Dilute Nitride GaInNAsSb for Next Generation Optical Communications

Collins, Xiao and Marshall, Andrew and Hayne, Manus (2021) Dilute Nitride GaInNAsSb for Next Generation Optical Communications. PhD thesis, Lancaster University.

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Abstract

Current technology for the light detection in telecommunication revolves around InGaAs based on InP substrates. However, there is interest in switching to GaAs substrates to take advantage of the cheaper and larger substrates as well as the lattice-matched AlxGa1−xAs material system. Therefore, this thesis reports on the use of the dilute nitride alloy GaInNAsSb lattice matched to GaAs that can detect at wavelengths important to telecommunications such as 1.55µm. Molecular beam epitaxy (MBE) was used to grow GaInNAsSb p-i-n and n-i-p photodetectors on GaAs substrates with the lowest band gap reaching 0.64eV, far below the necessary band gap required for detection at 1.55µm (0.8eV) and that for InGaAs (0.74eV). However, due to the high defect concentration typically found in dilute nitride alloys, a post-growth thermal anneal was applied to the samples which has the effect of blueshifting the band gap by approximately 40meV. The effect of a post-growth anneal on the band gap, dark current, unintentional doping concentration and quantum efficiency is investigated in order to find the most optimal temperature. This temperature is found to be related to an intrinsic dopant type switch in the GaInNAsSb as one of the samples with the lowest dark current density (2.2mAcm −2 at −5V) is one that is annealed at 735◦C, just before a switch in intrinsic type from n-type to p-type. This is further investigated with a much narrower annealing range centred around 735◦C. While this narrow annealing range did not result in a consistent type switch temperature for all samples, admittance spectroscopy indicated a large increase in a p-type defect concentration demonstrating the potential for the anneal to worsen the performance of the photodiode at sufficiently high temperatures. Furthermore, it was found that annealing at temperatures close to 730◦C results in a much better photodetector than as-grown devices, which can be quantified by comparing the quantum efficiency at 1.55µm where the annealed sample reaches 38% as compared to the 3% in the as-grown device.The 3dB bandwidth was measured on the most optimally annealed photodetectors. In this measurement, the highest bandwidth reached is 9.2GHz at −18V which is limited by the resistance of the photodetector and can be further improved using Ohmic contacts with a lower resistance. Finally, a discussion on the integration of GaInNAsSb with AlGaAs in a separate absorption and multiplication avalanche photodiode (SAM APD) is held. Impact ionisation in GaInNAsSb and its effect on noise is discussed, as well as the design of a SAM APD with a very thin AlGaAs multiplication layer of 50nm. In this study, such SAM APDs are grown with a GaAs absorber instead of GaInNAsSb to allow for any corrections required in the important charge sheet layer, however in future studies GaInNAsSb can be used instead of GaAs as an absorber.