Electron and optical microscopy

A Normarski differential interference microscope was used to measure the thickness of bulk epitaxial layers (GaSb, InAs and InxGa1-xSb). These epitaxial layers were cleaved, dipped

into a Murakami etchant solution (10 g sodium hydroxide, 10 g potassium ferrycyanide and 100 mL de-ionised water) for about 6 seconds and rinsed in de-ionised water prior to being looked at under the microscope. The etchant preferentially etches the interface between the epitaxial layer and the substrate and thus leaves a line that can be seen under the microscope.

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The microscope gives uncertainty that is better than 10 % in the measured thickness hence this microscope can only be used to measure thicknesses of 1.0 µm and above. Another Normarski microscope was also used for investigating the surface morphology of the epitaxial layers.

A JEOL JSM 2100 LAB6 transmission electron microscope with an atomic resolution of 1.4 Å and an accelerating voltage of 80 to 200 kV was used to study the structural properties of the SLS’s in cross-section. This system was also used to confirm the validity of the thicknesses of the SLS’s obtained by simulating the HR-XRD spectra. Focused ion beam scanning electron microscopy was used to cut and polish samples leaving (110) faces that were used for cross sectional images. This system was also used for high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging of the cross sections of the SLS’s. HAADF-STEM images were used to study the interfaces in the InAs/InxGa1-xSb SLS’s.

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Chapter 6

Growth of GaSb and In

Ga

Sb

6.1 Introduction

The growth of complex structures such as InAs/InxGa1-xSb SLS’s requires a thorough

understanding of the growth of its constituent layers (InAs and InxGa1-xSb). This chapter

discusses the MOVPE growth and characterization of GaSb and InxGa1-xSb. The growth of

InAs is not included in this discussion as it has already been extensively studied in our laboratory (Vankova, 2005).

The selection of the parameters (i.e. the growth temperature, nominal V/III ratio and the vapour composition) for the growth of the layers discussed in this section was based on the issues raised in Chapter 4. These include the melting point of the material to be grown, the decomposition temperature and the vapour pressure of the organometallic precursors.

The results presented here have been marred by issues of repeatability, which are more pronounced for ternary layers. This was noticed to worsen for longer time periods between the growth of different series. Included at the beginning of Section 6.2 and 6.3 is a plot of x as a function of the V/III ratio and xv, respectively, for different series grown over a period of

two years. Also included are discussions on what is expected based on previous reports in the literature with regard to the influence of the V/III ratio on the indium incorporation efficiency. The level of sensitivity of these properties of InxGa1-xSb to changes in the V/III

ratio is compared to that of GaSb grown at the same temperature.

This chapter includes discussions on the influence of the V/III ratio on the surface morphology, structural quality and optical properties of the thin films (Section 6.2.1 and 6.2.2). The influence of the vapour composition (xv) during the growth of InxGa1-xSb on the

above-mentioned properties will be discussed in Section 6.3. The influence of substrate orientation and the position of the substrate on the susceptor on the incorporation efficiency of indium into GaSb is presented in Section 6.4.

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6.2 Influence of V/III ratio on surface morphology, crystal structure,

optical properties and indium solid content of GaSb and In

Ga

Sb

Figure 6.1 shows a plot of x as function of the V/III ratio for various series of InxGa1-xSb

layers grown at 510 °C and 550 °C, with xv of 0.2, 0.4 and 0.5. The series grown at 550 °C

(depicted by solid symbols) were grown without the Epison III gas flow controller. In general, all the series show a sub-linear increase in the solid composition upon an increase in the V/III ratio. The samples grown at 510 °C have much higher indium mole fractions in the solid than those grown at 550 °C. Studies of the dependence of the incorporation efficiency of indium on the growth temperature have shown that lower growth temperatures yield enhanced incorporation. This has been attributed to the different growth rates of the binaries that are thermally activated. (Juang et al., 1991, Bougnot et al., 1986). As expected, the layers grown with xvof 0.5 have higher indium content in the solid compared to those grown

with xvof 0.4 and xvof 0.2 at the same temperature and V/III ratio.

Figure 6.1 Plot of x vs. V/III for several series of In_xGa_1-xSb layers grown over a period of three years at 510 °C and 550 °C, using x_v of 0.2, 0.4 and 0.5.

The dependence of x on the V/III ratio can be explained as follows: when the V/III ratio is less than unity at the growth interface, competition between the group III species (which are abundant) for antimony species (which is in short supply) will lead to the preferential incorporation of the group III element which yields a thermodynamically more stable binary,

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in this case GaSb. The incorporation efficiency of indium will thus be reduced. Even though no reports have been presented on the study of the influence of the V/III ratio on the solid composition of III-III-V materials, studies on the dependence of composition on the V/III ratio for III-V-V (like GaSb1-xAsx (Cherng et al., 1984)) can be used as a model for the

current study, as their behaviour is expected to be analogous.