Characterisation of GaAsBi based semiconductors

GaAsBi alloys have recently attracted much attention due to its large bandgap reduction, temperature insensitivity behaviour and giant spin-orbit splitting properties. These special characteristics enable enormous potential for GaAsBi in applications such as shortwave infrared optoelectronics, thermoelectricity and photovoltaics. Two conditions have restricted high-quality GaAsBi growth. First, a low growth temperature at ~400°C while maintaining the As/Ga atomic flux ratio close to stoichiometry is required. Second, the lattice mismatch with the substrate will inevitably lead to strain relaxation in thick structures. Therefore, there are very few reports of GaAsBi device with >400nm thick intrinsic region from the literature. In this work, four systematic series of bulk GaAsBi p-i-n and n-i-p samples with varying i-region thickness and bismuth content were grown using molecular beam epitaxy. The optical and electrical properties of those samples were characterised by various techniques. From the optical characterisations, the devices grown with thick structures show higher strain relaxation, which was confirmed by cross-hatch patterns observed by Nomarski microscopy and X-ray diffraction. The bismuth content of each wafer obtained from valence band anti-crossing model and XRD simulations match well. Current-voltage measurements show good diode behaviour with a clear relationship between the dark current and bismuth content. Comprehensive multiplication measurements were performed in those four sets of GaAsBi diodes at room temperature. A large disparity between the electron and hole ionisation coefficients has been found. Further analysis on the onset of multiplication of 400nm thick p-i-n and n-i-p with different bismuth content showed that adding Bi into GaAs significantly reduces the hole ionisation coefficient while the electron ionisation coefficient remains virtually unchanged. This is the first time such a peculiar discovery was observed in a dilute III-V alloy system and is attributed to the effect bismuth has on the spin-orbit splitting energy. This new and significant find will provide a new route to enhance the α/β ratio, making GaAsBi a promising material for low noise avalanche photodiodes.