A study of GaSB-based resonant cavity photodetectors

The design and modelling of resonant cavity-enhanced [RCE] photodetectors, where the active region is incorporated between two mirrors, has been considered using GaSb as the cavity material grown on GaAs substrates. These type of structures offer the possibility of highly efficient, wavelength selective photodetectors whilst allowing the use of much thinner active regions as compared to the conventional non-resonant structures. The antimonide material system has a carrier concentration of 1x1016cm-3 when undoped which presents a problem when considering photodiodes which require large active regions in order to achieve high efficiency; and hence the use of a resonant cavity structure is advantageous. Schottky and p-n structures have both been considered as potential diodes. In the form of conventional non-resonant devices these are then used as reference structures for the cavity detectors. Non-resonant p-n junction structures showed the best efficiency measurements with η~10% at -IV reverse bias and a corresponding D*[λ,= 1700nm]=4.93xl09cmHz 1/2W-1, the highest D* in this structure was 1.46x10 10cmHz1/2W-1 at 0V. Quarter-wave mirror stacks have been grown, using MBE, with measured reflectivties >95% at 1.68μm using GaAs/A1As layers. Bulk GaSb was grown on top of the DBR by MOCVD to form the cavity; and the air-semiconductor interface was used as the top reflector. Measured optical spectra show an increase in the photocurrent by a factor of ~4 at λ=1748nm and by a factor of ~3 at λ= 1631nm over non-resonant structures with the same active region thickness. The best quantum efficiency for an RCE structure was measured at λ= 1631nm where η~10% at -0.3V reverse bias with D*=1.71x109cmHz1/2W-1 and the best D* was achieved at the same wavelength where D*[λ= 1631nm]=4.49x10 10cmHz1/2W-1 at a bias of 0V. The main limit to the achievement of high η and D* is the poor dark current characteristics of all the structures considered in this thesis. Reduction of this parameter should, hopefully, result in an increase in the signal-to-noise ratio and an improvement in η and D*