Optimisation of GaAsBi Based Semiconductors

GaAsBi has recently attracted much attention due to its large band gap reduction, a less temperature dependence of the band gap, and the giant spin orbiting properties. The large band gap reduction of GaAsBi is explained by valence band anti-crossing (VBAC) model. It has been proposed that a resonant energy state is introduced in the valence band, and the interaction between that state and the original valence states leads to a splitting of the valence band into two sub-bands and therefore a reduction of the band gap.
Molecular beam epitaxy (MBE) has been implemented to grow GaAsBi layers. Two growth conditions should be satisfied to get bismuth incorporated into GaAs: the growth temperature should be low enough (usually lower than 400 °C), and the As:Ga atomic flux ratio should be near stoichiometry. Four parameters can affect the final bismuth incorporation: growth rate, As:Ga atomic flux ratio, Bi:Ga atomic flux ratio, and growth temperature. The relationships have been investigated in the previous research, and also explored in this thesis.
Absorption properties are the key properties of an optoelectronic device. Through measuring the photoresponse of GaAsBi based p-i-n heterojunction devices, the absorption coefficient as a function of the incident light energy is obtained. The results reveal that the absorption coefficient follows the square law of the Tauc relation, which indicates that the material is a direct band gap material. The diffusion length is a combination of the lifetime and the mobility of carriers, and it is the diffusion length that directly reflects the performance of carrier transportation. Especially in a device which requires high absorption of photons such as a solar cell, a long diffusion length becomes even more important. In this thesis, a model has been established to calculate the diffusion length of GaAsBi based on the photocurrent measurements, and results show that the diffusion length is around 1 μm.
Photoluminescence (PL) measurements are used to decide the bismuth content based on the relation between the bismuth content and the band gap obtained from VBAC. Temperature dependence of the band gap of GaAsBi is also investigated using PL measurements. The s-shape of the PL peak position against the temperature reveals the existence of localized states. The source of localized states is claimed to be from bismuth clusters in some papers. Low temperature behaviours of GaAsBi have further been investigated using low temperature current-voltage measurements. It seems that the results also reflected the existence of localized states.
For low noise avalanche photodiodes (APDs), it is important that the electron-initiated and hole-initiated impact ionization coefficients α and β are very different in magnitude. Photo-multiplication measurements were taken in this thesis to investigate the impact ionization property of GaAsBi p-i-n and n-i-p diodes. A relatively large ionization coefficient ratio α/β for GaAsBi heterojunctions was obtained, which shows a promising use of GaAsBi as low-noise electron-initiated APDs.