Low resistance visible wavelength distributed Bragg reflectors

Abstract

This dissertation reports the first application of semiconductor heterojunctions with naturally occurring small energy band offsets to distributed Bragg reflector (DBR) structures for the purpose of lowering their series resistance. Selecting DBR layer pairs with small valence or conduction band offsets eliminates the need for complicated structural grading schemes typically used to smooth out the interface energy barriers, and thus achieves low resistance DBRs using simple stacks of quarter wavelength thick layers. Low resistance, highly reflective DBRs are used in vertical cavity surface emitting lasers, which are attractive light sources for many visible wavelength applications. The target wavelength for this work is 650 nm. From theoretical band offset calculations using the Van de Walle model solid theory, heterojunctions of mixed arsenide and phosphide III-V semiconductor single crystal alloys were selected for the low band offset DBRs: Al0.90Ga0.10As/Al0.14Ga0.38In0.48P for p-type and AIAs/AI0.10Ga0.42ln0.48P for n-type. Theoretical specific contact resistance calculations showed that the low offset structures have smaller resistances than AlInP/AlGalnP DBRs which previously have been investigated for visible VCSELs. The epitaxial films for this work were grown on GaAs substrates by gas source molecular beam epitaxy. Bulk AlGaAs and AlGalnP layers were grown to qualify their structural and electrical characteristics, and this knowledge was applied to grow the DBR samples. Double crystal x-ray diffraction measurements showed the DBRs were high quality, single crystal structures. Reflectance measurements on 40 period DBRs determined each had a peak of 99% at locations within 4% of the target wavelength. The low band offset DBRs were processed and their specific contact resistances determined using the transmission line model. The measured values of 2.75 x 10-1 Ω·cm2 for the p-type DBR and 2.58x 10-5 Ω·cm2 for the n-type DBR were within experimental uncertainty of their theoretical values. These results compare very favorably with numbers reported in the literature for AlGaAs DBRs that employ complicated structural grading schemes. On a per period basis, the low valence band offset p-type DBR resistance is comparable to or lower than previous reports, while the low conduction band offset n-type DBR is an order of magnitude lower than the best previously reported result.