Reflectance Mapping of Semiconductor Distributed Bragg Reflectors

We describe the design, construction and performance evaluation of a spectrally resolved reflectance mapping setup. The setup is capable of mapping a full 2 inch diameter semiconductor wafer, with a best spatial resolution of 50μm. The operating wavelength range is from 0.35μm to 1.05μm with spectral resolution of ~1nm. We have used this setup for reflectance mapping studies on distributed Bragg reflectors (DBRs). The DBRs studied were made of AlAs/Al0.5Ga0.5As layers on GaAs substrates and were designed for peak in reflectivity around 633nm (He-Ne laser wavelength), at normal incidence. The maps at a fixed wavelength of 633nm showed an increase and then a decrease in the absolute reflectivity value along a particular direction in the samples. This can occur due to a change in the thickness of the DBR layers, arising from a variation in the growth rate along the gas flow direction in a horizontal Metal Organic Vapor Phase Epitaxy reactor. To confirm this hypothesis, we measured the full reflectivity spectrum at different points on the samples along the direction of variation in the peak reflectivity. This data showed a regular shift in the wavelength position of the high reflectivity band of the DBR along this direction. With the above hypothesis we were able to simulate this experimental data, for which we adopted the transfer matrix method that is used to calculate the reflectivity spectrum of multi-layered dielectric films. This analysis showed that the DBR layer thickness increased at the rate of 0.78nm/mm and 0.69nm/mm for the AlAs and the AlxGa1-xAs layers respectively along the gas flow direction.