Alhassni, Amra
(2023)
Investigation of Structural and Optical Properties of Dilute GaAsBi and InGaAsBi Semiconductor Nanostructures Grown by Molecular Beam Epitaxy.
PhD thesis, University of Nottingham.
Abstract
This thesis investigates the optical and structural properties of GaAs1-xBix thin epitaxial layers and self-assembled InGa(Bi)As quantum dots (QDs) grown on conventional (100) GaAs substrates by Molecular Beam Epitaxy (MBE). The GaAs1-xBix epilayers were grown at different substrate temperatures. The InGa(Bi)As QDs were formed via the Stranski-Krastanov (S-K) growth mode using bismuth as a surfactant.
Photoluminescence (PL) measurements showed that the GaAs1-xBix PL spectra exhibit two different behaviours depending on the growth temperature, namely, red and blue shift were observed as the growth temperature increases from 300 0C to 325 0C, and from 325 0C to 365 0C, respectively. Moreover, the Bi composition in the studied samples were determined and calculated from the PL data. The results showed that Bi incorporation into the GaAs host lattice is very sensitive to the growth temperature and varied from 2.3% to 4.7%, and from 4.7 % to 2.8% for a growth temperature in the range 300 0C - 3250C and 325 0C - 365 0C, respectively. These findings were supported by Scanning Electron Microscopy (SEM) results which showed that the samples with the highest surface concentrations of droplets are those with the lowest concentrations of Bi (samples grown at TG= 300 0C, 310 0C and 365 0C). This means that for these growth temperatures a lower concentration of Bi was incorporated into the GaAs structure. However, the sample with the highest concentration of bismuth (4.7%) which was grown at 325 0C, showed a lower number of both surface droplets and self aligned trailing nanotracks. These results are also consistent with Raman measurement which demonstrated that as the content of Bi increases, first there is a slight redshift and then a blueshift of the longitudinal optical (LO) phonon peak, which can be explained by the Bi-induced tensile and/or compressive stress. The optimum growth temperature for maximum Bi incorporation was found to be 325 0C (4.7%). The integrated PL intensity as a function of inverse temperature confirmed two types of defects. The first type is related to lattice disorder and the other related to Bi clusters.
The effect of gamma radiation dose on the structural and optical properties of dilute GaAs1-xBix thin epitaxial layers grown at different substrate temperatures by MBE on conventional (100) GaAs was also investigated. This study investigates the interaction of gamma radiation with GaAs1-xBix III-V semiconductor alloys, which have enormous potential use in ionizing radiation detectors that can be monitored through both optical and electrical measurements. From Raman measurements, it was found that the concentration of holes increased when the samples were irradiated. This result is in good agreement with the PL results, which showed that the intensity of the main peak increases after irradiation, indicating that the optical properties have improved for all samples. Furthermore, the X-ray diffraction
(XRD) data demonstrated that for irradiated GaAs1-xBix samples, their crystallographic quality was slightly worse after irradiation. This is due to the fact that radiation induces several types of defects, including structural defects. This result is consistent with PL results, which demonstrated that GaAs1-xBix samples have the largest PL full width at half maximum (FWHM) for all irradiated samples. This finding demonstrates that irradiated samples have worse quality compared to non-irradiated samples.
The effects of gamma radiation dose (30 kGy and 50 kGy) on self-assembled InGaAs/GaAs QDs formed at various growth temperatures (TG = 510 0C, 482 0C, 450 0C) with and without exposure to bismuth flux have been investigated. The PL results showed that for irradiation dose of 30 kGy, the QDs PL emission energy exhibit a blue shift of around 10 meV for sample grown without Bi, however, no blue shifts are detected in the PL of QDs for all samples grown with Bi surfactant at different growth temperatures. Interestingly, the PL emissions of QDs and wetting layers disappeared after the irradiation dose was increased to 50kGy for a sample grown without Bi. In contrast, for samples grown with a Bi surfactant the PL QDs emissions were still observed, however, their intensities were reduced by factors between 1.5 to 2. In general, gamma radiation treatment has better effect on the QDs samples grown under a Bi flux than the samples grown without Bi. The particular radiation dose of 30 kGy resulted in an improvement of the optical properties of all samples grown with Bi as a surfactant, as evidenced by a large increase in the QDs PL intensity after radiation. Furthermore, a growth temperature of 510 0C for InGaAs QDs was found to be optimal for both as-grown and gamma irradiated samples in terms of optical efficiency.
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