D. A. Evans

Modification of GaAs Schottky diodes by thin organic interlayers

Control of the interfacial potential barrier for metal∕n‐GaAs diodes has been achieved using thin interlayers of the organic semiconductor, tin phthalocyanine (SnPc). The current–voltage (I–V) characteristics for organic-modified Ag∕S:GaAs diodes indicate a change from rectifying to almost ohmic behavior as the thickness of the SnPc interlayer is increased. Modeling reveals thermionic emission to be the dominant transport mechanisms for all diodes (ideality factors, n<1.3). Unlike other organic interlayers in similar device structures, SnPc reduces the effective barrier height by influencing the space charge region of the GaAs. The change in barrier height deduced from the I–V characteristics [(0.26±0.02)V] is similar to the band-bending measured using core-level photoelectron spectroscopy for SnPc growth on the S-passivated n‐GaAs(001) surface [(0.22±0.04)eV] and is much larger than previously reported for other similar systems.

Determination of the optical band-gap energy of cubic and hexagonal boron nitride using luminescence excitation spectroscopy

Using synchrotron-based luminescence excitation spectroscopy in the energy range 4–20 eV at 8 K, the indirect Γ–X optical band-gap transition in cubic boron nitride is determined as 6.36 ± 0.03 eV, and the quasi-direct band-gap energy of hexagonal boron nitride is determined as 5.96 ± 0.04 eV. The composition and structure of the materials are self-consistently established by optically detected x-ray absorption spectroscopy, and both x-ray diffraction and Raman measurements on the same samples give independent confirmation of their chemical and structural purity: together, the results are therefore considered as providing definitive measurements of the optical band-gap energies of the two materials.

A comparison of S-passivation of III–V (001) surfaces using (NH4)2Sx and S2Cl2

Abstract Soft X-ray photoelectron spectroscopy has been applied to compare the effect of two S-passivating etchants (S 2 Cl 2 and (NH 4 ) 2 S x ) on the (001) surfaces of GaAs and InP. Detailed analysis of substrate and adsorbate core level emission peaks reveal the similar nature of S III V bonding in each case. Prior to in-situ annealing, the semiconductor surface is covered by an amorphous layer, and above around 400°C, a stable S-terminated surface is obtained in each case. Variations in the relative intensities of spectral features are more sensitive to process and temperature differences than to the etchants used. For S-terminated GaAs and InP surfaces, a high binding energy component is observed in the substrate Ga 3d and In 4d core level emission spectra corresponding to surface S Ga(In) bonding. The S 2p core level spectra contain two components related to the surface and sub-surface S atoms. As S bonding on the annealed GaAs S surface is not present above 400°C.

Synchrotron radiation studies of inorganic-organic semiconductor interfaces

Abstract Organic semiconductors (polymers and small molecules) are widely used in electronic and optoelectronic technologies. Many devices are based on multilayer structures where interfaces play a central role in device performance and where inorganic semiconductor models are inadequate. Synchrotron radiation techniques such as photoelectron spectroscopy (PES), near-edge X-ray absorption fine structure (NEXAFS) and X-ray standing wave spectroscopy (XSW) provide a powerful means of probing the structural, electronic and chemical properties of these interfaces. The surface-specificity of these techniques allows key properties to be monitored as the heterostructure is fabricated. This methodology has been directed at the growth of hybrid organic–inorganic semiconductor interfaces involving copper phthalocyanine as the model organic material and InSb and GaAs as the model inorganic semiconductor substrates. Core level PES has revealed that these interfaces are abrupt and chemically inert due to the weak bonding between the molecules and the inorganic semiconductor. NEXAFS studies have shown that there is a preferred orientation of the molecules within the organic semiconductor layers. The valence band offsets for the heterojunctions have been directly measured using valence level PES and were found to be very different for copper phthalocyanine on InSb and GaAs (0.7 and −0.3 eV respectively) although an interface dipole is present in both cases.

Copper phthalocyanine on InSb(111)A?interface bonding, growth mode and energy band alignment

Copper phthalocyanine on InSb(111)A—interface bonding, growth mode and energy band alignment, D.A. Evans, H.J. Steiner, S. Evans, R. Middleton, T.S. Jones, S. Park, T.U. Kampen, D.R.T. Zahn, G. Cabailh and I.T. McGovern, J. Phys.: Condens. Matter, 15, S2729–S2740, (2003)

GaN Cleaning by Ga Deposition, Reduction and Re‐Evaporation: An SXPS Study

The Ga deposition, reduction, and re-evaporation technique commonly used to produce clean n-GaN surfaces and Ag–GaN interface formation on the resultant surface, have been investigated by Soft X-ray Photoelectron Spectroscopy (SXPS) and current–voltage measurements. SXPS studies have indicated that Ga deposition produces a band-bending of ΔEk = + 1.0 eV to higher kinetic energy. Our results show this shift to be a partially reversible process: re-evaporation of the deposited Ga resulted in a Fermi shift of ΔEk = — 0.6 eV to lower energy. Ag deposition did not cause any further Fermi shift, indicating that the Fermi level is pinned (2.2 ± 0.2) eV above the valence band edge, possibly as a consequence of the cleaning procedure itself. Current voltage (I–V) measurements have shown a barrier height of 0.77 eV and an ideality factor of 1.6. Metal induced gap states and the unified defect model are discussed as possible barrier formation mechanisms.

Interface chemistry and band bending induced by Pt deposition onto GaP(110)

Abstract The Pt/GaP(110) interface has been studied by core and valence level photoemission using synchrotron radiation. The results are characteristic of a reactive interface, where the GaP substrate is disrupted by the deposited platinum layer resulting in an increasing reacted Ga emission and a strong attenuation of the substrate Ga emission. The detailed analysis of band bending shows that Pt, being a high work-function material, nevertheless has a final pinning position which is close to that of other materials. The value for the Schottky barrier inferred from the photoemission data, Φ b n is 1.56 eV. We observe a distortion of the equilibrium band arrangement by the incident photons, giving rise to a surface photovoltage even at room temperature. This effect can strongly influence the determination of surface band bending.

Electronic properties of interfaces between perylene derivatives and GaAs(001)surfaces

The adsorption of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,-dimethyl-3,4,9,10-perylenetetracarboxylic diimide (DiMe-PTCDI) on differently treated n-doped GaAs(100) surfaces was investigated using high-resolution photoemission spectroscopy. The chemical interaction between the molecules and the semiconductor substrate is found to be weak; core level photoemission spectra show no additional chemically shifted peaks, indicating the absence of any covalent/ionic bond formation. Only a sharpening of the core level spectra is observed for a coverage lower than one monolayer and this is attributed to a reduction of inhomogeneous band bending at the surface. This is interpreted in terms of preferential sticking of the organic molecules to surface defects. The energy offset between the occupied states in the substrate and the organic film is directly derived from ultraviolet photoemission spectroscopy measurements. Interface dipoles are found to form according to the electron affinities of the substrates and PTCDA films at the interfaces and, consequently, the vacuum level alignment rule does not hold. For vanishing interface dipole the lowest unoccupied molecular orbital of PTCDA is found to align with the conduction band minimum of GaAs resulting in electron affinity of 4.12 eV for PTCDA. This provides an energy gap in the range of 2.44–2.55 eV, which is larger than the onset of optical absorption. The same procedure is applied to DiMe-PTCDI layers.

Direct observation of Schottky to Ohmic transition in Al-diamond contacts using real-time photoelectron spectroscopy

Evans D A, Roberts O R, Vearey-Roberts A R, Langstaff D P, Twitchen D J and Schwitters M 2007 Direct observation of Schottky to ohmic transition in Al-diamond contacts using realtime photoelectron spectroscopy Appl. Phys. Lett. 91 132114 doi:10.1063/1.2790779

Zincblende–CdSe on GaSb(110): Characterization of epitaxial growth and electronic structure

Substrate‐stabilized pseudomorphic growth offers the chance to study the electronic structure of a particular semiconductor in different crystal structures, and to investigate the influence of structural differences on bulk and surface states. We have grown layers of CdSe in the zincblende modification on cleaved GaSb(110) surfaces by molecular beam epitaxy. The growth mode and structure of the overlayer were studied by means of low energy electron diffraction and photoemission using synchrotron radiation. The attenuation of substrate core level intensities with CdSe deposition indicates layerwise growth. Interface reaction leads to the liberation of Sb, which floats on the growth front, and the formation of a Ga–Se compound, as signaled by changes in substrate and overlayer core level line shape. The valence band offset for this lattice‐matched interface system is 1.09 eV, such that the heterojunction is of the staggered type, in agreement with predictions based on the dielectric midgap energy model.