Transcript PowerPoint 簡報

Part 3. Semiconductor Materials for Optoelectronic
Application
 The major semiconductor materials used for
optoelectronic applications are III-V and
II-V group. Group V materials are used in some cases,
but mostly in indirect applications.
 The reason for group III-V to be popular in
optoelectronic applications is due to the fact that most
III-V materials are direct bandgap semiconductors,
which is an necessary condition for efficient conversion
of electric energy to light emission.
 The integration of light source with the photonic devices
is desired => photonic devices of III-V system.
 III-V semiconductor materials:
Column III elements: Al, Ga, and In and column V
elements: N, P, As, Sb. In general, nitride is not
included in typically claimed III-V compound
semiconductor (discussed separately).
 The main optical process of importance in
optoelectronic applications are reflection,
waveguiding, diffraction, absorption, emission, and
electrooptics and nonlinear optical effect!
 parameters to express above properties are refractive
index (n), absorption coefficient (), and direct
bandgap energy (Eg or E). Some are indirect
bandgap materials with indirect bandgap energy
(EX or EL)
Solid line:
direct bandgap
materials
Dotted line:
indirect bandgap
materials
Matched system to
reduce the strain
effect and epitaxial
growth defects!
III-V Material systems with important optoelectronic applications
Materials substrate Lattice matched Important strained Main optoelectronic
system
members
members
applications
AlGaAs
GaAs
GaInAsP
/InP
InP
AlGaInAs
/InP
AlGaInP
GaAs
AlxGa1-xAs
AlAs
Ga0.47In0.53As
GaxIn1-xAsyP1-y
x=0.47y; 0 y 1
InP
InP
Ga0.47In0.53As
(AlxGa1-x)0.47In0.53As
0x1
Al0.48In0.52As
GaAs GaAs
Ga0.5In0.5P
(AlxGa1-x)0.5In0.5P
0x1
Ga1-xInxAs
0  x  0.25
Ga1-xInxAs
0.4  x  0.6
InAsxP1-x
0  x  0.2
Ga1-xInxAs
0.4  x  0.6
Ga1-xInxAs
0  x  0.25
Ga1-xInxP
0.4  x  0.6
Emitter and modulators
0.75 m    1.1 m
Detectors:
0.4 m    1.1 m
Optoelectronic devices
at  = 1.3 m and
 = 1.55 m
Optoelectronic devices
at  = 1.3 m and
 = 1.55 m
Red emitter
III-V Material systems with important optoelectronic applications
Materials substrate Lattice matched Important strained Main optoelectronic
system
members
members
applications
AlGaAsSb
/GaInAsSb
/GaSb
GaSb
GaAsP
GaAs
InP
GaSb
AlxGa1-xAsySb1-y
x = 12y; 0 x 1
AlxIn1-xAsySb1-y
x = 1.1y; 0 x 1
GaAs
GaP
Emitter and Detectors
 ~ 2-3 m
GaAsP
Visible LED’s
 Quantum Wells and Strained Materials:
The Optical properties of a semiconductor are altered
by quantum size effects; at least one of the
dimensions of material is on the order of De Broglie’s
wavelength of an electron:  = h/p; if p ~ eV =>
 = ~ a few nm;
 1D confinement: quantum wells; structures
consisting of a thin well materials sandwiched
between two layers of a barrier materials
 2D confinement: quantum wires; structures
consisting of a thin and narrow well materials
surrounded by barrier materials
 3D confinement: quantum dots; nano-size particles
in a barrier materials.
The quantum confinement => allowed electron and
hole states are quantized in the well region => energy
required to generate e-h pair or radiation emitted from
the process of e-h pair recombination is modified
=> wavelength tuning of the radiation
(used in LED or laser applications)
A B
 As a light source, the efficiency of the source is
strongly influenced by defects (line defects, etc) =>
crystal A and B should be grown as perfect as
possible (epitaxial system)
 Typically, A and B will not have the same lattice
constant => strained system => stability issue of the
system (relaxation and chemical aspect) and critical
thickness for the epitaxial growth become important!
 Strained epitaxial semiconductors can be used in
high speed electronic applications: HEMTs, HBTs!
 Typically, the resonator have to be constructed with
the same kind of material system used for light
source (for possible epitaxial relation) => easier to
integrate without affecting the quality of the active
materials.
 Optical efficiency determined by
(a) bandgap type: direct or indirect
(b) minority carrier lifetime: t ~ 1ms (for Si,Ge),
t ~ 1ns (for GaAs)
 Introduction of impurities in some indirect bandgap
materials=> efficient recombination; e.g.
isoelectronic N impurities in GaP
 Epitaxy: process of depositing thin layers of singlecrystal compounds onto a single crystal substrate.
Substrate
Device
Testing
Epitaxy
Device
Packaging
Material
Characterization
Reliability
Screening
Key Technology areas
Device
Processing
Final Packaged
Device
 Epitaxial growth techniques: liquid phase epitaxy
(LPE) (for low end product); metal organic vapor
phase epitaxy (MOVPE), and molecular beam
epitaxy (MBE).
 Light-Emitting Devices:
p
hn
n
+V
-V
Holes
Electrons
Active Layer
(e.g. GaAs)
Cladding Layers (e.g. GaAlAs)
Conduction
band
Efn
Efn
hn
Valence
band
Double heterostructure p-n
junction.
Energy E = hn or
E(eV) = 1.24/(m)
Refractive
Index
 Spontaneous emission: random recombination of
electrons and holes => light is emitted at a
wavelength corresponding to the band energy, but
random in phase => light emitting diodes (LEDs).
 Stimulated emission: a photon of light traveling
through the semiconductor interacts with the
electron and hole population to cause the radiative
recombination of another electron-hole pair => light
is emitted at a wavelength corresponding to the band
energy with the same phase => lasers.
 Light is absorbed within the semiconductor to
produce electron-hole pairs => photodetectors.
Light with energy smaller than the bandgap of the
semiconductor will not be absorbed.
Structures of LED (Important ones):
Light output
n
Dome LED
ohmic
contacts
diffused
p-type
Light output
p type
epitaxial layer
n type
substrate
Planar LED
ohmic contacts
 Dome and planar LED are used in most display
devices where the interest is in extracting the
maximum amount of light from the device. =>
light is emitted in all directions and using a lens
arrangement to focus the light.
 Burrus and edge-emitting LED are used mainly in
optical fiber communication systems.
Burrus LED
Multimode
optical fiber
250 m
~ 250 m
Epoxy
resin
Metal tab
Etched
well
50m
50 m
n-AlGaAs
p-GaAs
p-AlGaAs
p+-GaAs
SiO2
n-GaAs substrate
Metal contact
SiO2
Gold stud
Metal contact
~ 50 m
Primary lightemitting region
Edge-emitting
LED
p+-AlGaAs
p-AlGaAs
AlGaAs (Active layer)
n-AlGaAs
n-GaAs
Carrier confinement layers :
p-AlGaAs and n-AlGaAs
60
T
Output power is typically linear
with the drive current.
Advantages of these devices: ease of
modulation, long lifetime, low cost,
and high yield
0
0 Current (mA)100
* Semiconductor Lasers:
Laser has a much narrower
spectral range and a much
more intense light output (at
least 100 times more intense
than LED)
Relative optical power
Optical Power Output (W)
 Operation: apply a suitable voltage (5V) => a
forward current of between 5 and 100 mA.
Diode
Laser
LED
< 3 nm
~ 75 nm
Wavelength (nm)
 Create a lasing cavity that acts as an extremely high
Q resonator. The cavity is usually created by the
formation of mirrors at each end of the laser device.
 Mirrors: cleaving along the crystallographic plane
[(110)] => abrupt refractive index change at the
semiconductor-air interface (refreactivity ~ 0.33).
 The semiconductor between the two mirrors forms
the laser cavity.
 A high rate of stimulated emission => optical gain g.
For lasing to occur a further threshold condition
must be met which is that the round trip gain of a
photon is greater than unity.
Light output (mW)
7
0
Slope gives
external
efficiency 
0
T
Ith
Current (mA)
Lasing
emission
40
Spontaneous
emission
 In general, a reduction in the threshold current, an
increase in the total light output, and an increase in
the external quantum efficiency all leads to
improved lasing devices
Some Laser Structures:
Classical buried
heterostructure
(BH) Laser
Double channel
planar burried
heterostructure
(DCPBH) Laser
Distributed
feedback
(DFB)laser
Simple oxide stripe DH
(double Heterostructure)
 Present designs are almost exclusively the double
heterostructure type with many variations used to
constrain the device to operate in a single lateral
(transverse) mode.
 DFB: single longitudinal mode operation can also
be achieved in order to obtain extremely narrow
linewidth emission by using diffraction gratings
placed adjacent to the active layer of the laser
* Optical detectors:
A device that changes its properties by the absorption
of light. A great variety of different types of optical
detector ranging from thermal and pneumatic
detectors to pyroelectric detectors. The most
important devices are semiconductor photodiodes.
p
hn
n
Electric field
-V
Distance (x)
Applying a suitable reverse bias
voltage to a simple p-n junction
+V
=> create an electric field profile
=> separation the photo-generated
electron-hole pairs (absorption of
light within the semiconductor).
Speed of response is determined by
the device capacitance => governed
by the thickness of depletion region
=> small area device and low doped
active regions for low capacitance,
i.e. high speed.
 Noise: low noise could be obtained by minimizing
any leakage current (typically surface leakage
current) by having a large band gap materials on the
surface.
 Integration of different materials in a single chip is
challenging. A lot of issues are related to materials
science: such as selective area epitaxy (SAE), ion
beam etching for the formation of laser cavities and
waveguide components, the depositing of insulating
materials, and metallization methodology .
 AlGaAs Materials system:
GaAs: direct bandgap materials
AlAs: indirect bandgap materials
E: direct bandgap;
EX: indirect bandgap
E (eV )  1.423  1.36 x  0.22 x 2
EX (eV )  1.906  0.207 x  0.55 x 2
For effective light emission
the x < 0.4in GaxAl1-xAs
Refractive index of
GaxAl1-xAs
First Brillouin zone of diamond structure
kz
X
L

X
kx
X
ky
 GaxIn1-xAsyP1-y Materials system:
RT Eg and refractive index
of GaxIn1-xAsyP1-y for
x = 0.47y, 0  y  1;
(lattice matched to InP!)
RT absorption spectra of Ga0.47In0.53As, GaInAsP/InP (g =
1.3m), and InP. Solid lines: experimental data; dashed lines:
a fit to  = constant  (E - Eg)1/2.
1/2
indirect
h
 AlGaInAs/InP Materials system:
RT Eg of AlxGa1-xIn0.53As
E g (eV)  0.76  0.49 x  0.2 x 2
RT refractive index of
AlxGa1-xIn0.53As
 AlGaInP Materials system: main application is red
diode lasere
EX : indirect
E : direct
RT Eg, refractive index, and
absorption coefficient of
(AlxGa1-x)0.52In0.48As
(match GaAs)
E (eV)  1.89  0.64 x
EX (eV)  2.25  0.09 x
 GaSb Materials system: two quaternaries, AlGaAsSb,
GaInAsSb lattice matched to GaSb substrate
 AlxGa1-xAsySb1-y = (GaSb)x(AlAs0.083Sb0.917)1-x
 Ga1-xInxAsySb1-y = (GaSb)1-x(InAs0.911Sb0.089)x
2.5
2.0
RT Eg of
AlxGa1-xAsySb1-y and
Ga1-xInxAsySb1-y;
at RT and x = 0, EL
(indirect) > E =>
GaSb is barely a direct
bandgap materials!
Solid lines: AlxGa1-xAsySb1-y; y = 0.083x
Dashed lines: Ga1-xInxAsySb1-y; y = 0.911x
AlxGa1-xAsySb1-y: E
Eg (eV)
1.5
AlxGa1-xAsySb1-y: EX
Ga1-xInxAsySb1-y:EX
AlxGa1-xAsySb1-y: EL
1.0
Ga1-xInxAsySb1-y:EL
0.5
0.0
0.0
Ga1-xInxAsySb1-y:E
0.2
0.4
0.6
composition, x
0.8
1.0
AlxGa1-xAsySb1-y is
nearly indirect in the
whole range!
RT refractive index of
AlxGa1-xAsySb1-y/GaSb
RT refractive index of
Ga1-xInxAsySb1-y/GaSb
 GaAsP Materials system:
RT Eg of GaAsxP1-x;
Crossover of directindirect is ~ x = 0.5
E g (eV)  1.428  1.125 x  1.952 x 2
Absorption coefficient
of GaAsxP1-x
 II-VI semiconductor materials:
 Wide spectrum of energy gaps (Eg) => wide range of
optoelectronic properties; ranging from the far
infrared to the UV
 Large Eg difference => large band offset => adds
variety and flexibility to bandgap engineering
 Compare to III-V, II-VI semiconductors have stronger
polarity (bonds have more ionic characteristics and
less covalent characteristics).
 Magnetic ions (Mn++ and Fe++) can be easily
incorporated => magnetic semiconductors
 II-VI semiconductors are mainly prepared using MBE
or MOCVD.
Energy gaps and lattice constants for cubic group IV,
III-V, and II-VI semiconductors.
 ZnSe based blue-green LED; the importance is
diminishing, due to the successful development of
GaN.
 Material issue is more complicated than group IV and
III-V semiconductor;
 doping is one important issue in II-VI semiconductor;
n-type doing is easier than p-type doping (for ZnSe
two promising dopants Ga and Cl); the difficulty in
doping any II-VI semiconductors arises intrinsically
from the size of the energy gap itself, large gap
require high energy to shift the Fermi surface =>
enough to promote compensation through defect
formation
 Ohmic contacts in another major problem.
ideally one wish to use metals with work functions
above the bottom of the conduction and on the n-type
semiconductor and below the top of the valence band
on the p-type semiconductor; as gap  => harder to
find proper metals => overcome the problem by (a)
heavily dope the semiconductor layer to which the
contact is to be made; (b) a graded alloy to move the
top of the valence band close to the metal
workfunction.
 Diluted magnetic
B
AB
semiconductor
A
graded
heterostructures; e.g.
ZnSe/Zn0.9Fe0.1Se, magnetic field => conduction band
and valence band are different for carriers with
different angular momentum!
DMS
DMS
-1/2
ZnSe
hh
+1/2
-3/2
Type II
semiconductor
+3/2
Type I
semiconductor
 So far the real application of II-VI is not that much,
however the system provides rich variety in
phenomenon for academic studies;
 SiC and GaN as optoelectronic Materials:
 The need to operate devices at high temperature =>
studies of wide bandgap semiconductors (SiC, GaN,
and diamond)
 The need for denser optical storage (light with shorter
wavelength) => blue laser or even UV laser (AlGaN).
Zinc-blende and wurtzite SiC
and GaN lattice constants vs.
the energy gap.
SiC 2H and SIC 6H:
polymorph
 SiC:
 a family of close-packed materials which exhibit a
1-D polymorphism (called polytypism)
SiC 6H
A B C A B C A
SiC 2H
A B C A B
 3C-SiC: cubic zinc-blende structure (3periodicity;
C: cubic)
 SiC substrate crystal growth: commercial substrates
were grown by sublimation growth technique; SiC
is transported in the vapor phase to a SiCseed
crystal held at a lower temperature
 Sublimated SiC must diffuse
Seed SiC 1800oC
through porous graphite under
dT/dx ~ 20 K/cm
carefully controlled thermal and
pressure gradients to form high
Poly SiC 2000oC
quality single crystal 6H SiC
 SiC thin film epitaxy: Nishino, et. al. => the clean
Si substrate is exposed to a C-containing gas at 
growth temperature => a thin monocrystalline 3C
SiC on Si; template for epitaxial SiC growth =>
abundant defects: antiphase domain boundary,
misfit dislocations, microtwins, stacking faults,etc.
 SiC LEDs:
Spectrum of SiC LED.
 SiC Photodiodes:
Temperature dependent
responsivity of a SiC
photodiode.
SiC efficiency as a function
of bias.
 GaN:
 Grown by CVD, MOCVD, ECR-CDV, etc.
 The best substrate is sapphire (Al2O3). No nitride
substrate available => on sapphire a buffer layer
were prepared (typically AlN); the buffer were
initially amorphous => converts to single crystal
during subsequent growth; recently, low
temperature GaN as buffer layer was used, but
AlN seems to produce better results!
 Polytypism in nitride: typical structure for GaN,
AlN, and InN are wurtzite (2H), metastable
zinc-blende structure can also be formed
 Doping (p-type dopant: Mg)and ohmic contact
(Au/Ni, Ti/Al) are important issues for GaN.
 GaN-based LEDs:
EL spectra of Nichia
InGaN/GaN LED
Output power and quantum
efficiency of Nichia GaN
LED
Output power comparison of
Nichia InGaN, GaN LED,
Sanyo SiC and Cree SiC
(solid circle) LED.
Refractive indices of
important nitrides
Important semiconductor materials for optoelectronics
Materials Type
Substrate
Si
SiC
Ge
GaAs
IV
IV
IV
III-V
Si
SiC
Ge
GaAS
AlGaAs
III-V
GaAS
GaInP
III-V
GaAlInP
III-V
GaP
III-V
GaAsP
III-V
InP
III-V
InGaAs
III-V
InGaAsP III-V
InAlAs
III-V
InAlGaAs III-V
GaSb/GaAlSb II-VI
CdHgTe II-VI]
ZnSe
II-VI
ZnS
II-VI
GaAs
GaAS
GaP
GaP
InP
InP
InP
InP
InP
GaSb
CdTe
ZnSe
ZnS
Devices
Wavelength range(m)
Detectors, Solar cells
Blue LEDs
Detectors
LEDs, Lasers, Detectors, Solar
Cells, Imagers, Intensifiers
LEDs, Lasers,
Solar Cells, Imagers
Visible Lasers, LEDs
Visible Lasers, LEDs
Visible LEDs
Visible LEDs
Solar Cells
Detectors
Lasers, LEDS
Lasers, Detectors
Lasers, Detectors
Lasers, Detectors
Long wavelength Detectors
Short wavelength LEDs
Short wavelength LEDs
0.5-1
0.4
1-1.8
0.85
0.67-0.98
0.5-0.7
0.5-0.7
0.5-0.7
0.5-0.7
0.9
1-1.67
1-1.6
1-2.5
1-2.5
2-3.5
3-5 and 8-12
0.4-0.6
0.4-0.6
Commercial Applications of Optoelectronic Devices
Materials
GaAs/AlGaAs
Devices
Detectors, Infrared
LEDs and Lasers
InP/InP
InP/InGaP
Solar cell
Infrared LEDs,
Lasers (1-1.6m)
InP/InGaAs
1-1.67m Detectors
InGaAlAs/InGaAs 1.67-2.4m Detectors
GaAs/GaInP/
0.5-0.7m LEDs
GaInAlP
and Lasers
Si
Detectors and
Solar Cells
Ge
Detectors
SiC
Blue LEDs
GaSb/GaAlSb/InSb Long wavelength
detectors/smitters
ZnSe/ZnS
Visible LEDs
Applications
Remote control TV, etc., video disk
players, range-finding, solar energy
conversion, optical fiber communication
systems (local networks), image
intensifiers
Space solar cell
Optical fiber communications
(long-haul and local loop)
Optical fiber communications, instrumentation
Military applications, medicine, sensor
Displays, control, compact disk players, laser
printers/scanners, optical disk memories,
laser medicine equipment
Solar energy conversions, e.g. watches,
calculators, cooling, heating, detectors
Detectors
Displays, optical disk memories, etc.
Infrared imaging, night vision sights, missile
seekers, other military applications
Commercial applications (R&D stages only)