Semiconductor Lasers with Gratings Defined by Electron Beam Lithography
This thesis deals with the fabrication and characterization of semiconductor lasers with gratings defined by electron-beam lithography (EBL). Both distributed-feedback (DFB) lasers for fiber-optic communication and grating-surface-emitting (GSE) lasers suitable for a number of novel applications, from free-space communication to optical processing, are treated.
In the first part of the thesis, the technical limitations of EBL for grating fabrication in general and DFB grating fabrication in particular as well as its possibility to enhance the performance of DFB lasers by advanced grating designs are studied. Small-area, high- resolution EBL patterning is associated with field-stitching errors. The statistical nature of these errors and their effect on the single-mode (SM) stability of DFB lasers have been investigated. It is shown that the field-stitching accuracy is sufficient to produce high-quality gratings for DFB lasers if certain precautions are taken considering exposure conditions and laser design. A study of the spectral characteristics of DFB lasers with different grating configurations has also been carried out. It was found that good AR-coating of the cleaved facets and very accurate processing control is necessary in order to benefit from the l/4-shifted and multiple-phase-shifted (MPS) DFB laser structures as compared to regular unshifted DFB lasers. The MPS structure was shown to be effective to enhance the SM performance of DFB lasers with strong coupling and to obtain narrow linewidths. Finally, a new technique for the realization of a nonuniform coupling coefficient in DFB lasers is demonstrated. It enables a large variation and precise control of the coupling strength.
The second part of the thesis presents GSE lasers with record low threshold current densities (below 100 A/cm2) and an improved lateral mode control in broad devices. The low threshold current densities were the result of high-reflectivity and low-absorption gratings in conjunction with lateral index guiding and current confinement in the gain section. The devices contained a strained-layer InGaAs/BlGaAs single-quantum-well and a thin etch-stop layer to allow precise control of the grating position relative to the active layer. Uniform and well-defined rectangular surface- gratings were fabricated using EBL and chemically-assisted ion-beam etching. A study of the dependence of the threshold current and power-current linearity on the detuning of the Bragg wavelength from the gain peak is also presented. A strong dependence of the linearity on the detuning was found and the lasers showed good linearity at the wavelength yielding the lowest threshold current. Finally, GSE lasers with integrated dry-etched corner reflectors were fabricated. Well behaved lateral modes were observed in 70 µm wide lasers, emitting beams with a low divergence in both the lateral and longitudinal directions at low current injection.