Examination of the Growth and Evaporation of Gallium - (001) Using Reflection High Energy Electron Diffraction.
Abstract
Reflection high energy electron diffraction (RHEED) was demonstrated to be an quantitative in-situ probe for crystal growth studies. RHEED was used on Molecular Beam Epitaxy (MBE) grown Gallium Arsenide (GaAs) (001) surfaces to assess the role of the instrument and of surface imperfections on the diffraction pattern. Limitations were placed on the instrumental uncertainties and the maximum resolvable distance was estimated. Once the instrument had been characterized, RHEED was used during MBE growth to study both disorder and the role of surface steps on GaAs (001). During growth, oscillations in the intensity and width of the RHEED beams were observed. The period of the oscillations was equal to the monolayer deposition time and resulted from competition between step growth and nucleation. The amount of step growth and nucleation could be changed by varying the growth conditions or the crystal misorientation. The relative amounts of each could be determined by analyzing the diffraction profiles during growth. This was done using the two-level step model of Lent and Cohen. Excellent agreement was found for both angle of incidence and coverage dependence. Fits of the diffracted beam profiles were used to measure the diffusion length on the surface during growth as a function of Ga arrival rate, As(,4) pressure, crystal misorientation, and temperature. The measured diffusion lengths were incorporated into a model which describes how the intensity oscillations behave as growth conditions and crystal misorientation is varied. Similar oscillations in the RHEED intensity were found to take place with no incident Ga flux at higher temperatures. These correspond to layer-by-layer evaporation of the surface. The evaporation rate (measured from the oscillation period) was found to obey the equilibrium thermodynamic law of mass action for the dissociation of GaAs; K(,p) = P(,Ga) P(,AS(,2))(' 1/2). The evaporation of Ga from GaAs and Al(,x)Ga(,1-x)As films manifest itself in reduced growth rates and increased mole fraction X. These two quantities were measured during high temperature growth and confirm that Ga is evaporating from the surface.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1985
- Bibcode:
- 1985PhDT........31V
- Keywords:
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- Engineering: Electronics and Electrical; Physics: Condensed Matter