1. Laser III Device Design & Materials Selection
EBB424E
Dr Zainovia Lockman

2. 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?

3. 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.

4. 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 > 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).

5. Junction (active region and mode volume)
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

6. Threshold Current Density Definition
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.

7. Threshold Current Density The typical output spectrum
Stimulated Emission
Optical power 
Optical Power
laser
Spontaneous Emission I
Optical power 
LED
JTH

8. 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

9. The Heterojunction Laser Single & Double
Metal contact (+)
GaAs sandwiched between the higher band gap AlGaAs
n GaAlAs
N GaAs
1m
p GaAs
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-p-P
N-n-p-P

10. Homojunction laser

11. Carriers & Photons Confinement
N-Ga1-xAlxAs|p-GaAs|P-Ga1-xAlxAs
N |ACTIVE LAYER|P
GaAs and GaAlAs:
Have different refractive index
nGaAlAs < nGaAs
Have different Eg
Eg (GaAlAs) > 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) > Eg(GaAs)  Photons produced in GaAs will not be absorbed by GaAlAs.

12. 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)

13. Double Heterojunction StrpeLaser Diode

14. 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 < 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 m is preferred.
Refer to Wilson page 216 (figure 5.33) or see the next slide

15. Typical Exam Question  on GaAs/GaAlAs
Eg(x) = x (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.

16. Band Gap Engineering To answer: What other system can be used?

17. 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

18. Cladding Layer and Separate Confinement Heterostructure

19. Preparation for Next week (Monday)
Test on Laser and LED