Characteristic of 850-nm InGaAs/AlGaAs Vertical-Cavity Surface-Emitting Lasers

1
Characteristic of 850-nm InGaAs/AlGaAs
Vertical-Cavity Surface-Emitting Lasers

• Masters thesis of Yuni Chang
• SpeakerHan-Yi Chu
• National Changhua University of Education

2
Introduction

• Although the InGaAsP lasers operating at 1.3 and
  1.5 mm have long been used for long-haul optical
  fiber communication, laser diodes operating near
  850 and 650 nm find important applications in
  short-distance PMMA-based optical fiber
  communication.
• The laser beam emitted from a vertical-cavity
  surface-emitting laser is circular in shape and
  hence can be easily coupled into an optical fiber
  with a coupling efficiency up to 90.
• The vertical-cavity surface-emitting laser
  usually have small diverging angle, low threshold
  current, single laser mode output.

3

• The vertical-cavity surface-emitting lasers
  operating in the spectral range near 850 nm
  usually utilize GaAs/AlGaAs as the active layer
  materials.
• In a vertical-cavity surface-emitting laser, the
  distributed Bragg reflectors (DBR) are usually
  used as laser mirrors.
• The laser cavity consists of the active region
  and two spacer regions (n spacer and p spacer ).
  In order to improve the carrier confinement and
  optical confinement, the bandgap energy of the
  spacers is usually higher than that of the
  active region.
• In order to maximize the overlap of the electrons
  inside the active region and the standing wave, a
  laser cavity of one wavelength in length is
  usually utilized.

4
Simulation and Discussion
Al0.15Ga0.85As p-doping21018 (cm-3)

Al0.9Ga0.1As p-doping21018 (cm-3)
Al0.6Ga0.4As undoped
GaAs/Al0.35Ga0.65As or In0.1Ga0.9As/Al0.35Ga0.65As
3-Quantum wells undoped
Al0.6Ga0.4As undoped
Al0.9Ga0.1As n-doping11018 (cm-3)
Al0.15Ga0.85As n-doping11018 (cm-3)
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• The DBR mirrors consist of Al0.9Ga0.1As /
  Al0.15Ga0.85As that has an index of refraction
  and thickness of 2.97/71.55 and 3.5/60.7 nm
  respectively.
• Since the use of indium in well of the
  In0.1Ga0.9As/Al0.35Ga0.65As active layer will
  result in a decrease of the bandgap energy, the
  width of the quantum well needs to be smaller
  than that of the GaAs/Al0.35Ga0.65As.
• The widths of the quantum wells of
  GaAs/Al0.35Ga0.65As and In0.1Ga0.9As/Al0.35Ga0.65A
  s active layers are assumed to be 120Å and 40Å
  respectively.
• In the In0.1Ga0.9As/Al0.35Ga0.65As active region,
  the well is under a compressive strain of 0.7.

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Spectra of Spontaneous Emission Rate

In addition to the emission peak at 850 nm, there
exists emission peaks at shorter wavelength (at
815 nm for GaAs/Al0.35Ga0.65As and 807 nm for
In0.1Ga0.9As/Al0.35Ga0.65As) that are due to the
transitions from the second energy level in the
conduction band to the second energy level in the
valence band.
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Energy band diagrams of the (a)
GaAs/Al0.35Ga0.65As and (b) In0.1Ga0.9As/Al0.35Ga0
.65As VCSEL under study.

8
Distribution of electron concentration near
active region at 330K and 2.0 mA.

9
Stimulated Recombination Rate

10
Laser output power as a function of input current

• The In0.1Ga0.9As/Al0.35Ga0.65As and
  GaAs/Al0.35Ga0.65As quantum well structures have
  a threshold current of 0.37 mA and 0.77 mA.

11
Mode spectra of the GaAs/Al0.35Ga0.65As VCSEL at
0.56 mA (below the laser threshold) and 1.18 mA
(above the laser threshold)

12
Side Mode Suppression Ratio

• The main mode to first side mode suppression
  ratio increases dramatically near the laser
  threshold.
• In0.1Ga0.9As/Al0.35Ga0.65As VCSEL has a larger
  main mode to first side mode suppression ratio
  than the GaAs/Al0.35Ga0.65As VCSEL for all
  current input levels, which is beneficial for
  single laser mode operation.

13
Characteristic Temperature

• These curves can be best-fitted with the
  following formula to corresponding characteristic
  temperature

• A high characteristic temperature is desired for
  a VCSEL since the threshold current is less
  sensitive to temperature.

14
Threshold current of the InXGa1-XAs/Al0.35Ga0.65As
VCSEL as a function of temperature for different
indium composition,x.

• From the results, the characteristic temperature
  increases when the indium composition in
  InXGa1-XAs/Al0.35Ga0.65As increases.

15
Conclusion

• The strained InGaAs/AlGaAs VCSEL has a higher
  stimulated recombination rate, a lower threshold
  current , a higher main-side mode suppression
  ratio, and a higher characteristic temperature,
  which might be owing to its narrower well width
  and smaller carrier effective masses.