I. M. Vitomirov

Photoelectron attenuation lengths and thermal broadening in GaAs(110)

Abstract High resolution photoemission spectra from cleaved GaAs(110) have been used to determine the attenuation length as a function of photoelectron kinetic energy in the solid. The data are well described by an empirical functional form with a minimum escape depth of 4.77 A at an energy of 32 eV. These results are compared with those from studies of InSb(110) and GaP(110). In addition, constant resolution photoemission has been used to evaluate thermal broadening of core-level line shapes from 50–300 K over a range of photon energies from 40–190 eV. This effect is relevant in high resolution studies of temperature dependent chemistry on metal-semiconductor interfaces and heterojunctions.

Ag and Co cluster deposition on GaAs(110): Fermi level pinning in the absence of metal‐induced gap states and defects

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 di...

Temperature‐dependent interface morphology and Schottky barrier evolution for Au/InP(110)

Results of extensive high‐resolution synchrotron radiation photoemission studies reveal significant differences between Au/InP(110) interfaces formed at 300 and at 60 K. At 300 K, Au atom deposition results in substantial substrate disruption and, for coverages ≥2 A, a thickening Au overlayer containing intermixed In and P atoms. For atom deposition at 60 K, the amount of substrate disruption is not significantly changed, but surface segregation of the released In and P atoms is largely, though not entirely, inhibited. These reults show that the distinctly different temperature‐dependent Schottky barrier evolution on n‐ and p‐type InP(110) cannot be attributed to thermally controlled reaction since there is little such control. Low‐temperature changes in Fermi‐level pinning for Au coverages at 2–4 A appear related to the development of metallic character in the overlayer.

Abruptness of Au‐Si contacts with thin CoSi2 interlayers

High‐resolution synchrotron radiation photoelectron spectroscopy has been used to study Au/Si and Au/CoSi2/Si interface formation at room temperature. Our results show Au‐Si intermixing, the absence of a well defined Au‐Si compound, and surface segregation of small amounts of Si to the Au surface. An interlayer formed by the deposition of ≤3 A of Co has relatively small effect on this Au‐Si atomic profile. Intermixing is abruptly quenched, however, when the Co deposition exceeds 3.5 A, and a Au film free of Si can grow on the CoSi2 layer. These results demonstrate the effectiveness of CoSi2 layer as a barrier against Au‐Si intermixing and identify the critical coverage of Co needed to passivate the Si surface.

A 20–350 K variable‐temperature sample holder with sample interchangeability

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.

Energies and symmetries in interface formation: In/GaP(110) and Ga/InP(110)

We have used high‐resolution synchrotron radiation photoemission to study adatom–substrate interactions and growth morphologies for In/GaP(110) and Ga/InP(110). Room‐temperature experiments reveal extensive adatom clustering, but also substrate disruption and cation segregation for both interfaces, with greater amounts for Ga/InP(110). Ga deposition in InP(110) at 60 K also results in substrate disruption, but with kinetic trapping of the released In atoms close to the interface and a greater tendency toward layer‐by‐layer growth. In constrast, the deposition of preformed metallic Ga clusters shows no evidence for substrate disruption. We conclude that atom condensation and coalescence are responsible for disruption, with different activation barriers being present for cluster deposition and atom deposition because of the details of surface and interface bonding. For Ga deposition on n‐type InP(110) at 60 K, the appearance of states at the Fermi level is correlated to changes in band bending. Metal cluste...

Interface growth with atoms and preformed clusters: Morphology and Schottky barrier variations for Au/InP(110)

With synchrotron radiation photoemission, we contrast the morphology and the Schottky barrier obtained when Au atoms are condensed onto InP(110) at 300 and ∼60 K to what is obtained when preformed, metallic Au clusters are deposited. Atom by atom deposition at either temperature leads to substrate disruption and Fermi level pinning 0.75 eV below the conduction‐band minimum (CBM). Deposition of preformed Au clusters induces almost no disruption and a pinning position 0.42 eV below the CBM. Differences reflect the dependence upon the process, and therefore the energetics, of bringing dissimilar atoms in contact.

Interface formation by atom and cluster deposition: Novel electronic and structural properties

High‐resolution synchrotron radiation photoemission has been used to examine changes induced at semiconductor interfaces related to sample temperature and whether atoms or metal clusters are deposited. Atom deposition at 300 and 60 K for Au/n‐InP(110) and Co/n‐GaAs(110) results in substrate disruption, different rates of Fermi‐level movement, but the same final Fermi‐level pinning positions. A novel method has been developed for the deposition of preformed clusters onto atomically clean surfaces. Such deposition produces minimal substrate disruption for Au/InP and no disruption for Co/GaAs but the apparent unrelaxation of the surface in the vicinity of the cluster. Cluster deposition also leads to unique pinning positions of 0.42 and 0.32 eV below the conduction‐band minimum, respectively, that do not change as the amount of metal changes from 0.05 to 30 A. These positions are far from the charge neutrality point expected for metal induced gap states, even though up to ∼85% of the surface has a fully meta...

Fermi level movement for n- and p-GaAs interfaces: effects of temperature and dopant concentration

Photoemission studies demonstrate that temperature and dopant concentration dependent movement of the surface Fermi level is controlled by coupling between adatom‐induced and bulk states. At a low temperature for lightly doped n‐ or p‐GaAs, initial band bending inhibits tunneling and EF remains near the band edges until the onset of metallicity. For heavy doping, greater band bending reflects a thinner depletion region. Thermal cycling for 20≤T≤300 K for low coverages demonstrates that band bending is reversible.

Cluster-Assembled Interfaces

Publisher Summary Cluster deposition allows the formation of abrupt, nearly-ideal metal–GaAs interfaces. This procedure produces large, fully-metallic clusters on the GaAs surface without any observable substrate disruption. Interfaces formed in this manner show intriguing E F evolution, that is, metal and coverage independent for n-GaAs but weakly dependent on the metal for p-GaAs. The Fermi level energies are quite distinct from those observed for interfaces formed by direct atom deposition for n-GaAs but coincide better for p-GaAs. The differences are related to loss of surface relaxation around the clusters and the creation of new bonding configurations involving the clusters. No evidence of MIGS or conventional defect levels is seen for preformed metal cluster deposition. The details of the Fermi level evolution are not completely understood at this time, but the abrupt disruption-free nature of the interfaces make them particularly attractive for metal-semiconductor junction modeling. Certainly, these results demonstrate the importance of the energetics associated with atom condensation and bond formation. Specifically, comparison of results for Ag clusters grown spontaneously on GaAs during 300 K atom deposition and those deposited with the cluster technique shows dramatically different band bending.