摘要:

Surface photovoltage (SPV) spectroscopy has been used to investigate electronic surface state distributions in the forbidden band of ultrahigh vacuum (UHV) cleaved clean and gas covered (O2, H2, H2O, H2S) GaAs(110) surfaces. On cleanp‐type surfaces a correlation is found between the cleavage quality and the absolute height of the SPV for h/ω≳1.5 eV. This effect is related to surface states near midgap which are due to crystallographic defects. A similar surface state distribution is also observed after O2adsorption. Exposure to atomic hydrogen (H) causes surface states at about 0.1 eV below the conduction band or near 0.1 eV above the valence band edge. The results of H2O adsorption are interpreted in terms of empty surface states derived from a molecular H2O level; H2S adsorbs in a two‐stage process causing two different types of extrinsic surface states within the forbidden band.

ISSN:0022-5355    

DOI:10.1116/1.570190

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

摘要:

Recent work on the III–V surfaces has indicated that adsorbate coverages ≲0.01 monolayers can significantly influence the surface electronic structure. The use of secondary ion mass spectrometry (SIMS) to study sorption phenomena on (110) UHV‐cleaved surfaces of InP at levels below 0.01 monolayers is reported here. The analyses reveal that significant quantities of adsorbates can accrue on a surface cleaved and stored in UHV, with a very rapid initial contamination of the freshly cleaved surface. For ∠0.1 L exposure to UHV total contaminant coverages ≳10−3monolayers are observed. The surface chemistry appears complex, with phosphorus oxides being the predominant species. The oxidation is apparently due to a dissociation/adsorption reaction with CO, with the additional possibility of adsorption of volatile phosphorus oxides initially desorbed from the sides of the cleaved boule. The above sorption phenomena are believed to be associated with a relatively small number of highly active surface sites, for example at cleavage steps. Further exposure to UHV up to ∠ 10 L results in partial saturation in the surface contamination coverage. Exposure of this surface to O2(4×105L) leads to further oxidation and possible chemisorption of O2onto P dangling bonds. The SIMS spectra contain evidence for an increasing level of P–In bond breaking with UHV and O2exposure, and this is associated with contamination‐induced surface disorder.

ISSN:0022-5355    

DOI:10.1116/1.570191

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

ISSN:0022-5355    

DOI:10.1116/1.570192

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

摘要:

Using contact potential difference measurements and photoemission yield spectroscopy, the surface state distribution in the band gap has been obtained forn‐ andp‐type GaP(110) surfaces. All cleavages ofn‐type samples exhibit a band bending of approximately 0.5 eV. Correlatively, a small density of occupied surface states is found in the vicinity of the Fermi level. From comparison with calculations involving different kinds of surface reconstructions, it is thought to correspond to the tail of an intrinsic surface state band, mainly empty. Another surface state band, the shape and magnitude of which depend on the cleavage, is detected near the valence band maximum. Upon low oxygen exposure, while the band bending increases forn‐type samples, the surface state distribution in the gap is modified, new states appearing close to the valence band top. Possible explanations involving surface relaxation or missing surface atoms are suggested.

ISSN:0022-5355    

DOI:10.1116/1.570193

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

摘要:

While the oxidation stages of silicon beyond one monolayer have been extensively documented by photoemission and Auger experiments, the question as to whether initial chemisorption of oxygen is molecular or dissociative is controversial. We have carried out theoretical calculations using the extended tight‐binding method simulating both O2and O overlayer adsorption on Si(111) surface. Comparison of published and our new uv photoemission spectra with our calculated density of states indicates a better agreement with the dissociated oxygen model than with the molecular one. We have also studied the x‐ray photoemission of oxygen on Si(111) from submonolayer to several monolayers of oxide (∠ 8Å). Our data is consistent with initial dissociative adsorption but incompatible with molecular adsorption.

ISSN:0022-5355    

DOI:10.1116/1.570194

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

摘要:

Surface extended x‐ray absorption fine structure (EXAFS) measurements are reported above the OKabsorption edge (∠535 eV) for oxygen chemisorbed on Si(111). For the investigated initial oxidation stage—characterized by a Si 2pchemical shift of 2.5 eV—we find theO–Si bond length to be closely the same (within 0.05 Å) as in bulk SiO2. Comparative analysis of the relative EXAFS amplitude with that for SiO2provides information on the oxygen bonding geometry on the Si(111) surface.

ISSN:0022-5355    

DOI:10.1116/1.570195

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

摘要:

The vibrational frequencies of Si–H groups, SiH, SiH2, and SiH3, in substituted silane molecules, and hydrogenated amorphous solids, display shifts in frequency that can be correlated with the relative electronegativities of the silicon nearest neighbors, i.e, the substituting atoms or groups in the molecules, and the hydrogen second neighbors in the amorphous networks. In this paper, the empirical relations are extended to include a description of Si–H vibrations on semiconductor surfaces.

ISSN:0022-5355    

DOI:10.1116/1.570196

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

摘要:

A general framework about the influence of reactive particles on surface electronic structure and barrier formation at low coverages (ϑ?10−2) is deduced from experimental results obtained under thermodynamic equilibrium conditions on a suitable compound semiconductor ’’model surface.’’ Adsorption isotherms enable a rough classification of weak and strong electronic interactions into physisorption, chemisorption, and surface reaction steps, which depend on temperatures and partial pressures. (1) Covalent bonding of particles on ionic compound semiconductor surfaces, characterized by small partial charges formally attributed to the adsorption complex, may lead to extremely large dipole effects even at very low coverages [e.g.,dipole moments ≳ 15 Debye may formally be attributed to the adsorption complex ZnO (101̄0)/CO2,chemat ϑ<10−2]. Correspondingly, drastic changes are found in surface atom relaxation. Sticking coefficients are close to unity. (2) Ionic bonding, characterized by pronounced band bending, Sticking coefficients are extremely low for ’’acceptor type’’ adsorption.(3) Point defects at compound semiconductor surfaces are, for entropy reasons, thermodynamically stable at high temperatures. We present for the first time quantitative results on equilibrium concentrations of surface point defects [oxygen vacancies at ZnO(101̄0)], which cause surface Fermi level pinning at extremely low coverages (ϑ<10−3).

ISSN:0022-5355    

DOI:10.1116/1.570197

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

摘要:

A preliminary analysis of the nature of the defect causing Fermi‐level pinning on the (110) faces of the (III–V) semiconductors is presented. Attention is focused on the cases of vicinal faces. Steps along the (110), (11̄1) and (001) directions are considered as well as clusters of steps. It is first shown that the high density of states observed on small disorientation angle faces can be understood only for steps along the (11̄0) direction. The absence of LEED patterns for steps on the (110) faces is discussed: it may suggest that clusters of steps must be chosen preferentially to single steps. One is then led to discriminate between steps with metallic and nonmetallic atom edges. To do this the energy dependent effective interactions method is applied to provide an estimation of the step levels of gallium arsenide for every configuration. The results are discussed: it is proposed that one of the most likely geometry might be a cluster of steps with nonmetallic atom edges, each atom bearing only one dangling bond.

ISSN:0022-5355    

DOI:10.1116/1.570132

出版商:American Vacuum Society    

年代:1979

数据来源: AIP

摘要:

The work function of cleaved GaAs(110) surfaces was measured in dependence of temperature between room temperature and 400 °C. Surfaces cleaved fromp‐type and tellurium‐dopedn‐type crystals exhibit a reversible behavior, while silicon‐dopedn‐type samples showed an irreversible decrease of the work function after the first heating cycles. In the presence of cleavage steps the work function and its temperature dependence are determined by step‐induced surface states via band bending and by step‐induced changes of the surface dipole.

ISSN:0022-5355    

DOI:10.1116/1.570133

出版商:American Vacuum Society    

年代:1979

数据来源: AIP