Investigation of Structural and Optical Properties of Dilute GaAsBi and InGaAsBi Semiconductor Nanostructures Grown by Molecular Beam Epitaxy

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

Investigation of Structural and Optical Properties of Dilute GaAsBi and InGaAsBi Semiconductor Nanostructures Grown by Molecular Beam Epitaxy

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

Effect of thermal annealing on the optical and structural properties of (311)B and (001) GaAsBi/GaAs single quantum wells grown by MBE

The effect of Furnace Annealing (FA) and Rapid Thermal annealing (RTA) on the structural and optical properties of GaAs1 − xBix/GaAs single quantum wells grown on (001) and (311)B substrates by molecular beam epitaxy was investigated. The structural properties were investigated by high-resolution x-ray diffraction (HR-XRD) and Transmission Electron Microscopy. The Bi concentration profiles were determined by simulating the HR-XRD 2θ−ω scans using dynamical scattering theory to estimate the Bi content, lattice coherence, and quality of the interfaces. The Bi composition was found to be similar for both samples grown on (001) and (311)B GaAs substrates. However, the simulations indicate that the Bi composition is not only limited in the GaAsBi quantum well (QW) layer but also extends out of the GaAsBi QW toward the GaAs barrier. Photoluminescence (PL) measurements were performed as a function of temperature and laser power for samples with a nominal Bi composition of 3%. PL spectra showed that (001) and (311)B samples have different peak energies at 1.23 eV and 1.26 eV, respectively, at 10 K. After RTA at 300 °C for 2 min, the PL intensity of (311)B and (001) samples was enhanced by factors of ∼2.5 and 1.75, respectively. However, for the (001) and (311)B FA samples, an enhancement of the PL intensity by a factor of only 1.5 times could be achieved. The enhancement of PL intensity in annealed samples was interpreted in terms of PL activation energies, with a reduction in the alloy disorder and an increase in the Bi cluster

Structural And Optical Properties Of n-Type and p-Type GaAs(1−x)Bix Thin Films Grown By Molecular Beam Epitaxy On (311)B GaAs Substrates

In this paper, we report on the structural and optical properties of n-type Si-doped and p-type Be-doped GaAs(1−x)Bix thin films grown by molecular beam epitaxy on (311)B GaAs substrates with nominal Bi content x=5.4%. Similar samples without Bi were also grown for comparison purposes (n-type GaAs and p-type GaAs). X-ray diffraction, micro-Raman at room temperature, and photoluminescence (PL) measurements as a function of temperature and laser excitation power (PEXC) were performed to investigate their structural and optical properties. X-ray diffraction results revealed that the Bi incorporation in both n-type and p-type doped GaAsBi was similar, despite that the samples present remarkable differences in the number of Bi related defects, non-radiative centers and alloy disorder. Particularly, our results evidence that the Bi-related defects in n- and p-doped GaAsBi alloys have important impact on the differences of their optical properties

Investigation of the Effect of Substrate Orientation on the Structural, Electrical and Optical Properties of n-type GaAs1-xBix Layers Grown by Molecular Beam Epitaxy

Current-Voltage (I-V), Capacitance-Voltage (C-V), Deep Level Transient Spectroscopy (DLTS), Laplace DLTS, Photoluminescence (PL) and Micro-Raman techniques have been employed to investigate the effect of the orientation of the substrates on the structural, electrically and optically active defects in dilute GaAs1−xBix epilayers structures having a Bi composition x = ~5.4%, grown by Molecular Beam Epitaxy (MBE) on (100) and (311)B GaAs planes. X-ray diffraction results revealed that the in-plane strain in the Ga(As,Bi) layer of the samples grown on (100)-oriented substrate (−0.0484) is significantly larger than that of the samples grown on (311)B-oriented substrate. The substrate orientation is found to have a noticeable impact on the Bi incorporation and the electrical properties of dilute GaAsBi Schottky diodes. The I-V characteristics showed that (100) Schottky diodes exhibited a larger ideality factor and higher barrier height compared with (311)B samples. The DLTS measurements showed that the number of electrically active traps were different for the two GaAs substrate orientations. In particular, three and two electron traps are detected in samples grown on (100) and (311)B GaAs substrates, respectively, with activation energies ranging from 0.12 to 0.41 eV. Additionally, one hole trap was observed only in sample grown on (100) substrates with activation energy 0.24 eV. The observed traps with small activation energies are attributed to Bi pair defects. The photoluminescence (PL) and Raman spectra have evidenced different compressive strain which affects considerably the optical properties. Furthermore, the PL spectra were also affected by different contributions of Bi- related traps which are different for different substrate orientation in agreement with DLTS results

Dilute GaAs1−xBix epilayers with different bismuth concentrations grown by Molecular Beam Epitaxy: A promising candidate for gamma radiation sensor applications

Radiation interaction studies are very important for exploring the technological applications of new materials in radiation environments. This work reports the effect of gamma radiation dose on the structural and optical properties of dilute GaAs1−xBix epitaxial layers grown with different Bismuth contents by MBE on (1 0 0) GaAs substrates. The influence of radiation has been studied by X-Ray Diffraction (XRD), Raman spectroscopy, and photoluminescence (PL) measurements. The samples were also characterized by Scanning Transmission Electron Microscopy (STEM) and Scanning Electron Microscopy (SEM. Gamma radiation (γ-) was found to influence the optical properties of GaAs1−xBix epitaxial layers. From Raman measurements it was found that the concentration of holes increased when the samples were irradiated. This result is in good agreement with photoluminescence 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 XRD data revealed that for irradiated GaAs1−xBix samples, the crystallographic quality of the samples was slightly changed after irradiation. This result is consistent with the results of photoluminescence measurements, which demonstrated that the GaAs1−xBix samples exposed to 50 kGy dose showed an increase in photoluminescence and full width at half maximum for all irradiated samples

Effect of bismuth surfactant on the structural, morphological and optical properties of self-assembled InGaAs quantum dots grown by Molecular Beam Epitaxy on GaAs (001) substrates

In this work, we have investigated the effect of Bi surfactant on structural, morphological and optical properties of 5 monolayers self-assembled InGaAs quantum dots (QDs) grown on GaAs (001) substrates at various growth temperatures (435, 467 and 495 °C) by Molecular Beam Epitaxy. Two types of InGaAs QDs samples grown with and without exposure to bismuth were studied using Atomic Force Microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy and Photoluminescence (PL). Our results have demonstrated that Bi-mediated growth provides improved control of several properties of InGaAs QDs including an enhancement of the QD PL peak intensity by 1.7 times as compared to InGaAs/GaAs control sample grown without Bi. In addition, a red-shift of the PL peak energy of about 40 meV was also observed when the InGaAs QDs were grown by using Bi evidencing that Bi surfactant affects considerably the size of QDs. Furthermore, the QDs grown with Bi surfactant exhibited a higher degree of size uniformity as demonstrated by the observation of narrower Full Width at Half Maximum (FWHM) of the PL peaks. We have also shown that both Bi surfactant and substrate temperature play an important role to control the density of InGaAs QDs. The QD density decreased from 8.9 × 1010 cm−2 (control sample) to 2.0 × 1010 cm−2 for the sample grown at the lowest temperature of 435 °C under Bi flux. All these approaches to control and improve the properties of self-assembled QDs are important for device applications that require high optical efficiency and low QD density