Laser III Device Design & Materials Selection

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Laser IIIDevice Design Materials Selection

• EBB424E
• Dr Zainovia Lockman

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Laser 3- Lecture Layout

• By the end of the course you would be able to
  answer the following questions

• What is homojunction laser?
• What is heterojunction laser?
• Explain the principles of heterojunction laser.
• Sketch a typical stripe geometry laser diodes.
• What is quantum well laser?

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Introduction

• In the pervious lectures you have been explain
  about two important conditions for designing
  laser
• Optical Gain
• Medium which possess the desired energy level
  structure to support laser action ? in the case
  of diode laser this will be the active region of
  the p-n junction
• To establish a population inversion in a laser
  system ? the forward bias current supplied to the
  diode laser.
• 2. Optical Feedback
• Homojunction laser with one end cleaved and the
  other roughned. This is to achieve the optical
  feedback (optical gain) of the laser system.
  Such system is often termed Fabry-Perot Cavity.

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Threshold Current Density

• Consider a diagram showing the active region and
  mode volume of a semiconducting laser

Mode volume, thickness, d
p
Active region, thickness, t
n

• Recall that when forward biased, with eV gt Eg of
  the material, electrons (from degenerately doped
  n) and holes (from degenerately doped p) will be
  injected across the junction to create population
  inversion.
• The population inversion is created in a region
  called active region. Radiative transition may
  occur resulted in stimulated emission when the
  photon is absorbed by the electrons in the
  conduction band.
• The radiation generated will be spread out in the
  vicinity of the active region and is almost
  confined in the thin layer shown above (mode
  volume).

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Schematic construction of a homojunction GaAs
diode laser.
Metal contact ()
Cleaved end (110) Natural crystal planes of the
junction so that the end faces are parallel
p GaAs
The laser beam output
n GaAs
Junction (active region and mode volume)
Metal contact (-)
Roughened end

• The carriers in the active region increases
  refractive index of GaAs
• The refractive index increment is only 0.02,
  hence is not a good dielectric waveguide
• The beam therefore can be spread out to the
  surrounding region mode volume
• Vigorous pumping is therefore needed to enhance
  lasing
• The threshold current for the pumping action
  exceeds 400Amm-2

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Threshold Current DensityDefinition

• If the injected carrier concentration become
  large enough, the stimulated emission can exceed
  absorption so optical gain can be achieved in the
  active region. With appropriate configuration to
  achieve optical feedback, laser oscillation
  occurs when gain exceeds losses.
• For significant gain, a high current density is
  necessary.
• The onset of lasing is characterised by the a
  specific injection current known as the Threshold
  Current
• Since the simple homojunction laser has high
  threshold current, it is considered not
  efficient.
• The onset of laser action at the threshold
  current density is indicated by an abrupt
  increase in radiance of the emitting region,
  leading to marked decrease in spectral width.

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Threshold Current Density The typical output
spectrum
Stimulated Emission
laser
Optical Power
Spontaneous Emission
I
LED
JTH
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In conclusion about the homojunction laser.

• The main problem with the homojunction laser
  diode is that the threshold current density, Jth
  is far too high for practical applications.
• JTH increases with temperature, too high at room
  temperature, not continuous but pulsed laser
  output.
• Homojunction laser has
• Poor optical
• Less carrier confinement
• If Jth is low improve rate of stimulated
  emission improve efficiency of optical cavity
• To get low Jth
• Confined carriers in a narrow region ? carrier
  confinement
• Build dielectric waveguide around the optical
  gain region (increase photon concentration hence
  stimulated emission) ? photon confinement
• How do we achieve that?
• heterostructured laser diodes

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The Heterojunction LaserSingle Double
Metal contact ()
GaAs sandwiched between the higher band gap AlGaAs
n GaAlAs
N GaAs
p GaAs
1?m
n GaAlAs
p GaAlAs
p GaAs
P GaAlAs
Metal contact (-)
GaAs sandwiched between the higher band gap
AlGaAs. GaAs is the active region where lasing
takes place
N-n-p-P
N-p-P
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Homojunction laser
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Carriers Photons Confinement

• N-Ga1-xAlxAsp-GaAsP-Ga1-xAlxAs
• N ACTIVE LAYERP
• GaAs and GaAlAs
• Have different refractive index
• nGaAlAs lt nGaAs
• Have different Eg
• Eg (GaAlAs) gt Eg(GaAs)
• Band gap difference ? forms barriers for e and h
  to diffuse from GaAs to the sandwich layers of
  GaAlAs ? CARRIER CONFINEMENT
• Step difference in the refractive index ?
  waveguide (Optical/Photons Confienment)
• Eg (GaAlAs) gt Eg(GaAs) ? Photons produced in GaAs
  will not be absorbed by GaAlAs.

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Stripe Geometry DHJ Laser

• Features
• Oxide layer or high resistive layer (produced by
  proton bombardment) between metal contact and the
  semiconductor.
• Restrict current along the junction into narrow
  stripe (few microns)
• Small JTH with high Power ? continuous operation
• Used largely in Optical Fibre Communication
• The configuration is shown to you in Wilson page
  217 for DHJ with oxide that isolate the metal
  contact to the GaInAsP (figure 2.17)

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Double Heterojunction StrpeLaser Diode
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Materials Criteria Selection

• To date GaAs and GaAlAs are largely used.
• Advantages of AlGaAs/GaAs system is that
• GaAs is direct band gap material
• Ga1-xAlxAs is direct when x lt 0.45
• Lattice match between Ga1-xAlxAs GaAs is very
  small (0.1) therefore epi growth can be achieved
• The band gaps of both materials can be
  manipulated to produce SH or DH junctions lasers
  for high optical and carrier confinemnts
• For optical fibre communication, wavelength of
  1.1-1.6?m is preferred.
• Refer to Wilson page 216 (figure 5.33) or see the
  next slide

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Typical Exam Question ? on GaAs/GaAlAs

• Eg(x) 1.424 1.247x (eV) ? Empirical
  relationship
• Calculate the band gap if GaAlAs is to be used as
  emitter for fibre optics communication at
  wavelength 1.4?m.
• Calculate compositions of the GaAlAs ternary
  alloy for peak emission at wavelength 1.4?m.

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Band Gap EngineeringTo answer What other system
can be used?
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Quantum Well Lasers

• Structure similar to the DH laser except
  thickness of active layer is very small (10-20nm)
• E.g. narrow Eg GaAs sandwich between larger band
  gap GaAlAs
• With this configuration, density of states near
  the bottom of the conduction band and the top of
  the valance band increased significantly the
  hence enhance the population inversion
• Better population inversion, smaller active layer
  hence JTh is smaller.
• BUT, in single quantum well (SQW) extreme
  narrowness of the active region created poor
  optical confinement.
• So Solve by Multiple Quantum Well Structure
  (MQW)
• SQW can be coupled to produce the MQW
• Overall active region is now thicker
• Carriers which are not captured in one well can
  be captured by the second well etc.
• MQW has JTH higher than SQW ( 1mA) but the more
  optical power due to better optical confinement

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Cladding Layer and Separate Confinement
Heterostructure
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Preparation for Next week (Monday)

• Test on Laser and LED