator than a semiconductor
because of is large bandgap of 5.47 eV at room temperature, or graphite, which is a
semimetal. Though C is not a semiconductor, Si and C can form the IV–IV
compound semiconductor SiC, which has many different structural forms with
different bandgaps. Si and Ge can be mixed to form the IV–IV alloy semiconductor
Si x Ge 1-x . These group IV crystals and IV–IV compounds are indirect-gap materials.
The most important semiconductors for photonic devices, however, are the
III–V compound semiconductors, which are formed by combining group III
elements, such as Al, Ga, and In, with group V elements, such as N, P, As, and Sb. A
binary compound consists of two elements. There are more than ten binary III–V
semiconductors, such as GaAs, InP, AlAs, and InSb. Different binary III–V
compounds can be alloyed with varying compositions to form mixed crystals of
ternary compound alloys and quaternary compound alloys. A ternary III–V
compound consists of three elements, two group III elements and one group V
element, such as Al x Ga 1-x As, or one group III element and two group V elements,
such as GaAs 1-x P x . A quaternary III–V compound consists of two group III elements
and two group V elements, such as In 1-x Ga x As 1-y P y .
A III–V compound can be either a direct-gap or an indirect-gap material. A
III–V compound with a small bandgap tends to be a direct-gap material, whereas one
with a large bandgap tends to be an indirect-gap material.
Among the III–V compounds, the nitrides are quite unique. The binary nitride
semiconductors AlN, GaN, and InN, as well as their ternary alloys such as InGaN,
are all direct-gap semiconductors. These direct-gap semiconductors form a complete
series of materials that have bandgap energies ranging from 1.9 eV for InN to 6.2 eV
for AlN, corresponding to the spectral range from 650 to 200 nm. Therefore, the
nitride compounds and their alloys cover almost the entire visible spectrum and
extend to the ultraviolet region. They are particularly important for the development
of semiconductor lasers, light-emitting diodes, and semiconductor photodetectors in
the blue, violet, and ultraviolet spectral regions.
Page 15

ECE 101
Introduction to Photonic Devices
D, direct gap; I, indirect gap.
Page 16

ECE 101
Introduction to Photonic Devices
In the following, the properties of two important systems, namely, Al x Ga 1-x As
lattice matched to a GaAs substrate (the lattice constants) and In 1-x Ga x As y P 1-y lattice
matched to an InP substrate (the lattice constants), are summarized.
4.3.2 Al x Ga 1-x As / GaAs:
Over the entire composition range of 0 ≤ x ≤ 1, Al x Ga 1−x As is closely, though
not perfectly, lattice matched to GaAs. Because this ternary compound is an alloy of
indirectgap AlAs and direct-gap GaAs, it is a direct-gap semiconductor for small
values of x in the range 0 ≤ x < 0.45 but becomes an indirect-gap semiconductor for
large values of x in the range 0.45 < x ≤ 1. Its bandgap in electron volts at 300 K as a
function of the composition parameter x can be described by:
5.3.3 In 1-x Ga x As y P 1-y / InP:
The In 1-x Ga x As y P 1-y quaternary compounds that are lattice matched to InP are
directgap semiconductors over the entire lattice-matched composition range of
0 ≤ y ≤ 1 and x = 0.47y. At 300 K, the bandgap in electron volts as a function of the
composition parameter y is given by:
Page 17

ECE 101
Introduction to Photonic Devices
EXAMPLE: An InGaAsP quaternary compound that is lattice matched to InP
at 300 K has a bandgap optical wavelength of λg = 1.223 μm. Find the energy of its
bandgap. What is the composition of this quaternary compound?.
Assignments:
P. 1 Does the ternary compound Al 0.3 Ga 0.7 As have a direct or an indirect
bandgap?. What are its bandgap E g and the corresponding optical wavelength λg?.
P. 2 Answer the questions asked in P.1 for Al 0.7 Ga 0.3 As.
P. 3 The quaternary compound In 0.61 Ga 0.39 As 0.83 P 0.17 is lattice matched to InP at
300 K. Is it a direct-gap or an indirect-gap semiconductor? What are its bandgap
E g and the corresponding optical wavelength λg?.
P. 4 Find the compositions of the two InGaAsP quaternary compounds that are
both lattice matched to InP at 300 K and have bandgap optical wavelengths of
λg = 1.007 and 1.095 μm, respectively.
Page 18