Surface properties of clean, and with adsorbed oxygen, surfaces of CdTe (110), {111}, and (100) and of CdSe {0001} studied by electron-energy-loss spectroscopy and Auger-electron spectroscopy

Surface electronic states and oxidation properties of CdTe (110), {111}, and (100) surfaces and CdSe {0001} surfaces have been studied with low-energy electron-loss spectroscopy, Auger-electron spectroscopy, and low-energy-electron diffraction. The surface losses due to the transitions from the Cd-$4d$ core level locate the empty Cd-derived surface states at 2.5 [for the (110) and (111) plane] or 3.0 eV [for the (100)] in CdTe and 2.7 eV in CdSe, with respect to the valence-band maximum. The losses appearing at 61 and 66.2 eV in CdSe are identified as due to the transitions from the Se-$3d$ core level to the Se-derived surface states lying at about 8 and 13 eV above the valence-band maximum. No corresponding Te-derived empty surface states, i.e., transitions from the Te-$4d$ level to Te-derived surface states, have been detected for CdTe. Oxygen uptake on the ordered surface is very slow when the surface is exposed to molecular oxygen; the initial sticking coefficients are ${10}^{-13\mathrm{}}$ for CdTe (100), less than ${10}^{-14\mathrm{}}$ for CdTe (110), and ${10}^{-14\mathrm{}}$ for CdSe (0001). Oxidation is stimulated drastically by electron-beam irradiation on the surface exposed to molecular oxygen and bulklike oxide overlayers are formed. Upon oxidation of CdTe surfaces, the shifted Te-$4d$ loss due to Te${\mathrm{O}}_2$ occurs at rather early stages of oxidation and grows in magnitude with oxygen uptake, and the loss spectrum from the heavily oxidized surface resembles that of Te${\mathrm{O}}_2$, while formation of neither a CdO nor a Cd element is evident. These findings indicate that the oxygen adsorbs preferentially on surface Te atoms by breaking the back bonds at the initial stages of oxidation, and then oxidation proceeds into the bulk, forming the oxide overlayer composed entirely of Te${\mathrm{O}}_2$. In the case of the CdSe (0001) surface, oxygen adsorbs only on the surface Se atoms without breaking the back bonds at the initial stages of oxidation, and then it seems to adsorb with a breaking of the back bonds, and both selenium oxide and cadmium oxide are formed, although part of the selenium oxide sublimes away from the surface, leaving the oxide overlayer rich in CdO.