Conclusions:

From the preparatory work conducted with respect to the antimony valved source, a method has been found for establishing a relationship between the valve position and antimony BEP. The gallium and antimony sources were calibrated from observation of RHEED intensity oscillations, providing access to accurate V:III ratios from measured BEP values. Due to the lack of observed oscillations over a sufficient range of temperatures, the incorporation kinetics of antimony were not measured during this investigation. The lack of oscillations arises from a transition to step- flow growth where there is no significant change in surface roughness with time.

TEM analysis suggests that thermal oxide desorption from GaSb substrates is not complete, despite samples showing ordered RHEED patterns during cleaning. This has been observed by other researchers, resulting in the supplier from which the substrates were purchased utilising a thinner protective oxide layer than has historically been the case.* While the apparatus was not available during the course of this investigation, it would be desirable in the future to try H-atom cleaning, and use AES to determine the composition of the surface in situ.

Homoepitaxial GaSb growth has showed a range of morphological characteristics, including pyramidal, rippled and worm-like surfaces, depending on growth temperature and V:III ratio. Of the samples grown, none was observed to exhibit a completely flat surface. For the production of GaNxSb1-x films, which requires

growth temperatures lower than are employed for the binary compound, worm-like morphology will occur if the growth conditions of the ternary are the same as GaSb.

* _{Private communication with Wafertech}

In heteroepitaxial GaSb growth on GaAs the quality of the grown film is strongly dependent upon the thickness of deposition and, under the growth conditions used, produces a film with a large density of dislocations at the interface. These dislocations have been observed to propagate through the film and shows that the conditions used are not suited to high quality buffer layer growth. A thick GaSb buffer improves surface flatness, but removes advantages that growth upon GaAs provides, such as infrared characterisation or a low carrier concentration substrate.

There were GaSb/GaAs interfacial regions without threading dislocations. This suggests that misfit dislocations can relieve strain close to the interface, permitting growth of a defect-free GaSb layer or alloy, as has been reported for III-V heteroepitaxy.50,51 If so, a buffer layer may not be required although, in practice, a planarising layer of some description would be required for reproducible pyrometric temperature determination.

While there is potential for the growth of buffer-free samples using an interfacial misfit (IMF) array, time constraints precluded the investigation and optimisation of this method. In consideration of this and the results of chapter 3 which showed a rise in growth temperature during (In,Ga)Sb deposition on GaAs, the results here indicate that growth of GaNxSb1-x should initially be tried using GaSb substrates, utilising a

thin buffer layer to obtain a planar surface for temperature determination by pyrometry.

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5

Chapter 5

“They always say time changes things, but you actually have to change

them yourself.”

Andy Warhol (1928 - 1987)

GaNxSb1-x samples were grown under systematically varied conditions to determine the relationship between the growth conditions and the N incorporation, as determined by HRXRD.

Changes in nitrogen mole fractions as a function of both temperature and growth rate have been established, allowing nitrogen content to be controlled during growth upon GaSb substrates.

The maximum value of nitrogen incorporation observed was 2.6%. This represents the highest reported value for substitutional nitrogen content at the time of writing.