A. K. Wahi

EFFECT OF ANNEALING SB/INP(110) INTERFACES AND SCHOTTKY BARRIER FORMATION OF AG ON ANNEALED SB/INP(110) SURFACES

The effect of annealing one monolayer of Sb on p‐InP on the surface Fermi level position and the band bending due to Ag deposition on these well‐ordered surfaces have been studied using photoemission spectroscopy. The adsorption of one monolayer of Sb on p‐InP gives a Fermi level position 0.85 eV above the valence band maximum (VBM). However, with increasing annealing temperature, the band bending decreases and recovers to nearly the flatband condition above 200 °C. The Fermi level movement of annealed InP shows a correlation with the surface stoichiometry of phosphorus and indium. Ag deposition on these annealed Sb/p‐InP interfaces gives an anomalously low band bending of 0.5 eV above the VBM.

Large Schottky barrier heights on n-InP : a novel approach

An unusually high Schottky barrier height (SBH) of 0.82 eV has been observed at Au/n‐InP(110) interfaces with one monolayer of Sb as an interlayer using photoemission techniques. For Au deposited on clean cleaved InP(110), Au‐In alloying that occurs with increasing Au coverage destroys interfacial perfection. The resulting Fermi level position lies ∼ 0.5 eV below the conduction band minimum, as is common for metal interfaces with clean cleaved InP. However, for InP surfaces first passivated with one monolayer of Sb, this Au–In alloying is completely inhibited. An abrupt interface results in an increased possibility of a low interface defect density. This represents a possible way to control of SBH and produces high SBH for n‐InP.

Band bending at Al, In, Ag, and Pt interfaces with CdTe and ZnTe (110)

Band bending behavior and interfacial chemistry for Al, In, Ag, and Pt overlayers on vacuum‐cleaved p‐CdTe and p‐ZnTe (110) have been studied using ultraviolet photoelectron spectroscopy (UPS) and x‐ray photoelectron spectroscopy (XPS). These metals provide a range of metal–substrate reactivities: Al reacts strongly with Te, Ag moderately, and In minimally, with no evidence seen for In reaction on ZnTe. Pt exhibits strong alloying behavior with both Cd and Zn. All four metals are found to yield Schottky barriers on CdTe and ZnTe, with a narrow range of final Fermi level positions, Efi=Ef−EVBM, observed on CdTe, from 0.9 to 1.05±0.1 eV, and on ZnTe from 0.65 to 1.0±0.1 eV. The prediction of the MIGS model that a difference in barrier height exists for two semiconductors dependent upon their band lineup (valence band offset) is examined and found to agree with experiment for Ag, Pt, and Al, but not for In. For the highly reactive Al, no evidence for the overlayer metallicity required for metal‐induced gap s...

Photoenhancement mechanism for oxygen chemisorption on GaAs(110) using visible light

Visible light from an argon ion laser (514.5 nm, 3 W/cm2) is seen to increase oxygen chemisorption on cleaved GaAs(110) surfaces up to a final coverage between one and two monolayers. Using photoemission spectroscopy to measure the oxygen coverage after simultaneous exposure of the surface to oxygen and light, we have determined that oxygen uptake for photoenhanced exposures is independent of sample temperature and doping type. In addition, significantly less enhancement is seen for weakly bound oxidizing molecules (N2O) relative to the effects with molecular oxygen. These results are explained by a photoenhancement mechanism in which energy is released in a surface recombination event, possibly in the form of nonthermal phonons, causing physisorbed gas molecules to dissociate and thereby overcoming a major rate limiting step of the reaction in the dark. This reaction mechanism is supported by calculations of the surface recombination rates and free carrier densities at the surface which show that only the recombination rate is correlated with enhanced oxygen uptake. Other mechanisms and experimental data are also discussed.

Use of low temperature to reduce intermixing at metal:HgCdTe contacts

The metal/HgCdTe interfaces formed by evaporating Ag and Al onto cleaved HgCdTe substrates held at 170 K in ultrahigh vacuum (p<1×10−10 Torr) exhibit greatly reduced intermixing between the metal and substrate to the extent, in the case of the Ag/HgCdTe interface, that no detectable intermixing occurs. The incremental metal depositions used in this study ranged from tenths of monolayers at submonolayer coverages to several monolayers at thicker coverages. The evolution of these interfaces was investigated using the surface sensitive techniques of photoemission spectroscopy and low‐energy electron diffraction. In addition to the reduced intermixing, unusual band bending behavior in the semiconductor at very low metal coverages is observed for these interfaces formed at low temperature. In the low‐coverage regime (<5 monolayers), the bands are seen to bend from the position after metal deposition back towards the initial cleaved position after time for the Ag case and after exposure to x rays in the Al case...

CdTe and ZnTe metal interface formation and Fermi-level pinning

Interfacial morphology and Fermi‐level pinning behavior at the interfaces of Al, Ag, and Pt with UHV‐cleaved CdTe and ZnTe have been studied using x‐ray photoelectron and ultraviolet photoemission spectroscopies. Results are compared to metal/HgCdTe interface formation, where the weak Hg–Te bond and consequent ease of Hg loss strongly influence semiconductor disruption and metal–semiconductor intermixing. For Al/CdTe, we observe a case where the strong Al–Te reaction yields a significantly more extensive Al–Te reacted region than has been observed for HgCdTe. The Al/ZnTe interface is observed to be more abrupt than Al/CdTe. The final Fermi‐level pinning positions Efi=Ef−Evbm for Al, Ag, and Pt on p‐type CdTe and p‐ZnTe have been determined. Efi is found to be roughly the same for both CdTe and ZnTe, with the value for ZnTe lying ∼0.2 eV closer to the valence‐band maximum for all three metals. From these results, one would expect Schottky barriers of about the same height for these metals on p‐CdTe and p‐Z...

One monolayer of Sb or Bi used as a buffer layer preventing oxidation of InP

One monolayer of Sb or Bi has been used as a buffer layer to protect InP surfaces against the oxidation. We have used photoemission spectroscopy to estimate the oxide fraction of In and the surface Fermi level position. We find that one monolayer of Sb or Bi reduces the oxidation of underlying InP at least by more than two orders of magnitude. For passivated surfaces with one monolayer of Sb or Bi, the surface Fermi level remains pinned at the conventional pinning level 0.45 eV below the conduction‐band minimum (CBM) even for oxygen exposures above 1×107 L, whereas without the Sb or Bi overlayers, the surface Fermi level is near the CBM for the same O2 exposure. Core‐level studies indicate that the Sb or Bi overlayer must be itself oxidized before there is strong oxidation of the InP.

Photoemission spectroscopy of ordered overlayers on GaP (110)

Interfaces between UHV-cleaved GaP (110) and overlayers of Sn, Sb, and Bi have been studied using soft X-ray photoemission spectroscopy. The substrate core level attentuation indicates that these three interfaces exhibit Stranski-Krastanov growth. The overlayer core level spectra (Sn 4d, Sb 4d, and Bi 5d) exhibit two components for coverages up to one monolayer, indicating that the adatoms form a zigzag chain overlayer as has been reported at the Sb/GaAs interface. The Ga 3d and P 2p surface components disappear as the first monolayer develops, and the remaining bulk components are used to determine various lineshape parameters of these core levels. The Schottky barrier formation at these interfaces has also been examined. The band bending at these interfaces occurs chiefly for coverages exceeding one monolayer, when islands are forming on the ordered layer. For all three of these interfaces the final surface Fermi level position is in the neighborhood of 1.0eV above the valence band maximum.

Interfacial chemistry of metals on CdTe and ZnTe (110)

We consider interfacial chemistry and band bending behavior for Al, In, Ag, and Pt overlayers on vacuum‐cleaved p‐CdTe and p‐ZnTe (110) in a comparative study using ultraviolet photoelectron spectroscopy and x‐ray photoelectron spectroscopy. A range of metal–substrate reactivities have been considered: Al reacts strongly with Te, Ag moderately, and In minimally, with no evidence seen for In reaction on ZnTe. Pt exhibits strong alloying behavior with both Cd and Zn. These results for the binaries are compared to metal/HgCdTe interface formation. We find that Hg loss can significantly influence the extent of reaction and/or intermixing for these overlayers, with resulting disruption either inhibiting or facilitating chemical interaction. Reaction and intermixing for Al, Ag, and Pt overlayers on CdTe and ZnTe indicate these interfaces are not ideal. The possible role of defects at these four metal/CdTe and metal/ZnTe interfaces is considered, and provides a consistent explanation for the final Fermi level po...

Fermi‐level movement at metal/HgCdTe contacts formed at low temperature

In an effort to reduce the disruption to the HgCdTe surface upon metal overlayer formation over that observed at room temperature, interfaces formed at 100 K between HgCdTe and the three overlayer metals, Ag, Al, and Pd, are investigated. The metals were deposited onto the [110] cleaved HgCdTe surfaces in UHV in increments ranging from submonolayer at low coverages to tens of monolayers at high coverages, and the formation of the interfaces were monitored using photoemission spectroscopy subsequent to each deposition. The primary experimental observation of this study is that deposition of all three of these metals onto HgCdTe substrates held at reduced temperatures causes the surface Fermi level (Ef) to move from its initial cleaved position, typically located at or near the conduction‐band minimum, into the conduction band, and this movement is correlated with the absence of the movement of the overlayer metal into the semiconductor. This phenomenon is observed at the lowest coverages (submonolayer) for...