C. E. McCants

The advanced unified defect model for Schottky barrier formation

The advanced unified defect model (AUDM) for GaAs proposed in this paper can be looked upon as a refinement of the unified defect model (UDM) proposed in 1979 to explain Fermi level pinning on 3–5 compounds due to metals or nonmetals. The refinement lies in identifying the defect producing pinning at 0.75 and 0.5 eV above the valence band maximum as the AsGaantisite. Since the AsGaantisite is a double donor, a minority compensating acceptor is necessary. This is tentatively identified as the GaAsantisite. The concentration of As excess or deficiency due to processing or reactions at interfaces is particularly emphasized in this model. A wide range of experimental data is discussed in terms of this model and found to be in agreement with it. This includes the original data on which the UDM was based as well as more recent data including Fermi level pinning on the free-GaAs(100) molecular-beam epitaxy surface, Schottky barrier height for thick (∼ 1000 A) Ga films on GaAs, and the LaB6Schottky barrier height on GaAs(including thermal annealing effects). Of particular importance is the ability of this model to explain the changes in Schottky barrier height for Al and Au on GaAs due to thermal annealing and to relate these changes to interfacial chemistry.

The advanced unified defect model and its applications

The advanced unified defect model (AUDM) is presented for GaAs interfaces with metals, other semiconductors, and insulators. The key defect is the ASia antisite, which is also responsible for EL-2 and semi-insulating GaAs. The energy levels of the Asca antisite (0.75 and 0.52 eV) correspond well with the previously identified energy levels (0.75 and 0.5 eV) of the unified defect model (UDM). Using the AUDM, it is shown that a wide range of experimental data can now be explained. The Fermi level position on GaAs(001) MBE surfaces and its dependency on As or Ga are explained. The changes in Schottky barrier height of LaB6/GaAs, AI/GaAs, and Au/GaAs on annealing are explained in terms of Asia antisite density near the interface being increased or decreased due to the annealing. For A1 and Au this is correlated with the metal/semiconductor interfacial chemistry, and a predictive capability is developed in terms of net increase or decrease of As at the interface due to this reaction. The Fermi level pinning behavior at low temperature is also explained in terms of this model. True understanding of the surfaces and interfaces of 3-5 semiconductors is still in its infancy. In 1979 the unified defect model (UDM) was proposed to explain the behavior of interfaces of 3-5 compounds with metals and nonmetals. The key to this model was a suggestion, based on a wide range of experimental data, that the Fermi level at such interfaces could be determined by 3-5 lattice defects. In 1979 very little was known about 3-5 defects in the UDM. These were assigned to As and Ga vacancies. Much more is known today about defects in GaAs. As a result we present an advanced unified defect model (AUDM) in which the key defect is the same Asia antisite which provides EL-2 and semi-insulating GaAs. Based on this, a number of experimental observations which have given difficulty in the past can now be explained.

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.

Experimental results examining various models of Schottky barrier formation on GaAs

The mechanism of Schottky barrier formation on GaAs and other semiconductors seems to be getting more, rather than less, controversial. Therefore, it is important to examine new data as it becomes available. Four types of data will be mentioned. The emphasis will be on (1) the metal–GaAs chemical reaction products at the interface and their correlation (or lack thereof) with barrier heights and (2) the lack of evidence for discontinuity in Schottky barrier height as one goes from photoemission spectroscopy (PES) pinning results (thickness of order 1–1000 A metal films). Also mentioned are (3) changes in barrier height for Al, Au, and Ag on diodes annealed up to 500 °C and (4) the effect on barrier height of growing ∼20 A of native oxide on the GaAs before depositing a 1000‐A metal layer. These data will be presented for discussion and evaluated in terms of the various models of Schottky barrier formation now under discussion which appear most consistent with the unified defect model.

Comparative uptake kinetics of N2O and O2 chemisorption on GaAs(110)

Using surface sensitive photoemission we have measured oxygen uptake versus gas exposure to N2O and O2 on cleaved GaAs(110) surfaces in the coverage range from 0 to 1.5 monolayers, focusing on the reaction variables of surface temperature and visible light illumination. As would be expected given the greater ease of dissociation for N2O compared with O2 (bond strengths of 1.7 and 5.1 eV, respectively), we found that N2O reacts much more readily than O2 at room temperature in the dark, with sticking coefficients at 0.4 ML of 3×10−7 and 7×10−10, respectively. This observation is also supported by a truer measure of reactivity, the chemisorption activation energy, which at 0.15±0.08 eV for N2O is only one-third of the value 0.47±0.02 eV for O2 when corrected for the molecular physisorption energies. These results confirm the fact that the breakup of oxygen molecules physisorbed on the GaAs surface is a major rate-limiting step in the chemisorption reaction. The similarity which we have observed between the kinetics of N2O and optically enhanced O2 uptake furthermore suggests strongly that it is that step—the breakup of O2—which is assisted by visible light.

Chemical and electrical properties at the annealed Ti/GaAs(110) interface

We present the results of a photoemission study of the annealed Ti/GaAs(110) interface, using soft x‐ray (SXPS), x‐ray (XPS), and ultraviolet (UPS) photoelectron spectroscopies to monitor the substrate and overlayer core levels and the valence band for Ti thicknesses between 6.7 and 20 monolayers and annealing temperatures between 200 and 425 °C. The SXPS data imply the formation of a stable Ti–As region near the Ti/GaAs interface that increases in width with temperature. Concomitant to this, the binding energy of the Ga changes from its room‐temperature position towards elemental Ga. The XPS data suggest the presence of elemental Ga at or near the overlayer surface. The UPS data suggest the possibility of cluster formation on the overlayer surface. In situ current–voltage (I–V) measurements were also made on thick‐film systems fabricated using similar methods to those used in the photoemission studies [i.e., cleaving, depositing metal (∼1000 A), and annealing in UHV]. The movement of Ti–As reaction produ...

Kinetic study of Schottky barrier formation of In on GaAs(110) surface

The kinetics of the Schottky barrier formation of In on GaAs(110) surfaces has been studied by photoemission. A temperature dependence of the profile of the n‐GaAs(110) surface band bending versus In coverage has been observed. At liquid nitrogen temperature (LNT), the band bending attenuation is observed compared with a room and higher temperature band bending profile. Temperature dependence of the In growth mode is also observed. Spectroscopic evidence indicates that the GaAs band bending is correlated with the In metallic cluster growth. The experimental results are discussed in terms of the unified defect model. A possible transition from Bardeen limit (with pinning centers) to Schottky picture (without pinning centers) of the Schottky barrier formation of In on GaAs(110) is also discussed.

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.

Temperature-dependent pinning at the Al/n−GaAs (110) interface

It is shown that at the Al/n‐GaAs(110) interface grown in ultrahigh vacuum at −80 °C the Fermi level remains unpinned at least up to a 3 monolayer coverage. In contrast, at room temperature the pinning near midgap is established after a deposition of approximately 1 monolayer of Al. The low‐temperature behavior is correlated with the growth of a more uniform overlayer which inhibits cluster and defect formation. This result provides a critical test of models of Schottky barrier formation.

Cr on GaAs (110): The effect of electronegativity on the Schottky barrier height

Cr deposited in sequential steps on atomically clean GaAs(110) surfaces shows strong chemical reactivity at the interface with Cr‐arsenide formation. Soft x‐ray and ultraviolet photoemission spectroscopies have been employed to elucidate the room temperature chemistry and the Fermi level pinning behavior of this system. It was recently reported that the Pd/n‐GaAs(110) system has a Schottky barrier height of ∼0.9 eV, is highly reactive, and is characterized by similar adatom arsenide formation. The distinction here is that Cr/n‐GaAs(110) has a barrier height of ∼0.7 eV. Thus the Schottky barrier height appears to be independent of the nature of the reaction. An examination of the trends in the electronegativity (Pauling’s scale) shows a significant correlation to the Fermi level pinning. We found that low electronegativity (∼1.6) metals like Cr pin at the 0.7 eV level (below the conduction band minimum) which normally acts as an acceptor. The Fermi level of higher electronegativity (∼2.0) materials, e.g., Pd, Cu, Ag, and Au, is observed to pin at the 0.9 eV level which normally acts as a donor. This is exactly what one expects for charge transfer between the semiconductor and the metal using electronegativity as a basis. Further confirmation is found in the examination of changes in the ionization potential threshold of the Cr/GaAs system with metal deposition.