TITULO ONCE

3.3.1

Atomic Force Microscopy study

Atomic Force Microscopy (AFM) has been used as a routine technique to study the surface dot morphology, and dot density for InxGa1-xAs/GaAs QDs [17]. To optimize the QD size and density for our solar cell structures, three QD structures were grown with the only variation of dot growth time.

The surface morphology of the In0.5Ga0.5As QDs with the growth time of 3.3, 3.4, 3.6 s were measured using AFM and presented in Figure 3.2. Similar to what has been observed by other studies [1, 2, 5, 18], the dot density and size varies as a function of the amount of material deposited. When the dot growth time is increased from 3.3 to 3.4 s, the dot density is increased. However further increase of growth time to 3.6 s does not lead to increase of the density, indicating that saturation point is reached at a deposition time of ~ 3.4 s. Meanwhile, the QDs average size is increased with growth time. Also it is found that more clusters of larger dots are formed when the growth time is increased to 3.6 s (see Figure 3.2 (c)), which leads to more defects in the quantum dot layers [1, 4].

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Figure 3.2: AFM images of QDs grown with different growth time: (a) 3.3 s, (b) 3.4 s, (c) 3.6 s. For all images the scales are 1 μm × 1 μm. The height profiles are shown by the

colour bar.

The quantum dot density in a single layer that was grown for 3.3 s, 3.4 s, and 3.6 s were counted by ImageJ (an open source image processing program designed for scientific multidimensional images) [19] and listed in Table 3.1 below. The highest QD density is obtained from 3.4 s sample, the density is estimated at 4.5 × 1010 cm-2 for Figure 3.2 (b).

Table 3.1: Surface dot density for QDSC structures grown with different dot growth time.

Growth time/s Quantum dot density/cm-2

3.3 3.9 × 1010

3.4 4.5 × 1010

3.6 3.2 × 1010

3.3.2

Photoluminescence

The room temperature photoluminescence (PL) spectra from these three QD samples with different dot deposition times are shown in Figure 3.3. For each sample, there are two distinct PL peaks originating from the QD and wetting layers respectively, indicating the high optical quality of our QDSC structures with a dominant radiative recombination process.

During the 2D-3D transition process the emission peak from the wetting layer (WL) (take 3.3 s sample for example) is blue-shifted from that expected for the nominal composition of In0.5Ga0.5As materials [8]. This is mainly due to the quantum confinement effect for the ultra-thin wetting layer (~6 ML). Also, inter-diffusion between the InGaAs WL and GaAs layer changes the composition of the thin WL and lead to lower content of In, and thus a shorter

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wavelength. The other peak at longer wavelength comes from quantum dots that have been formed. As more WL material transforms into QDs with the increase of dot growth time, the QDs grow in size with the intensity of QD peak becomes stronger and that of the wetting layer becomes weaker. The increase in the ratio of PL intensity of QDs to WL with dot growth time can be seen clearly from Figure 3.3. Also, the peak position of QDs is red-shifted with longer deposition time after 3.4 s, which is due to the formation of larger QDs which have lower ground state energy. In addition, the full width at half maximum (FWHM) becomes slightly larger for longer growth time (FWHM values of QDs are 85 nm, 92 nm and 103 nm for 3.3 s, 3.4 s and 3.6 s samples respectively), indicating that the QDs size distribution is less uniform beyond certain deposition time.

Figure 3.3: Room temperature photoluminescence spectra of In0.5Ga0.5As/GaAs QDs

grown with different QD growth times.

Based on the above comparison from Figure 3.2 and Figure 3.3, 3.4 s was chosen as the growth time for the QDSC samples investigated in this work, since it provides the highest QD density and minimum cluster defects.

3.3.3

Transmission Electron Microscopy

As mentioned earlier, in order to achieve sufficient light absorption, multiple stacked QD layers is normally required in QDSC structures. However, due to

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the strain-driven self-organizing growth process and thus the accumulated strain with the increasing number of layers, the QD growth conditions are extremely critical for achieving defect-free multi-layer stacked QD structures. There is the tendency for dislocations to form in one layer and propagate throughout the whole multi-layer structure, which may lead to a large V- shaped defects and thus much degraded device performance [20]. To further evaluate the material quality of our QDSC structures, cross-sectional transmission electron microscopy (TEM) analyses were carried out. Also, bright field scanning transmission electron microscope (STEM) was used for structural observation. Since growths were carried out on (100) wafers, the cross-sectional STEM and TEM images are close to one of the <110> directions.

Cross-sectional bright field STEM image for the 3.4 s sample is shown in Figure 3.4. Well defined layer structure is clearly displayed, which is in good agreement with the nominal structural design shown in Figure 3.1 (b). The structure consisting of the different layers are clearly visible. GaAs, InGaAs and AlGaAs layers appear gray, black and white, respectively, corresponding to their composition and relative average atomic weights [21].

Figure 3.4: Bright field cross-sectional STEM image showing the whole quantum dot solar cell structure, in which the different layers are clearly observed.

Furthermore, Figure 3.5 shows the higher magnification TEM images of the QD region. It can be seen that the 10 QD/WL layers are well aligned with no V-shaped or other types of line defects observed in the structure. In addition, high resolution image in Figure 3.5 (c) indicates that no dislocations exist

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across the GaAs barrier and InGaAs QD/WL interfaces. The strain accumulation is properly controlled by current growth conditions.

Figure 3.5: (110) cross-sectional TEM view of the 10-layer QDSC at three different magnifications, where (c) is the high resolution image.