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Author

Tatsuya Okada

Date of Award

3-1996

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Materials Science and Engineering

Supervisor

Professor G.C. Weatherly

Abstract

Heteroepitaxial growth of In₁_ᵪGaᵪAsᵧP₁_ᵧ alloy thin films on InP(001) substrates using molecular beam epitaxy (MBE) is technologically important for the fabrication of optoelectronic devices. Recent progress in MBE, for example the use of hydride gas sources such as AsH₃ and PH₃ has improved the control of the P to As ratio and added more versatility to this thin film growth technique. In this thesis, transmission electron microscopy (TEM) studies of some unique growth-related phenomena occurring in In₁_ᵪGaᵪAsᵧP₁_ᵧ films grown with gas-source MBE are described. The experiments were intended to understand the growth mechanism of the films and the characteristics of structural "defects", rather than to obtain practical knowledge which can directly be applied to the manufacturing of optoelectronic devices.

The initial stage of strain relaxation in compressively-strained InAsᵧP₁_ᵧ ternary alloy films was studied. The compressive misfit strain in 190Å thick films ranged from -0.96% to -2.43% (0.30≤y≤0.77). Both TEM and cathodoluminescence imaging revealed the existence of new types of misfit dislocations aligned close to [100] and [010] directions generated from heterogeneous dislocation sources at the InAsᵧP₁_ᵧ/ InP (001) interfaces. The new types of misfit dislocations were found to be pure edge in character, formed probably by slip on {011} planes. Contrast profiles were computed for misfit dislocations parallel and adjacent to one of the free surfaces of a plan-view TEM foil by taking into account the significantly modified elastic fields associated with such misfit dislocations. The calculated profiles were useful when Burgers vectors of misfit dislocations generated from dislocation source were analyzed.

A composition modulation occurring only along the [110] direction with a small wavelength (100Å) was repeatedly observed in 3000Å thick In₁_ᵪGaᵪAsᵧP₁_ᵧ alloy films having a wide range of nominal composition being lattice-matched to InP. This modulation is different from the "classical" spinodal decomposition along elastically soft [100] and [010] directions with long-wavelength (1000-1500Å), commonly observed in In₁_ᵪGaᵪAsᵧP₁_ᵧ films grown by liquid phase epitaxy. A simple free energy argument including the effect of constraint from the InP substrate suggests that the observed modulation is not directly caused by the minimization of free energy but is driven by kinetic processes occurring at the surface of growing films.

The morphological instability of a strained film surface was also studied. Cross-sectional TEM investigation revealed the coexistence of prominent composition modulation and surface undulation in tensile-strained In₁_ᵪGaᵪAsᵧP₁_ᵧ films. The surface morphology of the strained films showed a clear correlation with the degree of the composition modulation, occurring predominantly along the [110] direction. When the film was under tension, the surface undulation developed at relatively low strains (as low as 0.5%); on the other hand, films with the same magnitude of compressive strain did not exhibit any surface undulation. In addition, the surface undulation was composition dependent; it was enhanced as the degree of composition modulation increased. These features distinguish the present morphological instability induced by the inherent composition modulation from the "classical" three-dimensional island growth mode of compressively strained films having strains larger than 2%.

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