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
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 J1mwere 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.