NOVELTY - Forming thin film (505) of predetermined thickness on substrate (510) involves a) moving substrate to first reaction space; b) contacting substrate with a first precursor of Group III element in first reaction space to form a first metal layer having submonolayer coverage; c) contacting first metal layer with hydrogen-containing species; d) moving substrate to a second reaction space; e) contacting substrate with a second precursor of Group V element in second reaction space to form a second layer; and f) repeating steps (a)-(e) until the thin film with predetermined thickness is formed.    USE - For forming thin film (preferably Group III-V film) of predetermined thickness on substrate. The thin film is gallium nitride thin film, aluminum nitride thin film, indium nitride thin film, aluminum gallium nitride thin film, aluminum indium nitride thin film, indium gallium nitride thin film, or indium gallium aluminum nitride thin film; or epitaxial layers of gallium nitride, aluminum nitride, indium nitride, aluminum gallium nitride, aluminum indium nitride, indium gallium nitride, or indium gallium aluminum nitride (claimed). The thin film obtained by the method used in various applications such as photovoltaic solar cell and light emitting diode (LED) devices and systems (e.g. GaN based violet laser diodes), field effect transistors (FET) (such as metal-oxide semiconductor FET (MOSFET) and metal semiconductor FET (MESFET) devices), and transistors (e.g. high electron mobility transistors (HEMT)).    ADVANTAGE - The method enables rapid repetition of sub-monolayer deposition of thin films; and provides thin film having high quality. The thin films obtained by the present method have defect (e.g. misfit dislocations, such as threading or edge dislocation) densities of 106 to 1010 dislocations/cm2; and have a root mean square of the height differences of less than or equal to 0.5 nm, or less than or equal to 3 nm, or less than or equal to 5 nm, or less than or equal to 10 nm as measured by atomic force microscopy (AFM). The full-width at half maximum of the (0002) or (1012) crystallographic reflections of Group III-V thin films formed according to present method have less than or equal to 100 arcseconds, or less than or equal to 400 arcseconds, or less than or equal to 600 arcseconds.    DESCRIPTION OF DRAWING(S) - The figure shows a structure having a Group III-V thin film formed over a substrate.    Thin film (505)    Substrate (510)