J. R. Waldrop

Measurement of ZnSe–GaAs(110) and ZnSe–Ge(110) heterojunction band discontinuities by x‐ray photoelectron spectroscopy (XPS)

X‐ray photoelectron spectroscopy was used to study the growth and energy‐band alignment of ZnSe–GaAs(110) and ZnSe–Ge(110) heterojunctions. The ZnSe–GaAs heterojunctions were formed by growing ZnSe on GaAs(110). Growth temperatures were varied to produce both epitaxial and nonepitaxial interfaces. For ZnSe grown at ∠300u2009°C on GaAs(110), the valence‐ band discontinuity ΔEv was 0.96 eV; for ZnSe deposited at room temperature and crystallized at ∠300u2009°C, ΔEv is 1.10 eV. The Ge–ZnSe(110) interfaces were formed by depositing Ge(ZnSe) on ZnSe(Ge)(110) at room temperature, followed by ∠300u2009°C crystallization. The corresponding ΔEv’s were 1.52 and 1.29 eV, respectively. Our measured ΔEv values for epitaxial heterojunctions are compared with the predictions of theoretical models. Our results demonstrate that substantial interface structure dependent contributions to ΔEv can occur at Ge–ZnSe(110) and GaAs– ZnSe(110) heterojunctions.

Measurement of semiconductor heterojunction band discontinuities by x‐ray photoemission spectroscopy

Accurate knowledge of the band discontinuities at a heterojunction interface and the factors that affect their magnitude are of both fundamental and practical interest. The application of x‐ray photoemission spectroscopy (XPS) to the direct, contactless, and quantitative measurement of the valence band discontinuity (ΔEv) at abrupt heterojunctions is discussed. The topics covered include a description of a method to achieve precise measurement, results of ΔEv measurements for many heterojunction pairs selected from the lattice‐matched semiconductor series Ge, GaAs, ZnSe, CuBr, and AlAs, and a comparison of experiment to models that predict ΔEv. Examples are given to illustrate the effect on ΔEv of such preparation‐dependent parameters as growth sequence and interface crystallographic orientation.

Correlation of GaAs surface chemistry and interface Fermi‐level position: A single defect model interpretation

XPS measurements of core‐level binding energies have been used to determine the interface Fermi‐level position in (100) and (110) n‐and p‐type GaAs samples as a function of surface treatment. A variation in interface Fermi‐level position of up to 0.7 eV has been observed. The difference in interface Fermi‐level position for n‐ and p‐type samples subjected to identical surface treatments remains nearly constant at ≊0.3 eV. A single defect model (SDM) is suggested which appears to provide the simplest explanation for the observed interface‐potential variations.

XPS measurement of GaAs–AlAs heterojunction band discontinuities: Growth sequence dependence

We report the direct measurement, by x‐ray photoemission spectroscopy, of the valence‐band discontinuity, ΔEv, for two types of abrupt GaAs–AlAs (110) heterojunctions grown by molecular beam epitaxy: (i) those formed by growth of GaAs on AlAs, and (ii) those grown in the reverse sequence, AlAs on GaAs. The ΔEv at GaAs–AlAs interfaces is, on average, 0.25 eV larger than at AlAs–GaAs interfaces. The ΔEv for GaAs–AlAs heterojunctions was found to average 0.4 eV; the corresponding ΔEv for AlAs–GaAs heterojunctions averaged 0.15 eV. The 0.25 eV difference in average ΔEv value that we observe for the two types of interface demonstrates that the energy‐band discontinuities depend on growth sequence in the GaAs–AlAs heterojunction system.

Protection of molecular beam epitaxy grown AlxGa1−xAs epilayers during ambient transfer

A method for the protection of reactive compound semiconductor surfaces during transfer from a molecular beam epitaxy apparatus through ambient for further processing or experimental characterization is demonstrated for GaAs and AlAs. This method is likely to be applicable to other compound semiconductors.

Interfacial chemical reactivity of metal contacts with thin native oxides of GaAs

X‐ray photoemission spectroscopy (XPS) was used to investigate the chemical reactivity and band‐bending variation of native oxide covered surfaces of GaAs (100) during Schottky‐barrier formation. The GaAs surfaces prior to metal deposition had thin ∠10 A overlayers of either As2O3 and Ga2O3 or only Ga2O3. A variety of metals, some chemically inert (Au, Cu, and Ag) and some chemically reactive (Al, Mg, Cr, and Ti), were studied on both types of oxide surfaces. The chemical reactions occurred at room temperature and were well predicted by bulk thermodynamic free energies of formation.

The mechanism of Schottky‐barrier formation in polyacetylene

An investigation of metal‐polyacetylene contacts by using x‐ray photoemission spectroscopy is reported. For undoped p‐type polyacetylene, Mg metal formed a rectifying contact with a Schottky‐barrier height of ≳0.6 eV; Au metal formed a pure ohmic contact. Changes in band bending in the polyacetylene with metal deposition were directly observed by x‐ray photoemission spectroscopy and correlated with I‐V transport measurements. Our results indicate that the mechanism for Schottky‐barrier formation in polyacetylene is the electrostatic match of work functions at the metal‐polyacetylene interface and that there are no intrinsic or extrinsic filled interface states within the polyacetylene band gap.

Refractory metal contacts to GaAs: Interface chemistry and Schottky‐barrier formation

A survey of the interface chemistry of the refractory metals W, Ta, Re, Ir, and Mo during room‐ temperature Schottky‐barrier contact formation to GaAs by using x‐ray photoemission spectroscopy (XPS) is reported. The metals were deposited onto clean n‐type GaAs (100) surfaces within the XPS system by two methods: Evaporation and plasma sputtering. For each metal a distinct interfacial reaction which produced GaAs dissociation and formation of a new metal arsenide is observed. Refractory metals are in general not chemically inert in contact to GaAs and nonabrupt interfaces ∠10 A in width are formed. XPS was also used to correlate interface chemistry with measurement of Schottky‐barrier height during contact formation. XPS measured barrier heights ranged from 0.9–0.7 eV, in the order W, Ir, Mo, Ta, and Re.

Reactivity and interface chemistry during Schottky‐barrier formations: Metals on thin native oxides of GaAs investigated by x‐ray photoelectron spectroscopy

The room‐temperature interfacial chemical reactions of overlayers of several diverse metals (Au, Cu, Al, Mg, Cr, and Ti) with thin native oxide films (∼10 A) on GaAs (100) surfaces were investigated with x‐ray photoelectron spectroscopy (XPS). The reactivity of these metals with the native oxides of GaAs ranged from inert to complete reduction for the oxides and is well predicted by bulk thermodynamic free energies of formation. Variations in band bending during Schottky‐barrier formation were monitored by XPS. The implication of the observed interface chemistry for Schottky‐barrier modeling is discussed.

Schottky barrier behavior in polycrystal GaAs

The results of a study of the effects of grain boundaries on the electronic properties of Schottky barriers formed in polycrystal GaAs are reported. The samples investigated were fabricated on n‐type (Sn doped) GaAs grown by MBE on large grain polycrystal n+‐GaAs substrates. Al layers were deposited in situ in the MBE chamber immediately following the growth of the epilayers and 10 mil Schottky dots were subsequently isolated photolithographically. The diodes characterized fell into three categories: (i) diodes which were located entirely within a single grain and exhibited ideal, n=1 behavior, (ii) diodes which straddled a grain boundary but whose electrical characteristics were indistinguishable from the single crystal diodes, and (iii) diodes which straddled a grain boundary and whose forward current contained an excess leakage component. The data are explained in terms of a model in which the Fermi level is pinned by localized bandgap states due to dangling bonds at the grain boundary and the resultin...