We report a room temperature soft x-ray photoemission spectroscopic study of electronic barrier formation and chemistry at Al, Ag, and Au contacts with horizontal Bridgman-grown GaAs(100) surfaces, prepared by etching, heat-cleaning in situ, and As-capping. As reported previously, we observe a pronounced chemical reaction with formation of dissociated Ga for Al, a minimal interface reaction for Ag, and interdiffusion for Au. However, we find significant differences in the interface electronic barrier heights between the present metal/melt-grown GaAs(100) contacts and those made on cleaved (110) surfaces as well as on melt-grown (100) surfaces. We obtain Schottky barrier heights of 0.62, 0.85, and 1.03 eV respectively for Al, Ag, and Au/GaAs interfaces covering a range of 0.41eV—in contrast to the 0.2–0.3 eV range obtained earlier for both cleaved (110) and heat-cleaned (100) surfaces as well as the 0.7 eV range observed for metal/molecular beam epitaxy GaAs(100) interfaces. This intermediate behavior underscores the dependence of barrier height variation on different growth and interface processing techniques.
We have used Auger electron diffraction with high angular resolution to measure elastic strain at a pseudomorphic metal-metal interface. Shifts in the position of the $\mathrm{Cu} {L}_{3}{M}_{4,5}{M}_{4,5}$ Auger intensity maximum along the [101] direction betray expansion of the Cu lattice normal to the Cu/Ni(001) interface resulting from the Cu-Ni lattice mismatch. In the pseudomorphic regime (up to 14 \AA{} of Cu), the Cu lattice constant perpendicular to the interface has been determined to be 3.71 \ifmmode\pm\else\textpm\fi{} 0.03 \AA{} while the lattice constant is 3.52 \AA{} in the plane of the interface (the lattice constant of Ni). Thus, the unit-cell volume of Cu is 46.0 \ifmmode\pm\else\textpm\fi{} 0.04 ${\mathrm{\AA{}}}^{3}$ in the pseudomorphic overlayer, compared to a bulk value of 47.0 ${\mathrm{\AA{}}}^{3}$. Above 14 \AA{}, the lattice constant perpendicular to the interface drops as a result of dislocation generation and the relief of elastic strain. The critical coverage at which strain relief begins and the dependence of strain on coverage are in good agreement with simple classical models.
Synchrotron-radiation photoemission studies of the formation of Al/GaAs(110) interfaces have been performed as a function of substrate temperature for 60\ensuremath{\le}T\ensuremath{\le}300 K for n- and p-type doped samples. The results show temperature-dependent changes in surface Fermi-level position, surface morphology, and the distribution of released substrate atoms in the overlayer. Detailed examination shows a separation in energy of \ensuremath{\sim}1.0 eV for the Al 2p binding energy for n- and p-type GaAs at low coverage. This equals the difference in band bending for the two substrates and demonstrates that the adatom energy reference is an intrinsic level of the semiconductor, not the Fermi level. Substrate band bending approaches its final value when ${E}_{F}$ becomes the energy reference for the overlayer, and this occurs at the onset of metallic overlayer behavior. Temperature-dependent band bending observed below \ensuremath{\sim}1 monolayer can be understood in terms of the coupling of an adsorbate energy level to the semiconductor via steady-state tunneling and thermionic emission. The high-coverage results at all temperatures are consistent with metallicity.
A simple design is presented for an ultrahigh-vacuum-compatible sample holder capable of maintaining temperatures between 20 K and ∼350 K. The design involves a copper sample holder (tank) mounted on a closed-cycle He refrigerator. Mechanical, thermal, and electrical contact between the sample and the tank is obtained by melting and resolidifying Ga in a recess in the tank. Temperature regulation is achieved through simultaneous action of the cold stage and a filament heater attached to the tank. This allows in situ sample exchange without compromise of ultimate vacuum or temperature.
The steps associated with intentionally misoriented GaAs(100) surfaces produce interface charge states that can substantially alter the Schottky barrier height. These interface states are located near midgap in energy with density increasing in nearly one-to-one proportion to the density of step-related bonding sites. This detailed correlation between vicinal step features and deep-level densities demonstrates and gauges the systematic interface electronic perturbation associated with off-axis growth.
A novel method of forming interfaces makes it possible to produce abrupt, defect-free metal/semiconductor boundaries. The procedure involves the growth of metal clusters on Xe buffer layers condensed on clean surfaces, and Xe sublimation brings the clusters into contact with the pristine surface. Clusters of Co and Ag produce unique, nearly coverage-independent Fermi level positions ∼0.32 and 1.0 eV below the conduction-band minimum for n- and p-GaAs (110), respectively. Detailed line shape analysis of the photoemission spectra shows no evidence for cluster-induced disruption or conventional defect formation. Instead, the EF position appears to be related to intrinsic surface states swept into the gap as a result of surface unrelaxation around the metal clusters. The results are contrasted to those for metal atom deposition at 60 and 300 K. For Co atom-by-atom deposition, substantial temperature dependences are observed in the Schottky barrier evolution, despite nearly equivalent adatom-induced surface disruption at both temperatures. Ag atom deposition at 300 K and Ag cluster deposition produce similar morphologies, but quite different EF positions. From the Ag results, it appears that cluster deposition produces strain at the interface, strain which is much less for atom deposition where the higher mobility of Ag allows better accommodation to the surface structure.
The surface stabilities of La1.85Sr0.15CuO4 and YBa2Cu3O6.9 have been studied for samples exposed to ultrahigh vacuum, oxygen, ∼1500 eV electron bombardment, and zero order synchrotron radiation. Oxygen chemisorption saturates by 10 L and induces energy states ∼1.6 eV above EF. High energy electron bombardment eliminates this structure for La1.85Sr0.15CuO4 but not for YBa2Cu3O6.9. The effects of monochromatic photon irradiation for uv or x‐ray beams are negligible, but surface modifications are observed when the sample is exposed to the zero‐order white beam generated by the Aladdin synchrotron radiation source. The surface modifications are related to BaCO3 residuals, probably in grain boundaries of the polycrystalline samples.
