W. G. Petro

Effect of low‐intensity laser radiation during oxidation of the GaAs(110) surface

Considerable interest in the use of laser irradiation as a means of processing (annealing, cleaning, alloying, etc.) semiconductor materials, as well as the importance of the oxygen surface chemistry on III‐V semiconductors, has led us to study the effect of low‐intensity (?3 W/cm2) laser radiation on the oxidation behavior of (110) surfaces of GaAs cleaved in UHV (<10−10 Torr). The oxygen sticking probability in the submonolayer coverage range has been increased by a factor of 103 (from a probability of ∠10−9 to ∠10−6) by uniform irradiation of the GaAs surface with a continuous wave Ar+ laser (λ = 5145 A) during the oxygen exposure. We find the data cannot be explained in terms of either heating of the surface or excitation of the oxygen by the laser radiation; it appears that the most likely explanation of the phenomena is an increase in the density of electrons and/or holes at the surface. A limiting step in the oxygen uptake process is the breakup of the oxygen molecule; this dissociation would be in...

Annealing of intimate Ag, Al, and Au–GaAs Schottky barriers

Using valence‐band and core‐level photoemission spectroscopy (PES) and electrical device measurements, the effects of annealing Ag, Al, and Au on n‐type (110)GaAs Schottky diodes fabricated in ultrahigh vacuum have been studied. PES was used to monitor the annealing‐induced changes in the interface Fermi level position and the chemical nature of the metal–semiconductor interface for submonolayer and several monolayer coverages. Barrier height determinations were also performed using current–voltage (I–V) and capacitance–voltage (C–V) device measurements on annealed thick metal film metal–semiconductor junctions. The results of this study show that the annealing‐induced microscopic changes in the electronic and chemical structure of the metal–semiconductor interface can be strongly correlated with the macroscopic changes in the electrical properties of thick film metal–semiconductor Schottky diodes.

Optically enhanced low temperature oxygen chemisorption on GaAs(110)

Optically enhanced oxidation of GaAs is promising both for insight into fundamental mechanisms of oxygen chemisorption and for possible application in GaAs direct write. For atomically clean GaAs(110), we have found that sample temperature is a key parameter in determining the increase of oxygen uptake under low‐intensity argon ion laser illumination (1 W/cm2 at 2.41 eV) relative to the ‘‘dark’’ uptake. Although cooling to 185 K suppresses the dark oxygen adsorption rate, the oxygen uptake with optical irradiation is the same at both 185 and 400 K. The ratio of laser‐assisted to dark uptake for 108 L exposure then approaches the enhancement necessary for spatially selective oxidation, showing an increase from 2.5 at room temperature to well over 10 at 185 K. After ruling out sample heating and direct oxygen gas excitation, we suggest that the light‐induced increase in uptake is due to electronic excitations in the GaAs. To explain the enhancement, one also needs to assume a weakly adsorbed precursor state...

Annealing of intimate Au‐GaAs Schottky barriers: Thick and ultrathin metal films

Using valence‐band and core‐level photoemission spectroscopy (PES) and electrical device measurements, the effects of annealing on Au:n‐type (110) GaAs Schottky diodes fabricated in ultrahigh vacuum have been studied. Similiar trends in the annealing‐induced changes in the barrier height of Au:n‐type GaAs were found for 0.2 and 15 monolayer coverages as determined by PES and for thick film coverages (1000 A) as determined by current‐voltage (I‐V) and capacitance‐voltage (C‐V) measurement techniques. In each case, the barrier height was found to be stable for temperatures between 30 and 200 °C and between 300 and 500 °C; while a gradual decrease in the barrier height was found for annealing temperatures of 200–300 °C. These changes are correlated with the formation of a Au‐Ga rich layer at the interface during anneals at 200 to 300 °C. Leakage currents were found to dominate the I‐V characteristics in the devices which were annealed above the Au‐Ga eutectic temperature. These peripheral leakage currents we...

Photoemission studies of the effect of temperature on the Au–GaAs(110) interface

We have performed soft x‐ray photoemission spectroscopy (SXPS) measurements, using synchrotron radiation as the excitation source, to study cleaved n‐type GaAs(110) surfaces covered with Au overlayers ranging in thickness from 0.3 monolayer (ML) to 80 ML. At room temperature our results show both band‐bending effects and significant Au intermixing, with a continuous, monotonic shift of the Au 4f core levels toward their bulk positions with increasing Au coverage. Observation of the Ga 3d and As 3d core levels indicates a preferential As segregation to the surface at the highest Au coverages. The effect of heating (up to 500 °C) on a relatively thick (15 ML) Au overlayer has been studied using valence band as well as core level emission. These results indicate an increase in the Ga and As concentrations with evidence of metallic Ga formation. These findings have an important bearing on our understanding of Schottky barrier formation as well as the particular behavior of the Au–GaAs interface.

Photoemission studies of the Au–InP(110) interface

We report new photoemission studies of the Au–InP (110) interface for both n‐ and p‐type samples using synchrotron radiation in the energy range 80 to185 eV. By slowly increasing Au coverages and by studying core level structures and valence band spectra we have been able to differentiate between two coverage intervals. The lower coverage interval (θ<1 ML) is dominated by band bending with no measurable chemical changes and with a total measured Schottky barrier height of 0.6 eV. In the higher coverage interval (1<θ≤37 ML), we observed changes in the In 4d and P 2p core level shapes and a monotonic shift of the Au 4f to lower binding energies with increased Au coverage. Evidence of intermixing of Au with the P and In atoms indicates the formation of a nonabrupt interface. These results are at variance with the view that Au is a nonreactive metal on InP (110).

Fermi energy pinning behavior and chemical reactivity of the Pd/GaAs (110) interface

The room‐temperature growth of the Pd/n‐GaAs (110) intimate interface has been studied by soft x‐ray core level photoemission spectroscopy. It is shown that Pd reacts with the GaAs surface forming an arsenide compound in which phase segregated Ga metal is included. In addition some As is segregated on the surface of this composite. Using deconvolution of the different core level contributions, the Schottky barrier height of this system is experimentally determined to be 0.9±0.05 eV. The pinning is attributed to the deeper level of the unified defect model.

High Schottky barriers on and thermally induced processes at the Au– GaAs(110) interface

New photoemission measurements show higher Schottky barrier heights (≳1.3 eV) on atomically clean GaAs(110) surfaces at a Au coverage of about 25 monolayers. It is suggested that this effect is due to the movement of Au into the semiconductor; at room temperature it creates acceptor states near the valence band maximum (VBM) that cause the Fermi level at the surface (Efs ) to move close to the VBM. We found that heating of the GaAs(110) surface (above 100 °C) covered with a small amount of Au (0.2 monolayer) causes Efs to move back to its original position (on the clean surface before Au deposition). The heating process is found to greatly inhibit the formation of large barrier heights due to the removal of defect states from the surface region.

Noble metal interactions with the InP(110) surface

We present details of the interfacial chemical reactions and intermixing observed for Cu, Ag, and Au overlayers on the clean cleaved InP(110) surface using soft x‐ray photoemission spectroscopy (SXPS). Analysis of the P 2p and In 4d core levels and the P LVV Auger line reveals a range of chemical interactions: Cu–P metallic bonding with phase segregation of metallic In for Cu overlayers, Ag islanding with a small amount of In segregation for Ag overlayers, and elemental P precipitation with Au–In alloying for Au overlayers. Deconvolution of the substrate and reacted layer components of the core level spectra is used to determine the Fermi level pinning positions for Ag and Au on n‐ and p‐type InP, and these values are compared to theoretical calculations of native defect energy levels.

Chemical reactions at the noble and near‐noble metal/InP interfaces: Comparison to Si and GaAs

The room‐temperature chemical reactions of noble (Cu, Ag, Au) and near‐noble transition (Ni, Pd) metals with the vacuum cleaved InP (110) surface have been studied with surface sensitive photoemission spectroscopies. It has been shown that the chemical reactions at these interfaces are closely similar to the reactions taking place on silicon substrates. In particular all metals that react strongly with Si to produce silicides (Cu, Ni, Pd) also form stable phosphides. This reaction is accompanied by phase segregation of metallic In. In addition, for Au (intermixing without a stable compound with P) and Ag (very weak reaction with substrate; growth of metallic Ag islands) the reactions with both InP and Si are qualitatively identical. It has been found that for GaAs the reactions with noble and near‐noble metals, though weaker than for InP, follow the same pattern.