Ellipsometry, reflectance and modulated spectroscopy of bulk and multi-layer semiconductor structures

Vertical-cavity surface-emitting lasers (VCSELs) are complex multi-layer structures whose operating characteristics are highly sensitive to variations in layer thickness and composition. They contain an active region of one or more quantum wells sandwiched between highly reflecting mirrors. Non-destructive optical characterisation techniques including reflectance spectroscopy, spectroscopic ellipsometry and photomodulated reflectance have been used to examine these structures and various components of them. In addition, the relatively novel technique of photomodulated spectroscopic ellipsometry has been examined in comparison with photomodulated reflectance in the characterisation of bulk, multilayer and quantum well material. The distributed Bragg reflectors of VCSELs provide the high reflectance required over a selected wavelength range. Optical measurements were used to determine important information concerning layer thicknesses and compositions, which were confirmed with X-ray diffraction and transmission electron microscopy. The techniques were also used to provide important information concerning growth and uniformity, which could be readily applied for feedback to growers or for device fabrication. Novel reflectance and photomodulated reflectance measurements made on a range of laser structures designed to operate over a range of wavelengths from 650 nm to 1 μm were used to examine the characteristics of the reflectors and the active region of the lasers. The cavity mode observed clearly indicates the lasing wavelength, and the interaction of the cavity and quantum well has been interpreted using new lineshapes. The cavity mode and quantum well resonance observed in photomodulated reflectance has been shown to provide a clear indication of where devices can be fabricated successfully from non-uniform material. The identification of the cavity and quantum well features has also enabled important information concerning the changes in structure and therefore in device performance with temperature and pressure. Measurements have also been able to provide important information to explain the variation in performance of some devices. (author)