2003/04/071 Characteristic of 850-nm InGaAs/AlGaAs Vertical-Cavity Surface-Emitting Lasers Master’s thesis of Yuni Chang Speaker:Han-Yi Chu National Changhua University of Education

2003/04/072 Introduction Although the InGaAsP lasers operating at 1.3 and 1.5  m 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.

2003/04/073 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.

2003/04/074 Simulation and Discussion Al 0.15 Ga 0.85 As p-doping=2×10 18 (cm -3 ) Al 0.9 Ga 0.1 As p-doping=2×10 18 (cm -3 ) Al 0.6 Ga 0.4 As undoped GaAs/Al 0.35 Ga 0.65 As or In 0.1 Ga 0.9 As/Al 0.35 Ga 0.65 As 3-Quantum wells undoped Al 0.6 Ga 0.4 As undoped Al 0.9 Ga 0.1 As n-doping=1×10 18 (cm -3 ) Al 0.15 Ga 0.85 As n-doping=1×10 18 (cm -3 )  20  30

2003/04/075 The DBR mirrors consist of Al 0.9 Ga 0.1 As / Al 0.15 Ga 0.85 As 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 In 0.1 Ga 0.9 As/Al 0.35 Ga 0.65 As 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/Al 0.35 Ga 0.65 As. The widths of the quantum wells of GaAs/Al 0.35 Ga 0.65 As and In 0.1 Ga 0.9 As/Al 0.35 Ga 0.65 As active layers are assumed to be 120Å and 40Å respectively. In the In 0.1 Ga 0.9 As/Al 0.35 Ga 0.65 As active region, the well is under a compressive strain of 0.7%.

2003/04/076 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/Al 0.35 Ga 0.65 As and 807 nm for In 0.1 Ga 0.9 As/Al 0.35 Ga 0.65 As) that are due to the transitions from the second energy level in the conduction band to the second energy level in the valence band.

2003/04/077 Energy band diagrams of the (a) GaAs/Al 0.35 Ga 0.65 As and (b) In 0.1 Ga 0.9 As/Al 0.35 Ga 0.65 As VCSEL under study.

2003/04/078 Distribution of electron concentration near active region at 330K and 2.0 mA. (a) (b)

2003/04/079 Stimulated Recombination Rate (a) (b)

2003/04/0710 Laser output power as a function of input current The In 0.1 Ga 0.9 As/Al 0.35 Ga 0.65 As and GaAs/Al 0.35 Ga 0.65 As quantum well structures have a threshold current of 0.37 mA and 0.77 mA.

2003/04/0711 Mode spectra of the GaAs/Al 0.35 Ga 0.65 As VCSEL at 0.56 mA (below the laser threshold) and 1.18 mA (above the laser threshold) (b) (a)

2003/04/0712 Side Mode Suppression Ratio The main mode to first side mode suppression ratio increases dramatically near the laser threshold. In 0.1 Ga 0.9 As/Al 0.35 Ga 0.65 As VCSEL has a larger main mode to first side mode suppression ratio than the GaAs/Al 0.35 Ga 0.65 As VCSEL for all current input levels, which is beneficial for single laser mode operation.

2003/04/0713 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.

2003/04/0714 Threshold current of the In X Ga 1-X As/Al 0.35 Ga 0.65 As VCSEL as a function of temperature for different indium composition,x. From the results, the characteristic temperature increases when the indium composition in In X Ga 1-X As/Al 0.35 Ga 0.65 As increases.

2003/04/0715 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.