One of the parameters influencing the properties of the structures studied in this thesis is the mole fraction ππ. This is basically the molar ratio of the organometallic source in the vapour stream and is given by:
ππ = ππππππ πππ»π»2 = ππππππππ π π π π ππππ. πππ»π»2πππ»π»2 πππ»π»2 , 3.13
where πππ»π»2 and πππ»π»2 are hydrogen density and molar mass, respectively.
3.1.4.3 V/III ratio
The molar ratio of the group V to group III species in the source vapour is one of the important parameters that influences the growth process and ultimately the structure and elemental composition of the deposited crystal. The V/III ratio is defined as the ratio of the molar flow rate of group V element(s) to the molar flow rate of group III element(s) in the vapour phase and can be mathematically represented by:
ππ πΌπΌπΌπΌπΌπΌ =
ππππ
πππΌπΌπΌπΌπΌπΌ, 3.14
where nIII and nV are the molar flow rates of the group III and group V element source, measured in mol/min.
3.1.5 Growth procedure
Detailed values of parameters used in this study will be given in the relevant results and discussion chapters later in the thesis. The substrates used were semi-insulating (100) GaAs substrates, misorientated by 2Β° towards (111)B. Pieces of substrate (~1x1 cm2) were cleaved from a wafer and then blown clean with dry nitrogen. No further cleaning of the substrate was required as it was bought βepi-readyβ. Subsequently, the cleaned substrate was placed on a molybdenum susceptor, loaded into the horizontal quartz reactor and purged with hydrogen for 10 minutes. This was followed by annealing at 600 Β°C for
45 ten minutes in order to remove any surface oxide. The annealing was done with a tBAs overpressure to prevent substrate surface deterioration. Following this the susceptor temperature was increased to 650 Β°C while maintaining the tBAs overpressure and a 100 nm GaAs buffer layer was grown for 840 s by introducing TEGa. A similar procedure was followed for bulk GaAs samples but a longer growth time was used. After the growth of the buffer, the susceptor temperature was set to a chosen value for the growth of either bulk GaSb or GaSb QDs. In order to avoid an arsenic background, the reactor was purged for 30 s prior to GaSb dot growth. Dots or bulk GaSb were grown by simultaneously introducing TEGa and TMSb into the reactor. For un-capped dot samples or bulk GaSb, no further layer was grown and the reactor heater was switched off. For capped samples, dot growth was immediately followed by growth of either a 10 nm GaAs cold cap, deposited at the dot growth temperature, or by growth of a GaAs cap at a chosen higher temperature. For cold capped samples, after depositing the cold cap, the susceptor temperature was increased to a chosen higher temperature before the remaining GaAs was deposited. The reactor heater was then switched off and the samples cooled down to 250 Β°C with a tBAs overpressure. Subsequent cool down to room temperature was performed with only hydrogen flowing through the reactor.
3.2 Sample characterisation 3.2.1 SPM
3.2.1.1 Introduction
The morphology of uncapped GaSb/GaAs dot samples was investigated by scanning probe microscopy (SPM) to study the shape, size and areal density.
SPM forms images of surfaces using a physical probe that scans the sample. An image of the surface being scanned is obtained by mechanically moving the probe (which is attached to a cantilever) in a raster scan of the specimen, line by line, and recording the probe-surface interaction as a function of position.The forces between the probe tip and the sample are measured from
46 the deflection of the cantilever when used in atomic force microscopy (AFM) mode of operation. The deflection moves a laser spot that reflects into an arrangement of photodiodes. This offers a 3D visualisation of the surface morphology. The working principle is demonstrated in Figure 3.3.
Figure 3.3: Working principle of SPM in AFM mode.
3.2.1.2 SPM procedure
A Bruker Dimension FastScan SPM was used to scan the surface features of the uncapped dot samples, the substrate and buffer layers. The mode of operation used was Tapping mode in air AFM. NanoScope software (version 1.40) was used to manipulate the SMP images to analyse these features. The areal dot density was analysed by the particle analysis function. Feature sizes were analysed by the section profile function of the software, while the shape was visually analysed by viewing the image in 2D and 3D. A typical image of an uncapped dot sample measured in this study in 2D and 3D is displayed in Figure 3.4 (a) and (b), respectively. Figure 3.5 displays a line profile of a section of the dots in Figure 3.4 obtained using the NanoScope software to demonstrate a typical analysis process.
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Figure 3.4: SPM images of typical uncapped GaSb QSs grown in this study. Images are displayed in (a) 2D to show the base shape, dot density and distribution and in (b) 3D to show the dot shape.
Figure 3.5: SPM line profile obtained (using the section function of the NanoSocope software) across a segment of the sample surface to reveal size and height.
3.2.2 TEM
3.2.2.1 Introduction
Information about the interfaces, as well as composition and morphology of capped samples, was obtained by high resolution scanning transmission electron microscopy (STEM). Transmission Electron Microscopy (TEM) uses an electron beam to interact with a sample to form an image on a photographic plate or specialized camera. High-Resolution TEM (HRTEM)
48 looks at the interference of the electron beam interacting with the sample, rather than the absorbance of the beam as in ordinary TEM. This gives a higher resolution, which is beneficial when studying nano-scale samples.
3.2.2.2 Experimental procedure
A double Cs-corrected JEOL ARM 200F transmission electron microscope operated at 200 kV and fitted with a Dual EELSTM mode Gatan GIF Quantum ERSTM spectrometer was used to study the structural properties of the GaSb/GaAs QD samples in cross-section. High angle annular dark field (HAADF) STEM imaging, sensitive to atomic number variations within the sample, bright field STEM imaging and electron energy loss spectroscopy (EELS) Spectrum Imaging (SI), were used to characterize the samples. A convergence semi-angle of 21.4 mrad was used for the STEM probe and a collection semi-angle of 54.3 mrad for the EELS signal collection. Sample preparation was done by removal of a specimen lamella from the near surface region of a sample using a FEI Nanolab 650 focused ion beam scanning electron microscope. Figure 3.6 displays an example of a TEM image of the samples produced in this study.
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Figure 3.6: Typical image of a GaSb/GaAs QD sample produced in this work. The image was acquired in High Angle Annular Dark Field (HAADF) STEM mode.
3.2.3 Photoluminescence 3.2.3.1 Introduction
Photoluminescence (PL) spectroscopy is a valuable technique used for sample characterisation in this study. As a tool that is used to study optical transitions in semiconductors, PL spectroscopy can provide indirect information of the heterointerface morphology in QDs and QWs. It can also serve as a probe for the structural and chemical perfection of the heterointerface. In this study PL was used to routinely evaluate the formation of the GaSb structures embedded in GaAs to ascertain incorporation. It was also used to study the carrier dynamics in the GaSb/GaAs QD structures.
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3.2.3.2 PL procedure
Photoluminescence spectra were collected using a fully automated Czerny- Turner type monochromator (1 meter focal length), a Nd:YAG diode-pumped laser (532 nm line) for excitation and a liquid nitrogen cooled Ge diode. The samples were mounted in a closed cycle He cryostat and the PL spectra measured between 10 K and RT.
An example of a 10 K PL spectrum showing optical transitions in a QD sample produced in this study is displayed in Figure 3.7. The weak and narrow peak at 829 nm, the broad band at ~ 950 nm and the broad band at ~ 1150 nm are due to transitions in the GaAs, the GaSb WL/QW and GaSb QDs, respectively. 800 900 1000 1100 1200 1300 1400 GaSb QD GaSb WL/QW
Int
ens
ity
(
A
rb.
uni
ts
)
Wavelength (nm)
GaAs B-EFigure 3.7: An example of a typical 10 K PL spectrum of a GaSb/GaAs QD sample fabricated in this study, showing emission from the intended QDs embedded in the GaAs matrix, as well as the GaSb WL and the host GaAs matrix.
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