Laser Diodes

Laser Diodes (LD) consist of semiconducting materials e.g. GaAs or Si, which are doped with electron donors and electron acceptors in different layers. Doping atoms can be Al and In among many others. At the interface between layers with electron depletion and electron excess, electrons and holes can recombine and emit light if a voltage is applied between these layers. Coherent light emission out of the LD is obtained if more light is amplified by recombination than lost through the reflecting outer surfaces of the LD (facets) and by absorption in the LD. The properties of the LD i.e. emission wavelength, linewidth, power, stability, lifetime depend on the dispersion functions of refractive index n and extinction k of all materials of the LD and on the thickness of each layer on the LD. In order to control the quality of the LD those dispersion functions and thickness shall be measured. Especially important for the function of the LD are the optical properties of the anti-reflection coating on the emitting surface of the LD. In a quantum well laser the interface of the light emission is only a few nm thin. Spectroscopic ellipsometry is the ordinary method to measure these optical properties on large semiconducting wafers. Imaging ellipsometry is needed to measure on those tiny LDs, because high lateral resolution of a few μm is necessary.

Back