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Croydon, W. F. (1985) MBE growth and investigation of (001) GaAs surfaces using SIMS. Doctoral thesis, City of London Polytechnic.
An MBE system has been designed and constructed. After identification and elimination of various problems, high mobility, Si-doped , n-tYfe GaAs has been grown with electron concentration from ~ 5 x 10[to the power of]15 cm[to the power of]-3 (u 77 ~ 30,000 cm[squared]/Vs) to a solubility limit at ~ 7 x 10[to the power of]18 cm[to the power of]-3. Unintentionally doped material is high mobility p-type ~ 10[to the power of]15 cm[to the power of]-3 (u77 ~ 8000 cm[squared]/Vs) , with carbon acceptors identified as the major impurity . Differently reconstructed surfaces have been grown and transferred under UHV conditions to a separate chamber for SIMS analysis.
The low- dose SIMS measurements also identified various deficiencies in the system and enabled improvements to be made throughout the project. Sampling of selected mass peaks enabled SIMS spectra to be obtained with an ion dose of only 5 x 10[to the power of]11 cm[to the power of]-2. The sensitivity of various secondary ion species to the primary ion dose was also measured. Clean surfaces showed reproducible differences only in higher secondary ion yields from the (2 x 4) As stabilised surface, which correlates with the higher work function of this reconstruction, and a higher relative GaAs+ ion yield from the same surface, associated with a more 'bulk-like ' reconstruction. The Ga2+ signal sometimes showed a large increase with primary ion dose. This may be due to variable quantities of randomly adsorbed arsenic or to ion induced damage, but also dependent on the incident direction of the primary ion beam. Negative SIMS spectra showed only contaminant species.
No great differences were apparent in oxygen uptake on the differently reconstructed surfaces. There are indications that details of the exposure regime may be important, and of a higher initial sticking coefficient on the (4 x 6) Ga stabilised surface, but the results are not easily interpretable. A tentative deduction is that after initial adsorption, possibly at defect sites, the oxide is formed by a process of nucleation and growth, involving oxidation of both gallium and arsenic atoms, followed by the development of a more highly oxidised surface layer at higher oxygen exposures. Several interesting effects such as mass fractionation, ion-bombardment induced species and primary ion current density effects have also been observed and merit further attention.
Various experiments have been suggested as a continuation of this project, but directed as much towards understanding the basic processes of secondary ion emission as in applying the results to determining the GaAs surface structure.
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