The interaction of hydrogen (H) and a ZnO(0001)-O surface has been investigated using a temperature programmed desorption (TPD) technique. When the surface is exposed to atomic hydrogen below 400 K, hydrogen is adsorbed on the surface. As the hydrogen exposure increases, bulk diffusion of hydrogen takes place. The existence of surface and bulk hydrogen has been confirmed using X-ray photoelectron spectroscopy (XPS). When the ZnO surface dosed with hydrogen is heated, surface hydrogen is desorbed at 432 K and bulk hydrogen is evolved at ∼539 K. Diffusion of hydrogen into the ZnO bulk is an activated process, and the activation energy is estimated to be 0.19 eV.

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The thermal reactions of surface and subsurface hydrogen atoms on ZnO have been investigated. When the (0001) and ð10 10Þ surfaces of ZnO are exposed to atomic hydrogen at 370 K, surface-bound hydrogen atoms are formed on both surfaces. As the hydrogen exposure increases, H atoms diffuse into the bulk of ZnO. Upon heating, recombinative desorption of surface H atoms occurs at $450 K on both surfaces. Bulk H atoms also evolve as H 2 molecules. In the case of ZnO(0001), bulk hydrogen is completely removed below 750 K. Evolution of bulk hydrogen on the ð10 10Þ face of ZnO takes place at much higher temperatures. Our observations confirm that the migration of H atoms in the ZnO crystal lattice takes place mostly along the c-axis.

The atomic layer deposition (ALD) of ZrO2 was conducted in ultrahigh vacuum (UHV) conditions. The surface was exposed to ZrCl4 and H2O in sequence and the surface species produced after each step were identified in situ with X-ray photoelectron spectroscopy (XPS). ZrCl4 is molecularly adsorbed at 140 K on the SiO2/ Si(111) surface covered with OH groups. When the surface is heated to 300 K, ZrCl4 loses two Cl atoms to produce ZrCl2 species. Remaining Cl atoms of ZrCl2 species can be completely removed by exposing the surface to H2O at 300 K followed by heating to 600 K. The layer-by-layer deposition of ZrO2 was successfully accomplished by repeated cycles of ZrCl4 dosing and H2O treatment.

Thermal reactions of CH3OH on a ZnO (0001) surface with and without coadsorbed atomic hydrogen have been investigated using a temperature-programmed desorption (TPD) technique. Both H2 and CH2O desorb at 510 and 580 K during CH3OH decomposition on ZnO(0001). When H atoms are adsorbed on ZnO(0001), the recombinative desorption of H2 takes place at around 450 K. In the process of CH3OH decomposition on ZnO(0001), H2 desorption is not observed until the surface temperature reaches 510 K. These observations indicate that surface-bound H atoms are not produced up to 510 K. When CD3OD and H are coadsorbed, the desorption of both H2 and HD is observed at 445 K. HD should be formed by the exchange reaction between CD3O–D and H on the surface, which indicates that the CD3O–D bond is partially broken to form an associative CD3···D complex. We suggest two different pathways for the formation of CH2O from CH3OH on ZnO(0001). At 510 K, CH2O and H2 are formed from the surface reaction of CH3O and H. Desorption of CH2O at 580 K is related to the complete decomposition of CH3O.