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nitrides. For obtaining atomic reactive nitrogen, the N2 molecules are dissociated by the radio-frequency (rf)-plasma or the electron cyclotron resonance (ECR) method. Although the rf-plasma source is the most popular and the rf-radical source produces considerably fewer ions than an ECR source due to the higher plasma pressures [Hug95], it is well known that ion damage can still induce during the epitaxy [Pow93]. Some techniques to avoid such ion damage by an ion tapping system using the static electric field, have been tried [Bot95, Mol94, Iwa96]. The other serious problems induced by the plasma may be some contamination such as oxygen or carbon dioxide.

MOMBE is also one of the potential growth techniques where the advantages of both MOVPE and MBE can be utilized. Film can be grown relatively at low temperature by MOMBE and premature reaction of the precursors, a serious problem in MOVPE, is minimized due to the large mean free path of gaseous molecules.

2.4.3 Atomic Layer Deposition (ALD)

Atomic Layer Deposition (ALD) can be considered as a special mode of CVD. It is a surface deposition process that can be used for the controlled growth of epitaxial films, and the fabrication of tailored molecular structures on the surfaces of solid substrates. ‘Monatomic layers’ can be grown in sequence which is a characteristic feature of ALD. Therefore, the desired coating thickness can be produced simply by counting the number of reaction sequences in the process. The surface reconstruction of the monolayer formed in the reaction sequence will influence the saturation mechanism and the saturation density of the precursor.

The ALD reaction sequences are normally perform in an ‘effective overdosing’ condition to ensure a complete saturation of the surface reaction to form the monoatomic layer. Furthermore, such effective overdosing’ condition also provides good conformal